Experimental investigation of NOx emission and ash-related issues in ammonia/coal/biomass co-combustion in a 25-kW down-fired furnace

Co-firing ammonia in coal units is a promising approach for the phasedown of coal power. In this paper, we demonstrate the feasibility of burning ammonia with coal and biomass in a 25- kW down-fired furnace with a swirl-stabilized burner. Ammonia is injected from the central tube at thermal ratios ranging from 0 to30% and can be completely burnt out in most co-firing cases. We investigate the NOx emission, unburnt carbon in fly ash, particulate matter formation and ash deposition behaviors when co-firing NH3 with either SH lignite coal or the coal/biomass blend. With a fixed air staging ratio, the NOx emission increases linearly with the NH3 fuel ratio. By increasing the percentage of secondary air, the emitted NOx can be reduced to 300 ppm with an NH3 thermal ratio of 30%. The unburnt carbon is affected by NH3 addition in a complex manner. With a 30% (thermal) NH3 addition, the unburnt carbon increases from 0.4% to 5.6% for the SH coal mainly due to a temperature drop, but decreases from 2.2% to 0.7% for the SH coal/biomass blend. As for the ash-related issues, the addition of NH3 to either coal or coal/biomass blend is found to alleviate both the fouling intensity and the ultrafine particulate matter formation ability. This is a major advantage over biomass combustion.