Ha Chan, Wang Sibo, Qin Jiang, Wang Cong, Liu Zekuan.
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001).
Abstract In order to solve the problem of endothermy in the reformer, the catalytic combustion reaction is coupled in the reactor, and the heat of the reforming reaction is supplied by the combustion reaction, which can improve the thermal efficiency of the system. However, due to the different chemical reaction rates of the two reactions, the matching degree of the endothermic reaction affects the hydrogen production efficiency. Only by strengthening the process coupling and studying the heat matching between the catalytic combustion chamber and the reforming chamber can the integrated reactor with compact structure and high energy efficiency be fabricated. In order to solve this problem, this paper carried out relevant experimental research, and the optimal coupling scheme of the two reaction chambers under different flow directions and different wall coating methods of the catalytic combustion chamber was carried out, and the results showed that no matter which integration method was chosen, the temperature of the front section of the reformer should be high and the wall temperature should be uniformAmong them, the vertical arrangement mode has great advantages, and the hydrogen production content can reach more than 74%;When the catalytic combustion chamber uses metal foam as the catalyst carrier, the hydrogen production content can reach more than 60%.
0 INTRODUCTION.
With the development of the economy, the use of fossil energy is increasing, but due to the non-renewable nature of fossil energy, the energy security situation has become increasingly severe, and countries around the world are actively carrying out the transformation of energy strategy. Therefore, some new energy technologies, such as solar energy, tidal energy, wind energy, biomass energy, hydrogen energy, etc., have been widely used in recent years. Among them, proton exchange membrane fuel cell (PEMFC) technology is the core technology for hydrogen energy utilization. PEMFC, which uses hydrogen energy as raw material, has become an important energy conversion device because of its advantages of zero carbon emissions, high energy conversion efficiency, and noiseless operation.
In recent years, proton exchange membrane fuel cells have gained acceptance for commercial applications. However, the stringent requirements for hydrogen sources limit the adoption of PEMFC. At present, PEMFC still uses high-purity hydrogen as fuel, which is not only expensive to produce, but most importantly, has a very low volumetric energy density. Therefore, in order to improve the commercial application value of PEMFC and give full play to the advantages of PEMFC, the primary problem at present is to obtain a hydrogen source with high volumetric energy density and easy to produce in the field. The conventional hydrogen production methods mainly include: low-temperature liquid hydrogen storage, high-pressure gaseous hydrogen storage, metal hydride hydrogen storage and liquid fuel reforming hydrogen production. Liquid fuel reforming hydrogen production technology has a higher volume hydrogen storage density, which is an ideal means of hydrogen supply for mobile vehicles and ship equipment, that is, hydrocarbons such as alcohols and hydrocarbons undergo chemical reactions under the action of catalysts to generate small molecule products such as hydrogen. In terms of the fuel for hydrogen production, methanol has very obvious advantages compared to other fuels. Methanol is liquid at room temperature and pressure, and can be prepared from renewable biomass. In terms of the molecular composition of the fuel, the hydrogen-to-carbon ratio of methanol is 4, and the hydrogen content in the product is high, and it is not easy to deposit carbon. Therefore, the development of efficient reforming technology using methanol as raw material is of great significance to solve the hydrogen source problem of PEMFC.
There are four main reactions used to produce hydrogen from methanol: steam reforming (MSR), partial oxidative reforming of methanol (POM), autothermal reforming of methanol (ATRM), and methanol cracking (MD). In the above reactions, MSR is widely used due to its highest hydrogen yield, but the reaction is endothermic and requires external auxiliary heating to meet the reaction demand, so the dynamic response characteristics of the reforming reactor are poor. Compared with MSR, the PoM reaction can be heated to the reaction temperature after ignition of methanol, which can be started quickly, so it has good dynamic response characteristics. However, autothermal reforming requires a vigorous combustion reaction, which makes it difficult to control the temperature in the reactor, and the rapid combustion reaction will cause sintering or carbon deposition of the catalyst. In order to avoid the above problems, the self-heating reactor that couples the absorption and exothermic reactions in the same reactor has been developed, that is, the reaction coupling on the same catalyst is separated, so that the endothermic reaction and the exothermic reaction occur on the adjacent sides of the reactor respectively, so that the methanol reforming reaction has a high hydrogen production content and fast response characteristics at the same time, and avoids the dependence on external heat sources.
In terms of the structural types of self-heating reactors, they are mainly divided into four categories, namely tubular self-heating reactors, plate self-heating reactors, microchannel self-heating reactors, and membrane reactors. Tubular self-heating reactors include tubular and casing reactors, and the tubular reactors are compact in structure, but they do not have advantages in system integration. The plate reactor separates the chambers of the suction and exothermic reactions through plates coated with catalysts on both sides, and heats the reforming through combustion and heat release, which is convenient for disassembly, amplification and integration. In recent years, with the application of microchemical technology, microchannel autothermal reforming reactors have been developed. Microchannel reactors can enhance heat and mass transfer and increase the specific surface area of the reactor, so more research on methanol reforming technology is focused on the development of microchannel reactors.
In order to improve the responsiveness characteristics of self-heating reactors, strengthen the coupling process of endothermic reactions, and improve the hydrogen production rate, relevant studies have been carried out by scholars. In the flow direction of the reactor, Hsueh et al. developed a numerical model of a plate reactor to provide the required heat for the reforming reaction of adjacent channels through methanol combustion, and the literature found that the RE number of the combustion chamber was higher than that of the reforming chamber to improve the methanol reforming conversion rate, and the methanol conversion rate under countercurrent conditions was about 10% higher than that of co-flow flow. In terms of the shape of the reactor flow channel, Hsueh Collaborators et al. used numerical simulation to study the effects of parallel channels and serpentine channels on the plate reformer and catalytic combustion chamber, and the literature found that the methanol conversion rate of the reformer and combustion chamber with serpentine channels was increased by 23% compared with that of parallel channels. In terms of the catalyst arrangement of the reactor, Herdem et al. used numerical simulation to study the heat distribution of the microchannel methanol steam reformer. It was found that the segmented catalyst layer reduced the mass of the catalyst by 25% and the methanol conversion rate by 90% compared to the continuous catalyst coatingTianqing Zheng et al. experimentally studied the effect of catalyst distribution in a microchannel-based methanol catalytic burner on the reforming effect, and found that the temperature of the combustion chamber can be controlled by changing the distribution of catalyst in the combustion chamber, thereby controlling the heat of the reforming reaction**. At the same time, in order to ensure the stability of the temperature in the reforming chamber and avoid the influence of hot spots in the catalytic combustion chamber on the reaction, Zheng et al. added an air cavity between the combustion chamber and the reforming chamber by numerical simulation, and studied the influence of the shape of the air chamber on the hydrogen production effect. The trapezoidal cavity with thick inlet and thin outlet can make the wall temperature of the reforming cavity more uniform and the hydrogen production effect better.
Through literature research, it can be seen that the above studies are very effective in improving the performance of self-heating reactors and can well improve the yield of hydrogen, but it can be found that the research on the current methanol thermally coupled integrated reactor is mainly based on numerical simulation, and the absorption and exothermic reactors all adopt the same catalyst loading mode, that is, both use foam metal or are coated on the wall, and different catalyst loading schemes can macroscopically regulate the reaction, providing a technical route for better heat matching. In the flow direction of the two reactive fluids, the mode of vertical flow is less studied. In summary, the on-site hydrogen preparation technology using methanol adsorption and exothermic coupling reactor is a reliable way to improve the hydrogen production rate and improve the system response speed. However, in such self-heating compact reformers, the main reason for performance degradation and failure is the presence of local temperature gradients in the coupling of chemical reactions in various transport processes and multifunctional materials. Therefore, temperature matching is essential for thermally coupled integrated reactors. To solve this problem, it is necessary to continue to explore efficient catalytic combustion and catalytic reforming integration schemes and strengthen process coupling. Therefore, in this paper, relevant experimental studies are carried out to determine the optimal coupling scheme of the two reaction chambers under different flow directions and different wall coating methods of the catalytic combustion chamber, and at the same time, combined with the microchannel reactor, the heat and mass transfer ability in the reaction process is further strengthened and the system response is accelerated.
In this paper, a microchannel methanol reforming reactor is designed and fabricated, and a test bench for the reforming reaction is built according to different integration methods.
Then, the wall temperature and the components of each substance in the product were detected by experiments, and the concentration of hydrogen in the products with different flow directions was explored, and the influence of different catalytic loading modes on the hydrogen concentration of the catalytic reforming chamber was explored, which provided a new integrated scheme for the subsequent methanol thermal coupling integrated reactor.
1 Experimental research.
Structural design of the reforming reactor.
The serpentine reactor has a longer flow than the straight channel reactor, which can increase the contact time between the catalyst and the fluid and improve the methanol conversion rate. In this paper, a zero-dimensional design is adopted, and the heat transfer coefficients are calculated by using countercurrent heat transfer and flow characteristic coefficients on both sides of the fluid, and the specific design process of the reformer is shown in the literature. Through several calculation iterations, the structural parameters of the serpentine channel reactor were obtained, and the size of the reactor channel designed in this paper was 1 mm 1 mm and the length was 1250 mm. The specific reactor structure diagram is shown in Figure 1.
Based on the three-dimensional design drawing of the serpentine channel reformer, the reformer is processed, the high-temperature alloy material is used, and the catalyst is coated, because the channel size of the microchannel reactor is small, the requirements for the catalyst are relatively high, and the nano-level powdered catalyst is used to coat the methanol reforming catalyst composed of Ni La2O3 CeO2 on the wall surface by wall coating. The physical diagram of the reformer is shown in Figure 2.
Structural design of catalytic combustion reactor.
The structure design of the catalytic combustion reactor was carried out by numerical simulation software, and the catalytic combustion surface mechanism file was imported. In order to better match the heat demand of the reformer, the combustion reactor also uses 5 channels, as shown in Figure 3. In order to optimize the coupling mode of the two reaction chambers of the catalytic combustion reaction and the catalytic reforming reaction in the same direction, vertical flow and different wall coating methods of the catalytic combustion chamber, the catalytic combustion chamber was simulated by using 5 heating rods with adjustable power, and the power of each heating rod was adjusted by calculating and determining the heat flow corresponding to the 5 channels before the experiment.
In the co-directional flow, the heating rod adopts the same arrangement as the flow channel of the catalytic reforming, and by changing the power of the heating rod, the forward flow with the flow of the reformed fuel gradually warms up or the reverse flow gradually cools down. In vertical flow, the heating rods are arranged perpendicular to the direction of the flow channel of the catalytic reforming, as shown in Figure 3.
Research on the integrated experimental scheme of methanol self-heating coupling reactor.
The experimental bench schematic diagram of the integrated research of methanol self-heating steam reforming reactor is shown in Figure 4, and the test system is mainly composed of four parts, namely: methanol solution supply system, fuel preheating system, fuel reforming system and product analysis system. Before the test bench is built, the specifications of the key components should be selected, such as the DC regulated power supply in the power supply cabinet with a maximum power of 20 kWThe constant flow pump adopts Elite P500+ high-pressure infusion pump, with a maximum flow rate of 500 mL min and a pressure resistance of 20 MPaThe mass flow meter adopts the micro mass flow meter produced by the German company Bronkhorst, with a maximum flow rate of 5000 g h and a withstand pressure of 01 mpa；The fluid temperature is measured with a diameter of 03 mm nickel-chromium-nickel-silicon armored thermocouple (type k), in order to improve the response speed of the system, the head of the armored thermocouple needs to be ground off to reduce the heat capacity of the thermocoupleThe differential pressure is measured using a Rosemount 3051 differential pressure transmitter from the United States, while the pressure measurement is measured by a 10 MPa range pressure transmitter from Shaanxi Mack.
During the experiment, the control power supply of the heating rod is first turned on and set according to the calculated power demand. When all temperatures in the test system are stabilized, a methanol solution is introduced. Before the fuel, pure methanol and purified water are mixed according to the water-carbon ratio, and the solution is pumped to the preheating pipe section through the solution pump, and copper electrodes are arranged at both ends of the preheating pipe, and the pipe is electrically heated to reach the reforming temperature of the fuel. The inlet of the preheating pipe is connected to the positive pole of the heating power supply, so an insulating terminal is arranged at the inlet. Pressure sensors and armored thermocouples are arranged at the inlet and outlet of the preheating tube to detect the pressure difference and temperature difference between the inlet and outlet of the preheating tube, so as to ensure that the fluid temperature of the outlet can meet the needs of the reforming reaction. The surface of the microchannel reformer is welded with a diameter of 03 mm NiC-NiSi K thermocouple for monitoring the wall temperature distribution of the reformer. Pressure sensors are also arranged at the inlet and outlet of the reformer, as well as armored thermocouples, which are used to calculate the flow resistance in the reactor. After the reaction, the high-temperature gas flow is cooled through the condensate tank, and when the gas is not detected, the product is directly discharged to the waste liquid storage tank, and when the gas needs to be detected, the three-way valve is opened to detect the flow rate of the gas production, and then the gas composition is detected by the gas chromatograph, and the reforming effect can be judged according to the gas detection results. The detectors used in the gas chromatograph are thermal conductivity detector (TCD) and hydrogen flame ionization detector (FID), according to the external standard method, that is, the standard gas is used to calibrate the peak time and peak area of the component before detection, and the gas component content in the experimental product is calculated according to the standard gas.
According to the principle of the test bench, the test bench for methanol reforming was designed and built, and the physical diagram of the test bench is shown in Figure 5.
2 Results and Discussion.
In order to realize the efficient coupling and integration of the fuel evaporation chamber, the fuel reforming chamber and the catalytic combustion chamber, and to increase the hydrogen production rate of the methanol reforming reaction while reducing the volume of components, it is very important to study the heat matching between the catalytic combustion chamber and the reforming chamber for the integration of the whole system. In this paper, through the thermocouple on the surface of the reaction chamber and the content of each component in the product, several integration schemes of catalytic combustion and catalytic reforming reaction chamber are proposed, and the optimal coupling scheme of the two reaction chambers under different flow directions and different wall coating methods of the catalytic combustion chamber is determined.
Effect of different flow modes on the performance of the coupled integrated reactor.
The fluid in the catalytic combustion chamber provides heat for the catalytic reforming through the combustion reaction. The chemical reactions in the two reaction chambers will be affected by the wall temperature and product concentration, which will lead to the mutual influence and mutual restriction of the chemical reactions in the upper and lower chambers. In order to develop a more efficient integration method, the catalytic combustion chamber and catalytic reforming chamber arranged in the same direction, and the catalytic combustion chamber and catalytic reforming chamber arranged vertically were experimentally studied.
The effect of the co-arrangement on the performance of the reactor.
The heating chamber is arranged in the same direction as the catalytic reforming chamber, and the heating chamber and the reforming chamber are arranged in layers, as shown in Figure 6, and the temperature measurement points are arranged on the reforming chamber along the direction of fluid flow. In order to select the inlet and outlet layout of the catalytic combustion chamber, the following scheme is adopted: heating along the reforming fluid and cooling along the reforming fluid. The power distribution of the heating channel in the hot plate is shown in Figure 7, and the total power controlled during the experiment is 580 W.
After adjusting the wall temperature according to the relevant power, when the wall temperature of the reformer is stable, the gas collection begins, and the gas composition is detected by the gas chromatograph. The final measured relative hydrogen content is shown in Figure 8. It can be seen that a large hydrogen production rate can be obtained by the arrangement of cooling along the reforming reaction channel, and the volume content of hydrogen in the product can reach 6015%, while the arrangement along the reforming reaction flow channel produces a lower hydrogen content of about 4044%。It can be found that under the action of chemical reaction, the arrangement of counterflow does not achieve better reaction effect, so the flow channel layout of the self-heating reformer cannot be arranged and optimized according to the traditional heat exchanger. Due to the existence of chemical reactions, there is a deviation from the conclusion that the countercurrent heat transfer effect of traditional heat exchangers is better.
Combined with the further analysis of the wall temperature, it can be seen that the cooling arrangement scheme should have a high wall temperature at the entrance, but due to the violent chemical reaction, a large amount of heat is absorbed, so the temperature at the entrance is low, and the wall temperature and heating power have an opposite trend, this is because, with the progress of the reaction, methanol participates in the reaction, the hydrogen content will increase, so the wall temperature will gradually change from low to high under the action of high heating power.
But in general, the cooling arrangement has a higher wall temperature before point 7, and in the second half of the channel, the temperature will be lower than the other option. The first half not only has a high methanol concentration, but also good caloric properties**. In the wall temperature distribution map, it can be found that the temperature of the No. 5 measurement point has decreased significantly, and combined with Figure 6, it can be seen that the No. 5 is located at the inflection point of the reactor channel, at this time, the flow direction has changed, which promotes the mass transfer of methanol, increases the chemical reaction rate, and the temperature decreases due to the heat absorption of the reaction. After the No. 9 measurement point, the wall temperature increases, which is due to the fact that methanol has basically participated in the reaction, there is no chemical reaction in the reactor, and the wall temperature begins to increase under the action of the heating rod.
Therefore, combined with the hydrogen production rate, the following conclusions can be drawn: the factors affecting the conversion rate of chemical reactions mainly include reaction temperature, reactant concentration, residence time, etc. The arrangement along the reactor cooling scheme has a high reactant concentration and reaction temperature in the first part of the reformer, and in the second half of the reactor, the temperature is low and the concentration is low, resulting in a very high hydrogen production rate. Therefore, for the reforming reaction, the high temperature in the first half can promote the chemical reaction, while in the second half, the concentration is lower, and even if the reaction temperature is very high, the reaction cannot be fully carried out without the timely replenishment of methanol. Therefore, in the co-directional arrangement, the heating along the reaction channel (counter-current arrangement) does not have enough heat** in the early stage of the reaction, and not enough methanol** in the late stage of the reaction, so the overall conversion rate is relatively low. Cooling along the reaction channel (downstream arrangement) is even more advantageous.
The effect of vertical arrangement on reactor performance.
The experiment also uses a heating chamber arranged perpendicular to the catalytic reforming chamber, and the heating chamber and the reforming chamber are arranged in layers, as shown in Figure 3. Similarly, for the inlet and outlet arrangement of the selected catalytic combustion chamber, the following scheme was adopted: heating along the reaction channel and cooling along the reaction channel. The power distribution of the heating channel of the hotplate is shown in Figure 7. The total control power during the experiment was 580 W.
The experimentally measured relative hydrogen content is shown in Figure 9. It can be seen that the arrangement of heating along the reforming reaction channel can obtain a large hydrogen content, which can reach more than 74%, while the arrangement of cooling along the reforming reaction channel can obtain a large hydrogen content, which can reach more than 64%. Combined with the wall temperature variation Figure 9, it can be seen that the heating along the reforming reaction channel is arranged in such a way that the temperature in the first half of the channel is high and the temperature in the second half is low, so it also has a high hydrogen content in the product. Therefore, for the vertical arrangement, the reaction temperature plays a dominant role in the inlet section of the reformer, and the high concentration and high reaction temperature can promote the chemical reaction. In the second half of the reactor, the reactant concentration plays a dominant role. Similar to the conclusion of co-directional flow.
At the same time, it can be found that the vertical flow channel layout scheme is lower than the same direction layout scheme, and the temperature fluctuation caused by the heating arrangement and cooling arrangement is lower, so in the product, the vertical arrangement scheme can obtain higher hydrogen content.
In summary, it can be concluded that for the methanol autothermal reforming reactor, the first half of the reaction temperature should be guaranteed, and at the same time, the vertical layout scheme is more advantageous in the layout scheme, so the vertical arrangement and heating along the reaction channel should be preferred.
Effect of catalyst coating on the performance of the coupled integrated reactor.
For the catalyst coating method of the catalytic combustion chamber, the wall coating technology is often used. However, in recent years, metal foam loading technology has gradually attracted attention due to its higher catalyst specific surface area and good thermal conductivity. For the catalytic combustion chamber of metal foam, due to the good thermal conductivity of metal foam, the wall temperature is uniform, which is approximately normal wall temperature. Therefore, the normal wall temperature analogue foam metal loading method is adopted.
Comparison of metal foam and wall coating under co-directional flow.
The hydrogen content in the catalytic cavity under co-directional flow using metal foam or wall coating technology is compared, and the hydrogen content is shown in Figure 10. According to the experimental results, it can be found that the loading scheme of metal foam not only has a uniform wall temperature, but also has a high hydrogen content in the product, which can reach 54%, while the hydrogen content of the wall coating combined with heating scheme is only 40%. However, the hydrogen content of the wall coating combined with cooling is 59%, which is higher than that of the metal foam solution. Therefore, the self-heating reformer with metal foam as the catalyst carrier has certain advantages, compared with the traditional wall coating, the hydrogen content can be increased by 28%, but after the optimization of the reaction chamber arrangement, that is, under the high methanol concentration, timely heat replenishment (downstream arrangement) can effectively increase the hydrogen content, and the optimized self-heating reactor content can.
Vertical flow of metal foam compared to wall coating.
Based on the above, it can be seen that a vertical arrangement scheme can achieve better hydrogen production due to the uniform wall temperature. Further, under vertical flow, the hydrogen concentration of the catalytic combustion chamber using foam metal and wall coating technology is compared, and the hydrogen content is shown in Figure 11, it can be seen that the hydrogen content of the metal foam load scheme is increased under vertical flow, and the hydrogen concentration is increased to about 63%, which is almost the same as the arrangement of wall coating combined with cooling, because under the vertical arrangement scheme, the wall temperature between the reactors is very uniform. Therefore, it can be seen that a uniform wall temperature is important to increase the hydrogen production rate of the reforming reaction. It was also found that the wall coating combined with the heating solution had the highest hydrogen content, which could further increase the hydrogen concentration by 15%. Timely heat replenishment in the inlet section can effectively improve the conversion rate of methanol and increase hydrogen production.
In summary, it can be seen that under vertical flow, the wall coating scheme combined with the heating arrangement scheme has the best effect, and the vertical arrangement scheme can obtain a higher hydrogen yield. In the arrangement of the reaction chamber of the self-heating coupling reactor, the chamber of the endothermic reaction and the exothermic reaction can be arranged vertically, and at the same time, a relatively uniform wall temperature is maintained during the coupling, and the heat at the inlet is also crucial for the whole reaction.
3 Conclusion.
Catalytic combustion can use the heat energy released by the combustion of residual fuels such as H2 and CO in the tail gas to provide the required energy for the evaporation and reaction of the methanol aqueous solution at the inlet of the fuel cell, which improves the energy utilization efficiency of the system on the one hand, and reduces the emission of pollutants in the tail gas on the other hand, which is an efficient and environmentally friendly technical means. Therefore, in order to realize the efficient coupling and integration of fuel evaporation chamber, fuel reforming chamber and catalytic combustion chamber, it is very important to study the heat matching between the catalytic combustion chamber and the reforming chamber for the integration of the whole system. The fluid in the catalytic combustion chamber provides heat for the catalytic reforming through the combustion reaction, and the chemistry in the two reaction chambers.
The reaction will be affected by the wall temperature and product concentration, which will lead to the mutual influence and mutual restriction of the chemical reactions in the upper and lower cavities. The following conclusions were obtained through experiments:
1) By comparing the co-directional arrangement and the vertical arrangement, it can be found that the vertical arrangement has great advantages, the hydrogen concentration in the product can reach more than 74%, while the hydrogen production rate of the co-flow can only reach 60%, and the vertical flow also has a low CO content. Therefore, when the catalytic combustion chamber is integrated with the reforming chamber, it should be arranged vertically.
2) Regardless of the integration method chosen, the temperature of the front section of the reformer should be high so that a high hydrogen production rate can be achieved. This is because, in the inlet section of the reformer, the reaction temperature plays a dominant role, and in the second half of the reactor, the reactant concentration plays a dominant role. Therefore, when the arrangement in the same direction is adopted, the cooling arrangement;When a vertical arrangement is adopted, the heating arrangement.
3) The loading scheme of metal foam can make the wall temperature uniform, and at the same time have a good heat and mass transfer effect, the three-dimensional structure of the carrier can increase the specific surface area, increase the methanol area of the catalyst and the fluid, when the copper foam is used as the combustion catalyst carrier, the hydrogen yield can reach 63%, and the surface temperature of the self-heating reformer can be stable after using the copper foam as the carrier.