Direct internal reforming

Another problem in direct reforming is the susceptibility of the nickel anode to catalyse the pyrolysis of methane and higher hydrocarbons (Eqs. (1) and (2)), which results in deleterious carbon deposition and subsequent build-up of 

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To model direct internal reforming in SOFC stacks additional source terms need to be added to the specie and energy equations in addition to including the equations for the extra species. The source terms account for a set of chemical reactions, the chemical mechanism, that is assumed to occur. A simple and commonly used mechanism for steam methane reforming on SOFC anode is ... 

Aguiar P., Adjiman C.S., Brandon N.P. (2005) Anode-supported intermediate-temperature direct internal reforming solid oxide fuel cell II Model-based dynamic performance and control. Journal of Power Sources 147, 136-147. 

Fig. 2.1. Working principle of the Molten Carbonate Fuel Cell (MCFC) with direct internal reforming (DIR).

Daox and Dared, can not only be used to adjust finite reaction rate constants of both reactions in the simulation, but each of these reactions can be eliminated by setting the specific Damkbhler number to zero. Thus, it is possible to simulate a pure reforming channel (Dam = 0) or an anode channel without direct internal reforming... 

In this chapter, three applications of this model are demonstrated. The comparison of different reforming concepts reveals the advantages of direct internal reforming (DIR) in the anode channel of the fuel cell. Moreover, with the help of the proposed model, the benefit of fuel cell cascades can be demonstrated and they can be compared to single cells. Results indicate that a considerable power increase can be expected, but the additional hardware required might offset any benefit in the case of smaller systems. The third application demonstrates that anode gas recycle can be simulated with this model, but it also reveals its limitations, as temperature effects are not considered. 

Boder M, Dittmeyer R (2006) Catalytic modification of conventional SOFC anodes with a view to reducing their activity for direct internal reforming of natural gas. J Power Sources 155 13... 

Morel B, Laurencin J, Bultel Y, Lefebvre-Joud F (2005) Anode-supported SOFC model centered on the direct internal reforming. J Electrochem Soc 152 A1382... 

In the direct internal reforming MCFC (DIR-MCFC) system, the direct reformation of fuel at the anode can give fuel saving of 20%, resulting in 12 /o improvement in fuel cell electrical efficiency. The schematic representation of the HR and DIR concepts is shown in Fig. 4. 

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

Gonjyo, Y. Matsumura, M. Tanaka, T. Performance of direct internal reforming molten carbonate fuel cells. Proceedings of the 26 ... 

Nakagawa N, Sagara H and Kato K (2001) Catalytic activity of Ni-YSZ-Ce02 Anode for the steam reforming in a direct internal-reforming solid oxide fuel cell. J Power Sources 92 88—95. 

Figure 6.16 Schematic of direct internal reforming in an MCFC.

In contrast to stationary applications, portable applications require frequent start and stop procedures. Therefore for SOFC, a robust cell design and adapted electrode-electrolyte assemblies are an important issue. Frequent thermal cycles between room temperature and an operation temperature of about 600-800 °C pose challenges to the layered system consisting of solid anode, ceranfic electrolyte and solid cathode with respect to thermal and mechanical stability. For several years, different approaches to developing tubular nficro SOFC have been undertaken but did not lead to a commercial product yet. As SOFC can be operated with pure hydrogen, reformate and hydrocarbons as fuel as well - the latter option means direct internal reforming at the anode catalyst — various investigations focused on reduced operation temperature and a parallel conversion of fuels [21]. 

Cell level models with varying dimensionality have been reported. To mention a few, 2D models are reported by Li et al. [88], Billigham et al. [89], and Keegan et al. [90], Burt et al. extended a ID model to simulate a cell stack using domain decomposition and parallel execution of the code [91]. Aguiar et al. have also reported ID model for direct internal reforming conditions [81,92]. 

Though the focus of this thesis is on direct internal reforming, the existing applications use some extend of pre-reformed fuel. Therefore, a systematic study to understand the influence of non-reformed and pre-reformed fuels on cell efficiency is carried out. It is well known that direct internal reforming can result in reduced cost and increased overall efficiency of the system. However, it is quite convincing from Fig. 7.19 that, the efficiency of the fuel cell is higher for pre-reformed fuel. Both efficiency and power density increases with extent of pre-reforming for both adiabatic and isothermal case (Fig. 7.20). 

Aguiar P, Adjiman CS, Brandon (2004) Anode-suppoited intermediate-temperature direct internal reforming solid oxide fuel cell I. Model-based steady-state performance. J Power Sources 138 120-136. 

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

Heldebrecht, P. and Sundmacher, K. (2005) Dynamic model of a cross-flow molten carbonate fuel cell with direct internal reforming. J. Electrochem. Soc., 152 (11), A2217-A2228. 

The direct internal reforming of pre-reformed natural gas has been demonstrated in SOFC systems, but the other approaches described above have mostly been implemented only in button cells. Thus, there is a need for stack-level sunulations and demcaistrations to more fully vet these approaches. 

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