Grant E. Dunham

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.

Heterogeneous oxidation of mercury in simulated post combustion conditions

Heterogeneous mercury oxidation was studied by exposing whole fly ash samples and magnetic, nonmagnetic, and size-classified fly ash fractions to elemental mercury vapor in simulated flue gas streams. Fly ash from sub-bituminous Wyodak–Anderson PRB coal and bituminous Blacksville coal were used. Scanning electron microscopy, X-ray diffraction, thermogravimetric analyses, and BET N2 isothermal sorption analyses were performed to characterize the fly ash samples. Mercury speciation downstream from the ash was determined using the Ontario Hydro method. Results showed that the presence of fly ash was critical for mercury oxidation, and the surface area of the ash appears to be an important parameter. However, for a given fly ash, there were generally no major differences in catalytic oxidation potential between different fly ash fractions. This includes fractions enriched in unburned carbon and iron oxides. The presence of NO2, HCl, and SO2 resulted in greater levels of mercury oxidation, while NO inhibited mercury oxidation. The gas matrix affected mercury oxidation more than the fly ash composition.

Flue gas effects on a carbon-based mercury sorbent

Coal is now the primary source of anthropogenic mercury emissions in the United States, accounting for 46%, or 72 tons/year, of the total U.S. Environmental Protection Agency (EPA) estimated 158 tons/year [U.S. Environmental Protection Agency, Mercury Study Report to Congress, EPA/600/P-94/002Aa, External Review Draft, Jan. 1995.]. Development of cost-effective mercury control for coal-fired boilers is a primary research need identified in the EPA Mercury Study Report to Congress [U.S. Environmental Protection Agency, Mercury Study Report to Congress, EPA/600/P-94/002Aa, External Review Draft, Jan. 1995.]. During combustion of mercury-containing fuels such as coal, the mercury is completely volatilized and is not controlled by conventional particulate control devices unless the solid material effectively traps the mercury through sorption mechanisms. Typically, this does not occur naturally to a significant degree by the collected ash material. However, a promising approach for mercury control is the injection of an effective sorbent upstream of the particulate control device. Since the amount of mercury in the gas stream from coal combustion is usually in the range of 5 to 10 μg/m3 (about 1 ppbv), only very small amounts of a sorbent may be necessary. A requirement is that the mercury be tightly bound in the sorbent, not desorbing upon exposure to ambient air or leaching under wet disposal conditions. On a worldwide basis, the projected increase in coal usage over the next two decades in China, India, and Indonesia will dwarf the current U.S. coal consumption of 1 billion tons/year [International Energy Outlook, U.S. Department of Energy, Energy Information Administration, Office of Integrated Analysis and Forecasting, Washington, DC, April 1998, DOE/EIA-0484(98).]. Therefore, in the United States, coal will be the dominant source of mercury emissions, and worldwide, coal may be the cause of significantly increased mercury emissions unless an effective control strategy is implemented. However, there is much uncertainty over the most technically sound and cost-effective approach for reducing mercury emissions from coal-fired boilers.

Effects of Sulfur Dioxide and Nitric Oxide on Mercury Oxidation and Reduction under Homogeneous Conditions

Abstract This paper is particularly related to elemental mercury (Hg0) oxidation and divalent mercury (Hg2+) reduction under simulated flue gas conditions in the presence of nitric oxide (NO) and sulfur dioxide (SO2). As a powerful oxidant and chlorinating reagent, Cl2 has the potential for Hg oxidation. However, the detailed mechanism for the interactions, especially among chlorine (Cl)-containing species, SO2, NO, as well as H2O, remains ambiguous. Research described in this paper therefore focused on the impacts of SO2 and NO on Hg0 oxidation and Hg2+ reduction with the intent of unraveling unrecognized interactions among Cl species, SO2, and NO most importantly in the presence of H2O. The experimental results demonstrated that SO2 and NO had pronounced inhibitory effects on Hg0 oxidation at high temperatures when H2O was also present in the gas blend. Such a demonstration was further confirmed by the reduction of Hg2+ back into its elemental form. Data revealed that SO2 and NO were capable of promoting homogeneous reduction of Hg2+ to Hg0 with H2O being present. However, the above inhibition or promotion disappeared under homogeneous conditions when H2O was removed from the gas blend.

LONG-TERM DEMONSTRATION OF SORBENT ENHANCEMENT ADDITIVE TECHNOLOGY FOR MERCURY CONTROL

Long-term demonstration tests of advanced sorbent enhancement additive (SEA) technologies have been completed at five coal-fired power plants. The targeted removal rate was 90% from baseline conditions at all five stations. The plants included Hawthorn Unit 5, Mill Creek Unit 4, San Miguel Unit 1, Centralia Unit 2, and Hoot Lake Unit 2. The materials tested included powdered activated carbon, treated carbon, scrubber additives, and SEAs. In only one case (San Miguel) was >90% removal not attainable. The reemission of mercury from the scrubber at this facility prevented >90% capture.

EFFECTS OF FLY ASH ON MERCURY OXIDATION DURING POST COMBUSTION CONDITIONS

Tests were performed in simulated flue gas streams using two fly ash samples from the electrostatic precipitators of two full-scale utility boilers. One fly ash was derived from a Powder River Basin (PRB) coal, while the other was derived from Blacksville coal (Pittsburgh No. 8 seam). The tests were performed at temperatures of 120 and 180 C under different gas compositions. Elemental mercury (Hg) streams were injected into the simulated flue gas and passed over filters (housed in a convection oven) loaded with fly ash. The Ontario Hydro method was used to determine the total amount of Hg passing through the filter as well as the percentages of elemental and oxidized Hg collected. Results indicated that substantial amounts of Hg oxidation did not occur with either fly ash, regardless of the temperature used for testing. When oxidation was observed, the magnitude of the oxidation was comparable between the two fly ashes. These results suggest that the gas matrix may be more important than the ash components with respect to the distribution of Hg species observed in gaseous effluents at coal-fired power plants.

Mercury Information Clearinghouse

The Canadian Electricity Association (CEA) identified a need and contracted the Energy & Environmental Research Center (EERC) to create and maintain an information clearinghouse on global research and development activities related to mercury emissions from coal-fired electric utilities. With the support of CEA, the Center for Air Toxic Metals{reg_sign} (CATM{reg_sign}) Affiliates, and the U.S. Department of Energy (DOE), the EERC developed comprehensive quarterly information updates that provide a detailed assessment of developments in the various areas of mercury monitoring, control, policy, and research. A total of eight topical reports were completed and are summarized and updated in this final CEA quarterly report. The original quarterly reports can be viewed at the CEA Web site (www.ceamercuryprogram.ca). In addition to a comprehensive update of previous mercury-related topics, a review of results from the CEA Mercury Program is provided. Members of Canadas coal-fired electricity generation sector (ATCO Power, EPCOR, Manitoba Hydro, New Brunswick Power, Nova Scotia Power Inc., Ontario Power Generation, SaskPower, and TransAlta) and CEA, have compiled an extensive database of information from stack-, coal-, and ash-sampling activities. Data from this effort are also available at the CEA Web site and have provided critical information for establishing and reviewing a mercury standard for Canada that is protective of environment and public health and is cost-effective. Specific goals outlined for the CEA mercury program included the following: (1) Improve emission inventories and develop management options through an intensive 2-year coal-, ash-, and stack-sampling program; (2) Promote effective stack testing through the development of guidance material and the support of on-site training on the Ontario Hydro method for employees, government representatives, and contractors on an as-needed basis; (3) Strengthen laboratory analytical capabilities through analysis and quality assurance programs; and (4) Create and maintain an information clearinghouse to ensure that all parties can keep informed on global mercury research and development activities.

MERCURY CONTROL WITH THE ADVANCED HYBRID PARTICULATE COLLECTOR

This project was awarded under U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) Program Solicitation DE-PS26-00NT40769 and specifically addresses Technical Topical Area 4-Testing Novel and Less Mature Control Technologies on Actual Flue Gas at the Pilot Scale. The project team includes the Energy & Environmental Research Center (EERC) as the main contractor; W.L. Gore & Associates, Inc., as a technical and financial partner; and the Big Stone Plant operated by Otter Tail Power Company, host for the field-testing portion of the research. Since 1995, DOE has supported development of a new concept in particulate control called the advanced hybrid particulate collector (AHPC). The AHPC has been licensed to W.L. Gore & Associates, Inc., and is now marketed as the Advanced Hybrid{trademark} filter by Gore. The AHPC combines the best features of electrostatic precipitators (ESPs) and baghouses in a unique configuration, providing major synergism between the two collection methods, both in the particulate collection step and in the transfer of dust to the hopper. The AHPC provides ultrahigh collection efficiency, overcoming the problem of excessive fine-particle emissions with conventional ESPs, and it solves the problem of reentrainment and re-collection of dust in conventional baghouses. The AHPC appears to have unique advantages for mercury control over baghouses or ESPs as an excellent gas-solid contactor. The objective of the original 5-task project is to demonstrate 90% total mercury control in the AHPC at a lower cost than current mercury control estimates. The approach includes bench-scale batch testing that ties the new work to previous results and links results with larger-scale pilot testing with real flue gas on a coal-fired combustion system, pilot-scale testing on a coal-fired combustion system with both a pulse-jet baghouse and an AHPC to prove or disprove the research hypotheses, and field demonstration pilot-scale testing at a utility power plant to prove scale-up and demonstrate longer-term mercury control. This project, if successful, will demonstrate at the pilot-scale level a technology that would provide a cost-effective technique to accomplish control of mercury emissions and, at the same time, greatly enhance fine particulate collection efficiency. The technology can be used to retrofit systems currently employing inefficient ESP technology as well as for new construction, thereby providing a solution to a large segment of the U.S. utility industry as well as other industries requiring mercury control. The scope of work was modified to include an additional sixth task, initiated in April 2003. The objective of this task is to evaluate the mercury capture effectiveness of the AHPC when used with elemental mercury oxidation additives, a spray dryer absorber, and novel baghouse sorbent inserts downstream of the fabric filter.

VALUE-ADDED SORBENT DEVELOPMENT

On a worldwide basis, the projected increase in coal usage over the next two decades in China, India, and Indonesia will dwarf the current U.S. coal consumption of 1 billion tons/year. Therefore, in the United States, coal will be the dominant source of mercury emissions, and worldwide, coal may be the cause of significantly increased mercury emissions unless an effective control strategy is implemented. However, there is much uncertainty over the most technically sound and cost-effective approach for reducing mercury emissions from coal-fired boilers. Several approaches are suggested for mercury control from coal-fired boilers, including enhancing the ability of wet scrubbers to retain mercury. However, many coal-fired boilers are not equipped with wet scrubbers. On the other hand, since almost all coal-fired boilers are equipped with either an electrostatic precipitator (ESP) or a baghouse, sorbent injection upstream of either an ESP or baghouse appears attractive, because it has the potential to control both Hg{sup 0} and Hg{sup 2+}, would appear to be easy to retrofit, and would be applicable to both industrial and utility boilers. Since mercury in the gas stream from coal combustion is present in only trace quantities, only very small amounts of sorbent may be necessary. If we assume a mercury concentration of 10 {micro}g/m{sup 3} and a sorbent-to-mercury mass ratio of 1000:1, the required sorbent loading is 10 mg/m{sup 3}, which is only 0.1% to 0.2% of a typical dust loading of 5-10 g/m{sup 3} (2.2-4.4 grains/scf). This amount of additional sorbent material in the ash would appear to be negligible and would not be expected to have an impact on control device performance or ash utilization. Accomplishing effective mercury control with sorbent injection upstream of a particulate control device requires several critical steps: (1) Dispersion of the small sorbent particles and mixing with the flue gas must be adequate to ensure that all of the gas is effectively treated in the short residence time (typically a few seconds) between sorbent injection and particle collection. (2) Assuming the sorbent particles can be injected and dispersed adequately, a second critical step is the mass transfer by diffusion of the mercury from the bulk flue gas to the particle surface within the available residence time. The ideal case would be to achieve sufficient mass transfer in the duct and not depend on additional transfer within the collection device. (3) Once the mercury molecules reach the surface of a sorbent particle, they will not be trapped unless sorption can occur at a rate equal to the rate of mass transfer by diffusion to the particle surface. Analysis by Rostam-Abadi and others concluded that only a very small surface area would theoretically be required to trap the mercury. The implication is that reactive surface sites are much more important than the amount of surface area. (4) Assuming the sorbent has the capacity and reactivity to trap the mercury that reaches the sorbent particles, the final critical step is long-term stability of the sorbed mercury.