Releasing behavior of mercury in coal during mild thermal treatment

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1 Indian Journal of Chemical Technology Vol. 22, November 2015, pp Releasing behavior of mercury in coal during mild thermal treatment Cheng Zhang*, Tingting Li, Dong Li, Ji Xia & Gang Chen* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei , People s Republic of China chengzhang@mail.hust.edu.cn Received 24 June 2013; accepted 5 February 2014 Mild coal thermal is one of the strategies for the emission control of mercury prior to coal utilization. Two Chinese coals have been selected to investigate the removal and emission behaviors of mercury in the process of coal mild thermal. Ontario Hydro Method (OHM) has been used to identify the emission behavior of different forms of mercury under rapid and temperature-programmable thermal treatments. The releasing behaviors of mercury associated with different matters in coal during thermal are identified by two-step acid washing pretreatment. The results indicate that Hg-silicate in coal is found to release between C. Hg-sulfide is found to release between C by mild thermal, however, oxidizing atmosphere has positive effect on lowering the releasing temperature for Hg-sulfide. Hg 0 is the major form of mercury emitted under N 2 atmosphere by coal thermal. Temperatures increasing from C under weak oxidizing atmosphere not only have significant effect on increasing the total mercury releasing ratio but also improving the proportion of Hg 2+ in the flue gas, especially for coal with high Hg-sulfide compounds. Keywords: Coal, Mercury, Association, Mild thermal, Pre-combustion emission control At present, the global mercury (Hg) emission is over 2500 Mg every year, and it is likely to increase in the future 1. The dangerous effects of Hg on the environment and human beings have been well established, and efforts to curtail atmospheric releases of Hg have begun in many countries of the developed world 2. Coal combustion is a major source of Hg emission due to the huge consumption of coal in coal-fired power plants 3. Extensive efforts have been made to control the emission of mercury, one approach is to reject mercury emission prior to coal utilization 4,5. Originally, mild thermal of coal before combustion is an efficient method to improve calorific value and thermal stability for low rank coals. However, it has recently been reported that hazardous element sulfur and some volatile toxic trace metals such as mercury could also be removed in this process Sulfur in coal have been validated to be partly removed by mild thermal, different sulfur removal efficiencies could be obtained via different atmospheres or temperatures in this process. Generally, hydrogen or oxidizing atmosphere can obtain a higher sulfur removal than inert atmosphere but also results in a lower char yield which is dissatisfied for economic concern 6,7. Xu et al. 8 and Iwashita et al. 10 have studied the mercury removal efficiency by mild pyrolysis and demonstrated that simple thermal at 380 could remove up to 80% of mercury in the original coal. It was approved by our previous research that weak oxidizing environment could significantly enhance the removal efficiencies of mercury in coal by mild thermal at optimal temperature 11. Ohki et al. 12 separated mercury in coal into two forms, one occurred in pyrite and related minerals which is hard to release during thermal, the other is present in more hydrophilic minerals (carbonate minerals, etc.) which is easily released. Mercury is emitted from coal combustion in either oxidized mercury (Hg 2+ ) or as elemental mercury (Hg 0 ) and either in the particulate or vapor phase 13. Extensive efforts have been made to increase the proportion of oxidized mercury in the flue gas which can be easily removed by the existing wet type Air Pollutants Control Devices (APCDs) like FGD, due to its high water-soluble property 14,15. As compared to the post-combustion controlling approaches for mercury in the flue gas, mild thermal treatment of coal is more easily to capture mercury released because that a concentrated gaseous mercury

2 ZHANG et al.: RELEASING BEHAVIOR OF MERCURY IN COAL DURING MILD THERMAL TREATMENT 329 could be obtained in the flue gas which has small amount than that from coal combustion. However, the fate of mercury emission characteristics during this process is not well established by now, especially for the emission behaviors of different forms of mercury. For better understanding the behavior of mercury emission from coal via mild thermal, it is important to realize the distribution and transferring mechanisms of different mercury speciation. Guizhou province in China is rich in coal resources. High mercury/sulfur content is the typical characteristic of coals in Guizhou. Twhinese bituminous coals (GZ and ZY) from Guizhou province were selected. The objective of this study is to characterize the releasing behaviors of mercury with different association in coal during mild thermal treatment. H 2 O 2 and 4%w/v KMnO 4 10%v/v H 2 SO 4 solution. After each test, solution in the impingers containing Hg 0 and Hg 2+ was moved out and then diluted and digested to reach the requirement for mercury determination, mercury concentration in the diluted solution was determined by cold vapor atomic fluorescence spectrometer. The char was also collected for mercury determination by DMA 80 mercury analyzer. The accuracy of DMA was evaluated by running the standard reference coal sample, and a relative standard deviation of DMA 80 at 9.7% was obtained. Thus, mercury in tar after thermal could be calculated by difference after we obtained the Hg 0 and Hg 2+ concentration in flue gas and Hg remaining in char. The tests were repeated for 3 times for each sample and the average value was given out. Experimental Section Determination of different forms of Hg and sulfur in coal The run of mine coals were air-dried at 40 C and crushed by a jaw crusher and then a pulverizer to finer than 250 µm for analysis. The sequential leaching was used to determine of different forms of Hg in raw coal, as described in more detail in our previous paper 11. The forms of sulfur in GZ and ZY coal samples were analyzed using ASTM D The results are given in Table 1. Determination of different forms of Hg by mild thermal The determination of different forms of Hg emitted from coal thermal was performed in a horizontal electric tube furnace by OHM which was recommended by EPA 16. The schematic of experimental setup is shown in Fig. 1. A horizontal flow-through quartz tube was used as the thermal chamber. Different working atmospheres were created by controlling the flow rates of N 2 and O 2 from cylinders. In each test, 2 g of coal sample was precisely weighed and placed in a ceramic sample holder centered in the middle of the quartz tube. The flow rate of carrier gas was kept constant at 2 L/min, unless otherwise stated. During the test, Hg 2+ emitted in the flue gas was absorbed by 1 mol/l KCl solution, Hg 0 was absorbed by 5%v/v HNO 3 10%v/v Samples Proximate analysis (wt%) Results and Discussion Table 1 Characteristics of the coal samples (air-dry basis) Ultimate analysis (wt%) Occurrence of mercury and sulfur in coal The results of the forms of mercury of sulfur in the coals are given in Table 1. For GZ coal, about half of total mercury was associated with sulfide minerals, we define this portion of mercury in coal as Hg-sulfide. However, for ZY coal, our previous study certified that most of mercury in ZY coal being associated with silicate-containing minerals 11, such as clays, we define this portion of mercury as Hg-silicate. Mercury leached out in the first two steps were defined as Hg-ion exchangeable and Hg-acid soluble, respectively. Mercury emission by rapid thermal The distribution of mercury in flue gas, tar and char for ZY and GZ coals at different temperatures under N 2 and 4%O 2 -N 2 atmospheres after rapid thermal Fig. 1 Schematic of the experimental setup used for thermal Percent of sulfur forms (%) Percent of Hg forms (%) Moist Ash C H S Sulfate Pyrite Organic Hg-ion Hg-acid Hg-sulfide Hg-silicate GZ ZY

3 330 INDIAN J. CHEM. TECHNOL., NOVEMBER 2015 for 20 min is shown in Fig. 2. The proportion of mercury emitted in the flue gas increased with the increasing of temperature. However, the percentage of mercury emitted in the tar did not show any significant change at different temperatures. The results also indicated that the removal of total mercury, including mercury emitted in flue gas and tar, increased with temperature increasing and could reach to more than 80% for both two coals when the temperature was higher than 400 C. A significant improvement upon total mercury removal was also observed in 4%O 2 -N 2 than in N 2 environment at 400 C. However, environments of thermal had a negligible effect on mercury removal when the temperature was higher than 500 C which could obtain the mercury removal efficiencies to about 90% for both two atmospheres. Rapid thermal experiments were performed at 400 C under different atmosphere to characterize the releasing of mercury as a function of residence time. The results are shown in Fig. 3. It can be observed that ZY coal has higher mercury removal efficiency than GZ coal in the first 5 min of thermal. It indicated that most of mercury in Hg-silicate form in ZY coal was not tightly bond with coal matrix, it was easier to break and release than Hg-sulfide which was the major form of mercury in GZ coal during rapid thermal. The results also demonstrated a sharp increasing of mercury removal in the first 20 min during thermal under different atmosphere. A slow increase was evident over 20 min with the increasing of residence time. The 4% oxygen containing atmosphere was observed to have a significant effect on mercury removal than N 2 gas at the same residence time, but only slight increase of mercury removal was found when the oxygen content increased from 4 to 10%. Mercury emission by temperature-programmable thermal For deep understanding the emission characteristics of different forms of mercury by thermal, the releasing of Hg 0 and Hg 2+ were determined by OHM through temperature programmable thermal under different atmospheres, the results are shown in Fig. 4. The emission of Hg 0 and Hg 2+ were observed in three temperature ranges, C, C and C under N 2 atmosphere. To further identification the releasing characteristics of mercury associated with different matters in coal, the same temperature-programmable thermal experiments were performed on acid treated coal. As shown in Fig. 5, mercury released within C disappeared in HCl treated coal during thermal indicates that mercury releasing with Fig. 3 The emission of total mercury in GZ(a) and ZY(b) coals as a function of residence time under different atmospheres Fig. 2 Distribution of mercury in flue gas, tar and char at different temperatures after rapid thermal

4 ZHANG et al.: RELEASING BEHAVIOR OF MERCURY IN COAL DURING MILD THERMAL TREATMENT C during mild thermal under N 2 atmosphere, as shown in reaction (1) and (2).... (1) FeS 2 FeS + S Fig. 4 Release intensity of Hg 0 and Hg 2+ during temperatureprogrammable thermal under different atmospheres (2) FeS2 CO FeS COS However, only two mercury emission peaks in two temperature range C and C were observed when the coal was thermal upgraded under oxidizing atmosphere, as shown in Fig. 4(b). The releasing of mercury was not observed under oxidizing atmosphere when the temperature was higher than 450 C. This observation indicated that mercury emission within C under N 2 atmosphere could probably shift to low temperature range and overlapped into the mercury emission peak within C under oxidizing atmosphere. The releasing of Hg 2+ was also found to have a distinct increasing in GZ coal within C, it was suggested to be mainly resulted from the releasing of Hg-sulfide in coal which happened within C under N 2 atmosphere. The results indicates that oxidizing atmosphere can facilitate and lower the temperature for the breaking of pyrite, as shown in reaction (3) and (4), and may also have the positive effect on the oxidizing of Hg 0 to Hg 2+ from Hg-sulfide in coal released during mild thermal. FeS + O o C FeS + FeSO + Fe ( SO ) + SO + Fe O... (3) FeS + O Fe ( SO ) + FeSO + SO + Fe O... (4) Fig. 5 Release intensity of Hg 0 (a) and Hg 2+ (b) of GZ raw coal and acid treated coal under N 2 atmosphere during temperature-programmable thermal C should belong to the releasing of ionexchangeable and acid soluble mercury in coal. Only one mercury releasing peak was observed within C from treated coal when the coal was treated by HCl+HNO 3. It further suggests mercury releasing during C should be from the Hg-silicate which still remained in coal after HCl+HNO 3 treated. Therefore, it is safe to consider that Hg-sulfide in coal suppose to release with the breaking of pyrite within The mechanism of mercury releasing by mild coal thermal should be different from coal combustion. Hg 0 (g) is the only stable form released out after coal combustion, part of Hg 0 (g) can be oxidized into Hg 2+ by the oxidants accompanied by the temperature decreasing of flue gas passing through different heating surfaces. For mild coal thermal, not all the mercury in coal was firstly transferred into Hg 0 (g) because the temperature was not high enough for the decomposing of some mercury compounds in coal. The possible mercury compounds evolved during thermal are listed in Table 2. It was believed from the results that the emissions of Hg 0 and Hg 2+ from coal thermal have the same temperature ranges. We have distinguished the association of mercury with different matters in coal, however, the existence of

5 332 INDIAN J. CHEM. TECHNOL., NOVEMBER 2015 Table 2 Possible mercury compounds evolving during thermal 18 Compounds (solid) HgS Hg 2 Cl 2 HgCl 2 HgO Hg 2 O HgSO 4 Element Hg Relevant properties (1 atm) Sublimes at Sublimes at 400 Boils at 302 Decompose at 500 Decompose at 100 Decompose Boils at 356 what kinds of mercury compounds in coal or the chemical bonds combination between mercury and other matters is still lack of sufficient evidences. It could be speculated that the emission of Hg 0 might come from the evaporation of element Hg in coal or the decomposition mercury compounds. The emission of Hg 2+ might come from the evaporation of direct sublimation from some mercury compounds, such as HgS and HgCl 2 in coal. HgS is always combined with pyrite in coal 17. For thermal under oxidizing atmosphere, the breaking of pyrite could be facilitated at low temperatures 11. Thus, the enhancement of pyrite breaking could also increase the releasing of HgS which combined with pyrite in coal and result in a increasing of Hg 2+ releasing under oxidizing atmosphere. On the other hand, the enhancement emission from Hg 0 to Hg 2+ in oxidizing atmosphere might also be resulted from the oxidization of Hg 0. Conclusion The study observed that most of mercury in Hg-silicate form in ZY coal is not tightly bond with coal matrix with respect to Hg-sulfide which was the major form of mercury in GZ coal. For mild thermal of coal, the releasing of ion-exchangeable and acid soluble mercury during mild thermal happen within C. Hg-silicate in coal is found to release between C. Hg-sulfide is found to release between C, however, oxidizing atmosphere is found to lower its releasing temperature. For temperature-programmable thermal under different atmospheres, Hg 0 is the major form of mercury emitted under N 2 atmosphere by coal thermal. Mercury removal display a sharp increasing under weak oxidizing atmosphere at 400 C, with respect to N 2 atmosphere. Further increasing of oxygen concentration from 4 to 10% could result in a slight increase in mercury removal but also could increase heat value loss which is not acceptable for economic concern. Increasing of temperatures from C and weak oxidizing atmosphere not only have significant effect on increasing the total mercury releasing ratio but also improving the proportion of Hg 2+ in the flue gas, especially for GZ coal with high Hg-sulfide content. The results also demonstrate that 4% O 2 -N 2 atmosphere and 400 C is the optimal option condition for mercury removal via thermal. Acknowledgement The project was financially supported by the National Natural Science Foundation of China (No ) and Natural Science Foundation of Hubei Province (2012FFB02602). References 1 Streets D G, Zhang Q & Wu Y, Environ Sci Technol, 43 (2009) Streets D G, Hao J M, Wu Y, Jiang J K, Chan M, Tian H Z & Feng X B, Atmos Environ, 39 (2005) Mercury Study Report tongress, Volume V: health effects of mercury and mercury compounds: EPA-452/R (U.S. Environmental Protection Agency, Washington, DC), Hoffart A, Seames W, Kozliak E, Riedinger S, Francini J & Carlson C, Fuel, 85 (2006) Luo G Q, Ma J J, Han J, Yao H, Xu M H, Zhang C, Chen G, Gupta R & Xu Z H, Fuel, 104 (2013), Qi Y Q, Li W, Chen H K & Li B Q, Fuel, 83 (2004) Qi Y Q, Li W, Chen H K & Li B Q, Fuel, 83 (2004) Xu Z H, Lu G Q & Chan Y O, Energy Fuels, 18 (2004) Guffey F D & Bland A E, Fuel Process Technol, 85 (2004) Iwashita A, Tanamachi S, Nakajima T, Takanashi H & Ohki A, Fuel, 83 (2004) Zhang C, Chen G, Gupta R & Xu Z H, Energy Fuels, 23 (2009) Ohki A, Sagayama K, Tanamachi S, Iwashita A, Nakajima T & Takanashi H, Powder Technol, 180 (2008) Galbreath K C & Zygarlicke C J, Fuel Process Technol, 65 (2000) Guo Y F, Yan N Q, Yang S J, Qu Z, Wu Q B, Liu Y, Liu P & Jia P P, Environ Sci Technol, 45 (2011) Yamaguchi A, Akiho H & Ito S, Powder Technol, 180 (2008) Standard Test Method for Elemental, Oxidized, Particle-Bound, and Total Mercury in Flue Gas Generated from Coal- Fired Stationary Sources: ASTM D6784 (ASTM International, West Conshohocken), Finkelman R B, Fuel Process Technol, 39 (1994) Weast R C, Handbook of Chemistry and Physics (CRC press, Cleveland, USA), 1984, 91.