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An Investigation on Mercury Association in an Alberta Sub-bituminous Coal† Cheng Zhang,‡ Gang Chen,*,‡ Ting Yang,‡ Guoqing Lu,§ Carol Mak,§ David Kelly,§ and Zhenghe Xu§ State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed August 20, 2006. ReVised Manuscript ReceiVed December 12, 2006
There are various mercury species in nature. Understanding mercury association with coal components could play a critical role in controlling mercury emission from coal utilization. In this study, an Alberta sub-bituminous coal was fractionated into various size and density fractions. The mercury and sulfur contents in each fraction were determined. To identify mercury association with coal components, the fractionated coal samples before and after low-temperature ashing were characterized by an X-ray diffractometer. Our study showed a clear correlation between the mercury and ash contents of coal. Such a relation was not observed between the mercury and sulfur contents. The potential to remove mercury prior to coal utilization by ash rejection using an air dense medium fluidized bed separator was demonstrated. At a combustible recovery of 81%, an ash rejection of 55% was achieved, leading to a mercury rejection of 56%.
1. Introduction Human activities, including the combustion of fossil fuels and incineration of the waste, gold mining, and other applications with mercury, have significantly increased the emission of mercury into our atmosphere. It has been estimated that the anthropogenic emission of mercury is about 60-80% of the global mercury emission and about 50% of anthropogenic mercury enters the global cycle.1 The transport and deposition of mercury by the atmosphere is a major reason that the mercury content in fish of remote lakes in North America is abnormally high.2 As a result, mercury as a global air pollutant source has increasingly become an environmental concern. The concentration of mercury in coal varies considerably, ranging from 0.02 to 1.0 µg/g of coal.3 A recent study4 concluded a worldwide average of mercury content in coal to be 0.1 ( 0.01 µg/g. On an ash basis, the mercury content averages 0.87 ( 0.08 and 0.62 ( 0.06 µg/g for bituminous and low-rank coals, respectively. Mercury, like most other trace elements, may exist in coal in different modes of occurrence. Although the speciation of this element in a given coal is not always known, mercury could occur in different coals as HgS, † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * To whom correspondence should be addressed. E-mail: gangchen@ mail.hust.edu.cn. ‡ Huazhong University of Science and Technology. § University of Alberta. (1) Dvonch, J. T.; Graney, J. R.; Keeler, G. J.; Stevens, R. K. Use of elemental tracers to source apportion Hg in South Florida precipitation. EnViron. Sci. Technol. 1999, 33, 4522-4527. (2) Watras, C. J.; Bloom, N. S.; Hudson, R. J. M.; Gherini, S.; Munson, R.; Claas, S. A.; Morrison, K. A.; Hurley, J.; Wiener, J. G. Sources and fates of mercury and methylmercury in Wisconsin lakes. In Mercury Pollution: Integration and Synthesis; Watras, C. J., Watras, W. J., Huckabee, J. W., Eds.; CRC Press, Inc.: Boca Raton, FL, 1994; pp 153-177. (3) Swaine, D. J. Trace Elements in Coal; Butterworths, London, U.K., 1990; pp 278-279. (4) Yudovich, Ya. E.; Ketris, M. P. Mercury in coal: A review: Part 1. Int. J. Coal Geol. 2005, 62, 107-134.
metallic mercury, associated with pyrite and sphalerite, or organically bound to coal matters.3 Mercury is of high volatility and becomes vaporized in the form of Hg0 and HgCl2 at high temperatures. As a result, mercury in coal is most likely to enter the flue gas during coal combustion and release into the atmosphere.5-9 Despite the low mercury content in coal, it has been recognized for years that coal-fired power plants in the United States are one of the largest sources of mercury emissions to the environment. As a consequence, on March 2005, the U.S. Environmental Protection Agency (EPA) issued the Clean Air Mercury Rule to permanently cap and reduce mercury emissions from coal-fired power plants. This rule makes the United States the first country in the world to regulate mercury emissions from utilities.10 Over the past few years, extensive efforts have been made to evaluate the quantity of mercury emitted from different sources and to reduce mercury pollution from coal.11-15 (5) Finkelman, R. B. Models of occurrence of potentially hazardous elements in coal: Levels of confidence. Fuel Process. Technol. 1994, 39, 21-34. (6) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control option for coal-fired power plant. Fuel Process. Technol. 2003, 82, 89-165. (7) Galbreath, K. C. Mercury transformations in coal combustion flue gas. Fuel Process. Technol. 2000, 66, 289-310. (8) Yewen, T.; Mortazavi, R.; Dureau, B.; Douglas, M. A. An investigation of mercury distribution and speciation during coal combustion. Fuel 2004, 83, 2229-2236. (9) Finkelman, R. B. Modes of occurrence of trace elements in coal. United States Geological Survey, Open File Report Number OFR-81-99, 1981; p 301. (10) U.S. Environmental Protection Agency (EPA). Mercury Study Report to Congress, Volume I: Executive Summary, Office of Air Quality Planning and Standards and Office of Research and Development, EPA452rR-97-003, December 1997. (11) Yudovich, Ya. E.; Ketris, M. P. Mercury in coal: A review: Part 2. Coal use and environmental problems. Int. J. Coal Geol. 2005, 62, 135165. (12) Wang, M.; Keener, T. C.; Khang, S. J. The effect of coal volatility on mercury removal from bituminous coal during mild pyrolysis. Fuel Process. Technol. 2000, 67, 147-161.
10.1021/ef060412e CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007
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Table 1. Results of the Proximate Analysis proximate analysis (wt % on an air-dry basis) moist ash VM FC 6.8
26.5
26.7
heating value (kJ/kg)
40.0
20 900
Table 2. Results of the Ultimate Analysis ultimate analysis (wt % on an air-dry basis) C
H
O
N
S
Hg content (ppm)
54.7
3.9
6.9
0.8
0.4
0.098
However, at present, there is no universally accepted mercury control technology for coal-fired utilities. One possible method of reducing mercury emission from coal utilization could be to clean the coal prior to its utilization. A number of physical coalcleaning methods are well-established and extensively used for reducing the ash-forming mineral matters and pyretic sulfur in coal prior to its combustion, to increase its calorific value and minimize problems of slagging and fouling, associated with the presence of mineral matters. The efficiency of these methods for reducing mercury content in coal prior to its combustion depends upon the association of this element with coal components. The mercury removal efficiency achieved thus far depended highly upon the type of coal and the cleaning procedures employed. The percentages of mercury rejection ranged from 1 to 99%. This uncertainty underlines the need for a better understanding of mercury behavior in coal cleaning. In this study, Alberta sub-bituminous coal was selected to study the correlation between mercury and minerals or sulfur content. Xu et al. at the University of Alberta had already studied mercury release characteristics from Alberta sub-bituminous coal during thermal upgrading.16 They established that thermal upgrading of sub-bituminous coal could be a viable option for mercury removal before combustion. In our work, the coal sample was separated into different size and density fractions by dry screening and the float-sink method, respectively. The mercury content of coal in each size and density fractions was determined. To identify mineral phases that are associated with mercury, the coal sample was ashed at two different temperatures. The resultant ash was characterized by an X-ray diffractometer (XRD). The aim of this study is to provide a better understanding of the mercury occurrence in Alberta subbituminous coal. We demonstrated a clear association of mercury with mineral matters and the potential to control mercury emissions from coal-fired power plants by mercury removal via coal cleaning prior to its combustion. 2. Experimental Section Samples. An Alberta sub-bituminous coal was used in this study. The results of proximate and ultimate analyses on this coal are given in Tables 1 and 2, respectively. Sample Preparation and Analysis: Size Fractionation. The run of mine coal was crushed by a jaw crusher to pass a 3.5 mesh screen (5.6 mm). The crushed samples were screened to -0.045, 0.045 × 0.25, 0.25 × 0.42, 0.42 × 1.00, 1.00 × 3.36, 3.36 × 5.66, (13) Sondreal, E. A.; Benson, S. A.; Pavlish, J. H. N.; Ralston, V. C. An overview of air quality III: Mercury, trace elements and particulate matter. Fuel Process. Technol. 2004, 85, 425-440. (14) Toole-O’Neil, B.; Tewalt, S. J.; Finkelman, R. B.; Akers, D. J. Mercury concentration in coal-unraveling the puzzle. Fuel 1999, 78, 4754. (15) Senior, C. L.; Helble, J. J.; Sarofim, A. F. Emissions of mercury trace elements and fine particles from stationary combustion sources. Fuel Process. Technol. 2000, 65-66, 263-288. (16) Xu, Z.; Lu, G.; Chan, O. Y. Fundamental study on mercury release characteristics during thermal upgrading of an Alberta sub-bituminous coal. Energy Fuels 2004, 18, 1855-1861.
Figure 1. Process diagram of an ADMFB separation system. Table 3. Volume Ratio of Heavy Liquids Required To Achieve Desired SGs volume ratio SG (g/cm3)
CCl4/C6H6
1.3 1.4 1.5 1.6 1.8
3:2 37:13 89:11
CHBr3/CCl4
1:49 21:79
and +5.66 mm size fractions for mercury analysis and to -1.00, 1.00 × 3.36, 3.36 × 5.66, and 5.66 × 22.6 mm size fractions for dry coal-cleaning tests using an air dense medium fluidized bed (ADMFB) separator to be described below. Density Fractionation. The float-sink method was used to fractionate coal samples into various density fractions using heavy liquids of specific gravity (SG) 1.3, 1.4, 1.5, 1.6, and 1.8 g/cm3, separately. The heavy liquids used in the test are carbon tetrachloride (CCl4), bromoform (CHBr3), benzene (C6H6), and their binary mixtures. The proportion of each liquid to achieve the desired density is given in Table 3. With these liquids, the coal in -5.6 mm size fractions was separated into six density fractions of -1.3, 1.3-1.4, 1.4-1.5, 1.5-1.6, 1.6-1.8, and +1.8 g/cm3. The samples were first placed in the liquid of lowest density (1.3 g/cm3). The floating particles in the liquid were collected, and the particles that sunk were placed in liquids of increasing density. The process was repeated to the heaviest liquid (1.8 g/cm3), in which particles that sunk were also collected as the heaviest density fraction. The collected particles in each density fractions were washed with hot water to remove heavy liquids and air-dried. ADMFB Separation. To test the potential of coal cleaning as a viable technology for precombustion mercury emission control, the coal sample in each size fraction was separated using an ADMFB separator as shown schematically in Figure 1. A vertical plexiglass cylinder of 20 cm inner diameter and 40 cm high with an air distributor was used as a separator. The air distributor is a metallic porous plate with an average pore size of 40 µm and thickness of 0.3 cm (Matt Corporation, Farmington, IL). The magnetite particles with a SG of 3.26 g/cm3 and mean diameter of 208 µm, purchased from Ward’s Natural Science Establishment (Rochester, NY), were used as fluidizing (separation) medium. The medium particles were placed on the air distributor to a desired height, measured using a metric ruler. The air volume flow rate through the air distributor was controlled by a valve and measured by a Blue-White flowmeter. The pressure of the compressed air was kept at a constant level of 40 psig. During fluidization, entrained particles in the exit air stream were collected using a Nederman’s Filter Box and the filtered air was then discharged into the atmosphere. In each tests, the medium particles were first suspended by the uplifting air to form a pseudofluid in the separator. After the fluidization of the bed was fully developed, the coal sample was introduced to the top of the fluidized bed and allowed to settle through the fluidizing bed. After an 8 min fluidization (stratification), the inlet air was turned off. When the fluidized bed was embedded with coal particles, it became packed again. The materials in the separator were collected into five height fractions along the column. The magnetic particles in each sample were separated from
Mercury in Alberta Sub-bituminous Coal the nonmagnetic coal particles by magnetic separation. The coal samples collected were then analyzed for ash content using a procedure described in American Society for Testing and Materials (ASTM) standard D3174. The performance of the ADMFB separator was evaluated by the combustible recovery and ash rejection after ashing the collected samples. The combustible recovery was calculated on the basis of the ratio of the combustible mass of the product divided by the combustible mass in the feed. It should be noted that the ash content values reported in this paper are on a dry basis. Coal Digestion and Mercury Analysis. For mercury analysis, coal samples in each size or density fractions were first pulverized to less than 0.25 mm prior to sample digestion. The coal samples were digested in a microwave digestion system, following the EPA standard 3052 method.17 This standard method is applicable to the microwave-assisted acid digestion of siliceous matrices and organic and other complex matrices, such as soil. Because there were no particular standard digestion methods satisfactory for coals, we adopted this digestion method by recognizing the similarities between coal and soil. On the basis of the EPA standard 3052 method, the digestion was performed in a mixture of 9 mL of HNO3, 3 mL of HF, and 2 mL of HCl. Recognizing the presence of a larger amount of solid organics in coal, we added 0.050 g of V2O5 and 2 mL of H2O2 as the catalyst and oxidant to assist digestion. The liquids obtained from the digestions were solid-free and therefore suitable to mercury analysis using PSA Millennium coldvapor atomic fluorescence spectroscopy (CVAFS). The detection limit of the CVAFS is 0.03 ng/mL, while the accuracy is better than 95%. Ashing. To reduce particle sizes, which was required for ash analysis using the ASTM Standard ASTM D 3174 method, the fractionated coal samples and those collected during ADMFB separation were pulverized to -0.2 mm by a Brinkmann-Retsch pulverizer. For coal samples that were coarser than 3.36 mm, a pulverizer from Braun Corp. (type UA) was used to first reduce particle sizes of samples. After homogenization, the pulverized samples were sampled into ceramic crucibles. The crucibles with the coal samples were dried in desiccators for 4 h and weighted precisely. The crucibles with the coal samples were then placed into an ashing furnace (Barstead-Thermolyne) and heated at 750 °C for 3 h. The crucibles with unburned contents were removed and weighted as soon as they were cooled to room temperatures in the desiccators. From mass balance, the ash content in each sample was calculated. Low-Temperature Ashing (LTA). To identify mercury association with mineral matter components, it is essential to characterize mineral matters in coal collected in various density and size fractions and those fractionated by the ADMFB separator. Because the presence of large-quantity organic solids is anticipated to interfere with accurate X-ray diffraction analysis and the organic removal using a conventional ashing muffle finance at high temperatures may cause solid-phase transformation of the original phases, organic (combustible) solid carbon in coal samples was first removed by LTA using a new LTA oven. The organic removal in this new LTA finance was based on irradiation of a sample with a flux of the activation oxygen atom at a temperature between 100 and 300 °C, during which solid organic carbon was oxidized without destroying the structure of minerals contained in the sample. In our experiment, the temperature for LTA was set at 100 °C to avoid alteration of the original mineral phases contained in the sample. As shown in Figure 2, the effectiveness of solid-carbon removal by LTA is clear by marginal differences between ash contents obtained using LTA at 100 °C and conventional ashing at 750 or 815 °C for a given set of samples. A slight overestimate of ash content by LTA is attributed to incomplete combustion of solid carbon or some inorganic matter at this temperature. It is also possible that certain types of solid-phase dehydration occurred during high-temperature ashing at 815 °C, leading to the loss of (17) U.S. Environmental Protection Agency (EPA). EPA standard 3052 method. Reversion 0, December 1996; Vol. 3052, pp 14-20.
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Figure 2. Mercury and ash contents of coal samples in different density fractions. Ashing was performed at 100 and 815 °C.
crystal water and hence an apparent weight loss. Nevertheless, such a minor difference is negligible on the interpretation of our results for the purpose of this paper. Sulfur Analysis. Total sulfur contents in various coal subsamples with different density were analyzed by the carbon/sulfur analyzer (EMIA, 320 V, Horiba, Japan) using the procedures described in the instrument user manual. Briefly, coal samples for sulfur analysis were pulverized to -0.25 mm using the Brinkmann-Retsch pulverizer. SRM 1632c bituminous coal purchased from the National Institute of Standards and Technology (Gaithersburg, MD) was used for calibration measurements. For each analysis, a precisely weighed coal sample around 0.08 g was placed in a new crucible. A total of 1.5 g of tungsten pellets and 0.3 g of tin granular were added to the crucible as accelerators for the analysis. The sulfur content of the coal sample was taken from the average value of three analyses. XRDs. The mineral matter samples after LTA were characterized using a PANalytical XRD. The XRD diffraction patterns were obtained by running 2θ from 2° to 90° with a step increment of 0.08° and a counting time of 1 s/step. When the XRD patterns of each ashed coal sample are compared to the standard diffraction patterns of known mineral phases, the major mineral phases in the original coal could be identified. The XRD background intensity was used to define the evolution of the content of the amorphous organic matter in coal.
3. Results and Discussions Ash Distributions: Ash and Mercury Distribution in Coal Samples of Different Densities. The results of low- and hightemperature ashing using coal samples obtained by float-sink density fractionation are shown in Figure 2. As anticipated, there is a clear increase in the ash content with an increase in the density of the sample. For example, the ash content in the coal sample of SG greater than 1.8 g/cm3 reached 74% in comparison with 10% for the coal of SG around 1.3 g/cm3. The sharp increase in ash content shows that the mineral components may assemble mainly in the density fraction of SG > 1.8 g/cm3. The mercury content in each of the coal samples of different density is shown by O in Figure 2. A corresponding increase in the mercury content with an increase in the density of the sample is evident. However, such a correlation is not observed when the sulfur content is considered, which will be discussed later. Ash and Mercury Distribution in Coal Samples of Different Particle Sizes. Figure 3 shows the results of ash and mercury analyses on coals of different sizes. It is interesting to note an increase in the ash content (b) with a decrease in the particle sizes of the sample, although no other physical separation was applied to the sample. From the results shown in this figure, solids in fine-size fractions appear to accumulate
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Figure 3. Mercury and ash contents of coal samples in different size fractions. Ashing was performed at 815 °C. Figure 5. Comparison between the sulfur and mercury contents in fractionated coal samples.
Figure 4. Correlation between the mercury and ash contents in coal samples fractionated into different size (0) and density (9) fractions.
Figure 6. X-ray diffraction patterns of a high ash content coal (68.5%) and its ashed residue.
more mineral matters, which may be linked with small grain sizes of mineral matters in coal. The mercury content (O) is seen to increase with a decrease in the particle sizes of the sample as well. It reaches 117.5 ppb in coal finer than 45 µm fractions, while it reaches only 19.8 ppb in coarse-size fractions of greater than 5.66 mm. The results appear to suggest that the mercury in coal is mainly concentrated in the mineral particles. Correlation between the Mercury and Ash Contents. The mercury and ash analyses on coal samples fractionated on the basis of particle size and density suggest a correlation between the mineral matter and mercury contents. To verify this correlation, the mercury content is plotted against the ash content of coal samples analyzed. The results in Figure 4 clearly show a linear relationship between these two contents. The linear relationship between the mercury and ash contents of coal observed in this study suggests that mercury in Alberta subbituminous coal is mainly associated with ash-forming mineral matters. This finding is in great contrast to the previous findings that mercury is mainly associated with sulfur-containing species, such as pyrite in coal.3,5 Correlation between the Mercury and Sulfur Contents. To test whether the mercury is indeed associated with the sulfur content in Alberta sub-bituminous coal, the mercury content is plotted against the sulfur content of fractionated coal samples in Figure 5. The absence of a positive correlation is evident. In fact, a decrease in the mercury content with an increase in the sulfur content appears to exist. Such an observation clearly rules out the possible association of mercury with sulfur in Alberta sub-bituminous coal. The absence of such a correlation is
understandable by considering an extremely low average sulfur content of 0.5% in Alberta sub-bituminous coal. Mineral XRD Analysis. To further identify the component of minerals in ash that is associated with mercury, the ashed samples were characterized with an XRD. To confirm whether LTA at 450 °C would alter the crystal structure of mineral components, a high ash content coal sample (68.5 wt % ash) obtained by density fractionation and its ashing product were first used to obtain XRD patterns, which are shown in Figure 6. A close inspection of the diffraction patterns revealed the identical diffraction patterns in terms of both peak positions and relative peak intensities, except for a reduction in the peak intensity and a significant shift of the diffraction peak from 2θ ) 18° to 21°. This peak arises from Na0.3Al2(Si,Al)4O10(OH)22H2O. The observed shift indicates dehydration of this component during ashing. This finding is consistent with the observed higher ash content at a lower ashing temperature for a given sample as shown by the results in Figure 2. Clearly, ashing of coal samples at 100 °C would not alter crystal phases. The diffraction patterns in Figure 6 reveal the richness of mineral components in original mineral matters. The principal components included quartz (SiO2), calcium sulfate anhydrite, paragonite, and surprisingly, chaoite, an incombustible solid carbon of crystal structures different from graphite and diamond. To provide a semiquantitative analysis, the ashed samples at 450 °C were doped with 10 wt % of galena (PbS), which was shown to feature distinct principal diffraction peaks. For a clear presentation, diffraction patterns from 2θ ) 8° to 32°, obtained with raw and fractionated coals of similar ash content of 20.6%,
Mercury in Alberta Sub-bituminous Coal
Figure 7. X-ray diffraction patterns of a raw coal sample and a fractionated coal sample containing an equivalent ash content of 20.6%.
Figure 8. Effect of fluidization air velocity on ADMFB separation using coal in 22.6 × 5.66 mm size fractions.
are shown in Figure 7. When normalized by the diffraction peak of galena at 2θ ) 30°, a similar content of chaoite in raw coal and in the fractionated coal of similar ash content is seen. From this result, mercury appears to be associated with chaoite. Chaoite is a mineral approved in 1968,18 named after Edward Ching-Te Chao, a Chinese-American petrologist. Former studies had shown that chaoite was found in shock-metamorphosed graphite gneisses and meteorites. From Hey’s index grouping (http://www.mindat.org/min-1207.html), we would expect some degree of association of mercury with chaoite. However, to draw a more definite conclusion toward the mercury association with chaoite, a systematic study would be needed. Removal of Mercury by Coal Cleaning. Although further studies are needed to derive a more definite conclusion on the mercury association with mineral components, it is nevertheless clear that mercury is tightly associated with mineral matters in Alberta sub-bituminous coal. This finding leads us to test whether dry coal cleaning could be a viable option for mercury emission control. For this purpose, coal samples in various size fractions were tested using our ADMFB separator. The results obtained with an Alberta sub-bituminous coal in 22.6-5.66 mm size fractions are shown in Figure 8. The separation was performed at different fluidizing velocities with a fixed medium height. For comparison, the results obtained using the floatsink method are also shown in this figure. It should be noted that the float-sink test represents an ideal separation performance. Figure 8 shows that, for a given separation medium, a fluidization air velocity of 6.01 cm/s is the optimal velocity for
Energy & Fuels, Vol. 21, No. 2, 2007 489
Figure 9. Correlation between the ash and mercury rejections for Alberta Seam 2 sub-bituminous coal.
separation, because the results obtained at this fluidizing velocity are the closest to the float-sink separation curve. The results obtained at 2.25 and 9.76 cm/s are far away from the floatsink curve, suggesting that operation at fluidization below or beyond the optimal fluidization air velocity would result in an inefficient separation. At a fluidization air velocity of 6.01 cm/ s, an ash reduction from 15.7 to 9.3% is achieved with 75.2% yield. This represents a combustible recovery of 80.9% and an ash rejection of 55.5%. After the ADMFB separation, the mercury contents of the coal sample were also analyzed. The mercury rejection result was generated in a similar fashion as ash rejection. The relationship between the mercury and ash rejections is shown in Figure 9. A clear linear relationship between the mercury and ash rejections was observed. The results not only confirm the association of mercury with mineral matters but also demonstrate the potential of mercury rejection via ash rejection by physical separations. In combination with the results in Figure 8, we could conclude a 56% mercury removal by ADMFB separation at 81% combustible recovery. The limit is set by the degree of mineral matter liberation from the organic matrix and the effectiveness of separation. Effective physical cleaning at finer sizes would help further improve ash rejection and hence mercury rejection without scarifying combustible recovery. Summary and Conclusions Through our recent research on the Alberta sub-bituminous coal, we established that samples in higher density or finer size fractions of mineral matter contents feature high mercury content. A clear linear correlation between the mineral matter and mercury contents was identified for Alberta sub-bituminous coal. Unlike most of the reported studies on bituminous coal of high sulfur contents, there was no clear correlation between the sulfur and mercury contents for the sub-bituminous coal studied by size and density fractionation methods. XRD analysis on ashing products and the coal sample of high mineral matter contents showed the fact that quartz (SiO2), calcium sulfate anhydrite, paragonite, and surprisingly, chaoite are principal components of ash-forming mineral matters. Mercury seems to be associated with chaoite in Alberta subbituminous coal to some extent, but further study is still needed to draw a definite conclusion. ADMFB separation applied to coarse Alberta sub-bituminous coal showed a 1:1 linear cor(18) El Goresy, A.; Donnay, G. A new allotriomorphic form of carbon from the Ries Crater. Science 1968, 161, 363-364.
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respondence between the ash rejection and mercury removal from the coal samples. At an 81% combustible recovery and 55% ash rejection, a mercury removal of 56% was achieved, demonstrating the potential of coal cleaning as a potential strategy for mercury emission control. Acknowledgment. The project was financially supported by the National Basic Research Program of China (2005CB724905)
Zhang et al. (973), the Natural Science Foundation of Hubei Province (2006ABC002), and the Programme of Introducing Talents of Discipline to Universities (“111” project), China. Z.X. also acknowledges the financial support from the NSERC-EPCOR-AERI Industry Research chair program and the Chinese Ministry of Science and Technology (MOST), Albert Innovation Science, and University of Alberta under the Joint Research Laboratory project. EF060412E
