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.