Optical Property Measurements of Mixed Coal Fly Ash and Particulate Carbon Aerosols Likely Emitted during Activated Carbon Injection for Mercury Emissions Control

Abstract

The most mature technology for controlling mercury emissions from coal combustion is the injection into the flue gas of powdered activated carbon (PAC) adsorbents having chemically treated surfaces designed to rapidly oxidize and adsorb mercury. However, carbonaceous particles are known to have low electrical resistivity, which contributes to their poor capture in electrostatic precipitators (ESPs), the most widely used method of particulate control for coal-fired power plants worldwide. Thus, the advent of mercury emissions standards for power plants has the potential for increased emissions of PAC. Our previous analyses have provided estimates of PAC emission rates resulting from PAC injection in the U.S. and extrapolated these estimates globally to project their associated climate forcing effect. The present work continues our examination by conducting the first comparative measurements of optical scattering and absorption of aerosols comprising varying mixtures of coal combustion fly ash and PAC. A partially fluidized bed (FB) containing fly ash-PAC admixtures with varying PAC concentrations elutriates aerosol agglomerates. A photoacoustic extinctiometer (PAX) extractively samples from the FB flow, providing measurements of optical absorption and scattering coefficients of fly ash (FA) alone and FA-PAC admixtures. Extracted aerosol samples from the FB flow provide particulate loading measurements, thermogravimetric analysis (TGA) provides estimations of the carbon content of the particulates collected from the FB emission, and SEM images of the collected aerosols provide qualitative insight into the aerosols’ size distributions and agglomeration state. Soot from an oil lamp flame provides a comparative benchmark. The results indicate that the increase of carbonaceous particles in the FB emissions can cause a significant linear increase of their mass absorption cross sections (MACs). Thus, widespread adoption of activated carbon injection (ACI) in conjunction with ESPs has the potential to constitute a new source of light absorbing particle emissions which can absorb light efficiently and potentially act like black carbon in the atmosphere