(53h) Production of Hydrogen By Methane Decomposition Using Fe-Supported Alumina Catalysts

Methane is the simplest hydrocarbon, and mainly, can be obtained from natural gas, petroleum and methane hydrate. However, methane is contained in biogas which can be obtained by fermentation of biomass such as excrement and sewage sludge. Much attention has been paid to useful utilization of biomass, since biomass is considered to be renewable and carbon neutral. Global warming potential of methane is 25 times higher than carbon dioxide. Emission of methane must be avoided to prevent global warming. Combustion is one of utilization methods of methane, but even by combustion, carbon dioxide, which also causes greenhouse effect, is emitted. Therefore, other method of methane utilization without emitting carbon dioxide is expected.

Hydrogen is expected to be clean energy source because combustion of hydrogen leads to formation of only water, and no carbon dioxide. As a method for production of hydrogen, steam-reforming of hydrocarbon has been known. However, this method leads to formation of carbon dioxide, and development of the method for production of hydrogen without formation of carbon dioxide is expected. As other method, decomposition of methane over catalysts has been studied quite extensively.

It is well known that Ni catalysts are effective for this reaction. However, Ni is toxic and somewhat expensive metal. On the other hand, Fe is nontoxic and cheap metal. Therefore, there will be many merits to use Fe as catalyst. For example, deposited carbon containing Fe after reaction can be disposed safely and easily.

We tried production of hydrogen and carbon by methane decomposition using Fe-supported alumina catalyst, and investigated the effects of flow rate of methane, reaction temperature and reduction treatment by methane and hydrogen on catalytic performance during methane decomposition reaction.

EXPERIMENTAL

Fe-supported alumina catalyst was obtained from The Japan Steel Works, LTD.. As characterization of catalyst before reaction, powder X-ray diffraction (XRD) analysis and X-ray Fluorescence (XRF) analysis were carried out. XRD analysis (Rigaku Corporation, MiniFlex600) was carried out to investigate the crystal structure of catalysts. The result of XRD analysis suggested that hematite (Fe₂O₃) and θ-alumina (Al₂O₃) coexist in Fe-supported alumina catalysts. XRF analysis (Rigaku Corporation, EDXL300) was also performed to determine the ratio of each element. From the result, content of Fe₂O₃ was calculated to ca. 55 wt. %.

Catalytic activity was measured with a fixed-bed reactor. Quartz reactor with inner diameter of 36 mm and length of 460 mm was used in lying condition. Catalyst weight was 0.2 g. Catalyst was mounted in quartz boat (25 mm width × 50 mm length × 12 mm depth) and the boat was set beside thermocouple in the quartz reactor. Reactor was heated to reaction temperature with heating rate of 10 ºC/min in image furnace. Before reaction, catalysts were treated by two kinds of gas conditions: 1) treatment in methane flow from start of heating, where concentration of methane was 100%, 2) treatment in methane and hydrogen flow below 650 ºC and only hydrogen supply was stopped at 650 ºC. Flow rate of methane was 20, 40 or 60 ml/min, and flow rate of hydrogen was 5.32 ml/min. Both of methane and hydrogen have activity for reduction of iron oxide. The reaction temperature was varied from 670 to 740 ºC, and constant during reaction. Between quartz reactor and gas chromatography (GC), glass tube willed with silica gel particles was connected in order to adsorb water formed during reduction of iron oxides.

Effluent gas was detected by GC (Shimadzu Corporation, Gas Chromatograph GC-8A) equipped with thermal conductivity detector (TCD) and packed column (GL Sciences Inc., Molecular Sieve 5A). Argon was used as carrier gas. Temperature of GC column was set to 50 ºC.

Under these conditions, change of rate of methane conversion with time-on-stream was observed. Mechanism of hydrogen production by decomposition of methane is considered to progress by following equation: CH₄ → C + 2H₂. No gaseous resultant other than hydrogen was detected. Methane conversion (%) was defined as follows: Methane conversion = (hydrogen concentration) / (200 − (hydrogen concentration)) × 100. Hydrogen concentration (%) was calculated from GC peak area. Basically, reaction time was fixed to 6 h.

Weight of catalyst before and after reaction was measured, and from these results, amount of deposited carbon was calculated. In this case, reaction was carried out until catalyst was almost deactivated.

RESULTS AND DISCUSSION

Primarily, time dependence on methane conversion was investigated in condition that flow rate of methane was 20 ml/min and only methane was supplied from start of heating. At 700 ºC, methane was scarcely converted even after 6 h from start of reaction. At 710 ºC, methane conversion was increased to 60.8 % after induction period of ca. 50 min. Above 720 ºC, induction period became shorter with increasing reaction temperature. Maximum values of methane conversion at 710, 720, 730 and 740 ºC were 60.8, 66.2, 68.7 and 69.0 %, respectively. Difference in methane conversion by reaction temperature was not so significant, suggesting that this reaction may reach equilibrium state. Equilibrium value is varied by temperature: the higher temperature leads to higher equilibrium value. It seems that reaction temperature of 700 ºC may not be sufficient temperature for reduction of Fe₂O₃ by methane with flow rate of 20 ml/min, since it is suggested that reduced FeO species have catalytic activity for methane decomposition. On the other hand, in all cases, methane conversion was gradually decreased after methane conversion reached maximum. The decrease from the maximum value to the value at 6 h after start of reaction at 710, 720, 730 and 740 ºC were 6.3, 11.5, 17.7 and 17.8 %, respectively. These results suggested that higher reaction temperature leads to larger decrease in methane conversion with time-on-stream.

In the condition of methane flow rate of 40 ml/min, maximum of methane conversion rate at each temperature was similar with maximum at corresponding temperature, respectively (i.e., quantity of methane conversion was twice as much as at flow rate of 20 ml/min). In this condition, even at 690 ºC, methane conversion was increased to 54.1%. Generally, induction period at methane flow rate of 40 ml/min was shorter than at 20 ml/min. After reaching the maximum of methane conversion, methane conversion was decreased with time-on-stream, and the decrease was larger for methane flow rate of 40 ml/min than for 20 ml/min, and was larger for higher reaction temperature than for lower temperature. More amount of methane flow and higher reaction temperature led to the larger decrease in methane conversion with time-on-stream.

Moreover, the effect of reduction treatment by mixture of methane and hydrogen below 650 ºC during heating was investigated. Here, flow rate of methane was 20 ml/min and flow rate of hydrogen was 5.32 ml/min. At 670 ºC, conversion of methane was extremely low, and at 680 ºC, methane conversion reached ca. 50%. Increasing reaction temperature led to higher methane conversion. In these cases, induction period was not observed, and decrease in methane conversion with time-on-stream was not significant. It is suggested that hydrogen has higher activity for reduction of iron oxide than methane, while both of methane and hydrogen have activity for reduction. Reduction using not only methane but also hydrogen led to appearance of catalytic activity even at lower temperature.

Change in weight of catalyst during reaction was investigated. Carbon deposition on catalyst during reaction led to increase in catalyst weight. Here, catalytic reaction was carried out until the catalysts were deactivated: flow rate of methane was 20, 40 or 60 ml/min and reaction temperature was 740 ºC. Only methane was supplied from start of heating. In the case of flow rate of 20 ml/min, Fe catalyst was deactivated after 22 h from start of reaction. Catalysts were deactivated after 14 and 6 h for methane flow rate of 40 and 60 ml/min, respectively. The ratios of catalysts weight of (after reaction)/(before reaction) were 16.70, 13.02 and 9.90 for 20, 40 and 60 ml/min, respectively. These results suggested that lower flow rate of methane enhanced carbon deposition in more degree. In the case of flow rate of 60 ml/min, maximum of methane conversion rate was lower than the cases of 20 and 40 ml/min, suggesting that, in this case, methane decomposition reaction did not reach equilibrium state.

Biogas obtained by fermentation of biomass contains carbon dioxide as well as methane. Therefore, effect of existence of carbon dioxide mixed with methane will be investigated.

CONCLUSIONS

In reaction of methane decomposition using Fe-supported alumina catalyst, higher reaction temperature led to slightly higher maximum in methane conversion, but led to larger decrease in methane conversion. Moreover, higher flow rate of methane did not affect the maximum in methane conversion rate so significantly, suggesting that this reaction reached equilibrium state. Higher flow rate led to larger decrease in methane conversion, shorter catalytic lifetime and less amount of deposited carbon. Presence of hydrogen to reduce iron oxide species led to appearance of catalytic activity even at low temperature without induction period.