In-situ study on K2CO3-catalyzed CO2 gasification of coal char

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In-situ study on K2CO3-catalyzed CO2 gasification of coal char: interactions and char structure evolution Huaili Zhu, Xingjun Wang, Fuchen Wang, and Guangsuo Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03255 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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In-situ study on K2CO3-catalyzed CO2 gasification of coal char: interactions and char structure evolution

Huaili Zhu, Xingjun Wang*, Fuchen Wang, Guangsuo Yu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education East China University of Science and Technology, Shanghai, 200237 (PR China)1 Abstract: To investigate the interactions between K2CO3 and coal char, a fixed-bed reactor was used to conduct catalytic pyrolysis and gasification of coal char. An in-situ Raman spectroscopy system was applied to characterize the evolution of char structure. Three different ranks of Chinese coals were deashed firstly and pyrolyzed to chars before experiment. In catalytic pyrolysis of coal char, the release of CO proved that reactions occurred between K2CO3 and char and the release of small amount of CO2 was connected with the oxygen content. In-situ Raman spectra results showed that the char structure order decreased with rising temperature for the production of intermediate. During gasification process, the char structure order decreased first and then increased attributed by the evaporation of K at high temperature. The ex-situ data revealed that the intermediate did not exit at room temperature. For better understanding of the true form of chars at high temperature, an in-situ Raman spectrometer is necessary. Keywords: Catalytic gasification, interaction, char structure, in-situ Raman spectroscopy

1 Introduction

Catalytic gasification of coal is a potential technique of coal clean utilization for its high *Corresponding authors. Tel.: +86-21-64252974; Fax: +86-21-64251312. E-mail: [email protected](Guangsuo Yu); [email protected](Xingjun Wang)

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gasification reactivity, low operating temperature and the selectivity to certain gas products. The metal-based catalyst, such as alkalis,[1, 2] alkaline earths[3-5] and transition metals,[6, 7] have been proved to be good catalysts in coal gasification. The most effective catalysts are alkali metal salts, especially K2CO3. The K2CO3 catalytic gasification of coal has been widely investigated since 1970s and 1980s,[8] and different catalytic mechanisms have been proposed including oxygen transfer mechanism[9] and reaction intermediate mechanism.[10] In oxygen transfer mechanism, the potassium is reduced firstly by carbon in coal and then oxidized by adsorption oxygen from gasification agents, which constitutes a redox cycle. The following equations illustrate the transformation of alkalis in a CO2-gasification process.[9]

M 2CO3 + 2C = 2 M + 3CO

(1)

2 M + CO2 = M 2 O + CO

(2)

M 2O + CO2 = M 2CO3

(3)

The alkalis is reduced in reaction (1) and oxidized in reaction (2). The sum of the three reactions is as follow:

C + CO2 = 2CO

(4)

But in gasification process, the alkalis do not actually exist, it might exist as a complex compounds with carbon (CnM) as equation (5) shown.

2 M + 2nC = 2Cn M

(5)

The complex compounds exist as different forms and have been studied for a long time,[11, 12] but the interactions between potassium and carbon and the reaction intermediates are still debatable. In pyrolysis/gasification process, the catalysts also have effect on char structure evolution, but how a catalyst would affect the char structural change is still being studied. Liu et.al[13] has used different catalysts including potassium in hydrogasification of coal and used Raman


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spectroscopy to characterize the evolution of char structure. The results show that alkalis can easily diffuse into the aromatic rings to form intercalation compounds with carbon (CnM) and the alkali catalysts changed the char hydrogasification pathways. Tay[14] had studied on catalytic gasification of a deashed Victorian brown coal and found that the catalyst tended to encourage the oxidation of char during gasification. The transformation of potassium in gasification has relationship with char structure evolution. Ash content in coal contains a plenty of alkali and alkaline earth metallic (AAEM) which have significant effect on char structural change and coal gasification reactivity of coal.[15-17] But, the high content of Si and Al in ash would also make the alkalis catalysts deactivation by react with alkalis to form alkali aluminosilicate, which is not water soluble salt and has no catalytic effect.[18, 19] To investigate the potassium catalyst effect on coal gasification, the influence of AAEM species, Si and Al should be eliminated firstly. An acid wash way can be used to remove the inherent minerals.[20, 21] Many gasification apparatus, such as fixed-bed reactor, fluidized-bed reactor and thermogravimetric analyzers (TGA), and analysis methods for characterization char structure including X-ray diffraction (XRD) and Raman spectroscopy have been used to study the effect of catalysts on coal pyrolysis or gasification and the char structural change.[22-24] Kopyscinski et.al[2] used

an in-situ X-ray diffraction (XRD) and thermogravimetric analyzers (TGA) to investigate the interactions between ash free coal and potassium catalyst in N2 and CO2 atmosphere and proposed a mechanism of potassium catalyzed CO2 gasification of coal. Ding et.al[21] investigated the Na2CO3 catalyzed pyrolysis and gasification of coal in an in-situ heating stage and found different gasification mechanisms of two coals. The in-situ techniques are now widely used to study the coal gasification for its advantage of intuitive observation.


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The aim of this work is to investigate the potassium catalytic effect on pyrolysis and CO2 gasification behaviors of ash free coal chars. In this study, three different Chinese coals, Zunyi anthracite (ZY), Shenfu bituminous (SF) and Yunnan lignite (YN), were deashed and pyrolyzed before experiment. K2CO3 was used as catalyst to study the transformation of potassium in catalytic pyrolysis and gasification. In addition, an in-situ Raman spectroscopy was used to characterize the evolution of char structure. 2 Experimental 2.1 Sample preparation Three different Chinese coals, Zunyi anthracite (ZY), Shenfu bituminous (SF) and Yunnan lignite (YN), were used in this work. The raw coal was pulverized and sieved to obtain a fraction sample of particle sizes from 75 to 125 um. The ash contents of coals are summarized in Table 1. On a dry basis, the ash contents are approximate 18.30 wt%, 8.06 wt% and 32.0 wt%, respectively. All the three coals have high content of alkali and alkaline earth metal (AAEM) which have catalytic effect on coal pyrolysis and gasification. To investigate the catalytic effect of K2CO3, it is better to remove the ash from coal before experiment to avoid possible interactions between inherent minerals and K2CO3 or coal. Aqueous HCl (~20%) and HF (~40%), obtained from Sinopharm Chemical Regent Co., Ltd, were used to remove the inherent minerals of coal. The acid-washing processes are as follows:[25, 26] Firstly, 50 g coal sample was put into 500 ml of aqueous HCl, and the mixture was stirred at normal temperature for 24 h using a magnetic stirrer. After filtering, the HCl-washed coal was put into 500 ml of aqueous HF and stirred for another 24 h. And then the acid-washed coal was washed using deionized water continuously to remove Cl-1which had influence on catalytic effect of K2CO3. After washing it for several times, the Cl-1 in


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the sample was almost removed by deionized water.[25] Finally, the demineralized coal was dried at 105 oC in a vacuum oven for 24 h. The demineralized coal char was prepared in a fixed-bed reactor in a constant flow rate of 1 L/min N2 atmosphere. The reactor was heated at a heating rate of 25 oC /min until the temperature reached to 900 oC and held for 1 h before cooling to room temperature. The coal chars were named as AF-ZY char, AF-SF char and AF-YN char, respectively. The catalyst loading was 10 wt% in this study,[27] which is the weight percent of the metal atoms reference to the amount of char. The method of catalyst loading was a solution impregnation method. The procedure of impregnating was as follows: a certain amount of catalyst powders (purity>99.99 wt%) were completely dissolved in 100 ML of deionized water to form solution, and then 10 g char was added into the solution; this mixture was stirred at 70–80 oC under N2 atmosphere using a magnetic stirrer until the liquid was transformed into a thickened mass. Finally, this thickened mass was dried at 105 oC in a vacuum oven for 24 h. The properties of raw coal and the demineralized coal char are given in Table 2 2.2 In-situ Raman spectroscopy experiment In-situ experiments were conducted by an in situ Raman spectroscopy system. [28] The system is mainly composed of four parts: a Thermofisher DXR Raman spectrometer, a heating stage (TS1500, Linkam), a temperature controller and an online computer system gathering sample spectrogram during different reaction phases continuously. The schematic diagram of the in-situ Raman spectroscopy system is shown in Fig. 1. A microscope equipped with a 50*lens was used to focus the excitation laser beam (514.5 nm exciting line of a Spectra Physics Ar-laser) on the sample and to collect the Raman signal in the backscattered direction. The beam was controlled to


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reach on the surface of char particles with a final laser power of ~2 mW and a spot diameter of 1 µm. The laser spot was much larger than the size of carbon micro-crystallites, ensuring the collection of the average information from a large number of the micro-crystallites in the char. The spectra were recorded in the wavenumber range of 1000–1800 cm-1 covering the first-order region. The acquisition time for each spectrum was 30 s. For the accuracy of experimental results, each experiment repeated three times as well. About 2 mg of corresponding sample was placed on the center surface of the heating stage which was heated at 25 oC /min to 105 oC and held for 30 min to drive off the moisture in the sample. Then the system was heated at the rate of 25 oC /min to the prescribed temperature (800 °C) under a continuous nitrogen flow of 100 mL/min. The Raman signal was collected at 20, 200, 400, 600 and 800 °C, respectively. Once the temperature reached to 800 °C, the N2 was switched to CO2 and the gasification started immediately. The evolution of char structure during the gasification process was also measured by collecting the Raman spectra continuously. As a comparison, the Raman spectra of ex-situ prepared chars in a fixed-bed reactor were also collected. 2.3 Experiments in a fixed-bed reactor A fixed-bed reactor system (as shown in Fig.2) depicted in an earlier work[27] was used to characterize the K2CO3 catalytic pyrolysis and gasification behavior of coal char. The system is primarily composed of four parts: gas feeding, reactor, controlling units, and analysis units. The reactor (0.05 m i.d. and 1.0 m height) is made of a special heat-resistant Inconel 625 alloy, designed to maintain up to 950 oC and 6 MPa. Metal supports and quartz sand were first added to the reactor, followed by an alundum tube (50 cm height), and 5 g samples in each run were placed


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in the middle area of the reactor. With the use of quartz and the alundum tube, the samples were not in direct contact with the metal tube, thus avoiding the catalytic effect of the metal tube on the pyrolysis or gasification of the coal. The furnace was heated at a rate of 25 oC /min in N2 atmosphere until it reached the predetermined temperature (800 °C). Then the N2 was switched to CO2 and the gasification started as the same process in the in-situ Raman spectroscopy system. Meanwhile, the flow rate of the outlet gas was measured by a flow meter, and the major gaseous products were quantitatively determined using an online non-dispersive infrared flue gas analyzer (Gasboard-3100). At the end of each experiment, the reactor was cooled to room temperature in N2 atmosphere. The remaining chars were collected for further analysis. Each experiment was repeated three times. The ex-situ chars were prepared as follows: in the pyrolysis process, chars were heated to 200, 400, 600 and 800 °C respectively and then cooled down to room temperature, the Raman spectra of those ex-situ char were collected at room temperature. In the gasification process, the chars were gasified for different times and cooled down to room temperature in N2 atmosphere, then the Raman spectra were collected as the same way of pyrolysis chars at room temperature. 3 Results and discussion 3.1 Catalytic pyrolysis of AF chars in fixed-bed reactor Chars were pyrolyzed in fixed-bed reactor and the product gases were measured by an infrared flue gas analyzer (Gasboard-3100). Results show that no gas products release from chars without K2CO3 addition because the volatiles in coal were almost removed in char prepared process at 900°C (not shown). But the catalysts loading chars release CO and a small amount of CO2. The amounts of gases at different pyrolysis temperatures are shown in Fig.3.


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It is obvious that the amount of CO is much more than that of CO2 and the product yields are connection with coal rank. The high rank of coal char, ZY char, produced fewer gases for its low reactivity. As a comparison, pure K2CO3 was pyrolyzed in N2 atmosphere under the same condition and no gas emission was observed as this temperature was far below its melting point of 891 oC[29] (not shown). So it could be inferred that K2CO3 reacted with char in heating stage with the production of CO and CO2. The following reactions were assumed to occur. ‫ܭ‬ଶ ‫ܱܥ‬ଷ + 2‫ܭ → ܥ‬ଶ ܱ − ‫ ܥ‬+ 2‫ܱܥ‬

(6)

‫ܭ‬ଶ ‫ܱܥ‬ଷ + 2‫ ܥ‬− ܱ → ‫ܭ‬ଶ ܱ − ‫ ܥ‬+ 2‫ܱܥ‬ଶ

(7)

‫ܭ‬ଶ ‫ܱܥ‬ଷ + 2‫ܭ → ܱ = ܥ‬ଶ ܱ − ‫ ܥ‬+ 2‫ܱܥ‬ଶ

(8)

Where the K2O-C is a complex intermediate produced in the pyrolysis process. The pure K2CO3 pyrolysis experiment reflected that the CO2 was not produced by the decomposition of K2CO3 but released from the char sample itself as the chars contained different amount of oxygen as shown in Table. 2. A part of stable oxygen containing functional groups remained in the char would react with K2CO3 with the release of CO2. Relationship between CO2 amount and oxygen content was shown in Fig. 4. It can be found that the CO2 production is proportional to mole ratio of O/C. The AF-YN char had high content of oxygen and produced more CO2 in pyrolysis. 3.2 In-situ catalytic pyrolysis Chars were pyrolysis in an in-situ Raman spectroscopy system and the Raman spectra of chars at different temperatures were collected and resolved by a curve fitting software, Origin8.5/Peak Fitting Module, into 4 Lorentzian bands (G, D1, D2 and D4) and 1 Gaussian band (D3 band). Fig. 5 shows curve-fitted results of SF char in the first-order region. It can be found that there are two characteristic peaks at 1350 cm-1 (D band or “defect” band) and 1580 cm-1 (G


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band or “Graphite” band) in each spectrum. The intensity of G band is stronger than that of D band. The D1 band is commonly called the defect band and appears at ~ 1350 cm-1 and can be attributed to the in-plane imperfections such as defects and heteroatoms and corresponds to a graphitic lattice vibration mode with A1g symmetry.[30-32] The D2 band at ~1620 cm-1 is always appearing as a shoulder on the G band, which represents the vibration mode of disordered graphitic lattices and corresponds to a graphitic lattice mode with E2g symmetry.[33, 34] The D3 band, at ~ 1500 cm-1, is suggested to originate from a sp2 band form of amorphous carbon including organic molecules and fragments of functional groups[35] and the D4 band at ~ 1200 cm-1 is still under debate for its attribution and is tentatively attributed to sp2-sp3 bonds or C–C and C=C stretching vibrations of polyene-like structures.[36, 37] The G band or ‘‘Graphite’’ band (~ 1580 cm-1) corresponds to an ideal graphitic lattice vibration mode with E2g symmetry.[34, 38] The variation of ID1/IG and IG/IAll ratios for six different chars with temperatures are shown in Fig. 6. ID1/IG is inversely proportional to the crystalline size,[30] the increase of ID1/IG means the decrease of char structure order. Fig.6a shows that the ID1/IG ratios of K-loading chars are obviously increase with the rising temperature, indicating the increase of disordered structure in char. Compared to chars without catalyst, the ID1/IG ratios of K2CO3-loading chars are higher, which means the addition of catalyst has significant catalytic influences on the char structural evolution during pyrolysis. For chars without catalyst, the ID1/IG ratios are increase slightly. Fig. 6b shows the IG/IAll ratios vary with temperatures. The IG/IAll ratio represents the graphite structure ratio in chars and can be used to characterize the highly ordered char structure. It can be found that all IG/IAll ratios decrease with the rising temperature indicating the decrease of graphite structure at high temperature.


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In addition to catalyst, the coal rank also has influence on the Raman results. It can be found that the AF-YN char has the highest ID1/IG ratio while the AF-ZY has the lowest ID1/IG ratio. The opposite trends are observed in Fig. 6b. It can be attributed to the high content of disordered structure in high rank coals. For comparison, the ex-situ prepared chars at different temperatures were also analyzed by a Raman spectrometer. The results are shown in Fig.7. The band area ratios of chars without catalyst have no significant change after pyrolyzed at different temperatures. Compared to the in-situ data, it can be inferred that the temperature has effect on the char structure. At high temperature, a part of ordered structure is cracked to produce active carbon which would react with gasification agents. It can also explain why char has higher reactivity at high temperature, because more disordered structure are produced. For chars with catalyst loading, the ID1/IG ratios increase slightly indicating the disordered structures are produced in the heat treatment process, in which the catalyst reacts with carbon and oxygen contained groups with CO and CO2 release. But the ID1/IG ratios of 800 oC treated K2CO3-loading chars are lower than that at 800 oC by in-situ analysis reflecting that the intermediates produced at high temperature would partially disappear at room temperature. 3.3 Gasification of chars Gasification behavior was measured in fixed–bed reactor. Char was heated to 800 oC and N2 was switched to CO2, then the gasification started immediately. The gas (mainly CO) release rate is shown in Fig. 8. It can be found that the gas release rate reaches the maximum value in a short time indicating high reactivity of chars at initial time. In the heating stage, active carbon was produced which could easily react with CO2 once contacting with it to produce CO. As the


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reaction progresses, the gas release rates gradually decreased meaning the decrease of gasification reactivity of chars. Before gasification with CO2, chars had the maximum concentration of active carbon and with the consumption of active carbon, the reactivity decreased. It can be found that the K2CO3-loading chars have higher gas release rates than that of chars without catalyst for the reason of more active carbon produced under the effect of catalyst in the pyrolysis process. The K combined with carbon to produce gasification intermediates which were active carbon and could easily react with CO2. At the same condition, high rank of coal char has low gasification reactivity indicating that the low rank coal char is easier to react with catalyst with the producing of intermediates. To investigate the evolution of chars during the gasification process, the in-situ gasification experiments were also conducted in the in-situ Raman spectroscopy system and the Raman spectra of each sample were collected. Fig.9 shows the variation of ID1/IG with gasification time at 800 oC. For the K2CO3 catalyzed samples, the ID1/IG ratios increase in initial time and then decrease soon. In initial time, it is a stage when intermediates are generated quickly via reaction between K2CO3 and char, so the ID1/IG ratio increases. Meanwhile, the slow evaporation of K in gasification leading the decrease of ID1/IG indicating the increase of char structure degree.[39, 40] But for the chars without catalyst, the ID1/IG ratios decrease directly from the beginning of gasification, which may attribute to the consumption of disordered carbon in gasification. It reveals that the uncatalyzed/catalyzed gasification of char undergoes different reaction progress. In addition, the ex-situ data were gathered and analyzed and the results was shown in Fig.10. The ID1/IG ratios of K2CO3 loading chars, which decrease directly, differ from that of in-situ data. It can infer that the intermediates produced at high temperature do not exist at room temperature,


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so the In pyrolysis, a remarkable increase in ID1/IG occurs in the K2CO3 catalyzed chars, indicating that the degree of disordered carbon content of the K2CO3 catalyzed chars rapidly increases, thus resulting in a decrease in IG/IAll (Fig. 6b). This result can be attributed to the fact that K is reduced by char at high temperature (eq.(6),(7)and (8)).[41] In this process, K diffuses into the aromatic rings to form intercalation compounds with carbon which is also assumed be an intermediate (K2O-C). The intermediates break down the C-C bonds making the large aromatic rings cracking to form small rings, thus leading the decrease of char structure order degree. For the chars without catalyst, the ID1/IG ratios increase for the cracking of char under high temperature. In the follow step, gasification, the intermediates react with gasification agent CO2 with the release of CO, which is a process of absorption oxygen by K2O-C. The reaction occurs as follow: ‫ܭ‬ଶ ܱ − ‫ ܥ‬+ ‫ܱܥ‬ଶ → ‫ܭ‬ଶ ܱଶ − ‫ ܥ‬+ ‫ܱܥ‬

(9)

In eq.(9), the reaction intermediate is oxidized, thus consisting a redox cycle. The ex-situ data prove that the reaction intermediate does not exist at room temperature. Researchers also found that K2CO3 could not exit at high temperature.[28] So it can infer that K participates the gasification process and the formation of intercalation compounds decreases the char structure order degree.

4 Conclusion

Three different ranks of coals and K2CO3 were used in this work to study the reactions between catalyst and coal char. An in-situ spectroscopy system was used to characterize the evolution of char structure in catalytic pyrolysis and gasification. The main conclusions are as follows:


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(1) The addition of K2CO3 can significantly improve the reactivity of chars. K2CO3 reacts with char to produce CO and CO2. And CO2 is mainly produced from the decomposition of oxygen containing functional groups. (2) K2CO3 is first reduced to produce intermediate which is an intercalation compounds making the decrease of char structure order degree. (3) The ex-situ data proved that the intermediate not exits at room temperature and the decrease of ID1/IG ratios in gasification is attributed to the slow loss of K at high temperature. (4) The in-situ Raman spectroscopy system can help understanding the true status of char structure at high temperature.

Acknowledgments This work was supported by the National Natural Science Foundation of China (NO. 21676091) and Fundamental Research Funds for the Central Universities (222201717018). References:

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Table captions Table 1 Ash compositions of coal Table 2 Proximate analysis and ultimate analysis of sample


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Table 1 Ash compositions of coal Constituent wt/%

Sample ZY

Al2O3

CaO

Fe2O3

SiO2

SO3

NaO

K2 O

MgO

30.23

0.69

2.26

58.10

0.37

2.27

1.56

0.44

SF

17.21

13.85

11.78

44.52

6.53

1.89

1.35

1.55

YN

16.09

17.28

11.23

34.27

13.67

0.37

1.01

3.78


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Table 2 Proximate analysis and ultimate analysis of sample Sample

Proximate Analysis/d (%)

Ultimate Analysis/daf (%)

Mole ratio

VM

FC

Ash

C

H

N

S

O*

O/C

ZY coal

7.89

73.81

18.30

94.83

2.44

1.34

0.87

0.52

-

SF coal

33.29

58.65

8.06

83.09

2.31

1.20

0.90

12.50

-

YN coal

43.22

24.58

32.20

63.92

5.33

1.63

1.62

27.50

-

AF-ZY char

0.87

96.00

1.53

96.11

0.95

1.68

0.94

0.32

0.0025

AF-SF char

0.17

99.09

0.74

95.96

1.02

1.40

0.40

1.22

0.0095

AF-YN char

0.22

98.99

0.79

92.36

1.97

1.73

1.82

2.12

0.0172

VM-volatile matter; FC-ﬁxed carbon; d-dry basis; daf-dry ash free basis; O*-calculated by difference


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Figure captions Fig. 1. Schematic diagram of in-situ Raman spectroscopy Fig. 2. Schematic diagram of fixed-bed reactor system Fig.3. The amounts of product gases at different pyrolysis temperatures Fig. 4 CO2 amount variation with mole ratio of O/C Fig. 5 Curve-fitted results of AF-SF char in the first-order region Fig. 6 Variation of ID1/IG and IG/IAll ratios with rising temperature Fig. 7 Variation of ID1/IG and IG/IAll ratios of ex-situ prepared chars with different temperatures Fig.8 CO release rate of different coal char in gasification Fig. 9 variation of ID1/IG with gasification time at 800 oC Fig. 10 variation of ID1/IG with gasification time at normal temperature


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Fig. 1. Schematic diagram of in-situ Raman spectroscopy


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Fig. 2. Schematic diagram of fixed-bed reactor system 1: mass flow controller; 2: check valve; 3: pressure gauge; 4: fixed-bed reactor; 5: thermocouple; 6: electric furnace; 7: liquid/gas separator; 8: valve; 9: pressure regulator; 10: gas washing bottle; 11: gas drying bottle; 12: flow meter; 13: online gas analyzer; 14: computer


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Fig.3. The amounts of product gases at different pyrolysis temperatures


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Fig. 4 CO2 amount variation with mole ratio of O/C


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Fig. 5 Curve-fitted results of AF-SF char in the first-order region


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Fig. 6 Variation of in-situ ID1/IG and IG/IAll ratios with rising temperature


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Fig. 7 Variation of ID1/IG and IG/IAll ratios of ex-situ prepared chars with different temperatures


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Fig.8 CO release rate of different coal char in gasification


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Fig. 9 variation of in-situ ID1/IG with gasification time at 800 oC


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Fig. 10 variation of ex-situ ID1/IG with gasification time at normal temperature


In-situ study on K2CO3-catalyzed CO2 gasification of coal char

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