Functional forms of nitrogen in coals and the release of coal nitrogen as NOx precursors (HCN and NH3)

The influence of coal nitrogen on the pyrolytic release of NO_x precursors, (HCN and NH₃), has been investigated for a range of Australian coals. Functional forms of nitrogen in the coals were determined by X-ray photoelectron spectroscopy. Pyrrolic-type nitrogen predominated (50-60% coal nitrogen), but pyridinic and quaternary forms were also detected. The proportion of pyridinic increased and that of quaternary decreased with increasing carbon contents of the coals. Four coals, ranging in rank from brown to bituminous and in nitrogen content from 0.6-2.0%, dry ash free basis, were pyrolyzed in a fluidized bed reactor at temperatures ranging from 500 to 1100°C, gas residence times of 0.3-0.5 s and particle heating rates of 10⁴ K s^-1. Formation of HCN and NH₃ occurred at higher temperatures than that required for maximum tar yields; cracking reactions of the tars are a probable source of HCN and NH₃. However, the results show that release of nitrogen from structures which are not volatilised as tar also occurs. The major nitrogen-containing components of the tars were identified and quantified by gas chromatography. Nitrogen contained in pyridinic groups was more stable than that in pyrrolic groups; thus, a higher pyrolysis temperature was necessary to release nitrogen as HCN and NH₃ from the pyridinic groups. For the range of coals and pyrolysis conditions studied here, however, nitrogen release (as a proportion of total coal nitrogen), in the form of NO_x precursors, did not depend on coal type or coal nitrogen content.