The purpose of this research was to advance the fundamental understanding of the chemistry of nitrogen in coals, in relation to nitrogen oxide formation in coal combustion. The structures of nitrogen in coal of different rank and model chars, and their transformation in pyrolysis, were investigated, as well as their influence on nitrogen oxide emissions in combustion. The method of transformation of different coal nitrogen structures during pyrolysis and combustion was explained, as well as the role of nitrogen primary structures in nitrogen oxide emission. The correlation of behavior of carbon and nitrogen structures was explained based on temperature-time sieve phenomenon, which may help in better design of burners and boilers for low-emission technologies of coal combustion.