Abstract:
A system and method for providing fuel mixes a first fuel with a second fuel at a mixing point to create a mixed fuel having a first calorie content. A control valve is located upstream of the mixing point. A process system downstream of the mixing point processes the mixed fuel to create a processed mixed fuel having a second calorie content. A first control signal is reflective of the first calorie content of the mixed fuel. A second control signal is reflective of the second calorie content of the processed mixed fuel. A third control signal is reflective of the operating level of the combustion engine. A controller connected to the control valve operates the control valve based on the first, second, and third control signals.

Description:
FIELD OF THE INVENTION 
     The present invention generally involves a system and method for controlling the calorie content of a fuel. More particularly, the present invention combines multiple fuels having different calorie contents to produce a processed mixed fuel having a desired calorie content. 
     BACKGROUND 
     Combustion engines rely on a stable fuel supply to initiate and maintain a desired combustion rate. For example, combustors on a gas turbine ignite fuel to generate combustion gases having a high temperature, pressure, and velocity. A fuel injection system supplies fuel to the combustors for ignition by a flame. At low power levels, the fuel injection system must provide fuel with a relatively high calorie content to maintain the combustion and avoid “blow out” of the flame. Conversely, “blow out” is less of a concern at high power levels, and fuel with a lower calorie content is more economical. 
     Possible fuels used by commercial combustion engines include blast furnace gas, coke oven gas, natural gas, and propane. The calorie content varies between each of these fuels. In addition, the calorie content for any particular fuel may vary, depending on the source of the fuel and the physical characteristics of the particular fuel, such as the purity, temperature, and pressure. For example, blast furnace gas and coke oven gas are by-products from the combustion of coke in the iron and steel industry; whereas, natural gas and propane are processed from naturally occurring underground deposits of methane and petroleum. The calorie content of blast furnace gas, also known as converter or LD gas, can vary between 700 kCal/m 3  and 950 kCal/m 3 . The calorie content of coke oven gas can vary between 3900 kCal/m 3  and 4400 kCal/m 3 . The calorie content of natural gas and petroleum often exceeds 4100 kCal/m 3 . 
     The unit cost of fuel generally increases as the calorie content of the fuel increases. Therefore, various systems and methods exist to reduce fuel costs by mixing less expensive, lower calorie content fuel with more expensive, higher calorie fuel to obtain a mixed fuel having a desired calorie content. 
     For example, U.S. Pat. No. 7,396,228 describes a fuel gas calorie control method and device that mixes multiple fuels having different calorie contents to obtain a mixed fuel having a desired calorie content. The system and method relies on the measured flow rate and measured calorie content of the constituent fuels to calculate and predict the resulting calorie content of the mixed fuel. 
     Various factors can effect the accuracy of the calculations used to predict the resulting calorie content of the mixed fuel. For example, an accurate flow measurement depends on the pressure of the supplied fuels, and the pressure of the supplied fuels may change over time. In addition, the constituent fuels are often supplied through large volume, low pressure piping, which further effects the accuracy of any flow measurement. 
     System processes may further change either the actual or desired calorie content of the mixed fuel. For example, the mixed fuel may be pressurized before introduction into the combustion engine, changing the calorie content of the mixed fuel. Moreover, the optimum or desired calorie content of the mixed fuel may change based on changes in the operating level of the combustion engine. 
     Therefore, the need exists for a fuel control system that does not rely on an accurate measurement of the constituent fuel flow to produce a mixed fuel having a desired calorie content. In addition, the need exists for a fuel control system that can adjust the calorie content of the mixed fuel to account for changes in the calorie content caused by subsequent processing of the mixed fuel after mixing. Lastly, the need exists for a fuel control system that can adjust the desired calorie content of the processed mixed fuel supplied to the combustion engine based on changes in the operating level of the combustion engine. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     One embodiment of the present invention is a system for providing fuel having a desired calorie content to a combustion engine over a range of operating levels. The system includes a first fuel supply pipe containing a first fuel. A second fuel supply pipe containing a second fuel connects to the first fuel supply pipe at a mixing point so that the first fuel mixes with the second fuel to create a mixed fuel having a first calorie content. A control valve is located in the second fuel supply pipe upstream of the mixing point. A process system downstream of the mixing point processes the mixed fuel to create a processed mixed fuel having a second calorie content. A first control signal is reflective of the first calorie content of the mixed fuel. A second control signal is reflective of the second calorie content of the processed mixed fuel. A third control signal is reflective of the operating level of the combustion engine. A controller connected to the control valve operates the control valve based on the first, second, and third control signals. 
     Another embodiment of the present invention is a system for providing fuel having a desired calorie content to a combustion engine over a range of operating levels. In this embodiment, the system includes a first fuel supply pipe containing a first fuel. A second fuel supply pipe containing a second fuel connects to the first fuel supply pipe at a mixing point so that the first fuel mixes with the second fuel to create a mixed fuel having a first calorie content. A control valve is located in the second fuel supply pipe upstream of the mixing point. A process system downstream of the mixing point processes the mixed fuel to create a processed mixed fuel having a second calorie content. A trim calorimeter downstream of the process system produces a trim control signal reflective of the second calorie content of the processed mixed fuel. A controller connected to the control valve adjusts the control valve based on the trim control signal from the trim calorimeter. 
     Another embodiment of the present invention is a method for providing fuel having a desired calorie content to a combustion engine over a range of operating levels. The method includes determining the desired calorie content of the fuel and mixing a first flow of fuel with a second flow of fuel to produce a mixed fuel having a first calorie content. The method further includes measuring the first calorie content of the mixed fuel and processing the mixed fuel to produce a processed mixed fuel having a second calorie content. The method also measures the second calorie content of the processed mixed fuel and adjusts the second flow of fuel based on the desired calorie content, the first calorie content of the mixed fuel, and the second calorie content of the processed mixed fuel. 
     Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which: 
         FIG. 1  is a simplified diagram of a fuel control system according to one embodiment of the present invention; and 
         FIG. 2  is a block diagram of a fuel flow controller according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. 
     Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
       FIG. 1  provides a simplified diagram of a fuel control system  10  according to one embodiment of the present invention. As shown, the fuel control system  10  generally includes multiple fuel sources  12 , piping  14  that connects the multiple fuel sources  12  to a combustion engine  16 , and instrumentation and control equipment to combine the multiple fuel sources  12  in the necessary proportions to produce a fuel having a desired calorie content. 
     The multiple fuel sources  12  are shown on the left side of  FIG. 1 . The fuel sources may include any fuel that can be used by the combustion engine  16 , such as blast furnace gas (BFG), coke oven gas (COG), natural gas (NG), and propane (P). Isolation valves  18  may be used to connect each fuel source to the piping  14 . The use of isolation valves  18  allows a specific fuel source to be either selected for use or isolated from the fuel control system  10 , depending on the availability of each fuel. 
     Each fuel source may be arranged according to the relative calorie content of the fuel. The less expensive and lower energy fuels (e.g., BFG) connect to the piping through a low energy supply line  20 . The more expensive and higher energy fuels (e.g., COG, NG, and P) connect to a high energy supply line  22 . The use of separate low  20  and high energy  22  supply lines allows the fuel control system  10  to select or receive multiple sources of both low and high energy fuels based on fuel availability. 
     The piping  14  that connects the multiple fuel sources  12  to the combustion engine  16  may be any manner of conduit, line, or transmission means for conveying fuel from a fuel source to a destination. The embodiment shown in  FIG. 1  includes no flow measurement devices in the piping  14  that connects the multiple fuel sources  12  to the combustion engine  16 ; however, alternate embodiments may include such devices if desired. 
     A control valve  24  connects the high energy supply line  22  to the low energy supply line  20  at a mixing point  26  so that the higher energy fuel mixes with the lower energy fuel to produce a mixed fuel. The mixing point  26  may include any structure for combining the low and high energy fuels, such as a tank or simply the piping where the low and high energy supply lines meet. The control valve  24  may be a variable flow orifice, throttle valve, regulator valve, or any equivalent structure for regulating a variable flow. The control valve  24  is generally installed on the supply line that carries the higher pressure fuel, which is the high energy supply line in  FIG. 1 . The position of the control valve  24  thus determines the amount of higher energy fuel that is mixed with the lower energy fuel to change the calorie content of the mixed fuel. For example, opening the control valve  24  increases the amount of higher energy fuel added to the mixed fuel, thus raising the calorie content of the mixed fuel. 
     A mixed fuel sensor  28  may be located downstream of the mixing point  26  to sample the mixed fuel and provide a mixed fuel control signal  30  reflective of the calorie content of the mixed fuel. The mixed fuel sensor  28  may be located as close to the mixing point  26  as reasonably practicable to allow for prompt measurement of the mixed fuel and correspondingly faster responses to transient changes in fuel demand. In addition, the fuel control system  10  is relatively cleaner downstream of the mixing point, so locating the mixed fuel sensor  28  downstream of the mixing point  26  facilitates maintenance on the mixed fuel sensor  28 . 
     The mixed fuel sensor  28  may be a high speed calorimeter, mass spectrometer, chromatograph, or any similar instrument capable of measuring the calorie content, hydrogen level, lower heating value (LHV), Wobbe index, or any other parameter of the mixed fuel to provide the mixed fuel control signal  30  reflective of the calorie content of the mixed fuel. In addition, multiple mixed fuel sensors  28  may be used to provide redundancy in the event an individual sensor fails. If multiple mixed fuel sensors  28  are used, additional circuitry  29  may be used to select the average, high, or low sensor to produce the mixed fuel control signal  30 . 
     The mixed fuel typically passes through one or more system processes to produce processed mixed fuel before reaching the combustion engine  16 . The system processes include any components that modify any physical characteristic or composition of the mixed fuel, such as a humidifier, dehumidifier, compressor, heat exchanger, and/or chemical addition station. As shown in  FIG. 1 , for example, the mixed fuel may pass through a compressor  32  and/or a heat exchanger  34  to regulate the pressure and/or temperature of the mixed fuel. Changes in the pressure and/or temperature of the mixed fuel produce corresponding changes in the calorie content of the processed mixed fuel on a volumetric basis. In addition, the addition or removal of noncombustible gases and liquid from the mixed fuel may also alter the calorie content of the processed mixed fuel on a volumetric basis. 
     A trim sensor  36  located downstream of the system processes samples the processed mixed fuel. The trim sensor  36  provides a trim control signal  38  reflective of the calorie content of the processed mixed fuel. The trim sensor  36  may be located as close to the combustion engine  16  as reasonably practicable to allow for measurement of the processed mixed fuel immediately before introduction into the combustion engine  16 . The trim sensor  36  may be a high speed calorimeter, mass spectrometer, chromatograph, or any similar instrument capable of measuring the calorie content, hydrogen level, lower heating value (LHV), Wobbe index, or any other parameter of the processed mixed fuel to provide the trim control signal  38  reflective of the calorie content of the processed mixed fuel. In addition, multiple sensors may be used to provide redundancy in the event an individual sensor fails. If multiple sensors are used, additional circuitry may be used to select the average, high, or low sensor to produce the trim control signal  38 . 
     A controller  40  determines the position of the control valve  24 . Many equivalent algorithms are possible to enable the controller  40  to accurately position the control valve  24  to produce the desired calorie content in the fuel.  FIG. 2  provides a block diagram of one algorithm according to one embodiment of the present invention. 
     As shown in  FIG. 2 , the controller  40  may receive a first control signal  42  from the mixed fuel sensor  28 , a second control signal  44  from the trim sensor  36 , and a third control signal  46  from the combustion engine  16 . The first control signal  42  from the mixed fuel sensor  28  may be the mixed fuel control signal  30  reflective of the calorie content of the mixed fuel. The second control signal  44  from the trim sensor  36  may be the trim control signal  38  reflective of the calorie content of the processed mixed fuel. The third control signal  46  from the combustion engine  16  may be reflective of the megawatt load, firing temperature, or other signal indicative of the operating level of the combustion engine  16 . 
     At block  48 , the controller  40  compares the first (mixed fuel) control signal  42  with the second (trim) control signal  44  to determine a trim value  50 . The trim value  50  is the change in calorie content between the mixed fuel and the processed mixed fuel. The controller  40  may adjust the first (mixed fuel) control signal to account for the transport time for the mixed fuel to travel from the mixed fuel sensor  28  to the trim sensor  36 , as shown by the LAG block in  FIG. 2 . This allows the controller  40  to more accurately compare the calorie content of the mixed fuel at the mixed fuel sensor  28  with the calorie content of that same mixed fuel after it has reached the trim sensor  36  as processed mixed fuel. 
     At block  52 , the controller  40  determines the desired calorie content  54  based on the third control signal  46 , which is reflective of the operating level of the combustion engine  16 . For some embodiments, the desired calorie content  54  may remain constant over the range of operating levels for the combustion engine  16 . For other embodiments, the desired calorie content  54  may change over the range of operating levels for the combustion engine  16 , depending on particular design considerations. 
     At block  56 , the controller  40  compares the desired calorie content  54  with the trim value  50  to determine an adjusted desired calorie content  58 . 
     At block  60 , the controller compares the adjusted calorie content  58  with the first (mixed fuel) control signal  42  to control the operation of the control valve  24 . If the adjusted calorie content  58  is greater than the first (mixed fuel) control signal  42 , the controller  40  opens the control valve  24  to increase the flow of high energy fuel thereby increasing the resulting calorie content of the fuel produced by the system  10 . Conversely, if the adjusted calorie content  58  is less than the first (mixed fuel) control signal  42 , the controller  40  closes the control valve  24  to decrease the flow of high energy fuel thereby decreasing the resulting calorie content of the fuel produced by the system  10 . 
     It should be appreciated by those skilled in the art that modifications and variations can be made to the embodiments of the invention set forth herein without departing from the scope and spirit of the invention as set forth in the appended claims and their equivalents.