Mechanism on the contribution of coal/char fragmentation to fly ash formation during pulverized coal combustion

In this paper, the correlations between coal/char fragmentation and fly ash formation during pulverized coal combustion are investigated. We observed an explosion-like fragmentation of Zhundong coal in the early devolatilization stage by means of high-speed photography in the Hencken flat-flame burner. While high ash-fusion (HAF) bituminous and coal-derived char samples only undergo gentle perimeter fragmentation in the char burning stage. Simultaneously, combustion experiments of two kinds of coals were conducted in a 25 kW down-fired combustor. The particle size distributions (PSDs) of both fine particulates (PM1-10) and bulk fly ash (PM10+) were measured by Electrical Low Pressure Impactor (ELPI) and Malvern Mastersizer 2000, respectively. The results show that the mass PSD of residual fly ash (PM1+) from Zhundong coal exhibits a bi-modal shape with two peaks located at 14 µm and 102 µm, whereas that from HAF coal only possesses a single peak at 74 µm. A hybrid model accounting for multiple-route ash formation processes is developed to predict the PSD of fly ash during coal combustion. By incorporating coal/char fragmentation sub-models, the simulation can quantitatively reproduce the measured PM1+ PSDs for different kinds of coals. The sensitivity analysis further reveals that the bi-modal mass distribution of PM1+ intrinsically results from the coal fragmentation during devolatilization.