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.     

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.     

Fuel flexibility study of an integrated 25 kW SOFC reformer system

The operation of solid oxide fuel cells on various fuels, such as natural gas, biogas and gases derived from biomass or coal gasification and distillate fuel reforming has been an active area of SOFC research in recent years. In this study, we develop a theoretical understanding and thermodynamic simulation capability for investigation of an integrated SOFC reformer system operating on various fuels. The theoretical understanding and simulation results suggest that significant thermal management challenges may result from the use of different types of fuels in the same integrated fuel cell reformer system. Syngas derived from coal is simulated according to specifications from high-temperature entrained bed coal gasifiers. Diesel syngas is approximated from data obtained in a previous NFCRC study of JP-8 and diesel operation of the integrated 25 kW SOFC reformer system. The syngas streams consist of mixtures of hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. Although the SOFC can tolerate a wide variety in fuel composition, the current analyses suggest that performance of integrated SOFC reformer systems may require significant operating condition changes and/or system design changes in order to operate well on this variety of fuels.     

An experimental study on the reaction characteristics of a coupled reactor with a catalytic combustor and a steam reformer for SOFC systems

Steam reforming performance in a coupled reactor that consists of a steam reformer and a catalytic combustor is experimentally investigated in this study. Endothermic steam reforming can occur through the absorption of heat from the catalytic combustion of the anode offgas in a heat-exchanging coupled reactor. The reaction characteristics were observed by varying parameters such as the inlet temperature of the catalytic combustor, the excess air ratio for the catalytic combustion, the fuel utilization rate in the fuel cells, and the steam-to-carbon ratio in the steam reformer. The reactor temperature and reformate composition were measured to analyze the performance of the reactor. The results show the potential applicability and design technologies of the coupled reactor for the fuel processing of high temperature fuel cells using an external reformer.     

Performance analysis of a SOFC under direct internal reforming conditions

This paper presents the performance analysis of a planar solid-oxide fuel cell (SOFC) under direct internal reforming conditions. A detailed solid-oxide fuel cell model is used to study the influences of various operating parameters on cell performance. Significant differences in efficiency and power density are observed for isothermal and adiabatic operational regimes. The influence of air number, specific catalyst area, anode thickness, steam to carbon (s/c) ratio of the inlet fuel, and extend of pre-reforming on cell performance is analyzed. In all cases except for the case of pre-reformed fuel, adiabatic operation results in lower performance compared to isothermal operation. It is further discussed that, though direct internal reforming may lead to cost reduction and increased efficiency by effective utilization of waste heat, the efficiency of the fuel cell itself is higher for pre-reformed fuel compared to non-reformed fuel. Furthermore, criteria for the choice of optimal operating conditions for cell stacks operating under direct internal reforming conditions are discussed.     

Stress intensity factors for internal and external cracks in pressurised thick-walled cylinders

Stress intensity factors for both internal and external semi-circular and semi-elliptical surface cracks in internally pressurised thick-walled cylinders of radius ratios between 2 and 3 are presented for a wide range of crack sizes. These solutions were obtained using the boundary integral equation (BIE) method for three-dimensional numerical stress analysis. Hoop strain distributions at the outer circumference of the cylinder are also presented for some external cracks, and shown to be useful for experimentally monitoring crack growth.     

An internal heat extraction system for solar ponds

An in-pond heat exchanger made by a reinforced polyethylene pipe is proposed. The advantages of the system with respect to commonly used external heat exchangers are discussed and the design criteria are presented. The cost analysis and a preliminary experimental investigation confirm the attractiveness of this very simple solution for extracting heat from solar ponds.     

Metrological analysis of the measurement system for a micro-cogenerative SOFC module

The need for environmental friendly energy conversion systems has been increasing over the last decade, also in relation to the world’s growth in energy demand. High Temperature Solid Oxide Fuel Cells (HT-SOFCs) represent a very promising technology since they can be directly fed with traditional fuels, other than hydrogen, and offer the possibility of cogeneration. Furthermore, the low dependence of their efficiency on the system size makes them suitable for distributed micro-scale energy conversion. However, SOFCs are still today in a development stage, and a great research effort is still required before commercialization becomes a reality. From this point of view, on-site experiments are crucial in order to deeply understand HT-SOFCs based cogeneration modules’ performance and applicability to residential energy sector. Even though some work is appearing in the recent literature, very often experimental data are not accompanied by measurement uncertainties analysis, which is an important aspect of any experimental campaign.In the present paper, on the basis of the on-board instruments metrological characteristics declared by the manufacturer, the a-priori uncertainty analysis of a cogenerative module based on HT-SOFCs is presented. Therefore, for each performance parameter, the value of the related uncertainty has been calculated. In the authors’ opinion, these quantities are very important for a proper evaluation of the performance of HT-SOFCs based cogeneration units and also for the essential validation of results obtained from fuel cells modeling.     

Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer

An innovative small-scale cogeneration system based on membrane reformer and PEM fuel cells is under development within the FluidCELL project. An experimental campaign has been carried out to characterize the PEM fuel cell and to define the operative conditions when integrated within the system. The hydrogen feeding the PEM is produced by a membrane reactor which in principle can separate pure hydrogen; however, in general, hydrogen purity is around 99.9%–99.99%. The focus of this work is the assessment of the PEM performance under different hydrogen purities featuring actual membrane selectivity and gases build-up by anode off-gas recirculation. Their effects on the cells voltage and local current distribution are measured at different conditions (pressure, humidity, stoichiometry, with and without air bleeding, in flow-through and dead-end operation). In flow-through mode, the cell voltage is relatively insensitive to the presence of inert gases (e.g. ?20 mV with inerts/H₂ from 0 to 20·10^?2 at 0.3 A/cm²), and resistant also to CO (e.g. ?35 mV with inerts/H₂ = 20·10^?2 and CO/H₂ from 0 to 20·10^?6 at 0.3 A/cm²), thanks to the Ru presence in the anode catalyst. Looking at the current density distribution on the cell surface, the most critical areas are the cathode inlet, likely due to insufficient air humidification, and the anode outlet, because of low hydrogen concentration and CO poisoning of the catalyst. Dead-end operation is also investigated using humid or impure hydrogen. In this case relatively small amount of impurities in the hydrogen feed rapidly reduces the cell voltage, requiring frequent purges (e.g. every 30 s with inerts/H₂ = 0.5·10^?2 at 0.3 A/cm²). These experiments set the basis for the management of the PEMFC stack integrated into the m-CHP system based on the FluidCELL concept.     

Performance analysis of a tubular solid oxide fuel cell with an indirect internal reformer

A 2‐D steady‐state mathematical model of a tubular solid oxide fuel cell with indirect internal reforming (IIR‐SOFC) has been developed to examine the chemical and electrochemical processes and the effect of different operating parameters on the cell performance. The conservation equations for energy, mass, momentum as well as the electrochemical equations are solved simultaneously employing numerical techniques. A co‐flow configuration is considered for gas streams in the air and fuel channels. The heat radiation between the preheater and reformer surface is incorporated into the model and local heat transfer coefficients are determined throughout the channels. The model predictions have been compared with the data available in the literature. The model was used to study the effect of various operating conditions on the cell performance. Numerical results indicate that as the cell operating pressure increases, the reforming reaction extends to a larger portion of the cell and the maximum temperature move away from the cell inlet. As a result, a more uniform temperature prevails in the solid structure which reduces thermal stresses. Also, at higher excess air, the rate of heat transfer to the air stream is augmented and the average cell temperature is decreased. Copyright © 2010 John Wiley & Sons, Ltd.     

High-entropy alloy anode for direct internal steam reforming of methane in SOFC

High-entropy alloy (HEA) anode and reforming catalyst, supported on gadolinium-doped ceria (GDC), have been synthesized and evaluated for the steam reforming of methane under SOFC operating conditions using a conventional fixed-bed catalytic reactor. As-synthesized HEA catalysts were subjected to various characterization techniques including N₂ adsorption/desorption analysis, SEM, XRD, TPR, TPO and TPD. The catalytic performance was evaluated in a quartz tube reactor over a temperature range of 700–800 °C, pressure of 1 atm, gas hourly space velocity (GHSV) of 45,000 h^?1 and steam-to-carbon (S/C) ratio of 2. The conversion and H₂ yield were calculated and compared. HEA/GDC exhibited a lower conversion rate than those of Ni/YSZ and Ni/GDC at 700 °C, but showed superior stability without any sign of carbon deposition unlike Ni base catalyst. HEA/GDC was further evaluated as an anode in a SOFC test, which showed high electrochemical stability with a comparable current density obtained on Ni electrode. The SOFC reported low and stable electrode polarization. Post-test analysis of the cell showed the absence of carbon at and within the electrode. It is suggested that HEA/GDC exhibits inherent robustness, good carbon tolerance and stable catalytic activity,` which makes it a potential anode candidate for direct utilization of hydrocarbon fuels in SOFC applications.     

Optimizing a steam-methane reformer for hydrogen production

By means of steam reforming, natural gas is converted to carbon dioxide and hydrogen. The reactions take place in reactor tubes which are covered with catalyst at the inside, where the reactive mixture flows. At the outside they are heated by combustion of natural gas with air.     

Influence of current densities in SOFC and PEFC stacks on a SOFC–PEFC combined system

A solid oxide fuel cell (SOFC)–polymer electrolyte fuel cell (PEFC) combined system was investigated by numerical simulation. Here, the effect of the current densities in the SOFC and the PEFC stacks on the system's performance is evaluated under a constant fuel utilization condition. It is shown that the SOFC–PEFC system has an optimal combination of current densities, for which the electrical efficiency is highest. The optimal combination exists because the cell voltage in one stack increases and that of the other stack decreases when the current densities are changed. It is clarified that there is an optimal size of the PEFC stack in the parallel-fuel-feeding-type SOFC–PEFC system from the viewpoint of efficiency, although a larger PEFC stack always leads to higher electrical efficiency in the series-fuel-feeding-type SOFC–PEFC system. The 40 kW-class PEFC stack is suitable for the 110 kW-class SOFC stack in the parallel-fuel-feeding type SOFC–PEFC system.     

Dynamic modeling of a methanol reformer—PEMFC stack system for analysis and design

Considerable effort has been devoted to the modeling of proton exchange membrane fuel cells (PEMFCs) as well as fuel processing units (FPUs). Many of these models consider only steady state analysis; the available dynamic models typically operate only in simple open loop configurations. However, a liquid fuel processor/PEMFC stack power unit for vehicular application will require tight integration and regulation of multiple units in order to function economically and reliably. Moreover, vehicular operation is inherently dynamic in nature, so traditional steady state process design approaches will be of limited value.This work addresses a minimum set of subcomponents necessary for modeling an overall vehicular power system. Additionally, the integration and control of these sub-units is addressed so that the unit can be operated as needed in a vehicular application by following a reference power trajectory. A number of design and operational parameters can be adjusted and the impact on system performance studied. Based on this preliminary analysis, heuristics are developed for optimal operation and design.     

Numerical evaluation of a parallel fuel feeding SOFC–PEFC system using seal-less planar SOFC stack

We propose a system that combines a seal-less planar solid oxide fuel cell (SOFC) stack and polymer electrolyte fuel cell (PEFC) stack. In the proposed system, fuel for the SOFC (SOFC fuel) and fuel for the PEFC (PEFC fuel) are fed to each stack in parallel. The steam reformer for the PEFC fuel surrounds the seal-less planar SOFC stack. Combustion exhaust heat from the SOFC stack is used for reforming the PEFC fuel. We show that the electrical efficiency in the SOFC–PEFC system is 5% higher than that in a simple SOFC system using only a seal-less planar SOFC stack when the SOFC operation temperature is higher than 973 K.     

SOFC／GT混合系统的计算机仿真

固体氧化物燃料电池(SOFC)/燃气轮机(GT)混合系统以其高效、低污染排放的优点受到各国尤其是西方发达国家的重视,被认为是解决21世纪能源与环境问题的关键技术之一.计算机仿真方法是目前研究SOFC/GT混合系统的主要方法之一.本文利用Aspen Custom Modeler 仿真平台对一回热器空气再热式SOFC/GT系统进行了仿真分析,给出了设计工况下混合循环各部件节点的状态参数值,分别就压比、燃料电池电流密度和燃料利用率对系统发电效率的影响进行了仿真分析.仿真结果表明,系统总发电效率随压比的关系曲线呈抛物线型,随着压比的增大,系统的发电效率先增大后减小.系统总发电效率与电流密度成反比关系,而与燃料利用率成正比;燃料电池的发电效率与电流密度和燃料利用率均成反比.     

Maximizing the efficiency of a HT-PEMFC system integrated with glycerol reformer

The efficiency and output power density of an integrated high temperature polymer electrolyte fuel cell system and glycerol reformer are studied. The effects of reformer temperature, steam to carbon ratio (S/C), fuel cell temperature, and anode stoichiometric ratio are examined. An increase in anode stoichiometric ratio will reduce CO poisoning effect at cell’s anode but cause lower fuel utilization towards energy generation. High S/C operation requires large amount of the energy available, however, it will increase anode tolerance to CO poisoning and therefore will lead to enhanced cell performance. Consequently, the optimum gas composition and flow rate is very dependent on cell operating current density and temperature. For example, at low current densities, similar efficiencies were obtained for all the S/C ratio studied range at cell temperature of 423.15 K, however, at cell temperature of 448.15 K, low S/C ratio provided higher efficiency in comparison to high S/C ratio. High S/C is essential when operating the cells at high current densities where CO has considerable impact on cell performance. Optimal conditions that provide the maximum power density at a given efficiency are reported.     

Combined pre-reformer/reformer system utilizing monolith catalysts for hydrogen production

The pre-reforming of higher hydrocarbon, propane, was performed to generate hydrogen from LPG without carbon deposition on the catalysts. A Ru/Ni/MgAl₂O₄ metallic monolith catalyst was employed to minimize the pressure drop over the catalyst bed. The propane pre-reforming reaction conditions for the complete conversion of propane with no carbon formation were identified to be the following: space velocities over 2400 h⁻¹ and temperatures between 400 and 450 °C with a H₂O/C₁ ratio of 3. The combined pre-reformer and the main reformer system with the Ru/Ni/MgAl₂O₄ metallic monolith catalyst was employed to test the conversion propane to syngas where the reaction heat was provided by catalytic combustors. Propane was converted in the pre-reformer to 52.5% H₂, 27.0% CH₄, 17.5% CO, and 3.0% CO₂ with a 331 °C inlet temperature and a 482 °C catalyst outlet temperature. The main steam reforming reactor converted the methane from the pre-reformer with a conversion of higher than 99.0% with a 366 °C inlet temperature and an 824 °C catalyst outlet temperature. With a total of 912 cc of the Ru/Ni/MgAl₂O₄ metallic monolith catalyst in the main reformer, the H₂ production from the propane reached an average of 3.25 Nm³h⁻¹ when the propane was fed at 0.4 Nm³h⁻¹.     

Off-design analysis of SOFC hybrid system

This paper sets out the results of mathematical modeling and numerical simulations of the off-design (part-load) operation of the solid oxide fuel cell hybrid system (SOFC-HS).     

Dynamic tank in series modeling of direct internal reforming SOFC

A dynamic tank in series reactor model of a direct internally reforming solid oxide fuel cell is presented and validated using experimental data as well as a computational fluid dynamics (CFD) model for the spatial profiles. The effect of the flow distribution pattern at the inlet manifold on the cell performance is studied with this model. The tank in series reactor model provides a reasonable understanding of the spatio‐temporal distribution of the key parameters at a much lesser computational cost when compared to CFD methods. The predicted V–I curves agree well with the experimental data at different inlet flows and temperatures, with a difference of less than ±1.5%. In addition, comparison of the steady‐state results with two‐dimensional contours from a CFD model demonstrates the success of the adopted approach of adjusting the flow distribution pattern at the inlet boundaries of different continuous stirred tank reactor compartments. The spatial variation of the temperature of the PEN structure is captured along with the distributions of the current density and the anode activation over‐potential that strongly related to the temperature as well as the species molar fractions. It is found that, under the influence of the flow distribution pattern and reaction rates, the dynamic responses to step changes in voltage (from 0.819 to 0.84 V), fuel flow (15%) and temperature changes (30 °C), on anode side and on cathode side, highly depend on the spatial locations in the cell. In general, the inlet points attain steady state rapidly compared to other regions. Copyright © 2017 John Wiley & Sons, Ltd.