Effect of rank, temperatures and inherent minerals on nitrogen emissions during coal pyrolysis in a fixed bed reactor

Pyrolysis of 11 coals with carbon contents of 77-93 wt.% (daf) and corresponding demineralized samples has been studied in a fixed bed quartz reactor with a heating rate of 20 K/min to examine rank, demineralization, temperature and inherent mineral species dependences of nitrogen distribution. Nitrogen mass balances fall within 92.5-104.6%. The results indicate that the chars derived from the coals with higher rank show larger nitrogen retention. Demineralization suppresses volatile nitrogen emission during coal pyrolysis, especially for low rank coals. Coal-N conversion to tar-N reaches the asymptotic values at 600 °C. HCN yields are lower than NH3 yields during coal pyrolysis. The trends in HCN and NH3 emissions are very similar and the yields reach the asymptotic value at about 1200 °C. N2 starts emitting at 600 °C, and as the temperature increases the conversion increases linearly with a corresponding reverse change of char-N. With the catalysts added, N2 formation is prompted with the sequence of Fe>Ca>K>Ti≫Na≫Si≈Al, meanwhile, char-N decreases correspondingly. Fe, Ca, K, Na, Si and Al increase coal-N conversion to NH3 with the sequence of Fe>Ca>K≈Na≫Si≈Al in the pyrolysis. Na addition prompts HCN formation; however, the presence of Ti and Ca decrease the HCN yields with small value. The other catalysts have no notable influence on HCN emission in the pyrolysis. Demineralization and Ti addition increase coal-N conversion to tar-N slightly whereas K, Ca, Mg, Na, Si and Al additions decrease tar-N yield weakly, other catalysts hardly influence tar nitrogen emission. N2 emits mainly from char-N with slight contribution of volatile nitrogen. The mechanism of different N-containing species formation and catalysts influence in the pyrolysis is also discussed in the paper.