The effects of various factors on NO_x reduction by urea solution injection in the fuel-rich zone (UIFR) under a reducing atmosphere at high temperature were experimentally investigated in a 330 MW tangentially pulverized coal-fired boiler. The experimental results indicated that the NO_x emission of the boiler could be effectively reduced by using the UIFR method, and the NO_x reduction efficiency was mostly affected by the operating load of the boiler, the air distribution condition, and the boiler operating oxygen content (BOOC). The higher the load was, the larger the optimal normalized stoichiometric ratio (NSR) and the lower the NO_x reduction efficiency became. As compared with the condition of conventional air distribution mode, the reducing atmosphere in the combustion zone could be enhanced under the condition of limiting air distribution (LAD) mode, which thus increased the NO_x reduction efficiency of UIFR and reduced the optimal NSR value. A low BOOC could further increase the NO_x reduction efficiency of UIFR. When the BOOC was, respectively, reduced to 1.71 and 1.85 vol % under 210 and 240 MW loads, the corresponding NO_x reduction efficiencies of UIFR reached 45.3 and 41.3% in the LAD mode, respectively. However, the low BOOC increased the CO emission concentration, which could be avoided by the combined use of UIFR and high-velocity over-fire air. These experimental results can provide guidance for the ultra-low NO_x emission of coal-fired boilers.

To explore methods of reducing NOx emission from pulverized coal boilers, the effects of injecting ammonia solution and pyrolysis gas into the furnace on NOx emission were experimentally investigated on a 75 t/h pulverized coal boiler. Results show that the deep air staging with 30% separated over fire air (SOFA) creates a high temperature and strong reducing atmosphere in the reducing zone, providing the prerequisites for NOx reduction by ammonia solution and pyrolysis gas. Compared with deep air staging itself, NOx emission can be reduced by 16.7% when ammonia solution is injected from the reducing zone with a normalized stoichiometric ratio of 2.0. However, NOx reduction efficiency is largely affected by its injection position. Similarly, NOx emission is decreased by 28.2% through injecting pyrolysis gas with its calorific value of 10% into the furnace, while a further increase of pyrolysis gas input will not increase NOx reduction efficiency. When ammonia solution and pyrolysis gas are simultaneously injected into the furnace under deep air staging conditions, the overall NOx reduction efficiency reaches 92.0% and NOx emission is decreased to 39.1 mg/m³. Considering the increasingly strict NOx emission standard, these findings can provide theoretical and practical guides to the future NOx reduction in pulverized coal boilers.