FINAL TECHNICAL REPORT

December 1, 1997, through November 30, 1998

Project Title: CORRELATE COAL/SCRUBBER PARAMETERS WITH Hg REMOVAL AND Hg SPECIES IN FLUE GAS

ICCI Project Number: 97-1/5.1A-8(2)

Principal Investigators: Matthew S. DeVito, William A. Rosenhoover, CONSOL Inc., Research & Development

Other Investigators: Subhash B. Bhagwat, Illinois State Geological Survey

Project Manager: Ron Carty, ICCI

ABSTRACT

The U.S. EPA is collecting information on mercury (Hg) emissions from coal-fired utilities to determine if Hg control for this source category is warranted. Presently, there is no proven Hg removal process for coal-fired utility boilers. The demonstration of multi-pollutant control by commercial flue gas desulfurization (FGD scrubbers) systems could provide utilities with an attractive control option that would help keep high-sulfur Illinois coal competitive with compliance coal. The objective of this study was to document the Hg reduction achieved by commercial FGD systems installed and operated by Illinois utilities firing Illinois coal. The simultaneous removal of HCl and HF also was determined.

Phase I (9/1/96 - 8/31/97) sampling results from two Illinois-based FGD-equipped coal-fired utility boilers showed 48% to 58% Hg removal across the scrubber and 56% to 72% total plant removal. Phase II (12/1/97 - 11/30/98) sampling from two additional Illinois-based FGD-equipped systems showed 46-57% Hg removal across the scrubber and 67-75% total plant removal. The average Hg removal across the FGD for the four units tested was 52%. The total system removal averaged 68%. The Hg removal due to adsorption on the ESP/bottom ash ranged from 7% to 13% for three of the plants. The remaining plant showed an almost 50% Hg reduction, which was a result of very high carbon in the ash and a lower operating ESP temperature. The material balance closures were within the data quality objectives and validate the gas phase Hg measurements. The data obtained in these studies will provide the Illinois coal and utility industries and emissions-regulating authorities valuable information on the effectiveness of wet FGD scrubbers for mercury removal.

A statistical analysis of the process parameters influencing Hg removal was conducted by the Illinois State Geological Survey (ISGS). Hg removal showed a strong correlation with scrubber slurry pH, with higher pH resulting in increased removal. A suspected correlation between Hg removal and the chlorine concentration of the coal was not apparent.

The average HCl removals for the Phase II test sites were 97% and 99%. The HF removals for these sites were >95%. This program showed that wet FGDs are effective for multi-pollutant control and can provide utilities with a valuable control option as air regulations become increasingly more stringent.

EXECUTIVE SUMMARY

The Environmental Protection Agency (EPA), the U.S. Department of Energy (DOE), the Electric Power Research Institute (EPRI), and their contractors have studied the partitioning of Hazardous Air Pollutants (HAPs) at coal-fired utility plants. The majority of the HAPs were captured with the fly ash in existing particulate collection devices (ESPs, fabric filters). The exceptions are the elements or compounds denoted as Class III (Hg, Se, HF, HCl) which, because of their volatility, are vapors at the exit of conventional particulate control equipment. Potential HAP control regulations could have an adverse effect on the Illinois coal and utility industries.

In a series of EPA Reports to Congress, EPA has identified potential health concerns related to Hg emissions from coal fired utilities. EPA has identified this source category as the largest national single source contributor of anthropogenic Hg emissions (~50 tpy for coal-fired utilities versus 150 total tpy from all anthropogenic sources). EPA is in the process of collecting additional Hg-related information from these sources to determine if Hg control is warranted. Currently, there is no demonstrated Hg control technology for coal-fired utilities. Carbon injection has been studied at the pilot scale with some limited success. However, the control costs for this technology have been estimated as high as $40,000 to $50,000 per lb of Hg removed.

Earlier studies at coal-fired utilities have shown that wet scrubbers can remove mercury. However, these studies show a wide variation in removal (0 to 100%), indicating the need for additional data to define the cause of this variation. The goal of this program was to develop more precise mercury removal data and other volatile HAPs, hydrofluoric acid (HF), and hydrochloric acid (HCl), by wet FGD systems. These data will supply the Illinois utility industry with information to evaluate SO₂ scrubbers as volatile HAP control devices. The data will be used to develop a model to predict HAP removal by SO₂ scrubbers.

In this program, CONSOL R&D and the Illinois State Geological Survey:

1. Determined the mercury and acid gas removal at four scrubber-equipped boilers firing Illinois coal (two in Phase I and two more in Phase II).

2. Completed correlation analysis of mercury removal with coal properties and/or scrubber operating parameters,

3. Evaluated the stability of the Hg contained in the FGD by-product solids.

The project team contacted the owner/operators of all of the scrubber-equipped boilers operating in Illinois. Four of the owners agreed to participate in the program. Two units were tested in Phase I (9/1/96 - 8/31/97) and the results of this program were previously reported. Two additional units were evaluated in June and July 1998. The results of this test work are documented in this report.

Phase I Results - Sites 1 and 2 (1996 testing previously reported)

The first sampling program was completed during the week of October 21, 1996, and the second sampling program was completed the week of October 28, 1996. Testing at both plants consisted of triplicate flue gas Hg measurements across the FGD scrubber. Process stream samples (i.e., feed coal, fly ash, bottom ash, limestone, FGD sludge/product, etc.) were obtained to conduct a mercury material balance to verify data quality.

The average Hg emissions for the two sites were 2.5 and 2.9 µg/m³ (0.005 and 0.004 lb/hr or ~30 lb/year). The Hg removals across the ESP and FGD system averaged 56% and 72% The average Hg removals across the FGDs were 48% and 58%. The Hg material balance closures for four tests conducted at the first plant ranged between 75% and 104%, and averaged 92%. The Hg material balance closures for three tests conducted at the second plant ranged from 108% to 125% and averaged 116%. Of the total Hg in the coal, 7% to 14% reported to the ESP ash and 35% to 70% to the FGD solids. No measurable Hg was found in the ash samples obtained upstream of the air preheater (bottom ash and economizer ash), or in the FGD slurry filtrate water.

The flue gas Hg speciation data indicate that the Hg fraction collected in the KCl impingers of the Ontario Hydro sampling train at the scrubber inlet is the species that is removed across the FGD system. In this method, the KCl impingers collect the oxidized fraction of the flue gas Hg (Hg⁺⁺). The inlet Hg speciation estimated from the KCl sampling method showed between 80% and 90% oxidized Hg at the ESP inlet. Outlet speciation data showed measurable particulate Hg emissions that are thought to be an artifact formation from a gas phase reaction of Hg with the filter media used in the sampling train.

The Illinois State Geological Survey conducted a literature search to identify available flue gas Hg sampling data from scrubbed units. It became evident, based on this literature search, that little reliable data existed. Statistical analyses performed on data obtained at the EPRI High Sulfur Test Center (HSTC) showed no statistical significance at the 95% level for Hg removal with any of the observed process parameters. The information collected in this study and other CONSOL in-house test data from an eastern coal-fired, scrubber-equipped utility were used to assist in this activity. A statistical analysis of these data showed no strong correlation between scrubber parameters and Hg removal. Some minor coal quality parameters appeared to correlate with Hg removal across the ESP.

The average HCl emissions at the two sites were 66 lb/hr and 26 lb/hr (~200 and ~80 tons per year). The average HCl removal across the FGD scrubber at both sites was ~82%. The HF emissions at the first site were below the detection limit of 2 lb/hr. The HF emissions at the second site averaged 6 lb/hr (~20 tons per year) with an average FGD scrubber removal of 79%. However, due to the sampling method used, we believe that the reported acid gas removal rates are biased low.

Phase II Results - Sites 3 and 4 (New Data)

The third sampling program was completed during the week of June 22, 1998, and the fourth sampling program was completed the week of July 14, 1998. Testing at the third plant consisted of four replicate flue gas Hg measurements at the ESP Inlet, the FGD inlet and the FGD outlet. Testing at the fourth plant consisted of triplicate flue gas Hg measurements at the ESP inlet and the FGD outlet. Process stream samples (i.e., feed coal, fly ash, bottom ash, limestone, FGD sludge/product, etc.) were obtained at both plants to conduct a mercury material balance to verify data quality.

The average Hg emissions for the two Phase II sites were 1.9 and 2.8 µg/m³ (0.0006 and 0.007 lb/hr). The total Hg removal across the entire system (ESP and FGD) averaged 75% and 67%. The average Hg removals across the FGDs were 46% and 57%. The Hg material balance closures for four tests conducted at the third plant ranged between 79% and 118%, and averaged 97%. The Hg material balance closures for three tests conducted at the fourth plant ranged from 99% to 108%, and averaged 104%. Of the total Hg in the coal, for Plant 3, 35% reported with the ESP ash and only 5% was associated with the FGD solids. However 13% of the Hg reported with the bottom ash, and another 17% was found in the pre-scrubber blowdown water. For Plant 4, 8% of the Hg reported with the ESP ash and 58% reported with the FGD solids.

As in the Phase I results, the flue gas Hg speciation data indicate that the Hg fraction collected in the KCl impingers of the Ontario Hydro sampling train at the scrubber inlet is the species that is removed across the FGD system. If it is assumed that the Hg adsorbed on the solid samples (ESP and bottom ash) and in the KCl impingers is in the oxidized form, then ~90% of the oxidized fraction is being removed by a combination of solid adsorption and removal across the FGD.

The information collected in this study and another CONSOL in-house test data from an eastern coal-fired, scrubber-equipped utility was added to the existing ISGS database for additional correlation analysis. A statistical analysis of these data showed a strong correlation between scrubber slurry pH and Hg removal where higher pH showed higher Hg removal.

The average HCl removal across the FGD scrubber at the third and fourth site were 99% and 97% respectively. The HF removals for these plants were >95% and >96% respectively.

The results of both the Phase I and II testing demonstrated that wet FGD systems installed and operated for SO₂ control are capable of removing 50-60% of the flue gas Hg. The total Hg removal ranged from 56% to 75% for the ESP-FGD combination. This information will provide utilities with a low-cost compliance option if EPA proposes a Hg emission reduction program.