Internal Reforming Reactions Research Articles

Three-dimensional Modeling of Internal Reforming SOFC with a Focus on the Interconnect Size Effect

A comprehensive 3D mathematical model is developed for planar solid oxide fuel cells (SOFCs) running on hydrocarbon fuels. The model fully considers the coupled gas transport, ionic conduction, electronic conduction, electrochemical reactions, internal reforming reaction, water gas shift reaction, and heat transfer. The model is validated by comparing the simulation results with literature data. Parametric simulations are conducted to investigate how the size of interconnect rib affect the various physical and chemical processes in the SOFC. It’s found that the interconnect rib greatly influences the gas diffusion in the porous electrode thus influences the electrochemical/chemical reactions. Optimal rib size is observed for internal reforming SOFCs. The results are useful for design of SOFC stack with hydrocarbon fuels.

Effect of Ni in the Anode on the Performance of Pulse-Jet Rechargeable Direct Carbon Fuel Cells

NTRODUCTION Solid Oxide Fuel Cells (SOFCs) operate at high temperature, and thus various fuels can be used due to internal reforming reactions of the fuel. Our research group previously developed Rechargeable Direct Carbon Fuel Cells (RDCFCs) in which the solid carbon deposited by thermal decomposition of hydrocarbon is used as fuel [1, 2]. We also developed Pulse Jet-Rechargeable Direct Carbon Fuel Cells (PJ-RDCFCs) as a new type of RDCFC [3, 4]. In a PJ-RDCFC, liquid hydrocarbon is supplied, thus enabling as fuel the use of hydrogen, lower hydrocarbons, and the solid carbon by thermal decomposition of the liquid hydrocarbon. The operating conditions of a PJ-RDCFC are broader because such conditions can be controlled by adjusting the fuel-supply interval time (T int), the amount in a single pulse (PJ vol), and the current density (j). The type of chemical species that react in a PJ-RDCFC can be selected by controlling these parameters [3]. Sulfur compounds contained in liquid fuel cause poisoning of the catalyst and increase the amount of deposited carbon, both of which cause catalyst degradation. Anode degradation can possibly be controlled, however, by adjusting the parameters in PJ-RDCFCs so that carbon-removal reactions occur selectivity [4]. Traditionally, Ni/GDC cermet anodes are used in PJ-RDCFCs, but the amount of Ni should be kept at a minimum because of the formation of NiCx. Only a slight difference existed between the power density (P) of RDCFCs with Ni and those without Ni, whereas the P of SOFCs with Ni/GDC was much smaller than those with GDC metal-free anodes [2]. In this research, to develop a metal-free anode that is tolerant against anode degradation, power generation characteristics of a PJ-RDCFC using a GDC metal-free anode and that using a Ni/GDC cermet anode were compared and the role of Ni in the anode was investigated. EXPERIMENTAL The electrolyte was a 8YSZ disk (0.25-mm thickness), the cathode was a (La0.8Sr0.2MnO3)/ScSZ composite, and the anodes were a Ni/GDC (Gd0.10Ce0.90O2-δ) composite and a GDC (Gd0.20Ce0.80O2-δ:Gd0.10Ge0.90O2-δ = 9:1) composite. Two different thicknesses of GDC were evaluated, 9- and 52-μm. The operating temperature was 900 °C and oxygen was introduced to the cathode at 60 sccm. Hydrogen was introduced to the anode at 200 sccm during the power generation of the SOFCs. In the power generation of a PJ-RDCFC, after 3 jet-pulses of isooctane as fuel followed by a 1-min wait, power generation at constant j was initiated. PJ vol was set to 1.2 μl. Charging was again carried out by pulse jetting at fixed T int (60 sec). After 15 power generation/charging cycles, pulse jetting was stopped. RESULT AND DISCUSSION Figure 1 shows the current density-terminal voltage (I-V) characteristics of the SOFCs fueled by hydrogen using (1) Ni/GDC (42-μm), (2) GDC (52-μm thickness) and (3) GDC (9-μm thickness). Their respective maximum P was (1) 225 mW/cm2, (2) 47.9 mW/cm2, (3) 132 mW/cm2 , and ohmic resistance was (1) 0.324 Ω/cm2, (2) 3.47 Ω/cm2, (3) 1.06 Ω/cm2. Lower electric conductivity in GDC leads to high ohmic resistance, and thus the maximum P using GDC anodes here was lower than using Ni/GDC. Ohmic resistance decreased, however, with the thinner GDC anode, resulting in a higher P. Figure 2 shows the power generation characteristics of PJ-RDCFCs using (1) Ni/GDC at j = 100 mA/cm2 and (2) GDC (9-μm thickness) at j = 15 mA/cm2. The average P were (1) 77.8 mW/cm2 and (2) 11.0 mW/cm2. The same V behavior was exhibited during each power generation of a single pulse (i.e., cyclic power generation), showing that steady power generation occurred using a GDC metal-free anode. With the Ni/GDC anode, V increased during a cyclic power generation, whereas with GDC, it increased just after pulse jetting and then decreased. In PJ-RDCFCs, V is affected by changes in the chemical species that contribute to the electrochemical and thermochemical reactions following increasing fuel utilization. Because Ni is thought to be mainly the catalyst of thermochemical reactions, the difference in power generation behavior with these two anodes apparently is due to the difference in such contribution by Ni to the thermochemical reactions. In conclusion, power generation of PJ-RDCFCs using a GDC metal-free anode was successful, and suggested that the difference in power generation characteristics is due to the presence of Ni, which is a catalyst of thermochemical reactions. REFERENCES [1]M. Ihara et al., Solid State Ionics, 175, 51 (2004) [2]Y. Tagawa et al., ECS Trans., 25 (2), 1133 (2009) [3] S. Sugiyama et al., ECS Trans., 58 (45), 21 (2014) [4] S. Hattori et al., ECS Trans., 64 (2), 211 (2014) Figure 1

Octane Steam Reforming and Electrochemical Reaction on Cu-GDC Cermet Anode of Internal Reforming Solid Oxide Fuel Cell

A porous cermet of Cu and gadolinium oxide-doped ceria (Cu-GDC), which, respectively, have excellent sulfur tolerance and oxygen ion supply property, was focused as an anode electrode for the octane-direct use SOFC. Influences of operating conditions, steam/carbon ratio and cell temperature, on the steam internal reforming reaction and electrochemical reaction on the Cu-GDC cermet anode electrode were investigated. In order to understand the influence of difference in branching of carbon chain, n-octane (linear alkane) and iso-octane (branched chain alkane) were used. By raising the S/C ratio, the carbon deposition is decreased but the fluctuation range of the OCV is increased. Mainly H2, CO, CO2, CH4, and C2H6 are generated in the reforming reaction of n-octane, and C3H8, CH3OH, and C4H10 are also generated in addition to those in the reforming reaction of iso-octane. By using the n-octane fuel, the carbon deposition is reduced. The rate-determining process is dissociation to H2 and CO.

Cracking Reaction of Octane Fuel and Electrochemical Reaction on Ni-YSZ Cermet Anode of Internal Steam Reforming Solid Oxide Fuel Cell

A porous cermet of Ni and yttria stabilized zirconia (Ni-YSZ) cermet was used as an anode electrode for the octane-direct use SOFC. Influences of operating conditions, steam/carbon ratio and cell temperature on the steam internal reforming reaction and electrochemical reaction on the anode electrode were investigated. In order to understand the influence of difference in branching of carbon chain, n-octane (linear alkane) and iso-octane (branched chain alkane) were used. By the internal steam reforming of n-octane, H2, CO, CO2, CH4, and C2H6 are generated. The reforming rate of n-octane is large, while the reforming of iso-octane is slow reaction. When the fuel utilization is low, H2 and CO preferentially contribute for the power generation. The carbon deposition rate is quite large, although the initial power generation performance is high. Especially, the carbon deposition is remarkable when iso-octane is used as the fuel.

Operational Aspects for Direct Coupling of Gas Turbine and Solid Oxide Fuel Cells

This contribution gives an overview of the theoretical and experimental investigations that have been carried out at DLR concerning the pressurized operation of SOFC. The influence of pressure on SOFC performance is analyzed and the influence of internal reforming reactions is evaluated. Furthermore, the influence of pressure on degradation mechanisms is theoretically analyzed. Results show that pressure can strongly influence the power density of SOFC. A decrease in SOFC performance caused by reformate gases is mainly due to temperature effects. Nickel oxidation and carbon deposition propensity are both pressure dependent and should be addressed when designing pressurized SOFC systems.

Performance degradation of solid oxide fuel cells due to sulfur poisoning of the electrochemical reaction and internal reforming reaction

Hydrogen sulfide is known to degrade the solid oxide fuel cell (SOFC) performance by adsorbing on the nickel anode catalyst. In this research, the mechanism underlying such SOFC degradation was evaluated based on a theoretical mathematical modeling approach and the sulfur coverage was calculated from a Temkin-like isotherm which is related to both temperature and hydrogen sulfide (H2S) concentration. The influences of the cell temperature, H2S concentration and electrochemical performance on both the sulfur coverage and cell polarization are studied in detail. Two specific models were considered to identify whether sulfur poisoning has a larger impact on cell performance through its effect on the electrochemical reaction or on the internal reforming reaction. It was found that sulfur poisoning has different effects on the hydrogen oxidation reaction and internal reforming reaction, leading to competing changes in cell performance with temperature and H2S concentration.

Comparison of humidified hydrogen and partly pre-reformed natural gas as fuel for solid oxide fuel cells applying computational fluid dynamics

A three-dimensional computational fluid dynamics (CFD) approach based on the finite element method (FEM) is used to investigate a solid oxide fuel cell (SOFC). Governing equations for heat, gas-phase species, electron, ion and momentum transport are implemented and coupled to kinetics describing electrochemical as well as internal reforming reactions. The model cell design is based on a cell from Ningbo Institute of Material Technology and Engineering in China and the electrochemical area-to-volume ratios are based on experimental work performed at Kyushu University in Japan.A parameter study is performed focusing on the inlet fuel composition, where humidified hydrogen, 30% pre-reformed natural gas (as defined by IEA) and 50% pre-reformed natural gas (as defined by Kyushu University) are compared. It is found that when 30% pre-reformed natural gas is supplied as fuel the air mass flow rate is halved, compared to the case with humidified hydrogen, keeping the inlet and outlet temperatures constant. The current density is decreased but the fuel utilization is kept at 80%. It is found that the cathode support layer has a significant oxygen gas-phase resistance in the direction normal to the cathode/electrolyte interface (at positions under the interconnect ribs), as well as an electron resistance inside the cathode (at positions under the air channel) in the same direction. The methane steam reforming reaction is shown, both according to the experiments and to the models, to proceed along the main flow direction throughout the cell.

Comparison between Kinetic Model and Equilibrium Model in Methane Fed SOFCs with Internal Reforming Reaction

SOFC is a clean and promising energy conversion device that has the advantage of fuel flexibility over other types of fuel cells. In the methane fed SOFCs with internal reforming reaction, direct internal reforming (DIR) reaction and water gas shift reaction (WGSR) occur in anode. To model internal reforming SOFCs, kinetic model and equilibrium model are normally utilized through reviewing previous researches. In order to investigate a more effective model to simulate the processes of electrochemical reactions, the chemical reactions and gas diffusion in the internal reforming SOFCs, comparison between kinetic model and equilibrium model will be conducted. The research results may provide a theoretical basis for future modeling in internal reforming SOFCs. Key words: SOFC, Direct Internal Reforming (DIR), Kinectic model, Equilibrium model

Thermal field in SOFC fed by CH4: Molar fractions effect

In the present work, the anode supported solid oxide fuel cell (AS_SOFC) is fed by air and fuel at cathode and at anode channels respectively. The fuel is a mixture of five components: methane (CH4), hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO) and steam (H2O), where a reforming phenomenon; an external or an internal one appears. At the SOFC anode side, an indirect or direct reforming phenomenon happens.The principal aim of this study is the visualization of the temperature fields under heat sources’ effect caused by the direct internal reforming reactions; the steam reforming reaction, the water gas shift reaction and the overall reforming reaction. The temperature gradient is discussed for several inlet fuel molar fractions.A program in FORTRAN language using the finite difference method is developed. The thermal fields’, in the plane perpendicular to the gas flow, is visualized by “Tec plot” program. This study requires a coupling conservation equations (mass, energy and species). The flows are governed by Darcy’s law. The results show that the fuel molar fractions cannot be ignored in the presence of direct internal reforming phenomenon.

SOFC Durability against Standby and Shutdown Cycling

To simulate realistic operating conditions in SOFC systems, we investigate the influence of thermal cycling on the performance of electrolyte-supported planar SOFCs. Thermal cycling is often associated with interruption of fuel supply, with three main modes; hot standby, cold standby, and shutdown. Cell performance degradation is most significant during shutdown cycles. Nickel oxidation and agglomeration are more pronounced when SOFCs are subjected to lower temperatures for longer periods of time, leading to significant performance degradation. Ostwald ripening at the anode leads to degradation as Ni grains increase in size with cycling. Ni particle precipitation on the anode zirconia grains and along electrolyte grain boundaries is found for the first time in shutdown cycling tests. When H2S is mixed with the fuel, the internal reforming reactions and electrode reactions are inhibited by sulfur poisoning of the Ni anodes, accelerating degradation. The SOFC cycling degradation mechanisms are discussed in detail.

In-Plane Distribution of Carbon Deposition on SOFCs

SOFC can directly utilize not only hydrogen but also various fuels such as city gas, kerosene, and others by internal reforming reaction on anodes. However, impurities and un-reformed hydrocarbon in fuels can cause degradation associated with e.g. carbon deposition. In this study, in-plane distribution of carbon deposition was evaluated using planar cells with an electrode area of 4 cm by 4 cm. The 2-dimensional distribution of deposited carbon has been visualized, where the carbon deposition was accelerated in the simultaneous presence of C3H8 and H2S as minor constituents in simulated pre-reformed CH4-based fuels.

Simulation of Surface Reactions and Multiscale Transport Processes in a Composite Anode Domain Relevant for Solid Oxide Fuel Cells

There are various transport phenomena (gas-phase species, heat, and momentum) occurring at different length scales in anode-supported solid oxide fuel cells (SOFCs), which are strongly affected by catalytic surface reactions at active triple-phase boundaries (TPBs) between the void space (for gas), Ni (catalysts for electrons), and YSZ (an electrolyte material for ions). To understand the multiscale chemical-reacting transport processes in the cell, a three-dimensional numerical calculation approach (the computational fluid dynamics (CFD) method) is further developed and applied for a composite domain including a porous anode, fuel gas flow channel, and solid interconnect. By calculating the rate of microscopic surface-reactions involving the surface-phase species, the gas-phase species/heat generation and consumption related to the internal reforming reactions have been identified and implemented. The applied microscopic model for the internal reforming reactions describes the adsorption and desorption reactions of six gas-phase species and surface reactions of 12 surface-adsorbed species. The predicted results are presented and analyzed in terms of the gas-phase species and temperature distributions and compared with those predicted by employing the global reaction scheme for the internal reforming reactions.

SOFC modeling considering hydrogen and carbon monoxide as electrochemical reactants

Solid oxide fuel cells (SOFCs) are promising as energy producing device with its high efficiency and flexibility in fuel choise. A fully coupled computational fluid dynamics (CFD) approach based on the finite element method, in two-dimensions, is developed to describe a solid oxide fuel cell (SOFC). Governing equations for, gas-phase species, heat, momentum, ion and electron transport are implemented and coupled to kinetics describing electrochemical and internal reforming reactions. Both carbon monoxide and hydrogen are considered as electrochemical reactants within the anode. The predicted results show that the current density distribution along the main flow direction depends on the local concentrations and temperature. A higher (local) fraction of electrochemical reactants increases the Nernst potential as well as the current density. For fuel mixtures without methane, the cathode air flow rate needs to be increased significantly to avoid high temperature gradients within the cell as well as a high outlet temperature.

The effect of electrolyte type on performance of solid oxide fuel cells running on hydrocarbon fuels

A two-dimensional model is developed to simulate the performance of methane fueled solid oxide fuel cells (SOFCs), focusing on the effect of electrolyte type on SOFC performance. The model considers the heat and mass transfer, direct internal reforming (DIR) reaction, water gas shift reaction (WGSR), and electrochemical reactions in SOFCs. The electrochemical oxidation of CO in oxygen ion-conducting SOFC (O-SOFC) is considered. The present study reveals that the performance of H-SOFC is lower than that of O-SOFC at a high temperature or at a low operating potential, as electrochemical oxidation of CO in O-SOFC contributes to power generation. This finding is contrary to our common understanding that proton conducting SOFC (H-SOFC) always performs better than O-SOFC. However, at a high operating potential of 0.8 V or at a lower temperature, H-SOFC does exhibit better performance than O-SOFC due to its higher Nernst potential and higher ionic conductivity of the electrolyte. This indicates that the proton conductors can be good choices for SOFCs at intermediate temperature, even with hydrocarbons fuels. The results provide better understanding on how the electrolyte type influences the performance of SOFCs running on hydrocarbon fuels.

Prediction of the performance of a solid oxide fuel cell fuelled with biosyngas: Influence of different steam-reforming reaction kinetic parameters

SOFCs are often designed to operate with specific fuels, quite often natural gas. CFD modeling is often used to arrive at efficient and safe SOFC designs. Therefore, when an SOFC is fed with different fuels, i.e., biosyngas, CFD can be used as a tool to predict whether the cell and stack will be safe and operate efficiently, and thus can give suggestions for the operation strategies for SOFCs. For that reason, a combined mass and heat transport model of an SOFC (single channel) has been developed for an anode-supported SOFC fed with biosyngas with special attention to the reaction kinetics of the direct internal reforming (DIR) reaction together with the water–gas shift reaction. An SOFC design jointly developed by ECN and Delft University of Technology is employed for the calculations. This work aims to predict the influence of different reforming reaction kinetic parameters on the cell performance by using an anode-supported intermediate temperature DIR planar solid oxide fuel single channel model, under co-flow operation. The DIR reaction of methane, the water–gas shift reaction and the electrochemical oxidation of hydrogen are being considered. As different reaction kinetic models are available in literature and employing them in CFD calculations will yield different results, a comparative analysis is carried out. Several cases were studied with a variety of DIR and water gas shift reaction kinetic parameters available from literature. For the different cases considered, the modeling results show differences in the current density distribution and temperature profile in the channel and in gas concentration profile along the channel. These differences are presented and discussed in detail. Predictions of the behaviors of internal reforming reaction in the reaction zone, and the possibilities of unwanted side reactions such as carbon deposition and Ni oxidation are given with constructive suggestions for future lab experiments.

The analyses of the start-up process of a planar, anode-supported solid oxide fuel cell using three different start-up procedures

Three start-up procedures for an anode-supported planar SOFC are proposed and investigated numerically in the present study. The first is to introduce the inlet fuel at the operation temperature after the heat-up process is completed. The second is to incorporate the anode-recycling mechanism into the start-up process. The third is to fix the difference between the inlet-fuel temperature and the cell minimum temperature. The numerical results obtained from the present study show that the effective maximum absolute temperature-gradient is exhibited in the early stage of the start-up process. For the present investigated SOFC configuration, the required start-up time for the case using methane is 3.2-fold longer than that using hydrogen. The effective maximum absolute temperature-gradient for the case using hydrogen is 2.2-fold larger than that utilizing methane. The endothermic internal reforming reaction of methane has a positive effect on the accommodation of the temperature uniformity during the start-up process. The anode-recycling mechanism significantly reduces the start-up time For the fixed-temperature-difference start-up procedure, a properly selected temperature difference may lead to a smaller effective maximum absolute temperature-gradient in the early stage and a shorter start-up time by accelerating the start-up pace in the later stage.

Performance comparison of the mass transfer models with internal reforming for solid oxide fuel cell anodes

In this work, models describing multicomponent gas diffusion process in an electrode of a porous solid oxide fuel cell (SOFC) anode coupled with internal reforming reactions were developed. The performances of three different types of models, the dusty-gas model (DGM), the binary-friction model (BFM) and the cylindrical pore interpolation model (CPIM), were compared in 1D. All these models take into account Knudsen diffusion and molecule–molecule diffusion can be used in transition region which is generally the case in a SOFC electrode. The developed models are able to predict the fuel components’ molar fraction distributions in the anode electrode, and the concentration overpotential. They are capable of simulating the internal reforming process for hydrocarbon fuel, such as natural gas, with kinetic models considering both methane-steam reforming (MSR), and water–gas shift reaction (WSR). The effects of pressure gradient, pore size, current density, are studied. It was found that three models give similar results in difference cases using the same “tuned” tortuosity factor (τ2). The difference caused using the isobaric assumption is negligible for the H2–H2O–Ar and CO–CO2 system, expect at small pore sizes (under 1μm) and high current density (above 1A/cm2). For a system fed with hydrocarbon fuel, the isobaric assumption will change the molar fraction distribution by up to 10% for different gas mixture components for the CPIM and the BFM, and up to 25% for the DGM at small pore sizes. However, the reaction rates for both MSR and WSR remain the same when the pressure variation is neglected.

Analysis of chemical, electrochemical reactions and thermo-fluid flow in methane-feed internal reforming SOFCs: Part I – Modeling and effect of gas concentrations

Direct internal reforming solid oxide fuel cells (DIR SOFCs) have complicated distributions of temperature and species concentrations due to various chemical and electrochemical reactions. The details of these properties are studied by a 3-D numerical simulation in this work. The simulation modeling used governing equations (mass, momentum, energy and species balance equations) generally suitable to porous medium with porosity variable of zero (solid), 0.3 (porous medium) and 1.0 (fluid). Chemical kinetics equations for the internal reforming and shift reactions based on the Langmuir–Hinshelwood model were incorporated. Hydrogen and carbon monoxide oxidations were considered both participating in electrochemical reactions. The experimentally measured current density–potential curves were compared with the simulation data to validate the code, which revealed that the simulation model was able to predict the dilution effect of nitrogen and the mass transfer under high current densities. It is found that the temperature dramatically declined near the fuel inlet with strong endothermic reactions, but it increased along the fluid flow with electrochemically exothermic reactions. A low steam-to-carbon ratio (SCR) led to high steam reforming and water gas shift reaction rates, which generated a greater amount of hydrogen. Therefore, current density increased with low SCR. The average current density due to carbon monoxide electrochemical oxidation varies from 205.3 A/m2 under an SCR of 2.0 to 47.6 A/m2 under an SCR of 4.0. The average current density due to hydrogen electrochemical oxidation was 5535.4 A/m2 under an SCR of 2.0, which was 27 times higher than that of carbon monoxide. The total current density ranged from 5740.8 A/m2 under an SCR of 2.0 to 2268.9 A/m2 under an SCR of 4.0.

ChemInform Abstract: Development of Multi‐Scale Models for Transport Processes Involving Catalytic Reactions in SOFCs

AbstractReview: 17 refs.

CFD Modeling: Different Kinetic Approaches for Internal Reforming Reactions in an Anode-Supported SOFC

Fuel cells are electrochemical devices that convert chemical energy into electricity. Solid oxide fuel cells (SOFCs) are a particularly interesting type because they can reform hydrocarbon fuels directly within the cell, which is possible, thanks to their high operating temperature. The purpose of this study is to develop an anode-supported SOFC theoretical model to enhance the understanding of the internal reforming reactions and their effects on the transport processes. A computational fluid dynamics approach, based on the finite element method, is implemented to unravel the interaction among internal reforming reactions, momentum, and heat and mass transport. The three different steam reforming reaction rates applied were developed and correlated with experimental studies found in the literature. An equilibrium rate equation is implemented for the water-gas shift reaction. The result showed that the reaction rates are very fast and differ quite a lot in size. The pre-exponential values, in relation to the partial pressures, and the activation energy affected the reaction rate. It was shown that the anode structure and catalytic composition have a major impact on the reforming reaction rate and cell performance. The large difference between the different activation energies and pre-exponential values found in the literature reveals that several parameters probably have a significant influence on the reaction rate. As the experiments with the same chemical compositions can be conducted on a cell or only on a reformer, it is important to reflect over the effect this has on the kinetic model. To fully understand the effect of the parameters connected to the internal reforming reaction, microscale modeling is needed.