Cathode off-gas

Hybrid power plants using high temperature fuel cells, and capture of CO2 from cathode off-gas. 

In order to simplify the system by reducing the number of turbo machines, the cathode off-gas is used for the combustion process. This measure leads to a high air ratio of 4.4 and a low adiabatic temperature of about 640 K. With regard to CH4 emissions, complete methane combustion requires a temperature of 723-773 K. 

Ahmed et al. [435] performed calculations to highlight the effect of the various operating parameters of an autothermal methane fuel processor on the overall water balance. The system considered by Ahmed et al. included an afterburner, which combusted the anode off-gas by cathode off-gas oxygen. Water was then recovered from the burner off-gas. As shown in Table 5.10, Section 5.4.1, the water balance improved when the S/C ratio was increased. 

Hydrogen conversion of 80% was assumed for the fuel cell. The unconverted hydrogen was then fed into the AFB downstream of an additional water separation (WS2). Part of the low temperature heat contained in the AFB off-gas, which had a temperature of 125 ° C downstream of HX- B was then removed by a second air cooler (about 1.8 kW of heat losses) and left the system at 70 °C. The heat formed by the oxidation reaction in the fuel cell was removed with the cathode air of the fuel cell. The cathode feed required humidification. Part of the water fed to the cathode was regained from the cathode off-gas (WS2) and fed back to the system. This improved the overall water balance. 

The electrical efficiency of the fuel cell was set to 60%. A catalytic afterburner was included in both system concepts. Residual hydrogen from the fuel cell anode was combusted therein with the cathode off-gas oxygen. 

A second generation system, as shown in Figure 5.61. It was composed of a microchannel oxidative steam reformer, which was supplied with water by a humidifier. The humidifier utilised cathode off-gas for humidification. Steam reforming was performed at a S/C ratio of 1.6 and O/C ratio 0.2. The microchannel steam reformer was coupled to an integrated catalytic burner The burner was... 

The start-up time demand of the second generation system was 5 min. The fuel processor efficiency was 70%, as for the first generation, while the overall system efficiency could be improved to 35% owing to the energy recovery by the coupled microdiannel reformer/bumer and the utilisation of cathode off-gas steam. 

Because waste gas from the anode is directly converted in the burner, all excess water from steam reforming is shuttled herein. The final molar water share in the exhaust gas depends on the air stoichiometry of the burner (Ab). Cathode off-gas and the burner flue gas are mixed in the exhaust in the center of the wing pod. To provide sufficient water for steam reforming at an S/C of 1.5, a share of 0.43 of the water in the... 

The steam reforming of hydrocarbons such as diesel has been demonstrated in MSRs whose mechanical stability has been proven at high temperature (750-850 °C). Most of the configurations consist of co-current flow diesel steam reforming combined with combustion of fuel cell anode and/or cathode off-gas surrogate. Full conversion was obtained in all cases. Power equivalent of these systems varied between 2-5 kW thermal energy of the hydrogen produced and 10 kW thermal input of the diesel feed [4,73]. However, the most advanced... 

Modem cells employ arrays of anodes (Ti02 coated with a noble metal) and cathodes (mild steel) spaced 3 mm apart and carrying current at 2700Am into brine (80-100gl ) at 60-80°C. Under these conditions current efficiency can reach 93% and 1 tonne of NaC103 can be obtained from 565 kg NaCl and 4535 kWh of electricity. The off-gas H2 is also collected. 

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. 

Initial experiments utilized cadmium as the cathode and CaCl2, 15 wt % CaF2, 5 wt % CaO as the salt. The cell was run at 700°C and a current density of about 0.5 amp/cm2. The cell potential was 2 volts. The reaction was monitored by taking filtered samples of the metal and salt phases and samples of the gas stream directly above the cell. The results indicate that during the electrolysis the calcium metal concentration in the cathode metal increased from 0 to 3 wt %. The CaO in the salt decreased from 5 to 1 wt % and CO2 and CO were found in the off gas. These results indicate the electrolysis proceeds as expected and, with optimization of the operating conditions, will serve as a means of recycling calcium metal thus reducing the waste volume from the reduction step. 

In this process, oxide fuel is dissolved in a molten chloride salt mixture through which Q2-HCI gas is flowing. Dissolved uranium and plutonium are then recovered as oxides by cathodic electrodeposition at 500 to 700°C. The process was demonstrated with kilogram quantities of irradiated fuel, with production of dense, crystalline UO2 or UO2-PUO2 reactor-grade material. Difficulties were experienced with process control, off-gas handling, electrolyte regeneration, and control of the plutonium/uranium ratio. Development has been discontinued. 

The charge transport process in the electrolyte is accomplished by carbonate ions (COs ) formed at the cathode from the reduction of oxygen and CO2. Therefore, CO2 must be added to the cathode flow in order to allow the formatiOTi of carbonate ions. This is normally done by adding anode off gas after post combustion to the cathode flow. CO2 management adds to the complexity of the MCFC system. 

An electrolytic technology for the preparation of ozone was based on glassy carbon as the anode material in concentrated fluoroboric acid (HBF ) [22-24]. The cells also employed a Pt-catalyzed, oxygen reduction cathode (gas diffusion electrode (GDE)) to lower the cell voltage (and hence the energy consumption) and to avoid the need to handle H2 off-gas from the cathode. It was necessary to operate with cooling, and the preferred temperature was 268 K. 

There is currently no information in the literature about a concentration difference of phosphoric add between the anode and cathode. A rough estimate of water distribution as a function of stoichiometric factor A, is given in Table 29.1. The estimate is based on the assumptions that the cell is operated with hydrogen and air and that no significant amount of water is accumulated inside the cell. The expected distribution from Table 29.1 agrees well with in-house experiments and literature data, where for a value of A(H2/air) = 2/2,15-20% of the product water was found in the anode off-gas [21]. Therefore, it can be reasoned that the concentration gradient of phosphoric add between the anode and cathode is negligible, as the MEA is very thin (100-300 (im) and water distribution should be fast at 160 °C. 

Other factors affecting the water balance are the hydrogen utilisation in the fuel cell anode and the oxygen stoichiometry on the cathode side. Increasing hydrogen utilisation requires a surplus of cathode air and consequently cathode stoichiometry needs to be increased. This dilutes the burner off-gas, which has a detrimental effect on the water balance of the fuel cell/fuel processor system [435]. 

The usual method applied for the acid leaching evaluation is to analyze the anodic and cathodic off-gasses for traces of vaporized acid. This is typically done by collecting the acid in water bubbler flasks. However, this does not necessarily provide a realistic picture of the severity, since only a single mechanism for the proposed acid loss is considered, i.e., the evaporation. Furthermore, it is apparent that not all of the escaped acid in the gas phase is carried all the way out with the fuel cell exhaust due to recondensation before collection. This conjecture is based on... 

At the cathode Hydrogen gas bubbles off, because of this reaction ... 

A portion of the anode-off gas can be recycled to the cathode for this purpose too. 

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