Micro-gasification: Cooking with gas from biomass - Amper

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Micro-gasification: Cooking with gas from dry biomass mango pips, cardboard, wood shavings, processed biomass briquettes or pelletized grasses, sawdust, lumber yard scrap (Photos Courtesy of WorldStove): Although not all are yet tested for use in a micro-gasifier for cooking, additional candidate fuels and biomass resources are discussed at http://www.gocpc.com/biomass-resources.html HERA – GIZ Manual Micro-gasification Version 1.01 January 2011 71
Micro-gasification: Cooking with gas from dry biomass 3.2 Factors influencing fuel properties 3.2.1 Moisture content What is the impact of moisture in the gasification process? Moisture reduces the net usable energy output of a fuel: Any moisture contained in a fuel is water that will consume 3,21 MJ of energy per kilogram (litre) of water to be evaporated in the process of heating the fuel from ambient to the pyrolysis temperature around 400°C. 18 This energy is not available for cooking, yet increases the weight of the fuel that needs to be provided to the stove. What is the right moisture content? There is no definite answer to this question. The fuel should be as dry as possible in tropical climates. A moisture content between 8 to 20 % seems to be best. Though some stoves with forced air claim to be able to burn fuel up to 30% moisture content, this is definitely not desirable, as too much energy will be wasted to dry the fuel. What happens if the fuel is too wet? Lighting wet fuel is much more difficult than lighting dry fuel. Some micro-gasifiers handle wet fuel better than others: o Micro-gasifiers with a flaming pyrolysis front have a limited tolerance for wet fuel, as the cooling effect caused by fuel moisture cools the flames in the zone of flaming pyrolysis: Imagine the evaporated steam, mixed with the combustible gases, acting as a fire-extinguisher putting out the flaming pyrolysis. In that case the ‗engine‘ of the wood-gas production comes to a halt and the micro-gasifier goes on to a slower oxidation mode known as ―smoldering pyrolysis‖. This oxidation mode does not generate enough useable heat to allow cooking, wastes the fuel, and has a negative effect on the emissions. o allo-thermal (retort) gasifiers, where pyrolysis is only caused by heat in the total absence of oxygen handle moist fuel better. They just lose fuel efficiency due to the energy expended during the fuel drying phase inside the retort. Super-heating water vapour causes cooling of the pyrolysis zone, resulting in less efficient use of the fuel and slower cooking times. Not enough information was found on the effect of higher moisture content in fuels on emissions. This is an open issue for further research. Conclusion: Moisture has negative effects on fuel economy and emissions. A substantial amount of the energy generated through the combustion of fuel is wasted due to the moisture present. The higher the moisture content of the fuel, the bigger the energy loss through evaporation and less energy value of the fuel is available for the intended purpose of fuel use like heating a cookpot. 18 An example with a detailed calculation can be found in the Annex to this module. HERA – GIZ Manual Micro-gasification Version 1.01 January 2011 72
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Micro-gasification: Cooking with gas from biomass - Amper

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