Correlate Fly Ash Capture of Hg With Ash Carbon Content and Flue Gas Temperature

FINAL TECHNICAL REPORT

November 1, 1998, through October 31, 1999

Project Title: CORRELATE FLY ASH CAPTURE OF Hg WITH ASH CARBON CONTENT AND FLUE GAS TEMPERATURE

ICCI Project Number: 98-1/1.2B-2

Principal Investigator: William A. Rosenhoover, CONSOL Inc., Research & Development

Project Manager: Ronald Carty, ICCI

ABSTRACT

In reports to Congress, the U.S. Environmental Protection Agency (EPA) has identified potential adverse health effects of mercury (Hg) emissions from coal-fired utilities. It has been determined that approximately one-third (50 tpy) of all U.S. anthropogenic Hg emissions (150-160 tpy) come from coal-fired utilities. Consequently, EPA is considering utility Hg emission regulations, which could have a costly effect on the Illinois coal and utility industries.

Currently, there is no universally accepted Hg control technology for coal-fired utilities. Studies have shown that fly ash can remove Hg, with the extent of removal increasing with an increase in fly ash carbon content and a decrease in flue gas temperature. However, the data base with fly ash is limited. This project was conducted to expand this data base. Variables studied were the fly ash carbon content (2 to 16 wt % C) and flue gas temperature (195 to 325 F).

The project team used CONSOL's coal combustion pilot plant (1.5 MM Btu/hr). The facility is designed to simulate the operation of an industrial or utility coal-fired boiler. The combustor fired an Illinois coal (CONSOL's Rend Lake Mine) during the test program. Fly ashes obtained from Illinois utility and industrial boilers were injected into the combustor duct and collected in an electrostatic precipitator (ESP). Mercury removals were measured across the duct and ESP. The flue gas temperature was controlled using both humidification with an in-duct atomization nozzle and the pilot plant heat exchanger. The research team used state-of-the-art solid and flue gas mercury sampling/analytical methods to determine mercury removal and speciation.

The research has shown that fly ash can capture a substantial portion of flue gas Hg. Mercury removals ranged from 30-100%. The effect of flue gas temperature on Hg capture varied depending on the amount of carbon (C) in the ash. With the low carbon ashes (2 to 8 wt % C), the Hg removals increased from 30-60% at 270 F to 80-90% at 200 F. With the high carbon ash (15 wt % C), Hg removal increased with increasing temperature: from 50-60% at 200 F to 80% at 320 F. The variation in temperature effect suggests different Hg capture mechanisms with different C ashes. With the exception of tests at 200 F, the Hg removals increased with the ash C content. At 200 F, the Hg removals (88-94%) were essentially constant with 3 wt % to 8 wt % C ashes. At 225, 275, and 325 F, the removals ranged from 40-50% with 2 wt % C to 95-100% with 12-13 wt % C. The C content of utility boiler fly ash varies widely. A substantial number of boilers produce fly ash with C in the 1-3 wt % range. However, the C content can be as high as 35-40 wt %, depending on boiler type and boiler conditions.

EXECUTIVE SUMMARY

Under the 1990 Clean Air Act Amendments (CAAA), the U.S. Environmental Protection Agency (EPA) conducted a comprehensive study of hazardous air pollutant (HAPs) emissions from coal-fired utility plants. Of the 189 elements or compounds identified as HAPs, the majority were found to be captured with fly ash in particulate collection systems (ESPs, fabric filters). The exceptions are the more volatile elements or compounds which pass through the emissions control equipment and are emitted to the atmosphere. One of these materials is mercury (Hg). EPA¹ claims to have established a plausible link between Hg and adverse health effects. As a result, EPA is considering regulations to limit utility mercury emissions. These regulations could adversely impact Illinois coal use through an increased use of natural gas and/or the use of non-Illinois coals.

Presently, there is no commercially accepted technology for removing Hg from coal-fired utility flue gas. Tests conducted by CONSOL^2,3 have shown that wet flue gas desulfurization systems remove up to 60% of the flue gas Hg at no extra cost. The majority of boilers do not have wet scrubbers, and must rely on ESPs for flue gas clean-up. These boilers would require dedicated Hg removal techniques. The dedicated Hg removal technologies would increase the cost to burn Illinois coal.

Lab^4,5 and pilot plant^6,7 studies have shown that fly ash has the capability to capture Hg; the mercury captured increased as the fly ash carbon content increased and the flue gas temperature decreased. If fly ash (already present in coal-fired flue gas) and flue gas humidification can be used to capture a significant portion of the Hg, this would provide a cost benefit to Illinois utilities, and help preserve Illinois coal markets. To expand the very limited data base on fly ash capture of Hg, CONSOL R&D conducted a Hg removal study in a pilot combustor equipped with flue gas humidification and an electrostatic precipitator. A correlation was developed between Hg removal and: 1) fly ash carbon content and 2) flue gas temperature.

The tests were conducted in CONSOL's 1.5 MM Btu/hr coal-fired combustor. The combustor is designed to simulate the operation of an industrial or utility boiler. For the test program, the combustor fired an Illinois coal (CONSOL's Rend Lake Mine located near Sesser, IL). Fly ashes obtained from Illinois industrial and utility boilers containing different carbon contents were injected into the combustor ductwork and collected in a pilot scale electrostatic precipitator (ESP). The Hg removal was measured across the combustor duct work (with 1.4 to 2.7 seconds flue gas residence time) and across the ESP. An in-duct atomization nozzle and combustor heat exchanger was used to control the flue gas temperature. The flue gas was sampled at the duct inlet and ESP exit to determine Hg removal across the pilot-scale ESP. The flue gas Hg concentration was determined using the Ontario Hydro ASTM Draft Method, which has been determined by DOE, EPRI,⁸ and others⁹ as the best method for determining both total and speciated mercury in flue gas. In addition to flue gas sampling, the injected fly ash and recovered ESP solids from each test were sampled and analyzed to confirm the Hg removals across the duct and ESP.

Various fly ashes obtained from Illinois industrial and utility boilers were evaluated in this study. The unburned carbon content (LOI) of the ash varied from 2-16 wt %. The low carbon represents a normal utility boiler operation, while the high carbon represents a low NOx burner operation. The use of low NOx burners has the disadvantage of generating fly ash with higher LOI, but this type of fly ash can be advantageous for Hg removal. For example, high carbon has a detrimental effect on the pozzolanic quality of fly ash, but can provide more active sites for absorption of Hg. Flue gas temperatures ranging from 195 F to 325 F were studied. A temperature of 325 F is normally found downstream of the air preheater without flue gas humidification. The temperatures less than 325 F represent varying degrees of flue gas humidification.

The results of this study show that fly ash, along with flue gas temperature reduction, is a potential Hg control technology. Mercury removals from 25-100% were obtained across the duct and ESP of the pilot combustor.

• The effect of flue gas temperature on Hg removal varied depending on the carbon content of the fly ash.

With the relatively low carbon (C) fly ashes (2 to 8 wt %), the duct/ESP Hg removal increased with decreasing flue gas temperature. In general, the removals increased from 30-60 % at 260-270 °F to 85-95% at 200 °F.

With 12 wt % C, the Hg removals (95-100%) were essentially constant over the temperature range of 220 to 320 °F.

With the high C ash (15 wt %), the Hg removal increased with an increase in temperature: 50-60% at 200 °F to 80% at 320 °F The opposite trend with temperature may suggest a temperature-dependent chemical reaction with the higher C ash.

• With the exception of tests at 200 °F, the Hg removal increased with an increase in fly ash C content.

- 200 °F: Mercury removals (88-94%) were essentially constant as the C content was increased from 3 wt % to 8 wt %.

- 220-230 °F: Removals increased from 45% with 2 wt % C to 99% with 13 wt % C.

- 265-275 °F: Removals increased from 50% with 2 wt % C to 98% with 12 wt % C.

- 320-325 °F: Removals increased from 40% with 2 wt % C to 95% with 12 wt % C.

Mercury speciation (Ontario Hydro flue gas sampling) results showed that fly ash will capture both oxidized and elemental mercury. As much as 80% of the Hg fraction collected in the potassium chloride (KCl) impingers, assumed to be oxidized Hg, was removed across the duct and ESP. As much as 70% of the Hg^o fraction was removed. The removals of oxidized and/or elemental Hg were actually higher than the values presented, since there was also a reduction in the particulate Hg (Hg captured by the sample train filter) from the duct inlet to the ESP exit. The particulate Hg can be oxidized or elemental Hg.

The results of this program show that fly ash, in combination with flue gas temperature reduction, has the capability to remove a substantial portion of flue gas Hg. These results, along with other fly ash studies,^4-7 provide the Illinois utility industry with a potential low-cost retrofit Hg control technology, which could be utilized in the event of EPA regulations on coal-fired Hg emissions.

References

1. U.S. EPA, "Mercury Report to Congress: SAB Review Draft", EPA-452R-96-001, U.S. Environmental Protection Agency, Research Triangle Park, NC, 1996.

2. DeVito, M. S. and Rosenhoover, W. A. 1997. "Flue Gas Mercury and Speciation Studies at Coal-Fired Utilities Equipped with Wet Scrubbers". Presented at EPRI 4th International Conference on Managing Hazardous Air Pollutants, November 1997, Washington, DC.

3. DeVito, M. S. and Rosenhoover, W. A. 1999. "Flue Gas Hg Measurements from Coal-Fired Boilers Equipped with Wet Scrubbers". Presented at Annual Meeting of the Air & Waste Management Association, June 1999, St. Louis, MO.

4. Hassett, D. J. and Eylands, K. E. 1999. "Mercury Capture on Coal Combustion Fly Ash", Fuel 78: 243-248.

5. Li, Z. and Hwang, J. 1997. "Mercury Distribution in Fly Ash Components". Presented at the 90th Annual Meeting & Exhibition of the Air & Waste Management Association, June 1997, Toronto, Ontario, Canada.

6. Sjostrom, S., Smith, J., Hunt, T., Chang, R., and Brown, T. D. 1997. "Demonstration of Dry Carbon-Based Sorbent Injection for Mercury Control in Utility ESPs and Baghouses". Presented at the 90th Annual Meeting & Exhibition of the Air & Waste Management Association, June 1997, Toronto, Ontario, Canada.

7. Schager, P., Hall, B. and Lindqvist, O. 1994. "The Retention of Gaseous Mercury on Flyashes". Mercury Pollution: Integration and Synthesis: 621-628.

8. Evaluation of Flue Gas Mercury Speciation Methods. 1997. EPRI TR-108988, Prepared by the Energy & Environmental Research Center for the Electric Power Research Institute, December 1997, Palo Alto, CA.

9. Curtis, K. E. and Lucas, F. 1997. "Comparison of Mercury Speciation Results Obtained with Ontario Hydro Technologies Sampling Method and the U.S. EPA Reference Method 29 at a Sub-bituminous Coal-Fired Power Plant". Presented at the 90th Annual Meeting & Exhibition of the Air & Waste Management Association, June 1997, Toronto, Ontario, Canada.