WOOD FUEL :

USE DRY WOOD

1 cubic metre of fresh felled timber weighs about 1 tonne, or 1000 kilogrammes and contains up to 600 kg of water.

Heat Energy comes from the dry wood; some is used up converting the water into steam.

BURN WOOD EFFICIENTLY

Under efficient, controlled conditions of burning, wood breaks down with air into ash, carbon dioxide and steam. Control of the air supply is critical. Burning occurs in three stages:

Pre-heating to 200ºC to oven dry wood and steam.

1. With primary air, wood burns at 200-500ºC to hydrocarbon gases and charcoal.

2. With hot secondary air, gases burn at 600-700ºC into carbon dioxide and steam.

3. With primary air, charcoal burns at 300-500ºC into carbon dioxide and ash.

If hydrocarbons are not all burnt they accumulate as soot and tar on the pot.

For efficient combustion remember the 3 Ts-Temperature to allow the reactions to occur, Turbulence to ensure there is good mixing, Time for the gases to remain in the combustion zone.

FIGURE

Comparative 'Effective Energy' for Various Stoves & Fuels where effective energy = calorific value x percentage heat utilized (PHU)

Stove	Fuel	Calorific x Value MJ/kg	PHU % Energy MJ/kg	Effective
Open fire	Wood	18	10 - 15	1.8 - 2.7
Improved woodstove	Wood	18	25 - 55	4.5 - 9.9
Traditional metal stove	Charcoal	29	15 - 20	4.4 - 5.8
Improved ceramic Stove	Charcoal	29	25 - 35	7.3 - 10.2
Gas stove, simple	LPG	46	55 - 65	25.3 - 29.9
Kerosene stove (wick type)	Kerosene	44	50 - 65	22.0 - 28.6