Modeling NO x release from a single coal particle I. Formation of NO from volatile nitrogen

Abstract

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 μm were modeled with gas temperatures from 1250 to 1900 K. Devolatilization 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 (≈ 10 μm), most of the fuel-nitrogen is released as hydrogen cyanide (up to 70%) prior to mixing with the bulk gas.