The coal-fired power plants are considered to be the major source of the anthropogenic mercury (Hg) emissions world-wide. In the United States (U.S.), the coal-fired power plants account for approximately 40% of the total anthropogenic Hg emissions. The elemental Hg (Hg0) emitted during the coal combustion process is hard to capture because of its volatility and insolubility in water. The oxidation of the Hg0 to compounds of oxidized Hg (Hg2+) which are efficiently removed using the wet scrubbers is regarded as a promising technology. In this study a bench-scale experimental system was designed and installed to remove the flue gas Hg0 using the enhanced oxidation and wet scrubbing technology. Hydrogen peroxide (H2O2) was used for the enhanced oxidation of the Hg0. The resulting Hg2+ was removed in a fixed packed bed (FPB) wet scrubber designed using HYSYS simulation. A modification of the Ontario Hydro Method (OHM) was used to collect the experimental samples which were analyzed for the Hg using Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) instrument. The experimental runs were conducted at 475oC with 260 ppm of H2O2 concentration and at 495oC with 300 ppm of H2O2 concentration in order to demonstrate the effect of temperature and H2O2 concentration on the oxidation of Hg0. The experimental study showed a statistically significant increase in the oxidation of Hg0 with increase in temperature and H2O2 concentration. The resulting Hg2+ was completely removed using the FPB wet scrubber. The experimental results were compared with the past kinetic model results. The experimental results showed lower oxidation of Hg0 when compared to the past kinetic model results. This could be due to the absence of the flue gas component chlorine (Cl2) which significantly contributed to the oxidation of Hg0 in the past kinetic model study. The addition of Cl2 and other flue gas components such as nitric oxide (NO) and sulfur dioxide (SO2) will further improve the insights for the enhanced oxidation of Hg0 using H2O2. A streamlined life cycle assessment (LCA) study was performed using the GaBi 6.0 PE Education LCA software to evaluate and compare the environmental performance in terms of the environmental impacts of this novel technology of H2O2 injection with the current technology of Cl2 injection used to oxidize the flue gas Hg0. The streamlined LCA study found that the production of Cl2 and the application of Cl2 injection technology showed more deposition of Hg2+ emissions in the fresh water, agricultural soil and the industrial soil. The production of H2O2 and the application of H2O2 injection technology showed more Hg2+ emission deposition in sea water and in the air. However, the deposition of Hg2+ emissions due to the application of H2O2 injection technology was not significantly higher when compared to Cl2 injection technology. The abiotic depletion potential (elements) was significantly higher for the production of Cl2 and for the application of Cl2 injection technology as compared to H2O2 production and application of H2O2 injection technology. The global warming potential (100 years) was twice for the production of H2O2 as compared to the Cl2 production. Most of the environmental impacts were similar for both the application of H2O2 injection and Cl2 injection technologies. A more detailed LCA study is recommended to benchmark the sustainability of these technologies.
July 27, 2016
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