| Boiling Point No. 15 - April 1991 |
by G Rossier & W Micuta of Renewable Energy Development Institute (REDI) ,Port au Prince, HAITI, November 1989 (editorial translation)
Production of Charcoal
Wood consists of cellulose,lignine and volatile substances, mainly water. During the process of production of charcoal (Pyrolysis), the volatiles are eliminated. The cellulose and lignine are decomposed by reactions of the following type;
Cn(H2O)m -> nC + mH2O 1. (cellulose) (charcoal) (steam)
Note: n = number of carbon atoms in molecule of cellulose m = number of oxygen atoms in molecule of cellulose
The reaction is endothermic (takes in heat). Inside the charcoal heap this heat is provided by partial combustion of some of the carbon.
C + O2 -> CO2 + 94 kcal (393 kj) 2. and
C +1/2 O2 -> CO + 26 kcal (109 kj) 3.
Both reactions are exothermic (give out heat to the environment).
The efficiency of pyrolysis of chemically pure cellulose is shown from the following;
(C6 H10 O5)n -> 6nC + 5nH2O n x 162g n x 72g + X x 90g (cellulose) (charcoal) (steam)
The yield ie, weight of charcoal/weight of cellulose = 72/ 162 = 45%
The last expression assumes pyrolysis of cellulose completely dry, in an oxygen free atmosphere and externally heated. This cannot be achieved in charcoal production . If the wood contains 50% volatiles (methanol, acetone, tars etc) and water, the yield becomes 4.5kg (22%)
In practice, the heat needed for pyrolysis is provided by the partial oxydisation of carbon-reaction (3) thus reducing the yield to 9 Kg (11 %) (half the carbon). Finally, part of the crown of the tree is not utilized for charcoal making. An FAO report - (FO.DP/HA 1/72/012.FAO Rome 1976) estimates this loss at 25% of the wood. The final yield therefore becomes about 12 (8%).
It follows therefore that artisanal charcoal production in the developing countries takes 8-12 kg of wood to produce 1 kg of charcoal.
Higher yields of say 5-6kg for 1 kg charcoal can only be achieved with efficient charcoal kilns which collect the gases produced and use the combustion of CO to provide the heat needed for pyrolysis. In a traditional charcoal heap most of the heat comes from reaction (3) which is less exothermic than (2) and consequently more of the charcoal has to be burnt to sustain pyrolysis.
A yield of one kg of charcoal per 10 kg of wood is normal, although there are many examples of higher yields. At best, 3/4 of the heat energy is lost in pyrolysis (see ODA report - BP article in this edition).
Calorific Value (CV) of Charcoal
Dry charcoal consists mainly of amorphous carbon and mineral ashes.
The CV of the charcoal in reaction (2) is 94kcal (393kj) per 12g of pure carbon ie, 7,830kcal/kg (32,730kj/kg). The amount of ash depends on the charcoal (wood) but assuming 5% the CV falls to 7,440kcal/kg/31,000kj/kg). These values are for top quality charcoal entirely carbonized. Partially calcified wood is often found in charcoal. In this case the CV is considerably reduced.
In addition some charcoal is lost as small pieces or dust in handling. During rain the charcoal becomes wet, increasing the weight, reducing the CV and wasting the heat needed to evaporate the moisture, as a result the CV is usually taken as 6,00()kcal/kg.
In an incandescent stove combustion tends to take place as follows;
CO2 -> 0 + 1/2O2 and at high temperature the equilibrium moves to the right. The burnt gases contain a considerable proportion of carbon monoxide which is colourless, without smell and very poisonous. Even in low concentrations in the air it destroys haemoglobin in the blood and at high concentrations it is lethal.
Multi-fuel Wood Stoves
REDI has introduced multi-fuel stoves with chimneys which have a thermal efficiency much higher than the traditional open fire stoves or even the most improved stoves. These stoves are designed to burn wood but can be used for all sorts of wastes such as bagasse, paper, cardboard etc. REDI has also developed less costly kerosene burners without reservoirs under pressure and so with less risk of explosions. The stoves can also be fired with butane, propane or methane gas burners.
Trials in November 1989 in Port au Prince showed a specific consumption of 50g of wood to bring 1 kg of water to the boil. Cooking times were greatly reduced. The long lightning up time for charcoal was cut down. The thermal insulation, low fuel consumption and the removal of burnt gases through the chimney mean that the cook suffers less from the heat and smoke. She is more comfortable and there is less risk to her health, than with the traditional Haitian charcoal stoves.
It is established that the use of wood in the form of charcoal causes a loss of three quarters of its calorific values. In addition the efficiency of charcoal stoves is moderate - 50-lOOg of charcoal are needed to boil l litre of water whereas a good wood stove has a specific consumption of only 50g/l. If the losses in producing the charcoal are also taken into account then the composition is even less favourable; 10kg of wood is needed to produce 1 kg of charcoal
Clearly a reduction in use of charcoal in domestic stoves will make a significant contribution to saving trees.
Unfortunately, there are many obstacles to the process of transition from charcoal fuel such as deeply rooted habits, lack of good multi-fuel stoves on the market, feeling of "social status" among some charcoal users and above all a powerful lobby from charcoal makers and sellers. A popular argument in favour of charcaol is lower transport cost as compared with firewood. This argument is less convincing is the calorific values of charcoal and that of wood transported are adjusted according to their fuel efficiency rate when burnt in good stoves, say 50% for wood and 25% for charcoal. In such a case the transport cost of one tonne of useful heat of wood or charcoal may be the same.