Ferritic steels have been recognized as candidates for their applications in heat exchangers used in Solid Oxide Fuel Cells? balance of plant. Combustion gases flowing through those heat exchangers can be very corrosive. Therefore, the National Energy Technology Laboratory determined materials performance of commercial S43000 stainless steel exposed to a simulated combustion gas at 800 °C. The exposure experiments were conducted on flat samples in the simulated combustion gas: 19vol% O2+6vol% H2O + 4vol% CO2 +71vol% N2 under isothermal conditions. After the experiment, the surface of a corroded sample was characterized by X-ray diffraction (XRD) to identify possible phases present in the scale, scanning electron microscopy (SEM) to determine microstructure of the oxide scales, and energy dispersive X-ray (EDX) or wavelength dispersive energy X-ray (WDX) spectroscopy to determine chemical composition in the scale and the metal substrate.
Reliable and affordable fuel cells are a centerpiece of the world?s energy future. They are energy conversion devices that generate electricity and heat by electrolyte. The chief characteristic of fuel cells is their ability to convert chemical energy to electrical energy without the need for combustion, thereby giving much higher conversion efficiencies than conventional methods, such as steam turbines.1 Cost remains the final obstacle that must be overcome for fuel cells to realize their full commercial potential. A major barrier for affordable high-temperature fuel cells is the cost of heat exchangers and other balance of plant (BOP) components. Advances in solid-state material manufacturing show promise for making solid oxide fuel cells (SOFC) applicable in any power application. Cost reduction can be achieved in component fabrication, materials used, and cell and stack designs. Balance of plant issues also present problems in the commercialization of fuel cell technology. Specifically for SOFC, air and fuel need to be heated and cooled at some stage of the process. This requires pumps, piping, heat exchangers, etc. in order to deliver useable electrical power.2 Currently, there are no economical commercial heat exchangers suitable for use with SOFC. Typical heat exchanger design criteria includes the transfer of heat energy from a hot (800-1000 °C), post combustion gas or hot depleted air from the SOFC system, as shown in Figure 1,3 to a cooler oxygen rich gas which, in turn, is supplied to the cathode side of the SOFC system. The sink (cold) fluid typically operates in the range from ambient to 300°C. For example, conventional heat exchangers can cost as much as $10,000 per unit. In order to meet the SECA goal of demonstration fuel cell systems at a cost of $400/kW, heat exchangers need to be in the $200 per unit for a 10 kW system. Significant improvements in cost are possible through implementation of advanced materials, designs, and process technology.4 For the heat exchange applications, resistance of metallic components to corrosion is critical. In this research, our focus was on corrosion resistance of SS43000 stainless steel potentially replacing more expensive and corrosion resistant N066255 in combustion gases derived from combusting natural gas reformate according to reaction (1):
(chemical equation available in full paper)
