Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. One form of a fuel cell consists of an anodic layer, a cathodic layer, and a dense ion conducting electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid-oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 ions catalytically by the cathode. The oxygen ions are conducted through the electrolyte and combine at the anode/electrolyte interface with hydrogen ions to form molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby electrons are transferred from the anode to the cathode. When hydrogen for the fuel cell is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode. Reformed gasoline and diesel oil are commonly used fuels in automotive fuel cell applications. However, other hydrogen-containing fuels for the reforming process such as, for example, JP8, natural gas, propane, synfuels, alkane alcohols, and coal based fuels may be used as well.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells. The outermost interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load. A typical prior art SOFC for use as an auxiliary power unit (APU) in a vehicle may comprise about 60 individual fuel cells and may generate, at full power, on the order of 5 kilowatts of electric power.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons as discussed above; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; and providing air for cooling the fuel cell stack.
A known shortcoming of a complete SOFC system is that it inherently has a relatively large thermal mass, and consequently, such a system is relatively slow in ramping up to full electric output. Electric output can't begin until the fuel cells are warmed to about 550° C., and a temperature of about 750° C. is required for full output and steady-state operation. The mass of the fuel cell stack along with the induced thermal stresses caused by heating the stack through heated air applied to the cathode dictate the length of time required to produce electricity. Even with known methods for preheating and forced heating of elements in an SOFC system, a prior art SOFC system requires on the order of sixty minutes, starting from ambient temperature, to begin producing usable amounts of electricity. Thus, costly and bulky additional energy storage systems would be required on an APU-powered vehicle to provide power during the fuel cell warm-up period, which causes dissatisfaction to users of such systems. In addition, long start-up times result in a reduction in operating efficiency, especially for intermittent, short-duration operation.
During these start-up periods, unwanted reactions of reformate fuel in the stack is possible, causing deposition of soot (coke) on the relatively cool anode surfaces. Such deposition is undesirable and can result in degradation and eventual failure of the fuel cell stack.
Steam is a known preventor of carbon (coke soot) forming reactions and a known cleaner of soot from anodes. Therefore, the anodes may be cleaned and prevented from coking by injection of hot steam into the fuel cell stack during warm-up to an operating temperature at which coking does not occur. One approach is to produce steam via vaporizing water from a separate water storage tank, and to then inject the steam into the fuel cell stack along with the reformate. Such a process is undesirable since it requires added complexity of apparatus and logic for storing, supplying, vaporizing, and replenishing water adjacent to the fuel cell system.
What is needed is a means for reducing the start-up period required to bring a large solid-oxide fuel cell system to operating temperature.
What is further needed is a means for preventing coke from depositing on the anodes of a solid-oxide fuel cell during the start-up period required to bring the fuel cell system to operating temperature.
What is still further needed is a means for providing hot steam to an SOFC fell cell stack to remove coke which may form on the anodes while the anodes are cool.
It is a principal object of the present invention to reduce the start-up time required for a solid oxide fuel cell system to begin producing electricity.
It is a further object of the invention to reduce the need for electrical storage systems in applications wherein a solid oxide fuel cell is an auxiliary electric power unit.
It is a still further object of the present invention to minimize coking of anodes during the start-up period required to bring a fuel cell system to operating temperature.
It is a still further object of the invention to remove coke deposits that have accumulated on the SOFC anodes.