Constance L. Senior

Gas-phase transformations of mercury in coal-fired power plants

Because mercury enters the food chain primarily through atmospheric deposition, exposure models require accurate information about mercury emission rates and mercury speciation from point sources. Since coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere, the speciation of mercury in coal-fired power plant flue gas is currently an active topic of research. We have demonstrated that the assumption of gas-phase equilibrium for mercury-containing species in coal-fired power plant exhaust is not valid at temperatures below approximately 800 K (500°C). Chlorine-containing species have been shown to be the most important for oxidation of elemental mercury in the post-combustion gases. The conversion of HCl to Cl2 in the flue gas of a coal-fired power plant is kinetically limited. Kinetic calculations of the homogeneous oxidation of elemental mercury by chlorine-containing species were carried out using global reactions from the literature. The levels of mercury oxidation, while of comparable magnitude to field observations, are still below the 40% to 80% oxidation typically observed in field measurements.

Emissions of mercury, trace elements, and fine particles from stationary combustion sources

Abstract The emissions of trace elements and mercury from stationary combustion sources are determined by the occurrence of these elements in fuels, the transformation of the elements into vapor and particles in furnaces, and the ability of the vapors and particles to penetrate the air pollution control devices (APCDs). For electrostatic precipitators (ESPs), in use at greater than 90% of coal-fired utility boilers in the US, the preferential escape of particles is in the 0.1–1.0 μm size. The major source of particles in this size range is the vaporization and condensation of the inorganic constituents in the parent fuel. Potentially toxic elements, although mainly confined to the particulate phase, may therefore show enhanced release to the environment as a result of preferential condensation on the surface of submicron particulate matter. Surface reaction with larger fly ash particles can reduce these emissions by redistributing the trace elements away from the difficult-to-capture submicron particulate. In contrast, mercury is the most volatile of the trace elements in coal and its escape is near complete when the mercury is in the elemental vapor form. Mercury emissions may be mitigated, however, by transformation to mercuric chloride, more readily captured either in scrubbers or by collection in the particulate form. In this paper, we present our recent research developments contributing to an improved understanding of the relative importance of factors determining trace element emissions. Significant progress that has been made in understanding and quantifying each of the processes governing the transformation of the inorganic constituents of fuels leading to promising results on the prediction of emissions given detailed characterization of fuels, the combustion conditions to which they are exposed, and the characteristics of APCDs.

Fixed-bed studies of the interactions between mercury and coal combustion fly ash

Sixteen different fly ash samples, generated from both pilot-scale and full-scale combustion systems, were exposed to a simulated flue gas containing either elemental mercury or HgCl2 in a bench-scale reactor system at the Energy and Environmental Research Center to evaluate the interactions and determine the effects of temperature, mercury species, and ash type on adsorption of mercury and oxidation of elemental mercury. The fly ash samples were characterized for surface area, loss on ignition, and forms of iron in the ash. While many of the ash samples oxidized elemental mercury, not all of the samples that oxidized mercury also captured elemental mercury. However, no capture of elemental mercury was observed without accompanying oxidation. Generally, oxidation of elemental mercury increased with increasing amount of magnetite in the ash. However, one high-carbon subbituminous ash with no magnetite showed considerable mercury oxidation that may have been due to unburned carbon. Surface area as well as the nature of the surface appeared to be important for oxidation and adsorption of elemental mercury. The capacity of the ash samples for HgCl2 was similar to that for elemental mercury. There was a good correlation between the capacity for HgCl2 and the surface area; capacity decreased with increasing temperature.

XAFS characterization of mercury captured from combustion gases on sorbents at low temperatures

A review is presented of data obtained by X-ray absorption fine structure (XAFS) spectroscopy on the speciation of mercury captured on a variety of sorbent materials from simulated combustion flue gases at low temperatures (<200 °C). Data for other key elements (S, Cl) are also presented. Implications for bonding mechanisms for the capture of mercury are discussed, including the relative importance of chemisorption and physisorption processes. Systematics in the parameters derived from mercury X-ray absorption near-edge structure (XANES) spectra indicate that mercury can be captured by bonding to I, Cl, S or O anionic species on the surfaces of carbonaceous and other sorbents, but only as ionic Hg2+. None of the observations made by XAFS spectroscopy is consistent with the capture of mercury in the elemental state, i.e. physisorption.

Vaporization of arsenic, selenium and antimony during coal combustion

Accumulation of toxic trace elements generated by coal-fired power stations presents a serious threat to the environment. Field testing and laboratory studies have revealed the existence of trace elements in submicron particles emitted from power stations. Arsenic, selenium, and antimony are present in the submicron particles presumably via a vaporization-condensation pathway although the volatilities of these elements are very different. Previous explanations include the volatility and the forms of occurrence of elements in coals. Based on well-controlled experimental studies for selected coals, this paper establishes the first quantitative physicochemical model for vaporization of arsenic, selenium, and antimony during coal pyrolysis and combustion. Advanced characterization methods found that the three elements are associated with pyrites in the coals burned for this study. The vaporization processes for these three elements consist of three consecutive processes: transport of molecules or atoms through the bulk pyrite liquid (melt) to the melt/gas interface, vaporization of elements at the surface of melts, and transport of molecules/atoms through the pores of the char to the atmosphere. The controlling step for vaporization of arsenic is diffusion through the melt. Diffusion processes in the melt and within the char pores together determine the vaporization rates for selenium and antimony.

Emissions and risks associated with oxyfuel combustion: State of the science and critical data gaps

Oxyfuel combustion is a promising technology that may greatly facilitate carbon capture and sequestration by increasing the relative CO2 content of the combustion emission stream. However, the potential effect of enhanced oxygen combustion conditions on emissions of criteria and hazardous air pollutants (e.g., acid gases, particulates, metals and organics) is not well studied. It is possible that combustion under oxyfuel conditions could produce emissions posing different risks than those currently being managed by the power industry (e.g., by changing the valence state of metals). The data available for addressing these concerns are quite limited and are typically derived from laboratory-scale or pilot-scale tests. A review of the available data does suggest that oxyfuel combustion may decrease the air emissions of some pollutants (e.g., SO2, NOx, particulates) whereas data for other pollutants are too limited to draw any conclusions. The oxy-combustion systems that have been proposed to date do not have a conventional “stack” and combustion flue gas is treated in such a way that solid or liquid waste streams are the major outputs. Use of this technology will therefore shift emissions from air to solid or liquid waste streams, but the risk management implications of this potential change have yet to be assessed. Truly useful studies of the potential effects of oxyfuel combustion on power plant emissions will require construction of integrated systems containing a combustion system coupled to a CO2 processing unit. Sampling and analysis to assess potential emission effects should be an essential part of integrated system tests. Implications: Oxyfuel combustion may facilitate carbon capture and sequestration by increasing the relative CO2 content of the combustion emission stream. However, the potential effect of enhanced oxygen combustion conditions on emissions of criteria and hazardous air pollutants has not been well studied. Combustion under oxyfuel conditions could produce emissions posing different risks than those currently being managed by the power industry. Therefore, before moving further with oxyfuel combustion as a new technology, it is appropriate to summarize the current understanding of potential emissions risk and to identify data gaps as priorities for future research.

Selenium Partitioning and Removal Across a Wet FGD Scrubber at a Coal-Fired Power Plant.

Selenium has unique fate and transport through a coal-fired power plant because of high vapor pressures of oxide (SeO2) in flue gas. This study was done at full-scale on a 900 MW coal-fired power plant with electrostatic precipitator (ESP) and wet flue gas desulfurization (FGD) scrubber. The first objective was to quantify the partitioning of selenium between gas and condensed phases at the scrubber inlet and outlet. The second objective was to determine the effect of scrubber operation conditions (pH, mass transfer, SO2 removal) on Se removal in both particulate and vapor phases. During part of the testing, hydrated lime (calcium hydroxide) was injected upstream of the scrubber. Gas-phase selenium and particulate-bound selenium were measured as a function of particle size at the inlet and outlet of the scrubber. The total (both phases) removal of Se across the scrubber averaged 61%, and was enhanced when hydrated lime sorbent was injected. There was evidence of gas-to-particle conversion of selenium across the scrubber, based on the dependence of selenium concentration on particle diameter downstream of the scrubber and on thermodynamic calculations.

Fundamentals of Mercury Oxidation in Flue Gas

The objective of this project is to understand the importance of and the contribution of gas-phase and solid-phase coal constituents in the mercury oxidation reactions. The project involves two experimental scales and a modeling effort. The team is comprised of University of Utah, Reaction Engineering International, and University of Connecticut. The objective is to determine the experimental parameters of importance in the homogeneous and heterogeneous oxidation reactions; validate models; and, improve existing models. Parameters to be studies include HCl, NOx, and SO{sub 2} concentrations, ash constituents, and temperature. This report summarizes Year 1 results for the experimental and modeling tasks. Experiments in the drop tube are just beginning and a new, speciated mercury analyzer is up and running. A preliminary assessment has been made for the drop tube experiments using the existing model of gas-phase kinetics.

Analysis of Halogen-Mercury Reactions in Flue Gas

Oxidized mercury species may be formed in combustion systems through gas-phase reactions between elemental mercury and halogens, such as chorine or bromine. This study examines how bromine species affect mercury oxidation in the gas phase and examines the effects of mixtures of bromine and chlorine on extents of oxidation. Experiments were conducted in a bench-scale, laminar flow, methane-fired (300 W), quartz-lined reactor in which gas composition (HCl, HBr, NO{sub x}, SO{sub 2}) and temperature profile were varied. In the experiments, the post-combustion gases were quenched from flame temperatures to about 350 C, and then speciated mercury was measured using a wet conditioning system and continuous emissions monitor (CEM). Supporting kinetic calculations were performed and compared with measured levels of oxidation. A significant portion of this report is devoted to sample conditioning as part of the mercury analysis system. In combustion systems with significant amounts of Br{sub 2} in the flue gas, the impinger solutions used to speciate mercury may be biased and care must be taken in interpreting mercury oxidation results. The stannous chloride solution used in the CEM conditioning system to convert all mercury to total mercury did not provide complete conversion of oxidized mercury to elemental, when bromine was added to the combustion system, resulting in a low bias for the total mercury measurement. The use of a hydroxylamine hydrochloride and sodium hydroxide solution instead of stannous chloride showed a significant improvement in the measurement of total mercury. Bromine was shown to be much more effective in the post-flame, homogeneous oxidation of mercury than chlorine, on an equivalent molar basis. Addition of NO to the flame (up to 400 ppmv) had no impact on mercury oxidation by chlorine or bromine. Addition of SO{sub 2} had no effect on mercury oxidation by chlorine at SO{sub 2} concentrations below about 400 ppmv; some increase in mercury oxidation was observed at SO{sub 2} concentrations of 400 ppmv and higher. In contrast, SO{sub 2} concentrations as low as 50 ppmv significantly reduced mercury oxidation by bromine, this reduction could be due to both gas and liquid phase interactions between SO{sub 2} and oxidized mercury species. The simultaneous presence of chlorine and bromine in the flue gas resulted in a slight increase in mercury oxidation above that obtained with bromine alone, the extent of the observed increase is proportional to the chlorine concentration. The results of this study can be used to understand the relative importance of gas-phase mercury oxidation by bromine and chlorine in combustion systems. Two temperature profiles were tested: a low quench (210 K/s) and a high quench (440 K/s). For chlorine the effects of quench rate were slight and hard to characterize with confidence. Oxidation with bromine proved sensitive to quench rate with significantly more oxidation at the lower rate. The data generated in this program are the first homogeneous laboratory-scale data on bromine-induced oxidation of mercury in a combustion system. Five Hg-Cl and three Hg-Br mechanisms, some published and others under development, were evaluated and compared to the new data. The Hg-halogen mechanisms were combined with submechanisms from Reaction Engineering International for NO{sub x}, SO{sub x}, and hydrocarbons. The homogeneous kinetics under-predicted the levels of mercury oxidation observed in full-scale systems. This shortcoming can be corrected by including heterogeneous kinetics in the model calculations.

A Computational Workbench Environment for Virtual Simulation of a Vision 21 Energyplex

In this paper we describe our progress toward creating a process workbench for performing virtual simulations of DOE Vision 21 energyplex systems. The workbench provides a framework for incorporating a full complement of models, ranging from simple heat/mass balance reactor models that run in minutes to detailed models that can require several hours to execute. Provided herein is an overview of a process workbench for a conventional PC power plant developed during the past year and our current efforts at developing a workbench for a gasifier based energyplex configuration.Copyright