Modeling NOx release from a single coal particle .1. Formation of NO from volatile nitrogen

For a single spherical coal particle, modeling has been undertaken of the processes of release of volatile fuel-nitrogen, and its subsequent conversion to nitric oxide during the devolatilization. The combustion conditions used were similar to those found in utility boilers. A finite volume numerical scheme was used to solve the equations of mass, species conservation, momentum and heat transfer about the devolatilizing particle. Particles in the size range 10-100 mu m were modeled with gas temperatures from 1250 to 1900 K. Dcvolatilization was described by two competing reactions; results are compared with other single particle models and experimental results reported in the literature. The influence of coal rank was examined by using a lignite, a sub-bituminous and a high volatile bituminous coal. The conversion efficiencies of volatile fuel-nitrogen to nitric oxide are presented. It was found that there can be a significant conversion of the volatile fuel-nitrogen to nitric oxide before the liberated volatiles have reached the oxygen supply in the bulk gas. Conversion efficiencies of volatile nitrogen to nitric oxide as high as 54% are predicted for conditions of high gas temperature and high oxygen concentrations. For small particles (approximate to 10 mu m), most of the fuel-nitrogen is released as hydrogen cyanide (up to 70%) prior to mixing with the bulk gas.