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Final Report: Development of Mercury Control Technologies
EPA Grant Number: R827649C017Subproject: this is subproject number 017 , established and managed by the Center Director under grant R827649
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: Center for Air Toxic Metals® (CATM®)
Center Director: Groenewold, Gerald
Title: Development of Mercury Control Technologies
Investigators: Pavlish, John H. , Benson, Steven A. , Galbreath, Kevin C. , Hassett, David J. , Heebink, Loreal V. , Holmes, Michael J. , Kong, Lingbu , Laudal, Dennis L. , Mibeck, Blaise , Miller, Stanley J. , Olson, Edwin S. , Ralston, Nicholas V.C. , Thompson, Jeffrey S. , Timpe, Ronald C. , Zygarlicke, Christopher J.
Institution: University of North Dakota
EPA Project Officer: Stelz, Bill
Project Period: October 15, 1999 through October 14, 2004
RFA: Center for Air Toxic Metals (CATM) (1998)
Research Category: Targeted Research
Description:
Objective:The objective of this research project was to address air toxic trace element emissions, which have become a matter of worldwide concern as well as a regulatory issue in the United States. The specific objective of this research project was to develop and disseminate critical information on air toxic metal compounds to support development and implementation of pollution prevention and control strategies that will reduce effectively air toxic metal emissions and releases to the environment.
Summary/Accomplishments (Outputs/Outcomes):Reactive Sorbents for In-Duct Mercury Capture
Because 80 to 90 percent mercury capture from coal combustion flue gases using carbon-based sorbents is attainable for some coals but can be cost-prohibitive in some applications, less costly, more effective sorbents are needed.
Test results from projects in this area showed that the effectiveness of in-duct injection of sorbents for mercury capture depends on the fineness and dispersion of the sorbent particles for achieving gas-phase diffusion of mercury from the bulk gas to the surface of the sorbent particles. A mathematical model developed to evaluate the limiting effects of diffusion and sorbent capacity can be used to establish the minimum number of particles and the corresponding amount of carbon of a specified size that would be needed to reduce the initial mercury concentration by any specified percentage. Variations in the diffusivity of mercury with temperatures between 120° and 180°C had minimal effect. Temperature has been shown, however, to have a significant effect on sorbent reactivity and capacity.
In laboratory tests on a packed bed of sorbent, where diffusion was not a limiting factor, the effectiveness of carbon-based sorbents was shown to be limited by gas-phase interactions with SO2 and NO2.
Oxidation Agents/Catalysts for Mercury Speciation and Control
Tests with selected coal ashes revealed that some of the mercury in a simulated flue gas stream could be removed by contact with the ash, possibly as a result of Hg0 oxidation by ash constituents and the reaction of the oxidized Hg with electron-rich species in the ash. Vanadate, a known oxidizing agent, has an oxidation potential sufficiently high to oxidize Hg0. Several lignite-based activated carbons (LACs) provided essentially 100 percent conversion of Hg0 to Hg2+ initially followed by complete capture at temperatures between 225° and 325°F (107° and 163°C). The captured oxidized mercury, however, was not retained for extended periods of exposure to simulated flue gas. Results of a full-factorial matrix of experiments varying flue gas species determined that the oxidized mercury was desorbed whenever a combination of SO2 and NO2 was present at concentrations that always will occur in a coal-fired boiler.
Activated Carbon (AC) Mercury Capture and Oxidation Mechanisms for Model Development
Several approaches have been tested to improve sorbent performance and to better understand the complex physical and chemical process by which sorbents absorb, oxidize, and retain mercury. Impregnation with various acids and ligands failed to improve the capacity of carbon sorbents. Similarly, impregnation with iron(III) and iron(IV) compounds showed no improvement. On the other hand, treatment with halogens showed significant improvement. Parametric testing has led to a two-step model that begins to explain mercury oxidation and capture. Bench-scale tests have been performed with AC in various combinations of flue gas components containing Hg0 and HgCl2.
Effects of Flue Gas Constituents on Mercury Capture
Tests were run to complete the full-factorial matrix of NOx, NO, H2O, SO2, and HCl impacts on mercury capture using HgCl2 injection. Carbon-based sorbents perform significantly differently depending on the concentration of acid gases, particularly HCl. Sorbent reactivity is delayed under low-acid gas conditions, indicating a sorbent-conditioning period. When HCl is not present in the flue gas, breakthrough of Hg in the fixed bed occurs sooner. After breakthrough, Cl disappears from the carbon along with mercury, indicating the release of a mercury chloride compound. NO2, SO2, HCl, and water all play important roles in forming different mercury species; CO2 appears to have no significant role. Under low-HCl conditions, the AC made at the Energy & Environmental Research Center (EERC) captured mercury for a longer period of time without breakthrough than did the Norit LAC. The absence of water also improved sorption capacity. As noted previously, the interaction of SO2 and NO2 results in the desorption of oxidized mercury captured on the sorbent. The mineral matter in most activated carbons does not appear to affect mercury capture.
Assessment of Various Catalysts to Oxidize Mercury in Flue Gas
Sorbents other than carbon also were tested. Testing of gold and vanadium/titanium catalysts for oxidation of mercury gave mixed results. Tests with gold samples showed minimal oxidation and a low mass balance until several days of capturing mercury to the saturation point, after which levels progressed from 80 to 100 percent oxidation quickly. Long-term catalyst tests in coal combustion flue gas showed that gold with the appropriate substrate and contact geometry initially captured then oxidized nearly 100 percent of the elemental mercury. A vanadium titanium catalyst was less effective, achieving 70 percent oxidation.
Bench-Scale Protocol Development
Advanced test protocols for testing mercury sorbents were developed, including evaluating several new approaches for impregnating carbon-based sorbents for enhanced mercury capture, evaluating catalysts for oxidizing Hg0 to improve capture in a wet scrubber, and evaluating bioleaching as a pretreatment approach for removing mercury from coal. Most of the bench-scale tests performed to evaluate sorbents have been run in the fixed-bed system, where breakthrough curves provide a comparison of relative capacity for different candidate sorbents. These tests are not useful for evaluating kinetic effects. The new entrained flow reactor (EFR) has been used to evaluate the effect of residence time on the level of mercury oxidation during coal combustion testing in the EERC pilot-scale combustion test facility. Results show that both lower temperature and longer residence time lead to a higher fraction of oxidized mercury. EFR tests have also demonstrated the effect of residence time and carbon injection rate on inflight capture of mercury.
Methods to Optimize Mercury Capture in Flue Gas Desulfurization Systems
Initial tests in the EERC conversion and environmental process simulator (CEPS) wet flue gas desulfurization (WFGD) unit resulted in HgCl2 being captured on the surface of the sampling tubing and the scrubber Plexiglas™ surfaces. Significant mass balancing errors resulted. The focus of the work shifted to oxidation of elemental mercury upstream of the WFGD. Both heterogeneous and homogeneous reactions were investigated and a mercury reaction pathway hypothesized.
An Assessment of Acid Wash and Bioleaching Pretreating Options to Remove Mercury from Coal
Chemical and biological pretreatment methods were investigated for removing mercury from a low-sulfur North Dakota lignite and a high-sulfur Pennsylvania bituminous coal. Variations on a two-step acid-washing procedure, however, removed between 60 percent and 90 percent of the mercury in lignite. Higher removals were obtained by using concentrated rather than dilute HCl in the second step and by increasing the temperature from 25° to 80°C. For lignite, a separate first-step treatment increased removals in the second step, possibly by swelling the coal; however, changing temperature, acid strength (including the use of pure water), and washing time in the first step had no statistically significant effect. For bituminous coal, a single washing step in concentrated acid removed 65 percent of the mercury, but prior washing in water had no positive effect. Attendant removals of sulfur ranged from 15 to 90 percent for lignite and 20 to 30 percent for the bituminous coal. The removals observed for mercury and sulfur, respectively, were not correlated, suggesting that these elements did not occur in similar forms. Because HgS is not soluble in hydrochloric acid, removals by acid washing suggest that a significant fraction of the mercury may have occurred in an organically bound form. Biotreatments were not effective. The mercury removal results of the bioleaching experiments presented show that, although a 7 to 8 percent average increase in mercury removal is shown for the bioleached coal samples compared to the control samples, this difference is not statistically significant.
Hydrothermal Pretreatment for Sulfur and Mercury Control
Hydrothermal pretreatment of coal in hot water at temperatures and pressures below supercritical conditions was investigated as a means of removing both sulfur and hazardous air pollutant precursors from coal before it is burned. No commercial coal-cleaning method has been shown yet to be effective for economically removing both organic sulfur and toxic trace metals from coal. The hydrothermal coal-cleaning method was investigated with the financial support of the Illinois Clean Coal Institute, Department of Energy, and Center for Air Toxic Metals. The process involves heating a coal-water slurry under pressure to a temperature approaching 370°C to extract sulfur and mineral constituents from the coal. The process successfully reduced the sulfur content of an Illinois bituminous coal from more than 3 percent to below 0.8 percent sulfur, which demonstrated that the extraction was effective in removing inorganic sulfur. The extraction breaks down sulfur-bearing minerals and leaves other mineral forms such as iron oxide and iron silicates in their place. For two coals tested, mercury was reduced by over 95 percent, arsenic by 85 percent, and selenium by 60 percent. The trace elements that were not removed appeared to concentrate in the altered minerals present in the product. Hydrothermal treatment was shown to be technically promising for removing both sulfur and toxic trace metals from coal. Preliminary economic analysis indicated that the method may be cost-effective for high-sulfur coals if it can meet the SO2 control requirement. The break-even cost of hydrothermal treatment was approximately $17/ton if a price of $150/ton of SO2 removed is assumed.
Development of an Advanced Technology for Mercury Capture
The sorbent regeneration process still is in the developmental stages but already has shown the ability to regenerate a spent sorbent and achieve effective and, in some cases, more effective results in removing mercury from flue gas.
The Effect of Ash Electrical Properties on Mercury Capture
Electron spin resonance analysis was performed to determine ash resistivity of three chemically similar ash samples collected from flue gas and having different concentrations of mercury. This method involved an indirect measurement of ash radicals by reacting an organic reagent with radicals, quenching the radicals in the ash while leaving an organic radical that gives an interpretable signal and, therefore, is useful in determining radical content of the ash.
Characterization of Coal-Derived Mercury Sorbents
Two ACs, high-calcium Norit LAC and high-sodium carbon, made at the EERC, were evaluated to determine their ability to sorb mercury and to determine the surface chemistry of the reacted sorbent particles after exposure to flue gas components.
Economics of Mercury Control Alternatives
The economics of several mercury control alternatives were evaluated to assist utilities in identifying least-cost technologies and to provide benchmarks for economic improvement. To date, coal cleaning, wet scrubbing, sorbent injection, and buying credits have been evaluated, either individually or in combination. A range of cost was determined for these options in relation to utility parameters, including boiler size, capacity factor, coal type, the level of SO2 control required, and the cost of SO2 control represented by the value of SO2 credits. Options were compared on the basis of achieving both a 1.2 lb/MMBtu SO2 emission level and 90 percent control of mercury. Some of the higher-priced options, such as wet scrubbing or hydrothermal pretreatment of coal, would be cost-effective if they could meet fully both the sulfur and mercury control requirements by themselves. If more than one control technology is required, lower-cost options that do not meet fully either requirement alone, such as conventional coal cleaning combined with purchase of SO2 credits, became more cost-effective. The evaluation concluded that no current technology can reliably achieve 90 percent control of mercury emissions as a stand-alone control option that can be applied broadly.
Supplemental Keywords:air, air quality, analysis, control, emissions, environment, hazardous, measurement, mercury, metals, modeling, pollutants, pollution, sampling, species, toxic, transformations,
,
Air, Scientific Discipline, Waste, RFA, Engineering, Chemistry, & Physics, Air Quality, Chemical Engineering, Analytical Chemistry, Incineration/Combustion, air toxics, Environmental Engineering, Environmental Chemistry, combustion contaminants, emissions contol engineering, combustion waste recovery, ambient air quality, ambient emissions, atmospheric models, mercury abatement technology, combustion technology, combustion control, emission control strategies, hazardous air pollutants, aerosol particles, trace metal emissions, chemical kinetics of incineration, air pollutants, air quality models, emission control technologies, mercury sorbents, atmospheric chemistry, mercury, metals, ambient metal species, air pollution control, mercury absorbtion, metal vapor emissions, wet scrubber system
Relevant Websites:
http://www.undeerc.org 
http://www.undeerc.org/catm/index.html
Progress and Final Reports:
2003 Progress Report
Original Abstract
Main Center Abstract and Reports:
R827649 Center for Air Toxic Metals® (CATM®)
Subprojects under this Center:
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R827649C001 Development And Demonstration Of Trace Metals Database
R827649C002 Nickel Speciation Of Residual Oil Ash
R827649C003 Atmospheric Deposition: Air Toxics At Lake Superior
R827649C004 Novel Approaches For Prevention And Control For Trace Metals
R827649C005 Wet Scrubber System
R827649C006 Technology Commercialization And Education
R827649C007 Development Of Speciation And Sampling Tools For Mercury In Flue Gas
R827649C008 Process Impacts On Trace Element Speciation
R827649C009 Mercury Transformations in Coal Combustion Flue Gas
R827649C010 Nickel, Chromium, and Arsenic Speciation of Ambient Particulate Matter in the Vicinity of an Oil-Fired Utility Boiler
R827649C011 Transition Metal Speciation of Fossil Fuel Combustion Flue Gases
R827649C012 Fundamental Study of the Impact of SCR on Mercury Speciation
R827649C013 Development of Mercury Sampling and Analytical Techniques
R827649C014 Longer-Term Testing of Continuous Mercury Monitors
R827649C015 Long-Term Mercury Monitoring at North Dakota Power Plants
R827649C016 Development of a Laser Absorption Continuous Mercury Monitor
R827649C017 Development of Mercury Control Technologies
R827649C018 Developing SCR Technology Options for Mercury Oxidation in Western Fuels
R827649C019 Modeling Mercury Speciation in Coal Combustion Systems
R827649C020 Stability of Mercury in Coal Combustion By-Products and Sorbents
R827649C021 Mercury in Alternative Fuels
R827649C022 Studies of Mercury Metabolism and Selenium Physiology