A theoretical model of the combustion of biomass is illustrated by the complete oxidation of giant brown kelp. Note that kelp, for which complete analytical data were available, is used here simply to illustrate the utility of the model, which is applicable to all biomass species. Based on the empirical formula derived from the elemental analysis of dry kelp at an assumed molecular weight of 100, the combustion stoichiometry is

C2.6iH4.63N0.i0S0.01O2 23 ash26.7 + 2.762502 —» 2.61C02 + 0.10NO2 + 0.01SO2

+ 2.315H20 + 26.7ash.

The experimentally measured ash content is assumed to be present in the original biomass and to be carried through the process unchanged. This is not strictly true since oxygen is chemically taken up as metal oxides are formed during standard ash determinations. The ash content is calculated as the difference between the weight of the residue after ashing the sample and the original sample weight, so it does not correspond to the actual ash-forming, metallic elements in the original, dry sample. But for purposes of illustrating the stoichiometry of complete combustion, this equation is adequate. The heat evolved by combustion of this particular sample of kelp is 12.39 MJ/kg (296.1 kcal/g-mol) with product water in the liquid state (Chapter 3). Since on the average, air is 20.95 mol % oxygen, the stoichiometric air requirement for complete combustion is 13.19 mol of air per mole of kelp, or an air-to — kelp mass ratio of 3.805. The ultimate concentration of C02 in the dry flue gas is 19.85 mol %.

Except for submerged combustion processes that are used for treatment of aqueous dissolved and suspended biosolids and a few other special combustion processes, the combustion of virgin and waste biomass involves solid fuels. Stoichiometric combustion data for four types of biomass, two coals, and one coke are compared in Table 7.2. Each of the biomass fuels is assumed to contain 15.0 wt % moisture. The stoichiometric air requirements are considerably less for biomass than for coals and cokes. The reason for this is that the C-to-H mass ratios of biomass are much less than those of fossil fuels. Also, most of the carbon in biomass is, effectively, already partially oxidized. Less oxygen is needed for complete oxidation. For the data in Table 7.2, it is assumed that organic nitrogen and sulfur in each solid fuel are oxidized to N02 and S02 and that nitrogen in air is inert. The calculated amounts of N02 and S02 formed on complete combustion are more than might be expected for a biomass fuel. The relatively high concentrations of organic nitrogen and sulfur in each biomass sample, except the pine wood sample, could potentially cause air pollution problems that require NOx and SOx removal from the combustion products before the flue gases are exhausted to the atmosphere. This will be discussed later. It is sufficient to state here that agricultural and forestry residues, wood chips, bagasse generated in sugarcane plantations, MSW, and RDF have been used as fuels for combustion systems for many years.