This chapter presents an overview of the interactions of mercury with activated carbon in flue gas atmospheres at moderate flue gas temperatures. Heterogeneous reactions of mercury at the carbon surface can account for the effectiveness of injected activated carbon powder as well as unburned carbon particulates in removing mercury from the flue gas stream. So we need to define these heterogeneous reactions and how the mercury is bound to the carbon surface.

The control of mercury in the air emissions from coal-fired power plants is an ongoing challenge. The native unburned carbons in fly ash can capture varying amounts of Hg depending upon the temperature and composition of the flue gas at the air pollution control device, with Hg capture increasing with a decrease in temperature; the amount of carbon in the fly ash, with Hg capture increasing with an increase in carbon; and the form of the carbon and the consequent surface area of the carbon, with Hg capture increasing with an increase in surface area. The latter is influenced by the rank of the feed coal, with carbons derived from the combustion of low-rank coals having a greater surface area than carbons from bituminous- and anthracite-rank coals.The chemistry of the feed coal and the resulting composition of the flue gas enhances Hg capture by fly ash carbons. This is particularly evident in the correlation of feed coal Cl content to Hg oxidation to HgCl2, enhancing Hg capture. Acid gases, including HCl and H2SO4 (at small concentrations) and the combination of HCl and NO2, in the flue gas can enhance the oxidation of Hg.In this presentation, we discuss the transport of Hg through the boiler and pollution-control systems, the mechanisms of Hg oxidation, and the parameters controlling Hg capture by coal-derived fly ash carbons. (C) 2009 Elsevier Ltd. All rights reserved.

Factors that influence kinetic reactivity and equilibrium between elemental mercury, carbon, and flue gas components have been the focus of numerous studies. This study pertains to recent bench-scale fixed-bed tests in which activated carbon was exposed to HgCl2 in a flue gas composition typical of an unscrubbed eastern bituminous coal. Results are discussed in light of a refined binding site model based on the zigzag carbene structures recently proposed for electronic states at the edges of the carbon graphene layers.

Recent advances in the mechanistic understanding of mercury oxidation on a carbon surface in flue gas are reviewed in this paper. Theoretical calculations were performed to determine whether the energetics are feasible for a proposed detailed model for oxidative addition of elemental mercury on a carbon edge structure. The results of the calculation show that mercury complexation with a carbenium ion formed at a zigzag edge carbon has a small positive ΔG, but attack of chloride on the complex will proceed with negative ΔG. The energetics rule out a direct covalent bond formation between mercury and the carbenium ion. Alternative concerted reaction models and double-charged models for the mechanism are also feasible but have not yet been computed.

For both the United States and Canada, testing has been under way for electric utilities to find viable and economical mercury control strategies to meet pending future mercury emission limits. The technology that holds the most promise for mercury control in low-chlorine lignite to meet the needs of the Clean Air Act in the United States and the Canada-Wide Standards in Canada is injection of treated activated carbon (AC) into the flue gas stream. Most of the treated carbons are reported to be halogenated, often with bromine. Under a previous multiyear project headed by the Energy & Environmental Research Center (EERC), testing was performed on a slipstream unit using actual lignite-derived flue gas to evaluate various sorbent technologies for their effectiveness, performance, and cost. Testing under this project showed that halogenated ACs performed very well, with mercury capture rates often {ge} 90%. However, differences were noted between treated ACs with respect to reactivity and capacity, possibly as a result of storage conditions. Under certain conditions (primarily storage in ambient air), notable performance degradation had occurred in mercury capture efficiency. Therefore, a small exploratory task within this project evaluated possible differences resulting from storage conditions and subsequent effects of aging that might somehow alter their chemical or physical properties. In order to further investigate this potential degradation of treated (halogenated) ACs, the EERC, together with DOE's National Energy Technology Laboratory, the North Dakota Industrial Commission (NDIC), the Electric Power Research Institute (EPRI), SaskPower, and Otter Tail Power Company, assessed the aging effects of brominated ACs for the effect that different storage durations, temperatures, and humidity conditions have on the mercury sorption capacity of treated ACs. No aging effects on initial capture activity were observed for any carbons or conditions in the investigation. As measured by the 50% breakthrough time, no changes in capacity were observed for Norit LH samples stored frozen and likely none for Norit LH samples stored under high humidity conditions. The major aging effects on capacity for the EERC brominated sample were seen as a decrease in capacity during the first week of storage under high humidity conditions. Storage of the Norit LH and the EERC 5% brominated samples under low humidity conditions resulted in slightly improved capacities. Storage of the 15% brominated sample under high humidity conditions also improved the capacity. Surface analysis using x-ray photoelectron spectroscopy (XPS) showed some migration of bromine to and from the surface occurred during storage, although no net loss of bromine was observed. The migration to the surface in the case of the 15% brominated carbon correlates with the increased capacity observed for this sample. The XPS elemental speciation data for the EERC brominated carbons showed a trend toward more bromide being formed at the expense of covalent bromine during storage. This was especially great for the 15% and the 5% stored in low humidity. The Norit LH samples did not show this increase in bromide concentration. This is consistent with the lack of change in capacity for the Norit LH during storage. The carbon speciation data showed generally more oxidization of carbon surfaces with storage, especially formation of carboxylate groups. The least increase in carboxylate groups was seen for the Norit LH stored under low humidity, which correlates with its increased capacity. In conclusion, the observed changes in surface chemistry can be related to the minimal changes in capacity observed, but the factors operate in different directions, so the relationships are complex. High-quality x-ray absorption fine structure spectra were obtained for most of the samples. Significant changes were observed in the x-ray absorption near edge structure and extended x-ray absorption fine structure spectra of the stored carbons, but proved difficult to interpret and correlate with structural changes.

The Energy & Environmental Research Center (EERC) has pursued a research program for producing activated carbon from North Dakota lignite that can be competitive with commercial-grade activated carbon. As part of this effort, small-scale production of activated carbon was produced from Fort Union lignite. A conceptual design of a commercial activated carbon production plant was drawn, and a market assessment was performed to determine likely revenue streams for the produced carbon. Activated carbon was produced from lignite coal in both laboratory-scale fixed-bed reactors and in a small pilot-scale rotary kiln. The EERC was successfully able to upgrade the laboratory-scale activated carbon production system to a pilot-scale rotary kiln system. The activated carbon produced from North Dakota lignite was superior to commercial grade DARCO{reg_sign} FGD and Rheinbraun's HOK activated coke product with respect to iodine number. The iodine number of North Dakota lignite-derived activated carbon was between 600 and 800 mg I{sub 2}/g, whereas the iodine number of DARCO FGD was between 500 and 600 mg I{sub 2}/g, and the iodine number of Rheinbraun's HOK activated coke product was around 275 mg I{sub 2}/g. The EERC performed both bench-scale and pilot-scale mercury capture tests using the activated carbon made under various optimization process conditions. For comparison, the mercury capture capability of commercial DARCO FGD was also tested. The lab-scale apparatus is a thin fixed-bed mercury-screening system, which has been used by the EERC for many mercury capture screen tests. The pilot-scale systems included two combustion units, both equipped with an electrostatic precipitator (ESP). Activated carbons were also tested in a slipstream baghouse at a Texas power plant. The results indicated that the activated carbon produced from North Dakota lignite coal is capable of removing mercury from flue gas. The tests showed that activated carbon with the greatest iodine number was superior to commercial DARCO FGD for mercury capture. The results of the activated carbon market assessment indicate an existing market for water treatment and an emerging application for mercury control. That market will involve both existing and new coal-fired plants. It is expected that 20% of the existing coal-fired plants will implement activated carbon injection by 2015, representing about 200,000 tons of annual demand. The potential annual demand by new plants is even greater. In the mercury control market, two characteristics are going to dominate the customer's buying habit-performance and price. As continued demonstration testing of activated carbon injection at the various coal-fired power plants progresses, the importance of fuel type and plant configuration on the type of activated carbon best suited is being identified.

The geometries and electronic energies of six singlet carbenes, with methyl and phenyl substituents, and the corresponding carbenium ions were obtained using several density functional theory (DFT) variants and the second-order Møller-Plesset method for electron correlation and compared with G3 results, with the aim to determine a relatively low-cost computational protocol that is sufficiently accurate for the specific molecules and ions of interest. Some additional calculations were performed at the CCSD(T) level. Results for diphenylcarbene, methylphenylcarbene, and their cations, which were not previously investigated by ab initio methods, are reported as are calculations on methylene, methylcarbene, dimethylcarbene, and phenylcarbene. The MPW3LYP/6-311+G(d,p) hybrid DFT level was found to give results that were in close agreement with those obtained using G3 theory, with a mean absolute deviation (MAD) of 1.76 kcal/mol for the calculated proton attachment energies (PAEs). Equilibrium geometries obtained with this method were compared with those obtained at the MP2/6-311G(d,p) level of theory, and bond lengths and bond angles had MADs of 0.005 A and 1.0 degrees, respectively. Harmonic vibrational frequencies of all the carbene molecules and the corresponding ions were computed to verify that the stationary points were true minima, to obtain zero-point corrected energies, to assist in infrared studies of the molecules. The recommended combination of method and basis set is expected to be a useful framework that uses modest amounts of computer resources to obtain usable thermochemical data on moderate-sized hydrocarbons and hydrocarbon cations, including coal-mimetic species.