06. Processes in cook stoves

What processes occur in cook stoves to produce biochar?

Organic matter used as fuel in stoves is converted thermally into syngas, solid residue (including biochar and ash) and liquid (including tars).

Commonly similar processes are used in large scale gasifiers, and are designed to maximise the gas production which can be captured and then used for energy. However these conditions are generally created in stoves using simple technology to maximise heat production for cooking.

Depending on the type of stove, a mixture of processes will occur at one time during use of the stove. Pyrolysis, gasification and incineration will occur to varying degrees and at different stages in the burn, and in different places within the stove.

Pyrolysis: Thermal cracking of organic matter occurs in the absence of air. Industrial processes can be divided into fast and slow pyrolysis. Slow pyrolysis is a slow heating rate and a lower temperature (generally 300 – 500oC). Carbonisation occurs, where carbon is a major product (as in biochar production) together with syn-gas and liquids.
Pyrolysis is chemical decompostion at high temperatures. The word comes from greek derived ‘pyro’ fire, and lysis ‘decomposition’. Unlike combustion water, oxygen or any other reagents are not required.

Gasification: A small amount of air is required but not enough to complete the burn.
In gasification cooks stoves, it is the syngas which is burned to produce the heat. The gas will mainly comprise of carbon monoxide.

Incineration: This is the process of full combustion in an oxygenated environment.

In stoves which produce biochar, pyrolysis and gasification will occur resulting in biochar production, then if incineration is allowed to follow, the biochar will turn into ash.

Power Plants A guide to Energy from Biomass (New Futures 16, January 2006, The Centre for Alternative Technology. Brian Horne)
Biochar, an emerging technology for climate change mitigation (UKBRC Working Paper 1)

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