Anode gas composition

Note When no further specification is given, data are from Dayton et al.,80n.i. = not indicated. a Tolerance level depends on anode gas composition and partial pressure of H2 these concentration limits increase when temperature increases, but they decrease at increasing pressures.75... 

Sulfur It is now well established that sulfur compounds in low ppm (parts per million) concentrations in fuel gas are detrimental to MCFCs (74,75,76,77,78). The tolerance of MCFCs to sulfur compounds (74) is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, gas cleanup). The principal sulfur compound that has an adverse effect on cell performance is H2S. At atmospheric pressure and high gas utilization (-75%), <10 ppm H2S in the fuel can be tolerated at the anode (tolerance level depends on anode gas composition and partial pressure of H2), and <1 ppm SO2 is acceptable in the oxidant (74). These concentration limits increase when the temperature increases, but they decrease at increasing pressures. 

EMF calculations were made by Archer and Sverdrup [9] for an 02-H2 cell as a function of anodic gas composition (H2-H20) expressed by the nQ/nH ratio where n0 and nH are the number of oxygen and hydrogen gram-atoms in the gas phase respectively, i.e. 

Primarily, the reforming reaction rate depends on the anode gas composition according to the following power law kinetics ... 

Electrolyte bath resistivity, at 950°C Current efficiency, 100 kA cells Normal energy efficiency Typical anode gas composition ... 

The above results were for SOFC buttrui cells, and more work is needed to demonstrate direct-methane utilization in stacks. One step towards this goal is simulation of stack operation. Such results indicate that in a tubular stack with a barrier layer over the first 30 % of the fuel-flow field, the anode-gas composition is maintained in the thermodynamically non-coking regime. Furthermore, the direct-methane feed case yielded overall stack performance nearly as good as the anode recycle case [33]. 

Lee CG, Hwang JY, Oh M, Kim DH, Lim HC (2008) Overpotential analysis with various anode gas compositions in a molten carbonate fuel cell. J Power Sources 179 467-473... 

Poisoning mechanisms depend on many factors, such as applied current density, inlet anodic gas composition, operating temperature and pressure. 

Table 7.3 Equilibrium composition for fuel gas and reversible cell potential calculated using the Nernst equation and assuming initial anode gas composition of 77.5% H2/19.4% C02/3.1% H2O at 1 atmosphere and cathode composition of 30% O2, 60% CO2, /10% N2...

The optoelectronic properties of the i -Si H films depend on many deposition parameters such as the pressure of the gas, flow rate, substrate temperature, power dissipation in the plasma, excitation frequency, anode—cathode distance, gas composition, and electrode configuration. Deposition conditions that are generally employed to produce device-quahty hydrogenated amorphous Si (i -SiH) are as follows gas composition = 100% SiH flow rate is high, --- dO cm pressure is low, 26—80 Pa (200—600 mtorr) deposition temperature = 250° C radio-frequency power is low, <25 mW/cm and the anode—cathode distance is 1-4 cm. 

Figure 14.16 the shows fuel cell stack performance of a 1 kWe atmospheric PEMFC stack using PtRu anodes, operating on various gas compositions. As can be clearly seen, already small concentrations of CO lead to a large decrease of fuel cell performance. An air-bleed of 1.5% air in hydrogen is able to mitigate this ef-... 

The MCFC provides a good example to illustrate the influence of the extent of reactant utilization on the electrode potential. An analysis of the gas composition at the fuel cell outlet as a function of utilization at the anode and cathode is presented in Example 10-5. The Nemst equation can be expressed in terms of the mole fraction of the gases (Xi) at the fuel cell outlet ... 

The addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the formation of CH4 is not favored. Carbon deposition can be reduced by increasing the partial pressure of H2O in the gas stream. The measurements (20) on 10 cm x 10 cm cells at 650°C using simulated gasified coal GF-1 (38% H2/56% CO/6% CO2) at 10 atm showed that only a small amount of CH4 is formed. At open circuit, 1.4 vol% CH4 (dry gas basis) was detected, and at fuel utilizations of 50 to 85%, 1.2 to 0.5% CH4 was measured. The experiments with a high CO fuel gas (GF-1) at 10 atmospheres and humidified at 163°C showed no indication of carbon deposition in a subscale MCFC. These studies indicated that CH4 formation and carbon deposition at the anodes in an MCFC operating on coal-derived fuels can be controlled, and under these conditions, the side reactions would have little influence on power plant efficiency. 

Table 6-5 Influence of Fuel Gas Composition on Reversible Anode Potential at 650°C... 

Partial pressure of gas compositions at anode (a) or cathode (c), atm. Polarization, V Current density, A/cm Cell impedance, Q-cm... 

Figure 28 shows the result of the kinetic in-cell investigation. For three anodic overpotentials there are plotted the obtained current densities versus gas composition, which simulates different degrees of hydrogen conversion. The gas composition changes from pure hydrogen (left) to H20/C02 = 1/1 mixtures. As predicted by Eq. (30), not only for complete conversion but also for vanishing conversion the current approaches zero as neither vapor nor carbon dioxide is present. 

Pure oxygen or air are generally used as the oxidant in SOFCs. The most frequently considered fuels are H2, CO, coal and light alkanes C H2 +2, mainly methane. The moderate oxidation of alkanes or coal by water vapor or carbon dioxide provides H2-C0-C02-H20 mixtures with various compositions. Then the thermodynamic cell EMF is a function of fuel gas composition in the close vicinity of the anode and can be calculated considering the following equilibria ... 

The first and the second law of thermodynamics allow the description of a reversible fuel cell, whereas in particular the second law of thermodynamics governs the reversibility of the transport processes. The fuel and the air are separated within the fuel cell as non-mixed gases consisting of the different components. The assumption of a reversible operating fuel cell presupposes that the chemical potentials of the fluids at the anode and the cathode are converted into electrical potentials at each specific gas composition. This implies that no diffusion occurs in the gaseous phases. The reactants deliver the total enthalpy J2 ni Hi to the fuel cell and the total enthalpy J2 ni Hj leaves the cell (Figure 2.1). 

The reversible voltage Vfcksv at constant anodic and cathodic gas compositions can be calculated by Equations (2.13) and (2.16)... 

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. 

For the calculation of the electrochemical reaction rates the cathode gas composition is required. Considering the MCFC as a black box, the anode feed gas is completely oxidized with air which is fed into the catalytic combustion chamber, either electro-chemically or in an ordinary combustion reaction. With this, the amount and composition of the exhaust gas is independent of the electric cell performance and can be calculated directly from the conditions of the anode feed and the air feed. According to the assumption of spatially concentrated conditions inside the cathode channel, the exhaust conditions correspond to the conditions in the cathode gas channel. Thus, the cathode gas composition is determined from a combustion calculus. 

The oxidation rate depends not only on the gas composition and the temperature parameter, but also on the electric potential difference between the electronically conductive part of the anode electrode and the ionically conductive electrolyte. Defining the electric potential of the solid part of the anode electrode as zero potential, the reaction rate depends on the electric potential in the electrolyte, other hand, the reduction reaction rate depends on the electric potential difference at the cathode electrode, which is the difference between the given cell voltage, Uceii, and the electrolyte potential, equilibrium constants are determined by the... 

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