Nitrogen evolution and soot formation during secondary coal pyrolysis

Economical NO_x control techniques used in pulverized coal furnaces, such as air/fuel staging, promote secondary reactions of the primary coal volatiles. Secondary reactions significantly influence the nitrogen transformations among different combustion products and the ultimate NO_x production. The major objectives of this study are to investigate the nitrogen evolution and soot formation mechanisms at high temperature, high heating rate conditions.; A CO/H₂/O₂/N₂ flame was operated under fuel-rich conditions in a flat flame reactor to provide a high temperature, oxygen-free post-flame environment to study secondary reactions of coal volatiles. Effects of temperature, residence time and coal rank on nitrogen evolution and soot formation were examined. Elemental compositions of the char, tar and soot were determined by elemental analysis, gas species distributions were determined using FTIR, and the chemical structure of the tar and soot was analyzed by solid-state 13C NMR spectroscopy.; Both temperature and residence time have a significant impact on the secondary reactions of tar. Coal-derived soot exhibited loss of aliphatic side chains and oxygen functional groups prior to significant growth in average aromatic ring size. Polymerization reactions accelerated at temperatures above 1400 K, leading to a larger and more interconnected cluster.; Experiments were performed on the model compounds of biphenyl and pyrene to study soot formation mechanisms for aromatic hydrocarbons. Ring opening reactions were shown to constitute the first step in the soot formation process for biphenyl, followed by ring size growth and cluster crosslinking. Little evidence of ring opening reactions was observed during the pyrolysis of pyrene.; A simple model was devised to describe the secondary reactions of coal volatiles based on the chemical structure analysis.; During secondary pyrolysis, an enrichment of nitrogen in tar was first observed, followed by a subsequent fast nitrogen release, finally decreasing at a much slower rate at high temperatures. The decay of the nitrogen functionalities in the tar is similar for all the coals in this study, indicating that reactivity of the tar nitrogen functionalities show very little rank dependence. As pyrolysis proceeded, the clusters in soot became larger and more interconnected, which retarded the further release of nitrogen. Some types of quaternary nitrogen are thought to be responsible for the earlier release of NH₃ than HCN at low temperatures. However, additional NH₃ can be formed through the interactions of HCN and other oxygen radicals in the gas phase or on a specific surface.