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Wood-gas stoves
[top] [end]Wood gas stovesHalf of humanity cooks over woodfires -- the poorer half. Nearly half the world's wood supply is used as fuel. Improved stoves save fuel and cut health-damaging indoor smoke pollution. One of the most recent prototypes of this improved stoves are the wood-gas stoves. Gasification is the cleanest, most efficient combustion method known. The wood-gas stoves can be started, operated and stopped with very low emissions and can use a wide variety of biomass fuels. These fuels had been called "Woodgas", because they can be easily made from wood or biomass. Various forms are: synthesis gas, typically 40% hydrogen, H2, 40% hydrogen, 3% methane, and 17% Carbon dioxide; producer gas, made by gasifying biomass with air (and therefore containing ~50% nitrogen); pyrolysis gas, similar to synthesis gas, but including lots or water and tar and accompanied by production of 10-30% charcoal. These produced gas which combusted so clean are used in chimney-less stoves to cook, without adverse effects.[top] [end]Principles of Downdraft Gasification for cooking[top] [end]GasificationAcording to Reed [2006], coal pyrolysis produces typically 80% fixed carbon, 20% gas and volatiles. The principle step then for coal gasification is getting that carbon to be a gas with either oxygen, CO2 or water2C + O2 forms 2 COi C + CO2 forms 2 COi C + H2O forms COi + H2 The first reaction is exothermic, while the last two are endothermic. So pass air/oxygen, CO2 and H2O through coal and it is produced COi + H2. Coal gasification was the principle form before 1940 and was practised at a large scale because of the need to remove sulfur and ash. During WW II however, biomass was the fuel of choice for small gasifiers to run cars, trucks and buses. [top] [end]Biomass GasificationWhen biomass is burned with insufficient air in a gasifier, it makes a "producer gas" containing primarily CO, H2, CO2, H2O and CH4. Downdraft gasifiers are tar-burning, char-making and are most suitable for biomass which contains 80% volatile material. Updraft, char-burning, tar-making, gasifiers are often used for coal which can be 80% char.In conventional downdraft gasifiers, air passes down through the fuel mass, then in the flaming pyrolysis zone burns the volatiles and tars while making charcoal and pyrolysis gas. The charcoal then further reduces the CO2 and H2O combustion products back to CO and H2 fuel. [top] [end]The Inverted Dowdraft GasifierIn inverted (top burning) downdraft gasification air passes up through the fuel and meets the flaming pyrolysis zone where the reaction generates charcoal and fuel gasWood-gas cooking stoves like the Swosthee wood-gas stoves in Malaysia are perhaps the best answer so far. These are gasifiers that produce gas from wood and then burn the gas, leaving ash and charcoal. They're clean, fast and efficient. They burn small pieces of wood, sticks, wood chips, corncobs or nutshells, producing a clean, blue flame and no smoke. A lot of cunning engineering has gone into the development of these stoves, and yet they're easily made from locally available materials -- even tin cans. Richard Boyt's low-tech wood-gas stove, made from 10 tin cans. Technically, they're called "inverted downdraft gasifier" stoves, operating on natural convection. How wood-gas stoves work: "A Wood-gas Stove For Developing Countries", by Tom B. Reed and Ronal Larson, Biomass Energy Foundation. 300g of sticks or chips burn for 30-45 minutes at high efficiency with low emissions. [top] [end]For more informationhttp://journeytoforever.org/at_woodfire.htm | |
Page created:
06 February 2004; Last edited:
19 June 2007; Version: 3 | |
Pagename: Wood-gasStoves @HEDON: JRAA | |
