COMBUSTION

Combustion of biomass can be considered as taking place in four stages. All the stages can. however, he taking place at the same time. For example

 a log of wood does not burn in one go. Similarly I stove that is re-charged during cooking will be experiencing different phases of combustion

simultaneously.

Stage 1.

In lighting the fire. supplying the initial heat source, the water content of the fuel is evaporated

off at about 100 °C.

Stage 2.

When the fuel temperatures reach between about 200°C and 350°C the volatile gases (compounds of carbon, hydrogen, and oxygen) are released.

Stage 3.

The volatile gases mix with oxygen and ignite at temperatures in excess of about 450 °C and burn with a yellowish flame radiating heat. Some of this heat is reabsorbed by the fuel releasing more volatiles. This process should become self- sustaining until all the volatiles have been released.

The volatiles need sufficient heat, oxygen, space and time to ignite. !f any of these are lacking the volatiles may leave the combustion chamber without igniting; the combustion will then be incomplete and inefficient; the fire will be smoky and may die altogether.

*  The volatiles and oxygen will mix more thorough if the air flow through the combustion chamber is slightly turbulent.

*  When all the volatiles have been released. charcoal remains. (Charcoal is mostly fixed carbon).  spaces.

Stage 4.

The charcoal burns (oxidizes). provided there is sufficient oxygen at the fire-bed. at temperatures around 800 C. The carbon monoxide produced reacts with oxygen (again provided oxygen is available) just above the fire bed. to give carbon dioxide. The charcoal: will usually continue to burn long after the volatiles have been used up. A charcoal fire requires oxygen both at the fire bed (primary air) and just above the coals (secondary air). If there is insufficient secondary air, the fire will give off carbon monoxide which can be dangerous to the stove user especially in enclosed spaces.

Note

 In a charcoal-burning stove only stage 4 occurs and hence the pot can be much closer to the fire bed than in a wood burning stove where the volatiles need space to mix and react with the oxygen. In a wood fire. if large quantities of charcoal remain after the fire has burnt out it indicates insufficient oxygen or heat at the fire bed.