Utilization of Natural Fuels in SOFCs

The major types of fuel for solid-oxide fuel cells (as the reactants being oxidized) are hydrogen and carbon monoxide. An important difference exists between these fuel cells and other types of fuel cells, in that various natural fuels or products of relatively simple processing of such fuels may also be utilized directly. 

As we know, the original aim of all work on fuel cells has actually been precisely the direct transformation of the chemical energy of natural fuels to electrical energy. In seeking solutions to this problem of a direct utilization of natural fuels in fuel cells, workers have encountered numerous difficulties that in many cases could practically not be overcome. These difficulties were associated with the very low rates of electrochemical oxidation of these fuels and with the presence of various contaminants that hinder and sometimes block these reactions completely. 

For this reason, the most realistic way of utilizing these natural fuels in a fuel cell includes their prior chemical (catalytic) conversion to other substances, primarily hydrogen, that are more readily oxidized electrochemically. In addition to conversion, the final processing product must also be freed carefully from all contaminants that could hinder the electrochemical reaction. These processes of conversion and purification are described in greater detail in Chapter 11. 

In view of the chief aim—of attaining a highly efficient utilization of the fuel s chemical energy—it will be obvious that installations combining conversion units, purification units, and fuel cells will not be advantageous, apart fi om their very large space requirements. Also, fuel conversion processes are usually 

The conversion processes proceed at elevated temperatures close to those of SOFC operation. Therefore, an important aspect of the work on SOFCs has been the attempt to build unified plants combining conversion processes and fuel cell operation. In this way, the heat from the fuel cells could be transferred directly to converters, with a much lower loss of thermal energy. This combination has been called internal reforming. 

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The German industry is pursuing the design of a natural gas fueled SOFC with a self-supporting structure of an electrolyte of 100 fim thickness and an area of 50 by 50 mm. Developmental steps of SOFC in the Siemens company have started with a 50 W unit in 1993, passed 370 W, 1.8 kW, 10.8 kW, and eventually reached 20 kW in 1996. The design specifications of a SOFC combined heat power plant (see Fig. 7-5) are an efficiency of > 50 %, a fuel utilization of > 90 %, a thermal cyclability of > 100 cycles, and an operational lifetime of > 40,000 hours. A 20 % pre-reforming of the natural gas fuel is required. Its construction is expected within the next 2-3 years [40]. 

The operation of SOFCs with LSCM anodes in sulfur-rich H2 leads to a decrease in cell performance in a short amoxmt of time. Liu reported a decrease in current density of 50% when exposing LSCM to 10% H2S/H2 at 950°C [66]. The decrease occurred over a period of 2 h, after which the performance stabilized. Ten percent H2S is much higher than the typical ppm levels of sulfur present in natural gas and represents attempts to utilize H2S as a fuel. However, even in H2 with 50 ppmv H2S, performance was observed to decrease significantly from 0.46 W/cm to 0.09 W/cm, again in 2 h, at 850°C and constant current density of 625 mA/cm [53]. The large deterioration of LSCM is due to La and Mn reacting with sulfur to form La202S, MnS, and a-MnOS [66]. Since Mn is the reactive site for fuel oxidation, the formation of these compositions is likely to have an adverse effect on anode performance. 

City gas (town gas) is used as the state-of-the-art fuel for the first-generation commercial SOFC systems. City gas, consisting mainly of CH4 from the natural gas, may be partially pre-reformed before supplying to the SOFC systems. Figure 6.25 shows the comparison of the current-voltage (1-V) characteristics between a H2-based fuel and a typical SOFC fuel (50% pre-reformed CH4 with S/C = 2.5) [261]. The cross section of a t3 pical SOFC used for this electrochemical measurement (see Fig. 6.25) is shown in Fig. 6.26. This simulated practical SOFC fuel for distributed cogeneration systems consists of the one-to-one mixture of fuUy reformed CH4 with S/C = 2.5 and non-reformed CH4 with S/C = 2.5. While too low S/C may result in carbon deposition, excessive amount of H2O causes lower OCV and may cause re-oxidation of Ni at high fuel utilization of the SOFC systems. 

In the short to medium term, therefore, it is appropriate to look at hydrocarbon fuels, with the environmental benefit arising from the much-improved efficiency of fuel utilization. Natural gas, for example North Sea gas CH4 (95%), C2H6 (4%), C4H10 (0.2%), and S (4 ppm), is clearly the most abundant and best-distributed fuel. The development of SOFC anodes capable of operating in natural gas, without suffering from carbon build up due to catalytic cracking, is still far from being achieved. 

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