A. Definition

Complete combustion (incineration, direct firing, burning) of biomass consists of the rapid chemical reaction (oxidation) of biomass and oxygen, the release of energy, and the simultaneous formation of the ultimate oxidation products of organic matter—C02 and water. Chemical energy is released, usually as radiant energy and thermal energy, the amount of which is a function of the enthalpy of combustion of the biomass. In the idealized case, stoichiometric amounts of biomass and oxygen are present and react so that perfect combus­tion occurs; that is, each reactant is totally consumed and only C02 and water are formed. Under normal conditions, such combustion does not occur with most carbon-containing solid fuels, including biomass.

When biomass is combusted under normal conditions, a flame is produced as visible radiation, provided oxidation occurs at a sufficient rate. By use of thermodynamic data, the theoretical temperature at which the products of combustion form under adiabatic, reversible conditions can be calculated. The theoretical flame temperature for the combustion of wood of various moisture contents with excess air is shown in Fig. 7.1 (Tewksbury, 1991). Green wood generally contains about 50% moisture by weight in the field, and excess air is used to promote complete combustion. It is apparent from Fig. 7.1 that both high fuel moisture levels and excess air significantly reduce the theoretical flame temperature. It is also apparent that to achieve maximum flame tempera­ture, dry fuel and small amounts of excess air are required. In actual practice, however, combustion is not adiabatic and the reactions that occur are irrevers­ible, so the actual flame temperature is less than the theoretical value.