Design of Carbonaceous Mercury Adsorbents from Waste Materials
Mercury is an air pollutant emitted from coal fired power plants. Once released into the environment, mercury undergoes conversion to organomercury compounds, which cause health concerns for both humans and animals. Many studies have been completed with the goal of reducing mercury emissions from flue gases of coal fired power plants using various types of sorbent and catalytic technologies. Mercury removal has most commonly been accomplished in full scale applications through injecting
... activated carbon-based sorbents into flue gas streams, including several commercial operations in North America. In particular, brominated activated carbon has been proven to be effective at improving the mercury removal efficiency. In order to reduce the cost associated with activated carbon injection, the research of this thesis studied an alternative carbon source, biomass ash waste, which is a by-product from combustion of waste wood for power generation. A chemical-mechanical bromination procedure was used to impregnate the wood ash with bromine (Br-Ash). The mercury capture performance of Br-Ash was found to be comparable to that of a commercial brominated activated carbon (Br-AC). Both Br-Ash and Br-AC captured mercury up to 390 o C. Bromine was found to be stable on the Br-Ash up to high temperatures, but leached considerably when exposed to water at all pH values and liquid to solid mass ratios. The mercury concentration in the leachate was very low at neutral pH and high liquid to solid mass ratios. However, at low or high pH values, the mercury concentration in the leachate was above the amount set by the Environmental Protection Agency (EPA) for classifying hazardous waste. Decreasing the liquid to solid mass ratio in the leach tests (from 20:1 to 2:1) further increased the concentration of mercury in the leachate. The high mercury concentration in this case was due to increased bromine concentration in the leachate. iii Based on these results, it was recommended to consider landfill conditions before disposal of the spent sorbent. In order to reduce environmental impact, the sorbent was re-designed to minimize the amount of Br required for mercury capture. The design of the new sorbent was based on studying the mercury removal mechanisms for Br-Ash compared to Br-AC using x-ray absorption spectroscopy (XAS). The mechanism of mercury capture by Br-Ash was proposed to involve oxidation of the mercury by the surface of the sorbent followed by binding to carbon near Br on the surface. In the case of Br-AC, the mercury was bound to sulfide groups that were not present on the Br-Ash. Understanding the mechanism of mercury capture led to the design of an optimal sorbent containing both Br and sulfide groups. Elemental sulfur was used to impregnate the wood ash, followed by bromination with a lower amount of Br (2 drops). Compared with sorbents containing only 2 drops Br (2D-Br-Ash) or sulfur (S-Ash), the combination of Br and sulfur (2D Br-S-Ash) significantly improved mercury capture. Optimum sulfur loading was achieved at a sulfur:carbon mass ratio of 1:20. The mercury capture mechanism of the 2D Br-S-Ash sorbent was also studied by XAS and was proposed to involve surface enhanced oxidation of mercury, followed by binding of the oxidized mercury to S, Br, or C on the surface of the sorbent. In addition, leach tests on the 2D Br-S-Ash sorbent showed a significant reduction of Hg and Br in the leachate at low liquid to solid ratios. Overall, a new type of carbon based sorbent containing both Br and S was designed with high mercury capture efficiency based on a study of mercury removal mechanisms. iv Preface The format of this thesis is a compilation of several papers; some of which are published and others are to be submitted for publication. Some of the papers and chapters are co-authored and the following list indicates the contributions for each paper. . in: Fuel. 2013. 108. p.54-59. Bisson was responsible for mercury pulse injection tests, thermogravimetric analysis, surface area measurements and manuscript preparation. Bisson prepared Figures 3-8. Liu and Yang were responsible for preparation of the Br-Ash and analysis of the sorbents tested at the power station, as well as preparing Figures 1, 2 and 9. Clark and Patel were responsible for arranging and conducting the tests in real flue gases of the power station, reviewing results and providing insight based on industrial perspective. Maham supervised the thermogravimetric analysis portion of the paper and provided insight into data interpretation as well as proofreading. Gupta and Xu supervised the project, assisted with data interpretation and proofread the manuscript prior to submission. Chapter 4: This chapter is to be submitted as "Potential Hazards of Brominated Carbon Sorbents for Mercury Emission Control," co-authored by Teresa M. Bisson and Zhenghe Xu. Bisson was responsible for experimental design, data collection and interpretation as well as manuscript preparation. Xu supervised the work and made suggestions to include x-ray photoelectron spectroscopy analysis. Xu also proofread the manuscript before submission. . in: Environ. Sci. Technol. 2012, 46, 12186−12193. Bisson was v responsible for conducting the mercury loading tests, analysis of the sorbents, and writing the sections pertaining to mercury loading and analysis, as well as the section regarding implications for Hg capture. MacLean was responsible for the x-ray absorption spectroscopy and x-ray fluorescence data collection and analysis, and was responsible for writing the corresponding sections. Hu supervised the x-ray absorption data collection and analysis, and was also responsible for proofreading the manuscript. Xu was responsible for supervising the mercury loading tests and analysis and proofread the manuscript before submission.