Micro-gasification: Cooking with gas from biomass - Amper

Micro-gasification: Cooking with gas from dry biomass
During the whole conversion process, temperatures increase from ambient temperature to
well above 800° Celsius, depending on local conditions.
In each step vapours and gases are released and the solids reduce in mass and volume.
If complete combustion is attained, emissions should be clean and only contain carbon dioxide
and water vapour. If combustion is not complete, then smoke and vapours composed of
unburned fuel and carbon monoxide will result.
1.1.1 Steps of biomass combustion
Once we know the conditions that influence combustion, we can use them to control and
optimise the process. Therefore let‘s take a moment to explore each step separately:
Step 1: Drying
The first change happens during drying. The amount of water transformed into vapour depends
on the moisture content of the raw fuel, which also determines the heat input needed
to evaporate all the water and the loss in mass and volume to get to dry fuel.
Step 2: Pyrolysis (Carbonisation)
Increased temperatures and absorbed heat eventually cause a complete decomposition of
the biomass, which separates into volatile gases and vapours, as a solid char remains behind.
1
The vapours contain various carbon-compounds with fuel value, referred to by the term
‗wood-gas‘. As the solid product of this stage is char, it is also referred to as Carbonisation 2 .
Pyrolysis can happen in the complete absence of oxygen, the regulating factor is heat.
In short: no heat input, no pyrolysis, no wood-gas generation and no fire.
Step 3: Char-gasification
Once char is formed, the next stage of the solid phase is to convert the carbon atoms to
gases and the non-carbon portion to ash.
This only happens if oxygen is available and reaches the char while it is still hot enough to
react. Then ‗char-gasification‟ occurs: oxygen reacts with the char solids, yielding carbon
monoxide, carbon dioxide and creating additional thermal energy.
The fraction of non-burnable solid mineral content of the char remains as ash 3 .
Note:
The regulating factor of char-gasification is the amount of available oxygen around the
hot char.
If the char is cooled and/or the oxygen supply is restricted, the conversion from char to ash
does not take place and the char will be conserved and no ash will be created.
Step 4: Gas-Combustion (see Figure 2)
The final stage of „gas-combustion‟ is where the gases are ‗burnt‘ (combusted) and the
bulk of the heat is released that can be used e.g. for cooking.
1 Some may have own experience with pyrolytic destruction of a slice of bread in a toaster: first it starts changing
colour from pale-white to golden-brown, while releasing an appetising smell. If left too long in the toaster, the
emerging volatiles will soon turn to thick biting smoke (‗wood-gas‘) while the bread will show various shades of
‗black‘ by charring and carbonisation. In the worst case it will come out as a lump of black char unfit for human
consumption.
2 Pyrolysis (Greek: decomposition by fire) and Carbonisation are like the flip-sides of the same coin, depending if
the focus is on the generation of wood-gas or the creation of char.
3 Ash contains only minerals that the plant once absorbed from the soil. Under normal circumstances completely
burnt ash should not contain any carbon or other substances with combustible value.
HERA – GIZ Manual Micro-gasification Version 1.01 January 2011
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