Dynamic modeling of high temperature PEM fuel cell start-up process

High temperature proton exchange membrane fuel cells (HT-PEMFCs) are considered to be the next generation fuel cells. Compared with standard low temperature proton exchange membrane fuel cells (LT-PEMFCs) the electrochemical kinetics for electrode reactions are enhanced by using a polybenzimidazole based membrane at an operation temperature between 160 °C and 180 °C. However, starting HT-PEMFCs from room temperature to a proper operation temperature is a challenge in application where a fast start of the fuel cell is required such as in uninterruptible power supply systems. There are different methods to start-up HT-PEMFCs. Based on a 3D physical model of a single HT-PEMFC, the start-up process is analyzed by comparing the start-up duration of the different start-up concepts. Furthermore, the temperature distribution in the HT-PEMFC is also analyzed. Finally, an optimal start-up method is proposed for the given cell configuration.     

Self-sustained operation and durability testing of a 300 W-class micro-structured LPG fuel processor

A miniaturized fuel processor for LPG has been developed and put into operation as compact hydrogen supply system for low power applications. The fuel processor consists of an integrated micro-structured evaporator and a micro-structured reformer both integrated with micro-structured catalytic burners, heat exchangers, and a micro-structured water-gas shift (WGS) stage. In the current paper, performance data of a coupled LPG steam reformer/catalytic burner are presented, which has been running stably over 1060 h with repeated start-up and shut-down cycles. On top of that, some performance data of complete LPG fuel processors will be shown, which have been operated up to 3500 h in combination with high temperature PEM fuel cell stacks. These fuel processing systems are capable to convert LPG with a nominal hydrogen production rate of 0.263 Nm³ h⁻¹. It could be demonstrated, that the micro-structured devices are not only compact but show also high reliability and durability.     

Methanol reformer integrated phosphoric acid fuel cell (PAFC) based compact plant for field deployment

Naval Material Research Laboratory (NMRL), based on the firm confidence of her core competence on material development, started an ambitious program on development of fuel cells for various Defense and non-Defense application in early nineties. The primary emphasis of this program is to develop phosphoric acid fuel cell (PAFC) based power plants integrated with hydrogen generators along with other accessories. In the process of development, it is understood that online generation of hydrogen from a liquid fuel is the key to success. Methanol, a liquid fuel, can be reformed easily with few side products and the resultant hydrogen rich reformer gas can be directly fed to a PAFC. Such configuration keeps the basic system simple and free of complicated filters and instrumentation.NMRL has developed a series of catalytic burners with high efficiency as the primary heat transfer source from the hot catalytic surface is based on conduction rather than convection as is done normally. Vaporizer is a coiled arrangement and reformer is hollow sections filled with Cu/Al₂O₃/ZnO catalyst, and the same is integrated with catalytic burners. Such arrangement is modular in nature and each reformer has hydrogen generation capacity of 90 lpm and start-up time is around half an hour. Modular design of reformer reactor allow them to used in different capacity plants such as a 2 kW plant configured with a reformer reactor with two vaporizer and 15 kW plant configured with seven nos. of reformer reactors and seven no. of vaporizer. The waste heat of the fuel cell and the same from the reformer burner flue is used to meet most of the reformer heat load. The catalytic burner of the reformer burns both waste hydrogen and methanol with very little excess air. PAFC being tolerant to CO (up to 1%) can be directly operated with the hydrogen rich reformer gas and the lean gas from the fuel cell is burnt into the reformer system.The raw DC output power is converted into either 100 VDC or 220 V single phase, 50 Hz sinusoidal AC power through appropriate power electronics. These configurations give overall efficiency of the plant to around 35-40 % based on LHV of Hydrogen. A battery bank is also incorporated to cater for the plant start-up and other temporary auxiliary power which get charged from the fuel cell output. Such configuration lead to the development of methanol reformer integrated PAFC based power plants of capacity ranging from 2 kW to 15 kW. The system is designed for continuous power production in the field. These plants are suitable for remote area, distributed power generation and application such as battery charging, domestic load etc.     

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.     

Fast starting fuel processor for automotive fuel cell systems

A fuel processor was constructed which incorporated two burners with direct steam generation by water injection into the burner exhaust. These burners with direct water vaporization enabled rapid fuel processor start-up for automotive fuel cell systems. The fuel processor consisted of a conventional chain of reactors: auto-thermal reformer (ATR), water gas shift (WGS) reactor and preferential oxidation (PrOx) reactor. The criticality of steam to the fuel reforming process was illustrated. By utilizing direct vaporization of water, and hydrogen for catalyst light-off, excellent start performance was obtained with a start time of 20 s to 30% power and 140 s to full power.     

Compact gasoline fuel processor for passenger vehicle APU

Due to the increasing demand for electrical power in today's passenger vehicles, and with the requirements regarding fuel consumption and environmental sustainability tightening, a fuel cell-based auxiliary power unit (APU) becomes a promising alternative to the conventional generation of electrical energy via internal combustion engine, generator and battery. It is obvious that the on-board stored fuel has to be used for the fuel cell system, thus, gasoline or diesel has to be reformed on board. This makes the auxiliary power unit a complex integrated system of stack, air supply, fuel processor, electrics as well as heat and water management. Aside from proving the technical feasibility of such a system, the development has to address three major barriers:start-up time, costs, and size/weight of the systems. In this paper a packaging concept for an auxiliary power unit is presented. The main emphasis is placed on the fuel processor, as good packaging of this large subsystem has the strongest impact on overall size.The fuel processor system consists of an autothermal reformer in combination with water–gas shift and selective oxidation stages, based on adiabatic reactors with inter-cooling. The configuration was realized in a laboratory set-up and experimentally investigated. The results gained from this confirm a general suitability for mobile applications. A start-up time of 30 min was measured, while a potential reduction to 10 min seems feasible. An overall fuel processor efficiency of about 77% was measured. On the basis of the know-how gained by the experimental investigation of the laboratory set-up a packaging concept was developed. Using state-of-the-art catalyst and heat exchanger technology, the volumes of these components are fixed. However, the overall volume is higher mainly due to mixing zones and flow ducts, which do not contribute to the chemical or thermal function of the system. Thus, the concept developed mainly focuses on minimization of those component volumes. Therefore, the packaging utilizes rectangular catalyst bricks and integrates flow ducts into the heat exchangers. A concept is presented with a 25 l fuel processor volume including thermal isolation for a 3 kW_el auxiliary power unit. The overall size of the system, i.e. including stack, air supply and auxiliaries can be estimated to 44 l.     

Metal membrane-type 25-kW methanol fuel processor for fuel-cell hybrid vehicle

A 25-kW on-board methanol fuel processor has been developed. It consists of a methanol steam reformer, which converts methanol to hydrogen-rich gas mixture, and two metal membrane modules, which clean-up the gas mixture to high-purity hydrogen. It produces hydrogen at rates up to 25 N m³/h and the purity of the product hydrogen is over 99.9995% with a CO content of less than 1 ppm. In this fuel processor, the operating condition of the reformer and the metal membrane modules is nearly the same, so that operation is simple and the overall system construction is compact by eliminating the extensive temperature control of the intermediate gas streams. The recovery of hydrogen in the metal membrane units is maintained at 70–75% by the control of the pressure in the system, and the remaining 25–30% hydrogen is recycled to a catalytic combustion zone to supply heat for the methanol steam-reforming reaction. The thermal efficiency of the fuel processor is about 75% and the inlet air pressure is as low as 4 psi. The fuel processor is currently being integrated with 25-kW polymer electrolyte membrane fuel-cell (PEMFC) stack developed by the Hyundai Motor Company. The stack exhibits the same performance as those with pure hydrogen, which proves that the maximum power output as well as the minimum stack degradation is possible with this fuel processor. This fuel-cell ‘engine’ is to be installed in a hybrid passenger vehicle for road testing.     

Optimal sizing of a fuel processor for auxiliary power applications of a fuel cell-powered passenger car

The present study considers the optimal sizing of a three-way hybrid powertrain consisting of a compact reformer, a compact battery and a low temperature PEM fuel cell stack serving as the main power unit. A simulation model consisting of the relevant characteristic parameters of the three power sources has been developed and has been used to study the fuel utilization features of the hybrid powertrain while going through the NEDC driving cycle with a given auxiliary power requirement. The optimality is based on minimizing fuel cost while having an assured range of 500 km under practical driving conditions and a further 100 km under reduced auxiliary power usage. It is shown that for performance characteristics of Toyota Mirai and for average auxiliary power consumption of 5 kW, a smaller NiMH battery size of 1.3 kWh together with a fuel processor of 5.6 kW constant output would be optimal with a further requirement of 25% more hydrogen and 33 kg of ethanol to be carried on-board. Substantial reductions in vehicle mass and fuel load can be achieved for more modest performance characteristics and auxiliary power consumption.     

Fuel effects on start-up energy and efficiency for automotive PEM fuel cell systems

This paper investigates the effects of various fuels on hydrogen production for automotive PEM fuel cell systems. Gasoline, methanol, ethanol, dimethyl ether and methane are compared for their effects on fuel processor size, start-up energy and overall efficiencies for 50 kW_e fuel processors. The start-up energy is the energy required to raise the temperature of the fuel processor from ambient temperature (20 °C) to that of the steady-state operating temperatures. The fuel processor modeled consisted of an equilibrium-ATR (autothermal), high-temperature water gas shift (HTS), low-temperature water gas shift (LTS) and preferential oxidation (PrOx) reactors. The individual reactor volumes with methane, dimethyl ether, methanol and ethanol were scaled relative to a gasoline-fueled fuel processor meeting the 2010 DOE technical targets. The modeled fuel processor volumes were, 25.9 L for methane, 30.8 L for dimethyl ether, 42.5 L for gasoline, 43.7 L for ethanol and 45.8 L for methane. The calculated fuel processor start-up energies for the modeled fuels were, 2712 kJ for methanol, 3423 kJ for dimethyl ether, 6632 kJ for ethanol, 7068 kJ for gasoline and 7592 kJ for methane. The modeled overall efficiencies, correcting for the fuel processor start-up energy using a drive cycle of 33 miles driven per day, were, 38.5% for dimethyl ether, 38.3% for methanol, 37% for gasoline, 34.5% for ethanol and 33.2% for methane assuming a steady-state efficiency of 44% for each fuel.     

Control and experimental characterization of a methanol reformer for a 350 W high temperature polymer electrolyte membrane fuel cell system

This work presents a control strategy for controlling the methanol reformer temperature of a 350 W high temperature polymer electrolyte membrane fuel cell system, by using a cascade control structure for reliable system operation. The primary states affecting the methanol catalyst bed temperature is the water and methanol mixture fuel flow and the burner fuel/air ratio and combined flow. An experimental setup is presented capable of testing the methanol reformer used in the Serenergy H3 350 Mobile Battery Charger; a high temperature polymer electrolyte membrane (HTPEM) fuel cell system. The experimental system consists of a fuel evaporator utilizing the high temperature waste gas from the cathode air cooled 45 cell HTPEM fuel cell stack. The fuel cells used are BASF P1000 MEAs which use phosphoric acid doped polybenzimidazole membranes. The resulting reformate gas output of the reformer system is shown at different reformer temperatures and fuel flows, using the implemented reformer control strategy. The gas quality of the output reformate gas is of HTPEM grade quality, and sufficient for supporting efficient and reliable HTPEM fuel cell operation with CO concentrations of around 1% at the nominal reformer operating temperatures. As expected increasing temperatures also increase the dry gas CO content of the reformate gas and decreases the methanol slip. The hydrogen content of the gas was measured at around 73% with 25% CO₂.     

Rapid start-up strategy of 1 kWe diesel reformer by solid oxide fuel cell integration

A rapid start-up strategy of a diesel reformer for on-board fuel cell applications was developed by fuel cell integration. With the integration with metal-supported solid oxide fuel cell which has high thermal shock resistance, a simpler and faster start-up protocol of the diesel reformer was obtained compared to that of the independent reformer setup without considering fuel cell integration. A reformer without fuel cell integration showed unstable reactor temperatures during the start-up process, which affects the reforming catalyst durability. By utilizing waste heat from the fuel cell stack, steam required at the diesel autothermal reforming could be stably provided during the start-up process. The developed diesel reformer was thermally sustainable after the initial heat-up process. As a result, the overall start-up time of the reformer after the diesel supply was reduced to 9 min from the diesel supply compared to 22 min without fuel cell integration.     

Pollutant emissions of burners for steam reformers for residential power supply

The cogeneration of heat and power by means of a fuel cell based CHP unit is a promising option for efficient residential power supply. For most applications natural gas is used as fuel. One main component of such a CHP unit is a fuel processor in order to generate hydrogen from the natural gas with hydrogen thermal power output of about 6 kW. Usually the steam reforming process is used for hydrogen production. In order to meet the heat demand of the endothermic steam reforming process the fuel processor is equipped with a burner, which has to work with natural gas during start up phase and mainly with the low calorific anodic off gas of the fuel cell stack during normal operation.The presented work is focused on aspects of the main pollutant emissions (carbon monoxide and nitrogen oxide) of burners integrated into the reformer. Experimental investigations of two different burners, which were developed and adapted to the steam reformer requirements, in a real fuel processor environment show, that it is possible to operate both burner concepts with high and low calorific gases with very low pollutant emissions in order to compete with emissions of current heating boilers, which are in the range of 15 mg kWh⁻¹ for CO and of 20 mg kWh⁻¹ for NO_x by adjusting suitable excess air ratios in the range of 1.2-1.4.But it is also demonstrated, that the efficiency of the fuel processor is influenced by the excess air ratio. An increase of the air ratio from 1.05 to 1.45 leads to an decrease of the efficiency from 80% to 76%. This results in a conflict of objectives between low pollutant emissions and high system efficiencies. The choice of a suitable burner concept and the definition of a suitable operation strategy can be based on the presented results. Additionally, aspects like fuel processor geometry, flame monitoring, pressure drop in the burner feed gas line as well as in the flue gas duct, investment costs and safety items have also to be considered for the burner selection.     

Multi-walled carbon nanotubes decorated by platinum catalyst for high temperature PEM fuel cell

In the literature, studies on platinum catalysts deposited on multi-walled carbon nanotube (Pt/MWCNT) have been mostly focused on low temperature fuel cell (LT-PEMFC) applications. In this study, we focus the synthesis and characterization of high temperature fuel cell (HT-PEMFC) performance of Pt/MWCNT in short and long term. The structural properties of the Pt/MWCNT electrocatalyst were analyzed by XRD, TGA, SEM and TEM measurements. The Pt/MWCNTs were also characterized by electrochemical measurements for durability estimation. Laboratory scale MEA with Pt/MWCNT was prepared by ultrasonic coating technique and has been tested in situ in single HT-PEMFC. Performance curves in dry Hydrogen/Air system were obtained that demonstrated performance comparable to commercial catalysts in that HT-PEMFC. The characterizations specified that the electrocatalytic and HT-PEMFC performance of the Pt/MWCNT catalysts are higher power density (0.360 W/cm²) than Pt/C (0.310 W/cm²) at 160 °C. The results obtained show that the synthesized catalysts are suitable for high temperature applications. In addition, the stability studies of MEAs prepared with Pt/MWCNT catalyst were performed by AST tests and compared with Pt/C based MEA.     

Hydrogen production with integrated microchannel fuel processor for portable fuel cell systems

An integrated microchannel methanol processor was developed by assembling unit reactors, which were fabricated by stacking and bonding microchannel patterned stainless steel plates, including fuel vaporizer, heat exchanger, catalytic combustor and steam reformer. Commercially available Cu/ZnO/Al₂O₃ catalyst was coated inside the microchannel of the unit reactor for steam reforming. Pt/Al₂O₃ pellets prepared by ‘incipient wetness’ were filled in the cavity reactor for catalytic combustion. Those unit reactors were integrated to develop the fuel processor and operated at different reaction conditions to optimize the reactor performance, including methanol steam reformer and methanol catalytic combustor. The optimized fuel processor has the dimensions of 60 mm × 40 mm × 30 mm, and produced 450sccm reformed gas containing 73.3% H₂, 24.5% CO₂ and 2.2% CO at 230–260 °C which can produce power output of 59 Wt.     

Hydrogen generation by alcohol reforming in a tandem cell consisting of a coupled fuel cell and electrolyser

The performance of a novel electro-reformer for the production of hydrogen by electro-reforming alcohols (methanol, ethanol and glycerol) without an external electrical energy input is described. This tandem cell consists of an alcohol fuel cell coupled directly to an alcohol reformer, negating the requirement for external electricity supply and thus reducing the cost of operation and installation. The tandem cell uses a polymer electrolyte membrane (PEM) based fuel cell and electrolyser. At 80 °C, hydrogen was generated from methanol, by the tandem PEM cell, at current densities above 200 mA cm⁻², without using an external electricity supply. At this condition the electro-reformer voltage was 0.32 V at an energy input (supplied by the fuel cell component) of 0.91 kWh/Nm³; i.e. less than 20% of the theoretical value for hydrogen generation by water electrolysis (4.7 kWh/Nm³) with zero electrical energy input from any external power source. The hydrogen generation rate was 6.2 × 10⁻⁴ mol (H₂) h⁻¹. The hydrogen production rate of the tandem cell with ethanol and glycerol was approximately an order of magnitude lower, than that with methanol.     

Start-up strategies of an experimental fuel processor

In this work, cold start-up of a methane fuel processor is explored. The experimental fuel processor is intended to provide hydrogen for a proton exchange membrane (PEM) fuel cell for the power generation (3 kWe). A dynamic model describing a series of reactors, the reformer, three water–gas shift reactors, and preferential reactor is constructed. Two important factors for rapid start-up are identified: speed of temperature front propagation and acceptable CO concentration. Steady-state analyses reveal that the fuel feed flow rate with fixed steam-to-carbon and air-to-carbon ratios is an ideal manipulated variable. Considering both large initial heat flux and gradual transition back to nominal operation, the shape of feed manipulation is determined. With the feed scenario available, the fuel processor start-up can be formulated as a constrained optimization problem and can be solved numerically. From optimization result, a heuristic is generated for rapid start-up of a fuel processor. This leads to a 25% improvement in the start-up time. Finally, issues of design modification are explored for further reduction in the start-up time.     

A diesel fuel processor for stable operation of solid oxide fuel cells system: II. Integrated diesel fuel processor for the operation of solid oxide fuel cells

Post-reforming experimental results for the complete removal of light hydrocarbons from diesel reformate are introduced in part I. In part II of the paper, an integrated diesel fuel processor is investigated for the stable operation of SOFCs. Several post-reforming processors have been operated to suppress both sulfur poisoning and carbon deposition on the anode catalyst. The integrated diesel fuel processor is composed of an autothermal reformer, a desulfurizer, and a post-reformer. The autothermal reforming section in the integrated diesel fuel processor effectively decomposes aromatics, and converts fuel into H₂-rich syngas. The subsequent desulfurizer removes sulfur-containing compounds present in the diesel reformate. Finally, the post-reformer completely removes the light hydrocarbons, which are carbon precursors, in the diesel reformate. We successfully operate the diesel reformer, desulfurizer, and post-reformer as microreactors for about 2500 h in an integrated mode. The degradation rate of the overall reforming performance is negligible for the 2000 h, and light hydrocarbons and sulfur-containing compounds are completely removed from the diesel reformate.     

A gasoline fuel processor designed to study quick-start performance

A fast-start capability is a key requirement for on-board fuel processors for automotive fuel cell systems operating on gasoline fuel. This paper reports on the design and fabrication of a suitable fuel processor having this capability and discusses estimates of the start-up fuel consumption for the laboratory unit. Also discussed are the start-up strategy and the results of a start-up simulation, which showed that the fuel processor can deliver 90% of the rated hydrogen capacity in 60 s, producing a product gas that contains >30% hydrogen and <50 ppm carbon monoxide. The start-up fuel consumption was estimated on the basis of the thermal mass of the fabricated components; the 10-kW_e laboratory unit was estimated to require >2.9 MJ of fuel energy.     

Experimental performance evaluation of an ammonia-fuelled microchannel reformer for hydrogen generation

Microchannel reactors appear attractive as integral parts of fuel processors to generate hydrogen (H₂) for portable and distributed fuel cell applications. The work described in this paper evaluates, characterizes, and demonstrates miniaturized H₂ production in a stand-alone ammonia-fuelled microchannel reformer. The performance of the microchannel reformer is investigated as a function of reaction temperature (450–700 °C) and gas-hourly-space-velocity (6520–32,600 Nml g_cat⁻¹ h⁻¹). The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH₃ conversion at 32,600 Nml g_cat⁻¹ h⁻¹ and 700 °C to generate an estimated fuel cell power output of 5.7 W_e and power density of 16 kW_e L⁻¹ (based on effective reactor volume). At the same time, reformer operation yielded low pressure drop (<10 Pa mm⁻¹) for all conditions considered. Overall, the microchannel reformer performed sufficiently exceptional to warrant serious consideration in supplying H₂ to fuel cell systems.     

A natural-gas fuel processor for a residential fuel cell system

A system model was used to develop an autothermal reforming fuel processor to meet the targets of 80% efficiency (higher heating value) and start-up energy consumption of less than 500 kJ when operated as part of a 1-kWe natural-gas fueled fuel cell system for cogeneration of heat and power. The key catalytic reactors of the fuel processor – namely the autothermal reformer, a two-stage water gas shift reactor and a preferential oxidation reactor – were configured and tested in a breadboard apparatus. Experimental results demonstrated a reformate containing ∼48% hydrogen (on a dry basis and with pure methane as fuel) and less than 5 ppm CO. The effects of steam-to-carbon and part load operations were explored.