Effect of Operational Variables on the Hydrogasification of Inner

Jul 16, 2013 - ABSTRACT: Hydrogasification of Inner Mongolian lignite semicoke was investigated at varying operational conditions: grain size of 0.15â...
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Effect of Operational Variables on the Hydrogasification of Inner Mongolian Lignite Semicoke Xiaokuo Ding,† Yongfa Zhang,*,† Tiankai Zhang,† Jian Tang,‡ Ying Xu,† and Jing Zhang† †

Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China ‡ Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China S Supporting Information *

ABSTRACT: Hydrogasification of Inner Mongolian lignite semicoke was investigated at varying operational conditions: grain size of 0.15−0.7 mm, hydrogen flow rate of 300−1500 mL/min, temperature of 700−900 °C, pressure of 0.1−5.0 MPa, and heating rate of 10−30 °C/min, using a self-developed high-temperature and high-pressure fixed-bed reactor with a design parameter of 1000 °C and 12 MPa. The reaction characteristics and mechanism of semicoke hydrogasification were discussed. The results showed that, at a grain size of semicoke of 1200 mL/min, the effects of internal and external diffusion resistances were basically eliminated; the increase of the pressure or heating rate exerted a positive influence on the hydrogasification of semicoke. The optimum reaction conditions were as follows: grain size of 0.25−0.35 mm, hydrogen flow rate of 1200 mL/min, temperature of 800 °C, and pressure of 3.0−4.0 MPa. During hydrogasification, the fracture of alkyl side chains, hydrogenation/methanation of carbon (e.g., C + 2H2 = CH4), and hydrogenation of oxygen-containing structures were accelerated. Three stages occurred during hydrogasification of Inner Mongolian lignite semicoke: hydropyrolysis stage, rapid hydrogasification stage, and slow hydrogasification stage. Moreover, the reaction characteristics of each stage were quite different because of distinct carbon structures.

1. INTRODUCTION With the increasing demands and prices of natural gas, coalbased synthetic natural gas (SNG) technology has attracted wide attention.1,2 Hydrogasification (C + 2H2 = CH4), with its thermal efficiency up to 70−80%, shows good prospects. In recent years, a lot of research has been performed on hydrogasification of bituminous coals and biomass.3−6 Lee et al. investigated the hydrogasification of bituminous coals and found that the carbon conversion and the methane contents were improved with the increase of the temperature, hydrogen pressure, or H2/coal ratio; the coal conversion of Datong bituminous coal was 48% with a concentration of CH4 of 31.2 vol %, at 800 °C, 7.0 MPa, and 0.5 of H2/coal ratio.7 Zhang et al. studied the effects of the H2/coal ratio and reaction time on the hydrogasification of Japanese sub-bituminous Taiheiyo coal and concluded that the process of hydrogasification was accelerated with the increase of the H2/coal ratio and the yield of methane was increased with the prolonging of the reaction time.8 In recent years, studies on the hydrogasification of lignite mainly focus on the development of catalyst. However, the study on the effect of operational varieties on the hydrogasification of lignite is comparatively rare. It was found that transition metals, such as iron and nickel, and alkali metals could promote the reaction rate of hydrogasification of lignite.9−11 Ohtsuka et al.12 held that iron markedly promoted the hydrogasification at 873 K and the coal conversion reached 76 wt % within 40 min. Murakami et al.13 pointed out that the peak of CH4 evolution appeared at 860−1010 K and the amount of CH4 evolution became larger with the increasing amount of exchanged nickel loading. Mısırlıoğlu et al.14 © 2013 American Chemical Society

investigated the hydrogasification of Turkey lignite char and maintained that the increase of the pressure and temperature was beneficial to the formation rate of methane and the hydrogenation activity of char was reduced with the increase of the carbonization temperature. The hydrogenation activity of the low-rank coal, such as lignite, is better than that of the high-rank coal, but the oxygen contents of the low-rank coal is as high as 15−25%, which leads to high hydrogen consumption during hydrogasification. It was found that, in comparison to raw lignite, the oxygen contents were greatly reduced and the hydrogenation activity was improved for pyrolyzed lignite semicoke. On the basis of this, a step conversion technology was put forward: production of semicoke with high hydrogenation activity by pyrolysis of lignite, coupled with the production of SNG by hydrogasification of semicoke.15,16 Inner Mongolia has lignite reserves of more than 150 billion tons, which account for more than 70% of total lignite reserves in China.17 In recent years, lignite pyrolysis technology has developed rapidly, but the majority of pyrolyzed semicoke is burnt to generate electricity at low use efficiency. The production of substitute natural gas by hydrogasification of lignite semicoke will not only achieve high-efficiency use of lignite semicoke but also ease the pressure on the natural gas supply; therefore, this technology shows high social and economic benefits. Hydrogasification of raw coal, biomass, or coke has been widely studied; however, there are few studies on Received: April 19, 2013 Revised: July 16, 2013 Published: July 16, 2013 4589

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Table 1. Proximate and Ultimate Analyses of Inner Mongolian Lignite and Semicoke proximate analysis (wad, %)

ultimate analysis (wdaf, %)

sample

moisture

ash

volatile matter

C

H

O

N

St

lignite semicoke

17.68 0.89

10.18 12.45

30.69 18.09

63.73 87.63

6.26 3.69

28.12 6.08

1.43 1.72

0.46 0.88

temperature measuring point and the semicoke bed was revised. The strong heat transfer took place between the outlet gas and stainlesssteel pipe, which made the rapid drop of product gas temperature; hence, the extra cooling unit was not necessary, and moreover, the cooling process exerted a slight influence on the product gas. To protect the gas chromatograph, a gas-purification equipment, with a glass tube including a CaCl2 bed of 40 cm height and a cotton bed of 10 cm height, was installed to remove the small amount of moisture in the outlet gas. Air in the reactor was first purged off by hydrogen (99.99%, Taiyuan, China), and then the reaction pressure was obtained by accelerating 1 MPa every 5 min; later, the semicoke was heated at a certain rate to the reaction temperature and kept thermostatic. The flow rate of inlet gas was controlled by the mass flow meter, and the back-pressure valve was used to guarantee the pressure stability of the system. The flow rate of outlet gas was measured by a soap foam flow meter, and gas chromatographies, including Shanghai Haixin GC-950 and Shanghai Linghua GC-9890A, were employed to detect the composition and contents of outlet gas. All of the pressure and temperature conditions for volume and flow rate data mentioned in this work, with exceptions noted, were 25 °C and 1 atm. 2.3. Analytical Apparatus. Gas chromatography GC-950 was equipped with a thermal conductivity detector (TCD), and its working conditions were high-purity hydrogen (99.999%, Beijing, China) as the carrier gas, gasification chamber temperature of 50 °C, and detection chamber and column/furnace temperatures of 40 °C. Channels A and B were contained in gas chromatography GC-950: channel A was accomplished using a 5 Å molecular sieve packed stainless-steel column (3.0 × 1000 mm), with a bridge current of 60 mA and precolumn pressure of 0.06 MPa, and was used to detect contents of CH4 and CO; channel B was accomplished using a GDX-502 packed stainless-steel column (3.0 × 2000 mm), with a bridge current of 60 mA and precolumn pressure of 0.08 MPa, and was used to detect the contents of CO2, C2H4, and C2H6. Gas chromatography GC-9890A was equipped with a TCD and used to detect contents of H2. Except for using high-purity nitrogen (99.999%, Beijing, China) as the carrier gas, other parameters were just the same as that of GC-950 channel A. The surface functional groups of semicokes were detected by a Bruker VERTEX70 infrared spectrometer (Germany) at a scanning range of 400−4000 cm−1, resolution of 4 cm−1, and scanning rate of 10 kHz. Semicoke and KBr, with amounts of 1 ± 0.05 and 100 ± 0.05 mg, respectively, were evenly mixed and pressed into disc. The crystal structures of semicokes were detected by a Daojin XRD6000 X-ray diffractometer (Japan), under the following testing conditions: Cu Kα radiation, voltage of 30 kV, scanning range of 5− 80°, scanning rate of 8°/min, and semicoke amount of 45.8−50.3 mg. 2.4. Calculation Method. The yield of tar was extremely small; therefore, the carbon conversion of semicoke X could be determined by eq 1.

hydrogasification of lignite semicoke, especially Inner Mongolian lignite semicoke. In this study, the effects of the grain size, hydrogen flow rate, reaction temperature, pressure, and heating rate on the hydrogasification of Inner Mongolian lignite semicoke were investigated using a self-developed hightemperature and high-pressure fixed-bed reactor.18 The reaction characteristics during hydrogasification and the reaction mechanism were discussed.

2. EXPERIMENTAL SECTION 2.1. Experimental Materials. In a nitrogen atmosphere, lignite was heated at 10 °C/min to 600 °C and then kept there for 30 min. Later, the semicoke was divided into four parts and broken into 0.7− 0.5, 0.5−0.35, 0.35−0.25, and 0.25−0.15 mm, respectively. The proximate and ultimate analyses of Inner Mongolian lignite and semicoke were shown in Table 1. From Table 1, it could be seen that, in comparison to lignite, the contents of moisture, volatile matter, and H and O elements of semicoke were reduced obviously but the contents of C, N, and S elements were increased, which resulted from the fact that the precipitation rates of C, N, and S elements were less than the weight loss rate of lignite. 2.2. Experimental Apparatus and Process of Semicoke Hydrogasification. Figure 1a showed the schematic diagram of

Figure 1. Schematic diagram of (a) hydrogasification of lignite semicoke and (b) concrete filling structure in the reaction tube: (1) H2, (2) N2, (3) partial pressure valve, (4) mass flow meter, (5) hightemperature and high-pressure reactor, (6) semicoke, (7) thermocouple, (8) temperature controller, (9) back-pressure valve, (10) gas purification, (11) gas chromatography, (12) computer, (13) soap foam flow meter, (14) reaction tube, (15) broken tiles (40−80 mesh), (16) stainless-steel filter screen (150 mesh), (17) semicoke, (18) broken tiles (100−120 mesh), and (19) stainless-steel filter screen (150 mesh). hydrogasification of lignite semicoke. The experimental apparatus was composed of a gas intake system with a partial pressure valve and mass flow meter included, a reaction system, which was mainly a hightemperature and high-pressure reactor, a control system with a temperature controller, a back-pressure valve, and a mass flow meter included, and a detection system, including gas chromatograph and soap foam flow meter. A reaction tube (21 mm inner diameter) was included in the high-temperature and high-pressure reactor. Inner Mongolian lignite semicoke, with its weight of 16 g and bed height of 7−8.5 cm, was put into the reaction tube, which was filled with filler on both ends to guarantee that the semicoke was in the constant temperature area, whose concrete filling structure could be seen in Figure 1b. The temperature measuring point of the thermocouple was installed outside of the reaction tube, whose height was the same as the middle of the semicoke bed. The temperature error between the

X = 12(VCH4 + 2VC2H6 + 2VC2H4 + VCO2 + VCO) /(1000 × 22.4mC)

(1) 3

The heating value q (MJ/Nm ) of the total gas product without hydrogen in 140 min was determined by eqs 2 and 3.

q = (CCH4 × 35.90 + CC2H6 × 64.35 + CC2H4 × 63.40 + CCO2 × 0 + CCO × 12.64)/100 Cx = Vx /(VCH4 + VC2H6 + VC2H4 + VCO2 + VCO) 4590

(2) (3)

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Figure 2. Influence of the grain size on the (a) variation of carbon conversion and (b) yields of gaseous products in 140 min.

Figure 3. Influence of hydrogen flow rate on the (a) variation of carbon conversion and (b) yields of gaseous products in 140 min. The formation rates of gaseous products Rx (mL g−1 min−1) were determined by eq 4.

R x = dVx /dt /mC = vcx /m C/100

thus relieving the impact of the temperature gradient on the reaction. With the grain size decreased, the diffusion path of gaseous molecules was shortened, the diffusion resistance was reduced, the mass transfer was enhanced, and the impact of internal diffusion resistance was weakened, all of which were beneficial for the hydrogasification of semicoke. When the grain size was 1200 mL/min, the reaction rate and carbon conversion were not obviously increased; meanwhile, the effect of external diffusion resistance was basically 4592

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Figure 6. Influence of heating rate on the (a) variation of reaction rate of carbon and (b) carbon conversion in 140 min.

methane was inhibited, which were beneficial to the increase of the reaction rate and carbon conversion. However, with the increase of the reaction pressure, the increasing amplitude of the methane equilibrium concentration was reduced as well as the increasing amplitude of carbon conversion. The increase of the reaction pressure also prolonged the retention time of gas products and inhibited the diffusion of hydrogen molecules and methane molecules,10 which were not good for the increase of the hydrogasification rate. It was shown in Figure 5b that, with the increase of the pressure, the yields of CH4, C2H6, and C2H4 were increased and the yields of CO and CO2 were reduced. Figure 5c showed that the concentration of alkane (CH4 + C2H6 + C2H4) and the heating value of gas products without hydrogen were increased with the increase of the reaction pressure. When the pressure increased from 0.1 to 2.0 MPa, the concentration of alkane and the heating value of gas products without hydrogen were increased from 39.6% and 18.87 MJ/Nm3 to 93.7% and 35.13 MJ/Nm3, respectively. At 5.0 MPa, the concentration of alkane and the heating value of gas products without hydrogen reached 95.1% and 35.77 MJ/Nm3. The increase of the reaction pressure could improve the carbon conversion, methane yield, and alkane concentration and heating value of gas products; however, the increasing amplitudes of carbon conversion and methane yield were gradually reduced. Therefore, the optimal pressure was selected as 3.0−4.0 MPa. 3.1.5. Effect of the Heating Rate. Figure 6 showed the influence of the heating rate on the (a) variation of the reaction rate of carbon and (b) carbon conversion in 140 min. Figure 6a showed that the hydrogasification of semicoke could be divided into two stages. With the increase of the heating rate, the reaction rate at the first stage was obviously increased and the difference of reaction rates between the two stages bacame more apparent. In Figure 6b, at the same time point, the carbon conversion rate at the rapid heating process was higher than that at the slow heating process. When the heating rate was increased from 10 to 30 °C/min, the maximum reaction rate was increased from 1.43 to 2.35 C %/min and the maximum methane contents in the outlet gases were increased from 27.1 to 39.4%. The increase of the heating rate could allow for the semicoke to be at a high temperature rapidly and accelerated the hydroconversion of structures with high hydrogenation activity (e.g., alkyl side chains, oxygencontaining functional groups, etc.). However, the skeleton carbon structures were in low hydrogenation activity; therefore,

chains in low-energy requirement; however, the aromatic rings were stable and hard to hydrogenate. It was found that the reaction active energy of hydrogasification was up to 100−200 kJ/mol;3,14,19 therefore, high energy was needed for the hydrogasification of carbon. The rise of the temperature could increase not only the atomic energies of C and H but also the motion frequency of H atoms, which increased the frequency of effective collision between H and C atoms and led to the increase of the reaction rate, carbon conversion, and yield of methane. C + 2H2 = CH4 (ΔH = −84.3 kJ/mol) was a strong exothermic reaction; therefore, the increase of the temperature was not beneficial for the formation of methane. Under the same pressure, with the increase of the reaction temperature, the equilibrium concentration of methane and the reaction equilibrium force decreased gradually and the decomposition of CH4 became easier, which inhibited the enhancement of carbon conversion and methane production.20 Between 800 and 900 °C, carbon conversion was not significantly increased with the increase of the temperature, which indicated that the promoting effects of the temperature on methane production and decomposition were almost balanced. Figure 4b showed that, with the increase of the temperature, the yield of CH4 was increased, while the yields of C2H6, C2H4, CO, and CO2 did not change much. Methane was the major product of semicoke hydrogasification; therefore, the yield of methane was increased with the increase of carbon conversion. C2H6, C2H4, CO, and CO2 were basically separated out before 700 °C; therefore, the effects of the final reaction temperature were not severe on the yields of C2H6, C2H4, CO, and CO2. 3.1.4. Effect of the Pressure. Figure 5 showed the influence of the pressure on the (a) variation of carbon conversion, (b) yields of gaseous products, and (c) alkane concentration and heating value of gas products in 140 min. It was shown in Figure 5a that, with the increase of the pressure, the carbon conversion was improved but the amplitude of increase was gradually reduced. When the pressure was increased from 0.1 to 1.0 MPa, carbon conversion was increased by 31.9% from 11.3 to 43.2%; when the pressure was increased from 1.0 to 5.0 MPa, the carbon conversion was increased by 29.5, 13.4, 6.2, and 4.0% at each increasement of 1.0 MPa. C + 2H2 = CH4 was a volume decrease reaction; therefore, the increase of the reaction pressure was beneficial for the formation of methane. With the increase of the pressure, the density of active hydrogen atoms was increased, the equilibrium concentration of methane and the reaction equilibrium force were increased, and the decomposition of 4593

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Table 2. Proximate and Ultimate Analyses of Semicokes with Different Hydrogenation Degrees proximate analysis (wad, %)

ultimate analysis (wdaf, %)

sample

moisture

ash

volatile matter

C

H

O

N

St

semicoke 60 min 70 min 80 min 90 min 110 min 140 min

0.89 1.12 0.89 1.05 0.96 1.23 0.95

12.45 19.52 22.23 26.31 30.68 45.63 77.92

18.09 12.56 9.87 6.25 2.05 1.32 0.68

87.63 94.50 97.11 98.21 98.61 98.87 99.09

3.69 2.10 1.60 1.01 0.78 0.63 0.49

6.08 1.89 0.28 0.20 0.13 0.11 0.10

1.72 0.81 0.48 0.24 0.22 0.20 0.16

0.88 0.70 0.53 0.34 0.26 0.19 0.16

the increase of the heating rate could not largely affect their reaction rate. 3.2. Characteristic Changes of Semicoke during Hydrogasification. Inner Mongolian lignite semicoke (16 g) was hydrogasificated at a grain size of 0.25−0.35 mm, hydrogen flow rate of 1200 mL/min, temperature of 800 °C, pressure of 4.0 MPa, and heating rate of 10 °C/min. The starting time for heating was marked as 0, and after 60 min of reaction, the inlet gas was changed to nitrogen and the backpressure valve was opened. Then, hydrogen in the reaction tube was purged off by nitrogen in 3 min. The reactor temperature was dropped to 50 °C after 2 h, and the rest of the semicoke was marked as “60 min”. Similarly, semicokes with different hydrogenation degrees (“70 min”, “80 min”, “90 min”, “110 min”, and “140 min”) were obtained. Semicokes with different hydrogenation degrees were stored in zip-lock bags separately after they were cooled. The characteristic changes during hydrogasification could be reflected by the differences of semicokes with different hydrogenation degrees. 3.2.1. Chemical Composition Analysis. Table 2 showed the proximate and ultimate analyses of semicokes with different hydrogenation degrees. Figure 7 showed the changes of the H/ C and O/C atom ratios of semicoke during hydrogasification.

structures, the hydrogenation activity of semicoke was gradually reduced. 3.2.2. Fourier Transform Infrared Spectroscopy (FTIR) Analysis. The FTIR of semicokes with different hydrogenation degrees was shown in Figure 8. The infrared spectra of

Figure 8. FTIR of semicokes with different hydrogenation degrees.

Table 3. Infrared Spectra of Functional Groups

Figure 7. Variation of H/C and O/C atom ratios with carbon conversion.

wavenumber (cm−1)

functional group

compounds

3690 3000−3500 2921, 2852 2358 1617 1454, 1385 1118 1035 550−850 538 471

N−H O−H, N−H C−H CO CC C−H CS SO C−H S−S S−H

−NH of aromatic ring −OH, −NH −CH3, −CH2 carboxyl, carbonyl CC of aromatic ring −CH3, −CH2 >CS >SO −CH of aromatic ring −S−S− −SH

functional groups were showed in Table 3. For the existence of different vibration types, absorption peaks might appear at different wavelengths for the same functional group; e.g., the deformation vibrations of −CH3 and −CH2 appeared at 1454 and 1385 cm−1, and the peaks of 2921 and 2852 cm−1 belonged to the stretching vibrations of −CH3 and −CH2. It was shown in Figure 8 that, before 800 °C, −CH3 and −CH2 deformation vibrations and stretching vibrations became weakened, which was led by the fracture of alkyl side chains. After 80 min of hydrogasification, the stretching vibrations of −OH and −NH and the absorption peak of hydrogen bonds formed from the association of −OH and N of pyridine or pyrrole structure (3000−3500 cm−1) were obviously weakened,

As shown in Table 2, with the progress of hydrogasification, the relative contents of H, O, N, and S in semicoke were gradually reduced, while the relative content of C was increased. Figure 7 showed that, with the progress of hydrogenation, the H/C atom ratio in semicoke was reduced and the O/C atom ratio was reduced from 0.052 to 0.002 in the first 70 min of hydrogasification. This result indicated that the carbon structures containing H, O, N, and S were more prone to hydroconversion, but with the consumption of these active 4594

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stretching vibrations of −NH in aromatic rings (3690 cm−1), CO stretching vibration (2358 cm−1), CS stretching vibration (1118 cm−1), SO stretching vibration (1035 cm−1), S−S characteristic absorption peak (538 cm−1), and S−H characteristic absorption peak (471 cm−1) largely disappeared (these heteroatom groups easily decomposed), and the existence of H2 could promote the fracture of these heteroatom groups and the formation of small molecules, and this was in accordance with the decrease of O, N, and S contents in semicoke. After 80 min of hydrogasification, −CH deformation vibration of the substituted aromatic ring (750−900 cm−1) and CC stretching vibration of the aromatic ring skeleton (1617 cm−1) were weakened, indicating that C−H in the aromatic ring structure was consumed, the absorption peak at 3000− 3500 cm−1 still existed and became weak, and the existence of heteroatoms promoted the hydrogenation of the aromatic ring. With the progress of the reaction, the CC stretching vibration of the aromatic ring skeleton became weakened; however, the peaks of −CH3 and −CH2 still existed, which may have resulted from the hydrogenation of the aromatic ring structure, and this was in accordance with the theoretical hypothesis by Espinal et al.22 After 140 min of hydrogasification, the carbon conversion was reached above 90% and the intensities of infrared absorption peaks almost disappeared. 3.2.3. X-ray Diffraction (XRD) Analysis. XRD patterns of the semicokes with different hydrogenation degrees were showed in Figure 9.

the hydrogasification of carbon;10−12 however, the formation of KFeO2 resulted in the loss of active catalytic components and then the decrease of the catalytic effect, which was one of the factors that lead to the decrease of the hydrogasification rate. 3.3. Reaction Characteristics and Mechanism of Semicoke Hydrogasification. The precipitation laws of gas products during hydrogasification and pyrolysis of lignite semicoke were investigated at a grain size of 0.25−0.35 mm, hydrogen flow rate of 1200 mL/min, temperature of 800 °C, pressure of 4.0 MPa, and heating rate of 10 °C/min. Figure 10 showed the (a) formation rates of gaseous products and (b) reaction rate of carbon and carbon conversion of semicoke during hydrogasification and (c) formation rates of CO and CO2 and (d) formation rates of CH4, C2H6, and C2H4 in hydrogasification and pyrolysis processes. Figure 10a showed that the hydrogasification process occurred in three stages: hydropyrolysis stage (before point A, 0−60 min), rapid hydrogasification stage (A−B, 60−80 min), and slow hydrogasification stage (after point B, 80−140 min). Figure 10b showed that the carbon contents consumed in each stage were 18.7, 27.6, and 53.7% of the total carbon contents in semicoke, respectively. FTIR analysis showed that the functional groups of heteroatoms and alkyl side chains were greatly reduced in the hydropyrolysis stage, which corresponded to the generation of CO2, CO, CH4, C2H6, and C2H4. As shown in panels c and d of Figure 10, in comparison to the pyrolysis process, the formation rates of CH4 and C2H6 in the hydropyrolysis stage were apparently improved and the formation rates of CO and CO2 were reduced. This was due to the acceleration of the alkyl fracture, hydrogenation/methanation of carbon (e.g., C + 2H2 = CH4), and hydrogenation of oxygen-containing structures under a high-pressure hydrogen atmosphere. The carbon structures consumed in the rapid hydrogasification stage showed high hydrogenation activity. Exposed-edge carbon atoms in the aromatic ring were generally believed to be the hydrogenation active sites.22,24 A lot of exposed C−H structures were contained in hydrogen-rich lowaromatic-ring structures, and they could act as active sites during hydrogasification. FTIR analysis indicated that C−H in the aromatic ring structure was obviously reduced in the rapid hydrogasification stage. The presence of O, S, or N atoms could lower the stability of aromatic rings; therefore, heterocyclic structures showed high hydrogenation activity too. Mısırlıoğlu et al. stated that the oxygen-containing structures were active sites during hydrogasification.14 During the slow hydrogasification stage, with the consumption of hydrogenation active structures, the densification degree of semicoke was increased, the number of active sites was reduced, and the hydrogenation of hydrogen-deficient skeleton carbon structures (e.g., microcrystalline structure) became the main reaction gradually; therefore, the reaction rate and methane concentration in gas products were rapidly reduced. It was widely accepted that the reaction rate was very high at the initial hydrogasification stage and dropped very soon with the increase of carbon conversion.1,21,25 Moseley and Paterson observed that, except for the evolution of the volatile, the rapid hydrogenation of carbon occurred at the rapid initial hydrogasification stage. 26 The experimental results were in accordance with that mentioned above.

Figure 9. XRD of semicokes with different hydrogenation degrees.

Figure 9 showed three diffraction peaks, appearing at 20.8°, 26.6°, and 31.3° of the semicoal XRD spectrum, attributed to SiO2 in ash, C(002), and KFeO2, separately. The peak C(002) denoted the degree of parallel and azimuthal orientation of the aromatic lamellae.23 In the first 90 min of hydrogasification, the intensity of the peak C(002) did not change much, but its intensity was decreased gradually after 90 min. This was because, at the early reaction stage, hydrogasification mainly occurred in non-crystalline structures, such as oxygencontaining functional groups, low-aromatic-ring structures, etc., while microcrystalline structures were not involved. After 90 min of hydrogasification, the non-crystalline structures were basically consumed, the hydrogasification of microcrystalline structures occurred, and the decrease of the size and quantity of microcrystalline structures led to the decrease of the peak C(002) intensity. After 80 min of hydrogasification, the KFeO2 diffraction peak appeared and its intensity increased gradually with the progress of the reaction. K and Fe were catalysts for 4595

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Figure 10. (a) Formation rates of gaseous products and (b) reaction rate of carbon and carbon conversion of semicoke during hydrogasification and (c) formation rates of CO and CO2 and (d) formation rates of CH4, C2H6, and C2H4 in hydrogasification and pyrolysis processes.

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4. CONCLUSION

ASSOCIATED CONTENT

S Supporting Information *

(1) Significant effects of the grain size and hydrogen flow rate were shown on the hydrogasification of semicoke. Under the conditions of grain size of 1200 mL/min, the effects of internal and external diffusion resistances were basically eliminated and the reaction was transferred from diffusion control to chemical reaction control. The increase of the temperature could promote both the formation and the decomposition of CH4, between 800 and 900 °C. The promoting effects of the temperature on methane formation and decomposition were almost balanced for Inner Mongolian lignite semicoke. With the increase of the reaction pressure, the carbon conversion and methane yield were increased but their increasing amplitudes were decreased gradually. The optimum reaction conditions were grain size of 0.25−0.35 mm; hydrogen flow rate of 1200 mL/min; temperature of 800 °C, and pressure of 3.0−4.0 MPa. Hydrogasification of Inner Mongolian lignite semicoke, for its high hydrogenation activity, can produce the outlet gas with high methane contents and heating values at relatively low temperature and pressure; therefore, it shows good industrial application prospect. (2) The fracture of alkyl side chains, hydrogenation/methanation of carbon (e.g., C + 2H2 = CH4), and hydrogenation of oxygen-containing groups/structures were promoted during hydrogasification. (3) Hydrogasification of Inner Mongolian lignite semicoke included three stages: hydropyrolysis of active functional groups, such as oxygencontaining functional groups, alkyl side chains, etc., rapid hydrogasification of carbon structures with high hydrogenation activity, for example, hydrogen-rich low-aromatic-ring and heterocyclic structures, and slow hydrogasification of hydrogen-deficient skeleton carbon structures.

Influence of methane concentration in feed gas on the equilibrium carbon conversion and yield of each gas product (Figures S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-0351-6018676. E-mail: yongfaz@yeah. net. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Grant 2012CB723105), the National Science and Techno logy Pillar Program (Grant 2012BAA04B03), and the Natural Science Foundation of China (Grant 51274147).

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NOMENCLATURE X = carbon conversion Vx = yield of gas product x, where x can be CH4, C2H6, C2H4, CO2, or CO (mL) mC = total mass of carbon in semicoke (g) q = heating value of gas products without hydrogen (MJ/ Nm3) Cx = concentration of x in total gas products without hydrogen (%) Rx = formation rates of gaseous products (mL g−1 min−1) v = flow rate of outlet gas (mL/min) cx = concentration of x in outlet gas with hydrogen (%) RC = reaction rate of carbon (C %/min) dx.doi.org/10.1021/ef4007092 | Energy Fuels 2013, 27, 4589−4597

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(24) Radović, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Importance of carbon active sites in the gasification of coal chars. Fuel 1983, 62, 849− 856. (25) Wen, C. Y.; Huebler, J. Kinetic study of coal char hydrogasification. Rapid initial reaction. Ind. Eng. Chem. Process Des. Dev. 1965, 4, 142−147. (26) Moseley, F.; Paterson, D. The rapid high temperature hydrogenation of coal chars. Part 1: Hydrogen pressures up to 100 atm. J. Inst. Fuel 1965, 288, 13−23.

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dx.doi.org/10.1021/ef4007092 | Energy Fuels 2013, 27, 4589−4597