The Impact of Operating Conditions on the Performance of a CH₄ Dry Reforming Membrane Reactor for H₂ Production

Akira Nishimura ^{1, *}, Satoshi Ohata ², Kaito Okukura ³, Eric Hu ⁴

1. Division of Mechanical Engineering, Graduate School of Engineering, Mie University , 1577 Kurimamachiya-cho, Tsu, Mie, 514-8507 Japan
2. Division of Mechanical Engineering, Graduate School of Engineering, Mie University , 1577 Kurimamachiya-cho, Tsu, Mie, 514-8507 Japan
3. Division of Mechanical Engineering, Graduate School of Engineering, Mie University , 1577 Kurimamachiya-cho, Tsu, Mie, 514-8507 Japan
4. School of Engineering, the University of Adelaide, Adelaide, SA 5005, Australia; E- M ail: eric.hu@adelaide.edu.au

* Correspondence: Akira Nishimura

Academic Editor: Alfonso Chinnici

Special Issue: Hydrogen Energy: Sustainable Production, Storage and Utilisation

Received: March 25, 2020 | Accepted: May 09, 2020 | Published: May 14, 2020

Journal of Energy and Power Technology 2020, Volume 2, Issue 2, doi:10.21926/jept.2002008

Recommended citation: Nishimura A, Ohata S, Okukura K, Hu E. The Impact of Operating Conditions on the Performance of a CH4 Dry Reforming Membrane Reactor for H2 Production. Journal of Energy and Power Technology 2020; 2(2): 008; doi:10.21926/jept.2002008.

© 2020 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

The fuel cell is a promising power generation technology as environmentally toxic gases such as NO_x, SO_x, and CO₂ are not emitted. H₂ is fuel for the fuel cell, produced from different resources. However, CO₂ would be emitted upon the production of H₂ from fossil fuels such as coal, oil, and natural gas. Therefore, H₂ production technology without CO₂ emission is important in a sustainable world.

In this study, H₂ is produced from biogas. Biogas is a gaseous fuel consisting of CH₄ (55–75 vol%) and CO₂ (25–45 vol%) [1], mainly produced upon fermentation of CH₄ by the action of anaerobic microorganisms on raw materials such as garbage, livestock excretion, and sewage sludge. Besides, the conversion of biogas to H₂ is carbon neutral, thus reduces carbon footprint. From a recent report by the World Bioenergy Association based on the data from the International Energy Agency (IEA) [2], the domestic supply of biogas globally reached 1.33 EJ in 2017 from 0.28 EJ in 2000. Therefore, biogas is a promising energy resource for the future.

Biogas is usually used as a fuel in the gas engine or micro gas turbine [3]. Since biogas contains CO₂ of approximately 40 vol%, the heating value is low compared to natural gas, which results in a decrease in the efficiency of power generation. Therefore, in this study, biogas is used in the dry reforming process to produce H₂, which is used as fuel for the fuel cells. Biogas dry reforming to produce H₂ was previously studied by several researchers [4–17]. The catalyst used in the process is important in evaluating the performance of biogas dry reforming. Though some unique catalysts such as Fe/Al₂O₃ [4], Rh/Al₂O₃ [5], Co-Ce/ZrO₂ [6], and Fe₂O₃/zeolite [7] are used, Ni-based catalyst is the most common one for the biogas dry reforming process [8–17], which is also used in this study. This study adopts a membrane reactor to improve the efficiency of biogas dry reforming. Generally, a membrane reactor is used to improve the CH₄ steam reforming by separating H₂ from the reaction space [18–20] as soon as it is produced. The synergetic effect of the membrane reactor on performing the reaction and separation in the same unit, their simplicity, and the possibility of advanced levels of automation and control offer an attractive opportunity to redesign industrial processes [21]. Generally, Pd based membrane is used to separate H₂ due to its high efficiency [21–23]. Some studies used the membrane reactor for CH₄ dry reforming [21–29]. The coupled effect of the membrane and catalyst on CH₄ conversion was discussed earlier [21]. It was reported that the dense Pd/Ag/Cu alloy membrane and Ni/CeZrO₂ catalyst together had the highest CH₄ conversion of 65.2% [22] among the investigated combinations. The Pd/Au alloy membrane used in the two-zone fluidized bed reactor [23,29] showed the CH₄ conversion and H₂ selectivity to be higher compared to the conventional fluidized bed reactor. The highest CH₄ conversion and H₂/CO ratio were approximately 80% and 1.8, respectively. Ni was not only used in the membrane reactor but also as a catalyst to promote the CH₄ dry reforming process in the same reactor [24]. Though the CH₄ conversion and H₂ recovery achieved up to 85% and 70%, respectively, the equilibrium conversion was not achieved due to the limited resistance time in the reactor. The numerical simulation using the double tube reactor model investigated the percentage of CH₄ conversion, H₂ concentration, and temperature in the CH₄ dry reforming membrane reactor [27]. It was found that high reformer temperature in the membrane reactor was not favorable for the high production of H₂. Besides, the impact of the number of layers of the membrane on H₂ separation, as well as CH₄ and CO₂ conversion in the dry reforming process was investigated by numerical simulation [30]. The CH₄ and CO₂ conversion decreased as the number of layers increased, although the inclination slightly changed after six layers of the membrane. The permeation of H₂ into the membrane increased as the number of layers increased. The gas separation membrane was also applied to the steam reforming of CH₄ [31] and C₂H₅OH [32]. CO₂ capture by dual-phase ceramic-carbonate was effective in obtaining highly pure H₂, and achieved higher CO₂ recovery compared to the conventional fixed bed reactor. The Pd/Cu membrane reactor with Ir/CeO₂ catalyst provided a high H₂ yield of 98.1% at 550 °C and a reforming pressure of 1300 kPa [32]. This membrane was used with Ru or Rh catalyst [33], and evaluated at 550 °C, resulting in the CH₄ and CO₂ conversion of 8% to 40%. These values were slightly higher than those obtained for a fixed bed reactor with the same amount of catalyst.

However, the impact of gas separation conditions such as feed ratio of the sweep gas, reactor pressure, and varying reaction temperature on the performance of biogas dry reforming reactor is currently unknown.

Therefore, this study aims to understand the effect of operating conditions such as feed ratio of the sweep gas, pressure in the reactor, and reaction temperature on the performance of biogas dry reforming. Since a pure Pd membrane has a relatively high solubility for carbon, it causes membrane degradation leading to the loss of permeability [34]. Therefore, the Pd/Cu alloy membrane was used in this study. The reaction scheme of CH₄ dry reforming is as follow:

CH₄ + CO₂ → 2CO + 2H₂ △H₂₉₈^o = 247 kJ/mol (1)

2. Experiment

2.1 Experiment Set-Up

Figure 1 illustrates the experimental set-up used in this study. It consists of a gas cylinder, mass flow controller (S48-32; produced by HORIBA METRON INC.), pressure sensor (KM31; by NAGANO KEIKI), valve, reactor, i.e., a combination of the reaction chamber and sweep chamber, and gas sampling tap. The reactor is in the furnace where the temperature is controlled by the far-infrared heater (MCHNNS1; produced by MISUMI). CH₄ gas with purity over 99.4 vol% and CO₂ gas with purity over 99.9 vol% are controlled by mass flow controller and mixed before passing through the reactor. The pressure of the mixed gas is measured by the pressure sensor. Ar gas having a purity over 99.99 vol% is controlled by the mass flow controller, and measured by the pressure sensor. This is fed as a sweep gas. The exhausted gas at the outlet of the reactor is suctioned by a gas syringe via the gas sampling tap, and its concentration is measured by FID gas chromatography and a methanizer. The minimum resolution of both the FID gas chromatograph and methanizer is 1 ppmV. The gas pressure at the outlet of the reactor is measured using the pressure sensor. The gas concentration and pressure are measured at the outlet of the reaction chamber and sweep chamber, respectively. A valve is installed next to the pressure gauge at the outlet of the reaction chamber to investigate the effect of pressure on the performance of the dry reforming of CH₄. The valve is placed 65 cm away from the outlet of the reaction chamber. Though its position would better be near the outlet, this distance is kept due to the heat resistance of the pressure sensor installed before the valve. When the valve is closed, the gas in the reaction chamber flows to the sweep chamber preferentially.

Figure 1 Schematic drawing of the experimental set-up.

Figure 2 illustrates the details of the reactor. The reactor consists of the reaction chamber, sweep chamber, and H₂ separation membrane. The reaction chamber and sweep chamber are made of stainless steel, having dimensions 40 mm × 100 mm × 40 mm. The volume of the reaction space is 16 × 10^–5 m³. Pure Ni catalyst having a porous structure is placed in the reaction chamber, which means the catalyst is 100 wt% of Ni. The average pore diameter of this catalyst is 1.9 mm. The weight of the Ni catalyst is 63.1 g. The Pd/Cu alloy membrane (Cu of 40 wt%; produced by Tanaka Kikinzoku Kogyo), which is installed between the reaction and separation chambers, causes H₂ separation. The thickness of the Pd/Cu alloy membrane is 20 mm. The temperatures at the inlet, middle, and outlet of the reaction and sweep chambers are measured by K-type thermocouples. The measured temperature and pressure are collected by the data logger (GL240; by Graphtec Corporation).

Figure 2 Schematic drawing of reactor.

Table 1 lists the experimental conditions. The molar ratio of the supplied CH₄:CO₂ varies at 1.5:1, 1:1, or 1:1.5 since this ratio simulates the biogas. The feed ratio of the sweep gas, which is defined as the flow rate of sweep gas divided by the flow rate of supply gas composed of CH₄ and CO₂, is set to 0.5, 1, or 2. The effects of these ratios, in the conditions when the valve is open or close and the reaction temperatures of 400 °C, 500 °C and 600 °C, were studied. The gas concentrations in the reaction and sweep chambers were evaluated by FID gas chromatograph, and methanizer (produced by GL Science). This study shows the average data of ten trials for different experimental conditions in the following figures. The distribution of each of the gas concentration was below 10%. H₂ and CO selectivity were evaluated. The thermal efficiency of the proposed reactor was also evaluated.

Table 1 Experimental condition.

2.2 Evaluation Procedure of Performance of the Proposed Reactor

In this study, the performance of the proposed reactor is evaluated in terms of gas concentration at the outlet of the reaction and sweep chambers, pressure difference between the gas and sweep chambers, H₂ and CO selectivity, and thermal efficiency. H₂ and CO selectivities are defined as follows:

S_H2 = C_{H2, out}/(C_{H2, out} + C_{CO, out}) × 100 (2)

S_CO = C_{CO, out}/(C_{H2, out} + C_{CO, out}) × 100 (3)

where S_H2 is the H₂ selectivity (%), C_{H2, out} is the concentration of H₂ at the outlet of the reaction and sweep chambers (ppmV), C_{CO, out} is the concentration of CO at the outlet of the reaction chamber (ppmV), and S_CO is the CO selectivity (%). The thermal efficiency is defined as follows:

η = Q_H2/(Q_react + Q_sweep) ×100 (4)

Q_H2 = {(m_CH4 + m_CO2)/(22.4×60}×(C_{H2, out}/10⁶)×(H_l×22.4×10³) (5)

Q_react = Q_CH4 + Q_CO2 (6)

Q_CH4 = {(m_CH4/(22.4×60)}×C_{p, CH4}×(T_react – T_room) (7)

Q_CO2 = {(m_CO2/(22.4×60)}×C_{p, CO2}×(T_react – T_room) (8)

Q_sweep = {(m_Ar/(22.4×60)}×C_{p, Ar}×(T_react – T_room) (9)

where η is the thermal efficiency (%), Q_H2 is the heating value of produced H₂ (W), m_CH4 is the flow rate of CH₄ at the inlet of the reaction chamber (NL/min), m_CO2 is the flow rate of CO₂ at the inlet of the reaction chamber (NL/min), H_l is the low heating value of H₂ (= 10.79) (MJ/Nm³), Q_react is the heat required for the reaction gases (W), Q_CH4 is the heat needed to preheat the supplied CH₄ (W), Q_CO2 is the heat needed to preheat the supplied CO₂ (W), C_{p, CH4} is the specific heat of CH₄ (kJ/(kmol · K)), T_react is the reaction temperature (K), T_room is the room temperature (K), C_{p, CO2} is the specific heat of CO₂ (kJ/(kmol · K)), Q_sweep is the heat needed to preheat sweep gas (W), m_Ar is the flow rate of Ar at the inlet of the sweep chamber (NL/min), and C_{p, Ar} is the specific heat of CO₂ (kJ/(kmol · K)).

3. Results and Discussion

3.1 Effect of Feed Ratio of Sweep Gas and Pressure in Reaction Chamber on H₂ Production

Figure 3, Figure 4, Figure 5 and Figure 6 compare the relationship between the concentration of CH₄, CO₂, H₂, and CO at the outlet of the reaction chamber, and the feed ratio of sweep gas, when the valve installed at the outlet is open or close, respectively. Figure 7 shows the comparison of the relationship between the concentration of H₂ at the outlet of the sweep chamber and feed ratio of sweep gas when the valve installed at the outlet of the reaction chamber is open or close, respectively. In these figures, the feed ratio of sweep gas is indicated by m_Ar/(m_CH4+m_CO2). The molar ratio of the supplied CH₄:CO₂ is 1.5:1. The reaction temperature is varied to 400 °C, 500 °C, or 600 °C.

According to Figure 4 and Figure 5, the concentration of CO₂ and H₂ in the reaction chamber are lower and higher, respectively, when the valve is close. Since the pressure difference between the reaction and sweep chambers is higher upon closing the valve, it is thought that H₂ separation is promoted, increasing the concentration of H₂. The increase in the H₂ concentration in the sweep chamber is shown in Figure 7. Though the concentration of CO₂ decreased, those of CH₄ (Figure 3) and CO (Figure 6) increased, when the valve is close. The following reactions [8,12] would take place:

CO₂ + H₂ → CO + H₂O △H₂₉₈^o = 41.2 kJ/mol (10)

CO₂ + 4H₂ → CH₄ + 2H₂O △H₂₉₈^o = -165.0 kJ/mol (11)

According to the reactions shown in Eq (10) and (11), CH₄ and CO consumption increased upon consumption of CO₂ and H₂.

Figure 3 Comparison of the relationship between the concentration of CH₄ at the outlet of the reaction chamber and feed ratio of sweep gas with the valve open or close (CH₄ : CO₂ = 1.5 : 1).

Figure 4 Comparison of the relationship between the concentration of CO₂ at the outlet of the reaction chamber and feed ratio of sweep gas with the valve open or close (CH₄ : CO₂ = 1.5 : 1).

Besides, the highest concentration of H₂ is obtained for m_Ar/(m_CH4+m_CO2) = 1, according to Figure 5, Figure 6 and Figure 7. It is revealed that the optimum m_Ar/(m_CH4+m_CO2) to produce H₂ is 1. This result agrees with a previous study [22]. According to this [22], since the main CH₄ dry reforming reaction, i.e., Eq (1) is reversible, introducing excess amounts of reactants could drive the reaction forward, resulting in the generation of more H₂. However, the produced H₂ could also react with the unreacted CO₂ in the reversible water-gas shift reactions, i.e., Eqs. (10) and (11). Therefore, higher CO₂ consumption was observed compared to that of CH₄. As a result, the highest concentration of H₂ was obtained when m_Ar/(m_CH4+m_CO2) = 1, as the reactions are balanced.

Figure 5 Comparison of the relationship between the concentration of H₂ at the outlet of the reaction chamber and feed ratio of sweep gas with the valve open or close (CH₄ : CO₂ = 1.5 : 1).

Figure 6 Comparison of the relationship between the concentration of CO at the outlet of the reaction chamber and feed ratio of sweep gas with the valve open or close (CH₄ : CO₂ = 1.5 : 1).

Figure 7 Comparison of the relationship between the concentration of H₂ at the outlet of sweep chamber and feed ratio of sweep gas with the valve open or close (CH₄ : CO₂ = 1.5 : 1).

Figure 8 and Figure 9 show the comparison of the relationship between the concentration of H₂ and CO at the outlet of the reaction chamber and m_Ar/(m_CH4+m_CO2) when the valve installed at the outlet of the reaction chamber is open or close, respectively. Figure 10 shows the comparison of the relationship between the concentration of H₂ at the outlet of the sweep chamber and m_Ar/(m_CH4+m_CO2) when the valve is open or close, respectively. The molar ratio of the supplied CH₄:CO₂ is 1:1. The reaction temperature is changed to 400 °C, 500 °C, and 600 °C.

Figure 8 Comparison of the relationship between the concentration of H₂ at the outlet of the reaction chamber and feed ratio of sweep gas with the valve is open or close (CH₄ : CO₂ = 1 : 1).

Figure 9 Comparison of the relationship between the concentration of CO at the outlet of the reaction chamber and feed ratio of sweep gas with the valve is open or close (CH₄ : CO₂ = 1 : 1).

According to Figures 8 and 10, the concentration of H₂ is higher when the valve is close. Since the pressure difference between the reaction and sweep chambers is higher upon closing the valve, it is thought that the H₂ separation is promoted, resulting in an increase in the concentration of H₂. Also, it is known from Figure 9 that the concentration of CO is higher when the valve is close. Since the molar ratio of the supplied CH₄:CO₂ = 1:1 is the same as the theoretical value as shown in Eq. (1), it is believed that the dry reforming progressed well. Besides, it is seen from Figures 8 and 10 that the highest concentration of H₂ is obtained for m_Ar/(m_CH4+m_CO2) = 1. It is revealed that the optimum m_Ar/(m_CH4+m_CO2) to produce H₂ is 1, and shows the same tendency when the molar ratio of supplied CH₄:CO₂ = 1.5:1.

Figure 10 Comparison of the relationship between the concentration of H₂ at the outlet of sweep chamber and feed ratio of sweep gas with the valve is open or close (CH₄ : CO₂ = 1 : 1).

Figure 11 and Figure 12 compare the relationship between the concentration of H₂ and CO at the outlet of the reaction chamber and m_Ar/(m_CH4+m_CO2), when the valve installed at the outlet of the reaction chamber is open or close, respectively. Figure 13 shows the comparison of the relationship between the concentration of H₂ at the outlet of the sweep chamber and m_Ar/(m_CH4+m_CO2) when the valve installed at the outlet of the reaction chamber is open or close, respectively. The molar ratio of the supplied CH₄:CO₂ is 1:1.5. The reaction temperature is changed to 400 °C, 500 °C or 600 °C.

According to Figures 11 and 13, the concentration of H₂ is higher when the valve is close. Since the pressure difference between the reaction and sweep chambers is higher when the valve is close, it is thought that H₂ separation is promoted, increasing the concentration of H₂. Additionally, it is seen from Figures 11 and 13 that the highest concentration of H₂ is obtained for m_Ar/(m_CH4+m_CO2) = 1. It is revealed that the optimum m_Ar/(m_CH4+m_CO2) to produce H₂ is 1, which is also the case when the molar ratio of the supplied CH₄:CO₂ = 1.5:1 and 1:1. It is obvious that m_Ar/(m_CH4+m_CO2) = 1 is the optimum ratio irrespective of the molar ratio of the supplied CH₄ : CO₂, where the reactions are balanced as described above.

According to Figures 3 to 13, the concentrations of produced H₂ and CO are low. This low conversion is thought to be caused by the use of pure Ni as the catalyst. In this study, pure Ni was used to evaluate the basic characteristics of the proposed membrane reactor as the first step to develop the reactor. According to the literature, noble metals such as Pd, Pt [21], and Ru, as well as the alloy of Ni and the other metals, e.g., Co, Cu, Fe, and Ce [28] showed better catalytic activity for the conversion of CH₄ and CO₂ compared to pure Ni. Therefore, this study used other catalysts, as mentioned above, to promote the CH₄ and CO₂ conversion in the proposed membrane reactor as the next step.

Figure 11 Comparison of the relationship between the concentration of H₂ at the outlet of the reaction chamber and feed ratio of sweep gas with the valve is open or close (CH₄ : CO₂ = 1 : 1.5).

Figure 12 Comparison of the relationship between the concentration of CO at the outlet of the reaction chamber and feed ratio of sweep gas with the valve is open or close (CH₄ : CO₂ = 1 : 1.5).

Figure 13 Comparison of the relationship between the concentration of H₂ at the outlet of sweep chamber and feed ratio of sweep gas with the valve is open or close (CH₄ : CO₂ = 1 : 1.5).

3.2 Evaluation on Performance of Proposed Reactor

Table 2 and Table 3 list the pressure differences between the reaction and sweep chambers when the molar ratio of CH₄:CO₂ = 1.5:1, 1:1, 1:1.5, and the valve at the outlet of the reaction chamber is open or close, respectively. It is seen from the data that the pressure differences are 0 kPa when the valve is open, irrespective of the molar ratio. Since the valve is open, the gases in the reaction chamber might flow out without H₂ separation by the Pd/Cu alloy membrane. This is further confirmed by the concentration of H₂ at the outlet of the sweep chamber, which is almost 0 ppmV, as shown in Figures 7, 10, and 13. When the valve is close, the pressure difference and concentration of H₂ at the outlet of the sweep chamber increase with the rise in the reaction temperature. In other words, closing the valve, i.e., increasing the pressure difference and reaction temperature, promotes the CH₄ dry reforming reaction.

Table 2 Pressure difference between the reaction chamber and the sweep chamber in the case of the molar ratio of CH₄ : CO₂ = 1.5 : 1, 1 : 1, 1 : 1.5, with the valve installed at the outlet of the reaction chamber is open (unit: kPa).

Table 3 Pressure difference between the reaction chamber and the sweep chamber in the case of the molar ratio of CH₄ : CO₂ = 1.5 : 1, 1 : 1, 1 : 1.5, without the valve installed at the outlet of the reaction chamber is open (unit: kPa).

Figure 14, Figure 15 and Figure 16 compare the relationship between H₂ or CO selectivity and m_Ar/(m_CH4+m_CO2) when the molar ratio of supplied CH₄:CO₂ = 1.5:1, 1:1, and 1:1.5, respectively with a closed valve. The reaction temperature is set to 400 °C, 500 °C, and 600 °C, respectively. The figures indicate that H₂ selectivity increases and CO selectivity decreases with an increase in the reaction temperature as the CH₄ dry reforming is an endothermic reaction [9,35]. According to the previous study [36], the H₂/CO ratio increases with increased reaction temperature because CH₄ dry reforming is an endothermic reaction, which is thermodynamically and kinetically favored at high temperatures.

Besides, these figures show that the CO selectivity is higher than that of H₂ at m_Ar/(m_CH4+m_CO2) = 2. If m_Ar/(m_CH4+m_CO2) is higher than 2, the biogas dry reforming in the reaction chamber requires longer reaction time.

Figure 14 Comparison of the relationship between H₂ or CO selectivity and feed ratio of sweep gas with the closed valve (CH₄ : CO₂ = 1.5 : 1).

Figure 15 Comparison of the relationship between H₂ or CO selectivity and feed ratio of sweep gas with the closed valve (CH₄ : CO₂ = 1 : 1).

Figure 16 Comparison of the relationship between H₂ or CO selectivity and feed ratio of sweep gas with the closed valve (CH₄ : CO₂ = 1 : 1.5).

Table 4 and Table 5 give a comparison of thermal efficiency under different operating conditions when the valve is open and close, respectively. The data show that the thermal efficiency with a closed valve is higher than that when it is open, irrespective of the reaction temperature, m_Ar/(m_CH4+m_CO2), and the molar ratio of the supplied CH₄:CO₂. Also, it is revealed that the thermal efficiency at m_Ar/(m_CH4+m_CO2) = 1 is higher than any other conditions.

Table 4 Comparison of thermal efficiency among different operation conditions with the open valve (unit: %).

Table 5 Comparison of thermal efficiency among different operation conditions with the close valve (unit: %).

In this study, the thermal efficiencies achieved were not high under different conditions. It is thought that effective H₂ separation was not obtained since the pressure difference between the reaction and sweep chambers was not high enough to extract H₂. As a result, there was no displacement of the thermodynamic equilibrium of the reaction, causing less production of H₂. It is necessary to improve the reaction performance of biogas dry reforming and H₂ separation. Since this study investigated pure Ni catalyst and Ni-based alloy catalyst such as Ni/Ce [10], Ni/La [17], Ni/ZnO-Al₂O₃ [37], the latter was found to have better performance and can be preferentially used for further studies. Also, the characterization of the H₂ separation membrane, such as composition, thickness, and metal type, is important to improve the efficiency of H₂ separation. Since this study investigated one type of H₂ separation membrane only, i.e., Pd/Cu alloy membrane (Cu of 40 wt%), the composition and thickness of the Pd/Cu alloy membrane will also be investigated in the future.

4. Conclusions

In this paper, the effect of operating conditions such as the feed ratio of the sweep gas, pressure in the reactor, and reaction temperature on the performance of biogas dry reforming was studied. The effects of the molar ratio of the supplied CH₄:CO₂, as well as m_Ar/(m_CH4+m_CO2) on the performance of biogas dry reforming were also investigated. The effects were evaluated by comparing the conditions when the valve was open and close at the outlet of the reaction chamber. The reaction temperature was set to 400 °C, 500 °C, and 600 °C, respectively. The following conclusions are obtained from the study:

1)

The final concentration of H₂ in the reaction chamber with closed valve is higher than that with the open valve, irrespective of the molar ratio of supplied CH₄:CO₂, since H₂ separation is promoted by the increase in the pressure difference between the reaction and sweep chambers.

2)

It is confirmed that the optimum m_Ar/(m_CH4+m_CO2) to produce H₂ is 1, irrespective of the molar ratio of supplied CH₄:CO₂.

3)

The H₂ selectivity increases and CO selectivity decreases with an increase in the reaction temperature, irrespective of the molar ratio of supplied CH₄:CO₂.

4)

CO selectivity is likely to be higher than H₂ selectivity at m_Ar/(m_CH4+m_CO2) = 2 since the separation speed would be higher compared to the reaction speed of biogas dry reforming.

5)

The thermal efficiency of the reactor with a closed valve is higher than that with the open valve, irrespective of the reaction temperature, m_Ar/(m_CH4+m_CO2), and the molar ratio of supplied CH₄:CO₂.

6)

It is revealed that the thermal efficiency at m_Ar/(m_CH4+m_CO2) = 1 is the highest due to maximum H₂ production.

Author Contributions

Akira Nishimura managed this study and wrote the paper; Satoshi Ohata carried out the experiment and collected the data; Kaito Okukura carried out the experiment and collected the data; Eric Hu refined the paper from the view point of native English speaker.

Funding

This work was financially supported by the joint usage/research program of the Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University.

Competing Interests

The authors have declared that no competing interests exist.