OK, so what really goes on inside the gasifier?
Depending on its type, a gasifier may operate at temperatures up to 2,600ºF, which is more than 12 times the boiling point of water, and pressures up to 1,000 pounds per square inch, gauge (psig), which is about 68 times that of standard atmospheric pressure. When the feedstock particles enter the gasifier and move through its zones they undergo three thermal and chemical processes – the first two in very rapid succession and the third more slowly:

• Pyrolysis (a chemical breakdown of complex compounds caused by heat) occurs as the particles heat up. Volatile substances such as tar, hydrogen, and methane are released and char, a combustible residue resembling charcoal, is produced, resulting in up to a 70 percent weight loss for coal. The properties of the feedstock determine the structure and composition of the char.
• Oxidation (controlled burning) then occurs as the volatile products and some of the char react with air or oxygen to form carbon dioxide and carbon monoxide. This type of reaction is known as exothermic, because it produces the heat.
• Reduction then occurs as the char reacts with the carbon dioxide and steam to produce carbon monoxide and hydrogen, with some methane, which together make up the gaseous mixture known as syngas. This type of reaction is known as endothermic, because it consumes the heat. Slag is left as a waste product.

The net result of all of the chemical reaction inside a gasifier is that gasification is exothermic – it gives off heat. However, the main product of gasification is the syngas.

[See diagrams of pyrolysis and gasification reactions at: http://en.wikipedia.org/wiki/Gasification.]

GASIFIER PROCESSES

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