Selective Noncatalytic Reduction of NO_x Using Ammonium Sulfate

Ammonium sulfate (AS) is of interest as an additive in stationary combustion plants for the simultaneous control of NOx (through selective noncatalytic reduction, SNCR) and deposition and corrosion (through sulfation of alkali chlorides). The SNCR performance of ammonium sulfate was evaluated through experiments in a laboratory-scale flow reactor.

Experiments with 5 and 10 wt % aqueous ammonium sulfate solutions, corresponding to AS/NO ratios above 1, yielded NO reductions up to 95% in a temperature interval of 1025-1075 °C. The results indicated that sulfur from ammonium sulfate is mainly released as SO3, even though SO2 is detected in increasing concentrations at temperatures above 1000 °C.

Addition of KCl to the SNCR process was shown to promote the reaction at lower temperatures, extending the temperature window for reduction by 50 °C. Furthermore, ammonium sulfate facilitated a high degree of KCl sulfation at or below 1000 °C, demonstrating the potential of using ammonium sulfate to simultaneously reduce NOx and corrosion in full-scale combustion plants.

The experiments were analyzed in terms of a detailed kinetic model. The SNCR experiments using ammonium sulfate were described satisfactorily by the model, although the NO reduction at the optimum was slightly underestimated. Also, the sulfation of KCl was captured well, but the promoting effect of KCl on SNCR with ammonium sulfate was greatly underestimated.

Possible reasons for this discrepancy were discussed.