Exergy and Economic Analysis of Catalytic Coal Gasifiers Coupled With Solid Oxide Fuel Cells

The National Energy Technology Laboratory (NETL) as well as Li et al. [1] have shown that integrating a catalytic coal gasifier with a solid oxide fuel cell (SOFC) can achieve high system efficiencies (∼60%) while capturing and sequestering >90% of the carbon dioxide. Integration of a catalytic gasifier with a SOFC is aided by the minimal exergy destruction inside a catalytic, steam-coal gasifier producing a high-methane content syngas, and the decreased exergy destruction in the SOFC due to the ability to operate at lower air stoichiometric flow ratios compared with a SOFC operating only on hydrogen. For a given temperature difference across the inlet and outlet of the fuel cell and for a given current density, a SOFC can be operated at a lower air stoichiometric ratio if there is a significant amount of methane in syngas and if the pressure of the fuel cell is above atmospheric pressure. Here, we present both an exergy analysis of one possible way of integrating a SOFC with a catalytic gasifier. The gasifier is a fluidized bed gasifier that uses ∼20%wt potassium carbonate along with the coal. Before entering the SOFC, carbon dioxide in the syngas is captured using lime, and then the anode tail gas from the SOFC is recycled back to the gasifier, similar to the configuration modeled by Li et al. [2]. We will present the exergy efficiency as a function of the pressure of the SOFC, and compare the exergy efficiency to other coal-based powerplants on the scale of 100–500 MWe. In addition, we use capital and other cost estimates from NETL [3] and others [4] to estimate the internal rate of return on investment (IRR) of various coal based fuel cell power plants, and compare the IRR of these plants with other fossil fuel based base load power plants. We also present the IRR of the catalytic gasification-SOFC power plant as a function of the pressure of the SOFC. Assuming recent fuel & electricity prices, a natural gas combined cycle (NGCC) power plant yields the highest value of rate of return on investment. However, our results suggest that, in the case of a CO2 tax near $30/t CO2, then three different configurations are equally viable economically: NGCC, advanced IGCC-CCS-EOR, and advanced IGFC-CCS-EOR that integrates a catalytic coal gasifier with a pressurized SOFC.

The fuel cell program at the United States Department of Energy (DOE) National Energy Technology Laboratory (NETL) is focused on the development of low-cost, highly efficient, and reliable fossil-fuel-based solid oxide fuel cell (SOFC) power systems that can generate environmentally friendly electric power with at least 90% carbon capture. NETL’s SOFC technology development roadmap is aligned with near-term market opportunities in the distributed generation sector to validate and advance the technology while paving the way for utility-scale natural gas (NG)- and coal-derived synthesis gas-fueled applications via progressively larger system demonstrations. The present study represents a part of a series of system evaluations being carried out at NETL to aid in prioritizing technological advances along research pathways to the realization of utility-scale SOFC systems, a transformational goal of the fuel cell program. In particular, the system performance of utility-scale NG fuel cell (NGFC) systems with and without carbon dioxide (CO2) capture is presented. The NGFC system analyzed features an external auto-thermal reformer (ATR) feeding the fuel to the SOFC system consisting of planar anode-supported SOFC with separated anode and cathode off-gas streams. In systems with CO2 capture, an air separation unit (ASU) is used to provide the oxygen for the ATR and for the combustion of unutilized fuel in the SOFC anode exhaust along with a CO2 purification unit to provide a nearly pure CO2 stream suitable for transport for usage in enhanced oil recovery (EOR) operations or for storage in underground saline formations. Remaining thermal energy in the exhaust gases is recovered in a bottoming steam Rankine cycle while supplying any process heat requirements. A reduced order model (ROM) developed at the Pacific Northwest National Laboratory (PNNL) is used to predict the SOFC performance. The ROM, while being computationally effective for system studies, provides other detailed information about the state of the stack, such as the internal temperature gradient, generally not available from simple performance models often used to represent the SOFC. Such additional information can be important in system optimization studies to preclude operation under off-design conditions that can adversely impact overall system reliability. The NGFC system performance was analyzed by varying salient system parameters, including the percent of internal (to the SOFC module) NG reformation—ranging from 0 to 100%—fuel utilization, and current density. The impact of advances in underlying SOFC technology on electrical performance was also explored.

Experimental and Thermo-Economic Analysis of Catalytic Gasification and Fuel Cell Power Systems

The fuel cell program at the United States Department of Energy (DOE) National Energy Technology Laboratory (NETL) is focused on the development of low-cost, highly efficient, and reliable fossil-fuel-based solid oxide fuel cell (SOFC) power systems that can generate environmentally-friendly electric power with at least 90 percent carbon capture. NETL’s SOFC technology development roadmap is aligned with near-term market opportunities in the distributed generation sector to validate and advance the technology while paving the way for utility-scale natural gas (NG)- and coal-derived synthesis gas-fueled applications via progressively larger system demonstrations. The present study represents a part of a series of system evaluations being carried out at NETL to aid in prioritizing technological advances along research pathways to the realization of utility-scale SOFC systems, a transformational goal of the fuel cell program. In particular, the system performance of utility-scale NG fuel cell (NGFC) systems with and without carbon dioxide (CO2) capture is presented. The NGFC system analyzed features an external auto-thermal reformer (ATR) feeding the fuel to the SOFC system consisting of planar anode-supported SOFC with separated anode and cathode off-gas streams. In systems with CO2 capture, an air separation unit (ASU) is used to provide the oxygen for the ATR and for the combustion of unutilized fuel in the SOFC anode exhaust along with a CO2 purification unit to provide a nearly pure CO2 stream suitable for transport for usage in enhanced oil recovery operations or for storage in underground saline formations. Remaining thermal energy in the exhaust gases is recovered in a bottoming steam Rankine cycle while supplying any process heat requirements. A reduced order model (ROM) developed at the Pacific Northwest National Laboratory (PNNL) is used to predict the SOFC performance. The ROM, while being computationally effective for system studies, provides other detailed information about the state of the stack, such as the internal temperature gradient, generally not available from simple performance models often used to represent the SOFC. Such additional information can be important in system optimization studies to preclude operation under off-design conditions that can adversely impact overall system reliability. The NGFC system performance was analyzed by varying salient system parameters, including the percent of internal (to the SOFC module) NG reformation — ranging from 0 to 100 percent — fuel utilization, and current density. The impact of advances in underlying SOFC technology on electrical performance was also explored.

Integrated gasification fuel cell (IGFC) power plants combining gasification and solid oxide fuel cell (SOFC) technologies are very promising for highly efficient and environmentally friendly power generation from coal. IGFC plant amenability to carbon capture for sequestration makes the technology more attractive given the increasing concern over global climate change caused by greenhouse gas emissions. With the support of the US Department of Energy and the National Energy Technology Laboratory, the Advanced Power and Energy Program has conducted a study to identify promising conceptual designs for IGFC plants. The most promising IGFC concept identified so far is a system with catalytic hydro-gasification, a pressurized (operating pressure of 10 bar) SOFC followed by a turbo-expander and a steam cycle. The design requirement for recycling de-carbonized anode exhaust back to the gasifier for hydro-gasification not only produces a synergistic integration of SOFC and gasification subsystems, but also makes carbon separation a natural result. The current analyses of this system show an efficiency of 58.4 per cent (coal higher heating value basis) while capturing 94 per cent of the CO2. Using this system as a baseline case, this work investigates the sensitivity of IGFC system performance on the extent of carbon capture. It is shown that the proposed IGFC system can achieve ultra-high carbon capture (>99 per cent) at small system efficiency expense while reducing carbon capture to below 90 per cent actually diminishes the system efficiency because less fuel is converted in the SOFC.

As a promising power generation technology, the solid oxide fuel cell (SOFC) system demonstrates high electrical efficiency and overall efficiency in cogeneration mode. Operating at high temperatures (> 680 ˚C), SOFCs cogenerate high-quality heat and can be integrated with different industry facilities. Furthermore, SOFC technology demonstrates high fuel tolerance, including hydrogen, natural gas, ammonia, LPG, etc. Unlike other fuel cell technologies, SOFC systems can also tolerate impurities in the fuel, e.g., CO2, N2 or even a small amount of O2. Besides, the compact system configuration makes SOFC system of great potential in the decentralized application.In the context of the Paris Agreement, more and more solutions to replace fossil fuels have been proposed, where biogas is considered as one promising alternative. Biogas generally contains several impurities, making its application limited in the fuel cell technology field. Removing all the impurities and making it compatible for all the fuel cell technologies would be expensive. Whereas, since SOFC has high tolerance in fuel impurities, keeping certain harmful impurities under safe thresholds with a biogas cleaning unit and injecting the purified biogas into the SOFC system will be less costly. In this case, a biogas-fed SOFC system integrated with a biogas cleaning unit can be considered as a promising renewable technology by avoiding fossil fuel use and converting biowaste into electricity and heat.Although biogas is produced from a renewable source, its use in SOFCs still generates CO2 emissions when it is fed with biogas. To realize carbon neutrality or even being carbon negative in a biogas-fed SOFC system, carbon capture (CC) technology is introduced to meet the net-zero carbon emission target in 2050. One conventional carbon capture storage (CCS) technology is the calcium looping system, which uses CaO/CaCO3 in a carbonation-calcination loop to separate and capture the CO2 from the exhaust gas. This type of technology is quite mature for large-scale applications and has been widely used in thermal power plants for the desulfurization and denitrification of exhaust. Novel technology based on sodium carbonate solution is capable of not only carbon capture, but also biogas cleaning. For the carbon capture utilization (CCU) technology, the Fischer-Tropsch (FT) synthesis can convert CO2 with hydrogen into syngas via electrochemical or thermochemical catalytically driven processes.Focusing on the decentralized power system, small-scale biogas-fed SOFC system integrated with a biogas cleaning unit and CC system should be considered with capacity ranging from 20 kW to 200 kW. As a cogeneration system, the integration should not be limited to mass exchange. The electricity consumed by the biogas cleaning unit and CC system can be fulfilled via the SOFC system. Based on the system temperature pattern, a bigger heat exchange network can be formulated between three parts, which can avoid extra heat input. In this case, the optimality of overall system efficiency and revenue can be achieved.In this study, one biogas-fed SOFC power system is proposed with capacity ranging from 20 kW to 200 kW. The techno-economic evaluation is carried out in two cases, with or without heat and electrical integration. Meanwhile, in different cases, the techno-economic analysis of different biogas cleaning technologies and CC technologies will be performed as well. Two operation modes of the SOFC system are considered: (1) hot recirculation mixed before the reformer, (2) cold recirculation mixed before the reformer. Without the biogas cleaning unit and the CC technology, the biogas-fed SOFC system's electrical efficiency can exceed 65% as is demonstrated in Figure 1. Furthermore, Detailed techno-economic analysis has been carried out by three indicators: capital expenditure (CAPEX), operating expenditure (OPEX) and levelized cost of electricity (LCOE). Integrated with biogas cleaning unit via only mass exchange, the LCOE of the biogas-fed system without CC technology ranges from around 0.11 EUR/kWh (20 kW) to 0.085 EUR/kWh (200 kW). The cost breakdown is shown in Figure 2(a). Based on the SOFC system techno-economic analysis, different CC technologies are added and compared. When the conventional CC technologies like Calcium looping (CaL) and Fischer-Tropsch (FT) synthesis are scaled down to 20-200 kW and applied, unaffordable LCOE prices (0.65 EUR/kWh for CaL and 5 EUR/kWh for FT) indicate that these large-scale conventional technologies are not feasible for our cases. However, a novel technology based on Sodium carbonate looping with 97% CC efficiency is rather promising and shows much lower cost. As is indicated in Figure 2(b), the LCOE cost of the whole system is only around 0.151 EUR/kWh at 200 kW system capacity, indicating that it is a techno-economically feasible solution for a carbon-neutral biogas-fed SOFC system. Figure 1

The National Energy Technology Laboratory (NETL) has developed a solid oxide fuel cell (SOFC) model based on commercial computational fluid dynamics (CFD) software. This new tool is being used to support the US DOE Solid State Energy Conversion Alliance Fuel Cell Program, which will require advanced fuel cell designs in order to meet the program goal of reaching $400/kW for small (∼5kW) systems. The NETL model combines a special SOFC electrochemical model with an electrical potential field model in the finite-volume commercial CFD code from Fluent Incorporated (Lebanon NH). Mass and energy sources and sinks resulting from the electrochemical reactions and electrical current flow are coupled to the fluid flow, chemical species transport, heat transfer, porous media flow, and gas phase chemistry capabilities available in the base CFD model. The NETL SOFC model has also been recently extended to model SOFC stacks with cells connected in electrical series. The model is able to predict detailed, spatially resolved current flow through the electrolyte and through all conducting media in three-dimensional SOFC cells and cell stacks. In conjunction with the SOFC model development program, NETL has an experimental facility in place to generate data for validation of the SOFC model. The experimental program includes collaboration with the University of Utah, a supplier of test specimens and preliminary cell performance data. Well-characterized SOFC test specimens are being tested in the NETL fuel cell test stands for single cell and short-stack arrangements. Anode-supported cells with controlled electrode microstructures, electrode thickness, and electrolyte thickness are being tested. Operating data from the test stands includes cell and stack polarization curves, temperature data, and chemical composition of reactant streams. Using NETL and University of Utah data, an extensive validation program is now underway for the NETL SOFC model. The model is being tested using a simple button-cell configuration. A parametric study of varying operating conditions, cell geometries and cell properties is being performed. Good agreement between predicted and measured cell performance has been observed and is presented. The model has also been applied to planar single cell and cell stack configurations to help in the design of NETL experimental test facilities.

This report summarizes the work performed by Hybrid Power Generation Systems, LLC (HPGS) under Cooperative Agreement DE-FC2601NT40779 for the US Department of Energy, National Energy Technology Laboratory (DoE/NETL) entitled ''Solid Oxide Fuel Cell Hybrid System for Distributed Power Generation''. The main objective of this project is to develop and demonstrate the feasibility of a highly efficient hybrid system integrating a planar Solid Oxide Fuel Cell (SOFC) and a gas turbine. A conceptual hybrid system design was selected for analysis and evaluation. The selected system is estimated to have over 65% system efficiency, a first cost of approximately $650/kW, and a cost of electricity of 8.4 cents/kW-hr. A control strategy and conceptual control design have been developed for the system. A number of SOFC module tests have been completed to evaluate the pressure impact to performance stability. The results show that the operating pressure accelerates the performance degradation. Several experiments were conducted to explore the effects of pressure on carbon formation. Experimental observations on a functioning cell have verified that carbon deposition does not occur in the cell at steam-to-carbon ratios lower than the steady-state design point for hybrid systems. Heat exchanger design, fabrication and performance testing as well as oxidation testing to support heat exchanger life analysis were also conducted. Performance tests of the prototype heat exchanger yielded heat transfer and pressure drop characteristics consistent with the heat exchanger specification. Multicell stacks have been tested and performance maps were obtained under hybrid operating conditions. Successful and repeatable fabrication of large (>12-inch diameter) planar SOFC cells was demonstrated using the tape calendering process. A number of large area cells and stacks were successfully performance tested at ambient and pressurized conditions. A 25 MW plant configuration was selected with projected system efficiency of over 65% and a factory cost of under $400/kW. The plant design is modular and can be scaled to both higher and lower plant power ratings. Integrated gasification fuel cell systems or IGFCs were developed and analyzed for plant sizes in excess of 200 MW. Two alternative integration configurations were selected with projected system efficiency of over 53% on a HHV basis, or about 10 percentage points higher than that of the state-of-the-art Integrated Gasification Combined Cycle (IGCC) systems.

The DOE Office of Fossil Energy Solid Oxide Fuel Cell (SOFC) Program is interested in the near-term commercialization of high-temperature SOFC and solid oxide electrolyzer cell (SOEC) technologies that are robust, reliable, and resilient. A recent report delivered to the United States Congress highlighted several development recommendations including the design of pilot-scale units, continued early stage research and development, increased industrial engagement, and the exploration of reversible operation of SOFC technology.The National Energy Technology Laboratory (NETL) SOFC research group currently addresses most of these recommendations. NETL’s in-house research efforts focus on the characterization, simulation, and mitigation of high temperature degradation of fuel cell components. The capstone of these efforts is NETL’s SOFC degradation modeling framework, which uses NETL supercomputing facilities to simulate SOFC performance degradation of thousands of possible electrode configurations experiencing multiple simultaneous degradation modes under a broad array of relevant operating conditions. The models for the different degradation modes are based upon experimental data (1) generated in-house, (2) referenced from available scientific publications, and (3) shared from collaborations with other industrial, academic, and national laboratory partners within the SOFC Program. The team then employs techniques in data analytics to select optimal electrodes to maximize the SOFC performance for given operating conditions. These modeling efforts guide electrode engineering efforts in-house and through external collaborations by identifying (1) which degradation modes contribute the most to overall performance degradation for given operating conditions and (2) which electrode features will have the greatest impact on lowering cell degradation and system costs. Additionally, the development on non-invasive in situ high temperature fiber optic sensors for temperature and gas composition measurement provides valuable data for inclusion in degradation models as well as informing technology development at the commercial scale. Finally, the wealth of experience gained in development degradation models and materials for reducing the cost of SOFC technology is being readily applied to reducing the cost of SOEC technology.NETL will report on its most recent progress in the field of SOFC and SOEC development, including degradation modeling, in situ fiber optic sensor development, electrode engineering, and relevant systems-level analyses.

Limits to Paris compatibility of CO2 capture and utilization

The pre-baseline configuration for an Integrated Gasification Fuel Cell (IGFC) system has been developed. This case uses current gasification, clean-up, gas turbine, and bottoming cycle technologies together with projected large planar Solid Oxide Fuel Cell (SOFC) technology. This pre-baseline case will be used as a basis for identifying the critical factors impacting system performance and the major technical challenges in implementing such systems. Top-level system requirements were used as the criteria to evaluate and down select alternative sub-systems. The top choice subsystems were subsequently integrated to form the pre-baseline case. The down-selected pre-baseline case includes a British Gas Lurgi (BGL) gasification and cleanup sub-system integrated with a GE Power Systems 6FA+e gas turbine and the Hybrid Power Generation Systems planar Solid Oxide Fuel Cell (SOFC) sub-system. The overall efficiency of this system is estimated to be 43.0%. The system efficiency of the pre-baseline system provides a benchmark level for further optimization efforts in this program.

Solid Oxide Fuel Cell (SOFC) has attracted huge scientific attentions lately, as it is a promising power production technology that can reduce user’s dependency on electricity-grid. SOFC system can generate electricity by using liquid or gaseous fuels. Small volume and external fuel storage make SOFC technology more compatible, and it can easily be adjusted for different industry power plant scales. Moreover, SOFC operates at high temperature (700 0C), and it cogenerates high-quality heat or steam, which can be used as a heat source within the system. As maximum fuel utilization for SOFC is around 85%, a burner is required for the combustion of unconverted fuels. The burner provides additional heat to the SOFC system, at the same time, it generates more CO2.In order to meet “net zero” emission target for greenhouse gases in the year 2050, it is essential to introduce Carbon Capture and Storage (CCS) technology that can retrieve most of the produced CO2 from emission intensive activities and store it permanently in nature (e.g, sequestration and mineralization), leading to an almost carbon neutral activity. The stored CO2 can also be used in methanation process, to convert it into green methane. By integrating an existing CCS technology with SOFC, and using biofuels as energy source, SOFC system can be considered as a carbon negative technology. The most common and widely used CCS technology is Chemical Absorption (CA). The CA process presents good retrofitting options. However, corrosion and degradation are major issues.The high efficiency of SOFC system is however penalised by the fact that the flue gases contain typically a mix of CO2 and N2, which makes it difficult and expensive to separate/capture the produced CO2. There is a possibility of directly injecting pure oxygen to the burner. For industrial scale, cryogenic distillation is commonly used air separation technology for producing O2. Cryogenic distillation is an energy intensive technology, and it is not suitable for small scale O2 production. In the recent times, other technologies such as Pressure Swing Adsorption (PSA) and membrane have evolved for O2 production from air. The non-cryogenic technologies for air separation are preferred to their lower operating cost and easier integration with other processes. PSA is usually suited for medium-range production capacity, and it can produce O2 with 94 % purity. However, it is still necessary to improve performance by reducing energy consumption. The energy consumption is 3.211 MJ/kg-O2 for PSA to produce oxygen. In the oxygen production process, energy consumption accounts for more than 90% of the operating cost.This study considers integration of a SOFC system with PSA, which consists of a SOFC stack, balance of plant components for SOFC system, PSA bed and auxiliary components for PSA. The PSA produces O2 from air, that is injected to the combustion chamber (or burner). In the integrated system, the energy needed for the PSA directly comes from SOFC system. The right amount of oxygen is used to complete the oxidation of the unconverted fuel from the anodic side of SOFC, and CO2 is separated automatically after water condensation. Figure 1 shows layout of integrated system. The performance of PSA is explored for different materials, different pressure ratio and temperature, to achieve minimum energy consumption. Moreover, heat integration has been studied for SOFC system, the integrated system has waste heat available, especially at the downstream of the burner. In order to valorize the waste heat of the system, a steam cycle has been integrated for producing extra amount of electricity, to improve the overall efficiency of the system. Finally, the performance of integrated system has been optimized for maximization of electrical efficiency and minimization of exergy distraction, via multi-objective optimization.In this study, the net electricity output from SOFC-PSA system is around 9 kW. The SOFC produces 9.8 kW electricity, and about 8% electricity is consumed by the PSA for producing O2 that is required for the combustion of unconverted fuel. The integrated system has an overall efficiency of more than 55%. A steam cycle has been integrated for producing extra amount of electricity, and it produces additional 2.5 kW of electricity. Hence, the overall efficiency of the integrated system reaches above 70% with automatic CO2 separation. Figure 1

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 power production. Most IGFC system analyses performed to-date have used nondimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work a quasi-two-dimensional (2D) 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. Activation polarization parameters previously used for high temperature electrolyte-supported SOFC result in cell performance that is much poorer than that observed for modern intermediate temperature anode-supported configurations; thus, a sensitivity analysis was conducted to identify appropriate parameters for modern SOFC modeling. Model results are shown for SOFC operation on humidified H2 and CH4 containing syngas, under coflow and counterflow 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 of SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed. The model can be implemented not only as a stand-alone SOFC analysis tool, but also a subroutine that can communicate and cooperate with chemical flow sheet software seamlessly for convenient IGFC system 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.

The U.S. Department of Energy, Office of Fossil Energy Stationary Fuel Cell Program