SOFC Fuel Processing

Fuel processing can be defined as the conversion of the raw primary fuel furnished to a fuel cell system into a fuel gas needed by the fuel cell [148,149]. SOFCs possess high fuel elasticity, because 

Methanol steam-reforming is normally carried out in the range between 250°C-350°C, depending on the reforming process, and the catalyst usually applied is Cu/ZnO [151], The chemical reactions taken into account in the steam-reforming of methanol, according to the literature, are the following ones [151-154]  

Ethanol is a natural renewable product normally produced from biomass, which is an important factor for near-zero carbon dioxide (C02) emissions. It is accessible, easy to transport, ecofriendly, nontoxic, and it can be transformed by catalytic reactions into hydrogen, which is important for fuel cell fuel processing [151], Section 9.8.10 discusses different processes for hydrogen production by the steam-reforming of ethanol. 

In spite of the problems created by carbonaceous fossil fuels, at present, it is a reality that the global profusion of these fuels will guarantee that they continue to be an important energy resource for some years to come [149], Natural gas, currently, is perhaps the most important fuel alternative for the many stationary fuel cells globally used [1,149], 

The Physical Chemistry of Materials Energy and Environmental Applications 

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In a real SOFC system, heat is exchanged within the SOFC in several ways including fuel processing, air preheating, flue gas cooling, etc. The excess air is commonly required to prevent overheating, while the conversion of hydrocarbons into hydrogen and carbon monoxide often absorbs heat. 

The necessary fuel processing of natural gas or other hydrocarbons and coal before its use in the SOFC changes the system design. The following investigations have been done for methane as the main component of natural gas to keep the calculations simple. A common type of fuel processing for hydrocarbons is the endothermic steam reforming process as shown for methane in Equation (2.105) with the heat demand Equation (2.106)... 

The SOFC can be modelled as one unit consisting of two parallel operating SOFCs fed with hydrogen and carbon monoxide. The irreversible effects including mixing are described by <+c < 1. The detailed reasons for these irreversibilities of the SOFC and other components are not necessary to understand the system s behaviour if they are considered properly in the system. The relation between work and heat within the single components and the temperatures of the heat sources and the heat sinks is the important issue here. The SOFC can be used as heat source of the fuel processing and evaporation. The required temperature levels are... 

The fuel processing operation of an SOFC critically depends on the anode structure and composition, since the electrochemical reaction can only take place at the three-phase boundary. If there is a breakdown in connectivity in any one of the three phases, the reaction cannot take place. Besides, if ions from the electrolyte cannot reach the reaction site, if the gas-phase fuel molecules cannot reach the reaction site, or if electrons cannot be removed from the reaction site, this site cannot contribute to the performance of the cell [5],... 

A 1 kW SOFC prototype based on tubular cells was designed, manufactured and tested in 1989. The cell had a working surface of 63 cm2 and a mean power 12.5 W or 0.2 W/cm2 at 900°C. 16 cells were assembled into a stack. The prototype consisted of 6 stacks. The prototype was fed with methane internal partial oxidation was used for fuel processing. The prototype worked at fuel utilization of 0.9 and had efficiency about 40%. 

Lindermeir, A, Kah, S, Kavurucu, S, Miihlner, M. On-board diesel fuel processing for an SOFC-APU—Technical challenges for catalysis and reactor design. Appl. Catal. Environ. 2007 70 488-497. 

Steam is required for fuel processing based on SRs and ATRs and oxygen is required for ATRs and POXs. For steam supply, there are three options (i) outside water supply with steam generation, (ii) a water pump with a steam generator providing steam from a water tank with a condenser at the system exhaust, and (iii) recychng of the SOFC anode outlet containing product water to the reformer. Options (i) and (iii) maintain water neutrality and are preferred if the system has fresh water supply limitations. 

Figure 33.36 is an example of a 300 kW aircraft SOFC APU design [70]. In this system design, the power generation subsystem consists of twin 150 kW hybrid SOFCs and the fuel processing subsystem consists of an ATR FR and a DS. A fuel heat exchanger (HEX) is used to cool the reformate gas stream. A portion of exhaust gases from the SOFC are recycled via the CGR and AGR blowers. 

Fuel flexibility One of the merits of SOFCs is fuel flexibility. Since the charged species associated with oxidants is transported in SOFCs, any kinds of fuels can be utilized in principles. Since nickel anodes are weak against carbon deposition or sulfur poisoning, fuels are used in SOFCs after fuel processing is made. Thus, fuel chemistry based on the thermodynamics is one of the major components in understanding SOFC systems. 

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