Anode Channel

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. 

A tool is required for the evaluation of possible combinations of different reforming concepts, and for the analysis of various flow sheet options such as gas recycling, cell cascading, and side feeding. This tool should be  

In this section, a mathematical model is introduced which fulfills these requirements. First, the complete model equations are given and their derivation is briefly indicated. In the second part, an accompanying diagram to this model is introduced, in which the simulation results can be displayed. 

The steady-state anode model is derived from a dynamic, spatially two-dimensional description of a single cross-flow MCFC in terms of dimensionless variables [7, 8] under the following assumptions  

In the following section, the component mass balances in the anode channel, some mixing rules, the equations for the cathode gas composition, the kinetics of the reforming and the electrochemical reactions and finally the equations for the fuel cell power are provided. 

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Velocity vectors and density contours in the anode channel at location A... 

Considering that at a time t, the H2/air-front, which divides the fuel cell into the power source (the H2/air cell) and the load (the air/air cell), is located at x (normalized anode channel length), conservation of charge yields ... 

Figure 11. Variation of water vapor concentration in the cathode and the anode channel of the PEMFC with the MPL under the counter-flow mode at a setting of 0.5 A/cm2 measured using the TDLAS system.

Figure 12 under the counter-flow mode at 0.5 A/cm2. The index with the MPL is larger than that without the MPL for an index of 0-1 as depicted in Fig. 12. This result also shows that the MPL enhances water back-transport from the cathode side to the anode side. However, when the index is negative, meaning that the internal water circulation from the anode channel to the cathode channel, the index with the MPL is slightly higher than the index without the MPL. Therefore, the MPL at the cathode suppressed water vapor absorption at the anode, which is explainable by membrane hydration attributable to the MPL at the cathode. Consequently, the MPL promotes membrane hydration, leading less internal water circulation from the anode to the cathode side.

Sheng, C. Y. and Dean, A. M. Importance of Gas-phase Kinetics within the Anode Channel of a Solid-oxide Fuel Cell, Journal of Physical Chemistry, 108, 3772, (2004). 

Some research groups working on the modeling of MCFC include the reforming reactions in their process models in different ways. He and Chen [1] and Yoshiba et al. [2] only consider the water-gas shift reaction in a spatially distributed anode channel. Due to its high rate, they assume the shift reaction to be in chemical equilibrium. Lukas and Lee [3] and Park et al. [4] also describe the water-gas shift reaction in equilibrium, but in addition they include the steam reforming reaction of methane as an irreversible reaction with a finite reaction rate. In particular, Park... 

The working principle of the MCFC is illustrated in Fig. 2.1. The anode is fed with a preheated mixture of desulfurized natural gas and steam at a steamxarbon (S/C) ratio of about 2.5. This feed is converted via steam reforming into a hydrogen-rich gas mixture at the reformer catalyst, which is placed inside the anode channel. Carbon monoxide is the byproduct of this reforming reaction. Simultaneously, the water-gas shift reaction transforms carbon monoxide into carbon dioxide and another hydrogen molecule ... 

In addition to hydrogen possibly being formed directly inside the anode channel, it can also be produced by other reforming steps. Figure 2.2 shows the three basic... 

The governing equations - that is, mainly the component and the total mass balances in the anode channels - are provided here in dimensionless form. The five ordinary differential equations (ODE) with respect to the spatial coordinate describe the development of the five unknowns in one single anode channel, namely the mole fractions, with i = CH4, H2O, H2, CO2, as well as the molar flow density inside the anode channel, y. Here, the Damkohler numbers, Da/, are the dimensionless reaction rate constant of the reforming and the oxidation reaction, respectively, the rj are the corresponding dimensionless reaction rates, and the v, j are the stoichiometric coefficients ... 

The mole fractions and the total molar flow rate at the anode channel inlet, %i in and Ti , serve as initial conditions ... 

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... 

To reduce the number of ODE even further, a physically motivated transformation is applied. The idea is that for a given composition of the feed gas (CEU/I-hO-mix-ture, characterized by the S/C ratio), the composition of the gas at any point in the anode channel or in any of the reforming units is described by only two states the extent of the reforming reaction, assigned re/, and the extent of the oxidation reaction, assigned fo,x. These variables are made dimensionless and normalized to unity, so they can be interpreted as follows ... 

Thus, we receive for the molar flow density inside the anode channel ... 

To calculate the required initial conditions for Eq. (20) and the molar flow into the anode channel, namely j, and f, some mixing rules are required. They result from the total and the component balances at the mixing point at the channel inlet ... 

All reactions are considered to be reversible, so the gas composition can reach equilibrium for one or both reactions in the anode channel. In Fig. 2.5, several equilibrium lines of the reforming reaction are displayed for different temperatures. On these lines, the rate of the reforming reaction equals zero. 

As indicated in Fig. 2.2, three different reforming concepts are available for high-temperature fuel cells. The steady-state anode model presented above allows a comparison of various combinations of reforming concepts. First, a system without a reforming catalyst inside the anode channel is considered - that is, a fuel cell without DIR. Three alternatives for fuel gas treatment are discussed ... 

In this application, all three reforming units are considered to be infinitely long, and thus equilibrium is reached in each unit. The effluent of the reforming unit is fed into the anode channel where only hydrogen oxidation takes place (Dare/= 0). 

In the open literature, one can find the proposal to recycle part of the anode exhaust gas back towards the anode inlet [5, 6], as shown in Fig. 2.3. Especially for pure DIR systems, this has certain advantages If fresh feed gas is fed into the anode channel, only low hydrogen contents will be present at the inlet, and consequently the electrode will not be used in this region. This can be amended by recycling a part of the exhaust, which still contains some hydrogen, back to the inlet Define the recycle flow via a recycle ratio, R, according to ... 

The interaction of these two processes can be described by a simple isothermal model, which is based on balances of mass and charge. The model describes the extent of the reforming and oxidation reactions along the anode channel. The essential simulation results can easily be displayed in a conversion diagram which is a phase diagram of the two dynamic state variables, namely the extents of two reactions. 

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. 

The characteristic feature is a large number of vertical anolyte channels (9 x 3 mm), on whose back foils of platinum as active material are attached on tantalum foils mounted on a supporting plate. This is cooled from behind to realize an anolyte temperature of 30-45 °C (to diminish acid-catalyzed hydrolysis of the peroxodisulfate anion formed). Graphite acts as a cathode block, separated from the anode channels by a microporous polymer diaphragm, preventing reduction processes at the cathode. 

These arguments are valid for the anode channel as well. For instance, the velocity of the flow in the gas-fed DMFC anode channel is given by... 

Consider the cathode channel (similar arguments are applicable to the anode channel of DMFC or hydrogen PEFC). For simplicity we assume that (1) the catalyst layer is thin enough, so that there are no voltage losses associated with proton transport across the layer and (2) the diffusion losses of oxygen in the backing and catalyst layers are negligible. 

The approach described in Sections 8.2.3 and 8.2.4.5.3 was used to construct quasi-2D (Q2D) analytical and semi-analytical models of PEFC [246, 247] and DMFC [248, 249], The Q2D model of a PEFC [246] takes into account water management effects, losses due to oxygen transport through the GDL, and the effect of oxygen stoichiometry. The model is fast and thus suitable for fitting however, the systematic comparison of model predictions with experiment has yet not been performed. Q2D approaches have been employed to construct a model of PEFC performance degradation [250], to explain the instabilities of PEFC operation [251, 252] and to rationalize the effect of CO2 bubbles in the anode channel on DMFC performance [253, 254],... 

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