An extremely large amount of basic research has been carried out on the combustion of solid fuels, including fossil, biomass, and inorganic solid fuels, to ascertain the mechanisms and kinetics of the process. Each category of fuel combusts under different conditions which are determined by a variety of intensive chemical and physical properties of the solids and external ambient factors. An empirical view of biomass combustion involves the evaporation of the high-energy volatiles such as the terpenes, which burn in the gas phase with flaming combustion (cf. Shafizadeh and DeGroot, 1976). The lignocellulosics in the solid biomass, under the influence of high temperature or a sufficiently

FLAME TEMPERATURE (°С)

FIGURE 7.1 Theoretical flame temperature vs wood moisture content and excess air. Adapted from Tewksbury (1991).

strong energy source, decompose to form pyrolysis products, which also burn in the gas phase with flaming combustion. The residual char burns at a lower rate by surface oxidation or glowing combustion. The cellulosics are converted mainly to combustible and noncombustible volatiles, including water and C02, while the lignins contribute mainly to the char fraction.

The temperature within the flame is a function of reaction time, combustion intensity, flame velocity, and energy transferred to the surroundings. The flame temperatures measured on combustion of acetylene, gasoline, hydrogen, and natural gas in air under controlled conditions are 2319, 2310, 2045, and 1880°C, respectively (Reed, 1983). The combustion of biomass does not reach these temperature levels because of the lower energy density of biomass and the mechanism of biomass combustion. Simplistically, the mechanism involved in the combustion of solid biomass can be viewed as a stepwise process, although all steps occur simultaneously in the combustion chamber. First, the increasing temperature dries the incoming, fresh biomass. The physically contained moisture in the biomass is vaporized. At about 150 to 200°C, thermal decomposition and devolatilization of the solid biomass then begins on the biomass surface and volatile organic compounds are evolved as a gas, which burns in the combustion chamber. (Note that the discussion of biomass pyroly­sis in Chapter 8 addresses some of the important research on the mechanisms and kinetics of biomass devolatilization which also apply to biomass combus­tion.) The remaining fuel components in the carbonaceous residue are com­busted by diffusion of oxygen to the solid surface at temperatures of about 400 to 800°C and greater. This temperature range is attained by absorption of radiant energy from the hot combustion products and the combustion chamber surfaces. Temperatures as high as 1500°C have been recorded when the incoming fresh fuel is dry and the combustion process is carefully con­trolled. The use of preheated air permits similar temperatures in some systems even with biomass that contains some moisture.