On trimodal particle size distributions in fly ash from pulverized-coal combustion

Combustion-generated fine particles, defined as those with aerodynamic diameters less than 2.5 mum, have come under increased regulatory scrutiny because of suspected links to adverse human health effects. Whereas classical theories regarding coal combustion suggest that mechanisms of ash vaporization and fragmentation lead to bimodal ash particle size distributions (PSDs), this paper presents experimental results supporting other existing hypotheses that three distinct ash modes may be more appropriate. This paper focuses on the existence and generality of a central mode, between approximately 0.7 and 3.0 mum diameter. This central mode is presumably caused by fragmentation mechanisms, but is still important from a health perspective, because a large portion is contained within the 2.5 mum particle size fraction. Presented here are experimental results from two different laboratory combustors and one industrial boiler, all burning pulverized coals. Use of a variety of particle-sampling and size classification methods, including electrical mobility, time-of-fight, and inertial (low-pressure impaction) methods, confirms that the central mode is not an artifact of the particle-sampling and -sizing methods used. Results from the combustion of 10 different coals consistently show that this central mode is significant for both high- and low-rank coals. Size-segregated elemental distributions of calcium, iron, and aluminum provide additional insight into mechanisms of formation of each mode. Field tests show that the central mode can be the major contributor to fine particle emissions leaving an electrostatic precipitator (ESP). The new experimental results presented here are interpreted in the light of complementary existing data and available theories from the literature.