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Energy & Fuels 2008, 22, 1444–1448
Production of Syngas via Partial Oxidation and CO2 Reforming of Coke Oven Gas over a Ni Catalyst Jianzhong Guo,†,‡ Zhaoyin Hou,*,† Jing Gao,† and Xiaoming Zheng*,† Institute of Catalysis, Department of Chemistry, Zhejiang UniVersity (Xixi Campus), 310028 Hangzhou, China, and Department of Chemistry, Zhejiang Forestry UniVersity, 311300 Lin’an, China ReceiVed July 9, 2007. ReVised Manuscript ReceiVed December 4, 2007
The partial oxidation and CO2 reforming of coke oven gas (COG) to syngas was investigated on differently sized Ni catalysts in a fluidized-bed reactor. It was found that the catalytic performance of Ni depends strongly on its particle size. The small-sized Ni catalyst exhibited higher activity and higher selectivity in the partial oxidation of COG. The conversion of CH4 was kept at 80.7% at a lower temperature (750 °C) and a wide space velocity (from 8000 to 80 000 h-1). CO2 reforming of COG is also an efficient route for syngas production. The H2/CO ratio in the COG-derived syngas could be controlled by manipulating the concentration of O2 or CO2 added in the feed. The yield of produced syngas increases with an increase in temperature.
1. Introduction The coke industry has gotten an enlarged utilization in China since 2000 because of the increasing price of crude oil; more than 243 million tons of coke was produced in 2005 in China. A huge amount of coke oven gas (COG), which contains H2 (∼58%), CH4 (∼26%), CO (∼6%), N2 (∼4%), CO2 (∼2.7%), C2+ (∼2.5%), O2 (∼0.5%), and S (∼0.02%), is released as a byproduct. Until now, only about 20% of the produced COG is simply utilized as fuel; and a large amount of COG is directly let off, which causes serious air pollution. Chemical utilization of COG and CO2 is of great interest in both economical and environmental protection aspects. At the same time, selective conversion of CH4 in the H2-rich COG is one of the challenging topics in catalysis research. Recently, several patents claimed that methanol,1,2 dimethyl ether,3 or long-chain hydrocarbons could be produced via the syngas derived from COG. However, mainly noncatalytic conversion was reported in the syngas production step in these literature reports, which needs a very high operation temperature, a high energy supply, and specially designed equipment. The production of hydrogen via the partial oxidation and steam reforming of COG was also reported without the use of a catalyst at higher temperature (1100–1400 °C).4 As the steam reforming of COG is a highly endothermic reaction, this process also requires a large energy input, a high O2/COG ratio (4/7), and a high H2O/COG ratio (2/5-3/7) in order to resist the carbon deposition. However, an overstoichiometric amount of O2 in the feed (when O2/CH4 > 0.5) is consumed mainly in the oxidation of CO to CO2 and the oxidation of H2 to H2O. The catalytic conversion of COG would depress the reaction temperature and would lessen the quantity of the heat supply; * To whom correspondence should be addressed. Telephone: +86-57188273283. Fax: +86-571-88273283. E-mail:
[email protected]. † Zhejiang University (Xixi Campus). ‡ Zhejiang Forestry University. (1) Guo, Y. J. CN 1373113A, 2002. (2) Fang, D. W.; Tong, J. F.; Zhao, X.; Guo, X. Y.; Feng, X. T.; Yu, Z. H.; Wang, F. C.; Fang, D. Y.; Tang, J. P. CN 1660734, 2005. (3) Yang, Q. S. CN 1876614, 2006. (4) Onozaki, M.; Watanabe, K.; Hashimoto, T.; Saegusa, H.; Katayama, Y. Fuel 2006, 85, 143–149.
the catalytic partial oxidation of COG to syngas has low energy requirements due to the contribution of the exothermic CH4 oxidation (eq 1). CH4 + 0.5O2 f CO + 2H2
∆H298o ) -27.3 kJ/mol (1)
At the same time, the CO2 reforming of COG is also very interesting because CO2 is an associated gas in COG. In addition, the H2/CO ratio in the COG-derived syngas via CO2 reforming is more suitable for the synthesis of methanol, dimethyl ether, and long-chain hydrocarbons. Ni is popularly reported as the active species for the partial oxidation and CO2 reforming of CH4 to syngas because of its higher catalytic activity, lower price, and abundant resources.5–8 The selective partial oxidation and CO2 reforming of CH4 in the H2-rich COG are challenging research work and are seldom reported. Recently, we found that small-sized Ni catalysts exhibited a higher activity and a high hydrogen selectivity in the combined CO2 reforming and partial oxidation of methane at a higher space velocity (90 000 h-1).9 In this study, we want to report an effective process for the H2-rich COG conversion to syngas via the selective partial oxidation and CO2 reforming at a high space velocity and a relatively lower temperature over differently sized Ni catalysts. The influence of temperature, O2/CH4 ratio, CO2/CH4 ratio, and space velocity (8000-240 000 h-1) over a small-sized Ni catalyst (4.5 nm) on methane conversion and H2/CO ratio in the produced syngas was investigated. 2. Experimental Section 2.1. Catalyst Preparation. Ni catalysts sized from 4.5 to 45.0 nm were prepared via direct impregnation of [Ni(en)3]2+ (en, ethylenediamine), [Ni(EDTA)]2- (EDTA, ethylenediaminetetraace(5) Hu, Y. H.; Ruckenstein, E. AdV. Catal. 2004, 48, 297–345. (6) Hu, Y. H.; Ruckenstein, E. Catal. ReV. Sci. Eng. 2002, 44, 423– 454. (7) Bradford, M. C. J.; Vannice, M. A. Catal. ReV. Sci. Eng. 1999, 41, 1–42. (8) Hou, Z. Y.; Yashima, T. Appl. Catal., A 2004, 261, 205–209. (9) Hou, Z. Y.; Gao, J.; Guo, J. Z.; Liang, D.; Lou, H.; Zheng, X. M. J. Catal. 2007, 250, 331–341.
10.1021/ef7003865 CCC: $40.75 2008 American Chemical Society Published on Web 04/05/2008
Syngas Production from Coke OVen Gas
Energy & Fuels, Vol. 22, No. 3, 2008 1445
Table 1. Properties of Ni/SiO2 Catalysts Prepared from Different Ni Precursorsa Ni diameter (nm) Ni precursor
Ni H2 H2 uptake (µmol/g of cat.) dispersion (%) adsorption XRDb
aq solution of Ni(NO3)2 aq solution of nickel(II) acetate ethanol solution of nickel(II) acetylacetonate aq solution of [Ni(EDTA)]2aq solution of [Ni(en)3]2+
7.4
1.81
44.5
45.0
19.5
4.81
16.8
16.0
54.6
13.46
6.0
8.5
the diluent gas) with a CH4/O2 molar ratio of 1.0/0.5 and a gas hourly space velocity (GHSV) of 80 000 h-1. In the CO2 reforming of COG, the added amount of CO2 was controlled at a CH4/CO2 ratio of 1/1. The effluent gas was measured with a wet-gas-flowmeter, cooled in an ice–water trap, and then analyzed with an online gas chromatograph (Shimadzu, GC-8A) equipped with a packed column (TDX-01) and a thermal conductivity detector. In the partial oxidation of COG, the conversion and the selectivity were calculated as follows: X(CH4) (%) )
77.5
19.11
4.3
6.0
87.3
21.49
3.8
4.5
a The samples were reduced in H at 750 °C for 1 h, with total H 2 2 uptake at 25 °C. b Reported in ref 9.
S(H2) (%) )
sample
surface area (m2 g-1)
pore volume (cm3 g-1)
SiO2 45.0 nm sized Ni catalyst 16.0 nm sized Ni catalyst 4.5 nm sized Ni catalyst
331 293 289 283
1.20 1.07 1.05 1.06
pore diameter (nm) 15.8 15.6 16.1 16.4
tic acid), nickel(II) acetylacetonate dihydrate (>99%), nickel(II) acetate, and Ni(NO3)2 · 6H2O onto a spherical SiO2 support. The loading of the Ni was controlled at 5 wt % of the support. The preparation procedure was described in previous works.9–12 2.2. Characterizations. The chemical adsorption of H2 was carried out in a quartz reactor, which was equipped with an online mass analyzer (OmniStar GSD301) in order to detect the dispersion of the Ni. Catalysts (100 mg) were first reduced with H2 at 750 °C for 1 h and then cooled to room temperature in N2; then, H2 (30 µL per pulse, carrier gas, N2 40 cm3 min-1) was injected into the reduced catalysts until no adsorption of H2 could be detected in the MS. The percentage of Ni dispersion and the average nickel crystallite diameter were calculated on the basis of the assumption of spherical crystallites of uniform size, according to ref 13. The physical properties of the commercial SiO2 and Ni/SiO2 with different Ni particle sizes were measured by nitrogen adsorption at liquid nitrogen temperature, using an OMNISORP 100 CX system (Coulter Co.). All samples were pretreated at 200 °C for 2 h under high vacuum. X-ray diffraction (XRD) spectra of the Ni/SiO2 with different Ni particle sizes were obtained by D8 Advance (Bruker, Germany) equipment using nickel-filtered Cu KR radiation operated at 40 kV and 40 mA. Diffraction data were recorded using continuous scanning with a rate of 0.02°/s. The deposited carbon was estimated via thermogravimetry (TG) (Perkin-Elmer model PE TAC7/DX) from 50 to 800 °C at a ramp of 10 °C min-1 in 10% O2/N2. Before the analysis, the sample was pretreated in N2 (99.999% pure) at 300 °C for 30 min. 2.3. Catalytic Reaction. The partial oxidation and CO2 reforming of COG were carried out in a fluidized-bed quartz tube (i.d. ) 12 mm) under atmospheric pressure. The reaction temperature was monitored and controlled in the middle of the fluidized catalyst bed. Prior to the reaction, the catalyst was reduced at 750 °C for 1 h in a H2 flow. In the partial oxidation of COG, O2 was added to COG (H2 (58%, v/v), CH4 (26%, v/v), CO (6%, v/v), and N2 as (10) Farago, M. E.; James, J. M.; Trew, V. C. G. J. Chem. Soc. A 1967, 89, 820. (11) Carriat, J. Y.; Che, M.; Kermarec, M.; Verdaguer, M.; Michalowicz, A. J. Am. Chem. Soc. 1998, 120, 2059–2070. (12) Cook, C. M.; Long, F. A. J. Am. Chem. Soc. 1951, 73, 4119–4121. (13) Mustard, D. G.; Bartholomew, C. H. J. Catal. 1981, 67, 186–206.
FCH4,in
× 100
FH2,out
(2)
× 100
(3)
FCO,out × 100 FCO,in + (FCH4,in - FCH4,out)
(4)
FH2,in + 2(FCH4,in - FCH4,out)
S(CO) (%) )
Table 2. Structure of the Support and Catalysts
FCH4,in - FCH4,out
Fi ) CiFtotal
(5)
In the CO2 reforming of COG, the conversions of CH4 and CO2 and the selectivity of H2 and CO were calculated as follows: X(CH4) (%) ) X(CO2) (%) ) S(H2) (%) )
FCH4,in - FCH4,out FCH4,in FCO2,in - FCO2,out FCO2,in
× 100
(6)
× 100
(7)
FH2,out FH2,in + 2(FCH4,in - FCH4,out)
× 100
(8)
S(CO) (%) ) FCO,out × 100 (9) FCO,in + (FCH4,in - FCH4,out) + (FCO2,in - FCO2,out) where X, S, and F are the conversion, the selectivity, and the gas flow rate, respectively; Ftotal is the total feed gas flow rate or effluent gas flow rate; Ci is the molar fraction of component i in the feed gas or reaction effluent gas, which is detected using gas chromatography (GC).
3. Results and Discussion 3.1. Partial Oxidation of Coke Oven Gas (COG). Structure of Ni/SiO2 Catalysts. Table 1 summarizes the properties of the Ni/SiO2 catalysts prepared from different Ni precursors detected by hydrogen adsorption. The percentage dispersion indicates that the Ni/SiO2 prepared via Ni complex precursors ([Ni(EDTA)]2- and [Ni(en)3]2+) was very well dispersed. The calculated sizes of the crystallites of Ni from both the hydrogenation adsorption and the XRD analysis are listed in Table 1. The structures of the support and the differently sized Ni catalysts detected via nitrogen adsorption are summarized in Table 2. The calculated surface areas of the Ni/SiO2 catalysts with different Ni particle sizes according to their adsorption isotherms ranged from 283 to 293 m2 g-1, which was slightly less than that of pure SiO2, while the total pore volume decreased obviously from 1.20 cm3 g-1 of pure SiO2 to 1.05 cm3 g-1 after loading of the Ni. Partial Oxidation of Coke OVen Gas (COG) on Differently Sized Ni Catalysts. The catalytic activity of differently sized Ni catalysts for the partial oxidation of COG at 750 °C and 80 000 h-1 is summarized in Figure 1. It can be found that the conversion of CH4 increases with the decrease in the Ni particle size. On the big-sized Ni catalyst (45.0 nm), the CH4 conversion
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Partial Oxidation of Coke OVen Gas (COG) at Different Reaction Temperatures. The influence of reaction temperature on the catalytic performance of the 4.5 nm sized Ni catalyst is depicted in Figure 2. It was found that the conversion of CH4 and the selectivity of H2 and CO increase from 34.9, 81.7, and 40.7% to 97.0, 100, and 90.2%, respectively, with the increasing reaction temperature. Oxygen is consumed completely at temperatures from 600 to 900 °C. These results indicated that the CH4 combustion reaction might be the major reaction at low temperatures. In the partial oxidation of COG to syngas, the increase in the CH4 conversion and the H2/CO selectivity is due to the fact that COG is a mixture of H2, CH4, and CO, and several reactions happened on the surface on the catalysts in this mixture. The following reactions might occur during the partial oxidation process of COG: Figure 1. Effect of Ni particle size on the catalytic activity of the partial oxidation of COG. Reaction conditions: temperature (T), 750 °C; gas hourly space velocity (GHSV), 80 000 h-1.
Figure 2. Effect of temperature on the partial oxidation of COG over the 4.5 nm sized Ni catalyst. Reaction conditions: GHSV ) 80 000 h-1.
is only 24.7%. However, the conversion of CH4 reaches 80.7% on the small-sized Ni catalysts (4.5 nm). The more important observation is that the selectivity of H2 increases gradually from 84.1 to ∼100%, and the selectivity of CO increases from 75.5 to 82.8% with a decreasing Ni particle size from 45.0 to 4.5 nm. These results infer that a small-sized Ni catalyst is more efficient for the selective partial oxidation of CH4 in the H2rich COG, while a bigger-sized Ni catalyst brings about complete oxidation of CH4 (and/or complete oxidation of the produced H2 and CO); furthermore, the 4.5 nm sized Ni catalyst exhibits a stable CH4 conversion in a H2-rich atmosphere at 80 000 h-1. Ruckenstein and Wang14 and Wei and Iglesia15–17 reported that the activity of supported Rh, Pt, and Ni catalysts for the partial oxidation of CH4 depended on their dispersion and particle size. While in the H2-rich reactant, the higher selectivity of a small-sized Ni catalyst for methane partial oxidation to syngas is seldom reported in previous works. We deduced that small-sized metal is more active for the activation of CH4, which might be attributed to the fact that coordinative unsaturated metal surface atoms prevalent in smaller clusters activate C-H bonds more effectively than atoms on lower-index surfaces. (14) (15) (16) (17)
Ruckenstein, E.; Wang, H. Y. J. Catal. 1999, 187, 151–159. Wei, J. M.; Iglesia, E. J. Catal. 2004, 225, 116–127. Wei, J. M.; Iglesia, E. J. Phys. Chem. B 2004, 108, 4094–4103. Wei, J. M.; Iglesia, E. J. Phys. Chem. B 2004, 108, 7253–7262.
CH4 + 2O2 f CO2 + 2H2O
∆H298o ) -802.6 kJ/mol (10)
H2 + 0.5O2 f 2H2O
∆H298o ) -241.8 kJ/mol (11)
CO + 0.5O2 f CO2
∆H298o ) -283.0 kJ/mol (12)
CH4 + 0.5O2 f CO + 2H2
∆H298o ) -27.3 kJ/mol
(13)
CH4 + H2O f CO + 3H2
∆H298o ) 206.2 kJ/mol
(14)
CH4 + CO2 f 2CO + 2H2
∆H298o ) 247.3 kJ/mol
(15)
CO2 + H2 f CO + H2O
∆H298 ) 116.0 kJ/mol
(16)
CH4 f C + 2H2
∆H298o ) 71.6 kJ/mol
(17)
o
These results fit well with the thermodynamic properties of reactions 10–17. Reactions 10-13 are exothermic processes that are favorable at low temperature, while reactions 14-17 are endothermic processes that are favorable at high temperature. These thermodynamic characteristic resulted in the increase of CH4 conversion and H2 selectivity. Heat released from reactions 10-13 can compensate for the energy needed in reactions 14-17. The H2/CO ratio decreased from 11.0 to 3.8 as the temperature increased from 600 to 900 °C; these results also confirm that higher temperature is favorable for the production of H2 and CO via partial oxidation of CH4 and/or CO2/H2O reforming of CH4. The decrease in the H2/CO ratio is more suitable for the synthesis of methanol, dimethyl ether, and long-chain hydrocarbons. Time on stream of the partial oxidation of COG over the 4.5 nm sized Ni catalyst is presented in Figure 3. The conversion of CH4 reached 80.7%, and no deactivation was detected in 10 h at the higher space velocity (80 000 h-1). Figure 4 shows the XRD spectra of the fresh and spent catalysts with 45.0 and 4.5 nm sized Ni. It was found that Ni remains in a high dispersion state, and no Ni agglomeration is detected either in the 4.5 nm catalyst or in the 45.0 nm catalyst. Characterizations of the spent catalyst indicated that no carbon deposited over the 4.5 nm sized Ni catalyst could be examined. These results could be attributed to their higher stability during the partial oxidation of COG. Partial Oxidation of Coke OVen Gas (COG) at Different O2/CH4 Ratios. Figure 5 shows the influence of the O2/CH4 ratio (GHSV ) 80 000 h-1) on the catalytic performance of the 4.5 nm sized Ni catalyst at 750 °C. CH4 conversion increased quickly from 32.1 to 94.5% with an increasing O2/CH4 ratio from 0.125 to 1.0. However, the selectivity of H2 and CO decreased continuously from 99.5 and 80.1% to 80.3 and
Syngas Production from Coke OVen Gas
Figure 3. Time on stream of the partial oxidation of COG over the 4.5 nm sized Ni catalyst. Reaction conditions: T ) 750 °C; GHSV ) 80 000 h-1.
Figure 4. XRD spectra of different sized Ni catalysts: (a) 4.5 nm fresh; (b) 4.5 nm spent (10 h); (c) 45.0 nm fresh; (d) 45.0 nm spent (10 h).
Figure 5. Effect of the O2/CH4 ratio on the partial oxidation of COG over the 4.5 nm sized Ni catalyst. Reaction conditions: T ) 750 °C; GHSV ) 80 000 h-1.
66.1% when the O2/CH4 ratio was higher than 0.5. In addition, there is a considerable decrease in the H2/CO ratio from 8.4 to 4.4 as the O2/CH4 ratio increases from 0.125 to 1.0. These results infer that superfluous O2 in the feed (when O2/CH4 > 0.5) is consumed mainly in the complete oxidation of CH4 (and/or complete oxidation of the produced H2 and CO).
Energy & Fuels, Vol. 22, No. 3, 2008 1447
Figure 6. Effect of the space velocity on the partial oxidation of COG over the 4.5 nm sized Ni catalyst (abscissa, logarithm coordinate). Reaction conditions: T ) 750 °C.
Figure 7. Effect of temperature on the CO2 reforming of COG over the 4.5 nm sized Ni catalyst. Reaction conditions: GHSV ) 80 000 h-1.
Partial Oxidation of Coke OVen Gas (COG) at Different Space Velocities. The influence of space velocity (O2/CH4 ) 0.50, molar ratio) on the catalytic performance of the 4.5 nm sized Ni catalyst at 750 °C is illustrated in Figure 6. The catalyst exhibits a constant CH4 conversion (>81%) and a constant H2 selectivity (∼100%) in a wide-ranged space velocity (from 8000 to 80 000 h-1). When the space velocity is further enhanced to 240 000 h-1, the CH4 conversion decreases continuously to 40.7%, and the selectivity of H2 and CO decrease slightly from ∼100 and 82.8% to 92.1 and 74.6%, respectively. The H2/CO ratio increases from 4.5 to 6.2. The slight decrease in the selectivity of H2 and CO at the high space velocity (240 000 h-1) may be attributed to the increased contribution of complete methane combustion (eq 10) and/or the decreased contribution of the reforming reactions (eqs 14 and 15). Though there is a decrease in the H2 selectivity at the high space velocity, the H2/CO ratio in the produced syngas increases. These results further confirmed that the complete methane combustion (eq 10) performs more favorably with increasing space velocity. 3.2. CO2 Reforming of Coke Oven Gas (COG) to Syngas. CO2 Reforming of Coke OVen Gas (COG) at Different Reaction Temperatures. Figure 7 shows the effect of the reaction temperature on the catalytic performance of the 4.5 nm sized Ni catalyst for CO2 reforming of COG. It can be found that the conversion of CH4 and CO2 and the selectivity of H2 increase with the increasing reaction temperature. The detected conver-
1448 Energy & Fuels, Vol. 22, No. 3, 2008
Figure 8. Time on stream of the CO2 reforming of COG over the 4.5 nm sized Ni catalyst. Reaction conditions: T ) 750 °C; GHSV ) 80 000 h-1.
Guo et al.
Figure 10. Effect of the CO2/CH4 ratio on the CO2 reforming of COG over the 4.5 nm sized Ni catalyst. Reaction conditions: T ) 750 °C; GHSV ) 80 000 h-1.
methane.18,19 In this contribution, the excellent coke resistance ability of the small-sized Ni catalyst during the CO2 reforming of COG could be attributed to the fact that the excessive H2 might eliminate the carbon deposition during the reaction. CO2 Reforming of Coke OVen Gas (COG) at Different CO2/ CH4 Ratios. Figure 10 presents the influence of the CO2/CH4 ratio on the CO2 reforming of COG performed in a fluidizedbed reactor on the 4.5 nm sized Ni catalyst. The conversion of CH4 increases continuously with the increasing amount of CO2 in the feed, while the conversion of CO2 decreases from 86.0 to 56.9%. The selectivity of H2 decreases slightly. That is, the H2/CO molar ratio in the product gas could be successfully controlled in the range of 1.7 to 2.6 by manipulating the relative concentrations of CO2 and CH4 in the feed, which is very important for the possible following synthesis of methanol, dimethyl ether, and long-chain hydrocarbons. Figure 9. TG result of the 4.5 nm sized Ni catalyst during CO2 reforming of methane for 10 h.
sions of CH4 and CO2 are 5.8 and 46.5%, respectively, at 600 °C, but these data increase to 92.2 and 93.4%, respectively, at 875 °C. The selectivity of H2 increased from 86.6 to 100%, while the selectivity of CO remained at 100% with an increase of temperature from 600 to 875 °C. The lower conversion of CH4 at 600 °C demonstrates that high temperature is required for CO2 reforming due to the thermodynamical limitation.5–8 The molar ratio of H2/CO decreased from 3.3 to 2.2 as the reaction temperature increased from 600 to 875 °C. These results also confirm that the water gas shift reaction (eq 16) might occur mainly at a lower temperature, and a higher reaction temperature favors the CO2 reforming of CH4. Time on stream of the CO2 reforming of COG over the 4.5 nm sized Ni catalyst is presented in Figure 8. The conversion of CH4 and CO2 reached to 66.7 and 78.5%, respectively, and the detected conversions only decreased slightly in 10 h at the higher space velocity (80 000 h-1). The characterization of the spent catalyst revealed that a small amount of carbon (
