Particulate carbon emissions from electrostatic precipitators used for mercury emissions control: operational factors and implications

Injection of powdered activated carbon (PAC) into the combustion flue gas of a coal-fired boiler is a leading approach to reducing anthropogenic mercury emissions. Small particle size and poor electrical properties are known to make carbonaceous particles more difficult to remove from a gas stream by electrostatic precipitation, by far the dominant particulate control technology for coal-fired boilers. The present analysis estimates PAC emissions from electrostatic precipitators (ESPs) and considers both the operational factors driving such emissions as well as their implications in terms of adsorbed mercury concentrations and their potential to act as a climate forcing agent similar to black carbon. The results of the analysis find the potential for PAC to increase particulate carbon emissions by tens of percent to over 150 % in the worst-case scenario considered. Such emissions could increase the contribution of coal combustion to total anthropogenic emissions of particulate carbon by several percentage points. Elevated levels of mercury on such emissions can translate into gas-phase-equivalent mercury concentrations approaching 1 ppb. The most important uncertainty influencing these results is the removal efficiency within ESPs of PM2.5-activated carbon particles.