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 NO_x 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 NO_x emission increases linearly with the NH₃ fuel ratio. By increasing the percentage of secondary air, the emitted NO_x 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.