|Boiling Point No. 04 - March 1983 (ITDG, 1983, 20 p.)|
by Siddhartha Bhatt
Central Power Research Institute,
Firewood, as such, does not take part in heat transfer, but the flames do. Firewood is converted to flames which transfer heat. A flame is a hot mass of burning carbon particles, when it cools down sufficiently the mass of residual gases are called flue gases.
The flame radiates heat in all directions. When the radiation is blocked by a solid wall (such as the inner surface of the combustion chamber) the wall takes up heat and conducts it away from the flame zone. If the radiation is not blocked it is lost to the environment. After the flame transfers heat to the vessel the hot flue gases moving into the environment carry away heat. The two main losses that can be identified are radiation, and stack losses.
The radiation loss can be subdivided into radiation to solid portions (walls, vessels, etc) and radiation to the rest of the environment.
In wood combustion the energy losses can be pictured in terms of wood lost per hour per area (for radiation), or wood lost per kg of wood input (for stack losses). The energy loss is converted on the basis that 1 kg of wood is equivalent to 1.42 MJ. Radiation lost to a stove wall is only 0.1 gm of wood per hour per sq cm. Direct radiation to the environment is 25g/hr/sq cm Thus a stove opening (for fuel feed or pot) loses 250 times more heat than that to a wall. It was this observation that led people who cooked on open fires to burn wood in a closed combustion chamber, which evolved into the traditional cookstove. Incidentally, the cookstove also provides wind protection to the flame and fire protection to the users.
The traditional cookstove has many openings to the combustion chamber - the fuel feed opening and the pot-hole(s) being the major ones. These are zones where severe heat losses take place. A conventional cookstove loses, from these two places, - 2 to 4 kg of wood per hour in energy losses. If these openings are closed or optimised, a 250-fold saving in the wall energy losses can be achieved and this can be about one third of the overall fuel consumption.
Now, about the gases that leave the stove as waste. These range in temperature from 80ºC to 400ºC and the loss is of the order of 70 to 440 gm of wood per kg of input wood. At 100ºC, 200ºC and 300ºC the losses will be 90 gm/kg, 210 gm/kg and 330 gm/kg of input fuel respectively.
The stack loss is linked to the radiative loss. When the latter is more the fuel consumption increases. The stack loss increases as fuel consumption increases.
The solution to minimising heat losses would be to plug all openings in the fire burning zone by using dampers and shields at the fuel door openings. As far as possible the fire must not be visible at all. An incidental advantage of this scheme is that fire hazards and inconvenience due to radiative heat is reduced. This solution may result in the shooting up of the stack gas temperature. The stack gas heat can be utilized by the old technique of using a multiple pot opening system. Dampers at the connections in between two pots is also suggested to control the stack losses.