Effect of mineral on sulfur behavior during pressurized coal pyrolysis

Effect of mineral on sulfur behavior during pressurized coal pyrolysis

Fuel Processing Technology 85 (2004) 863 – 871 www.elsevier.com/locate/fuproc Effect of mineral on sulfur behavior during pressurized coal pyrolysis ...

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Fuel Processing Technology 85 (2004) 863 – 871 www.elsevier.com/locate/fuproc

Effect of mineral on sulfur behavior during pressurized coal pyrolysis Quanrun Liu a, Haoquan Hu a,*, Qiang Zhou a, Shengwei Zhu a, Guohua Chen b a

Institute of Coal Chemical Engineering, Dalian University of Technology, 129 Street, Dalian 116012, PR China b Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, PR China Received 27 April 2003; received in revised form 10 October 2003; accepted 1 November 2003

Abstract Chinese Yima coal and its HCl/HF and HCl/HF/HNO3 treated samples were pyrolyzed in a fixed bed reactor under a pressure of 2 MPa. The effect of mineral on organic and pyrite sulfur transformation was investigated along with the effect of decomposition of pyrite on organic sulfur removal during pyrolysis. The results showed that the inherent mineral in coal has little effect on the decomposition of pyrite. The problems to hinder sulfur removal from coal during pyrolysis mainly include following three aspects: (I) large proportion of sulfide was produced from alkaline mineral matter in coal reacting with sulfur containing gas; (II) pyrite sulfur transformed into organic sulfur; (III) iron sulfide produced by decomposition of pyrite, which is more difficult to decompose at low temperature. D 2004 Elsevier B.V. All rights reserved. Keywords: Coal pyrolysis; Pyrite; Organic sulfur; Desulfurization

1. Introduction There are many studies on coal pyrolysis as a means of thermal desulfurization to produce a clean solid fuel from high sulfur coals. Unfortunately, sulfur removal during pyrolysis is only between 15% and 40%, and sulfur content in the char usually is higher than that of raw coal at mild pyrolysis conditions [1,2]. The extent of desulfurization is

* Corresponding author. Tel./fax: +86-411-3705226. E-mail address: [email protected] (H. Hu). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2003.11.031


Q. Liu et al. / Fuel Processing Technology 85 (2004) 863–871

influenced by many factors, such as pyrolysis conditions, the quantity of sulfur and the distribution of different forms of sulfur in the coal [3 – 6]. Interconversion between different forms of sulfur and sulfur fixation by minerals was also reported to affect the efficiency of desulfurization [7– 9]. The effect of mineral matter is believed to influence the amount and form of organic sulfur and inorganic sulfur but little such report is available in the published literatures. To enhance the efficiency of desulfurization during coal pyrolysis, it is necessary to understand the interaction between various forms of sulfur and the role of mineral in coal. Moreover, studies on pyrolysis and hydropyrolysis are tending to perform under pressured condition, thus it is also necessary to study the sulfur behavior during pressured pyrolysis. In this work, a high sulfur bituminous coal, Chinese Yima coal, was demineralized by HCl and HF and further treated by HNO3. The raw, HCl/HF treated (abbr. Dem) and HCl/ HF/HNO3 treated (abbr. Demp) samples of Yima coal were pyrolyzed at different temperatures in a pressurized fixed bed reactor to investigate the interaction between inorganic (mainly pyrite) and organic sulfur and the effect of inherent mineral on the behaviors of different sulfur forms.

2. Experimental 2.1. Sample preparation Yima coal sample with an original particle size of 40 to 80 mesh was further ground to  120 mesh before use. The demineralized sample (Dem) was prepared by the following procedures: raw coal was first mixed with 5 M HCl in plastic beaker at 60 jC for 45min. The coal – water mixture was filtered and mixed with HF (40%) for another 45 min followed by another filtration. Afterwards, the coal was well mixed with concentrated HCl. The coal was finally filtered and washed with hot distilled water until no chloride was detected by silver nitrate titration. After HCl/HF treatment, most carbonate, oxide and sulfate were removed, while pyrite and organic sulfur were almost unchanged. To further remove the pyrite from the demineralized sample obtained after HCl/HF treatment, Dem sample was treated again according to ASTM D2492-80. Briefly speaking, the Dem sample was mixed with 17% HNO3 and stirred at 60 jC for 45 min, then filtered and washed. The sample so obtained is denoted as Demp. The raw, Dem and Demp samples were dried in a vacuum oven at 80 jC for 24 h before pyrolysis experiment. Table 1 shows the proximate, ultimate and sulfur forms analysis of three samples and Table 2 is the ash composition of raw coal sample obtained by X-ray fluorescence analysis. X-ray diffraction (XRD) patterns of some char samples after pyrolysis were obtained on a DmaxrA X-ray diffraction analyzer with a Cu Ka radiation operated at 35/40 kV and 40/70 mA. 2.2. Pyrolysis and analysis Pyrolysis was performed in a vertical fixed bed reactor from 350 to 650 jC at a 50 jC interval. About 5 to 10 g coal sample was used in each experiment. High purity nitrogen

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Table 1 Analysis of coal samples Sample




Proximate analysis (wt.%, db) Ash V.M. F.C.

10.4 36.2 53.4

2.1 39.8 58.1

0.8 46.3 52.9

Ultimate analysis (wt.%, daf ) Carbon Hydrogen Nitrogen Sulfur Oxygena

61.6 4.9 1.2 1.2 31.1

61.4 4.6 1.2 1.1 31.7

60.5 4.4 1.2 0.9 33.0

Sulfur analysis (wt.%, db) Pyrite Organica Sulfate Total a

1.22 1.08 0.28 2.58

1.14 1.07 – 2.23

– 0.85 – 0.85

By difference.

was introduced from the top of the reactor to bring the volatile matter out. Its flow rate was regulated at 0.7 dm3/min (25 jC, 0.101MPa) by a mass flow controller operating at 2 MPa. The heating time from room temperature to the desired value for the reactor was about 10 min. The reactor temperature was held at that temperature for 30 min. Total sulfur in coal and char was determined by Coulomb method (Testing Standard of China: GB214-77) and pyrite sulfur content was determined by ASTM D2492-80. The organic sulfur content in coal or char was analyzed as follows: coal or char sample was washed by 5 M HCl and 2 M HNO3 according to ASTM method before filtered and dried. Then the sulfur content in coal or char, organic sulfur, was analyzed by Coulomb method directly. The organic sulfur content in coal or char sample can be determined by the following expression: So ¼ Wd  Sd =Wo where So is organic sulfur content in coal or char sample, Sd is sulfur content after treatment, Wd is the weight of coal or char sample after treatment, Wo is the original weight of coal or char sample. The transformation behavior of sulfate sulfur is not discussed here because Yima coal contains very low sulfate sulfur, see Table 1. Table 2 Main composition of coal ash (wt.%, db) SiO2





















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3. Results and discussion To compare the behavior of sulfur in various coal samples during pyrolysis, sulfur removal of every form was calculated by the following expression: ! Schar Wchar Sulfur removal ðwt:%Þ ¼ 1   100% Ssample Wsample where Schar and Ssample is content of a specific form of sulfur in the char and starting sample, Wchar and Wsample is mass of the char and starting sample, respectively. 3.1. Effect on total sulfur removal Removal of total sulfur from raw and Dem Yima coal samples as a function of temperature during pyrolysis is shown in Fig. 1. As expected, the total sulfur removal increases with increasing temperature for both raw and Dem samples. Before 450 jC, there is little difference between raw and Dem coals, but after that temperature, the total sulfur removal of Dem coal is higher than that of raw coal because the inherent mineral matter in the raw coal can trap the sulfur produced [10 – 12]. The reactions of the inherent minerals with sulfur containing gases (mainly H2S) are one of the major reasons for retention of sulfur in char. The reaction of H2S with calcite, CaCO3, is thermodynamically favorable at above 420 jC, while MgCO3 is not favorable until about 580 jC [10]. Yima coal contains abundant alkaline mineral, especially CaO, see Table 2. That is why in this study the mineral decreases the efficiency of desulfurization only occurring at relatively high temperature region. This tendency is more evident with pyrolysis temperature increasing. At the final temperature of 650 jC, the total sulfur removal is about 42% for raw coal and 50% for Dem coal.

Fig. 1. Total sulfur removal from raw and Dem samples during pyrolysis.

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3.2. Effect on pyrite sulfur removal Fig. 2 shows the pyrite sulfur removal and decomposition rate in raw and Dem samples during pyrolysis. For the two samples, the decomposition character of pyrite is almost the same, that is to say, the mineral in coal has little effect on pyrite decomposition during coal pyrolysis. Pyrite remains stable up to 400 jC, then started to loss sulfur quickly and the transformation of pyrite into ferrous sulfide occurred [3,11,13]. At the temperature of 650 jC, about 90% of pyrite in both raw and Dem samples decomposed. From the pyrite decomposition rate shown in Fig. 2, it can be seen that the maximum decomposition rate of pyrite is in the range of 450 to 550 jC and therefore most sulfur removal is observed in these temperatures. 3.3. Effect on organic sulfur removal Fig. 3 shows the removal of organic sulfur from Raw, Dem and Demp samples during pyrolysis as a function of temperature. Below 450 jC, the three samples share the similar trend of organic sulfur removal. This result implies that at low temperature range, organic sulfur decomposition is independent of the presence of inorganic sulfur and inorganic mineral matter in coal. The noticeable increase in the organic sulfur removal at this temperature region is mainly volatile organic sulfur, such as thiol, polysulfide and disulfide [10,14 – 16]. Organic sulfur removal in Demp sample continually increases at the whole pyrolysis temperature region. But for raw and Dem samples, organic sulfur removal begins to decrease, especially at 450 to 550 jC indicating that organic sulfur increases in pyrolytic char and other forms of sulfur might have changed into organic sulfur. Even at 650 jC, the organic sulfur removal is still lower than that at 450 jC, which indicates that the organic sulfur produced from other sulfur forms in pyrolysis is very stable. It might be thiophonic or condensed thiophenic sulfur [14,15].

Fig. 2. Pyrite sulfur removal and decomposition rate of raw and Dem samples during pyrolysis.


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Fig. 3. Organic sulfur removal from raw, Dem and Demp samples during pyrolysis.

Increasing organic sulfur in char of raw and Dem sample during pyrolysis may attribute to two aspects. One is that the sulfur produced from pyrite decomposition could react with organic matrix in coal and form new C –S organic sulfur bond, this can be confirmed by comparing the results between Demp and Dem samples because pyrite is the only difference between them. Another is that coal may produce chars with certain amounts of closed pores [9,17], such closed pores can trap H2S which can subsequently form new complex organic sulfide with surrounding organic matter. The treatment of coal with acid can open some of the closed pores, acid treated coal therefore has less amount of trapped H2S. This may be the reason that the organic sulfur removal of Dem sample is higher than raw sample after 450 jC during pyrolysis. From Fig. 3, it can be seen that the inorganic sulfur in coal changes to organic sulfur is another unfavorable factor that affects efficiency of desulfurization. As to Yima raw coal, only 20% organic sulfur can be removed by pyrolysis at 650 jC. 3.4. Effect on formation of sulfide sulfur As mentioned above, different sulfur forms in coal will convert from one to another and some sulfur will change into sulfide sulfur in the presence of mineral matter during pyrolysis. The proportion of sulfide sulfur formation in char is calculated by the following expression: Ssd;char Wchar  100%: St;sample Wsample where Ssd,char is sulfide sulfur content in the char, St,sample is total sulfur content in the starting sample. Formation of inorganic sulfide sulfur of raw and Dem samples was shown in Fig. 4. Sulfide sulfur from raw sample is much higher than that from Dem sample in the whole

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Fig. 4. Formation of sulfide sulfur of raw and Dem sample during pyrolysis.

pyrolysis temperature. The maximum formation rate of sulfide sulfur appears at temperature between 450 and 550 jC, corresponding to the maximum decomposition rate of pyrite. There are two routes for the formation of sulfide sulfur in char during pyrolysis: (I) decomposition of pyrite to iron sulfide; (II) a result of reactions involving sulfurcontaining gas with alkaline matter in the char [17,18]. Fig. 5 shows the XRD spectra of chars from raw and Dem samples obtained at 650 jC pyrolysis. The figure shows that FeS is the only sulfide in the char of Dem sample, but there are distinct peaks

Fig. 5. X-ray diffraction spectra of chars from raw and Dem samples formed at 650 jC pyrolysis.


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associated with CaS and CaSO4 besides FeS in the char from raw coal. Most of FeS may result from decomposition of pyrite, but CaS can only be attributed to the fixation of H2S by alkaline earth carbonate and alkaline oxides [10,19,20]. Clearly, sulfide sulfur in raw coal char is due to both the above reasons while sulfide sulfur in Dem coal only due to the decomposition of pyrite because of no other minerals present in the Dem coal. Yima coal contains a significant amount of alkali components, mainly CaO, which results in higher sulfide sulfur in char from raw sample than that from Dem sample of coal. There is no pyrite peak appearance in both samples, indicating that almost all pyrite have been decomposed during pyrolysis at 650 jC. Fig. 4 shows also that formation of sulfide might be the most serious problem of desulfurization during pyrolysis, about 25% total sulfur in final char is sulfide sulfur. Thus how to promote decomposition of sulfide and prevent from fixation of H2S by alkaline mineral matter will be one of important issues to improve efficiency of coal desulfurization by pyrolysis.

4. Conclusions That the alkaline matter in coal reacts with sulfur containing gas to form sulfide, pyrite sulfur transforms into organic sulfur and pyrite decomposes into iron sulfide are the main problems hindering the sulfur removal from coal during pyrolysis.

Acknowledgements The work was supported by fund of the National Key Fundamental Research and Development Project of China (G1999022101).

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