Performance of a PEMFC system integrated with a biogas chemical looping reforming processor: A theoretical analysis and comparison with other fuel processors (steam reforming,partial oxidation and auto-thermal reforming)

In this work, the performance of a PEMFC (proton exchange membrane fuel cell) system integrated with a biogas chemical looping reforming processor is analyzed. The global efficiency is investigated by means of a thermodynamic study and the application of a generalized steady-state electrochemical model. The theoretical analysis is carried out for the commercial fuel cell BCS 500W stack. From literature, chemical looping reforming (CLR) is described as an attractive process only if the system operates at high pressure. However, the present research shows that advantages of the CLR process can be obtained at atmospheric pressure if this technology is integrated with a PEMFC system. The performance of a complete fuel cell system employing a fuel processor based on CLR technology is compared with those achieved when conventional fuel processors (steam reforming (SR), partial oxidation (PO) and auto-thermal reforming (ATR)) are used. In the first part of this paper, the Gibbs energy minimization method is applied to the unit comprising the fuel- and air-reactors in CLR or to the reformer (SR, PO, ATR). The goal is to investigate the characteristics of these different types of reforming process to generate hydrogen from clean model biogas and identify the optimized operating conditions for each process. Then, in the second part of this research, material and energy balances are solved for the complete fuel cell system processing biogas, taking into account the optimized conditions found in the first part. The overall efficiency of the PEMFC stack integrated with the fuel processor is found to be dependent on the required power demand. At low loads, efficiency is around 45%, whereas, at higher power demands, efficiencies around 25% are calculated for all the fuel processors. Simulation results show that, to generate the same molar flow-rate of H₂ to operate the PEMFC stack at a given current, the global process involving SR reactor is by far much more energy demanding than the other technologies. In this case, biogas is burnt in a catalytic combustor to supply the energy required, and there is a concern with respect to CO₂ emissions. The use of fuel processors based on CLR, PO or ATR results in an auto-thermal global process. If CLR based fuel processor is employed, CO₂ can be easily recovered, since air is not mixed with the reformate. In addition, the highest values of voltage and power are achieved when the PEMFC stack is fed on the stream coming from SR and CLR fuel processors. When a H₂ mixture is produced by reforming biogas through PO and ATR technologies, the relative anode overpotential of a single cell is about 55 mV, whereas, with the use of CLR and SR processes, this value is reduced to ∼37 and 24 mV, respectively. In this way, CLR can be seen as an advantageous reforming technology, since it allows that the global process can be operated under auto-thermal conditions and, at the same time, it allows the PEMFC stack to achieve values of voltage and power closer to those obtained when SR fuel processors are used. Thus, efforts on the development of fuel processors based on CLR technology operating at atmospheric pressure can be considered by future researchers. In the case of biogas, the CO₂ captured can produce additional economical benefits in a ‘carbon market’.     

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.     

A 75-kW methanol reforming fuel cell system

A 75-kW methanol reforming fuel cell system, which consists of a fuel cell system and a methanol auto-thermal reforming fuel processor has been developed at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS). The core of the fuel cell system is a group of CO tolerant PEMFC stacks with a double layer composite structured anode. The fuel cell stacks show good CO tolerance even though 140 ppm CO was present in the reformate stream during transients. The auto-thermal reforming (ATR) fuel cell processor could adiabatically produce a suitable reformate without external energy consumption. The output of hydrogen-rich reformate was approximately 120 N m³ h⁻¹ with a H₂ content near 53% and the CO concentrations generally were under 30 ppm. The fuel cell system was integrated with the methanol reforming fuel processor and the peak power output of the fuel cell system exceeded 75 kW in testing. The hydrogen utilization approached 70% in the fuel cell system.     

Final step for CO syngas clean-up: Comparison between CO-PROX and CO-SMET processes

The performance of the CO preferential oxidation (PROX) process was compared with the CO selective methanation (SMET) one, both applied as the last clean-up process step of a fuel processor unit (FPU) to remove CO from syngas. The FPU was completed with the reformer (autothermal reformer ATR or steam reformer SR) and a non-isothermal water gas shift (NI-WGS) reactor. Furthermore, the reforming of different hydrocarbon fuels, among those most commonly found in service stations (gasoline, light diesel oil and compressed natural gas) was examined. The comparison, in terms of different FPU configurations and fuels, was carried out by a series of steady-state system simulations in Aspen Plus^®. From the obtained data, the performance of CO-PROX was not very different from that of CO-SMET, making it complex to give a definitive answer on the best FPU scheme. The most promising fuel processor with respect to performance is a chain of ATR, NI-WGS and CO-SMET. However, maintaining the same chain of clean-up reactors, the FPU with SR instead of ATR could also be a satisfactory choice. Even if there are lower efficiencies and H₂ specific production compared to the ATR-based FPU, the SR-based one does not produce a syngas with the high N₂ concentration typical of the ATR-based FPU. The syngas dilution by nitrogen is somehow detrimental for the stack efficiency, when syngas feeds PEM-FCs, since it lowers the polarization curve.     

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).     

A review of biomass-derived fuel processors for fuel cell systems

Fuel cell coupled with biomass-derived fuel processor can convert renewable energy into a useful form in an environmental-friendly and CO₂-neutral manner. It is considered as one of the most promising energy supply systems in the future. Biomass-derived fuels, such as ethanol, methanol, biodiesel, glycerol, and biogas, can be fed to a fuel processor as a raw fuel for reforming by autothermal reforming, steam reforming, partial oxidation, or other reforming methods. Catalysts play an important role in the fuel processor to convert biomass fuels with high hydrogen selectivity. The processor configuration is another crucial factor determining the application and the performance of a biomass fuel processing system. The newly developed monolithic reactor, micro-reactor, and internal reforming technologies have demonstrated that they are robust in converting a wide range of biomass fuels with high efficiency. This paper provides a review of the biomass-derived fuel processing technologies from various perspectives including the feedstock, reforming mechanisms, catalysts, and processor configurations. The research challenges and future development of biomass fuel processor are also discussed.     

Fuel cell feed system based on H2 production by a compact multi-fuel catalytic ATR reactor

Hydrogen fuel cells seem the most viable solution to the pollution reduction and the energy growing demand. Very compact and small size production plant for distribute H₂ production may reduce hydrogen transport and storage difficulties. Due to the high reactor compactness and thermal self-sustainability, the auto-thermal reforming (ATR) reaction of gaseous and liquid hydrocarbons can be the optimal solution. Fossil hydrocarbons like methane, gasoline and diesel still remain the favourite feed for catalytic auto-thermal reformer, due to the widespread existing delivery pipelines and the high energy density. Unfortunately, due to the different characteristics of liquid and gaseous fuels, it's very difficult to realize a multi-fuel processor characterized by high performances in terms of thermal efficiency and hydrogen yield, and, up to now, very low number of papers dealing with multi-fuel reformers is present in the literature.     

Temperature and hydrogen flow rate controls of diesel autothermal reformer for 3.6 kW PEM fuel cell system with autoignition delay time analysis

In this paper catalyst temperature and hydrogen flow rate controls are an area of interest for autothermal reforming (ATR) of diesel fuel to provide continuous and necessary hydrogen flow to the on-board fuel cell vehicle system. ATR control system design is important to ensure proper and stable performance of fuel processor and fuel cell stack. Fast system response is required for varying load changes in the on-board fuel cell system. To cope with control objectives, a combination of PI and PID controllers are proposed to keep the controlled variables on their setpoints. ATR catalyst temperature is controlled with feedback PID controller through variable OCR (oxygen to carbon ratio) manipulation and kept to the setpoint value of 900 °C. Additionally diesel auto-ignition delay time is implemented through fuel flow rate delay to avoid complete oxidation of fuel. Hydrogen flow rate to the fuel cell stack is kept to setpoint of required hydrogen flow rate according to fuel cell load current using PI controller. An integrated dynamic model of fuel processor and fuel cell stack is also developed to check the fuel cell voltage. Product gas composition of 35, 18 and 4% is achieved for hydrogen, nitrogen, and carbon dioxide, respectively. The results show fast response capabilities of fuel processor following the fuel cell load change and successfully fulfills the control objectives.     

Self-sustained operation of a kWe-class kerosene-reforming processor for solid oxide fuel cells

In this paper, fuel-processing technologies are developed for application in residential power generation (RPG) in solid oxide fuel cells (SOFCs). Kerosene is selected as the fuel because of its high hydrogen density and because of the established infrastructure that already exists in South Korea. A kerosene fuel processor with two different reaction stages, autothermal reforming (ATR) and adsorptive desulfurization reactions, is developed for SOFC operations. ATR is suited to the reforming of liquid hydrocarbon fuels because oxygen-aided reactions can break the aromatics in the fuel and steam can suppress carbon deposition during the reforming reaction. ATR can also be implemented as a self-sustaining reactor due to the exothermicity of the reaction. The kW_e self-sustained kerosene fuel processor, including the desulfurizer, operates for about 250 h in this study. This fuel processor does not require a heat exchanger between the ATR reactor and the desulfurizer or electric equipment for heat supply and fuel or water vaporization because a suitable temperature of the ATR reformate is reached for H₂S adsorption on the ZnO catalyst beds in desulfurizer. Although the CH₄ concentration in the reformate gas of the fuel processor is higher due to the lower temperature of ATR tail gas, SOFCs can directly use CH₄ as a fuel with the addition of sufficient steam feeds (H₂O/CH₄ ≥ 1.5), in contrast to low-temperature fuel cells. The reforming efficiency of the fuel processor is about 60%, and the desulfurizer removed H₂S to a sufficient level to allow for the operation of SOFCs.     

Development of integrated reformer systems for syngas production

Precision Combustion, Inc. (PCI) has developed autothermal reformer (ATR) units based on its patented Microlith^® technology and demonstrated stable, coke-free operation using JP-8 consisting of up to 70 ppm_w sulfur for 1100 h with complete fuel conversion and reforming efficiency of ∼85%. The feasibility of operating ATR reformers with distillate fuels containing higher sulfur (up to 400 ppm_w) has also been demonstrated for 55 h, while producing syngas (i.e., H₂ and CO) at >80% reforming efficiency and at complete fuel conversion. Reformer test results, demonstrating water neutral operation, catalyst sulfur tolerance, removal of sulfur to <1 ppm_v, and maintenance of higher hydrocarbons to levels acceptable for fuel cell stacks, provide a measure of the ATR performance that can be expected under realistic conditions with readily available fuels. The results also give valuable insights to the system design and operation strategy for integrated power generation systems.     

Reforming system for co-generation of hydrogen and mechanical work

The paper describes a new design for a reforming system for converting hydrocarbon fuels into pure hydrogen. The system is based on an autothermal reforming (ATR) reactor operating at elevated pressures followed by membrane-based hydrogen separation. The high-pressure membrane discharge stream is combusted and expanded through a turbine generating additional power. Process simulation modeling illustrates the effect of pressure and other operating parameters on system performance and demonstrates a system reforming efficiency approaching 80%. When coupled with a PEM fuel cell and an electrical generator, fuel to electricity efficiency is above 40%. Other anticipated benefits of the system include compact size, simplicity in control and fast start up.     

Bio-fuel reformation for solid oxide fuel cell applications. Part 3: Biodiesel–diesel blends

The auto-thermal reforming (ATR) performance of diesel blended with biodiesel (e.g., B5, B10, B20, B40, and B80) was investigated and compared to pure diesel and biodiesel ATR in a single-tube reformer with ceramic monolith wash-coated rhodium/ceria–zirconia catalyst. The initial operating condition of the ATR of all studied fuels was set as total moles of oxygen from air, water, and fuel per mole of carbon (O/C) = 1.47, moles of water to carbon (H₂O/C) = 0.6, and gas hourly space velocity = 33,950 h⁻¹ at 1223 K reformer temperature, to achieve the same syngas (H₂ + CO) production rate. A direct photo-acoustic micro-soot meter was applied to quantify the dynamic evolution of carbon formation and a mass spectrometer was used to measure the gas composition of reformer effluents. The blends with more biodiesel content were found to have a lower syngas production rate and reforming efficiency, and require more air and higher reformer temperature to avoid carbon formation. Strong correlations between ethylene and solid carbon concentration were observed in the reformation of all the fuels and blends, with more biodiesel content tending to have higher ethylene production. This study is one component of a three-part investigation of bio-fuel reforming, also including fuel vaporization and reactant mixing (Part 1) and biodiesel (Part 2).     

Investigations on the behaviour of 2 kW natural gas fuel processor

In this paper, the first experimental investigations on a pre-commercial natural gas steam reformer have been presented. The fuel processor unit contains the elements as follows: desulfurizer, steam reformer reactor, CO shift converter, CO preferential oxidation (PROX) reactor, steam generator, burner and heat exchangers.The fuel processor produces 45 Nl/min of syngas in which the hydrogen concentration is about 75 vol.% and the other chemical species are nitrogen, carbon dioxide and methane. The CO concentration is below 1 ppmv, so that this reforming system is suitable for the integration with a PEM fuel cell stack.The experimental activity has been conducted in a test station, properly designed to measure the behaviour of the fuel processor. The laboratory test facility is equipped by a National Instruments Compact DAQ real-time data acquisition and control system running Labview™ software. Several measurement instruments and controlling devices have been installed. Furthermore, a gas chromatograph is used to measure the product gas composition during the tests.The aim of this work has been to analyze the behaviour of this pre-commercial steam reforming unit during its operation cycle in different operating conditions (full and partial loads) in order to study its integration with a PEM fuel cell for developing a high efficiency microcogeneration system for residential applications.     

Developing fuel map to predict the effect of fuel composition on the maximum efficiency of solid oxide fuel cells

At any given cell operating condition, a fuel map can be developed to predict the effect of a fuel containing carbon, hydrogen, oxygen and inert gas atoms on the maximum cell efficiency (MCE) of solid oxide fuel cells (SOFCs). To create a fuel map, a thermodynamic model is developed to obtain the fuels that would yield identical MCE for SOFCs. These fuels make a continuous curve in the ternary coordinate system. A fuel map is established by developing continuous fuel curves for different MCEs at the same operating condition of a cell and representing them in the carbon-hydrogen-oxygen (C-H-O) ternary diagram. The graphical representation of fuel maps can be applied to predict the effect of the fuel composition and fuel processor on the MCE of SOFCs. As a general result, among the fuels that can be directly utilized in SOFCs, at the same temperature and pressure, the one located at the intersection of the H-C axis and the carbon deposition boundary (CDB) curve in the C-H-O ternary diagram provides the highest MCE. For any fuel that can be indirectly utilized in SOFCs, the steam reforming fuel processor always yields a higher MCE than auto-thermal reforming or partial oxidation fuel processors at the same anode inlet fuel temperature.     

Technical design and economic evaluation of a PEM fuel cell system

The main objectives of this study are to develop the economic models and their characterization trends for the common unit processes and utilities in the fuel cell system. In this study, a proton electrolyte membrane fuel cell (PEMFC) system is taken as a case study. The overall system consists of five major units, namely auto-thermal reformer (ATR), water gas shift reactor (WGS), membrane, pressure swing adsorber (PSA) and fuel cell stack. Besides that, the process utilities like compressor, heat exchanger, water adsorber are also included in the system. From the result, it is determined that the specific cost of a PEM fuel cell stack is about US$ 500 per kW, while the specific manufacturing and capital investment costs are in the range of US$ 1200 per kW and US$ 2900 per kW, respectively. Besides that the electricity cost is calculated as US$ 0.04 kWh. The results also prove that the cost of PEM fuel cell system is comparable with other conventional internal engine.     

Performance evaluation of a fuel processing system based on membrane reactors technology integrated with a PEMFC stack

The development of fuel cells is promised to enable the distributed generation of electricity in the near future. However, the infrastructure for production and distribution of hydrogen, the fuel of choice for fuel cells, is currently lacking. Efficient production of hydrogen from fuels that have existing infrastructure (e.g., natural gas, gasoline or LPG) would remove a major drawback to use fuel cells for distributed power generation.The aim of this paper is to define the better operating conditions of an innovative hydrogen generation system (the fuel processing system, FP) based on LPG steam reforming, equipped with a membrane shift reactor, and integrated with a PEMFC (Proton Exchange Membrane Fuel Cell) stack of 5 kWel.With respect to the conventional hydrogen generation systems, the use of membrane reactors (MRs) technology allows to increase the hydrogen generation and to simplify the FP-PEMFC plant, because the CO removal system, needed to reduce the CO content at levels required by the PEMFC, is avoided.Therefore, in order to identify the optimal operating conditions of the FP-PEMFC system, a sensitivity analysis on the fuel processing system has been carried out by varying the main operating parameters of both the reforming reactor and the membrane water gas shift reactor. The sensitivity analysis has been performed by means of a thermochemical model properly developed.Results show that the thermal efficiency of the fuel processing system is maximize (82.4%, referred to the HHV of fuels) at a reforming temperature of 800 °C, a reforming pressure of 8 bar, and an S/C molar ratio equal to 6. In the nominal operating condition of the PEMFC stack, the FP-PEMFC system efficiency is 36.1% (39.0% respect to the LHV).     

On-board reforming of biodiesel and bioethanol for high temperature PEM fuel cells: Comparison of autothermal reforming and steam reforming

In the 21st century biofuels will play an important role as alternative fuels in the transportation sector. In this paper different reforming options (steam reforming (SR) and autothermal reforming (ATR)) for the on-board conversion of bioethanol and biodiesel into a hydrogen-rich gas suitable for high temperature PEM (HTPEM) fuel cells are investigated using the simulation tool Aspen Plus. Special emphasis is placed on thermal heat integration. Methyl-oleate (C₁₉H₃₆O₂) is chosen as reference substance for biodiesel. Bioethanol is represented by ethanol (C₂H₅OH). For the steam reforming concept with heat integration a maximum fuel processing efficiency of 75.6% (76.3%) is obtained for biodiesel (bioethanol) at S/C = 3. For the autothermal reforming concept with heat integration a maximum fuel processing efficiency of 74.1% (75.1%) is obtained for biodiesel (bioethanol) at S/C = 2 and λ = 0.36 (0.35). Taking into account the better dynamic behaviour and lower system complexity of the reforming concept based on ATR, autothermal reforming in combination with a water gas shift reactor is considered as the preferred option for on-board reforming of biodiesel and bioethanol. Based on the simulation results optimum operating conditions for a novel 5 kW biofuel processor are derived.     

A reforming system for co-generation of hydrogen and mechanical work from methanol

The paper describes a reforming system for converting methanol into pure hydrogen. The system is based on an autothermal reforming reactor operating at elevated pressures followed by membrane-based hydrogen separation. The high-pressure membrane retentate stream is combusted and expanded through a turbine generating additional power. Process simulation illustrates the effects of the system operating parameters on performance and demonstrates system reforming efficiency up to ∼90%. When coupled with a PEM fuel cell and an electrical generator, overall fuel to electricity efficiency can be >48% depending upon the efficiency of a PEM fuel cell stack.     

Reforming options for hydrogen production from fossil fuels for PEM fuel cells

PEM fuel cell systems are considered as a sustainable option for the future transport sector in the future. There is great interest in converting current hydrocarbon based transportation fuels into hydrogen rich gases acceptable by PEM fuel cells on-board of vehicles. In this paper, we compare the results of our simulation studies for 100 kW PEM fuel cell systems utilizing three different major reforming technologies, namely steam reforming (SREF), partial oxidation (POX) and autothermal reforming (ATR). Natural gas, gasoline and diesel are the selected hydrocarbon fuels. It is desired to investigate the effect of the selected fuel reforming options on the overall fuel cell system efficiency, which depends on the fuel processing, PEM fuel cell and auxiliary system efficiencies. The Aspen-HYSYS 3.1 code has been used for simulation purposes. Process parameters of fuel preparation steps have been determined considering the limitations set by the catalysts and hydrocarbons involved. Results indicate that fuel properties, fuel processing system and its operation parameters, and PEM fuel cell characteristics all affect the overall system efficiencies. Steam reforming appears as the most efficient fuel preparation option for all investigated fuels. Natural gas with steam reforming shows the highest fuel cell system efficiency. Good heat integration within the fuel cell system is absolutely necessary to achieve acceptable overall system efficiencies.     

Comparison of steam and autothermal reforming of methanol using a packed-bed low-cost copper catalyst

Small-scale reformers for hydrogen production via steam and autothermal reforming of hydrocarbon feedstocks can be a solution to the lack of hydrogen distribution infrastructure. A packed-bed reactor is one possible design for such purpose. However, the two reforming processes of steam and autothermal methods have different characteristics, thus they have different and often opposite design requirements. In implementing control strategy for small-scale reformers, understanding the overall chemical reactions and the reactor physical properties becomes essential. This paper presents some inherent features of a packed-bed reactor that can both improve and/or degrade the performance of a packed-bed reactor with both reforming modes.The high thermal resistance of the packed bed is disadvantageous to steam reforming (SR), but it is beneficial to the autothermal reforming (ATR) mode with appropriate reactor geometry. The low catalyst utilization in steam reforming can help to prevent the unconverted fuel leaving the reactor during transient by allowing briefly for higher reactant fuel flow rates. In this study, experiments were performed using three reactor geometries to illustrate these properties and a discussion is presented on how to take advantages of these properties in reactor design.