Abstract:
A method for determining an optimal combustion interval during start-up of a hydrocarbon catalytic reformer under various conditions of temperature, fuel type, and combustion fuel flow rate. An initial catalyst temperature is measured and an algorithm is used to calculate a rate of heating of the catalyst by combustion based upon heat content of the fuel, selected fuel flow rate, and heat capacity and mass of the catalyst and reformer passages. From the initial temperature and the heating gradient, an optimal combustion interval is inferred through the algorithm and used to terminate combustion, initiate a combustion quench interval, and change over the fuel flow rate and mixture from combustion to reforming.

Description:
TECHNICAL FIELD 
     The present invention relates to methods for operating hydrocarbon catalytic reformers; more particularly, to reformer start-up control conditions; and most particularly, to a strategy and algorithm for calculating a reformer combustor burn time for heating the reformer catalyst to a minimum reforming temperature. 
     BACKGROUND OF THE INVENTION 
     Reformers for catalytically oxidizing hydrocarbons to produce hydrogen and carbon monoxide fuels are well known. Such reformers are used as fuel generators for downstream fuel cell systems in known fashion. Catalytic reforming requires an elevated catalyst temperature that at steady-state is typically between about 650° C. and 800° C. The reforming temperature then is maintained either by exothermic reforming or by endothermic reforming in the presence of hot exhaust recycled from the fuel cell system. 
     At start-up from an ambient temperature, the catalyst must be heated to a minimum temperature of about 500° C. before reforming can begin. One method for rapidly heating the catalyst is to combust oxygen and hydrocarbon fuel in an inline combustor ahead of the reformer and to pass the combustor exhaust through the reformer and then past the fuel cell anodes. In this practice, the combustor is operated optimally at a fuel-lean fuel:air ratio, whereas reforming is operated optimally at a very fuel-rich condition. Thus, it becomes of great importance to know when the catalyst surface reaches a temperature sufficient to support catalysis, in order to change over the mixture from combustion to reforming. If the changeover is too early, the catalyst temperature will be too low, and non-reformed hydrocarbons will be passed to the anodes, causing coking of the anodes and efficiency loss of the fuel cell system. If the changeover is too late, the reformer catalyst durability will be negatively impacted and the potential for pre-ignition in the reformer will be increased. 
     Obviously, a temperature probe at the catalyst surface could indicate when a suitable surface temperature has been reached. However, in practice such a location is not especially robust or practical and can also interfere with proper flow of gases through the reformer. Instead, a temperature probe typically is disposed within the ceramic elements of the reformer, which serves to protect the probe but also insulates it significantly, creating serious hysteresis between actual surface temperatures and measured temperatures during periods of rapid temperature change in the reformer. 
     One approach to dealing with this problem is to simply determine empirically how long it takes for the surface to reach the required minimum reforming temperature, and is then program the system controller to change the mixture after that time period. However, the length of time will depend upon the thermal state of the catalyst at start-up; the system may have been shut down only recently, in which case the reformer may still be quite warm, thus shortening the required combustion time. Indeed, if the reformer temperature is still sufficiently high to permit reforming, no combustion at all may be needed or desired. Also, the rate of heating will depend upon the latent combustive heat value of the fuel source being used, as well as the heat capacity and mass of the catalyst. Thus, neither a simple time instruction nor catalyst internal temperature measurement is adequate to determine when to change the entering mixture from combustion to reforming. 
     What is needed in the art is an improved means of estimating when to terminate combustion and change over to reforming. 
     It is a principal object of the present invention to change over a hydrocarbon reformer from combustion to reforming when the surface temperature of the catalyst exceeds a predetermined value. 
     SUMMARY OF THE INVENTION 
     Briefly described, a method in accordance with the invention is useful in determining an optimal combustion interval during start-up of a hydrocarbon catalytic reformer under various temperature conditions. An initial catalyst temperature is measured and an algorithm is used to calculate a temperature rise in the catalyst mass. From the initial temperature and the heating gradient, a combustion interval is calculated and used to terminate combustion, to initiate a combustion quench interval, and to change over the fuel flow rate from combustion to reforming. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a simplified fuel cell system including a hydrocarbon reformer controlled in accordance with the invention; and 
         FIG. 2  is an idealized graph showing a typical heating curve in accordance with the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a simplified fuel cell apparatus  10  for generating electrical energy from catalytic combustion of hydrogen includes a fuel cell assembly  12 , a catalytic hydrocarbon reformer  14 , and an electronic control module  16  (ECM) for controlling the flow of hydrocarbon fuel  18  and air  20  into reformer  14  via respective control valves  22 , 24 . Reformer  14  includes a combustion chamber  26 , a reforming section  28  containing catalytic elements  29 , and a temperature probe  30  that sends temperature signals to ECM  16 . Combustion chamber  26  includes an igniter  32  controlled by ECM  16 . 
     ECM  16  may include a computing environment operable to perform tasks or instructions in accordance with pre-programmed software constructs including algorithms, execution instructions or sequences, computations, software code modules, interface specifications or the like. It will be understood and appreciated that the functions performed by ECM  16  could be implemented in a computing environment such as a personal computer (PC) or other computing device. Such a computer may also include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as program modules, data structures, computer readable instructions, or other data. Computer storage media may include, but is not limited to, Read Only Memory (ROM), Random Access Memory (RAM), flash memory, Electrically Erasable Programmable Read-Only Memory (EEPROM), or other types of memory, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, CD-ROM, digital versatile disks (DVD) or other optical disk storage, or any other medium which can be used to store the desired information and which can be accessed by computer. Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. It will be understood that combinations of any of the above should also be included within the scope of computer readable media. 
     In operation, ECM  16  receives a temperature signal from temperature probe  30  with respect to a general temperature condition with the catalyst in reforming section  28 . As noted above, during intervals of changing temperatures within reformer  14 , values from probe  30  lag instantaneous and actual temperature of the functional surfaces of catalytic elements  29 . ECM  16  interrogates probe  30 , and if the indicated temperature is below a predetermined value, for example, 500° C., at which reforming can take place in elements  29 , ECM trims valves  22  and  24  to admit a predetermined flow of fuel  18  and air  20 , at a predetermined ratio, into combustor  26 . Admitted fuel and air are mixed in combustor  26  and are ignited by igniter  32 , creating a hot exhaust that passes  34  into reforming section  28  where it heats elements  29 . The spent exhaust passes further  36  into fuel cell assembly  12  and thence is discarded  38 . In the prior art, when probe  30  indicates a predetermined temperature value, ECM  16  shuts off fuel flow through valve  22  and initiates a short quench interval to extinguish combustion in combustor  26 , then adjusts valves  22  and  24  to provide a predetermined mixture ratio and flow rate suitable for reforming by catalytic elements  29 . 
     Referring to  FIG. 2 , first and second heating curves  40 , 42  are shown for system  10  shown in  FIG. 1 , representing the time during which combustor  26  is in operation to heat elements  29  to a threshold temperature for reforming to commence. Curves  40 , 42  both begin at an actual ambient temperature of 25° C. on the surface of catalytic elements  29 , and reach 500° C. at elapsed times t 1  and t 2 , respectively. Although curves  40 , 42  may represent actual conditions on the same system  10 , having identical catalyst heat capacities and thermal mass, heating is slower under curve  42  either because the combustion fuel flow is lower, or because the fuel is different and has a lower latent heat value, or both. Further, for curve  40 , the elapsed time from t 0  to t 1  is a function of starting at 25° C. However, if the reformer is still warm from a previous use, for example, still at 200° C., the elapsed time for combustion is only t 1 -t 3 . Thus, in any start-up of system  10 , it is imperative to known the thermal status of the reformer. 
     Referring still to  FIG. 2 , as noted above, because of the location of probe  30 , a time lag exists between the actual temperature of elements  29  (curve  40 ) and the temperature response of probe  30  (curve  40 ′), corresponding to t 0 ′-t 0 . Assuming that after this lag the response curve parallels the actual heating curve at any given moment, it is seen that the actual temperature is about 550° C. by the time that probe  30  reports 500° C., that is 50° C. higher and t 1 ′-t 1  later than necessary to begin reforming. 
     In accordance with the present invention, a linear algorithm of the form y=m×+b is provided for ECM  16  to estimate the slope of curve  40  at any given moment and thereby calculate when t 1  will occur:
 
 T   P   =T   I   +∫[FLHV×FF /( kC   CAT   ×k   MASS )] dt   (Eq. 1)
 
where T P  equals the predicted temperature (y) at any time after t 0  equals the initial temperature T I  (the intercept b) plus the integral of the fuel latent heat value FLHV times the fuel flow rate FF divided by the constant heat capacity of the catalyst kC CAT  times the constant “important” mass k MASS  of the catalyst and surroundings (the slope m), all times the change in time dt (x) from t 0 . Note that, in operation, fuel flow rate FF is varied by control valve  22  as conditions require, as described above. Therefore, the calculated slope m ((FLHV×FF)/(kC CAT ×k MASS )) will not be constant but instead will vary in relation to the varied flow rate FF. Note also that values for FLHV, kC CAT , and k MASS  can be readily established in a laboratory by one of ordinary skill in the art without undue experimentation, as can be the fuel flow FF delivery curve of valve  22 .
 
     The value of probe  30  in this invention is to establish T I , assuming that when the system is started up at any given temperature the reformer is at thermal equilibrium and T I  is in fact a close measure of the actual surface temperature of catalyst  29 . Beyond that time, the algorithm substantially follows curve  40  rather than curve  40 ′ to arrive at the desired reforming initiation temperature of 500° C. at time t 1 . 
     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.