An experimental study of the release of nitrogen from coals pyrolyzed in fluidized-bed reactors

Abstract

Nitrogen release has been measured from a suite of Australian, German, and U.S. coals pyrolyzed under rapid heating conditions in two bench scale reactors. The first reactor was a single-stage fluidized bed reactor: the second was a two-stage reactor in which a fluidized bed was coupled with a tubular reactor. The two-stage reactor enabled cracking reactions of the volatiles to be studied in isolation from reactions of the char. The results show that the gas-phase N-containing species, HCN and NH3, are formed from both brown and bituminous coals. In addition, HNCO is also observed from bituminous coals: HNCO is a plausible precursor for NH3 formation. At high temperatures low-volatile coals give lower yields of HCN and NH3 than high-volatile coals. This observation, however, does not imply that tar cracking is the only source of HCN and NH3. In fact, the results obtained in the two-stage reactor suggest that a significant proportion of the HCN is released from the char and that tar cracking is not a significant source of NH3. A major part of the NH3 is produced by interactions of N-containing species with the char. These results, together with previous studies of N release, demonstrate that there is no simple relationship between N functionality in coals and the composition and rate of release of the nitrogen.