Thermally integrated fuel processor design for fuel cell applications

Effective thermal integration could enable the use of compact fuel processors with PEM fuel cell-based power systems. These systems have potential for deployment in distributed, stationary electricity generation using natural gas. This paper describes a concept wherein the latent heat of vaporization of H₂O is used to control the axial temperature gradient of a fuel processor consisting of an autothermal reformer (ATR) with water gas shift (WGS) and preferential oxidation (PROX) reactors to manage the CO exhaust concentration. A prototype was experimentally evaluated using methane fuel over a range of external heat addition and thermal inputs. The experiments confirmed that the axial temperature profile of the fuel processor can be controlled by managing only the vapor fraction of the premixed reactant stream. The optimal temperature profile is shown to result in high thermal efficiency and a CO concentration less than 40 ppm at the exit of the PROX reactor.     

Development and design of experiments optimization of a high temperature proton exchange membrane fuel cell auxiliary power unit with onboard fuel processor

In this work, the concept development, system layout, component simulation and the overall DOE system optimization of a HT-PEM fuel cell APU with a net electric power output of 4.5 kW and an onboard methane fuel processor are presented.A highly integrated system layout has been developed that enables fast startup within 7.5 min, a closed system water balance and high fuel processor efficiencies of up to 85% due to the recuperation of the anode offgas burner heat. The integration of the system battery into the load management enhances the transient electric performance and the maximum electric power output of the APU system.Simulation models of the carbon monoxide influence on HT-PEM cell voltage, the concentration and temperature profiles within the autothermal reformer (ATR) and the CO conversion rates within the watergas shift stages (WGSs) have been developed. They enable the optimization of the CO concentration in the anode gas of the fuel cell in order to achieve maximum system efficiencies and an optimized dimensioning of the ATR and WGS reactors.Furthermore a DOE optimization of the global system parameters cathode stoichiometry, anode stoichiometry, air/fuel ratio and steam/carbon ratio of the fuel processing system has been performed in order to achieve maximum system efficiencies for all system operating points under given boundary conditions.     

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.     

Experimental and theoretical analysis of a natural gas fuel processor

In this study, a natural gas fuel processor was experimentally and theoretically investigated. The constructed 2.0 kWth fuel processor is suitable for a residential-scale high temperature proton exchange membrane fuel cell. The system consists of an autothermal reformer; gas clean-up units, namely high and low-temperature water-gas shift reactors; and utilities including feeding unit, burner, evaporator and heat exchangers. Commercial monolith catalysts were used in the reactors. The simulation was carried out by using ASPEN HYSYS program. A validated kinetic model and adiabatic equilibrium model were both presented and compared with experimental data. The nominal operating conditions which were determined by the kinetic model were the steam-to-carbon ratio of 3.0, the oxygen-to-carbon ratio of 0.5 and the inlet temperatures of 450 °C for autothermal reformer, 400 °C for high-temperature water-gas shift reactor and 310 °C for low-temperature water-gas shift reactor. Experimental results at the nominal condition showed that the performance criteria of the hydrogen yield, the fuel conversion and the efficiency were 2.53, 93.5% and 82.3% (higher heating value-HHV), respectively. The validated kinetic model was further used for the determination of 2–10kWthermal fuel processor efficiency which was increasing linearly up-to 86.3% (HHV).     

Cold start simulations of a gasoline based fuel processor for mobile fuel cell applications

The cold start behaviour of the gas processing unit is one crucial issue for the use of gasoline based fuel reformers for mobile fuel cell systems. In this contribution different cold start strategies for a mobile fuel reformer based on gasoline are presented and discussed. The simulation studies are based on 1-d, dynamic multiphase models for both an autothermal gasoline reformer (ATR) and a thermally integrated reforming unit consisting of an ATR, a heat exchanger and a high-temperature-shift-reactor (HTS). Setup and geometric parameters for both models correspond to pilot stage systems considered by DaimlerChrysler.Results on the reactive heat-up of the ATR by partial and total oxidation of gasoline show the impact of the air/fuel-ratio and the thermal load on the cold start duration. The use of the reformat during the rapid start-up of the ATR is mainly limited by the availability of steam for autothermal operation. Due to the high thermal capacities of the system, the whole reforming unit requires much longer time for the cold start. Especially the slow convective heat-up of the HTS restricts the conversion of CO and the subsequent use of the reformat in the fuel cell. Several options for the acceleration of the cold start were investigated. Both a simple λ-control strategy and the reactive heat-up of the HTS by (partial) oxidation of the reformat with injected air reduce the cold start time significantly. With these measures a hydrogen-rich reformat with acceptable CO-concentration is available within two minutes. Moreover, the cold start time can be further reduced, if the HTS is heated up electrically to their ignition temperature at the beginning of the cold start. Thereby the CO-conversion in the HTS already starts in the first minute and, depending on the availability of steam for the feed stream, a cold start of the reforming unit below one minute seems to be possible.     

Bio-fuel reforming for solid oxide fuel cell applications. Part 2: Biodiesel

Biodiesel is considered as a renewable hydrogen source for solid oxide fuel cells (SOFCs). This study contributes to a fundamental understanding of biodiesel autothermal reforming (ATR), which has not yet been widely explored in the open literature. Ultra-low sulfur diesel (ULSD) ATR is established as a baseline for this analysis. This work applies a micro-soot meter based on a photo-acoustic method to quantify the condensed carbon from a single-tube reactor, and uses a mass spectrometer to measure the effluent gas composition under different operating conditions (reformer temperature, steam/carbon ratio, oxygen/carbon ratio, and gas hourly space velocity). The key objective is to identify the optimum operating environment for biodiesel ATR with carbon-free deposition and peak hydrogen yield. Thermodynamic analysis based on the method of total Gibbs free energy minimization is used to evaluate the equilibrium composition of effluent from the reformer. The experimental investigations complimented with this theoretical analysis of biodiesel ATR enable effectively optimizing the onboard reforming conditions. 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–diesel blends (Part 3).     

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.     

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.     

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.     

Diesel fuel processor for PEM fuel cells: Two possible alternatives (ATR versus SR)

There are large efforts in exploring the on-board reforming technologies, which would avoid the actual lack of hydrogen infrastructure and related safety issues. From this view point, the present work deals with the comparison between two different 10 kW_e fuel processors (FP) systems for the production of hydrogen-rich fuel gas starting from diesel oil, based respectively on autothermal (ATR) and steam-reforming (SR) process and related CO clean-up technologies; the obtained hydrogen rich gas is fed to the PEMFC stack of an auxiliary power unit (APU). Based on a series of simulations with Matlab/Simulink, the two systems were compared in terms of FP and APU efficiency, hydrogen concentration fed to the FC, water balance and process scheme complexity. Notwithstanding a slightly higher process scheme complexity and a slightly more difficult water recovery, the FP based on the SR scheme, as compared to the ATR one, shows higher efficiency and larger hydrogen concentration for the stream fed to the PEMFC anode, which represent key issues for auxiliary power generation based on FCs as compared, e.g. to alternators.     

Experimental investigation on a methane fuel processor for polymer electrolyte fuel cells

This study presents experimental study on a novel methane fuel processing system for hydrogen (H₂) production. The unit includes into a single package the autothermal reformer, the CO shift converter, the preferential oxidation reactor and the internal heat exchangers. Effects of operative conditions, related to the H₂ productivity, on the performances, were investigated experimentally, in order to evaluate the integration of the fuel processor with a Polymer Electrolyte Fuel Cell (PEFC) system for residential applications. The sensitivity analysis showed that the overall performance is strongly dependent upon the operative conditions considered.     

Biodiesel fuel processor for APU applications

Tail pipe emission reduction, increased use of renewable fuels and efficient supply of auxiliary power for road vehicles using fuel cells have been the main drivers of the European project BIOFEAT (biodiesel fuel processor for a fuel cell auxiliary power unit for a vehicle). Within the project a biodiesel fuelled heat integrated fuel processor for 10 kW_e capacity has been designed and constructed. Demonstration tests showed a high quality reformate with less than 10 ppm of CO and a gross efficiency of 87%.     

Fuel processors for fuel cell APU applications

The conversion of liquid hydrocarbons to a hydrogen rich product gas is a central process step in fuel processors for auxiliary power units (APUs) for vehicles of all kinds. The selection of the reforming process depends on the fuel and the type of the fuel cell.For vehicle power trains, liquid hydrocarbons like gasoline, kerosene, and diesel are utilized and, therefore, they will also be the fuel for the respective APU systems.The fuel cells commonly envisioned for mobile APU applications are molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Since high-temperature fuel cells, e.g. MCFCs or SOFCs, can be supplied with a feed gas that contains carbon monoxide (CO) their fuel processor does not require reactors for CO reduction and removal. For PEMFCs on the other hand, CO concentrations in the feed gas must not exceed 50 ppm, better 20 ppm, which requires additional reactors downstream of the reforming reactor.This paper gives an overview of the current state of the fuel processor development for APU applications and APU system developments. Furthermore, it will present the latest developments at Fraunhofer ISE regarding fuel processors for high-temperature fuel cell APU systems on board of ships and aircrafts.     

Heat exchanger design for autothermal reforming of diesel

The increasing electrification of vehicles for passenger and heavy duty transport requires the deployment of efficient, low-emission power sources. Auxiliary Power Units (APUs) based on fuels cells offer an excellent solution, especially for supplying power during idling mode. For urban transport applications, gaseous hydrogen appears to be the best fuel option, whereas long-distance applications are better served by a liquid energy carrier. The autothermal reforming of liquid fuels such as diesel presents a simple and efficient method for producing hydrogen for fuel cell APUs. Heat integration for steam generation and air pre-warming are the key elements to a compact autothermal reformer design. With the aid of intense CFD simulations, a reformer construction was achieved with the high power density of 3.3 kW_th/l. Experimental validation indicates high hydrogen concentrations of between 32 and 36%, depending on diesel quality. In combination with already existing results, the newest autothermal reformer (ATR) generation enables the set-up of a complete APU system, fulfilling the U.S. Department of Energy (DOE) targets for fuel cell-based APUs.     

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.     

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.     

A portable fuel processor for hydrogen production from ethanol in a 250 Wel fuel cell system

This paper describes the design, manufacturing, operation, and optimization of a mobile ethanol reformer fuel cell system for off-grid power production. The reformer system includes an autothermal reformer, high and low temperature shift reactors, a selective methanation reactor, and a tail gas combustor. A minimum amount of process controls and little internal heat integration kept system architecture simple.     

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.     

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.