Micro-gasification: Cooking with gas from biomass - Amper

Micro-gasification: Cooking with gas from dry biomass
released by the combustion reactions supplies the necessary thermal energy to continue
the fire and make it self-sustaining.
When designing a device to control the burning of biomass and regulate the rate of heat
generation, it is important to note that the drying and pyrolysis stages are controlled by regulating
the amount of HEAT that reaches the solid biomass, while the later steps of chargasification
and vapour combustion depend on the availability of OXYGEN.
The two red horizontal arrows in Figure 2 symbolise that char-gasification produces some
radiating heat. Combustion also radiates heat towards the biomass fuel. These sources of
heat continue the initial endothermic steps and generate more wood-gases, sustaining the
fire in the form of the yellow and blue flames above the ―burning‖ wood.
1.1.2 The ‘uncontrolled’ open fire
In this photo we can detect all
these stages of a ‗burning‘ process
in an open fire, happening
simultaneously in a rather uncontrolled
manner: unburnt raw
fuel (left), yellow flames (centre)
indicating wood-gas combustion,
red-glowing embers and charred
black wood partially covered by
grey-white ash (right).
A stick lying across has the left
end unburnt, a black charred
transition zone and the right end
covered with ash.
Smoke is the result of incomplete
combustion, most visible to the
left, where there are no flames.
1.1.3 Improving control in a gasifier device
A biomass gasifier is the broad term for a device that turns solid biomass into gas that can
subsequently be burnt in a controlled manner. Unlike in the open fire, the gas-generation is
controllably separate in space and time from the gas-combustion, like shown in the next
figure. While open fires and most conventional cook-stoves are regulated by the fuel supply,
most gasifiers are controlled by the air supply.
Gasifiers offer the potential to deliberately optimise the frame-conditions of each conversion
step. By controlling the inputs heat and air, an exceptionally clean combustion of biomass
can be achieved. The major challenge is to get the right amounts of air to the right places.
The step of char-gasification can be suppressed, if the hot char does not get exposed to
sufficient air. In this case the combustible gases are predominately generated by pyrolysis
HERA – GIZ Manual Micro-gasification Version 1.01 January 2011
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