Hydrogen Production from Simulated Hot Coke Oven Gas by Using

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Energy & Fuels 2009, 23, 414–421

Hydrogen Production from Simulated Hot Coke Oven Gas by Using Oxygen-Permeable Ceramics Hongwei Cheng, Yuwen Zhang,* Xionggang Lu, Weizhong Ding,* and Qian Li Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai UniVersity, Number 275 Mailbox, 149 Yanchang Road, Shanghai 200072, People’s Republic of China ReceiVed September 11, 2008. ReVised Manuscript ReceiVed NoVember 14, 2008

Hydrogen production from simulated hot coke oven gas (HCOG) was investigated in a BaCo0.7Fe0.2Nb0.1O3-δ (BCFNO) membrane reactor combined with a Ni/Mg(Al)O catalyst by the partial oxidation with toluene as a model tar compound under atmospheric pressure. The reaction results indicated that toluene was completely converted to H2 and CO in the catalytic reforming of the simulated HCOG in the temperature range from 825 to 875 °C. Both thermodynamically predicated values and experimental data showed that the selective oxidation of toluene took precedence over that of CH4 in the reforming reaction. At optimized reaction conditions, the dense oxygen-permeable membrane has an oxygen permeation flux around 12.3 mL cm-2 min-1, and a CH4 conversion of 86%, a CO2 conversion of 99%, a H2 yield of 88%, and a CO yield of 87% have been achieved. When the toluene and methane were reformed, the amount of H2 in the reaction effluent gas was about 2 times more than that of original H2 in simulated HCOG. The results reveal that it is feasible for hydrogen production from HCOG by reforming hydrocarbon compounds in a ceramic oxygen-permeable membrane reactor.

Table 1. Composition of HCOG

1. Introduction High-efficiency, large-scale, and low-cost technologies for producing hydrogen are urgently demanded to meet the needs of supplying sufficient clean energy for power, transportation, and other applications.1-5 Hydrogen production from coke oven gas (COG) is such a technology and attracting increasing attention.6,7 Once hot coke oven gas (HCOG) generated at 800 °C in coke ovens is cooled, its residual gaseous form is COG. It is used as a source material for hydrogen production or as a fuel after separation and recovery of tar. Table 1 shown the typical composition of HCOG.8 The main components of COG are 54-59% hydrogen and 24-28% methane. Currently, hydrogen separation from COG by pressure swing adsorption (PSA) is an industrial process. The PSA is a physical separation technology and can separate the original hydrogen in COG from other components. However, besides the original hydrogen, many energetic components, such as methane, CO, and other hydrocarbon compounds (tar), exist in HCOG. The hydrocarbon compounds account for about 30 wt % of HCOG. Theoretically, by the high-temperature desulfurization, partial oxidation reforming of hydrocarbon compounds, and water-gas-shift reac* To whom correspondence should be addressed. Telephone/Fax: +8621-56338244. E-mail: [email protected] (Y.Z.); [email protected] (W.D.). (1) Weinert, J. X.; Liu, S. J.; Ogden, J. M.; Ma, J. X. Int. J. Hydrogen Energy 2007, 32, 4089–4100. (2) Mueller-Langer, F.; Tzimas, E.; Kaltschmitt, M.; Peteves, S. Int. J. Hydrogen Energy 2007, 32, 3797–3810. (3) Wang, L. S.; Murata, K.; Matsumura, Y.; Inaba, M. Energy Fuels 2006, 20, 1377–1381. (4) Swami, S. M.; Abraham, M. A. Energy Fuels 2006, 20, 2616–2622. (5) Chun, Y. N.; Song, H. W.; Kim, S. C.; Lim, M. S. Energy Fuels 2008, 22, 123–127. (6) Joseck, F.; Wang, M.; Wu, Y. Int. J. Hydrogen Energy 2008, 33, 1445–1454. (7) Murray, M. L.; Seymour, E. H.; Rogut, J.; Zechowska, S. W. Int. J. Hydrogen Energy 2008, 33, 20–27. (8) Kirton, P. J.; Ellis, J.; Crisp, P. T. Fuel 1991, 70, 1383–1389.

COG composition (mol %) H2 CH4 CO CO2 N2 O2 CnHm H2S

54-59 24-28 5.5-7 1-3 3-5 0.3-0.7 2-3 0.01-0.5

tar composition (wt %) benzene naphthalene toluene p-xylene, m-xylene phenanthrene indene phenol others

33.53 17.11 8.85 4.69 3.34 2.79 1.72 27.97

tion, the amount of hydrogen produced is more than that of original hydrogen in the HCOG. The process of hydrogen production from HCOG is much more suitable for the individual coking plants, in which there is no facility for recovery of tar and separation of original hydrogen. The HCOG from those individual coking plants exists in a form of a waste after combustion, especially in China, where the coke tonnage of above 2 billion tons ranks as the first in the world and the corresponding amount of COG has reached 744 billion N m3 in recent years. There have been few reports on how to effectively use HCOG for hydrogen production in the past few years. Li et al.9,10 reported that Ni/Al2O3 catalyst and Indonesian natural limonite ore catalyst had excellent performance for light fuel gas production from nascent coal volatile hydropyrolysis. Miura et al.11 studied tar thermal pyrolysis and steam reforming in simulated HCOG for synthesis gas production at temperatures above 1000 °C. Guo et al.12 investigated the partial oxidation and CO2 reforming of simulated COG to syngas in a fluidized bed reactor and found that small-sized Ni catalyst exhibited (9) Li, L. Y.; Morishita, K.; Takarada, T. J. Chem. Eng. Jpn. 2006, 39, 461–468. (10) Li, L. Y.; Morishita, K.; Takarada, T. Fuel 2007, 86, 1570–1576. (11) Miura, K.; Kawase, M.; Nakagawa, H.; Ashida, R.; Nakai, T.; Ishikawa, T. J. Chem. Eng. Jpn. 2003, 36, 735–741. (12) Guo, J. Z.; Hou, Z. Y.; Gao, J.; Zheng, X. M. Energy Fuels 2008, 22, 1444–1448.

10.1021/ef8007618 CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

Hydrogen Production from HCOG

Energy & Fuels, Vol. 23, 2009 415

Figure 1. Schematic diagram of the membrane system and gas flow direction.

higher activity and selectivity in the reforming reaction. Purwanto et al.13 proposed a simple concept for the hydrogen production process from COG, and Shen et al.14 developed a new route for hydrogen production from CH4-rich gas, which was separated from HCOG by a prism membrane. Recently, our group have achieved hydrogen amplification of simulated COG by reforming of methane in a ceramic oxygen-permeable membrane reactor.15 This technology could significantly reduce the energy and the cost for hydrogen production by eliminating the need for an individual oxygen production plant. In this study, the Ni/Mg(Al)O catalyst prepared by the homogeneous precipitation method using urea hydrolysis was first used in the catalytic reforming of simulated HCOG in a ceramic oxygen-permeable membrane BaCo0.7Fe0.2Nb0.1O3-δ (BCFNO) reactor with toluene as a model tar compound under atmospheric pressure. The influences of reaction temperature, toluene content in feed gas, and air flow rate on reaction effluent gas composition were examined. The performance of the membrane reactor and the Ni/Mg(Al)O catalyst under the condition of the simulated HCOG was also investigated. 2. Experimental Section 2.1. Materials Preparation. The BCFNO powder was obtained by the same method as that described in our previous work.15 The experimental procedure used to prepare Ni/Mg(Al)O catalyst by the homogeneous precipitation method using urea hydrolysis was illustrated as follows: 0.048 mol of Ni(NO3)2 · 6H2O, 0.232 mol of Mg(NO3)2 · 6H2O, 0.12 mol of Al(NO3)3 · 9H2O, and 2.76 mol of urea (the molar ratios of urea/NO3- ) 3:1) were dissolved in deionized water, and the solution was stirred at 95-100 °C for 10 h, followed by aging at the same temperature for 12 h without stirring. Then, the precipitate was filtered, rinsed, and dried at 110 °C overnight. The dried solid was calcined at 850 °C for 5 h to obtain the Ni/Mg(Al)O catalyst. 2.2. Samples Characterization. The changes of the morphology of the membranes were observed using a scanning electron microscope (SEM, JEOL JSM-6700F). Their compositions before and after the reaction were determined using an energy-dispersive X-ray spectroscope (EDXS, OXFORD INCA). (13) Purwanto, H.; Akiyama, T. Curr. AdV. Mater. Processes 2005, 18, 272. (14) Shen, J.; Wang, Z. Z.; Yang, H. W.; Yao, R. S. Energy Fuels 2007, 21, 3588–3592. (15) Zhang, Y. W.; Li, Q.; Shen, P. J.; Liu, Y.; Yang, Z. B.; Ding, W. Z.; Lu, X. G. Int. J. Hydrogen Energy 2008, 33, 3311–3319.

X-ray diffraction (XRD, Rigaku D/Max-2550) was used to characterize the phase evolution of the membranes and catalysts. A transmission electron microscope (TEM, JEOL JEM-200CX) operating at 100 kV was employed to investigate the distribution of metal particle size and carbon deposition on the surface of the catalyst. A thermogravimetric analyzer (TGA, SDT Q600) was used to detect the coke deposited on catalysts after the catalytic reaction. About 10 mg of the used catalyst was loaded into a platinum pan and kept under an argon atmosphere at 50 °C for 2 h. Then, the thermogravimetric temperature-programmed oxidiation (TG-TPO) experiment was carried out with a 10 mol % O2/Ar flow rate of 30 mL/min and a ramp of 10 °C/min in a range from 50 to 1100 °C. After TG-TPO, the sample was cooled to 50 °C in an argon flow rate of 30 mL/min and kept for 2 h, and immediately, the thermogravimetric temperature-programmed reduction (TG-TPR) experiment was carried out under a 10 mol % H2/Ar atmosphere. 2.3. Experimental Setup. Figure 1 shows the schematic diagram of the oxygen-permeable membrane reactor and the gas flow chart. One side of the membrane was exposed to compressed air, and the other side was exposed to the simulated HCOG, which consisted of simulated COG (57.9% H2, 31.5% CH4, 7.4% CO, and 3.2% CO2) and toluene. Toluene was selected as a model tar compound just as it was extensively used in other literature,16,17 owing to the fact that toluene contains both aromatic moiety (phenyl) and aliphatic moiety (methyl) and actually exists in tar. The gas flow rates were controlled by mass flow controllers. Toluene was charged through a liquid pump. Before the reaction, the reaction mixture was preheated in a stainless-steel vessel filled with scrap iron at 300 °C. To avoid the condensation of the reactant, the pipeline was held at 150 °C. Hydrocarbon compounds were determined by a Varian CP 3800 gas chromatograph (GC) with a flame ionization detector (FID). The analysis of H2, CH4, CO, and CO2 was performed by another Varian CP 3800 GC with a thermal conductivity detector (TCD). Prior to the start of a test, the discoid membrane was sealed to the reactor with a silver seal. The effective inner surface area of the discoid membrane was around 1.3 cm2. A total of 0.5 g of 20-40 mesh Ni/Mg(Al)O catalyst was directly placed on the membrane. A K-type thermocouple was placed near the air side of the membrane to monitor the reaction temperature. The oxygen permeability of the discoid sample was determined from the content of CO and CO2 in the reacted gas, and the amount of H2O was evaluated from the balance of hydrogen before and after the reaction. The flow rate of outlet gas was measured by a soap(16) Srinakruang, J.; Sato, K.; Vitidsant, T.; Fujimoto, K. Fuel 2006, 85, 2419–2426. (17) Zhang, R. Q.; Wang, Y. C.; Brown, R. C. Energy ConVers. Manage. 2007, 48, 68–77.

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membrane flowmeter. The toluene, CH4, and CO2 conversions and the H2 and CO yields were calculated as follows:

toluene conversion, % )

CH4 conversion, % )

CO2 conversion, % )

H2 yield, % )

CO yield, % )

out Fin toluene - Ftoluene

Fin toluene in out - FCH FCH 4 4 in FCH 4 in out - FCO FCO 2 2 in FCO 2

FHout2 - FHin2 in 2FCH + 4Fin toluene 4

× 100

× 100

× 100

× 100

out in FCO - FCO in in FCH + FCO + 7Fin toluene 4 2

× 100

out where Fin i and Fi were the flow rates of the inlet and outlet gas i, respectively.

3. Results and Discussion 3.1. Performance of Membrane Reactor under the Condition of the Simulated HCOG. The catalytic partial oxidation reaction of the simulated HCOG was studied in a diskshape BCFNO membrane reactor packed with Ni/Mg(Al)O catalyst. To study the difference of the reforming of simulated COG with and without toluene, when the reforming reaction of simulated COG was stable for 3 h, toluene was injected into the feed gas. Then, the effect of the reaction temperature on the reformation of the simulated HCOG was investigated. All of the results were given in Figure 2. When the simulated COG without toluene was fed to the membrane reactor at 875 °C within the first 3 h, the average conversions of CH4 and CO2 were 92.4 and 71.2%, while the average yields of H2 and CO were 91.1 and 90.8%, respectively. In the following 3 h, after toluene addition into the simulated COG at the same temperature, the average oxygen permeation flux increased from 10.5 to 12.3 mL cm-2 min-1 (Figure 2a). Simultaneously, the conversion of CH4 reduced to 86.4%, and the yields of H2 and CO decreased to 88.2 and 86.8%, respectively, while the conversion of CO2 increased to 99.0%, as shown in Figure 2b. When the toluene was injected into the reactor, the selective oxidation reaction between eqs 1 and 2 happened 0.3C7H8(TLU, g) + 1.65O2(g) ) 2.1CO(g) + 1.2H2O(g) 0 35 K875 (1) °C ) 2.4 × 10

1.1CH4(g) + 1.65O2(g) ) 1.1CO(g) + 2.2H2O(g) 0 30 K875 (2) °C ) 8.2 × 10

The equilibrium constant of eq 1 is 2.9 × 104 times larger than that of eq 2.18 This indicates that the oxidation of toluene takes precedence over CH4 thermodynamically under the experimental conditions. Although the oxygen permeation flux was increased, the increment of oxygen supplied by the membrane was not enough for the partial oxidation of toluene. As a result, the conversion of CH4 decreased. At the same time, the oxidant was not sufficient in the reaction system and CO2 was consumed (18) ESM software (2005) HSC chemistry 5.1. Accessed Aug 2, 2005.

Figure 2. (a) Effect of the temperature on the oxygen permeation flux and (b) conversion and yield of the reforming reaction. Reaction conditions: simulated COG flow rate, 98.6 mL/min (STP); toluene flow rate, 1.4 mL/min (STP); membrane thickness, 1.0 mm; air flow rate, 300 mL/min (STP).

dramatically through the dry reforming reaction (CH4 + CO2 ) 2CO + 2H2 and/or C7H8 + 7CO2 ) 14CO + 4H2). The effect of the reaction temperature on the reforming of the simulated HCOG can be seen from II to IV regions in Figure 2. Obviously, the performance of the membrane reactor was strongly affected by the operating temperature. When the reaction temperature decreased from 875 to 825 °C, the conversions of CH4 and CO2 dropped from 86.4 to 70.3% and from 99.0 to 91.5%, while the yields of H2 and CO reduced from 88.2 to 77.7% and from 86.8 to 71.8%, respectively. For a dense ceramic membrane, the composition of simulated HCOG and air flow rate were kept unchanged. The oxygen permeation flux can be changed as a function of temperature, and this change will effect the reactant conversions and syngas yields. Furthermore, the variation in the product composition could influence the oxygen partial pressure at the reforming side, which will alter the oxygen permeation flux in turn. This is a self-consistent process between the oxygen permeation and the catalytic partial oxidation reaction. In our experiments, when the process of temperature decreased from 875 to 825 °C, the changes of the reforming reactions resulting from the oxygen permeation flux decreased from 12.3 to 9.9 mL cm-2 min-1. The toluene was completely converted in the whole process. A comparison of the experimental values and thermodynamically predicted ones was summarized in Table 2. The experimental data, such as the oxygen permeation fluxes (JO2), the conversions of toluene and CH4, and the yields of H2 and CO



Table 2. Comparison of the Experimental and Thermodynamically Predicted Values temperature (°C) 875 850 825 a

reforming gases simulated simulated simulated simulated

COG COG + toluene COG + toluene COG + toluene

JO2 [mL (STP) cm-2 min-1] 10.5 12.3 11.4 9.9

toluene conversiona (%) a 100 100 100

CH4 conversiona (%)

H2 yielda (%)

CO yielda (%)

b

a

b

a

b

a

b

100 100 100

93.4 93.2 91.6 89.6

92.4 86.4 79.4 70.3

91.6 91.3 89.9 86.9

91.1 88.2 85.6 77.8

92.2 81.4 74.5 64.1

90.7 86.8 80.7 71.8

a, thermodynamic calculation; b, experimental results.

Figure 3. Variation of components in simulated COG with and without toluene before and after reforming at different temperatures. Reaction conditions: simulated COG flow rate, 98.6 mL/min (STP); toluene flow rate, 1.4 mL/min (STP); membrane thickness, 1.0 mm; air flow rate, 300 mL/min (STP).

were average ones over the corresponding different reaction temperatures. The predicted thermodynamic values were obtained by using the Gibbs energy minimization method.15 As is evident from Table 2, it can be seen that the results of the experiment were very close to the thermodynamically predicated ones. Both experimental data and thermodynamically predicated values of CH4 conversion decreased when toluene was injected into the reactor. The variations in the components of simulated COG with and without toluene before and after reforming at different temperatures were shown in Figure 3. It can be seen that the amounts of H2 and CO after reforming decreased gradually but the amount of CH4 behaved in a contrary trend with decreasing temperature. It is also clearly observed that the amount of H2 in the outlet gas was about 2 times higher than that of original H2 in simulated COG and the average amounts of H2 and CO were 109.8-116.9 and 38.9-45.5 mL/min at different reaction temperatures, respectively. Actually, the amount of hydrocarbon compounds in HCOG is much more than that of toluene used in the present experiment. Through the process of reforming and water-gas-shift reaction, the hydrocarbon compounds in HCOG and CO in the reaction effluent gas can be converted to hydrogen and the total amount of hydrogen in the final products could be obviously increased. 3.2. Effect of the Toluene Content in Feed Gas on the Membrane Reaction. Figure 4 shows the influence of the toluene content in feed gas on the catalytic reforming reaction of the simulated HCOG at 875 °C. When the toluene content in feed gas increased from 1.4 to 4.7 mol % (4.7 mol % toluene in the feed gas is equivalent to the simulated tar in HCOG), the oxygen permeation flux increased gradually from 12.2 to 13.9 mL cm-2 min-1 (Figure 4a). Simultaneously, the conversions of toluene and CH4 reduced from 100 to 73.9% and from 85.5 to 37.5%, and the yields of H2 and CO decreased from 88.0 to 46.4% and from 86.5 to 54.5%, respectively, while the conversion of CO2 was kept above 99.0%, as shown in Figure 4b.

Figure 4. (a) Effect of the toluene content in feed gas on the oxygen permeation flux and (b) conversion and yield of the reforming reaction. Reaction conditions: simulated COG flow rate, 98.6 mL/min (STP); membrane thickness, 1.0 mm; air flow rate, 300 mL/min (STP); temperature, 875 °C.

According to the Wagner equation,19 the oxygen permeation flux through a membrane is affected by the thickness of the membrane, operation temperature, and the oxygen partial gradient across the membrane. It was also reported that the oxygen permeation flux strongly depends upon the oxidation reaction rate and reactant flow rate.20 Under the experimental conditions, with the thickness of the membrane and the operation temperature kept invariable, it can be concluded that the increase of the oxygen permeation flux mainly resulted from the increase of the oxygen partial gradient across the membrane. Therefore, the oxygen permeation flux was augmented with the addition of toluene content because of the significantly faster oxidation reaction with toluene than with methane, and the flow rate of the reactant increased with the increase of the toluene content in feed gas. However, the increment of oxygen supplied by the membrane was not enough for the partial oxidation of the (19) Xu, S. J.; Thomson, W. J. Chem. Eng. Sci. 1999, 54, 3839–3850. (20) Akin, F. T.; Lin, J. Y. S. J. Membr. Sci. 2004, 231, 133–146.


Figure 5. (a) Effect of the air flow rate on the oxygen permeation flux and (b) conversion and yield of the reforming reaction. Reaction conditions: simulated COG flow rate, 98.6 mL/min (STP); toluene flow rate, 1.4 mL/min (STP); membrane thickness, 1.0 mm; temperature, 850 °C.

toluene and CH4. Therefore, most of CO2 was consumed, owing to the dry reforming and the conversions of toluene and CH4, and the yields of H2 and CO decreased. In fact, the lower ratio of the reaction area to the total reactor volume is one of the disadvantages in using a disk-shaped membrane in experiments.15,21,22 If the oxygen permeation flux increased remarkably by improving the performance of the membrane and the catalysts or the improved configuration designs of the membrane reactors were used, the remainder of toluene and CH4 will be completely converted. A tubular membrane reactor for reforming hydrocarbon compounds in HCOG is being developed in our research group. 3.3. Effect of the Air Flow Rate on the Membrane Reaction. Figure 5 shows the influence of the air flow rate on the catalytic reforming reaction of the simulated HCOG at 850 °C. Obviously, at the beginning, the membrane reaction changed and was subsequently kept stable with the increase of the air flow rate. When the air flow rate increased from 50 to 300 mL/ min, the oxygen permeation flux increased from 5.9 to 11.7 mL cm-2 min-1. However, when the air flow rate was higher than 300 mL/min, its influence on the membrane reaction became negligible and the oxygen permeation flux remained about 11.7 mL cm-2 min-1 (Figure 5a). This indicated that, at the reaction temperature of 850 °C, the exchange of oxygen on (21) Harada, M.; Domen, K.; Hara, M.; Tatsumi, T. Chem. Lett. 2006, 35, 968–969. (22) Dong, H.; Shao, Z. P.; Xiong, G. X.; Tong, J. H.; Sheng, S. S.; Yang, W. S. Catal. Today 2001, 67, 3–13.

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the air side of the membrane was the rate-controlling step of oxygen permeation when the air flow rate was below 300 mL/ min. Hence, the oxygen permeation flux increases with the increase of the air flow rate. While the air flow rate was above 300 mL/min, its influence on the membrane reaction was negligible. Therefore, the air flow rate was kept constant at 300 mL/min for the other experiments in this study. As shown in Figure 5b, toluene was completely converted in the air flow rate range from 50 to 400 mL/min. The behaviors of the conversion of CH4 and the yields of H2 and CO were consistent with that of the oxygen permeation flux. They increased drastically with the increase of the air flow rate from 50 to 300 mL/min and changed slightly when the air flow rate was above 300 mL/min. The yields of H2 and CO increased mainly owing to the increasing conversion of CH4. When the air flow rate was lower than 150 mL/min, CO2 was completely consumed. However, the conversion of CO2 was down to around 95% when the air flow rate became higher. 3.4. Characterization of the Used Membrane and Catalyst. For practical applications, the technical innovation on the catalytic partial oxidation of hydrocarbon compounds by using an oxygen-permeable ceramic membrane reactor is strongly dependent upon the development of the mixedconducting membrane material having the potential for high oxygen permeability and stability under reducing atmosphere and a new type of catalyst having high activity and suppression of carbon deposition. The fresh and used membrane in the different reaction temperatures were characterized by SEM, EDXS, and XRD. Figure 6 shows the SEM images of the fresh and used BCFNO membranes. For the fresh membrane, ceramic grains with clear boundaries were visible as shown in parts a and b of Figure 6. It can be seen that the morphology of the used membrane surface exposed to the reforming gas and air permeation side of the membrane were all compact, although they were different. SEM images of the cross-section view (parts d and f of Figure 6) also displayed the structural changes on both surfaces of the BCFNO membrane, which were only up to several micrometers in depth. Table 3 showed the EDXS results of the fresh and used BCFNO membrane. The compositions of both the top and cross-section of the fresh BCFNO membrane were identical, approximately Ba/Co/Fe/Nb ) 51: 33.9:10:5. Segregation of the constituent metals was not observed on the top of a fresh membrane. After the experiment, the content of Ba increased and Co decreased on the air side of the used membrane, whereas the content of Ba and Co enriched on the permeation side. The increase in the Ba content on the air side was similar to the results of our previous work15 and literature.23 The XRD patterns of the fresh and used membranes were shown in Figure 7. In comparison to the fresh membrane, some new phases, such as BaCO3 and Co2O3, formed on the used membrane surfaces. However, a typical perovskite structure was still kept in the middle section of the membrane (Figure 7c). In the region of the Ba enrichment on both membrane surfaces, BaCO3 was the main phase detected by XRD (parts b and d of Figure 7). Figure 7b confirmed that Co2O3 was the main phase in the region of Co enrichment on the membrane surface exposed to the reforming gas. The elemental composition of positions 4 and 7 in Table 3 were similar to that in the middle section of the membrane. This revealed that the decomposition of the BCFNO material occurs on the membrane surface only several micrometers in depth. The cross-section morphology of (23) Ikeguchi, M.; Mimura, T.; Sekine, Y.; Kikuchi, E.; Matsukata, M. Appl. Catal., A 2005, 290, 212–220.



Figure 6. SEM images of the fresh and used BCFNO membranes. a and b are surface and cross-section of the fresh membrane, respectively; c and e are the air and permeation sides of the used membrane, respectively; and d and f are cross-sections near the air and permeation sides of the used membrane, respectively. The dark broken line frames in the images show the areas of the EDXS analysis. Table 3. EDX Results of the Fresh and Used BCFNO Membrane membrane

position (number)

Ba (mol %)

Co (mol %)

Fe (mol %)

Nb (mol %)

fresh

top (1) cross-section (2) air surface (3) air cross-section (4) permeation surface (5) permeation surface (6) permeation cross-section (7)

50.33 51.71 73.34 52.10 98.09 29.41 53.29

35.06 32.79 21.48 32.86 1.91 49.53 28.35

9.75 10.33 0 10.08 0 10.82 12.00

4.86 5.17 5.18 4.96 0 10.24 6.36

used

the used membrane was different compared to the results of our previous study. In our previous study, the structure changes run deeper into the bulk of the BCFNO membrane up to over 100 µm after the reaction when the Ni/La2O3-γAl2O3 catalyst was used and the maximum of the oxygen permeation flux was only 7.5 mL cm-2 min-1.15 This difference maybe resulted from the higher oxygen permeation flux of 12.3 mL cm-2 min-1 when the new type of Ni/Mg(Al)O catalyst was used in combination with the membrane. The destruction process progresses into the membrane up to a certain depth, corresponding to the lowest value for the oxygen activity, where the oxide is thermodynamically stable.24,25 The increase of the oxygen permeation flux might restrain the destruction to some extent.

XRD patterns of the precursor, fresh, reduced, and used Ni/ Mg(Al)O catalysts in the different reaction temperatures were shown in Figure 8. The characteristic reflections of the interbasal planes (003), (006), (012), (015), (018), (110), and (113) of crystalline layered double hydroxides (LDHs) were clearly seen (Figure 8a) as deposited by the homogeneous precipitation method using urea hydrolysis. After calcined at 850 °C for 5 h, a spinel (Ni,Mg)Al2O4 with three characteristic reflection peaks at 2θ ) 18.9°, 30.7°, and 59.8° and NiO-MgO solid solution, showing reflections at 2θ ) 37.2°, 43.2°, and 62.7° were (24) Lein, H. L.; Wiik, K.; Grande, T. Solid State Ionics 2006, 177, 1587–1590. (25) Bouwmeester, H. J. M. Catal. Today 2003, 82, 141–150.


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Figure 7. XRD patterns of the fresh and used BCFNO membranes: (a) fresh membrane, (b) permeation side of the used membrane, (c) bulk of the used membrane, and (d) air side of the used membrane.

Figure 9. TEM images of the Ni/Mg(Al)O catalyst: (a) reduced and (b) used.

Figure 8. XRD patterns of the Ni/Mg(Al)O catalyst: (a) as synthesized, (b) calcined, (c) reduced, and (d) used.

identified (Figure 8b). After the reaction, the peaks corresponding to Ni crystallites at 44.6°, 51.8°, and 76.4° were a bit sharper compared to those before the reaction, indicating that the sintering of nickel clusters occurred during the reaction procedure. These are in accordance with the TEM analysis, as shown in Figure 9. From Figure 8d, the crystalline phases of C were observed at 25.8°, showing that carbon was formed in the reaction. TEM images of the reduced and used Ni/Mg(Al)O catalyst in the different reaction temperatures were shown in Figure 9. On a reduced catalyst, there were many Ni particles with a high decentralization and uniform size distribution. However, it can be seen that the TEM image changed after the reaction. Most of the small Ni particles were retained without carbon deposition. However, some whisker carbon was observed on a few of the large Ni particles. The whisker diameter was mostly centered at about 20 nm, and each whisker had a Ni particle standing on the tip.26 A well-known fact is that whisker carbon cannot cover metallic sites despite the deposition of large amounts of carbon, thus resulting in much lower deactivation rates than those expected if metal-covering carbon deposits occurred.27 The carbon deposition on the catalyst used in the different reaction temperatures was investigated by TG-TPO and TG(26) Tomishige, K.; Chen, Y. G.; Fujimoto, K. J. Catal. 1999, 181, 91– 103. (27) Pompeo, F.; Nichio, N. N.; Ferretti, O. A.; Resasco, D. Int. J. Hydrogen Energy 2005, 30, 1399–1405.

Figure 10. TG result of the used Ni/Mg(Al)O catalyst at different reaction temperatures.

TPR methods (presented in Figure 10). The result of TG-TPO showed that the used catalyst exhibited a slight weight augment from 150 to 750 °C and sharp weight loss from 750 to 900 °C. The diversification of the weight was attributed to the Ni metal oxidation and removal of deposited carbon. Subsequent TGTPR results show a sharp weight loss between 800 and 1000 °C corresponding to the reduction of nickel oxide. Therefore, the total weight loss in both TG-TPO and TG-TPR experiments was the amount of carbon deposited on the catalysts. It is revealed that a small amount of carbon deposits on the used catalyst (6.0 wt %), which could be contributed to its slight deactivation, and the excessive H2 eliminate the carbon deposition during the reaction.12 4. Conclusion Hydrogen amplification of simulated COG by partial oxidation of hydrocarbon compounds in a simulated HCOG with toluene as a model tar compound was achieved using a BCFNO


membrane reactor combined with a Ni/Mg(Al)O catalyst. The selective oxidation between toluene and CH4 had happened, and the oxidation of toluene takes precedence over CH4. Toluene was completely converted, and the amount of H2 in the outlet gas was about 2 times more than that of original H2 in simulated COG when the toluene content in feed gas was 1.4 mol %. The increases of the toluene content in feed gas and air flow rate were beneficial to improve the oxygen permeation flux. The Ni/Mg(Al)O catalyst used in the membrane reactor possessed good catalytic activity and resistance to coking in the simulated HCOG atmosphere. On the basis of the dense ceramic membrane technology, the method of catalytic partial oxidation of


hydrocarbon compounds in HCOG provides an alternative for hydrogen production. Acknowledgment. The financial support received from the National High Technology Research and Development Program of China (Grant 2006AA11A189), the Science and Technology Commission of Shanghai Municipality (Grant 07DZ12036), the National Engineering Research Center for Advanced Steel Technology (NERCAST) (Grant 050209), and the Graduate Innovation Fund of Shanghai University (Grant A.16-0110-07-005) is gratefully appreciated. EF8007618

Hydrogen Production from Simulated Hot Coke Oven Gas by Using

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