In the early 1980's there was a great deal of interest in the nitrogen content of coal liquids. Although interest in these products has lessened in recent years, they have provided corroborative evidence for the functionality of nitrogen structures to be found in solid coal. Modern instrumental techniques, principally X-ray photoelectron spectroscopy, have been used to study nitrogen in solid coal. The dominant nitrogen form in solid coal is pyrrolic in which nitrogen is bound in five-membered rings. The next most important form is pyridnic nitrogen which increases with coal rank.
Pyridnic nitrogen is bound in six-membered rings.
At present, it is not possible to predict with any confidence the emissions of nitrogen oxides from coal combustion. It seems logical to assume that the amount of nitrogen in the coal, and the way in which it is bound into the coal structure, would affect the amount and distribution of nitrogen oxide emissions. Unfortunately, it is not yet possible to link nitrogen functionality directly with the formation of nitrogen oxides in coal combustion. The suggestion that the nitrogen functionality exerts an indirect effect, possibly related to rank, seems to be the best explanation at present. The relationship of the coal nitrogen with emissions of nitrogen oxides is an area in which further research is needed, especially if more effective in-furnace abatement techniques are to be developed.