OCR Text |
Show div (pu<t> - rgrad<t» = S~ (1 ) and are solved for ct> in axisymmetric cylindrical coordinates for gas velocity, pressure, enthalpy, mixture fraction, mixture fraction variance and combustion gas concentrations. A Lagrangian approach is used to track the pf particles through the fumace. The authors have added nitrogen chemistry equations which are solved for nitric oxide, HeN and ammonia concentrations. The model described here is based on the reaction pathways depicted in Figure 1. The kinetics for the reaction of the nitrogen species are taken from De Soete [16], Glarborg et al. [17] and Miller and Bowman [18]. Although not essential for a correct combustion simulation, drying kinetics have been incorporated to describe the evaporation of the 11 % of water in the sub-bituminous coal. NO Chemistry The following assumptions were employed. 1. The primary process in nitrogen conversion is that volatile nitrogen goes either to HeN or NH3. Quarternary nitrogen is assumed to be converted directly to NH3, as detennined from the fuel volatile content [10], see Figure 1. The remaining volatile nitrogen is released either as HeN or NH3, the ratio being detennined from the O/N data of Aho et al. [12] as shown in Figure 2. 2. The major pathway for NO formation is via HeN oxidation, although HeN can also reduce previously-formed NO. Reactions R1 and R2 comprise the simple chemistry originally suggested by De Soete [16]. HCN + O2 -+ NO+ ... (R1) HCN + NO -+ N2 + ... (R2) 5 |