Circular Catalytic Hydrogen/Methanol Plate Burner with Stackable Clover Channels Supporting Rapid Start-Up and Stable Operation for Highly Efficient Reformer System

Abstract

This study proposes a platinum catalytic plate burner with a clover-shaped microchannel design to reduce the maximum temperature difference (ΔT_max) and improve long-term hydrogen production (HP) performance in an autothermal methanol steam reforming (ATMSR) microreactor. The burner integrates with a plate reformer within a cylindrical adiabatic container. By optimizing catalyst arrangement and incorporating a parallel clover-type microchannel design, thermal gradients inside the burner are minimized, enabling better operation conditions for the plate reformer. Three Pt catalyst gradients (50/50, 40/60, and 30/70) reduce ΔT_max from 48.2 °C and 38.3 °C to 25.8 °C. Additionally, the startup time to 250 °C is reduced from 35, 25, and 14 min, respectively. The integration of the plate burner and reformer with the 30/70 catalyst type shows a higher methanol conversion rate (98%), better hydrogen yield, and lower CO selectivity compared to the 50/50 type. Long-term testing for 30 h shows a low catalyst degradation rate, making it suitable for sustained operation.

1. Introduction

As global warming, air pollution, and fossil fuels shortages are considered serious environmental issues today, researchers and scientists have been dedicated to discovering clean energy sources. Fuel cells that use hydrogen to generate energy offer a method of replacing fossil fuels and reducing carbon emissions [1,2,3,4,5].

In recent years, methanol has been widely studied as a means of portable hydrogen. Methanol can be easily transported as a liquid fuel compared to hydrogen stored in high-pressure gas tanks. Although there are many methods for reforming methanol, due to its lower CO concentration, higher hydrogen production, and reasonable reaction temperature range, hydrogen produced by methanol steam reforming (MSR) reaction is a decent method that can directly provide reformed hydrogen to fuel cells [6,7,8]. For example, Simon Lennart Sahlin and others integrated a methanol reforming reaction with a 5 kW high-temperature PEM fuel cell system. Their system started up a reformer within 45 min, achieving an overall system efficiency of between 27% and 30% [9]. However, due to the endothermic reaction of MSR, a heat source is required in the reforming system. As a heat source, catalytic combustion is a safer and cleaner way to burn fuels such as hydrogen [10] or methanol [11,12], which can start combustion at 30 °C, often using platinum as a catalyst in the combustion chamber. Since the flameless reaction temperature is relatively low, the NO_x emissions during catalytic combustion are also not significant [13]. In addition, the hydrogen off-gas produced by the fuel cell can be burned and used as a heat source to provide heat for the steady-state endothermic methanol steam reforming reaction [14]. Ha Chan et al. [15] illustrated that the utilization of fuel cell exhaust hydrogen gas can significantly improve the the efficiency of the fuel cell system. In the presence of a highly efficient catalyst, ignition can occur at room temperature, which avoids the preheating process of hydrogen, speeds up the start-up time, and reduces the fuel cell system’s reliance on electric heating.

On the other hand, T. Zheng et al. integrated a micro-catalytic hydrogen burner and a methanol reformer [16]. In their reformer system, a porous support with a flat plate structure was used as the burner. The system adopts a stacked configuration design, which not only saves space, but also improves heat transfer efficiency. Through theoretical calculations, it was found that when fuel enters the combustion chamber from top to bottom, this shape results in the most uniform temperature distribution and reaction gas concentration. Experimental results show that compared with the unoptimized carrier, the temperature variation can reach 62 °C, while after optimization, the temperature change is reduced to 23.8 °C. Under uniform heating, the CO selectivity and long-term performance of the methanol reforming reaction can be improved.

In addition to integrating with hydrogen burners, Y. Wang et al. also integrated reformers with methanol burners [17]. Methanol burners can burn methanol at room temperature to replace electric heaters or high-pressure tanks for hydrogen storage. This automatic heating system can operate at 350 W PEMFC for over 1 h. But the burner does not use the exhaust gas of the fuel cell, and its volume is also quite large for a hundred-watts system.

Methanol burners are also widely used in hydrogen fuel cell vehicles [15]. Methanol burners as heat supply are not only important supports for ATMSR hydrogen production, but also dominate the hydrogen production performance of ATMSR microreactor. Especially in plate methanol reformers heated by methanol burners, the temperature changes provided by the burners will greatly affect the efficiency of the overall reformer catalyst performance [18]. Catalysts in high-temperature regions tend to deactivate faster, thereby reducing the hydrogen production efficiency of the ATMSR microreactor and the performance of the reformer system [19,20].

We have previously designed a 10 W-class reformer system [21]. The reformer and burner are made of copper and have good thermal conductivity. The flow channel of the reformer is a Swiss-roll channel with a length of 354 mm and a depth of 2 mm. The flow channel of the combustion chamber has a length of 220 mm and a depth of 2 mm, as shown in Figures S1 and S2 [21]. By adjusting the air flow during hydrogen combustion, the burner can heat up to 300 °C in 10 min and maintain it at 300 °C for 20 h. When using methanol as fuel, the burner can reach up to 280 °C in 5 min. Additionally, after a start-up situation, hydrogen from the fuel cell exhaust can be supplied to the combustor to maintain the reformer temperature. Through this mixed combustion strategy, we can not only provide a self-starting system but also eliminate the waste of unreacted hydrogen in the fuel cell [22].

Based on this 10 W system, we expanded the gas production capacity of the reformer system to 100 times to the KW level by adopting a parallel clover-type microchannel design for the reformer as shown in Figure S1. To better serve the enlarged reformer, we redesigned the burner channel into a cloverleaf channel, which maintains its previous catalytic length and flow resistance due to similar fluid channel lengths. This burner, using an evenly distributed catalyst, can be integrated directly into the reformer using vertical stacking and providing direct heat, reaching a start-up temperature of 250 °C in 40 min. After 18 h of continuous operation, with a methanol feed amount of 1.8 mL/min, the fuel conversion rate, system degradation rate, and energy efficiency can approach 95%, 0.17%/h, and 72%, respectively. However, uneven reaction rates between the burner inlet and outlet resulted in a maximum temperature difference of 49 °C, resulting in poor start-up time, CO selection, and long-term operational performance of the reformer system.

Therefore, in this research, the optimization of a clover type combustor as a heat supply for the 1 kw-reformer system is realized by re-designing the density and arrangement of the catalyst: the number of catalysts at the fuel inlet is reduced and gradually increased towards the outlet with three different gradient ratios. This approach mitigates the reaction non-uniformity caused by pressure drop reduction, enhances overall heat supply uniformity, and achieves a more uniform overall temperature resulting in highly efficient reactions.

2. Materials and Methods

2.1. Design of the Circular Catalytic Hydrogen/Methanol Plate Burner

The burner is made of aluminum (AL6061) with excellent thermal conductivity (151–202 W/m·K) and is designed with a cloverleaf-shaped circular channel. It has a diameter of 180 mm and a total of 16 inlets and 16 outlets. The total length of the channel is approximately 310 mm, with a width of 4 mm, and a depth of 2 mm, placed on an aluminum plate with a thickness of 12 mm, as shown in Figure 1.

2.2. Catalyst Preparation

After mixing deionized water with polyvinyl alcohol (PVA) creating a 2wt% PVA aqueous solution, the mixture was placed on a hot plate and heated to 90 °C until the PVA completely dissolved in water, which was then cooled back to room temperature for later use. Next, the commercial platinum catalyst (78-1661, Sigma-Aldrich Inc., St. Louis, MI, USA) was mixed with the 2wt% PVA to form a 5wt% catalyst mixture, which was then subjected to 1 h of ultrasonic agitation in an ultrasonic shaker. The catalyst mixture was subsequently placed in an electromagnetic stirrer, set at a rotational speed of 600 rpm, and stirred for 6 h. The detailed preparation process is shown in Figures S2 and S4–S6.

2.3. Methanol Combustion Test

The Experiment setup of methanol combustion test is shown in Figure 2. When applying Methanol as combustion fuel, a bubbler is added in the system to evaporate methanol into air and to mix completely with air. Air was supplied into burner using an air bottle and mass flowmeter, and enters into the burner after uniform stirring and mixing. The reactant and burnt gas of the combustion is analyzed using a Gas Chromatograph (GC1000 & GC3000, China Chromatography, New Taipei City, Taiwan) to measure the conversion rate of Methanol. The temperature of the burner is measured with T-type thermal couple (MX100, Yokogawa, Tokyo, Japan) inserted to three temperature measurement points of the burner chamber plate.

3. Results

3.1. Catalyst Arrangement and Reaction Performance Investigation for Methanol Catalytic Combustion

Figure 3 shows a diagram of a methanol combustor with different catalyst arrangements. Here, the 5% Pt content of the Pt/Al₂O₃ catalyst was coated through an adhesive PVA on a nickel foam with 50 ppi and dimensions of 5 mm × 5 mm × 10 mm. To achieve a better temperature uniformity for high reformer performance, there are three different arrangements of the catalyst in the burners with a gradient between entrance and exhaust region as 50/50, 40/60, and 30/70, respectively, as shown in Figure 3. The 50/50-type structure employs a uniform arrangement of the catalyst. The 40/60-type structure slightly reduces the number of catalysts at the combustor inlet and gradually increases it towards the outlet. The 30/70-type structure, compared to the 40/60-type structure, significantly reduces the number of catalysts at the inlet and concentrates them more towards the combustor outlet.

The reaction performance of the catalytic combustors with different catalyst arrangements was studied under a fixed 9000 sccm air flow and a 1.6 mL/min methanol consumption rate. Figure 4 shows the infrared thermography of methanol combustion using catalytic combustors with different catalyst arrangements. Compared to the 50/50-type and 40/60-type, the 30/70-type demonstrated not only a more uniform temperature distribution but also a larger high temperature coverage area located at the center of the burner, suggesting a more uniform and more efficient heat source for heating up the reformer.

In addition to Infrared thermography for qualitatively verifying the temperature distribution among the burners, there are also six thermal couples arranged for temperature monitoring to the burner, as per the locations shown in Figure 5a. Figure 5b–d shows the thermal distribution on the combustion reaction chamber plate with a fixed methanol flow rate of 1.6 mL/min and 9000 sccm under different catalyst arrangements (50/50 type, 40/60 type, and 30/70 type). Compared to 50/50 type and 40/60 type supports, the 30/70 type exhibited a lower maximum temperature difference (△T_max) among six locations on the combustion reaction chamber plate. The △T_max values for 50/50 type, 40/60 type, and 30/70 types were measured as 48.2 °C, 38.3 °C, and 25.8 °C, respectively. Compared to the 50/50 type structure (Figure 5e), the △T_max of the 30/70 type support was reduced by 22.4 °C. In Figure 5b–d, it can also be observed that the startup times for the 50/50 type, 40/60 type, and 30/70 type are 35 min, 25 min, and 14 min, respectively. Compared to the 50/50 type structure, the startup time for the 30/70 type was shortened by 21 min. These comparisons verify that using the 30/70 type catalyst can effectively decrease both the temperature difference and startup time on the combustion reaction chamber plate, thus suggest a more efficient heat source for supporting reformers to achieve better performance.

3.2. Integration with Plate Reformer

The integration of the plate reformer system with the burner is shown in Figures S3 and S7. Figure 6 shows the hydrogen production performance of the self-thermal methanol steam reforming microreactor, integrated with the clover plate burner, using different catalyst arrangements under various methanol flow rates (2–6 mL/min) at a fixed temperature of 250 °C. Compared with the 50/50 type (A type in the figure) design, the reaction chamber plate for the 30/70 type (C type in the figure) exhibited a better methanol steam reforming reaction performance. Especially compared with the 50/50 type, the 30/70 type not only increases the methanol conversion rate by 10.2%, but also increases the H2 flow rate by 10% at a methanol–water mixed injection rate of 6 mL/min and 250° operation temperature, while CO selectivity decreased by 1.7%. The results show that when the heat distribution ΔT_max of the burner plate is small (or more uniform), it is more conducive to improving the methanol steam reforming reaction efficiency of the SRM reformer.

Figure 7 shows the 30 h long-term performance of the ATMSR system integrated with the 30/70 type burner. After optimizing the heating system, it can be observed that it provides heat more evenly to the reformer for the reforming reaction, thus reducing the occurrence of local hotspots. When continuously supplying 4 mL/min of methanol solution for 30 h, the methanol conversion rate of the reformer decreased from an initial 97.1% to 92.3%, and hydrogen conversion rate of the burner decreased from 99.9% to 97.5%, providing a degradation rate of 0.16%/h and 0.08%/h for reformer and burner, respectively. This is much lower than our previous reformer systems containing similar catalysts and integrated with 50/50 type burner (reformers: ranging from 97 to 89.3%, burners: ranging from 98.6 to 95.6%) [22]. The hydrogen conversion rate of the burner system remained above 95% even after 30 h of continuous operation, demonstrating decent long-term performance.

Figure 8 shows the particle agglomeration situations after long-term reactions with different catalyst arrangements in the combustor. As shown in Figure 8, the particle size of the catalyst at the front and back end of the 50/50 Type combustor reaction is 24.9 nm and 3. 0 nm, respectively, with more severe sintering and aggregation at the front end, but not much utilization of catalyst at the back end. However, for the 30/70 type combustor, the particle size of the catalyst at the front and back ends is 19.9 nm and 13.2 nm, respectively, demonstrating a more uniform catalyst preservation/utilization throughout the reaction channel for long term operation.

4. Discussion

According to the heating pattern obtained with the 50/50 catalyst configuration, as shown in Figure 4 (left) we found that a violent reaction occurred after the fuel entered the reactor, causing the fuel gas to rapidly heat up and expand. This results in increased flow resistance at the reactor inlet and uneven reaction rates as well as uneven heating. Therefore, after the catalyst configuration is changed to the 30/70 type, the reaction rate when the gas reached the front end of the catalyst will be reduced, thereby reducing the sudden expansion of the reaction gas, temperature, and back pressure. The amount of the catalyst will gradually increase along the channel and gently enhance the reaction, providing a more uniform reaction rate compared to the 50/50 type, resulting in a more stable heating pattern and more even heat distribution.

The Transmission Electron Microscope (TEM) images also show that in the unoptimized burner (50/50 type), there is obvious sintering of the platinum catalyst between the front and rear of the reactor; while in the improved design (30/70 type), the sintering conditions of the front and rear catalysts are similar. Judging from the results of the sintering condition of the catalyst, the large temperature difference within the reactor resulted in uneven sintering state of the catalyst. This may accelerate catalyst deactivation at the reactor inlet and shorten combustor catalyst life.

In terms of MSR system integration—the integration among an evaporator, reformer and burner—using a 50/50 burner and the methanol–water flow rate at 2 and 4 mL/min allows the evaporator to fully vaporize the methanol–water solution and then enter the reformer system for reaction. However, at a higher flow rate of 6 mL/min, some non-evaporated methanol aqueous solution was observed in the evaporator during the experiment, resulting in a methanol conversion rate reduced to only 88%. In comparison, when using the 30/70 type burner, the burner’s higher fuel conversion rate provides enough heat for the evaporator to maintain an approximately 98% methanol conversion rate even at a flow rate of 6 mL/min.

In addition, in terms of CO selectivity, the 30/70 burner helps the reformer reduce the formation of local hot spots when integrated, thereby reducing the CO concentration. The reduction in CO concentration helps reduce the risk of fuel cell poisoning when the reformer system integrated into fuel cell systems in the future.

Finally, we compared our research with recent related studies, as shown in Table 1. We compared the Burner ΔT_max, Burner hydrogen conversion rate, Reformer methanol conversion rate, and Reformer carbon monoxide selectivity, respectively. According to the comparison in Table 1, it can be observed that for ΔT_max, Reference [15] outperforms ours; however, for burner hydrogen conversion rate and reformer methanol conversion rate, our study is superior to the other two. In addition, for Reformer carbon monoxide selectivity, Reference [16] outperforms ours, indicating that their hydrogen production is more compatible with fuel cells. This comes from the reformers containing different catalysts, and it indicates that our reformer requires some more improvement for the catalyst in the future.

5. Conclusions

In this research, we proposed a platinum catalytical plate burner with a clover-shaped microchannel design, which can be stacked with a plate reformer for integration within a cylindrical adiabatic container. By investigating three different catalyst placements (50/50, 40/60, and 30/70), we optimized the temperature uniformity of the combustion chamber to achieve a low temperature difference (ΔT_max) of less than 25 °C, improved the stability of the long term operation of a methanal steam reforming system with high performance. The catalyst state after the 30 h long-term operation also illustrates the relevant results of transmission electron microscopy (TEM) analysis: optimized combustion chamber (30/70 type) vs. unmodified Combustion chambers (50/50 type) exhibited more uniform sintered catalyst particles throughout the fluid passage, helping to extend catalyst life. Additionally, when integrated with the Methanol Steam Reforming (MSR) system, the CO concentration in the reformer can be effectively reduced by 1.7% with a catalyst degradation rate of 0.16%/min, enabling highly efficient operation.

Author Contributions

Conceptualization, F.-G.T.; methodology, F.-G.T.; validation, W.-H.H.; investigation, W.-H.H.; resources, F.-G.T.; writing—original draft, W.-H.H. and H.-J.L.; writing—review and editing, F.-G.T.; visualization, X.-J.W.; supervision, F.-G.T.; project administration, F.-G.T.; funding acquisition, F.-G.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. Design of the Circular Catalytic Hydrogen/Methanol Plate Burner. (a) Schematic top view of the circular plate burner. (b) 3D design of the plate burner. (c) Fabricated burner of Aluminum plate.

Figure 2. Experiment setup of methanol combustion test.

Figure 3. Structural diagram of catalytic combustor with different catalyst arrangement designs: 50/50-type (left); 40/60-type (middle); 30/70-type (right).

Figure 4. Infrared thermography of methanol burners with different catalyst arrangements.

Figure 5. Temperature monitoring of the burners with different catalyst arrangements. (a) The locations for thermal couple measurements, and temperature measured results of burner startup process for, the colored dots in the figure represent the lines of the corresponding colors in subfigures (b)–(d); (b) 50/50 type; (c) 40/60 type; (d) 30/70 type; (e) the comparison of the maximum temperature differences △Tmax among different burner types.

Figure 5. Temperature monitoring of the burners with different catalyst arrangements. (a) The locations for thermal couple measurements, and temperature measured results of burner startup process for, the colored dots in the figure represent the lines of the corresponding colors in subfigures (b)–(d); (b) 50/50 type; (c) 40/60 type; (d) 30/70 type; (e) the comparison of the maximum temperature differences △Tmax among different burner types.

Figure 6. Reaction performance of the ATMSR integrated with burners of different catalyst arrangements under various fueling rates (2, 4, and 6 mL/min of methanol solution). (a) Methanol conversion; (b) H2 flow rate; (c) CO selectivity.

Figure 7. Long-term HP performance of an ATMSR reactor integrated with a 30/70 type burner for 30 h reaction.

Figure 8. Particle agglomeration of combustion reaction catalysts after long-term operation: (a,c) front and (b,d) back end particle agglomeration of 50/50 type (upper figure), and 30/70 type (lower figure).

Table 1

Comparison of the performance of various burners when integrated with methanal steam reformer.

	This Work	[15]	[16]
Burner ΔTmax	25.8 °C	20 °C	28.7 °C
Burner hydrogen conversion rate	99.9%	98.21%	NA
Reformer methanol conversion rate	Decrease from 97.1% to 92.3% (after 30 h)	NA	Decrease from 90% to 57.79% (after 20 h)
Reformer carbon monoxide selectivity	7.58%	NA	4.02%

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app142412069/s1, Figure S1: Swiss-roll reformer; Figure S2: Swiss-roll burner; Figure S3: Cloverleaf Counterflow Micro-channel Plate Gas Flow Structures; Figure S4: Burner solvent; Figure S5: Nickel mesh; Figure S6: Burner catalyst preparation process; Figure S7: Integrated autothermal methanol steam reforming system.

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