Impact of Membrane Thickness on Characteristics of Biogas Dry Reforming Membrane Reactor Using Pd/Cu Membrane and Ni/Cr/Ru Catalyst

Abstract

The aim of the present study was to clarify the influence of the thickness of the Pd/Cu membrane on the characteristics of biogas dry reforming (BDR) with aNi/Cr/Ru catalyst. We also clarified the impact of the reaction temperature, the molar ratio of CH₄:CO₂, the differential pressure between the reaction and sweep chambers, and the introduction of a sweep gas on the characteristics of a BDR reactor with a Pd/Cu membrane and a Ni/Cr/Ru catalyst. Through this study’s results, we clarify that the concentration of H₂ in the reaction chamber and the sweep chamber increases with the increase in the reaction temperature. In addition, this study clarifies that the highest concentration of H₂ in the reaction chamber and the sweep chamber can be obtained with a molar ratio of CH₄:CO₂ = 1.5:1. This study also clarifies that the highest concentration of H₂ can be obtained with a thickness of 40 μm, a molar ratio of CH₄:CO₂ = 1.5:1, and a differential pressure between the reaction chamber and the sweep chamber of 0 MPa without a sweep gas, which was 4890 ppmV in the reaction chamber and 38 ppmV in the sweep chamber. Under these conditions, CH₄ conversion, H₂ yield, and thermal efficiency were 75.0%, 0.214%, and 2.92%, respectively.

1. Introduction

Global warming is a serious problem effecting the whole world. Many promising procedures have been proposed to solve this problem. According to the literature [1], CO₂ hydrogenation is one technology with the potential to solve the problem of global warming. Na-FeZrOx-9 has been developed, achieving an up to 60% CO₂ conversion, and could help to solve the problem of global warming. Another procedure involves the use of green H₂, e.g., H₂ production via a renewable energy source. This study focuses on biogas dry reforming (BDR) as a procedure to produce green H₂. Biogas is a fuel that generally consists of CH₄ (55–75 vol%) and CO₂ (25–45 vol%) [2]. Biogas is formed through fermentation by means of the action of anaerobic microorganisms on raw materials, e.g., garbage, livestock excretion, and sewage sludge. In 2020, 1.46 EJ of biogas was produced in the world [3], which is five times greater than the amount produced in 2000 [3]. Consequently, we except the amount of biogas produced to continue to increase in the near future.

Generally speaking, biogas is used as a gas fuel for gas engines or micro gas turbines [4]. However, its power generation efficiency is low compared to natural gas due to its lower heating value, which is caused by the inclusion of CO₂. In an effort to solve this problem, this study developed a system consisting of a BDR reactor with a solid oxide fuel cell (SOFC) [5,6,7]. CO, which is a by-product of BDR, can be utilized as fuel for SOFCs. Since SOFCs can serve as co-generation systems, their total energy conversion efficiency is higher than that of existing power generation systems, i.e., gas engines and micro gas turbines.

Many studies on BDR have been reported [8,9,10,11,12]. This study focuses on the selection of a catalyst since this choice influences the performance of a BDR reactor. According to a literature survey, Ni-based catalysts have been investigated for BDR [8,9,10,11,12]. A Ni/Al₂O₃ catalyst synthesized via conventional wet impregnation achieved CH₄ conversion of approximately 100% and CO₂ conversion of approximately 95% at 850 °C [8]. CH₄ conversion and CO₂ conversion increased with an increase in the reaction temperature from 700 °C to 850 °C. The CO₂-rich condition (CO₂/CH₄ = 1.2) was slightly better from the viewpoint of conversion, and it induced a significant reverse water gas shift reaction (RWGS), which provided a low H₂/CO ratio and H₂ selectivity. Ni₃Co supported on Al₂O₃ achieved CH₄ conversion of 59.8% and 96.2% at 600 °C and 750 °C, respectively [9]. In addition, this catalyst achieved CO₂ conversions of 24.4% and 45.4% at 600 °C and 750 °C, respectively. Though significant carbon formation caused the deactivation of the catalyst, carbon formation was reduced at a higher reaction temperature. NiCeZrO₂ prepared by the sol–gel method achieved CH₄ conversion of 50% and CO₂ conversion of 50% at 800 °C [10]. An increase in the molar ratio of CH₄:CO₂ to 1.5 resulted in the formation of coke, which was suppressed by the addition of water. Ni/MgO, prepared by a wet impregnation method, achieved CH₄ conversion of approximately 70% and CO₂ conversion of approximately 85% at 800 °C [11]. In an investigation on the impact of a reaction temperature ranging from 300 °C to 800 °C, the highest CH₄ conversion, as well as the highest CO₂ conversion, was obtained at 800 °C. ANi-La/SBA-16 catalyst prepared by a hydrothermal process achieved CH₄ conversion of approximately 95% and CO₂ conversion of approximately 93% at 750 °C [12]. CH₄ conversion and CO₂ conversion increased with an increase in the reaction temperature from 600 °C to 750 °C, achieving CH₄ conversion and CO₂ conversion of approximately 70% at 600 °C.

Ru-based catalysts have also been investigated [13,14,15]. The ARu/Ni catalyst prepared by wet impregnation achieved CH₄ conversion of approximately 78% and CO₂ conversion of approximately 72% at 750 °C [13]. When the ratio of Ru to Ni was changed from 0 to 2, a ratio of 0.8 achieved the best performance. Ru supported on a binary La₂O₃-SiO₂ catalyst prepared by incipient wetness impregnation achieved a CH₄ conversion of 5.3% and a CO₂ conversion of 5.5% at 550 °C [14]. Ru-Ni/MgAl₂O₄ prepared by a wet impregnation method achieved CH₄ conversion of 93% and CO₂ conversion of 96% at 750 °C [15]. When the reaction temperature decreased from 750 °C to 600 °C, CH₄ conversion and CO₂ conversion decreased to 65% and 73%, respectively.

Although some Ni-based catalysts have been examined, a Ni/Cr catalyst has not been investigated well, with the exception of the authors’ previous study [6].

Another important issue to consider is operation at lower temperatures in order to improve the thermal energy efficiency of BDR, since BDR is an endothermic reaction. This study proposes the use of a membrane reactor as an effective procedure to achieve this purpose. H₂ production can be improved by means of providing a non-equilibrium state and H₂ separation from the reaction site [6]. Though some Pd alloy membranes, i.e., Pd/Ag and Pd/Au, have been commercialized and adopted for research, the cost of Pd/Cu is less than that of Pd/Ag, Pd/Au, and pure Pd. This study proposes cost as an important aspect to consider in the industrial application of the presented system. Consequently, this study selected Pd/Cu as a H₂ separation membrane. The authors’ previous study using the Ni/Cr/Ru catalyst [6] used a Pd/Cu membrane whose thickness was only 20 μm. However, there has not yet been a study examining the influence of the thickness of the Pd/Cu membrane on the characteristics of BDR using the Ni/Cr/Ru catalyst. This study proposes that there will be the optimum thickness of the Pd/Cu membrane with which to obtain the highest performance of the membrane reactor using the Ni/Cr/Ru catalyst proposed. This hypothesis arises from the fact that the H₂ separation rate of the H₂ separation membrane and the H₂ production rate of the catalyst are thought to be important to realizing the non-equilibrium state of BDR.

The authors would like to summarize the originality and the novelty of this study as follows: As explained above, Ni-based catalysts have been well investigated [8,9,10,11,12], and Ni/Ru-type catalysts have also investigated [13,14,15]. These catalysts demonstrated a better performance compared to the pure Ni catalyst. However, there is no report on the use of the Ni/Cr/Ru catalyst apart from the previous study conducted by the authors [6]. There has been no investigation into the impact of the thickness of the H₂ separation membrane (Pd/Cu membrane) on the performance of the membrane reactor using the Ni/Cr/Ru catalyst. This study thus proposes that there is an optimum thickness of the Pd/Cu membrane with which to obtain the highest performance of the membrane reactor using the Ni/Cr/Ru catalyst. Therefore, the originality and the novelty of this study are clear.

Consequently, the aim of the present study is to reveal the influence of the thickness of the Pd/Cu membrane on the characteristics of BDR using the Ni/Cr/Ru catalyst. The influence of reaction temperature, the molar ratio of CH₄:CO₂, the differential pressure between the reaction chamber and the sweep chamber, and the introduction of a sweep gas on the characteristics of the BDR reactor with the Pd/Cu membrane and Ni/Cr/Ru catalyst is investigated. The molar ratio of CH₄:CO₂ = 1.5:1 simulates a biogas in this study.

2. Experiment

2.1. Experimental Set-Up

Figure 1 shows the schematic drawing of the experimental set-up of this study. The experimental set-up consists of a gas cylinder, mass flow controllers (S48-32, produced by HORIBA METRON INC. Beijing, China), pressure sensors (KM31), valves, a vacuum pump, a reactor composed of a reaction chamber and a sweep chamber, and gas sampling taps [6]. The reactor is placed in a self-order-made electric furnace. We control the temperature in the electric furnace using far-infrared heaters (MCHNS1, produced by MISUMI, Tokyo, Japan). CH₄ gas with a purity over 99.4 vol% and CO₂ with a purity over 99.9 vol% are controlled by means of mass flow controllers and mixed before flowing into the reaction chamber [6]. We measure the pressure of the mixed gas at the inlet of the reaction chamber with pressure sensors. We control the Ar gas, with a purity of over 99.99 vol%, by means of a mass flow controller, employ it as a sweep gas, and measure its pressure via a pressure sensor [6]. We suction the exhausted gases at the outlet of the reaction chamber and sweep chamber by means of a gas syringe with gas sampling taps. We measure the concentration of the sampled gas with a TCD gas chromatograph (GL Science, Tokyo, Japan) [6]. The minimum resolution of the TCD gas chromatograph and the methanizer is 1 ppmV. We measure the gas pressure at the outlet of the reactor using a pressure sensor. We measure the gas concentration and pressure at the outlet of the reaction chamber and the sweep chamber, respectively [6]. We install the valves at the outlet of the reaction chamber and close them when we flow the reaction gas, which consists of CH₄ and CO₂ into the reaction chamber. As a result, H₂ in the reaction chamber after contacting the Ni/Cr/Ru catalyst penetrates through the H₂ separation membrane, i.e., the Pd/Cu membrane, and flows into the sweep chamber. No H₂ flows out of the reaction chamber unless it penetrates through the H₂ separation membrane.

Figure 2 shows a detailed drawing of the reactor in this study. The reactor consists of a reaction chamber, a sweep chamber, and a H₂ separation membrane, i.e., the Pd/Cu membrane [6]. The reaction chamber and the sweep chamber are made of stainless steel, having the size of 40 mm × 100 mm × 40 mm [6]. The volume of the reaction space is 16 × 10⁻⁵ m³ [6]. We charge a porous Ni/Cr/Ru (composition: Ni: 69.2 wt%; Cr: 29.6 wt%; Ru: 1.2 wt%) in the reaction chamber [6]. This study has adopted the commercial Ni/Cr/Ru catalyst produced by Sumitomo Electric Toyama Co., Ltd. (Tokyo, Japan). According to the brochure of manufacture of Ni/Cr/Ru catalyst (Sumitomo Electric Toyama Co., Ltd.), the Ni/Cr/Ru catalyst is produced by the following processes: (i) foam resin; (ii) conductive treatment; (iii) electrodeposition; (iv) heat disposal; and (v) alloying treatment. The mean hole size of the Ni/Cr/Ru catalyst adopted in the present study is 1.95 mm. According to the manufacture’s brochure, the porosity of the Ni/Cr/Ru catalyst used in this study is 0.93. The weight of Ni/Cr/Ru catalyst is 66.3 g.

Figure 3 shows a photograph charged in the reaction chamber in this study [6]. We selected a Pd/Cu membrane (Cu: 40 wt%, produced by Tanaka Kikinzoku, Tokyo, Japan) as a H₂ separation membrane [6]. The thicknesses of the Pd/Cu membrane were 20 μm, 40 μm, and 60 μm. We measured the temperatures at the inlet, the middle, and the outlet of the reaction chamber and the sweep chamber using K-type thermocouples. We controlled the initial operation temperature and set it by a far-infrared heater and thermocouples. We saved the measured temperature and pressure data using a data logger (GL240, Graphic Corporation, Tokyo, Japan) [6].

Table 1. Parameters of the experimental conditions [6].

Parameters	Values
Initial reaction temperature [°C]	400, 500, and 600
Pressure of supply gas [MPa]	0.10
Differential pressure between the reaction chamber and the sweep chamber [MPa]	0, 0.010, and 0.020
Molar ratio of provided CH4:CO2 (flow rate of provided CH4:CO2 [NL/min])	1.5:1, 1:1, and 1:1.5 (1.088:0.725, 0.725:0.725, 0.725:1.088)
Feed ratio of sweep gas to supply gas [−]	0 (W/O), 1.0 (W)

shows the experimental parameters applied in the present study. The molar ratio of the provided CH₄:CO₂ was changed by 1.5:1, 1:1, and 1:1.5 [6]. The molar ratio of CH₄:CO₂ simulated biogas in the present study. In this study, the authors have designed a reactor which can produce the amount of H₂ necessary to conduct 100 W class of SOFC when the biogas dry reforming is performed theoretically. From the authors’ previous study [16], the feed ratio of sweep gas to supply gas, which is defined as the flow rate of the sweep gas divided by the flow rate of the supply gas composed of CH₄ and CO₂, was set at 1.0. This is the optimum feed ratio of sweep gas to supply gas according to the authors’ previous study [16]. We investigate the impact of the installation of sweep gas. It can be necessary to separate H₂ from the membrane reactor with a sweep gas and to extract the H₂ from the sweep gas for the purpose of applying the system proposed by the authors. We have to consider that the additional energy and equipment are necessary to install the system suggested by this study in the actual industry. On the other hand, the additional energy and equipment are not necessary if we can separate H₂ without a sweep gas from the membrane reactor. Consequently, this study has conducted a comparison of the presence and the absence of sweep gas. We changed the differential pressure between the reaction chamber and the sweep chamber by 0 MPa, 0.010 MPa, and 0.020 MPa [6]. This study measures and confirms the differential pressure using the pressure sensors installed at the outlet of the reaction chamber and the sweep chamber. As to the differential pressure between the reaction chamber and the sweep chamber, this study carries out the experiment at a differential pressure of 0.030 MPa and a temperature of 600 °C. According to the result, the hole was made within the Pd/Cu membrane. Consequently, this study reports the results below 0.020 MPa. We changed the initial reaction temperature by 400 °C, 500 °C, and 600 °C [6]. We measured the initial reaction temperature by means of thermocouples before flowing the mixed gas of CH₄ and CO₂ and the sweep gas into the reactor. The temperature of the reaction chamber decreased by approximately 3 °C during the experiment because of the endothermic reactions, which are shown above. The gas concentrations at the outlet of the reaction chamber and the sweep chamber were detected by means of the FID gas chromatograph (GC320; GL Science) and methanizer (MT221; GL Science) [6]. We show the mean data of five trials for each experimental condition investigated in this study in the following figures. In this study, the distribution of each gas concentration is below 10%.

2.2. Assessment Factor to Evaluate the Performance of BDR Mmbrane Ractor in This Study

This study conducts an evaluation on the performance of the suggested BDR membrane reactor by investigating gas concentration at the outlet of the reaction chamber and the sweep chamber. This study evaluates CH₄ conversion (X_CH4), CO₂ conversion (X_CO2), H₂ yield (Y_H2), H₂ selectivity (S_H2), and CO selectivity (S_CO) using these data. This study defines these assessment factors as follows:

X_CH4 = (C_{CH4, in} − C_{CH4, out})/(C_{CH4, in}) × 100

(1)

X_CO2 = (C_{CO2, in} − C_{CO2, out})/(C_{CO2, in}) × 100

(2)

Y_H2 = (1/2)(C_{H2, out})/(C_{H4, in}) × 100

(3)

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

(4)

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

(5)

where C_{CH4, in} indicates the concentration of CH₄ at the inlet of the reaction chamber [mol], C_{CH4, out} indicates the concentration of CH₄ at the outlet of the reaction chamber [mol], C_{CO2, in} indicates the concentration of CO₂ at the inlet of the reaction chamber [mol], C_{CO2, out} indicates the concentration of CO₂ at the outlet of the reaction chamber [mol], C_{H2, out} indicates the concentration of H₂ at the outlet of the reaction chamber and the sweep chamber [mol], and C_{CO, out} indicates the concentration of CO at the outlet of the reaction chamber [mol].

Additionally, we evaluate H₂ recovery (H) and permeation flux (F) as follows:

H = (C_{H2, out, sweep})/(C_{H2, out, sweep} + C_{H2, out, react}) × 100

(6)

$$F={\displaystyle \frac{P\left(\sqrt{P_{react,ave}}-\sqrt{P_{sweep,ave}}\right)}{\delta }}\times 100$$

(7)

where C_{H2, out, sweep} means the concentration of H₂ at the outlet of the sweep chamber [mol], C_{H2, out, react} indicates the concentration of H₂ at the outlet of the reaction chamber [mol], P indicates the permeation factor [mol/(m∙s∙Pa^0.5)], P_{react, ave} indicates the average pressure of the reaction chamber [MPa], P_{sweep, ave} indicates the average pressure of the sweep chamber [MPa], and δ indicates the thickness of the Pd/Cu membrane [m].

Furthermore, this study also evaluates the thermal efficiency of the membrane reactor (η), which is defined as follows:

$$\eta ={\displaystyle \frac{Q_{H2}}{\left(W_{R.C.}+W_{S.C.}+W_P\right)}}\times 100$$

(8)

where Q_H2 means the heating value of produced H₂ based on the lower heating value [W], W_R.C. means the amount of pre-heat of supply gas for the reaction chamber [W], W_S.C. means the amount of pre-heat of the sweep gas for the sweep chamber [W], and W_p means the pump power to provide the differential pressure between the reaction chamber and the sweep chamber [W].

3. Results and Discussion

3.1. Impact of the Thickness of the Pd/Cu Membrane on Each Gas Concentration in the Reaction Chamber and the Concentration of H₂ in the Sweep Chamber, Changing the Initial Reaction Temperature and the Differential Pressure Between the Reaction Chamber and the Sweep Chamber with and Without a Sweep Gas

Figure 4 and Figure 5 show the impact of the thickness of the Pd/Cu membrane on each gas concentration in the reaction chamber and the concentration of H₂ in the sweep chamber, changing the initial reaction temperature (it is described as a reaction temperature later) and the differential pressure between the reaction chamber and the sweep chamber without a sweep gas, respectively. In addition, Figure 6 and Figure 7 show the impact of the thickness of the Pd/Cu membrane on each gas concentration in the reaction chamber and the concentration of H₂ in the sweep chamber, changing the reaction temperature and the differential pressure between the reaction chamber and the sweep chamber with a sweep gas, respectively. Moreover, the molar ratio of CH₄:CO₂ is 1.5:1 in these figures. This study shows the results with the gas concentration in the unit of ppmV in these figures.

In this study, the reaction scheme of CH₄ dry reforming (DR) is described as follows:

CH₄ + CO₂ ↔ 2CO + 2H₂ + 247 kJ/mol.

(9)

Additionally, the following reaction schemes can be considered in this study:

CO₂ + H₂ ↔ CO + H₂O + 41 kJ/mol

(10)

CO₂ + 4H₂ ↔ CH₄ + 2H₂O − 164 kJ/mol

(11)

CH₄ + H₂O ↔ CO + 3H₂ + 206 kJ/mol

(12)

where Equation (10) indicates a reverse water gas shift reaction (RWGS), Equation (11) indicates a methanation reaction (MR), and Equation (12) indicates a steam reforming of CH₄ (SR). As to the carbon deposition, the following reaction schemes are considered in this study:

CH₄ ↔ C + 2H₂ + 75 kJ/mol

(13)

2CO ↔ C + CO₂ − 173 kJ/mol

(14)

CO₂ + 2H₂ ↔ C + 2H₂O − 90 kJ/mol

(15)

CO + H₂ ↔ C + H₂O − 131 kJ/mol

(16)

According to Figure 4 and Figure 6, it is seen that the concentration of H₂ in the reaction chamber increases with the increase in the reaction temperature irrespective of the thickness of Pd/Cu membrane, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. Since DR, RWGS, and SR are endothermic reactions, as shown in Equations (9), (10), and (12), the reaction progresses well with the increase in the reaction temperature from the theoretical kinetic study [17].

On the other hand, it is seen from Figure 5 and Figure 7 that the concentration in the sweep chamber increases with the increase in the reaction temperature irrespective of the thickness of Pd/Cu membrane, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. Since the concentration of H₂ in the reaction chamber is larger at higher reaction temperatures, the driving force to penetrate through the Pd/Cu membrane becomes larger because of the high H₂ partial differential pressure between the reaction chamber and the sweep chamber, i.e., a high concentration difference of H₂ between the reaction chamber and the sweep chamber. Consequently, a higher concentration of H₂ in the sweep chamber is obtained.

Considering the influence of the thickness of the Pd/Cu membrane on the concentrations of H₂ in the reaction chamber and the sweep chamber, the largest concentrations of H₂ in the reaction and the sweep chamber are obtained mainly for the thickness of 40 μm among the investigated conditions, i.e., the reaction temperature, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. It can be thought that H₂ might be penetrated through the H₂ separation membrane when the thickness of the H₂ separation membrane is thinner. However, the optimum thickness of the Pd/Cu membrane is 40 μm in this study. If the H₂ separation rate of the Pd/Cu membrane is larger than the reaction rate of the Ni/Cr/Ru catalyst used in this study, the impact of H₂ separation, providing a non-equilibrium state in DR, is not sufficiently effective [6]. This study proposes that it can be necessary to match the H₂ production rate of the Ni/Cr/Ru catalyst and the H₂ separation rate of the Pd/Cu separation membrane to obtain the non-equilibrium state of DR [6]. When using the thicker membrane, i.e., with a thickness of 60 μm, the H₂ separation rate is too low due to the big resistance necessary to separate H₂. On the other hand, the H₂ separation rate is too fast when using the thinner membrane, i.e., with a thickness of 20 μm, due to small resistance necessary to separate H₂. Consequently, this study proposes that the optimum thickness of the Pd/Cu membrane is 40 μm in this study.

3.2. Impact of the Thickness of the Pd/Cu Membrane on Each Gas Concentration in the Reaction Chamber and the Concentration of H₂ in the Sweep Chamber, Changing the Molar Ratio and the Differential Pressure Between the Reaction Chamber and the Sweep Chamber with and Without a Sweep Gas

Figure 8 and Figure 9 show the impact of the thickness of the Pd/Cu membrane on each gas concentration in the reaction chamber and the concentration of H₂ in the sweep chamber, changing the molar ratio and the differential pressure between the reaction chamber and the sweep chamber without a sweep gas, respectively. In addition, Figure 10 and Figure 11 show the impact of the thickness of the Pd/Cu membrane on each gas concentration in the reaction chamber and the concentration of H₂ in the sweep chamber, changing the molar ratio and the differential pressure between the reaction chamber and the sweep chamber with a sweep gas, respectively. From these figures, the molar ratio of CH₄:CO₂ was changed to 1.5:1, 1:1, and 1:1.5, and the differential pressure between the reaction chamber and the sweep chamber was changed by 0 MPa, 0.010 MPa, and 0.020 MPa. Moreover, the reaction temperature is 600 °C in these figures.

We can see from Figure 8 and Figure 10 that the largest concentration of H₂ in the reaction chamber is obtained for the molar ratio of CH₄:CO₂ = 1.5:1 among the investigated molar ratios irrespective of the thickness of Pd/Cu membrane, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. We can claim that this tendency matches with the authors’ previous studies investigating Ni, Ni/Cr, and Ni/Cr/Ru catalysts [6]. The reaction mechanism to explain why the largest concentration of H₂ is obtained for the molar ratio of CH₄:CO₂ = 1.5:1 can be explained as follows [6]: (i) H₂ is produced via the reactions shown in Equations (9) and (13); (ii) the produced H₂ is consumed via the reaction shown in Equation (10), resulting in CO being produced; (iii) a part of the CO produced via the reactions shown in Equations (9) and (10) is consumed via Equation (14); and (iv) the H₂O produced by the reactions explained by Equations (10) and (11) is consumed during Equation (12).

In addition, it is seen from Figure 9 and Figure 11 that the concentration of H₂ in the sweep chamber is the highest for the molar ratio of CH₄:CO₂ = 1.5:1 among the investigated molar ratios regardless of the thickness of Pd/Cu membrane, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. Since the concentration of H₂ is the reaction chamber in the case of the molar ratio of CH₄:CO₂ = 1.5:1 is higher compared to the other molar ratios, the driving force to penetrate H₂ through the Pd/Cu membrane becomes higher because of the high H₂ partial pressure between the reaction chamber and the sweep chamber, i.e., a large concentration difference of H₂ between the reaction chamber and the sweep chamber. Consequently, the higher concentration of H₂ in the sweep chamber is obtained in the case of the molar ratio of CH₄:CO₂ = 1.5:1.

Moreover, we can see from Figure 8, Figure 9 and Figure 10 that the concentrations of CO, CH₄, and CO₂ are larger than those of H₂ in the reaction chamber and the sweep chamber. We consider the reactions displayed by Equations (9)–(16). The following reactions steps might have to be carried out [6]:

(i)

H₂ is produced via Equations (9) and (13).

(ii)

The produced H₂ is consumed via Equations (10) and (11), resulting in the production of CO, CH₄, and H₂O.

(iii)

The produced CO is consumed via Equations (14) and (16), resulting in the production of C, CO₂, and H₂O.

Consequently, this study proposes that the concentrations of CO, CH₄, and CO₂ are higher than those of H₂ in the reaction chamber and the sweep chamber. After all experiments, including 54 experimental conditions, were carried out for each thickness of the Pd/Cu membrane, the weights of the Ni/Cr/Ru catalyst for thicknesses of 20 μm, 40 μm, and 60 μm have increased by 0.108 g, 0.017 g, and 0.315 g, respectively, indicating that a carbon is produced. Although the amount of produced carbon was small, a carbon deposition was observed in this study. Additionally, regarding H₂O production, as explained by Equations (10) and (11), we confirmed this through naked eye observation using a gas bag, as in the previous study [6].

Considering the impact of the thickness of the Pd/Cu membrane, the largest concentration of H₂ is obtained for a thickness of 40 μm mainly among the investigated conditions, i.e., the molar ratio of CH₄:CO₂, the differential pressure between the reaction chamber and the sweep chamber, and the existence, or not, of a sweep gas. Since the concentration of H₂ is higher in the case of 40 μm, the above-described reaction steps (ii) and (iii) might not occur strongly.

As discussed above, it can be thought that H₂ might be penetrated through the H₂ separation membrane when the thickness of the H₂ separation membrane is thinner. However, the optimum thickness of the Pd/Cu membrane is 40 μm in this study. If the H₂ separation rate of the Pd/Cu membrane is larger than the reaction rate of the Ni/Cr/Ru catalyst used in this study, the impact of H₂ separation, providing a non-equilibrium state in DR, is not sufficiently effective [6]. This study proposes that it can be necessary to match the H₂ production rate of the Ni/Cr/Ru catalyst and the H₂ separation rate of the Pd/Cu separation membrane to obtain the non-equilibrium state of DR [6]. Therefore, this study claims that the optimum thickness of Pd/Cu membrane is 40 μm.

As to the impact of the thickness of the Pd/Cu membrane on the stability of the membrane, it is thought that the stability improves with the increase in the thickness of the Pd/Cu membrane, resulting in an increase in durability of the membrane. However, this study has not confirmed that the damage difference among the different thicknesses of the Pd/Cu membrane after 250 h of experiments for each thickness of Pd/Cu membrane.

3.3. Comparison of Assessment Factors for the Investigated Experimental Conditions

To examine the characteristics of the suggested membrane reactor with the Ni/Cr/Ru catalyst and the Pd/Cu membrane,

Table 2. Comparisons of CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO selectivity, H₂ recovery, permeation flux, and thermal efficiency (reaction temperature: 600 °C; pressure difference: 0 MPa; molar ratio of CH₄:CO₂: (a) 1.5:1; (b) 1:1; (c) 1:1.5).

Thickness of Pd/Cu Membrane [μm]	Sweep Gas	CH4 Conversion [%]	CO2 Conversion [%]	H2 Yield [%]	H2 Selectivity [%]	CO Selectivity [%]	H2 Recovery [%]	Permeation Flux [mol/(m2·s)]	Thermal Efficiency [%]
(a)
20	W/O	67.1	−93.6	0.259	0.933	99.1	0.933	0	3.55
	W	71.8	−101	0.167	0.649	99.4	1.34	0	1.46
40	W/O	75.0	−106	0.214	0.766	99.2	1.29	0	2.92
	W	77.1	−108	0.411	1.57	98.4	0.771	0	3.59
60	W/O	69.1	−97.2	5.72 × 10−2	0.238	99.8	1.02	0	0.783
	W	72.3	−102	0.132	0.552	99.4	0.944	0	1.16
(b)
20	W/O	64.9	−59.4	0.241	0.812	99.2	0.290	0	2.76
	W	69.8	−64.5	0.167	0.557	99.4	0.120	0	1.22
40	W/O	79.9	−74.6	0.148	0.585	99.4	1.08	0	1.69
	W	79.7	−74.2	0.236	1.030	99.0	1.27	0	1.71
60	W/O	81.2	−76.0	8.80 × 10−2	0.425	99.6	0.682	0	1.00
	W	76.2	−71.0	0.103	0.483	99.5	0.776	0	0.750
(c)
20	W/O	80.0	−49.0	0.129	0.660	99.3	0.291	0	1.18
	W	75.5	−46.0	0.116	0.561	99.4	0.216	0	0.677
40	W/O	88.2	−54.6	2.21 × 10−2	0.102	99.9	1.13	0	0.200
	W	79.3	−48.6	4.49 × 10−2	0.239	99.8	0.557	0	0.261
60	W/O	80.4	−49.4	1.39 × 10−2	7.41 × 10−2	99.9	0.900	0	0.126
	W	84.2	−51.9	2.71 × 10−2	0.151	99.8	0.462	0	0.158

,

Table 3. Comparisons of CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO selectivity, H₂ recovery, permeation flux, and thermal efficiency (reaction temperature: 600 °C; pressure difference: 0.010 MPa; molar ratio of CH₄:CO₂: (a) 1.5:1; (b) 1:1; (c) 1:1.5).

Thickness of Pd/Cu Membrane [μm]	Sweep Gas	CH4 Conversion [%]	CO2 Conversion [%]	H2 Yield [%]	H2 Selectivity [%]	CO Selectivity [%]	H2 Recovery [%]	Permeation Flux [mol/(m2·s)]	Thermal Efficiency [%]
(a)
20	W/O	70.4	−98.9	0.171	0.638	99.4	1.17	5.00 × 10−4	2.34
	W	71.0	−99.8	0.160	0.624	99.4	0.938	5.00 × 10−4	1.40
40	W/O	72.1	−101	0.315	1.16	98.8	0.953	2.50 × 10−4	4.31
	W	74.2	−104	0.186	0.751	99.2	1.43	2.50 × 10−4	1.62
60	W/O	72.9	−103	0.126	0.549	99.5	0.595	1.67 × 10−4	1.73
	W	70.5	−99.2	7.47 × 10−2	0.311	99.7	0.781	1.67 × 10−4	0.653
(b)
20	W/O	70.8	−65.6	0.128	0.595	99.4	7.80 × 10−2	5.00 × 10−4	1.47
	W	68.9	−63.7	0.115	0.509	99.5	8.70 × 10−2	5.00 × 10−4	0.841
40	W/O	76.3	−71.1	0.118	0.570	99.4	1.19	2.50 × 10−4	1.34
	W	76.1	−70.9	7.82 × 10−2	0.330	99.7	0.384	2.50 × 10−4	0.571
60	W/O	80.1	−75.0	4.97 × 10−2	0.240	99.8	1.01	1.67 × 10−4	0.565
	W	80.1	−75.0	4.40 × 10−2	0.200	99.8	0.682	1.67 × 10−4	0.320
(c)
20	W/O	83.2	−51.2	4.01 × 10−2	0.202	99.8	0.312	5.00 × 10−4	0.366
	W	86.7	−53.6	2.34 × 10−2	0.117	99.9	0.535	5.00 × 10−4	0.136
40	W/O	86.6	−53.5	2.32 × 10−2	0.115	99.9	1.08	2.50 × 10−4	0.210
	W	86.3	−53.4	1.59 × 10−2	8.36 × 10−2	99.9	0	2.50 × 10−4	9.32 × 10−2
60	W/O	82.9	−51.1	2.47 × 10−2	0.140	99.9	0.506	1.67 × 10−4	0.225
	W	83.0	−51.2	1.68 × 10−2	9.51 × 10−2	99.9	0.743	1.67 × 10−4	9.77 × 10−2

and

Table 4. Comparisons of CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO selectivity, H₂ recovery, permeation flux, and thermal efficiency (reaction temperature: 600 °C; pressure difference: 0.020 MPa; molar ratio of CH₄:CO₂: (a) 1.5:1; (b) 1:1; (c) 1:1.5).

Thickness of Pd/Cu Membrane [μm]	Sweep Gas	CH4 Conversion [%]	CO2 Conversion [%]	H2 Yield [%]	H2 Selectivity [%]	CO Selectivity [%]	H2 Recovery [%]	Permeation Flux [mol/(m2·s)]	Thermal Efficiency [%]
(a)
20	W/O	68.2	−95.6	0.151	0.611	99.4	0.661	7.07 × 10−4	2.08
	W	70.8	−99.6	0.132	0.508	99.5	0.567	7.07 × 10−4	1.16
40	W/O	79.8	−113	0.273	1.12	98.9	0.885	3.54 × 10−4	3.74
	W	75.4	−106	0.151	0.601	99.4	1.16	3.54 × 10−4	1.31
60	W/O	73.9	−104	8.90 × 10−2	0.349	99.7	0.749	2.36 × 10−4	1.22
	W	75.6	−107	5.85 × 10−2	0.257	99.7	0.997	2.36 × 10−4	0.510
(b)
20	W/O	69.1	−63.9	9.12 × 10−2	0.384	99.6	0.110	7.07 × 10−4	1.05
	W	69.9	−64.7	8.87 × 10−2	0.396	99.6	0.113	7.07 × 10−4	0.649
40	W/O	78.9	−73.8	8.91 × 10−2	0.405	99.6	1.46	3.54 × 10−4	1.01
	W	79.8	−74.6	6.17 × 10−2	0.265	99.7	2.11	3.54 × 10−4	0.442
60	W/O	76.2	−71.1	4.75 × 10−2	0.228	99.8	1.05	2.36 × 10−4	0.539
	W	80.0	−74.9	4.40 × 10−2	0.248	99.8	0.682	2.36 × 10−4	0.320
(c)
20	W/O	79.7	−48.9	1.41 × 10−2	6.62 × 10−2	99.9	0.885	7.07 × 10−4	0.128
	W	81.8	−50.4	1.35 × 10−2	6.51 × 10−2	99.9	0.926	7.07 × 10−4	7.82 × 10−2
40	W/O	83.0	−51.2	1.50 × 10−2	8.68 × 10−2	99.9	1.67	3.54 × 10−4	0.135
	W	82.5	−50.8	1.35 × 10−2	6.64 × 10−2	99.9	0.923	3.54 × 10−4	7.85 × 10−2
60	W/O	85.8	−53.0	1.67 × 10−2	9.63 × 10−2	99.9	0.748	2.36 × 10−4	0.152
	W	86.6	−53.6	1.64 × 10−2	9.60 × 10−2	99.9	0.762	2.36 × 10−4	9.52 × 10−2

show the comparisons of the CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO₂ selectivity, H₂ recovery, permeation flux, and thermal efficiency for the different thicknesses of the Pd/Cu membrane, the molar ratios of CH₄:CO₂, and the differential pressures between the reaction chamber and the sweep chamber. The reaction temperature is 600 °C in this study.

From Table 2, Table 3 and Table 4, it is seen that the CO₂ conversion indicates a negative value from −46.0% to −113%, irrespective of investigated conditions. This study proposes that the reason for the negative CO₂ conversion is caused by Equation (14). The CO consumed by Equation (14) is produced by biogas DR, i.e., as in Equation (9). We can think that a reaction consuming CH₄ and producing CO₂ occurs [6]. In addition, it can be seen from Table 2, Table 3 and Table 4 that the CO selectivity is much higher than the H₂ selectivity, irrespective of investigated conditions, which is similar to the findings of the authors’ previous studies [6]. On the other hand, we can see from Table 2, Table 3 and Table 4 that the CH₄ conversion indicates a positive value from 67.1% to 88.2%. It can be thought that CH₄ is consumed by DR, as in Equation (13). It can be seen in Figure 5, Figure 7, Figure 9, and Figure 11 that H₂ is moved to the sweep chamber. However, it can also be seen in Figure 4, Figure 6, Figure 8, and Figure 10 that some of the H₂ produced remains in the reaction chamber, indicating that not all of the H₂ moves to the sweep chamber by penetrating through the Pd/Cu membrane. Consequently, this study claims that the following reaction mechanism can be explained [6]: (i) H₂ is produced by the reactions shown in Equations (9) and (13); (ii) the produced H₂ is consumed by the reaction shown in Equation (10), resulting in the production of CO; (iii) a part of the CO produced by the reactions shown in Equations (9) and (10) is consumed during the reaction shown in Equation (14); and (vi) the H₂O produced during the reactions shown in Equations (10) and (11) is consumed via the process presented in Equation (12). In addition, CH₄ conversion and CO selectivity increase with the increase in the thickness of the Pd/Cu membrane, while the H₂ yield decreases. This study proposes that the reason why CH₄ conversion increases is due to the conduction of Equation (11), which produces CH₄ consumption, i.e., decreasing H₂. Moreover, it is thought that the produced H₂ is consumed as in Equation (10), which produces CO. As a result, CO selectivity becomes larger. When the thickness of the Pd/Cu membrane increases, it is thought that the performance of H₂ separation would be lower due to the increase in the penetration resistance of the Pd/Cu membrane. As a result, the H₂ remaining in the reaction chamber is consumed by the reactions, excluding biogas DR.

As discussed before, the production of carbon, as well as that of H₂O, was confirmed. In this study, the amount of H₂O produced could not be measured experimentally and accurately. However, the previous numerical simulation of biogas DR, which was conducted by the authors [18], reported the characteristics of H₂O production. It was reported that H₂O of 0.3 mol/m³ was produced in the case of CH₄:CO₂ = 1.5:1 at 600 °C, where the porosity of the catalyst set for the numerical simulation matched, approximately, with that used for the authors’ previous experimental study [19]. Under this condition, it was reported that H₂ of 3.5 mol/m³, CO of 3.8 mol/m³, and C of 0.1 mol/m³ were produced. In calculating the H₂O selectivity for these products, i.e., H₂O, H₂, CO, and C, via the below equation, the H₂O selectivity was 3.9%.

S_H2O = (C_{H2O, out})/(C_{H2O, out} + C_{H2, out} + C_{CO, out} + C_{C, out})×100

(17)

From the investigation in the present study, the largest concentration of H₂ was obtained for the thickness of 40 μm, the molar ratio of CH₄:CO₂ = 1.5:1, and the differential pressure between the reaction chamber and the sweep chamber of 0 MPa, without a sweep gas, which is 4890 ppmV in the reaction chamber and 38 ppmV in the sweep chamber, respectively. We reveal in Table 2, Table 3 and Table 4 that CH₄ conversion, H₂ yield, and thermal efficiency are 75.0%, 0.214%, and 2.92%, respectively, under this condition. Though the CH₄ conversions obtained in this study are competitive to the previous studies [8,9,10,11,12], the H₂ yield and the thermal efficiency are still low. Therefore, this study suggests the following approaches to improving the performance of the suggested membrane reactor with the Ni/Cr/Ru catalyst and the Pd/Cu membrane: (i) to optimize the catalyst shape and composition (the pore size and the weight ratio of Ni, Cr, and Ru); (ii) to optimize the thickness and the composition of the Pd/Cu membrane; and (iii) to match the H₂ separation rate of Pd/Cu membrane and the H₂ production rate of the Ni/Cr/Ru catalyst, deciding the optimum operation condition. The authors would like to investigate these subjects in the near future.

In addition, according to the investigation undertaken in this study, according to Table 2, Table 3 and Table 4, the highest H₂ recovery—which means the process efficiency—of 2.11% was obtained in the case of CH₄:CO₂ = 1.5:1 at 600 °C for the pressure difference between the reaction chamber and the sweep chamber of 0.020 MPa. On the other hand, the studies reported that the H₂ yields were 20% at 1 bar and 650 °C using the Pd/Ag membrane [20]; 30% at 101.3 kPa and 450 °C with a sweep gas of 10 mL/min using PdAg/NaA-Pd/PSS [21]; and 33% at 550 °C in the case of CH₄:CO₂ = 1.1 using the Pd/Ag/Cu membrane [22]. Comparing the H₂ recovery obtained in this study with those reported in previous studies, the H₂ recovery obtained in this study is low. Consequently, this study proposes the optimization of the H₂ separation membrane, e.g., the chemical and physical properties and geometries, i.e., thickness. And the investigation of a novel catalyst, e.g., Ni/Ru/Ir, is also necessary. These are the proposed future developments of this project.

4. Conclusions

We have conducted this investigation to reveal the influence of the thickness of the Pd/Cu membrane on the performance of BDR using the Ni/Cr/Ru catalyst. We have also investigated the influence of reaction temperature, the molar ratio of CH₄:CO₂, the differential pressure between the reaction chamber and the sweep chamber, and the introduction of a sweep gas on the characteristics of the BDR reactor with the Pd/Cu membrane and Ni/Cr/Ru catalyst. As a result, we can draw the following conclusions:

(i)

We reveal that the concentration of H₂ in the reaction chamber increases with an increase in the reaction temperature irrespective of the thickness of the Pd/Cu membrane, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. The reaction progresses well with a rise in the reaction temperature since DR, RWGS, and SR are endothermic reactions.

(ii)

We reveal that the concentration in the sweep chamber increases with an increase in the reaction temperature irrespective of the thickness of Pd/Cu membrane, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. When the concentration of H₂ in the reaction chamber is larger at higher reaction temperature, the driving force to penetrate through the Pd/Cu membrane is larger because of the high H₂ partial differential pressure between the reaction chamber and the sweep chamber.

(iii)

It is revealed that the largest concentration of H₂ in the reaction chamber and that in the sweep chamber are obtained for a molar ratio of CH₄:CO₂ = 1.5:1, irrespective of the thickness of Pd/Cu membrane, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas.

(iv)

The highest concentration of H₂ is mainly obtained for the thickness of 40 μm among the investigated conditions, i.e., the reaction temperature, the molar ratio of CH₄:CO₂, the differential pressure between the reaction chamber and the sweep chamber, and the existence of a sweep gas. This study claims that the optimum thickness is 40 μm.

(v)

It is clarified that the highest concentration of H₂ is obtained for the thickness of 40 μm, the molar ratio of CH₄:CO₂ = 1.5:1, and the differential pressure between the reaction chamber and the sweep chamber of 0 MPa without a sweep gas, which is 4890 ppmV in the reaction chamber and 38 ppmV in the sweep chamber, respectively. Under these conditions, the CH₄ conversion, H₂ yield, and thermal efficiency are 75.0%, 0.214%, and 2.92%, respectively.

(vi)

In the near future, the following objectives can be considered: (i) to optimize the catalyst shape and composition (i.e., the pore size and the weight ratio of Ni, Cr, and Ru); (ii) to optimize the thickness and the composition of the Pd/Cu membrane; and (iii) to match the H₂ separation rate of the Pd/Cu membrane and the H₂ production rate of the Ni/Cr/Ru catalyst, determining the optimum operation condition.