Determination of the forms of calcium present in coal chars by Ca

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1 Determination of the forms of calcium present in coal chars by Ca K-edge XANES with Synchrotron Radiation * LIU Lijuan( 刘利娟 ) 1, LIU Huijun( 刘慧君 ) 2, CUI Mingqi( 崔明启 ) 1,1), HU Yongfeng( 胡永峰 ) 3, ZHENG Lei( 郑雷 ) 1, ZHAO Yidong( 赵屹东 ) 1, MA Chenyan( 马陈燕 ) 1,XI Shibo( 席识博 ) 1,YANG Dongliang( 杨栋亮 ) 1, GUO Zhiying( 郭志英 ) 1, WANG Jie ( 王杰 ) 2,2), 1 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing , P. R. China 2 Department of Chemical Engineering for Energy, East China University of Science and Technology, Shanghai , China 3 Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0X4, Canada Abstract This work is concerned with the Ca transformations during the pyrolysis of Ca(OH) 2 or CaCO 3-added coals. Ca K-edge X-ray absorption near edge structure (XANES) spectroscopy was applied to determine the forms of Ca in chars prepared from the pyrolysis of Ca-added coal. Results showed that Ca(OH) 2 and CaSO 4 existed in both the Ca(OH) 2-added chars and the CaCO 3-added chars, while CaS and CaO only existed in the chars prepared from the Ca(OH) 2-added coal. Moreover, it was found that carboxyl Ca was formed during pyrolysis for either the Ca(OH) 2-added coal or the CaCO 3-added coals. Keywords: char; pyrolysis; calcium; K-edge XANES PACS: ag 1 Introduction Sulfur in coal is generally unfavorable for the utilization of coal, because it forms the environmental pollutant. Capture of sulfur-containing gases during coal utilization was a common way to reduce the emissions of sulfur-containing gases. Ca-based additives, such as CaO, CaCO 3 and Ca(OH) 2 have been widely used as an sulfur adsorbent during coal pyrolysis and gasification [1-5]. It is well known that the efficiency of sulfur retention differs with Ca-based additives. Guan et al. observed with the use of XRD that Ca(OH) 2 had a higher efficiency of capturing sulfur than CaCO 3 [2]. XRD analysis is widely available and sometimes effective to determine Ca-based minerals in raw coal and chars, but it is difficult to determine non-crystalline calcium including the calcium associated with the organics. XANES spectroscopy is element-selective and local-structure sensitive, and it is very powerful to determine a diversity of calcium species in coal and other materials. Several groups adopted Ca K-edge XANES to determine Ca speciation qualitatively and quantitatively [6-12]. A few studies focused on the changes of Ca species during coal pyrolysis and gasification [6,9,11,12] and the Ca speciation in raw coal and fractions, by means of Ca K-edge XANES [10]. Although CaCO 3 and Ca(OH) 2 have been often used as Ca-based additives to capture sulfur during pyrolysis of high-sulfur coal, there still is lack of information regarding the transformation of Ca, especially non-crystalline calcium, during the pyrolysis process. In this study, Ca K-edge XANES was used to determine Ca species in chars prepared from CaCO 3 added and * Supported by National Program on Key Basic Research Project (2010CB ) and National Natural Science Foundation of China ( and ) 1) cuimq@ihep.ac.cn 2) jwang2006@ecust.edu.cn

2 Ca(OH) 2 added coals. 2 Experimental The coal used in this study was a high-sulfur bituminous coal from Chongqing city in China. The proximate analysis of the raw coal and the X-ray fluorescence analysis of the ash composition showed that the raw coal contained 3.84% sulfur and 0.28% Ca. Experiments of pyrolysis were conducted on a tubular fixed-bed reactor. Before and during the pyrolysis, N 2 was flowed from the bottom to the top of the reactor at a flow rate of 200 ml/min to carry out the pyrolysis under an inert atmosphere. The sample was heated from room temperature to the predetermined temperature at 15 C /min. Three types of coal samples, raw coal, Ca(OH) 2 -added coal and CaCO 3 -added coal, were used. For simplicity, the CaCO 3 -added coal is abbreviated to CC coal, and Ca(OH) 2 -added coal to CH coal, and the chars prepared from them to CC char and CH char, respectively. The weight percentages of CaCO 3 in CC coals were 5%, 10%, and 15%, and that of Ca(OH) 2 in CH coals were also 5%, 10%, and 15%. The mass ratios of the inherent Ca in the raw coal and the added Ca in CC coals were 1:7.4, 1:15 and 1:25, respectively, while that in CH coals were 1:10, 1:20 and 1:34, respectively. Most experiments of Ca K-edge XANES spectra were collected at beamline 4B7A at Beijing Synchrotron Radiation Facility. Some were conducted at soft X-ray micro-characterization beamline (SXRMB) at Canadian Light Source. The spectra of the same sample conducted on both beamlines agreed with each other. The energy was calibrated at the white line of calcium sulfate at eV at both beamlines. Ca K-edge XANES spectra of Ca-added chars and Ca-containing reference compounds were all carried out in total electron yield (TEY) mode. The Ca-containing reference compounds included carboxyl-bound Ca (calcium acetate), and inorganic minerals (CaCO 3, Ca(OH) 2, CaSO 4, CaO and CaS). The spectra of Ca-added chars were normalized and fitted using Athena program package [13]. 3 Results and discussion The content of Ca in raw coal was only 0.28%. The amounts of Ca(OH) 2 and CaCO 3 added to raw coal were 5%, 10% and 15%. Since the content of coal in raw coal was small as compared to the loading of Ca(OH) 2 and CaCO 3 in CH coal and CC coal, we ignored the influence of the internal calcium in coal. We did not show the Ca species in raw coal and the chars derived from raw coal. Fig. 1 shows Ca K-edge XANES of the CC char prepared from CaCO 3 -added coal at 600 C. It could be seen that the CC char contained CaCO 3, together with calcium acetate, Ca(OH) 2, and CaSO 4. The fitting results demonstrated that CaCO 3, carboxyl-bound Ca, Ca(OH) 2, and CaSO 4 accounted for 27±5%, 24±5%, 21±5% and 28±5% of the total Ca in CC char, respectively. It could be inferred that even at 600 C, a majority of CaCO 3 was decomposed under the condition of coal pyrolysis, with the formation of carboxyl-bound Ca, Ca(OH) 2 and CaSO 4. CaCO 3 could be converted to carboxyl-bound Ca, which had not been reported before. The formation of carboxyl-bound was likely due to the reaction of CaCO 3 with the carbon matrix of coal during pyrolysis, although the mechanism of the conversion of CaCO 3 to carboxyl-bound Ca during pyrolysis merits further investigating. Ca(OH) 2 accounted for one fifth of total Ca in CC char. The formation of CaSO 4 could be due to the reaction of Ca with the sulfur-containing gases released during pyrolysis and oxygen in coal. No CaO was found in CC char prepared at 600 C, probably because the char sample well contact air leading to

3 hydration of CaO to Ca(OH) 2. Also, CaS was not detected in the CC char prepared at 600 C. Fig. 1 Ca K-edge XANES of CC char prepared at 600 C Fig. 2 shows Ca K-edge XANES of CH char prepared from Ca(OH) 2 -added coal at 600 C. In CH char, Ca occurred in the forms of Ca(OH) 2, carboxyl-bound Ca, CaCO 3, CaO, CaSO 4 and CaS. The fitting results indicated that Ca(OH) 2 only accounted for 18±5% of total Ca in CH char, and carboxyl-bound Ca, and CaCO 3 accounted for 42±5%, 23±5% of total Ca in CH char, respectively. Ca(OH) 2 could be also converted to carboxyl-bound Ca. The formation of carboxyl-bound Ca appeared to be more effective for CH coal than CC coal. CaSO 4 also formed in CH char, but the percentages of CaSO 4 of total Ca in CH char was less than 5%, which was smaller than that in CC char. CaO was present in CH char, because Ca(OH) 2 extensively decomposed above 500 C to form CaO, which was might be only re-hydrated during the sample storage and use. Some presence of CaS was detected in CH char, differing from no presence of CaS in the corresponding char sample derived from CC coal. This suggested a more effective sulfur-retention reaction between Ca(OH) 2 and H 2 S released by the coal pyrolysis, and this reaction formed CaS. The formation of CaS was consistent with the results of sulfur K-edge XANES in Liu et al. [5]. The formation of CaCO 3 in CH char could be due to the reaction of Ca(OH) 2 and CO 2. Fig. 2 Ca K-edge XANES of CH char prepared at 600 C Fig. 3 shows Ca K-edge XANES of CH char prepared at 900 C. It indicated that Ca in CH char prepared at 900 C existed still in the form of carboxyl-bound Ca, CaCO 3, Ca(OH) 2, CaSO 4 CaO, and CaS, which was the same as that in CH char prepared at 600 C. It was of great curiosity that the content of carboxyl-bound Ca in CH char did not change during pyrolysis from 600 C to 900 C, suggestive of the dispersion of calcium on the surface of char, by some chemical combination with oxygenated groups. This type of calcium may be closely related to the catalytic activity of calcium

4 during the coal pyrolysis and char gasification. CaCO 3 in CH char decreased above 600 C and reduced to less than 10% of total Ca in CH char prepared at 900 C, while Ca(OH) 2 increased which was consistent with the result of the XRD [5]. CaSO 4 was still less than 5%. More CaS formed at 900 C and its content was up to 12% of total Ca in the CH char, indicating that increasing temperature from 600 C to 900 C increased the reaction of calcium with H 2 S. It was reasonable that the content of CaO was little changed. Fig. 3 Ca K-edge XANES of CH char prepared at 900 C 4 Conclusion In this study, Ca K-edge XANES has been successfully determined Ca species in chars. Ca K-edge XANES results showed that CaCO 3 and Ca(OH) 2 could be converted to carboxyl-bound Ca during pyrolysis of Ca-added coal. All CaCO 3, Ca(OH) 2 and CaSO 4 existed in both type of chars, and CaSO 4 in CC char was higher than in CH char. However, CaS and CaO were found in CH char, but not in CC char, indicating Ca(OH) 2 in CH coal was easily converted to CaO and also easily reacted with the released H 2 S to form CaS during pyrolysis. The authors acknowledge the staff at Canadian Light Source (CLS) for their support, and the CLS is financially supported by CFI, NRC, NSERC, OIT, and University of Saskatchewan. References 1 Maes I I, Gryglewicz G, Yperman J, et al., Fuel, 1997, 76: GUAN R G, LI W and LI B Q, Fuel, 2003, 82: WANG J, YAO Y H, CAO J Q, et al., Fuel, 2010, 89: Lin S, Harada M, Suzuki Y, et al., Energy & Fuels, 2003, 17: LIU L J, LIU H J, CUI M Q, et al. Fuel, revised 6 Huggins F E, Huffman G P, Shah N, et al., Fuel, 1988, 67: Yamashita H, Nomura M and Tomita A, Energy & Fuels, 1992, 6: Cazorla-Amoros D, Linares-Solano A, Salinas-Martinez de Lecea C, et al., Energy & Fuels, 1993, 7: Huffman G P, Huggins F E, Levasseur A A, et al., Fuel, 1989, 68: Huggins F E, Seidu L B A, Shah N, et al., International Journal of Coal Geology, 2009, 78: Huggins F E, Shah N, Huffman G P, et al., Fuel, 1988, 67: Huffman G P, Huggins F E, Jenkins R G, et al., Fuel, 1986, 65:

5 13 Ravel B and Newville M, Journal of Synchrotron Radiation, 2005, 12:

POINT SOURCES OF POLLUTION: LOCAL EFFECTS AND IT S CONTROL Vol. II - Clean Coal Technologies - Bingjiang Liu

POINT SOURCES OF POLLUTION: LOCAL EFFECTS AND IT S CONTROL Vol. II - Clean Coal Technologies - Bingjiang Liu CLEAN COAL TECHNOLOGIES Bingjiang Liu Department of Environmental Sciences and Engineering, Tsinghua University, Beijing, P. R. China Keywords: Clean coal technologies, pollution control, sulfur dioxide,