9th International Conference on Coal Science September 7-12, 1997, Essen, Germany Evaluation of Precombustion Control of Mercury Emissions from Coal Combustion by Mild Pyrolysis in a Rijke Pulse Combustor by T.C. Keener*, A.T. Wolf* and S.J. Khang** *Department of Civil and Environmental Engineering **Department of Chemical Engineering University of Cincinnati Cincinnati, Ohio U.S.A.

Title III of the 1990 Clean Air Act Amendments Air toxics will be controlled to the maximum extent technically possible (maximum available control technology, MACT). A plant that emits over 10 tons/year of any one of the listed toxic compounds, or emits 25 tons/year (or more) of any combination of these pollutants will have to comply to the control standards. Mercury poises the greatest environmental health risk from coal combustion of the compounds listed. Regulations controlling the release of mercury will most likely be promulgated in the future for coal burning power plants

RESEARCH OBJECTIVES Evaluate the fate of mercury present in coals during mild pyrolysis. Address the potential of controlling the release of mercury and its compounds to the atmosphere by means of adsorption/ chemisorption to solid sorbents. The release of mercury from the coal structure prior to combustion offers the greatest potential for separation of its compounds from the gases and vapors which evolve during mild pyrolysis.

Background Information Coal combustion has been estimated to account for over 8 % of the mercury emissions to the atmosphere2. The plume of a coal-fired power plant has been reported to contain gaseous mercury in excess of 1000 ng Hg/m3 within a few kilometers of the source.3 Investigations into the fate of mercury during coal combustion have shown that approximately 10% of the original mercury content was found to be in the fly ash. ref. 2 Sittig, M. (1975),3 Lindberg, S.E. (1980)

Background Information, con’t. Estimates of worldwide air emissions of mercury from the burning of coal range from 140-2720 tons/year4. The proposed study is one of the viable methods for reduction in air toxics emissions from combustion gases produced by burning coal. -ref. 4 Airey, 1982

MILD PYROLYSIS OF COAL Coal is heated in the absence of oxygen at low temperature (<550oC), low pressure (about 1 atm.) and usually a low heating rate. Much of the coal component does not devolatilize, and negligible destruction to the coal matrix occurs. The total heating value of coal does not significantly decrease. It has been shown to be an economically and technically viable method of desulfurization and denitrification.

MILD PYROLYSIS OF COAL The coal undergoes a series of chemical and physical changes and is converted into char, tar and light gases. Gaseous compounds include CO, CO2, H2, hydrocarbons, nitrogen compounds, sulfur containing species, and volatile metals. Factors which affect the yield and composition of the volatile fraction1: coal rank, coal composition, particle size, temperature, heating rate, atmosphere, pressure, sample size and type of reactor. ref: 1Peter, J.J.T. and Jacob, M.(1988)

Temperature Regions in Coal Pyrolysis

Experimental Methods Samples of one discrete size (-4 mesh) of the two coals were studied using the mild pyrolysis reactor in conjunction with the Rijke combustor. Test were conducted at steady combustion conditions (~25% EA), with temperatures above the grate ranging from 925 - 985 oC (~12 g/min coal feed rate). U.S. EPA Method 29 was used to determine Hg concentrations in the flue gas, off-gas, bottom ash and lime. ASTM Method D3684-94 was used to determine the Hg concentration in the coal and char.

Experimental Methods, con’t.

Dual Screw Feeder Reactor The University of Cincinnati

Rijke Pulse Combustor Dimensions: Length 3.05m (10’) Dia. 13.97cm (5.5”) Bed @ L/4

Results of Desulfurization Tests 1.00 Results of Desulfurization Tests 0.10 400 C 425 C 450 C 475 C 0.01 2 3 4 5 6 Residence Time, minutes "Coal Desulfurization by Mild Pyrolysis in a Dual-Auger Coal Feeder,” L. Lin, S.J. Khang and T.C. Keener, in press, Fuel Processing Technology, 1997.

MERCURY IN COAL Boiling point (elemental form) of 356oC. It has been shown to be released from coals at low temperatures.5 Mercury is listed as a calcophile element, those which will commonly form sulfides, and has been shown to be in association with both Sphalerite (ZnS)6 and Pyrite (FeS2 )7. 5 Karr, C., Jr. (1978) 6Miller, W.G. (1974) 7 Swaine, D.J. (1977)

Relationship of Coal Mercury to Coal Sulfur

Hg Values of Five Ohio Coals Coal Location Best Value(ppm) Belmont Co. 0.13 Belmont Co. 0.16 Coshocton Co. 0.15 Harrison Co. 0.46 Jefferson Co. 0.26 (Commercially washed sample)

Coal Properties* Ohio #6A (Carroll Co., OH) Pittsburgh #8 (Washington Co., PA) As Determined Dry Basis %Ash 9.37 9.89 13.32 13.67 % Fixed Carbon 48.19 50.86 50.58 51.90 %Volatile Mat. 37.19 39.25 33.56 34.43 %Nitrogen 1.62 1.64 1.33 1.36 %Sulfur 3.66 3.86 1.34 1.37 %Oxygen (difference) 7.36 7.77 6.87 7.05 % Moisture 5.25 2.54 Hg Conc. (ppm) 0.48±0.04 0.21±0.07 kJ/kg 30,408 29,899 *Obtained from the Penn State Coal Sample Bank

Mercury Removal vs Time 80 60 Xmax @400 C Xmax @ 350 C Mercury Removal vs Time 40 20 Lower Freeport Coal 350 C 400 C 2 4 6 8 10 Residence Time, minutes

Mercury Removal vs Temperature and Residence Time Freeport #6a Coal 80 Residence Time 2 minutes 4 minutes 60 8 minutes 10 minutes 40 20 200 300 400 500 600 Temperature, C Mercury Removal vs Temperature and Residence Time Freeport #6a Coal

600 C 500 C 400 C 300 C 0.1000 0.0100 -12001.5/T k = 547,984 e 2 0.0010 R = 0.93 4 E = 9.978 (10 ) kJ/kg mol Lower Freeport Pittsburgh #8 0.0001 1.000E-3 1.200E-3 1.400E-3 1.600E-3 1.800E-3 1/T, K

Change in Heat Content Due to Pyrolysis 0.40 0.30 0.20 0.10 0.00 -0.10 300 400 500 600 Temperature, C

Tests Summary

Mercury Removal in Reactor * * Mercury Removal in Reactor

Effect on Stack Gas Hg Concentration

Overall Mercury Mass Balance

Conclusions Mild pyrolysis offers potential for precombustion removal of mercury from coal. Mercury removal is dependent on residence time and temperature Over 50% of mercury may be removed at a temperature of 430 oC with a residence time of 4 minutes; higher removals may be obtained at longer residence times. Combustion conditions of bituminous coal tested not affected by mild pyrolysis at temperatures below 470 oC