Suppressed Nitrogen Evolution From Coal-Derived Soot And Low-Volatility Coal Chars

摘要

This laboratory study uses a novel furnace to resolve nitrogen evolution during the three stages of pulverized coal combustion: primary devolatilization, secondary pyrolysis, and combustion. The behavior of six coals depicts continuous rank variations, as well as suppressed nitrogen evolution from low volatility coals. During primary devolatilization of any coal, aromatic compounds in tar and oils are virtually the only shuttles for nitrogen out of the coal matrix. The small amounts of HCN observed while primary devolatilization winds down probably come from the char, because char particles are significantly hotter than tar in these experiments. Secondary pyrolysis promotes additional HCN evolution from both char and tar. Moreover, substantial fractions of the volatile-nitrogen from primary devolatilization is reincorporated into a carbonaceous soot matrix for all coal types. The fraction of coal nitrogen incorporated into soot remains constant, even while soot yields dramatically increase. Incorporation of nitrogen into soot has the potential to substantially reduce the amount of coal nitrogen amenable to aerodynamic NOx abatement strategies for coals with large tar yields. This potential limitation is compounded by another limitation for low volatility coals. Whereas one-half to two-thirds of the coal-nitrogen is expelled by thermal decomposition from other coal types, only 30 to 40% is expelled from low volatility samples. This tendency suggests that nitrogen functionalities become much more refractory as their surrounding aromatic domains become more extensive, either in high rank coals or soot. For such systems, our measurements indicate that the only way to expel the nitrogen is to burn it away.