Experimental study on nitrogen transformation in combustion of pulverized semi-coke preheated in a circulating fluidized bed.

A series of pulverized semi-coke combustion experiments were carried out to investigate the characteristics of fuel-N transformation and the effect of operating conditions in a circulating fluidized bed (CFB) and a down-fire combustor (DFC) on fuel-N transformation. The fuel-N transformation takes place in two stages: first, the fuel-N in pulverized semi-coke is partly converted to N2 and NH3 in a circulating fluidized bed; second, the fuel-N in preheated semi-coke and NH3 is converted to NOx or N2 in a down-fire combustor. The fuel-N in the preheated semi-coke and the NH3 concentration in the syngas both decreased with increasing λCFB in the preheating process. The distribution of the secondary air determined the paths of NH3 conversion in the reducing zone. The air staging of the tertiary air promoted NH3, which was generated for fuel-N, converting to N2 in a complete combustion process. With this process, the NOx emissions were effectively limited. The lowest NO emission from combustion of pulverized semi-coke was 50 mg/m3 (about 60 mg/m3 NOx) (at 6 vol % free O2), which satisfied the emission standard of air pollutants for thermal power plants in China.