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Energy & Fuels 2008, 22, 1142–1147
Cracking of Simulated Oil Refinery Off-Gas over a Coal Char, Petroleum Coke, and Quartz Yuan Zhang,†,‡ Jin-hu Wu,*,† and Dong-ke Zhang§ Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China, Graduate School of Chinese Academy of Sciences, Beijing 100039, China, and Centre for Fuels and Energy, Curtin UniVersity of Technology, GPO Box U1987, Perth, WA6845, Australia ReceiVed NoVember 14, 2007. ReVised Manuscript ReceiVed January 5, 2008
The cracking of oil refinery off-gas, simulated with a gas mixture containing methane (51%), ethylene (21.4%), ethane (21.1%), and propane (6.5%), over a coal char, petroleum coke, and quartz, respectively, has been studied in a fixed bed reactor. The experiments were performed at temperatures between 850 and 1000 °C and at atmospheric pressure. The results show that the conversions of all species considered increased with increasing temperature. Ethane and propane completely decomposed over all three bed materials in the temperature range investigated. However, the higher initial conversion rates of methane and ethylene cracking at all temperatures were observed only over the coal char and not on the petroleum coke and quartz, indicating a significant catalytic effect of the coal char on methane and ethylene cracking. Methane and ethylene conversions decreased with reaction time due to deactivation of the coal char by carbon deposition on the char surface and, in the later stage of a cracking experiment, became negative, suggesting that methane and ethylene had been formed during the cracking of ethane and propane.
1. Introduction During petroleum refining processing, a number of waste streams of gaseous hydrocarbons are generated from operation units including the processes of delayed coking, thermal cracking, fluid catalytic cracking, etc. These gases, also referred to as refinery off-gas, are rich in low carbon hydrocarbons, alkanes such as methane, ethane, and propane, and alkenes such as ethylene and propylene. The dry refinery off-gas is generally used as a fuel, and the amount in excess of utility fuel requirements of the refinery is usually flared, presenting a waste of a valuable resource. A low cost and environmentally friendly means to utilize this resource is therefore highly desired. Production of syngas (H2 + CO) for synthesis of chemicals, synthetic fuel, or hydrogen is considered a worthwhile approach. The low C hydrocarbons as found in the refinery off-gas can be reformed to produce syngas by means of steam reforming, carbon dioxide reforming, and partial oxidation reforming. Recently, a new concept of combined coal gasification and gas reforming in a fluidized-bed reactor for utilization of coal-bed methane has been proposed and proved feasible on the laboratory scale.1 Under moderate temperature (about 1000 °C) and atmospheric pressure conditions, high once-through methane conversion with favorable quality of the syngas was achieved. It was also demonstrated that the coal char studied has a beneficial catalytic activity for the cracking and reforming of methane.2–5 Furthermore, the process has the advantages of an adjustable ratio of H2/CO of the syngas * Corresponding author. Tel.: +86 351 4031362. E-mail address:
[email protected]. † Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. § Curtin University of Technology. (1) Wu, J. H.; Fang, Y. T.; Wang, Y.; Zhang, D. K. Energy Fuels 2005, 19, 512–516. (2) Sun Z. Q.; Wu, J. H.; Wang, Y.; Zhang, D. K. Methane Cracking over a Chinese Coal Char in a Fixed-Bed Reactor. 5th Asia-Pacific Conference on Combustion; Adelaide, Australia, July 17–20, 2005; pp 401–404.
to suit further downstream processing and avoiding the use of an expensive metal catalyst. Extending the previous work, we have proposed the concept of coconversion of the refinery offgas and a solid fuel, either a coal char or petroleum coke, both of which are also abundantly available at relatively low costs, to produce syngas. Such a process is likely to involve cracking, steam reforming, and carbon dioxide reforming of the refinery off-gas and the shift reactions. The present contribution investigates the cracking of refinery off-gas over a coal char, petroleum coke, and quartz. Hydrocarbon cracking on carbon materials has attracted increasing research interests in recent years.2,4,6–16 Muradov6–8 studied methane cracking to produce hydrogen with activated carbon, carbon black, graphite, carbon fiber, and carbon nanometer pipe as the catalysts with the activated carbon and (3) Sun Z. Q.; Wu, J. H.; Wang, Y.; Zhang, D. K. Methane and Carbon Dioxide Reactions over a Chinese Coal Char in a Fixed-Bed Reactor. 5th Asia-Pacific Conference on Combustion; Adelaide, Australia, July 17–20, 2005; pp 437-440. (4) Sun, Z. Q.; Wu, J. H.; Haghighi, M.; et al. Energy Fuels 2007, 21, 1601–1605. (5) Haghighi, M.; Sun, Z. Q.; Wu, J. H.; et al. Proc. Combust. Inst. 2007, 31, 1983–1990. (6) Muradov, N. Energy Fuels 1998, 12, 41–48. (7) Muradov, N. Int. J. Hydrogen Energy 2001, 26, 1165–1175. (8) Muradov, N. Catal. Commun. 2001, 2, 89–94. (9) Lee, E. K.; Lee, S. Y.; Han, G. Y.; et al. Carbon 2004, 42, 2641– 2648. (10) Bai, Z. Q.; Chen, H. K.; Li, B. Q.; Li, W. J. Anal. Appl. Pyrolysis 2005, 73, 335–341. (11) Bai, Z. Q.; Chen, H. K.; Li, W.; Li, B. Q. Int. J. Hydrogen Energy 2006, 31, 899–905. (12) Bai, Z. Q.; Chen, H. K.; Li, W.; Li, B. Q. J. Fuel Chem. Technol. 2006, 34, 66–70. (13) van der Vaart, D. R. Combust. Flame 1988, 71, 35–39. (14) Hesketh, R. P.; Davidson, J. F. Combust. Flame 1991, 85, 449– 467. (15) Ross, D. P.; Yan, H. M.; Zhang, D. K. Combust. Flame 2001, 124, 156–164. (16) Ross, D. P.; Yan, H. M.; Zhang, D. K. Fuel 2004, 83, 1979–1990.
10.1021/ef700680d CCC: $40.75 2008 American Chemical Society Published on Web 02/19/2008
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Table 1. Proximate and Ultimate Analyses of Binxian Coal, Binxian Coal Char, and Petroleum Coke proximate analysis (% w/w) Mad
Aad
Vad
ultimate analysis (% w/w) Cad
Had
Oad
Nad
Sad
Binxian coal 5.02 13.89 27.67 66.77 3.91 9.16 0.99 0.26 Binxian raw char 1.04 12.7 2.08 81.82 1.04 2.09 0.89 0.42 petroleum coke 0.53 1.02 1.35 91.82 1.26 2.68 1.02 1.67
carbon black being identified as better catalysts. Lee et al.9 observed that the initial conversion rate of methane cracking on carbon black is lower than that on activated carbon but the carbon black shows a better steady-state performance. Bai et al.10–12 found that the activated carbon can catalyze methane cracking but methane conversion rate decreases with time. van der Vaart,13 Hesketh and Davidson,14 and Ross, et al.15,16 also found that coal char can improve the rate of pyrolysis of propane in their respective studies of the coal combustion and gasification with propane used to simulate the coal volatile matter.
which required 9 g of coal char, 9.5 g of petroleum coke, or 18 g of quartz. Nitrogen is continuously flowed at 200 mL min-1 through the reactor while the furnace is heated to a desired temperature between 850 to 1000 °C. Once a set temperature is reached and stabilized, the gas stream is switched to the mixture of the simulated refinery off-gas and N2 at a volumetric ratio of 1:9 (refinery offgas:N2), also at a flow rate of 200 mL min-1. The exit stream from the reactor is sampled periodically and analyzed using two Shimadzu gas chromatographs (GC-14C): one analyzes the N2, H2, and CH4 using a thermal conductivity detector (TCD) and a Φ 3 mm × 3 m carbon molecular sieve column with argon as the carrier gas while the other analyzes C1-C4 hydrocarbons using a flame ionization detector (FID) and a Φ 0.32 mm × 30 m Rt-QPLOT column with N2 as the carrier gas. In addition, the surface structures of the coal char before and after the cracking experiments are also investigated using a JEOL JSM-6360LV scanning electronic microscope (SEM), and the specific surface area and pore structure properties are determined using a TriStar 3000 physical adsorption apparatus using N2 adsorption at 77 K.
3. Results and Discussion 2. Experimental Details The refinery off-gas simulated using a gas mixture containing methane (51%), ethylene (21.4%), ethane (21.1%), and propane (6.5%) was employed in the present study. A coal char produced by pyrolyzing Binxian bituminous coal in nitrogen at 900 °C for 30 min, petroleum coke, provided by Shijiazhuang Oil Refinery (Hebei Province, P.R. China), and quartz were used as the bed materials over which the cracking of the refinery off-gas was studied, respectively. The bed materials were sieved to a size fraction of 0.355–0.63 mm for the experimentation. Table 1 shows the proximate and ultimate analyses of the raw coal and the coal char as well as the petroleum coke. The refinery off-gas cracking experiments were carried out in a vertical quartz tube fixed bed reactor as shown in Figure 1. The reactor has a diameter of 25 mm (i.d.) and a height of 620 mm, respectively, and is housed in an electrically heated furnace. The refinery off-gas is diluted with N2 with the aid of mass flow controllers to achieved the desired ratios, the gas mixture is then passed into the reactor from the top, and the effluent exits the reactor from the bottom, as illustrated in Figure 1. The high rate of nitrogen dilution of the simulated refinery off-gas employed in the experiments is a strategy employed in this research to minimize the diffusion effect on the cracking reactions to be investigated and to provide a “trace” for quantitative analysis of the other species during the experimentation as it can be safely assumed that nitrogen is neither consumed nor produced in the cracking of the refinery off-gas. In a typical experimental run, a bed material is weighed and placed into the quartz tube reactor to a bed depth of ca. 40 mm,
Figure 1. Schematic of the experimental system: (1) mass flow controllers, (2) mixing chamber, (3) temperature controller, (4) quartz tube reactor, (5) electrically heated furnace.
“Hydrogen balance” before and after the cracking of refinery off-gas was employed as a check for the mass balance and therefore the validity of an experiment. The numbers of hydrogen moles are calculated based on the concentrations of hydrogen containing species and the constant volumetric flow rate of N2 before and after the experiment and expressed as the molar ratio of the hydrogen element in the input and exit streams. Hydrogen balance, rather than carbon balance, was chosen to check the mass balance because the reaction system involves “phase changes” of the carbon from gas to solid due to carbon deposition, which cannot be accurately accounted for in the current experimental setup. The hydrogen balance results for typical experiments using the three different bed materials, respectively, at 900 °C and using the coal char at different temperatures between 850 and 1000 °C are shown in Figure 2. It is evident that the hydrogen balances were all around 1, given reasonable instrumental errors, suggesting that the experimental results were reliable. At 900 °C, the conversion of propane on all three bed materials reached 100% and did not change with the reaction time, indicating that the propane has been completely converted through thermal cracking without the need for a catalyst. Ethane cracking exhibited a similar trend but slightly less than 100% conversion. Figure 3 shows the change in ethane conversion with reaction time when refinery off-gas was cracked over the coal char, petroleum coke, and quartz, respectively, at 900 °C, reaching above 97% conversion and essentially remaining constant, indicating that the ethane can also be cracked without a catalyst. Methane conversions during the refinery off-gas cracking over the coal char, petroleum coke, and quartz at 900 °C are shown in Figure 4. Methane conversion on the coal char was initially significantly higher and remained higher than when the cracking was performed on the petroleum coke and quartz. The initial conversion of methane on the coal char reached about 23%, gradually decreased as the reaction progressed, and finally stabilized at a steady-state value of about -16%. However, the methane conversions on the petroleum coke and quartz were quite low, at ca. -22%, and basically remained constant during the course of the experiments. The negative conversion of methane during the cracking of the refinery off-gas indicates that methane was formed, rather than consumed, from the cracking of propane and ethane as well as possibly ethylene. Ethylene conversions as a function of time over the coal char, petroleum coke, and quartz at 900 °C are given in Figure 5.
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Figure 2. Hydrogen balance during the refinery off-gas cracking over a bed of the coal char, petroleum coke, and quartz at 900 °C (left) and over a bed of the char at different temperatures (right) (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
Figure 3. C2H6 conversion during the refinery off-gas cracking over the coal char, petroleum coke, and quartz at 900 °C (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
Figure 4. CH4 conversion during the refinery off-gas cracking over a bed of the coal char, petroleum coke, and quartz at 900 °C (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
Ethylene conversion on the coal char was seen to be higher than those on the petroleum coke and quartz, while the conversion rates over the petroleum coke and the quartz were basically the same. The initial conversion of ethylene on the coal char was as high as 72%, while it was only about 10% over the petroleum coke and essentially zero over quartz. The conversion gradually decreased as the reaction progressed and leveled off at -11% on the char and -17% on the petroleum coke and quartz. The negative ethylene conversions observed in the later stages of the experiments suggest that ethylene had also been formed from the cracking of ethane and propane. An overwhelming observation of the three different types of bed materials is that the fresh char induced significantly higher conversions of methane and ethylene during the cracking than
Figure 5. C2H4 conversion during the refinery off-gas cracking over a bed of the coal char, petroleum coke, and quartz at 900 °C (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
the petroleum coke and quartz, although the petroleum coke also showed a slightly higher activity than quartz. The results in Figures 4 and 5 suggest that the coal char has a catalytic effect on the cracking of the lower hydrocarbons, similar to our earlier observations on methane cracking.2,4 However, this catalytic activity of the char is rapidly lost as the reaction progresses, due to carbon deposition,4 becoming similar to those of the petroleum coke. Figure 6 gives a photographic account of severe carbon deposition in the quartz tube reactor before and after an experiment. Figure 7 shows the SEM images of the char before and after the refinery off-gas cracking experiment. It can be seen that the fresh char has a very clean porous structure with clear edges and the spent char surface is covered with amorphous carbon spheres deposited on the char surface. As the cracking of the refinery off-gas continued on the char, the deposited carbon grew on the char surface and some lumps and needle like aggregates gradually formed. The lumps and needles are composed of many nanosized carbon spheres as illustrated in Figure 8. The BET surface areas and pore characteristics of the coal char, petroleum coke, and quartz before and after 2 h of experiment of the refinery off-gas cracking at 900 °C and with a total flowrate of 200 mL min-1 (with 10% refinery off-gas and 90% N2 dilution) are detailed in Table 2. It is evident that the total surface area and micropore area of the coal char were greatly reduced after the 2 h cracking experiment. The total surface area decreased more than 8 times from 6.49 to 0.82 m2 g-1while the micropore area decreased 27 times from 2.46 to 0.09 m2 g-1. Accordingly, the micropore volume was significantly reduced, by some 2 orders of magnitude, from 1.12 × 10-3 to 3.5 × 10-5 cm3 g-1. In the meantime, the average pore diameter increased from 1.84 to 23.73 nm. These changes in the char
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Figure 6. Photographs showing severe carbon deposition in the quartz tube reactor before (left) and after (right) a refinery off-gas cracking experiment at 900 °C (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
Figure 7. SEM images of a char before (left) and after (right) a refinery off-gas cracking experiment, showing carbon deposition on the char surface (experimental conditions are the same as those for Figure 6).
Figure 8. High-resolution SEM images of carbon deposits on a char surface after a refinery off-gas cracking experiment, showing aggregates of carbon spheres (experimental conditions are the same as those for Figure 6).
surface and structural characteristics indicate that the deposited carbon generated by cracking of the refinery off-gas, primarily propane and ethane, covered the char surface and blocked the pore channels, especially the micropores. We have already shown in an earlier piece of work4 that the coal char has a profound catalytic effect on methane cracking. A similar effect has also been observed on ethylene cracking on the same coal char in the present experimentation. We have also demonstrated that the petroleum coke and quartz, which have very low total surface areas and no micropores (Table 2), show no catalytic effect on methane or ethylene cracking. These observations suggest that the cracking of methane and ethylene are more related to the micropore surface area and pore volume of the char, although the catalytic nature of the char surface is not known at this stage.
Table 2. Variations in the Surface Properties of the Coal Char, Petroleum Coke, and Quartz before and after a Refinery Off-Gas Cracking Experiment at 900 °C for 2 ha
sample fresh coal char spent coal char fresh petroleum coke spent petroleum coke fresh quartz spent quartz a
total micropore surface area, area, m2 g-1 m2 g-1
micropore volume, cm3 g-1
average pore diameter, nm
6.49 0.82 2.89
2.46 0.09 1.52
1.12 × 10-3 3.50 × 10-5 6.96 × 10-4
1.84 23.73 7.18
0.45
N/A
N/A
28.50
0.20 0.13
N/A N/A
N/A N/A
N/A N/A
Experimental conditions are the same as those for Figure 6.
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Figure 9. H2 yields (left) and C balance (right) during the refinery off-gas cracking over the char, petroleum coke, and quartz at 900 °C (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
Figure 10. C2H6 conversion during the refinery off-gas cracking over the char at different temperatures (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
Figure 11. CH4 conversion during the refinery off-gas cracking over a bed of the char at different temperatures (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
Figure 9 compares H2 yields and the gas phase carbon balance during the refinery off-gas cracking at 900 °C over the char, petroleum coke, and quartz, respectively. The H2 yield is defined as the number of moles of molecular hydrogen in the exit stream divided by the equivalent total number of moles of hydrogen in the hydrocarbons in the inlet stream, expressed as a percentage which is calculated from eq 1 below. Y )
CH2,out 2CCH4,in + 2CC2H4,in + 3CC2H6,in + 4CC3H8,in
×
CN2,in CN2,out (1)
where “C ” represents the concentrations of H2, N2, CH4, C2H4, C2H6, and C3H8 and subscripts “in” and “out” refer to reactor inlet and outlet streams, respectively.
Figure 12. C2H4 conversion during the refinery off-gas cracking over a bed of the char at different temperatures (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
As the deposited carbon from the cracking was not measured, so the carbon balance was indicated by the ratio of the number of carbon atoms in the gas phase products to that in the feed stream. A lower gas phase carbon balance ratio indicates greater carbon formation and higher conversion of the refinery off-gas cracking. The results presented in Figure 9 also clearly show that the char plays a catalytic role in the cracking of the lower hydrocarbons (namely, methane and ethylene in this study); the more completely the lower hydrocarbons are cracked, the more molecular hydrogen will be generated. When the char was used as the bed material, the hydrogen yield gradually decreased to a stead-state value. The initial and maximum hydrogen yield of 67% was achieved over the fresh char, while the maximum hydrogen yields were only 37% and 30%, primarily due to the cracking of the higher hydrocarbons (propane and ethane) over the petroleum coke and quartz, respectively. Accordingly, the formation of carbon deposition was more severe on the char than on the petroleum coke and quartz, as indicated by the carbon balance data. With the char as the bed material, the carbon balance ratio rapidly increased from 32% to an equilibrium value of 68%, while the carbon balance ratio only increased from 66% to slightly above 72% over the petroleum coke and the quartz, respectively. Since propane and ethane conversions showed little difference when the three bed materials were used, the differences in the H2 yields and the gas phase carbon balance ratios are clearly attributed to the different abilities of the bed materials to crack the lower hydrocarbons (methane and ethylene). This further proves that the char catalyzes the cracking of methane and ethylene while the petroleum coke and quartz do not.
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Figure 13. H2 yields and carbon balance during the refinery off-gas cracking over a bed of the char at different temperatures (CH4 5.1%, C2H4 2.14%, C2H6 2.11%, C3H8 0.65%, N2 90%, total flow rate 200 mL min-1).
The effect of reaction temperature on the cracking of the refinery off-gas over the char was further investigated in detail. Propane conversion during the refinery off-gas cracking experiments was not sensitive to temperature. At all temperatures between 850 and 1000 °C, propane was always completely converted and the conversion did not change with the reaction time. Ethane conversion, on the other hand, was slightly dependent on the reaction temperature. Figure 10 shows that ethane conversion over the char was about 90% at 850 °C and increased to over 99% at 1000 °C. The higher the reaction temperature, the higher the ethane conversion. Both propane and ethane conversions were not sensitive to the bed materials either, nor to reaction time as previously shown in Figure 2. These observations on the conversions of propane and ethane suggest that their cracking is primarily a diffusion-controlled thermal cracking process and does not requires a surface to activate the breakage of the bonds within these molecules. Note that, at 1000 °C, due to severe carbon deposition causing blockage of the reactor after some 40 min of operation, the results are considered not reliable after that point. Therefore, only the first five data points (within the first 30 min of the experiment) for the 1000 °C runs are presented in Figures 10-13 below. However, both the initial and steady-state methane conversions during the refinery off-gas cracking over the char are strongly dependent on the reaction temperature as shown in Figure 11. The initial methane conversion reached about 43% at 1000 °C with a steady-state conversion near zero. Note that methane can be produced from the cracking of propane and ethane as well as ethylene and the methane conversion levels presented here depend on the rate at which methane is decomposed and the rate at which methane is formed from the cracking of the higher hydrocarbons present. It is also noted from Figure 11 that the higher the reaction temperature, the higher the methane conversion. Figure 12 shows ethylene conversions during the refinery offgas cracking over the char at different temperatures. It can be seen that the ethylene conversion was also initially high and, as the reaction progressed, gradually decreased to lower values, depending on the temperature. At 900 °C the steady-state ethylene conversion was about zero, suggesting that the rate of ethylene cracking and the rate of ethylene production (from propane and ethane cracking) reached a steady state. When the temperature was increased from 850 to 1000 °C, the ethylene conversion was increased rapidly. The steady state ethane
conversion was about -70% at 850 °C, while at 1000 °C it reached 68%, indicating that the conversion of ethylene is very sensitive to the reaction temperature. H2 yields and the gas phase carbon balance ratios during the refinery off-gas cracking at different temperatures are shown in Figure 13. The hydrogen yield was always initially high and gradually reduced with reaction time to a steady-state level at a given temperature. The gas phase carbon balance ratio was always initially low and increased to a steady-state value with time and, as H2 yield increased at higher temperatures, so did the carbon deposition. The equilibrium gas phase carbon balance ratio decreased as the reaction temperature was increased. 4. Conclusions The cracking of oil refinery off-gas over a coal char, petroleum coke, and quartz has been studied using a fixed bed reactor between 850 and 1000 °C and atmospheric pressure. Propane conversion reached 100% over all three bed materials within the temperature range investigated. Ethane conversion only showed slight dependence on temperature and was not sensitive to the bed materials either. These results suggest that the cracking of propane and ethane above 850 °C is essentially a thermal cracking process without the need of a catalyst. However, both methane and ethylene conversions were dependent on the type of bed material, reaction temperature, and time. The conversions of methane and ethylene were the net results of their decomposition and formation from the cracking of propane and ethane. The conversions of methane and ethylene were always initially high but decreased with reaction time over the coal char; however, little conversion of methane and ethylene was observed over the petroleum coke and quartz. This suggests that the coal char has a catalytic effect on the cracking of methane and ethylene and this catalytic effect is attributed to the micropore structure of the char. The deposition of carbon from the cracking of the refinery off-gas deactivated the char for methane and ethylene conversion by blocking the micropore structure of the char. Acknowledgment. This work has been supported under the Natural Science Foundation of China (Project: 50628404) and the Outstanding Overseas Chinese Talent Funds Scheme of Chinese Academy of Sciences (CAS). EF700680D
