Azra Selimovic

Combined solid oxide fuel cell and gas turbine systems for efficient power and heat generation

Abstract The Department of Heat and Power Engineering at Lund University in Sweden has been conducting theoretical studies of combined SOFC and gas turbine (SOFC/GT) cycles. The overall goal is an unbiased evaluation of performance prospects and operational behaviour of such systems. The project is part of a Swedish national program on high-temperature fuel cells. Results of continuous studies started earlier by authors are presented. Recent developments in modelling techniques has resulted in a more accurate fuel cell model giving an advantage over previous system studies based on simplified SOFC models. The fuel cell model has been improved by detailed representation of resistive cell losses, reaction kinetics for the reforming reaction and heat conduction through the solid part of the cell. This SOFC model has further been confirmed against the literature and integrated into simulation software, Aspen Plus™. Recent calculations have focused on a system with external pre-reforming and anode gas recirculation for the internal supply of steam. A reference system, sized at 500 kW, has also been analyzed in variants with gas turbine reheat and air compression intercooling. In addition, knowledge of stack and system behaviour has been gained from sensitivity studies. It is shown that the pressure ratio has a large impact on performance and that electrical efficiencies of more than 65% are possible at low pressure ratios.

Artificial neural network simulator for SOFC performance prediction

This paper describes the development of a novel modelling tool for evaluation of solid oxide fuel cell (SOFC) performance. An artificial neural network (ANN) is trained with a reduced amount of data generated by a validated cell model, and it is then capable of learning the generic functional relationship between inputs and outputs of the system. Once the network is trained, the ANN-driven simulator can predict different operational parameters of the SOFC (i.e. gas flows, operational voltages, current density, etc.) avoiding the detailed description of the fuel cell processes. The highly parallel connectivity within the ANN further reduces the computational time. In a real case, the necessary data for training the ANN simulator would be extracted from experiments. This simulator could be suitable for different applications in the fuel cell field, such as, the construction of performance maps and operating point optimisation and analysis. All this is performed with minimum time demand and good accuracy. This intelligent model together with the operational conditions may provide useful insight into SOFC operating characteristics and improved means of selecting operating conditions, reducing costs and the need for extensive experiments.

Networked solid oxide fuel cell stacks combined with a gas turbine cycle

An improved design of fuel cells stacks arrangement has been suggested before for MCFC where reactant streams are ducted such that they are fed and recycled among multiple MCFC stacks in series. By networking fuel cell stacks, increased efficiency, improved thermal balance, and higher total reactant utilisation can be achieved. In this study, a combination of networked solid oxide fuel cell (SOFC) stacks and a gas turbine (GT) has been modelled and analysed. In such a combination, the stacks are operating in series with respect to the fuel flow. In previous studies, conducted on hybrid SOFC/GT cycles by the authors, it was shown that the major part of the output of such cycles can be addressed to the fuel cell. In those studies, a single SOFC with parallel gas flows to individual cells were assumed. It can be expected that if the performance of the fuel cell is enhanced by networking, the overall system performance will improve. In the first part of this paper, the benefit of the networked stacks is demonstrated for a stand alone stack while the second part analyses and discusses the impact networking of the stacks has on the SOFC/GT system performance and design. For stacks with both reactant streams in series, a significant increase of system efficiency was found (almost 5% points), which, however, can be explained mainly by an improved thermal management.

Design and Off-Design Predictions of a Combined SOFC and Gas Turbine System

In the present study design and off-design performance of a combined SOFC/GT system is investigated. For the evaluation a process simulation tool, Aspen Plus®, was used, completed with a user model representing a SOFC component of planar design. The fuel cell model is a detailed mathematical and physical description thereby avoiding performance curves. A recuperated SOFC/GT system, in size of around 500 kW, with natural gas as fuel, was analysed. Design calculations and sensitivity studies resulted in high electric efficiencies at low pressure ratios. Recent work has focused on off-design calculations investigating different ambient conditions and part load behaviour of this system. Thanks to the small size of the gas turbine, the operational mode is assumed flexible, allowing variation of both rotor speed and of turbine inlet temperature (TIT). In the calculations a simplified compressor map and a choked expander were considered. A high part load efficiency of the system was found exceeding the design point efficiency by as much as three percentage points.Copyright

Intermediate Temperature SOFC in Gas Turbine Cycles

Operational temperature around 800°C is desirable for solid oxide fuel cells (SOFC) due to alleviation of many serious problems, associated with high temperature, i.e., high degradation rate and cost of balance of plant components along with the need for expensive ceramic interconnect. This paper is concerned with the performance of hybrid cycles employing the intermedium temperature SOFC and a gas turbine. The calculations are performed with Aspen Plus® for a system in a size of 500 kW, using methane as fuel. The simulation tool is completed by a mathematical model of the fuel cell. Cell geometry is chosen to represent the type of cells developed at Riso National Laboratory. For the stand alone SOFC, introduction of the metallic interconnect gave an overall performance improvement. A maximum electric efficiency of more than 70% for the system was calculated at low pressure ratios.Copyright

A Dynamic Model of an Atmospheric Solid Oxide Fuel Cell System for Stationary Power Generation

This paper focuses on the transient behavior of a solid oxide fuel cell system used for stationary power production. Dynamic modelling is applied to identify the characteristic time scales of the system components when introducing a disturbance in operational parameters of the system. The information on the response of the system may be used to specify the control loops needed to manage the changes with respect to safe component operation. The commercial process modelling tool gPROMS is used to perform the system simulations. The component library of the tool is completed with dynamic models of a fuel cell stack and a pre-reformer. The other components are modelled for steady state operation. For the fuel cell a detailed dynamic model is obtained by writing the constitutive laws for heat transfer in the solid part of the cell and conservation of heat and mass in the air and fuel channels. Comprehensive representation of resistive cell losses, reaction kinetics for the reforming and heat conduction through the solid part of the cell is also included in the model. The pre-reformer is described as a dynamic pseudo-homogeneous one-dimensional tubular reactor accounting for methane steam reforming and water-gas shift reaction. The differences in the transient behavior of the system components and their interaction are investigated under load changes and feed disturbances. Copyright (Less)

Computational Analysis of O2 Separating Membrane for a CO2-Emission-Free Power Process

The increased demand for clean power in recent years has led to the development of various processes that include different types of CO2 capture. Several options are possible: pre-combustion concepts (fuel de-carbonization and subsequent combustion of H2 ), post-combustion concepts (tail-end CO2 capture solutions, such as amine scrubbing), and integrated concepts in which combustion is carried out in pure a O2 or oxygen-enriched environment instead of air. The integrated concepts involve the use of oxygen-, hydrogen-, or CO2 -separating membranes resulting in exhaust gas containing CO2 and water, from which CO2 can easily be separated. In contrast to traditional oxygen pumps, where a solid oxide electrolyte is sandwiched between two gas-permeable electrodes, a dense, mixed ionic-electronic conducting membrane (MIECM) shows high potential for oxygen separation without external electrodes attached to the oxide surface. Models for oxygen transport through dense membranes have been reported in numerous recent studies. In this study, an equation for oxygen separation has been integrated into a steady-state heat and mass transfer membrane model. Oxygen transfer through a porous supporting layer of membrane is also taken into account. The developed FORTRAN code has been used for numerical investigation and performance analysis of the MIECM and the oxygen transport potential over a range of operating conditions. Preliminary results indicate that a non-uniform temperature distribution, for a given set of oxygen inlet boundary conditions has considerable impact on the oxygen flux and membrane efficiency. Since the implementation of detailed membrane models in heat and mass balance calculations for system studies would result in excessive calculation time, results from this study will be utilized for the generation of correlations describing the oxygen transfer as a function of operating parameters such as temperature and partial pressure. This modeling approach is expected to improve the accuracy of system studies.Copyright