Abstract:
A method of operating a fuel cell system to produce electrical power that includes a hydrocarbon or alcohol fuel feed stock containing water vapor or steam being reformed in the fuel cell or in a separate reformer with the output gas from the fuel cell going to a water gas shift reactor to convert a portion of the carbon monoxide to carbon dioxide and hydrogen. A portion of the carbon dioxide then being removed to yield a hydrogen rich gas that is piped back into the solid oxide fuel cell or the molten carbonate fuel cell in concert with the reformed or unreformed fuel feed stock. A system for performing the method is also provided.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority benefit of US Provisional Application Ser. No. 62/157,637 filed 6 May 2015; the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention in general relates to hydrogen reforming and fuel cells, and in particular, to the reforming of fuels in conjunction with their use in solid oxide or other high temperature fuel cells. 
       BACKGROUND OF THE INVENTION 
       [0003]    The reliability of high-temperature solid oxide fuel cells (SOFCs) has improved over the last ten years to the point that they are attractive options for electric power generation in some automobiles, airplanes, and auxiliary power supplies. When SOFCs, or to a lesser extent molten carbonate fuel cells (MCFCs), are operated at high temperatures they tolerate high concentrations of carbon monoxide and sulfur in the form of SOx and hydrogen sulfide, and are capable of a limited degrees of in-situ reforming—something that is advantageous from the stand-point of fuel logistics. It is far easier to transport fuel-energy as a liquid fuel or as methane gas, than as a large volume of hydrogen gas. There are however a few problems with in-situ reforming however, and a major one is the danger of coking, a problem that gets worse when dealing with the more-desirable, heavier fuels, e.g. gasoline and jet fuel JP-8. Another problem is that, at high temperatures, the carbon tends to leave the fuel cell as carbon monoxide instead of as carbon dioxide. Carbon monoxide is toxic, and the emission thereof represents an energy inefficiency. 
         [0004]    Thus, there exists a need for a system containing a SOFC or MCFC that reforms carbon-based fuels to generate usable energy at high efficiency and has reduced levels of carbon monoxide emissions and a lower tendency for coking. 
       SUMMARY OF THE INVENTION 
       [0005]    A method of operating a fuel cell system to produce electrical power which includes a carbon-based fuel feed stock containing water vapor or steam being reformed to produce hydrogen in a solid oxide fuel cell or a molten carbonate fuel cell or in a reactor upstream of same. The fuel-hydrogen is reacted in the fuel cell with an oxygen or air steam to produce electrical power and an output gas containing carbon monoxide and water. The output gas from the fuel cell is then reacted in a water gas shift reactor to convert a portion of the carbon monoxide to carbon dioxide and a hydrogen-rich reacted gas output. A portion of the carbon dioxide and water are then removed, ideally via membrane, with the remaining, hydrogen-rich gas being returned to the solid oxide fuel cell or the molten carbonate fuel cell in concert with the reformed or unreformed fuel feed stock. 
         [0006]    A version of the method is shown in  FIG. 1 . A fuel feed to a solid oxide fuel cell (SOFC) or molten carbonate fuel cell (MCFC) is placed in thermal communication with a regenerative heat exchanger and mixed with a hydrogen-rich gas, In the fuel cell, the mixture produces electricity to power a load. The output gas from the fuel cell is cooled and fed into a water gas shift reactor followed by a membrane or other means that extracts carbon dioxide and other waste from the stream exiting the water gas shift reactor. The water gas shift reactor converting carbon monoxide to carbon dioxide and also generating a hydrogen rich gas that includes carbon dioxide. The extracted carbon dioxide leaves the system at near atmospheric pressure, and the non-extracted hydrogen rich gas is recycled via piping to the fuel cell by way of a blower, fan, or Venturi. The fuel feed, in this case, being a hydrocarbon or alcohol fuel stock. Exemplary loads include an airplane, a ship, a submarine, a truck, a train, and a bus; as well as stationary applications such as buildings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention is further illustrated with respect to the following figure that is intended to be exemplary of the specific aspects of the present invention, but should not be construed as limiting the appended claims to those aspects shown in the figure. 
           [0008]      FIG. 1  is a schematic of flows for a power generation apparatus operating according to the inventive method described. The green lines in the figure showing the flow of unreformed fuel and of water or steam; the blue lines show the flow of air to and from the fuel cell; and the black lines show the flow of reformed fuel and/or fuel cell output gases. Many of the components in the figure are optional. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    The present invention has utility as a SOFC or MCFC system and a process for operation thereof that is more efficient than conventional systems. The inventive system relies on the use of a water-gas shift reactor and a CO 2 -extraction membrane. 
         [0010]    It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4. It is also to be understood that, where a series of components are listed, these are only descriptive. An actual system could contain more or fewer components and would still operate substantially the same. 
         [0011]    An inventive system is depicted in  FIG. 1  generally at  10 , a fuel feed stock  12 , e.g. stored in a fuel tank  14 . Fuels operative as feed stock  12  herein illustratively include propane, methane, gasoline, military diesel fuel (JP-8), ethanol, methanol, combinations thereof, by themselves or mixed with water. The fuel feed stock  12  enters fuel pump  15  and is conveyed by piping  16 , heated and perhaps heated by exhaust  17  and mixed with steam or partially boiled water to produce a heated fuel water/steam mix  18 . This mix is heated further in a fuel heat exchanger (HX)  20  and is heated there to a temperature typically between 450 and 800° C. The heated mixture  22  enters steam reformer  25  or directly into the SOFC or MCFC  30 . In some inventive embodiments the steam reformer  25  is coupled with a sulfur removal stage  26 . Regardless of whether the reformer or sulfur removal stage  26  is present, the fuel mix  29  enters a SOFC or MCFC  30 . Typically the fuel will be mixed with a heated recycle flow  28 . The operating pressure for the fuel cell  30  will typically be 45-150 psi (3-10 atm). As such, a pump is shown for the fuel  15  and for the water recycle  35 . Operating at elevated pressures allows the process to exhaust most of the product CO 2  to atmosphere by use of a passive membrane. Attractive solid oxide fuel cells for this application are provided by Versa Systems, Acumetrics, Protonex, Siemens and Bloom Energy. Molten Carbonate fuel cells are provided by Fuel Cell Energy. Typical MOFC and SOFC operating temperatures here range from 600 to 1000° C. 
         [0012]    As shown in  FIG. 1 , the steam reformer  25  is not adiabatic, but is intimately attached to the SOFC or MCFC  30 . A result of the attachment is that heat is provided to the reformer  25  from the SOFC or MCFC  30 . As with much in  FIG. 1 , it will be appreciated that this is not necessary but is a specific inventive embodiment. The reformer could be essentially adiabatic, or for some versions of this invention, all the fuel reformation could take place in the fuel cell itself. A recycle stream of hydrogen-rich gas  28  is joined with the reformed mixture  29  prior to entering the SOFC or MCFC  30 . Pressure for the recycle is provided via a blower, fan, or Venturi, a blower  19  being shown in  FIG. 1 . This hydrogen-rich recycle flow  28  adds to the power generated by the SOFC or MCFC  30 . It also adds to the heat generated by the SOFC or MCFC  30  and to the amount of water in the SOFC or MCFC  30 . Typical ratios of fuel mixture to hydrogen-rich gas volume to fuel mix vapor volume range from 1:3 to 3:1 on an slpm basis. The net effect of the inclusion of hydrogen-rich gas  28  with the fuel mixture  22  or  29  is to increase the power output per unit of fuel gas  22 , going to the SOFC or MCFC  30 . It is also predicted that adding the recycle flow  28  will reduce the tendency for coking in the fuel cell  30  by providing heat and water for steam reforming. 
         [0013]    The cathode of the SOFC or MCFC  30  has a feed of oxygen-rich gas  31 . This gas is typically air. Here it is shown to enter the fuel cell  30  through a rectifying air heat exchanger (HX)  32 . This heat exchanger  32  heats the oxygen-rich gas before it enters the fuel cell  30 , and cools the exhaust gas  33  flowing away from the fuel cell  30 . Typical oxygen-rich gases  31  include air, pure oxygen, or air mixed with CO2 (common with MCFC). The feed oxygen source  33  may be provided at a variety of pressures ranging from ambient air pressure to more than 10 atm; a blower,  34  is shown. In some inventive embodiments, the exhaust gas  33  or a portion thereof is used to provide compression energy to the fuel pumps  15 ,  35  or to the blowers  19 ,  34 . 
         [0014]    The hydrogen-rich gas  28  is produced via a water-gas-shift reactor (WGS)  40  operating in conjunction with at least one heat exchanger  20 ,  37 , and a carbon dioxide extraction membrane  42 . Most versions of invention will need at least one heat exchanger  20 , 37  for the fuel exhaust  36  because it is expected that the fuel cell  30  will operate at a higher temperature than is desirable for feed to the WGS  40 . At these higher temperatures, the SOFC or MCFC  30  generates considerable carbon monoxide in the output gas  36 . By cooling the output gas  36  and sending it to the WGS  40 , we convert some of the CO to CO2 and H2. REB Research has made and sold water-gas shift reactors. 
         [0015]    It is now necessary to remove the waste water and CO2 produced in the process. As shown in  FIG. 1 , this is done via a knockout drum  41  and/or a carbon dioxide specific membrane  42 . The use of these means allows the process to discard the majority of the water and CO2 in the recycle stream  44  without discarding the majority of the hydrogen and without significantly decreasing the pressure of the hydrogen-rich recycle gas  44 ,  45 . By removing the CO2 and H2O using  41  and  42 , we expect to find that the recycle flow,  45  has a higher H 2 -concentration than the feed gas  44 , and that it is relatively low in temperature, and at pressure where it can be easily sent back to the fuel cell  30 , e.g. via a low-cost blower  19 . This is a lower cost, lower energy option than if a hydrogen-specific membrane were to be used instead. Several types of CO 2 -extracting membranes  42  are available. Most will also extract water and H 2 S. Use of the membrane  42  to extract water could allow the system to operate without the water knockout  41 . 
         [0016]    The membrane  42  operative herein illustratively includes polymers such as polyacetylenes polyaniline poly (arylene ether)s, polyarylates, polycarbonates, polyetherimides, poly (ethylene oxide), polyimides, poly(phenylene ether), poly(pyrrolone)s and polysulfones; carbon; silicas; zeolites; mixed-matrix; hybrid membranes; and facilitated transported membranes as detailed in H. Yang et al. Journal of Environmental Sciences 20(2008) 14-27. The Polaris membrane from MTR Inc. is a silicone polymers, e.g. dimethylsiloxane, that is exemplary of a membrane  42 . It shows H 2 -CO 2  selectivity of about  30  with an operating temperature, ≦50° C. The use of Polaris and similar CO 2 -extracting membranes requires that the output from the WGS  40  must be cooled from a typical WGS temperature of 200-450° C. to 50° C. or below. A cooling heat exchanger  17  is shown. Since water will condense at 50° C. a water knock-out  41  is shown. It is appreciated that a membrane  42  operating at higher temperatures, e.g. 125° C., or above could preclude the use of the knockout  41 , or the need for a heat exchanger boiler  37 . In such cases we may use membranes, one to extract CO 2  the other for water, or we may rely on the single membrane  42  to extract both CO 2  and H 2 O. 
         [0017]    A key operating issue involves heating in response to load changes. One heating technique, useful once the SOFC or MCFC  30  is at operating temperature, is to adjust the voltage at the SOFC or MCFC  30 . The lower the voltage, the lower the efficiency, the more chemical energy is available for heat. At steady state operation, the aim is to operate the SOFC or MCFC  30  at 0.8-1.0 Volts. The rectifying heat exchangers, particularly  20  and  34  allow one to produce electricity at these voltages while providing sufficient heat to reform most hydrocarbon fuels. Without the heat exchangers and the hydrogen-rich recycle, steam-reforming would be too endothermic to allow efficient in-situ reforming with such efficient power generation. 
         [0018]      FIG. 1  shows a separate reforming reactor  25  in thermal contact with the FC  30 . Though it is appreciated that reforming functionality is available in current MCFCs and SOFCs, it is the inventor&#39;s expectation that a separate reformer  25  will be advantageous because it provides additional room for catalyst. The separate reformer  25  is shown to be in thermal contact with the SOFC or MCFC  30  because most fuel reforming requires heat.  FIG. 1  shows water being recycled to the fuel stream through the use of the knockout  41  and the water recycle pump  35 . There is also a water HX boiler,  37  that may or may not be included in all embodiments of the water recycle. 
         [0019]    The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention.