Catalytic Gasification of Crushed Coke and Changes of Structural

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Catalytic Gasification of Crushed Coke and Changes of Structural Characteristics Boyang Bai, Qingjie Guo,* Yankun Li, Xiude Hu, and Jingjing Ma State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, Ningxia 750021, People’s Republic of China ABSTRACT: The influence of the catalyzer on crushed coke gasification reactivity and its structural changes during steam gasification were studied in the present paper. Crushed coke samples were loaded with three kinds of catalysts, CaO, Na2CO3, and Fe2O3, and then dried at 950 °C. The coke gasified with steam in thermogravimetric analyser was used to study the influence of catalyzer loading on the gasification reactivity. Coke samples under different reaction times were prepared using a tube-type furnace. A specific surface area and porosity analyzer, an X-ray diffraction (XRD) apparatus, and a Raman spectra apparatus were used to characterize the structure of coke. A comparison of the reactivity of coke loaded with different kinds of catalyzers revealed that the coke with Na2CO3 exhibited higher gasification reactivity, and the optimum load of Na2CO3 on the coke was 5%. Brunauer−Emmett−Teller data showed that the addition of Na2CO3 can effectively corrode the pores on the surface of the coke to produce many small holes on the surface of the coke and widen the aperture of the coke. When Na2CO3 is added to the channel of the coke, the smaller porosity of the coke becomes more crowded. Coke loaded with Na2CO3 has a larger average pore diameter, which is more beneficial to the reaction with steam. Raman spectra analysis showed that the degree of graphitelike structures in the graphite layer increases as the reaction proceeds and that Na2CO3 inhibits the growth of large aromatic ring structures and decreases the content of graphite-like crystal structures. The XRD results suggested that some of the sodium atoms are inserted into the coke matrix and distort the orientation of carbon crystallites, thus effectively hindering the trend of carbon graphitization in the process of gasification. The XRD analysis result is in accordance with the results of Raman analysis. improve the gasification reaction rate. To find suitable catalysts, researchers have investigated almost all of the periodic table of elements. At present, the catalysts with known higher activity are mainly alkali metals, such as K2CO3 and Na2CO3, alkaline earth metals, such as CaO and BaSO4, transition metals, such as Fe2O3 and Ni(NO3)2, etc. Different catalysts have different catalytic abilities.6−8 Alkali metal has good catalytic activity but is easily

1. INTRODUCTION The development and utilization of energy are major problems facing mankind. Currently available energy mainly includes oil, natural gas, coal, nuclear energy, and renewable energy. In 2016, the global coal production was approximately 7.35 billion tons, of which China, as a major producer of coal, accounted for approximately 45.7% of the coal production, and a portion of the coal is used as a raw material for the preparation of coke. Coke is an important raw material in the metallurgical industry, but during transport and handling, approximately 8−10% of the coke will turn into char dust (the maximum size is less than 20 mm). There are at least 50 million tons of crushed coke produced annually in China. Limited by the quality and grain size of metallurgical coke, a large number of crushed coke can only be treated as low-order fuel or secondary coking materials for coking raw materials. Gasification technology is considered to be a promising option for the clean utilization of coal as a result of its high efficiency and flexibility.1 The gasification technology will convert coke into synthesis gas for further processing of methanol, alkene, and other chemical products, not only increasing the added value of coke but also solving the problem of low coke price and no market demand, with obvious environmental and economic benefits. However, crushed coke has the characteristics of low gasification activity and a high gasification initiation temperature. Therefore, improving the gasification rate of coke is a difficult problem. In recent years, many typical gasification techniques have been developed, such as pyrolysis, catalytic pyrolysis, oxygen/steam gasification, and catalytic steam gasification.2−5 The catalytic gasification technology of coal is used to promote the coal gasification reaction by adding a catalyst, which can significantly © XXXX American Chemical Society

Table 1. Proximate and Ultimate Analyses of Crushed Coke proximate analysis (wt %)

ultimate analysis (wt %)

M

A

V

FC

C

H

O

N

S

LHV (MJ kg−1)

0.43

13.94

1.33

84.30

85.15

0.26

0.18

1.07

1.24

26.17

Figure 1. Flow diagram of the experimental apparatus. Received: January 16, 2018 Revised: February 12, 2018 Published: February 13, 2018 A

DOI: 10.1021/acs.energyfuels.8b00192 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 2. Thermal weight loss diagram of crushed coke.

Figure 3. Carbon conversion rate of crushed coke with different catalysts.

Figure 4. Gasification rate of crushed coke with different catalysts. B


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Coal gasification using agents such as CO2 or H2O is another important step in gasification technology. With steam as the gasifying agent, the content of hydrogen in the product gas can be increased, the calorific value of the syngas can be increased, and the additional value of crushed coke will be increased; thus, steam is selected as the gasifying agent for the gasification reaction. A large number of studies have found that CaO, K2CO3, Na2CO3, Fe2O3, and other cheap catalysts have good catalytic activity in a steam atmosphere.10,12−17 The carbon structure is also an important factor affecting gasification activity. Chang et al. found that the gasification activity of biochars has an important relationship with the carbon structure,18 Li et al. have studied the influence of metal ions on the structure of demineralized lignite char, and the result shows that metal ions inhibit transition to graphitization.19 Unfortunately, information concerning the effect of the catalyst on the carbon structure of crushed coke during gasificaton is limited. To solve the problem of the slow gasification rate of crushed coke, the key objectives of the present study were to investigate suitable catalysts and the change of the structure of crushed coke during the reaction. These investigations can be classified

volatilized at high temperatures and easily reacts with minerals in coal ash, such as kaolin, which affect its activity. Alkaline earth metals play a very important role in the process of steam gasification of graphite; however, the activity will decrease as a result of sintering problems, and the activity is poor at high temperatures. The iron catalyst of the transition metals can obviously accelerate the gasification reaction rate of lignite but easily reacts with trace elements (such as S) in coal, resulting in deactivation. Different coals and gasification agents influence the activity of the catalyst. Matsuoka et al.9 indicated that the degree of graphitization is high and that the char with smaller porosity is unfavorable for adhesion of the catalyst. Yeboah et al. showed that the catalytic activity of K2CO3 is stronger than that of Na2CO3 in a CO2 atmosphere,10 but Kwon et al. showed that the catalytic activity of Na2CO3 is stronger than that of K2CO3 under a steam atmosphere.11 The pore structure and ash content of coke have a certain influence on the addition of the catalyst, and the gasification agent has a certain influence on the activity of the catalyst. Therefore, it is very important to select a suitable and cheap catalyst to improve the gasification activity of crushed coke.

Figure 5. Carbon conversion rate of crushed coke with different loading catalysts.

Figure 6. Gasification rate of crushed coke with different loading catalysts. C


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the M-coke as a catalyst (denoted Na-coke, Fe-coke, and Ca-coke). First, a certain amount of Na2CO3, Fe(NO3)3, and CaO was dissolved in a small amount of deionized water; the solution was added to the corresponding amount of M-coke, heated at 90 °C, stirred for 2 h, then placed in a dry oven, and dried at 110 °C. To eliminate the influence of Na2CO3 and Fe(NO3)3 decomposition, all samples were heated in a tube furnace using an argon atmosphere at 950 °C for 30 min. 2.2. Experimental Setup and Procedure. 2.2.1. Steam Gasification Experiment. The gasification reaction characteristics of the M-coke with the catalyzer were determined with a simultaneous thermal analyzer (STA449 F3 Jupiter, Netzsch Instruments, Germany). Previous basic experiments showed that the starting temperature of M-coke was approximately 950 °C; therefore, the reaction temperature of the gasification experiment was 950 °C. Approximately 15 mg (±0.2 mg) of M-coke was loaded into the ceramic pan of the thermogravimetric analyser, heated from ambient to 950 °C at 10 °C/min, and then maintained at this temperature for 1 h to ensure sufficient conversion of coal. The heating process was performed in an argon atmosphere (60 mL/min, 99.999%), and the constant temperature process used steam as the gasifying agent.

into two aspects. First, the gasification characteristics of crushed coke with three types of cheap catalysts (CaO, Na2CO3, and Fe2O3) and different loads were investigated using thermogravimetric analyser (TGA) to choose a suitable cheap catalyst and loading capacity. Second, the coke before and after loading the catalyst with different reaction times was prepared and characterized by X-ray diffraction (XRD), Raman spectroscopy, Brunauer−Emmett−Teller (BET), and scanning electron microscopy (SEM) to investigate the effect of the catalyst on the coke structure in the reaction process.

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Crushed coke from the Ningxia Qinghua Group was selected as the fuel, referred to as M-coke, whose proximate analysis and ultimate analysis results are listed in Table 1. The M-coke particles were sieved into the size range of 100−150 μm. The catalyst was loaded on the M-coke by impregnation, and 3 wt % Na2CO3, Fe2O3, and CaO (percentage mass of metallic ion) were impregnated on

Figure 7. Reaction conversion rate of coke at different reaction times.

Figure 8. Pore size distribution of coke. D


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Energy & Fuels When the reactor temperature rose to 150 °C, the steam was pumped into the reactor by a micro pump. The steam flow rate was 3 g/h, and the steam pressure was 70%. The carbon conversion ratio (x) and gasification rate (R) of coke were calculated as follows:

x=

(m0 − mt ) (m0 − mash)

(1)

R=

dx dt

(2)

loaded into the ceramic pan and placed at one end of the quartz tube, which was sealed, blown with argon for 30 min (200 mL/min, 99.999%), and then heated from ambient to 950 °C at 10 °C/min. After a constant temperature was reached, the ceramic pan was pushed into the central area of the furnace. After 10 min, the argon was switched to steam (3 g/min, with a steam partial pressure of 70%); when the pressure was stable for 10 s, the timing began. After the reaction time was reached, the steam was switched to argon. Meanwhile, the sample boat was pulled to the end of the quartz tube, and the quartz tube was immediately removed from the tube furnace and placed in air. To investigate the effect of Na2CO3 addition on the M-coke at different reaction times, M-coke and Na-coke were prepared under different reaction times (5, 10, 15, 20, 25, 30, 35, 40, and 45 min). 2.2.3. Characterization of Crushed Coke. The change in the surface morphology of the coke was assessed using a scanning electron microscope (Quanta 250/Quanta 430, FEI NanoPorts, Hillsboro, OR, U.S.A.).

where m0, mt, and mash are the initial quality of the coke, the coke mass at gasification time t, and the ash and residual catalyst mass, respectively. 2.2.2. Coke Sample Preparation. The M-coke sample was prepared using a tube-type furnace, which is shown in Figure 1. Coke gasified at different reaction times in the steam gasification process was prepared using this device. Approximately 5 g of coke was

Figure 9. Variation of pore structure parameters of coke during steam gasification (a, M-coke; b, Na-coke). E


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Energy & Fuels The resolution of the scanning electron microscope is 1 nm, and the magnification is from 2 × 104 to 8 × 105. The carbon structures of coke were assessed using a Raman spectrometer (DXR, Thermo Fisher Scientific, Waltham, MA, U.S.A.) consisting of a 532 nm excitation laser and a confocal microscope. The coke samples were spread on a glass slide, and four regions were chosen randomly for analysis. Spectra were recorded over the range of 800−2000 cm−1, and all four spectra were averaged to analyze the heterogeneity of coke. A specific surface area and porosity analyzer (ASAP2020-HD883FLEX, Micromeritics, Norcross, GA, U.S.A.) was used to determine the pore structure. The samples were measured at −196 °C using liquid nitrogen as the adsorbate. The pore structure parameters, such as the

specific surface area, pore size, and pore volume, were calculated using the BET model and Barrett−Joyner−Halenda (BJH) model. The microcrystalline structure was analyzed using an X-ray diffractometer (D/max 2200 PC03030502, Rigaku, Japan). Cu target radiation was used, with a voltage of 40 kV, a current of 100 mA, Ra of 0.3 mm, Da of 1°, scanning range 2θ of 10−80°, a continuous scan type, and a measurement accuracy of 0.02.

3. RESULTS AND DISCUSSION 3.1. Catalyst for Catalytic Gasification of Crushed Coke. M-coke has the characteristics of a high gasification temperature;

Figure 10. continued F


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in 1 h of reaction time at 950 °C, the carbon conversion rate was only 44.8% and the maximum gasification rate was 1.65%/min. With the addition of CaO, there is little effect on the gasification process and the carbon conversion and maximum gasification rate show almost no change; this result shows that the addition of CaO leads to almost no change in the gasification reactivity of the coke. With the addition of Fe2O3, there is a certain reaction in the atmosphere of nitrogen because Fe2O3 has a solid−solid reaction with coke. The carbon conversion and maximum gasification rate are higher, demonstrating that Fe2O3 has a certain catalytic effect on the coke gasification reaction, but the catalytic ability is weak.

therefore, experiments were carried out in a thermogravimetric analyser to determine the starting temperature of its gasification. Figure 2 shows a thermal weight loss diagram of M-coke with temperature variation under a steam atmosphere. M-coke begins to gasify at approximately 920 °C; therefore, to ensure that the M-coke can be gasified, all subsequent gasification experiments were carried out at 950 °C. Figures 3 and 4 show the carbon conversion rate and gasification rate with the change of time in the process of the M-coke gasification reaction with different catalysts supported. It can be seen that the reaction activity of the steam of the M-coke is low;

Figure 10. SEM analysis of coke (a, M-coke; b, Na-coke). G


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area and pore volume show a trend of first rising and then decreasing, and the trend for the average pore diameter is the opposite. The increase of the surface area and pore volume of the coke is caused by the opening of closed holes and the reaction with steam and extension of holes on the surface of the coke. A number of micropore sizes increase and merge during the reaction, resulting in the surface area and pore volume decline at 40−45 min. Na-coke has a smaller pore volume and surface area and a larger average pore diameter because, when Na2CO3 is added to the channel of the coke, the smaller porosity of the coke becomes more crowded and, at the same time, Na2CO3 effectively enlarges the focal aperture through erosion. Steam more easily reacts with the pores with larger sizes; therefore, Na-coke has better gasification performance. The pore structure change of Na-coke is completely different from that of M-coke during 1 h of the gasification reaction. The surface area first increases and then decreases because the surface area increases gradually with the gasification reaction in the early stage, but the surface area decreases in the later stage as a result of the hole collapse, which results in a decline in the rate of gasification. As the reaction continues, the surface area increases. This is due to the continued erosion of steam, but the reaction rate is less related to the surface area. The results show that the addition of Na2CO3 accelerates the reaction process of steam and coke by increasing the pore diameter of the coke. Figure 10 shows the SEM analysis of coke. As time goes on, the surface of M-coke gradually becomes rough and porous, proving that the steam reacts with the coke on the surface and corrodes it.

However, after adding Na2CO3, the carbon conversion rate and maximum gasification rate were significantly increased, to 83.06% and 6.164%/min, respectively, proving that Na2CO3 can effectively improve the gasification activity of the coke. Many studies have shown that the catalyst will plug the pore structure of the focal surface, and the excess catalyst will not be fully exposed to the active surface of the coke; thus, the catalyst will have a maximum load.20,21 Figures 5 and 6 show the carbon conversion rate and gasification rate with time in the process of the M-coke gasification reaction for different loading catalysts. It can be clearly seen from Figures 5 and 6 that, when the Na2CO3 load is 1%, the carbon conversion rate and maximum gasification rate are increased at the same temperature compared to M-coke. As the load of the catalyst increases, the carbon conversion rate and maximum gasification rate of M-coke increase. However, when the load is raised from 5 to 6%, this has little effect on the gasification activity of M-coke, and the curves in Figures 4 and 5 almost coincide. Increasing the load to 10% resulted in a decrease in the carbon conversion rate and maximum gasification rate. It can be concluded that the optimal load of Na2CO3 for this M-coke is approximately 5%, and the continuous increase of the load does not increase the efficiency of coke gasification. Li et al.21 carried out many catalytic coal char gasification studies with various catalysts and different loads and found that, when Na2CO3 is used as a catalyst for catalytic gasification of char, its saturated load is approximately 25%, which is much higher than the 5% Na2CO3-saturated load in the catalytic gasification of M-coke found in this paper. This difference occurs because M-coke is prepared at a high temperature; therefore, the specific surface area is smaller and the pore structure is not developed. Thus, a small amount of catalyst will plug the pores on the surface of the M-coke and push the load of the catalyst to the limit. 3.2. Effect of Na2CO3 on the Structure of Crushed Coke. At present, research on the catalytic mechanism of alkali metal catalysts considers alkali metals to be eroded on the surface of coal to increase the active surface area and active sites of the coal surface to increase the reaction rate of coal gasification.22,23 At the same time, it is found that the catalyst can change the structure of the coke.18,24,25 Therefore, in this paper, a series of characterizations (BET, SEM, XRD, and Raman) were carried out to characterize the carbon structure of the coke and investigate the effect of the addition of Na2CO3 on the structure of M-coke in the reaction process. Figure 7 shows the reaction conversion rate of coke at different reaction times (0−45 min, sampling every 5 min). 3.2.1. Pore Structure. The pore structure is an important factor affecting the reactivity of coke gasification.26,27 Figure 8 shows the pore size distribution of coke. It can be seen that M-coke contains a large amount of micropores, while few mesopores are detected, and the main pore size of M-coke is approximately 3.8 nm. However, the pore size distribution of 1.1 and 5.7 nm pores increased obviously after the addition of Na2CO3, and the pore size distribution of 3.8 nm pores was reduced. This phenomenon occurred because Na2CO3 effectively erodes the surface, thus causing many 1.1 nm micropores on the coke surface; at the same time, as a result of erosion, some 3.8 nm pores are merged into 5.7 nm pores. It is proven that the addition of Na2CO3 can effectively corrode the pores on the surface of the coke to make many small holes on the surface of the coke and widen the aperture of the coke. The changes in the BET surface area and pore size at different reaction times for coke are shown in Figure 9. For the M-coke, during 1 h of gasification, the surface area is 5−100 m2/g, which is a lower range for coal; this may be the reason for the lower gasification reactivity of M-coke. The surface

Figure 11. Raman spectra of coke (a, M-coke; b, Na-coke). H


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ranging from 0 to 45 min at 950 °C. The G band at ∼1580 cm−1 has been attributed to the ideal graphitic lattice (E2g symmetry) stretching mode.29,30 The spectral bands of other observational spectra are typical representatives of the disordered graphite lattice (D bands). The D1 band at ∼1350 cm−1 has been associated with a vibration mode involving graphene layer edges (A1g symmetry)28,29,31 or alternatively with an A2 transition.32,33 The D2 band at ∼1620 cm−1 has been attributed to a vibration mode involving surface graphene layers (E2g symmetry).29,33 The high signal intensity between the two observed peaks has been allocated to a D3 band at ∼1500 cm−1 that is associated with the amorphous carbon content of coke (organic molecules, fragments, and functional groups).28,34,35 Moreover, the weak bands at ∼1180 cm−1 (D4) have been assigned to vibrations of disordered graphite lattices (A1g symmetry), sp2- and sp3-hybridized carbon bonds, C−C and CC stretching vibrations of polyenes, and ionic impurities.29,36 The parameters ID1/IG, ID3/IG, IG/Iall, and La represent the degree of graphite-like structures in the graphite layer (the smaller the ID1/IG, the larger the graphitization degree), the amorphous carbon fraction, the content of graphite-like crystal structures, and the crystallite size, respectively. La can be described as

With the addition of Na2CO3, the surface of the coke changed from smooth to rough, and it can be clearly seen that the fragments attached to the surface of the coke; with the passage of time, the pores on the M-coke surface maintain a larger aperture. Consistent with the results of BET analysis, Na2CO3 eroded the surface of the coke, increasing the average pore diameter of the coke and the rate of gasification. Meanwhile, Na2CO3 attached to the surface of the coke, thus reducing the specific surface area. 3.2.2. Raman Spectrum. The first-order Raman spectra of coke are generally characterized by two broad and strongly overlapping peaks with intensity maxima near 1580 cm−1 (G or “graphite” peak) and 1350 cm−1 (D or “defect” peak). According to the experimental observation and theoretical calculation, five bands corresponding to the different vibration modes of the crystal or molecular structure in the sample materials are presented. Sadezky et al.28 tested and compared nine different band combinations and demonstrated that the best fit to the Raman spectra of a wide range of coke samples was obtained with four Lorentzian bands (G, D1, D2, and D4) and one Gaussian band (D3); thus, in this paper, the Raman spectra of coke over the range of 800−2000 cm−1 were curve-fitted with five bands, D1, D2, D3, D4, and G, using PeakFit, version 4.12. Figure 11 shows spectral curve-fitting diagrams of the Raman spectrum and the evolution of the spectra of coke at different steps of the gasification process, that is, after reaction times

La = C(λ)(ID1/IG)−1

(3)

C(λ) = C0 + λC1

(4)

Figure 12. Changes of ID1/IG, ID3/IG, IG/Iall, and La during the process of coke gasification (a, ID1/IG; b, ID3/IG; c, IG/Iall; and d, La). I


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Energy & Fuels where C(λ) is the calibration factor of the wavelength, C0 is −12.6 nm, and C1 is 0.033 nm. The range of the effective wavelength is 400−700 nm; 532 nm was used in this study. The data in Figure 12 show the changes in ID1/IG, ID3/IG, IG/ Iall, and La. For the M-coke, as the reaction proceeds, ID1/IG and ID3/IG show a downward trajectory. The degree of graphite-like structures in the graphite layer is increasing, and the amorphous carbon fraction is reduced gradually. The amorphous carbon structure and defect structure change to the ordered crystal sp2 carbon atom; that is, the graphitization transformation occurs. At the same time, IG/Iall and La showed a rising trend with time, proving that the content of graphite-like crystal structures and the crystallite size are magnified. This occurs because the aromatic structure is greatly consolidated, making the carbon structure of coke more orderly, thus promoting the process of graphitization and coke crystallites merging with each other, which makes the size of the coke crystallites increase. Studies have shown that the

lower the degree of graphite-like structures of chars, the better the reactivity.9,37 From the aforementioned knowledge that, with the gasification reaction, the graphite-like crystal structure and the crystallite size of the coke become increasingly larger, the coke has a lower reactivity and the gasification rate will become slower with the gasification reaction. However, for the Na-coke, as the reaction proceeds, ID1/IG, ID3/IG, IG/Iall, and La showed relatively stable trajectories. Na2CO3 promotes the decomposition of large aromatic rings into small aromatic rings, simultaneously hindering the formation of large aromatic rings in the coke particles and coke crystallites merging with each other; thus, further graphitization of the coke during the reaction is prevented. In comparison, during the reaction, Na-coke has higher ID1/IG and ID3/IG and lower IG/Iall and La than the coke. The higher ID1/IG means that various forms of structural defects increased with the introduction of Na2CO3, and the lower IG/Iall and La and the higher ID3/IG reflect that partial large polyaromatic ring

Figure 13. XRD spectra of coke (a, M-coke; b, Na-coke). J


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Energy & Fuels structures transform into amorphous carbon structures under catalytic action. In other words, Na-coke has a smaller degree of graphitization, crystallite size, and content of graphite-like crystal structures; moreover, its amorphous carbon fraction is higher. This seems to provide evidence that added Na2CO3 will be beneficial to the steam gasification process of metallurgy coke because it inhibits the conversion from amorphous carbon to ordered carbon in the gasification process and there is a close relationship between the amorphous carbon structure and the reaction site. 3.2.3. XRD. Figure 13 shows the XRD spectra and the fitting diagram of coke. Coke has two characteristic peaks: the (002) peak (22−35°) and (100) peak (42−45°). In general, the narrower and higher the (002) peak, the better the orientation of the aromatic layer, and the narrower and higher the (100) peak, the larger the size of the aromatic layer. The intensity of the two characteristic peaks dramatically declined with Na2CO3 loading, and the peaks of crushed coke were sharper and narrower than those of Na-coke, which proved that the loading of Na2CO3 can effectively destroy the aromatic lamellar structure of coke. For further analysis, the XRD spectra of coke over the examined 2θ range were curve-fitted with three bands, 002, γ, and 100, using PeakFit, version 4.12. The (002) peak is due to the stacking of the graphite basal planes. The band on the left-hand side of the (002) peak, at approximately 22°, is the γ band, which is related to the packing of the saturated structure, such as aliphatic side chains, leading to the asymmetry of the (002) peak. Three parameters of the carbon crystallite structure were calculated using the Bragg equation and Scherrer equations24,38 d002 =

Lc =

La =

λ 2 sin(θ(002))

(5)

0.89λ cos(θ(002))

(6)

1.84λ β100 cos(θ(100))

(7)

β002

where d002 is the interplanar spacing of two aromatic microcrystalline layers, λ is 1.5406 Å, Lc and La are the average crystallite height and average diameter of the microcrystalline layer, respectively, β(002) and β(100) are the full width at half maximum of the (002) and (100) peaks, respectively, and θ(002) and θ(100) are the corresponding scattering angles, respectively. All of the calculated values of the parameters for the carbon crystallite structure are shown in Figure 14. For the coke, as the reaction proceeds, the value of d002 gradually declines. On the other hand, both Lc and La are gradually rising. It can be concluded that the aromatic structure is aggrandized, and the coke is proceeding in the direction of graphitization. Na-coke has a higher d002 and lower La and Lc, and these three parameters are all stable during the reaction process, demonstrating that Na2CO3 can hinder the further growth of the carbon microcrystalline size and the graphitization process of the coke. From the XRD analysis of coke samples, it can be seen that part of Na2CO3 is present in the form of NaC64 in the coke, and the results indicate that some of the sodium atoms are inserted into the coke matrix and distort the orientation of carbon crystallites. In addition, studies have shown that residual functional groups on coke would also destroy the parallelism of the layers in the basic structural units and the constancy of the interval between layers, resulting in a lower intensity of (002) and (100) peaks.39 In summary, sodium atoms affected the stability of stacking and hindered the graphitization process by entering into aromatic

Figure 14. Calculated values of parameters for the carbon crystallite structure (a, d002; b, Lc; and c, La).

layers; therefore, sodium improved the steam gasification reactivity of crushed coke. The result is in accordance with the results of Raman analysis.

4. CONCLUSION The following conclusions can be drawn from the above study: (1) Na2CO3 as a catalyst can effectively improve the steam gasification rate of crushed coke, and the Na2CO3 load of 5% has the best value. (2) The addition of Na2CO3 can effectively K


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Energy & Fuels

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corrode the pores on the surface of coke to produce many small holes on the surface of the coke and widen the aperture of the coke, and Na-coke has a larger average pore diameter, which is more beneficial to the reaction with steam. (3) The degree of graphite-like structures in the graphite layer increases as the reaction proceeds. Na2CO3 inhibits the growth of large aromatic ring structures and decreases the content of graphite-like crystal structures. Some of the sodium atoms are inserted into the coke matrix and distort the orientation of carbon crystallites, thus effectively hindering the trend of carbon graphitization in the process of gasification. The XRD analysis result is in accordance with the results of Raman analysis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Boyang Bai: 0000-0001-7740-2129 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the National First-Rate Discipline Construction Project of Ningxia (NXYLXK2017A04), the Key Research and Development Program of Ningxia (2016BY005), and the Ningxia Scientific and Technological Innovation Team for Coal Cleaning and Utilization.

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Catalytic Gasification of Crushed Coke and Changes of Structural

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