In addition to the energy released by combustion, the rate of combustion is also important in the design of combustion systems. Occasionally, biomass combustion power plants have been observed to be underdesigned in terms of boiler volume and Ž grate area for the rated capacity. Typical design heat release rates expressed per unit . grate area for a stoker fired travelling grate combustor are in the range of 2 to 4 MW Ž. y2 Ž. thermal m . A whole tree combustion concept not yet built utilizes a design based y2 wx on a heat release rate of 6 MW m , but this also uses a very deep fuel bed 22 . Some circulating fluidized bed furnaces firing biomass have heat release rates approaching 10 MW my2. The rates at which biomass fuels burn depends on a number of physical wx phenomena which have been set out in some detail by Kanury 23 . Two predominant factors are the rates of heat transfer and the kinetic rates of reaction. Particle size Ž. dominates the influence of heat transfer, with small, thin thermally thin particles Ž. heating rapidly and coarser, thicker thermally thick particles heating more slowly. Combustion occurs both in the gas phase with the burning of volatile materials released through pyrolysis of the fuel upon heating, and heterogeneously in the solid phase as char oxidation. The burning of volatiles is generally quite rapid and follows as fast as volatiles are released, the oxidation of the char occurs much more slowly. The residence time of the particle in the furnace and the environment of the particle are therefore important to the total conversion attained through combustion, as well as the emissions from the combustor.