APPROPRIATE TECHNOLOGIES FOR AN EMANCIPATING COOPERATION: THE DEVELOPMENT OF AN EFFECTIVE IMPROVED COOK-STOVE
APPROPRIATE TECHNOLOGIES
FOR AN EMANCIPATING COOPERATION:
THE DEVELOPMENT OF AN EFFECTIVE
IMPROVED COOK-STOVE
Simone Pietro Parmigiani
Simone Pietro Parmigiani
UNIVERSITÀ DEGLI STUDI DI BRESCIA FACOLTÀ DI INGEGNERIA Dottorato di Ricerca in Tecnologie e Sistemi Energetici per l'Industria Meccanica Settore Scientifico Disciplinare: ING-IND/10 XXIV Ciclo
Appropriate technologies for an emancipating cooperation: the development of an effective improved cook-stove
Dottorando: Simone Pietro Parmigiani Relatore: Prof. Adriano Maria Lezzi Coordinatore di Dottorato: Prof. Gian Paolo Beretta
Anno Accademico 2010-2011
INDEX ITALIAN SUMMARY PREFACE INTRODUCTION Part 1 BIRD-EYE OVERVIEW.................................................................1 Chapter 1: ENERGY OUTLOOK.................................... ..2 1.1 Global scale...........................................................2 1.2 Less technologically developed countries.......... ..4 1.3 Heat, electricity & mechanical energy..................8 1.4 Possible primary energy mix.............................. 10 Chapter 2: STRATEGIC REQUIREMENTS FOR A SUSTAINABLE DEVELOPMENT..................................12 2.1 Resources deployment........................................ 12 2.2 Other environmental impacts..............................15 2.3 Social & economic impact..................................17 Chapter 3: THE ROLE OF APPROPRIATE TECHNOLOGIES: EMANCIPATION........................... 18 3.1 Self-sustainability............................................... 19 3.2 Local acceptability.............................................. 19 3.3 Low-power & efficiency.....................................20 3.4 Easy is not necessarily simple............................ 21
Part 2. DOWN-TO-EARTH...................................................................... 23 Chapter 1: BACKGROUND............................................. 23 1.1 Each setting has it's own needs...........................26 1.2 Vertical approach & horizontal approach........... 27 1.3 Project needs in Chad......................................... 28 1.3.1 Context & issues.................................. 28 1.3.2 'Centrafricain stove' dissemination...... 33 1.4 Further issues...................................................... 34 1.4.1 Safety................................................... 34 1.4.2 The need of a chimney......................... 35 1.4.3 Alternative fuels...................................36 1.4.4 Rice husk..............................................37 1.5 Effectiveness & emancipation............................ 39 1.6 Technologies....................................................... 40 1.6.1 Heat from biomass gasification........... 40 1.6.2 Electricity & mechanical power with Stirling engines............................ 42 Chapter 2: EFFECTIVE IMPROVED COOK-STOVE DEVELOPMENT...............................................................45 2.1 State of the art.....................................................45 2.2 Early stages.........................................................50 2.2.1 Rice husk gasifier.................................50 2.2.2 Crude earth cook-stove........................ 53 2.3 Rice husk burner................................................. 56 2.3.1 Materials & construction..................... 56 2.3.2 Water Boiling Test............................... 60 2.3.3 Temperature profile..............................62 2.3.4 Chimney emissions and draft...............62 2.3.5 Indoor air contamination......................64 2.3.6 Ashes & bio-char..................................64
Chapter 3: THE ROAD TO 'MY CHUBBY COOKSTOVE & MY LITTLE COOKSTOVE'........................................... 66 Chapter 4: RESULTS.........................................................117 4.1 Affordability, reliability & safety........................117 4.2 Efficiency............................................................ 119 4.3 Emissions............................................................123
CONCLUSIONS.............................................................................125 Impact & Dissemination.......................................................125 Alliances & Networks.......................................................... 126 The Lack of Development Tools.......................................... 127
ANNEX 1: STIRLING ENGINES................................................129
PUBLICATIONS, CONFERENCES AND SEMINARS............ 143
ACKNOWLEDGEMENTS...........................................................145
REFERENCES............................................................................... 147
ITALIAN SUMMARY Per la cottura dei cibi, circa un terzo della popolazione mondiale non ha alternative all’uso di legna o carbone. I problemi più frequentemente connessi sono il rapido esaurirsi della risorsa e la scarsa efficacia delle tecnologie utilizzate per il suo sfruttamento, spesso solo tre pietre per sorreggere la pentola sotto la quale viene acceso il fuoco. Con l’espressione ‘scarsa efficacia’ si include sia l’efficienza termica del processo, ma anche la praticità d’uso e soprattutto l’impatto sulle persone e sull’ambiente. Infatti, l’inquinamento dell’aria domestica è una delle prime cause di mortalità nei paesi in via di sviluppo e si stima che causi circa 1,5-2 milioni di decessi all’anno, mentre problematiche come la deforestazione e la desertificazione sono spesso legate a consumi di legna non sostenibili. In questa tesi, dopo una attenta introduzione alle problematiche legate alla necessità di sviluppare tecnologie appropriate per uno sviluppo sostenibile, sono elencati i vari stadi della sperimentazione di un nuovo disegno di stufa-da-cucina che ha come scopo il recupero di biomasse alternative. Ad esempio, i gusci dei chicchi di riso, chiamati lolla, sono uno scarto agricolo molto diffuso in molti paesi a risorse limitate, ma difficilmente sfruttabile con sistemi tradizionali. Questa ricerca è quindi volta all’individuazione e allo sviluppo di tecnologie atte alla valorizzazione di questa biomassa di scarto. Il processo di combustione/gassificazione che permette lo sfruttamento energetico della lolla di riso è ottenuto con una semplice stufa realizzata con mattoni in terra cruda. La struttura cilindrica é completata da un contenitore interno per il combustibile, realizzato con rete metallica, e da un camino, il cui tiraggio permette sia il processo di gassificazione che l’evacuazione dei fumi dall’ambiente
domestico. Affidabilità e buone prestazioni della stufa sono state ottenute con un percorso di ricerca, incentrato sullo studio dell'efficienza e delle emissioni, che hanno permesso di meglio comprendere i principi di funzionamento del sistema energetico. Lo studio è completato con un’analisi degli effetti del tiraggio, vero motore di questo sistema a lolla. Inoltre, con l'ultima configurazione presentata, si sono ottenuti ottimi dati preliminari per quanto riguarda l'efficienza e le emissioni, garantendo un minor impatto sull’ambiente e sulla salute umana.
PREFACE This research was begun developing a system where gasification of biomass was used to heat an external combustion engine. The project, among the winners of the Rotary Enfasi Award 2009, aimed to develop new simple technologies to address the energy-access challenge in developing countries. Much time has been initially dedicated to the modeling, sizing and the prototype building of a Ringbom-Stirling engine for electrical energy generation. Bringing such technology to a reliable configuration requires quite an effort, both in terms of time and money. To heat the Stirling engine, a gasifying system was also studied and developed. Initially, its use as a cook-stove, was considered a secondary service for the energy system, but investigating deeper the argument of energy-access in developing countries, it has become clear that technologies related to cooking were much more urgently needed. This situation has arised because cooking matters are typically left to the single household choices, not being targeted by governmental policies. The focus of the research has then been shifted mostly on the cookstove, while the development of the Stirling engine has been left momentarily aside, once proper performances for heat generation are assured by the gasifying system.
INTRODUCTION A large part of the international cooperation community is focusing its attention on the targets set by the Millennium Development Goals, initially established in 2000 by the United Nations. Those statements and targets are easily sharable, but there's also some debate and criticism. The goals are well-structured and they set targets that are addressing a large part of the most urgent needs in developing countries, but are missing out on some fundamental aspects. Goal 1 Goal 2 Goal 3 Goal 4 Goal 5 Goal 6 Goal 7 Goal 8
Eradicate extreme poverty and hunger Achieve universal primary education Promote gender equality and empower women Reduce child mortality rates Improve maternal health Combat HIV/AIDS, malaria, and other diseases Ensure environmental sustainability Develop a global partnership for development
The titles of the 8 Millennium Development Goals One of the main criticisms is that they are overly-ambitious statements, not addressing adequately how to fulfill the targets that are so accurately set within each goal. There is no specific MDG focusing on energy and also in the detailed targets, set by each goal, energy is neglected, but it is clear that energy access is crucial to the achievement of all the goals. In September 2010, Ban Ki Moon, United Nations Secretary General said: “Universal energy access is a key priority on the global development agenda. It is a foundation to all MDGs.� The UN Advisory Group on Energy and Climate Change has called for adoption of the goal of universal access to modern energy services by 2030. Nevertheless, the efforts in delivering the prime energy service, 'clean cooking technologies', are very low and in the meantime many MDG targets may be missed just for this reason.
Part 1. BIRD-EYE OVERVIEW
The scope of this thesis is the development of emancipating technologies to address the energy-access challenge in low-income countries. Worldwide the basic energy needs are for electricity and heat, used mainly for cooking and heating water; for this last scope biomass is the only available and reliable source for almost half of the Earth's population. Three-stone fires are still largely used and the research, design and dissemination of healthier and more efficient cook-stoves is key to an effective cooperation. Early research on this topic dates in the 50's but diffuse awareness of this problem is quite recent. Something might be changing although, for example, the Global Alliance for Clean Cookstoves was launched in 2010 at the Clinton Global Initiative forum. Led by the UN Foundation, the Alliance is a public-private partnership of more than 60 national governments, UN agencies, private companies and non-governmental organizations.
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Chapter 1: ENERGY OUTLOOK 1.1 Global scale Disparity in energy consumption between industrialized and developing countries is enormous, the World's Total Primary Energy Supply (TPES) for 2008 was 12267 Mtoe. OECD countries (Organisation for Economic Co-operation and Development) account for 5422 Mtoe or 44% of the total, but are made up by only 1190 million people or 18% of the Earth's total population. Excluding those countries, the former Soviet Union and the Middle-East, the remaining part of Asia, Latin America and Africa, accounting for 4962 million people (74% of the Earth's total population), uses only 4771 Mtoe (39% of the World's TPES). Africa alone accounts for 984 million people (15%) and uses only 655 Mtoe (5% of World's TPES). Data for TPES/population highlights this difference very clearly. As can be seen in Figure 1 most of the Earth's population lives below the World's average, while just a little share is living on much higher standards with respect to this average. If all the Earth was living on the same level as OECD countries, the total TPES would be over the unrealistic number of 30000 Mtoe. (source for all the above data is IEA, 2010a) 2.7 billion people worldwide, 40% of the Earth's total population, rely on solid fuels to fulfill their cooking needs and a large part of those people use this resource with three-stone fires or other inefficient traditional stoves. This problem is shooting up, due to many simultaneous causes. Population growth is the first reason, but also recession and the rising cost of modern liquid fuels are bringing many household back to using biomass as a 'cheaper' alternative (IEA, 2010b). As indicated in Figure 3 most of the people relying on the traditional use of biomass live in Sub-Saharan Africa or Asia. 2
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1.5 billion people worldwide, 22% of the Earth's total population, lack access to electricity. Electrification is a priority on most agendas for developing countries, but often the low-reliability and low-power don't allow to solve many of the critical issues that need to be addressed. 5 4.5
OECD; 1190
4 FORMER SOVIET UNION; 285
TPES/Pop (toe/capita)
3.5 3
MIDDLE EAST; 199
2.5 NON-OECD EUROPE; 53
2
CHINA; 1333
1.5 1
LATIN AMERICA; 462
WORLD; 6688
AFRICA; 984
ASIA; 2183
0.5
Population (million)
0
Figure 1. Total primary energy supply per capita in toe/capita, bubbles are proportional to the relevant population (in millions). In the box, OECD member countries. Founding states are marked with dark blue. Data source: IEA, 2010a. 6000
TPES (Mtoe)
5000 4000 3000 2000 1000 0 OECD
MIDDLE EAST
FORMER SOVIET UNION
NON-OECD EUROPE
CHINA
ASIA
LATIN AMERICA
AFRICA
Figure 2. Total Primary Energy Supply by regions. Data source: IEA, 2010a. 3
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It's quite interesting that a significant share of the Earth's population have some kind of access to electricity, but still must rely on biomass for cooking. Population growth in developing countries will hence require more solid fuels for primary needs, as feasible alternatives are not foreseeable in the near future. Energy scenarios to be faced in the future may be dramatic, if energy development does not change it's approach. Bridging this energy gap, possibly with alternatives to conventional fossil fuels, is necessary to avoid wars over resources. Developing countries can not follow the same path of industrialized countries, they must leapfrog to cleaner technologies and alternative fuels.
Figure 3. Number and share of population relying on the traditional use of biomass for cooking (IEA, 2010b)
1.2 Less technologically developed countries Of all worldwide biomass users, only 27% cook on improved and efficient stoves, but figures are even worst if the focus is set on SubSaharan Africa, where this percentage drops to 6% (Legros, 2009). 4
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Meeting the Millennium Development Goals would require a huge effort on giving access to modern liquid fuels for cooking or disseminating improved biomass-fueled technologies to several million households. As it is clear from Figure 4 the actual trend is highly not-compatible with MDG targets. In particular data for South Asia show that in this area the problem is quickly getting worse. The figure referring to the electricity access (Fig. 5) is less alarming, except for Sub-Saharan Africa, where very little has been made. In Sub-Saharan Africa (including South Africa) the electrification rate is just 31% and 80% of the population still cooks on three-stone fires or traditional stoves (IEA, 2010b). Those families use up to 2 tons/year of wood for cooking due to those rudimentary or inefficient stoves.
Figure 4. Progress toward MDG compatible targets, Percentage of people without access to modern fuels for cooking. Source: PRACTICAL ACTION, 2010. It is quite illuminating that where governments have seriously implemented development policies, significant results have been obtained. Electrification involves big investments and it is mostly a government-led process, while households usually make decisions 5
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about their own cook-stove; policies often keep themselves out of this issue. Few countries have set targets on eradicating traditional use of biomass and hence very little progress in this field has been achieved. It must be outlined how the traditional use of biomass is not criticized just because of the use of biomass itself, but because it implies a poor combustion with poor performances for the required task.
Figure 5. Progress toward MDG compatible targets, Percentage of people without access to electricity. Source: PRACTICAL ACTION, 2010. Table 1 shows the total data for the lack of access to electricity and the traditional use of biomass for cooking. The problem is mainly rooted in Sub-Saharan Africa and in Developing Asia, as these two areas sum up to almost the global data. Population density in those regions can be quite high and ensuring modern energy access to all those persons is a big challenge.
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NUMBER OF PEOPLE RELYING ON THE TRADITIONAL USE OF BIOMASS FOR COOKING
Developing countries*
587 585 799 8 404 387 31 1438
657 653 1937 423 855 659 85 2679
World**
1441
2679
Africa Sub-Saharan Africa Developing Asia China India Other Asia Latin America
*Includes Middle East countries. **Includes OECD and transition economies.
Table 1. Number of people without access to electricity and relying on the traditional use of biomass, 2009 (million). Source: IEA, 2010b. The Universal Modern Energy Access Case (UMEAC) developed by the IEA calculates what is necessary for universal access to modern energy services by 2030 and interprets the MDGs targets on these issues as meaning that no more than one billion people should be without access to electricity by 2015 and no more than 1.7 billion should still be using traditional biomass for cooking on open fires or primitive stoves. Investments needed to follow this strategy would be way lower than those connected with fossil fuel subsidies to developing countries (IEA, 2010b). Efforts are shown in Table 2. 2015 Rural
2030 Urban
Rural
Urban
100% access to the grid
100% access, of which 30% connected to the grid and 70% either mini-grid of off-grid
100% access to the grid
Access to electricity
Provide 257 million people with electricity access
Access to Clean Cooking Facilities
Provide 800 million 100% access to LPG people with access to Provide 200 million stoves (30%), biogas LPG stoves (30%), people with access to systems (15%), or biogas systems (15%) LPG stoves advanced biomass or advenced biomass cookstoves (55%) cookstoves (55%)
100% access to LPG stoves
Table 2. Targets for the UMEAC. Source: IEA, 2010b. 7
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1.3 Heat, electricity & mechanical energy Basic needs for human living that require energy could be synthetically summarized as (PRACTICAL ACTION, 2010): • • • • • • •
lighting cooking water heating space heating cooling information and communication earning a living
The last entry accounts for all the power (mainly mechanical) necessary for all energy-dependent activities such as processing, running workshops or other small appliances. But other energy is also required by many services as water sanitation and health-care, for example; not considering at all transports or water pumping-andsupply or irrigating, primarily because those tasks mostly depend on other instances, but still, when achieved, are energy-demanding. Many energy services overlap in terms of source, technology and usage. The first example for this is the traditional use of biomass: 'open fires'. With a single fuel and almost no technology, they provide energy for cooking, lighting, space heating and also have the secondary use of 'social gathering point'. It must be underlined that all those services are very poorly delivered by an 'open fire', but still need to be replaced when a new technology is introduced because modern stoves, otherwise highly desirable, mostly lack these 'auxiliary' services.
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Electrification programs are developing quite fast and could provide the necessary power to address some of the cited energy demands. Even if many grids are very unreliable and available power is often insufficient, electric energy can be very useful to providing lighting, cooling and access to information and communication devices. Growing needs of developing countries must be met using renewable energy as much as possible, good practices are key to sustainability. Efforts are being made to implement 'clean' technologies in grid, minigrid and off-grid solutions in many electrification programs. In particular off-grid solutions in rural contexts often use locally available renewable sources. Most of those plants are mini-hydro, mini-wind, or photovoltaics. All these technologies have the advantage of having no fuel cost, but they have high upfront cost and hence their success is often highly dependent on the role of governmental policy. Cooking, water and space heating, although, will not be able to rely on electricity, because of their high energy consumption. Also access to mechanical power suffers from the same difficulties and is often totally disregarded, while being of crucial importance for any manufacturing activity. As it is clear from Table 3 if electricity won't meet those needs, very little else is being done. Just biomass is left as the only available and reliable resource to survive, but biomass is a bizarre renewable. It can actually be used in a renewable manner and a quite 'clean' combustion can be achieved, but it definitely is not a renewable or a 'clean' source if deforestation problems arise and smokes from incomplete combustion stagnate in living environments, so technology and specific-research, in particular, can play a significant role in enabling these practices.
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Electricity Modern fuels Improved cook-stoves Mechanical power
Developing countries (total) 68 17 11 5
Of which: Sub-Saharan Africa 35 13 7 5
Table 3. Number of developing countries with energy-access targets Source: WORLD BANK, 2011. Gasification processes, beside being key to a 'clean'1 combustion of any solid fuel, allow the deployment of new kind of biomasses, previously disregarded as energy resources. Many agricultural residues are useless in open fires, but could be gasified and used to generate heat for cooking or other purposes. Gasification technologies could also be very useful for the development of biomass powered internal or external combustion engines that could be implemented as stand alone systems to deliver mechanical energy or to be used as electric generators. In some contexts this could represent a more convenient solution to conventional renewable systems.
1.4 Possible primary energy mix In most industrialized countries the economy driver has been cheap energy from fossil fuels and today the discovery of a new oil reserve might often have more market-impact than a great technological innovation. It's a matter of 'inertia'. We must learn from this and with such a fast growing population, primary energy needs in developing countries must be addressed with a different approach. Technology innovation can lead to a different primary energy mix, allowing the economic system to develop more sustainably with respect to the ecosystem. 10
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Biomass and solid fuels are the only affordable energy resources for the poorest households and this will probably be true for a few more decades (IEA, 2009). LPG and other 'cleaner' cooking fuels become available only with higher income-levels, which are not extensively expected for large part of the Earth's population. Anyway, as income rises, electrification rate rises, but often modern fuel accessibility for cooking does not, depending mainly on governmental policies and logistic issues. On the other side, electrification programs alone will not solve the problem of cooking fuels, electricity is almost always too expensive or scarce to be used for cooking purposes. As a consequence, issues such as resource deployment or indoor air pollution will not be addressed. These last issues alone, call for the necessity of developing more efficient and effective technologies for all the households that will continue to rely on biomasses for primary needs. Allowing the use of alternative biomasses in an efficient and 'clean' burning cookstove is a way to face the problem of absolute scarcity and relative scarcity. The goal is to find a substitution for what is no longer available. A more efficient technology is useful because it lessens the fuel consumption, but reducing is often not enough and a new, 'cleaner', resource, a substitution to what has been traditionally used, can bring a great relieve on a stressed environment.
1
The terms 'clean' and 'cleaner' are used in a broad sense. Mainly considering the user's perspective, rather than absolute emissions.
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Chapter 2:STRATEGIC REQUIREMENTS FOR A SUSTAINABLE DEVELOPMENT 2.1 Resources deployment Insufficient and inappropriate energy-access in overcrowded contexts often leads to very high harvesting levels of biomass, especially wood. This mainly happens around towns and cities or when communities are displaced because of wars or diseases or again in regions heavily affected by climate change. The surroundings of many African settlements have no more trees, due to intensive collection of wood, often for charcoal production, a very common practice to allow an easier transport of the fuel from rural to urban areas. This kind of resource management can transform biomass in a non-renewable and highly non-sustainable resource. Deforestation arising from these practices is one of the greatest environmental problems in very poor regions. It's a vicious circle, poor people will use cheap wood for cooking, but this wood-collection is among primary causes of deforestation, which can lead to a lower quantity of available land for food production or, even worse, to desertification (REPUBBLIQUE DU TCHAD, 2004). In return, resources are less and less and people might be displaced as a consequence. The collection of biomass in rural areas is some times more sustainable (Fig. 6), as people collect 'dead' wood from the trees, possibly not cutting the live plant, in order to conserve it for the future, but this can happen only where resources are sufficient for the population. Chances are that such availability might be more and more rare. Very large cities have grown and are still growing (Fig. 7) in many developing countries, representing a big challenge for energy-access. Logistically, modern liquid fuels as LPG are the only possible solution 12
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for these settlements, but still affordability-issues for these kind of fuels pose a big question mark on how people will cook a hot meal at least once a day. Local availability is key to a sustainable use of any resource, and this will have to be achieved mostly without the use of fossil fuels, which in history have been very useful as a mean of postponing the problem. Many kind of agricultural residues could represent a useful energy alternative. Traditionally many of them are not used for energy generation in developing countries, but are locally available.
Figure 6. A rural settlement in southern Chad. Image: Francesco Vitali. 13
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Figure 7. Image of the fast growing suburb of Dhaka, Bangladesh. Source: http://www.skyscrapercity.com/showthread.php? t=602088&page=5 For example, rice husk is one of the most largely available residues worldwide (ParĂŠ, 2011), but it's size and physical properties are responsible for a very poor combustion, hence requiring special technologies for it's deployment. In addition, it's low density and low heating value don't allow a convenient transport of this resource, but it could still represent an excellent ingredient for a 'good' primary energy mix. Its use in rural context could be massive in rice producing regions, a few in Africa but many in China, India and South-Eastern Asia. 14
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2.2 Other environmental impacts Resource deployment is not the only problem that arises from this excessive use of wood. Technology research has to focus on efficiency to lower specific consumption, but also emissions need to be held down to contain environmental pollution. The use of traditional stoves has a relevant effect on air quality especially because of black carbon (soot) emitted with the typically incomplete combustion that takes place in a three-stone fire. Biomass use can be CO 2 neutral if burned trees or crops are planted again, but otherwise the global impact can be high, because of the large number of households involved. If we accept that climate change is influenced by human activity, we must anyway acknowledge that the poorest are not among the main contributors, but surely are the ones who suffer more from its most extreme consequences. Another fundamental issue is indoor air pollution (Emmelin, 2007). Only Âź of households that rely on biomass for cooking use a chimney or a smoke hood. The first step to be taken is to introduce ventilation and to improve the kitchen layout. This could be just a matter of habits, but other instances are some times more relevant. Traditional huts, where three-stone fires where originally lit, used to have strawroofs, that allowed the smoke to slowly leave the living environment. 'Cheaper & modern' metal sheets (Fig. 7) are now much more desirable, but they increase stagnation, bringing along a much higher exposure to harmful smokes especially for women and their children. Also candles and low efficiency lanterns emit smoke, but the rate is quite low with comparison to stoves, the worse lanterns being comparable with the best stoves (PRACTICAL ACTION, 2010). Annually between 1.5 and 2 millions deaths (more than 4000 per day) are in relation with indoor air pollution due to cooking fuels (IEA, 15
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2010b; WHO and UNDP, 2009), but there is also evidence for many other respiratory illnesses and eye diseases, like cataract for example, that are directly related to this issue (WHO, 2002).
Figure 8. Premature annual deaths from household air pollution and other diseases. Source: IEA, 2010b.
Figure 9. Premature annual deaths from household air pollution and other diseases in 2004. Source: WHO (2006). 16
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The problem is largely diffused, and kills as much as HIV/AIDS, tuberculosis or malaria, but it seems to be kept in much less consideration, maybe because it's locally constrained. Viruses and other diseases can be much more frightening, because of their infective characteristics and it is clear, comparing data in Figure 8 and Figure 9, that the issue of indoor air pollution is not being addressed adequately.
2.3 Social & economic impact In a social perspective, cooking fuels are for the poorest households a big cost in terms of money or fatigue. Wood collection (Fig. 10) is a strenuous work that can take hours and comes with many hazards, included human assault. Energy poverty can be addressed promoting a more efficient use of traditional fuels, but it could prove even more effective to introduce the use of locally available alternative fuels. Usually new technologies are needed to achieve this, but they must be acceptable by the population and affordable in terms of upfront cost and use. The ultimate goal should be not to need much financial support or strong policies to disseminate the new technology. It should just fit the context. Possibly this should also create local opportunities like enterprises to build, disseminate and maintain the technology, besides eventually developing a market for the new fuels. During dissemination projects, local populations should be trained in doing this, but the kind of implemented technology must allow it. Where possible this should be kept in high consideration as early as at the design stage, considering it as a constraint.
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Chapter 3:THE ROLE OF APPROPRIATE TECHNOLOGIES: EMANCIPATION Cooperation projects and the relevant research are often not successful. This is true because of several reasons. There are many variables that can not be controlled, but, among barriers and drivers of cooperation, some are more fundamental than others. Neo-colonial arrogance in teaching, for example, is a big barrier, while the aim of development should be to enable people to have choices. Emancipation arising from a new technology can hence be an important driver.
Figure 10. Daily wood-collection can require long walks.
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3.1 Self-sustainability The first concern of cooperation should be not to create dependencies, otherwise many efforts could be vane. Dependencies can be of many kinds, economical or logistical, social or cultural. Local populations should not feel prostration because they depend on others for resources or technology. To achieve this, some kind of compromise needs to be accepted. For example, leapfrogging to 'clean' technologies is desirable, but the jump has not to land too far from local habits. This can prove to be almost impossible some times, but it's important to grab this opportunity when it's at reach. Many cooperation projects have failed because the implemented technologies where not self-sustainable in some perspective. Projects are often limited in time, while some kind of follow-up is almost always necessary. The local population must then be made familiar with the newly introduced systems. When a cooperation project ends, the users are left with the technology, but they must 'own' it, they have to understand it and therefore be able to build it, maintain it, repair it and possibly improve it.
3.2 Local acceptability Usually donors make the decisions regarding the more urgent needs and relevant projects, but local people should also be considered as a part of the decision making group. This is a base-requirement for a project's success. Technology must be regarded as a need by the local population, especially the end-users. The call for a new or different service is fundamental for success in delivering new systems and the community has to be actively involved at many stages in order to reach the ultimate goal of a project. This kind of driver can also be rather important to address those problems that are not felt as urgent by the population, but still are. An example for this is indoor smoke, 19
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often disregarded as a critical problem. It can be rather difficult sometimes to introduce a chimney or a smoke-hood by itself, but if a new system that allows to exploit cheap or eventually no-cost fuel, needs a chimney, this could be more easily accepted. For the same reason a balance between affordability and desirability must be found. High-end design is not necessary, but still aesthetics, besides functionality, must be considered.
3.3 Low-power & efficiency Emancipating development must follow a smart path and should educate to low power use and to avoid wastefulness, promoting efficient solutions also for small and diffuse needs. An example for this is lighting. When lighting is provided by electricity, energy-saving technologies as LEDs or fluorescent lamps should be promoted, saving a lot of energy with respect to other available solutions. Often, the problem it's not about having much energy, but about the services you can obtain from that energy. This kind of approach can be very useful in scaling up new solutions to allow more and more people to benefit from it. On the other hand, an important problem could also arise. If a new technology is more efficient and energy is more available, this might bring much more consumptions. The new system would not then be slowing down the degradation of the environment, hence it might not be so effective. This brings up the problem of effectiveness vs. efficiency. Efficiency is almost always a primary interest in research, but especially in this case the focus should be more on effectiveness and on fitting the context. The maximum efficiency with these fixed constraints is the one that should be sought for. Energy-access is urgent and 'clean' technologies are needed, but still who is on the path of development must evolve gradually and cooperation should not just deliver few contenting comforts, but should show how to move forward. 20
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3.4 Easy is not necessarily simple Appropriate technologies must be easy to use, but this does not necessarily mean that only simple systems can fulfill the task. A typical example for this are cell phones, which are quite popular among many poor communities. Phones allow to keep contacts with relatives living far away and this is, of course, highly requested. Also computers represent a good example as they are very useful as teaching means. A cell phone or a PC cannot be simple at all, but still they are easy to use and they have proved to be quite appropriate in some cases. These are clearly particular examples, which have been possible because of the low energy consumption required by those devices, but for other primary energy services the problem is slightly different. Easiness of use for a new technology can still derive from a rather sophisticated engineering, but for cooking purposes, for instance, technically intermediate solutions can be more appropriate than high-end ones, being able to reach a larger share of the population.
In the following part the development of an emancipating technology is reported according to the following structure: • • •
Background of cook-stove research and of the problem's setting Rice-husk burner prototype design and development Performance test results
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Part 2. DOWN-TO-EARTH Chapter 1: BACKGROUND Improved stoves research dates back to the 1950s, and it was focused mostly on efficiency. In the meantime many projects have been implemented but few of them succeeded. Cook-stoves can perform very well in a laboratory setting, but being not effective in the actual context. This was a fundamental lesson to be learned. Among many mistakes, a frequent error has been not to care about the follow-up programs of many short-term projects. Communities that were not provided with an adequate market development for the new technology have not been able to keep the new cook-stoves in proper conditions for operation. Another issue has often been the quick deterioration of new systems, due mostly to components and construction quality. The case of the Lorena Stove (Fig. 11) in Central and South America is emblematic. The Lorena stove is an enclosed stove of crude earth construction, with a chimney built onto it. The name Lorena comes from the combination of the two Spanish words lodo and arena (mud and sand), as the building material is basically a mix of the two. This bulky stove was designed with the mistaken belief that crude earth would act as insulation; there was a basic misunderstanding of the
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difference between insulation and mass. A higher thickness (and henceforth mass) leads to a higher thermal inertia, not a higher insulation. This problem and a poor quality in the duplicates, reproduced in loco, have led to many cases in which the new stove had higher fuel consumption, wood in this case, than traditional systems. It is also very important to ensure a proper reproduction process for a new stove model as well as a proper installation and maintenance. Governmental or Non-Governmental Organizations, that actually disseminate the new cook-stoves, are usually in charge for these actions. Nonetheless, also technology development can address part of these issues implicitly with appropriate design solutions, when possible. Besides these considerations, past programs show that in order to be effective and useful a stove needs to be durable, affordable and safe.
Figure 11. One of many versions of the Lorena Stove. There are many cook-stove models around, based on biomass-fueled systems, but also other alternative options must be considered.
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At this point it is necessary to introduce short definitions of different levels of technology related to cook-stoves (Fig. 12). •
Traditional cook-stove: is referred to rudimentary stove models as three-stone fires.
•
Improved cook-stove: is referred to models that (with respect to traditional cook-stoves) raise efficiency, remove or lessen the smoke and lower fuel consumption.
•
Advanced biomass cook-stove: is referred to more expensive models that guarantee some standard levels for security, efficiency, emissions and durability.
•
Effective improved cook-stove: is referred to models that are close to Advanced biomass cook-stoves in terms of performances, but that are cheaper and that can be manufactured locally.
The last entry could play a very significant role for those who are not expected to get access to modern cooking fuels in the next few years. In the optimistic event of meeting the targets set in Table 2, at least 1.5 billion people would still be using traditional cook-stoves in 2015 and this part of the population may probably be the one living in the poorest regions of the Earth. Obviously, governmental organizations will address the growing urban needs first, but still rural communities, with limited resources, are quite numerically important in several countries. There are no clear targets for new stove technologies, but a rather relevant guideline has been outlined in a 2011 US Department of Energy (DOE) meeting on biomass cook-stoves. Proposed reasonable 25
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targets are a 90% emission reduction and a 50% fuel savings (DOE, 2011). Recently the 'Lima Consensus', a document redacted at the 2011 “Partnership for Clean Indoor Air” (PCIA, 2011) conference in Lima, Peru, has outlined the need for creating a larger consensus around standards and testing. Part of the procedures have been set, but detailed and definitive agreed procedures are expected in the next year.
Figure 12. Clockwise, a three stone fire; an improved cook-stove; an advanced biomass cook-stove; an effective improved cook-stove.
1.1 Each setting has it's own needs To face this kind of problem the “one size fits all” approach is decidedly inappropriate. It is more realistic to accept that 1000 stove 26
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models are needed in order to reach the needs of 1000 contexts. This should not appear discouraging. It is just a direct consequence of bringing people's needs upfront in the list of priorities. With the expression “different setting� many variables are involved, but it is useful to point out some of them. There is a large number of cooking practices that have to be supported with different rates of heat-supply; there are different available and reliable fuels; there are also differences in cultures and markets for dissemination. Distinctions can also be appropriate in a household perspective and, for this reason, different levels of affordability might require different models, besides considering the option of having many cooking devices for different cooking purposes. (DOE, 2011)
1.2 Vertical approach & horizontal approach Flexibility in technology selection is needed in order to quickly deliver energy services. For this reason technical research should be guided by on-field research and by implementation programs. Some important constraints might arise from non-technical considerations. Taking them into account, as early as possible in the development process, can lead to a significant final benefit. Let's consider the so-called vertical and horizontal research approach. They are both needed and both are essential to each other in order to establish an iterative cycle of feedback and improvement, based on technology improvement, field validation and users acceptance. The vertical approach requires research to confront with actual demands, developing the technology to fit the context and address a specific need, while the horizontal approach allows research to seek for new options or general schemes, that could be subsequently adapted with a vertical approach. The vertical approach is complementary to the horizontal one for feedback information, in return of a wider and more effective variety of technologies to choose among. 27
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Sometimes it can be effective to clearly distinguish these two phases, even if often they are carried out by the same researcher. In the vertical approach a practical vision is more appropriate, there is a need to understand non-scientific issues and 'translate' them in technology appropriateness, while in the horizontal approach there are possibilities for technically high-level collaborations among Universities and/or Research institutions, promoting the use of more sophisticated technologies and to better investigate innovation opportunities.
1.3 Project needs in Chad The cue for this research has come from a previous project (20082011) of the Italian NGO ACRA in collaboration with CeTAmb, the University of Brescia “Research and Documentation Center for Appropriate Technologies for Environmental Management in Developing Countries�. This project was aimed to reducing the energy-access problem in Chad, especially in the capital city of N'Djamena (Vitali, 2009).
1.3.1 Context & issues Besides standing to benefit from its recently-acquired status as an oilexporting state, Chad is one of the poorest countries of the Earth and it is ranked as one with the highest corruption level. Hence, how the new revenues, deriving from the recently discovered fossil fuel reservoirs, will be spent is quite controversial and, in any case, the poorest households will not probably benefit much.
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Figure 13. The Logone Valley, south of N'Djamena, Chad. To better understand how low the level of opportunities is in this country two information are probably paradigmatic. Data for Chad's energy balance are not even listed in the 2010 Key Energy World Statistics from IEA, while all other neighboring states are. Moreover, Chad is ranked 2nd (after Somalia) in the 2011 Failed State List (FFP, 2011), issued each year by the Fund For Peace, which analyzes the following topics: Social indicators • • • •
Demographic pressures. Massive movement of refugees and internally displaced peoples. Legacy of vengeance-seeking group grievance. Chronic and sustained human flight.
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Economic indicators • •
Uneven economic development along group lines. Sharp and/or severe economic decline.
Political indicators • • • • • •
Criminalization and/or delegitimization of the state. Progressive deterioration of public services. Widespread violation of human rights. Security apparatus as "state within a state". Rise of factionalized elites. Intervention of other states or external factors.
In the “2011 Country Profile Series” Chad is so described: “Chad is currently threatened by regional and domestic instability. Rebel forces remain a destabilizing force in the country, though cross-border attacks between Sudanese and Chadian militias have decreased following a peace agreement between the two countries. Around half a million refugees and 'internally displaced persons' remain in the eastern region of Chad. The humanitarian crisis has been exacerbated by continued pressure on food and water supplies in the region. Although Chad’s oil revenues have the potential to contribute to poverty reduction, they are just as likely to be siphoned off by corrupt officials, perpetuating the Deby government’s illegitimacy and lack of accountability. The UN’s MINURCAT mission that was stationed in Chad (and the Central African Republic) withdrew from the country at the end of 2010, at the request of the Chadian government.” (FFP, 2011)
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Figure 14. Chad Soil Occupation. Source: REPUBBLIQUE DU TCHAD, 2004. In this very severe context (Fig. 14) the most urgent energy-access issue to address is cook-stove improvement and dissemination. Regardless of all the above cited state defaults or failures, people will look for at least one hot meal per day, but the fuel availability in this African region is very limited and problematic. In Chad the excessive deployment of the soil has led to deforestation and local desertification (REPUBBLIQUE DU TCHAD, 2004). This 31
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has happened because of many reasons, land has been cleared of its spontaneous vegetation (Fig. 15) for agricultural use or for burningwood gathering and charcoal production, besides other natural or more global causes. Locally, desertification advance-speed can be of some kilometers per year, hence, starting from January 2009, the Chadian government has imposed the total ban of wood and charcoal use in the capital city of N'Djamena (1 million people). Environment Minister Ali Souleiman Dabye said that more than 60% of Chad's natural tree cover had been lost because of the indiscriminate cutting of trees for charcoal production and that Chadians must be aware of this problem.
Figure 15. Landscape south of N'Djamena. Image: Francesco Vitali. As a result many households have been left without the possibility of cooking, since modern liquid fuels are not affordable by almost 80% of the urban population. Families have been forced to burn furniture, cow dung, rubbish and roots of plants in order to eat a hot meal. Charcoal and wood have become available on the black-market and prices had rose snappishly (BBC, 2009). 32
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1.3.2 'Centrafricain stove' dissemination Priorities must be well-handled in these delicate situations, different kinds of interventions are needed to achieve short-term and long-term targets. In order to address this urgent situation, CeTAmb has been involved in a project, with the Italian NGO ACRA, to lower fuel consumption as a short-term action to deliver immediate benefit. This has been done disseminating an existing and already widely used technology, the 'Centrafricain stove' (Fig. 16). This model of improved cook-stove is very simple and locally reproducible, giving good performances with 50% fuel saving in actual use with reference to three-stone fires (Vitali, 2010). However, this does not solve the problem, it is only slowing down the process. Nevertheless, it is granting some time to prepare more suited middle-to-long term interventions, bringing some immediate relieve in a highly problematic context.
Figure 16. End of training in ACRA's 'Centrafricain stove' dissemination project in Chad. 33
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Besides consumption, the 'Centrafricain stove', from a technological point of view, is not much more than a three-stone fire. Though it does ensure some heat insulation with a lateral metal and clay containment and it does provide some aeration with a vent grate underneath. This allows a better combustion and a better heat transfer, but still it does require wood as a fuel. The more complete combustion occurring in the 'Centrafricain stove' also lowers harmful emissions, but does not remove them from the cooking environment.
1.4 Further issues Long-term solutions should also address other issues that are related with the definition of an adequate energy-access. To achieve this, it is also necessary to develop different systems to face such diffused and varied energy-requirements. Therefore, the only sustainable path could be facilitating the access to LPG-stoves in the urban settlements and enabling the rural communities to use alternative biomasses as much as possible, but relying on efficient and 'clean' technologies (IEA, 2010b).
1.4.1 Safety A crucial topic in stove development is safety. This aspect is fundamental for a successful technology. Three-stone fires offer optional services, as lighting for example, but at the cost of safety. Not providing a biomass cook-stove with an enclosed combustion chamber can be very dangerous for users, especially children that can get in contact with the open fire. Heating is less influenced by this factor since some heat will be released in the cooking environment by almost
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any improved stove's structure. Other important safety issues are the temperature reached by the stove's external surface and, possibly, ensuring the absence of cutting edges, spikes or other dangerous elements. Tipping is another frequent accident in stove use and a stable structure is hence very important.
1.4.2 The need of a chimney Removing smoke from the living environment is essential because of the health benefits that are related to this issue (Smith-Sivertsen, 2009), but introducing chimneys can sometimes prove to be a quite difficult task. Many drawbacks can arise from this device, some are just based on user's habits and do not have a sound basis, others are more technically pertinent. For instance, food aroma might be different if the meal does not get “smoked�. However, this topic can be solved with proper information about the pro's and con's of using such technology. It is much more important to pay a careful attention to the real problems concerning the introduction of chimneys when there are no local habits about them. Dissemination projects need to point out that these devices demand proper installation, cleaning and maintenance. Positioning must be made carefully because of the particular atmospheric conditions. Rainstorms or wind should not disturb the functioning of the chimney, but also excessive direct sun can be a problem with metal chimneys. If the top of the metal duct is too hot at ignition, the chimney will not draft. Other very important safety issues are related with the flammable structures that chimneys need to cross. If only few huts get burnt because of 'the new cook-stoves and their chimneys', than many efforts in reducing indoor air pollution could be vane. After being well placed, a chimney also requires timely cleaning 35
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and it should be frequently checked for wears or leakages to be promptly repaired. These are basic concepts that are not obvious in many cultures. Therefore, specific efforts must be planned to emphasize these topics when chimneys are introduced for the first time.
1.4.3 Alternative fuels Intensive wood use can bring too much pressure on fragile environments, so it is important to stop using this resource when environmental degradation is evident, but still people must be left with some feasible option and not just bans. New fuels are needed to fill the gap between the three-stone and LPG users. Even in the most optimistic scenario, part of the urban population will still be using biomass and this market could better be targeted with processed fuel for logistical reasons. Possibly briquetting of alternative biomasses can be investigated, to substitute the production of charcoal from wood (Mazz첫, 2007). On the other side, rural contexts are more suitable to promote the use of alternative unprocessed fuel, as storing and distribution of much less-dense biomasses can prove easier. Gasification processes allow the use of many biomass resulting from agricultural residues. These potential fuels are otherwise lost for heat generation, unless proper technologies are developed and introduced. These biomasses include peanut shells, coconut and rice husk, corn stalks and stover, sugar cane bagasse, wheat chaff and many others. Also some dedicated cultures could be appropriate, but they should not be in competition with food-production on fertile soils; pigeon peas, flax, bamboo and switchgrass are examples of very drought resistant species that are particularly suited to fight desertification and that could represent an alternative energy source. (Roth, 2011) 36
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1.4.4 Rice husk The Logone valley (Fig. 13), south of N'Djamena, is a rice producing region in Chad at the border with Cameroon. Rice husk is highly abundant and it is not used (Fig. 17). Huge piles of husk are gathered during milling and they are subsequently set on fire just for disposing. Those piles burn very slowly producing no flame and filling the air with dense and dark smoke. It must really appear totally useless for energy services.
Figure 17. Available rice husk near Bongor, Chad. Following the CeTAmb-ACRA project some evaluations of possible alternatives have been formulated. At first, an attempt at briquetting this residuals was made to understand if densification could be useful for recovering them as a possible fuel (Mazz첫, 2010). This path has 37
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not been viable since rice husk is very elastic and flexible. It is also very light so it is quite difficult to grind it and so far the only way to obtain a briquette is the addition of high quantities (more than 50% in weight) of binding material. Investigation on the possible use of this unprocessed fuel in ad-hoc cook-stoves for the local rural communities was then started, as a collaboration between the Dept. of Mechanical Engineering and CeTAmb. The following investigations have been carried out in close collaboration with Francesco Vitali, a PhD candidate from CeTAmb, in charge of the project with the Italian NGO ACRA. Rice husk (Fig. 18) is a very particular fuel. It has a very low bulk density, variable between 73 kg/m3 for loose husk to 145 kg/m3 for vibrated husk. Average surface-volume ratio is around 4000 m2/m3, while the thermal conductivity value is around 0.036 W/m째C, comparable to that of many insulating materials (IRRI, 2011). This adds complications to heating up this biomass for gasification purposes.
Figure 18. A close view of rice husk. 38
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Characterization of this fuel has been carried out by IPC Golgi, a chemistry-based high-school in Brescia that is member of one of CeTAmb's collaboration projects. Heating Values for this biomass have been obtained by the students of the high-school, under the teacher's guidance, using Mahler's calorimetric bomb (Tab. 4).
Rice Husk
Higher Heating Value
Lower Heating Value
9.16 MJ/kg
8.37 MJ/kg
Table 4. Heating Values for rice husk (IPC Golgi). The values reported in the current literature are slightly higher, ranging between 12 and 14 MJ/kg for the LHV (Permchart, 2009; Chungsangunsit, 2010) but it is not always clear if the datum is referred to husk with or without the internal lining, that can be useful for animal feeding. Rice husk for energy production is typically without this thin 'oily' skin and Heating Values can be slightly lower for this reason. Nevertheless, for any further calculation the value of 12,54 MJ/kg has been used (Pare', 2011), since the value obtained by the high-school students seemed much too low.
1.5 Effectiveness & emancipation Designing a new technology for particular contexts demands some accurate preliminary analysis, setting some criteria to be used as guidelines during the development stages. Some constraints need to be fixed for ensuring the desired results. Topics as safety or reliability are quite easily addressed as those requirements are indisputable, but efficiency, for instance, is a much more arguable issue. Efficiency is almost always related to the adopted technological level and hence to 39
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price and affordability in most cases. Equity may then become a more relevant argument while making these choices. If effectiveness is sought and emancipation should be the consequence, some compromise must be accepted for the applicable technological level and henceforth its efficiency. Other basic requirements are the local availability of most of the materials and the access to most of the necessary construction techniques. In this manner the users can be enabled to easily maintain their new technology. In addition, if the wears due to the daily use are repairable by the end-users, also durability issues are implicitly targeted.
1.6 Technologies 1.6.1 Heat from biomass gasification Heat from agricultural residues is not always obtainable in a simple way and this does not foster the deployment of such a source for energetic purposes in poor contexts. The chemical, physical and morphological characteristics of a specific biomass may require adhoc technologies to make it suitable for heat generation (Reed, 1996). The separation and hereafter the control over the various phases of the combustion process may be of great help. Direct combustion of fine residues is often prevented by an insufficient air distribution within the stacked biomass, but also wood undergoes an incomplete combustion in traditional fires. However, it is possible to obtain heat from biomasses decoupling the gas-generating processes that require very little oxygen from the subsequent combustion of the gases, that requires more of it. Biomasses always undergo a series of changes during any combustion
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process, but how these phases are arranged makes a great difference for the overall efficiency of the combustion process. The most important phases are: • •
• •
drying, which requires heat to eliminate water vapor from the biomass; pyrolysis, which requires heat to brake molecules and release combustible vapors known as 'wood-gas', leaving solid hot char (rich in carbon); gasification, which requires oxygen to extract the carbon from the char, producing 'char-gas'; ashes are left; gas combustion, which requires ignition and oxygen to mix with the gases; heat, carbon dioxide and water vapor are yielded.
Each of these phases have their own characteristics and needs, so they are very difficult to be controlled in an open fire or in many directcombustion devices, where all these processes happen simultaneously, overlapping both in space and time. The first two phases require heat, while the latter ones are mainly regulated by the supplied oxygen. Separating these stages allows a better control over each one of them, making the whole process more efficient. The combustion of any biomass benefits from this, but for some residues, like rice husk, it might be the only viable exploitation method. Besides supplying a sufficient amount of heat to start up the system, enabling the onset of the first two stages, the greatest challenge is to supply the proper amount of air (hence oxygen) in the right places, at the right time. For cooking purposes with simple systems it is advisable to burn the gases close to where they have been originated, so they do not loose heat. However, with appropriate layouts, these gases could also be burnt in a separate location or be stored. 41
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In an open fire the combustion is always incomplete and a large amount of smoke is produced. Gasification processes enable a “cleaner� combustion, but this still requires a delicate balancing of the various stages to obtain usable heat and limited emissions, which will soar up if, for example, not enough oxygen is available for the final combustion or if the system cools down. The design of some gasifying systems may not perform totally, or in part, the char gasification phase, yielding as final product, besides the ashes, some bio-char. It is important to point out that this material has been proved to be very useful as soil structural support (Criscuoli, 2011). When mixed with the soil, small residual carbon-structures resist, retaining water and other nutriments, therefore helping the plant growing, while boosting production even for impoverished soils (Roth, 2011).
1.6.2 Electricity & mechanical power with Stirling engines Rural electrification has become a primary goal for developing strategies over the last 20 years. Mainly two different sizes are being targeted, single household stand-alone systems or larger community systems, both grid connected or powering a local mini-grid. Low power electricity production can be quite useful for numerous services, as powering information and communication systems, but also for providing lighting, with low-power devices as Light Emitting Diodes (LEDs) or fluorescent lamps. As a matter of facts, providing the lighting for the ambient where the light was previously ensured by the open fire, that is not any more available because of the use of an
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enclosed combustion chambers, introduced with new cooking technologies. Last, but not least, electricity is also used to be converted in mechanical energy, although this process requires, in general, powerful plants. In particular contexts biomasses could be used also for producing electrical energy. Gases produced by gasification systems can be converted in electrical energy, or mechanical energy, with internal or external combustion engines. Using an engine based on the Otto-cycle requires a very good cleaning of the gases, prior to their injection, while Stirling engines could be powered with the heat provided by the combustion of less refined gases. Maintenance would than require more cleaning, rather than filter substitutions. Stirling engines could also benefit from a more simple architecture, with respect to endothermic engines, but much research is still necessary to design a simple and reliable machine (Walker, 1996). In addition, production costs would be considerably higher than the ones for more complex but conventional engines, which are also available in many different sizes, while Stirling engines could more realistically be a low-power solution. Nonetheless, this kind of research, carried out with a horizontal approach, could be proved to be useful for many other settings, introducing small size co-generation, for example in workshops where both heat and mechanical power (or electricity) are needed. Nonetheless, besides starting investigating this topic (see annex 1), the focus has subsequently been set on the cook-stove development, judging energy-access for cooking more urgent than energy-access for electricity. In addition, the latter is often addressed by Governmental policies, being developed at a society-scale, while cooking matters are at a house-hold scale and, therefore, often disregarded.
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Figure 19. A schematic diagram of the traditional Lo-Trau stove (above) and reproductions of traditional Japanese stoves (below). 44
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Chapter 2: EFFECTIVE IMPROVED COOK-STOVE DEVELOPMENT 2.1 State of the art Some technologies to exploit rice husk or other fine residues for cooking purposes have already been developed, but the research on this topic and the efforts in delivering effective solutions are quite limited with respect to the possible impact of such energy-recovery tecniques. The Lo-Trau stove was developed and used in Vietnam starting from the 50's, in Vietnamese Lò means 'stove' and Trau means 'rice husk', but its design is quite versatile and it actually runs also with many different fine residues. This stove comes both in a fixed version and in a more recent portable one. The stove design resembles the one of even older stoves used in Japan many centuries ago as visible in Figure 19. The portable version of the Lo-Trau stove is characterized by a conical grate to hold the husks and needs timely tapping of the fuel and removal of the ashes, but it is capable of providing continuous operation, which could be quite useful, especially for other noncooking tasks. Other improved designs of this layout include the 'Turbo Mayon' stove, implemented by REAP-Canada (REAP, 2011), which has optimized the air-flows and the geometric proportions of the Lo-Trau design. This is defined as a quasi-gasifier stove, since it does not perform gasification strictly, but still relies on it for operation while developing a bluish flame (Fig. 20). 45
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Figure 20. The Turbo Mayon Stove. Source: REAP, 2011.
The best example of well operating rice husk gasifying cook-stove is the Belonio stove (Belonio, 2005), developed in the Philippines by Alexis T. Belonio starting in 2003. It is basically a cylindrical reactor filled with the husks and lit from above. An electrical fan provides the necessary air and the flame produced is steady. Also emissions are excellent, as no smoke exits from the stove. This is a quite efficient stove but has a higher technological level and needs an electricity connection or at least batteries to operate. It is hence suitable for settings where some progress in energy-access has already been made. The most recent model (Fig. 21) has an additional feeding system to allow continuous use. The stove costs Euro 40-45 (Belonio, 2011). 46
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Figure 21. The latest version of the Belonio Stove. Source: Belonio, 2011.
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The reported data and performances of this stove are: • • • • • • • • •
Reactor Diameter (m): 0.12 Heat Output (kWt): 1.19 Fuel Requirement (kg/hr): 1.1 Ignition Time (min): 1 Time to Boil 2 liters of water (min): 14 Fan Size: 6 cm x 6 cm Fan Input Voltage (Volt): 12 DC Fan Wattage (Watts): 3 Overall Dimension: L x W x H (m):0.4 x 0.4 x 0.75
Figure 22. The ECHO Stove. Source: PRACTICAL ACTION, 2010. Another particular design is the one of the ECHO stove (Fig. 22). It is made of 28 bricks and is intended to run on sawdust, but also rice husk is suitable. Two ducts are inserted in the stove during loading operations, then they are carefully removed before ignition, leaving 48
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aeration channels through the stacked biomass. The solution is acknowledged as useful where many residues are available and require disposal, but it emits smoke in the cooking environment, so ventilation is strictly necessary. Very recently Small Red Tile, a group of Georgia Tech engineers and industrial designers, has also started developing a rice husk gasifier (Fig. 23) based on natural draft (SMALL RED TILE, 2011). Many other designs are available for different fuels, but in recovering rice husk with simple energy-systems the above examples are the most known and considered.
Figure 23. A prototype by Small Red Tile (Georgia Tech). Source: SMALL RED TILE, 2011. 49
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All these solutions have their own pro's and con's, but it is evident that most of them are completely made of metal-parts, besides the ECHO stove that has other evident limitations as the combustion emission in the environment directly surrounding the stove. More self-sustainable and locally reproducible solutions are not given for this type of fuel (Roth, 2011; Belonio, 2005).
2.2 Early stages The appropriate cook-stove technology needed for contexts like the Logone Valley in Chad is not available. Nevertheless, the basic principles for operation are quite clear, so another, even simpler, design has been sought for. The most challenging problem to solve is how to guarantee adequate air flows and draft, without using precisely machined and assembled metal-sheets or externally forced air for aeration. The preferable construction material is crude earth since it is widely available and familiar for many end-users, facilitating maintenance and repairs. Moreover, the necessary draft for operation can be provided naturally by a chimney, which accomplishes a second important task, in particular, avoiding dispersion of harmful smokes in the living environments.
2.2.1 Rice husk gasifier Preliminary experiments were carried out building a down-draft gasifier. This system was made of a stainless-steel cylindrical reactor, insulated with an abundant vermiculite filling. Air was forced laterally from the upper part of the reactor, while gas could exit from a lower lateral opening. A top cap allowed the recharging of the biomass and a lower cap allowed ash removal. Air was forced with a low-power CPU
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fan and the fan speed could be regulated with a potentiometer. Thermocouple probes where inserted in the reactor in order to study temperature ranges for this kind of process. A schematic picture of the system is shown in Figure 24.
Figure 24. The down-draft gasifier's scheme. This experience has helped to understand the possible conditions to be considered for all the subsequent development stages, besides proving that gas could be obtained in such a simple manner from such a problematic source as rice husk. Results have been quite useful. Temperatures in the reactor vary abruptly, going from up to 1000 째C to less than 200 째C in just 40 cm. Functioning has not been very reliable, but, sometimes, a powerful flame has been obtained as visible in Figure 25. 51
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There are many different designs for biomass gasifiers and understanding the various possible configurations and processes is a key issue for developing new layouts. A technological overview on different systems allows to outline the main distinguishing points (Roth, 2011): • • • • •
the flow direction (up-draft, down-draft or cross-draft); draft origin (electrical fan or blower, natural or chimney induced draft); the feedstock; combustion location (closed-coupled, separated or postponed by cleaning & stocking the gas); by-products (ashes or bio-char).
Figure 25. The flame from the gasifier's outlet. When smoke is produced in the right conditions, a lighter is enough to ignite it at the exit, where it finds oxygen to burn.
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2.2.2 Crude earth cook-stove As the first attempt to use crude earth for the structure of the stove, the simplest geometrical shape was chosen, trying to bring some gasification processes inside a simple cubic combustion chamber made of mud-bricks, the outer dimensions of each side of the stove being approximately 80 cm (Fig. 26). A lateral entrance for air was placed at the bottom of the structure and a metal net kept the biomass in the outer part of the combustion chamber, much similarly to the ECHO stove layout, but forcing draft, which was induced by the chimney placed on the top of the stove (Fig. 27). The use of an auxiliary fan was discarded because of the absence of electricity in the given context, even if the possibility to use Thermo Electric Generators, could have been intriguing, but much lowering the selfsustainability of the resulting stove.
Flue gas Gas Metal net
Secondary air Flame
Fuels
Primary air
Figure 26. An early sketch of the stove and its working principle. Heat is generated in the central part, while gases can laterally exit the biomass and burn just below the pot. 53
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The top of the stove was initially made with a metal plate and subsequently with a crude earth casting. Both of them proved not to be suitable, the first in terms of reliability in the sealing of the combustion chamber and the second in terms of durability.
Figure 27. The first prototype of crude-earth cook-stove. Clockwise, the outer structure, the complete system and the inner combustion chamber. Primary and secondary air ducts are visible. In the first runs the heat generation did not often seem useful for cooking, as a large amount of biomass was needed to achieve low demanding tasks, but nevertheless it was clear that this process could be much improved. The first intervention has been the introduction of 54
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a secondary air channel that conducted extra air in the combustion chamber roughly 10 cm below the pot, allowing the combustion of gases in the desired zone. This has greatly improved performances, which were anyway low in an absolute figure, and emissions, which were still quite smoky. Other alternative fuels, besides rice husk, were tested in this initial phase, both briquetted (an example is visible in Figure 28) and unprocessed grass and leaves. Almost all of them worked, even if many different problems arose with each different biomass. Most of these runs produced a high amount of dark smoke, but sometimes quite transparent emissions were leaving, encouraging further research (Zappa, 2009).
Figure 28. Briquetted leaves are tested with a large central flame made with some wood.
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2.3 Rice husk burner The previous experiences led to the understanding that if gasification of rice husk was to be obtained, some more complex arrangements were necessary. Allowing for this design just the use of few materials and no auxiliary system, the only possibility left has been to investigate the configuration of a structure that induces the appropriate air flows and heat distribution in the combustion chamber, with the necessary draft provided by the chimney. The pot is sealed in its position, sinking a little bit into the stove. This helps the draft and allows a better heat transfer to the pot, besides ensuring benefits regarding safety issues, since no flame escapes, surrounding the pot and possibly reaching the handles. Two sizes have been investigated, a smaller one for single house-holds and a larger one for institutional targets. Successful solutions, tested on one size, have been mutually replied on the other one.
2.3.1 Materials & construction Mud-bricks have been used to build the structures of all the tested prototypes. Commercial mud-bricks from two different companies, Brioni and Conluto, were used, even if this project is in close collaboration with researchers from the University of Brescia that are carrying out investigation on mud-bricks production methods (Brocchetti, 2009) and on structural, thermal and acoustic properties of bricks made of crude earth (Settura, 2012). The used mud-bricks are not extruded, they are obtained from simple casting and the result is quite porous, its density being regulated by gravity during the drying phase. Extruded mud-bricks are much more resistant but require a higher amount of raw materials and are much less insulating
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than the porous ones. Further investigation on mixing rice husk, or its ashes, to the dough are planned, since it is possible that these could improve the insulating behavior of the material. Crude earth is a traditional material that has been used for ages in many developing countries. The possible mixes are many and different characteristic can be obtained by varying the quality of the ingredients along with their proportions. Also the brick production techniques greatly influence the final result, but mainly the diverse qualities of the available clay have the main influences. Each clay has its optimal mix for obtaining optimal results, but the basic composition of the medley is always: • • • •
clay sand organic fibers (grass, straw, etc.) water
Gross typical proportions in volume are: 5 parts of clay, 10 parts of sand, 1 part of fibers and 3 parts of water. This last quantity is the most discretionary one. A more wet mixture will have better binding properties, but will shrink much more, leaving cracks after drying, especially in the wedges. Building a vertical wall has much less of these problems, but assembling a complex structure such as a cookstove is quite different. Hence, some practice is needed in order to obtain the desired results. In the prototypes some slight cracks opened, especially in the wedges, during drying, but those are easily repairable before the first start-up. Other larger cracks opened during transportation from inside to outside the laboratory. Prototypes were built on pallets in order to move them outside for testing, but the vibrations in those transports 57
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caused many cracks to open in the stove. In a fixed use, as it is intended for these cook-stoves, those cracks are not expected. Nonetheless, they have not been repaired, but once, as they could resemble wear due to intensive and prolonged use. The effect of those cracks has been not noticeable, both on functioning reliability and on performances.
Figure 29. Preparation of the dough and building the structure. Construction of the structure is made by wetting the bricks and arranging them in the desired positions, binding them with some mortar obtained with the same mix of the mud-bricks as is visible in Figure 29. The finished structure has then to dry for a time ranging from 4-5 days in hot-dry weather to 2-3 weeks in cold-humid conditions. The structure lays on four bricks and the wall of the combustion chamber is built upon them, leaving four lateral entrances for the air. The top of the smaller stove is made just by carefully closing the main structure with some dough around the pot and leaving the whole to dry with the pot in it. The larger stove has a removable top made using the lower bottom of an oil barrel, this has been cut about 5 cm high and 58
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with a central opening for the pot. It has been subsequently filled with the same mixture, keeping the pot above the central hole similarly to how it is done for the smaller stove. Oil barrels are largely available in many developing countries, but similar structures could also be realized with metal sheets, which are also needed for other small parts of the prototypes. To arrange the internal distribution of the biomass, of the heat and of the air flows, some ducts have been made with metal nets or, others, with short metal pipes. Different kinds of metal-nets have been used, a cheap closely-weaved net has been used for the central duct, while other two, larger meshed, types have been used for the bottom grate and the metal basket. The last kind used is quite similar to the one found in Chad by Francesco Vitali (Vitali, 2010) and has hence been preferred. In the latest versions, only this kind of net is being used. A closer mesh for the central duct is obtained by rolling up the net three times, resulting in a triple layer. Commercial metal-flues have been used as chimneys for these prototypes, because of their convenience for laboratory testing. Those kind of pipes may not easily be available in many contexts, but, once the desired drafting characteristics are defined, the smoke-stack's structure could also be made with mud-bricks or with ceramic mountable ducts, reproducing mostly the same effect of the metalflues used for the prototypes, but using locally available materials. For the technology to be disseminated, a construction of the crude earth structure from casting is also advisable as this would also address an important barrier in cooperation projects. Metal casts can be precisely produced in selected workshops and subsequently used to build the stoves in-situ, thus ensuring precise dimensions and proportions. 59
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2.3.2 Water Boiling Test To assess the energetic performances of the prototypes the Water Boiling Test (WBT) approach has been followed, but the prescribed procedures have not been fulfilled. This is due to the operation mode of the developed cook-stove prototypes, which are batch fed, while the WBT is defined for stoves where the fuel is accessible and can then be added or removed (Bailis, 2007). In these rice husk burners the fuel is charged before ignition and once the system is lit up, it is intended to run until the fuel ends. In the latest versions, the internal air-flow configuration is obtained with the use of a metal-net basket that contains all the rice husk, thus allowing the removal of the fuel at any time. However, anticipated removal of the fuel is not suggested, as the energy still available at the end of the cooking phase could be used for other services, as heating water for sanitary purposes. The energy efficiency of the stove has been calculated over the entire time of operation, because dividing the boiling phase contribution from the simmering one, as required by the WBT procedures, has proved not to be feasible. Efficiency, η , is calculated in the cooking perspective of heat transferred to the water in the pot, hence it is not a combustion efficiency, since the energy transferred to the stove is considered as a loss. η=
c w⋅mw⋅Δ T +mev⋅hfg mc⋅LHV c +mrh⋅LHV rh
The first term at the numerator accounts for the energy relevant to heating the water, where c w is the water's specific heat, 4.19 kJ/ (kg·K), mw is the mass of the water in kg and Δ T is the difference between the boiling temperature and the initial temperature in °C or K. The second term accounts for the energy needed for the phase change 60
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of the evaporated part, mev is the mass of evaporated water in kg and h fg is the specific enthalpy of vaporization at atmospheric pressure, 2257 kJ/kg. At the denominator the total primary energy introduced in the stove with the biomass and the start up fuel is considered. mrh is the mass of rice husk, mc is the mass of charcoal, both in kg, while LHV rh and LHV c are the respective Lower Heating Values, 12540 kJ/kg and 25000 kJ/kg. Other performance data, besides the boiling time, are obtained upon calculations averaged over the duration of the single run. The mean power is calculated as: Power=
mcâ‹…LHV c +mrhâ‹…LHV rh TD s
where TD s is the total duration of the run in seconds. The specific consumption is calculated as:
specific consumption=
mrh mw
The burning rate is calculated as:
burning rate=
m rh TD min
where TD min is the total duration of the run in minutes.
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To perform these measurements few instruments are needed, a watch or chronometer, a portable digital scale (the one used during our tests has a 0.02 kg precision) and at least one thermocouple to measure the water temperature.
2.3.3 Temperature profile Temperature profiles have been measured in the most significant points and the insight of the inner thermal conditions has been linked with the visual observations noted during the runs. In the last runs, adding the relevant emission data has been a key to a better understanding of the processes and to improve the internal structure. Temperatures have been recorded in all the runs using “type k� thermocouples. These have been connected to digital readers and the data were recorded manually for the smaller stove where only two points (water and flame) have been measured, while for the larger stove, a data acquisition system from National Instruments, controlled by a Labview program, has been used, allowing to collect temperature data up to eight points, water, flame, flue and one-to-six points in the stacked biomass.
2.3.4 Chimney emissions and draft At the early stages of the project no measurement for emissions has been carried out, nonetheless observation of the smoke leaving the chimney has always been useful, because the first part of development has been aimed to achieve a reliable functioning of the stove with, possibly, little smoke emission. As soon as this target was reached, a 62
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more detailed analysis of the flue gases was started. The data recorded for emission, 1 m above the combustion chamber outlet, include flue temperature and concentration values for CO and O 2, while only in few runs NO and NOx data were recorded. The values for these pollutants were quite low, hence these additional measurements were not performed further. Sulfur is almost not present in rice husk, hence SOx have not been measured. The portable gas analyzers, used during the experimental runs, have been Testo 350 MXL (TESTO, 2011) and Tecnocontrol Boston (TECNOCONTROL, 2011). Draft pressure is a critical parameter in the design of these particular systems and its values, measured in the lower part of the chimney, have been used to understand how much the draft changes during the various phases of operation and what draft pressure is needed for the most critical ones. This observation has been carried out using an analogical manometer (Fig. 30), filled with a special oil, showing the pressure difference between inside and outside the chimney.
Figure 30. The analogical device to measure draft-pressure.
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2.3.5 Indoor air contamination Indoor air pollution measurements were also carried out to investigate the concentration of CO in the cooking environment. Most of the smokes are evacuated outside by the chimney, but some may escape from possible cracks in the stove structure, or from the pot sealing, which might loosen a little bit during use. Smoke happened to escape in few occasions when draft was insufficient, while with higher draft pressure values, this did not happen, yielding to much better conditions, as the ones obtained with the latest versions of the stove. For this measurement a Crowcon Gasman analyzer has been used (CROWCON, 2011).
2.3.6 Ashes & Bio-char Ashes from rice husk are usually in the range of 15%-18%, with respect to the initial mass, and they are used in many industrial application for their thermal-resistant properties. That led us to the idea of possibly mixing them with crude earth for use in the construction of the stoves.
Element Cd Cu Sample 1 0.95 11 (mg/Kg) Sample 2 1.375 22.5 (mg/Kg) Mean value 1.162 16.75 (mg/Kg) Italian law 3 150 Limits (mg/kg)
Zn
Pb
Cr
Ni
199
41
1.42
3.55
161
43
1.52
3.7
180
42
1.47
3.62
600
100
100
60
Table 5. Chemical analysis of rice husk ashes. (IPC Golgi) 64
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The other possibility is to use those ashes as soil fertilizer. Chemical analysis of the ashes was carried out by the students of IPC Golgi high-school, yielding excellent results, as visible in Table 5. Those ashes were left in the stove for one day before removing them for analysis. This allowed to collect cold ashes, but has also assured a complete combustion, as deduced from their white appearance. Another possible residue, alternate or mixed to ash, is bio-char. If the process is stopped when most of the biomass is still very hot, for example removing the basket and quenching the biomass in water, totally black matter is left. This carbon-rich residue, besides having still a quite significant residual combustible component, is very useful for agriculture, since it can be mixed with the soil having the effect of locally retaining water and nutrients to be available for the growing plants. When farming in dry environments, a consistent part of water and nutrients are lost to the adjacent or lower part of land not reached by the roots and not involved in feeding the crops, while bio-char use greatly reduces this effect. Furthermore, bio-char does not dissolve and those properties gained by the soil last for many years. Some examples of meterials very similar to bio-char, as 'Terra-preta', have lasted thousand of years in Brazil (Criscuoli, 2011). Also known as 'Amazonian black earth', this kind of soil derives from the habit of ancient farmers to add charcoal to improve the soil performances. The effect of burying these carbon-structures, almost permanently, could result in more than a carbon-neutral combustion, typically reached with the combustion of well-managed biomass, but rather more a carbon-negative process, since part of the carbon absorbed with CO 2 from the plant during growth, is not released in the combustion process, but instead it is trapped in the soil. This could be proved to be very important for possible dissemination projects financed with carbon credits.
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Chapter 3: THE ROAD TO 'MY CHUBBY COOKSTOVE' & 'MY LITTLE COOKSTOVE' During construction and subsequent testing, the cook-stoves have inspired liking in many students and colleagues, hence the large version and the smaller one have been given the chatty names of 'My chubby cook-stove' (MCC) and 'My little cook-stove' (mlc). The first burner prototype has been built in the large version in February 2010. It is somehow inspired by the previous crude-earth cook-stove prototype (Fig. 27), as far as materials and basic arrangements are concerned, but the working processes are quite different (Fig. 31). The base of the stove is made of a metal net that is positioned on four bricks, allowing additional aeration of the rice husk. A little fraction of the induced air flow, passes directly through the rice-husk, while most of it flows through the central duct. This has been decided to help the gasification process. The external structure is always made with mud-bricks (Fig. 32), but they are positioned to form a cylindrical structure, to improve heat distribution in the combustion chamber and help a more even consumption of the fuel. As a matter of fact, large amounts of unburnt biomass were always found in the corners of the previous square-based stove. Another important improvement, has been positioning the bricks vertically and not flat, hence a lower number of bricks (28 instead of 48) has been needed to build a stove of similar dimensions. This also greatly helps in diminishing the thermal inertia of the stove that with the much bulkier previous solution had seemed too much influent. To avoid the risk of developing the flame in the chimney, instead of under the pot, the secondary air inlet has been offset with respect to the chimney of 30째.
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Flue gas
Secondary air
Biomass
Flame Primary air Metal net Figure 31. The schematic section of the first version of MCC.
The outer dimensions of the stove are: • • • • •
external mean diameter......... 60 cm internal mean diameter …..... 50 cm stove height........................... 60 cm chimney height...................... 200 cm chimney diameter.................. 9 cm
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Figure 32. The structure of MCC. The pot sinking in the crude-earth top is visible, as the secondary air duct and one of the lower air-intakes.
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The central duct has always been made with metal net, but the shape has been switched to cylindrical and the section has been greatly reduced. This induces better performances and lowers the quantity of the fuel needed for start-up, wood or charcoal. The top also derives from the previous experiences. The metal top had proved to be solid, but quite dangerous because very hot and poor in providing wind protection in outdoor tests. The crude-earth top, instead, proved to be fragile but was very nice in sealing the pot (half the way up to the handles) and its outer temperature was quite low, never seeming possibly harmful. The desired combination of qualities has been achieved by using an oil barrel's bottom, filled with some crude-earth. The exposed part of the top is quite cold and also the barrel's external ring does not get too hot, even if does warm up to 60-70 째C under some conditions. This measure has been carried out sporadically when the stove was at full power and the structure started to heat up a little bit. The crude-earth proved to be an excellent material for safety purposes, since it slowly accumulates heat during the cooking phase and only after an hour or so, when usually the fuel is almost finished the external part of the structure starts to heat up and to release heat in the cooking environment. The chimney's inlet is embedded in the crude-earth structure at the upper end of the combustion chamber and is not fixed to the top any more. A simple butterfly valve is positioned in the lower part of the chimney and gives the possibility of reducing the draft. In this phase of the development the chimney's dimensions and proportions have been kept fixed and the research has focused mainly on the air flows and the heat conditions inside the combustion chamber. The study of the draft and the chimney has been postponed once a proper combustion processes was achieved, appearing as secondary tasks with respect to finding a possibly working arrangement. 69
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The focus has been then set mainly on efficiency, acquiring temperature values to gain some insight in the various phases of the process and hence reaching better proportions between the main air flows involved in the combustion process: the primary air coming from the central duct, the secondary air coming from the upper horizontal duct and the one flowing through the biomass and allowing for gasification. Emissions have not been analyzed in these first runs, even if a qualitative observation of the smoke leaving the chimney has been helpful for understanding some of the occurring conditions. Start-up procedures require to fill the outer part of the stove with the biomass (Fig. 33), while in the central duct some charcoal or wood is placed. The top is positioned and subsequently the start-up fuel is lit from above. At this point the combustion chamber gets closed by placing the pot in its seat.
Figure 33. On the left, the look through the pot hole before ignition. On the right, the combustion chamber with only ashes left. After few sporadic runs, where operation had seemed quite successful, investigation on different sizes for the primary and secondary air ducts was conducted in winter 2011 and it has led to the understanding that a stronger secondary air-flow was needed for a better combustion, besides providing a lot of useful information for the further 70
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Water
Rice husk
Coal
Efficiency
Specific consumption
Burning Rate
cm 10.0 10.0 13.0 13.0 7.5 13.0 13.0 10.0 10.0 10.0 10.0 10.0 13.0 13.0 7.5 10.0 7.5 7.5
cm 4.2 3.3 4.2 3.3 4.2 3.3 4.2 3.3 4.2 3.3 4.2 3.3 4.2 3.3 4.2 3.3 4.2 3.3
l 5.00 5.00 5.00 5.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00
kg 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00
kg 0.100 0.100 0.200 0.200 0.160 0.200 0.280 0.140 0.100 0.100 0.100 0.100 0.140 0.270 0.140 0.100 0.110 0.070
% 6% 5% 6% 5% 5% 5% 4% 5% 7% 5% 6% 5% 6% 4% 6% 6% 5% 4%
g/l 1200 1200 1200 1200 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000
g/min min kW 80.00 75 17.28 74.07 81 16.00 62.50 96 13.93 58.25 103 12.98 75.00 80 16.51 73.17 82 16.31 85.71 70 19.58 76.92 78 16.82 63.83 94 13.78 61.22 98 13.22 74.07 81 16.00 68.97 87 14.89 67.42 89 14.75 61.22 98 13.94 61.86 97 13.53 64.52 93 13.93 60.61 99 13.13 68.97 87 14.75
Mean Power
Secondary Air diameter
Date 03/12/10 03/12/10 04/01/11 04/01/11 12/01/11 12/01/11 13/01/11 13/01/11 17/01/11 17/01/11 19/01/11 19/01/11 25/01/11 25/01/11 26/01/11 26/01/11 08/02/11 08/02/11
Total Duration
MCC
Primary Air diameter
developments. Three diameters (7.5 cm, 10 cm and 13 cm) were tested for the primary air duct, while only two (3.3 cm and 4.2 cm) for the secondary air metal-ducts, for a total of six configurations. The results for the efficiencies and other adapted WBT indicators are listed in Table 6 and in Figure 34 the temperature profiles of these tests are shown. Temperatures have been acquired in the water, in the flame, in the rice husk (in a central position) and at the outlet of the chimney. This campaign has been carried out in collaboration with Fabio Magagnini (Magagnini, 2011).
Table 6. Main parameters of the winter runs with MCC. Efficiency and other adapted WBT indicators. 71
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3-12-10 900
Øc =10 cm Øs=4,2 cm
Temperature [°C]
800 700 600 500 400
Trh
300
Tflame
200
Twater
100
Tflue
0 0.00
0.28
0.57
1.26
time [min]
Temperature [°C]
3-12-10 1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =10 cm Øs =3,3 cm
Trh Tflame Twater Tflue
0.28
0.57
1.26
time [min]
Temperature [°C]
4-1-11
1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =13 cm Øs=4,2 cm Trh Tflame Twater Tflue 0.28
0.57 time [min]
72
1.26
1.55
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4-1-11 Temperature [°C]
900 800 700
Øc =13 cm Øs =3,3 cm
600 500 400 300
Trh Tflame
200 100 0 0.00
Twater Tflue 0.28
0.57
1.26
1.55
time [min]
Temperature [°C]
12-1-11 1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =13 cm Øs=3,3 cm Trh Tflame Twater Tflue 0.14
0.28
0.43
0.57
1.12
1.26
time [min]
Temperature [°C]
12-1-11
900 800 700 600 500 400 300 200 100 0
Øc =7,5 cm Øs =4,2 cm
Trh Tflame Twater Tflue 0.00
0.14
0.28
0.43
0.57
1.12
1.26
time [min]
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13-1-11 900
Øc =13 cm Øs =4,2 cm
Temperature [°C]
800 700 600 500 400
Trh
300 200
Tflame
100
Tflue
0 0.00
Twater
0.14
0.28
0.43
0.57
1.12
time [min]
Temperature [°C]
13-1-11
1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =10 cm Øs=3,3 cm
Trh Tflame Twater Tflue 0.14
0.28
0.43
0.57
1.12
1.26
time [min]
Temperature [°C]
17-1-11
1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =10 cm Øs=4,2 cm
Trh Tflame Twater Tflue 0.28
0.57 time [min]
74
1.26
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Temperature [°C]
17-1-11
900 800 700 600 500 400 300 200 100 0 0.00
Øc =10 cm Øs =3,3 cm Trh Twater Tflame Tflue 0.28
0.57
1.26
time [min]
18-1-11
900
Øc =13 cm Øs=4,2 cm
Temperature [°C]
800 700 600 500 400
Trh
300 200
Tflame
100
Tflue
0 0.00
Twater
0.28
0.57
1.26
1.55
time [min]
Temperature [°C]
18-1-11 900 800 700 600 500 400 300 200 100 0 0.00
Øc =7,5 cm Øs=3,3 cm
Trh Tflame Twater Tflue 0.28
0.57
1.26
time [min]
75
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Temperature [°C]
19-1-11
1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =10 cm Øs=4,2 cm
Trh Tflame Twater Tflue 0.14
0.28
0.43
0.57
1.12
1.26
time [min]
Temperature [°C]
19-1-11
1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =10 cm Øs=3,3 cm
Trh Tflame Twater Tflue 0.28
0.57
1.26
time [min]
25-1-11
900
Øc =13 cm Øs =4,2 cm
800 Temperature [°C]
700 600 500 400
Trh
300
Tflame
200
Twater
100
Tflue
0 0.00
0.28
0.57 time [min]
76
1.26
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25-1-11
800
Øc =13 cm Øs=3,3 cm
Temperature [°C]
700 600 500 400
Trh
300
Tflame
200
Twater
100
Tflue
0 0.00
0.28
0.57
1.26
time [min]
26-1-11
900
Øc =7,5 cm Øs=4,2 cm
800 Temperature [°C]
700 600 500 400
Trh
300
Tflame
200
Twater
100
Tflue
0 0.00
0.28
0.57
1.26
time [min]
Temperature [°C]
26-1-11
1000 900 800 700 600 500 400 300 200 100 0 0.00
Øc =10 cm Øs=3,3 cm
Trh Tflame Twater Tflue 0.28
0.57
1.26
time [min]
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8-2-11
900
Øc =7,5 cm Øs =4,2 cm
800 Temperature [°C]
700 600 500 400
Trh
300
Tflame
200
Twater
100
Tflue
0 0.00
0.28
0.57
1.26
time [min]
8-2-11 900
Øc =7,5 cm Øs=3,3 cm
Temperature [°C]
800 700 600 500 400
Trh
300
Tflame
200 100
Twater
0 0.00
Tflue 0.28
0.57
1.26
time [min]
Figure 34. Temperature profiles for the winter runs with MCC. Trh stands for rice husk temperature, acquired at mid-height and mid-depth.
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The initial peak in flame temperature is due to the charcoal combustion and some gasification of the adjacent rice husk. The second one, where present, is due to gas ignition. Sometimes this is revealed by the thermocouple in the rice husk. The fuel lowers during the run and the flame develops just above it. The thermocouple, that was in the rice husk, is now exposed and detects the flame temperature at mid height in the combustion chamber. The temperature below the pot is often lower in these circumstances. This is mainly due to the large dimensions of the stove and is, of course, very bad for cooking purposes. Repeated peaks in flame temperature, where present, are due to the addition of charcoal or wood in the central duct, not only to gas ignition. The chimney valve was occasionally closed in some runs, especially when the flue gases have been very hot. The draft induced by the chimney is hence reduced to provide a better combustion and extend the duration of the run. Ebullition in general, but especially vigorous boiling, is probably influenced from the weather conditions, often the water temperature did not exceed 95 째C especially when 5 liters of water have been loaded in the pot. Most of the runs have then been carried out with 3 liters instead of 5. The WBT procedure also requires not to use a lid and also this constraint is much more penalizing in a cold weather rather than in a hot one, besides considering that initial temperature of the water heated in these runs usually is around 10-12 째C, instead of 25 째C that are assumable for hotter climates. Besides acquiring the data, many useful observations were made in these runs and one of the most relevant ones has been that oxygen is insufficient when the gas production starts to be vigorous. The secondary air, provided by the horizontal duct, is enough for the initial 79
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phase when high temperature zones are small and less husk is hence involved in the gasification process, but much more air is needed to burn the large amount of gas produced in the second phase, when a larger part of the fuel is involved in the process. The combustion has then been helped sometimes by lifting the pot a little, the gas ignite thanks to the abundant fresh air entering from the pot's hole. This part of the run is typically quite powerful and often the water in the pot reaches boiling temperature again, sometimes one hour after the initial water temperature peak. Occasionally the stoves were moved after being charged and the vibrations have induced the rice husk to settle more. The effects of this raised density seemed yielding better performances, so in other occasions some manual compression of the fuel in the combustion chamber has been tested, but this did not prove useful. Hereafter, fuel has been usually vibrated after being charged. These tests have been carried out outdoor in winter with very low temperatures, the stoves have been kept under a roof, but otherwise they have been exposed to the outdoor conditions. The initial temperature of the stove structure, which was close to 0 째C, and the high humidity stored in the mud-bricks probably can explain, at least partially, why the thermal conditions for gasification are reached so slowly. Sometimes additional charcoal has been needed to reach temperatures necessary for gasification. More charcoal was also used for the large primary duct with respect to the smaller ones that contained less of it. In previous preliminary runs, held in summer, the smaller duct worked very well, but probably contained too little charcoal to contrast the difficult ambient conditions in winter. The smaller duct proved excellent also in the test conducted in situ by Francesco Vitali, who has reproduced the stove (Fig. 35) in the hot conditions of Chad, using only locally available materials and 80
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Mean Power
min 82
Burning Rate
% 6%
Specific consumption
Boiling Time
kg kg 8.70 none
Efficiency
l 5.00
Coal
Date March 2010
Rice husk
MCC
Water
spending roughly 50 $ for the whole prototype. Resulting performances are perfectly comparable with those obtained in Brescia. The data for these runs is shown in Table 7 (Vitali, 2012).
g/l g/min kW 1720 110 22.12
Table 7. Main parameters of the in situ runs with the MCC prototype in Chad. Efficiency and other adapted WBT indicators. The data are averaged on three runs.
Coal
Efficiency
Specific consumption
Burning Rate
l 3.00 3.00 3.00 3.00
kg 6.00 6.00 6.00 6.00
kg 0.080 0.080 0.080 0.080
% 6% 5% 4% 6%
g/l 2000 2000 2000 2000
g/min min kW 69.77 86 14.97 69.77 86 14.97 60.61 99 13.00 61.22 98 13.14
Mean Power
Rice husk
cm 3.3 4.2 3.3 4.2
Total Duration
Water
Date cm 18/05/11 7.5 18/05/11 10.0 19/05/11 10.0 19/05/11 7.5
Secondary Air diameter
MCC
Primary Air diameter
This configuration has subsequently been replied in few late spring runs to quantify the possible influence of the ambient conditions, not resulting in a significant difference. Efficiency values have not been much different from the ones obtained in winter. Data for these runs are shown in Table 8.
Table 8. Main parameters of the comparative spring runs with MCC. Efficiency and other adapted WBT indicators. 81
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Figure 35. The stove's prototype, realized in situ by Francesco Vitali.
82
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High power and high fuel consumption in the runs performed with MCC, besides the still quite influent thermal inertia, have suggested the idea of investigating a smaller size stove and this has led to the construction of mlc. As a first attempt, down-sizing from the larger stove has been performed, new outer dimensions of the stove are: • • • • •
external mean diameter.….... 40 cm internal mean diameter.......... 30 cm stove height........................... 45 cm chimney height...................... 200 cm / 175 cm chimney internal diameter..... 4 cm
Figure 36. The first version of mlc at start-up. This design has no top and the pot sinks directly in the stove and also the chimney is incorporated in the structure. The intake of the flue is positioned as high as possible, above the pot's bottom. Hence, better directing the flame to the desired spot. The internal central duct has been made with the same metal net, but with a 5 cm internal diameter. 83
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Also a larger 7.5 cm diameter was tested but the stove did not work properly, producing large amount of smoke and never heating much the water in the pot. This model of the stove in the various configurations holds a quantity of rice husk ranging between 1 and 1.5 kg, depending on the configuration tested. Hence, the quantity of water was also reduced to 2 liters, at least for the first runs. Despite the weather being considerably hotter, with respect to the previous winter tests on MCC, the problem of taking the water to the boiling temperature was not solved. Wind also was found to have a very negative influence on the water temperature. A blow of fresh breeze in a sunny day at noon could induce a 3-4 째C drop in the water. Anyway, on the first tests, performances were considered encouraging, since thermal inertia was greatly lower and heat generation was more confined and closer to the pot in the smaller combustion chamber. Both power and fuel consumption have been significantly lowered, bringing thermal efficiency up to an interesting 8%, but it was clear that the stove did not hold enough rice husk to boil 3 liters of water, as ultimately intended. The goal was boiling 3 liters of water (and evaporating almost one of them) using 1 kg of rice husk, with a thermal efficiency around 20%. This target was set, comparing total costs of wood-use and rice-husk-use (Fig. 37), in the Logone Valley, Chad (Vitali, 2012).
Figure 37. Cost comparison between use of wood and rice husk. Source: Vitali, 2012. 84
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Figure 38. The second version of mlc. The lower ducts are visible, together with some tangled-up metal wire exiting from the secondary air duct.
85
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Another version has then been realized (Fig. 38), being slightly larger and allowing the pot to sink more deep into the stove, down to the pot's handles to provide as much wind protection as possible and to increase the surface exposed to the flame. Another issue to be addressed was the long start-up process, so 5 auxiliary small ducts, with a 0.8 cm inner diameter, have been placed in the bottom of the structure, allowing extra air from outside to enter directly in the middle of the rice husk, helping a fast onset of the gasification processes. Some kind of cross-gasification is induced from these elements and the results have been quite good. Most of the attention has been set on the secondary air, since, from previous experiments, this was the most relevant issue to be solved. In fact the lack of oxygen denied the proper combustion of the produced gases. Cooling, induced from excessive secondary air, is a very relevant drawback to be evaluated for setting the proper flow rate. Hence, some preheating of this air has been tested. Preheating has been realized inserting a metallic component in the inner end of the secondary air duct. This part of the combustion chamber is always at high temperatures, above 400 째C for most of the run, so, when this element is warm enough, it can release heat to the air flowing through it. A tangled-up metal wire has been tested first, also to induce turbulence in the secondary air flow, but the pressure drop caused by this element lowered too much the flow-rate. Another attempt has then been made with a rolled metal net, which induces a more laminar, but intense, flow. The effect of these solutions have not been very relevant, as also closing the secondary air inlet in some phases (at start-up or at the end) did not prove successful. Another issue is ash removal that is not very easy with this lay-out, a metal hook is needed for raising the lower grate and allowing the 86
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ashes to fall down. Ashes need then to be removed through the lower air entrances. Results for these runs are shown in Table 9 while temperature profiles are shown in Figure 39.
Specific consumption
kg 0.10 0.10 0.10 0.10 0.10 0.10 0.05 0.10 0.10 0.10 0.10 0.10 0.10 0.10
% 14% 10% 9% 10% 12% 12% 14% 15% 14% 11% 11% 11% 13% 11%
min 15* na na 29* na na 32* na na na na na na na
g/l 680 440 700 600 590 670 660 407 433 363 630 407 393 580
g/min min 20.30 67 19.13 69 23.73 59 20.69 58 24.58 48 20.00 67 18.86 70 18.21 67 14.61 89 15.59 68 17.26 73 16.49 74 13.72 86 23.84 73
Mean Power
Boiling Time
kg 1.36 1.32 1.40 1.20 1.18 1.34 1.32 1.22 1.30 1.06 1.26 1.22 1.18 1.74
Total Duration
Efficiency
l 2.00 3.00 2.00 2.00 2.00 2.00 2.00 3.00 3.00 2.92 2.00 3.00 3.00 3.00
Burning Rate
Coal
Date 18/03/11 30/03/11 06/04/11 13/04/11 14/04/11 20/04/11 21/04/11 05/05/11 06/05/11 10/05/11 12/05/11 13/05/11 20/05/11 15/06/11
Rice husk
mlc
Water
Temperatures have been acquired manually with a digital reader only in the water and in the flame because of setting conditions. Some water temperature data are missing because the thermocouple has been sometimes removed from the water.
kW 4.86 4.60 5.67 5.04 6.01 4.80 4.24 4.43 3.52 3.87 4.18 4.01 3.35 5.55
Table 9. Main parameters of the spring runs with mlc. Efficiency and other adapted WBT indicators. (* boiling times for 2 liters of water)
87
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18-3-2011 900
Temperature [째C]
800 700 600 500
T flame
400
T w ater
300 200 100 0 0
20
40
60
80
Time [min]
30-3-2011
Temperature [째C]
700 600 500 400
T f lame
300
T w ater
200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
6-4-2011
Temperature[째C]
700 600 500 400
T flame
300
T w ater
200 100 0 0
10
20
30
40
Time [min]
88
50
60
70
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13-4-2011 900
Temperature [째C]
800 700 600 500
T flame
400
T w ater
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
14-4-2011 800
Temperature [째C]
700 600 500 T flame
400
T water
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
20-4-2011 Temperature [째C]
600 500 400 T flame
300
T w ater
200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
89
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21-4-2011 700
Temperature [째C]
600 500 400
T f lame T w ater
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
5-5-2011 450
Temperature [째C]
400 350 300 250
T f lame
200
T w ater
150 100 50 0 00.00
00.14
00.28
00.43
00.57
01.12
01.26
Time [min]
6-5-2011 Temperature [째C]
600 500 400 T f lame
300
T w ater
200 100 0 00.00
00.14
00.28
00.43
00.57
Time [min]
90
01.12
01.26
01.40
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10-5-2011 700
Temperature [째C]
600 500 400
T f lame T w ater
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
12-5-2011
Temperature [째C]
700 600 500 400
T f lame T w ater
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
13-5-2011 700
Temperature [째C]
600 500 400
T flame T w ater
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
01.26
Time [min]
91
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20-5-2011 700
Temperature [째C]
600 500 400
T flame T w ater
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
01.26
01.40
Time [min]
15-6-2011 900
Temperature [째C]
800 700 600 500
T f lame
400
T w ater
300 200 100 0 00.00
00.14
00.28
00.43
00.57
01.12
Time [min]
Figure 39. Temperature profiles for the spring runs with mlc.
The overall conclusion of this experimental campaign has been that the extra lateral air, from the 5 lower lateral ducts, greatly helps the gasification process, but this does not happen every time. Runs with a second high peak in flame temperature often yielded good performances. The next step has then been trying a completely new internal structure, that could foster this effect of inducing a larger air flow through the rice husk. 92
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A basket made of metal net has been built to hold the fuel, the diameter being slightly smaller than the combustion chamber, therefore leaving a gap between the external part of the basket and the interior wall of the crude-earth structure. The central duct is also fixed to the basket. This solution greatly helps both loading of fuel and removal of ashes, besides allowing removal of all fuel at any time. This would also be quite important for a timely quenching of the rice husk to provide bio-char (Fig. 40).
Figure 40. The metal basket after a run with quenched bio-char and some ash. 93
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Preliminary, two sizes have been tested in mlc, a smaller one and a medium one, which hold roughly 0.3 kg and 0.6 kg of rice husk respectively. Subsequently, a larger version of the metal basket has been made to hold about 1 kg of rice husk, since the efficiency of the preliminary runs was close to the value needed for the original goal of boiling 3 liters of water with 1 kg of rice husk.
Figure 41. The two options of the outer air flow in the gap. This kind of configuration has then been tested both in mlc and MCC, but with slight differences, besides capacity (1 kg of rice husk for mlc and 3-3.5 kg for MCC). It has been immediately clear that the flow induced by the chimney was mainly divided among the central duct and the outer gap (2-3 cm in mlc and 4-5 cm in MCC) . This latter flow could prove useful in two different ways (Fig. 41). It could draft a lot of secondary air, if the lower inlet of the gap is left open, or it could force a more abundant flow through the rice husk if the bottom of the gap was sealed. The first configuration has been tested on MCC, while the second one on mlc. Preliminary testing on mlc in July 2011 has proved the chimney to be suitable for the small and medium basket, but it drafted an insufficient amount of air with respect to the 94
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quantity of rice husk held by the larger one. Hence, a new prototype with a much larger chimney has been built, doubling the internal diameter from 4 cm to 8 cm.
% min g/l g/min 15% na 227 18.89 16% na 200 19.35 10% na 360 19.15 19% 25 273 17.45 16% 22 287 22.63
Mean Power
Burning Rate
Boiling Time Specific consumption
kg 0.05 0.05 0.10 0.10 0.10
Efficiency
kg 0.34 0.60 0.90 0.82 0.86
Total Duration
Date l 15/07/11 small 1.50 15/07/11 medium 3.00 22/07/11 large 2.50 09/11/11 large 3.00 14/11/11 large 3.00
Coal
Basket Size
Rice husk
mlc
Water
These experiments (9/11 and 14/11) proved to be successful as from the data reported in Table 10, that clearly show the possibility of reaching quite high efficiency values also with the larger basket. Temperature profiles for these runs are shown in Figure 42.
min 18 31 47 47 38
kW 5.11 4.72 4.89 4.53 5.83
Table 10. Main parameters of the first runs with the metal basket in mlc. Efficiency and other adapted WBT indicators.
95
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15-7-2011 800
Temperature [째C]
700 600 500 T flame
400
T w ater
300 200 100 0 0.00
0.02
0.05
0.08
0.11
0.14
0.17
0.20
Time [min]
15-7-2011
Temperature [째C]
700 600 500 400
T f lame T w ater
300 200 100 0 00.00.00
00.07.12
00.14.24
00.21.36
00.28.48
00.36.00
Time [min]
22-7-2011 700
Temperature [째C]
600 500 400
T flame T water
300 200 100 0 0.00
0.07
0.14
0.21
0.28
Time [min]
96
0.36
0.43
0.50
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9-11-2011 900
Temperature [째C]
800 700 600 500
T flame
400
T w ater
300 200 100 0 00.00
00.07
00.14
00.21
00.28
00.36
00.43
Time [min]
14-11-2011 800
Temperature [째C]
700 600 500 T flame
400
T water
300 200 100 0 00.00
00.07
00.14
00.21
00.28
00.36
00.43
Time [min]
Figure 42. Temperature profiles for the first runs with the metal basket in mlc. For the last run also a graph of emissions is shown. At a certain point data for CO and H2 went out of scale. 97
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Water
Rice husk
Coal
Efficiency
Specific consumption
Burning Rate
cm 10.0 7.5 10.0 7.5 7.5 10.0 7.5 7.5 7.5 7.5 7.5
cm basket basket basket basket basket basket basket basket basket basket basket
l 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 5.00
kg 3.00 3.06 2.96 3.06 3.32 3.10 3.46 3.24 3.40 3.16 3.38
kg 0.100 0.100 0.150 0.150 0.100 0.150 0.120 0.100 0.150 0.100 0.100
% 5% 7% 9% 9% 6% 6% 7% 7% 9% 7% 5%
g/l 1000 1020 987 1020 1107 1033 1153 1080 1133 1053 676
g/min min kW 37.50 80 8.36 31.88 96 7.10 50.17 59 11.54 35.17 87 8.07 51.88 64 11.49 45.59 68 10.45 42.72 81 9.54 36.00 90 7.99 34.34 99 7.81 35.91 88 7.98 71.91 47 15.92
Mean Power
Secondary Air diameter
Date 12/08/11 12/08/11 16/08/11 16/08/11 24/08/11 25/08/11 02/09/11 23/09/11 27/09/11 11/10/11 25/10/11
Total Duration
MCC
Primary Air diameter
The other configuration has been tested on MCC. The lower entrance of the lateral gap between the basket and the crude-earth has been left completely open, allowing an abundant and continuous air-flow for supplying oxygen for combustion. The basket has been supported by two thin metal rods, positioned where the bottom grate was in the former version. These runs did not proved to be very successful. Reached efficiencies are higher than those obtained by the earlier versions of MCC, but missing the initial expectations. Probably, the very intense cold-air flow through the lateral gap has a too much influential cooling effect. Also the draft, provided by the chimney (9 cm diameter), is probably insufficient. Data for these runs, acquired in collaboration with Marco Lorandi, are visible in Table 11, while temperature profiles are shown in Figure 43.
Table 11. Main parameters of the runs with the metal basket in MCC. Efficiency and other adapted WBT indicators.
98
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12-8-2011 800
100
700
90 80 70
500
60
400
50
300
40
Water Temperature [°C]
Temperature [°C]
600
30
200
20
100
T flame T flue T rh T water
10
0 0.00
0.14
0.28
0.43
0.57
1.12
1.26
0 1.40
time [min]
900
100
800
90
700
80 70
600
60
500
50
400
40
300
30
200
20
100
10
0 0.00
0.14
0.28
0.43
0.57
1.12
1.26
Water Temperature [°C]
Temperature [°C]
12-8-2011
T flame T flue T rh T water
0 1.40
time [min]
900
100
800
90
700
80 70
600
60
500
50
400
40
300
30
200
20
100
10
0 0.00
0.14
0.28
0.43
0.57
1.12
1.26
Water Temperature [°C]
Temperature [°C]
16-8-2011
T flame T flue T rh T water
0 1.40
time [min]
99
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16-8-2011 800
100
700
90
Temperature [°C]
70
500
60
400
50
300
40 30
200
20
100 0 0.00
Water Temperature [°C]
80
600
T flame T flue T rh T water
10 0.14
0.28
0.43
0.57
1.12
1.26
0 1.40
time [min]
24-8-2011 800
100
700
90
Temperature [°C]
70
500
60
400
50
300
40 30
200
20
100 0 0.00
Water Temperature [°C]
80
600
T flame T flue T rh T water
10 0.14
0.28
0.43
0.57
0 1.12
time [min]
900
100
800
90
700
80 70
600
60
500
50
400
40
300
30
200
20
100
10
0 0.00
0.14
0.28
0.43
time [min]
100
0.57
0 1.12
Water Temperature [°C]
Temperature [°C]
25-8-2011
T flame T flue T rh T water
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2-9-2011 800
100
700
90 80 70
500
60
400
50
300
40 30
200
20
100 0 0.00
Water Temperature [°C]
Temperature [°C]
600
T flame T flue T rh T water
10 0.14
0.28
0.43
0.57
0 1.12
time [min]
900
100
800
90
700
80 70
600
60
500
50
400
40
300
30
200
20
100
10
0 0.00
0.14
0.28
0.43
0.57
Water Temperature [°C]
Temperature [°C]
23-9-2011
T flame T rh T water
0 1.12
time [min]
900
100
800
90
700
80
600
70 60
500
50
400
40
300
30
200
20
100
10
0 0.00
0.14
0.28
0.43
0.57
1.12
1.26
Water Temperature [°C]
Temperature [°C]
27-9-2011
T flame T rh T water
0 1.40
time [min]
101
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11-10-2011 800
100
700
90
Temperature [째C]
70
500
60
400
50
300
40 30
200
20
100 0 0.00
Water Temperature [째C]
80
600
T flame T rh T water
10 0.14
0.28
0.43
0.57
1.12
1.26
0 1.40
time [min]
Figure 43. Temperature profiles for the runs with the metal basket inside MCC. To be tested in mlc (Fig. 45), another improvement was attained by mean of a thin metal plate to serve as gas concentrator. It has been placed on top of the metal basket, while a 15 cm wide hole at the center allows the concentration of the flow raising from the central duct. The external diameter is as wide as the basket (24 cm diameter). This metal plate better divides the draft-induced air flow in the two components, one flowing directly through the central duct, while the other fraction is forced to flow through the rice husk and then in the lateral gap between the basket and the crude-earth structure, as visible in Figure 44. The split flow is supposed to work in a dual mode in the two different phases. During the first phase, just after start-up, heat is released in the central duct by the charcoal. That heats up the adjacent rice husk starting the gasification process. Gases from this rice husk are released in the central duct and they burn just under the pot together with the charcoal, while the other external flow dries all of the other rice husk, drafting through it. This phase lasts about 20 minutes. 102
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Figure 44. The metal plate, used as gas concentrator, seen through the pot hole, at the end of a run. Black and white ashes are visible. In the second phase, the charcoal and the adjacent rice husk are exhausted, but the remaining fuel is ready for gasification. At this point all the draft through the husk is producing gas, since the necessary temperature conditions are reached, and the air flow through the central duct now serves as secondary air to provide all those gases with enough oxygen for combustion. When this phase starts, the stove begins emitting a slight 'sparkling' noise because flue temperatures raise and hence the draft. Smokes also start being white, instead of transparent, indicating a poor combustion, probably due to an inappropriate oxygen-mixing level. It is useful at this point to slightly close the chimney valve to reduce the draft and to slow down the air flow a little bit, allowing more time for a more complete combustion of the gases. Smoke, usually, turns again transparent at this point and the run lasts a little longer. 103
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Figure 45. The most recent version of mlc.
Date Run l kg kg % min 19/12/11 1 3.00 0.86 0.05 16% na 19/12/11 2 3.00 1.08 0.05 11% na 19/12/11 3 3.00 0.98 0.05 15% na 28/12/11 1 3.02 1.00 0.05 18% 27 28/12/11 2 2.98 0.94 0.05 19% 27 28/12/11 3 3.00 1.04 0.05 18% 28
g/l 287 360 327 331 315 347
g/min min 17.92 48 20.00 54 13.80 71 20.83 48 17.74 53 14.86 70
Mean Power
Total Duration
Burning Rate
Specific consumption
Boiling Time
Efficiency
Coal
Rice husk
mlc
Water
Performances for these runs are visible in Table 12 , while the relevant emissions and temperature profiles are shown in Figure 46. In these runs the conditions of the previous tests, held on 9/11 and 14/11, have been replied with very little changes.
kW 4.18 4.57 3.18 4.79 4.10 3.40
Table 12. Main parameters of the runs with the metal basket inside mlc. Efficiency and other adapted WBT indicators. 104
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800
100
700
90 80
Temperature [째C]
600
70
500
60
400
50
300
40 30
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100
Water Temperature [째C]
19-12-2011 Run 1
T flame T flue T water
10
0 0.00
0.14
0.28
0.43
0 1.12
0.57
Time [min]
25
0,9 0,8
20
0,7
0,5 0,4
10
CO [%]
O2 [%]
0,6 15
O2 CO
0,3 0,2
5
0,1 0 0.00
0.07
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0.36
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Time [min]
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800
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Temperature [째C]
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Water Temperature [째C]
19-12-2011 Run 2
T flame T flue T water
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0.57
Time [min]
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20
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0 0.00
0.07
0.14
0.21
0.28 Time [min]
106
0.36
0.43
0.50
0 0.57
CO [%]
O2 [%]
0,7 15
O2 CO
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19-12-11 Run 3
800
100
700
90
Temperature [째C]
70
500
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10 0.14
0.28
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0 1.12
Time [min]
25
1,6 1,4 1,2 1
15
0,8 10
0,6
CO [%]
O2 [%]
20
O2 CO
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5
0,2 0 0.00
0.14
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0.43
0.57
0 1.12
Time [min]
107
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28-12-11 Run 1
100
700
90 80
Temperature [째C]
600
70
500
60
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50
300
40 30
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20
100 0 0.00
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T flame T flue T water
10 0.14
0.28
0.43
0.57
0 1.12
Time [min]
25
0,9 0,8 0,7 0,6
15
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Time [min]
108
0.57
0 1.12
CO[%]
O2 [%]
20
O2 CO
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800
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Temperature [째C]
600
70
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Water Temperature [째C]
28-12-11 Run 2
T flame T flue T water
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0.43
0.57
0 1.12
Time [min]
25
1,4 1,2
20 15
0,8
10
0,6
CO[%]
O2 [%]
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Time [min]
109
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800
100
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90 80
Temperature [째C]
600
70
500
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100 0 0.00
Water Temperature [째C]
28-12-11 Run 3
T flame T flue T water
10 0.14
0.28
0.43
0.57
0 1.12
Time [min]
25
1,2 1 0,8
15
0,6 10
O2 CO
0,4
5 0 0.00
CO [%]
O2 [%]
20
0,2
0.14
0.28
0.43
0.57
0 1.12
Time [min]
Figure 46. Temperature profiles and emissions for the runs with the metal basket inside mlc. 110
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Little changes have been tried in these runs, but it is quite clear that this configuration is particularly suitable to exploit rice husk, since the main target has been reached. Besides performances, this last lay-out has proved to be quite reliable, without any need for user intervention besides the initial ignition and, on occasion, the flue valve regulation. The runs held on 19-12-11 have been dedicated to adjusting the bottom air-intake dimensions. Of the four available inlets, each of 7080 cm2, three were closed in the second run and two in the third one. This last run has been the most promising one and hence, two of the four air-intakes have been left closed in the runs held on the 28-12-11. These three runs have been performed in the same conditions, waiting some time between them, about one hour, to allow some cooling of the structure. External surfaces, in this time, cool down almost to ambient temperature, but probably some heat is still retained in the crude-earth structure and released very slowly, not heating much the external surface. The only variation in these runs has been the flue valve regulation. In the first run, the valve has been closed (45째) 17 minutes after ignition, as soon as flue temperature had started raising, while in the second and third run the valve has been closed (45째) after 22 minutes. Further investigation on this effect is needed to optimize the combustion of this second operation-phase, possibly introducing some automatic regulation, based on drafting pressure. Simple mechanical systems for this kind of problem are commercially available in industrialized countries, but the working principle could be adapted to even more simple configurations. The appropriateness of this kind of solution is questionable, since it would require some accurate calibration and maintenance. The overall configuration of these last runs is now being implemented also in MCC, for that a new series of tests is planned, investigating the effect of different draft conditions by changing the dimensions of the 111
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MCC
Primary Air diameter
Secondary Air diameter
Water
Rice husk
Coal
Efficiency
Specific consumption
Burning Rate
Total Duration
Mean Power
bottom air inlets and of the chimney, having this shown a quite relevant effect on mlc. A largely excessive draft, induced by a 11 m high chimney, has been tested with this new configuration, with drafting pressure reaching values up to 60 Pa, quite far from the usual 8-15 Pa range of the previous runs. These runs have had very low efficiencies and 3 kg of rice husk lasted quite little time, much less than 60 minutes, but brought internal temperatures to previously unreached values of 1200 째C. Draft needs hence to be further investigated with high attention for these possible dangerous conditions. Temperatures profiles (acquired according to the scheme in Figure 47) are shown in Figure 48 and performances are listed in Table 13.
Date 31/10/11 04/11/11 25/11/11 19/12/11
cm 7.5 7.5 7.5 10.0
cm basket basket basket basket
l 3.00 3.00 3.00 3.00
kg 3.30 3.36 3.38 3.22
kg 0.120 0.150 0.150 0.150
% 2% 4% 4% 3%
g/l 1100 1120 1127 1073
g/min 86.8 101.8 73.5 100.6
min 38 33 46 32
kW 19.47 23.17 16.72 22.98
Table 13. Main parameters of the runs with excessive draft in MCC. Efficiency and other adapted WBT indicators.
Figure 47. Positions of the thermocouples in the rice husk. 112
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31-10-2011 100
1200
90
70
800
60 50
600
40 400
30
Water Temperature [째C]
80 T flame
Water Temperature [째C]
Temperature [째C]
1000
T flame
T rh L1H3 T rh L1H2 T rh L1H1 T rh L2H3 T rh L2H2 T rh L2H1 T w ater
20
200
10 0 00.00
0 00.07
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time [min]
4-11-2011 100
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80 70
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T rh L1H3 T rh L1H2 T rh L1H1 T rh L2H3 T rh L2H2 T rh L2H1 T w ater
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time [min]
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25-11-2011 100
1200
90
70
800
60 50
600
40 400
30
Water Temperature [째C]
80 T flame
Water Temperature [째C]
Temperature [째C]
1000
T flame
T rh L1H3 T rh L1H2 T rh L1H1 T rh L2H3 T rh L2H2 T rh L2H1 T w ater
20
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10 0 00.00
0 00.07
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00.50
time [min]
19-12-2011 100
1200
90
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80 70
800
60 50
600
40 400
30 20
200
10 0 00.00
0 00.07
00.14
00.21
00.28
00.36
time [min]
Figure 48. Temperature profiles for the runs with excessive draft in MCC. 114
T rh L1H3 T rh L1H2 T rh L1H1 T rh L2H3 T rh L2H2 T rh L2H1 T w ater
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Helpful
Harmful
to achieving the objective
to achieving the objective
Strengths
Weaknesses
Internal origin
Reliable and user-independent operation
Heat exposed metal parts may Adaptability to different kind of incur in early deterioration local cooking preparations
Efficiency comparable to other Material resistance to daily use Chimney maintenance required improved cook-stoves has to be tested Use of non-traditionally exploitable resource
Not continuous feeding; batch loading limits duration
Firepower regulation has to be studied
Smoke withdrawal through the chimney
Optimization of draft and air intakes
Affordability
Crude-earth mixture compositions
Easy and cheap maintenance
Construction from casting
Opportunities External origin
To be investigated
Threatens
Mlc may increase the access of Rice husk is difficult to Rice husk local availability has local population to a wider transport and to store due to its to be investigated before energy technology portfolio low bulk density dissemination Contrasts high costs of wood and other fuels
Rice husk has low LHV
Wood is becoming everyday less accessible
Use of wood is strongly rooted in the daily cooking habits
Support from governmental household energy strategy
Recovers waste biomass Currently rice husk has no cost
Table 14. SWOT analysis of mlc.
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Chapter 4: RESULTS The primary goals that had been set at the beginning of this research program have been reached almost completely. A simple and effective configuration for the combustion chamber has been picked out, three liters of water are being boiled with one kilogram of rice husk and this is achieved with a cheap system, requiring few materials and skills for reproduction. Much research is still needed to optimize the performances and ease the dissemination problems, but the overall cook-stove lay-out is judged ready for the first on-field trials. An overall summary is shown in Table 14.
4.1 Affordability, reliability & safety Local reproduction cost of this stove model is quite low. Cost can vary from one context to another, but a very low cost for the crude-earth mixture components is often assumable in rice producing regions. Only the cost of the simple metal-net and for the chimney, if not made with mud-bricks, should be considered. Supposed costs should be lower than 15$ for mlc. (Vitali, 2012) Besides being locally reproducible, costs are lower with respect to advanced biomass cookstove and, hence, this cook-stove might not need much financial support during the dissemination projects. Besides these considerations, the operation of the stove in the final configuration has proved to be reliable, as long as the start-up procedure is followed properly, and does not require further interventions. More prolonged testing is, of course, necessary to asses performances in time, as well as the wearing of the components, 117
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especially of the metal net used for the inner basket. Very high temperatures are reached and the use of low-quality metal-nets has to be supposed. When analyzing the possible wears in time and, hence, the needs for maintenance, the metal-basket is probably the most exposed component. The quality of this element is the most crucial one to be considered for dissemination. The life-span of a basket is still to be evaluated, but, probably, the inner duct will need to be replaced more often than the remaining basket structure.
Test SHARP EDGES AND POINTS COOKSTOVE TIPPING CONTAINMENT OF FUEL OBSTRUCTIONS NEAR COOKING SURFACE SURFACE TEMPERATURE HEAT TRANSMISSION TO SURROUNDINGS TEMPERATURE OF OPERATIONAL CONSTRUCTION CHIMNEY SHIELDING FLAMES SURROUNDING COOKPOT FLAMES EXITING FUEL CHAMBER, CANISTER, OR PIPES
Individual Value Rating Best 4 Good 3 Fair 2 Poor 1
Overall Rating Best Good Fair Poor
Value
mlc 1 2 3 4 5 6 7 8 9 10
4 4 4 3 2 4 4 2 4 4
Total Score 93 ≤ Tot ≤ 100 84 ≤ Tot ≤ 92 76 ≤ Tot ≤ 83 25 ≤ Tot ≤ 75
Table 15. Summarized safety scores for mlc. 118
Weight
Safety has been evaluated during the last runs according to Iowa State Safety Rating protocol, as suggested by the Lima Consensus (PCIA, 2011). This protocol evaluates many topics, involved in safety and gives an overall rating. In Table 15 a summary of the scores obtained is shown.
x x x x x x x x x x
1.5 = 3 = 2.5 = 2 = 2 = 2.5 = 2 = 2.5 = 3 = 4 = Total
6 12 10 6 4 10 8 5 12 16 89
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4.2 Efficiency Since the final task is cooking, efficiency has been calculated and used in this perspective. Besides indicating the level of combustionperformance, the efficiency value also reflects how well the heat is transferred inside the pot, so when reading efficiency values these two main contributors should be considered. Obviously, both of them are effects that are sought. The development has been following a positive trend both for mlc and for MCC, as is clearly visible in Figure 49. 21% 19%
Efficiency
17% 15% 13% 11% 9%
05/11/11
31/12/11
22/10/11
11/08/11
17/12/11
08/10/11
28/07/11
03/12/11
24/09/11
14/07/11
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10/09/11
30/06/11
27/08/11
16/06/11
13/08/11
30/07/11
16/07/11
02/07/11
18/06/11
04/06/11
21/05/11
07/05/11
26/03/11 16/12/10
23/04/11
12/03/11 02/12/10
09/04/11
26/02/11 18/11/10
7%
10% 9%
Efficiency
8% 7% 6% 5% 4%
20/10/11
06/10/11
22/09/11
08/09/11
25/08/11
02/06/11
19/05/11
05/05/11
21/04/11
07/04/11
24/03/11
10/03/11
24/02/11
10/02/11
27/01/11
13/01/11
30/12/10
3%
Figure 49. Efficiency values of all recorded runs with mlc (top) and MCC (bottom). The black line is a linear best fit of the data. 119
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In the case of mlc another important result has been achieved, greatly lowering the fuel consumption. In Figure 50 only the runs where vigorous boiling has been obtained are reported. In the first ones, vigorous boiling was obtainable only lowering the water quantity, 1.5 l or 2 l, and the amount of needed rice husk was slightly higher.
Specific Consumption (g/l)
800 700 600 500 400 300
12/01/12
29/12/11
15/12/11
01/12/11
17/11/11
03/11/11
20/10/11
06/10/11
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02/06/11
19/05/11
05/05/11
21/04/11
07/04/11
24/03/11
10/03/11
24/02/11
200
Figure 50. Values for specific consumption. Only runs where vigorous ebullition has been achieved are reported.
The burning rate, also, appears to be connected with efficiency. The most efficient runs with mlc have proved to burn the fuel a little bit slower, hence reducing the burning rate, but being much more effective in delivering the heat inside the pot. These effects should be particularly sought, because they prolong the useful-operation time and the heat transfer, with, substantially, the same initial fuel charge. This trend is shown in Figure 51. This effect is much less clear in the runs with MCC, but this result has been probably obtained in mlc by adapting a good proportion between fuel quantity and draft. This optimization has still to be done in MCC.
120
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28,00
Burning Rate (g/min)
26,00 24,00 22,00 20,00 18,00 16,00 14,00 12,00 10,00 8%
10%
12%
14%
16%
18%
20%
Efficiency
Figure 51. The Burning Rate values plotted against the efficiency in all the mlc runs. The black line is a linear best fit of the data. Visual observations during the runs have suggested that a reliable and efficient operation were closely linked. This had seemed to be induced also by charging the right amount of start-up fuel, charcoal in the case. Analyzing the data, separately for each different stove size, this effect is clearly visible, as shown in Figure 52. This is not due to the influence of the charcoal term in the used efficiency formula; typically, doubling or halving the charcoal amount, in a calculation, influences the final result by 1%. This could also possibly be linked with the amount of charged rice husk. The above results seem to confirm what visual observation had suggested: mlc works well with 0.05 kg of charcoal and MCC with 0.15 kg of charcoal. Since mlc holds roughly 1 kg of rice husk and MCC has been charged with 3 kg in all the last runs, a rate of 0.05 g of charcoal per each kilogram of rice husk, seems to be optimal.
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20%
Efficiency
18% 16% Runs
14%
Average
12% 10% 8% 0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,10
0,11
0,12
Start-up Coal Quantity (kg)
10% 9% 8% Efficiency
7% 6% 5% 4% 3% 2% 1% 0% 0,000
0,050
0,100
0,150
0,200
0,250
0,300
Start-up Coal Quantity (kg)
Figure 52. Charcoal amount vs efficiencies. Runs with mlc (top); runs with MCC (bottom). Finally, efficiency values may not seem very high, but when performances are compared with those obtained by other technologies (up to 60% for LPG stoves), the quality, availability and dependability of the used fuel should be considered.
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4.3 Emissions Besides permitting a better understanding of the various phases of operation, the emissions data have been acquired in the chimney to evaluate the impact on environment of the proposed solution. Data for CO contamination has also been acquired, at a 1 m distance from the stove, to confirm that the induced draft eliminates most of the indoor air pollution related to the use of the stove. Both outputs have been satisfactory. There are no laws on emissions in most of the contexts where this solution could be implemented, but, nevertheless, the issue is highly important, especially with regard to the number of possible users. Where limits on emissions are set, they are frequently related to the used fuel, however it is quite hard to find references for small systems, powered with rice husk. Hence, values for CO emissions from the stove have been calculated and compared to the European limits set by norm UNI EN 303-5 for heating and cooking systems under 50 kW powered with solid fuels, ie coal, wood and derived products. This norm sets as upper limit (in the worse class) for CO concentration 25000 mg/Nm3 (at 10 % O2), which is complied with, in the runs held on 28-12-2011. For this calculation the data from the analyzer have been averaged on the whole duration of the run and this value has then been proportionally rescaled with reference to a 10% O 2 content, as required by the norm. The results of this procedure are shown in Table 16. Date 28/12/11 28/12/11 28/12/11
Run 1 2 3
CO mg/Nm3 16500.00 14237.06 11135.56
Table 16. Averaged values for CO in the flue gases. The values are referred to a 10 % O2 content. 123
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It is quite difficult to find limits for NO x emissions from low power systems, so comparisons have been made with limits required for larger applications, typically wood-chip fueled co-generation plants. NOx had been previously measured during the runs held on 31-102011 and 4-11-2011. The results from these runs are shown in Table 17, the values are referred to a 13% O2 content in the flue gases and are complying the Italian limit set for plants up to 6 MW, which is 500 mg/Nm3 (D.Lgs. 152/2006). Hence, these “costly� data have no longer been acquired. Date 31/10/11 11/04/11
NOx mg/Nm3 301.76 364.40
Table 17. Averaged values for NOx in the flue gases. The values are referred to a 13 % O2 content.
CO [ppm]
Furthermore, indoor air pollution has been evaluated measuring CO concentration in the living environment. Values have been acquired continuously for the duration of the three runs held on 28-12-2011, also between the runs. Hence, collecting the measurement also when the stove gets opened. Results show that the values of concentration are always way lower than the limits set by the indoor air quality guidelines, issued by WHO (2005) which are 100 ppm and 30 ppm, respectively averaged on 15 minutes and 60 minutes. Figure 53 shows that limits are largely complied with, in every single reading. 30 20 10 0 8.24
9.36
10.48
12.00
13.12
14.24
15.36
time [h:min]
Figure 53. Indoor CO concentration values acquired during the runs held on 28-12-2011. 124
CONCLUSIONS IMPACT & DISSEMINATION The first MDG calls for eradication of hunger and extreme poverty, but without setting any specific target on household energy-access or, more specifically, on cook-stoves. Lowering indoor air pollution is the other main target, the entire household would benefit from this, primarily raising health conditions for women and children. It is widely agreed that these issues are of central relevance, but in the panorama of available technologies, 'clean' solutions are available only starting from an intermediate income-level. In fact, advancedbiomass-cookstoves are often not suitable for the most critical contexts, which are left with just a few little improvements that can be readily implemented, as is evident from the N'Djamena example. Data provided by the Chadian Ministry for Agricultural Activities about rice production in 2010, in the Logone Valley, 9553 tons were produced (79% in the dry season and 21% in the rainy season). This results in a by-production of husk of 1911 tons. 3613 people were identified as producer, resulting in an average production per producer per year of 2.64 tons of rice and 0.53 tons of husk. This could fuel roughly 500 runs of mlc per producer per year, providing a complementary alternative to traditional fuels currently used by
CONCLUSIONS______________________________________________________
producers’ families. In more general terms, social and economic consequences of freeing up time from wood collection could prove of paramount importance. 'Scaling up stove and fuel programs can provide a transformative tool for sustainable development' is stated in the last relevant report by the World Bank for Development (WORLD BANK, 2011). The US Department of Energy is also starting to pay a more careful attention to these issue and a large program on biomass cook-stoves is under consideration, involving the academia, the research community and the private sector (DOE, 2011). Nevertheless, forecasts for possible large dissemination projects of new technologies are quite difficult, because of the wide variety of settings and, mostly, because of the dependence upon governmental strategies. For smaller-scale dissemination, there are many conceivable financing tools to be used, as Clean Development Mechanism (CDM) for example, which have proved to be successful when applied ethically, but at the present time only 3 cook-stove projects are registered (WORLD BANK, 2011). The role of effective-improved-cookstoves, as the one presented in this work, could prove quite appropriate, being cheaper and more self-sustainable.
ALLIANCES & NETWORKS In order to be effective these projects need a multi-sectorial approach. Understanding the interconnected effects of technologies, policies and programs with the given setting requires many different competences. With this approach, collaborations among researchers and more wide 126
______________________________________________________CONCLUSIONS
alliances can play a particularly useful role. It is under this condition that it is helpful to separate a more (context-oriented) vertical approach from the (process-oriented) horizontal one. Today the community of researchers working on this topic is quite small, with respect to the possibly involved beneficiaries, but mostly it is poorly linked, hence early in the learning curve for both design and dissemination of new effective technologies. The Global Alliance for Clean Cook-stoves could be quite helpful in overcoming this situation, as also the ground-agreements stated in the 'Lima consensus' for widely agreed targets could induce more confrontation in the cookstove-developing community. In July 2011 the Italian community working on cook-stoves, biomass gasification and bio-char has started the 'Smart-Cooking' network, following the Smart-Cooking workshop held in Varese, Italy. 'Fuoco perfetto' is another Italian initiative, focusing mostly on dissemination and media documentation (FUOCO PERFETTO, 2011).
THE LACK OF DEVELOPMENT TOOLS Modeling is almost completely missing in this field, but hopefully this gap will be filled, as the international community is starting to stress this issue for future research activities. The availability of such informations would influence many fundamental decision, as combustion chamber design, or chimney dimensions, to induce a desired draft effect. Also heat transfer, emissions and gasification/combustion processes, could be better addressed, for example investigating the trade-off between cleaner combustion and 127
CONCLUSIONS______________________________________________________
heat transfer. If the pot is positioned closer to flame heat transfer can be eased, but this flame interruption, may cause incomplete combustion and hence worse emissions. The ultimate goal of these efforts could be the achievement of a sizing software, integrating, besides all of the above models, the performances of the construction materials. It could be quite useful if a design tool is made available to quickly address the optimization of a generic stove model to the particular cooking habits, fuels and local material resources for a given context.
128
APPENDIX 1 – STIRLING ENGINES
History The invention of Stirling machines and so-called hot-air engines dates even before the official birth of thermodynamics (1824 - year of publication of Sadi Carnot's "Reflexions"). These machines have been among the first heat engines to be invented, just after the steam engine. The latter has usually been used for high-power systems, but bringing along a high risk for possible explosions of the boilers, while the Stirling engines occupied the niche of low-power, not presenting risks of explosion, but being slow and bulky machines, because they worked at atmospheric pressure. The first hot-air engine (Fig. 54) was built by the Scottish brothers Robert and James Stirling in 1816, even if the patent was filed only for the so-called "heat economizer", ie the regenerator, which represented the true heart of the machine. The first prototype, built by the Stirling Brothers, is a single-cylinder, singleacting and had a long displacer piston: the regenerator was not a physically separate entity, but consisted of the long gap that resulted between the displacer and the cylinder wall, where the air was forced to pass at each cycle. Hot air engines have been used considerably during the last century and for a long time it seemed that they could replace the steam engine for the production of mechanical energy. This engine, however, was soon abandoned because of the frequent breakdowns of the hot exchangers that were in direct contact with the flames, as the materials of the time did not guarantee sufficient heat resistance. Thanks to great advances in material-technology, especially for heat exchangers and regenerators, the Stirling engine can now overcome the main obstacles for the achievement of high performances. However, Stirling engines have been replaced by internal combustion
Appendix 1 __________________________________________________________
engines and electric motors, which did not have the same technological difficulties (seals, dry lubrication, heat-resistant materials and a low power to weight ratio). Novel interest for this engine was due to the special needs of the military industry in the late '30s, Gloeilampenfabrieken Philips in Eindhoven (Holland) commissioned its own technicians to create innovative small generators, that could power radios and other electrical equipment to be used in areas not served by grid electricity. Until the '50s the Philips Stirling generator was still produced in quite large numbers. Silent operation was the main advantage. Though dating its origin at the beginning of the 19 th century, this type of engine has been recently rediscovered and is subject of extensive investigation because of his multi-fuel adaptability and also because of the remarkable performance progress due to modern materials, which have allowed to overcome some of its severe limitations and poor operational performances.
Figure 54. The first Stirling engine (patent of 1816) A: cylinder, B: furnace, C:-separator piston (displacer), D: power plunger, E: cold space, F: hot space 130
__________________________________________________________Appendix 1
The revival of the Stirling machine is recent history, the first energy crisis of 1973 suggested the urgency of developing new options, now largely enabled by modern materials and technologies, as the use of new temperature-resistant steels and alloys or the use of other internal fluids, instead of air, operating at higher values of mean working pressure.
Working principle The term Stirling machine is used to address a wide range of fluid machines in which the fluid itself (usually a gas) undergoes a close and regenerative thermodynamic cycle, close to the ideal Stirling cycle (consisting of two isotherms and two isochores transformations). This ideal cycle has the same efficiency as the Carnot cycle (Walker, 1973) and this can be a great advantage for a simple machine. The engine encloses a working fluid that is heated at one end and cooled on the other side. In its most simple configuration it has a power piston that varies the internal volume and a displacer that forces the working fluid against the hot end or the cold end. The combined motion of the power piston and of the displacer, at first, compress the working fluid and allows its heating, then induces expansion and the cooling, allowing the extraction of useful work from the engine (Fig. 55), since compressing a cold fluid requires less energy than the one obtained by the expansion of the fluid, when hot. Power densities depend mainly on the kind of fluid used and on the mean internal pressure, but even in more technologically advanced products this figure is pretty low (Picchi, 2009). For power requirements one major issue of hot-air engines is the need to pressurize the working fluid (Senft, 1991), which usually induces to enclose the entire engine in one big pressurized vessel. Helium or 131
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hydrogen, at mean working pressures up to 20 MPa, are used in some high-end solutions but, still, these machines are quite big with respect to internal combustion engines. This generates two of the main hot-air engine's problems, sealing and friction. These problems usually lead to high-tech solutions which require expensive materials and construction techniques. For development cooperation purposes, the mean internal pressure should not be very high, but this doesn't solve the problem. For these contexts air should be used as the working fluid of the engine and the mean internal pressure should be kept between 0.3 – 1 MPa (Senft, 1991), resulting in a very big engine, with respect to it's power output, but allowing the use of much simpler and cheaper components.
Figure 55. The four main phases of the cycle: compression, heating, expansion and cooling. There are many possible configurations for the Stirling engine, mainly depending on how the cold and hot end of the engine are placed, in the same or in separate cylinders (Walker, 1980). The three main configurations are called alpha (two pistons in two distinct cylinders), beta (piston and displacer in the same cylinder) and gamma (piston and displacer in two distinct cylinders). The engine shown in Figure 54 is a beta configuration, while the one in Figure 56 is a gamma.
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Context In non-grid-connected locations of developing countries the small needed amount of electric energy is usually generated with internal combustion engines, requiring a regular supply of fossil fuels. High power is typically not necessary, 1kWe is enough for a refrigerator in a remote medical settlement, for a small incubator in a village farm or for few appliances in a workshop. Hence, the Stirling engines, besides its intrinsic capability of adapting to various fuels, appears as an appropriate solution, also due to its mechanical simplicity which is absolutely necessary for an easy operation and maintenance on site and, eventually, also for local reproduction. This solution could be useful to generate both electricity and heat using a locally available source in alternative to the simultaneous use of coal, gasoline and wood. It is almost always possible to find some residual biomass from local production and agricultural residues are often burned just for disposal, while their energetic exploitation could relieve the local ecosystem of some of its deployment (Anderson, 2004). Besides considering the comparison to internal combustion engines, Stirling engines may also be more convenient than Organic Rankine Cycles (ORC), when low thermal powers are available. Indeed, systems below the 30 kWe power-threshold could benefit from the adoption of external combustion engines, especially if performances are comparable, with values for efficiency being typically around 10% for a small ORC.
Ringbom architecture The Ringbom architecture (Fig. 56) is one of many variations of the
133
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Stirling engine and drops the mechanical linkage of the displacer to the crankshaft, relying only on the internal pressure's variations to drive the displacer's motion, thanks to a rod, connected to the displacer, that exits the engine (Senft, 1993). The external atmospheric pressure, acting on the rod, is enough only if the engine is not pressurized, otherwise an auxiliary spring will be needed to compensate a higher mean internal pressure, avoiding the need for a pressurized external vessel of the whole engine (Olbermann, 1989). This may appear as an extra problem in the sizing and developing stage of the prototype, but could then result in a cheaper and easier-tooperate machine, which is one of the main goals for appropriate technologies.
Figure 56. The prototype by Ossian Ringbom. Efficiency performance is hard to determine in the development stage, especially because this type of engine do not have a standardized 134
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power model that can be considered trustworthy (Arquès, 1994; Senft, 1985). The ideal engine has a Carnot efficiency, that for large temperature gaps can reach a very high value, but this does not mean that very high efficiencies can be reached with the present technology at a reasonable cost, not even in a developed country's perspective. The overall electrical efficiency that can be targeted is, reasonably, below 10%, which is quite low on an absolute basis, but could be evaluated differently considering that the fuel is typically costless (economically and environmentally) and that technologies and production could be not so expensive.
Metal-bellows One of the main drawbacks of the Stirling engine are sealing concerns and the relevant friction losses. High technology solutions have obviated this limit rising the power density through the choice of selected and highly pressurized gases (Henry, 2008). This increases the rotational speed and the power of the engine without increasing dimensions, thus reducing the friction losses' impact on performances. The 'air engine', as the one in discussion here, has other needs (Organ, 2007; 2009). These engines are typically quite large and have a very low power density, so a different approach is necessary. To be appropriate for use in developing countries, air is the only working fluid that can be realistically used and only a low pressurization of this gas can be admitted. To obviate the friction problem, that in these conditions can completely compromise the correct operation of the engine, metal bellows have been investigated in order to avoid the whole sealing concern. Metal bellows can be used instead of the typical cylinder-piston arrangement. This eliminates the need of sealing between moving 135
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parts, since there is no slit for the working fluid to pass through. It also removes most of the friction losses, that are so crucial in a bigdimension and low-power engine. The bellows also work as a spring and this compensates, at least in part, the difference between the mean internal pressure and the external pressure that acts on the displacer's rod and that triggers it's motion in the Ringbom configuration (Senft, 1993). Where it is not possible to completely compensate all the pressure difference with this effect, an auxiliary spring can still be placed at the external end of the displacer's rod. The use of hydro-formed metal bellows has been implemented initially only on the displacer's rod side (Fig. 57), while the main advantage would be achieved only when a metal bellows (or perhaps a high temperature silicone bellows) will be placed also on the cold end of the engine, thus eliminating the whole power-piston/cylinder arrangement, which is the main responsible for friction losses. This larger component has to be linked to the crankshaft much like a traditional piston, but will be developed only in a second phase, since some insight on metal bellows used in this unconventional way has to be gathered, testing the smaller one. The shape of the bellows, the number and extension of the bends, the thickness of the metal are all crucial issues for a correct operation, but the most critical problem is related to the pressure difference that the bellows (and the relevant welding) have to withstand in motion. Commercially available bellows are built for other purposes and it is hence difficult to find moving bellows that withstand a high pressure differential across it's wall. This innovative component has to be tested experimentally, since it is very difficult to foresee how the metallurgical, structural and technical problems sum up. Commercially available bellows, with a low spring constant, only bear 136
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a pressure difference of about 0.1 MPa, while very hard ones can withstand high pressure dierences. A very important aspect is the development of bellows that have a good compliance and that, at the same time, are able to bear pressures in the order of 0.3-1.0 MPa.
Figure 57. A detail of the metal bellows placed on the displacer's rod.
Mathematical model The thermodynamics and geometry of the engine have been analyzed through a mathematical model. Starting from the sinusoidal motion of the power piston and the displacer's position, the mathematical model calculates the volume variation of the hot, cold and dead regions inside the engine and hence defines the internal pressure of the working fluid, as a function of the piston's and displacer's position. Setting the dynamic equilibrium of the forces that act on the displacer and considering also the viscous damping effect of the limit-stops that bound the displacer's motion, it is possible to obtain the motion equation for the displacer, which can be numerically integrated to give the displacer's position. Subsequent calculations provide the 137
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thermodynamic cycle shown in Figure 59 and an estimate of the ideal's machine output power together with it's efficiency.
Figure 58. The section of the Ringbom-Stirling engine. Image: Daniele Zani. The model has been implemented (Arquès, 1994; Senft, 1985) to set the main parameters and to size the components of the RingbomStirling engine, whose section is shown in Figure 58. This model has been defined under the following assumptions: 1. 2. 3. 4.
The working fluid is a perfect gas. The working fluid's mass is constant. The pressure is instantly constant in space. The internal working volumes are isothermal regions in space and time. 5. The power piston's motion is sinusoidal. 6. The displacer's motion is bound upwards and downwards by limit-stops, modelled as viscous dampers. 7. The forces acting on the displacer are gravity, pressure and the elastic return of the spring. 138
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The parameters set that has been chosen for the prototype leads to an output power of about 1.5 kW and an 18% efficiency, considering air as working fluid and a 0.3 MPa mean internal pressure (Invernizzi, 2010). These results are very far from what is possible to really achieve from the prototype engine, shown in Figure 60, but still these calculations are extremely important for the sizing of the components of the Ringbom version of the Stirling engine, mainly because through the balancing of the various parameters it is possible to define the displacer's motion, which is the key to a proper operation. When the displacer reaches the hot or the cold end it has to come to a full stop before it starts to move backwards, this 'stop' is represented by the flat part in the displacer's position graph, which is clearly visible in Figure 59. This short stop should help both the heating and the subsequent cooling of the fluid. The result is a displacer that moves in phase with the power piston, but without any mechanical linkage to the power piston's crankshaft. This happens only below a certain rotational frequency, which is just above 12 Hz (720 rpm) for this prototype, while the flat part in the displacer's motion graph is still evident. Approaching this upper limit the flat part shrinks and eventually disappears. At higher rotational speeds the displacer starts moving irregularly making useful power extraction impossible.
139
12
150
10
125
8
100
6
75
4
50
2
25 0
0.03
0.06
0.09
0.12
0.15
0.18
0.21
0.24
0.27
Piston displacement [mm]
Displacer displacement [mm]
Appendix 1 __________________________________________________________
0.3
−2
− 25
−4
− 50
−6
− 75
−8
− 100
− 10
− 125
− 12
− 150
time [s] Displacer (red line) Piston (blue line)
6 5.5 5
Pressure [bar]
4.5 4 3.5 3 2.5 2 1.5 1 −4 6×10
−4
8.4×10
1.08×10
−3
−3
1.32×10
−3
1.56×10
−3
1.8×10
−3
2.04×10
2.28×10
−3
−3
2.52×10
−3
2.76×10
−3
3×10
Volume [m3]
Figure 59. Results from the numerical simulation, (above) the power piston's and the displacer's positions in function of time; (below) the thermodynamic cycle in the pV plane. 140
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Prototype In the first prototype metal bellows have been used only on the displacer side, while a traditional piston/cylinder, with a jacketed water-cooling system, has been placed on the cold side of the engine. As soon as some experience is gathered through the development of the displacer's metal bellows, also the power cylinder-piston could be substituted by bellows. The mechanical prototype has been built and motion is satisfactory, but heat exchangers are still missing for the engine to operate.
Figure 60. The mechanical prototype.
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142
___________________________________Publications, Conferences and Seminars
PUBLICATIONS, CONFERENCES AND SEMINARS
“PROGETTAZIONE DI UN MOTORE RINGBOM-STIRLING PER LA PRODUZIONE DI ENERGIA ELETTRICA NEI PAESI IN VIA DI SVILUPPO”, 1° Congresso Nazionale del Coordinamento della Meccanica Italiana, Palermo, 20-22 June, 2010.
“DISTRIBUTED ELECTRICITY GENERATION USING AN EXTERNAL COMBUSTION RINGBOM-STIRLING ENGINE HEATED BY AGRICULTURAL WASTES”, XXVIII UIT Congress on Heat Transfer, Brescia, 21-23 June, 2010.
“A BIOMASS POWERED RINGBOM-STIRLING ENGINE FOR DEVELOPING COUNTRIES: A LOW-BUDGET SOLUTION FOR DISTRIBUTED ELECTRICITY GENERATION” International Conference on Renewable Energies and Power Quality (ICREPQ’10), Granada (Spagna), dal 23 al 25 Marzo, 2010.
“A
PROTOTYPE OF A RICE HUSK STOVE AS AN APPROPRIATE TECHNOLOGY FOR HOUSEHOLD ENERGY IN THE LOGONE VALLEY (CHAD – CAMEROUN)”, II Congresso nazionale CUCS, “La cooperazione universitaria e la sinergia con la società civile e le imprese”, Padova, 15-16 september 2011.
143
Publications, Conferences and Seminars__________________________________
“ESPERIENZA DI RICERCA: GENERAZIONE DISTRIBUITA DI ENERGIA ELETTRICA DA BIOMASSE DI SCARTO" Energia nei Paesi a risorse limitate: tecnologie appropriate per la valorizzazione energetica di biomasse e rifiuti organici, V Corso di aggiornamento, CeTAmb, Brescia, Italy, 8-9 July, 2010.
“ANALYSIS OF TECHNOLOGIES ENERGY EFFICIENCY” Appropriate Technologies for Environmental Management in Developing Countries, CeTAmb Summer School, Brescia, Italy, June 20-24, 2011. “RESEARCH AND DEVELOPMENT OF AN APPROPRIATE RICE HUSK BURNING STOVE” Smart Cooking - International Workshop on Improved stoves, Gasifier stoves and other cooking technologies for cooperation projects, Varese, Italy, June 30 – July 1, 2011.
144
____________________________________________________Acknowledgements
ACKNOWLEDGEMENTS The research discussed in this work has been carried out in close collaboration with Francesco Vitali and 'CeTAmb'. We wish to thank him and all 'CeTAmb' staff for their competence and support. We wish to thank Adriano Maria Lezzi for his continuative support, as well as Costante Invernizzi, Angelo MazzÚ, Mariagrazia Pilotelli and Valerio Villa for their precious suggestions. A special thankyou also goes to many other DIMI members, including Ph.D. Colleagues. We wish to thank also some students, both graduate and undergraduate, that have helped in the project with their stage or thesis, Michele Fontana, Daniele Zani, Fabio Zappa, Fabio Magagnini and Marco Lorandi. The research discussed in this work is part of a project that has been among the winners of the 'Rotary Enfasi Award 2009' and has been substantially funded by Rotary International – Districts 2030, 2040, 2050, 2070, 2080, 2090, 2100, 2110, 2120. We wish to thank Edoardo Mattei and 'Torneria Automatica' for his help in the realization of the Stirling engine prototype. We wish to thank Sandro Bani and 'ANFUS' (Associazione Nazionale FUmisti e Spazzacamini) for the help in the emission testing set-up for the cook-stoves and drafting suggestions. We wish to thank Salvatore Inturri, 'IPC Golgi High-School' and the relevant students for the help in the characterization of rice husk and its ashes. We wish to thank Carlo Cardana and 'Riso Ticino' for his help in providing the rice husk, used in the experimental campaigns. The biggest thank-you goes to all my family, that always supports me and to all the friends that always incourage me. Last, but not least, thanks Alessandra, for bearing me during the preparation of this work. 145
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