Randall Gemmen

Issues for low-emission, fuel-flexible power systems

Modern power generation systems can produce clean, economical energy. Gas turbines, modern reciprocating engines and fuel cells may all play a role in new power production, both for electric power and mechanical drive applications. Compared to their counterparts of even a decade ago, new power systems have significantly reduced pollutant emissions. However, the careful balance between low emissions and operating performance often requires that system performance be optimized on a single fuel. Thus, for example, a gas turbine designed to produce low emissions on natural gas may not easily achieve the same emission goals on a different gaseous fuel. This paper reviews the various issues associated with changes in gaseous fuel composition for low-emission turbines, reciprocating engines and fuel cells.

Single crystalline La0.5Sr0.5MnO3 microcubes as cathode of solid oxide fuel cell

The efficiency of solid oxide fuel cells (SOFCs) is heavily dependent on the electrocatalytic activity of the cathode toward the oxygen reduction reaction (ORR). In order to achieve better cathode performance, single crystalline La0.5Sr0.5MnO3 (LSM) microcubes with the {200} facets have been synthesized by the hydrothermal method. It is found that the LSM microcubes exhibit lower polarization resistance than the conventional polycrystalline La0.8Sr0.2MnO3 powder in air from 700 °C to 900 °C. The ORR activation energy of the LSM microcubes is lower than that of the conventional powder. The ORR kinetics for the microcubes is limited by the charge transfer step while that for the conventional powder is dominated by the oxygen adsorption and dissociation on the cathode surface.

Development of Dynamic Modeling Tools for Solid Oxide and Molten Carbonate Hybrid Fuel Cell Gas Turbine Systems

This paper describes some generic solid oxide and molten carbonate hybrid fuel cell gas turbine systems and dynamic modeling tools that are being developed to simulate the performance of these and other hybrid fuel cell systems. The generic hybrid systems are presented to introduce issues and technical development challenges that hybrid fuel cell gas turbine systems must address and to provide a platform for the development of the dynamic modeling tools. The present goals are to develop dynamic models for the basic components of solid oxide and molten carbonate fuel cell gas turbine hybrids, ensure their reliability, and obtain a basic understanding of their performance prior to integration into a complete hybrid system model. Preliminary results for molten carbonate and solid oxide fuel cell types are presented. These results provide understanding of some of the operational characteristics of fuel cells, and indicate the complexity of the dynamic response of fuel cell hybrid components. For the fuel cell models, generic planar designs are analyzed showing voltage and current behavior following step changes in load resistance and steady state performance curves. The results provide confidence in each of the model’s reliability, enabling them to be integrated for hybrid system simulation. Results from the integrated simulations will provide guidance on future hybrid technology development needs.Copyright

Performance Comparison of Internal Reforming Against External Reforming in a Solid Oxide Fuel Cell, Gas Turbine Hybrid System

Solid Oxide Fuel Cell (SOFC) developers are presently considering both internal and external reforming fuel cell designs. Generally, the endothermic reforming reaction and excess air through the cathode provide the cooling needed to remove waste heat from the fuel cell. Current information suggests that external reforming fuel cells will require a flow rate twice the amount necessary for internal reforming fuel cells. The increased airflow could negatively impact system performance. This paper compares the performance among various external reforming hybrid configurations and an internal reforming hybrid configuration. A system configuration that uses the reformer to cool a cathode recycle stream is introduced, and a system that uses interstage external reforming is proposed. Results show that the thermodynamic performance of these proposed concepts are an improvement over a base-concept external approach, and can be better than an internal reforming hybrid system, depending on the fuel cell cooling requirements.

TECHNICAL DEVELOPMENT ISSUES AND DYNAMIC MODELING OF GAS TURBINE AND FUEL CELL HYBRID SYSTEMS

This paper describes safety issues important to the operation of combined fuel cell and gas turbine (hybrid) systems, and provides motivation for building dynamic modeling tools to support their development. It also describes two models — a steam reformer and a fuel cell — that will be used to investigate the dynamic performance of a hybrid system. The present goals are to develop dynamic models for these two components, ensure their reliability, and obtain a basic understanding of their performance prior to integration into a complete hybrid system model. Because of the large physical domain to be analyzed in the integrated hybrid system, both reformer and fuel cell models are simplified to a one-dimensional system of equations. Model results are presented for a tubular, counterflow steam reformer showing methane conversion and temperature behavior during initial startup, and following several step change perturbations. For the fuel cell model, a generic planar type is analyzed showing voltage and current behavior following step changes in load resistance and fuel input. The results provide confidence in each model’s reliability, enabling them to be integrated for hybrid system simulation. Results from the integrated simulations will provide guidance on future hybrid technology development needs.Copyright

Characterization of Air Flow Management and Control in a Fuel Cell Turbine Hybrid Power System Using Hardware Simulation

Air flow management and control in a fuel cell gas turbine hybrid power system is evaluated using the Hybrid Performance (Hyper) hardware simulation facility at the National Energy Technology Laboratory (NETL), U.S. Department of Energy. The Hyper facility at NETL is a hardware simulation of a fuel cell gas turbine hybrid power system capable of emulating systems in the range of 300kW to 900kW. The hardware portion is comprised of a modified single-shaft gas turbine, a high performance exhaust gas recuperator, several pressure vessels that represent the volumes and flow impedances of the fuel cell and combustors, and the associated integration piping. The simulation portion consists of a real time fuel cell model that is used to control a natural gas burner which replicates the thermal output of a solid oxide fuel cell. Thermal management in the fuel cell component of the hybrid system, especially during an imposed load transient, is improved through the control of cathode air flow. This can be accomplished in a fuel cell turbine hybrid by diverting air around the fuel cell system. Two methods for air flow control are presented in the paper. In this paper, the use of bleed air by-pass and cold air by-pass are characterized quantitatively in terms of compressor inlet flow, process limits, system efficiency and system performance.Copyright

A test device for premixed gas turbine combustion oscillations

This paper discusses the design and operation of a test combustor suitable for studying combustion oscillations caused by a commercial-scale gas turbine fuel nozzle. Aside from the need to be conducted at elevated pressures and temperatures, it is desirable for the experimental device to be flexible in its geometry so as to provide an acoustic environment representative of the commercial device. The combustor design, capabilities, and relevant instrumentation for such a device are presented, along with initial operating experience and preliminary data that suggests the importance of nozzle reference velocity and air temperature.

Development of controls for dynamic operation of carbonate fuel cell-gas turbine hybrid systems

Hybrid fuel cell/gas turbine (FC/GT) systems have been shown through experiment and simulation to be highly efficient technologies with low emissions. Maintaining efficient, low emission, and safe operation, whether during disturbances or regular operational transients, is a challenge to both understand and address. Some likely disturbances can arise from changes in ambient temperature, fuel flow variations induced by supply pressure disturbances, fuel composition variability, and power demand fluctuations. To gain insight into the dynamic operation of such cycles and address operating challenges, dynamic modeling tools have been developed at two different laboratories. In this paper these models are used to simulate the dynamic operation of an integrated MCFC/GT hybrid system and to subsequently develop and test control strategies for the hybrid power plant. Two control strategies are developed and tested for their ability to control the system during various perturbations. Predicted fuel cell operating temperature, fuel utilization, fuel cell and GT power, shaft speed, compressor mass flow and temperatures throughout the FC/GT system are presented for the controlled response to a fuel cell voltage increase in order to show the effect of a load decrease.Copyright

Validation and Application of a CFD-Based Model for Solid Oxide Fuel Cells and Stacks

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

Influence of Radiative Heat Transfer on Variation of Cell Voltage Within a Stack

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.Copyright