The moisture content of green biomass can be quite high and can adversely affect the combustion process. If the moisture content is excessive, the combus­tion process may not be self-sustaining and supplemental fuel must be used, which could defeat the objective of producing energy by biomass combustion for captive use or market. High moisture can also cause incomplete combustion, low overall thermal efficiencies, excessive emissions, and the formation of products such as tars that interfere with operation of the system. Predrying of the fuel or blending it with dry fuel to reduce the equivalent moisture content before combustion may be necessary in these cases. Woody biomass fuels containing 10 to 20 wt % moisture are generally preferred for conventional biomass combustion systems. This moisture content range permits a close approach to complete combustion without incurring the costs of further bio­mass drying and allows temperatures in the combustion chamber to reach 750 to 1000°C. As already mentioned, lower moisture contents in biomass fuels can facilitate attainmant of even higher combustion temperatures.

Another factor in biomass combustion is fuel particle size and particle size distribution. The furnace design often determines the optimum ranges of these parameters. But in general, the smaller the fuel particles, the more rapid and complete the combustion process. The larger particles require longer residence times in the combustion chamber at a given temperature. In commercial sys­tems, the capital and operating costs of fuel particle size reduction and predry-

ing are weighed against their beneficial effects on combustion and furnace design and costs.