Predicting the evolution of fuel nitrogen from various coals

Abstract

Nitrogen release from most coal types occurs by three mechanisms: tar shuttling, HCN production from char-N, and combustion of char-N. This modeling study extends FLASHCHAIN to characterize the first two N-release mechanisms for heating rates from 5 to 10 4 K/s, temperatures to 1550 K, pressures from vacuum to several atmospheres, and 13 coals of rank from sub-bituminous through low volatile bituminous. Based primarily on the accuracy of predicted tar yields, the agreement among predicted and observed evolution histories of tar-N, HCN, and char-N and the nitrogen contents of tar is within experimental uncertainty throught this domain. Under conditions of rapid heating, tar shuttling is the only mechanism for nitrogen release as long as tar is being expelled, but for slower heating conditions, it is overlapped by HCN production from char-N. The predictions correctly depict the rank dependence of nitrogen evolution, exhibiting a broad maximum for hv bituminous coals before falling off for low volatility coals. Tar shuttling is almost entirely responsible for this trend. Also, the predicted nitrogen contents of tar increase throughout primary devolatilization as cumulative tar yields grow, in accord with measured values. HCN production from char-N has an extremely broad thermal response that is well represented with a very broad distribution of activation energies. Predicted char-N and HCN evolution histories for numerous coals based on the same energy distribution are within experimental uncertainty of observed levels for several coal types, although indications that HCN production becomes slower for coals of progressively higher rank are also considered.