Abstract:
A fuel cell hybrid power generation system and method which is to be used in a gas distribution system in which a higher pressure gas is transported/distributed and reduced to a lower pressure gas for a gas distribution or transmission line and a pre-heater is used to heat the higher pressure gas before it is reduced in pressure. More particularly, the fuel cell hybrid power generation system has an energy recovery generator which is responsive to the pre-heated higher pressure gas and adapted to reduce the gas pressure of the pre-heated higher pressure gas to produce the lower pressure gas and while simultaneously generating an electrical output. A fuel cell power plant is also included in the power generation system and is adapted to generate an electrical output while producing waste heat. The fuel cell power plant is further adapted to make the waste heat available to the pre-heater so as to enable the pre-heater to heat the higher pressure gas. An electrical assembly in the power generation system is responsive to the electrical outputs of the energy recovery generator and the fuel cell power plant and generates a combined electrical output.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    This invention relates to fuel cell systems and, in particular, fuel cell hybrid power plant systems and methods for gas distribution systems. 
         [0002]    In conventional gas distribution systems, the natural gas being supplied by utilities such as, for example, Enbridge, Inc., an assignee of the subject application, is carried over extra-high pressure transmission and distribution pipelines. This extra-high pressure gas is reduced in pressure for distribution at a lower pressure which is typically 50-80 psig from an upstream pressure which is usually in the range of two to twenty times higher than the downstream pressure. These extra-high pressure pipelines deliver gas to city gate stations, or within urban centers, to district stations, which reduce the gas pressure so that the gas can then be distributed at lower pressures to natural gas users or consumers. The city gate or district stations are usually referred to as “pressure let down stations,” or “pressure reducing stations” and they must provide the necessary reduction of the extra-high pressure gas to the desired lower pressures. 
         [0003]    The reduction in gas pressure is typically accomplished at each pressure let down station through pressure reducing valves. Accompanying the reduction in pressure is a refrigerant effect attributable to constant enthalpy expansion. This effect is similar to the cooling experienced when any gaseous compound (propane, compressed air, etc.) experiences a combination of significant pressure reduction and high volumetric flows. A physical manifestation of this can be noticed with the operation of a propane barbeque, where propane under pressure in the storage cylinder experiences a pressure reduction when the gas exits the cylinder. Under high flow conditions this refrigerant effect produces a cold exterior on the storage cylinder, which under extreme conditions can result in frost accumulating on the cylinder. 
         [0004]    As above-stated, the same chilling or refrigerant effect occurs on natural gas pipelines where large gas flows undergo significant pressure drops. This cooling effect on gas pipelines is undesirable as it can cause heavy frost formation which can negatively affect pipeline system integrity and/or create movement of pavement near any pipelines which are within municipal road allowances. This significant cooling can also create control problems with hydrates (moisture) in the conveyed gas or fuel. To eliminate these concerns, gas utilities typically pre-heat the extra-high pressure gas before it is delivered to the pressure let down stations. This is usually accomplished by passing the gas through a pre-heater or heat exchanger having a thermal heat transfer fluid loop (typically a glycol loop) which is supplied heat via natural gas boilers. The heated heat fluid of the heat transfer fluid loop of the pre-heater heats the extra-high pressure gas sufficiently so that as its pressure is reduced in the let down station, the temperature of the gas is maintained above freezing, i.e., above 32° F. or 0° C. 
         [0005]    As can be appreciated, the need to heat the ultra-high natural gas before delivery to the let down stations requires considerable energy and, therefore, reduces the overall efficiency of the gas distribution system. Also, the reduction in the gas pressure at the let down stations generates significant energy which to date has been untapped and wasted. 
         [0006]    It is, therefore, an object of the present invention to provide a system and method for use in gas distributions systems which aims to provide an efficient and cost effective approach to reducing the pressure of gas supplied from extra-high pressure gas distribution/transmission lines. 
         [0007]    It is also an object of the present invention to provide a system and method of the above type in which the energy generated in reducing the gas pressure is utilized to enhance system efficiencies. 
         [0008]    It is also an object of the present invention to provide a system and method of the above type which utilizes components which contribute lesser amounts of contaminants to the atmosphere. 
       SUMMARY OF THE INVENTION  
       [0009]    In the embodiments of the invention to be described hereinafter, the above and other objectives are realized in a fuel cell hybrid power generation system and method usable in a gas distribution system in which an extra-high pressure gas is transported/distributed and then reduced to a lower pressure for a gas distribution or transmission line and a pre-heater is used to heat the extra-high pressure gas before it is reduced in pressure. More particularly, the fuel cell hybrid power generation system has an energy recovery generator which is responsive to the pre-heated extra-high pressure gas and is adapted to reduce the gas pressure of the pre-heated extra-high pressure gas to produce the lower pressure gas while generating an electrical output. A fuel cell power plant is also included in the power generation system and is adapted to generate an electrical output while producing waste heat. The fuel cell power plant is further adapted to make the waste heat available to the pre-heater so as to enable the pre-heater to heat the extra-high pressure gas. An electrical unit or assembly in the power generation system is responsive to the electrical outputs of the energy recovery generator and the fuel cell power plant and generates a combined electrical output. Optionally, the fuel cell power plant is additionally adapted to utilize some of the lower pressure (or higher pressure) gas as the fuel supply for the plant. 
         [0010]    In certain embodiments of the invention, the energy recovery generator is in the form of a rotary expansion device which reduces the pressure of the extra-high pressure gas by expansion resulting in a mechanical output which drives an electrical generator. In some of these embodiments, the fuel cell power plant utilizes fuel cell modules adapted to internally reform the input fuel supplied by the lower pressure gas. Optionally, the fuel cell modules may include stacks of internally reforming fuel cells and further, optionally, these fuels cells can be internally reforming molten carbonate fuel cells. 
         [0011]    Also, in some embodiments the combined electrical output of the electrical assembly is made available to an electrical grid and/or loads. Additionally, in certain embodiments, the pre-heater utilizes a heat exchanger with a thermal heat transfer fluid loop for heating the extra-high pressure gas and the power plant flue gas includes oxidant exhaust gas which forms at least a portion of the waste heat of the power plant. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  shows a fuel cell hybrid power generation system used in a gas distribution system; 
           [0014]      FIG. 2  shows the fuel cell power plant and electrical assembly of the generation system of  FIG. 1  in greater detail; 
           [0015]      FIG. 3  shows the power conditioning system of the electrical assembly of  FIG. 2  in greater detail; and 
           [0016]      FIG. 4  shows the energy recovery generator of the system of  FIG. 1  in greater detail. 
       
    
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  shows schematically a fuel cell hybrid power generation system  10  used in conjunction with a gas distribution system  100 . The gas distribution system  100  includes an extra-high pressure gas transmission or distribution line  101 , which typically transports natural gas, at an extra-high pressure, to one or more pressure let down stations  102 . At the let down station  102 , the extra-high pressure gas is coupled from the line  101  and is reduced to a lower pressure, typically about 50-60 psig. The lower pressure gas is then coupled from the let down station  102  to one or more gas distribution lines  103  which deliver or make the gas available for delivery to user locations. 
         [0018]    At the let down station  102 , the fuel cell hybrid power generation system  10  provides the pressure reduction of the of the extra-high pressure gas supplied to the station from the line  101 . The power generation system  10  employs a fuel cell power plant  11  which, in the case shown, is supplied gas from the distribution line  103  and is adapted to utilize this gas as fuel supply gas for the plant. More particularly, using this fuel supply gas and an oxidant supply gas, the fuel cell power plant  11 , through electrochemical conversion, generates an electrical output which is fed to an electrical assembly or unit  13 . 
         [0019]    The electrochemical conversion process in the fuel cell power plant  11  also generates near-zero emission waste heat. This heat is provided to a pre-heater unit  14  which is used to heat the extra-high pressure gas from the line  101  before the gas is subject to pressure reduction by the fuel cell hybrid power generation system  10 . In the illustrative case, the pre-heater is shown as included in the gas distribution system  100 . Alternatively, the pre-heater  14 , if not provided in the distribution system  100 , can be included in the power generation system  10 . 
         [0020]    Once the extra-high pressure gas is pre-heated, the gas is coupled through appropriate lines or piping to an energy recovery generator  12  in the generation system  10 . The energy recovery generator  12  reduces the pressure in the pre-heated extra-high pressure gas to the desired lower pressure and delivers the gas to the distribution line  103 . Concurrently with and as a result of this pressure reduction, the energy recovery generator also generates an electrical output which is additionally fed to the electrical assembly  13 . 
         [0021]    The assembly  13  conditions and optimizes the electrical outputs of the fuel cell power plant  11  and the energy recovery unit  12 . The system then makes the combined output available to one or more electrical loads and/or or to a power grid for eventual use by consumers. 
         [0022]    With the gas distribution system  100  of  FIG. 1  and, in particular, the pressure let down station  102  implemented by use of the fuel cell hybrid generation system  10 , the overall efficiency of the gas distribution system  100  is improved. In particular, the reduction in gas pressure is accompanied by the generation of useful electrical power in the energy recovery generator  12  and this electrical power is coupled to the assembly  13  for eventual use. The electrical power from the energy recovery generator  12  is further supplemented by the steady and constant electrical power output of fuel cell power plant  11  so the resultant combined electrical power output from the assembly  13  can be reliably maintained at a desired minimum level. Also, the waste heat from the fuel cell power plant  11  is utilized in the pre-heater  14  so that the need to use a boiler for this purpose is lessened or completely replaced. Moreover, the near zero emission waste heat of the fuel cell power plant acts as an offset to any boiler emissions. Assembly  13  provides for additional system controls which optimize the emission reductions and system fuel efficiencies. 
         [0023]      FIG. 2  shows a schematic diagram of a fuel cell power plant  11  and electrical assembly  13  which is usable in certain embodiments of the invention. A shown, the power plant  11  includes a plurality of fuel cell modules I IA which form a DC power section. Each of the modules I IA is adapted optionally to operate directly with natural gas, methane or other hydrocarbon fuels as a fuel supply with little or no reforming; these gases can then be directly reformed within the fuel cell modules themselves. To this end, each module  11 A can include one or more internally reforming stacks of fuel cells. Examples of usable internally reforming fuel cells are molten carbonate internally reforming fuel cells. 
         [0024]    With the fuel cell modules reforming the fuel supply gas, the need for separate reforming units within the power plant  11  or elsewhere in the system is lessened. Direct reforming in the power plant  11  is also of significant benefit when used with the gas distribution system  100 , since the lower pressure natural gas from the distribution line  103  can then be used as fuel supply gas for the fuel cell modules  11 A. 
         [0025]    Each fuel cell module  11 A also produces waste heat. This waste heat includes oxidant exhaust gas and, as shown in  FIG. 2 , is coupled from the fuel cell modules  11 A to a heat recovery unit  11 B. The heat recovery unit  11 B exhausts this waste heat as flue gas and, as above-discussed, this flue gas is used by the pre-heater  14  for pre-heating the extra-high pressure gas from the line  101 . Prior to being exhausted, a portion of the waste heat is used to process the fuel supply and water supply after these supplies have been treated in a fuel and water treatment unit  11 C also included in the power plant  11 . The latter unit includes fuel clean-up and other fuel treatment reactors (e.g., de-oxidation reactors for peak shaving natural gas) for treating the fuel and water. 
         [0026]    The treated fuel and water are processed to produce a fuel/steam mixture at an appropriate temperature for delivery to the fuel cell modules  11 A. To this end, the unit  11 B includes packaged catalytic reactors and cold oxidant (air) supply blowers with associated local controls. 
         [0027]    A fuel cell power plant which can be used as the power plant  11  in  FIG. 2  is currently manufactured by one of the assignees of the subject application, FuelCell Energy, Inc. under product designation DFC® 3000. Other power plants manufactured by such assignee under product designations DFC®1500 and DFC300MA can also be adapted for use as the power plant  11 . 
         [0028]    Also, while internally reforming molten carbonate fuel cells are examples of one-type of fuel cell usable in the fuel cell modules  11 A, the principles of the invention are intended to apply to all types of fuel cells. Thus, it is within the contemplation of the invention to use other types of both high and low temperature fuel cells, whether internally reforming or non-internally reforming in the modules  11 A. Examples of usable fuel cells include, but are not limited to, the following: solid oxide fuel cells, phosphoric acid fuel cells and PEM fuel cells. 
         [0029]    If the modules  11 A contain non-internally reforming fuel cells, additional reforming equipment may have to be added to the fuel cell power plant  11  or elsewhere in the system to reform the fuel supply before it is fed to the fuel cell modules. 
         [0030]    Looking now at the electrical assembly  13 , as shown in  FIG. 2 , it includes a power conditioning unit  13 A, a system control unit  13 B and an electrical power plant  13 C. The power conditioning system  13 A is shown in more detail in  FIG. 3  and includes a DC to AC converter  13 AA which converts the DC output of the fuel cell modules  11 A into an AC output. As seen in  FIG. 3 , the electrical output of the energy recovery unit  12  is also fed to the output of the DC to AC converter  13 AA where it is combined with the output of the converter. The combined output is then supplied to AC loads and/or to a grid unit  201 . The latter unit, in turn, couples the AC output to a power line system  202  for eventual consumer use. 
         [0031]    It should be noted that the power conditioning system  13 A can be configured in other forms than that shown in  FIG. 3 . Thus, for example, instead of the AC output of the energy recovery generator  12  being supplied to the output of the DC to AC converter  13 AA, it could have instead been converted from an AC output to a DC output and then combined with the output of the fuel cell modules at the input of the DC to AC converter  13 AA. Another alternative would be to include a DC to DC converter at the output of the fuel cell modules  11 A to raise the level of the DC output prior to the output being fed to the input of the converter  13 AA. A third alternative, particularly, for smaller energy recovery generators, would be to provide a DC output from the energy recovery generator and then combine this output with the output of the fuel cell modules  11 A at the input of the DC to AC converter  13 AA. 
         [0032]    The electrical power plant  13 C of the assembly  13  includes equipment for supplying power to the heat recovery unit and other equipment in the fuel cell power plant  11 . As shown, the plant  13 C derives this power from a portion of the AC power from the energy recovery generator  12 . Although not shown, additional power can be derived from a portion of the fuel cell module electrical output. The plant  13 C also includes for backup a battery-supported uninterruptible power supply to maintain power for the control system and other plant components during voltage dips or interruptions. 
         [0033]    The control system unit  13  C includes the basic controls for controlling the various components of the system. More particularly, the energy recovery generator&#39;s power profile is similar to wind power generation with variable output tied to varying resource availability from the recovered pipeline gas energy. The coupled electricity from the fuel cell stacks of the fuel cell power plant allows the fuel cell hybrid power generation system  10  firm its own generation profile with an optimized control system in assembly  13 . The control system allows operating parameters for the fuel cell hybrid power hybrid generation system to be established on one of three operating modes including:
       (i) optimized electricity yield for maximum annual electricity production; or   (ii) optimized fuel efficiencies, where output from the energy recovery generator is prioritized; or   (iii) optimized emission reductions, where output from the fuel cell power plant is prioritized and the energy recovery generator output is matched to the available thermal output of the fuel cells of the fuel cell power plant.       
 
         [0037]      FIG. 4  shows an energy recovery generator  12  useful with certain embodiments of the invention and, in particular with those using the fuel cell power plant shown in  FIG. 2 . As illustrated, the unit  12  includes a rotary expansion device (turbo expander or reciprocating expander)  12 A and an electrical generator  12 B driven by the mechanical output of the expansion device. The mechanical energy of the expansion device is derived from the expansion of the extra-high pressure gas with the resultant reduction in pressure. This drives the generator to result in an electrical output (AC or DC). A turbo expander manufactured by Cryostar under product designation TG-200/60-EX is an example of a turbo expander that can be used for rotary expansion device  12 A of  FIG. 3 . The generator  12 B, in turn, can be a unit manufactured by Alsthom under the product designation F2RTCN450L2C. 
         [0038]    It is also within the contemplation of the invention to use other energy recovery generators for the generator  12 . Thus, for example, other expansion recovery machinery currently produced as commercial, early commercial, demonstration, or prototype units from various manufacturers, or ones under development but not currently produced, could also be used. Any device which permits the recovery of waste energy from the let down of gas pressure at the pipeline pressure let down station  102  for the conversion of the energy into useful electricity generation can be used for the energy recovery generator  12 . 
         [0039]    The pre-heater system  14  shown in  FIG. 1  can be standard pre-heater including a heat exchanger with a standard liquid to gas, or gas to gas, thermal heat transfer fluid loop. In such case, the heat exchanger receives the waste heat from the power plant  11  to heat the thermal fluid loop. The heated thermal fluid then proceeds around its loop path and gives up its heat to the extra-high pressure gas to provide the desired pre-heating of the gas. The thermal fluid then continues around its path to be heated by the waste heat again and the process continues so that the extra-high pressure gas is continuously pre-heated as it passes through the heat exchanger. 
         [0040]    A shown in  FIG. 1 , the system  100  may also include a boiler  104  and let down valves  105 . These components are provided as backups to the fuel cell hybrid generation system  10  and can be brought in service to provide heat into the pre-heater  14  and to provide pressure reduction for the gas in the line  101  during the maintenance of or other interruptions in the power generation system  10 . 
         [0041]    In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments, which represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and the scope of the invention. Thus, for example, while in the embodiment of the invention shown in  FIG. 1 , the fuel cell power plant is supplied fuel gas from the low pressure gas distribution line  103 , it can also be supplied fuel gas at a higher pressure from the line  101  or be supplied fuel gas from a seal leakage system that is designed or built into the energy recovery generator  12 . It is also noted that the pressures and pressure ranges given herein for the extra-high pressure gas and the lower pressure gas are intended to be illustrative examples only and the invention is not intended to be limited thereby and is intended to cover any and all systems in which there is a reduction and/or let down of gas pressure from a higher to a lower pressure.