Patent Publication Number: US-2006000142-A1

Title: Precise oxygen to carbon ratio control in oxidation reformers

Description:
TECHNICAL FIELD  
      This invention relates to precise and reliable control of oxygen to carbon ratio in a catalytic partial oxidizer reformer (CPO) or an autothermal reformer (ATR), determining air and fuel flow rates in close proximity to the reformer, compensating for non-linear fuel or air flow versus fuel or air valve position and non-linear air flow versus air blower speed, and preventing water condensation corruption of flow measurements.  
     BACKGROUND ART  
      It is well known that catalytic partial oxidation reformers (CPOs) and autothermal reformers (ATRs) typically utilized to produce hydrogen-rich gas, referred to as “reformate”, require precise control of oxygen to carbon ratio in the fuel gas, and must have the level of sulfur therein reduced to below about 25 parts per billion by volume (ppbv) in order to avoid sulfur poisoning of the catalysts in subsequent fuel processing reactors to provide low COX gas called “syngas”.  
      As is known, the ratio of oxygen to carbon in a CPO should be about 0.62. Ratios higher than about 0.65 lead to higher temperature and resulting catalyst damage and causes reduced hydrogen concentration; ratios of less than 0.50 lead to reduced hydrogen production as well as elevated levels of methane.  
      Maintaining this ratio is extremely difficult during transients, that is, when there are increases or decreases in the fuel flow command. As an example, an increased fuel flow ahead of a hydrogen desulfurizer (HDS) will not show up downstream of the HDS immediately, since much of the initial increase in flow is absorbed in pressurizing the HDS due to its large volume. Therefore, a flow transmitter ahead of the HDS will be indicating to the air flow channel a higher flow of fuel than is actually reaching the CPO. This will cause a higher O 2 /C ratio in the CPO and thus reduced production of hydrogen in the CPO and possibly damage the CPO catalyst; conversely in a down transient of fuel flow.  
      Another hindrance to maintaining a proper O 2 /C ratio is the susceptibility of flow transducers to water condensation. Turbine type flow transducers have a tendency to drag causing false low flow indications, particularly in the presence of moisture, and they tend to “free-wheel” during down transients, due to inertia, causing false high flow indications. In an air flow path, moisture is always present and tends to condense on the air flow transducer. The air flow control responds to this false decrease in air flow indication by increasing the blower speed, resulting in excessive O 2 , CPO catalyst overheating, and possible catalyst damage.  
      Moisture also confounds thermal dispersion type flow meters; vortex-shedding type meters are not stable during fast transients. In short, no measurement technology has demonstrated the ability to accurately measure fuel and air flows during fast transients in the presence of moisture.  
     DISCLOSURE OF INVENTION  
      Objects of the invention include: more precise and reliable control of oxygen to carbon ratio in a hydrocarbon fuel reforming system employing an HDS; more precise and reliable control over the flow of fuel and air in a hydrocarbon fuel reformer; longer CPO and ATR catalyst life as a consequence of improved O 2 /C control; reduction of excessive oxygen as a consequence of fuel transients; improved tracking of air flow as a function of fuel flow in a reformer; more precise and reliable measurement of air flow and fuel flow in a reformer; ensuring that the fuel/air mix is fuel-rich during increasing fuel or decreasing fuel transients and improved control of hydrogen and carbon monoxide concentrations in the reformate produced by a hydrocarbon fuel reformer.  
      This invention is predicated in part on the realization that the  10  flow of fuel provided to a hydrocarbon fuel reformer such as a CPO or an ATR, and the commensurate amount of air flow, should be measured in as close proximity to the entrance of the reformer as possible. The invention is predicated in part on the realizations that the flow of fuel or air through a fuel or air valve is not a linear function of the positioning of the valve and that the flow of air produced by a blower is not a linear function of the speed of the blower. This invention is also predicated in part on the realization that water condensation from humidified air corrupts conventional air flow measurements.  
      According to the present invention, in a hydrocarbon fuel reformer system employing a reformer, such as a CPO or an ATR, the flow of fuel, used to determine the commensurate flow of air, is measured by a differential pressure sensor responsive to fuel flow changes at the inlet of the reformer. In accordance with the invention in one form, the flow is determined by a differential pressure transducer connected across a small orifice or laminar restriction for better resolution of flow in fuel flow transients. In another form, the invention measures fuel flow as a function of pressure drop across the HDS.  
      In accordance with the invention, the flow of air to the hydrocarbon fuel reforming system employing a CPO or ATR is determined by a differential pressure transducer responsive to air flow changes in close proximity to the reformer. In accordance with the invention in one form, the flow is determined by a differential pressure transducer connected across a small orifice or laminar flow restriction in the air flow between the air blower and the reformer. In another form, the invention measures air flow by a differential pressure transducer across a small orifice or laminar flow restriction that is fabricated as an integral part of the reformer air inlet. In this form, the orifice or laminar restriction will become heated by conduction of heat from the reformer vessel and will thereby resist water condensation from humidified air.  
      According to another aspect of the invention, the fuel flow command is adjusted to accommodate the non-linearity between the amount of fuel flow through a valve as a function of the position of the valve; this is applicable also to air flow when a valve is used to control air. In further accord with this aspect of the invention, an air blower command is compensated to accommodate the lack of linearity in the volume of air which a blower will provide as a function of the speed of the blower.  
      According further to the invention, differential pressure is measured across a flow restraint disposed integrally with the inlet of a hydrocarbon fuel reformer, in close thermal communication therewith.  
      Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a simplified functional schematic diagram illustrating a first generation of a CPO reformer being fed air and hydrocarbon fuel through an HDS, employing aspects of the invention.  
       FIG. 2  is a simplified functional schematic diagram of a CPO reformer receiving fuel from an HDS, employing additional aspects of the present invention.  
       FIG. 3  is a chart of fuel flow as a function of fuel valve position.  
       FIG. 4  is a chart of position command to the fuel valve as a function of the linearized fuel command.  
       FIG. 5  is a chart of air flow as a function of air blower speed.  
       FIG. 6  is a chart of speed command to an air blower as a function of the linearized air command.  
       FIGS. 7 and 8  are fragmentary diagrams of variants of  FIG. 3 .  
       FIGS. 9-11  are charts of hydrogen production, oxygen to carbon ratio, and carbon monoxide at the exit of a CPO, all as a function of time.  
       FIG. 12  is a side elevation cross section of a heated, laminar restriction of the invention. 
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION  
      In a first generation of the invention shown in  FIG. 1 , hydrocarbon fuel, such as liquid petroleum gas or natural gas, flows in a conduit  11  through a valve  12  and a flow metering transducer  13 , over a conduit to a fuel preheater  14  and then to a hydrogen desulfurizer (HDS)  15 . The HDS typically will have a noble metal catalyst, such as platinum or palladium, on which sulfur compounds, such as thiophene, are reduced to hydrogen sulfide, which is then captured by a sorbent, such as zinc oxide. Low sulfur fuel is fed over a fuel flow path, such as a conduit  17 , to the mixer  18  at the inlet of a reformer, such as a CPO  19  or an ATR, where it is mixed with a controlled flow of air in a conduit  53  (described hereinafter). The CPO output in a conduit  20  comprises reformate gas which, for methane feed stock, is roughly 37% hydrogen, 14% CO, 4% CO 2  and traces of other gases.  
      The reformate may be further processed, such as in a water-gas shift reactor and a preferential CO oxidizer to make syngas suitable for use, for instance, in a fuel cell. To make the hydrogen desulfurization process go forward, additional hydrogen is required in the HDS, which may be provided by the syngas resulting from the aforementioned downstream processing, such as in a line  23 , as illustrated in copending U.S. patent application Ser. No. 10/731,291, filed Dec. 9, 2003. This forms no part of the invention and is not described further.  
      The flow transducer  13 , which may comprise a turbine or other type of transducer, provides a signal on a line  24  as feedback to a summing junction  25 , the positive input of which is a fuel flow command on a line  26 . The error signal on a line  28  is provided to a proportional/integral gain  29 , the output of which on a line  30  controls the positioning of the valve  12 . So long as the flow indicated by the flow transducer  13  is the same as the flow dictated by the fuel flow command on the line  26 , the valve will not be moved. Any variation in the fuel flow from the commanded amount will cause a commensurate adjustment in the position of the valve  12 .  
      To ensure that conservative amounts of oxygen are always commanded as described hereinbefore, a minimum selecting function  31  selects the least of the fuel flow command on line  26  and the actual fuel flow on line  24  to apply over a line  32  to a fuel/air schedule  33  which is a fixed indication of the amount of air required for the amount of fuel flowing in order to provide the correct oxygen-to-carbon ratio. Thus, during an up-transient in the fuel flow command, the air fuel schedule will respond to the actual fuel flow, which will be lower than the command. During a down-transient, the schedule  33  will respond to the command, which will be lower than the actual flow. The output of the schedule  33  on a line  39  is applied to a summer  40 , the negative input of which is on a line  41  from an air flow transducer  42 , which may also be of the turbine or other type. The air flow error signal on a line  46  is provided to a proportional/integral gain  47 , the output of which on a line  48  controls the speed of an air blower  49 , which receives humidified air over a conduit  50 . The output of the flow transducer  42  passes to the mixer  18  of the CPO  19  in an air flow path, such as a conduit  53 .  
      Referring to  FIG. 2 , according to a major aspect of the present invention, pressure indicative of fuel flow is measured at the inlet to the CPO  19 . Because the temperature downstream of the HDS  15  is on the order of 350° C. (662° F.), traditional flow meters, such as a turbine flow meter  13  ( FIG. 1 ), cannot be utilized because they would be damaged by the heat. Therefore, a small laminar flow restriction, venturi or orifice  13   a  is provided and a differential pressure transducer  13   b  provides a signal on a line  24   a  indicative of the difference in pressure on opposite sides of the orifice  13   a . A schedule  13   c  of fuel flow as a function of pressure differential provides a signal on a line  24   b  indicative of flow through the orifice.  
      Another aspect of the present invention is illustrated in  FIGS. 3 and 4 . In  FIG. 3 , the fuel flow in grams per second is plotted as a function of fuel valve position. It is readily apparent that at the more open fuel valve positions, there is a smaller incremental flow of fuel per incremental increase in position. To compensate for that, a linearization function  58  ( FIG. 4 ) provides a position command to the valve  12  as a function of fuel command from the PI function  29 , as illustrated. That is, for higher fuel commands the position commands are incrementally greater. Stated alternatively, the concave up gain command of  FIG. 4  compensates for the concave down flow/position relationship of  FIG. 3 . The net result is that the fuel flow is a linear function of the valve position command. Although illustrated in  FIG. 2  as a separate function  58 , the linearization may be accomplished together with the proportional/integral gain  29  in a single function using gain scheduling.  
      If desired, the schedule  33  may be made responsive to some selected function  31   a , other than the minimum selecting function  31  depending on the application of the invention. Or, it could be just the fuel flow command on line  26  if desired, while utilizing other aspects of the invention; this results in faster air response because the air controller does not wait for measured changes in the fuel flow. This approach requires that the integral and proportional gains of PI  29  and PI  47  are tuned to give about the same setpoint tracking (servo) response, which can provide very tight control of oxygen to carbon ratios on both up transients and down transients.  
      According to the invention, the non-linear relationship of air flow as function of air blower speed is illustrated in  FIG. 5 . Therefore, a linearization function  60 , has a gain of speed command to the blower on the line  48   b , as a function of air command from the proportional integral function  47  on the line  48 , as is illustrated in  FIG. 6 . Thus, the concave down linearization of  FIG. 6  compensates for the concave up air flow versus air blower speed of  FIG. 5 . In other embodiments as shown in  FIG. 8 , the air flow may be controlled by a valve, in which case linearization may be as described with respect to  FIGS. 3 and 4 .  
      In  FIG. 2 , a small venturi or orifice  42   a  is provided with a differential pressure transducer  42   b  feeding a schedule  42   c  to provide the air flow signal on the line  41   a . This is in accordance with another aspect of the invention, which eliminates the inaccuracies of turbine and other type flow meters.  
      As illustrated in  FIG. 8 , instead of sensing the fuel pressure differential across the orifice  13   a , the fuel pressure differential may be sensed by a transducer  13   d  across the hydrogen desulfurizer  15 , if desired. However, measurement with the orifice  13   a , as seen in  FIG. 2  or laminar flow restriction, described with respect to  FIG. 12 , are preferred.  
      In  FIGS. 9-11 , performance of  FIG. 1  is illustrated in dotted lines and performance of the invention,  FIG. 2 , is illustrated in solid lines.  FIGS. 9-11  compare hydrogen production, oxygen-to-carbon ratio, and CPO exit CO, respectively. The illustrations refer to hydrogen production demanded by a load-following fuel cell power plant as the fuel cell power plant responds to a load increasing at about 20 kilowatts per second, from about 30 kW electric power output to about 150 kW electric power output of the fuel cell.  FIG. 9  illustrates that there is a slight increase in hydrogen production during the first five seconds of the transient, which is the most critical time period for load-following fuel cell power plants.  
      The most notable benefit of the invention is illustrated in  FIG. 10 , where the oxygen/carbon ratio is seen to not vary by more than about 0.01. This is in contrast with an unacceptably excessive ratio of about 0.72 when the features of the invention are not employed.  FIG. 1   2  illustrates that the maximum peak in the production of carbon monoxide during the transient is slightly less with the invention than in the prior art.  
      In another aspect of the invention, the air flow is measured by measuring pressure drop across an orifice or laminar flow restriction that is integrated with the CPO air inlet. This will heat the orifice or laminar flow restriction and prevent water condensation and associated corruption of air flow measurements. In  FIG. 12 , a laminar flow restriction  56  is threaded into a flange  57  of the CPO  19 , and provided with insulation  58 . This keeps the laminar flow restriction  56  at a sufficiently high temperature to eliminate (or nearly so) condensation which would otherwise corrupt flow measurements. The orifice  13   a  ( FIG. 2 ) could be so disposed.  
      The aforementioned patent application is incorporated herein by reference.  
      Thus, although the invention has been shown and described with respect to 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 therein and thereto, without departing from the spirit and scope of the invention.