Carbon startup-shutdown

During aging and startup/shutdown, the authors found that poisoning by carbonate-type species was not a major issue. Rather, by HR-TEM and CO chemisorption, the authors concluded that sintering of Pt occurred, with clusters growing from about 1.5 nm to 2.7 nm over the aging period. 

Use materials that are applicable over the full range of operating conditions such as normal, startup, routine shutdown, emergency shutdown, and draining the system. For example, carbon steel may be acceptable for normal operating conditions but may be subject to brittle fracture at low temperatures under abnormal conditions (as in the case of a liquefied gas). Cold water, of less than 60°F, during hydrotest may cause brittle fracture of carbon steel. 

The cooler part of the combustion air preheater - the tail end of the flue-gas-heat-recovery train - is more likely to corrode due to sulfur dioxide (S02) condensing from the flue gas. In this area, cast iron or glass will resist the acid attack. Carbon steel preheater tubes, joined with 1.5 to 2 meters of SS 304 tubes at the cold end of the tube sheet, can ensure reasonable service life. Typically, the flue gas temperature to the stack is maintained above the dew point of S02 to prevent condensation. During startup and shutdown, condensation of S02 will occur88. 

Yu, P.T. et ah. The impact of carbon stability on PEM fuel cell startup and shutdown voltage degradation, ECS Trans., 3, 797, 2006. 

Carbon dioxide removal systems can be made environmentally acceptable by properly specifying the instrumentation, water wash of the overhead vapors, efficient use of demisters, routine control of the solvent physical properties and chemical composition, and care in startup and emergency shutdown... 

The design of the reactor internals has not been addressed yet, but they likely will be made of graphite or carbon composites to accommodate the high-core outlet temperature required by the NGNP (1000°C). It is possible that carbon-insulated metallic alloy will be used for the core support structure, although this has not been evaluated yet. Control rods will be required to provide for reactor startup, normal operation, and shutdown. The munber and placement of control rods has not been evaluated yet, but the rods will be constructed from carbon composites for the drive shafts and absorber casing and boron carbide or other high-temperature absorber for the neutron absorber. The control rod drive mechanisms will be located above the reactor enclosure head. 

In addition to being present throughout the steam system at startup, air enters whenever there is a shutdown that allows steam to condense and pull a vacuum. Also, it is constantly present if the deaerator is malfunctioning and feedwater conditioning-chemical dosage is inadequate. Carbon dioxide is also present in steam produced from water that is not completely demineralized. Besides the adverse effect of these gases on heat transfer, their presence leads to... 

Combined with the presence of the Pt catalyst, which accelerates this reaction, measurable CO2 production occurs above 1.0 V. While this indicates that slow corrosion will occur at open circuit potential, the rate is still relatively low, but is enough to contribute to degradation over longer lifetimes. However, the key driver for corrosion of the carbon support is the increased potentials that typically occur during shutdown or startup, when the cathode potential can go up to 1.5 V or higher [461. 

It was demonstrated that idle and OCV operation resulted in a 2-5% carbon weight loss after 2000 h of continuous operation while shutdown/startup cycle exhibited a 5-10% weight loss at the end of cell operation. Fuel starvation corroded carbon support quickly since greater than 25% carbon weight loss was observed during the first 100 h of cell operation. 

The carbon cathode bottom blocks are the most important part of the cathode construction, and this statement is not an overestimation. A single crack that reaches the surface of the block at startup may cause shutdown of the cell within the first months of service. On the other hand, the expected service life of the cell is calculated on the basis of the bottom wear. That is why the quality of carbon cathode blocks is given special attention (Fig. 2.18). 

Within the 5,500 operational hours of automotive PEMFC systems, the stack could sit for a long time at idle condition, i.e., when no power is supplied to the propulsion system, and the only power drawn from the fuel cell stack is that required to support the fuel cell system andllaries (e.g., cooling pumps). This results in cathode potentials of approximately 0.90V (Mathias et al. 2005). This idle time could total over several thousand hours depending on the particular usage profile. The stack could also be at open circuit voltage (OCV) condition, where no current is drawn from the stack and the eathode potential increases to approximately 0.95 V. This can occur just before fuel ceU system shutdown, immediately after startup, as weU as in the case of battery/fuel ceU hybrid systems in which the battery provides the electrical load under low power or idle conditions. The accumulated time at OCV condition for 38,500 start/stop cycles could be over lOOh (38,500 cycles x 10 s per cycle). Figure 8 shows the predicted carbon weight loss as a function of time at these two potentials, based on the carbon corrosion... 

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