FUELS BRIQUETTES
Briquetting : a forgotten opportunity Diego Cattaneo - University of Brescia
The briquetting process is an intermediate transformation technology between chipping and pelletisation. Used in the context of a balanced wood-energy sector it may be a convenient opportunity with low investment costs.
Italy
Oleodynamic “briquettes machine” used for the tests experiences.
In the current fossil fuel era, biomass and in particular wood biomass, is more and more important thanks to its “clean” energy characteristics and to new industrial uses for the production of electricity. Technology and the market have now turned towards the use of wood fuels recovered from wood industry residues and from forest management. It is now necessary to programme a complete wood-energy sector to cut the costs of recovering the wood
and to obtain a valid, selected, guaranteed product. To this end the briquetting process is a possible alternative to both chipping, which does not allow for real commercial standardisation of this wood product, and pellets, which require high industrial investment for their production.
USES FOR BRIQUETTES The most frequent applications for t h i s t y p e o f f u e l a re o f b o t h a domestic and industrial nature; from fireplaces or stoves to boilers generating hot water and steam, from wine distilleries as a fuel to use in the production process, to textile industries, farms and gasification for the production of electricity.
THE BRIQUETTING PROCESS
DIEGO CATTANEO
Until recently the briquette was considered the pellet’s “poor relation” because, although energy-consuming transformation processes are necessary to obtain the end product, it does not have the same characteristics of availability such as ease of distribution (deliverable by tanker) and a wide range of combustion systems on offer even for individual homes. Briquetting is the compressing of materials into small logs with a diameter of between 5 and 10 cm and of any length depending on the briquette technology used (screw, piston, oil-pressure). T h e m a t e r i a l s a re s i m p l y c o m -
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pressed without the addition of glues or additives. The briquetting processes have developed in two directions: Europe and the USA have chosen the path of mechanical compression (hydraulic or pistons) while the East has preferred worm screw pressing.
CHEMICAL PROCESSES AND FORCES IN PLAY The densification of the biomass at h i g h p re s s u re d e t e r m i n e s t h e mechanical bond and the increased adhesion between particles, forming the intermolecular bonds in the contact zone. Chemically, cohesion takes place in the following way: the divided solids (chopped biomass or stems) attract the atoms and free molecules in the surrounding atmosphere; the surfaces of the materials form absorption layers which are no longer free to move and remain in close contact or penetrate; the state of absorption is conditioned by the lignin produced at high temperatures induced by pressure; the a p p l i c a t i o n o f h i g h p re s s u re s increases the area and micro-particles of contact. It is important to point out that, as well as the process described above, Van der Waals forces also come into play. They form further chemical bonds, particularly if the biomasses are powdered, which helps the densification of the materials. The resistance of the resulting agglomerate depends only on the type of interaction and on the characteristics of the materials.
FUELS BRIQUETTES MECHANISMS OF HYDRAULIC COMPRESSING
Briquettes with a hole in their centers: the combustion remarkably improved thanks to the greater surface exposed to the flame and the circulating air inside of the hole.
DIEGO CATTANEO
The compressing of the material takes place using a piston powered by an (electric or internal combustion) engine with an oil-pressure system at a high pressure. The operating temperature is over 200°C with pressures varying depending on the engine and the piston used. Generally the material used must have a residual humidity of no more than 14% and be no more than 30mm long to allow it to fall easily into the loading system. Using a three-piston briquette press (machine for textile fibres) it is possible to use materials of less than 3040cm in length avoiding the chopping stage and obtaining identical production outputs. This variation requires an initial vertical pressing of the biomass followed by horizontal. The small log obtained from the briquette process has a density of 1-1.4 kg/dm3, is cylindrical with a varying diameter of between 50 and 70mm and length of between 50 and 100mm for each run of the piston. Using a longer stroke piston and cylinder, the briquette can reach a length of 300mm. Pressing reduces the humidity of the material. The operation of loading can be either manual or automated using a hopper with a volume of 0.5-1m 3 equipped with a mechanical or electric feeler capable of stopping the system when the loaded material drops below a set level. Inside the hopper there is a paddle wheel system which guides the material into the loading box. The required power varies from 5.5-37 HP with an absorption of 15-40 Kwh /t depending on the model, production rate and use (production for medium small or industrial use). A production of 1800 Kg/h is possible with a power of 37 Kw. The work cycle is alternating, synchronous with the movement of the two loading and pressing pistons, and allows the extrusion of a briquette every 15 seconds. The hydraulic oil guarantees a speed of 7 cycles per minute against the 270 cycles per minute of the process of the mould and counter mould press (see below). The slowness of the operation helps reduce wear from friction between the mating parts. Studies on briquetting in Italy have
concentrated on hydraulic systems; the best operating combinations between briquette process times, lower heating power and energy balance were obtained using shrubs (olive cuttings, hedge trimmings or vines pruning) or residues with a high oil or resin content (lavender, mint, coconut). Some tests have made it possible to obtain briquettes even with a humidity level of about 22% in the plant material. The technology’s strong points are its versatility with regard to the materials it can use, the high level of humidity permitted, the limited power required and the uniformity of the briquette. The downside is that the use of virgin biomasses is still at an experimental stage.
TECHNOLOGY USING MOULD AND COUNTER MOULD The mould and piston technology, known also as hammer (made up of a hydraulic cylinder) and mould technology, is being developed above all in Eastern countries. The biomass is pushed into the mould by a reciprocating piston; thanks to high pressure the mass is compressed to obtain a compact product. The standard diameter of briquettes produced using this machine is about 60mm. The energy required by the machine to have a productive capacity of 700 Kg /h is 25 KW. The hammer moves about 270 times per minute
during the process. The positive aspect of this method of briquette manufacturing is the limited wear of the mechanical systems while the widespread use is restricted by high machinery costs, the lack of plants built in Europe and the need for an excessive drying process.
SCREW COMPRESSING MECHANISMS The combined effects of the friction caused to the surface of the container and of the heat due to the internal friction in the rotation of the material and the screw (600 r.p.m.) cause an increase in temperature in the system which helps to heat the biomass; the conical screw forces it through the orifice forming a briquette of the desired shape. During this stage maximum pressure is obtained. In the screw briquette press three separate zones can be identified: feeding, transport (on a belt to drop material into the silo) and the actual briquetting in which the greatest pressure and temperatures are reached (about 250°C). Of fundamental importance in this type of briquetting are the acceptable humidity (8-9%) of materials and the cutting of the biomass which must be as thin as possible. The briquettes obtained do not keep the i r e x tr u d e d sha pe a nd , a s a result, do not seem to be suitable for lengthy storage. Density of the materials in the finished briquette is 1-1.4 kg/dm3 and, thanks to the operating system, there are no
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FUELS BRIQUETTES Effect of temperature on density of briquette as pressure increases
Domestic heating with briquettes.
limits to the length of the log. They are cylindrical with a central hole (photograph 2) obtained using a pin placed in the centre of the extrusion orifice. The powers used are 60 kWh/t with a variable production depending on the speed of rotation of the screw and on the size of the material used; production varies from 2800 - 3600 Kg/h depending on the biomass treated. In this process the biomass is continually extruded from one or more screws through a tapered mould which is externally heated to reduce friction; the high temperature, due to the application of high pressures, fluidises the lignin present in the biomass which acts as a binding agent. In conclusion, the advantages of the screw briquette press are: mature technology, a uniform final product with a high volumetric density and a carbonised external surface facilitating self-ignition and preserving the briquette from humidity upon entry, the briquette’s central hole improves combustion for the passage of air during the first stage of combustion. Disadvantages: the need for materials with very low humidity, the need for high power and the wear and tear of the feeding screw.
CIMAJ
EFFECT OF PREHEATING
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The preheating of plant residues increases the life of the machine’s screw, reduces fuel consumption and increases the final production of the compactor. In laboratories it has been observed that the work necessary for densification can be reduced by almost a half by preheating the unprocessed plant material (Graph 1). It is obvious that for some materials an increase in pressure at a certain temperature increases density; it is more interesting to analyse the following graph which highlights the increase in density of the material as the temperature increases with the same load inside a hydraulic press. This means production is maintained at the same product density. In conclusion, the heating of the product brings about a very import a n t re d u c t i o n i n t h e re q u i re d powe r ( 15- 20%) with obvious energy savings and less wear of machinery.