Wood Components:

Wood consists of three main components: cellulose, lignin and water. The cellulose and lignin and some other materials are tightly bound together and make up the material we call wood. The water is adsorbed or held as molecules of water on the cellulose/lignin structure. Air dry or "seasoned" wood still contains 12-18% of adsorbed water. Growing, freshly cut or "unseasoned" wood contains, in addition, liquid water to give a total water content of about 40 to 100% expressed as a percentage of the oven dry weight of the wood.

Wood Charcoal Carbonization Flow:

1. The first step in carbonization in the kiln is drying out of the wood at 100°C or below to zero moisture content. The temperature of the oven dry wood is then raised to about 280°C. The energy for these steps comes from partial combustion of some of the wood charged to the kiln or pit and it is an energy absorbing or endothermic reaction.

2. When the wood is dry and heated to around 280°C, it begins to spontaneously break down to produce charcoal plus water vapour, methanol, acetic acid and more complex chemicals, chiefly in the form of tars and non-condensible gas consisting mainly of hydrogen, carbon monoxide and carbon dioxide. Air is admitted to the carbonising kiln or pit to allow some wood to be burned and the nitrogen from this air will also be present in the gas. The oxygen of the air is used up in burning part of the wood charged.

3. Low and Middle Temperature Charcoal: The spontaneous breakdown or carbonization of the wood above a temperature of 280°C liberates energy and hence this reaction is said to be exothermic. This process of spontaneous breakdown or carbonization continues until only the carbonised residue called charcoal remains. Unless further external heat is provided, the process stops and the temperature reaches a maximum of about 400°C. This charcoal, however, will still contain appreciable amounts of tarry residue, together with the ash of the original wood. The ash content of the charcoal is about 3-5%; the tarry residue may amount to about 30% by weight and the balance is fixed carbon about 65-70%.

4. High Temperature Charcoal Process: Further heating increases the fixed carbon content by driving off and decomposing more of the tars. A temperature of 500°C gives a typical fixed carbon content of about 85% and a volatile content of about 10%. The yield of charcoal at this temperature is about 33% of the weight of the oven dry wood carbonised - not counting the wood which was burned to carbonise the remainder. Thus the theoretical yield of charcoal varies with temperature of carbonization due to the change in its content of volatile tarry material. 

Temperature effect on yield and composition of final charcoal

The Usage of Final Charcoal: 

Daily life:  Cooking ,BBQ,House heating. Fuel in other industries.Agriculture : Improve the quality of the soil.

Industry: Reprocessed into activated carbon. Construction bricks with clay.

Fuel in industries such as steel and iron factories, ceramics, etc.

Biogas: Widely used as fuel.Greenhouse agriculture heating.Fuel for heating or drying.

For the generator to get electricity.

Wood Vinegar and Wood Tar: Great materials in shipbuilding and paint industry.Raw materials in chemical and medicine field. Plant nutrition regulates auxin or insecticide.

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