Reforming internal

Internal reforming may also be utilized with an SOFC (see below). When operating with higher-order hydrocarbon fuels, such as propane, butane or naphtha, the addition of a pre-reformer may be necessary to prevent the buildup of unacceptable levels of carbon deposits within the cell structure. 

With high-temperature fuel cells (SOFC, MCFC), it is possible to convert the free energy of methane (or hydrogen and other fuels) directly into electric energy with the theoretical voltage expressed by the Faraday equation (AG =-n FE) where n is the number of electrons participating in the electrochemical reaction, E the voltage and the Faraday constant. It 

The maximum useful work which can be obtained by combustion of natural gas is equal to the exergy of methane being 830 kJ/mol (1 bar, 25 C), as shown in Table 2.3. The LHV is almost the same, 802 kJ/mol, since the entropy change is very small. This means that almost as much energy can be obtained as heat as would have been obtained as work if the process had been carried out reversibly. The subsequent conversion of the thermal energy to mechanical energy is limited by the Carnot efficiency. 

For methanol (with AS° 0), rjideai is above 1 and increases with temperature. However, the advantage of using methanol is partly lost because methanol is manufactured from natural gas with an efficiency of approximately 66% (LHV) (refer to Section 2.6). 

These ideal effieiencies show the upper limits of the practieal efficiencies using the eorresponding fuels. The ideal voltage in the eell is calculated from AG , but this reversible voltage is reduced due to the actual activities of the reactants and products in the cell as expressed by the Nemst equation, and by the polarisation on the cathode and anode and the internal resistance of the electrolyte when current is drawn from the cell. 

The advantage of converting the hydrocarbons in the fuel cell rather than converting the fuel to hydrogen first is evident. If steam reforming of natural gas takes place in an externally fired reformer, there is a loss in efficiency because of the high temperature created in the flame, which is not utilised fully for work because the waste heat ean only be recovered via the Carnot cycle as shown in Example 2.3. 

Straight rib Active area Thickness 0.020 Pitch -0.200 Height -0.100  

Electrolyte plate Cathode Current collector Separator 

This carbon deposition can be inhibited by adding steam to the fuel. This reaction (Eq. 9.2) is strongly endothermic in nature, whereas fuel cell reactions in Eqs. (9.3) and (9.4) are slow exothermic reactions. This can create instability in the coupling between fast endothermic and slow exothermic reactions. 

A self-sustaining internal reformer is the need of the hour, where heat from the stack is sufficient for the endothermic requirement of hydrocarbon reforming. During fuel cell reaction oxidation of hydrogen at the anode (Eq. 9.3) water is produced, which can be reintroduced along with hydrocarbon fuel in the form of steam instead of continuously adding water to the system. This is shown in Fig. 9.9. 

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Steam reforming of CH is commonly carried out at 750 to 900°C, thus at the lower operating temperature of MCFCs a high activity catalyst is required. The internal reforming of methane in IRMCFCs, where the steam-reforming reaction... 

Fuel cells such as the one shown on Fig. 3.4a convert H2 to H20 and produce electrical power with no intermediate combustion cycle. Thus their thermodynamic efficiency compares favorably with thermal power generation which is limited by Carnot-type constraints. One important advantage of solid electrolyte fuel cells is that, due to their high operating temperature (typically 700° to 1100°C), they offer the possibility of "internal reforming" which permits the use of fuels such as methane without a separate external reformer.33 36... 

Development of Anode Catalyst for Internal Reforming of CH4 by CO2 in SOFC System... 

In this work, the catalytic reforming of CH4 by CO2 over Ni based catalysts was investigated to develop a high performance anode catalyst for application in an internal reforming SOFC system. The prepared catalysts were characterized by N2 physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). 

Figure 1 shows the effects of reaction temperature on the conversions of CO2 and CH4 over Ni-YSZ-Ce02 and Ni-YSZ-MgO catalysts. It was found that the Ni-YSZ-Ce02 catalyst is showed higher catalytic activity than the Ni-YSZ-MgO catalyst at temperature range of 650 850 Ti and the maximum activity was observed at above 800 °C, the optimum temperature for internal reforming in SOFC system [5]. In our previous work, it was identified that Ni-YSZ-MgO catalyst was deactivated with reaction time, however Ni-YSZ-Ce02 showed stable catalytic activity more than Ni-YSZ-MgO catalyst imder tiie tested conditions [6]. 

Fig. 2. Effects of temperature on the catalytic performance in the internal reforming of CH4 by CO2 over ESC of SOFC system.

The internal reforming of CH4 by CQzin SOFC system was performed over an ESC (electrolyte st rported cell) prepared with Ni based anode catalysts. Figure 5 diows the performance of voltage and power density with current density over various ESC (Ni based anodes I YSZ (LaSr)Mn03) at SOOC when CH4 and CO2 were used as reactants. To improve the contact between single cell and collector, different types of SOFC reactor were used [5]. In the optimized reactor (C), it was found fliat die opai-... 

Fig. 5. The performance of voltage and power density with current density in the internal reforming of CH4 by CX>20ver ESC (anodes I YSZ I (LaSr)Mn03) of SOFC syston.

The effects of total flow rate of fuels (CO2/CH4 = 1) on the impedance in the internal reforming of CH4 by C02 0ver ESC (NiO-YSZ- Ce02 I YSZ I (LaSr)Mn03) of SOFC system are represoited in Figure 6. It was comsidaed fliat the total resistance was dependent on the total flow rate because the conva sions of CO2 and CH4 over ESC were affected by contact time in the internal reforming system. 

The revenue from carbon taxes can be used into the economy to reduce income taxes or levies on labor or capital investment. This may be part of a national or international reform of the taxation systems with the effects to shift the tax burden from "goods" like labor to "bads" like pollution. 

Fuel cell Fuel Gas purity required Tolerated inert compounds Internally reformable compounds... 

Product Heat Management Process Gas + Independent Cooling Medium Process Gas + Electrolyte Calculation Process Gas + Independent Cooling Medium Internal Reforming -1-Process Gas Internal Reforming -1-Process Gas Internal Reforming -1-Process Gas... 

Figure 1-3 External Reforming and Internal Reforming MCFC System Comparison...

The focus of the utility demonstrations and FCE s fuel cell development program is the commercialization of 300 kilowatt, 1.5 megawatt, and 3 megawatt MCFC plants. Characteristics of the FCE 3 megawatt internal reforming commercial MCFC plant are as follows (17) ... 

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