Evaluation of the optimal fuel characteristics for efficient NO reduction by coal reburning

Thirteen coals and a char, with volatile matter ranging from 7.9 to 50.4 wt% (daf) and nitrogen content from 1.2 to 2.1 wt% (daf), were evaluated as reburning fuels in an isothermal drop-tube reactor system. The NO reduction efficiency by coal reburning decreased with increasing primary-zone or secondary (reburning)-zone air:fuel stoichiometry. The NO reduction efficiency was also nearly proportional to the extent of carbon burnout in the reburning zone and hence to coal reactivity, and generally increased with decreasing coal rank. With a high primary-zone NO concentration (similar to 650 ppmv) and reburning-zone fuel-rich conditions, the NO reduction efficiency could be improved by increasing the rate of input of reburning-coal volatile nitrogen into the reburning zone. A comparison of the reburning performance of a partly devolatilized char with that of the parent high-volatile bituminous coal showed that the parent coal always achieved a higher NO reduction than the char. A mechanistic model is proposed to determine the relative contributions Of volatiles and char to the total NO reduction observed. (C) 1997 Elsevier Science Ltd.