Micro-gasification: Cooking with gas from biomass - Amper

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Micro-gasification: Cooking with gas from dry biomass 3.2.2 Particle size and particle size distribution What is the impact of fuel size on the gasification process? The size of the fuel in the fuel-bed determines how easily gases can flow and travel through the fuel-bed, whether this is incoming air or outgoing wood-gas or char-gas. The fuel size also dictates how fast the heat from the flaming pyrolysis conducts down the fuel stack. The fuel size and shape is not generally critical to a stove operation, but the fuel has to be in the range of the acceptable properties to allow use of proven stove designs without modification. Any new fuel needs to be tested and the operating conditions adjusted for the particular properties of a new fuel source. What is a suitable size of fuel for a micro-gasifier? What is „too small or too big‟? In general, granular and ‗chunky‘ material, which will enable an appropriate and steady gasflow through the fuel-bed in the combustion chamber is preferred for a natural draft device. In general particle sizes seem to work better when the minimum is 6 mm length for the smallest dimension,, so the dimensions should preferably be between 6x6x6 mm - and 60x60 mm x height of the combustion chamber (like a big briquette or straw bundle, that can be placed vertically in the combustion chamber). A rule-of-thumb by Hugh McLaughlin says that the average of the lengths in all 3 dimensions of a fuel particle should be less than 10% of the diameter of the fuel container. In other words, if a fuel container is 15 cm in diameter, the particles should not exceed an average of 15 mm length over the 3 dimensions. Too small particles will block the gas-flow. The restrictive effect of fine particles like sawdust or rice husks on flow can be overcome either by forced convection through a fan or a blower, or (less desirably) with more draft through a tall chimney. Large chunks of fuel create three problems. First, a thick object takes longer for the pyrolysis to reach the center of that biomass particle. The initial pyrolysis leaves behind a layer of charcoal that actually insulates the center of the particle. As air continues to reach that part of the pyrolysis front, this favors the conversion from char to ash and can result in a considerable increase in temperature (which may cause physical stress on the material of the fuel chamber). It adversely affects the char yield, which is an unwanted effect if char should be saved for further use. It is also harder to get the flaming pyrolysis front to progress compactly and uniformly down a bed of larger particles, with the result that the end of the burn is less precise, resulting in some uncarbonized material in the center of the lowest large particles and other sections of char being burned to ash. The second problem is that too big chunks create big spaces between the fuel, which are filled with air under normal conditions. Unrestricted air-flow might lead to excessive primary air in the fuel-bed. The particle size distribution defines the ‗pore space‘ between the fuel and therefore the ease of gas passage: Gas will follow the ‗open corridors‘ that are not blocked. So if fine particles block gas passage in a particular zone of the fuel bed, the gas will find its alternative ways through other zones in the fuel bed with bigger gaps between bigger fuel chunks. This will lead to a very uneven flaming pyrolysis front, likely to result in smoke and incomplete fuel use. Third, when big chunks create cavities between them and finer particles rest above, ignited finer material can fall by gravity in the cavity beneath the pyrolysis front. This will cause an abrupt increase in the gas production, which often cannot be combusted with the available secondary air. The result is undesirable smoke. Paul Anderson suggests that the longest dimension of the fuel particles should not be greater than 25% of the diameter of the fuel container, so that fuel that is casually dropped into the container can fill in the voids between the particles. HERA – GIZ Manual Micro-gasification Version 1.01 January 2011 73
Micro-gasification: Cooking with gas from dry biomass An exception to this is the use of longer fuels that are intentionally placed (not casually dropped) vertically into the fuel container. Examples include segments of bamboo, bundles of grasses, and some stick-wood that is not excessively contorted. These vertical piles often have many long channels for the primary air. In this case, a second type of fuel that is smaller can be added to the top and (usually with some shaking) loosely fill those channels, preventing any ignited fuel from dropping to lower positions. In general, the initial difficulties about fuel selection and loading are soon overcome when local people gain experience and have their own preferred fuels and procedures. Conclusion: a fuel should be reasonably uniform to prevent blockages and unequal movement of the pyrolysis front, as this may create smoke. However: Micro-gasifiers have a big comparative advantage in the range of fuel sizes, including those fuels that are otherwise too small to easily be burnt cleanly in other stove models. HERA – GIZ Manual Micro-gasification Version 1.01 January 2011 74
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Micro-gasification: Cooking with gas from biomass - Amper

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