Energy Fuels 2010, 24, 53–57 Published on Web 07/28/2009
: DOI:10.1021/ef900473p
Mode of Occurrence and Thermal Stability of Mercury in Coal† Vladimir Strezov,* Tim J. Evans, Artur Ziolkowski, and Peter F. Nelson Graduate School of the Environment, Department of Environment and Geography, Macquarie University, Sydney NSW 2109, Australia Received May 18, 2009. Revised Manuscript Received July 8, 2009
Mode of occurrence of mercury in three coals was studied using sequential selective extraction, physical separation, and thermal treatment methods. Mercury speciation in the selected coals was determined using a five step sequential selective extraction method showing associations of mercury fractions with the organic and mineral structure in the analyzed coals. One of the coals was further subjected to thermal treatment with results revealing release of the organo-complexed mercury from coal at temperatures below 400 °C, while the mercury fraction associated with the coal mineral lattice was found to be the more stable form, released at temperatures of 600 °C. The mercury content was also determined for a selected range of coal density fractions showing strong correlation between mercury concentration in coal and the coal density fractions. The heaviest density fraction of the coal exhibited the largest ash as well as the highest pyritic sulfur content. Previous studies have established a strong relationship between mercury and the pyritic sulfur in coal, which was also confirmed in this work.
influenced by coal type. Particulate-bound mercury (Hgp) can be removed with the existing PM control equipment. The oxidized form (Hg2þ), on the other hand, can be successfully removed using wet desulfurization devices, whereas the elemental form (Hg0) is difficult to capture and is generally emitted to the atmosphere from the stack. Therefore, knowledge of mercury species in the flue gas provides an understanding of the amount of mercury that can be successfully captured with the existing emission control technologies. Difficulties in controlling Hg0 emissions have prompted research and development of new mercury retention technologies based on adsorption of mercury on activated carbons, primarily impregnated with sulfur, chlorine, or iodine.3 The principle of mercury control with activated carbon is based on injection of powdered sorbent in the flue gas at a location before the particulate matter control unit (ESP or FF). In the flue gas, mercury binds to the sorbent, after which the mercury-rich activated carbon is collected in the particulate matter control unit. Activated carbon injection has several disadvantages, such as high cost and low applicable temperature range. If flue gas temperatures at the point where sorbent is injected are increased from 100 to 170 °C, then the mercury capture efficiency using activated carbons will deteriorate by 20-90%.4,5 Furthermore, Rubin et al.6 noted that the use of activated carbon for mercury control would further increase the concentration of flue gas particulate matter by 9%. Precombustion pyrolytic removal of mercury from coal is another option for mercury control, initially proposed by
Introduction Coals contain small quantities of mercury with a potential to be emitted to atmosphere when coal is thermally processed. There is widespread acceptance that, during thermal treatment of coal, all mercury originally present in the coal matrix is converted to the very volatile elemental form of mercury Hg0, which is insoluble in water and difficult to capture.1 In this form very high levels of emissions to atmosphere are possible, approaching 100% of the mercury in coal. Postcombustion reactions of the mercury, which take place as the combustion gases cool, result in substantial oxidation of Hg0 to Hg2þ.2 Compared to elemental mercury, the oxidized form of mercury is more easily captured by air pollution control devices and is more soluble in scrubbing solutions.1 Due to the intrinsic properties and behavior of mercury under combustion conditions, its emission control and management is still the major environmental and technical challenge. There is a vast range of scientific and industrial investigations focused on developing successful mercury retention controls, and they can be grouped into options for precombustion mercury removal and options for postcombustion emissions control. Postcombustion mercury control can be achieved, to some extent, with existing emission control units, such as fabric filters (FF), electrostatic precipitators (ESP), selective catalytic reduction (SCR), and wet scrubbers, currently applied to control emissions of particulate matter (PM), NOx, and SO2. The amount of mercury successfully captured with these units depends directly on the mercury speciation and is strongly
(3) Uddin, M. A.; Yamada, T.; Ochiai, R.; Sasaoka, E. Energy Fuels 2008, 22, 2284–2289. (4) Dunham, G. E.; Miller, S. J.; Laudal, D. L. Investigation of Sorbent Injection for Mercury Control in Coal-Fired Boilers. Final Report for EPRI and DOE FETC; 1998. (5) Hargis, R. A.; O’Dowd, W. J.; Pennline, H. W. 17th Annual Pittsburgh Coal Conference, Pittsburgh, PA, Session 19a, Paper 1; 2000. (6) Rubin, E.; Berkenpas, M. B.; Farrell, A.; Gibbon, G. A.; Smith, D. N. Air and Waste Management Association 94th Annual Meeting and Exhibition, Chicago, IL, 2001; p 14.
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Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: vstrezov@ gse.mq.edu.au. (1) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Fuel Process. Technol. 2003, 82, 89–165. (2) Senior, C. L.; Helble, J. J.; Sarofim, A. F. Fuel Process. Technol. 2000, 65-66, 263–288. r 2009 American Chemical Society
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Energy Fuels 2010, 24, 53–57
: DOI:10.1021/ef900473p
Strezov et al. Table 1. Coal Properties
proximate analysis (wt %, ad)
C01 C02 C03
ultimate analysis (wt%, daf)
moist
ash
VM
FC
C
H
N
S
O
Hg (ng/g)
13.9 14 1.9
4.4 6.5 13.1
37.1 33.7 9.5
44.6 45.8 75.5
61.8 60.1 76.7
4.01 3.35 3.03
0.86 1.08 1.62
0.4 0.88 0.7
32.93 34.59 17.95
61.6 15.5 19.4
Merriam et al.7 The aim of this method is to utilize the low volatilization point of mercury and promote its removal with low-temperature coal pyrolysis. Previous work,8 however, identified that mercury removal by mild pyrolysis is highly coal type dependent, and its success is influenced by the proportion of strongly bound mercury present in the parent coal. Although a number of previous publications have already dealt with identification of the mode of occurrence of mercury in the coal, the release mechanisms of individual mercuric forms and species from coal with temperature are still not well understood. Hence, understanding of the mode of occurrence of mercury in coal, as well as the stability of mercury release with temperature, is highly important to predicting mercury behavior and speciation during combustion and, with that, improving feasibility of the mercury control options. The aim of this work is to investigate mode of occurrence of mercury in coal and the effect of temperature on the release of different mercury forms.
Figure 1. Effect of temperature on mercury release from coals.
were placed in ultraclean Teflon pressure vessels containing 15 mL of nitric acid, 5 mL of hydrofluoric acid, and 3 mL of hydrochloric acid. The vials were shaken and placed in an oven set at 95 °C for 12 to 15 h. Upon cooling the samples were diluted to 50 mL with a mixture of 0.07 M BrCl dissolved in 4 M HCl. After shaking, the samples were allowed to settle before aliquoting for analysis. Aliquots of the digest ranging from 0.01 to 5.0 mL were analyzed using SnCl2 to reduce the reactive Hg2þ form of mercury to Hg0, and the sample train was purged with nitrogen for 20 min at 300 mL/min onto a gold-coated sand trap.9 The mercury collected on the analytical trap was thermally desorbed into the atom cell of the CVAFS detector. The detection limit using this method was 0.2 ppb of mercury in the original coal. Sequential Selective Extraction. Mercury speciation in coal was determined using the sequential selective extraction method described previously by Bloom et al.9 Solutions used for leaching extractions were deionized water (mercury fraction F1), deionized water with a mixture of acetic and hydrochloric acid with total solution of pH 2 (F2), 1 M KOH (F3), 12 M HNO3 (F4), and aqua regia (F5). The solutions were tested for mercury prior to leaching, and very low concentrations of less than 10 ng/L of mercury were detected in all of the applied solutions. The analytical procedures were conducted using ultraclean sample handling to avoid contamination.
Experimental Section Coal Samples. Three coal samples with properties shown in Table 1 were applied in this study. Coal C01 is from USA, and C02 and C03 are from Australian origin. The first two coals (C01 and C02) are sub-bituminous coals, and C03 is semianthracite. Coal Pyrolysis. To monitor mercury stability and release from coal with temperature, the selected samples were heated separately under argon atmosphere in a fixed bed pyrolyser at a heating rate of 50 °C/min. Approximately 2.5 g of the original coal was heated to selected temperatures with the holding time at the ultimate temperature set to 20 min. The holding temperatures applied in the study ranged from 100 to 800 °C with increments of 100 °C. The heated coal samples were then cooled to room temperature and pulverized in an ultraclean tungsten carbide ball mill. The crushed samples were subjected to quantitative analysis for the remaining mercury content. Density Separation. The coal samples were first homogenized using density separation sink-float technique in order to reduce variability in maceral and mineral matter concentrations of the individual particles. The sink-float separation mediums used for density separation in this study were a mixture of white spirit (specific gravity (SG) of 0.77) and perchloroethylene (SG = 1.60). Six different sample fractions with densities of F1.30, S1.30-F1.35, S1.35-F1.40, S1.40-F1.45, S1.45-F1.50, and S1.50 were produced for the study. Mercury Measurements in Coals and Chars. Total mercury contents in the coals and chars were determined by coldvapor atomic fluorescence spectrometry (CVAFS). Pulverized coal and char samples weighing approximately 0.25 g
Results The effect of temperature on mercury retained in the coal samples is shown in Figure 1. Mercury in coal chars was found to decrease with temperature. At 200 °C the mercury retained in the coal chars were in the range of 30% for C02, 45% for C01, and 65% for C03 of the initial mercury concentration. The variability in the decrease of mercury concentration in the three coals with temperature is most likely due to the differences in the intrinsic moisture content in the samples, coal properties, as well as different nature of mercury binding in the coal matrix. Concentrations of up to 4.5% of initial
(7) Merriam, N. W.; Grimes, R. W.; Tweed, R. E. Process for Low Mercury Coal, US Patent No. 5,403,365, 1995. (8) Strezov, V.; Morrison, A.; Nelson, P. F. Energy Fuels 2007, 21, 496–500.
(9) Bloom, N. S.; Preus, E. M.; Katon, J.; Hiltner, M. Anal. Chim. Acta 2003, 479, 233 248.
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Energy Fuels 2010, 24, 53–57
: DOI:10.1021/ef900473p
Strezov et al.
Table 2. Sequential Selective Extraction Fractions, ng/g, on As Received Basis fraction
F1
F2
F3
F4
F5
description
water-soluble
pH 2 extractable
organo complexed
strong complexed
cinnabar
0.25 0.18
