Abstract
Fuel cells, especially the high temperature polymer electrolyte membrane (HT-PEM) fuel cell, are attracting more attention in backup power, transportation and stationary applications. The HT-PEM fuel cells convert hydrogen directly into electricity, making it possible to have a higher efficiency than conventional generators. Nevertheless, technical problems in hydrogen storage and transportation are still the essential obstacles to the widespread adoption of hydrogen powered fuel cell systems. As an alternative method, the methanol steam reforming (MSR) process has been considered promising for the in-situ hydrogen generation in HT-PEM fuel cell systems.
The hydrogen-rich gas produced in-situ by the MSR process has a significant impact on the performance and durability of the HT-PEM fuel cell stack. Hence, the performance of the reformer should be studied. Special attentions should be paid to the contents of carbon monoxide and methanol in the reformate. The experiments for the reformer provide us the direct and concise results of the system. However, it can be costly and time-consuming, especially when multiple variables should be tested. The tested results could also be affected by certain objective factors such as unnoticed catalyst deactivation, errors in measuring devices (e.g., incorrect fitting of the measuring scale), and inappropriate research design. The mathematical modelling of packed bed reactors allows a deep insight into the chemical reactions and transport processes in the reactor.
A multi-tubular packed-bed MSR reformer for hydrogen production is designed to be integrated with a 5 kW HT-PEM fuel cell stack in this study. A co-current stream of burner gas passes by the shell side of the reformer acting as an external heat source for the catalyst bed. Simultaneously, the reactants flow through the tubular packed-bed reactor with MSR reactions occurring on the commercial CuO/ZnO/Al2O3 based catalyst. As the reactants flow through the packed bed, complex physical and chemical phenomena at different scales occur in the reactor. Several assumptions are made to simplify the modeling of packed bed reactors.
The one-dimensional pseudo-homogeneous model considers the reforming reactions and the axial transfer phenomena in the reactor. Since no mixing in the axial direction and complete mixing in the radial direction are assumed, this model is referred as an ideal plug-flow model. The effectiveness factor in this model is calculated as a function of the Thiele modulus. The pressure drop in the axial direction through the catalyst bed is also considered, though it generally does not significantly affect the overall reformer performance. Hence, based on the one-dimensional model, the effect of operating parameters such as the steam to carbon ratio (S/C), the contact time (Wcat/FCH3OH) and the inlet temperature of burner gas can be investigated. According to the simulation results of the one-dimensional model, the increase in the inlet temperature of burner gas and Wcat/FCH3OH is observed to improve the methanol conversion of the reformer. Simultaneously, the CO concentration in the reformate gas is also increased due to the enhanced temperature in the catalyst bed. Additionally, a higher S/C value leads to a lower methanol conversion due to the increased heat consumption for evaporating and heating the additional steam. This will also reduce the amount of CO in the MSR process.
The one-dimensional pseudo-homogeneous model can be extended to a two-dimensional pseudo-homogeneous model by additionally considering the radial gradients of temperature and concentrations in the reactor tube. For a tubular reactor with rather non-uniformly distributed temperature and concentration in the radial direction, this two-dimensional model can make more accurate predictions of the reformer performance. Moreover, the two-dimensional model can be used to estimate the hot-spot temperature and the local catalyst deactivation when the reactor operates at relatively high temperature. Nevertheless, it requires slightly more computation time than the one-dimensional model. According to the simulated temperature distributions in the catalyst bed, the lowest temperature appears at the centre of the reactor tube, and the hot spot is generally formed at the location about 3 cm from the reactor entrance near the tube wall. The increase in the inlet temperature of the burner gas significantly enhances the hot spot temperature of the catalyst bed. In addition, the tube diameter is found to be the most important factor that affects the difference of the simulation results between the one-dimensional model and the two-dimensional model. Besides, this difference between these two models becomes less noticeable when the methanol conversion of the reformer is approaching 100%.
Due to the porous structure of the catalyst, not all the catalytically active surface are exposed to the bulk conditions. Therefore, the reaction-diffusion process within a catalyst particle is modelled considering the interphase and intraparticle transfer resistances. Therefore, the effectiveness factor can be calculated by solving the intraparticle mass and energy balance equations. This method provides an insight into the interaction between the intrinsic kinetics and the heat and mass transport characteristics of the porous catalyst pellet. According to the simulation results in the particle scale, the interphase and intraparticle heat and mass transfer resistances are enhanced with the increasing bulk fluid temperature and the catalyst diameter. Therefore, at the inlet conditions of the reactor, it can be observed that the effectiveness factors for the MSR and MD reactions decrease with the increasing burner temperature and particle diameter. Another method used to estimate the effectiveness factor in this study is by regarding the effectiveness factor as a function of the Thiele modulus. This Thiele modulus-effectiveness factor method requires shorter computation time but is less accurate for the calculation of effectiveness factors.
The hydrogen-rich gas produced in-situ by the MSR process has a significant impact on the performance and durability of the HT-PEM fuel cell stack. Hence, the performance of the reformer should be studied. Special attentions should be paid to the contents of carbon monoxide and methanol in the reformate. The experiments for the reformer provide us the direct and concise results of the system. However, it can be costly and time-consuming, especially when multiple variables should be tested. The tested results could also be affected by certain objective factors such as unnoticed catalyst deactivation, errors in measuring devices (e.g., incorrect fitting of the measuring scale), and inappropriate research design. The mathematical modelling of packed bed reactors allows a deep insight into the chemical reactions and transport processes in the reactor.
A multi-tubular packed-bed MSR reformer for hydrogen production is designed to be integrated with a 5 kW HT-PEM fuel cell stack in this study. A co-current stream of burner gas passes by the shell side of the reformer acting as an external heat source for the catalyst bed. Simultaneously, the reactants flow through the tubular packed-bed reactor with MSR reactions occurring on the commercial CuO/ZnO/Al2O3 based catalyst. As the reactants flow through the packed bed, complex physical and chemical phenomena at different scales occur in the reactor. Several assumptions are made to simplify the modeling of packed bed reactors.
The one-dimensional pseudo-homogeneous model considers the reforming reactions and the axial transfer phenomena in the reactor. Since no mixing in the axial direction and complete mixing in the radial direction are assumed, this model is referred as an ideal plug-flow model. The effectiveness factor in this model is calculated as a function of the Thiele modulus. The pressure drop in the axial direction through the catalyst bed is also considered, though it generally does not significantly affect the overall reformer performance. Hence, based on the one-dimensional model, the effect of operating parameters such as the steam to carbon ratio (S/C), the contact time (Wcat/FCH3OH) and the inlet temperature of burner gas can be investigated. According to the simulation results of the one-dimensional model, the increase in the inlet temperature of burner gas and Wcat/FCH3OH is observed to improve the methanol conversion of the reformer. Simultaneously, the CO concentration in the reformate gas is also increased due to the enhanced temperature in the catalyst bed. Additionally, a higher S/C value leads to a lower methanol conversion due to the increased heat consumption for evaporating and heating the additional steam. This will also reduce the amount of CO in the MSR process.
The one-dimensional pseudo-homogeneous model can be extended to a two-dimensional pseudo-homogeneous model by additionally considering the radial gradients of temperature and concentrations in the reactor tube. For a tubular reactor with rather non-uniformly distributed temperature and concentration in the radial direction, this two-dimensional model can make more accurate predictions of the reformer performance. Moreover, the two-dimensional model can be used to estimate the hot-spot temperature and the local catalyst deactivation when the reactor operates at relatively high temperature. Nevertheless, it requires slightly more computation time than the one-dimensional model. According to the simulated temperature distributions in the catalyst bed, the lowest temperature appears at the centre of the reactor tube, and the hot spot is generally formed at the location about 3 cm from the reactor entrance near the tube wall. The increase in the inlet temperature of the burner gas significantly enhances the hot spot temperature of the catalyst bed. In addition, the tube diameter is found to be the most important factor that affects the difference of the simulation results between the one-dimensional model and the two-dimensional model. Besides, this difference between these two models becomes less noticeable when the methanol conversion of the reformer is approaching 100%.
Due to the porous structure of the catalyst, not all the catalytically active surface are exposed to the bulk conditions. Therefore, the reaction-diffusion process within a catalyst particle is modelled considering the interphase and intraparticle transfer resistances. Therefore, the effectiveness factor can be calculated by solving the intraparticle mass and energy balance equations. This method provides an insight into the interaction between the intrinsic kinetics and the heat and mass transport characteristics of the porous catalyst pellet. According to the simulation results in the particle scale, the interphase and intraparticle heat and mass transfer resistances are enhanced with the increasing bulk fluid temperature and the catalyst diameter. Therefore, at the inlet conditions of the reactor, it can be observed that the effectiveness factors for the MSR and MD reactions decrease with the increasing burner temperature and particle diameter. Another method used to estimate the effectiveness factor in this study is by regarding the effectiveness factor as a function of the Thiele modulus. This Thiele modulus-effectiveness factor method requires shorter computation time but is less accurate for the calculation of effectiveness factors.
| Originalsprog | Engelsk |
|---|---|
| Vejledere |
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| Udgiver | |
| ISBN'er, elektronisk | 978-87-7573-914-1 |
| DOI | |
| Status | Udgivet - 2022 |
Bibliografisk note
PhD supervisors:Assoc. Prof. Samuel Simon Araya, Aalborg University
Prof. Søren Knudsen Kær, Aalborg University