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Calcium species evolution mechanism during coal pyrolysis and char gasification in H2O/CO2 Yonghui Bai a, *, Xuhao Yang b, Fan Li b, Jiaofei Wang a, Xudong Song a, Min Yao c, Guangsuo Yu a, d a
State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan, 750021, China State Key Laboratory Breeding Base of Coal Science and Technology Co-Founded by Shanxi Province and Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China c CHN Energy Ningxia Coal Industry Co., Ltd., Yinchuan, 750000, China d Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai, 200237, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2019 Received in revised form 25 November 2019 Accepted 3 December 2019 Available online xxx
The release mechanism of Ca during coal pyrolysis and char gasification in H2O, CO2 and their mixtures was studied. Sequential chemical extraction was used to determine the modes of occurrence of Ca in coal and char. The released Ca from coal pyrolysis and char gasification were captured and analyzed by activated carbon adsorption and X-ray photoelectron spectroscopy (XPS). Model compounds CaS and CaSO4 were adopted to further reveal the released form of Ca under different atmospheres. The results indicate that Ca in coal is mainly released as CaCl2 during the pyrolysis process, and the possible migration mechanism of Ca during pyrolysis was proposed. Ca in coal is mainly released in the form of CaCl2, CaCO3, and CaSO4 during the gasification, and Ca is released as CaCl2 under all conditions. In addition, Ca will be released as CaCO3 under CO2 atmosphere, as CaSO4 under H2O and H2O/CO2 atmospheres at 800 C and 900 C, released as CaSO4 under all conditions at 1000 C. This is closely related to the formation of CaO2 intermediates during the gasification process. © 2020 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Calcium Release mechanism Pyrolysis Gasification H2O/CO2
1. Introduction Calcium, a common and relative high-content metal element in coal, has important influences on coal pyrolysis and gasification characteristics. The effects of Ca in coal on the pyrolysis mainly show in the yields and composition of three-phase products of liquid, solid and gas. For gas-phase products, Ca can promote the pyrolysis of coal to produce gaseous small molecules, which can increase pyrolysis gas yield and change its composition [1,2]. The studies [3,4] have shown that Ca can reduce tar yields of coal pyrolysis, and it is believed that Ca is mainly responsible for the catalysis of tar cracking and poly-condensation, resulting in tars to convert to larger molecules (coke) and smaller molecules (gas and light tar). Ca can also increase the yield of char, mainly by promoting the poly-condensation of coal tar, and would also have impacts on char physical characteristics, increasing the pore volume and specific surface area, thus improving the gasification
* Corresponding author. E-mail address: [email protected] (Y. Bai).
reactivity [5]. Ca, which is the most important in-situ alkaline earth metal catalyst in coal, also affects the coal gasification process. Whether it is the gasification reaction of CeH2O or CeCO2, it can exhibit excellent catalytic activity. As to the investigations on Ca release during pyrolysis and gasification, it is far less than that of alkali metals Na and K. However, there are also some studies have done in this area. Li et al. [6] studied the pyrolysis of Victoria brown coal and found that the inert or reducing atmospheres is helpful to the release of Ca. Li et al. [7] revealed that the Ca release rate under CO2 gasification conditions is higher than that under H2O and H2O/ CO2 conditions at either 800 or 900 C. Our previous work also performed the char gasification in H2O/ CO2 mixtures and found that the char reactivity in the mixed H2O/ CO2 atmosphere is much higher than the sum of that in the two pure gases in specific conditions, due to the synergistic effect [8] between H2O and CO2, Ca played a main catalytic contribution to the synergy [9]. Further studies have found that CaO is the main active component that leads to the synergy, the synergistic effect was vanished at higher temperatures (>900 C), which is attributed to the sintering of CaO at high temperature [10,11]. Also, we
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Please cite this article as: Y. Bai et al., Calcium species evolution mechanism during coal pyrolysis and char gasification in H2O/CO2, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.002
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acetylene flame test. The forms of calcium presents in char samples and adsorbed activated carbon were tested using a Thermo ESCALAB 250XI (US Thermo Fisher Scientific) X-ray photoelectron spectrometer, curve fitting analysis was performed using the (20% Gaussian-Lorentzian) curve fitting function in the XPS PEAK software. The gasification reactions of the Ca model compounds were conducted using a thermo gravimetric analyzer (NETZSCH STA 449F3). The 10 mg sample was placed uniformly on a plate and heated to 800 C at a rate of 10 C/min under a continuous Ar flow of 40 mL/min. Once the temperature reached to 800 C, the valve of the Ar flow was closed, and 200 mL/min of the gasification agent was injected, the ratios of the gasification agents were as follows: (i) Ar60% þ CO2 40%, (ii) Ar 60% þ H2O 40%, (iii) Ar 60% þ H2O 20% þ CO2 20%.
discover that a positive interaction between H2O and CO2 in developing char pore structure during the gasification was exist, resulting in that H2O creates porous structure on char surface to promote the diffusion of CO2 further into the pore structure [12]. Obviously, the formation and conversion of Ca species during char gasification is vital important to the understanding of synergistic effect. However, as the preliminary procedure, pyrolysis will influence the gasification greatly. So, further research is needed on the occurrence of Ca in coal and its release during pyrolysis and gasification. This is of great significance to the explanation of synergistic effect, development and utilization of high-alkaline coal. 2. Experimental 2.1. Coal sample
3. Results and discussions
A low rank bituminous coal, rich in Ca, from Yining County in Xinjiang, China was used in this study (abbreviated as YN). The proximate and ultimate analyses of YN were shown in Table 1. And the ash analyses of the coal were shown in Table 2.
3.1. Release mechanism of Ca during pyrolysis Forms of Ca in coal char prepared at different temperatures during pyrolysis are shown in Fig. 1. The results shown that HClsoluble form was the predominant form of Ca in YN raw coal, accounting for 60.6% of the total Ca, while the rest Ca presented as water-soluble form and ion-exchangeable soluble form which accounted for 25.2% and 10.6%, respectively. The stable form content was particularly low. It could also be found that the contents of water-soluble Ca and HCl-soluble Ca in chars decreased significantly with increasing temperature. The content of ionexchangeable Ca was almost kept constant. Moreover, the content of stable form in chars was positive related to the increase of temperature. The water-soluble form and HCl-soluble form transformed basically into gas phase and while another small part transformed into stable form which was different with the results obtained by Wang et al. [15] and Zhao et al. [16], they found that small part Na in water-soluble form and ion-exchangeable soluble form transformed into stable form. The main reason for the different phenomena is that the forms of Ca and Na in coal are different. In addition, the carboxylate of Na has only one carboxyl group connected to coal matrix, but the carboxylate of Ca has two carboxyl groups, which is relatively stable and is not easily to release or conversion. Moreover [13], our previous work indicates that the total release ratio of Ca during pyrolysis gradually increases as the temperature rise. This means that the Ca content in the sample is greatly reduced by pyrolysis, resulting in the presence of Ca in the sample in a steady state which is in accordance with the mentioned discussion about the release of different mode Ca species. XPS is an energy spectrum that uses the X-ray to excitate the sample electron, mainly for the analysis of the sample surface elements. Zhang et al. [17] used XPS to analyze the form of K in the char loaded with K2CO3 and KCl, and explained its catalytic effect on coal gasification. The XPS standard spectrum of Ca has two peaks of 2p1/2 and 2p3/2 [18]. The spacing between the two peaks is 3.5. When Ca combined with different anions to form compounds, the position of peaks also different. The type of compound can be determined based on the position of the 2p3/2 peak.
2.2. Char preparation The coal was pulverized and sieved to particle size between 1.0 and 1.4 mm. Pyrolysis and gasification experiments were conducted in a designed quartz fixed-bed reactor. The schematic diagram and detailed description of experimental conditions have been provided in our previous work [13]. In addition, approximately 5 g of activated carbon was placed below the coal sample to absorb the Ca species released into gas phase during reactions. Activated carbon was placed between the sintered plate and the coal sample. 2.3. Sequential chemical extraction and microwave digestion Sequential chemical extraction was used to determine the modes of occurrence of Ca in coal and char, Benson et al. [14] divided AAEMs into four forms: water-soluble form, ionexchangeable form, HCl-soluble form, and stable form with the sequential chemical extraction method. The extraction process was described as following: A 1.000 g char sample with particle size less than 0.150 mm was immersed into 30 mL ultrapure water, stirred at 60 C for 4 h, filtered and washed (the water-soluble form of Ca). Then leaching with 0.1 mol/L NH4Ac (the ion-exchangeable form of Ca) and 0.1 mol/L HCl (the HCl-soluble form of Ca). The sample after the last extraction procedure was digested using microwave digestion (the stable form of Ca). A 0.050 g sample was dissolved by 6 mL HNO3, 2 mL HF and 2 mL H2O2 in a microwave digestion systemeMDS-6G. After digestion, the solution was heated to 170 C to removal the residual acid, and then diluted to 100 mL with ultrapure water and kept in dry and cool place before being used. 2.4. Samples analysis The quantification of Ca in the coal and char was measurement by AA240FS Atomic Absorption Spectroscopy (AAS), using air-
Table 1 Proximate and ultimate analyses of the coal sample. Proximate analysis (wt. %)
Ultimate analysis (wt. %), daf
Mad
Aad
Vad
FCad
C
H
O*
N
S
12.8
3.1
24.4
59.7
79.2
3.9
16.0
0.6
0.3
Note: ad, air-dried basis; daf, dry and ash-free basis; *by difference.
Please cite this article as: Y. Bai et al., Calcium species evolution mechanism during coal pyrolysis and char gasification in H2O/CO2, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.002
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Table 2 Chemical compositions of the coal ash. Ash chemical composition(wt. %) SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
Others
34.7
4.7
5.7
29.2
11.5
0.2
11.0
0.2
2.1
0.8
Fig. 1. Forms of Ca in coal char prepared at different pyrolysis temperature. Fig. 3. XPS patterns of Ca in char prepared at 1000 C.
The XPS patterns of Ca2p in YN raw coal are shown in Fig. 2. It can be seen that the spectrum of Ca in the raw coal is very messy, because the forms of Ca in raw coal are very complicate, there are not only calcium in the form of inorganic salts such as CaCl2, CaCO3 and calcium aluminosilicate, but also organic calcium in the form of carboxylate. The different forms of Ca have different peak positions in the XPS spectrum, lead to the superposition of peaks. The XPS patterns of Ca 2p in char prepared at 1000 C are shown in Fig. 3, compared to the raw coal, the spectrum line become neat. The reason is that pyrolysis causes Ca in the coal to be released and converted, and the form of Ca remaining in the char is much simpler than that in the raw coal. The XPS patterns of Ca in gas
phase adsorbed by activated carbon during pyrolysis at 1000 C are shown in Fig. 4, the spectrum line is more regular than the raw coal and char, and the peaks of 2p1/2 and 2p3/2 are also obvious. This indicates that the form of Ca released into the gas phase during pyrolysis is relatively single, compared with the standard spectrum, the absorbed Ca species in the activated carbon is CaCl2. The watersoluble form of Ca in raw coal is dominated in CaCl2, which are easily volatilizes and releases during the pyrolysis process. There will also be some other forms of Ca react to become Ca2þ during pyrolysis and combine with Cl released from the gas phase to form CaCl2.
Fig. 2. XPS patterns of Ca in YN raw coal.
Fig. 4. XPS patterns of Ca in gas phase adsorbed by activated carbon in pyrolysis at 1000 C.
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According to the structure of coal and representative functional groups in coal, the possible migration mechanism of Ca during pyrolysis are proposed and shown in Fig. 5. As shown in the coal macroscopic structure, the blue dots, yellow dots, gray dots, and red dots represents water-soluble form of Ca, ion-exchangeable form of Ca, HCl-soluble form of Ca and stable form of Ca, respectively. As most of the inorganic metal elements in the raw coal, a small part of Ca is on the coal surface, most of them are in the pore structure inside the coal. The Ca in the coal will be released into the gas phase with the pyrolysis volatiles during pyrolysis, and the remaining Ca remained in the pyrolysis char. Also, a series of complex reactions involved Ca will occur during the pyrolysis process. The watersoluble, ion-exchangeable, and HCl-soluble forms of Ca are relatively easy to release while stable form of Ca are hard to release. The water-soluble form of Ca is the most easily released and the release amount is the most of the four existing forms of Ca, as shown in the dotted line ellipse. The water-soluble form of Ca mainly exists as CaCl2 and most of them will volatilize during the pyrolysis process. With increasing pyrolysis temperature, the cracking reactions of oxygen-containing functional groups in the coal occur because of the poor thermal stability of carboxyl groups. The ion-exchangeable form of Ca in coal mainly existing as carboxyl Ca and all the carboxyl groups will decompose during this process. The positions shown by the red arrows will break during the pyrolysis process. However, the ion-exchangeable form of Ca remained and the content is almost keeping constant according to the abovementioned results from Fig. 1, this is mainly because of the reactions in the solid line ellipse. The carboxyl Ca in coal is connected with two carboxyl functional groups, during pyrolysis, the carboxyl group is cracking and resulting in the generation of CO2. Ca that attached to the carboxyl group does not separate from the coal structure or release into gas phase. Instead, linked to the aromatic structure after the carboxyl group is broken, still existing in the ionexchangeable form state. Ca originally associated with eCOO
group maybe bonded to char matrix (CM), the reactions are as follows [19,20]: (eCOOeCaeOOCe) þ (eCM) ¼ (eCOOeCaeCM) þ CO2
(1)
(eCOOeCaeCM) þ (eCM) ¼ (CMeCaeCM) þ CO2
(2)
Compared with eCOOeCaeOOCe structures in coal, the proportion of carboxyl Ca attached to the straight hydrocarbon structure is relatively less. The carboxyl Ca will change to the form of Ca2þ after the carboxyl group broken, then produces CaO with O. CaO will further react with Al2O3, SiO2 or aluminosilicate to produce calcium aluminosilicate. However, the dominant role of the Ca content is the CMeCaeCM structure, so the content of ionexchangeable form of Ca in the coal chars nearly keeps constant after pyrolysis. The HCl-soluble forms of Ca in coal is mainly exist as CaCO3, and part of them decomposes into Ca2þ and CaO during pyrolysis, Ca2þwill release into the gas phase, and CaO will reacts further to form stable form of Ca, but most of them transform to stable form of Ca during pyrolysis process. The stable form of Ca is a high thermally stable mineral formed by the combination of CaO and aluminosilicate, which is difficult to decompose and release during pyrolysis. However, a small part of the water-soluble form of Ca and most of the HCl-soluble form of Ca will be converted to stable form, the possible chemical reaction equations are shown in Table 3. 3.2. Release mechanism of calcium during gasification The same method was used to absorb the Ca species that released into gas phase during gasification. The XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under CO2, H2O, and H2O/CO2 atmospheres at 800 C are shown in Fig. 6. It can be seen that the Ca adsorbed in the activated carbon under H2O
Fig. 5. Migration mechanism diagram of Ca during pyrolysis.
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Table 3 Main reactions of CaO during pyrolysis. Number
Reaction
1 2 3 4 5 6 7 8 9
CaO þ SiO2/CaO$SiO2 2CaO þ SiO2/ 2CaO$SiO2 3CaO þ SiO2/ 3CaO$SiO2 3CaO þ 2SiO2/ 3CaO$2SiO2 CaO þ Al2O3/CaO$Al2O3 3CaO þ Al2O3/ 3CaO$Al2O3 CaO þ Al2O3þ 2SiO2/CaO$Al2O3$2SiO2 2CaO þ Al2O3þ SiO2/ 2CaO$Al2O3$SiO2 3CaO þ Al2O3þ 3SiO2/ 3CaO$Al2O3$3SiO2
and H2O/CO2 atmospheres are CaCl2 and CaSO4, while the adsorbed Ca in the activated carbon under CO2 atmosphere are CaCl2 and CaCO3. The XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under CO2, H2O and H2O/CO2 at 900 C are shown in Fig. 7, the rule at this temperature is the same as 800 C. The XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under CO2, H2O and H2O/CO2 at 1000 C are shown in Fig. 8, the Ca absorbed in activated carbon under three atmospheres are CaCl2 and CaSO4. It can be concluded that Ca in coal char has been released in the form of CaCl2 during the gasification. In addition to CaCl2, Ca in the CO2 atmosphere has been released as CaCO3, and released as CaSO4 in H2O and H2O/CO2 atmospheres at 800 C and 900 C. At 1000 C, Ca has been released in the form of CaCl2 and CaSO4 under three atmospheres, the temperature has become the main factor that affecting the release and the influence of atmosphere become weak. Ca in coal char is mainly released in the three forms of CaCl2, CaCO3, and CaSO4 during the gasification. Previous studies have shown [21] that under strong oxidizing atmospheres, such as O2, Ca and S in coal are more easily to react to form CaSO4. The gasification agents CO2 and H2O used in this experiment are weak oxidizing atmospheres, and the possibility of CaS generation will be greater. But the results from XPS analysis indicate that CaSO4 but not CaS is the main product. In order to prove that the release of Ca during gasification is in the form of CaSO4 but not CaS, the TG analysis of CaS and CaSO4 under different atmospheres was analyzed. The weight loss curves of CaSO4 under different atmospheres are shown in Fig. 9. CaSO4 has good thermal stability and does not react after water loss between 200 C and 300 C under three atmospheres. After the introduction of gasification agent, the mass increases. This is because CaSO4 adsorbs a part of the gasification agent and makes its own mass increase. And it can be inferred that the amount of CaSO4 adsorbed on H2O is greater than that on CO2 at 800 C. CaSO4 can decompose when the temperature exceed 1100 C. This shows that CaSO4 produced would not decomposed by further reactions under this gasification conditions. The weight loss curve of CaS under different atmospheres is shown in Fig. 10. The gasification agent was purged until the temperature reaches 800 C. The mass of CaS has been reduced and fluctuate during the heating stage, because the hydroscopicity of CaS is very high. The mass of the reactants increases when the CO2 was purged, because CaS could react with CO2 to form CaO. The reaction equations are (3) and (4). The reactions are different, the former produces COS, the latter produces CO and SO2, but CaS has been converted to CaO. And CaO immediately reacts with CO2 to produce CaCO3, so the mass increased. When H2O or H2O/CO2 was purged, the mass of the reactants reduce because CaS reacts with H2O in a series of reactions such as reactions (5), (6), (7), and produce CaO finally. These three reactions are continuous, so the mass is reduced. Moreover, the mass under the H2O/CO2 atmosphere is decreased more than that under the H2O atmosphere.
Fig. 6. XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under (a) CO2, (b) H2O, and (c) H2O/CO2 at 800 C.
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Fig. 7. XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under (a) CO2, (b) H2O, and (c) H2O/CO2 at 900 C.
Fig. 8. XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under (a) CO2, (b) H2O, and (c) H2O/CO2 at 1000 C.
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process. Ca is more stable and difficult to release compared with Na, but some of them would also be released as CaCl2. The Ca in the gasification reaction is mainly CaO. CaO forms intermediates of metal oxide CaO2 at high temperature. CaO2 has higher reactivity and reacts more easily with gasification agents or gas-phase products compared with CaO. At the same time, CaO2 is more hardly to combine with the carbon matrix of coal char compared with that of CaO, and is more likely to release during the reaction. When the gasification temperature is 800 C and 900 C, Ca in coal char will also be released as CaCO3 under CO2 atmosphere. In addition to CaO reacts with CO2 to produce CaCO3, CaO2 also reacts with CO2 to produce CaCO3. The CaCO3 released into the gas phase is produced by the CaO2, but the CaCO3 produced by the CaO remains in the coal char. Ca in coal char is released as CaSO4 under H2O and H2O/CO2 atmospheres. Sulfur in char during gasification will produce SO2 and H2S etc. And SO2 will react with CaO2 to produce CaSO4, or CaO2 reacts with H2O to form Ca (OH)2, then Ca (OH)2 reacts with SO2 further to form CaSO4. The CaSO4 produced will be released into the gas phase during the reaction. At the same time, CaO in coal char will also produce CaSO4 through the latter route. However, CaO has lower reactivity than CaO2. CaO2 and CaO will form a competitive situation, so CaSO4 produced by CaO is less and does not release. That is H2O plays a leading role and Ca in coal char will be released as CaCl2 and CaSO4 under H2O and H2O/CO2 atmospheres at this temperature. There is no difference in the release of Ca under the three atmospheres at 1000 C. This is because the effect of temperature on Ca migration has exceeded the influence of the atmosphere and has become the dominant factor. The released CaSO4 is formed by the reaction of SO2 in the gasphase and CaO2. The amount of released Ca decrease significantly at 1000 C compared to 800 C and 900 C, the amount of released CaCl2 and CaSO4 also decreased. According to the facts that the release rate, release form, and the compositions of ash have almost no difference under different atmospheres at 1000 C [13], it can be conclude that the influence of the atmosphere on Ca migration is very weak, and the temperature plays a major role.
Fig. 9. TG analysis of CaSO4 under different atmospheres.
4. Conclusions
Fig. 10. TG analysis of CaS under different atmospheres.
CaS þ CO2 / CaO þ COS
(3)
CaS þ 3CO2 / CaO þ 3CO þ SO2
(4)
CaS þ H2O / Ca(SH) (OH)
(5)
Ca(SH) (OH) þ H2O / Ca(OH)2 þ H2S
(6)
Ca(OH)2 / CaO þ H2O
(7)
The results indicate that Ca adsorbed during the gasification cannot be existing in the form of CaS. Once CaS is formed, it will further react and produce to CaO finally under this gasification conditions. But CaSO4 produced is thermally stable and does not decompose, so Ca in the char could be released as CaSO4 rather than CaS. Ca in coal char will be released as CaCl2 during the gasification process. Ca 2þ produced during the reaction or existing as a reaction intermediate could combine with Cl in coal to form CaCl2 and releases to the gas phase. In addition, the gasification reaction temperature is 800 Ce1000 C which reached the melting point of CaCl2, and the molten CaCl2 is easily released during the gasification. This is similar to the results of Kosminski et al. [22]. They believe that Na in coal was released as NaCl during the gasification
The release mechanism of Ca during YN coal pyrolysis and char gasification in CO2, H2O and H2O/CO2 atmospheres were investigated, and the main conclusions are shown as following: Ca in coal is mainly released as CaCl2 during the pyrolysis process. The water-soluble form of Ca is dominated by CaCl2, and most of them will be released during the pyrolysis. The ion-exchangeable form of Ca mainly exists as carboxyl Ca and linked to the aromatic structure after the carboxyl group is broken during the pyrolysis, still existing in the ion-exchangeable form, so the content is almost keep constant. The HCl-soluble form of Ca is mainly exists as CaCO3, part of CaCO3 decomposes to Ca2þ and CaO during the pyrolysis process, then Ca2þ is released into the gas phase and CaO can further reacts with Si and Al to form stable form of Ca. Ca in coal is mainly released in the form of CaCl2, CaCO3, and CaSO4 during the gasification, and Ca is released as CaCl2 under all conditions. In addition, Ca will be released as CaCO3 under CO2 atmosphere, as CaSO4 under H2O and H2O/CO2 atmospheres at 800 C and 900 C. Ca was released as CaSO4 under all three atmospheres at 1000 C. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21968024), and the project of Key Research Plan of Ningxia (2019BCH01001).
Please cite this article as: Y. Bai et al., Calcium species evolution mechanism during coal pyrolysis and char gasification in H2O/CO2, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.002
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Calcium species evolution mechanism during coal pyrolysis and char gasification in H2O/CO2 Yonghui Bai a, *, Xuhao Yang b, Fan Li b, Jiaofei Wang a, Xudong Song a, Min Yao c, Guangsuo Yu a, d a
State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan, 750021, China State Key Laboratory Breeding Base of Coal Science and Technology Co-Founded by Shanxi Province and Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China c CHN Energy Ningxia Coal Industry Co., Ltd., Yinchuan, 750000, China d Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai, 200237, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2019 Received in revised form 25 November 2019 Accepted 3 December 2019 Available online xxx
The release mechanism of Ca during coal pyrolysis and char gasification in H2O, CO2 and their mixtures was studied. Sequential chemical extraction was used to determine the modes of occurrence of Ca in coal and char. The released Ca from coal pyrolysis and char gasification were captured and analyzed by activated carbon adsorption and X-ray photoelectron spectroscopy (XPS). Model compounds CaS and CaSO4 were adopted to further reveal the released form of Ca under different atmospheres. The results indicate that Ca in coal is mainly released as CaCl2 during the pyrolysis process, and the possible migration mechanism of Ca during pyrolysis was proposed. Ca in coal is mainly released in the form of CaCl2, CaCO3, and CaSO4 during the gasification, and Ca is released as CaCl2 under all conditions. In addition, Ca will be released as CaCO3 under CO2 atmosphere, as CaSO4 under H2O and H2O/CO2 atmospheres at 800 C and 900 C, released as CaSO4 under all conditions at 1000 C. This is closely related to the formation of CaO2 intermediates during the gasification process. © 2020 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Calcium Release mechanism Pyrolysis Gasification H2O/CO2
1. Introduction Calcium, a common and relative high-content metal element in coal, has important influences on coal pyrolysis and gasification characteristics. The effects of Ca in coal on the pyrolysis mainly show in the yields and composition of three-phase products of liquid, solid and gas. For gas-phase products, Ca can promote the pyrolysis of coal to produce gaseous small molecules, which can increase pyrolysis gas yield and change its composition [1,2]. The studies [3,4] have shown that Ca can reduce tar yields of coal pyrolysis, and it is believed that Ca is mainly responsible for the catalysis of tar cracking and poly-condensation, resulting in tars to convert to larger molecules (coke) and smaller molecules (gas and light tar). Ca can also increase the yield of char, mainly by promoting the poly-condensation of coal tar, and would also have impacts on char physical characteristics, increasing the pore volume and specific surface area, thus improving the gasification
* Corresponding author. E-mail address: [email protected] (Y. Bai).
reactivity [5]. Ca, which is the most important in-situ alkaline earth metal catalyst in coal, also affects the coal gasification process. Whether it is the gasification reaction of CeH2O or CeCO2, it can exhibit excellent catalytic activity. As to the investigations on Ca release during pyrolysis and gasification, it is far less than that of alkali metals Na and K. However, there are also some studies have done in this area. Li et al. [6] studied the pyrolysis of Victoria brown coal and found that the inert or reducing atmospheres is helpful to the release of Ca. Li et al. [7] revealed that the Ca release rate under CO2 gasification conditions is higher than that under H2O and H2O/ CO2 conditions at either 800 or 900 C. Our previous work also performed the char gasification in H2O/ CO2 mixtures and found that the char reactivity in the mixed H2O/ CO2 atmosphere is much higher than the sum of that in the two pure gases in specific conditions, due to the synergistic effect [8] between H2O and CO2, Ca played a main catalytic contribution to the synergy [9]. Further studies have found that CaO is the main active component that leads to the synergy, the synergistic effect was vanished at higher temperatures (>900 C), which is attributed to the sintering of CaO at high temperature [10,11]. Also, we
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acetylene flame test. The forms of calcium presents in char samples and adsorbed activated carbon were tested using a Thermo ESCALAB 250XI (US Thermo Fisher Scientific) X-ray photoelectron spectrometer, curve fitting analysis was performed using the (20% Gaussian-Lorentzian) curve fitting function in the XPS PEAK software. The gasification reactions of the Ca model compounds were conducted using a thermo gravimetric analyzer (NETZSCH STA 449F3). The 10 mg sample was placed uniformly on a plate and heated to 800 C at a rate of 10 C/min under a continuous Ar flow of 40 mL/min. Once the temperature reached to 800 C, the valve of the Ar flow was closed, and 200 mL/min of the gasification agent was injected, the ratios of the gasification agents were as follows: (i) Ar60% þ CO2 40%, (ii) Ar 60% þ H2O 40%, (iii) Ar 60% þ H2O 20% þ CO2 20%.
discover that a positive interaction between H2O and CO2 in developing char pore structure during the gasification was exist, resulting in that H2O creates porous structure on char surface to promote the diffusion of CO2 further into the pore structure [12]. Obviously, the formation and conversion of Ca species during char gasification is vital important to the understanding of synergistic effect. However, as the preliminary procedure, pyrolysis will influence the gasification greatly. So, further research is needed on the occurrence of Ca in coal and its release during pyrolysis and gasification. This is of great significance to the explanation of synergistic effect, development and utilization of high-alkaline coal. 2. Experimental 2.1. Coal sample
3. Results and discussions
A low rank bituminous coal, rich in Ca, from Yining County in Xinjiang, China was used in this study (abbreviated as YN). The proximate and ultimate analyses of YN were shown in Table 1. And the ash analyses of the coal were shown in Table 2.
3.1. Release mechanism of Ca during pyrolysis Forms of Ca in coal char prepared at different temperatures during pyrolysis are shown in Fig. 1. The results shown that HClsoluble form was the predominant form of Ca in YN raw coal, accounting for 60.6% of the total Ca, while the rest Ca presented as water-soluble form and ion-exchangeable soluble form which accounted for 25.2% and 10.6%, respectively. The stable form content was particularly low. It could also be found that the contents of water-soluble Ca and HCl-soluble Ca in chars decreased significantly with increasing temperature. The content of ionexchangeable Ca was almost kept constant. Moreover, the content of stable form in chars was positive related to the increase of temperature. The water-soluble form and HCl-soluble form transformed basically into gas phase and while another small part transformed into stable form which was different with the results obtained by Wang et al. [15] and Zhao et al. [16], they found that small part Na in water-soluble form and ion-exchangeable soluble form transformed into stable form. The main reason for the different phenomena is that the forms of Ca and Na in coal are different. In addition, the carboxylate of Na has only one carboxyl group connected to coal matrix, but the carboxylate of Ca has two carboxyl groups, which is relatively stable and is not easily to release or conversion. Moreover [13], our previous work indicates that the total release ratio of Ca during pyrolysis gradually increases as the temperature rise. This means that the Ca content in the sample is greatly reduced by pyrolysis, resulting in the presence of Ca in the sample in a steady state which is in accordance with the mentioned discussion about the release of different mode Ca species. XPS is an energy spectrum that uses the X-ray to excitate the sample electron, mainly for the analysis of the sample surface elements. Zhang et al. [17] used XPS to analyze the form of K in the char loaded with K2CO3 and KCl, and explained its catalytic effect on coal gasification. The XPS standard spectrum of Ca has two peaks of 2p1/2 and 2p3/2 [18]. The spacing between the two peaks is 3.5. When Ca combined with different anions to form compounds, the position of peaks also different. The type of compound can be determined based on the position of the 2p3/2 peak.
2.2. Char preparation The coal was pulverized and sieved to particle size between 1.0 and 1.4 mm. Pyrolysis and gasification experiments were conducted in a designed quartz fixed-bed reactor. The schematic diagram and detailed description of experimental conditions have been provided in our previous work [13]. In addition, approximately 5 g of activated carbon was placed below the coal sample to absorb the Ca species released into gas phase during reactions. Activated carbon was placed between the sintered plate and the coal sample. 2.3. Sequential chemical extraction and microwave digestion Sequential chemical extraction was used to determine the modes of occurrence of Ca in coal and char, Benson et al. [14] divided AAEMs into four forms: water-soluble form, ionexchangeable form, HCl-soluble form, and stable form with the sequential chemical extraction method. The extraction process was described as following: A 1.000 g char sample with particle size less than 0.150 mm was immersed into 30 mL ultrapure water, stirred at 60 C for 4 h, filtered and washed (the water-soluble form of Ca). Then leaching with 0.1 mol/L NH4Ac (the ion-exchangeable form of Ca) and 0.1 mol/L HCl (the HCl-soluble form of Ca). The sample after the last extraction procedure was digested using microwave digestion (the stable form of Ca). A 0.050 g sample was dissolved by 6 mL HNO3, 2 mL HF and 2 mL H2O2 in a microwave digestion systemeMDS-6G. After digestion, the solution was heated to 170 C to removal the residual acid, and then diluted to 100 mL with ultrapure water and kept in dry and cool place before being used. 2.4. Samples analysis The quantification of Ca in the coal and char was measurement by AA240FS Atomic Absorption Spectroscopy (AAS), using air-
Table 1 Proximate and ultimate analyses of the coal sample. Proximate analysis (wt. %)
Ultimate analysis (wt. %), daf
Mad
Aad
Vad
FCad
C
H
O*
N
S
12.8
3.1
24.4
59.7
79.2
3.9
16.0
0.6
0.3
Note: ad, air-dried basis; daf, dry and ash-free basis; *by difference.
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Table 2 Chemical compositions of the coal ash. Ash chemical composition(wt. %) SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
Others
34.7
4.7
5.7
29.2
11.5
0.2
11.0
0.2
2.1
0.8
Fig. 1. Forms of Ca in coal char prepared at different pyrolysis temperature. Fig. 3. XPS patterns of Ca in char prepared at 1000 C.
The XPS patterns of Ca2p in YN raw coal are shown in Fig. 2. It can be seen that the spectrum of Ca in the raw coal is very messy, because the forms of Ca in raw coal are very complicate, there are not only calcium in the form of inorganic salts such as CaCl2, CaCO3 and calcium aluminosilicate, but also organic calcium in the form of carboxylate. The different forms of Ca have different peak positions in the XPS spectrum, lead to the superposition of peaks. The XPS patterns of Ca 2p in char prepared at 1000 C are shown in Fig. 3, compared to the raw coal, the spectrum line become neat. The reason is that pyrolysis causes Ca in the coal to be released and converted, and the form of Ca remaining in the char is much simpler than that in the raw coal. The XPS patterns of Ca in gas
phase adsorbed by activated carbon during pyrolysis at 1000 C are shown in Fig. 4, the spectrum line is more regular than the raw coal and char, and the peaks of 2p1/2 and 2p3/2 are also obvious. This indicates that the form of Ca released into the gas phase during pyrolysis is relatively single, compared with the standard spectrum, the absorbed Ca species in the activated carbon is CaCl2. The watersoluble form of Ca in raw coal is dominated in CaCl2, which are easily volatilizes and releases during the pyrolysis process. There will also be some other forms of Ca react to become Ca2þ during pyrolysis and combine with Cl released from the gas phase to form CaCl2.
Fig. 2. XPS patterns of Ca in YN raw coal.
Fig. 4. XPS patterns of Ca in gas phase adsorbed by activated carbon in pyrolysis at 1000 C.
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According to the structure of coal and representative functional groups in coal, the possible migration mechanism of Ca during pyrolysis are proposed and shown in Fig. 5. As shown in the coal macroscopic structure, the blue dots, yellow dots, gray dots, and red dots represents water-soluble form of Ca, ion-exchangeable form of Ca, HCl-soluble form of Ca and stable form of Ca, respectively. As most of the inorganic metal elements in the raw coal, a small part of Ca is on the coal surface, most of them are in the pore structure inside the coal. The Ca in the coal will be released into the gas phase with the pyrolysis volatiles during pyrolysis, and the remaining Ca remained in the pyrolysis char. Also, a series of complex reactions involved Ca will occur during the pyrolysis process. The watersoluble, ion-exchangeable, and HCl-soluble forms of Ca are relatively easy to release while stable form of Ca are hard to release. The water-soluble form of Ca is the most easily released and the release amount is the most of the four existing forms of Ca, as shown in the dotted line ellipse. The water-soluble form of Ca mainly exists as CaCl2 and most of them will volatilize during the pyrolysis process. With increasing pyrolysis temperature, the cracking reactions of oxygen-containing functional groups in the coal occur because of the poor thermal stability of carboxyl groups. The ion-exchangeable form of Ca in coal mainly existing as carboxyl Ca and all the carboxyl groups will decompose during this process. The positions shown by the red arrows will break during the pyrolysis process. However, the ion-exchangeable form of Ca remained and the content is almost keeping constant according to the abovementioned results from Fig. 1, this is mainly because of the reactions in the solid line ellipse. The carboxyl Ca in coal is connected with two carboxyl functional groups, during pyrolysis, the carboxyl group is cracking and resulting in the generation of CO2. Ca that attached to the carboxyl group does not separate from the coal structure or release into gas phase. Instead, linked to the aromatic structure after the carboxyl group is broken, still existing in the ionexchangeable form state. Ca originally associated with eCOO
group maybe bonded to char matrix (CM), the reactions are as follows [19,20]: (eCOOeCaeOOCe) þ (eCM) ¼ (eCOOeCaeCM) þ CO2
(1)
(eCOOeCaeCM) þ (eCM) ¼ (CMeCaeCM) þ CO2
(2)
Compared with eCOOeCaeOOCe structures in coal, the proportion of carboxyl Ca attached to the straight hydrocarbon structure is relatively less. The carboxyl Ca will change to the form of Ca2þ after the carboxyl group broken, then produces CaO with O. CaO will further react with Al2O3, SiO2 or aluminosilicate to produce calcium aluminosilicate. However, the dominant role of the Ca content is the CMeCaeCM structure, so the content of ionexchangeable form of Ca in the coal chars nearly keeps constant after pyrolysis. The HCl-soluble forms of Ca in coal is mainly exist as CaCO3, and part of them decomposes into Ca2þ and CaO during pyrolysis, Ca2þwill release into the gas phase, and CaO will reacts further to form stable form of Ca, but most of them transform to stable form of Ca during pyrolysis process. The stable form of Ca is a high thermally stable mineral formed by the combination of CaO and aluminosilicate, which is difficult to decompose and release during pyrolysis. However, a small part of the water-soluble form of Ca and most of the HCl-soluble form of Ca will be converted to stable form, the possible chemical reaction equations are shown in Table 3. 3.2. Release mechanism of calcium during gasification The same method was used to absorb the Ca species that released into gas phase during gasification. The XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under CO2, H2O, and H2O/CO2 atmospheres at 800 C are shown in Fig. 6. It can be seen that the Ca adsorbed in the activated carbon under H2O
Fig. 5. Migration mechanism diagram of Ca during pyrolysis.
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Table 3 Main reactions of CaO during pyrolysis. Number
Reaction
1 2 3 4 5 6 7 8 9
CaO þ SiO2/CaO$SiO2 2CaO þ SiO2/ 2CaO$SiO2 3CaO þ SiO2/ 3CaO$SiO2 3CaO þ 2SiO2/ 3CaO$2SiO2 CaO þ Al2O3/CaO$Al2O3 3CaO þ Al2O3/ 3CaO$Al2O3 CaO þ Al2O3þ 2SiO2/CaO$Al2O3$2SiO2 2CaO þ Al2O3þ SiO2/ 2CaO$Al2O3$SiO2 3CaO þ Al2O3þ 3SiO2/ 3CaO$Al2O3$3SiO2
and H2O/CO2 atmospheres are CaCl2 and CaSO4, while the adsorbed Ca in the activated carbon under CO2 atmosphere are CaCl2 and CaCO3. The XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under CO2, H2O and H2O/CO2 at 900 C are shown in Fig. 7, the rule at this temperature is the same as 800 C. The XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under CO2, H2O and H2O/CO2 at 1000 C are shown in Fig. 8, the Ca absorbed in activated carbon under three atmospheres are CaCl2 and CaSO4. It can be concluded that Ca in coal char has been released in the form of CaCl2 during the gasification. In addition to CaCl2, Ca in the CO2 atmosphere has been released as CaCO3, and released as CaSO4 in H2O and H2O/CO2 atmospheres at 800 C and 900 C. At 1000 C, Ca has been released in the form of CaCl2 and CaSO4 under three atmospheres, the temperature has become the main factor that affecting the release and the influence of atmosphere become weak. Ca in coal char is mainly released in the three forms of CaCl2, CaCO3, and CaSO4 during the gasification. Previous studies have shown [21] that under strong oxidizing atmospheres, such as O2, Ca and S in coal are more easily to react to form CaSO4. The gasification agents CO2 and H2O used in this experiment are weak oxidizing atmospheres, and the possibility of CaS generation will be greater. But the results from XPS analysis indicate that CaSO4 but not CaS is the main product. In order to prove that the release of Ca during gasification is in the form of CaSO4 but not CaS, the TG analysis of CaS and CaSO4 under different atmospheres was analyzed. The weight loss curves of CaSO4 under different atmospheres are shown in Fig. 9. CaSO4 has good thermal stability and does not react after water loss between 200 C and 300 C under three atmospheres. After the introduction of gasification agent, the mass increases. This is because CaSO4 adsorbs a part of the gasification agent and makes its own mass increase. And it can be inferred that the amount of CaSO4 adsorbed on H2O is greater than that on CO2 at 800 C. CaSO4 can decompose when the temperature exceed 1100 C. This shows that CaSO4 produced would not decomposed by further reactions under this gasification conditions. The weight loss curve of CaS under different atmospheres is shown in Fig. 10. The gasification agent was purged until the temperature reaches 800 C. The mass of CaS has been reduced and fluctuate during the heating stage, because the hydroscopicity of CaS is very high. The mass of the reactants increases when the CO2 was purged, because CaS could react with CO2 to form CaO. The reaction equations are (3) and (4). The reactions are different, the former produces COS, the latter produces CO and SO2, but CaS has been converted to CaO. And CaO immediately reacts with CO2 to produce CaCO3, so the mass increased. When H2O or H2O/CO2 was purged, the mass of the reactants reduce because CaS reacts with H2O in a series of reactions such as reactions (5), (6), (7), and produce CaO finally. These three reactions are continuous, so the mass is reduced. Moreover, the mass under the H2O/CO2 atmosphere is decreased more than that under the H2O atmosphere.
Fig. 6. XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under (a) CO2, (b) H2O, and (c) H2O/CO2 at 800 C.
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Fig. 7. XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under (a) CO2, (b) H2O, and (c) H2O/CO2 at 900 C.
Fig. 8. XPS patterns of Ca in gas phase adsorbed by activated carbon in gasification under (a) CO2, (b) H2O, and (c) H2O/CO2 at 1000 C.
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process. Ca is more stable and difficult to release compared with Na, but some of them would also be released as CaCl2. The Ca in the gasification reaction is mainly CaO. CaO forms intermediates of metal oxide CaO2 at high temperature. CaO2 has higher reactivity and reacts more easily with gasification agents or gas-phase products compared with CaO. At the same time, CaO2 is more hardly to combine with the carbon matrix of coal char compared with that of CaO, and is more likely to release during the reaction. When the gasification temperature is 800 C and 900 C, Ca in coal char will also be released as CaCO3 under CO2 atmosphere. In addition to CaO reacts with CO2 to produce CaCO3, CaO2 also reacts with CO2 to produce CaCO3. The CaCO3 released into the gas phase is produced by the CaO2, but the CaCO3 produced by the CaO remains in the coal char. Ca in coal char is released as CaSO4 under H2O and H2O/CO2 atmospheres. Sulfur in char during gasification will produce SO2 and H2S etc. And SO2 will react with CaO2 to produce CaSO4, or CaO2 reacts with H2O to form Ca (OH)2, then Ca (OH)2 reacts with SO2 further to form CaSO4. The CaSO4 produced will be released into the gas phase during the reaction. At the same time, CaO in coal char will also produce CaSO4 through the latter route. However, CaO has lower reactivity than CaO2. CaO2 and CaO will form a competitive situation, so CaSO4 produced by CaO is less and does not release. That is H2O plays a leading role and Ca in coal char will be released as CaCl2 and CaSO4 under H2O and H2O/CO2 atmospheres at this temperature. There is no difference in the release of Ca under the three atmospheres at 1000 C. This is because the effect of temperature on Ca migration has exceeded the influence of the atmosphere and has become the dominant factor. The released CaSO4 is formed by the reaction of SO2 in the gasphase and CaO2. The amount of released Ca decrease significantly at 1000 C compared to 800 C and 900 C, the amount of released CaCl2 and CaSO4 also decreased. According to the facts that the release rate, release form, and the compositions of ash have almost no difference under different atmospheres at 1000 C [13], it can be conclude that the influence of the atmosphere on Ca migration is very weak, and the temperature plays a major role.
Fig. 9. TG analysis of CaSO4 under different atmospheres.
4. Conclusions
Fig. 10. TG analysis of CaS under different atmospheres.
CaS þ CO2 / CaO þ COS
(3)
CaS þ 3CO2 / CaO þ 3CO þ SO2
(4)
CaS þ H2O / Ca(SH) (OH)
(5)
Ca(SH) (OH) þ H2O / Ca(OH)2 þ H2S
(6)
Ca(OH)2 / CaO þ H2O
(7)
The results indicate that Ca adsorbed during the gasification cannot be existing in the form of CaS. Once CaS is formed, it will further react and produce to CaO finally under this gasification conditions. But CaSO4 produced is thermally stable and does not decompose, so Ca in the char could be released as CaSO4 rather than CaS. Ca in coal char will be released as CaCl2 during the gasification process. Ca 2þ produced during the reaction or existing as a reaction intermediate could combine with Cl in coal to form CaCl2 and releases to the gas phase. In addition, the gasification reaction temperature is 800 Ce1000 C which reached the melting point of CaCl2, and the molten CaCl2 is easily released during the gasification. This is similar to the results of Kosminski et al. [22]. They believe that Na in coal was released as NaCl during the gasification
The release mechanism of Ca during YN coal pyrolysis and char gasification in CO2, H2O and H2O/CO2 atmospheres were investigated, and the main conclusions are shown as following: Ca in coal is mainly released as CaCl2 during the pyrolysis process. The water-soluble form of Ca is dominated by CaCl2, and most of them will be released during the pyrolysis. The ion-exchangeable form of Ca mainly exists as carboxyl Ca and linked to the aromatic structure after the carboxyl group is broken during the pyrolysis, still existing in the ion-exchangeable form, so the content is almost keep constant. The HCl-soluble form of Ca is mainly exists as CaCO3, part of CaCO3 decomposes to Ca2þ and CaO during the pyrolysis process, then Ca2þ is released into the gas phase and CaO can further reacts with Si and Al to form stable form of Ca. Ca in coal is mainly released in the form of CaCl2, CaCO3, and CaSO4 during the gasification, and Ca is released as CaCl2 under all conditions. In addition, Ca will be released as CaCO3 under CO2 atmosphere, as CaSO4 under H2O and H2O/CO2 atmospheres at 800 C and 900 C. Ca was released as CaSO4 under all three atmospheres at 1000 C. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21968024), and the project of Key Research Plan of Ningxia (2019BCH01001).
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Please cite this article as: Y. Bai et al., Calcium species evolution mechanism during coal pyrolysis and char gasification in H2O/CO2, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.002