SOFC systems

SOFCs cannot function at temperatures lower than around 700°C, beyond which the standard materials used as electrolytes become conductive of 0 ions. Also, the stack needs to be placed in an adiabatic enclosure which is raised to the stack s nominal temperature before it can produce electrical power. Thus, the system cannot be started up at ambient temperature. The nominal operating temperature is between 750 and 850°C approximately, depending on the manufacturers of the stacks. 

SOFC stacks respond badly to rapid and significant changes in the operating conditions. Also, any modification of the reference points, the fuel flowrate, oxidant flowrate, current or temperature of the chamber needs to be made in stages, followed by a period of thermal stabilization so as to preserve the lifetime of the stack. 

SOFC systems can be supplied with hydrogen or with a gas from the reforming of a hydrocarbon with a high carbon monoxide content, because the high operating temperature means the CO can be oxidized into CO2 in the anodic compartment. Supply with a reformed hydrocarbon is the most usual at present, because hydrogen distribution infrastmctures are not yet in place in most cases. 

The anode is made of a nickel-based cermet, which needs to be reduced the first time the temperature of the stack is raised. This is done by washing the anode with a reducing gas (e g. a mixture of nitrogen and hydrogen) the first time the stack s temperature is raised. 

The only possible mode of operation for the anode, therefore, is an open anode with a high factor of overstoichiometry. At the anode s output, the effluent is therefore a mixture which is still very rich in fuel. This gas is then recycled into the system, and different forms can be envisaged  

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Development of Anode Catalyst for Internal Reforming of CH4 by CO2 in SOFC System... 

In this work, the catalytic reforming of CH4 by CO2 over Ni based catalysts was investigated to develop a high performance anode catalyst for application in an internal reforming SOFC system. The prepared catalysts were characterized by N2 physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). 

To reduce the formation of carbon deposited on the anode side [2], MgO and Ce02 were selected as a modification agent of Ni-YSZ anodic catalyst for the co-generation of syngas and electricity in the SOFC system. It was considered that Ni provides the catalytic activity for the catalytic reforming and electronic conductivity for electrode, and YSZ provides ionic conductivity and a thermal expansion matched with the YSZ electrolyte. 

Figure 1 shows the effects of reaction temperature on the conversions of CO2 and CH4 over Ni-YSZ-Ce02 and Ni-YSZ-MgO catalysts. It was found that the Ni-YSZ-Ce02 catalyst is showed higher catalytic activity than the Ni-YSZ-MgO catalyst at temperature range of 650 850 Ti and the maximum activity was observed at above 800 °C, the optimum temperature for internal reforming in SOFC system [5]. In our previous work, it was identified that Ni-YSZ-MgO catalyst was deactivated with reaction time, however Ni-YSZ-Ce02 showed stable catalytic activity more than Ni-YSZ-MgO catalyst imder tiie tested conditions [6]. 

Fig. 2. Effects of temperature on the catalytic performance in the internal reforming of CH4 by CO2 over ESC of SOFC system.

The internal reforming of CH4 by CQzin SOFC system was performed over an ESC (electrolyte st rported cell) prepared with Ni based anode catalysts. Figure 5 diows the performance of voltage and power density with current density over various ESC (Ni based anodes I YSZ (LaSr)Mn03) at SOOC when CH4 and CO2 were used as reactants. To improve the contact between single cell and collector, different types of SOFC reactor were used [5]. In the optimized reactor (C), it was found fliat die opai-... 

The effects of total flow rate of fuels (CO2/CH4 = 1) on the impedance in the internal reforming of CH4 by C02 0ver ESC (NiO-YSZ- Ce02 I YSZ I (LaSr)Mn03) of SOFC system are represoited in Figure 6. It was comsidaed fliat the total resistance was dependent on the total flow rate because the conva sions of CO2 and CH4 over ESC were affected by contact time in the internal reforming system. 

Solid oxide fuel cell (SOFC) systems can reach electrical efficiencies of over 50% when using natural gas, diesel or biogas. When combined with gas turbines there can be electrical efficiencies of 70%, for small installation as well as large. In a fuel cell system, these efficiencies can be kept at partial loads as low as 50%. Conventional technologies must run at close to full load to be most efficient. 

NOx and SOx emissions from SOFC systems are negligible. They are typically 0.06-g/kWhe and 0.013-g/kWhe (kilo-watt hours electrical). 

The benefits and feasibility of hybrid systems have been established with conceptual studies and small-scale demonstrations fueled with natural gas. If large-scale, greater than 100-MW, fuel cell/turbine hybrid systems are to become a reality a reduction in fuel cell costs and scalability to larger units is required. The SECA program demonstrated 3 to 10 kW SOFC systems with costs of less than 800/kW in 2005. 

Hydrogen can be separated from the flue gas at low cost in high-temperature fuel cells. A SOFC system may be able to cogenerate hydrogen for about 3.00 per kg which can match gasoline. Since these fuel cells could be part of the fueling station, there would be no need for a hydrogen delivery infrastructure. 

Portable DMFCs seem close to commercialisation. They are likely to be followed by stationary MCFC and SOFC systems for decentralised heat and power generation. More research is still needed before PEMFCs are ready for commercialisation in the transport sector. 

Robustness requires a low sensitivity to what might cause permanent damage or degradation of the SOFC system and, hence, the cell - for excursions outside the normal operating window, contaminants in the fuel and the air, thermal and reduction-oxidation (redox) cycling of the anode. 

Additionally, nickel is a well established steam-reforming catalyst. An ideal SOFC system operated on natural gas applies internal steam reforming, i.e., the reforming of the methane takes place in the anode compartment of the stack. This type of system is favored for system simplicity and costs (no external reformer), and for system efficiency because the heat generated by the cell reaction is directly used by the reform reaction, and hence the cooling requirements of the stack (by air at the cathode side) are significantly reduced. 

Based on the system requirements discussed above, fuel cell APUs will consist of a fuel processor, a stack system and the balance of plant. Figure 1-13 lists the components required in SOFC and PEM based systems. The components needed in a PEM system for APU applications are similar to that needed in residential power. The main issue for components for PEM-based systems is the minimization or elimination of the use of external supplied water. For both PEM and SOFC systems, start-up batteries (either existing or dedicated units) will be needed since external electric power is not available. 

Detailed cost and design studies for both PEFC and SOFC systems at sizes ranging from 5kW to 1 MW were made that point to the fundamental differences between PEFC and SOFC technology that impact the system design and by implication the cost structure. These differences will be discussed in the following paragraphs. 

Figure 1-14 shows a simplified layout for an SOFC-based APU. The air for reformer operation and cathode requirements is compressed in a single compressor and then split between the unit operations. The external water supply shown in figure 1-14 will most likely not be needed the anode recycle stream provides water. Unreacted anode tail gas is recuperated in a tail gas burner. Additional energy is available in a SOFC system from enthalpy recovery from tail gas effluent streams that are typically 400-600°C. Current thinking is that reformers for transportation fuel based SOFC APUs will be of the exothermic type (i.e. partial oxidation or autothermal reforming), as no viable steam reformers are available for such fuels. 

Figure 1-14. Simplified System process flow diagram of pre-reformer/SOFC system... 

The main difference in SOFC stack cost structure as compared to PEFC cost relates to the simpler system configuration of the SOFC-based system. This is mainly due to the fact that SOFC stacks do not contain the type of high-cost precious metals that PEFCs contain. This is off-set in part by the relatively complex manufacturing process required for the manufacture of the SOFC electrode electrolyte plates and by the somewhat lower power density in SOFC systems. Low temperature operation (enabled with electrode supported planar configuration) enables the use of low cost metallic interconnects which can be manufactured with conventional metal forming operations. 

One of the key challenges with small-scale SOFC systems is to overcome heat losses. The higher the heat losses are, the more recuperation is required to maintain the fuel cell within an acceptable operating temperature range and hence to ensure good performance. 

Figure 1-16. Projected cost structure of a SkWnet APU SOFC system. Gasoline fueled POX reformer, Fuel cell operating at 300mW/cm, 0.7 V, 90 % fuel utilization, 500,000...

Natural gas systems with endothermic steam reformers often make use of the residual fuel from the anode in a reformer burner. Alternatively, the residual fuel could be combusted prior to a gas expander to boost performance. In an MCFC system, the residual fuel often is combusted to maximize the supply of CO2 to the cathode while at the same time providing air preheating. In an SOFC system, the residual fuel often is combusted to provide high-temperature air preheating. 

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