Modelling of tubular-designed solid oxide fuel cell with indirect internal reforming operation fed by different primary fuels

Mathematical models of an indirect internal reforming solid oxide fuel cell (IIR-SOFC) fed by four different primary fuels, i.e., methane, biogas, methanol and ethanol, are developed based on steady-state, heterogeneous, two-dimensional and tubular-design SOFC models. The effect of fuel type on the thermal coupling between internal endothermic reforming with exothermic electrochemical reactions and system performance are determined. The simulation reveals that an IIR-SOFC fuelled by methanol provides the smoothest temperature gradient with high electrochemical efficiency. Furthermore, the content of CO₂ in biogas plays an important role on system performance since electrical efficiency is improved by the removal of some CO₂ from biogas but a larger temperature gradient is expected.Sensitivity analysis of three parameters, namely, a operating pressure, inlet steam to carbon (S:C) ratio and flow direction is then performed. By increasing the operating pressure up to 10 bar, the system efficiency increases and the temperature gradient can be minimized. The use of a high inlet S:C ratio reduces the cooling spot at the entrance of reformer channel but the electrical efficiency is considerably decreased. An IIR-SOFC with a counter-flow pattern (as based case) is compared with that with co-flow pattern (co-flow of air and fuel streams through fuel cell). The IIR-SOFC with co-flow pattern provides higher voltage and a smoother temperature gradient along the system due to superior matching between heat supplied from electrochemical reaction and heat required for steam reforming reaction; thus it is expected to be a better option for practical applications.     

Modeling of IT-SOFC with indirect internal reforming operation fueled by methane: Effect of oxygen adding as autothermal reforming

Mathematical models of an Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) with indirect internal reforming operation (IIR-SOFC) fueled by methane were developed. The models were based on a steady-state heterogeneous two-dimensional tubular-design SOFC. The benefit in adding oxygen to methane and steam as the feed for autothermal reforming reaction on the thermal behavior and SOFC performance was simulated. The results indicated that smoother temperature gradient with lower local cooling at the entrance of the reformer channel can be achieved by adding a small amount of oxygen. However, the electrical efficiency noticeably decreased when too high oxygen content was added due to the loss of hydrogen generation from the oxidation reaction; hence, the inlet oxygen to carbon (O/C) molar ratio must be carefully controlled. Another benefit of adding oxygen is the reduction of excess steam requirement, which could reduce the quantity of heat required to generate the steam and eventually increases the overall system performance. It was also found that the operating temperature strongly affects the electrical efficiency achievement and temperature distribution along the SOFC system. By increasing the operating temperature, the system efficiency increases but a significant temperature gradient is also detected. The system with a counter-flow pattern was compared to that with a co-flow pattern. The co-flow pattern provided smoother temperature gradient along the system due to better matching between the heat supplied from the electrochemical reaction and the heat required for the steam reforming reaction. However, the electrical efficiency of the co-flow pattern is lower due to the higher cell polarization at a lower system temperature.     

Modeling of SOFC with indirect internal reforming operation: Comparison of conventional packed-bed and catalytic coated-wall internal reformer

In the present work, mathematical models of indirect internal reforming solid oxide fuel cells (IIR-SOFC) fueled by methane were developed to analyze the thermal coupling of an internal endothermic reforming with exothermic electrochemical reactions and determine the system performance. The models are based on steady-state, heterogeneous, two-dimensional reformer and annular design SOFC models. Two types of internal reformer i.e. conventional packed-bed and catalytic coated-wall reformers were considered here. The simulations indicated that IIR-SOFC with packed-bed internal reformer leads to the rapid methane consumption and undesirable local cooling at the entrance of internal reformer due to the mismatch between thermal load associated with rapid reforming rate and local amount of heat available from electrochemical reactions. The simulation then revealed that IIR-SOFC with coated-wall internal reformer provides smoother methane conversion with significant lower local cooling at the entrance of internal reformer.     

Numerical analysis of output characteristics of tubular SOFC with internal reformer

In the solid oxide fuel cell (SOFC) system, the internal reforming of raw fuel will act as an efficient cooling system. To realize this cooling system, a special design of the internal reformer is required to avoid the inhomogeneous temperature distribution caused by the strong endothermic reforming reaction at the entrance of the internal reformer. For this purpose, a tubular internal reformer with adjusted catalyst density can be inserted into the tubular SOFC stack. By arranging this, the raw fuel flows along the axis of the internal reformer to be moderately reformed and returns at the end of the internal reformer as a sufficiently reformed fuel.In this paper, the output characteristics of this configuration are simulated using mathematical models, in which one-dimensional temperature and molar distributions are computed along the flow direction. By properly mounting the catalyst density in the internal reformer, the temperature distribution of the cell stack becomes moderate, and the power generation efficiency and the exhaust gas temperature are higher. Effects of other operating conditions such as fuel recirculation, fuel inlet temperature, air recirculation and air inlet temperature are also examined under the condition where the maximum temperature of the stack is kept at 1300 K by adjusting the air flow rate. Under this condition, these operating conditions exert a considerable effect on the exhaust temperature but have a slight effect on the efficiency.     

A transient study of double-jacketed membrane reactor via methanol steam reforming

A non-isothermal unsteady-state model was established to simulate methanol steam reforming using a double-jacketed Pd membrane reactor. At steady state, a self-sustained membrane reactor was achieved by the oxidation of residual methanol and hydrogen from reformer for endothermic steam reforming. The molar fractions of species and reformer temperature were analyzed under co-current operation between oxidation and reformer sides. The start-up of reformer was simulated under two conditions: (1) The catalyst temperature was lower than the influent temperature and (2) The catalyst temperature was higher than influent temperature. Condition 1 yielded higher methanol conversion and reformer temperature than condition 2 at steady state. Moreover, the instability of species can be minimized on condition 1 during start-up. The fluctuation of membrane reactor at steady state was also studied. Two strategies were compared to analyze the reformer response when temporary extra hydrogen was required. The results showed that increasing inlet methanol outperformed increasing reformer temperature.     

Design of a methanol reformer for on-board production of hydrogen as fuel for a 3 kW High-Temperature Proton Exchange Membrane Fuel Cell power system

The method of Computational Fluid Dynamics is used to predict the process parameters and select the optimum operating regime of a methanol reformer for on-board production of hydrogen as fuel for a 3 kW High-Temperature Proton Exchange Membrane Fuel Cell power system. The analysis uses a three reactions kinetics model for methanol steam reforming, water gas shift and methanol decomposition reactions on Cu/ZnO/Al₂O₃ catalyst. Numerical simulations are performed at single channel level for a range of reformer operating temperatures and values of the molar flow rate of methanol per weight of catalyst at the reformer inlet. Two operating regimes of the fuel processor are selected which offer high methanol conversion rate and high hydrogen production while simultaneously result in a small reformer size and a reformate gas composition that can be tolerated by phosphoric acid-doped high temperature membrane electrode assemblies for proton exchange membrane fuel cells. Based on the results of the numerical simulations, the reactor is sized, and its design is optimized.     

Performance improvement of methanol steam reforming system with auxiliary heat recovery units

The thermal energy of a methanol steam reforming system is balanced with heat-up by a methanol burner, heat absorption by an evaporator, and an endothermic reforming reactor. As the thermal energy of a methanol steam reformer is delicately controlled, its thermal efficiency is significantly improved. In this study, three different system configurations are compared, namely, (1) a reference methanol steam reformer with an external evaporator, (2) a methanol steam reformer with an internal evaporator and type-1 auxiliary heat recovery unit (AHRU) with a heat source gas, and (3) a methanol steam reformer with an internal evaporator and type-2 AHRU with a heat source gas and reformed gas. These three configurations are analyzed, and the two heat recovery units are investigated. Results show that the internally evaporated methanol steam reformer efficiently converts the methanol to a hydrogen-rich mixture as exit gases are utilized to heat up the inlet methanol/water mixture.     

Autothermal reforming of n-dodecane and desulfurized Jet-A fuel for producing hydrogen-rich syngas

Catalytic reforming is a technology to produce hydrogen and syngas from heavy hydrocarbon fuels in order to supply hydrogen to fuel cells. A lab-scale 2.5 kW_t autothermal reforming (ATR) system with a specially designed reformer and combined analysis of balance-of-plant was studied and tested in the present study. NiO–Rh based bimetallic catalysts with promoters of Ce, K, and La were used in the reformer. The performance of the reformer was studied by checking the hydrogen selectivity, CO_x selectivity, and energy conversion efficiency at various operating temperatures, steam to carbon ratios, oxygen to carbon ratios, and reactants' inlet temperatures. The experimental work firstly tested n-dodecane as the surrogate of Jet-A fuel to optimize operating conditions. After that, desulfurized commercial Jet-A fuel was tested at the optimized operating conditions. The design of the reformer and the catalyst are recommended for high performance Jet-A fuel reforming and hydrogen-rich syngas production.     

Numerical investigation of a multichannel reactor for syngas production by methanol steam reforming at various operating conditions

A novel multichannel reactor with a bifurcation inlet manifold, a rectangular outlet manifold, and sixteen parallel minichannels with commercial CuO/ZnO/Al₂O₃ catalyst for methanol steam reforming was numerically investigated in this paper. A three-dimensional numerical model was established to study the heat and mass transfer characteristics as well as the chemical reaction rates. The numerical model adopted the triple rate kinetic model of methanol steam reforming which can accurately calculate the consumption and generation of each species in the reactor. The effects of steam to carbon molar ratio, weight hourly space velocity, operating temperature and catalyst layer thickness on the methanol steam reforming performance were evaluated and discussed. The distributions of temperature, velocity, species concentration, and reaction rates in the reactor were obtained and analyzed to explain the mechanisms of different effects. It is suggested that the operating temperature of 548 K, steam to carbon ratio of 1.3, and weight hourly space velocity of 0.67 h⁻¹ are recommended operating conditions for methanol steam reforming by the novel multichannel reactor with catalyst fully packed in the parallel minichannels.     

Thermodynamic analysis and heat integration for hydrogen production from bio-butanol for SOFC application: Steam reforming vs. autothermal reforming

Thermodynamic analysis of hydrogen production by steam reforming and autothermal reforming of bio-butanol was investigated for solid oxide fuel cell applications. The effects of reformer operating conditions, e.g., reformer temperature, steam to carbon molar ratio, and oxygen to carbon molar ratio, were investigated with the objective to maximize hydrogen production and to reduce utility requirements of the process and based on which favorable conditions of reformer were proposed. Process flow diagram for steam reforming and autothermal reforming integrated with solid oxide fuel cell was developed. Heat integration with pinch analysis method was carried out for both the processes at favorable reformer conditions. Power generation, electrical efficiency, useful energy for co-generation application, and utility requirements for both the processes were compared.     

Bio-fuel reformation for solid oxide fuel cell applications. Part 1: Fuel vaporization and reactant mixing

A new configuration of a mixing chamber integrated with a customized porous nozzle has been developed to completely vaporize heavy hydrocarbon fuels (e.g., diesel, biodiesel) and achieve homogenous mixing of fuel/air/steam. This proposed configuration suppresses hydrocarbon thermal pyrolysis and solid carbon formation in the fuel vaporization step. The porous nozzle promotes the micro-explosion of emulsified fuel and accelerates secondary atomization to reduce the droplet size. The mixing chamber with customized nozzle was integrated in a single-tube reformer system in order to analyze its effect on diesel and biodiesel auto-thermal reforming (ATR). It has been demonstrated that the customized nozzle not only improved the hydrogen production rate and the reforming efficiency, but it also stabilized the chemical reactions within the reformer and prevented the reactor inlet from high temperature sintering. For diesel ATR, this mixing chamber–reformer combination enabled operation at relatively low reformer temperature without forming solid carbon. This study is one component of a three-part investigation of bio-fuel reforming, also including biodiesel (Part 2) and biodiesel–diesel blends (Part 3).     

Externally reformed solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid systems fueled by methanol and di-methyl-ether (DME)

Solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid power plants integrate a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200–500 kW). The SOFC–MGT systems currently developed are fueled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and di-methyl-ether (DME) (200–350 °C) is significantly lower than that of natural gas (700–900 °C), the reformer can be sited outside the stack. External reforming in SOFC–MGT plants fueled by methanol and DME enhances efficiency due to improved exhaust heat recovery and higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio (SCR)) must be carefully chosen to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fueled hybrid plant is approximately 240 °C, giving efficiencies of about 67–68% with a SOFC temperature of 900 °C (the efficiency is about 72–73% at 1000 °C). Similarly, for DME the optimum reforming temperature is approximately 280 °C with efficiencies of 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around one for methanol and 1.5–2 for DME.     

Numerical simulation of configuration and catalyst-layer effects on micro-channel steam reforming of methanol

A micro-reactor with eight non-parallel channels is proposed to improve the performance of micro-channel steam reforming of methanol. The widths of some channels in the micro-reactor vary gradually along the reactor length direction. The Zn-Cr/CeO₂-ZrO₂ catalyst is coated in the reformer with a certain porosity and permeability. The effects of micro-reactor structures and catalyst-coated manners on several factors are studied, including temperature distributions, velocity distributions, reactant concentrations and the methanol conversion rate. The results indicate that such a structure with a certain entrance inclination angle and channel inclination angle guarantees flow distribution uniformity in each reforming channel. Flow distribution uniformity is conducive to the increase of the methanol conversion rate. Besides, in order to measure strengths and weaknesses of different catalyst-coated manners, a wall-coated reformer and a packed-bed reformer are studied respectively. It is found that compared to the packed-bed reformer, the temperature and the methanol conversion rate in wall-coated reformer are far higher. It is necessary to find an optimal catalyst thickness that is able to reduce the CO concentration because the catalyst thickness can affect CO concentration in the product gases indirectly. The optimal inclination angles and the catalyst thickness are proposed based on the simulating results.     

Comparison between 1D and 2D numerical models of a multi-tubular packed-bed reactor for methanol steam reforming

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.     

Thermo-fluid dynamics modelling of steam electrolysis in fully-assembled tubular high-temperature proton-conducting cells

Electrolysis based on renewable energies offers a promising carbon-free solution for hydrogen generation and storage. The recent developments of proton ceramic electrolysis cells operating at intermediate temperatures bear promise of superior energy efficiency compared to oxide ion conducting electrolytes. Here, a proton ceramic Single Engineering Unit (SEU) design is optimized for steam electrolysis using a computational fluid dynamics (CFD) model implemented in a COMSOL Multiphysics software. The SEU is an all-in-one tubular cell arrangement that constitutes the smallest electrolysis unit and enables efficient, adaptable pressurized hydrogen generation. The parametrical modelling study is conducted for two adiabatic operation scenarios with distinct steam conversion rates and tested for multiple key parameters, namely internal and external chamber pressures and inlet stream temperature. The modelling results show that low steam conversions enable operation at higher current densities and that the thermoneutral voltage for a fixed steam conversion is highly sensitive to the process conditions and operation modes. The increment of the pressure of the generated hydrogen implies a reduced production rate at thermoneutral voltage, although it can be compensated with an enhanced steam pressure or a reduced inlet temperature. Additionally, the introduction of a porous medium as the SEU current collector in the steam chamber enhances heat transport within this chamber. The area specific resistance of the system determines the current density, enforcing an adaption of the area of the electrolyser to satisfy the target hydrogen production and energy efficiency. The resulting proposed SEU design and adapted operational parameters allow effective delivery of pressurized dry hydrogen for a wide range of conditions and applications.     

Transport phenomena in a steam-methanol reforming microreactor with internal heating

An experimental and theoretical study of steam reforming of methanol is carried out in a packed-bed microreactor with internal heating. Experimental results of the methanol conversion and carbon monoxide concentration in an internally heated reformer are compared with those of an externally heated reformer. Higher methanol conversion and carbon monoxide concentration are obtained for internal heating at the same conditions. The results show the conversion efficiency of methanol and CO concentration increase with increasing internal heating rate over the range of operating conditions. A correlation for the conversion efficiency of methanol has been obtained as a function of the internal heating rate and a dimensionless time parameter which represents the ratio of the characteristic time of the methanol flow to the time for chemical reaction.     

Performance predictions of a tubular SOFC operating on a partially reformed JP-8 surrogate

This paper uses chemically reacting flow models to explore the effect of upstream JP-8 steam reforming on the performance of a tubular, anode-supported, solid-oxide fuel cell. In all cases studied in this paper, a steam–carbon ratio of 3 is used for the reformer inlet. However, by varying the reformer temperature, the methane concentration in the reformate stream can be varied. In this study methane mole fractions are varied between 0 and 20%, on a dry basis. The methane mole fraction is found to have a substantial effect on fuel-cell efficiency, power density, and heat-release profiles. The paper also explores the effects of internal reforming chemistry and electrochemical charge transfer on the gas-phase kinetics and propensity for deposit formation. A detailed reaction mechanism is used to describe methane steam reforming on Ni within the anode, while a detailed gas-phase mechanism is used to predict the gas-phase composition in the fuel channel.     

Optimization of Jet-A fuel reforming for aerospace applications

This paper focuses on the optimization of Jet-A fuel reforming for use with solid-oxide fuel cells in aerospace applications. Because of the specialized operating conditions and reforming requirements, a broad range of reforming inlet conditions need to be considered. Both equilibrium calculations for reforming of a Jet-A surrogate and zero-dimensional modeling with detailed chemistry for reforming of a kerosene surrogate are performed over a wide range of conditions with varying inlet temperature, operating pressure, steam-to-carbon ratio, and oxygen-to-carbon ratio. While equilibrium calculations provide some insight into the efficiency of the final reformer, the kinetics modeling can account for the finite residence time of the gas within the reformer. Calculations using finite-rate gas-phase chemistry indicate that the most efficient mode of reforming is achieved using a short-contact partial oxidation reactor operating with minimal water addition. Certain factors to consider for the development of a future catalytic reformer, such as local hot-spots and coke deposition on the catalyst, are also discussed.     

Methanol fuel processor and PEM fuel cell modeling for mobile application

The use of hydrocarbon fed fuel cell systems including a fuel processor can be an entry market for this emerging technology avoiding the problem of hydrogen infrastructure. This article presents a 1 kW low temperature PEM fuel cell system with fuel processor, the system is fueled by a mixture of methanol and water that is converted into hydrogen rich gas using a steam reformer. A complete system model including a fluidic fuel processor model containing evaporation, steam reformer, hydrogen filter, combustion, as well as a multi-domain fuel cell model is introduced. Experiments are performed with an IDATECH FCS1200™ fuel cell system. The results of modeling and experimentation show good results, namely with regard to fuel cell current and voltage as well as hydrogen production and pressure. The system is auto sufficient and shows an efficiency of 25.12%. The presented work is a step towards a complete system model, needed to develop a well adapted system control assuring optimized system efficiency.     

Combustion characteristics of anode off-gas on the steam reforming performance

The combination of steam reforming and HT-PEMFC has been considered as a proper set up for the efficient hydrogen production. Recycling anode off-gas is energy-saving strategy, which leads to enhance the overall efficiency of the HT-PEMFC. Thus, the recycling effect of anode off-gas on steam-reforming performance needs to be further studied. This paper, therefore, investigated that the combustion of anode off-gas recycled impacts on the steam reformer, which consists of premixed-flame burner, steam reforming and water-gas shift reactors. The temperature rising of internal catalyst was affected by lower heating value of fuels when the distance between catalyst and burner is relatively short, while by the flow rate of fuels and the steam to carbon ratio when its distance is long. The concentration of carbon monoxide was the lowest at 180 °C of LTS temperature, while NG and AOG modes showed the highest thermal efficiency at LTS temperature of 220–300 °C and 270–350 °C, respectively. The optimum condition of thermal efficiency to maximize hydrogen production was determined by steam reforming rather than water gas shift reaction. It was confirmed that the condition to obtain the highest thermal efficiency is about 650 °C of steam reforming temperature, regardless of combustion fuel and carbon monoxide reduction. The difference of hydrogen yield between upper and lower values is up to 1.5 kW as electric energy with a variation of thermal efficiency. Hydrogen yield showed the linear proportion to the thermal efficiency of steam reformer, which needs to be further increased through proper thermal management.