Autothermal reforming of gasoline for fuel cell applications: Controller design and analysis

In this work, we address the control system options available to an autothermal reforming (ATR) reactor. The targeted application is within an on-board fuel processor for a hydrogen-fed low-temperature fuel cell. The feedback controller employs air feed rate as the manipulated variable and a measurement of catalyst temperature as the control variable. Disturbances include significant fluctuations in the measured temperature as well as large throughput changes, owning to the on-board application. Our investigation includes an analysis of a simple feedback configuration as well as feed-forward control structure. It is concluded that the feedback only method is insufficient for the unique challenges associated with on-board operation, which include fast start-up and quick load changes. While the feed-forward configuration improves performance, we found a fair amount of sensitivity with respect to model mismatch. The general conclusion is that some form of advanced control will be needed to meet the stringent performance requirements of the on-board fuel processor application.     

Internal reforming of hydrocarbon fuels in tubular solid oxide fuel cells

The application of heterogeneous catalysis has an important role to play in the successful commercial development of solid oxide fuel cell (SOFC) technology. In this paper, we present an SOFC that combines a catalyst layer with a conventional anode, allowing internal reforming via partial oxidation (POX) of fuels such as methane, propane, butane, biomass gas, etc., without coking and yielding stable power output. The catalyst layer is fabricated on the anode simply by catalyst support coating and reforming catalyst impregnation. The composition and microstructure of catalyst support layer as well as the catalyst composition was easily tailored to meet the demand of in situ reforming. The usage of catalyst layer as an integrated part of the traditional SOFC will provide a simple low-cost power-generating system at substantially higher fuel efficiency and faster start-ups, and may accelerate the application of SOFCs through the direct use of hydrocarbon fuels.     

Detailed kinetic modelling of non-catalytic ethanol reforming for SOFC applications

Ethanol is a particularly attractive alternative fuel for automotive and stationary applications. Due to its high hydrogen content, ethanol can also be utilized for hydrogen production in SOFC systems. The present study assesses the potential of non-catalytic ethanol reforming using a detailed chemical kinetic approach. A recently developed comprehensive detailed mechanism for ethanol oxidation, pyrolysis and combustion is implemented and validated against data from ethanol reformers. Comparisons between computations and experimental major and intermediate species data are shown to be satisfactory. Chemical aspects of the fuel reforming process are thoroughly investigated through rate-of-production and sensitivity analyses with particular emphasis on syngas and potential carbonaceous deposit formation. An assessment of ethanol as a primary fuel versus conventional fuels with similar hydrogen content is also numerically performed. It is shown that ethanol features higher conversion efficiency to syngas than methane. Soot precursor chemistry is shown to be largely dependant both on fuel and reactor operating conditions. Finally, the work demonstrates the limitations of the thermodynamic equilibrium approach.     

Microplasma reforming of hydrocarbons for fuel cell power

The implementation of a microplasma approach for small scale reforming processes is explored as an alternative to more standard catalyst-based processes. Plasmas are a known approach to activating a chemical reaction in place of catalysts, and microplasmas are particularly attractive owing to their extremely high electron and power densities. Their inherent compactness gives them appeal for portable applications, but their modularity leads to scalability for higher capacity. We describe the realization of experimental microplasma reactors based on the microhollow cathode discharge (MHCD) structure by silicon micromachining for device fabrication. Experiments were carried out with model hydrocarbons methane and butane in the reactors within a microfluidic flow and analytical setup. We observe several key phenomena, including the ability to liberate hydrogen from the hydrocarbons at temperatures near ambient and sub-Watt input power levels, the tendency toward hydrocarbon decomposition rather than oxidation even in the presence of oxygen, and the need for a neutral carrier to obtain conversion. Mass and energy balances on these experiments revealed conversions up to nearly 50%, but the conversion of electrical power input to chemical reaction enthalpy was only on the order of 1%. These initial, exploratory results were recorded with devices and at process settings without optimization, and are hence promising for an emerging, catalyst-free reforming approach.     

Assessment of the reactor network approach for integrated modelling of an SOFC system

Incorporation of Solid-oxide fuel cells (SOFC) into hybrid systems with CHP capabilities is an attractive option for clean and efficient decentralised electricity generation. SOFC system operation on practical liquid fuels requires an efficient preparation system for the formation of a homogeneous reformate mixture. This can be accomplished with the use of a stabilized cool flame vapouriser (SCFV) combined with a thermal partial oxidation (T-POX) reformer, and such systems are already under development. The successful and efficient thermochemical operation of an SOFC system requires an accurate determination of the optimum conditions for each constituent component (e.g. fuel processing unit, fuel cell stack, off-gas burner) and for the integrated system. The present work demonstrates a computational methodology for the thermochemical assessment of a novel SOFC system operated on liquid fuels. Simulations have been performed, both at component and system levels, using a reactor network approach, involving a simplified flow and mixing representation, while retaining full detailed chemistry. Computations are performed at a component level with reactor networks specially formulated for the SCFV and the T-POX reactors, derived on the basis of CFD calculations, coupled with detailed kinetic mechanisms for n-heptane, a reasonable diesel fuel surrogate. Model predictions are compared against experimental data, wherever possible. The individual components are integrated at a system level and parametric analyses are performed so as to determine optimum conditions for efficient and clean operation.     

Advanced tubular solid oxide fuel cells with high efficiency for internal reforming of hydrocarbon fuels

Solid oxide fuel cells (SOFCs) constitute an attractive power-generation technology that converts chemical energy directly into electricity while causing little pollution. NanoDynamics Energy (NDE) Inc. has developed micro-tubular SOFC-based portable power generation systems that run on both gaseous and liquid fuels. In this paper, we present our next generation solid oxide fuel cells that exhibit total efficiencies in excess of 60% running on hydrogen fuel and 40+% running on readily available gaseous hydrocarbon fuels such as propane, butane etc. The advanced fuel cell design enables power generation at very high power densities and efficiencies (lower heating value-based) while reforming different hydrocarbon fuels directly inside the tubular SOFC without the aid of fuel pre-processing/reforming. The integrated catalytic layered SOFC demonstrated stable performance for >1000 h at high efficiency while running on propane fuel at sub-stoichiometric oxygen-to-fuel ratios. This technology will facilitate the introduction of highly efficient, reliable, fuel flexible, and lightweight portable power generation systems.     

Direct methane solid oxide fuel cell working by gradual internal steam reforming: Analysis of operation

A solid oxide fuel cell was designed to be operated with pure hydrocarbons, without additive or carrier gas, in order to bring technological simplifications, cost reductions and to extend the fuel flexibility limits. The cell was built-up from a conventional cell (LSM/YSZ/Ni-YSZ), to which was added a Ir-CeO₂ catalyst layer at the anode side and an original current collecting system. The cell was first operated with steam in gradual internal reforming (GIR) conditions (R = [H₂O]/[CH₄] < 1) with carrier gas at the anode. The optimal operating parameters were determined in terms of flow rates, cell potential, and fuel utilisation. The cell was finally operated with pure dry methane at 900 °C at 0.6 V yielding current density of about 0.1 A cm⁻² at max power for 120 h. Small but abrupt deterioration of the performances was observed, but no carbon deposition. Electrical and chemical analysis of this degradation are provided.At total, the fuel cell was operated for more than 200 h in pure dry methane, demonstrating that gradual internal reforming actually occurred efficiently in the anode compartment, which make possible operation without reforming agent such as H₂O or CO₂ for other hydrocarbon fuels.     

Diesel steam reforming with a nickel-alumina spinel catalyst for solid oxide fuel cell application

Liquid hydrocarbons (LC) are considered as fuel cells feed and, more particularly, as solid oxide fuel cell feed. Cost-effective LC-reforming catalysts are critically needed for the successful commercialization of such technologies. An alternative to noble metal catalysts, proposed by the authors in a previous publication, has been proven efficient for diesel steam reforming (SR). Nickel, less expensive and more readily available than noble metals, was used in a form that prevents deactivation. The catalyst formulation is a Ni-alumina spinel (NiAl₂O₄) supported on alumina (Al₂O₃) and yttria-stabilized zirconia (YSZ).SR of commercial diesel was undertaken for more than 15 h at high gas hourly space velocities and steam-to-carbon ratios lower than 2. Constant diesel conversion and high hydrogen concentrations were obtained. Ni catalyst characterization revealed no detectable amounts of carbon on the spinel catalyst surface Ni. The effect of catalyst composition (Ni concentration and YSZ presence) was studied to understand and optimize the developed catalyst. Two phenomena were found to be influenced by relative catalyst composition: water-gas-shift vs reforming reaction extent, and concentration of light hydrocarbons in products.     

Ethanol internal steam reforming in intermediate temperature solid oxide fuel cell

This study investigates the performance of a standard Ni-YSZ anode supported cell under ethanol steam reforming operating conditions. Therefore, the fuel cell was directly operated with a steam/ethanol mixture (3 to 1 molar). Other gas mixtures were also used for comparison to check the conversion of ethanol and of reformate gases (H₂, CO) in the fuel cell. The electrochemical properties of the fuel cell fed with four different fuel compositions were characterized between 710 and 860 °C by I-V and EIS measurements at OCV and under polarization. In order to elucidate the limiting processes, impedance spectra obtained with different gas compositions were compared using the derivative of the real part of the impedance with respect of the natural logarithm of the frequency.Results show that internal steam reforming of ethanol takes place significantly on Ni-YSZ anode only above 760 °C. Comparisons of results obtained with reformate gas showed that the electrochemical cell performance is dominated by the conversion of hydrogen. The conversion of CO also occurs either directly or indirectly through the water-gas shift reaction but has a significant impact on the electrochemical performance only above 760 °C.     

Hydrogen production by autothermal reforming of LPG for PEM fuel cell applications

Indirect partial oxidation, or oxidative steam reforming, tests of a bimetallic Pt–Ni catalyst supported on δ$\delta$-alumina were conducted in propane–n  -butane mixtures (LPG) used as feed. _H2${\mathrm{H}}_2$ production activity and _H2/CO${\mathrm{H}}_2/\mathrm{CO}$ selectivity were investigated in response to different S/C, C/_O2$\mathrm{C}/{\mathrm{O}}_2$ and W/F ratios. It was confirmed that higher steam content in the reactant stream increases both the activity and the _H2/CO${\mathrm{H}}_2/\mathrm{CO}$ selectivity of the process. Low residence times created a positive impact on catalyst activity not only for hydrogen but also for carbon monoxide production due to the increased amount of fresh hydrocarbon in the feed stream. Hence, the highest selectivity level was obtained at intermediate residence times. The response of the system to C/_O2$\mathrm{C}/{\mathrm{O}}_2$ ratio was found to depend on the available steam content due to the complex nature of IPOX. The Pt–Ni catalyst was very prone to catalyst deactivation at low S/C ratios accompanied by high C/_O2$\mathrm{C}/{\mathrm{O}}_2$ ratios, but this problem was not encountered at high S/C ratios. A comparison of catalyst performance for different propane-to-n-butane ratios in the LPG feed indicated that the Pt–Ni catalyst has slightly better activity and selectivity at higher n-butane contents at the expense of becoming more sensitive to coke deposition.     

Technological aspects of an auxiliary power unit with internal reforming methanol fuel cell

Increasing source runtime, speeding up the transient response, while minimizing weight, volume and cost of the power supply system are key requirements for portable, mobile and off-grid applications of fuel cells. In this respect, Internal Reforming Methanol Fuel Cell (IRMFC) modules were designed, constructed and tested based on an innovative double reformer (DRef) configuration and metallic bipolar plates (BPPs) with unique arrangement. Recently developed cross-linked Advent TPS^® high-temperature membrane electrode assemblies (MEAs) were employed for fuel cell operation at 210 °C. Taking into account the requirement for a light-weight and low-volume stack, Cu-based methanol reforming catalyst were supported on carbon papers, resulting in ultra-thin reformers. The proposed configuration offered a significant decrease in the weight and volume of the whole power system, as compared with previous voluminous foam-based modules. Moreover, specifically designed bipolar plates were made of coated Al-metal alloys, which proved to be stable in the strong acidic environment at elevated temperatures. The prototype 32MEAs-32DRef IRMFC stack of 100 W including home-made insulation casing, was integrated for operation at 200–210 °C and at 0.2 A cm⁻², demonstrating the functionality of the unit. A power output of 100.7 W (3.14 W per cell; 0.114 W cm⁻²) was achieved in the last run following several on-off cycles. The volumetric power density of the IRMFC stack including insulation and casing is around 30 W per lt, being among the highest reported either in the case of portable or stationary applications. Overall, the observed stability of reformers and bipolar plates was satisfactory within the timeframe of the work undertaken. Specific targets for improvement of the efficiency were identified, and the main drawback had to do with low thermal and mechanical stability of the membranes under start-up/shut-down transient operation.     

Evaluation of fuel diversity in Solid Oxide Fuel Cell system

Operability of Solid Oxide Fuel Cell (SOFC) on numerous fuels has been widely counted as a leading advantage in literature. In a designed system, however, switching from a fuel to another is not practically a straightforward task as this causes several system performance issues in both dynamic and steady-state modes. In order to demonstrate the system fuel diversity capabilities, these consequences must be well-evaluated by quantifying the characteristic measures for numerous fuel cases and also potential combinations. From this viewpoint, the numerical predictive models play a critical role. This paper aims to investigate the performance of a SOFC system fed by various fuels using a demonstrated system level model. Process configuration and streams results of a real-life SOFC system rig published in literature are used to validate the model. The presented model is capable not only of capturing the system performance measures but also the SOFC internal variable distributions, allowing the multiscale study of fuel switching scenarios. The fuel change impacts on the system are simulated by considering various fuel sources, i.e., natural gas, biogas, and syngas. Moreover, applications of simulated fuel mixtures are assessed. The modelling results show significant concerns about fuel switching in a system in terms of variation of efficiencies, stack internal temperature and current density homogeneity, and environmental issues. Moreover, the results reveal opportunities for multi-fuel design to address the operation and application requirements such as optimisation of the anode off-gas recycling rate and the thermal-to-electrical ratio as well as the system specific greenhouse gases, i.e., g-CO_x/Wh release.     

Modeling and dynamics of an autothermal JP5 fuel reformer for marine fuel cell applications

In this work, a dynamic model of an integrated autothermal reformer (ATR) and proton exchange membrane fuel cell (PEM FC) system and model-based evaluation of its dynamic characteristics are presented. The ATR reforms JP5 fuel into a hydrogen rich flow. The hydrogen is extracted from the reformate flow by a separator membrane (SEP), then supplied to the PEM FC for power generation. A catalytic burner (CB) and a turbine are also incorporated to recuperate energy from the remaining SEP flow that would otherwise be wasted. A dynamic model of this system, based on the ideal gas law and energy balance principles, is developed and used to explore the effects of the operating setpoint selection of the SEP on the overall system efficiency. The analysis reveals that a trade-off exists between the SEP efficiency and the overall system efficiency. Finally the open loop system simulation results are presented and conclusions are drawn on the SEP operation.     

A complete miniaturized microstructured methanol fuel processor/fuel cell system for low power applications

A complete miniaturized methanol fuel processor/fuel cell system was developed and put into operation as compact hydrogen supplier for low power application. The whole system consisting of a micro-structured evaporator, a micro-structured reformer and two stages of preferential oxidation of CO (PROX) reactor, micro-structured catalytic burner, and fuel cell was operated to evaluate the performance of the whole production line from methanol to electricity. The performance of micro methanol steam reformer and PROX reactor was systematically investigated. The effect of reaction temperature, steam to carbon ratio, and contact time on the methanol steam reformer performance is presented in terms of catalytic activity, selectivity, and reformate yield. The performance of PROX reactor fed with the reformate produced by the reformer reactor was evaluated by the variation of reaction temperature and oxygen to CO ratio. The results demonstrate that micro-structured device may be an attractive power source candidate for low power application.     

Detailed kinetic modelling of the T-POX reforming process using a reactor network approach

Solid-oxide fuel cells (SOFCs) are particularly attractive since they offer clean and efficient decentralized electricity generation and can be incorporated into hybrid systems with CHP capabilities. However, small scale SOFC systems operated with hydrocarbon fuels require external reforming. A very promising reforming technology involves partial oxidation (POX) in an inert porous material (T-POX reformer). The present work provides extensive numerical simulation of a prototype T-POX reformer operating with methane. Computations are performed using a reactor network approach incorporating full detailed chemistry and results are successfully compared against experimentally determined hydrocarbon species data. Computational results are further used to identify the elementary kinetic pathways for hydrocarbon fuel partial oxidation, molecular growth and pollutant formation as well as to identify optimum reformer operating conditions.     

Thermodynamic study of hydrogen production from crude glycerol autothermal reforming for fuel cell applications

This study presents a thermodynamic analysis of hydrogen production from an autothermal reforming of crude glycerol derived from a biodiesel production process. As a composition of crude glycerol depends on feedstock and processes used in biodiesel production, a mixture of glycerol and methanol, major components in crude glycerol, at different ratios was used to investigate its effect on the autothermal reforming process. Equilibrium compositions of reforming gas obtained were determined as a function of temperature, steam to crude glycerol ratio, and oxygen to crude glycerol ratio. The results showed that at isothermal condition, raising operating temperature increases hydrogen yield, whereas increasing steam to crude glycerol and oxygen to crude glycerol ratios causes a reduction of hydrogen concentration. However, high temperature operation also promotes CO formation which would hinder the performance of low-temperature fuel cells. The steam to crude glycerol ratio is a key factor to reduce the extent of CO but a dilution effect of steam should be considered if reforming gas is fed to fuel cells. An increase in the ratio of glycerol to methanol in crude glycerol can increase the amount of hydrogen produced. In addition, an optimal operating condition of glycerol autothermal reforming at a thermoneutral condition that no external heat to sustain the reformer operation is required, was investigated.     

Dimethoxymethane and trimethoxymethane as alternative fuels for fuel cells

The electrooxidation of dimethoxymethane (DMM) and trimethoxymethane (TMM) was studied at different platinum-based electrocatalysts deposited onto a titanium mesh substrate by thermal decomposition of chloride precursors. Half-cell tests showed an increase in oxidation current for the methoxy fuels at the platinum electrode with the alloying of ruthenium and tin. Increase in reaction temperature and reactant concentration showed an increase in current density for the mesh-based anodes similar to carbon-supported catalysts. Single fuel cell tests, employing the titanium mesh anode with PtRu and PtSn catalysts showed maximum power densities up to 31 mW cm⁻² and 48 mW cm⁻² for 1.0 mol dm⁻³ aqueous solutions of DMM and TMM, respectively at 60 °C using oxygen.     

A reduced 1D dynamic model of a planar direct internal reforming solid oxide fuel cell for system research

A reduced 1D dynamic model of a planar direct internal reforming SOFC (DIR-SOFC) is presented in this paper for system research by introducing two simplifications. The two simplification strategies employed are called Integration and Average, respectively. The present model is evaluated with a detailed 1D SOFC model, which does not introduce the two simplifications, and a lumped parameter (i.e., 0D) SOFC model. Results show that under the operating conditions investigated the accuracy of the reduced model is not significantly compromised by the two simplifications in prediction of the outlet gas flow rates and molar fractions, the outlet temperatures, and the cell voltage, while its computational time is significantly decreased by them. Moreover, it is quite simple in form. Therefore, the reduced SOFC model is attractive for system research. Compared with the lumped model, the reduced SOFC model is an improvement with regard to accuracy because it takes into account the spatially distributed nature of SOFCs to a certain extent. The discretized node number for solving the reduced model can be taken as an adjustable parameter in modeling, and is determined according to specific modeling requirements.     

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.     

Catalytic steam reforming of methane,methanol, and ethanol over Ni/YSZ: The possible use of these fuels in internal reforming SOFC

This study investigated the possible use of methane, methanol, and ethanol with steam as a direct feed to Ni/YSZ anode of a direct internal reforming Solid Oxide Fuel Cell (DIR-SOFC). It was found that methane with appropriate steam content can be directly fed to Ni/YSZ anode without the problem of carbon formation, while methanol can also be introduced at a temperature as high as 1000 °C. In contrast, ethanol cannot be used as the direct fuel for DIR-SOFC operation even at high steam content and high operating temperature due to the easy degradation of Ni/YSZ by carbon deposition. From the steam reforming of ethanol over Ni/YSZ, significant amounts of ethane and ethylene were present in the product gas due to the incomplete reforming of ethanol. These formations are the major reason for the high rate of carbon formation as these components act as very strong promoters for carbon formation.