Biogas en

Biomass cogeneration plants and composting plants

BIOGAS and Composting PRIMARY MATERIALS AND FUNDAMENTAL PRINCIPALS

BIOMASSES USED IN ENERGY PRODUCTION There are several types of primary materials which can be used in a biogas energy production system:

Primary material from industry and agriculture food industry waste: •

meat processing

•

fish processing

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milk and cheeses: curds and whey

•

fruit processing - citrus fruits

•

canned products

•

juices from fermentation processes

•

starch

•

sugar

•

coffee processing

•

grape and wine processing

•

waste waters: organic sludge

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Biomass cogeneration plants and composting plants

Non- food industrial waste

•

farmed or natural seaweed

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black liquor from paper manufacturing

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scrap wood

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textiles

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paper industry

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waste waters from the organic chemistry sector

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others

Other Sources: Specific agricultural crops: corn, sorghum, cereals in general, farm waste such as cornstalks, straw, branches etc.

Crops grown specifically for energy production: poplar, miscanthus, cane, willow, kenaf, cardoon etc.

It is important that the planning of primary materials occurs in compliance with the law, guarantees sufficient space for the management of primary materials and access to transport.

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Biomass cogeneration plants and composting plants

DESCRIPTION OF THE PROCESS

Microbiological degradation technology using organic material with a high-content of volatile organic substances (which determines its level of putrescence), enables the use of these primary materials for the production of biogas in a cogeneration power plant which produces electrical and heat energy.

Residual heat from the cogenerator is decoupled from the process and used separately as heated water (90°C) or via subsidiary techniques where it can be used either as steam or cooled.

Substances resulting from anaerobic digestion can be further stabilized at the bottom of the digester in a composting plant for the production of high agronomicvalue organic stabilized fertilizer. In this way it is also possible to avoid the costly task of stocking such odorous material.

Biogas is produced through a microbiological conversion process, in the absence of oxygen. This process involves the breaking down of complex organic substances (lipids, proteids and gluceides) present in plants and animal byproducts, by micro-organisms.

The resulting gas is as versatile as natural gas and usually contains 50 - 70% methane. The remaining proportion consists of CO2.

The heat of combustion of the gas varies according to its methane content but on average is approximately 23,000 kj/nm3. Biogas produced in this way is then cleaned, accumulated and can be used to fuel gas boilers coupled with turbines for the production of electrical energy, in combined cycle power stations or internal combustion engines.

Biogas can also be upgraded to bio methane through the separation of the nonmethane part.

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Biomass cogeneration plants and composting plants

Flow Chart of Integrated System

ANAEROBIC DIGESTION=>BIOGAS=>COGENERATION=>ELECTRICAL ENERGY=> HEAT AND HOT WATER SLUDGE THICKENNING=>LIQUID=>PURIFICATION PLANT=>SEPRATION OF SLUDGE=>PURIFIED WATER=>PRESSED SLUDGE=>COMPOSTING=>ORGANIC FERTILIZER SCRAP WOOD AND MATERIAL SLUDGE=>COMPOSTING=>ORGANIC FERTILIZER

SOLID

ORGANIC=>

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PRESSED

Biomass cogeneration plants and composting plants

HYDROLYSIS AND DIGESTION The methanization process: Prior to this phase, all physical and chemical conditions are created inside the digester in order to begin the biogas cycle.

Methanogenic bacteria use organic acids and salts produced in previous phases and transform them directly into methane and carbon dioxide. Such bacteria are characterized by a slow level of growth, solely under anaerobic conditions. 70% of the methane produced derives from the fermentation of acetic acid by acetoclastic methanogenic bacteria; the remaining 30% may derive from the breakdown of carbon dioxide by H2 oxidizing bacteria or the breakdown of any methanol which may have been produced during the first phase. Methanization reactions are catalysed by enzymes in proteins. Acids, alcohols and salts can all be broken down through a variety of reactions: -

acid fermentation: CH3COOH → CH4 + CO2

-

breakdown of carbon dioxide: 4CH3CH2OH + CO2 → 3CH4 + 3CH3COOH

Each of the aforementioned reactions produces energy which is used by bacteria for their vital functions. At the end of the biomass fermentation process, the biogas produced is collected in special gas tanks and waste material from digestion, containing organic material which has not been fully stabilized; this substance can be used as fertilizer. Its complete stabilization through an anaerobic microbiological system inside a composting plant turns it into a high quality and agronomic value soil improver.

The continuous digester can be compared to the digestive system, into which organic materials are regularly fed at one end, and where waste is extracted at the other end. Bacteria on the inside are continuously fed and a constant temperature is maintained.

All available primary materials must be introduced with a liquid effluent which ensures that anaerobic bacteria avoid contact with free oxygen.

It is important that material introduced into a continuous digester contains a diversified proportion of organic perishable substances and vegetable fibres, in order to heighten the digester's performance and production.

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Biomass cogeneration plants and composting plants

CLEANING THE GAS

Dehumidified and purified gas is used in the gas engines in cogeneration plants.

Upon passing through the purification phase, BIOGAS is sent to the cogenerator.

Here, it is fed into the gas engines via a control system, where it is combusted. It is possible to reduce the CO and NOx content of resulting waste gas, through the use of an oxidizing catalyst or a thermojet, within prescribed limits. A catalyser and induction silencer are usually included. Cogeneration heat exchangers are present along the path of waste gas in order to cool the engine and for mixing, and they can be activated upon the manager's request.

THE USE OF GAS The environmental policy process which also concerns the energetic valorisation of biomasses, activated in the wake of the Kyoto conference for the reduction of atmospheric pollution and greenhouse gases (methane is one of the most important), has increased interest in the reuse of biogas. The same can be said for recent EU regulation n. 1774/2002 regarding animal by-products, which identifies anaerobic digestion as a process enabling such by-products to be recycled as fertilizers. There are many benefits: an improvement in the environmental sustainability of animal husbandry farms, extra income from green energy, a reduction of environmental problems linked to emissions and odours, and an improvement in the agronomic utilization of fertilizing elements present in liquid wastes.

After undergoing necessary treatments, biogas can be used in different ways: a) for the generation of heat (or cold); b) for the cogeneration of electrical and thermal energy in bi o tri generation. c) as a bio fuel (BIOMETHANE) or to replace methane after purification and upgrading processes.

This production chain also produces digested sludge which can be used in farming or as a primary material for the production of Compost, a high organic-content substance used for the fertilization of land which can be of excellent quality particularly in the case of phyto-suppressive compost.

The compost and fertilizer production chains are extremely valuable in agronomic, environmental and economic terms

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Biomass cogeneration plants and composting plants

Digested sludge is produced from the same processes used to extract biogas and can be applied in agronomy or even better as a primary material for the production of high quality organic soil improver (compost).

Digested sludge is used as soon as it exits the digesters and is further stabilized in the form of quality organic fertilizer (high content of nutrients for soil), of high agronomic and environmental value insofar as the stabilized product can be sold in bulk or packaged in a variety of ways in MD or abroad where it is highly appreciated.

This is a high organic content product and is an excellent soil improver/fertilizer and can be used to produce phyto-suppressive compost.

The compost and organic fertilizer production process is interesting in economic terms if we consider return and possible markets, aside from its capacity to heighten rates of transformers in the sale of electrical energy.

The solid part of digested sludge, together with ammonium sulphate derived from the stripping of ammonia from liquid in circulation, can be spread in fields as a natural soil improver. However, storage problems arise insofar as it is not a biologically stabilized material and as such produces odours as well as the proliferation of undesirable insects and animals.

Composting ensures a high agronomic and phyto-suppressive value fertilizer derived from digested sludge. The process involves further fermentation, this time aerobic, entirely controlled and confined to a technical premise with special ventilation systems: the building is an integral and technological part of the plant.

Biomass accumulations are periodically moved using special equipment and aired in order to ensure sufficient air flow for material undergoing fermentation: oxygenation improves stabilization and avoids at the same time the production of odours.

Highly fermentable primary materials and masses under production are always continuously supplied and kept inside closed buildings and slightly below ground level.

Fresh air is allowed to enter, however air may only exit via a conduit leading to a biofilter which eliminates impurities or residual organic substances.

Dumping and storage areas are also located within the building, therefore there are absolutely no unstable biological materials outdoors in order to avoid any problems concerning odour, insects or vermin.

Technologies applied for the movement and management of humidity as well as for the regulation of oxygen in masses, ensures that no odorous molecules are produced; all air is passed through a bio-filter as described here below.

Stabilized material must be stored indoors for a further 30-90 days.

Then, this odourless and completely stable material can be spread in the fields as a high quality natural fertilizer, phyto-suppressive or commercialized in bulk or packaged in a variety of ways.

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Biomass cogeneration plants and composting plants

Bio-filters are large enough to effectively filter all plant parts. The bio-filter is a tub which can be underground or at ground level with an adequate thickness of filtering and biologically active mass through which air slowly passes in order to be purified through the elimination of odorous molecules. The odour of the end product is similar to humus.

CHECKS

1. Plant checks are automatic and computer assisted via a PLC; only the manager receives control functions. All fluxes are automatically controlled so that optimal reaction temperatures are maintained and the desired electrical power is produced. All modules comprise individual control cabins which are connected through power, control and IT connections to the central control in the control room. Electrical grids for the management of the entire plant and power divisors are appropriately located. The plant is entirely automated. For functional control and actions required in case of anomaly, there is a PC located in the observatory which enables the monitoring of all values and which is used to intervene on all commands by remote control. An independent electrical power plant for the start up and plant controls is also present. A remote control interface is also installed for management via internet.

2. The entire process is monitored, not only in terms of instruments but also via a series of chemical, physical and microbiological laboratory checks in order to optimize every stage of the process.

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