Volatile matter content of the fuel
The amount of volatiles in the feedstock determines the necessity of special measures (either in design of the gasifier or in the layout of the gas cleanup train) in order to remove tars from the product gas in engine applications.
In practice the only biomass fuel that does not need this special attention is good-quality charcoal.
The volatile matter content in charcoal however is often underestimated and in practice may be anything from 3 to 30 percent or more. As a general rule if the fuel contains more than 10 percent volatile matter it should be used in downdraught gas producers, but even in this case the method of charcoal production should be taken into account. Charcoal produced in large scale retorts is fairly consistent in volatile matter content, but large differences can be observed in charcoal produced from small scale open pits or portable metal kilos that are common in most developing countries.