Direct combustion of solid biomass occurs through evaporation combustion, decomposition combustion, surface combustion, and smoldering combustion. Components in the biomass which have a relatively simple structure and a low fusion temperature, fuse and evaporate when heated, and burn by reacting with oxygen in the gas phase. This is called evaporation combustion. The heavy oils present in the biomass first decompose due to the high temperatures encountered during combustion. The gas produced from thermal decomposition by heating reacts with oxygen in gas phase, flames, and then burns. This is called decom­position combustion. The char which remains after these forms of combustion, burns by surface combustion. Smoldering combustion is the thermal combustion reaction at temperature lower than the ignition temperature of the volatile com­ponents of the reactive fuels such as wood. If the ignition is forced to smoke, or temperature exceeds ignition point, flammable combustion occurs. In industrial direct combustion of biomass, decomposition combustion and surface combustion are the main forms of combustion [8].