Indirect internal reforming

During indirect reforming the SOFC unit has a special provision for a separate catalyst which reforms hydrocarbon for producing gas which is oxidised at the anode. The scheme of an indirect reforming path is shown in Fig. 9.11. 

Compared with direct reforming, the efficiency is less and also indirect reforming is a little more complex but is still economical, efficient, and easy rather than using an external reformer. 

Consequently, the majority of designs currently being developed employ a separate catalyst within the SOFC stack, upstream of the anode, to indirectly reform the majority of the hydrocarbon fuel, with some residual hydrocarbon reforming occurring directly on the anode. 

Depending on the temperature and the steam to methane ratio, the water gas shift reaction (Eq. (7)) (AH = —42 kj mol ) can occur, whereby some of the CO is converted to CO2, with production of one mole of hydrogen for every mole of CO converted  

The H2 and CO are then electrochemically oxidised to H2O and CO2 (Eqs. (5) and (6)) at the anode by oxide ions electrochemically pumped through the solid electrolyte. 

Conventional steam reforming catalysis has been extensively studied and reviewed over the last two decades [38-41]. Generally, in steam reforming catalysis, steam to carbon ratios of around 2.5-3 are used, i.e. well in excess of the stoichiometric requirement of Eq. (3), such that the equilibrium of the water gas shift reaction (Eq. (7)) lies to the right to maximise H2 production, and minimise carbon deposition through hydrocarbon pyrolysis (Eqs. (1) and (2)) and the Boudouard reaction (Eq. (8))  

Carbon dioxide, formed by the water gas shift reaction (Eq. (7)) and by electrochemical oxidation of carbon monoxide (Eq. (6)), present in the exit gas leaving the anode, can be recirculated in the fuel supply at the cell inlet. It is well known that CO2 can act as an oxidant for hydrocarbons in the presence of a suitable catalyst, so called dry reforming (Eqs. (9) and (10) for methane and a general higher hydrocarbon, respectively) [38,39,42,43]  

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T. Okado, et al., "Study of Temperature Control in Indirect Internal Reforming MCFC Stack," presented at 25th lECEC, pp. 207-212, 1990. 

Aguiar, P., Chadwick, D. and Kershenbaum, L. (2002) Modelling of an indirect internal reforming solid oxide fuel cell, Chemical Engineering Science 57, 1665-1677. 

Nishino, T., Iwai, H. and Suzuki, K. (2006) Comprehensive numerical modeling and analysis of a cell-based indirect internal reforming tubular SOFC, Journal of Fuel Cell Science and Technology 3, 33 14. 

The indirect internal reformer (HR) is situated within the cell stack in separate reforming channels, where only the reforming reaction takes place. This concept features energetic coupling with the exothermic oxidation process. The main advantage is that no external heat exchanger is required, as the separator plate between HR and anode channel fulfills this function. The HR can be seen as an external reformer operating at fuel cell temperature. 

Fig. 4 Schematic representation of (A) direct internal reforming (DIR) and (B) indirect internal reforming (HR) MCFC concepts.

Figure 3.3 Schemes of (a) conventional external reforming MCFC, (b) direct internal reforming, and (c) indirect internal reforming...

WGS, water gas shift reaction SMR, steam methane reforming reaction HR, indirect internal reforming. 

Many of the models can be used to describe not only the behavior of single cells, but also that of a whole stack. This extension of the model equations has not been discussed here but, as indicated in Table 28.1, this has been reahzed with many models. Other extensions and modifications, such as the application ofequihbrium assumptions with regard to the reforming reactions, the modeling of a catalytic burner between the anode exhaust and the cathode inlet, or the addition of model equations describing an indirect internal reforming reactor, are frequently apphed in MCFC models. For the sake of brevity, they have not been discussed in detail in this chapter. 

Autothermal or steam reforming of methane was considered in thermodynamic calculations by Cavallaro and Freni for reformers, which were integrated into a molten carbonate fuel cell [43]. Direct or indirect internal reforming is possible within the molten carbonate fuel cell. The reforming may be performed either by the anode itself or by a dedicated catalyst in the anode compartment in analogy with the solid oxide fuel cell, as has been explained above. Direct reforming of alcohol fuels is also possible in molten carbonate fuel cells [44], whereas processing of liquid hydrocarbons requires a pre-reformer. 

There are two approaches to internal reforming. Indirect internal reforming (HR) the reforming reaction takes place in channels or compartments within the stack that are adjacent to the anode compartments, the heat generated in the cell is transferred to the reforming channels, and the product from the reforming is fed to the anode channels. 

Kim H, Cho JH, Lee KS (2013) Detailed dynamic modeling of a molten carbonate fuel cell stack with indirect internal reformers. Euel Cells 13 259-269... 

Pfafferodt M, Heidebrecht P, Sundmacher K, Wuerteirbeiger U, Bednatz M (2008) Multiscale Simulation of the Indirect Internal Reforming Unit (HR) in a Molten Carbonate Fuel Cell (MCFC). Ind Eng Chem Res 47 4332- 341... 

Freni S, Aquino M, Passalacqua E (1994) Molten carbonate fuel cell with indirect internal reforming. J Power Sources 52 41-47... 

Miyazaki M, Okada T, Ide H, Matsumoto S, Shinoki T, Ohtsuki J (1992) Development of an indirect internal reforming molten carbonate fuel cell stack. In Proceedings of the Intersociety Energy Conversion Engineering Conference, 27th(3), vol 3. 287-3.292... 

Figure 8.4 Schematic representation of direct and indirect internal reforming.

Reforming As discussed, since the MCFC produces waste heat and steam at the anode like the SOFC and can use CO as fuel, an excellent opportunity exists for internal reformation of the fuel gas. There are two types of internal reformation practiced, indirect internal reformation (IRR) and direct internal reformation (DIR). In IRR, the fuel gas is mixed with water vapor, heated inside the stack with waste heat, and reformed over a catalyst bed into a hydrogen-CO mixture. The reformed mixture then enters the active fuel cell area. In DIR, the fuel gas is reformed directly in the active area flow fields. Although the DIR approach is more compact and can theoretically be used to remove a significant portion of the waste heat from the stack, IRR allows the use of different catalysts specifically for reformation, prolonging system life [32]. 

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