WOOD CHARCOAL

Wood charcoal which is called simply charcoal is made by carbonization (destructive distillation, i.e. heating in absence of air) of wood at 600C.
Physico-chemical changes during wood carbonization
Following four stages are involved in the carbonization of wood:
- When the temperature reaches 100-120C, initial decomposition of wood takes place resulting in the formation of little distillate gas containing acetic acid and water.
- Active distillation of wood takes place upto 350C till the process is exothermic producing liquid products (like acetic acid, methyl alcohol, pyroligneous acid, tar etc.) and gaseous products containing Carbon monoxide, carbon dioxide, nitrogen, hydrogen, hydrocarbons etc.
- From 350 to 600C, slow evolution of residual volatile matters (i.e. gases) from the wood/charcoal left in 3rd stage.
Products in wood carbonization
Charcoal is the solid product left after the carbonization of wood. Hot gases are cooled to separate wood gas and liquid into two layers. Upper layer of the liquid is prodigious acid and the lower level is wood tar. Pyroligneous acid is an aqueous solution of acetone, methyl alcohol, and acetone and wood spirit mainly. Wood tar can be fractionated to separate many chemicals. Besides it is used as a supplementary plant fuel. Normally matured dense wood gives dense charcoal on carbonization.
Typical product yields in wood carbonization
Product Yield % air dried
Charcoal 30
Pyrolignious acid 38
- Acetic acid 8
- Wood spirit 15
- Water 15
Wood tar 10
Wood gas 22
Scheme of wood carbonization
Carbonization of wood is done in open pit (primitive method, now obsolete), kilns or metal retorts. Pits and kiln are located in forests and retorts in factories.
In open pit carbonization wood is burned in large heaps with restricted air. Yield of charcoal is 20%, which is of inferior quality. Besides, gas and liquid by-products are lost to the atmosphere, as they cannot be recovered. Even the charcoal that we get from domestic wood burning ovens also comes in this category.
In charcoal kiln also, charcoal (of better yield and quality) is the only product as gases and liquid by-products are not recovered. Kiln is parabolic in shape having typical dimension, radius=3 meters, height=2.5 meters and capacity=30m3 of stocked wood.
Wood is stacked on the ground with one verticals central passage acting s the chimney and a horizontal passage at the bottom for introducing fire to the center. The kiln is covered with thick layers of grass, leaves etc. and then plastered with a mixture of earth soil and charcoal dust. Initial firing is done with grass and twigs and then the wood is partly burned to supply heat for the process. Carbonization time is 7-10 days. After that fire is extinguished with water and the kiln is allowed to cool for a week are least before the charcoal is taken out.
Carbonization in metal retorts is done at a low temperature of 350C. Retorts may be of four types namely:
-Externally fired (heated) batch retorts
-Externally fire continuous vertical retorts
-Internally heated batch vertical r retorts
-Internally heated continuous vertical retorts.
Most recent design is internally heated retorts which use-forced recalculation of heated inert gases evolved during carbonization. Besides, it employs efficient mechanical handling of wood and charcoal and has high thermal efficiency.
Characteristics of products of low temperature carbonization of wood are:
- Charcoal yield is high (35%)
- Ash content of charcoal is low (below 2.5%)
- Volatile matter in charcoal is high (up to 15%); hence it can be easily ignited and burns at low rates.
- Heating value of charcoal is high (7500Kcal/kg)
- Charcoals produced have high vapor adsorption capacity.
High temperature carbonization of wood.
It is mainly carried out for the production of town gas and chemicals besides for charcoal. Carbonization temperature is 1000-1200C. Besides, town gas (which can be used for heating of domestic ovens) the valuable liquid chemicals like creosote (used as a wood preservative), turpentine, light & heavy oils are produced. Product characteristics are:
- Charcoal yield= 25%
- Heating value of charcoal = 8000 kcal/kg
- Gas yield=850 Nm3 gas/ton dry wood
- Gross heating value of gas = 3000kcal/Nm3
- Composition of gas: CO2 =13% CmHn (unsaturated hydrocarbons) =2%
CO=24%, CH4=15%, N2 =1%, H2=45%.
Uses of charcoal
- Because of its large specific surface area (150-450 m2 /gm) and light and porous nature, it is used for removal of obnoxious and coloring materials from solutions, gases, vapors, petroleum products etc. By adsorption on its surface.
- It can be used as a feedstock for gasification to make producer gas, which is used for domestic and industrial heating. During Second World War, this producer gas was used as a fuel in road vehicles in many countries.
- It is used as a clean and smooth burning fuel in domestic heating ovens but it is a costly fuel.
- Previously it was being used for metallurgical furnaces but now it has been replaced by coke. In blast furnace using charcoal instead of coke, the charcoal consumption can be upto 1-ton charcoal/ton pig iron for capacity of the blast furnaces of,000 tons pig iron per year.
- It is used very widely as fuel for blacksmith’s and metalworker forge furnaces/ovens.
- It is raw material for the manufacture of carbon disulphide.
- It is mainly used as a domestic fuel in developing countries.
Composition of charcoal.
A typical composition of charcoal is given below:
C=80% O2&N2=15%,H2=2% and Ash =3%
Merits of charcoal as a fuel
- It has a very high specific surface area compared to coal (20-200m2/gm coal)
- Its ash content is very low (below 3%)
- Its calorific value is high (6500-8000 kcal/kg).
Demerits of charcoal as a fuel
Its mechanical strength is very poor, hence it gets crushed to powder in operation, which is easily swept away in a current of gases, and also it may prevent the proper flow of gases in the furnace.