Cathode exhaust, stream

The experimental setup shown in Figure 2.3 was used to measure the amount of liquid water and water vapor in the cathode exhaust stream at the stack exit under various operating conditions. 

A five-cell stack (with a structure identical with that of the 30-cell stack) and a column of Drierite (anhydrous CaSO (W.A. Hammond Dryerite) used for trapping water vapor in the stack cathode exhaust stream were placed inside a convection oven set at the stack operating temperature. A T-valve was used to separate the water vapor and liquid water by gravity. The liquid water was directed to the outside of the oven with a peristaltic pump set at a flow rate of 1 mLmin , which was slightly higher than the flux of liquid water from the stack cathode exit but significantly lower than the cathode feed rate. The liquid water thus separated from the stack cathode exhaust stream was collected vdth a conical beaker placed... 

On the other hand, high pressure determines a higher energy consumption associated to the compressor, then a CEM can be used to recover some energy from the pressurised cathode exhaust stream (Fig. 4.3b). This solution adds complexity to the on-board power plant and can be usefully applied in medium large-size fuel cell power trains (10-100 kW). 

Back diffusion of water depends on water concentration on both sides of the membrane, water diffusivity through the membrane, and membrane thickness. Because water concentration is not uniform, it is not easy to explicitly calculate back diffusion for the entire cell or a stack of cells. For the sake of mass balance, back diffusion may be expressed as a fraction, p, of electroosmotic drag. When P = 1, back diffusion is equal to electroosmotic drag, that is, there is no net water transport across the membrane. The coefficient p may be determined experimentally by carefully condensing and measuring water content in both anode and cathode exhaust streams. 

SOFC cathode air stream co2 + h2o Combustion of remaining fuel in SOFC anode exhaust gas... 

Anode exhaust gas SOFC containing unconverted fuel h2o + co2 h2 Combustion of SOFC anode off-gas with cathode air stream (SOFC-GT concept). Recycling of fuel or for PEM, and so on. 

Another usable by-product is the process exhaust air of aPEM-FC or HT-PEM-FC which accumulates at the cathode side. Because half of the oxygen in the intake air reacts with the hydrogen only 10.5 % oxygen remains in the exhaust stream. The remaining gas of the exhaust is mostly nitrogen. This oxygen concentration is so low that in such an atmosphere the majority of substances cannot bum [20]. Since no further reactants than water is in the exhaust gas, it is not poisonous. Further explanations are in Chap. 8. 

In practice is not simple to determine whether the current reduction in the cell is due to alcohol crossover, incomplete oxidation, or both. A way to quantify the contribution of both effects is to measure the methanol concentration in the anode exhaust (for determining the amount of methanol oxidized and permeated), and the concentration of methanol and CO2 in the cathode exit. Part of the CO2 at the cathode exhaust is due to parasitic oxidation of methanol at the cathode, while the rest is a consequence of CO2 crossover from the anode. The last can be determined by a half-ceU experiment by flowing hydrogen through the cathode, to avoid methanol oxidation, in such a way that all the CO2 measured in the cathode outlet stream must have crossed the membrane from the anode [29]. 

Ambient air (stream 200) is compressed in a two-stage compressor with intercooling to conditions of approximately 193 °C (380 °F) and 8.33 atmospheres (122.4 psia). The majority of the compressed air (stream 203) is utilized in the fuel cell cathode however, a small amount of air is split off (stream 210) for use in the reformer burner. The spent oxidant (stream 205) enters a recuperative heat exchange before entering a cathode exhaust contact cooler, which removes moisture to be reused in the cycle. The dehumidified stream (stream 207) is again heated, mixed with the small reformer air stream, and sent to the reformer burner (stream 211). The reformer burner exhaust (stream 300) preheats the incoming oxidant and is sent to the auxiliary burner, where a small amount of natural gas (stream 118) is introduced. The amount of natural gas required in the auxiliary burner is set so the turbine shaft work balances the work required at the compressor shaft. The cycle exhaust (stream 304) is at approximately 177 °C (350 F). 

Mass spectrometry of liquid samples of the cathode outlet stream is another way of determining the methanol crossover flux. For mass spectrometric measurements of methanol crossover, a clear description of the respective system conld be achieved by measuring the background methanol signal of a cell filled with distilled water and equipped with the membrane sample, and subseqnently adding well-adjusted portions of aqueous or pure methanol to this liquid [25]. The slopes of mass signal vs. time curves are typical for diffusion-controlled processes and with the help of the calibration lines, the diffusion coefficient of methanol through the membrane can be calculated. Online analysis of the cathode exhaust gas with multipurpose electrochemical mass spectrometry can also be employed to determine methanol permeability. However, as mentioned, the assumptions that the entire permeated methanol is converted to CO and that there is no anodic CO contribution are contentious. 

In reaction (1.39), the anode is sulphided and then gets reduced to nickel. The poisoning due to H2S interferes with the water gas shift reaction equilibrium. Hence, chromium is used in the anodes as it acts as a sulphur-tolerant catalyst. As CO2 is required at the cathodes, it is supplied by anode gas recychng. Hence, the sulphur may also contaminate the cathode as it may be present in the exhaust stream of the anode. The sulphur upon entering the cathode may react with the carbonate ions to produce alkali sulphates which are transported towards the anode via the electrolyte. On reaching the anode, the sulphate S04 is reduced to S. ... 

Combined Brayton-Rankine Cycle The combined Brayton-Rankine cycle. Figure 9-14, again shows the gas turbine compressor for the air flow to the cell. This flow passes through a heat exchanger in direct contact with the cell it removes the heat produced in cell operation and maintains cell operation at constant temperature. The air and fuel streams then pass into the cathode and anode compartments of the fuel cell. The separate streams leaving the cell enter the combustor and then the gas turbine. The turbine exhaust flows to the heat recovery steam generator and then to the stack. The steam produced drives the steam turbine. It is then condensed and pumped back to the steam generator. 

The Sr-Ce oxides were stable at 600-800°C when H2O was added in the streams of anode and cathode. The addition of H2O was necessary in order to maintain protonic conduction through the ceramic electrolyte [4]. However, it is known that Sr-Ce oxides decomposed to SrCOs and CeO2 at high CO2 atmosphere [12]. Therefore, the use of the Sr-Ce oxide is not proper under the condition of CO2 atmosphere. In the present study, gas composition detected in the exhaust from the anode was CH4, H2, H2O, and CO. No CO2 was observed throughout the present study. This may be because the reaction temperature was higher than 600°C, and a sufficient amount of H2O was supplied to the anode and cathode. There was high possibility of production of CO2 at lower temperatures of 300-500°C [7]. The use of a compact Ni electrode in place of the porous Ni/SiO2 electrode may be proper if CO2 is... 

The system concept has only two input streams for air and propane and one common output stream. Air is used as oxygen carrier for the cathode and the burner and serves in addition as cooling medium for the stack. As propellant gas for the ejector the already pressurised propane fuel is used. To reduce heat up time during system start up an additional propane supply to the burner is integrated, enabling supplementary energy supply. Nevertheless, at steady state operation propane is supplied only to the ejector. The only output stream is the exhaust gas from the burner. Figure 1 shows the flowchart. 

The oxidant is introduced via a central AI2O3 injector tube and fuel gas is supplied to the exterior of the closed-end cathode tube. In this arrangement, the AI2O3 tube extends to the closed end of the tube, and the oxidant flows back past the cathode surface to the open end. The fuel flows past the anode on the exterior of the cell and in a parallel direction (co-flow) to the oxidant gas. The spent gases are exhausted into a common plenum, where any remaining fuel reacts. The heat generated preheats the incoming oxidant stream and drives an expander. One... 

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