Re-Emission of Mercury from Activated Carbon in Coal Combustion Flue Gas

During the combustion of coal, mercury (Hg) is released and emitted from the stack into the atmosphere. After the release of Hg into the environment, it can react and form methyl mercury, which is the form of Hg that bio-accumulates in organisms such as fish and poses a significant health hazard to humans. Activated carbon (AC) has been shown to be a viable sorbent to capture Hg from coal combustion flue gas and is currently used commercially in power plants. However, untreated AC is not a very effective sorbent when compared to chemically modified/promoted AC treated with substances such as sulfur or halide salts. This study investigates Hg capture using a commercially available chemically modified activated carbon.

Currently there are concerns that AC has the capability of re-emitting already adsorbed Hg during unit start up due to ramps in temperature, sulfur trioxide (SO₃) concentration, or both. While the release of Hg is known to occur, the cause of re-emission is not well understood. The objective of this study is to investigate the factors that might have an impact on the re-emission of Hg from AC under a flue gas environment. In this study, the AC samples were exposed to Hg in flue gas composed of either only combustion products (N₂, O₂, CO₂ and H₂O) or a simulated flue gas (combustion products with NO and SO₂) to evaluate its capture capabilities. After being saturated with Hg in the flue gas mixture, the AC samples were subjected to a SO₃ concentration ramp (2 to 20ppm), a temperature ramp (from 105 °C to 205 °C) and both SO₃ and temperature ramp in order to understand how these conditions influence Hg re-emission. Temperature was found to be the main factor impacting re-emission resulting in the release of up to 40% of the Hg fed to the reactor. Ramping the SO₃ concentration independently had a negligible effect on emission/reemission, while ramping both the SO₃ concentration and temperature together showed a similar trend to that of the temperature ramp only.