Novel Approaches For Prevention And Control For Trace Metals

EPA Grant Number: R827649C004
Subproject: this is subproject number 004 , 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: Novel Approaches For Prevention And Control For Trace Metals
Investigators: Mann, Michael D. , Keuther, Kaleb , Kozliak, Eugene , Pavlish, John H. , Sternberg, Steven , Timpe, Ronald C.
Institution: University of North Dakota
EPA Project Officer: Chung, Serena
Project Period: October 15, 1999 through October 14, 2002
RFA: Center for Air Toxic Metals (CATM) (1998) RFA Text |  Recipients Lists
Research Category: Targeted Research

Objective:

Two novel technologies for prevention and control of HAPs are being explored. The first is a precombustion process that removes trace metals, chlorine, and other HAP precursors from the fuel and/or waste prior to incineration using hydrothermal treatment. Three modes of hydrothermal treatment are being investigated during the program: subcritical water extraction, steam extraction (subcritical pressure and supercritical temperature), and supercritical water extraction. This process has a wide range of applications, including preparation of premium- rade fuels, pollution prevention in waste incinerators, treatment of contaminated soils, and processing spent sorbents. The second innovative technology involves a novel application of microorganisms in a fiber-based trickle bioreactor to remove trace metals and volatile organics from the flue gas stream. The bioreactors are novel, in that they use a polymer fiber support for the microorganisms, which improves the efficiency of removal and reduces the required quantity of cell mass required per given volume of contaminated air. In addition to proving the technical viability of these innovative techniques for preventing and controlling HAPs, a preliminary assessment of the economic viability will be performed and the results compared to commercially available alternatives.

The primary purpose of this project is to demonstrate the technical viability of two novel treatment methods to simultaneously remove air toxic metals and other HAPs. While the focus of the program will be on mercury, other pollutants that should be affected by both of these processes include lead, arsenic, selenium, cadmium, chlorine, fluorine, sulfur, and volatile organics. The general goals of the program are listed briefly below, with specific objectives for each task presented with the scope of work for that task. Goals are as follows:

  • To demonstrate the effectiveness of hydrothermal treatment as a pretreatment method for controlling trace metals and chlorine from medical wastes, municipal solid wastes, and other waste materials while simultaneously producing an upgraded fuel.

  • To investigate the capabilities of bioreactors to remove contaminants (volatile metals and organic compounds) from airstreams using polymer fibers to support particular strains of microorganisms.

  • To compare cost and effectiveness of various metal emission prevention and control technologies that are currently being researched or used commercially.
  • Approach:

    This project comprises three discrete tasks. The first task focuses on the use of hydrothermal treatment as thermal washing of contaminated waste and fossil fuels to remove associated trace elements from the solid material. The second task examines the use of biofilters for the removal of volatile metals and organics from stacks. The third task compares the cost-effectiveness of these and other HAP prevention and control technologies. The specific objective of the Environmental Aspects of Hydrothermal Treatment task is to determine the amount of toxic metals and chlorine that can be removed from a variety of high-volume waste streams using sub- and supercritical water. Bench-scale extractions are being performed using a multigram apparatus capable of thermally treating up to 20 g of solid material at temperatures and pressures both above and below the critical conditions of water. Experiments consist of loading approximately 10 g into the tubing reactor, preheating the furnaces to selected conditions, and flowing water through the fixed bed for a set period of time. Extractions are at 250?C and 800 psi (subcritical), 400?C and 2100 psi (steam conditions), and 400?C and 3300 psi (supercritical conditions) for each waste or solid fuel. Water/steam flows through the solids for approximately 30 min at the specified conditions prior to depressurization and sample collection. From the extractions, the solid residual is analyzed to determine how much mercury, cadmium, lead, and chlorine were extracted from the solid to the aqueous phase. Water and gas samples from those tests showing good mercury removal are also analyzed.

    Metal and VOC (volatile organic compound) Removal from Gas Streams by Novel Filter-Based Trickle-Bed Bioreactors focuses on proof of concept, rather than process optimization, to investigate the capabilities of bioreactors to remove mercury and VOCs from airstreams. As part of this, the quantities of Hg2+ and Hg" that the microorganisms can take up per unit cell mass and the breakthrough time of mercury through the bioreactor filters are being determined for use in preliminary full-scale reactor design. The test sequence begins with incubating the microorganisms and inoculating them on the biofilter. Once the filters have been impregnated with the biomass, the filters are placed in the trickle-bed bioreactor. Three filters in series will be used to facilitate the collection of design information. The filters will be exposed to humidified air spiked with Hg" and HgCl2,, a selected VOC, and HCl. The biomass is not sensitive to the other normal flue gas constituents [9]; therefore, to decrease the cost and complexity of the experiments, these will not be added to the synthetic gas mixture. The concentrations of trace metals and VOCs leaving the reactor will be measured to determine the effectiveness of the treatment method. Each of the biofilters will be digested and analyzed upon completion of each individual test to provide input for a mass balance. The experimental test matrix is designed to allow determination of the total amount of metals that can be adsorbed by the biomass and of the time-dependent removal of metals using the biofilters.

    The goal of the economic analysis is to compare cost and effectiveness of various metal emission prevention and control technologies that are currently being researched or used commercially. Mercury control methods are the focus of the study. The approach has been to build models that are based on the major equipment costs. Fixed percentages of the equipment costs are used to calculate the other components of capital cost. Operating costs include raw materials and additional labor that are required due to the control technology. The review is utilizing available information in the literature, including process layouts, capital and operating costs, and level of control. Vendors and users are being contacted to get a generic technology description, costs, and levels of control. The cost for the two technologies being investigated in Tasks 1 and 2 and other EERC-developed technologies will be estimated by comparing them to existing technologies where data exist and/or by pricing major components to determine critical parameters. Sensitivity analyses are being performed to determine the impact of varying critical process parameters. The technologies being reviewed include conventional and advanced physical cleaning, hydrothermal treatment, activated carbon injection, fixed-bed adsorbers, flue gas condensers, and wet and dry scrubbing. A technology summary will be prepared for each control method evaluated, and the technologies will be compared in clear, concise tables and graphs.

    Progress

    The use of hydrothermal treatment for removal of mercury and other HAPS has been the focus of one of the major tasks under the CATM Year 5 program. To date, small batch experiments have been carried out on two sewage sludges (high and low levels of chlorine), RDF, petroleum coke, and raw crude oil. Additional work has been performed on two Illinois Basin coals under the CATM JSRP with ICCI. In addition, oxidizing agents, including acetic acid, formic acid, and hydrogen peroxide, have been used for several tests.

    Available results show that almost complete removal of both chlorine (>99.9%) and mercury (89.6%-99.4%) can be accomplished with the sewage sludges. Results from the hydrothermal treatment of the high-chlorine sewage sludge are shown in the following table. Essentially all of the chlorine is removed for all processing conditions tested. Under subcritical conditions, most of the chlorine is transferred to the aqueous phase, while at supercritical conditions, almost half of the chlorine is either released in the gas phase or has reacted with the metal walls of the reactor. Additional work needs to be done to close the chlorine balance and to determine the extent of any corrosion that may occur at the supercritical conditions.

    To accomplish high levels of mercury removal from the sewage sludge, supercritical temperature but subcritical pressure (steam) is required. In all three cases, a significant fraction of the mercury is released in the gas phase. A zeolite or carbon filter would be used in an integrated system to concentrate the mercury and remove it from the gas phase. Although no leaching tests have been performed, it is anticipated that this material would be suitable for disposal.

    For the RDF case, the mercury appears to be bound rather tightly to the inorganic fraction and remains with the solid residue. Lead also follows this pattern. With the RDF, especially at the supercritical conditions, the organic fraction can be separated from the residual solids and could potentially be used as a clean-burning liquid fuel. It is speculated that the chlorine will remain in the water phase; therefore, the dioxin&ran issue will be resolved in addition to the trace metals through hydrothermal treatment. For the RDF, since the mercury and other HAPS appear to be stable in the solid fraction even under the most stringent treatment conditions, the hydrothermal treatment of RDF may generate a solid material that can be safely disposed.

    Extractions were performed with petroleum coke and crude oils. The premise behind the choice of these materials was that the hydrothermal processing conditions could potentially open the porphyrin groups that are the typical sites for vanadium, nickel, and other metals such as mercury. If these metals could be released from the porphyrins, then they could be extracted from the organic residue. Preliminary results, however, showed that hydrothermal processing, even at supercritical conditions, was not sufficient to remove any of the metals from the petroleum coke. These samples were further treated with H2O2, acetic acid, and formic acid, in the hope that these oxidizing agents would attack the sites binding the metals and provide a mechanism to release them from the organic matrix. No significant improvement was noted in metal reduction. The tests with the crude oils were equally unsuccessful in removal of nickel and vanadium from the organic matrix.

    Work performed under a jointly sponsored project by CATM, DOE, and ICC1 showed that both sulfur and mercury can be reduced from bituminous coals. Compliant coal was produced on the bench scale from a 3.5% sulfur coal. In addition, the following reductions in HAPs were noted: 99% mercury, 85% arsenic, 29% selenium, and 53% chlorine. The upgraded product had a heating value of 14,475 Btu/lb. Solids recovery for the process was only 68%; however, the energy recovery was significantly higher at 93%.

    Work on mercury removal through trickle-bed biofiltration focused primarily on reactor design and establishing a viable growth of microorganisms. The most difficult task, the design and construction of a functioning bench-scale trickle bed bioreactor, has been completed. The prototype is currently undergoing shakedown testing with few complications encountered. Construction of the remaining four bioreactors is under way based on the successful design and operation of the prototype. The first prototype reactor will be ready for mercury remediation trials in early February.

    The focus of the economic evaluation has been primarily on the coal cleaning. The cost- effectiveness of conventional coal cleaning, froth floatation, selective agglomeration, and hydrothermal treatment have been preliminarily compared. The primary cost of coal cleaning incurred by the utility company is an incremental increase in the price of fuel. This can range from $2 to over $30 per ton of coal. Coal cleaning, however, also offers a number of benefits to the utility in the form of lower SO2 emissions, lower operating and maintenance costs resulting from reduced ash content, and lower waste disposal costs. In analyzing the cost of mercury reduction using coal-cleaning methods, credits were given to offset these benefits. Based on the conservative estimates of $100/ton SO2 reduced and $20/ton for waste disposal, the incremental cost of using cleaned coal for mercury control was estimated, with results shown in the figure above. The mercury removal costs are shown as a function over the typical range of coal-cleaning costs for each of the technologies considered. The calculations are based on mercury removals of 21% for conventional cleaning, 55% for froth floatation, 68% for selective agglomeration, and 90% for hydrothermal treatment. The results show that conventional coal cleaning is very cost-effective, based on a $/lb mercury removed. However, since conventional cleaning is only effective at removing low levels of mercury, it probably is not a viable method for mercury control. Of the advanced coal-cleaning methods, hydrothermal treatment shows the most promise, both in terms of achievable mercury levels and in cost. Mercury removal levels of 90% and greater have been demonstrated with Illinois Basin coal, with the incremental cost of mercury removal ranging from $40,000 to $84,00O/lb Hg removed. This is within the range of costs of $4920 to $70,000/1b Hg removed presented by EPA in its recent report to Congress as the projected cost of mercury control in utility applications. Future work at the EERC will focus on optimizing the hydrothermal treatment process and developing more accurate cost estimates.

    Rationale:

    Environmentally sound disposal of municipal and industrial wastes has become a major issue in the past decade [1,2]. The volume of these wastes, including refuse, medical wastes, hazardous wastes, and sewage sludge has continued to grow annually, and traditional disposal methods (landfilling or ocean dumping) are becoming less acceptable because of cost and environmental concerns. Incineration in modern high-efficiency combustors is being employed for a growing fraction of waste streams to achieve significant reduction in refuse volume, but has been coupled with an increase in the complexity and efficiency of air pollution controls to limit incinerator emissions. Emission control for these systems, however, is more challenging than for a coal-fired plant because of HCl, HF, dioxin, furans, and much higher concentrations of the heavy metals (mercury, lead, cadmium, arsenic, chromium, nickel, and zinc) emissions from typical waste burning plants [3]. Consequently, precombustion technologies have an even greater potential for pollution control for waste-fired systems than for coal-fired systems.

    The use of hydrothermal treatment has shown success at extracting sulfur and associated metals. Based on preliminary tests at the EERC, organically associated sulfur and metals have been extracted from coal using subcritical water; thus conditions at or above supercritical pressure may not be necessary. Work currently being conducted at the EERC under support from ICC1 has confirmed that in excess of 80% of total sulfur and up to 99% of the mercury can be removed using water at 400?C and 2300 psig, much below the critical pressure of 3200 psig [4]. Other researchers have reported the beneficial extraction properties of subcritical water or hydrothermal treatment for organics and/or metals.

    Traditional methods for air purification (adsorption, incineration, and catalytic combustion) have inherent disadvantages. Adsorption requires periodic regeneration of the adsorbent. Incineration and catalytic combustion require high temperatures (150?-500?C, depending on the catalyst and pollutants), and thermal methods do not remove metals [5]. Biological methods of air purification, based upon the ability of certain bacteria to degrade toxic organic compounds to produce carbon dioxide and water, may be considered an alternative to the traditional methods [6]. The ability of many microorganisms to adsorb metals onto their cell surface has also been observed [7]. These metals become a permanent part of the cell structure and are, therefore, not desorbed to rerelease the metal pollutant. The metals accumulate in the cell biomass and require periodic harvesting of the biomass for removal [7]. The removed metal-laden biomass can be treated for recovery of the metal. Preliminary experiments using polymer fibers as supports for bacterial cell immobilization in the trickle-bed bioreactor show removal efficiencies ranging from 70% to 100% for various pollutants(toluene, ethyl acetate, undecane, styrene, ethanol, etc.). The substrate concentrations varied from 10 to 300 mg/m3 and the space velocity of air from 2500 to 20,000 hr-1. In other experiments, microorganisms were shown to accumulate metals in cell mass at concentrations 1000-fold above the water concentration. Total amounts of metal accounted for l%-2% of the dry cell mass [7,8]. It is suspected that similar results will be possible in contaminated airstreams.

    An estimation of the cost of these and other control technologies are needed as control strategies are developed. The EPA in its recent Mercury Study Report to Congress presented cost estimates for a variety of technologies. However, a more detailed economic analyses, with special emphasis on cost-sensitivity analysis, will help determine critical parameters to be evaluated in full-scale test burns, help direct research activities, allow the benefits of co-control to be evaluated, and allow potential cost reductions resulting from technology improvements to be evaluated.

    Supplemental Keywords:

    RFA, Scientific Discipline, Air, Toxics, Waste, Sustainable Industry/Business, National Recommended Water Quality, Chemical Engineering, air toxics, cleaner production/pollution prevention, Remediation, Environmental Chemistry, HAPS, Chemistry and Materials Science, Incineration/Combustion, Engineering, EPCRA, 33/50, Engineering, Chemistry, & Physics, Environmental Engineering, mercury , supercritical water extraction, air pollutants, cadmium & cadmium compounds, Chlorine, mercury, Arsenic Compounds (inorganic including arsine), hazardous air pollutants, lead, lead & lead compounds, air pollution control, pollution control technologies, steam extraction, emission controls, sulfur, toxic metals, Fluorine, Selenium, waste incineration, mercury & mercury compounds, sub critical water extraction, tannery waste, pollution prevention, trace metals, arsenic, Cadmium Compounds

    Progress and Final Reports:

  • 2000
  • 2001
  • Final

  • 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