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.

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

UNLABELLED: 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, NO(x), 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.

Mercury capture by native fly ash carbons in coal-fired power plants.

The control of mercury in the air emissions from coal-fired power plants is an on-going 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 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.

Chemistry of Trace Inorganic Elements in Coal Combustion Systems: A Century of Discovery.

Coal fueled the Industrial Revolution and the global expansion of electrification in the 20th century. In the 21st century, coal use has declined in North America and Europe, but continues to increase in Asia. Coal contains many of the elements of the Periodic Table, in percent-levels or in trace amounts (ppm, ppb). The impact of many of these elements on the environment via air and water discharges from coal-fired plants has been studied with decades of research on their chemical transformations within combustion systems and on their fates upon reintroduction into the environment. The transformations of the trace elements present in coal burned during combustion can be categorized as thermal volatilizations from the coal in the furnace; thermal decomposition of trace element compounds inside the coal; encapsulation inside ash structures through high-temperature vitrification; oxidation of the trace elements with the myriad species contained in flue gas through gas phase (homogeneous) reactions or catalytic (gas-solid) reactions; adsorption and/or reactions with active sites on entrained fly ash particulates contained in the flue gas; and absorption into solutions. These transformations can, in many cases, impact the fraction of these trace elements that are removed by various pollution control devices compared to the fraction released into the environment. The sampling and measurement of trace elements, in the inlet coal, outlet flue gas, aqueous scrubber solutions, and ash matrices, represents a significant challenge. This review focuses on the behavior of trace elements in industrial coal combustion systems with an emphasis on what has been learned over the past century uniquely related to the use of coal in boilers for electricity and heat production. Key accomplishments in measurement, modeling and control of trace element emissions in coal-fired systems are highlighted.

Oxidation of mercury across selective catalytic reduction catalysts in coal-fired power plants.

A kinetic model for predicting the amount of mercury (Hg) oxidation across selective catalytic reduction (SCR) systems in coal-fired power plants was developed and tested. The model incorporated the effects of diffusion within the porous SCR catalyst and the competition between ammonia and Hg for active sites on the catalyst. Laboratory data on Hg oxidation in simulated flue gas and slipstream data on Hg oxidation in flue gas from power plants were modeled. The model provided good fits to the data for eight different catalysts, both plate and monolith, across a temperature range of 280-420 degrees C, with space velocities varying from 1900 to 5000 hr(-1). Space velocity, temperature, hydrochloric acid content of the flue gas, ratio of ammonia to nitric oxide, and catalyst design all affected Hg oxidation across the SCR catalyst. The model can be used to predict the impact of coal properties, catalyst design, and operating conditions on Hg oxidation across SCRs.

Confounding effects of aqueous-phase impinger chemistry on apparent oxidation of mercury in flue gases.

Gas-phase reactions between elemental mercury and chlorine are a possible pathway to producing oxidized mercury species such as mercuric chloride in combustion systems. This study examines the effect of the chemistry of a commonly used sample conditioning system on apparent and actual levels of mercury oxidation in a methane-fired, 0.3 kW, quartz-lined reactor in which gas composition (HCl, Cl2, NOx, SO2) and quench rate were varied. The sample conditioning system included two impingers in parallel: one containing an aqueous solution of KCl to trap HgCl2, and one containing an aqueous solution of SnCl2 to reduce HgCl2 to elemental mercury (Hg0). Gas-phase concentrations of Cl2 as low as 1.5 ppmv were sufficient to oxidize a significant fraction of the elemental mercury in the KCl impinger via the hypochlorite ion. Furthermore, these low, but interfering levels of Cl2 appeared to persist in flue gases from several doped rapidly mixed flames with varied post flame temperature quench rates. The addition of 0.5 wt% sodium thiosulfate to the KCl solution completely prevented the oxidation from occurring in the impinger. The addition of thiosulfate did not inhibit the KCl impinger's ability to capture HgCl2. The effectiveness of the thiosulfate was unchanged by NO or SO2. These results bring into question laboratory scale experimental data on mercury oxidation where wet chemistry was used to partition metallic and oxidized mercury without the presence of sufficient levels of SO2.

Selenium and arsenic speciation in fly ash from full-scale coal-burning utility plants.

X-ray absorption fine structure spectroscopy has been used to determine directly the oxidation states and speciation of selenium and arsenic in 10 fly ash samples collected from full-scale utility plants. Such information is needed to assess the health risk posed by these elements in fly ash and to understand their behavior during combustion and in fly ash disposal options, such as sequestration in tailings ponds. Selenium is found predominantly as Se(IV) in selenite (SeO3(2-)) species, whereas arsenic is found predominantly as As(V) in arsenate (AsO4(3-)) species. Two distinct types of selenite and arsenate spectra were observed depending upon whether the fly ash was derived from eastern U.S. bituminous (Fe-rich) coals or from western subbituminous or lignite (Ca-rich) coals. Similar spectral details were observed for both arsenic and selenium in the two different types of fly ash, suggesting that the postcombustion behavior and capture of both of these elements are likely controlled by the same dominant element or phase in each type of fly ash.