A numerical investigation of the effect of flue gas recirculation on the evolution of ultra-fine ash particles during pulverized coal char combustion

Flue gas recirculation (FGR) has been widely employed for NOx reduction. However, because PM removal in flue gas is never 100% efficient, a gradually accumulated particle concentration (particularly the ultra-fine particles, smaller than 1.0 μm in aerodynamic diameter) may occur in the furnace, affecting PM emission and power plant operation. Experimental measurements cannot provide detailed information on the evolution of ultra-fine particle (including formation, and time-dependent growth and number size distribution). In this work, an improvement over our previous effort to model ultrafine particle formation during coal char combustion is developed and applied to elucidate the effects of various FGR parameters on ultrafine particle formation, including FGR ratio, dust removal efficiency, and particle size distribution in FGR. Model results show that FGR without recirculated particles results in the nucleation of fewer but larger particles. However, the presence of diluting FGR gases results in a low particle collision frequency and consequently yields small particle size and high number density after coalescence. The recirculated particles not only provide available surface area for condensable mineral vapors but also take part in coalescence via collision; consequently, larger particles are formed at a lower overall number density. The ultra-fine particle size first increases slowly with an increase in dust removal efficiency, but then drops rapidly after reaching a peak; An increase in recirculated particle size and a decrease in FGR results in increased particle size. For all cases, the particle number density shows opposite trends to particle size. Overall, the results derived here provide meaningful guidelines for practical application.