Concurrent removal of elemental mercury and SO2 from flue gas using a thiol-impregnated CaCO3-based adsorbent: a full factorial design study.

Mercury (Hg) emitted from coal-based thermal power plants (CTPPs) can accumulate and bio-magnify in the food chain, thereby posing a risk to humans and wildlife. The central idea of this study was to develop an adsorbent which can concurrently remove elemental mercury (Hg0) and SO2 emitted from coal-based thermal power plants (CTPPs) in a single unit operation. Specifically, a composite adsorbent of CaCO3 impregnated with 2-mercaptobenimidazole (2-MBI) (referred to as modified calcium carbonate (MCC)) was developed. While 2-MBI having sulfur functional group could selectively adsorb Hg0, CaCO3 could remove SO2. Performance of the adsorbent was evaluated in terms of (i) removal (%) of Hg0 and SO2, (ii) adsorption mechanism, (iii) adsorption kinetics, and (iv) leaching potential of mercury from spent adsorbent. The adsorption studies were performed using a 22 full factorial design of experiments with 15 ppbV of Hg0 and 600 ppmV of SO2. Two factors, (i) reaction temperature (80 and 120 °C; temperature range in flue gas) and (ii) mass of 2-MBI (10 and 15 wt%), were investigated for the removal of Hg0 and SO2 (as %). The maximum Hg0 and SO2 removal was 86 and 93%, respectively. The results of XPS characterization showed that chemisorption is the predominant mechanism of Hg0 and SO2 adsorption on MCC. The Hg0 adsorption on MCC followed Elovich kinetic model which is also indicative of chemisorption on heterogeneous surface. The toxicity characteristic leaching procedure (TCLP) and synthetic precipitation leaching procedure (SPLP) leached mercury from the spent adsorbent were within the acceptable levels defined in these tests. The engineering significance of this study is that the 2-MBI-modified CaCO3-based adsorbent has potential for concurrent removal of Hg0 and SO2 in a single unit operation. With only minor process modifications, the newly developed adsorbent can replace CaCO3 in the flue-gas desulfurization (FGD) system.