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Sulfur transformation during the pyrolysis of coal with the addition of CaSO4 in a fixed-bed reactor
Accepted Manuscript Title: Sulfur transformation during the pyrolysis of coal with the addition of CaSO4 in a fixed-bed reactor Authors: Xin Jia, Qinhui Wang, Long Han, Leming Cheng, Mengxiang Fang, Zhongyang Luo, Kefa Cen PII: DOI: Reference:
S0165-2370(16)30145-0 http://dx.doi.org/doi:10.1016/j.jaap.2017.01.016 JAAP 3940
To appear in:
J. Anal. Appl. Pyrolysis
Received date: Accepted date:
16-3-2016 16-1-2017
Please cite this article as: Xin Jia, Qinhui Wang, Long Han, Leming Cheng, Mengxiang Fang, Zhongyang Luo, Kefa Cen, Sulfur transformation during the pyrolysis of coal with the addition of CaSO4 in a fixed-bed reactor, Journal of Analytical and Applied Pyrolysis http://dx.doi.org/10.1016/j.jaap.2017.01.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sulfur transformation during the pyrolysis of coal with the addition of CaSO4 in a fixed-bed reactor Xin Jiaa, Luoa, a
Qinhui Wang*a,
Long Hanb,
Leming Chenga, Mengxiang Fanga, Zhongyang
Kefa Cena
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering,
Zhejiang University, Hangzhou, 310027, P.R. China b
Institute for Energy and Power Engineering, Zhejiang University of Technology, Chaowang
Road 18, Hangzhou 310014, China * Corresponding author: E-mail address: [email protected], Phone: +86-571-87952802.
1
Highlights (1) Higher H2S and COS yields could be obtained by the presence of CaSO4 at high temperature. (2) CaSO4 could reduce tar sulfur at high temperatures. (3) High proportions of CaSO4 could promote the decomposition of organic sulfur in the char at high temperatures.
(3) A significant increase of sulfide sulfur was observed with the addition of CaSO4 above 600oC.
Abstract: The present paper was devoted to investigate the sulfur transformation during pyrolysis of the mixture of coal and CaSO4 in a fixed bed reactor at a temperature range of 500-800oC. The results indicated that the presence of CaSO4 could promote the evolution of H2S and COS at high temperature, which should be due to the higher decomposition rate of organic sulfur and the reactions between CaSO4 and pyrolysis products, e.g., H2 and CO. Moreover, when the blending ratio(CaSO4/coal) exceeded 20%, CaSO4 could greatly promote the decomposition of organic sulfur at 800 oC. The mechanism of organic sulfur decrease due to CaSO4 was proposed. In comparison of raw coal, sulfide sulfur was significantly increased with the addition of CaSO4 above 600 oC, which should be mainly due to CaS formation through CaSO4 decomposition. Key words: CaSO4, sulfur transformation, coal pyrolysis, H2S, organic sulfur Introduction Coal is the major energy resource in China, and the main utilization of the coal is direct combustion. However, sulfur of the coal gives rise to serious pollution. Based on this, the coal staged conversion process coupling the coal pyrolysis and the residue char combustion is proposed [1-4] and it can obtain high desulfurization efficiency. The principle of the coal staged conversion 2
process is illustrated in Fig.1. High temperature circulating ash from circulating fluidized bed (CFB) boiler as heat carrier is transported to the pyrolyzer to provide heat for coal pyrolysis. The coal is firstly fed to the pyrolyzer to incur pyrolysis by interaction with circulating ash, producing gas, tar and char. The char is then sent to CFB boiler to generate electricity and heat. Thus, it can realize the poly-generation of gas, tar, electricity and heat in this system. What’s more, some sulfur is transformed to the gas and tar during coal pyrolysis and it is easier for sulfur removal from coal gas and tar than that from flue gas, so it can reduce DeSOx burden for coal staged conversion process, compared with direct combustion. The interaction of coal and ash occurs in the pyrolysis of the coal staged conversion process, making it clearly distinguished from the traditional coal pyrolysis process. Since the heat for pyrolysis was provided by high circulating ash, much high temperature circulating ash was needed to satisfy the pyrolysis temperature and the blending ratio of ash to coal usually ranged from 5:1 to 9:1[5]. There were some literature studies on pyrolysis product distribution by mixing coal ash. Liang et al.[5] studied the effect of coal ash on the pyrolysis product, indicating that increasing blending ratios of coal ash to coal could increase gas yield. The investigation of Ran et al. [6]showed that the coal ash could greatly reduce the gas yield. The effect of coal ash on pyrolysis behavior was not clearly understood, which was mainly due to the complex components of the coal ash. Therefore, to reveal the effect of coal ash, it was great necessary to study the effect of main components of coal ash on the pyrolysis. It was likely to predict the pyrolysis behavior based on the composition of coal ash and by understanding how the main components of coal ash affected pyrolysis behavior. The coal ash was made up of SiO2, Fe2O3, CaSO4, etc. As the first step, the sulfur behavior of the mixture of coal and CaSO4 was studied. Since CaO or CaSO3 was 3
widely used for desulfurization in CFB combustion, the sulfur of coal is mainly captured in the ash as a form of CaSO4 and the proportion of CaSO4 in the ash might be relatively high( Example: CaSO4 content in XLT lignite used in this paper was 34% by weight). Considering the blending ratio of coal ash to coal and CaSO4 content in the coal ash, the proportion of CaSO4 to coal might be rather high. CaSO4was proved to be a promising oxygen carrier candidate for chemical looping process, so a lot of studies had been carried out about CaSO4 decomposition under reducing atmosphere, e.g., H2 and CO. Sulfur-containing gas, like SO2, H2S and COS, were also released in this process. The possible reactions were presented as Eqs 1-6 [7]. The majority of literature on CaSO4 decomposition by reducing gases mainly focused at high temperatures (usually above 850 oC). The study about CaSO4 decomposition by reducing gas at pyrolysis temperature (usually 500-800 o
C) and CaSO4 behavior during coal pyrolysis was less. It was reported that most CaSO4 can be
converted to CaS at 800oC during coal pyrolysis through solid-solid reaction mechanism (Eqs 7 and 8) and inherent minerals of the coal could promote CaSO4 decomposition in our previous study [8]. Setyawati et al. [9] studied sulfate behavior in the pyrolysis of the mixture of pure CaSO4 and lignite free of minerals, finding that about 36% CaSO4 was decomposed. However, the reports about how CaSO4 affected sulfur behavior in the coal pyrolysis were rarely seen. 4H2+CaSO4 → CaS +4H2O 4H2+CaSO4 → CaO+ H2S + 3H2O H2+CaSO4 → CaO+ SO2 + H2O 4CO + CaSO4 → CaS +4 CO2 CO+CaSO4 →CaO + CO2 +SO2
(1) (2) (3) (4) (5)
4CO+CaSO4 →CaO + 3CO2 +COS
(6)
2C+CaSO4 → CaS +2CO2 4C+CaSO4 → CaO +3CO2 + COS
(7) (8)
4
Therefore, the main aim of this paper was to give a comprehensive investigation of sulfur behavior during co-pyrolysis of coal and CaSO4. It included sulfur behavior of both coal and CaSO4 during pyrolysis.
2. Experimental 2.1sample The coal used in this study was Xiaolongtan lignite, the coal was ground and sieved with a particle size less than 0.15 mm. The proximate analysis, ultimate analysis, sulfur forms and ash constituents are summarized in Table 1 and Table 2. Powdered CaSO4 used in this study was purchased from Sinopharm, with a particle size of less than 1 mm and >97% purity. The coal was dried at 105 oC for 12 h and CaSO4 was dried at 200 oC for 6 h. Prior to the experiment, 8 g coal and a certain proportion of CaSO4 were placed in a sealed container and the sealed container was shaken violently for 10 min, and in this case the coal and CaSO4 could be mixed well. The proportions of CaSO4 / coal used in this study were 10%, 50%, 100% by weight. 2.2 Apparatus and procedure The pyrolysis experiments were carried out in a normal horizontal tube furnace. Fig. 2 shows the schematic flow diagram of experimental unit. The pyrolyzer was a quartz tube of 40 mm i.d and 600 mm long. Prior to each run, 200 ml/min N2 was introduced to purge the system for 40 min. 8 g coal with a certain proportion of CaSO4 was uniformly spread in a quartz contained and placed in the center of the tube furnace and then heated at 20 oC/min to the desired temperatures (500, 600, 700 and 800 oC) and kept for 30 min under 200 ml/min N2 atmosphere. The liquid products were captured by a thimble filter in a tar trap cooled by a bath of ice/ethanol and the cold trap and 5
connection line was washed with acetone. The gas was collected in a gas bag and measured by a gas chromatography equipped with flame photometric detector. Considering the nitrogen produced from coal pyrolysis was insignificant compared with nitrogen flow rate in the carrier gas, VN2
N2 tracing method was used for the calculation of gases yields: V=
CN2
, where VN2 represented
the total volume of carrier gas (N2) introduced to this system during the whole experiment, CN2 represented the concentration of N2. The sulfur content of tar was determined by SN analyzer (KY-3000SN), which could measure the sulfur and nitrogen content for liquid samples. Different forms of sulfur were analyzed according to GB/T 215-2003. The Gladfelter and Dickerhoof method [10] was used to analyze the forms of sulfur in char.
3. Results and discussions CaSO4 itself was a form of sulfur and the other forms of sulfur, like sulfurous sulfur and sulfide sulfur, might be not only from the coal, but also from CaSO4. In addition, the total sulfur fed in each experiment for different proportions of CaSO4 also showed huge differences. Considering the above, it was unreasonable to represent different forms of sulfur using the relative proportion (%) like other literatures. The absolute sulfur amount of different forms was used to show sulfur transformation in this paper. The absolute sulfur content was 206.3 mg, 394.5 mg, 1147.3 mg and 2088.5 mg when the proportions of CaSO4 were 0, 10%, 50% and 100% respectively. 3.1 Sulfurous gas evolution The yield of H2S by the both presence and absence of CaSO4 is shown in Fig. 3. During pyrolysis of raw coal, the evolution of H2S was mainly occurred below 600 oC, which was mainly 6
due to the decomposition of pyrite and non-stable organic sulfur [11,12]. Above 600 oC, the release of H2S was quite tiny, which was mainly due to the fixation of alkaline mineral matter [13,14].The effect of CaSO4 on H2S depended greatly on the temperature. In other words, H2S yield appeared to be little changed with the addition of CaSO4 at low temperatures below 600 oC. However, at high temperatures above 600 oC, H2S was monotonically increased with increasing the proportions of CaSO4. For example, the yield of H2S was increased from 45.7 mg for raw coal to 51.8 mg for 100% CaSO4 at 800 oC. This suggested the presence of CaSO4 facilitated H2S release at high temperatures. The increase of H2S might be not only derived from coal through the decomposition of the other forms of sulfur, but also derived from CaSO4 through the reactions between CaSO4 and pyrolysis gas, i.e., H2 and CH4 [7,15,16]. It was proven that a higher organic sulfur decomposition rate was observed due to the addition of CaSO4 (see below), so this might be the possible reason that caused an increase of H2S, for it was well recognized that organic sulfur decomposition was an important source for H2S. To confirm whether H2S could be produced through the reactions between pyrolyzed gas and CaSO4, 8 g pure CaSO4 was heated at 20 oC/min from room temperature to the desired temperatures (500, 600, 700 and 800 oC) and kept for 30 min under simulated pyrolyzed gas atmosphere (i.e. 25% H2, 25% CO, 25% CO2 and 25% CH4 by volume). The release of H2S and COS are shown in Table 3. It could be seen the release of H2S was almost negligible below 600 oC, however, much H2S was formed above 600 oC, which indicated that Eq. 2 could take place above 600 oC. The amount of H2S produced through Eq. 2 was 22.685 mg at 800 oC (Table 3), while the total H2S content for raw coal was 45.2 mg during pyrolysis. This meant Eq. 2 played an important part in H2S evolution. Only about 1.13% CaSO4 was converted to H2S during the reactions between simulating pyrolysis gas and CaSO4 at 800 oC. 7
Although Eq.2 only played a little role in CaSO4 decomposition, it could greatly change H2S evolution. It was because the sulfur in CaSO4 was far more than the amount of H2S. Therefore, we could believe that both the organic decomposition and the reactions between CaSO4 and volatile matter were responsible for the H2S increase. The variation of COS as a function of temperatures and proportions of CaSO4 is shown in Fig.4. During the pyrolysis of raw coal, the main evolution of COS occurred at low temperatures below 600 oC [14]. It was well known that COS was mainly derived from the direct decomposition of some organic sulfur of coal, the reactions of sulfur from decomposition of pyrite and CO or CO2, and the secondary reactions between H2S and CO or CO2 [17]. The effect of CaSO4 on COS was quite similar to that on H2S, namely, CaSO4 had little effect on COS at low temperatures, but could significantly promote COS evolution at 800 oC. The yield of COS for 100% CaSO4 (5.28 mg) was even twice that for raw coal (2.61 mg) for raw coal. Apart from the three routes above mentioned, Eqs. 6 and 8 might also be important sources for producing COS at high temperatures by the presence of CaSO4 [7,15,16]. About 1.092 mg COS was produced for pure CaSO4 under simulating pyrolysis gas atmosphere (Table 3), which indicated that Eq. 6 could take place, thus enhancing COS yield. To study the relationship of Eq. 8 and the increase of COS, the char (obtained as follows: the coal was pyrolyzed at 900 oC for 30 min and it turned out that almost no gas was produced during pyrolysis of the char) was pyrolyzed at 800 oC with the addition of CaSO4 to measure the COS release. The result showed that about 0.97 mg COS was produced during the pyrolysis of the mixture of char and CaSO4. This suggested that Eq. 8 could take place, which was responsible for COS release in this case, because Eq. 6 could hardly take place due to the lack of CO. What’s more, CaSO4 could promote organic sulfur decomposition 8
and it was well recognized that the direct decomposition of organic sulfur was an important source for COS. So the decomposition of organic sulfur due to CaSO4 also should be responsible for COS increase. In a summary, the increase of COS at high temperature was not only derived from the coal though the organic sulfur decomposition, but also derived from CaSO4 through the reactions between CaSO4 and pyrolysis gas or carbon of the char. 3.2 Tar sulfur Fig. 5 compares tar sulfur in the presence and absence of CaSO4. Tar sulfur seemed to be less affected at low temperatures (below 600 oC). However, high proportions of CaSO4 (50% and 100% CaSO4) could reduce tar sulfur above 600 oC. On one hand, CaO could be formed in the process of CaSO4 decomposition at high temperatures through Eqs 2, 6 and 8. Many studies showed that CaO could catalyze the decomposition of tar sulfur, thus reducing tar sulfur [13,18]. On the other hand, CaSO4 could enhance tar sulfur decomposition. The mechanism of organic sulfur decrease in the tar might be also similar to that in the char, for the basic properties of organic sulfur in the tar were similar to organic sulfur in the char. The mechanism of the organic sulfur decrease in the char would be discussed in detail later. 3.3 Sulfur release during the pyrolysis of coal and CaSO4 It was difficult to compare sulfur removal of the coal with different proportions of CaSO4 by measuring the sulfur content of the mixture. Since the sulfur of the mixture during the pyrolysis was converted to gaseous sulfur and tar sulfur, sulfur release also could reflect the sulfur content of the mixture after pyrolysis according to sulfur equilibrium and sulfur release could be determined as follows: Sulfur release=nH2S + nCOS + +ntar−sulfur 9
Where nH2S, nCOS and ntar-sulfur represented the total amount of H2S, COS and tar sulfur produced in the whole experiment respectively. Fig. 6 compares sulfur release of the coal with different proportions of CaSO4. It could be seen that the effect of CaSO4 on sulfur release was greatly dependent on the temperatures. At low temperatures (< 600 oC), sulfur release was little-changed with the addition of CaSO4. However, when the temperature continued to rise (>600 oC), higher sulfur release was obtained by the presence of CaSO4. Elevating temperature and the proportion of CaSO4 was beneficial to desulfurization for the mixture of coal and CaSO4. CaSO4 went decomposition through Eqs. 1-8 during pyrolysis, H2S and COS were produced in this process. This will enhance sulfur release, but this part was mainly derived from CaSO4 rather than coal. In addition, CaSO4 could greatly enhance organic sulfur decomposition, although most organic sulfur was converted to sulfide sulfur, some organic sulfur could be transferred to the gas, thereby increasing sulfur release of the coal, which would be discussed below. 3.4. Pyritic sulfur distribution Fig. 7 displays the data of pyritic sulfur in the presence and absence of CaSO4. It was observed that almost all of pyrite was fully decomposed at 600 oC in all cases and pyritic sulfur was not detected above 600 oC [19]. It was also noted that there was almost no change in pyritic sulfur distribution between raw coal and different proportions of CaSO4. This meant the effect of CaSO4 on pyrite sulfur was quite small. It was reported that CaSO4 began to decompose above 600 oC [8], whereas pyritic had been fully decomposed in this temperature range. So it was easily understood that CaSO4 did not greatly affect the distribution of pyritic sulfur. 3.5. Organic sulfur distribution in the char 10
Fig. 8 shows the variation of organic sulfur in the char with temperatures and blending ratios. It could be seen that organic sulfur showed a remarkable increase with the temperature from 500 o
C to 700 oC. It was well recognized that the pyrite sulfur could be converted to organic sulfur
during the pyrolysis [20] and the secondary reactions between H2S and organic matrix also could generate new organic sulfur. Therefore, the increase of organic sulfur was mainly because H2S generated in the pyrolysis (including from the pyritic sulfur decomposition) was captured by the active organic matrix to form new organic sulfur. Above 700 oC, organic sulfur began to decrease, but the decrease was not remarkable for raw coal. This was mainly because the organic sulfur was very stable (mainly existed in thiophene form) and did not have an apparent decomposition even at high temperatures. The organic sulfur appeared to be scarcely affected by the presence of CaSO4 at 500-700 oC. The effect of CaSO4 on organic sulfur at 800 oC was strongly dependent on the blending ratios (CaSO4 / coal). A low proportion of CaSO4 did not cause a significant change of organic sulfur. However, a significant decrease of organic sulfur was observed in the case of 50% CaSO4 and 100% CaSO4. The content of organic sulfur was significantly declined from 63.86 mg for raw coal to 25.11 mg for 50% CaSO4 to 21.66 mg for 100% CaSO4.This suggested only two conditions (high temperatures and high ratios of CaSO4/coal) were fulfilled, could organic sulfur be effectively removed. The results were fairly interesting, because stable organic sulfur was difficult to remove and such high removal rate for organic sulfur was rarely obtained. Considering blending ratio was an important parameter influencing organic sulfur removal, the variation of organic decomposition rate with more blending ratios was further determined. The results showed that the content of organic sulfur declined significantly from 59.62 mg to 25.93 mg with blending ratio increasing 11
from 20% to 30% CaSO4. This meant that organic sulfur reduced sharply when the ratio (CaSO4 / coal) exceeded 20%. To investigate the role of pyrolyzed gas and tar in reducing organic sulfur, the char obtained at 900 oC with 100% CaSO4 was pyrolyzed. The results showed that the organic sulfur for char was 57.39 mg and that for the mixture of char and CaSO4 was only 15.21 mg. The decrease of organic sulfur for raw coal (42.2 mg) was approximately the same with the char (42.18 mg) by the presence of CaSO4, which indicated that the decrease of organic sulfur with the addition of CaSO4 was mainly due to the char rather than pyrolyzed gas and tar. It had been reported that the carbon in the coal was responsible for CaSO4 decomposition in our previous study and the carbon in the char was remarkable reduced with the addition of high proportions of CaSO4 at 800 oC and the total carbon content declined from 0.236 mol for raw coal to 0.186 mol for 50%CaSO4 to 0.156 mol for 100% CaSO4, which was mainly due to Eqs 7 and 8 [8]. It was likely that CaSO4 could affect organic sulfur by changing the carbon content of the char. Since the S atom was bond to carbon atoms in organic sulfur and the organic sulfur depended heavily on the carbon atoms of the organic sulfur of the coal, when the carbons of organic sulfur were oxidized by CaSO4, the organic sulfur might be affected. Suppose that the blending ratio of CaSO4 was high enough and all carbon in the char was oxidized by CaSO4, organic sulfur would not exist in the char. When the carbon in the tar and the char reacted with CaSO4, C-C bonds were broken. When the carbon of the organic sulfur in the char and the tar reacted with CaSO4, the breakage of C-S bonds occurred simultaneously along with the cleavage of C-C bonds in organic sulfur of the char and the tar. The result obtained in this study was quite similar to that obtained by Liu [21]. He investigated sulfur behavior under 2% O2 atmosphere and noticed that not only C-C bonds but 12
also C-S bonds in the char was broken by oxygen, thus increasing desulfurization ratio especially in organic sulfur removal. In liu’s study, C-C bonds in the char were broken by O2, whereas in this study they were broken by CaSO4. Therefore, the mechanism of organic sulfur decreased in this study was similar to that in Liu’s study, that’s, C-S bonds of organic sulfur was broken when C-C bonds of organic sulfur were broken by CaSO4 or O2, as a result, organic sulfur was considerably
reduced by high proportions of CaSO4.
and
were used to typify aromatic
sulfur and thiophene in the tar and in the char in this paper. It could be believed that the possible mechanism of organic sulfur decrease due to CaSO4 might be expressed as follows:
·S· radical might be produced in the process of organic sulfur decomposition and it could either react with H· to form H2S or combine with inherent minerals to form sulfide sulfur. It was observed that the main product of the organic sulfur decomposition was sulfide sulfur instead of H2S. The decrease of organic sulfur due to CaSO4 was about 42.17 mg, whereas the increase of H2S due to CaSO4 was only 6.1 mg. This demonstrated that most organic sulfur was converted to sulfide sulfur and only a small proportion of organic sulfur was transformed to H2S. This was mainly because H2S due to organic sulfur decomposition was also fixed by the alkaline mineral matter, which was quite similar to the results obtained at 800 oC during pyrolysis of raw coal, that’s, almost no H2S was released at high temperatures due to the fixation of mineral matters. A tiny proportion of organic sulfur also could be converted to COS. 13
Higher active carbon compounds of the coal with lower bond energy was more likely to react with CaSO4 compared with less higher active carbon. When the ratio of CaSO4 was below 20%, organic sulfur was little-changed. This indicated there must have been other compounds with higher reactivity of reacting with CaSO4 than aromatic sulfur and thiophene in the coal, and they would react with CaSO4 first. However, when the compounds with higher reactivity was consumed with increasing the proportion of CaSO4, the organic sulfur in the char, like aromatic sulfur and thiophene, were likely to react with CaSO4, thus significantly reducing organic sulfur. This might provide a reasonble explanation for why only high proportions of CaSO4 could promote the removal of organic sulfur.
3.6 Sulfide sulfur distribution The sulfide sulfur distribution with the addition of different proportions of CaSO4 is shown in Fig. 9. It could be seen that sulfide sulfur was greatly dependent on the temperatures and the proportions of CaSO4. During the pyrolysis of raw coal, as the temperature rose, sulfide sulfur presented a fast increase. Sulfide sulfur distribution increased from 27.89 mg to 86.17 mg with elevating temperature from 500 oC to 800 oC. As many literature observed, sulfide sulfur for raw coal was mainly derived from two aspects: (I) decomposition of pyrite to iron sulfide; (II) the secondary reactions between sulfur containing gas and alkaline matter in the char [22]. The presence of CaSO4 had little effect on sulfide sulfur at low temperatures, but could remarkably enhance sulfide sulfur formation above 600 oC. Sulfide sulfur was increased from 86.17 mg for raw coal to 1456.92 mg for 100% CaSO4. This was mainly due to CaS formation through CaSO4 decomposition. It was proved that CaSO4 began to decompose above 600 oC and most CaSO4 was 14
converted to CaS at 800 oC [8], which could explain why a significant increase of sulfide sulfur could be obtained by the presence of CaSO4. In addition, most organic sulfur was converted to sulfide sulfur at 800 oC, which had been mentioned previously. This also caused an increase of sulfide sulfur to some extent.
Conclusions: Coal with different proportions of CaSO4 was pyrolyzed in a fixed-bed reactor to study the sulfur transformation during pyrolysis of the mixture of coal and CaSO4. The following conclusions could be drawn: (1) Higher H2S and COS yields could be obtained by the presence of CaSO4 at high temperature. This was mainly because H2S and COS could be produced through the reactions: 4H2+CaSO4 → CaO+ H2S + 3H2O, 2C+CaSO4 → CaO+ CO2+COS and 4CO+CaSO4 → CaO + 3CO2 +COS. What’s more, the decomposition of organic sulfur was also responsible for the increase of H2S and COS. (2) High proportions of CaSO4 could promote the decomposition of organic sulfur both in the tar and in the char at high temperatures. This was because C-S bonds are broken of stable organic sulfurs in tar and char along with breaking C-C bonds of organic sulfur when the carbon reacted with CaSO4, thus reducing the organic sulfur in the tar and in the char. (3) A significant increase of sulfide sulfur was observed with the addition of CaSO4 above 600 oC, which was mainly because a great deal of CaS was formed due to CaSO4 decomposition.
Acknowledgements The authors appreciate financial supports by U.S.-China Clean Energy Research Center on Advanced Coal Technology Consortium (2016YFE0102500 ) and the National Nature Science Foundation of China (51276160).
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16
Fig. 1.The principle of the coal staged conversion process
Fig. 2. Schematic of the experimental apparatus
17
Fig.3. Effect of CaSO4 on H2S distribution
Fig. 4. Effect of CaSO4 on COS distribution
18
Fig. 5. Effect of CaSO4 on tar sulfur distribution
Fig. 6. Effect of CaSO4 on sulfur release
19
Fig. 7. Effect of CaSO4 on pyrite sulfur distribution
Fig. 8. Effect of CaSO4 on organic sulfur distribution
20
Fig. 9. Effect of CaSO4 on sulfide sulfur distribution
21
Table 1. Main characteristics of samples Sample
XLT a, *
Proximate analysis/wt%, ad
Ultimate analysis, wt%, ad
M
V
A
Fc
C
H
N
Oa
2.68
44.02
18.19
35.11
48.72
4.64
1.24
22.02
Sulfur forms, wt%, ad
St
Ss
Sp
So*
2.51
0.83
0.32
1.36
by difference ;St, total sulfur; Sp, pyritic sulfur; Ss, sulfate sulfur; and So, organic sulfur
Table 2. Analysis of ash compositions in raw coals (wt%) Compound
SiO2
Fe2O3
Al2O3
CaO
MgO
K2O
Na2O
SO3
Othersa
XLT
33.47
8.09
12.04
19.11
3.42
0.76
0.16
20.01
2.94
a
by difference Table 3. The release of H2S and COS for pure CaSO4 under simulated pyrolyzed gas atmosphere Temperature(oC)
500
600
700
800
H2S(mg) COS(mg)
0.084 0.078
0.121 0.11
15.052 0.744
22.685 1.092
22
S0165-2370(16)30145-0 http://dx.doi.org/doi:10.1016/j.jaap.2017.01.016 JAAP 3940
To appear in:
J. Anal. Appl. Pyrolysis
Received date: Accepted date:
16-3-2016 16-1-2017
Please cite this article as: Xin Jia, Qinhui Wang, Long Han, Leming Cheng, Mengxiang Fang, Zhongyang Luo, Kefa Cen, Sulfur transformation during the pyrolysis of coal with the addition of CaSO4 in a fixed-bed reactor, Journal of Analytical and Applied Pyrolysis http://dx.doi.org/10.1016/j.jaap.2017.01.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sulfur transformation during the pyrolysis of coal with the addition of CaSO4 in a fixed-bed reactor Xin Jiaa, Luoa, a
Qinhui Wang*a,
Long Hanb,
Leming Chenga, Mengxiang Fanga, Zhongyang
Kefa Cena
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering,
Zhejiang University, Hangzhou, 310027, P.R. China b
Institute for Energy and Power Engineering, Zhejiang University of Technology, Chaowang
Road 18, Hangzhou 310014, China * Corresponding author: E-mail address: [email protected], Phone: +86-571-87952802.
1
Highlights (1) Higher H2S and COS yields could be obtained by the presence of CaSO4 at high temperature. (2) CaSO4 could reduce tar sulfur at high temperatures. (3) High proportions of CaSO4 could promote the decomposition of organic sulfur in the char at high temperatures.
(3) A significant increase of sulfide sulfur was observed with the addition of CaSO4 above 600oC.
Abstract: The present paper was devoted to investigate the sulfur transformation during pyrolysis of the mixture of coal and CaSO4 in a fixed bed reactor at a temperature range of 500-800oC. The results indicated that the presence of CaSO4 could promote the evolution of H2S and COS at high temperature, which should be due to the higher decomposition rate of organic sulfur and the reactions between CaSO4 and pyrolysis products, e.g., H2 and CO. Moreover, when the blending ratio(CaSO4/coal) exceeded 20%, CaSO4 could greatly promote the decomposition of organic sulfur at 800 oC. The mechanism of organic sulfur decrease due to CaSO4 was proposed. In comparison of raw coal, sulfide sulfur was significantly increased with the addition of CaSO4 above 600 oC, which should be mainly due to CaS formation through CaSO4 decomposition. Key words: CaSO4, sulfur transformation, coal pyrolysis, H2S, organic sulfur Introduction Coal is the major energy resource in China, and the main utilization of the coal is direct combustion. However, sulfur of the coal gives rise to serious pollution. Based on this, the coal staged conversion process coupling the coal pyrolysis and the residue char combustion is proposed [1-4] and it can obtain high desulfurization efficiency. The principle of the coal staged conversion 2
process is illustrated in Fig.1. High temperature circulating ash from circulating fluidized bed (CFB) boiler as heat carrier is transported to the pyrolyzer to provide heat for coal pyrolysis. The coal is firstly fed to the pyrolyzer to incur pyrolysis by interaction with circulating ash, producing gas, tar and char. The char is then sent to CFB boiler to generate electricity and heat. Thus, it can realize the poly-generation of gas, tar, electricity and heat in this system. What’s more, some sulfur is transformed to the gas and tar during coal pyrolysis and it is easier for sulfur removal from coal gas and tar than that from flue gas, so it can reduce DeSOx burden for coal staged conversion process, compared with direct combustion. The interaction of coal and ash occurs in the pyrolysis of the coal staged conversion process, making it clearly distinguished from the traditional coal pyrolysis process. Since the heat for pyrolysis was provided by high circulating ash, much high temperature circulating ash was needed to satisfy the pyrolysis temperature and the blending ratio of ash to coal usually ranged from 5:1 to 9:1[5]. There were some literature studies on pyrolysis product distribution by mixing coal ash. Liang et al.[5] studied the effect of coal ash on the pyrolysis product, indicating that increasing blending ratios of coal ash to coal could increase gas yield. The investigation of Ran et al. [6]showed that the coal ash could greatly reduce the gas yield. The effect of coal ash on pyrolysis behavior was not clearly understood, which was mainly due to the complex components of the coal ash. Therefore, to reveal the effect of coal ash, it was great necessary to study the effect of main components of coal ash on the pyrolysis. It was likely to predict the pyrolysis behavior based on the composition of coal ash and by understanding how the main components of coal ash affected pyrolysis behavior. The coal ash was made up of SiO2, Fe2O3, CaSO4, etc. As the first step, the sulfur behavior of the mixture of coal and CaSO4 was studied. Since CaO or CaSO3 was 3
widely used for desulfurization in CFB combustion, the sulfur of coal is mainly captured in the ash as a form of CaSO4 and the proportion of CaSO4 in the ash might be relatively high( Example: CaSO4 content in XLT lignite used in this paper was 34% by weight). Considering the blending ratio of coal ash to coal and CaSO4 content in the coal ash, the proportion of CaSO4 to coal might be rather high. CaSO4was proved to be a promising oxygen carrier candidate for chemical looping process, so a lot of studies had been carried out about CaSO4 decomposition under reducing atmosphere, e.g., H2 and CO. Sulfur-containing gas, like SO2, H2S and COS, were also released in this process. The possible reactions were presented as Eqs 1-6 [7]. The majority of literature on CaSO4 decomposition by reducing gases mainly focused at high temperatures (usually above 850 oC). The study about CaSO4 decomposition by reducing gas at pyrolysis temperature (usually 500-800 o
C) and CaSO4 behavior during coal pyrolysis was less. It was reported that most CaSO4 can be
converted to CaS at 800oC during coal pyrolysis through solid-solid reaction mechanism (Eqs 7 and 8) and inherent minerals of the coal could promote CaSO4 decomposition in our previous study [8]. Setyawati et al. [9] studied sulfate behavior in the pyrolysis of the mixture of pure CaSO4 and lignite free of minerals, finding that about 36% CaSO4 was decomposed. However, the reports about how CaSO4 affected sulfur behavior in the coal pyrolysis were rarely seen. 4H2+CaSO4 → CaS +4H2O 4H2+CaSO4 → CaO+ H2S + 3H2O H2+CaSO4 → CaO+ SO2 + H2O 4CO + CaSO4 → CaS +4 CO2 CO+CaSO4 →CaO + CO2 +SO2
(1) (2) (3) (4) (5)
4CO+CaSO4 →CaO + 3CO2 +COS
(6)
2C+CaSO4 → CaS +2CO2 4C+CaSO4 → CaO +3CO2 + COS
(7) (8)
4
Therefore, the main aim of this paper was to give a comprehensive investigation of sulfur behavior during co-pyrolysis of coal and CaSO4. It included sulfur behavior of both coal and CaSO4 during pyrolysis.
2. Experimental 2.1sample The coal used in this study was Xiaolongtan lignite, the coal was ground and sieved with a particle size less than 0.15 mm. The proximate analysis, ultimate analysis, sulfur forms and ash constituents are summarized in Table 1 and Table 2. Powdered CaSO4 used in this study was purchased from Sinopharm, with a particle size of less than 1 mm and >97% purity. The coal was dried at 105 oC for 12 h and CaSO4 was dried at 200 oC for 6 h. Prior to the experiment, 8 g coal and a certain proportion of CaSO4 were placed in a sealed container and the sealed container was shaken violently for 10 min, and in this case the coal and CaSO4 could be mixed well. The proportions of CaSO4 / coal used in this study were 10%, 50%, 100% by weight. 2.2 Apparatus and procedure The pyrolysis experiments were carried out in a normal horizontal tube furnace. Fig. 2 shows the schematic flow diagram of experimental unit. The pyrolyzer was a quartz tube of 40 mm i.d and 600 mm long. Prior to each run, 200 ml/min N2 was introduced to purge the system for 40 min. 8 g coal with a certain proportion of CaSO4 was uniformly spread in a quartz contained and placed in the center of the tube furnace and then heated at 20 oC/min to the desired temperatures (500, 600, 700 and 800 oC) and kept for 30 min under 200 ml/min N2 atmosphere. The liquid products were captured by a thimble filter in a tar trap cooled by a bath of ice/ethanol and the cold trap and 5
connection line was washed with acetone. The gas was collected in a gas bag and measured by a gas chromatography equipped with flame photometric detector. Considering the nitrogen produced from coal pyrolysis was insignificant compared with nitrogen flow rate in the carrier gas, VN2
N2 tracing method was used for the calculation of gases yields: V=
CN2
, where VN2 represented
the total volume of carrier gas (N2) introduced to this system during the whole experiment, CN2 represented the concentration of N2. The sulfur content of tar was determined by SN analyzer (KY-3000SN), which could measure the sulfur and nitrogen content for liquid samples. Different forms of sulfur were analyzed according to GB/T 215-2003. The Gladfelter and Dickerhoof method [10] was used to analyze the forms of sulfur in char.
3. Results and discussions CaSO4 itself was a form of sulfur and the other forms of sulfur, like sulfurous sulfur and sulfide sulfur, might be not only from the coal, but also from CaSO4. In addition, the total sulfur fed in each experiment for different proportions of CaSO4 also showed huge differences. Considering the above, it was unreasonable to represent different forms of sulfur using the relative proportion (%) like other literatures. The absolute sulfur amount of different forms was used to show sulfur transformation in this paper. The absolute sulfur content was 206.3 mg, 394.5 mg, 1147.3 mg and 2088.5 mg when the proportions of CaSO4 were 0, 10%, 50% and 100% respectively. 3.1 Sulfurous gas evolution The yield of H2S by the both presence and absence of CaSO4 is shown in Fig. 3. During pyrolysis of raw coal, the evolution of H2S was mainly occurred below 600 oC, which was mainly 6
due to the decomposition of pyrite and non-stable organic sulfur [11,12]. Above 600 oC, the release of H2S was quite tiny, which was mainly due to the fixation of alkaline mineral matter [13,14].The effect of CaSO4 on H2S depended greatly on the temperature. In other words, H2S yield appeared to be little changed with the addition of CaSO4 at low temperatures below 600 oC. However, at high temperatures above 600 oC, H2S was monotonically increased with increasing the proportions of CaSO4. For example, the yield of H2S was increased from 45.7 mg for raw coal to 51.8 mg for 100% CaSO4 at 800 oC. This suggested the presence of CaSO4 facilitated H2S release at high temperatures. The increase of H2S might be not only derived from coal through the decomposition of the other forms of sulfur, but also derived from CaSO4 through the reactions between CaSO4 and pyrolysis gas, i.e., H2 and CH4 [7,15,16]. It was proven that a higher organic sulfur decomposition rate was observed due to the addition of CaSO4 (see below), so this might be the possible reason that caused an increase of H2S, for it was well recognized that organic sulfur decomposition was an important source for H2S. To confirm whether H2S could be produced through the reactions between pyrolyzed gas and CaSO4, 8 g pure CaSO4 was heated at 20 oC/min from room temperature to the desired temperatures (500, 600, 700 and 800 oC) and kept for 30 min under simulated pyrolyzed gas atmosphere (i.e. 25% H2, 25% CO, 25% CO2 and 25% CH4 by volume). The release of H2S and COS are shown in Table 3. It could be seen the release of H2S was almost negligible below 600 oC, however, much H2S was formed above 600 oC, which indicated that Eq. 2 could take place above 600 oC. The amount of H2S produced through Eq. 2 was 22.685 mg at 800 oC (Table 3), while the total H2S content for raw coal was 45.2 mg during pyrolysis. This meant Eq. 2 played an important part in H2S evolution. Only about 1.13% CaSO4 was converted to H2S during the reactions between simulating pyrolysis gas and CaSO4 at 800 oC. 7
Although Eq.2 only played a little role in CaSO4 decomposition, it could greatly change H2S evolution. It was because the sulfur in CaSO4 was far more than the amount of H2S. Therefore, we could believe that both the organic decomposition and the reactions between CaSO4 and volatile matter were responsible for the H2S increase. The variation of COS as a function of temperatures and proportions of CaSO4 is shown in Fig.4. During the pyrolysis of raw coal, the main evolution of COS occurred at low temperatures below 600 oC [14]. It was well known that COS was mainly derived from the direct decomposition of some organic sulfur of coal, the reactions of sulfur from decomposition of pyrite and CO or CO2, and the secondary reactions between H2S and CO or CO2 [17]. The effect of CaSO4 on COS was quite similar to that on H2S, namely, CaSO4 had little effect on COS at low temperatures, but could significantly promote COS evolution at 800 oC. The yield of COS for 100% CaSO4 (5.28 mg) was even twice that for raw coal (2.61 mg) for raw coal. Apart from the three routes above mentioned, Eqs. 6 and 8 might also be important sources for producing COS at high temperatures by the presence of CaSO4 [7,15,16]. About 1.092 mg COS was produced for pure CaSO4 under simulating pyrolysis gas atmosphere (Table 3), which indicated that Eq. 6 could take place, thus enhancing COS yield. To study the relationship of Eq. 8 and the increase of COS, the char (obtained as follows: the coal was pyrolyzed at 900 oC for 30 min and it turned out that almost no gas was produced during pyrolysis of the char) was pyrolyzed at 800 oC with the addition of CaSO4 to measure the COS release. The result showed that about 0.97 mg COS was produced during the pyrolysis of the mixture of char and CaSO4. This suggested that Eq. 8 could take place, which was responsible for COS release in this case, because Eq. 6 could hardly take place due to the lack of CO. What’s more, CaSO4 could promote organic sulfur decomposition 8
and it was well recognized that the direct decomposition of organic sulfur was an important source for COS. So the decomposition of organic sulfur due to CaSO4 also should be responsible for COS increase. In a summary, the increase of COS at high temperature was not only derived from the coal though the organic sulfur decomposition, but also derived from CaSO4 through the reactions between CaSO4 and pyrolysis gas or carbon of the char. 3.2 Tar sulfur Fig. 5 compares tar sulfur in the presence and absence of CaSO4. Tar sulfur seemed to be less affected at low temperatures (below 600 oC). However, high proportions of CaSO4 (50% and 100% CaSO4) could reduce tar sulfur above 600 oC. On one hand, CaO could be formed in the process of CaSO4 decomposition at high temperatures through Eqs 2, 6 and 8. Many studies showed that CaO could catalyze the decomposition of tar sulfur, thus reducing tar sulfur [13,18]. On the other hand, CaSO4 could enhance tar sulfur decomposition. The mechanism of organic sulfur decrease in the tar might be also similar to that in the char, for the basic properties of organic sulfur in the tar were similar to organic sulfur in the char. The mechanism of the organic sulfur decrease in the char would be discussed in detail later. 3.3 Sulfur release during the pyrolysis of coal and CaSO4 It was difficult to compare sulfur removal of the coal with different proportions of CaSO4 by measuring the sulfur content of the mixture. Since the sulfur of the mixture during the pyrolysis was converted to gaseous sulfur and tar sulfur, sulfur release also could reflect the sulfur content of the mixture after pyrolysis according to sulfur equilibrium and sulfur release could be determined as follows: Sulfur release=nH2S + nCOS + +ntar−sulfur 9
Where nH2S, nCOS and ntar-sulfur represented the total amount of H2S, COS and tar sulfur produced in the whole experiment respectively. Fig. 6 compares sulfur release of the coal with different proportions of CaSO4. It could be seen that the effect of CaSO4 on sulfur release was greatly dependent on the temperatures. At low temperatures (< 600 oC), sulfur release was little-changed with the addition of CaSO4. However, when the temperature continued to rise (>600 oC), higher sulfur release was obtained by the presence of CaSO4. Elevating temperature and the proportion of CaSO4 was beneficial to desulfurization for the mixture of coal and CaSO4. CaSO4 went decomposition through Eqs. 1-8 during pyrolysis, H2S and COS were produced in this process. This will enhance sulfur release, but this part was mainly derived from CaSO4 rather than coal. In addition, CaSO4 could greatly enhance organic sulfur decomposition, although most organic sulfur was converted to sulfide sulfur, some organic sulfur could be transferred to the gas, thereby increasing sulfur release of the coal, which would be discussed below. 3.4. Pyritic sulfur distribution Fig. 7 displays the data of pyritic sulfur in the presence and absence of CaSO4. It was observed that almost all of pyrite was fully decomposed at 600 oC in all cases and pyritic sulfur was not detected above 600 oC [19]. It was also noted that there was almost no change in pyritic sulfur distribution between raw coal and different proportions of CaSO4. This meant the effect of CaSO4 on pyrite sulfur was quite small. It was reported that CaSO4 began to decompose above 600 oC [8], whereas pyritic had been fully decomposed in this temperature range. So it was easily understood that CaSO4 did not greatly affect the distribution of pyritic sulfur. 3.5. Organic sulfur distribution in the char 10
Fig. 8 shows the variation of organic sulfur in the char with temperatures and blending ratios. It could be seen that organic sulfur showed a remarkable increase with the temperature from 500 o
C to 700 oC. It was well recognized that the pyrite sulfur could be converted to organic sulfur
during the pyrolysis [20] and the secondary reactions between H2S and organic matrix also could generate new organic sulfur. Therefore, the increase of organic sulfur was mainly because H2S generated in the pyrolysis (including from the pyritic sulfur decomposition) was captured by the active organic matrix to form new organic sulfur. Above 700 oC, organic sulfur began to decrease, but the decrease was not remarkable for raw coal. This was mainly because the organic sulfur was very stable (mainly existed in thiophene form) and did not have an apparent decomposition even at high temperatures. The organic sulfur appeared to be scarcely affected by the presence of CaSO4 at 500-700 oC. The effect of CaSO4 on organic sulfur at 800 oC was strongly dependent on the blending ratios (CaSO4 / coal). A low proportion of CaSO4 did not cause a significant change of organic sulfur. However, a significant decrease of organic sulfur was observed in the case of 50% CaSO4 and 100% CaSO4. The content of organic sulfur was significantly declined from 63.86 mg for raw coal to 25.11 mg for 50% CaSO4 to 21.66 mg for 100% CaSO4.This suggested only two conditions (high temperatures and high ratios of CaSO4/coal) were fulfilled, could organic sulfur be effectively removed. The results were fairly interesting, because stable organic sulfur was difficult to remove and such high removal rate for organic sulfur was rarely obtained. Considering blending ratio was an important parameter influencing organic sulfur removal, the variation of organic decomposition rate with more blending ratios was further determined. The results showed that the content of organic sulfur declined significantly from 59.62 mg to 25.93 mg with blending ratio increasing 11
from 20% to 30% CaSO4. This meant that organic sulfur reduced sharply when the ratio (CaSO4 / coal) exceeded 20%. To investigate the role of pyrolyzed gas and tar in reducing organic sulfur, the char obtained at 900 oC with 100% CaSO4 was pyrolyzed. The results showed that the organic sulfur for char was 57.39 mg and that for the mixture of char and CaSO4 was only 15.21 mg. The decrease of organic sulfur for raw coal (42.2 mg) was approximately the same with the char (42.18 mg) by the presence of CaSO4, which indicated that the decrease of organic sulfur with the addition of CaSO4 was mainly due to the char rather than pyrolyzed gas and tar. It had been reported that the carbon in the coal was responsible for CaSO4 decomposition in our previous study and the carbon in the char was remarkable reduced with the addition of high proportions of CaSO4 at 800 oC and the total carbon content declined from 0.236 mol for raw coal to 0.186 mol for 50%CaSO4 to 0.156 mol for 100% CaSO4, which was mainly due to Eqs 7 and 8 [8]. It was likely that CaSO4 could affect organic sulfur by changing the carbon content of the char. Since the S atom was bond to carbon atoms in organic sulfur and the organic sulfur depended heavily on the carbon atoms of the organic sulfur of the coal, when the carbons of organic sulfur were oxidized by CaSO4, the organic sulfur might be affected. Suppose that the blending ratio of CaSO4 was high enough and all carbon in the char was oxidized by CaSO4, organic sulfur would not exist in the char. When the carbon in the tar and the char reacted with CaSO4, C-C bonds were broken. When the carbon of the organic sulfur in the char and the tar reacted with CaSO4, the breakage of C-S bonds occurred simultaneously along with the cleavage of C-C bonds in organic sulfur of the char and the tar. The result obtained in this study was quite similar to that obtained by Liu [21]. He investigated sulfur behavior under 2% O2 atmosphere and noticed that not only C-C bonds but 12
also C-S bonds in the char was broken by oxygen, thus increasing desulfurization ratio especially in organic sulfur removal. In liu’s study, C-C bonds in the char were broken by O2, whereas in this study they were broken by CaSO4. Therefore, the mechanism of organic sulfur decreased in this study was similar to that in Liu’s study, that’s, C-S bonds of organic sulfur was broken when C-C bonds of organic sulfur were broken by CaSO4 or O2, as a result, organic sulfur was considerably
reduced by high proportions of CaSO4.
and
were used to typify aromatic
sulfur and thiophene in the tar and in the char in this paper. It could be believed that the possible mechanism of organic sulfur decrease due to CaSO4 might be expressed as follows:
·S· radical might be produced in the process of organic sulfur decomposition and it could either react with H· to form H2S or combine with inherent minerals to form sulfide sulfur. It was observed that the main product of the organic sulfur decomposition was sulfide sulfur instead of H2S. The decrease of organic sulfur due to CaSO4 was about 42.17 mg, whereas the increase of H2S due to CaSO4 was only 6.1 mg. This demonstrated that most organic sulfur was converted to sulfide sulfur and only a small proportion of organic sulfur was transformed to H2S. This was mainly because H2S due to organic sulfur decomposition was also fixed by the alkaline mineral matter, which was quite similar to the results obtained at 800 oC during pyrolysis of raw coal, that’s, almost no H2S was released at high temperatures due to the fixation of mineral matters. A tiny proportion of organic sulfur also could be converted to COS. 13
Higher active carbon compounds of the coal with lower bond energy was more likely to react with CaSO4 compared with less higher active carbon. When the ratio of CaSO4 was below 20%, organic sulfur was little-changed. This indicated there must have been other compounds with higher reactivity of reacting with CaSO4 than aromatic sulfur and thiophene in the coal, and they would react with CaSO4 first. However, when the compounds with higher reactivity was consumed with increasing the proportion of CaSO4, the organic sulfur in the char, like aromatic sulfur and thiophene, were likely to react with CaSO4, thus significantly reducing organic sulfur. This might provide a reasonble explanation for why only high proportions of CaSO4 could promote the removal of organic sulfur.
3.6 Sulfide sulfur distribution The sulfide sulfur distribution with the addition of different proportions of CaSO4 is shown in Fig. 9. It could be seen that sulfide sulfur was greatly dependent on the temperatures and the proportions of CaSO4. During the pyrolysis of raw coal, as the temperature rose, sulfide sulfur presented a fast increase. Sulfide sulfur distribution increased from 27.89 mg to 86.17 mg with elevating temperature from 500 oC to 800 oC. As many literature observed, sulfide sulfur for raw coal was mainly derived from two aspects: (I) decomposition of pyrite to iron sulfide; (II) the secondary reactions between sulfur containing gas and alkaline matter in the char [22]. The presence of CaSO4 had little effect on sulfide sulfur at low temperatures, but could remarkably enhance sulfide sulfur formation above 600 oC. Sulfide sulfur was increased from 86.17 mg for raw coal to 1456.92 mg for 100% CaSO4. This was mainly due to CaS formation through CaSO4 decomposition. It was proved that CaSO4 began to decompose above 600 oC and most CaSO4 was 14
converted to CaS at 800 oC [8], which could explain why a significant increase of sulfide sulfur could be obtained by the presence of CaSO4. In addition, most organic sulfur was converted to sulfide sulfur at 800 oC, which had been mentioned previously. This also caused an increase of sulfide sulfur to some extent.
Conclusions: Coal with different proportions of CaSO4 was pyrolyzed in a fixed-bed reactor to study the sulfur transformation during pyrolysis of the mixture of coal and CaSO4. The following conclusions could be drawn: (1) Higher H2S and COS yields could be obtained by the presence of CaSO4 at high temperature. This was mainly because H2S and COS could be produced through the reactions: 4H2+CaSO4 → CaO+ H2S + 3H2O, 2C+CaSO4 → CaO+ CO2+COS and 4CO+CaSO4 → CaO + 3CO2 +COS. What’s more, the decomposition of organic sulfur was also responsible for the increase of H2S and COS. (2) High proportions of CaSO4 could promote the decomposition of organic sulfur both in the tar and in the char at high temperatures. This was because C-S bonds are broken of stable organic sulfurs in tar and char along with breaking C-C bonds of organic sulfur when the carbon reacted with CaSO4, thus reducing the organic sulfur in the tar and in the char. (3) A significant increase of sulfide sulfur was observed with the addition of CaSO4 above 600 oC, which was mainly because a great deal of CaS was formed due to CaSO4 decomposition.
Acknowledgements The authors appreciate financial supports by U.S.-China Clean Energy Research Center on Advanced Coal Technology Consortium (2016YFE0102500 ) and the National Nature Science Foundation of China (51276160).
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Fig. 1.The principle of the coal staged conversion process
Fig. 2. Schematic of the experimental apparatus
17
Fig.3. Effect of CaSO4 on H2S distribution
Fig. 4. Effect of CaSO4 on COS distribution
18
Fig. 5. Effect of CaSO4 on tar sulfur distribution
Fig. 6. Effect of CaSO4 on sulfur release
19
Fig. 7. Effect of CaSO4 on pyrite sulfur distribution
Fig. 8. Effect of CaSO4 on organic sulfur distribution
20
Fig. 9. Effect of CaSO4 on sulfide sulfur distribution
21
Table 1. Main characteristics of samples Sample
XLT a, *
Proximate analysis/wt%, ad
Ultimate analysis, wt%, ad
M
V
A
Fc
C
H
N
Oa
2.68
44.02
18.19
35.11
48.72
4.64
1.24
22.02
Sulfur forms, wt%, ad
St
Ss
Sp
So*
2.51
0.83
0.32
1.36
by difference ;St, total sulfur; Sp, pyritic sulfur; Ss, sulfate sulfur; and So, organic sulfur
Table 2. Analysis of ash compositions in raw coals (wt%) Compound
SiO2
Fe2O3
Al2O3
CaO
MgO
K2O
Na2O
SO3
Othersa
XLT
33.47
8.09
12.04
19.11
3.42
0.76
0.16
20.01
2.94
a
by difference Table 3. The release of H2S and COS for pure CaSO4 under simulated pyrolyzed gas atmosphere Temperature(oC)
500
600
700
800
H2S(mg) COS(mg)
0.084 0.078
0.121 0.11
15.052 0.744
22.685 1.092
22