FuelCell Energy is developing solid oxide fuel cell (SOFC) technology for very efficient, economically-viable, coal-to- electricity power plants utilizing synthesis gas (syngas) from a coal gasifier (1). One of the key objectives is implementation of an innovative system concept in design of a multi-MW power plant to achieve an electrical efficiency exceeding 50% based on the high heating value (HHV) of coal, exclusive of power requirements for CO2 compression. The system is also designed to remove at least ninety percent of carbon in the syngas, for sequestration as CO2. Combined with CO2 separation technology, the Integrated Gasification Fuel Cell (IGFC) power plant is expected to achieve near-zero emissions to the environment. Development of this technology will significantly advance the nation’s energy security and independence interests (through effective utilization of the nation’s vast coal reserves), address pollution and greenhouse gas concerns, and help enhance the nation’s economic growth.
Introduction
FuelCell Energy (FCE) is in Phase I of a multiphase program to develop fuel cell technology for very efficient coal-to-electricity power plants with near-zero emissions. The program is being carried out through a cooperative agreement with the Department of Energy Office of Fossil Energy’s Solid State Energy Conversion Alliance (SECA) program. FuelCell Energy, Inc. (FCE), Versa Power Systems (VPS), WorleyParsons Group Inc., and the Gas Technology Institute (GTI) are collaborating in this project to develop economically-viable multi-megawatt (MW) solid oxide fuel cell (SOFC)-based power plant systems for utilization of synthesis gas (syngas) from a coal gasifier (1). One of the key objectives is implementation of an innovative system concept in design of the multi-MW power plant to achieve an electrical efficiency exceeding 50% based on coal’s high heating value (HHV), inclusive of coal gasification and carbon separation processes but exclusive of power requirements for carbon dioxide compression. The coal-based power plant is targeted to have a cost of $400/kW, based on 2002 dollars, for the SOFC power block, exclusive of the coal gasification and gas clean-up costs. One of the system design criteria is the removal of at least ninety percent of carbon in the syngas, for carbon sequestration. Combined with a carbon dioxide separation technology, the Integrated Gasification Fuel Cell (IGFC) power plant is expected to achieve near-zero emissions to the environment. Development of this technology will significantly advance the nation’s energy security and independence interests (through effective utilization of the nation’s vast coal reserves), address pollution and greenhouse gas concerns, and help enhance the nation’s economic growth.
Program Plans
The program is organized into three phases with specific tasks. Phase I of the program is focused on preliminary engineering design and cost analysis of multi-MW power plant systems – a baseline power plant (>100 MW in size). In parallel, the focus is also on cell and stack development. This includes the scale-up of existing SOFC cell area and stack size (number of cells), performance improvements, and manufacturing capacity increase. Phase I deliverable is the test demonstration of a SOFC stack building block unit that is representative of a MW-class module, on simulated coal syngas.
Upon successful completion of Phase I and selection by DOE to continue, Phase II of the program will focus on the development of cell repeat unit sizes sufficient to form the basis of fuel cell stacks that are technically and economically viable for aggregation into a ≥250kW fuel cell module. A sufficient number of repeat units shall be assembled to form the Phase II stack test article.
Upon successful completion of Phase II and selection by DOE to continue, Phase III of the program will focus on modularization of the Phase II stack building block units into a MW-size module that will serve as the building block for a ≥100MWe Integrated Gasification Fuel Cell (IGFC) system. The Phase III program will include the design, fabrication, and testing of a ≥250kW fuel cell module as well as the design, fabrication, and test of a proof-of-concept (POC) multi-MW power plant. The POC system demonstration will establish the commercial viability of SOFC for large scale coal-based power generation applications.
Achievement of the fuel cell system cost and performance targets is realized by a multi-faceted approach, involving: performance improvements of the fuel cell active components, scale-up of fuel cell stacks, and innovative developments of stack module and system concepts (2). FCE utilizes the planar cell and stack technology of its SOFC partner, Versa Power Systems Inc (VPS), for all its SOFC development programs. Development of the advanced fuel cell and stack components with enhanced durability and performance is being carried out by VPS. This includes scale-up of existing SOFC cell area and stack size. VPS has well-established processes, quality control procedures, and equipment for the manufacture of small to intermediate size cells and stacks. This serves as a solid base for cell area and stack size scale-up. Increased manufacturing capacity is also being pursued by VPS as a part of the SOFC technology development objective. The efforts by FCE, WorleyParsons, and GTI are focused on system development, design, and cost analysis as well as the evaluation of effects of coal contaminates on the fuel cell performance.
Baseline Power Plant System Development
At the system level, innovative cycle configurations were developed based on the existing commercial gasification/syngas clean-up technology platforms. Figure 1 shows a top level block flow diagram for the baseline large-scale coal based SOFC power plant. As shown, the power plant consists of three major subsystems or islands. The gasification island includes a coal gasifier and the supporting subsystems/components. An air separation unit is required to provide oxygen for the gasifier. The design basis utilizes a bituminous coal fuel such as Illinois No. 6. After removal of particulates, sour syngas from the gasifier is cleaned and processed in the gas cleanup and sequestration island. Sulfur and other contaminants/impurities such as mercury are removed to the levels necessary to yield the fuel gas suitable for feeding the SOFC power modules downstream. A CO2 separation subsystem can be a part of the syngas clean-up island. The gas is processed to remove enough CO2 to enable the required level of carbon capture from the syngas. Some of the CO2 removal can be accomplished downstream of the fuel cell (post-processing). The sweet syngas is then utilized for power generation in the SOFC power island. The fuel cell operates at atmospheric pressure and the waste heat from fuel cell is utilized in a combined cycle which may consist of a gas turbine, steam turbine, or both. Supplemental power is also generated by expanding the high-pressure syngas in a fuel gas expander upstream of the fuel cell. The plant is well integrated to achieve a high electrical efficiency. Thermal integration to recover waste heat and utilize it for generating the process steam required by the plant is an example of the close coupling between the gasification, clean-up, and SOFC Power Islands. Conceptual engineering design and analysis for multi-MW systems are being conducted for power plants in the size range of 400-500 MW. The design efforts have been focused on developing system configurations, performing system simulations, and conducting system and cost analysis to most effectively support the reliability, efficiency, carbon capture, and cost targets of the multi-MW IGFC system.
Figure 1. Coal Based Integrated Gasification Fuel Cell Power Plant Block Flow Diagram.
Major gasifier types such as moving bed, entrained flow, and fluid bed gasifiers were considered. In addition to well-established and proven gasification technologies, advanced gasification technologies including the transport reactor, which may offer high cold gas efficiencies (>84%), were also considered for an alternate long-term design. Review of the commercial coal gasification technologies was accomplished using a gasifier selection matrix that supports the program objectives. The ConocoPhillips (COP) entrained-flow gasifier was selected for integration into the commercial-based IGFC system. It was the highest ranking commercial gasifier based on high CGE (cold gas efficiency), meeting the system carbon separation requirement, low auxiliary power loads and low steam demand.
Syngas clean-up and carbon separation technologies were evaluated in conjunction with the various gasifiers. Acid gas removal processes based on chemical, physical, and hybrid solvents were considered. Only commercially available systems were considered for the baseline system in Phase I. The removal technologies that were evaluated for the AGR included: Selexol, Rectisol, Ucarsol Amine Process, FLEXSORB and Sulfinol. The evaluation and analysis also included: COS removal, Hg (mercury) removal, and sulfur polishing. The conclusion of the analysis was that a double stage Selexol AGR is the most cost-effective system that meets the requirements of Sulfur removal to low levels and ⩾ 90% carbon separation for the baseline system.
Figure 2. Configuration Option B Featuring Steam Turbine: Use of Steam Bottoming Cycle Increases the Overall Power Plant Efficiency.
Several co-generation alternatives including a bottoming steam turbine only or indirect and direct-fired gas turbines in conjunction with a steam turbine were evaluated to ensure a synergistic fit with SOFC module requirements. The system concepts considered also included approaches for high fuel utilization and stack thermal management. Three configuration options - one featuring an indirectly heated gas turbine (configuration A1), one with a direct-fired gas turbine, and one utilizing a steam turbine/bottoming cycle (configuration B) - were analyzed. Figure 2 shows the simplified process flow diagram for configuration B. This represents the SOFC power island section of the power plant. Heat and mass balances and plant performance evaluation/analysis were conducted for all three configurations.
Effort is currently directed to the development of high methane (in syngas), high efficiency cycles based on future advanced technology. Catalytic gasifiers offering high cold gas efficiency with low auxiliary load (low O2 consumption) and low steam demand are being considered. Warm (or humid) gas cleanup technology offering the benefits of reduced thermal losses and water retention (by syngas, useful for shift/SOFC operation downstream) is also being considered. Water quench-based syngas cooling is applied as it is compatible with humid gas cleaning and SOFC water needs. Figure 3 shows a simplified block flow diagram of a typical system. The configuration employs oxycombustion of SOFC anode off-gas for CO2 capture. The post-fuel cell capture of CO2 derives benefits from the CO shift reaction occurring in the SOFC.
Figure 3. Block Flow Diagram of Advanced Cycle: Coal-based SOFC System Features High Methane Syngas and High Efficiency.
Much progress has been made in concept development, engineering design, and cost analysis for the baseline power plant system, with special attention to the coal-gas clean- up system and turbine combined cycle technology for maximum efficiency with minimum cost. Preliminary cost analysis of the SOFC Multi-MW Baseline Power Plant shows a clear path to achieving the SECA cost goals. Preliminary system simulations and engineering analysis, conducted jointly with the WorleyParsons Group, has led to a Baseline SOFC power plant system design with greater than 50% electrical efficiency based on coal HHV. Table 1 shows a performance comparison of three baseline systems: one with an indirectly-heated gas turbine (configuration A), another with a steam bottoming cycle (configuration B), and the third utilizing advanced technology options such as catalytic gasification.
Table 1. Baseline Integrated Gasification Fuel Cell (IFGT) System Summary
SOFC Technology Development
VPS has also made significant progress in the area of cell and stack development. SOFC cell manufacturing scale-up from 156cm2 to greater than 1000cm2 size has been successfully demonstrated. Production capacity for scaled-up 625cm2 (550cm2 cell active area) components has been validated. This validation is based on the manufacture of over 1000 cell components meeting component design specifications and targeted production yields. Performance repeatability of scaled-up, 625cm2 size components has been validated with several single cell and short stack tests. These results indicate no significant electrochemical performance loss due to cell scale-up to an active cell area of 550cm2. Efforts are well underway at VPS to enhance stack manufacturing capacity to 500 kW/year and cell manufacturing capacity to 1,000kW/year.
Electrochemical performance enhancement is critical to the overall program objectives (cost, life, and efficiency). Materials development focused on improving baseline cell electrochemical performance via several parallel approaches of materials modifications and microstructure optimizations. Major performance improvement breakthroughs have been achieved as shown in Figure 4. Performance enhancements of 9% and 17% were achieved at operating temperatures of 750°C and 700°C, respectively. This improvement enables cell operation at higher power density, lower temperature, higher operating voltage, or a combination of these factors.
Figure 4. Solid Oxide Fuel Cell Electrochemical Performance Enhancement
Five stack building blocks (64 cells each) will be assembled into a 50-kW stack tower and 20 stack towers will be assembled into a single MW-size module to be used for the multi-MW power plant systems, as illustrated in Figure 5. This design configuration has undergone significant computational modeling analysis at FCE, VPS and Pacific Northwest National Laboratories (PNNL). This analysis resulted in uniform flow through the module gas delivery system and acceptable temperature distribution among and within the individual towers. The vessel, base, insulation, and tower compression system designs were conceptualized using FCE’s current commercial MW technology.
Figure 5. MW-scale SOFC Stack Module: Contains Twenty 50-kW Stack Towers
Summary
Innovative system concepts have been developed for a multi-MW Integrated Gasification Fuel Cell (IGFC) power plant based on both commercially-available and advanced technologies. A baseline system with net electrical efficiency of greater than 50% (coal HHV), exclusive of power requirements for CO2 compression, including >90% CO2 capture has been successfully modeled. Significant breakthroughs in SOFC electrochemical performance and degradation characteristics has been achieved. Continued effort will be focused on IGFC system development utilizing advanced technologies for high efficiency and lower cost. SOFC development will continue with a focus on design and testing of larger-scale fuel cell towers and modules.
Source: Hossein Ghezel-Ayagha, Jody Doyona, Jim Walzaka, Stephen Jollya, Dilip Patela, Allen Adriania, Peng Huanga, Keith E. Davisa, David Staufferb, Vladimir Vaysmanb, Sunitha Asapub, Brian Borglumc, Eric Tangc, Randy Petrid, Chakravarthy Sishtlae
a FuelCell Energy, Inc., 3 Great Pasture Road, Danbury, Connecticut
b WorleyParsons Group Inc., 2675 Morgantown Road, Reading, Pennsylvania
c Versa Power Systems, 4852 52nd Street, SE Calgary, Alberta, Canada
d Versa Power Systems, 8392 Continental Divide Road, Suite 101, Littleton
e Gas Technology Institute, 1700 South Mount Prospect Road, Des Plaines
a FuelCell Energy, Inc., 3 Great Pasture Road, Danbury, Connecticut
b WorleyParsons Group Inc., 2675 Morgantown Road, Reading, Pennsylvania
c Versa Power Systems, 4852 52nd Street, SE Calgary, Alberta, Canada
d Versa Power Systems, 8392 Continental Divide Road, Suite 101, Littleton
e Gas Technology Institute, 1700 South Mount Prospect Road, Des Plaines
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