Bimodal fly ash size distributions and their influence on gas-particle mass transfer during electrostatic precipitation

Electrostatic precipitators (ESPs) have previously been demonstrated to achieve substantial (up to 60-70%) removal efficiency of mercury from coal-fired power plants (CFPPs). However, a high degree of scatter exists in the pilot- and full-scale data, suggesting an incomplete understanding of the mechanism by which mercury is adsorbed within an ESP, particularly by native fly ash. The present analysis explores the influence of bimodal particle size distributions (PSDs) on the gas-particle mass transfer underlying mercury adsorption by fly ash within an ESP. The analysis is motivated by the recent discovery by other investigators of bimodal fly ash PSDs resulting from coal combustion. Results of the present analysis show that, relative to similar monomodal PSDs, bimodal PSDs exhibit greatly increased gas-particle mass transfer potential during electrostatic precipitation. For bimodal PSDs, gas-particle mass transfer potential increases with increasing particle mass in the second mode and decreasing geometric mean diameter and geometric standard deviation of the second mode. A supplemental analysis compares the mercury removal potential of native fly ash, injected fly ash, and injected powdered activated carbon (PAC) during their collection within an ESP, using representative mercury adsorption capacities and particle mass loadings for each. Results showed only marginal differences in mercury removal efficiency between the three sorbents. (C) 2006 Elsevier B.V All rights reserved.