Article pubs.acs.org/EF
Catalytic Destruction of a Surrogate Organic Hazardous Air Pollutant as a Potential Co-benefit for Coal-Fired Selective Catalytic Reduction Systems Chun Wai Lee,*,† Yongxin Zhao,‡,§ Shengyong Lu,∥ and William R. Stevens⊥,# †
National Risk Management Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, North Carolina 27711, United States ‡ ARCADIS U.S., Incorporated, Durham, North Carolina 27713, United States ∥ State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ⊥ Oak Ridge Institute for Science and Education, Oak Ridge, Tennessee 37830, United States ABSTRACT: Catalytic destruction of benzene (C6H6), a surrogate for organic hazardous air pollutants (HAPs) produced from coal combustion, was investigated using a commercial selective catalytic reduction (SCR) catalyst for evaluating the potential cobenefit of the SCR technology for reducing organic HAP emissions. Bench-scale experiments were performed using simulated coal combustion flue gases under a broad range of SCR reaction conditions. C6H6 was added at 1 or 17 ppm into the flue gas mixtures with different concentrations of sulfur dioxide (SO2), nitrogen oxide (NO), hydrogen chloride (HCl), and ammonia (NH3) to simulate the combustion of bituminous and sub-bituminous coals. The destruction of the C6H6 across the catalyst was measured by a total hydrocarbon analyzer and a resonance-enhanced multiphoton ionization time-of-flight mass spectrometer (REMPI−TOFMS) for the experiments with high (17 ppm) and low (1 ppm) concentrations of C6H6, respectively. The operating parameters of the SCR process, including the space velocity, temperature, and concentration of C6H6, were found to have a significant impact on the destruction of C6H6. The constituents of the flue gas had very little impact on the destruction, suggesting that the significant additional co-benefit of destruction of trace organic HAPs provided by the SCR process may be applicable to a wide variety of coals under different firing conditions. Destruction of C6H6 with high efficiencies is likely to occur concurrently with the reduction of NO during the SCR process without indication of carbon deposition on the catalyst.
1. INTRODUCTION The Mercury and Air Toxics Standards (MATS) promulgated by the United States Environmental Protection Agency (U.S. EPA) require existing and new coal-fired electricity-generating units (EGUs) to reduce the emissions of hazardous air pollutants (HAPs).1 The standards established numerical emission limits for mercury (Hg), particulate matter (PM) as a surrogate for toxic non-mercury metals, and hydrochloric acid (HCl) as a surrogate for all toxic acid gases. The standards also specify work practices, instead of numerical values, to limit the emissions of organic HAPs, including dioxins and furans. Because dioxins and furans are formed as a result of inefficient incomplete combustion, the work practice standards require that annual performance tests be conducted for individual units, including inspection, adjustment, maintenance, and repair, to ensure optimal combustion. In addition, the Cross-State Air Pollution Rule (CSAPR) requires states to improve air quality significantly by reducing power-plant emissions that cross state lines and contribute to ozone and fine-particle pollution in other states.2 The CSAPR requires 28 states to reduce their annual emissions of SO2 and their annual and/or seasonal emissions of nitrogen oxides (NOx) to assist in the attainment of the National Ambient Air Quality Standards (NAAQS) for ozone and fine particles. With the implementation of the new MATS and CSAPR, coal-fired power plants are expected to comply with the new standards and rules through a range of © 2016 American Chemical Society
strategies, including upgrading their existing emission control devices and/or the installation of new control equipment. Because the new standards and rules require the reduction of more air pollutants, a multi-pollutant control approach, which provides the co-benefit of using existing control devices to remove additional air pollutants, becomes an attractive compliance strategy. Selective catalytic reduction (SCR) is the most effective postcombustion NOx control technology (with typical reductions of 80−90%) that has been used by coal-fired power plants for meeting stringent NOx emission limits. SCR converts flue gas NOx into nitrogen gas (N2) and water via a catalyzed reaction with ammonia using a catalyst with a formulation of V2O5− WO3/TiO2. The combination of SCR with the widely used wet-scrubber technology for SO2 emission control offers a mercury removal co-benefit.3 The co-benefit is due, in large part, to the vanadium component of the SCR catalyst, which also provides active sites that dissociate HCl,4 a minor constituent of the flue gas produced by combustion of coal. In sharp contrast to molecular HCl, the more reactive dissociated HCl species is capable of converting elemental mercury (Hgo) into water-soluble HgCl2, which can be Received: September 10, 2015 Revised: January 11, 2016 Published: February 8, 2016 2240
DOI: 10.1021/acs.energyfuels.5b02058 Energy Fuels 2016, 30, 2240−2247
Article
Energy & Fuels
Figure 1. Down-flow SCR reactor system.
that existed in the coal combustion flue gases is important for reducing emissions of organic HAPs from coal-fired EGUs. Catalytic destruction has been considered a better approach than activated carbon adsorption for reducing the emissions of organic HAPs because the adsorption generates hazardous solid wastes that present a secondary waste-disposal issue.16,17 The low-temperature catalytic destruction of dioxins using a proprietary catalyst with a vanadium/titanium formulation has shown promise for reducing organic HAPs in laboratory tests that were conducted by Liljelind et al.17 A catalyst with a formulation of V2O5−WO3/TiO2 has been tested for the destruction of a wide variety of volatile organic compounds (VOCs), including benzene, toluene, methylcyclohexane, mono- and dichlorobenzenes, chlorophenol, trichloroethane, and trichloroethylene, with high destruction efficiencies at temperatures below 350 °C.18 Other studies also found that vanadium-based catalysts are reactive for the destruction of 1,2dichlorobenzene.19,20 Because vanadium is the primary active component of commercial SCR catalysts, it is of interest to explore additional co-benefits of SCR installed in coal-fired EGUs to reduce trace emissions of organic HAPs. Because no additional equipment is required, the catalytic destruction of organic HAPs by SCR will provide a large number of SCRequipped, coal-fired EGUs, with the significant co-benefit of reducing the emissions of organic HAPs at minimal cost. On the basis of the above discussion, it is apparent that research to examine the co-benefit provided by SCR units for the reduction of the emissions of organic HAPs is imperative. A bench-scale flow reactor was used in our research to simulate the operation of SCR units under the conditions of firing bituminous or sub-bituminous coal. A resonance-enhanced multiphoton ionization time-of-flight mass spectrometer (REMPI−TOFMS) monitor, developed in our laboratory, was used to measure trace concentrations of C6H6 (e.g., 1 ppm). To improve the accuracy of the C6H6 measurements for tests with low C6H6 concentrations, we used a REMPI−TOFMS for real-time measurement of C6H6, instead of using the THC analyzer. Equation 1 was used to assess the efficiency of the catalytic destruction of C6H6
destruction of benzene (C6H6) as a surrogate organic HAP. Benzene was selected as the surrogate of organic HAPs because it is a trace toxic compound that contains the aromatic ring structure that has been found in the emissions from coal-fired power plants.8 Results generated by the bench-scale tests were further analyzed using a standard least-squares model to evaluate the significance of each individual operating parameter and their cross effects. The objective of this study was to gain an understanding of how the operating conditions of the SCR unit and the type of coal influenced the destruction of organic HAPs.
2. EXPERIMENTAL SECTION 2.1. Down-Flow SCR Reactor. A bench-scale down-flow reactor was used to investigate the catalytic destruction of C6H6 under various operating conditions of the SCR units in the field. Details of the reactor, which has been used to evaluate the co-benefit of SCR for Hgo oxidation, are available elsewhere.6 A sample of a honeycomb SCR catalyst was placed inside the reactor. The sample was cut from a commercial NOx reduction SCR catalyst made in the U.S. that has a propriety formulation of the oxides of vanadium and tungsten supported on titanium oxide (V2O5−WO3/TiO2). The sample was sulfated prior to the SCR tests. Figure 1 shows the schematic diagram of the cylindrical reactor. The reactor, constructed of Pyrex glass, was 3100 mm in length and 40 mm in diameter. The reactor was maintained at a defined temperature by three temperature-controlled electric furnaces to mimic the operation of SCR units in coal-fired EGUs. A quartz tube housed in a temperature-controlled electric furnace was used as the methane (CH4) burner, where CH4 was combusted to provide some of the simulated coal combustion flue gas constituents, including carbon dioxide (CO2), water vapor (H2O), nitrogen (N2), and oxygen (O2). Other flue gas constituents, including sulfur dioxide (SO2), nitrogen oxide (NO), hydrogen chloride (HCl), and benzene (C6H6), were added to the flue gas at the top of the reactor at controlled rates. NH3 gas was provided into the reactor separately to achieve a NH3/NO ratio of 0.9, through a port located between the first and second heating furnaces, to minimize its reaction with HCl or SO2, to prevent the formation of “sticky” ammonia salts. The long entrance region upstream of the catalyst provided an adequate length (>2000 mm) for complete mixing of the components of the simulated flue gas. Two honeycomb (3 × 3 cells, 8.25 mm pitch) catalyst samples, 300 and 600 mm in length, were used for achieving two different SVs for the C6H6 destruction tests. Under a constant flue gas flow rate of 14 L/min through the short and long catalysts, the corresponding SVs were 4000 and 2000 h−1, respectively. Table 1 summarizes the experimental conditions employed in the C6H6 destruction tests.
C6H6 destruction (%) = {([C6H6]inlet − [C6H6]outlet )/[C6H6]inlet } × 100
where [C6H6]inlet and [C6H6]outlet represent the concentrations of C6H6 at the inlet and outlet of the catalyst, respectively. 2.2. REMPI−TOFMS for Trace Benzene Measurement. The continuous, online measurement of low concentrations of aromatic compounds (e.g.,
