Further development of a microchannel steam reformer for diesel fuel

This paper presents results from the ongoing optimisation of a microchannel steam reformer for diesel fuel which is developed in the framework of the development of a PEM fuel cell system for vehicular applications. Four downscaled reformers with different catalytic coatings of precious metal were operated in order to identify the most favourable catalyst formulation. Diesel surrogate was processed at varying temperatures and steam to carbon ratios (S/C). The reformer performance was investigated considering hydrogen yield, reformate composition, fuel conversion, and deactivation from carbon formation. Complete fuel conversion is obtained with several catalysts. One catalyst in particular is less susceptible to carbon formation and shows a high selectivity.     

Coupled operation of a diesel steam reformer and an LT- and HT-PEFC

Polymer electrolyte fuel cells (PEFC) combined with diesel fuel processors offer a great potential for auxiliary power units (APU) in mobile applications. In a joint research project with partners from industry, Oel-Waerme-Institut GmbH is developing an integrated modular fuel cell system for mobile power generation in caravans and yachts. The system includes a steam reforming fuel processor that allows the operation of low-temperature (LT-) as well as high-temperature (HT-) PEFC.     

Experimental and computational investigations of a compact steam reformer for fuel oil and diesel fuel

The present work describes the optimisation of a compact steam reformer for light fuel oil and diesel fuel. The reformer is based upon a catalytically coated micro heat exchanger that thermally couples the reforming reaction with a catalytic combustion. Since the reforming process is sensitive to reaction temperatures and internal flow patterns, the reformer was modelled using a commercial CFD code in order to optimise its geometry. Fluid flow, heat transfer and chemical reactions were considered on both sides of the heat exchanger. The model was successfully validated with experimental data from reformer tests with 4 kW, 6 kW and 10 kW thermal inputs of light fuel oil. In further simulations the model was applied to investigate parallel flow, counter flow and cross flow conditions along with inlet geometry variations for the reformer. The experimental results show that the reformer design allows inlet temperatures below 773 K because of its internal superheating capability. The simulation results indicate that two parallel flow configurations provide fast superheating and high fuel conversion rates. The temperature increase inside the reactor is influenced by the inlet geometry on the combustion side.     

Fast start-up of a diesel fuel processor for PEM fuel cells

Fuel cell systems based on liquid fuels are particularly suitable for auxiliary power generation due to the high energy density of the fuel and its easy storage. Together with industrial partners, Oel-Waerme-Institut is developing a 3 kW_el PEM fuel cell system based on diesel steam reforming to be applied as an APU for caravans and yachts. The start-up time of a fuel cell APU is of crucial importance since a buffer battery has to supply electric power until the system is ready to take over. Therefore, the start-up time directly affects the battery capacity and consequently the system size, weight, and cost.     

Numerical analysis of a 20-kWe biogas steam reformer in PEMFC applications

This study supports the development of a 20-kW_e biogas reformer for PEMFC applications. Biogas contains a high concentration of carbon dioxide (CO₂) in the feedstock, which can act as a reforming agent but increases the carbon monoxide (CO) concentration in the product. The feedstock composition and the external heat supply impact both the methane (CH₄) conversion and the CO concentration in the product; thus, a numerical model is developed to investigate these impacts. As the CO₂ content in the feedstock or the external heat supply increases, both the CH₄ conversion and the CO concentration in the product increase. A high CO₂ concentration or a high operating temperature stimulates the reforming reactions but also shifts the equilibrium to favor CO production. However, the performance can be improved by controlling the heat supply method. Using a more uniform temperature distribution at approximately 650 °C, the CO concentration decreases while the CH₄ conversion increases. This finding suggests the importance of temperature control in biogas steam reforming.     

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.     

Study on a compact methanol reformer for a miniature fuel cell

A compact methanol reformer for hydrogen production has been successfully fabricated, which integrated one reforming chamber, one water gas shift reaction chamber, two preheating chambers and two combustion chambers. It can be started-up at room temperature by the combustion of liquid methanol in the combustion chamber within 7 min without any external heating. The cold start response of the methanol reformer has been investigated using different parameters including methanol and air supply rate, and the experiments revealed that the optimum methanol and air flow rate were 0.55 mL/min and 3 L/min respectively. The results indicated that this methanol reformer can provide a high concentration of hydrogen (more than 73%) and the system efficiency is always maintained above 74%. It is further demonstrated in more than 1600 h continuous performance that the reformer could be operated autothermally and exhibited good test stability.     

Comparison of compact reformer configurations for on-board fuel processing

Two compact reformer configurations in the context of production of hydrogen in a fuel processing system for use in a Proton Exchange Membrane Fuel Cell (PEMFC) based auxiliary power unit in the 2–3 kW range are compared using computer-based modeling techniques. Hydrogen is produced via catalytic steam reforming of n-heptane, the surrogate for petroleum naphtha. Heat required for this endothermic reaction is supplied via catalytic combustion of methane, the model compound for natural gas. The combination of steam reforming and catalytic combustion is modeled for a microchannel reactor configuration in which reactions and heat transfer take place in parallel, micro-sized flow paths with wall-coated catalysts and for a cascade reactor configuration in which reactions occur in a series of adiabatic packed-beds, heat exchange in interconnecting microchannel heat exchangers being used to maintain the desired temperature. Size and efficiency of the fuel processor consisting of the reformer, hydrogen clean-up units and heat exchange peripherals are estimated for either case of using a microchannel and a cascade configuration in the reforming step. The respective sizes of fuel processors with microchannel and cascade configurations are 1.53 × 10⁻³ and 1.71 × 10⁻³ m³. The overall efficiency of the fuel processor, defined as the ratio of the lower heating value of the hydrogen produced to the lower heating value of the fuel consumed, is 68.2% with the microchannel reactor and 73.5% with the cascade reactor mainly due to 30% lower consumption of n-heptane in the latter. The cascade system also offers advanced temperature control over the reactions and ease of catalyst replacement.     

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.     

Biogas steam reformer for hydrogen production: Evaluation of the reformer prototype and catalysts

This work aims to investigate a biogas steam reforming prototype performance for hydrogen production by mass spectrometry and gas chromatography analyses of catalysts and products of the reform. It was found that 7.4% Ni/NiAl₂O₄/γ-Al₂O₃ with aluminate layer and 3.1% Ru/γ-Al₂O₃ were effective as catalysts, given that they showed high CH₄ conversion, CO and H₂ selectivity, resistance to carbon deposition, and low activity loss. The effect of CH₄:CO₂ ratio revealed that both catalysts have the same behavior. An increase in CO₂ concentration resulted in a decrease in H₂/CO ratio from 2.9 to 2.4 for the Ni catalyst at 850 °C, and from 3 to 2.4 for the Ru catalyst at 700 °C. In conclusion, optimal performance has been achieved in a CH₄:CO₂ ratio of 1.5:1. H₂ yield was 60% for both catalysts at their respective operating temperature. Prototype dimensions and catalysts preparation and characterization are also presented.     

Impact of gas-phase reactions in the mixing region upstream of a diesel fuel autothermal reformer

The use of diesel fuel to power a solid oxide fuel cell (SOFC) presents several challenges. A major issue is deposit formation in either the external reformer, the anode channel, or within the SOFC anode itself. One potential cause of deposit formation under autothermal reforming conditions is the onset of gas-phase reactions upsteam of the catalyst to form ethylene, a deposit precursor. Another potential problem is improper mixing of the fuel, air, and steam streams. Incomplete mixing leads to fuel rich gas pockets in which gas phase pyrolysis chemistry might be accelerated to produce even more ethylene. We performed a combined experiment/modeling analysis to identify combinations of temperature and reaction time that might lead to deposit formation. Two alkanes, n-hexane and n-dodecane, were selected as surrogates for diesel fuel since a detailed mechanism is available for these species. This mechanism was first validated against n-hexane pyrolysis data. It was then used to predict fuel conversion and ethylene production under a variety of reforming conditions, ranging from steam reforming to catalytic partial oxidation. Assuming that the reactants are perfectly mixed at 800 K, the predictions suggest that a mixture must reach the catalyst in less than 0.1 s to avoid formation of potentially troublesome quantities of ethylene. Additional calculations using a simple model to account for improper mixing demonstrate the need for the components to be transported to the catalyst on a much shorter time scale, since both the relatively lean and relatively rich regions react faster and rapidly form ethylene.     

Performance improvement of diesel autothermal reformer by applying ultrasonic injector for effective fuel delivery

Technology for the reforming of heavy hydrocarbons, such as diesel, to supply hydrogen for fuel cell applications is very attractive and challenging due to its delicate control requirements. The slow reforming kinetics of aromatics contained in diesel, sulfur poisoning, and severe carbon deposition make it difficult to obtain long-term performance with high reforming efficiency. In addition, diesel has a critical mixing problem due to its high boiling point, which results in a fluctuation of reforming efficiency. An ultrasonic injector (UI) have been devised for effective diesel delivery. The UI can atomize diesel into droplets (∼40 μm) by using a piezoelectric transducer and consumes much less power than a heating-type vapourizer. In addition, reforming efficiencies increase by as much as 20% compared with a non-UI reformer under the same operation conditions. Therefore, it appears that effective fuel delivery is linked to the reforming kinetics on the catalyst surface. A 100-W_e, self-sustaining, diesel autothermal reformer using the UI is designed. In addition, the deactivation process of the catalyst, by carbon deposition, is investigated in detail.     

Kinetic model of a plate fin heat exchanger with catalytic coating as a steam reformer of methane,biogas, and dimethyl ether

The paper presents an analysis of a compact plate fin heat exchanger with catalytic coating. The unit is used to convert various fuels into hydrogen‐rich gas, which is then fed into the anodic compartments of a solid oxide fuel cell (SOFC) stack. The study looks at the fuel processor for methane, biogas, and dimethyl ether (DME). In the first phase, the reaction kinetics model was based on data from the literature. This was followed by tuning and validation of the numerical model using data collected during an experimental campaign. Four values of the steam to carbon ratio (2.0, 2.5, 3.0, and 3.5) were used to analyze the performance of the heat exchanger, which was investigated in a temperature range of 500°C to 750°C. The experimental data are compared with predictions of the model implemented in Aspen HYSYS modeling software and discussed. It was found that the relative prediction error of the simulator does not exceed 3.5%.     

Design of linear controllers applied to an ethanol steam reformer for PEM fuel cell applications

This paper focuses on the design of a controller for a low temperature ethanol steam reformer for the production of hydrogen to feed a protonic exchange membrane (PEM) fuel cell. It describes different control structures for the reformer and treats the control structure selection of this multiple input multiple output (MIMO) system. For each considered control structure, decentralised 2 × 2 controllers with proportional integral (PI) control actions in each control loop are implemented. The tuning of the PI parameters and the performance evaluation of the different controllers are based on a non-linear simulation model. For the validation and comparison of the considered controllers, the dynamic response for different setpoint changes and initial conditions is analysed, as well as the behaviour of the controlled system against disturbances.     

Heat flux analysis of a cylindrical steam reformer by a modified Nusselt number

A numerical method is used to investigate a steam reformer. The reactor is assumed as a porous medium, because it is filled with catalysts of a packed-bed type, and a pseudo-homogeneous model is incorporated for a chemical reaction model. The steam reforming (SR) reaction, water–gas shift (WGS) reaction, and direct steam reforming (DSR) reaction are assumed to be dominant reactions in the steam reformer. The difference in temperature between the inside and outside of the reactor is a driving force in heat transfer, and is affected by the amount of heat adsorption by an endothermic reaction. A modified Nusselt number (Nu_M) can represent the heat transfer rate of the endothermic reactor, and thus Nu_M can be used to describe the performance of the steam reformer. The SR reaction rate is sufficiently activated when Nu_M around the inlet region is greater than 10, and fuel conversion exceeds 0.9 when the difference in Nu_M value between the inlet area and outlet area is greater than 5. The correlation between fuel conversion and operating conditions has also been studied by using Nu_M.     

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.     

Genetic algorithm-based strategy for the steam reformer optimization

The steam reforming reaction is widely used for obtaining hydrogen. The reforming reaction has a strong endothermic character, which means it requires a considerable and continuous heat supply to proceed. Due to the process character, a highly non-uniform temperature field develops inside the reactor. It has a consequence in large temperature gradients, leading to the catalyst degradation and a reduced lifetime of the reforming unit. The aim of the presented research is to unify the temperature field developing in the reactor, for easier control of the process and extension of the reformer's life expectancy. A conventional plug-flow reactor consists of a cylindrical pipe body filled with catalyst. The presented methodology included optimizing the catalyst distribution in the reactor to acquire the most uniform temperature field possible. A genetic algorithm is selected as an optimization technique for finding the most advantageous alignment of the catalyst. It is an example of evolutionary algorithms, basing on rules similar to natural selection. The algorithm generates a random, initial population of reactors and the reforming simulation is executed for each of them. The computation results are then evaluated and ranked using predefined fitness functions. The ranked reactors parameters' are further recombined with selection probability based on the fitness values, until a whole new population is created and the algorithm's loop restarts. The higher the fitness value of a specific reactor, the higher are the chances of passing its segments composition to the proceeding generation. The fitness computation leaves a vast space for improvements, as it may be computed based on many different process' parameters. This work focuses on distinguishing differences in the algorithm performance, depending on the formula for fitness calculation. The algorithm's converging speed, overall fitness values of specimens and optimization results were investigated and compared. The results show that the algorithm with an updated fitness calculation procedure performs considerably better. Only about 50% of computational time was required, to acquire results of the same quality for the presented numerical cases, when comparing with the previously prepared procedure.     

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

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

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.