Abstract:
A technique is disclosed of modified shut-down of a fuel cell power plant ( 14 ) having a fuel cell stack assembly ( 26 ) contained with and supplying electrical power to a mobile vehicle ( 12 ). The vehicle characteristically proceeds at intervals to a station ( 10 ) containing one or more resources ( 20, 20 A,  20 B,  20 C, etc) utilized by the fuel cell power plant, for resupply thereof. One such, resource provided at/by the station is electrical energy ( 20 A), aid operation of the fuel cell power plant ( 14 ) is controlled to utilize the availability of that electrical energy ( 20 A) to maintain, an active protective Hydrogen On condition for greatly extended intervals, for example many hours to several days or longer, via a Power On mode of operation. The Power On mode maintains at least a low level of hydrogen introduction and circulation sufficient to maintain a predetermined presence, e.g., pressure, of hydrogen at the fuel cell stack assembly ( 26 ).

Description:
BACKGROUND  
       [0001]    The disclosure relates generally to fuel cell power systems and more particularly to fuel cell power plants suited for mobile usage, as in transportation vehicles. More particularly still, the disclosure relates to operation of such feel cell power systems and the fuel cell power plan(s) thereof, into and/or in, mode(s) other then normal operating modes to enhance durability, economy, and/or the like. 
         [0002]    Fuel cell power plants for mobile/transportation usage are increasingly being employed in automobiles and in larger carriers, such as bosses. Those power plains typically employ numerous PEM-type fuel cells arranged in one or more stack(s), or stack assembly(s) (CSA), for the generation of electrical power to both power electrical motors to directly propel the vehicle, and to provide energy for storage in au energy storage system such as a battery, or bank of batteries. 
         [0003]    A PEM feel cell employs a membrane-type electrolyte in which the membrane is a proton exchange membrane, or polymer electrolyte membrane, (PEM). The membrane is disposed between anode and cathode electrodes respectively. The catalyzed cathode and anode serve to induce the desired electrochemical reactions. Reactants, typically art oxidant seen as oxygen or air and a fuel seen as hydrogen, are flowed through respective cathode and anode flow fields over respectively opposite sides of the membrane to obtain the requisite electrochemical reaction. 
         [0004]    It is known that the startup and shutdown mechanisms associated with operation of a fuel cell stack have a critical, typically adverse, impact on the durability and life of the stack. Degradation of a fuel cell stack results from oxidation of catalyst support material, as well as high temperatures and high (electrical) potentials. This impact is more pronounced in the application of fuel cell power plants for transportation purposes, because of the increased number/frequency of startups and shutdowns as compared to typically long-duration operation in stationary applications, and is of particular impact with respect to such power plants in large transport vehicles, such as busses. 
         [0005]    A number of different techniques have been proposed or employed to help mitigate the degradation mechanisms associated with the startup/shutdown cycle, with a preferred approach being one of hydrogen stabilization, sometimes referred to as a “Hydrogen On” (or “H 2  On”) approach in the “Hydrogen On” approach, oxygen is removed from the air side volume of the fuel cell stack (including the oxidant flow fields in the cells, inlet and outlet manifolds, plumbing, etc), and hydrogen levels are stabilized in both the cathode and anode flow fields, thus preventing or minimizing at least high voltages in the cells. Typical hydrogen stabilization is accomplished in a shutdown procedure which includes (a) blocking the input of fresh air into the stack, (b) removing the service load and connecting the stack in a resistive auxiliary load, (e) performing cathode recycle by conducing cathode exhaust to the inlet of a/the cathode blower, which remains on, while continuing in provide fresh hydrogen to the cells, with fuel recycle on, and with fuel purge (exhaust) blocked, and continuing this until sufficient hydrogen has been provided to react with all of the residual oxygen in the air side volume, and (d) closing off the inlet of fresh hydrogen to the fuel cells while continuing to provide cathode and feel recycling (recirculation) until shutdown is completed. One representative example of such a system is disclosed in U.S. Pat. No. 8,142,950 issued Mar. 27, 2012 to Reiser et al, and assigned to the owner of the present application. Another similar representative example is disclosed in U. S. Patent Application Publication No. US 2011/0223495 A1 published Sep. 15, 2011 by M. L., Perry and assigned to the owner of the present invention. 
         [0006]    It will be noted that the Hydrogen On stabilization approach is employed during the process of shutting down and may relatively quickly terminate the supply of air to the stack, but may continue to rely for a limited interval on the delivery and/or recirculation of hydrogen to the stack. While the passivation benefits of the Hydrogen On condition may conduce for some interval during the actual shutdown state, that interval is typically limited to a few hours, e.g., less than 16 hours. It has been deemed impractical from both logistical and durability standpoints to repeatedly restart, briefly operate, and again shutdown and H 2 -passivate the fuel cell power plant for the purpose of extending the duration of the H 2  On passivation benefits. Such limits on the duration of the Hydrogen On condition correspondingly limit the benefits derived from such mode of operation, especially for extended intervals during which the power plant is normally shutdown. 
       SUMMARY 
       [0007]    Various transportation vehicles do, or are required from time to time to, proceed to a station, garage, “shed”, or terminal for various types of storage and/or maintenance. While at such stations, the vehicle may receive a recharge of various resources such as hydrogen fuel, and its feel cell power plant has heretofore typically been in a shutdown condition for intervals that may extend from several hours to several days. However, it is recognized herein that by utilizing one or more of the station&#39;s relatively unlimited resources, including at least electrical power, and by modifying the normal shutdown process to include a so-called Power On mode, it is possible for the protective Hydrogen On condition to remain active for greatly extended intervals. 
         [0008]    A method or technique is disclosed of modified shutdown of, or for, a fuel cell power plant contained with and supplying electrical power to a mobile vehicle. The vehicle characteristically proceeds at intervals to a station containing one or more resources utilized by the fuel cell power plant, for resupply thereof One such resource provided at/by the station is electrical energy, and operation of the fuel cell power plant is modified to utilize the availability of that electrical energy to maintain an active protective Hydrogen On condition for greatly extended intervals, for example of several days or longer, via a so-called Powder On mode of operation. 
         [0009]    The Power On mode maintains a passivating flow of hydrogen, directly and/or by recirculation, to or within the fuel cell power plant, and is typically entered via modification of an otherwise conventional shutdown procedure. Whereas initial aspects of a conventional shutdown procedure are or may be, retained, at some preselected phase or condition in that procedure at which most of the H 2  passivation will have been attained, the procedure may be modified in response to the presence of at least external electrical power. The procedure is modified to maintain a hydrogen flow, directly and/or by recirculation, to thereby maintain and extend the duration of hydrogen passivation to intervals greater than 16 hours, or 30 hours, or indeed to many days. Regulation of hydrogen flow may be in response to a sensed pressure of hydrogen, be example at or near an inlet for the hydrogen to the fuel cell of the fuel cell power plant, and involve providing hydrogen when the sensed pressure thereof decreases to soma threshold, for example 2 kPa. 
         [0010]    While the provision of the Power On mode of operation, depends initially upon the availability of electrical power at the station to extend the otherwise limited duration of H 2  On passivation, additional benefits may also be derived during the Power On mode by supplying to the vehicle and/or the feel cell power plant other resources from the station, such as supplemental H 2  and/or supplemental coolant and/or cooling capacity. 
         [0011]    The foregoing features and advantages of the present disclosure will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings am not necessarily to scale. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0013]      FIG. 1  is a simplified diagram illustrating a part of a station or terminal at which one or more vehicles having a met ceil power plant are, or may be, connected to external resources in accordance with the present disclosure; 
           [0014]      FIG. 2  is a generalized diagram of a vehicle of  FIG. 1 , depleting in block form various generalized components of the fuel cell power plant; 
           [0015]      FIG. 3  is a schematic diagram view of relevant portions of the fuel cell power plant of  FIG. 2 . illustrating some of the components and their interconnections in greater detail; 
           [0016]      FIG. 4A  illustrates a generalized functional flow diagram of the initial portion of the Hydrogen On shutdown process prior to determination of a bower On enabled state; 
           [0017]      FIG. 4B  is a continuation of the generalized functional flow diagram began in  FIG. 4A , illustrating, alternatively, completion of the shutdown process or entry into and maintenance of the Power On process; and 
           [0018]      FIG. 5  is a simple functional pictorial representation of the determination of a Power On enabled stare. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         [0020]    Referring to  FIG. 1 , there is depicted a portion of a station  10 , referred to variously also as a terminal, shed, garage, etc., at which, one or more fuel cell-powered vehicles  12  maybe temporarily located, as for periodic storage, maintenance, and/or replenishment of required resources. The vehicles  12  characteristically contain a fuel cell power system (FCPS)  14 , a limited on-board supply of hydrogen  16 , an electric rumor drive  18 , and various vehicle auxiliary loads  19 . While the vehicle  12  might be any of various types of fuel cell-powered vehicles, including but not limited to automobiles, pucks, and busses, the disclosure is particularly directed to vehicles such as busses which routinely proceed to a station  10  for overnight weekend, or longer, storage or housing, and which may obtain replenishment at the terminal  10  of one or more “external” resources  20 . Those resources  20  include at least electrical power  20 A, and may optionally also include hydrogen  20 B, coolant and coolant processing  20 C, and/or other resources. Such resources  20  are, or may be, conveniently available at a station  10 , and in quantities that are normally unlimited relative to their limited quantities as housed on the respective vehicles  12 . For illustrative purposes in  FIG. 1 , each and all of the resources  20  are depicted as being connected to a respective vehicle  12  via a gross conduit  22 , which in actuality will be various independent conduits for the materials, entities, properties, and functions conveyed. It will be understood, though not separately shown, that for the transfer of electrical power there will be appropriate electrical cables/connectors/contactors, etc. The same would be true for the optional provision of hydrogen and/or coolant and coolant processing. Importantly also, the gross conduit  22  is presumed to embrace the transfer of signal information between the vehicle  12  and the station  10  with respect to the resources  20 . 
         [0021]    Referring to  FIG. 2 , the vehicle  12 , and particularly the fuel cell power system  14 , is depicted in somewhat greater detail. The FCPS  14  importantly includes a fuel cell power plant (FCPP)  24 . It may also include an electrical energy storage system (PSS)  25 . The FCPP  24  is capable of electrochemically providing electrical energy in a known manner. The ESS  25  may be a rechargeable battery, or battery system. Either, or both, of the FCPP  24  and the ESS  25  is/are capable of providing electrical energy to the vehicle motor drive to propel the vehicle  12 . The ESS  25  may conveniently be recharged, to varying extents, by dynamic braking of die vehicle and/or by operation of the FCPP  24 . Moreover, the ESS  25  is depicted in  FIG. 2  as being connected to also supply electrical energy to the FCPP  24 . This capacity allows some limited, operation of limited duration of various electrically-powered equipment that comprise part of the FCPP  24 , even during times such as shutdown when the FCPP  24  is not normally supplying electrical energy, and for purposes of restart. 
         [0022]    Considering the FCPP  24  in greater detail, it is generally comprised of a fuel cell stack assembly (CSA)  26 ; a fuel processing system (FPS)  28  for processing the hydrogen fuel delivered from on-board source  16  and delivered to and/or recirculated from/to the CSA  26 ; an air processing, system (APS)  29  for receiving an oxidant such as air from a source designated  30  and delivered to and/or recirculated from/to the CSA  26 ; a thermal management system (TMS)  32  for managing the thermal requirements of the CSA  26 ; a voltage limiting device (VLD)  34  for regulating and/or limiting the output voltage of the CSA  26 ; and a controller  36  for regulating the numerous processes and functions of at least the FCPP  24 , and typically also those of the ESS  25 . the vehicle auxiliary loads  19 , and, the interface between the station resources and the vehicle  12 , particularly the FCPP  24 . 
         [0023]    Reference is now made to  FIG. 3  which depicts the relevant components of the FCPP  24  in greater detail. The fuel cell stack assembly  26  is comprised of multiple fuel cells, here collectively shown as a single cell, each having a cathode  38  and an anode  40  with an electrolyte  42  between them. In the illustrated embodiment, the electrolyte is a proton exchange membrane (PEM). In an exemplary embodiment, the reactant gas flow fields (well known and not shown) are disposed in bipolar plates, at least one of which has coolant passages  44  on a side of the plate opposite to the side having reactant gas flow field channels. These bipolar plates can either be solid or micro-porous, hydrophilic water transport plates, both of which are well known. There may or may not be cooler plates Interspersed or at intervals between bipolar plates. 
         [0024]    Fuel is provided, from the onboard source  16  of hydrogen, through a pressure control valve  46 , and a fuel flow control valve  48  to inlets  50  of the anodes. The exits  52  of the flow fields transfer hydrogen-depleted exhaust through a conduit  54  that leads to a remotely controllable anode exhaust valve  56 , which in turn is operatively connected to a mixing box  73 . The anode exhaust valve  56  may be slightly opened continuously, or operated periodically on a poise-width modulation basis, as necessary to eliminate contaminants and hunts such as nitrogen, and/or for mixture with exhaust air for possible recycle. A conduit  57  also connected to conduit  54  connects the anode exhaust to an anode recycle pump  58 , which might be an ejector, the output of which in a conduit  59  is connected with the inlets  50  of the anodes for anode fuel recycle. The anode recycle pump may also be an electrochemical hydrogen pump which relies upon passing a direct current through the feel cell in a known manner and provides both recirculation and purification of the exit hydrogen. 
         [0025]    Air is provided in a conduit  60  through an air inlet valve  62  to an air blower  64 , which provides air through an air control valve  65  to inlets  66  of the cathodes  38 . The cathode exits  67  are connected through a conduit  68  and an exhaust valve  69  to exhaust and includes a mixing box  73  where exhausted process air is mixed with exhausted feel so as to reduce flammability before discharging to the atmosphere, as well as for possible recycle to the cathodes  38 . The exhaust valve  69  is remotely operable for variably controlling the exhaust flow of not only the cathodes  38  but also the anodes  40 , including partially and fully closed, states for facilitating cathode recycle flow. 
         [0026]    To perform hydrogen stabilization in the anodes and cathodes at shutdown, cathode recycle is provided in this embodiment by a conduit  70  which is connected through a remotely operable feedback valve  71  and a conduit  72  to the inlet of air blower  64 . Still further, a hydrogen transfer means in communication between the flow path of anodes  40  and the flow paths of cathodes  38  selectively permits transfer of hydrogen feel between those flow fields during shutdown. In this embodiment, the hydrogen transfer means comprises the hydrogen-containing exhaust from the anodes  40  being connected to the mixing box  73  via conduit  54  and anode exhaust valve  56 , for mixing with air exhausted from the cathodes  38  and then recycled via conduits  70  and  72 . Alternatively, or additionally, other hydrogen transfer means may comprise a hydrogen transfer valve  74 . here depicted in broken line form, secured in fluid communication between the anode inlets  50  and a lower-pressure point in the cathode recycle path, such as at the inlet to the air blower  64  leading to the cathode inlets  66 . Other alternate means for such hydrogen, transfer might take the form of an electrochemical hydrogen pump which relies upon passing a direct current through the fuel cell in a known manner, or simply by relying upon relatively slow diffusion of hydrogen across the PEM electrolyte  42 . 
         [0027]    The thermal management system  32  is depicted only in a generalized form, but it will be understood that it may take any of several well known forms for condensing or otherwise cooling moisture or liquid coolant exiting the cell stack assembly  26  and selectively returning some portion to the coolant passages  44 . If the cell stack assembly  26  is cooled by conduction and convection of sensible heat into circulating coolant, the thermal management system will typically include a fan-cooled heat exchanger and liquid circulating pump, as well as an accumulator. If the cell stack assembly  26  employs evaporative cooling, the cathode exhaust may be condensed by a condenser and the resulting liquid stored in an accumulator for selective use in the stack. 
         [0028]    A pressure transducer  76  is operably connected with the hydrogen flow path for sensing and indicating the pressure of hydrogen therein as a facet of the Power On routine or procedure to be hereinafter described in greater detail. The hydrogen pressure transducer  76  is conveniently connected to the hydrogen inlet stream substantially at or near the anode inlets  50 , and has a pressure measuring range sufficient to embrace the pressures hereinafter described. 
         [0029]    Referring briefly to the portion of  FIGS. 2 and 3  designated VLD  34 , there is depicted in  FIG. 3  an external circuit  78  that receives electrical current generated by the CSA  26  for supply to a primary load designated  80  when a primary switch  82  is closed (shown open here) and the CSA is in normal operation. That primary load designated  80  will typically include at least the vehicle motor drive  18 , and may additionally include all or some of the vehicle auxiliary loads  19 , but would not normally, though may, include the various loads required to operate the FCPP itself. The VLD  34  additionally includes an auxiliary load  84 , here depicted as being of a variable or remotely adjustable resistance, that does not receive electrical current during normal operation of the CSA  26  and an auxiliary load switch  85  is open. The auxiliary load  84  is intended to reduce and/or limit CSA  26  output voltage during the shutdown procedure, as known and later described. A diode  86  may be connected in series with the auxiliary load  84  to terminate current flow through the latter when cell output voltage drops below a pre-selected value. A voltmeter  87  or similar voltage sensing device is connected across the electrical output terminals of the CSA  26  to provide an indication of voltage for purposes of control and regulation. The voltage limiting function may alternatively or additionally be provided by the ESS  25 , for example as disclosed I U.S. Pat. No. 7,790,303 by S. J. Fredette and assigned to the owner of the present invention. 
         [0030]    Although no separate connections are depicted, the controller  36  of the FCPP  24  is understood to he operatively connected with the various elements under its control and forming part of the following description of operation. The controller  36  may take any of a variety of well known forms capable of performing the functions hereinafter described, with examples ranging from simple manual operation of some or all valves and connections, etc, to programmed electronic digital processor control via hardwired and/or remote wireless connection. The ability to sense actual connectivity with one or more of the resources  20 , though not shown, is presumed as known and understood, and may include sensors associated with physical contact, detection of electrical current, hydrogen, and/or coolant flow, etc. 
         [0031]    Attention is now directed additionally to  FIGS. 4A ,  4 B, and  5  for an understanding of the process of operation in a Power On mode, including the initial process of shutting down and diverting from that process, prior to reaching the full shutdown state, to a branch process of Power On which maintains an active Hydrogen On condition for an extended interval. These Figs. functionally depict the higher level operations of that process, and the accompanying description correlates the hardware of  FIGS. 1-3  with the flow diagram(s) of  FIGS. 4A ,  4 B, and  5 . 
         [0032]    During normal operation of the FCPP  24 , primary switch  82  is closed and electricity is being provided to at least the primary load  80 , which may include vehicle drive motor  18 . The oxidant blower  30  and the anode recycle pump  58  are on. The air inlet valve  62  and the cathode exhaust valve  69  are open, as are the hydrogen inlet valve  48  and anode exhaust valve  56 . Generally, air is flowing through the cathode fields and being exhausted, and hydrogen is flowing through the anode fields, with some being exhausted via anode exhaust valve  56 , mixing box  73 , and cathode exhaust valve  69 , and some being recycled (recirculated) to the anode fields. 
         [0033]    Shutting down the FCPP  24  will typically occur when the vehicle  12  is at rest, and in the present instance may be located at station  10 . Whether the FCPP  24  folly shuts down or alternately enters the Power On mode is determined by whether or not the vehicle  12  is operatively connected to one or more of the resources  20  at station  10 . 
         [0034]    Step  100  in  FIG. 4A  represents the initiation of the shutdown process, and is accomplished by a switch or ignition signal “off” or other method (not shown). This in tarn is followed at step  102  by the removal of the primary load  80  and the enabling of the voltage limiting device  34 . More specifically, step  102  involves opening switch  82 , and closing switch  85  that connects auxiliary load  84 , which in torn may be a variable load device. 
         [0035]    Next, the cathode oxygen is depleted at step  104  by closing the air inlet valve  62  and the cathode exhaust valve  69 ; running the air blower  64  at maximum, speed for some preselected interval, for example in the range of 3 to 300 seconds; monitoring the voltage output of the CSA  26 , as via voltmeter  87 ; and when the voltage drops sufficiently, as for example to a preselected level between about 0.1 and 0.7 volts, proceeding to step  106 . 
         [0036]    At step  106 , hydrogen is accumulated in the CSA  26  by reducing the speed of the air blower  64  to somewhere in the range of 20%-70% of its maximum speed; and maintaining the air inlet valve  62  and the cathode exhaust valve  69  in the closed state. During this phase, hydrogen will migrate and/or be transported from the anodes  40  to the cathodes  38  where it reacts with the oxygen, thus serving to further deplete the oxygen. The direct transport of hydrogen comes, in the illustrated, embodiment, at least partly front the recycle to the cathodes  38  of anode exhaust hydrogen mixed with cathode exhaust air at mixing box  73 , which assumes the anode exhaust valve  56  to remain at least partly open. 
         [0037]    As an adjunct or extension of the process describe for step  106 , the next step  108  acts to reduce cathode oxygen concentration oven further by perhaps reducing the speed of air blower even lower in the range presented with respect to step  106 ; maintaining the air inlet valve  62  and the cathode exhaust valve  69  in the closed state; and changing/varying the resistance of the auxiliary load  84  of the VLD  34  to a lower value to increase current draw. As in step  106 , hydrogen will continue to migrate and/or be transported from the anodes  40  to the cathodes  38  where it reacts with the oxygen, thus serving to further deplete the oxygen. Then, shortly after entering step  108  of the shutdown procedure (e.g., 0.1s to 1.0s), the program sequence queries whether or not it is to enter the Power On mode. This is done at decision step  110  by conducting a Power_On signal cheek. 
         [0038]    Referring briefly to  FIG. 5 , there is depicted a functional representation of a symbolic Power On switch  111 , here shown closed, for setting in step  112   a  Power On Sag either to a “1” (True) state or to a “0” (False) state. The True state is reflective that the Power On mode is, or is to be, enabled, and may be attained symbolically by closure of switch  111  manually by an operator and/or automatically upon actual operative connection of the FCPP  24  to at least the electrical power  20 A at the station  10 . To the extent others of the external resources  20 , such as hydrogen  20 B and/or coolant resources  20 C are to be utilized in addition to the electrical power, appropriate signals indicating their respective connections may also be used or required to establish the Power On enabled, or True, state. 
         [0039]    Returning to the flow diagram, now at  FIG. 4B , it will he noted that the Power On signal cheek of decision step  110  presents either a True or a False response alternative, depending upon which flag was set in step  112  of  FIG. 5 . If the Power On mode flag had been set to a “0”, indicating False, at block  114 , typically because the vehicle  12  and its FCPP  24  are not actually connected to the electrical power resource  20 A, then the shutdown routine is continued at step  116  by stopping the fuel and air supplies. This is done by shutdown of the air blower  64 ; and closing, or maintaining closed, the hydrogen inlet valve  48 , the air inlet valve  62 , the air recirculation valve  71 , and the cathode exhaust valve  69 . 
         [0000]    Following completion of these steps the fuel cell power plant  24 , and particularly the CSA  24 , will he deemed, shutdown, as reflected at function block  118 . 
         [0040]    If, however, the Power On mode flag had been set to a “1” indicating True, at block  120 , typically because the vehicle  12  and its FCPP  24  are actually connected to the electrical power resource  20 A, then the Power On mode is entered as reflected in function step  122  at which the recirculation of air and hydrogen fuel is continued. Viewing the Power On mode in greater detail, the electrical power is now supplied to the FCPP  24  from the main power bus resource  20 A of the station  10  to assure adequate power for the various primps, blowers, valve actuators, etc. for an expended, potentially indefinite, period that may comprise many hours, e.g., 16 or 30, or days, or even weeks, being limited only by limits on the availability of other resources such as hydrogen. In this Power On mode, the air blower speed is reduced from the prior maximum, typically to less than 30% of that maximum speed, while continuing to recirculate cathode exhaust mixed with anode exhaust as the anode exhaust valve  56  continues to remain partly open. The pressure sensor  46  monitors the anode inlet  50  pressure and, via operation, of the controller  36 , serves to activate the hydrogen inlet valve  48  in a manner that maintains the pressure above some minimum threshold, as for example 2 kPa, but below about 10 kPa as determined by the characteristics of the stack and cell materials including crossover sensitivity and sealing properties. Similarly, the recirculation of the hydrogen exhaust from the anodes  40  back to the anode inlets  50  is continued via recycle pump or ejector  58 . 
         [0041]    As noted, the Power On mode may fee continued for a lengthy period, however for a variety of reasons it may become desirable or necessary to terminate it. One prime example is the need to restart the FCPP  24  for normal operation, another example being a cessation of the availability or connection with, the supplied external resource. Accordingly, the program routine which defines and controls the Power On mode includes provision far periodically returning to the Power On decision block  110  to cheek that the Power On state should, or does, remain enabled, if at any time the check of the Power On state reveals the False, or “0”, condition, the program routine of function block  122  immediately jumps, shown by broken line  125 , to the shutdown routine depicted and described with respect to step  116  to complete a shutdown operation. 
         [0042]    Although the disclosure has been described and illustrated with respect to the exemplary embodiments thereof it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the disclosure.