Air cathode exhaust

Figure 2.3 Schematic diagram of the experimental setup for measuring amounts of liquid water and water vapor in the air cathode exhaust at a close point of cathode exit of an operating DMFC stack.

Figure 2.5 Amount of liquid water in the air cathode exhaust at cathode exit point of an operating DMFC stack as a function of stack operating temperature and air feed actual stoichiometry.

Hence the maximum air feed actual stoichiometry is a function of water vapor partial pressure corresponding to the air exhaust release temperature (pw) and the total pressure on the air cathode exhaust (ptotai)- Figure 2.8 shows the maximum air feed actual stoichiometry calculated from Equation 2.5, using water vapor partial pressure from the CRC Handbook of Chemistry and Physics [D.R. Lide (ed.), 72nd edn, 1991-92], as a function of air cathode exhaust release temperature. 

Figure 2.8 Maximum allowed air feed actual stoichiometry for a DMFC power system as a function of air cathode exhaust release temperature.

The spent fuel is completely combusted in the anode exhaust converter. This flue gas mixture is fed directly to the fuel cell cathode. The cathode exhaust has significant usable heat, which is utilized in the fuel cleanup and in steam generation. The residual heat can be utilized to heat air, water, or steam for cogeneration applications. Design parameters for the IR-MCFC are presented in Table 9-9. Overall performance values are presented in Table 9-10. 

The results show that, at temperatures below 60 °C and an air feed stoichiometry below three, the cathode exhaust is fully saturated (nearly fully saturated at 60 °C) with water vapor and the exhaust remains saturated after passing through a condenser at a lower temperature. In order to maintain water balance, all of the liquid water and part of the water vapor in the cathode exhaust have to be recovered and returned to the anode side before the cathode exhaust is released to the atmosphere. Because of the low efficiency of a condenser operated with a small temperature gradient between the stack and the environment, a DMFC stack for portable power applications is preferably operated at a low air feed stoichiometry in order to maximize the efficiency of the balance of plant and thus the energy conversion efficiency for the complete DMFC power system. Thermal balance under given operating conditions was calculated here based on the demonstrated stack performance, mass balance and the amount of waste heat to be rejected. 

Cathode Recycle. Air from the cathode exhaust is recycled to the cathode inlet and increases the air temperature from the compressor by mixing. As with the previous case, this method requires certain compressor airflow to achieve the required A T across the fuel cell. Two methods for achieving cathode recycle are commonly considered. A high temperature blower powered by an electric motor can be used to recycle the flow. Also, an ejector uses the compressor as the primary flow to recycle the cathode air. These comparisons will assume a blower is used. 

Heated fuel and water are reformed and isothermally oxidised at the MCFC anode. The unused fuel and depleted air from the anode are burnt with added air in a catalytic oxidiser, the output of which heats the cathode and supplies it with oxygen. The cathode exhaust heats the incoming fuel and water in a heat exchanger. The latter exhausts to desired users, for example steam generation or thermal process. 

Incoming air is humidified by a tubular humidifier made of many PEM tubules. It transfers some heat and moisture from the outgoing air from the cathode exhaust to the incoming air to make it hotter and humidified to a certain extent (e.g., 75%). 

Current Density (A cm ) HjO Produced (mmol cm S ) HjO Needed by Anode (nunol cm s ) HjO Needed Taken Out (mmol cm s ) 0. Needed atl Stoich. Ratio (mmol cm s ) Air Needed atl Stoich. Ratio (mmol cm s ) Air Remaining at 1 Stoich. (nunol cm s" ) Vapor% in Cathode Exhaust... 

Local hydrogen starvation is usually observed in the cell before cell reversal occurs. By measurement of the CO2 concentration in the cathode exhaust gas, carbon corrosion can be detected. However, it is extremely difficult to distinguish between CO2 evolution within the cell and its amount present in air due to low concentrations. Alternatively, critical conditions can be detected by integration of a reference electrode (see Section 20.2.1). Since the reference electrode has to be positioned within the starved region, several reference electrodes have to be used per cell in order to make detection of the starved region probable. This can only be realized in a laboratory cell and is not practical for automotive stacks. 

Figure 2 shows a simplified process flow diagram of the proposed system. Propane and anode offgas (AOG) are fed to the reformer. The AOG contains H2O, CO2 and heat from the electrochemical oxidation of the H2 and CO on the SOFC anode. That is used for endothermic steam- and dry-reforming of the propane. The reformer provides the fiiel gas for the SOFC stack. The remaining peut of the AOG is fed together with the cathode exhaust air... 

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). 

Figure 4.23 shows the cathode utilization required to achieve 100% water saturation of the cathode exhaust air from the product as a function of temperature and pressnre. It is evident that operation at elevated pressnre is essential to retain water in the membrane electrolyte when operating the fuel cell at reasonable gas utilization at elevated temperature. 

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