A Finite Volume SOFC Model for Coal-Based Integrated Gasification Fuel Cell Systems Analysis

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for energy production. Most IGFC system analyses performed to date have used non-dimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work, a one-dimensional finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Results are presented for SOFC operation on humidified hydrogen and methane-containing syngas, under co-flow and counter-flow configurations; detailed internal profiles of species mole fractions, temperature, current density and electrochemical performance are obtained. The effects of performance, fuel composition and flow configuration on SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed.

Introduction With the recent enhancement of renewable energy utilization, large scale energy conversion and storage become more important to control the surplus electric power due to the unsteady power generation. Reversible hydrogen energy storage system via solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) are expected as a high efficient device because of its high operating temperature (600℃~1000℃). It is possible to unify them to one device as reversible system (RSOFC/EC), but it is necessary to understand the mechanism of FC/EC reactions and design the device with a good balance between FC and EC in order to apply them for an actual energy system. While SOFC reaction models and many electrochemical data of electrodes have been developed and reported by many researchers, there are few reports to investigate the reaction model of SOEC. Thus, it is possible to predict the performance of SOEC based on a comprehensive SOFC/EC models and experimental data of SOFC, because the reactions of SOFC and SOEC are a reversible relationship and the equal value of IR loss can be applied. However, the comprehensive SOFC/EC electrochemical model has not been established. A series of experimental data of anodic and cathodic current density vs. the potential should be investigated at various conditions to develop the SOFC/EC model. In this work, a series of anodic and cathodic electrode performance was taken using a SOFC in various conditions. Furthermore, attempting to predict SOEC performance from SOFC performances, we proposed and discussed a new model of electrolysis based on our developed SOFC model. Experimental The Disks of 8 mol% Y2O3-ZrO2 (YSZ; 0.25 mm thickness, 20 mm diameter) were used as the electrolyte and the anode was a Ni/YSZ or Ni/GDC composite, and the cathode was a La0.8Sr0.2MnO3/ScSZ composite. We measured the dependence of the current density of anodes on oxygen activity with wide range to get both power generation and electrolysis data for various gas composition (PH2=0.3~0.99 atm, PH2O=0.01, 0.03, 0.05 atm). Results and Discussion 1. Method to fit parameters on the SOFC/EC model We have developed the electrochemical reaction model on the fuel electrode (=anode) based on the equilibrium of competitive adsorption and oxygen activity, ao , specified by anode potential (Eq.1, Table.1) [1]. The SOFC model was constructed by assuming competitive adsorption equilibrium of H2, H2O and O on Ni surfaces at three-phase boundary and the rate-determining step to be Langmuir-type reactions of H with O (Table.1). We obtained an equation of the electrical current density, i, as a function of the oxygen activity (ao ) calculated from the anode potential which is simply measured from experiments. To confirm effectiveness of the model, we fitted the model to actual experimental data described by i vs. ao . From the equations of model on Table.1, each current density of SOFC (ia ) and SOEC(ic ) are described as Eq.2. To draw the plot of i vs. ao on the model, the rate constants (ka, kc ) and the equilibrium constants (KO, KH, KH2O ) need to be defined as reliable value. Thus, from the equilibrium potential vs. 1 atm O2 (Eq.3), we defined the rate constant of kc as Eq.4. When the kc became a dependent variable of ka , then we fitted ia to power generation experimental data described by i vs. ao so that the values of KHKO/KH2O were fitted approximately. Finally, i c model could be drawn on the plot of i vs. ao and the maximum value of i c was obtained by Eq.5 which is independent of ao and Eq.6 was obtained using Eq.4. 2. Experimental results and parameter fitting on the SOFC/EC model As the expreimental results, the maximum power density of SOFC with Ni/YSZ was 588.5 mW/cm2 for FC mode at PH2=0.99 atm, PH2O=0.01 atm, and ic(max) was 0.285 A/cm2 for EC mode at PH2O=0.03 atm. However, ic(max) calculated by the model (Eq.6) was 0.050 A/cm2. Also, in case of SOFC with Ni/GDC, ic(max) was 0.400 A/cm2 for EC mode at PH2O=0.03 atm, while ic(max) calculated by the model (Eq.6) was 0.055 A/cm2. The electrolysis current density measured by our RSOFC/EC showed 6 times higher than model values approximately. This result suggested the possibility that additional parallel reaction path was existed. In this presentation, we will discuss the possible electrochemical paths to explain the disagreement between the model and the experimental data. Finally, the comprehensive SOFC/EC model will be proposed to predict EC performance using SOFC data.

Introduction For many potential mobile robot applications, current power supply technology (such as the lithium ion battery technology) is in fact a key limiting factor due to its low energy density. Solid oxide fuel cells (SOFCs) are a promising alternative and can overcome limitations of current battery technology. This is because SOFCs can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density such as gasoline and so on.Ni-YSZ cermet is the most promising anode material for SOFCs because of its low cost, chemical stability, and closely matched thermalexpansion coefficients with the dense YSZ electrolyte. Ni is an excellent catalyst for hydrocarbon fuel reforming and cracking, which leads to rapid carbon deposition inside the SOFC anode and causes coking and deactivation1). In principle, the carbon deposition on Ni can be depressed by adding steam in the hydrocarbon fuel2,3), whereas few detailed investigations of SOFCs that operate on gasoline fuel have been reported.In this work, we studied the effect of steam content and demonstrated the direct operation of SOFCs by octane which is a main ingredient of gasoline. In the case of the Ni-YSZ anode, the operation temperature must be high in order to avoid sulfur poisoning4 -6 ), because most commercially available fuels contain sulfur. Therefore, in this study, the SOFCs operation was conducted at high temperature of 950 oC. Experimental Surface-polished YSZ pellets of 2 mm in thickness were used for the electrolyte. Pt cathode electrode and Pt reference electrode were prepared on the surface and side of the pellets, respectively, by firing Pt paste at 1300 oC. Then, NiO powder and GDC powder (NiO/YSZ=6/4 (wt)) were thoroughly mixed in a vacuum using the planetary ball mill (200 rpm, 12 h), and made into NiO-YSZ pastes using ethyl-cellulose and organic solvent. The NiO-YSZ paste was screen-printed in diameter of 8 mm on the YSZ pellets and sintered at 1200 oC for 1 h.Octane and steam were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Octane bubbler temperature was 30 oC, and Steam/Carbon ratio (S/C) were controlled by changing the temperature of the water bubbler from 40 to 70 oC. Electrochemical properties (open circuit voltage, anode electrode resistance, I - V and I - P characteristics, stable power generation property) were measured over a temperature range of 950-700 oC. During the constant current generation, exhaust gas was analyzed by gas chromatography. Results and Discussion Figure 1 shows I - V and I - P characteristics for the cells at 950 oC in wet octane. Maximum power density of 35 mW/cm2 was obtained by S/C of 8.9. The power density decreased with S/C quickly.Figure 2 shows the time course of cell voltage at a drawn current density of 5.2 mA/cm2 for S/C of 3.5. The cell voltage was stable. That is, carbon deposit does not arise even in the cell of the Ni-YSZ anode in low S/C of 3.5 when the operation temperature is high of 950 oC. References 1) S. Mclntosh and R.J. Gorte, “Direct Hydrocarbon Solid Oxide Fuel Cells,” Chemical Reviews, 104 (2004) 4845- 4865.2) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: I. Equilibrium Compositions and Reforming Conditions,” Journal of Electrochemical Society, 850 (7) (2003) A878-A884.3) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: II. The C-H-O Ternary Diagrams,” Journal of Electrochemical Society, 850 (7) (2003) A885-A888.4) K. Sasaki, K. Susuki, A. Iyoshi, M. Uchimura, N. Imamura, H. Kusaba, Y. Teraoka, H. Fuchino, K. Tsujimoto, Y. Uchida, N. Jingo, “H2S Poisoning of Solid Oxide Fuel Cells,” J. Electrochem. Soc., 153 (2006) A2023–A2029.5) P. Lohsoontorn, D.J.L. Brett, N.P. Brandon, “Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells,” Journal of Power Sources, 175 (2008) 60–67.6) H. Kishimoto, Y-P Xiong, K. Yamaji, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, “Stability of Ni base anode for direct hydrocarbon SOFCs, ” Journal Chemical Engineering of Japan, 40 (13) (2007) 1178-1182.

Recent approach based heterogeneous comprehensive learning Archimedes optimization algorithm for identifying the optimal parameters of different fuel cells

Simulation of a tubular solid oxide fuel cell stack using AspenPlusTM unit operation models

The National Energy Technology Laboratory (NETL) has undertaken a review of coal gasification technologies that integrate with solid oxide fuel cells (SOFC) to achieve system efficiencies near 60% while capturing and sequestering >90% of the carbon dioxide [1–2]. One way to achieve an overall system efficiency of greater than 60% is in a power plant in which a catalytic coal gasifier produces a syngas with a methane composition of roughly 25% on a dry volume basis and this is sent to a SOFC, with CO2 capture occurring either before or after the SOFC. Integration of a catalytic gasifier with a SOFC, as opposed to a conventional entrained flow gasifier, is improved due to (a) decreased exergy destruction inside a catalytic, steam-coal gasifier producing a high-methane content syngas, and (b) decreased exergy destruction in the SOFC due to the ability to operate at lower air stoichiometric flow ratios. For example, thermal management of the SOFC is greatly improved due to the steam-methane reforming in the anode of the fuel cell. This paper has two main goals. First, we converted the levelized cost of electricity (LCOE) estimates of various research groups into an average internal rate of return on investment (IRR) in order to make comparisons between their results, and to underscore the increased rate of return on investment for advanced integrated gasification fuel cell systems with carbon capture & sequestration (IGFC-CCS) compared with conventional integrated gasification combined cycle (IGCC-CCS) systems and pulverized coal combustion (PCC-CCS) systems. Using capital, labor, and fuel costs from previous researchers and using an average price of baseload electricity generation of $61.50 / MW-hr, we calculated inflation-adjusted IRR values of up to 13%/yr for catalytic gasification with pressurized fuel cell and carbon dioxide capture and storage (CCS), whereas we calculate an IRR of ∼4%/yr and ∼2%/yr for new, conventional IGCC-CCS and PCC-CCS, respectively. If the carbon dioxide is used for enhanced oil recovery rather than for saline aquifer storage, then the IRR values improve to 16%/yr, 10%/yr, and 8%/yr, respectively. For comparison, the IRR of a new conventional IGCC or PCC power plant without CO2 capture are estimated to be 11%/yr and 15.0%/yr, respectively. Second, we conducted an exergy analysis of two different configurations in which syngas from a catalytic gasifier fuels a SOFC. In the first case, the CO2 is captured before the SOFC, and the anode tail gas is sent back to the catalytic gasifier. In the second case, the anode tail gas is oxy-combusted using oxygen ion ceramic membranes and then CO2 is captured for sequestration. In both cases, we find that the system efficiency is greater than 60%. These values compare well with previous system analysis [1–4]. In future work, we plan to calculate the IRR of these two cases and compare with previous economic analyses conducted at NETL [1–2].

Commercialization of solid oxide fuel cell (SOFC) systemsrequiresimproved SOFC performance and durability, which is highly dependenton the coupling of the SOFC stack with other auxiliary components,SOFC stack configuration, and electrode microstructure. Optimizationof SOFC systems at the system/stack/cell/electrode scale via experimentationis expensive and challenging, whereas numerical modeling can be fastand cost-effective. Although many excellent reviews on SOFCs havebeen published, the previous articles lack practical problem-orientedliterature classification and do not cover new emerging models, suchas artificial intelligence (AI) assisted models, heterogeneous models,and so on. These models are important for accelerating the solutionof large-scale multiphysics models and describing mesoscopic electrodebehaviors. In this review, a top-down approach is adopted that cantruly guide SOFC system/stack/cell/electrode design to meet targetedapplications. Another distinct feature of this review is the inclusionof the latest developments in SOFC modeling. This review offers athorough summary and in-depth analysis of an extensive collectionof research on SOFC simulations, classifying the models into distinctcategories based on their varying scales, and serves as a valuabletool to assist researchers in selecting the most suitable models fordiverse research objects.

This research focuses on the performance analysis and control of a solid oxide fuel cell (SOFC) with direct internal reforming using methane as fuel. Firstly, the steady state analysis is performed to design the optimal operational condition for this system. The direct internal reforming of methane in SOFC can utilize the heat from the exothermic electrochemical reaction to produce hydrogen-rich gas, leading to an increasing system efficiency. However, the coupling of the endothermic reforming and electrochemical reactions results in a complicated dynamic response. Therefore, the effect of input variables on the cell temperature and cell voltage is analyzed to investigate the dynamic behavior that is important for design a controller. It is found that the cell operating temperature and cell voltage are dependent on the fuel and air inlet temperature as well as the current density. Next, to achieve the efficient control system, the control structure design of the SOFC is considered to identify good controlled variables and manipulated variables. The concept of a controllability analysis is applied to the control system design of the SOFC for the selection of input-output pairings by considering the relative gain array (RGA). The result shows that the inlet molar flow rates of air and fuel are manipulated variables to control the cell temperature and the content of fuel, respectively. Finally, the conventional and advanced control techniques are designed and implemented to control the cell temperature, the content of methane and the cell voltage. In this research, the model predictive control (MPC) that is a model-based control strategy is proposed for SOFC control. In general, MPC requires the accurate and reliable model of the process to be controlled. However, the SOFC model is complicated and involves uncertain parameters. Therefore, an off-line robust MPC algorithm is developed and employed for controlling the SOFC. The robust MPC algorithm based on linear time-varying (LTV) and linear parameter varying (LPV) systems is also studied. The simulation results show that under the model uncertainty, the proposed robust MPC can control the SOFC and guarantee the stability of the SOFC. The robust MPC algorithm using the LPV model of the SOFC can achieve a better control performance because the model parameters are on-line updated and used in the control calculation.

Ammonia has been studied as a potential energy vector and suitable fuel for Solid Oxide Fuel Cells (SOFCs). The use of ammonia could guarantee high electrical efficiency, with a carbon free and high energy density fuel. This study analyzes the potential of a power generation system based on a SOFC fueled by NH3. Experimental tests on a SOFC short stack were developed using both pure and diluted ammonia. The tests demonstrated the operation of the concept and highlighted possible advantages in terms of thermal equilibrium. Decomposition of ammonia introduces an endothermic reaction that permits heat absorbance in the anode and a better control of stack temperature. Based on tests results, a thermodynamic model of a complete system was designed and studied. The results demonstrate that the ammonia fueled SOFC is more efficient than an equivalent hydrogen fueled one, due to the cooling effect of internal reactions that reduces ancillaries energy consumptions related to cathode air flow.

2D thermal-fluid modeling and parametric analysis of a planar solid oxide fuel cell

One way to increase the value of renewable energy is to use a high efficiency reversible solid oxide fuel cell (SOFC) as an energy storage device. The utilization of SOFC for distributed generation (DG) requires the development of inverters that convert variable DC power into AC power. This paper addresses the integration of the SOFC with a power electronic inverter as a distributed generation device. The SOFC model and power inverter blocks are implemented in MATLAB/Simulink with three competing inverter control schemes. Hardware tests are required in the future and the purpose of this research is to verify a stable SOFC power system.

Steady-state and dynamic models of solid oxide fuel cells based on Satin Bowerbird Optimizer

Solid oxide fuel cell (SOFC) has excellent fuel flexibility for various fuels. Despite some drawbacks like storage and transportation, hydrogen stands up as the best fuel for SOFC. Hydrogen fuel is diluted non-reactive gas species before it is supplied to the SOFC. In this study, a quasi-three-dimensional SOFC model with real microstructure is used to analyse the effect of the diluted fuel mixture. The hydrogen fuel is diluted with nitrogen and a small amount of steam. The mole amount of hydrogen within fuel mixtures is kept constant. On the other hand, the air that is supplied to the air channel of the SOFC remains unchanged. It is found that the cell that is supplied with the highest concentration of hydrogen has the highest performance due to its high partial pressure of hydrogen within the fuel mixture. Such a high partial pressure promotes a low anode concentration loss. Also, the cell that is supplied with a low hydrogen concentration is unable to benefit from its high average cell temperature as its performance is drained by the low partial pressure of hydrogen within the fuel mixture.

SummaryA design study was undertaken of a 100 kW natural gas solid oxide fuel cell (SOFC)/gas turbine (GT) hybrid system to evaluate the impacts of the reforming process under a broad range of SOFC fuel utilizations. An equilibrium reformer model and a one‐dimensional SOFC model are employed to perform system study for this hybrid, while balance of plant model is built in Ebsion to simulate the associated performance of GT cycle. Fifty design cases with varied external fuel reforming temperatures are analyzed based on different SOFC fuel utilization levels. With the decrease of reforming temperature, optimal fuel utilization in terms of efficiency shift from 90% with external reforming to 60% with internal reforming. Highest efficiency up to 74% was achieved with internal reforming, but this came with 1275 K cathode inlet air temperature and 1673 K turbine inlet temperature, which no current fuel cell and few GTs can tolerate. High fuel pre‐reforming rates provided a higher design flexibility and showed opportunities of efficiency improvement with appropriate thermal integration strategy. And the feasibility of achieving high efficiencies in the hybrid system with thermally integrated external reforming will be explored in our future work.Novelty StatementBoth fuel reforming process and fuel utilization could affect the heat and energy balance in the hybrid gas solid oxide fuel cell (SOFC)‐gas turbine (GT) system. However, the optimal design condition considering different fuel reforming approaches and SOFC fuel utilization levels was not determined in the literature. In addition, appropriate integration strategies, used to balance the heat and power between the two subsystems (SOFC and GT), are imperative under different fuel reforming approaches and SOFC fuel utilization levels. But, the feasibility of those thermal integration strategies has not been examined in existing studies. This work studied the coupling effects of SOFC fuel utilization and fuel reforming process on system performance of a natural gas hybrid SOFC/GT cycle. Highest efficiency up to 74% was achieved with internal reforming, but this came with 1275 K cathode inlet air temperature and 1673 K turbine inlet temperature, which no current fuel cell and few GTs can tolerate. High fuel pre‐reforming rates provided a higher design flexibility and showed opportunities of efficiency improvement with appropriate thermal integration strategy.

The increase of DG penetration in the distribution networks makes more and more interesting the investigation of security issues which are typical of the large transmission grids (transient stability, voltage stability and small signal stability) as well as the study of the DG responses to load changes in the grid. This paper presents the model of a Solid Oxide Fuel Cell (SOFC) dedicated to perform dynamic simulations on MV distribution networks. In particular in the present paper transient stability and load following issues are tackled. The first part of the paper is aimed at describing the model of the SOFC which is then used in the test system. As also a gas turbine is inserted into the test system, a brief look at the gas turbine model is mandatory. The aforementioned models include both the slow dynamics (GT prime mover, electrochemical system time constants for SOFC) necessary to study the DG response to load variations and the fast dynamics (e.g the power converter control and protection systems for SOFCs) necessary to study the transient stability related issues. The results of the analysis (carried out on a test MV distribution network) are reported and some conclusions are drawn.