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Kinetics studies and structure characteristics of coal char under pressurized CO2 gasification conditions
Fuel 146 (2015) 103–110
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Fuel journal homepage: www.elsevier.com/locate/fuel
Kinetics studies and structure characteristics of coal char under pressurized CO2 gasification conditions Lang Liu a,b, Yan Cao b,⇑, Qingcai Liu a a b
College of Materials Science & Engineering, Chongqing University, Shapingba, Chongqing 400044, China Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, KY 42101, USA
h i g h l i g h t s Porous structures of char had major impacts on char CO2 gasification. Porous structures quickly developed and rapidly collapsed during the reaction. The graphite crystallites experienced limited change in CO2-gasification process.
a r t i c l e
i n f o
Article history: Received 20 November 2014 Received in revised form 29 December 2014 Accepted 1 January 2015 Available online 12 January 2015 Keywords: CO2 gasification Kinetics Char characterizations High-pressures
a b s t r a c t The combined effects of surface area, pore structure, degrees of graphitization and graphite crystallites on the char gasification kinetics were studied under elevated pressure and temperature conditions using a High-Pressure Thermo-Gravimetric Analyzer (HP-TGA) and characterization methods, such as BET, XRD, Raman spectroscopy, FTIR and SEM. The BET and SEM results indicated that the porous structures of the chars developed quickly until the carbon conversion reached 0.9, and then rapidly collapsed to the end of the whole carbon conversion. The changes in char structures were in good agreement with the gasification rate. XRD, Raman and FTIR analysis provided evidence demonstrating an increased ratio of graphite to amorphous carbon in bone-structure of chars, which was attributed to the consumption of the amorphous carbon and the retention of the graphite carbon during the gasification process. The collected data implied that the contributions of surface area and pore volume of the char on its kinetics were much greater than its graphite crystallites during the char CO2 gasification. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Coal gasification is usually divided into two steps: coal pyrolyzation and char gasification. Because the rate of coal pyrolysis is much higher than char gasification, char gasification is usually considered as the rate-determining step in the overall coal gasification process. Water and carbon dioxide are two regular gasification agents, and water has been widely used as a gasification agent, but is recently limited to reach in the energy development sector, and alternatively, using CO2 as a gasification agent has been paid recent attention [1–3]. Understanding the kinetics of the CO2 char gasification under elevated temperature and pressure is helpful to better organize many industry processes, such as the conventional coal gasification and the novel chemical looping combustion and
⇑ Corresponding author. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.fuel.2015.01.002 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
gasification (CLC&G) for syngas production and power generation through an integrated gasification combustion cycle (IGCC). Many factors can influence char reactivity, such as coal rank, particle size, ash minerals, operational temperature and pressure and the partial pressure of the CO2 [4–8] and the char structure [9–11], among which the importance of char structure is not particularly well understood [12]. Due to the change of the char structure during char gasification, such as the porous structure [13], graphite single crystal ordering (‘‘crystallites’’) [14], and the degree of graphitization [15], the gasification process is very complicated [2,15–19]. The variations of char porous structures and graph crystallites during the char gasification are directly relative to the char kinetics, and further to process economics in the coal gasification, especially using CO2 as a gasification agent. Li et al. [10] studied the changes in coal char structure under pyrolysis and gasification conditions by the Raman spectra, and also acquired detailed information about the skeletal structures of the chars. Tay and Li [19] studied the structural features of gasified char under 800 °C in a fluidized-bed/fixed-bed reactor by
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FT-Raman/IR spectroscopy, and the results showed that the char structures changed drastically during the gasification process. Lorenz et al. [20] studied the role of char surface structure development in pulverized coal combustion under ambient pressure, and results showed that a larger surface area of the plug flow reactor char was due to a developed microspore structure, which was developed during devolatilization. Most of the published literature have focused on the variations of the porous structure and the degree of graphitization during the char gasification under ambient pressure [10,17,21]. Few previous studies have focused on the variations of char porous structure, graphite crystallites, and the degree of graphitization during the char gasification under pressurized CO2 conditions. Also the combined effects of surface areas, pore structure, degrees of graphitization and graphite crystallites on the char gasification kinetics have seldom been reported before. Many physical instruments can be ideal tools to understand derivations and variations of char structures during the char gasification process, such as BET for determinations of specific surface areas and pore structures of chars, X-ray diffraction (XRD) and Raman spectroscopy for the determinations of amorphous carbon and graphite carbons, and Fourier Transform Infrared spectroscopy (FTIR) for the determination of bonding between carbon and other major coal elements, that have carbon functional groups. This study mainly provided an overview of char structure changes during CO2 gasification by applying joint analytical measures to characterize char structures, but only focused on one single coal char. The emphasis of this paper was to track variations of the char structure and carbon crystallites using CO2 as a gasification agent under pressurized conditions, in view of the whole char gasification process. The major instrumentation involved included BET, XRD, Raman spectroscopy, FTIR and SEM. The char samples were obtained in a pressurized thermo-gravimetric analyzer under a CO2 environment. The temperatures were controlled isothermally in a range between 950 and 1150 °C, while allowing the pressure to vary between ambient and 2.0 MPa.
120 min. The combined gas flow rates of N2 and CO2 were 1000 mL/min. Based on the assumption that the ash decomposition was negligible [22], fractional carbon conversion was calculated by Eq (1). The carbon conversion efficiency (x) of char defined as the mass ratio of the gasified char at any time to the initial char can be expressed as follow.
x¼
w0 w 100% w0 wash
ð1Þ
Reaction rate (q) was calculated from mass ratio versus time profiles using Eq (2).
q¼
dx ð1 xÞdt
ð2Þ
where w0 is the initial mass of char, w is the instantaneous char mass at reaction time t, and wash is the mass of ash. 2.2. Char characterizations 2.2.1. BET The BET specific surface areas and pore volumes of chars were obtained using the N2 adsorption isotherms at 77 K by a Micromeritics ASAP 2020 apparatus in the relative pressure range between 0.07 and 0.35. The specific surface areas and pore volumes of chars were calculated by the BET equation. Prior to the analysis, the char samples were degassed at 150 °C for 3 h. 2.2.2. XRD For each of char samples, X-ray diffraction curves were obtained in a Varian Cary 100 UV–Vis spectrophotometer (200–800 nm) equipped with the DRA-CA-30 diffuse reflectance accessory using the Cu KR radiation (k = 0.1542 nm). The spectra were recorded in the 2h range of 20–70° with a scanning speed of 0.3° min1. The interlayer spacing of each sample was calculated by Bragg [23] equations according to the diffraction peak of 002 crystal plane.
2. Experimental 2.1. Materials
2.2.3. Raman Raman spectra of char samples were collected using a DXR Raman microscope (Thermo Scientific), including a 780-nm excitation laser and a confocal microscope with 3 objectives. The laser power at the sample surface was controlled at about 6 mW. The laser spot diameter reaching the sample was about 3 lm with specific resolution of 5 cm1. During the measurement, the char sample was spread out on a glass slide. About 5–10 particles were randomly chosen and analyzed. The spectra were recorded in the range of 800–2000 cm1, covering first-order bands. The acquisition time for each spectrum was 60 s.
The proximate analysis of the Kentucky Bituminous coal used in this paper is summarized in Table 1; the diameter of the selected char particles was approximately 200 lm. Char preparation and gasification processes were carried out consecutively in a pressurized thermo-gravimetric analyzer (TGA-HP150S). The TGA-HP 150s model is specially configured for gasification and related studies. It includes an integrated steam generator and a doublewalled reaction chamber. The reaction chamber is characterized as follows, the maximum weight capacity is 25 g, the weighing accuracy is ±0.0002%, the weighing precision is ±30 lg, the sensitivity is 10 lg, and the upper temperature and pressure limit is 1200 °C and 5.0 MPa, respectively. In the char preparation process, 500 mg coal was placed in a crucible boat, and then pressured and heated to design conditions under a nitrogen atmosphere. Thereafter, the gas was switched to CO2 with a molar concentration of 25–100% (balance nitrogen) and the gasification process was allowed to proceed. Finally, the sample was held isothermal for
2.2.4. FTIR The IR spectra were recorded from 4000 to 650 cm1 (with a resolution of 2 cm1 and 20 averaged scans) on a Perkin Elmer Spectrum One FTIR spectrometer with a UATR attachment using KBr pellets.
Table 1 Proximate and ultimate analysis of the Kentucky Bituminous Coal (wt%). Proximate analysis
Coal
Ultimate analysis (dry basis)
Moisture
Fixed Carbon
Volatiles
Ash
C
H
N
O
S
5.06
49.46
35.02
10.46
65.52
4.52
1.43
13.94
3.57
L. Liu et al. / Fuel 146 (2015) 103–110
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2.2.5. SEM High Resolution Scanning Electron Microscopy (HRSEM) experiments were conducted on a Hitachi S-5200 field emission scanning electron microscope. The microscope was operated between 5 and 25 kV. To prepare the samples for examination, silver paste was applied to an SEM stub. The samples were sprinkled onto the paste, and then coated with gold. 3. Results and discussion 3.1. Effects of operational conditions on char reactivity The gasification reactivity of the char using CO2 as gasification agent, in response to variation in the concentration of carbon dioxide, temperature and pressure, are shown in Figs. 1–3. Fig. 1 shows carbon conversion efficiency versus time as the CO2 concentration varied between 25% and 100% (by mole) at 2.0 MPa and 1050 °C. It can be observed that the carbon conversion efficiency was sensitive to the variations in CO2 concentrations. At such operational conditions, the increase of CO2 led to the rise of the carbon conversion throughout the char gasification process. The effects of temperature on the char gasification were pretty straightforward, as shown in Fig. 2. The elevation of gasification temperature generally resulted in increasing carbon conversion efficiency under constant pressure and constant CO2 concentration. As shown in Fig. 3, the effects of pressure on gasification were similar to that of temperature, the increase in gasification pressure resulted in an increase in the carbon conversion efficiency, which was consistent with published studies [3,22]. Fig. 4 shows the gasification rate versus carbon conversion efficiency using 100% pure CO2 as a gasification agent and at different operating conditions. It was observed that the gasification rate experienced an initially slow increase, followed by a rapid increase, and finally a decrease corresponding to the carbon conversion efficiency. It was well known that the char structural parameters, such as specific surface area and crystallites structure, varied significantly during the carbon conversion under varying gasification conditions [20,24,25].
Fig. 2. The effect of temperature on gasification rate at 25% CO2 by mole and 2.0 MPa.
3.2. Evolution of char structure during CO2 gasification To distinguish the structure changes of the char during gasification, the morphologies, pore size, and graphite crystal features of the char samples were characterized by BET, FTIR, Raman, XRD
Fig. 1. The effect of CO2 concentration on gasification rate at 2.0 MPa and 1050 °C.
Fig. 3. The effect of pressure on gasification rate at 25% CO2 by mole and 1150 °C.
Fig. 4. The reaction rate versus carbon conversion during char gasification at 100% CO2 by mole.
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and SEM. The selected char samples were prepared under different carbon conversions but at constant conditions of 100% CO2, 2.0 MPa, and 1050 °C. The structural parameters of the char samples included the initial surface area, the in-plane strain and the interplanar spacing, d002, ratio of perfect graphite structures to graphite structures with some defects, Ig/Id, and carbon functional groups. Measurements of BET specific surface areas and pore volume of the samples were presented in Fig. 5. The surface areas of the chars were significantly increased from 40 to 380 m2 g1 when the carbon conversion increased from 0 to 0.9. A similar trend was also observed in pore volumes of chars. Further increases in carbon conversion efficiency led to a decrease of both surface area and pore volume in the gasified char. It appears that the porosity of the chars was developed in favor of generating more micro-pores when the carbon conversion efficiency of selected coal chars varied from 0 to 0.9. The char gasification not only led to pore growth, but also new pore formation when the carbon conversion efficiency was blow 0.9. When the carbon conversion efficiency was over 0.9, pores started to coalesce and collapse, leading to a disappearance of the available pore for the continuous gasification reactions. The BET analysis of chars at different gasification stages indicated that the gasification reactions occurred within pores with the pores widening and new pore inception, thus pores disappeared at the final stage of char gasification. Fig. 6(a) shows the XRD patterns of the prepared char samples. The peaks, which appear at around 27° and 50° were attributed to 0 0 2 and 1 0 0 crystalline structures of graphite carbon in the char samples, respectively [26]. The parameter of the in-plane strain and the interplanar spacing, d002 [23] (XRD peak at 27°) was mostly used to extract the structural information of the char samples from the XRD patterns. The intensity of the graphite peak (0 0 2) and the values of d002 were presented in Fig. 6(b) and (c). These figures show that a graphite peak around 27° was observed during the whole char gasification process, and the intensity of the graphite peak increased with carbon conversion. The results suggest that amorphous carbon in the char was preferentially gasified. Fig. 6(c) showed that the interlayer spacing (d002), was essentially unchanged during the char-CO2 gasification, which suggests that the average size of the crystallites did not change remarkably. This trend was similar to the study described earlier [17], and which indicates that the structure of the perfect graphite changed minimally during the char gasification process. These results imply that graphite carbon is relatively inert in the CO2 gasification atmosphere [27]. The Raman spectra for samples can be designated roughly as a disordered carbon band (D-band) and a graphite band (G-band) at
Fig. 5. Change in surface area and pore volume during char gasification.
approximately 1350 cm1 and 1580 cm1 [19], respectively. Fig. 7(a) presented a typical Raman spectroscopic profile between 800 and 2000 cm1 for each char sample. In this paper, averaging the multiple Raman spectra for each sample was performed to manage the heterogeneity of char particles. As can be seen in Fig. 7, the intensity of the D and G band showed a prominent increase with carbon conversion, attributed to that the aromatic ring and oxygen-containing groups condensed during the first stage of gasification. It is believed that, as the char becomes more aromatized, the light absorptivity of the char would increase which leads to a decrease of the Raman intensity, meanwhile, oxygen-containing groups also affect the observed Raman intensity of the char and thus, increased the intensity of the Raman peak [28]. When the carbon conversion reaches 0.6, the intensity of the D band decreased, which was attributed to the breakdown of the aromatic rings and oxygen-containing groups. The ratio R1, is a very important parameter indicating the char structural information [16,19], and is defined as:
R1 ¼
Ig Id
ð3Þ
where Ig and Id are referred to as the intensities of G-band and Dband, respectively. Larger values of R1 denote less disorder or amorphous carbon in the chars [29]. The values of R1 for the char samples were given in Fig. 7(b). This ratio increased with carbon conversion efficiencies, which indicated that the relative amounts of large disordered or amorphous carbon decreased with carbon conversion efficiencies. This trend was likely caused by the consumption of the amorphous carbon, while preserving the perfect graphite structures, which have not changed during the char gasification (Fig. 6(c)). In order to get more information about the char structure change during the gasification, FTIR was carried out to study changes in the carbon functional groups during the char-CO2 gasification. The FTIR spectra of chars are given in Fig. 8. The spectra of char samples were characterized by four principal bands at wavenumbers of 2350 cm1, 1269 cm1, 1033 cm1, and 750 cm1, respectively. The band at 2350 cm1 is assigned to the presence of CO2 [30], the band at 1269 cm1 to the aromatic CO– or phenolic –OH stretching [31], the band at 1033 cm1 to the C–O stretch for the O–CH3 and C–OH [31], and the band at 750 cm1 to aromatic C–H out of plane vibrations [32]. Due to char gasification with CO2, the presence of the band at 2350 cm1 for CO2 adsorption was obvious for the prepared char samples as the conversion efficiency varied between 0.21 and 0.83. Additionally, there were no significant peaks at the 2350 cm1 band in the char samples at both initial and final stages of CO2 gasification, revealing limited CO2 adsorption. A similar trend was observed in other FTIR curves relative to other major FTIR peaks, such as those at 1269 cm1, 1033 cm1 and 750 cm1. It is natural to see the occurrences and changes of FTIR peaks corresponding to bonds between C and O, due to the active adsorption and desorption on the surface of chars. However, was interesting to see that similar development and diminishing of C–H band, not only occurred, but also actively changed under varying CO2 gasification conditions. This could be attributed to internal hydrogen constituents accessing and reacting with char during the gasification process. The evolutionary paths of all examined carbon functional groups showed an initial increase in absorbance during the early stages of gasification, which is most likely due to pore structure development of chars, followed by a decline leading to disappearance. The pore growth and opening of enclosed pores in the early stages of the char gasification in which more carbon functional groups were released or generated from such pores facilitated an increasing trend in absorbance of those carbon functional groups. However, at the final stage of the
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Fig. 6. Diffractograms and d002 obtained for the char samples in the 2h range of 20–70°.
Fig. 7. Typical Raman spectra and the Ig/Id of char samples.
Fig. 8. FTIR spectra of the char samples.
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Fig. 9. SEM images of char samples prepared at different gasification stages.
char gasification, the pores coalesced and collapsed, which resulted in the functional groups correspondingly diminishing. To gain better insight into the changes of char morphology during char gasification, the char samples were subject to observations using SEM. Fig. 9 shows the SEM images of the prepared char samples at different gasification stages. The SEM images clearly reveal sizes, surface roughness and smoothness, irregularities in shape and structures of visible surface pores of the char samples. In the initial stage of the char gasification, the chars were not smooth, but coarse with many small embossed features, which were identified to be the parts of a small surface bulge on the char surface. Using the SEM in micro-meter resolutions, there was evidence of
the pores growth and the opening of closed pores during the initial gasification stage. It was rationally inferred that the pores in submicro and nano-meter levels experienced similar developments. The initial pore opening on the char surface, presented an avenue whereby additional pores underneath the char surface were accessible to the gasification agent. This pore opening process moved forward until carbon conversion reached around 0.9. This action enabled greater char area into the gasification process which resulted in an increase in gasification rates until the carbon conversion efficiency reached around 0.9. There was also a secondary event on the surface of chars, in that the surface of the char gradually became smooth and the irregular bulges disappeared. This
L. Liu et al. / Fuel 146 (2015) 103–110
phenomenon started happening as the carbon conversion efficiency reached 0.6, and was significant when the carbon conversion efficiency exceeded 0.8. In the final stage of gasification when the carbon conversion efficiency was above 0.9, the surface of the chars was more like regular rock with disappearing visible surface pores. The sizes of the char particles also showed a decreasing trend during the char gasification process, which resulted in the reaction interface moving progressively inward. It would be logical to predict that the internal pores also experienced the similar trends of coalesce and collapse, which led to disappearance of the pore surface area available at the final stage of the gasification process. These results are consistent with results from BET analysis and reaction rates of gasification of chars. Characterization of char samples, at different gasification stages, indicated that the developed surface areas, pore structures, and the graphite crystalline affected char kinetics. The variations in the surface area and pore structure of gasified chars have deterministic effects on char kinetics throughout the whole char gasification process. The graphite crystallites had minimal effects on char kinetics until the final stage of char gasification, when the carbon conversion efficiency was above 0.9 in this study. This observation has been evidenced by char characterizations using XRD, Raman and FTIR. Raman analysis of chars provided the substantiation regarding an increased ratio of graphite to amorphous carbons in bone-structure of chars. This was also evidenced by XRD in the peak heights of the graphite crystalline (in Fig. 6(b)); especially as the carbon conversion efficiency reached above 0.9, where the majority amorphous carbon has been consumed and graphite carbon was the only major carbon type in the char residues. FTIR spectroscopy was used to determine the change of carbon functional groups during the char gasification process. A joint effort, using XRD, Raman and FTIR, better presented the occurrence and development of the degrees of graphitization, graphite single crystal ordering (‘‘crystallites’’), and the carbon functional groups. Future works will be extended to cover major factors other than char itself, such as coal rank and operational conditions, such as temperature, pressure, and the partial pressure of CO2. The aforementioned factors maybe syndetic, however, it will be more complicated to address the variations of char kinetics. Contrastively, another major kinetics-affecting factor, the porous structures experienced, pretty straight forward development until the carbon conversion reached 0.9, and then quickly collapsed for the remainder of the carbon conversion.
4. Conclusions Char gasification using CO2 was conducted at elevated pressures and temperatures using a high pressure thermo-gravimetric analyzer (HP-TGA). The TGA results show that the gasification rate experienced an initially slow increase, followed by a rapid increase, and finally a sharp decrease corresponding to the proceeding carbon conversion. The structural and graphite crystalline features of the char particles were analyzed throughout the pressurized reaction. The results show variations of surface area and pore structure of gasified chars which have rate determining effects on char kinetics throughout the entire char gasification process. The formation and development of graphite crystallites had minimal effects on char kinetics. The gasified char had fewer carbon functional groups until the carbon conversion efficiency reached 0.6 for the selected coal char. The amorphous carbon was consumed during char CO2 gasification; however, graphite carbon experienced limited change. Contrastively, another major kinetics-affecting factor, the porous structures developed quickly until the carbon conversion reached
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0.9, and then rapidly collapsed for the remainder of the carbon conversion.
Acknowledgements This work was performed under KEEC funds (Kentucky Energy and Environment Cabinet Research Fund 2013-2014, PON2 127 1300002875 1), the authors also thank China Scholarship Council for the financial support.
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Kinetics studies and structure characteristics of coal char under pressurized CO2 gasification conditions Lang Liu a,b, Yan Cao b,⇑, Qingcai Liu a a b
College of Materials Science & Engineering, Chongqing University, Shapingba, Chongqing 400044, China Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, KY 42101, USA
h i g h l i g h t s Porous structures of char had major impacts on char CO2 gasification. Porous structures quickly developed and rapidly collapsed during the reaction. The graphite crystallites experienced limited change in CO2-gasification process.
a r t i c l e
i n f o
Article history: Received 20 November 2014 Received in revised form 29 December 2014 Accepted 1 January 2015 Available online 12 January 2015 Keywords: CO2 gasification Kinetics Char characterizations High-pressures
a b s t r a c t The combined effects of surface area, pore structure, degrees of graphitization and graphite crystallites on the char gasification kinetics were studied under elevated pressure and temperature conditions using a High-Pressure Thermo-Gravimetric Analyzer (HP-TGA) and characterization methods, such as BET, XRD, Raman spectroscopy, FTIR and SEM. The BET and SEM results indicated that the porous structures of the chars developed quickly until the carbon conversion reached 0.9, and then rapidly collapsed to the end of the whole carbon conversion. The changes in char structures were in good agreement with the gasification rate. XRD, Raman and FTIR analysis provided evidence demonstrating an increased ratio of graphite to amorphous carbon in bone-structure of chars, which was attributed to the consumption of the amorphous carbon and the retention of the graphite carbon during the gasification process. The collected data implied that the contributions of surface area and pore volume of the char on its kinetics were much greater than its graphite crystallites during the char CO2 gasification. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Coal gasification is usually divided into two steps: coal pyrolyzation and char gasification. Because the rate of coal pyrolysis is much higher than char gasification, char gasification is usually considered as the rate-determining step in the overall coal gasification process. Water and carbon dioxide are two regular gasification agents, and water has been widely used as a gasification agent, but is recently limited to reach in the energy development sector, and alternatively, using CO2 as a gasification agent has been paid recent attention [1–3]. Understanding the kinetics of the CO2 char gasification under elevated temperature and pressure is helpful to better organize many industry processes, such as the conventional coal gasification and the novel chemical looping combustion and
⇑ Corresponding author. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.fuel.2015.01.002 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
gasification (CLC&G) for syngas production and power generation through an integrated gasification combustion cycle (IGCC). Many factors can influence char reactivity, such as coal rank, particle size, ash minerals, operational temperature and pressure and the partial pressure of the CO2 [4–8] and the char structure [9–11], among which the importance of char structure is not particularly well understood [12]. Due to the change of the char structure during char gasification, such as the porous structure [13], graphite single crystal ordering (‘‘crystallites’’) [14], and the degree of graphitization [15], the gasification process is very complicated [2,15–19]. The variations of char porous structures and graph crystallites during the char gasification are directly relative to the char kinetics, and further to process economics in the coal gasification, especially using CO2 as a gasification agent. Li et al. [10] studied the changes in coal char structure under pyrolysis and gasification conditions by the Raman spectra, and also acquired detailed information about the skeletal structures of the chars. Tay and Li [19] studied the structural features of gasified char under 800 °C in a fluidized-bed/fixed-bed reactor by
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FT-Raman/IR spectroscopy, and the results showed that the char structures changed drastically during the gasification process. Lorenz et al. [20] studied the role of char surface structure development in pulverized coal combustion under ambient pressure, and results showed that a larger surface area of the plug flow reactor char was due to a developed microspore structure, which was developed during devolatilization. Most of the published literature have focused on the variations of the porous structure and the degree of graphitization during the char gasification under ambient pressure [10,17,21]. Few previous studies have focused on the variations of char porous structure, graphite crystallites, and the degree of graphitization during the char gasification under pressurized CO2 conditions. Also the combined effects of surface areas, pore structure, degrees of graphitization and graphite crystallites on the char gasification kinetics have seldom been reported before. Many physical instruments can be ideal tools to understand derivations and variations of char structures during the char gasification process, such as BET for determinations of specific surface areas and pore structures of chars, X-ray diffraction (XRD) and Raman spectroscopy for the determinations of amorphous carbon and graphite carbons, and Fourier Transform Infrared spectroscopy (FTIR) for the determination of bonding between carbon and other major coal elements, that have carbon functional groups. This study mainly provided an overview of char structure changes during CO2 gasification by applying joint analytical measures to characterize char structures, but only focused on one single coal char. The emphasis of this paper was to track variations of the char structure and carbon crystallites using CO2 as a gasification agent under pressurized conditions, in view of the whole char gasification process. The major instrumentation involved included BET, XRD, Raman spectroscopy, FTIR and SEM. The char samples were obtained in a pressurized thermo-gravimetric analyzer under a CO2 environment. The temperatures were controlled isothermally in a range between 950 and 1150 °C, while allowing the pressure to vary between ambient and 2.0 MPa.
120 min. The combined gas flow rates of N2 and CO2 were 1000 mL/min. Based on the assumption that the ash decomposition was negligible [22], fractional carbon conversion was calculated by Eq (1). The carbon conversion efficiency (x) of char defined as the mass ratio of the gasified char at any time to the initial char can be expressed as follow.
x¼
w0 w 100% w0 wash
ð1Þ
Reaction rate (q) was calculated from mass ratio versus time profiles using Eq (2).
q¼
dx ð1 xÞdt
ð2Þ
where w0 is the initial mass of char, w is the instantaneous char mass at reaction time t, and wash is the mass of ash. 2.2. Char characterizations 2.2.1. BET The BET specific surface areas and pore volumes of chars were obtained using the N2 adsorption isotherms at 77 K by a Micromeritics ASAP 2020 apparatus in the relative pressure range between 0.07 and 0.35. The specific surface areas and pore volumes of chars were calculated by the BET equation. Prior to the analysis, the char samples were degassed at 150 °C for 3 h. 2.2.2. XRD For each of char samples, X-ray diffraction curves were obtained in a Varian Cary 100 UV–Vis spectrophotometer (200–800 nm) equipped with the DRA-CA-30 diffuse reflectance accessory using the Cu KR radiation (k = 0.1542 nm). The spectra were recorded in the 2h range of 20–70° with a scanning speed of 0.3° min1. The interlayer spacing of each sample was calculated by Bragg [23] equations according to the diffraction peak of 002 crystal plane.
2. Experimental 2.1. Materials
2.2.3. Raman Raman spectra of char samples were collected using a DXR Raman microscope (Thermo Scientific), including a 780-nm excitation laser and a confocal microscope with 3 objectives. The laser power at the sample surface was controlled at about 6 mW. The laser spot diameter reaching the sample was about 3 lm with specific resolution of 5 cm1. During the measurement, the char sample was spread out on a glass slide. About 5–10 particles were randomly chosen and analyzed. The spectra were recorded in the range of 800–2000 cm1, covering first-order bands. The acquisition time for each spectrum was 60 s.
The proximate analysis of the Kentucky Bituminous coal used in this paper is summarized in Table 1; the diameter of the selected char particles was approximately 200 lm. Char preparation and gasification processes were carried out consecutively in a pressurized thermo-gravimetric analyzer (TGA-HP150S). The TGA-HP 150s model is specially configured for gasification and related studies. It includes an integrated steam generator and a doublewalled reaction chamber. The reaction chamber is characterized as follows, the maximum weight capacity is 25 g, the weighing accuracy is ±0.0002%, the weighing precision is ±30 lg, the sensitivity is 10 lg, and the upper temperature and pressure limit is 1200 °C and 5.0 MPa, respectively. In the char preparation process, 500 mg coal was placed in a crucible boat, and then pressured and heated to design conditions under a nitrogen atmosphere. Thereafter, the gas was switched to CO2 with a molar concentration of 25–100% (balance nitrogen) and the gasification process was allowed to proceed. Finally, the sample was held isothermal for
2.2.4. FTIR The IR spectra were recorded from 4000 to 650 cm1 (with a resolution of 2 cm1 and 20 averaged scans) on a Perkin Elmer Spectrum One FTIR spectrometer with a UATR attachment using KBr pellets.
Table 1 Proximate and ultimate analysis of the Kentucky Bituminous Coal (wt%). Proximate analysis
Coal
Ultimate analysis (dry basis)
Moisture
Fixed Carbon
Volatiles
Ash
C
H
N
O
S
5.06
49.46
35.02
10.46
65.52
4.52
1.43
13.94
3.57
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2.2.5. SEM High Resolution Scanning Electron Microscopy (HRSEM) experiments were conducted on a Hitachi S-5200 field emission scanning electron microscope. The microscope was operated between 5 and 25 kV. To prepare the samples for examination, silver paste was applied to an SEM stub. The samples were sprinkled onto the paste, and then coated with gold. 3. Results and discussion 3.1. Effects of operational conditions on char reactivity The gasification reactivity of the char using CO2 as gasification agent, in response to variation in the concentration of carbon dioxide, temperature and pressure, are shown in Figs. 1–3. Fig. 1 shows carbon conversion efficiency versus time as the CO2 concentration varied between 25% and 100% (by mole) at 2.0 MPa and 1050 °C. It can be observed that the carbon conversion efficiency was sensitive to the variations in CO2 concentrations. At such operational conditions, the increase of CO2 led to the rise of the carbon conversion throughout the char gasification process. The effects of temperature on the char gasification were pretty straightforward, as shown in Fig. 2. The elevation of gasification temperature generally resulted in increasing carbon conversion efficiency under constant pressure and constant CO2 concentration. As shown in Fig. 3, the effects of pressure on gasification were similar to that of temperature, the increase in gasification pressure resulted in an increase in the carbon conversion efficiency, which was consistent with published studies [3,22]. Fig. 4 shows the gasification rate versus carbon conversion efficiency using 100% pure CO2 as a gasification agent and at different operating conditions. It was observed that the gasification rate experienced an initially slow increase, followed by a rapid increase, and finally a decrease corresponding to the carbon conversion efficiency. It was well known that the char structural parameters, such as specific surface area and crystallites structure, varied significantly during the carbon conversion under varying gasification conditions [20,24,25].
Fig. 2. The effect of temperature on gasification rate at 25% CO2 by mole and 2.0 MPa.
3.2. Evolution of char structure during CO2 gasification To distinguish the structure changes of the char during gasification, the morphologies, pore size, and graphite crystal features of the char samples were characterized by BET, FTIR, Raman, XRD
Fig. 1. The effect of CO2 concentration on gasification rate at 2.0 MPa and 1050 °C.
Fig. 3. The effect of pressure on gasification rate at 25% CO2 by mole and 1150 °C.
Fig. 4. The reaction rate versus carbon conversion during char gasification at 100% CO2 by mole.
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and SEM. The selected char samples were prepared under different carbon conversions but at constant conditions of 100% CO2, 2.0 MPa, and 1050 °C. The structural parameters of the char samples included the initial surface area, the in-plane strain and the interplanar spacing, d002, ratio of perfect graphite structures to graphite structures with some defects, Ig/Id, and carbon functional groups. Measurements of BET specific surface areas and pore volume of the samples were presented in Fig. 5. The surface areas of the chars were significantly increased from 40 to 380 m2 g1 when the carbon conversion increased from 0 to 0.9. A similar trend was also observed in pore volumes of chars. Further increases in carbon conversion efficiency led to a decrease of both surface area and pore volume in the gasified char. It appears that the porosity of the chars was developed in favor of generating more micro-pores when the carbon conversion efficiency of selected coal chars varied from 0 to 0.9. The char gasification not only led to pore growth, but also new pore formation when the carbon conversion efficiency was blow 0.9. When the carbon conversion efficiency was over 0.9, pores started to coalesce and collapse, leading to a disappearance of the available pore for the continuous gasification reactions. The BET analysis of chars at different gasification stages indicated that the gasification reactions occurred within pores with the pores widening and new pore inception, thus pores disappeared at the final stage of char gasification. Fig. 6(a) shows the XRD patterns of the prepared char samples. The peaks, which appear at around 27° and 50° were attributed to 0 0 2 and 1 0 0 crystalline structures of graphite carbon in the char samples, respectively [26]. The parameter of the in-plane strain and the interplanar spacing, d002 [23] (XRD peak at 27°) was mostly used to extract the structural information of the char samples from the XRD patterns. The intensity of the graphite peak (0 0 2) and the values of d002 were presented in Fig. 6(b) and (c). These figures show that a graphite peak around 27° was observed during the whole char gasification process, and the intensity of the graphite peak increased with carbon conversion. The results suggest that amorphous carbon in the char was preferentially gasified. Fig. 6(c) showed that the interlayer spacing (d002), was essentially unchanged during the char-CO2 gasification, which suggests that the average size of the crystallites did not change remarkably. This trend was similar to the study described earlier [17], and which indicates that the structure of the perfect graphite changed minimally during the char gasification process. These results imply that graphite carbon is relatively inert in the CO2 gasification atmosphere [27]. The Raman spectra for samples can be designated roughly as a disordered carbon band (D-band) and a graphite band (G-band) at
Fig. 5. Change in surface area and pore volume during char gasification.
approximately 1350 cm1 and 1580 cm1 [19], respectively. Fig. 7(a) presented a typical Raman spectroscopic profile between 800 and 2000 cm1 for each char sample. In this paper, averaging the multiple Raman spectra for each sample was performed to manage the heterogeneity of char particles. As can be seen in Fig. 7, the intensity of the D and G band showed a prominent increase with carbon conversion, attributed to that the aromatic ring and oxygen-containing groups condensed during the first stage of gasification. It is believed that, as the char becomes more aromatized, the light absorptivity of the char would increase which leads to a decrease of the Raman intensity, meanwhile, oxygen-containing groups also affect the observed Raman intensity of the char and thus, increased the intensity of the Raman peak [28]. When the carbon conversion reaches 0.6, the intensity of the D band decreased, which was attributed to the breakdown of the aromatic rings and oxygen-containing groups. The ratio R1, is a very important parameter indicating the char structural information [16,19], and is defined as:
R1 ¼
Ig Id
ð3Þ
where Ig and Id are referred to as the intensities of G-band and Dband, respectively. Larger values of R1 denote less disorder or amorphous carbon in the chars [29]. The values of R1 for the char samples were given in Fig. 7(b). This ratio increased with carbon conversion efficiencies, which indicated that the relative amounts of large disordered or amorphous carbon decreased with carbon conversion efficiencies. This trend was likely caused by the consumption of the amorphous carbon, while preserving the perfect graphite structures, which have not changed during the char gasification (Fig. 6(c)). In order to get more information about the char structure change during the gasification, FTIR was carried out to study changes in the carbon functional groups during the char-CO2 gasification. The FTIR spectra of chars are given in Fig. 8. The spectra of char samples were characterized by four principal bands at wavenumbers of 2350 cm1, 1269 cm1, 1033 cm1, and 750 cm1, respectively. The band at 2350 cm1 is assigned to the presence of CO2 [30], the band at 1269 cm1 to the aromatic CO– or phenolic –OH stretching [31], the band at 1033 cm1 to the C–O stretch for the O–CH3 and C–OH [31], and the band at 750 cm1 to aromatic C–H out of plane vibrations [32]. Due to char gasification with CO2, the presence of the band at 2350 cm1 for CO2 adsorption was obvious for the prepared char samples as the conversion efficiency varied between 0.21 and 0.83. Additionally, there were no significant peaks at the 2350 cm1 band in the char samples at both initial and final stages of CO2 gasification, revealing limited CO2 adsorption. A similar trend was observed in other FTIR curves relative to other major FTIR peaks, such as those at 1269 cm1, 1033 cm1 and 750 cm1. It is natural to see the occurrences and changes of FTIR peaks corresponding to bonds between C and O, due to the active adsorption and desorption on the surface of chars. However, was interesting to see that similar development and diminishing of C–H band, not only occurred, but also actively changed under varying CO2 gasification conditions. This could be attributed to internal hydrogen constituents accessing and reacting with char during the gasification process. The evolutionary paths of all examined carbon functional groups showed an initial increase in absorbance during the early stages of gasification, which is most likely due to pore structure development of chars, followed by a decline leading to disappearance. The pore growth and opening of enclosed pores in the early stages of the char gasification in which more carbon functional groups were released or generated from such pores facilitated an increasing trend in absorbance of those carbon functional groups. However, at the final stage of the
L. Liu et al. / Fuel 146 (2015) 103–110
Fig. 6. Diffractograms and d002 obtained for the char samples in the 2h range of 20–70°.
Fig. 7. Typical Raman spectra and the Ig/Id of char samples.
Fig. 8. FTIR spectra of the char samples.
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Fig. 9. SEM images of char samples prepared at different gasification stages.
char gasification, the pores coalesced and collapsed, which resulted in the functional groups correspondingly diminishing. To gain better insight into the changes of char morphology during char gasification, the char samples were subject to observations using SEM. Fig. 9 shows the SEM images of the prepared char samples at different gasification stages. The SEM images clearly reveal sizes, surface roughness and smoothness, irregularities in shape and structures of visible surface pores of the char samples. In the initial stage of the char gasification, the chars were not smooth, but coarse with many small embossed features, which were identified to be the parts of a small surface bulge on the char surface. Using the SEM in micro-meter resolutions, there was evidence of
the pores growth and the opening of closed pores during the initial gasification stage. It was rationally inferred that the pores in submicro and nano-meter levels experienced similar developments. The initial pore opening on the char surface, presented an avenue whereby additional pores underneath the char surface were accessible to the gasification agent. This pore opening process moved forward until carbon conversion reached around 0.9. This action enabled greater char area into the gasification process which resulted in an increase in gasification rates until the carbon conversion efficiency reached around 0.9. There was also a secondary event on the surface of chars, in that the surface of the char gradually became smooth and the irregular bulges disappeared. This
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phenomenon started happening as the carbon conversion efficiency reached 0.6, and was significant when the carbon conversion efficiency exceeded 0.8. In the final stage of gasification when the carbon conversion efficiency was above 0.9, the surface of the chars was more like regular rock with disappearing visible surface pores. The sizes of the char particles also showed a decreasing trend during the char gasification process, which resulted in the reaction interface moving progressively inward. It would be logical to predict that the internal pores also experienced the similar trends of coalesce and collapse, which led to disappearance of the pore surface area available at the final stage of the gasification process. These results are consistent with results from BET analysis and reaction rates of gasification of chars. Characterization of char samples, at different gasification stages, indicated that the developed surface areas, pore structures, and the graphite crystalline affected char kinetics. The variations in the surface area and pore structure of gasified chars have deterministic effects on char kinetics throughout the whole char gasification process. The graphite crystallites had minimal effects on char kinetics until the final stage of char gasification, when the carbon conversion efficiency was above 0.9 in this study. This observation has been evidenced by char characterizations using XRD, Raman and FTIR. Raman analysis of chars provided the substantiation regarding an increased ratio of graphite to amorphous carbons in bone-structure of chars. This was also evidenced by XRD in the peak heights of the graphite crystalline (in Fig. 6(b)); especially as the carbon conversion efficiency reached above 0.9, where the majority amorphous carbon has been consumed and graphite carbon was the only major carbon type in the char residues. FTIR spectroscopy was used to determine the change of carbon functional groups during the char gasification process. A joint effort, using XRD, Raman and FTIR, better presented the occurrence and development of the degrees of graphitization, graphite single crystal ordering (‘‘crystallites’’), and the carbon functional groups. Future works will be extended to cover major factors other than char itself, such as coal rank and operational conditions, such as temperature, pressure, and the partial pressure of CO2. The aforementioned factors maybe syndetic, however, it will be more complicated to address the variations of char kinetics. Contrastively, another major kinetics-affecting factor, the porous structures experienced, pretty straight forward development until the carbon conversion reached 0.9, and then quickly collapsed for the remainder of the carbon conversion.
4. Conclusions Char gasification using CO2 was conducted at elevated pressures and temperatures using a high pressure thermo-gravimetric analyzer (HP-TGA). The TGA results show that the gasification rate experienced an initially slow increase, followed by a rapid increase, and finally a sharp decrease corresponding to the proceeding carbon conversion. The structural and graphite crystalline features of the char particles were analyzed throughout the pressurized reaction. The results show variations of surface area and pore structure of gasified chars which have rate determining effects on char kinetics throughout the entire char gasification process. The formation and development of graphite crystallites had minimal effects on char kinetics. The gasified char had fewer carbon functional groups until the carbon conversion efficiency reached 0.6 for the selected coal char. The amorphous carbon was consumed during char CO2 gasification; however, graphite carbon experienced limited change. Contrastively, another major kinetics-affecting factor, the porous structures developed quickly until the carbon conversion reached
109
0.9, and then rapidly collapsed for the remainder of the carbon conversion.
Acknowledgements This work was performed under KEEC funds (Kentucky Energy and Environment Cabinet Research Fund 2013-2014, PON2 127 1300002875 1), the authors also thank China Scholarship Council for the financial support.
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