Effects of reburning mechanically-activated micronized coal on reduction of NO_x: Computational study of a real-scale tangentially-fired boiler

The paper presents some results of numerical simulations of the effects of reburning mechanically-activated micronized coal on reduction of NO_x and unburnt carbon residual in a tangentially fired boiler of 500 t/h steam production. The conventional RANS approach was used to compute two-phase (reactive dispersed particles in gaseous medium) multi-component system, with some modifications related to particle heat transfer and their reactions. The comprehensive model was verified in simulation of the same boiler with conventional firing, as well as in an experimental pilot-scale combustor using micronized, non-activated and mechanically-activated coal. The comparison with the standard dust-coal firing showed that reburning reduces NO_x by 37.5%, but leads to an increase in heat loss due to unburnt carbon residual by 61%, primarily due to the imposed suboptimal stoichiometric ratio. Switching to reburning micronized coal reduced NO_x by 49% while heat loss was still substantial, 42% higher than in the conventional firing. Using mechanically-activated micronized coal of the same granulation brought only marginal further improvement in NO_x reduction (to 50%), but to a remarkable decrease in heat loss, only 3.3% higher than in the case without reburning. The simulations were performed for a set of plausible operating parameters (stoichiometric ratios, flow rates of reburn coal, flue gas recirculation, overfire air), which all need to be optimized to achieve maximum effects. Nevertheless, the simulations demonstrated that reburning of activated micronized coal is a feasible option to achieve significant NO_x reduction without being penalized with excessive unburnt fuel/heat residual.