This review presents methanol as a potential renewable alternative to fossil fuels in the fight against climate change. It explores the renewable ways of obtaining methanol and its use in efficient ...energy systems for a net zero-emission carbon cycle, with a special focus on fuel cells. It investigates the different parts of the carbon cycle from a methanol and fuel cell perspective. In recent years, the potential for a methanol economy has been shown and there has been significant technological advancement of its renewable production and utilization. Even though its full adoption will require further development, it can be produced from renewable electricity and biomass or CO2 capture and can be used in several industrial sectors, which make it an excellent liquid electrofuel for the transition to a sustainable economy. By converting CO2 into liquid fuels, the harmful effects of CO2 emissions from existing industries that still rely on fossil fuels are reduced. The methanol can then be used both in the energy sector and the chemical industry, and become an all-around substitute for petroleum. The scope of this review is to put together the different aspects of methanol as an energy carrier of the future, with particular focus on its renewable production and its use in high-temperature polymer electrolyte fuel cells (HT-PEMFCs) via methanol steam reforming.
Methanol as a hydrogen carrier can be reformed with steam over Cu/ZnO/Al2O3 catalysts. In this paper a comprehensive pseudo-homogenous model of a multi-tubular packed-bed reformer has been developed ...to investigate the impact of operating conditions and geometric parameters on its performance. A kinetic Langmuir-Hinshelwood model of the methanol steam reforming process was proposed. In addition to the kinetic model, the pressure drop and the mass and heat transfer phenomena along the reactor were taken into account. This model was verified by a dynamic model in the platform of ASPEN. The diffusion effect inside catalyst particles was also estimated and accounted for by the effectiveness factor. The simulation results showed axial temperature profiles in both tube and shell side with different operating conditions. Moreover, the lower flow rate of liquid fuel and higher inlet temperature of thermal air led to a lower concentration of residual methanol, but also a higher concentration of generated CO from the reformer exit. The choices of operating conditions were limited to ensure a tolerable concentration of methanol and CO in H2-rich gas for feeding into a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) stack. With fixed catalyst load, the increase of tube number and decrease of tube diameter improved the methanol conversion, but also increased the CO concentration in reformed gas. In addition, increasing the number of baffle plates in the shell side increased the methanol conversion and the CO concentration.
A method for simultaneous heat and multicomponent mass transfer incorporated with the volume of fluid surface tracking method was developed in a two-dimensional inclined channel. The process in the ...channel includes direct contact condensation of hydrocarbon mixtures with and without noncondensable gas, and distillation effect is also considered. Interfacial transport was performed by a multicomponent phase change model in kinetic forms considering the assumption of thermodynamic equilibrium at the vapor–liquid or vapor/gas–liquid interface using Peng–Robinson equations. The shear-stress transport k–ω turbulence model damped near the vapor–liquid or vapor/gas–liquid interface was used. The hydrocarbon mixtures in both phases were described by five pseudo-components, and Stefan–Maxwell equations were used to describe diffusional interactions in the multicomponent system. Parametric studies were performed to investigate further the model with various boundary conditions. Simulations for a binary system were also performed for a preliminary validation. For the liquid phase, similar trends of the Sherwood numbers were found between the results by simulations and predicted by the Penetration Theory. For the vapor phase, good agreement was observed between the results by empirical correlation and simulations.
The kinetics of methanol synthesis remains debatable for various reasons, such as the lack of scientifically conclusive agreement about reaction mechanisms. The focus of this paper is on the ...evaluation of the intrinsic kinetics of the methanol synthesis reaction based on CO2 hydrogenation and the associated reverse water–gas shift as overall reactions. The industrial methanol synthesis catalyst, Cu/ZnO/Al2O3/MgO, was used for performing the kinetic studies. An optimal kinetic model was assessed for its ability to predict the experimental data from differential to integral conditions, contrary to the typical fitting of only the integral conditions’ data (common practice, as reported in the literature). The catalyst testing and kinetic evaluations were performed at various temperatures (210–260 °C) and pressures (40–77 bar), and for different stoichiometric numbers (0.9–1.9), H2/CO2 ratios (3.0–4.4) and carbon oxide ratios (0.9–1.0), in an isothermal fixed bed reactor, operated in a plug-flow mode. Experiments with CO in the feed were also generated and fitted. Different literature kinetic models with different assumptions on active sites, rate-determining steps, and hence, model formulations were fitted and compared. The original Seidel model appeared to fit the kinetic data very well, but it has twelve parameters. The modified model (MOD) we propose is derived from this Seidel model, but it has fewer (nine) parameters—it excludes CO hydrogenation, but it takes into consideration the morphological changes of active sites and CO adsorption. This MOD model, with three active sites, gave the best fit to all the data sets.
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In this study, a comparative study was conducted on the three reactor types (the adiabatic, water-cooled and gas-cooled reactor) employed for the traditional syngas to methanol (STM) ...process to investigate their potential applications to the STM process with the CO2-rich feed gas or the CO2 hydrogenation to methanol (CTM) process. The temperature profiles in the axial and radial directions particularly the hot-spot temperatures, operating conditions and methanol yields for the reactors have been investigated using the thermodynamic analysis, the CFD method and the pseudo-homogeneous model. The capital costs for CTM process with the three reactor types have also been evaluated. Compared with the traditional STM process, the STM process with the CO2-rich feed gas and CTM process exhibited reduced hot-spot temperatures. The simulation results showed that the single-bed adiabatic (without internal cooling) reactor and the gas-cooled reactor exhibited potentials for the CTM process, where the hot-spot temperatures the hot-spot temperatures in the reactors can be within the typical operating temperature range (e.g., 220–280 °C) for the catalyst. Regarding the comparison of the three reactor types for the CTM process, the water-cooled reactor showed advantages in terms of efficient heat removal, low hot-spot temperature and relatively wide range of inlet temperature for control. The adiabatic reactor and the gas-cooled reactor demonstrated a relatively low and medium performance, and also a relatively low and medium capital cost, respectively, which indicates the potentials of the two reactor types in a small-scale CTM process.
The hydrogen-rich gas produced in-situ by methanol steam reforming (MSR) reactions significantly affects the performance and endurance of the high-temperature polymer electrolyte membrane (HT-PEM) ...fuel cell stack. A numerical study of MSR reactions over a commercial CuO/ZnO/Al2O3 catalyst coupling with the heat and mass transfer phenomena in a co-current packed-bed reactor is conducted. The simulation results of a 1D and a 2D pseudo-homogeneous reactor model are compared, which indicates the importance of radial gradients in the catalyst bed. The effects of geometry and operating parameters on the steady-state performance of the reactor are investigated. The simulation results show that the increases in the inlet temperature of burner gas and the tube diameter significantly increase the non-uniformity of radial temperature distributions in reformer tubes. Hot spots are formed near the tube wall in the entrance region. The hot-spot temperature in the catalyst bed rises with the increase in the inlet temperature of burner gas. Moreover, the difference in simulation results between the 1D and 2D models is shown to be primarily influenced by the tube diameter. With a methanol conversion approaching 100% or a relatively small tube diameter, the simplified 1D model can be used instead of the 2D model to estimate the reactor performance.
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•Development of a 2D model for the MSR in a multi-tubular packed-bed reactor.•Comparison of simulation results between 1D and 2D reactor models.•Prediction of the temperature distribution inside a reactor tube.•Studying the effects of geometry and operating conditions on reformer performance.•Optimization of operating conditions for integration with a 5 kW HT-PEMFC stack.
•Dynamic simulations of the MSD process for pilot-scale green methanol production.•Energy analysis for the MS, MD and MSD processes under load change operations.•Energy efficiency of 75.4%–77.1% for ...the MSD process under load change operations.•Surrogate modeling for the MSD process by the NARX model with good accuracy.
Green methanol production, based on intermittent renewable energy sources, requires a more flexible operation mode and close integration with other sections, such as, the electrical grid and electrolysis processes. In this study, methanol synthesis and distillation processes (MSD) for pilot-scale green methanol production (corresponding to 22,236 tons/year) were investigated by dynamic modeling, focusing on energy analysis and dynamic characteristics during load change (LC) operations. The dynamic simulation results with a ramp rate of 50% load / h indicated energy efficiencies of 87.7% (at full-load) and 90.2% (at half-load) for the methanol synthesis process, 86.8% (full-load) and 82.4% (half-load) for the methanol distillation process, and 77.1% (full-load) and 75.4% (half-load) for the MSD process. Relatively small fluctuations were achieved with a ramp time of 1 h for the LC operations. Based on the constructed dynamic model, a surrogate modeling for the MSD process was conducted using the nonlinear autoregressive exogenous model (NARX) model, which exhibited good accuracy with the evaluated performance for the testing data of the root-mean-square error (RMSE) = 3.09 × 10-5, mean absolute error (MAE) = 2.30 × 10-4, and R2 = 1.0. The constructed NARX model can be further integrated with models for other sections of the power-to-methanol process.
Green hydrogen is produced through different methods in the lab but only a few technologies are commercialized. Cost reduction is widely expected to compete with the existing carbon-emitting ...alternatives. This paper compares alkaline, proton exchange membrane, and solid oxide electrolysis cells as the dominant technologies. Economic analyses with scale-up effects show meaningful differences between PEM and alkaline electrolyzers as relatively settled methods and solid oxide as an immature technology. Monte Carlo simulations on grid-connected electrolysis using the Danish electricity market confirm that both PEM and alkaline electrolyzers can already produce hydrogen with less than 3 €/Kg if taxes and levies are removed. The price may even drop below 2 €/Kg after the mass adoption of all three technologies. Furthermore, if electricity is delivered at half prices, the levelized cost of hydrogen falls around 1 €/Kg. The capabilities for cost reduction after scaling-up are 33%, 34%, and 50% in alkaline, PEM, and solid oxide electrolyzers respectively while they could get intensified with subsidization to 56%, 59%, and 70%. The results indicate that solid oxide electrolyzers can be as economical as alkaline and PEM ones. However, grey hydrogen seems to remain unbeatable without subsidized electricity and/or carbon tax adjustments.
•Scale-up effects and technology developments can drop hydrogen prices below 2 €/Kg.•The electrolysis method becomes less important in large-scale deployments.•Large scaling and technology developments are not enough for cost competition.•Carbon regulations and subsidies are necessary to change grey hydrogen to green.
The paper presents a comprehensive review of the current status of integrated high temperature proton exchange membrane fuel cell (HT-PEMFC) and methanol steam reformer (MSR) systems. It highlights ...the advantages and limitations of the technology and outlines key areas for future improvement. A thorough discussion of novel reformer designs and optimizations aimed at improving the performance of the reformer, as well as different integrated MSR-HT-PEMFC system configurations are provided. The control strategies of the system operation and system diagnosis are also addressed, offering a complete picture of the integrated system design. The review revealed that several processes and components of the system should be improved to facilitate large-scale implementation of the MSR-HT-PEMFC systems. The lengthy system startup is one area that requires improvements. A structural design that is more compact without sacrificing performance is also required, which could possibly be achieved by recovering water from the fuel cell to fulfill MSR's water needs and consequently shrink the fuel tank. Reformer design should account for both heat transfer optimizations and reduced pressure drop to enhance the system's performance. Finally, research must concentrate on membrane materials for HT-PEMFC that can operate in the 200–300 °C temperature range and catalyst materials for more efficient MSR process at lower temperature should be investigated to improve the heat integration and overall system efficiency.
•The potential for HT-PEMFC and MSR in the future energy system was emphasized.•Recent advances in reformer design and optimization were presented.•Different integration methods for the MSR-HT-PEMFC system were analyzed.•Dynamics, control and diagnosis of integrated systems were discussed.•System lifetime, efficiency and challenges in future applications were elaborated.
A pseudo-homogeneous model for the methanol steam reforming process was developed based on reaction kinetics over a CuO/ZnO/Al2O3 catalyst and non-adiabatic heat and mass transfer performances in a ...co-current packed-bed reactor. A Thiele modulus method and an intraparticle distribution method were applied for predicting the effectiveness factors for main reactions and providing insights into the diffusion-reaction process in a cylindrical catalyst pellet. The results of both methods are validated and show good agreements with the experimental data, but the intraparticle distribution method provides better predictions. Results indicate that increases in catalyst size and bulk fluid temperature amplify the impact of intraparticle diffusion limitations, showing a decrease in effectiveness factors. To satisfy the requirements of a high temperature polymer electrolyte membrane fuel cell stack, the optimized operating conditions, which bring the methanol and CO concentrations to less than 1% vol in the reformate stream, are determined based on the simulation results.
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•One-dimensional pseudo-homogeneous model of a packed-bed methanol steam reformer.•Two methods to obtain approximate solutions of the catalytic effectiveness factor.•Validation of reaction kinetics and effectiveness factors with experimental data.•Effects of main operating parameters on the intraparticle heat and mass transfer.•Optimized conditions to obtain less than 1% of CH3OH and CO in reformate gas.
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