**4. Mechanisms of nitrogen oxide formation during coal combustion**

In the process of coal combustion, nitrogen oxides are produced during homo- and heterogeneous reactions of the air, volatile matter and char. In combustion gases, nitrogen oxides occur as NO, N2O and NO2 species. In Figure 3, a simplified scheme of the mechanism of their formation is presented. The basic component of nitrogen oxide emissions in high-temperature processes is NO. Complex mechanisms of nitrogen oxide formation during coal combustion have been classified using the source of nitrogen and divided into two basic groups (Zeldowicz, 1946; Fenimore & Jones, 1957; Fenimore, 1971; Johnsson et al., 1992; Bowman, 1973; Sarofim & Pohl, 1973; de Soete, 1975; Malte & Pratt, 1977; Levy et al., 1978; Fenimore & Fraenkel, 1981; Heyd, 1982; Miller et al., 1984; Glarborg et al., 1986; Fong & Peters, 1986; Niksa, 1988; Cheng et al., 1989; Miller & Bowman, 1989; Muzio et al., 1990; Glarborg et al., 1992; Williams et al., 1992; Tomeczek & Gradoń, 1997; Williams et al., 1997):


Fig. 3. A simplified scheme of nitrogen oxide formation during coal combustion

According to the above processes, the amounts of forming oxides depend on combustion conditions. High oxygen concentrations during the initial phase of coal combustion promote formation of fuel-nitrogen oxides (Attar and Hendrickson, 1982).
