Micro-gasification: Cooking with gas from biomass - Amper

Micro-gasification: Cooking with gas from dry biomass
1.1 Gasifying solid biomass for cooking
Understanding the difference between ―feeding an open fire‖ and ―controlling a combustion
process in a gasifier‖ is one starting point for understanding the way biomass and fire are
combined in cooking devices.
Let‘s start with a familiar example: Everybody has seen a burning candle: once lit, it proceeds
to slowly melt the wax and burn with a stable flame for a prolonged time. Notably,
wax burns by a multi-step process where it first melts, then travels as a liquid up the wick,
then vaporizes due to additional heat received by the wick. The flame provides heat to melt
additional solid wax at the top of the candle by both radiant heat and proximity. The vaporized
wax mixes with oxygen in the air – and the visible flame is present at the interface
where the wax vapours leaving the wick meet the oxygen in the air surrounding the flame.
Wood burns in much the same way as the wax in the candle, with a few specific differences.
Most of these differences are due to the fact that candles are made from highly refined wax,
and wood is a less pure fuel – but much more available and affordable than wax.
Wood and other solid biomass constitute, after all, the oldest cooking fuels. They are even
today the most prevalent source of cooking energy on the planet.
As in the case of the candle, also the burning of wood and other solid biomass is a sequence
of transformations – occurring in close proximity, but separated by small distances
in time and space, as shown in Figure 1.
Figure 1: Changes in Solid Fuel
The solid substances undergo changes determined by the presence of heat and oxygen:
1. as biomass is heated, it evaporates excess moisture and it‘s surface temperature increases,
2. at elevated temperatures, biomass pyrolyses (‗decomposes by fire‘) into combustible
vapours and a solid, known as ―char‖,
3. red hot ‖char‖ can be converted to ash if sufficient oxygen is available,
4. mixed with oxygen the vapours and gases generated can be combusted when ignited
HERA – GIZ Manual Micro-gasification Version 1.01 January 2011
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