Mercury control in municipal waste combustors and coal-fired utilities

Control of mercury (Hg) emissions from municipal waste combustors (MWCs) and coal-fired utilities has attracted attention due to current and potential regulations. Among several techniques evaluated for Hg control, dry sorbent injection (primarily injection of activated carbon) has shown promise for consistently removing high levels of Hg from MWC or coal flue gas. However, the performance in terms of amount of Hg removed per amount of sorbent varies widely between the MWC and coal-fired applications and from unit to unit. In this study, we have conducted bench-scale experiments under conditions simulating MWCs and coal-fired units to study Hg capture by dry sorbents. The effect of reaction temperature on the capture of different Hg species [Hg-0 and Hg(II)] by various types of dry sorbents was the focus of bench-scale tests. An attempt has also been made in this study to compare the bench-scale results with results obtained from pilot studies and to explain disparities in fuel, and unit-specific performance. Our investigations showed that the reaction temperature and Hg species strongly affect Hg control. The results obtained in this suggested the two following mechanisms for Hg capture: i) Capture of Hg-0 by activated carbons is limited by sorption kinetics, and ii) Capture of mercuric chloride (HgCl2) by activated carbons is limited by collision with carbon particles.