The heavy metal Hg is a global pollutant [1Chen, J.J.; Ren, J.L.; Zhong, Y.J.; Luo, Y.Y.; Ye, S.J. Quantum chemistry study of mercury adsorption. J. Chinese Soc. Power Eng., 2010, 30(12), 960-964., 2Seigneur, C.; Vijayaraghavan, K.; Lohman, K.; Karamchandani, P.; Scott, C. Global source attribution for mercury deposition in the United States. Environ. Sci. Technol., 2004, 38(2), 555-569.
[http://dx.doi.org/10.1021/es034109t] [PMID: 14750733] ], mercury emissions from human activities almost are coal combustion and coal-fired plants have been identified as the major anthropogenic source of mercury [3Yin, L.B.; Zhuo, Y.Q.; Xu, Q.S. Mercury emission from coal-fired power plants in China. Proc. Chin. Soc. Electrical. Eng., 2013, 33(29), 1-9.-5Yudovich, Y.E.; Ketris, M.P. Mercury in coal: a review Part 2. Coal use and environmental problems. Int. J. Coal Geol., 2005, 62(3), 135-165.
[http://dx.doi.org/10.1016/j.coal.2004.11.003] ]. The reaction and migration mechanisms of mercury in fly ash is of great significance to the effective control of Hg emissions. In China, the energy supply highly depends on coal combustion, and coal consumption is expected to increase dramatically in the near future [6Zhang, L.; Wong, M.H. Environmental mercury contamination in China: sources and impacts. Environ. Int., 2007, 33(1), 108-121.
[http://dx.doi.org/10.1016/j.envint.2006.06.022] [PMID: 16914205] -8Zhang, L.; Zhuo, Y.; Chen, L.; Xu, X.; Chen, C. Mercury emissions from six coal-fired power plants in China. Fuel Process. Technol., 2008, 89(11), 1033-1040.
[http://dx.doi.org/10.1016/j.fuproc.2008.04.002] ]. Although mercury concentration in unit mass of coal is quite low, extensive utilization of coal inevitably lead to massive emission of Hg. To restrict Hg emission, the governments announced a series of ruling that regulates the Hg emissions from coal-fired power plants in the world.

The three principal forms of Hg in post-combustion flue gas are gas phase elemental mercury (Hg⁰), gas phase oxidized mercury (Hg²⁺) and particulate bound mercury (Hg_p), respectively [9Galbreath, K.C.; Zygarlicke, C.J. Mercury transformations in coal combustion flue gas. Fuel Process. Technol., 2000, 65-66, 289-310.
[http://dx.doi.org/10.1016/S0378-3820(99)00102-2] -12Agarwal, H.; Romero, C.E.; Rosales, F.H.; Mendoza-Covarrubias, C. A global kinetic mechanism for the prediction of Hg oxidation by a chlorine species. Energ. Sci. Technol., 2012, 4(1), 41-54.]. Hg²⁺ is water soluble and it can be removed by wet flue gas desulfurization devices in progress of removing sulfur. Elemental Hg⁰ does not have water solubility and difficult to be removed. Hg_p is easily removed by dust control equipment, such as electrostatic precipitator (ESP) or bag house filters. Lopez-Anton et al. [13Lopez-Anton, M.A.; Yuan, Y.; Perry, R.; Maroto-Valer, M.M. Analysis of mercury species present during coal combustion by thermal desorption. Fuel, 2010, 89(3), 629-634.
[http://dx.doi.org/10.1016/j.fuel.2009.08.034] , 14Rumayor, M.; Diaz-Somoano, M.; Lopez-Anton, M.A.; Martinez-Tarazona, M.R. Mercury compounds characterization by thermal desorption. Talanta, 2013, 114, 318-322.
[http://dx.doi.org/10.1016/j.talanta.2013.05.059] [PMID: 23953477] ] demonstrated that HgCl₂, HgS, HgO, and HgSO₄ were the main Hg species in coal-fired ash. Due to the diverse thermal stabilities of different mercury species, the mercury speciation can be identified via analyzing their thermal stabilities. Presently, the widely-used technology for direct control of mercury in fly ash is adsorption of active carbon [15Vidic, R.D.; Siler, D.P. Vapor-phase elemental mercury adsorption by activated carbon impregnated with chloride and chelating agents. Carbon, 2001, 39(1), 3-14.
[http://dx.doi.org/10.1016/S0008-6223(00)00081-6] -18Fan, L.; Ling, L.; Wang, B.; Zhang, R. The adsorption of mercury species and catalytic oxidation of Hg0 on the metal-loaded activated carbon. Appl. Catal. A Gen., 2016, 520, 13-23.
[http://dx.doi.org/10.1016/j.apcata.2016.03.036] ], nevertheless, there are some crucial drawbacks, like high carbon-to-mercury proportion and high cost, constrain wide application of mercury captured by activated carbon [19Lee, S.H.; Park, Y.O. Gas-phase mercury removal by carbon-based sorbents. Fuel Process. Technol., 2003, 84(1), 197-206.
[http://dx.doi.org/10.1016/S0378-3820(03)00055-9] -21Lopez-Antón, M.A.; Tascón, J.M.; Martínez-Tarazona, M.R. Retention of mercury in activated carbons in coal combustion and gasification flue gases. Fuel Process. Technol., 2002, 77, 353-358.
[http://dx.doi.org/10.1016/S0378-3820(02)00054-1] ]. Also, other sorbents had been examined with excellent adsorption capacity to capture mercury [22Ghorishi, S.B.; Sedman, C.B. Low concentration mercury sorption mechanisms and control by calcium-based sorbents: application in coal-fired processes. J. Air Waste Manag. Assoc., 1998, 48(12), 1191-1198.
[http://dx.doi.org/10.1080/10473289.1998.10463752] , 23Ghorishi, S.B.; Singer, C.F.; Jozewicz, W.S.; Sedman, C.B.; Srivastava, R.K. Simultaneous control of Hg0, SO₂, and NOx by novel oxidized calcium-based sorbents. J. Air Waste Manag. Assoc., 2002, 52(3), 273-278.
[http://dx.doi.org/10.1080/10473289.2002.10470786] [PMID: 11924858] ], such as calcium-based sorbents and other chemicals as well as adsorption capacity of active carbon [24Vidic, R.D.; Siler, D.P. Vapor-phase elemental mercury adsorption by activated carbon impregnated with chloride and chelating agents. Carbon, 2001, 39(1), 3-14.
[http://dx.doi.org/10.1016/S0008-6223(00)00081-6] ]. In order to control the emission of Hg, lots of scholars have studied the relationship between the composition of fly ash and mercury capture. There is a large amount of literatures about factors that influence mercury capture have emerged in recent years [25Kostova, I.; Vassileva, C.; Dai, S.; Hower, J.C.; Apostolova, D. Influence of surface area properties on mercury capture behaviour of coal fly ashes from some Bulgarian power plants. Int. J. Coal Geol., 2013, 116, 227-235.
[http://dx.doi.org/10.1016/j.coal.2013.03.008] -30Tan, Z.; Xiang, J.; Su, S.; Zeng, H.; Zhou, C.; Sun, L.; Hu, S.; Qiu, J. Enhanced capture of elemental mercury by bamboo-based sorbents. J. Hazard. Mater., 2012, 239-240, 160-166.
[http://dx.doi.org/10.1016/j.jhazmat.2012.08.053] [PMID: 22995206] ], and some common views have been reached: the adsorption of mercury by fly ash is highly correlated with unburned carbon, ash particles, coal type and so on.

To understand with regard to the adsorption of mercury by sorbents, it should figure out the mercury retention mechanisms and stability of particulate mercury in ash, similarly, the identification of mercury species in ash is also important. This paper aims to obtain the mercury species and concentration in fly ash that was collected from five coal-fired power plants in China and clarify the thermal decomposition mechanism of mercury components and factors that influence the adsorption of mercury by ash.

Table 3 presents experimental results of ash samples, including Hg concentration, unburned carbon content and mean ash particle size. Different ash samples have diverse Hg concentration, unburned carbon content and mean ash particle size. The average mercury concentration of BD and GG ash are the highest, and correspond to the highest content of unburned carbon and the minimal particle mean size. ZH ash has the least mercury concentration and unburned carbon content, the particle mean size is relatively maximum.

Table 3
Analysis of the ash samples.

Fig. (1) shows correlation between unburned carbon content and adsorption of mercury in fly ash. With comparison and analysis, it can be found that the correlation is well between carbon content and mercury adsorb in ZH fly ash sample, the correlation coefficient R² is 0.97. The correlation is relatively poor of unburned carbon and mercury captured by fly ash in WF ash sample, the correlation coefficient R² is 0.66. The correlation coefficients of carbon content and mercury capture in other fly ash samples are between 0.66 and 0.97. If all samples are taken into account, the overall correlation coefficient R² is 0.83, which shows that there is a significantly positive correlation between unburned carbon content and mercury capture in fly ash sample. High carbon content in fly ash means that the ash samples contain more coal chars and unburned carbon particles. There are lots of active carbon sites and oxygen containing functional groups, like carboxyl group, hydroxyl group and alpha oxygen group. These active sites and functional groups can provide adsorption sites for particulate mercury. Therefore, the more carbon particles in fly ash, the more coal char with active sites and oxygen containing functional groups can capture a large amount of mercury particles.

3.2. Ash Particle and Adsorption of Mercury

The influence of mean ash particle size on mercury capture is shown in Fig. (2). Apparently, the correlation between ash size and mercury capture is relatively good, as represented by the wide scatter in data. It can be found that the smaller ash particles, the more mercury grains be captured. Lower triangular represents BD fly ash sample, distribution of these data is more extensive, the correlation coefficient R² is 0.67. Upper triangular represents WF ash sample, the correlation between mean particle size and mercury capture is good, the correlation coefficient R² reaches 0.86. Circles represent ZH ash sample, the mean size of these ash particles is much larger, the content of mercury be captured is little, and the correlation coefficient R² is 0.81. Blocks indicate GG fly ash samples, these ash samples with small particle size, thus, the content of mercury be adsorbed is quite large, the correlation coefficient R² is 0.83. Cross represent DS ash sample and the correlation coefficient R² is the largest, it can reach 0.98. Considering all these ash samples, the relation of mean ash particle size and mercury capture does appear positive correlation, the correlation coefficient R² is 0.60, thus, ash particle size is an important factor to influence the mercury capture. The pores of fly ash change and develop continuously in the process of electrostatic dust removing, the wider the pore distribution of ash, the more favorable to the mercury be captured by fly ash particles. If the mean ash particle size is larger, the distribution of particles is more dispersed and it is not conducive to the mercury captured. On the contrary, the smaller mean ash particles size, the more closer of the ash particles, thus, the specific surface area becomes greater, which can form aggregate with large pores and then capture mercury easily.

Fig. (1) Mercury captured by all of the ashes versus carbon in ash.

▼BD ash; ▲WF ash; ○ZH ash; ×DS ash; □GG ash

Fig. (2) Mercury captured by all of the ashes versus ash particle size.

▼BD ash;▲WF ash;○ZH ash; ×DS ash;□GG ash

Coal rank is an important factor in determining the configuration and type on unburned carbon in fly ash and components of gas. Low-rank coal does not undergo thermoplastic transitions, whereas bituminous coal generally expands and undergo thermoplastic transitions when temperature upon 300°C [36Gray, R.J.; DeVanney, K.F. Coke carbon forms: microscopic classification and industrial applications. Int. J. Coal Geol., 1986, 6(3), 277-297.
[http://dx.doi.org/10.1016/0166-5162(86)90005-4] ]. Coals of anthracite rank generally do not display thermoplastic properties. Jennifer et al. [37Wilcox, J.; Rupp, E.; Ying, S.C.; Lim, D.H.; Negreira, A.S.; Kirchofer, A.; Lee, K. Mercury adsorption and oxidation in coal combustion and gasification processes. Int. J. Coal Geol., 2012, 90, 4-20.
[http://dx.doi.org/10.1016/j.coal.2011.12.003] ] proposed that carbon from low-rank coals have high mercury capture efficiency. High-rank coals can produce lower carbon-content ash with less oxygen containing functional groups in unburned carbon, whereas high concentration of unburned carbon is usually found in ash produced from low-rank coals. Hower [38Hower, J.C.; Kostova, I.J. Comparative studies of mercury capture by Bulgarian and Kentucky fly ash carbons. Annual Meeting, 2008, , pp. 6-9.] demonstrated that low-carbon Bulgarian fly ash sourced from low-rank coals has a greater tendency to capture Hg than does bituminous-sourced Kentucky fly ashes. Table 1 shows that BD and GG coal rank are lower than others coals, ZH and DS coal are closely ranked, WF coal has the highest rank. Given that BD and GG ashes have high mercury concentration, and ZH ash has the least mercury concentration. Experimental results show that the mercury content level in fly ash corresponds to coal rank, it can also be considered that the lower coal rank, the more mercury may be captured by fly ash. The experimental results are consistent with Hower’s conclusions [38Hower, J.C.; Kostova, I.J. Comparative studies of mercury capture by Bulgarian and Kentucky fly ash carbons. Annual Meeting, 2008, , pp. 6-9.].

Adsorption of mercury by fly ash are not only influenced by physical adsorption, but also affected by chemical composition in fly ash. In order to clarify chemical component influence on mercury capture, XRF was applied to measure primary element in ash sample, and the detailed data had been listed in Table 4. There are primary element Si, Al, Fe, Ca, Ti, Na and K in ash samples. Pavlish et al. [39Pavlish, 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 options for coal-fired power plants. Fuel Process. Technol., 2003, 82(2), 89-165.
[http://dx.doi.org/10.1016/S0378-3820(03)00059-6] ] suggested that Fe oxides can be used as a catalyst to catalyze the reaction of HCl and Hg. Galbreath [40Galbreath, K.C.; Zygarlicke, C.J. Mercury transformations in coal combustion flue gas. Fuel Process. Technol., 2000, 65, 289-310.
[http://dx.doi.org/10.1016/S0378-3820(99)00102-2] ] demonstrated that element Ca bonded to active carbon can provide high reactivity sites to adsorb particulate mercury. To a certain extent, element S and Cl can promote the adsorption of mercury [41Yao, Y.; Velpari, V.; Economy, J. Design of sulfur treated activated carbon fibers for gas phase elemental mercury removal. Fuel, 2014, 116, 560-565.
[http://dx.doi.org/10.1016/j.fuel.2013.08.063] -43Xu, Y.; Zeng, X.; Luo, G.; Zhang, B.; Xu, P.; Xu, M.; Yao, H. Chlorine-Char composite synthesized by co-pyrolysis of biomass wastes and polyvinyl chloride for elemental mercury removal. Fuel, 2016, 183, 73-79.
[http://dx.doi.org/10.1016/j.fuel.2016.06.024] ]. In Table 4, it can be found that BD and WF ash have similar element Fe, Cl and S content, BD ash has the much higher element Ca and corresponds to high mercury in ash. The element Fe, Ca, S and Cl concentration in ZH ash are higher than that of DS ash, but ZH ash has less mercury content compared with DS ash. The element Fe, Ca and Cl contents in DS ash are similar to GG ash, the element S concentration in DS ash is higher than that of GG ash, but DS ash has less mercury content than that of GG ash. There is no significant correlation between element Fe, Ca, S, Cl concentration and mercury content in fly ash, it indicates that physical adsorption plays a major role in capturing mercury. Although chemical component may promote the adsorption of mercury, it has little influence than physical adsorption.

Table 4
XRF analysis of fly ash samples.

Fig. (3) shows the ratios of different Hg compounds in fly ash samples. The major Hg species in all of the ashes are HgCl₂, HgS (black, red), and HgO. Hg₂SO₄ and Hg₂Cl₂ are unstable, Hg₂Cl₂ easily break into Hg and HgCl₂ with light. Similarly, Hg₂SO₄ easily decompose to Hg and HgSO₄. The content of element bromine is little in coals, thus, HgBr₂ can be ignored.

The HgCl₂ concentrations in ashes are significantly different. WF coal has the highest Cl content, and the mean HgCl₂ ratios of WF coal ashes exceed 30%. By contrast, the Cl content of BD coal is much lower compared with other coal samples. As a result, the HgCl₂ ratios of BD coal ashes are the lowest at less than 5.5%. Given the information above, the high Cl content in coal can lead to a high HgCl₂ ratio in fly ash. From the Fig. (3) it can be observed that the ratios of HgO are little in all fly ash samples. The ratio of HgO in WF ash sample is about 5%, the highest ratio is ZH fly ash sample and it can reach 13%. The content of HgSO₄ is the least and it can be neglected.

Fig. (3) Ratios of different mercury compounds in the ashes.

Fig. (4a and b) show the release curves of Hg compounds in ten fly ash samples at four different temperatures. All ash samples were collected from the first ESP field under two loads of each power plants.

Fig. (4) Release of mercury species in fly ash at four temperatures.

Fig. (4a) shows the release curves of Hg compounds in different ash samples at the temperature of 80 °C. According to Lopez-Anton et al. [31Lopez-Anton, M.A.; Perry, R.; Abad-Valle, P.; Díaz-Somoano, M.; Martínez-Tarazona, M.R.; Maroto-Valer, M.M. Speciation of mercury in fly ashes by temperature programmed decomposition. Fuel Process. Technol., 2011, 92(3), 707-711.
[http://dx.doi.org/10.1016/j.fuproc.2010.12.002] ], the HgCl₂ captured by fly ash begins to release at 70 °C. Thus, the capture of HgCl₂ by ashes would be detected in the decrease in mercury concentrations when the ashes were heated at 80 °C. After heating 180 minutes, the content of released mercury is still less than 10%, it indicates that the amount of HgCl₂ is little in fly ash samples. A large number of studies on the reaction of Hg in flue gas have demonstrate that chlorine is the key element for the oxidation of Hg⁰ and generation of Hg_p, the lower element chlorine content in raw coal leads to low HgCl₂ concentration in fly ash. It indicates that the Cl content of raw coal is the key factor to affect HgCl₂ content in fly ash. Fig. (4b) shows the release curves of Hg compounds in ash samples at the heating temperature of 180 °C. According to Table 2, black cube HgS begins to release at temperature of 170 °C. Thus, the mercury released at heating temperature of 180 °C should be HgCl₂ and HgS (black). It can be found that the content of HgCl₂ and black cube HgS are different in diverse samples, these two kinds of mercury components decompose rapidly in the previous half of hour, and release completely after one hour. Finally, the remaining mercury contents in ash samples keep to a constant. Fig. (4c) shows the release curves of Hg compounds at the temperature of 210 °C. According to Table 2, HgO begins to release at temperature of 200 °C. Therefore, the mercury released at heating temperature of 210 °C should be HgCl₂, black cube HgS and HgO. In this Figure, it can be observed that nearly half of mercury compounds have be decomposed. The mercury components release rapidly in the previous hour, and after continuously heating for half an hour the mercury contents remain basically constant. The experimental results indicate that HgCl₂, black cube HgS and HgO contents represent half of the total mercury concentrations. Fig. (4d) shows the release profiles of Hg compounds in fly ash samples at the heating temperature of 250 °C. According to Table 2, the heating temperature 250 °C is higher than that of the mercury compounds decomposition except for HgSO₄. Therefore, when the fly ash samples are heated at 250 °C continuously, almost all of the mercury components will be released in ash samples. In Fig. (4d), all curves close to the X axis at 150 minutes, the amount of mercury compounds close to zero.

Meij et al. [44Meij, R. The distribution of trace elements during the combustion of coal. In: Environmental aspects of trace elements in coal; Dalway, J. S.;Fari, G.; Eds.; Kluwer Academic Publishers: Boston , 1995; pp. 111-127.] considered that the chlorine can influence the adsorption of mercury. Lee [45Lee, S.J.; Seo, Y.C.; Jurng, J.; Lee, T.G. Removal of gas-phase elemental mercury by iodine-and chlorine-impregnated activated carbons. Atmos. Environ., 2004, 38(29), 4887-4893.
[http://dx.doi.org/10.1016/j.atmosenv.2004.05.043] ] proposed that chlorine plays an important role in the sorption of mercury in fly ash. Kellie et al. [46Kellie, S.; Cao, Y.; Duan, Y.; Li, L.; Chu, P.; Mehta, A.; Pan, W.P. Factors affecting mercury speciation in a 100-MW coal-fired boiler with low-NOx burners. Energy Fuels, 2005, 19(3), 800-806.
[http://dx.doi.org/10.1021/ef049769d] ] suggested that high element chlorine in coal, which can generate high HCl concentration in flue gas and can promote formation of Hg²⁺. HCl is the exclusive form of halogen in flue gas, and it can promote Hg oxidation on the zigzag carbon edge site of the unburned carbon. Reactions of Hg and Cl are included in homogeneous and heterogeneous reaction. Homogeneous reaction mechanisms include reaction (3) to (6) [47Sliger, R.N.; Kramlich, J.C.; Marinov, N.M. Towards the development of a chemical kinetic model for the homogeneous oxidation of mercury by chlorine species. Fuel Process. Technol., 2000, 65, 423-438.
[http://dx.doi.org/10.1016/S0378-3820(99)00108-3] ].

Where the (g) represents gaseous phase.

Element mercury can be oxidized to gaseous HgCl by gaseous HCl and Cl₂, and then some gaseous HgCl be oxidized thoroughly to gaseous HgCl₂. HgCl₂ is less volatile and could begin to condense on the surface of fly ash particles when the temperature is not too high (below 140°C) in the ESP [44Meij, R. The distribution of trace elements during the combustion of coal. In: Environmental aspects of trace elements in coal; Dalway, J. S.;Fari, G.; Eds.; Kluwer Academic Publishers: Boston , 1995; pp. 111-127.]. As well as heterogeneous reaction mechanisms include reactions (7) to (10) [48Presto, A.A.; Granite, E.J. Survey of catalysts for oxidation of mercury in flue gas. Environ. Sci. Technol., 2006, 40(18), 5601-5609.
[http://dx.doi.org/10.1021/es060504i] [PMID: 17007115] ].

Where the (g) represents gaseous phase, the (ad) represents reactant be adsorbed to the solid surface.

In the process of heterogeneous reaction, firstly, gaseous elemental mercury and chlorine adsorb on ash particles, and then reaction occurs between mercury and chlorine via equation (9). Whether adsorbed HgCl releases depend on the bond type and energy between the HgCl and ash particles. Although Cl/Hg rate of WF coal is about 15 times higher than that of BD coal, the mercury concentration in WF ash is one-third of that in BD ash. Similarly, Cl/Hg rate of ZH coal is 14 times higher than that of BD coal, but mercury content in ZH ash is less than that of BD ash. Consequently, it can be found that there does not exist obvious correlation between chlorine content and mercury adsorbed by ash particles. This experimental results consistent with views of Rubel [49Rubel, A.M.; Hower, J.C.; Mardon, S.M.; Zimmerer, M.J. Thermal stability of mercury captured by ash. Fuel, 2006, 85(17-18), 2509-2515.
[http://dx.doi.org/10.1016/j.fuel.2006.05.007] ]. But high content of chlorine in coal can lead to increase of HgCl₂ content in ash.

A distinctive feature shown in Fig. (3) is with regard to the high HgS ratios in all of ash samples. The HgS ratios of BD ash samples, WF ash samples, ZH ash samples, DS ash samples and GG ash samples are 83.7% 70.0% 83.8% 88.5% and 88.0% respectively. The formation of HgS requires reduced S; however, the S in coal is supposed to be oxidized to SO₂ in an oxidation atmosphere, such as the coal combustion process. The S in coal can only transform into H₂S or other species containing reductive S in reductive processes, such as coal pyrolysis. Therefore, HgS can be formed through reactions (11) to (13) [50Morimoto, T.; Wu, S.; Uddin, M.A.; Sasaoka, E. Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H₂S. Fuel, 2005, 84(14), 1968-1974.
[http://dx.doi.org/10.1016/j.fuel.2005.04.007] ].

Where the (ad) represents elemental sulfur adsorbs on char surface.

H₂S can be oxidized to elemental sulfur by oxygen or SO₂ on surface of char, and then adsorbed sulfur can capture elemental mercury by forming Hg-S bond. Therefore, elemental sulfur can play a positive role in capturing mercury. Rubel et al. [49Rubel, A.M.; Hower, J.C.; Mardon, S.M.; Zimmerer, M.J. Thermal stability of mercury captured by ash. Fuel, 2006, 85(17-18), 2509-2515.
[http://dx.doi.org/10.1016/j.fuel.2006.05.007] ] demonstrated that the good correlation exists between sulfur and mercury capture. Hsi et al. [51Hsi, H.C.; Chen, S.; Rostam-Abadi, M.; Rood, M.J.; Richardson, C.F.; Carey, T.R.; Chang, R. Preparation and evaluation of coal-derived activated carbons for removal of mercury vapor from simulated coal combustion flue gases. Energy Fuels, 1998, 12(6), 1061-1070.
[http://dx.doi.org/10.1021/ef9801064] ] reported that unburned carbon derived from high sulfur coals have shown to have higher mercury adsorption capacities than those from low sulfur coals. Rostam-Abadi et al. [52Rostam-Abadi, M.; Chen, S.; Hsi, H.C.; Rood, M.J.; Carey, T.R.; Richardson, C.F.; Chang, R. Development and Evaluation of Low-Cost Sorbents for Removal of Mercury Emissions from Coal Combustion Flue Gas; EPRI Report TE-114043, 1998.] suggested that sulfur functional groups can increase the mercury adsorption capacity of active carbons. BD and GG coal have much higher sulfur content compared with other coals, similarly, BD and GG coal ashes have high mercury concentration. ZH coal have the least sulfur content and corresponds to the least mercury concentration in ash. The positive correlation between mercury and sulfur indicated that mercury maybe deposited on ash as a sulfur compound.

Promoting the transformation of Hg⁰ to Hg_p helps to control gaseous mercury emission. However, whether the mercury captured by ash still has a potential to pollute the environment depends on thermal and chemical stabilities. The experimental results indicate that there exists a good correlation between mercury capture and carbon particles. The unburned carbon content and adsorption of mercury show a significantly positive correlation. On the contrary, there is a clearly negative correlation between mean ash particle size and mercury capture, the smaller the mean size of ash particles, the more mercury can be captured by ash. Also, the experimental results indicate that the lower the coal rank, the more mercury can be adsorbed on ash particles. Physical adsorption plays a major role in the adsorption of mercury. High Cl content in coal does not absolutely result in capturing more mercury but it can lead to a high HgCl₂ ratio in particulate mercury. Element sulfur in coal is an important factor in adsorption of mercury by fly ash. The HgO concentration is less, and HgSO₄ ratio is negligible in all of ashes and the main formation of mercury in fly ash are HgCl₂ and HgS.