The formation of ammonia in air-blown gasification: Does char-derived NO act as a precursor?

Air-blown gasification of coal has received considerable attention worldwide and especially in the UK in conjunction with the Air-Blown Gasification Cycle, an integrated combined cycle which has been developed in recent decades. The cycle has the potential to produce electricity at higher efficiencies and lower emissions than conventional pf plants. However, the formation of ammonia in the gasifier has been identified as a potential problem, as it would contribute to NO_x emissions. The formation of ammonia has been studied in a laboratory-scale rig designed and constructed specifically for that purpose. The reactor was a fluidised bed of the spouted bed variety. NH₃, NO and HCN levels could be monitored at different bed heights to trace their formation and conversion. Four coals were gasified at 1000 °C and 0.2 MPa. It was found that the bulk of the NH₃ was formed in the spout and the NH₃ concentration increased progressively with reactor height. Formation of NO was relatively minor and, although both NO and HCN may act as precursors for NH₃ formation in the devolatilisation/partial combustion zone, no evidence was found to suggest they make a significant contribution to any subsequent increase in NH₃ emissions.