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Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures
Journal of Analytical and Applied Pyrolysis 104 (2013) 202–209
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Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures Yonghui Bai, Pei Wang, Lunjing Yan, Changlong Liu, Fan Li ∗ , Kechang Xie State Key Laboratory Breeding Base of Coal Science and Technology, Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 27 November 2012 Accepted 7 August 2013 Available online 22 August 2013 Keywords: CO2 Gas evolution Char structure Pyrolysis
a b s t r a c t The gas release properties and changes in the char structure during lump coal pyrolysis were investigated in Ar and CO2 atmosphere. Fixed bed reactor and gas chromatography were employed to generate char at four pressures (0.1–1.5 MPa in 0.5 MPa increments) and measure gas composition from room temperature to 700 ◦ C in 50 ◦ C increments, respectively. The influences of pressure and atmosphere on char structural changes were examined by FT-IR and XRD. The results indicate that CO2 atmosphere is favorable to the release of H2 O. The initial evolution temperature of gas species shows a notable dependency on atmosphere and pressure. Especially for CH4 , CO2 atmosphere can promote its emission in advance and make the evolution profile subdued within 0.5 MPa but hinder its evolution over the pressure range of 0.5 MPa, and this inhibitive effect is more evident at a higher pressure. The chars prepared in CO2 atmosphere have more cycloalkane and aliphatic structures, and the role of atmosphere is particularly evident when the pressure exceeds 0.5 MPa. There are more content of hydroxyl, secondary hydroxyl, olefinic C C bonds, aromatic C C structure left in chars prepared under CO2 atmosphere compared with those prepared under Ar atmosphere. The interplanar distance d0 0 2 indicates that Ar atmosphere is more helpful to the graphitization of char during pressurized coal pyrolysis, the stacking height Lc of char sample prepared in CO2 is always higher than that prepared in Ar atmosphere within 1.0 MPa, CO2 atmosphere is beneficial to the growth of crystal layer in vertical position. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The pressurized lump fixed bed gasification technology has showed a notable advantage in SNG production because there is a high content of CH4 in the gas product from gasifier [1]. The CH4 content in crude gas varied from 8% to 12% according to different operating parameters such as the pressure, temperature and the feeding coal properties. In fact, nearly all SNG plants in plan or in production in China use this technology to produce CO and H2 for CH4 synthesis. Because the gas and coal move counter-currently in this kind of gasifier, there is a temperature drop in the reactor [2–4]. As a result the combustion, gasification, pyrolysis and drying zones can be identified in the gasifier. The products from this gasifier include gas, liquid tar and solid coal ash which can be used for brick manufacturing [5]. In the past great attentions have been paid to the gasification process on tar elimination, for tar is undesirable because of various problems associated with condensation [6–8]. However [9], with increasing price of crude oil, precious chemical
∗ Corresponding author. Tel.: +86 351 6018076; fax: +86 351 6010482. E-mail address: [email protected] (F. Li). 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.08.003
material from gasification may be regarded as an advantage rather than a disadvantage. This kind of gasifier, however, has a higher steam to carbon ratio, controlling gasifier temperature lower than the coal ash fusion temperature. This would generate a large amount of waste water, which may bring about damage to the environment and human health. In this case, CO2 can be used partly in place of steam as gasification agent to [10]: (1) save large amount of steam and use CO2; (2) enable the adjustment of the H2 /CO ratio of the crude gas produced as required by downstream processes; and (3) enable the adjustment of gasifier temperature for Boudouard reaction that is also an endothermic reaction where even the heat carrier ability of CO2 is much higher than that of steam. Thus lump coal gasification research in coexistence of steam and CO2 is of great significance. Pyrolysis, as the preliminary process of coal gasification, plays a crucial role in determining gas product distributions and char structure, which is of vital importance to gasification reactivity. Therefore, prior knowledge of lump coal pyrolysis behavior in CO2 atmosphere is useful for a better understanding of coal gasification in coexistence of steam and CO2 [11–15]. Some research works have been conducted for pulverized coal gasification with mixtures of steam and CO2 , and pulverized coal pyrolysis under CO2 and inert atmosphere also have been reported
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[16–20]. From previous research [20,21], the effects of CO2 on coal pyrolysis are embodied in the following three ways: (1) the physical properties of CO2 are different with inert gas, which may influence the diffusion rate of volatile gas; (2) CO2 is a product of coal pyrolysis, which may influence the volatile composition and yield; (3) CO2 is not only a pyrolysis atmosphere but can react with coal char, which may make the char structure and gas evolution different. Knowledge of the effect of CO2 on gas and char structure evolution during pressurized lump coal pyrolysis is rather limited, so the main objective of this study is to both experimentally and theoretically clarify the effect of CO2 on gas and char structure evolution during pressurized lump coal pyrolysis. 2. Experimental 2.1. Initial samples The gases used are CO2 (99.99% purity), Ar (99.99% purity). The solid is a Chinese western bituminous coal from a reservoir located in Yining County, Xinjiang Province. 2.2. Char preparation and gas detection Pressurized pyrolysis tests were performed in a vertical fixed bed reactor, which has an inner diameter of 2.4 cm, maximum design pressure 3 MPa and temperature 800 ◦ C. Tests were conducted at pressures of 0.1, 0.5, 1.0, and 1.5 MPa in pure Ar and CO2 atmosphere, respectively, from room temperature to 700 ◦ C at a constant heating rate of 10 ◦ C/min. Approximately 50 g of lump sized coal sample (+5.0 to 6.0 mm) was placed in the flattemperature zone of the reactor. A K type thermocouple was located 0.3 cm above the sample pan to control the sample temperature and another one was located in the furnace wall for furnace temperature control. When the sample was in place the whole system was sealed and a sweep Ar or CO2 gas flow of 1000 ml/min was used to purge air out in the reactor for 30 min. Then according to different experimental conditions, the reactor was pressurized with Ar or CO2 to the goal pressure value and 500 ml/min Ar was introduced from the top of the reactor to bring the volatile matter out, then the furnace heating was started. The schematic diagram of the experimental system is shown in Fig. 1. After experiment, the char sample was collected from the reactor, which was marked with a label CAx Py . In the CAx Py , “C” stands for char, “x” indicates the
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atmosphere of the char preparation, and “y” is referred to as the system pressure of the pyrolysis system. For example, CAAr P1.5 stands for the char prepared at 1.5 MPa in Ar atmosphere with a heating rate of 10 ◦ C/min from room temperature to 700 ◦ C. The Proximate analytical value of the coal and its residue chars used in this study were analyzed by ISO11722:1999 (for moisture), ISO1171:1997 (for ash) and ISO562:1998 (for volatile). The ultimate analytical values were analyzed by ISO625-1996 (for C, H) and ISO333-1996 (for N), the properties of the raw coal and its resultant chars are shown in Table 1. The gas released during the pyrolysis was first pretreated for tar and moisture removal and then the stream that exists the back pressure regulator was analyzed with a gas chromatograph. The injector temperature was kept at 120 ◦ C. The CH4 was separated by using a TM-PLOTU capillary column (30 m × 0.53 mm × 20 m) and detected by a flame ionization detector (FID1). H2 and CO separation were performed using a TDX-01 packed column (1 m × 3 mm), H2 was detected by a thermal conductivity detector (TCD). CO was converted to CH4 by a Nickel furnace and then detected by FID2. N2 was used as the carrier gas. The oven temperature was programmed from 45 ◦ C (2 min) to 145 ◦ C at the heating rate of 8 ◦ C/min. The temperature of TCD, FID1 and FID2 are 120, 180, and 350 ◦ C, respectively. Product gases during coal pyrolysis were collected by gas bags from room temperature to 700 ◦ C in 50 ◦ C increments. The time to collect the gas sample is from Ti − 10 ◦ C to Ti + 10 ◦ C (Ti : 50, 100, 150,. . .,700 ◦ C). The accumulated concentration from temperature of Ti − 10 ◦ C to Ti + 10 ◦ C was used to evaluate the evolution concentration of each gas at temperature Ti . The evolution profile of gases was defined by the following formula: ri =
V % × (VTi +10 ◦ C − VTi −10 ◦ C ) mc Vm
(mmol/g)
where ri is the mol gas formed per gram coal at temperature Ti , V% is the volume percent of each gas, Vm is the mole volume of gas at the environmental temperature and the atmospheric pressure, VTi +10 ◦ C is the reading of volume value at temperatures of Ti + 10 ◦ C, mc is the weight of dry coal sample (given in g). 2.3. Char characterization FTIR spectrometry was conducted and recorded via Potassium bromide (KBr) pellet technique using a VECTOR70 FTIR spectrometer (BRUKER, Germany). Spectra for each sample were collected in
Fig. 1. Schematic diagram of the pressurized fixed bed reactor system (1, cylinder; 2, pressure reducing valve; 3, control valve; 4, pressure gauge; 5, mass flow controller; 6, furnace; 7, 8, thermocouples; 9, coal sample; 10, pressurized reactor; 11, temperature controller; 12, primary tar condenser; 13, secondary condenser; 14, back pressure regulator).
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Table 1 Proximate and ultimate analysis of raw coal and its resultant chars. RC
CAAr P0.1
Proximate analysis Moisture (wt%, ad) Volatile (wt%, daf) Ash (wt%, ad)
CAAr P0.5
CAAr P1.0
14.2 33.1 6.14
1.43 5.21 11.4
1.44 4.54 9.8
1.33 4.37 13.1
1.54 2.88 9.03
2.16 6.26 10.5
1.91 5.75 8.72
1.96 5.28 8.53
1.90 5.50 13.6
Ultimate analysis Carbon (wt%, daf) Hydrogen (wt%, daf) Oxygen (wt%, daf) Nitrogen (wt%, daf) Sulfur (total) (wt%, daf)
75.1 3.59 19.8 1.22 0.39
90.1 2.81 3.91 0.87 2.33
91.2 2.61 3.49 0.88 1.72
91.4 2.31 3.36 0.82 2.14
93.7 2.09 2.25 0.73 1.20
88.4 3.05 5.48 0.98 2.05
91.4 2.89 4.53 0.97 0.25
91.6 2.85 3.88 0.89 0.77
93.3 2.86 0.37 0.92 2.53
the mid-IR (400–4000 cm–1 ) region with a resolution of 4 cm–1 and scans of 16. From every sample, the spectrum of a similar thickness KBr wafer was subtracted to obtain high quality spectra. XRD pattern was recorded using a Rigaku Miniflex II diffractometer with Cu K␣1 radiation ( = 0.15406 nm) at 30 kV and 15 mA to scan over the angular 2 range of 5–80◦ with a scanning speed of 8◦ /min. The classical Bragg and Scherer equations were used to analyze the structural parameters d0 0 2 , La and Lc . d0 0 2 is the interplanar distance of two aromatic layers of microcrystalline (nm), Lc is the thickness of microcrystalline (nm), and La is the diameter of microcrystalline (nm). Details of the procedures used to fix the interplanar distance of two aromatic layers of microcrystalline, thickness of microcrystalline and diameter of microcrystalline have been reported elsewhere [22]. 2.4. TGA analysis The TG experiments were performed in the thermogravimetric analyzer (NETZSCH STA449F3), the maximum temperature error of the measurement is ±1 ◦ C and the mass precision is 1 g. Approximately 10 mg initial sample with particle size less than 200 m was fed into the Al2 O3 plate and heated from room temperature to 1000 ◦ C at a constant heating rate of 10 ◦ C/min under Ar or CO2 atmosphere at a constant flow rate of 50 ml/min. Repeated experiments were conducted to ensure the reproducibility of the experiments and accuracy of the data. 3. Results and discussion 3.1. Pyrolysis behavior of raw coal To obtain prior knowledge of pyrolysis behavior of the coal sample used in this study, thermogravimetric analysis was used to investigate the mass decay of coal as a function of temperature and atmosphere. Fig. 2 shows the mass decomposition of coal at a heating rate of 10 ◦ C/min under Ar and CO2 atmosphere. Three distinct regimes can be identified. The first stage occurs between ambient temperature and 250 ◦ C, the minor mass decay observed is mainly due to the release of H2 O and some absorbing gases such as CH4 , CO2 and N2 . The maximum mass decay rate happened at about 70 ◦ C, is 0.57%/min and 0.86%/min at Ar and CO2 atmosphere, respectively. And the mass loss rate under Ar atmosphere is always slower than that under CO2 atmosphere, the difference was well repeated by experiment. Therefore, CO2 atmosphere is favorable to the release of H2 O and absorbing gases. Zhang et al. discovered that [23], at the same temperature and balance pressure, the diffusion coefficient of CO2 is higher than that of CH4 , CO2 can not only be absorbed in the surface of coal pore structure, but it can be dissolved in the coal matrix to swell the matrix itself. It is also reported [24] that replacing the absorbed CH4 by absorbing CO2 can reduce coal surface energy. Therefore, it may be concluded that CO2 can displace the CH4
CAAr P1.5
CACO2 P0.1
CACO2 P0.5
CACO2 P1.0
CACO2 P1.5
absorbed in the coal matrix, which will cause the surface energy of the coal to reduce and the matrix to dissolve, and this is helpful to the release of H2 O and absorbing gases. The greatest fraction of mass loss occurs during the pyrolysis stage between 250 and 680 ◦ C, which is caused by the thermal decomposition of raw coal. The devolatilization rate in CO2 atmosphere first increases with increasing pyrolysis temperature and reaches a maximum at 454 ◦ C, and the maximum devolatilization rate is 0.86%/min. During this regime, replacing Ar with CO2 does not influence the corresponding temperature of mass loss rate peak, but will inhibit the volatile releasing rate during the temperature range of 250–550 ◦ C. The third stage covers a wide temperature range from 680 ◦ C to the final temperature and different weight loss curves were observed. The weight loss of coal sample is 41.12% in CO2 atmosphere pyrolysis from ambient temperature to 800 ◦ C, higher than 31.12% in Ar atmosphere, which is certainly caused by the gasification of coal with CO2 at 680 ◦ C. 3.2. Gas evolution characteristic under CO2 and Ar atmosphere Fig. 3 shows the evolution of the H2 from the pyrolysis of coal under Ar and CO2 atmosphere at four pressures. On the whole, increasing pyrolysis pressure inhibits the evolution of H2 regardless of the atmosphere. But Table 1 shows the volatile left in the char after pyrolysis decrease with increasing pyrolysis pressure, which is different with the finding to other results [25,26]. Yang et al. [27] got the conclusion that increasing pyrolysis pressure was helpful to the conversion of organic functional groups to light gases because of the cracking of aromatic and alkane components during coal pyrolysis under higher pressure. Moreover, Wang et al.’s [28] experiments showed that because of the higher gas density in pressurized condition, the heat transfer rate of the char and the gas molecular motion rate is higher than that in atmospheric pressure,
Fig. 2. TG and DTG curves of coal pyrolysis under different atmosphere.
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Fig. 3. Evolution of H2 with temperature under different coal pyrolysis conditions (Ar–H2 : H2 evolution as a function of temperature in Ar atmosphere).
which could promote the bond rupture of the molecular in the char. It has been clear that high pressure suppresses the formation and release of tar and shifts the molecular weight of tar to a lighter fraction. On the other hand, high pressure promotes secondary reactions, hence increases the total yield of light gases. Because tar is the predominant product of volatile matter, the total yields of volatile matter decrease significantly when pressure is increased [29,30]. It is maybe that the volatile evolved during pyrolysis has a longer residence time in the coal because of the relative large particles and increased pressures, the probability that secondary pyrolysis happens is increased, therefore, the volatile left in the char after pyrolysis decrease with increasing pyrolysis pressure. For Ar–H2 , with the pressure increases from 0.1 to 1.5 MPa in 0.5 increments, the initial evolution temperature lag is clearly visible, the temperature shift from 300 → 350 → 450 → 550 ◦ C. All of the profiles showed two consecutive evolution regimes with a transition temperature in the neighborhood of 500 ◦ C with the exception of 1.5 MPa. At ambient pressure the release of H2 concentration only has a slight increase before 550 ◦ C. Once the temperature exceed 550 ◦ C, the H2 evolved sharply because of the secondary pyrolysis reaction such as cracking, de-hydrogen and breaking up of bridge structures. H2 comes from the condensation of aromatic structures or the decomposition of heterocyclic compounds, processes that occur at a high temperature [31]. At elevated pressures, when the temperature exceeds 600 ◦ C, secondary pyrolysis is the critical factor that contributes to the formation of H2 . Increasing pressure has inhibitive effect on the evolution of H2 . For CO2 –H2 at 1.5 MPa, the initial temperature of H2 evolution is advanced by nearly 50 ◦ C and has one more stage when compared with Ar–H2. To Ar–H2 at 1.5 MPa, nearly all hydrogen was generated from the secondary pyrolysis. Fig. 3 also shows that H2 evolved in the temperature range of 250–550 ◦ C in CO2 atmosphere is less than that evolved in Ar atmosphere. Therefore, the slower mass decay in CO2 atmosphere described in Fig. 2 may be caused by the inhibition effect of the atmosphere. Table 1 shows a remarkable trend for the Ar pyrolyzed chars, with increasing pyrolysis pressure, the oxygen content in the char decreased gradually. In addition, the evolved H2 concentration shows the same trend, which means the evolved H2 has a notable dependency on the oxygen consuming content during pyrolysis. Fig. 4 shows the relationship between the evolved H2 concentration and the oxygen consuming content during pyrolysis in both Ar and CO2 atmosphere. Obviously, the oxygen content plays an important role in the H2 release, the higher the oxygen consumed
Fig. 4. Relationship between evolved H2 concentration in gas and the oxygen content left in the char.
during pyrolysis, the lower the H2 emitted. This may be attributed to the formation of H2 O, Arenillas et al. got the same results [32]. Oxygen content also influences CO formation at a high temperature. Especially for bituminous coals, the cleavage of alkyl aryl ethers has been found to be particularly important [31]. Fig. 5 shows that the evolution of CO at ambient pressure could be divided into three stages regardless of CO2 or Ar atmosphere. The first stage is for pyrolysis temperature before 200 ◦ C where the evolution of CO seems to be slow. The second stage is for pyrolysis temperature between 200 and 550 ◦ C where the CO concentration increases obviously with the rise of temperature to reach a maximum at 450 ◦ C, and CO concentration evolved in Ar and CO2 atmosphere are different, Ar atmosphere is prone to inducing more CO evolved. The third stage is for temperature between 550 ◦ C and 700 ◦ C, CO concentration regain increases sharply and this is much more evident in CO2 atmosphere. The formation of CO in the first stage is mainly due to the absorbing gases in the coal matrix. The second stage [33] is probably attributed to the cracking and reforming of thermolabile carbonyl and ether groups. According to other researchers [34], the production of CO is mainly attributed to aryl ether linkages. At
Fig. 5. Evolution of CO with temperature under different coal pyrolysis conditions.
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Fig. 6. Evolution of CH4 with temperature under different coal pyrolysis conditions.
Fig. 7. Variations of initial gas evolution temperature with pyrolysis pressure under Ar and CO2 atmosphere (Ar + H2 : initial evolution temperature of H2 as a function of pressure in Ar atmosphere).
3.3. Char structural evolution the third stage, increasing CO concentration in Ar atmosphere is probably from the scissions of diarylether groups and secondary reactions of decomposition of the char [35], and it is certain that CO2 gasification reaction happened at 680 ◦ C is an important factor that contributes to the evident CO concentration increase in CO2 atmosphere. In addition, increasing pressure shows hindrance function on CO evolution, especially when the pressure is within 1.0 MPa. Compared with the evolution performance of H2 , CO and CO2 , effects of pressure and atmosphere on CH4 evolution are much more complicated. Fig. 6 indicates that when the pressure is less than 0.5 MPa, CO2 atmosphere could promote the emission of CH4 in advance and made the evolution profile subdued compared with CH4 emission in Ar atmosphere. But when the pressure is higher than 0.5 MPa, the initial evolution temperature lag of CH4 is observed under CO2 atmosphere and the lag is more evident at a higher pressure. Before 500 ◦ C, it is probable that [33] the CH4 is mainly from the cracking of methoxyl group. After 500 ◦ C, the CH4 evolution might be attributed to the rupture of aromatic rings and secondary decomposition of some light gases. For Ar–CH4 at 0.1 MPa, CH4 evolution covers over a wider temperature range from 300 to 700 ◦ C, the CH4 concentration first increases and reaches a maximum in the neighborhood of 500 ◦ C. With the temperature increase further, the CH4 concentration decreases sharply until 640 ◦ C, then regain get a steep increase. For CO2 –CH4 at 0.1 MPa, the temperature range of CH4 evolution expands to 150–700 ◦ C, and all evolution peak shifts to low temperature region. It maybe that there has a competitive adsorption between CH4 and CO2 , and the adsorptivity of CO2 is higher than that of CH4 , which made more CH4 evolved slowly. To have a more detailed analysis about the initial evolution temperature of H2 , CO and CH4 , variations of initial evolution temperatures with pyrolysis pressure for each gas under Ar and CO2 atmosphere was obtained in Fig. 7. The initial evolution temperature reflects the thermal behavior of the coal during pyrolysis. To all curves, the pressure of 0.5 MPa seems to be a turning point pressure for the initial gas evolution temperature regardless of the atmosphere. Overall, within 1.5 MPa, the initial evolution temperature of H2 , CO and CH4 all shift to a higher temperature region with the exception of Ar + CH4 . Within 0.5 MPa, the initial evolution temperature changed marked with increasing pyrolysis pressure. When the pressure exceeds 0.5 MPa, this trend was weakened and even Ar + CH4 presents a reverse behavior.
3.3.1. FTIR analysis FTIR qualitative analysis is used to investigate the impacts of pressure and atmosphere on functional group formation. The FTIR spectra of chars prepared at different conditions are shown in Fig. 8. The spectra indicate that all chars contain a number of infrared absorption peak, such as hydroxyl, aliphatic hydrocarbon, olefinic and aromatic C C bond, olefinic and aromatic C H, etc. Regardless of the atmosphere, the band with strong intensity at 3427 cm−1 denotes the presence of large amounts of hydroxyl groups in the char, which can be attributed to OH bond stretching vibrations. The appearance of the peaks at 2920 and 2852 cm−1 are the asymmetrical and symmetrical stretching vibration of CH2 in the cycloalkane and aliphatic. Comparing a and b, it reveals that the chars prepared under CO2 atmosphere have more cycloalkane and aliphatic structure left, the role of atmosphere is particularly evident when the pressure exceeds 0.5 MPa [18]. Wang et al. also discovered that the replacement of N2 with CO2 does result in great changes in the surface chemistry for coal char. CO2 atmosphere increases the content of aliphatic C H groups and the concentration of methylene C H bands compared with N2 . The spectra in the range of 1000–1800 cm–1 involved abundant information about the C C and oxygen containing functional groups. As seen, the hydroxyl, secondary hydroxyl, olefinic C C bonds, aromatic C C bonds are the main atomic groups and structures in the char that result in the infrared absorption in this region. As revealed, all samples prepared in CO2 atmosphere have more content of C C, O H, and C O left in char compared with that in Ar atmosphere [36]. Wang et al. revealed that N2 atmosphere was helpful to the phenolic hydroxyl formation while Coal chars obtained in CO2 atmosphere have relatively lower phenolic hydroxyl content in char sample by performing the FTIR analysis of coal chars obtained in CO2 atmosphere and inert N2 atmosphere [37]. Minkova et al. compared and found the samples treated in a flow of reactive gas and found that samples treated in reactive gas have higher contents of oxygen-containing groups with basic and weakly acidic character. In both Ar and CO2 atmosphere increasing pressure has promotion effect on–CH2 formation within 0.5 MPa, but that effect shows a reverse trend over 0.5 MPa. 3.3.2. XRD analysis As one of the most frequently used techniques in studying microcrystalline structural, the XRD patterns can distinguish the
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Fig. 8. FTIR spectra of (a) the chars generated in Ar atmosphere at different pressures (b) the chars generated in CO2 atmosphere at different pressures.
pyrolysis conditions on the char structural change. Results of the raw coal and chars prepared at different temperatures are shown in Fig. 9. It can be seen that all patterns show two characteristic peaks 0 0 2 and 1 0 0 with the exception of RC. The char microcrystalline
structure is transferred to graphitic structure gradually after pressurized pyrolysis [38]. 0 0 2 peak indicates the degree of orientation of aromatic carbon net in microcrystal, 1 0 0 peak indicates the size of aromatic carbon net in microcrystal; with the 0 0 2 peak and 1 0 0 peak going to be narrower and higher, the synusia would
Fig. 9. XRD spectrum of chars prepared at different atmosphere and pressure.
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Fig. 10. Variation of crystallite structure parameters with pressure and atmosphere (a: d0 0 2 ; b: Lc ; c: La ).
have better degree of orientation while the aromatic nuclear have higher degree for condensation. To have a more detailed analysis about the crystalline structure information, Fig. 10 shows the variation of crystallite structure parameters with pressure and atmosphere. Fig. 10(a) indicates that with the pyrolysis pressure increased from 0.1 to 1.5 MPa, the values of interplanar distance d0 0 2 in both Ar and CO2 atmosphere all have a slightly and ordered decrease, from 0.3697 nm to 0.3649 nm in CO2 atmosphere and from 0.3692 nm to 0.3644 nm in Ar atmosphere. But the value is still greater than the d0 0 2 parameter of perfect graphite. In addition, the interplanar distance d0 0 2 of all chars prepared in CO2 atmosphere are greater than that prepared in Ar atmosphere. Therefore, compared with CO2 , Ar atmosphere is more helpful to the graphitization of coal char and increasing pyrolysis pressure is also contributed to the graphitization process [18]. Wang et al. analyzed the char structure by using Raman Spectroscopy, and also discovered that char got from CO2 has more disordered structures when compared with the char from inert N2 atmosphere. By contrast, Fig. 10(b) and (c) suggests that the crystallite diameter La and crystallite height Lc are all changed significantly with different pyrolysis atmosphere and increasing pyrolysis pressure. This shows that the growth of microcrystalline structure happened not only in crystal lattice but in crystal layer, and the variation trend shows a notable dependency on atmosphere and pressure. Fig. 10(b) reveals that the stacking height Lc of char sample prepared in CO2 atmosphere within 1.0 MPa are always higher than that prepared in Ar atmosphere, and it is favorable to the growth of crystal layer in vertical position. When the pressure exceeds 1.0 MPa, reverse result was observed. It can be reasoned that within 1.0 MPa, the preferential consumption of disordered carbon that lead to the great order in the char for the coal sample used can react with CO2 at 680 ◦ C. When the pressure exceeds 1.0 MPa, increasing pressure has an inhibit effect on carbon gasification reaction, which made the further growth of Lc become hard
[17]. Liu et al. also got the results that the gasification reaction is a very important factor during pyrolysis in CO2 atmosphere and CO2 was more beneficial for high-volatile coal to develop carbon active sites and amorphous carbon. Fig. 10(c) tells that the influence of pyrolysis pressure on crystallite diameter is largely depends on the atmosphere. In Ar atmosphere, La changed with pressure gradually. But in CO2 atmosphere, La is very sensitive to pressure variations. 4. Conclusions Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures were investigated in a pressurized fixed bed reactor. The gas evolved during experiment was detected by gas chromatography and the structural features of chars formed under Ar and CO2 atmosphere at 0.1, 0.5, 1.0, and 1.5 MPa were characterized with FTIR and XRD. The main conclusions obtained were summarized below: 1. TG and DTG results show that CO2 atmosphere is favorable to the release of H2 O and absorbing gases. The evolution of CH4 and CO2 show reverse trend at 0.1 MPa. It may be concluded that CO2 can displace the CH4 absorbed in the coal matrix, which will cause the surface energy of the coal to reduce and the matrix to dissolve, and this is benefit to the release of H2 O and absorbing gases. 2. The pressure of 0.5 MPa seems to be a turning point of pressure for the initial gas evolution temperature regardless of the atmosphere. Within 1.5 MPa, the initial evolution temperature of H2 , CO, CH4 all shift to a higher temperature region with the exception of Ar + CH4 . High H2 concentration is measured for T > 550 ◦ C at 0.1 MPa, possibly attributed to secondary pyrolysis of semicoke. Increased pressure shows hindrance function on CO evolution, especially when the pressure is lower than 1.0 MPa. It is an important reason that CO2 gasification reaction happened
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at 680 ◦ C contributes to the CO concentration increase evidently in CO2 atmosphere. 3. FT-IR spectroscopy of the coal chars showed obvious variations in the functional groups among the sample pyrolyzed at atmosphere pressure and all the samples obtained at high pressure. All samples prepared in CO2 atmosphere have more content of hydroxyl, secondary hydroxyl, olefinic C C bonds, aromatic C C bonds left in char compared with that in Ar atmosphere. In both Ar and CO2 atmosphere, pressure seems to have a promotion effect on CH2 formation within 0.5 MPa. The chars prepared under CO2 atmosphere have more cycloalkane and aliphatic structure. The role of atmosphere effect is particularly evident when the pressure exceeds 0.5 MPa. 4. Ar atmosphere is more helpful to the graphitization of coal char during pressurized coal pyrolysis. The growth of microcrystalline structure happened not only in crystal lattice but in crystal layer, and the growth trend shows a notable dependency on atmosphere and pressure. The influence of pyrolysis pressure on crystallite diameter largely depends on the atmosphere. The stacking height Lc of char sample prepared in CO2 atmosphere within 1.0 MPa are always higher than that prepared in Ar atmosphere. CO2 atmosphere is favorable to the growth of crystal layer in vertical position. Acknowledgements The authors gratefully acknowledge the financial support from National Natural Science Foundation (21176166), Shanxi Scholarship Council of China(2012-040), the National Basic Research Program of China (2012CB723105), Shanxi Provincial Education Department (20123028) and Sinkiang Kingho Group of China Kingho Energy Group. References [1] L. Delanie, N. Reinier, L. Dieter, Production of on-specification fuels in coalto-liquid (CTL) Fischer-Tropsch plants based on fixed-bed dry bottom coal gasification, Energy Fuels 24 (2010) 1479–1486. [2] J.R. Bunt, J.P. Joubert, F.B. Waanders, Coal char temperature profile estimation using optical reflectance for a commercial-scale Sasol-Lurgi FBDB gasifier, Fuel 87 (2008) 2849–2855. [3] J.R. Bunt, F.B. Waanders, Identification of the reaction zones occurring in a commercial-scale Sasol-Lurgi FBDB gasifier, Fuel 87 (2008) 1814–1823. [4] J.C. Van Dyk, M.J. Keyser, M. Coertzen, Syngas production from South African coal sources using, Int. J. Coal Geol. 65 (2006) 243–253. [5] R.H. Matjie, C. Van Alphen, Mineralogical features of size and density fractions in Sasol coal gasification ash, South Africa and potential by-products, Fuel 87 (2008) 1439–1445. [6] L. Devi, K.J. Ptasinski, F.J.J.G. Janssen, A review of the primary measures for tar elimination in biomass gasication processes, Biomass Bioenergy 24 (2003) 125–140. [7] J. Han, H. Kim, The reduction and control technology of tar during biomass gasification/pyrolysis: an overview, Renew. Sustain. Energy Rev. 12 (2008) 397–416. [8] Y. Cao, Y. Wang, J.T. Riley, W.P. Pan, A novel biomass air gasification process for producing tar-free higher heating value fuel gas, Fuel Process. Technol. 87 (2006) 343–353. [9] D.A. Bell, B.F. Towler, M.H. Fan, Coal Gasification and its Applications, first ed., Elsevier, Inc., Great Britain, 2011. [10] H.C. Butterman, M.J. Castaldi, CO2 as a carbon neutral fuel source via enhanced biomass gasification, Environ. Sci. Technol. 43 (2009) 9030–9037.
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Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures Yonghui Bai, Pei Wang, Lunjing Yan, Changlong Liu, Fan Li ∗ , Kechang Xie State Key Laboratory Breeding Base of Coal Science and Technology, Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 27 November 2012 Accepted 7 August 2013 Available online 22 August 2013 Keywords: CO2 Gas evolution Char structure Pyrolysis
a b s t r a c t The gas release properties and changes in the char structure during lump coal pyrolysis were investigated in Ar and CO2 atmosphere. Fixed bed reactor and gas chromatography were employed to generate char at four pressures (0.1–1.5 MPa in 0.5 MPa increments) and measure gas composition from room temperature to 700 ◦ C in 50 ◦ C increments, respectively. The influences of pressure and atmosphere on char structural changes were examined by FT-IR and XRD. The results indicate that CO2 atmosphere is favorable to the release of H2 O. The initial evolution temperature of gas species shows a notable dependency on atmosphere and pressure. Especially for CH4 , CO2 atmosphere can promote its emission in advance and make the evolution profile subdued within 0.5 MPa but hinder its evolution over the pressure range of 0.5 MPa, and this inhibitive effect is more evident at a higher pressure. The chars prepared in CO2 atmosphere have more cycloalkane and aliphatic structures, and the role of atmosphere is particularly evident when the pressure exceeds 0.5 MPa. There are more content of hydroxyl, secondary hydroxyl, olefinic C C bonds, aromatic C C structure left in chars prepared under CO2 atmosphere compared with those prepared under Ar atmosphere. The interplanar distance d0 0 2 indicates that Ar atmosphere is more helpful to the graphitization of char during pressurized coal pyrolysis, the stacking height Lc of char sample prepared in CO2 is always higher than that prepared in Ar atmosphere within 1.0 MPa, CO2 atmosphere is beneficial to the growth of crystal layer in vertical position. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The pressurized lump fixed bed gasification technology has showed a notable advantage in SNG production because there is a high content of CH4 in the gas product from gasifier [1]. The CH4 content in crude gas varied from 8% to 12% according to different operating parameters such as the pressure, temperature and the feeding coal properties. In fact, nearly all SNG plants in plan or in production in China use this technology to produce CO and H2 for CH4 synthesis. Because the gas and coal move counter-currently in this kind of gasifier, there is a temperature drop in the reactor [2–4]. As a result the combustion, gasification, pyrolysis and drying zones can be identified in the gasifier. The products from this gasifier include gas, liquid tar and solid coal ash which can be used for brick manufacturing [5]. In the past great attentions have been paid to the gasification process on tar elimination, for tar is undesirable because of various problems associated with condensation [6–8]. However [9], with increasing price of crude oil, precious chemical
∗ Corresponding author. Tel.: +86 351 6018076; fax: +86 351 6010482. E-mail address: [email protected] (F. Li). 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.08.003
material from gasification may be regarded as an advantage rather than a disadvantage. This kind of gasifier, however, has a higher steam to carbon ratio, controlling gasifier temperature lower than the coal ash fusion temperature. This would generate a large amount of waste water, which may bring about damage to the environment and human health. In this case, CO2 can be used partly in place of steam as gasification agent to [10]: (1) save large amount of steam and use CO2; (2) enable the adjustment of the H2 /CO ratio of the crude gas produced as required by downstream processes; and (3) enable the adjustment of gasifier temperature for Boudouard reaction that is also an endothermic reaction where even the heat carrier ability of CO2 is much higher than that of steam. Thus lump coal gasification research in coexistence of steam and CO2 is of great significance. Pyrolysis, as the preliminary process of coal gasification, plays a crucial role in determining gas product distributions and char structure, which is of vital importance to gasification reactivity. Therefore, prior knowledge of lump coal pyrolysis behavior in CO2 atmosphere is useful for a better understanding of coal gasification in coexistence of steam and CO2 [11–15]. Some research works have been conducted for pulverized coal gasification with mixtures of steam and CO2 , and pulverized coal pyrolysis under CO2 and inert atmosphere also have been reported
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[16–20]. From previous research [20,21], the effects of CO2 on coal pyrolysis are embodied in the following three ways: (1) the physical properties of CO2 are different with inert gas, which may influence the diffusion rate of volatile gas; (2) CO2 is a product of coal pyrolysis, which may influence the volatile composition and yield; (3) CO2 is not only a pyrolysis atmosphere but can react with coal char, which may make the char structure and gas evolution different. Knowledge of the effect of CO2 on gas and char structure evolution during pressurized lump coal pyrolysis is rather limited, so the main objective of this study is to both experimentally and theoretically clarify the effect of CO2 on gas and char structure evolution during pressurized lump coal pyrolysis. 2. Experimental 2.1. Initial samples The gases used are CO2 (99.99% purity), Ar (99.99% purity). The solid is a Chinese western bituminous coal from a reservoir located in Yining County, Xinjiang Province. 2.2. Char preparation and gas detection Pressurized pyrolysis tests were performed in a vertical fixed bed reactor, which has an inner diameter of 2.4 cm, maximum design pressure 3 MPa and temperature 800 ◦ C. Tests were conducted at pressures of 0.1, 0.5, 1.0, and 1.5 MPa in pure Ar and CO2 atmosphere, respectively, from room temperature to 700 ◦ C at a constant heating rate of 10 ◦ C/min. Approximately 50 g of lump sized coal sample (+5.0 to 6.0 mm) was placed in the flattemperature zone of the reactor. A K type thermocouple was located 0.3 cm above the sample pan to control the sample temperature and another one was located in the furnace wall for furnace temperature control. When the sample was in place the whole system was sealed and a sweep Ar or CO2 gas flow of 1000 ml/min was used to purge air out in the reactor for 30 min. Then according to different experimental conditions, the reactor was pressurized with Ar or CO2 to the goal pressure value and 500 ml/min Ar was introduced from the top of the reactor to bring the volatile matter out, then the furnace heating was started. The schematic diagram of the experimental system is shown in Fig. 1. After experiment, the char sample was collected from the reactor, which was marked with a label CAx Py . In the CAx Py , “C” stands for char, “x” indicates the
203
atmosphere of the char preparation, and “y” is referred to as the system pressure of the pyrolysis system. For example, CAAr P1.5 stands for the char prepared at 1.5 MPa in Ar atmosphere with a heating rate of 10 ◦ C/min from room temperature to 700 ◦ C. The Proximate analytical value of the coal and its residue chars used in this study were analyzed by ISO11722:1999 (for moisture), ISO1171:1997 (for ash) and ISO562:1998 (for volatile). The ultimate analytical values were analyzed by ISO625-1996 (for C, H) and ISO333-1996 (for N), the properties of the raw coal and its resultant chars are shown in Table 1. The gas released during the pyrolysis was first pretreated for tar and moisture removal and then the stream that exists the back pressure regulator was analyzed with a gas chromatograph. The injector temperature was kept at 120 ◦ C. The CH4 was separated by using a TM-PLOTU capillary column (30 m × 0.53 mm × 20 m) and detected by a flame ionization detector (FID1). H2 and CO separation were performed using a TDX-01 packed column (1 m × 3 mm), H2 was detected by a thermal conductivity detector (TCD). CO was converted to CH4 by a Nickel furnace and then detected by FID2. N2 was used as the carrier gas. The oven temperature was programmed from 45 ◦ C (2 min) to 145 ◦ C at the heating rate of 8 ◦ C/min. The temperature of TCD, FID1 and FID2 are 120, 180, and 350 ◦ C, respectively. Product gases during coal pyrolysis were collected by gas bags from room temperature to 700 ◦ C in 50 ◦ C increments. The time to collect the gas sample is from Ti − 10 ◦ C to Ti + 10 ◦ C (Ti : 50, 100, 150,. . .,700 ◦ C). The accumulated concentration from temperature of Ti − 10 ◦ C to Ti + 10 ◦ C was used to evaluate the evolution concentration of each gas at temperature Ti . The evolution profile of gases was defined by the following formula: ri =
V % × (VTi +10 ◦ C − VTi −10 ◦ C ) mc Vm
(mmol/g)
where ri is the mol gas formed per gram coal at temperature Ti , V% is the volume percent of each gas, Vm is the mole volume of gas at the environmental temperature and the atmospheric pressure, VTi +10 ◦ C is the reading of volume value at temperatures of Ti + 10 ◦ C, mc is the weight of dry coal sample (given in g). 2.3. Char characterization FTIR spectrometry was conducted and recorded via Potassium bromide (KBr) pellet technique using a VECTOR70 FTIR spectrometer (BRUKER, Germany). Spectra for each sample were collected in
Fig. 1. Schematic diagram of the pressurized fixed bed reactor system (1, cylinder; 2, pressure reducing valve; 3, control valve; 4, pressure gauge; 5, mass flow controller; 6, furnace; 7, 8, thermocouples; 9, coal sample; 10, pressurized reactor; 11, temperature controller; 12, primary tar condenser; 13, secondary condenser; 14, back pressure regulator).
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Table 1 Proximate and ultimate analysis of raw coal and its resultant chars. RC
CAAr P0.1
Proximate analysis Moisture (wt%, ad) Volatile (wt%, daf) Ash (wt%, ad)
CAAr P0.5
CAAr P1.0
14.2 33.1 6.14
1.43 5.21 11.4
1.44 4.54 9.8
1.33 4.37 13.1
1.54 2.88 9.03
2.16 6.26 10.5
1.91 5.75 8.72
1.96 5.28 8.53
1.90 5.50 13.6
Ultimate analysis Carbon (wt%, daf) Hydrogen (wt%, daf) Oxygen (wt%, daf) Nitrogen (wt%, daf) Sulfur (total) (wt%, daf)
75.1 3.59 19.8 1.22 0.39
90.1 2.81 3.91 0.87 2.33
91.2 2.61 3.49 0.88 1.72
91.4 2.31 3.36 0.82 2.14
93.7 2.09 2.25 0.73 1.20
88.4 3.05 5.48 0.98 2.05
91.4 2.89 4.53 0.97 0.25
91.6 2.85 3.88 0.89 0.77
93.3 2.86 0.37 0.92 2.53
the mid-IR (400–4000 cm–1 ) region with a resolution of 4 cm–1 and scans of 16. From every sample, the spectrum of a similar thickness KBr wafer was subtracted to obtain high quality spectra. XRD pattern was recorded using a Rigaku Miniflex II diffractometer with Cu K␣1 radiation ( = 0.15406 nm) at 30 kV and 15 mA to scan over the angular 2 range of 5–80◦ with a scanning speed of 8◦ /min. The classical Bragg and Scherer equations were used to analyze the structural parameters d0 0 2 , La and Lc . d0 0 2 is the interplanar distance of two aromatic layers of microcrystalline (nm), Lc is the thickness of microcrystalline (nm), and La is the diameter of microcrystalline (nm). Details of the procedures used to fix the interplanar distance of two aromatic layers of microcrystalline, thickness of microcrystalline and diameter of microcrystalline have been reported elsewhere [22]. 2.4. TGA analysis The TG experiments were performed in the thermogravimetric analyzer (NETZSCH STA449F3), the maximum temperature error of the measurement is ±1 ◦ C and the mass precision is 1 g. Approximately 10 mg initial sample with particle size less than 200 m was fed into the Al2 O3 plate and heated from room temperature to 1000 ◦ C at a constant heating rate of 10 ◦ C/min under Ar or CO2 atmosphere at a constant flow rate of 50 ml/min. Repeated experiments were conducted to ensure the reproducibility of the experiments and accuracy of the data. 3. Results and discussion 3.1. Pyrolysis behavior of raw coal To obtain prior knowledge of pyrolysis behavior of the coal sample used in this study, thermogravimetric analysis was used to investigate the mass decay of coal as a function of temperature and atmosphere. Fig. 2 shows the mass decomposition of coal at a heating rate of 10 ◦ C/min under Ar and CO2 atmosphere. Three distinct regimes can be identified. The first stage occurs between ambient temperature and 250 ◦ C, the minor mass decay observed is mainly due to the release of H2 O and some absorbing gases such as CH4 , CO2 and N2 . The maximum mass decay rate happened at about 70 ◦ C, is 0.57%/min and 0.86%/min at Ar and CO2 atmosphere, respectively. And the mass loss rate under Ar atmosphere is always slower than that under CO2 atmosphere, the difference was well repeated by experiment. Therefore, CO2 atmosphere is favorable to the release of H2 O and absorbing gases. Zhang et al. discovered that [23], at the same temperature and balance pressure, the diffusion coefficient of CO2 is higher than that of CH4 , CO2 can not only be absorbed in the surface of coal pore structure, but it can be dissolved in the coal matrix to swell the matrix itself. It is also reported [24] that replacing the absorbed CH4 by absorbing CO2 can reduce coal surface energy. Therefore, it may be concluded that CO2 can displace the CH4
CAAr P1.5
CACO2 P0.1
CACO2 P0.5
CACO2 P1.0
CACO2 P1.5
absorbed in the coal matrix, which will cause the surface energy of the coal to reduce and the matrix to dissolve, and this is helpful to the release of H2 O and absorbing gases. The greatest fraction of mass loss occurs during the pyrolysis stage between 250 and 680 ◦ C, which is caused by the thermal decomposition of raw coal. The devolatilization rate in CO2 atmosphere first increases with increasing pyrolysis temperature and reaches a maximum at 454 ◦ C, and the maximum devolatilization rate is 0.86%/min. During this regime, replacing Ar with CO2 does not influence the corresponding temperature of mass loss rate peak, but will inhibit the volatile releasing rate during the temperature range of 250–550 ◦ C. The third stage covers a wide temperature range from 680 ◦ C to the final temperature and different weight loss curves were observed. The weight loss of coal sample is 41.12% in CO2 atmosphere pyrolysis from ambient temperature to 800 ◦ C, higher than 31.12% in Ar atmosphere, which is certainly caused by the gasification of coal with CO2 at 680 ◦ C. 3.2. Gas evolution characteristic under CO2 and Ar atmosphere Fig. 3 shows the evolution of the H2 from the pyrolysis of coal under Ar and CO2 atmosphere at four pressures. On the whole, increasing pyrolysis pressure inhibits the evolution of H2 regardless of the atmosphere. But Table 1 shows the volatile left in the char after pyrolysis decrease with increasing pyrolysis pressure, which is different with the finding to other results [25,26]. Yang et al. [27] got the conclusion that increasing pyrolysis pressure was helpful to the conversion of organic functional groups to light gases because of the cracking of aromatic and alkane components during coal pyrolysis under higher pressure. Moreover, Wang et al.’s [28] experiments showed that because of the higher gas density in pressurized condition, the heat transfer rate of the char and the gas molecular motion rate is higher than that in atmospheric pressure,
Fig. 2. TG and DTG curves of coal pyrolysis under different atmosphere.
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Fig. 3. Evolution of H2 with temperature under different coal pyrolysis conditions (Ar–H2 : H2 evolution as a function of temperature in Ar atmosphere).
which could promote the bond rupture of the molecular in the char. It has been clear that high pressure suppresses the formation and release of tar and shifts the molecular weight of tar to a lighter fraction. On the other hand, high pressure promotes secondary reactions, hence increases the total yield of light gases. Because tar is the predominant product of volatile matter, the total yields of volatile matter decrease significantly when pressure is increased [29,30]. It is maybe that the volatile evolved during pyrolysis has a longer residence time in the coal because of the relative large particles and increased pressures, the probability that secondary pyrolysis happens is increased, therefore, the volatile left in the char after pyrolysis decrease with increasing pyrolysis pressure. For Ar–H2 , with the pressure increases from 0.1 to 1.5 MPa in 0.5 increments, the initial evolution temperature lag is clearly visible, the temperature shift from 300 → 350 → 450 → 550 ◦ C. All of the profiles showed two consecutive evolution regimes with a transition temperature in the neighborhood of 500 ◦ C with the exception of 1.5 MPa. At ambient pressure the release of H2 concentration only has a slight increase before 550 ◦ C. Once the temperature exceed 550 ◦ C, the H2 evolved sharply because of the secondary pyrolysis reaction such as cracking, de-hydrogen and breaking up of bridge structures. H2 comes from the condensation of aromatic structures or the decomposition of heterocyclic compounds, processes that occur at a high temperature [31]. At elevated pressures, when the temperature exceeds 600 ◦ C, secondary pyrolysis is the critical factor that contributes to the formation of H2 . Increasing pressure has inhibitive effect on the evolution of H2 . For CO2 –H2 at 1.5 MPa, the initial temperature of H2 evolution is advanced by nearly 50 ◦ C and has one more stage when compared with Ar–H2. To Ar–H2 at 1.5 MPa, nearly all hydrogen was generated from the secondary pyrolysis. Fig. 3 also shows that H2 evolved in the temperature range of 250–550 ◦ C in CO2 atmosphere is less than that evolved in Ar atmosphere. Therefore, the slower mass decay in CO2 atmosphere described in Fig. 2 may be caused by the inhibition effect of the atmosphere. Table 1 shows a remarkable trend for the Ar pyrolyzed chars, with increasing pyrolysis pressure, the oxygen content in the char decreased gradually. In addition, the evolved H2 concentration shows the same trend, which means the evolved H2 has a notable dependency on the oxygen consuming content during pyrolysis. Fig. 4 shows the relationship between the evolved H2 concentration and the oxygen consuming content during pyrolysis in both Ar and CO2 atmosphere. Obviously, the oxygen content plays an important role in the H2 release, the higher the oxygen consumed
Fig. 4. Relationship between evolved H2 concentration in gas and the oxygen content left in the char.
during pyrolysis, the lower the H2 emitted. This may be attributed to the formation of H2 O, Arenillas et al. got the same results [32]. Oxygen content also influences CO formation at a high temperature. Especially for bituminous coals, the cleavage of alkyl aryl ethers has been found to be particularly important [31]. Fig. 5 shows that the evolution of CO at ambient pressure could be divided into three stages regardless of CO2 or Ar atmosphere. The first stage is for pyrolysis temperature before 200 ◦ C where the evolution of CO seems to be slow. The second stage is for pyrolysis temperature between 200 and 550 ◦ C where the CO concentration increases obviously with the rise of temperature to reach a maximum at 450 ◦ C, and CO concentration evolved in Ar and CO2 atmosphere are different, Ar atmosphere is prone to inducing more CO evolved. The third stage is for temperature between 550 ◦ C and 700 ◦ C, CO concentration regain increases sharply and this is much more evident in CO2 atmosphere. The formation of CO in the first stage is mainly due to the absorbing gases in the coal matrix. The second stage [33] is probably attributed to the cracking and reforming of thermolabile carbonyl and ether groups. According to other researchers [34], the production of CO is mainly attributed to aryl ether linkages. At
Fig. 5. Evolution of CO with temperature under different coal pyrolysis conditions.
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Fig. 6. Evolution of CH4 with temperature under different coal pyrolysis conditions.
Fig. 7. Variations of initial gas evolution temperature with pyrolysis pressure under Ar and CO2 atmosphere (Ar + H2 : initial evolution temperature of H2 as a function of pressure in Ar atmosphere).
3.3. Char structural evolution the third stage, increasing CO concentration in Ar atmosphere is probably from the scissions of diarylether groups and secondary reactions of decomposition of the char [35], and it is certain that CO2 gasification reaction happened at 680 ◦ C is an important factor that contributes to the evident CO concentration increase in CO2 atmosphere. In addition, increasing pressure shows hindrance function on CO evolution, especially when the pressure is within 1.0 MPa. Compared with the evolution performance of H2 , CO and CO2 , effects of pressure and atmosphere on CH4 evolution are much more complicated. Fig. 6 indicates that when the pressure is less than 0.5 MPa, CO2 atmosphere could promote the emission of CH4 in advance and made the evolution profile subdued compared with CH4 emission in Ar atmosphere. But when the pressure is higher than 0.5 MPa, the initial evolution temperature lag of CH4 is observed under CO2 atmosphere and the lag is more evident at a higher pressure. Before 500 ◦ C, it is probable that [33] the CH4 is mainly from the cracking of methoxyl group. After 500 ◦ C, the CH4 evolution might be attributed to the rupture of aromatic rings and secondary decomposition of some light gases. For Ar–CH4 at 0.1 MPa, CH4 evolution covers over a wider temperature range from 300 to 700 ◦ C, the CH4 concentration first increases and reaches a maximum in the neighborhood of 500 ◦ C. With the temperature increase further, the CH4 concentration decreases sharply until 640 ◦ C, then regain get a steep increase. For CO2 –CH4 at 0.1 MPa, the temperature range of CH4 evolution expands to 150–700 ◦ C, and all evolution peak shifts to low temperature region. It maybe that there has a competitive adsorption between CH4 and CO2 , and the adsorptivity of CO2 is higher than that of CH4 , which made more CH4 evolved slowly. To have a more detailed analysis about the initial evolution temperature of H2 , CO and CH4 , variations of initial evolution temperatures with pyrolysis pressure for each gas under Ar and CO2 atmosphere was obtained in Fig. 7. The initial evolution temperature reflects the thermal behavior of the coal during pyrolysis. To all curves, the pressure of 0.5 MPa seems to be a turning point pressure for the initial gas evolution temperature regardless of the atmosphere. Overall, within 1.5 MPa, the initial evolution temperature of H2 , CO and CH4 all shift to a higher temperature region with the exception of Ar + CH4 . Within 0.5 MPa, the initial evolution temperature changed marked with increasing pyrolysis pressure. When the pressure exceeds 0.5 MPa, this trend was weakened and even Ar + CH4 presents a reverse behavior.
3.3.1. FTIR analysis FTIR qualitative analysis is used to investigate the impacts of pressure and atmosphere on functional group formation. The FTIR spectra of chars prepared at different conditions are shown in Fig. 8. The spectra indicate that all chars contain a number of infrared absorption peak, such as hydroxyl, aliphatic hydrocarbon, olefinic and aromatic C C bond, olefinic and aromatic C H, etc. Regardless of the atmosphere, the band with strong intensity at 3427 cm−1 denotes the presence of large amounts of hydroxyl groups in the char, which can be attributed to OH bond stretching vibrations. The appearance of the peaks at 2920 and 2852 cm−1 are the asymmetrical and symmetrical stretching vibration of CH2 in the cycloalkane and aliphatic. Comparing a and b, it reveals that the chars prepared under CO2 atmosphere have more cycloalkane and aliphatic structure left, the role of atmosphere is particularly evident when the pressure exceeds 0.5 MPa [18]. Wang et al. also discovered that the replacement of N2 with CO2 does result in great changes in the surface chemistry for coal char. CO2 atmosphere increases the content of aliphatic C H groups and the concentration of methylene C H bands compared with N2 . The spectra in the range of 1000–1800 cm–1 involved abundant information about the C C and oxygen containing functional groups. As seen, the hydroxyl, secondary hydroxyl, olefinic C C bonds, aromatic C C bonds are the main atomic groups and structures in the char that result in the infrared absorption in this region. As revealed, all samples prepared in CO2 atmosphere have more content of C C, O H, and C O left in char compared with that in Ar atmosphere [36]. Wang et al. revealed that N2 atmosphere was helpful to the phenolic hydroxyl formation while Coal chars obtained in CO2 atmosphere have relatively lower phenolic hydroxyl content in char sample by performing the FTIR analysis of coal chars obtained in CO2 atmosphere and inert N2 atmosphere [37]. Minkova et al. compared and found the samples treated in a flow of reactive gas and found that samples treated in reactive gas have higher contents of oxygen-containing groups with basic and weakly acidic character. In both Ar and CO2 atmosphere increasing pressure has promotion effect on–CH2 formation within 0.5 MPa, but that effect shows a reverse trend over 0.5 MPa. 3.3.2. XRD analysis As one of the most frequently used techniques in studying microcrystalline structural, the XRD patterns can distinguish the
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Fig. 8. FTIR spectra of (a) the chars generated in Ar atmosphere at different pressures (b) the chars generated in CO2 atmosphere at different pressures.
pyrolysis conditions on the char structural change. Results of the raw coal and chars prepared at different temperatures are shown in Fig. 9. It can be seen that all patterns show two characteristic peaks 0 0 2 and 1 0 0 with the exception of RC. The char microcrystalline
structure is transferred to graphitic structure gradually after pressurized pyrolysis [38]. 0 0 2 peak indicates the degree of orientation of aromatic carbon net in microcrystal, 1 0 0 peak indicates the size of aromatic carbon net in microcrystal; with the 0 0 2 peak and 1 0 0 peak going to be narrower and higher, the synusia would
Fig. 9. XRD spectrum of chars prepared at different atmosphere and pressure.
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Fig. 10. Variation of crystallite structure parameters with pressure and atmosphere (a: d0 0 2 ; b: Lc ; c: La ).
have better degree of orientation while the aromatic nuclear have higher degree for condensation. To have a more detailed analysis about the crystalline structure information, Fig. 10 shows the variation of crystallite structure parameters with pressure and atmosphere. Fig. 10(a) indicates that with the pyrolysis pressure increased from 0.1 to 1.5 MPa, the values of interplanar distance d0 0 2 in both Ar and CO2 atmosphere all have a slightly and ordered decrease, from 0.3697 nm to 0.3649 nm in CO2 atmosphere and from 0.3692 nm to 0.3644 nm in Ar atmosphere. But the value is still greater than the d0 0 2 parameter of perfect graphite. In addition, the interplanar distance d0 0 2 of all chars prepared in CO2 atmosphere are greater than that prepared in Ar atmosphere. Therefore, compared with CO2 , Ar atmosphere is more helpful to the graphitization of coal char and increasing pyrolysis pressure is also contributed to the graphitization process [18]. Wang et al. analyzed the char structure by using Raman Spectroscopy, and also discovered that char got from CO2 has more disordered structures when compared with the char from inert N2 atmosphere. By contrast, Fig. 10(b) and (c) suggests that the crystallite diameter La and crystallite height Lc are all changed significantly with different pyrolysis atmosphere and increasing pyrolysis pressure. This shows that the growth of microcrystalline structure happened not only in crystal lattice but in crystal layer, and the variation trend shows a notable dependency on atmosphere and pressure. Fig. 10(b) reveals that the stacking height Lc of char sample prepared in CO2 atmosphere within 1.0 MPa are always higher than that prepared in Ar atmosphere, and it is favorable to the growth of crystal layer in vertical position. When the pressure exceeds 1.0 MPa, reverse result was observed. It can be reasoned that within 1.0 MPa, the preferential consumption of disordered carbon that lead to the great order in the char for the coal sample used can react with CO2 at 680 ◦ C. When the pressure exceeds 1.0 MPa, increasing pressure has an inhibit effect on carbon gasification reaction, which made the further growth of Lc become hard
[17]. Liu et al. also got the results that the gasification reaction is a very important factor during pyrolysis in CO2 atmosphere and CO2 was more beneficial for high-volatile coal to develop carbon active sites and amorphous carbon. Fig. 10(c) tells that the influence of pyrolysis pressure on crystallite diameter is largely depends on the atmosphere. In Ar atmosphere, La changed with pressure gradually. But in CO2 atmosphere, La is very sensitive to pressure variations. 4. Conclusions Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures were investigated in a pressurized fixed bed reactor. The gas evolved during experiment was detected by gas chromatography and the structural features of chars formed under Ar and CO2 atmosphere at 0.1, 0.5, 1.0, and 1.5 MPa were characterized with FTIR and XRD. The main conclusions obtained were summarized below: 1. TG and DTG results show that CO2 atmosphere is favorable to the release of H2 O and absorbing gases. The evolution of CH4 and CO2 show reverse trend at 0.1 MPa. It may be concluded that CO2 can displace the CH4 absorbed in the coal matrix, which will cause the surface energy of the coal to reduce and the matrix to dissolve, and this is benefit to the release of H2 O and absorbing gases. 2. The pressure of 0.5 MPa seems to be a turning point of pressure for the initial gas evolution temperature regardless of the atmosphere. Within 1.5 MPa, the initial evolution temperature of H2 , CO, CH4 all shift to a higher temperature region with the exception of Ar + CH4 . High H2 concentration is measured for T > 550 ◦ C at 0.1 MPa, possibly attributed to secondary pyrolysis of semicoke. Increased pressure shows hindrance function on CO evolution, especially when the pressure is lower than 1.0 MPa. It is an important reason that CO2 gasification reaction happened
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at 680 ◦ C contributes to the CO concentration increase evidently in CO2 atmosphere. 3. FT-IR spectroscopy of the coal chars showed obvious variations in the functional groups among the sample pyrolyzed at atmosphere pressure and all the samples obtained at high pressure. All samples prepared in CO2 atmosphere have more content of hydroxyl, secondary hydroxyl, olefinic C C bonds, aromatic C C bonds left in char compared with that in Ar atmosphere. In both Ar and CO2 atmosphere, pressure seems to have a promotion effect on CH2 formation within 0.5 MPa. The chars prepared under CO2 atmosphere have more cycloalkane and aliphatic structure. The role of atmosphere effect is particularly evident when the pressure exceeds 0.5 MPa. 4. Ar atmosphere is more helpful to the graphitization of coal char during pressurized coal pyrolysis. The growth of microcrystalline structure happened not only in crystal lattice but in crystal layer, and the growth trend shows a notable dependency on atmosphere and pressure. The influence of pyrolysis pressure on crystallite diameter largely depends on the atmosphere. The stacking height Lc of char sample prepared in CO2 atmosphere within 1.0 MPa are always higher than that prepared in Ar atmosphere. CO2 atmosphere is favorable to the growth of crystal layer in vertical position. Acknowledgements The authors gratefully acknowledge the financial support from National Natural Science Foundation (21176166), Shanxi Scholarship Council of China(2012-040), the National Basic Research Program of China (2012CB723105), Shanxi Provincial Education Department (20123028) and Sinkiang Kingho Group of China Kingho Energy Group. References [1] L. Delanie, N. Reinier, L. Dieter, Production of on-specification fuels in coalto-liquid (CTL) Fischer-Tropsch plants based on fixed-bed dry bottom coal gasification, Energy Fuels 24 (2010) 1479–1486. [2] J.R. Bunt, J.P. Joubert, F.B. Waanders, Coal char temperature profile estimation using optical reflectance for a commercial-scale Sasol-Lurgi FBDB gasifier, Fuel 87 (2008) 2849–2855. [3] J.R. Bunt, F.B. Waanders, Identification of the reaction zones occurring in a commercial-scale Sasol-Lurgi FBDB gasifier, Fuel 87 (2008) 1814–1823. [4] J.C. Van Dyk, M.J. Keyser, M. Coertzen, Syngas production from South African coal sources using, Int. J. Coal Geol. 65 (2006) 243–253. [5] R.H. Matjie, C. Van Alphen, Mineralogical features of size and density fractions in Sasol coal gasification ash, South Africa and potential by-products, Fuel 87 (2008) 1439–1445. [6] L. Devi, K.J. Ptasinski, F.J.J.G. Janssen, A review of the primary measures for tar elimination in biomass gasication processes, Biomass Bioenergy 24 (2003) 125–140. [7] J. Han, H. Kim, The reduction and control technology of tar during biomass gasification/pyrolysis: an overview, Renew. Sustain. Energy Rev. 12 (2008) 397–416. [8] Y. Cao, Y. Wang, J.T. Riley, W.P. Pan, A novel biomass air gasification process for producing tar-free higher heating value fuel gas, Fuel Process. Technol. 87 (2006) 343–353. [9] D.A. Bell, B.F. Towler, M.H. Fan, Coal Gasification and its Applications, first ed., Elsevier, Inc., Great Britain, 2011. [10] H.C. Butterman, M.J. Castaldi, CO2 as a carbon neutral fuel source via enhanced biomass gasification, Environ. Sci. Technol. 43 (2009) 9030–9037.
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