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close this book Boiling Point No. 28- August 1992
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Alternative Approach to Wood Combustion

A short extract from "Wood Combustion Studies Part 11", July 1991 from Wood Stoves Group, Eindhoven, Holland by A M Hasan and R Khan (pp 35)

Khan's paper is a detailed study of wood combustion with down draught air supply. This is very efficient but not yet practical for domestic cooking stoves (see also Boiling Point 21, 23 and 29 - December 1992).

"Wood basically consists of non-poisonous chemical elements viz. Carbon, Hydrogen and Oxygen. If it is burnt completely there should be no air pollution, because the end products of these reactions are water and carbon dioxide. But practically this never happens. When wood is heated to 250-300C, about 80% of the original dry mass of wood is expelled in the form of gases, vapours and finely divided droplets, leaving behind charcoal. To ensure complete combustion, sufficient amounts of air should be supplied both to the volatiles and the charcoal.

In a conventional up-draught stove the primary air enters below the grate and moves upward through the fuelbed consisting primarily of charcoal. During that time, oxygen in the air meets with the charcoal heterogeneously at its surface and forms CO (Krishna Prasad et al., 1984). This CO migrates into the spaces between the particles and reacts with oxygen homogeneously and forms CO2 and heat. Again this CO2 reacts heterogeneously with the carbon surface and forms CO, which is an endothermic reaction. As a result, there is reduction in temperature of the flue gases. Thus the conversion of CO2 into CO, which is dependent on the temperature and the time, will be less intense in the higher layers of the charcoal bed. Still the temperatures are high enough to cause pyrolysis of the fresh wood and the liberation of volatiles.

Fig 1 - Schematic of a 5kW Downdraft Stove


When the volatiles emerge from the top of the fuelbed, they hold gaseous and tarry decomposition products that derive from wood in the upper layers of the fuelbed and they contain no oxygen (conventional wisdom suggests that under such a design approach, the amount of air supplied at the bottom of the grate is only sufficient for the combustion of charcoal). The volatiles can only be burnt by supplying a sufficient amount of secondary air into the system. However, even then the efforts to bum it do not always meet with success, because (a) the air will further cool down the gas mixture and (b) the way in which the air is supplied does not necessarily guarantee its proper mixing with the fuelbed effluent. Incomplete burning will result in the deposit of tar and soot on the bottom of the utensil and large emissions of irritants, toxins and carcinogens in the kitchen environment.

The principle of operation of the downdraught stove, in contrast to conventional designs, is that the flow of air is in the same direction as the volatiles and fuel. A chimney for the stove is essential to provide the draught, which induces the liberated volatiles and air to flow downward through the fuelbed where they bum vigorously resulting in much higher temperatures (950-1100C) than during conventional burning (550-750C). It has been experimentally found that this mode of burning leads to a very good combustion for certain operating conditions. It is suggested that this is due to:

i) more homogeneous mixing of volatiles and air due to higher flow velocities than in conventional combustion;

ii) higher temperatures; and

iii) shorter time interval between reaction of air and volatiles which prevents condensation and polymerisation reactions resulting in products which are more difficult to convert to CO2 and H2O.