During combustion, the chemically bound carbon and hydrogen in the various organic components of biomass are oxidized. Incomplete combustion can result in excessive emissions of particulate matter and partially oxidized derivatives, some of which are toxic. Chemically bound nitrogen and sulfur that may be present in the biomass are oxidized to nitrogen and sulfur oxides—mostly sulfur dioxide, S02, but some sulfur trioxide, S03; and mostly nitric oxide, NO, but some nitrogen dioxide, N02. Air is the usual source of the oxidant, oxygen, for biomass combustion. Small amounts of nitrogen in the air are
also converted to nitrogen oxides at combustion temperatures according to the reactions

N2 + 02 —» 2NO 2NO + 02 2N02.

The equilibrium concentrations of NO formed from equimolar amounts of nitrogen and oxygen at various temperatures are shown in Table 7.1. It is evident that the higher the combustion temperature, the higher the NO concen­tration. The concentrations of chemically bound nitrogen and sulfur are zero to very low in most woody biomass species, but some biomass can contain relatively large amounts of these elements (Chapter 3). Elements such as chlorine, which can be present at relatively high concentrations in biomass such as MSW and RDF, but which are present in very small concentrations in woody biomass, are converted to chlorine compounds such as hydrogen chloride. Most of the chlorine derivatives are considered to be pollutants. Carbon monoxide, acid gases, unburned hydrocarbons, partially oxidized or­ganic compounds, polycyclic aromatic derivatives, trace metal oxides, nitrogen and sulfur oxides, chlorine derivatives, particulate carbon, and flyash are found in the flue gases of poorly controlled systems. The amounts of ash formed on oxidation of the metallic elements in biomass can be minor or major combus­tion products, depending on the composition of the biomass fuel. Biomass combustion systems should be designed to approach complete combustion under controlled conditions as closely as possible to extract the maximum amount of thermal energy, minimize undesirable emissions, and meet environ­mental regulations.

The stoichiometric amount of oxygen is the minimum amount needed for complete combustion of the fuel. A limited amount of excess oxygen is often

TABLE 7.1 Thermodynamic Equilibrium Concentrations of Nitric Oxide from Equimolar Amounts of Nitrogen and Oxygen”

43

89

251

500

1000

1630 5460

‘Tewksbury (1991).

used with solid fuels to promote complete combustion. Since ambient air contains about 79 mol % nitrogen and is the usual source of oxygen under normal conditions, nitrogen is a major constituent of the flue gas. The tempera­ture attained in the combustion chamber depends on the rate of heat release, its dissipation and transfer, and the quantity of combustion gases. So the increase in combustion temperature is substantially less with air as the oxygen source compared to pure oxygen because of dilution of the combustion gases with nitrogen. The air-to-biomass mass ratio is therefore an important parame­ter because it affects the rate of combustion and the final temperature of the combustion gases. Oxygen-enriched air and the use of small fuel particles or powders have been employed to maximize combustion temperatures.