Abstract:
A fuel control system for controlling the ratio of ring temperature adjustment in rings of a combustor in a gas turbine. The fuel control system consists of employing a computer to perform the steps of; defining an operational boundary inner ring temperature adjustments versus outer ring temperature adjustments that defines a safe operating region for the annular combustor of the gas turbine; calculating a point of operation of the inner ring temperature adjustment versus the outer ring temperature adjustment within a safety margin of the operational boundary, wherein nitrous oxide (NO x ) emission levels of the gas turbine is reduced; and regulating the ring temperatures to maintain a near global minimum point of operation, while maintaining normal operation of the gas turbine.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to fuel-air optimization in annular gas turbine combustors and more particularly concerns a system for continual on-line trimming of the fuel flow rate to rings of a annular combustor to optimize NOx emissions. 
     FIG. 1 shows a gas turbine combustor system  10  from a Dry Low Emissions (DLE) Industrial Engine, such as the GE LM6000™, which includes a compressor  2 , a combustor  3  and a turbine  4 . Fuel is mixed with compressed air from the compressor  2  and burned in the combustor  3 . The resulting flow of combustion products out of the combustor  3  drives the turbine  4 , which in turn drives a load (not shown) as well as the compressor  2 . The exhaust from the turbine  4  is eventually released to the atmosphere. One type of combustor commonly used today is the so-called annular combustor. One exemplary embodiment of the annular combustor comprises a plurality of separate rings, wherein each ring is connected to the compressor  2  and the fuel supply provides combustion products to drive turbine  4 . This combustor is fully described in U.S. Pat. No. 5,323,604. 
     FIG. 1 further shows one embodiment of a DLE type annular combustor  3  having three rings  5 , 7 , and  9 . The rings define a combustion chamber (not shown) to which a fuel-air mixture from an inner ring premixer  12 , a pilot ring premixer  16 , and an outer ring premixer  18  is injected. Compressed air enters each of the premixers  12 ,  16 , and  18  via an air line  13  and fuel enters via a fuel line  15 . A main valve  14 , also referred to as a pilot value, is disposed in the fuel line  15  to throttle the flow of fuel into each of outer ring premixer  12  and inner ring premixer  18 . Alternatively, the fuel and air may be directly injected into the combustion chamber without premixing. This results in near-stoichiometric, high temperature combustion which leads to copious production of varying combinations of oxides of Nitrogen, which are generally referred to as NO x . Premixing the fuel and air prior to combustion results in lean premixed combustion, which produces lower flame temperatures and thus lower NO x  emissions. Flame temperature in a ring of the combustor is proportional to the fuel-air-ratio in the operating region of a DLE type combustor, hence ring flame temperature and fuel-air-ratio are used interchangeably in the present specification. 
     Reducing emissions of harmful gases such as NO x  into the atmosphere is of prime concern. It is, therefore, desirable for gas turbine-based power plants burning natural gas to employ means for dramatically reducing NO x  emissions. Natural gas-fired gas turbines produce no measurable particulate exhaust of oxides of Sulfur (SO x ) and, if the combustion process is properly controlled, very little NO x  or Carbon Monoxide (CO). 
     There is a need for real time, on-line trimming of the fuel flow to each ring of a annular combustor in accordance with minimizing total NO x  emissions. There is an additional need for a trim system to carry out the real time, on-line trimming which is retrofittable to existing gas turbines. The trim system must be such that its failure will not affect the baseline operation of the gas turbine. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned needs are addressed by the present invention which provides apparatus and method for minimizing NO x  emissions of a gas turbine. The present invention comprises a fuel control system for controlling the ratio of ring temperature adjustment in at least one ring of an annular combustor in a gas turbine. The fuel control system consists of a computer to perform the steps of; 1) defining an operational boundary of inner ring temperature adjustment versus an outer ring temperature adjustment that defines a safe operating region for the gas turbine; 2) calculating a operating point of the inner ring temperature adjustments versus the outer ring temperature adjustment within a safety margin of the operational boundary, wherein NO x  emission levels of the gas turbine is substantially minimized; and 3) regulating each ring temperature adjustment to maintain a near global minimum point of operation while maintaining normal operating parameters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a conventional annular combustor based gas turbine; 
     FIG. 2 is a schematic representation of the controller having a NO x  regulator of the present invention; 
     FIG. 3 is a further schematic block diagram representation of a controller and a NO x  regulator of the present invention; 
     FIG. 4 is a further schematic block diagram of the NO x  regulator of FIG. 3; 
     FIG. 5 is a system level flow diagram of the method of regulating NO x  emmisions of the present invention; 
     FIG. 6 is a graphical illustration of the outer temperature reference adjustment versus the inner temperature reference adjustment of the present invention; 
     FIG. 7 is further detail of the flow diagram in FIG. 3 of the control process of the present invention; and 
     FIG. 8 is a graphical illustration of the arc that defines the global optimum operating point of the gas turbine of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention addresses the issue of reducing emissions of harmful NO x  gases produced by a gas turbine. NO x  emissions are very sensitive to combustor temperature in a gas turbine and the amount of NO x  produced is an exponential function of temperature. In this specification NO x  refers to various oxides of Nitrogen. The present invention is also adapted to operate the gas turbine to minimize high levels of acoustic noise, to eliminate turbine blow-out based on unacceptably lean fuel, and to reduce high levels of Carbon Monoxide (CO). 
     Referring to FIGS. 1 through 4, in which like elements correspond with like numbers, a controller  20  is coupled to an inner ring fuel valve  22 , a pilot ring fuel valve  14 , and an outer ring fuel valve  24  to regulate the flow of fuel from fuel supply  6  to respective premixers  18 , 16 , and  12  via an inner ring fuel signal  36 , an pilot ring fuel signal  40 , and an outer ring fuel signal  38 . A plurality of control signals are coupled from the turbine engine to controller  20 , including, for example, dynamic pressure P x  ( 26 ), (the dynamic peak-to-peak dynamic pressure of combustor  3 ), NO x  ( 34 ), inner ring temperature ( 30 ), outer ring temperature ( 28 ), and fuel valve position status ( 32 ). It is understood that the above identified control signals are only a representative subset of the control signals that may be used by controller  20  to control the gas turbine. It is also understood that signals P x  dynamic pressure  26 , outer ring temperature  28 , inner ring temperature  30 , and NO x  emission level  34 , may be calculated in the controller from other measured signals, and not necessarily measured directly by sensors. 
     FIG. 2 illustrates further detail of controller  20  which comprises a turbine controller  50  and a NO x  regulator  56 . Controller  20  further comprises an inner ring fuel adjuster  52 , outer ring fuel adjuster  54 , which inner ring adjuster  52  and outer ring adjuster  54  are respectively coupled to an inner ring fuel driver  58  and an outer ring fuel driver  62 . Pilot fuel driver  60  is coupled to turbine controller  50 . NO x  regulator  56  is coupled to inner ring fuel adjuster  52  and outer ring fuel adjuster  54 . Because of the connection arrangement, NO x  regulator  56  only has the ability to adjust the fuel flowing to inner ring fuel valve  22  and outer fuel ring valve  24 . Inner fuel driver  58  generates inner ring fuel signal  36 , and outer ring fuel driver  62  generates outer ring fuel signal  38 . Pilot ring fuel signal  40  is derived from the difference between the total fuel flow and the sum of inner fuel flow and outer fuel flow. 
     Main fuel valve  14  acts to regulate the total fuel flow to the combustor. Inner ring fuel valve  22  and outer ring fuel valve  24  is coupled to inner ring  5  and outer ring  9 . Further, the regulation of fuel flow by inner ring fuel value  22  and outer ring fuel valve  24  is limited by turbine controller  50 . Because of the limitation imposed on the fuel control system by the above description, design constraints of the gas turbine which define the dynamic operating range of combustor  3  is also controlled by turbine controller  50 . In the event that NO x  regulator  56  fails, turbine controller  50  continues to control the operation of the gas turbine in a fail safe mode. NO x  regulator  56  is designed to make adjustments only to the fuel flow of inner ring  5  and outer ring  9 . 
     In an alternative embodiment of the present invention NO x  regulator  56  is a stand alone control device  80  rather than being integrated into controller  20 , as illustrated in FIG.  3 . Inner ring fuel adjuster  52  and inner ring fuel valve driver  58  may also be integrated into a single adjuster and inner ring valve driver  72 . Correspondingly, outer ring fuel adjuster  54  and the associated driver  62  may be integrated into outer ring adjuster and driver  70 . In this embodiment NO x  regulator  56  comprises a micro-processor  86 , wherein micro-processor  86  is coupled to at least one digital-to-analog converter  84 , a digital latch  88  to read digital signal  32 , and an analog-to-digital converter  90 , as illustrated in FIG.  4 . Analog-to-digital converter  90  may also be coupled to an analog multiplexer  92  so as to read analog signals  28 ,  30 , and  34 . NO x  regulator  56  may alternatively comprise a digital signal processor having the above described digital and analog functions built in. 
     NO x  regulator  56  operates fuel valves  22  and  24  so as to control the fuel flow to inner ring premixer  18  and outer ring premixer  12 . By regulating fuel flow in this manner the amount of fuel from the outer ring  9 , pilot ring  7 , and inner ring  5  of combustor  3  is regulated. The fuel flow determines fuel-air-ratio in each ring of combustor  3 , which in turn determines the temperature in each ring. Because the temperatures in inner ring  5 , pilot ring  7 , and outer ring  9  determines the amount of NO x  produced by combustor  3 , NO x  regulator  56  acts to control NO x  emissions in combustor  3 , as is further described below. 
     Referring to FIGS. 5 through 7, in which like elements correspond to like numbers, a method for controlling inner ring fuel valve  22  and outer ring fuel valve  24  is described. FIG. 5, illustrates a process flow diagram  100  for determining the near optimum operating point to minimize NO x  emissions in a gas turbine. 
     First, a determination is made as to the operating mode of the gas turbine, that is, whether inner ring  5  and outer ring  9  are under control of turbine controller  50 . Step  102  . Typically, this data is provided to NO x  regulator  56  from turbine controller  50  via fuel valve status line  32  (FIG.  2 ). It is understood that the present invention operates in the mode where inner ring  5 , pilot ring  7 , and outer ring  9  are concurrently being controlled by turbine controller  50 , identified as the ABC mode. Other modes of operation are within the scope of the present invention, such as, modes AB and BC, wherein mode AB is defined as the mode when inner ring valve  22  is shut off and mode BC is defined as the mode when outer ring valve  24  is shut off. 
     Next, a specified number of gas turbine operating points are identified along an operation boundary  156  defined by combustion chamber temperatures at inner ring  5  and outer ring  9 . One exemplary illustration  150  of these operating points are shown, for example, by operating points  160 , 162 , 164 ,  166 , 168 , and  170 , illustrated in FIG.  6 . Each of operating points  160 , 162 , 164 , 166 , 168 , 170  was selected to be disposed along three planes which intersect at nominal operating point  158 . It is understood that any method for selecting the operating points may be utilized. The goal is to select a specified number of operating points disposed along operating boundary  156 . At each of these operating points NO x  levels are measured during normal operation of the gas turbine. The point selection and NO x  measurement process step is illustrated by step  104  of FIG.  5 . The number of boundary points identified is typically at least 6 so as to provide sufficient data points for a second order curve fit analysis to be conducted on the data points. A boundary point  156  is defined as an operating point of the gas turbine engine within a defined preferred operating region. The preferred operating region is defined as the region where a number of conditions are generally met, such as, when the dynamic operating range of combustor  3  is satisfied, when gas turbine acoustic noise is low, when the gas turbine is not in a blow-out range, when CO is low, and when NO x  is below a high upper limit. NO x  typically comprise about 25 parts per million volume exhaust gas. These conditions are generally met when dynamic pressure P x  is within a specified pressure range during operation, typically, between about zero and about 15,000 Pascals. The more specific dynamic pressure range is gas turbine specific, and is typically provided by the gas turbine manufacturer. The non-preferred operating region, which is any area not in the preferred operating region, is to be avoided. 
     FIG. 6 further provides a graphical illustration of the fuel temperature adjustment range of a typical annular combustor of a gas turbine. The range of inner ring temperature adjustment is from about negative 65 degrees Celsius to about positive 65 degrees Celsius above and below a nominal operating ring temperature  158  that is set by turbine control  50 . This inner ring temperature adjustment range is represented by the horizontal axis of FIG.  6 . The range of outer ring temperature adjustment is from about negative 65 degrees Celsius to about positive 65 degrees Celsius above and below nominal operating ring temperature  158 . This outer ring temperature adjustment range is represented by the vertical axis of FIG.  6 . 
     Next, the specified operating points  160 ,  162 , 164 , 166 ,  168 , and  170  are used to generate coefficients a 0  through a n  (in this example n=6) for a curve fit function that relates a resulting polynomial to NO x . This polynomial function is at least a second-order polynomial function having the form identified in equation 1. 
     
       
           NO   x   =a   0   +a   1   x+a   2   y+a   3   xy+a   4   x   2   +a   5   y   2   equation 1  
       
     
     the variable “x” corresponds with the inner ring temperature adjustment  172  and the variable “y” corresponds with the outer ring temperature adjustment  174 , illustrated in FIG.  6 . The curve fit function corresponds with step  106  illustrated in FIG.  5 . Equation 1 is used when NO x  regulator  56  is in the ABC mode. In the AB mode “x”=zero and in the BC mode “y”=zero. 
     Next, the “x” and “y” value where NO x  is minimized is calculated by taking a partial derivative of equation 1 with respect to “x” and a partial derivative with respect to “y”, represented by step  108  of FIG.  5 . The resulting point is defined as the global minimum  157  and is depicted in FIG.  6 . It is noted that the global minimum  157  may be in the non-preferred region. 
     Finally, the gas turbine inner and outer ring temperature adjustments are incremented so that the gas turbine is operating either at the global minimum  157  or near the global minimum  157  within the preferred operating region, step  110  of FIG.  5 . When the gas turbine is operating at near the global minimum NO x  emission levels are reduced in a range from about 10 percent to about 20 percent from levels occurring when the present invention in not utilized. In this specification near global optimum operating point is therefore defined as the operating point in which the NO x  emission levels are reduced in a range from about 10% to about 20% from nominal operating emission levels. The process for operation of the gas turbine at or near the global minimum is further described next. 
     Referring to FIGS. 6 and 7, in which like elements correspond to like numbers, a method for operating the gas turbine at or near global minimum  157 , is further described. 
     It is first appropriate to define an safety margin “r”, which is the margin as measured by inner ring temperature adjustment and outer ring temperature adjustment, in which the gas turbine operates within the preferred region. Safety margin “r” is typically about 15 degrees Celsius. It is understood that the safety margin is dependent upon the accuracy of the control system and the response of the gas turbine and may change as appropriate for any given gas turbine system. 
     Next, the inner ring temperature adjustment and outer ring temperature adjustment are incremented so that the gas turbine operation is adjusted starting at the nominal operating point  158  (FIG.  6 ), along a global margin line extending from nominal operating point  158  to global minimum  157 , as identified by step of FIGS. 5 and 7. At each increment it is determined whether the operating boundary  156  has been crossed, in which case the gas turbine would be operating in the non-preferred region  152 , as identified by step  118  of FIG.  7 . 
     If the operating boundary  156  has not been crossed the gas turbine is adjusted along global margin line by safety margin “r”, as identified by step  120  of FIG.  7 . If operating boundary  156  has again not been crossed it is assumed that the operation of the gas turbine at the safety margin is as close to the global minimum  157  as the safety margin will allow, as such, the gas turbine continues to operate at a inner ring temperature adjustment and outer ring temperature adjustment represented by the safety margin. These process steps are identified by blocks  122  and  126  of FIG.  7 . 
     If the operating boundary  156  was crossed after the gas turbine was incremented by the safety margin in step  120 , the gas turbine is reset to the operating point  171  just before the safety margin, which point is assumed to be near global optimum operating point  157 . These process steps are identified by blocks  122  and  124  in FIG.  7 . 
     Referring to FIGS. 7 and 8, in which like elements correspond to like numbers, the method for determining the minimum NO x  operating point for the gas turbine is further described. If after incrementing the gas turbine as identified in step  112  the operating boundary  156  is crossed, the respective inner and outer combustor ring temperature are reversed by the safety margin along the global margin line  159 , as identified by steps  118  and  128  in FIG.  7 . Next, in step  129  the fuel control system is exercised to generate at least an additional three points of operation wherein NO x  reading are taken to generate operating points  210 , 212 ,  214 , as illustrated in FIG. 8, so that an additional curve fit function may be calculated as a function of the angle θ subtended at  158  with respect to the line segment  210 - 158  using equation 2, as identified by step  130 . 
     
       
           NO   x   =b   0   b   1   θ+b   2 θ 2   equation 2  
       
     
     In equation 2, b 0 , b 1 , and b 2  are coefficients that are calculated using a least squares regression or curve fit function from NO x , values  210 , 212 , and  214 . Angle θ is any one of two specified angles (θ 1  and θ 2 ) defined by the set operating points  158 ,  210 , and  212  (θ 1 ), and  158 ,  210 , and  214  (θ 2 ), as illustrated in FIG.  8 . The purpose of this calculation is to determine a local minimum point of operation  204  along the arc defined by operating points  210 ,  212 , and  214  near the operating boundary  156 . The derivative with respect to the θ angle is determined and solved for NO x  equal zero. From this calculated value an angle θ corresponding to local minimum NO x  point  204  is identified, as illustrated by step  132 . A local minimum line  202  is next determined, wherein local minimum line  202  is defined as the line between nominal operating point  158  and the local minimum point  204  step  134 . The gas turbine is incremented from nominal operating point  158  along local minimum line  202  until the operating point  222  step  136 , which is a safety margin “r” away from the local minimum point  204  step  138 . Operating point  171  is the near global operating point of the gas turbine. The near global operating point is defined as the operating point at which NO x  emission levels are reduced in a range from about 10 percent to about 20 percent from nominal NO x  emission levels when the gas turbine is operating and the present invention is not utilized. 
     The methods described herein may be used to optimize the output of any chemical reactor which has several parameters that can be controlled and several output variables. The method described herein which reduce NO x  emissions by optimizing ring flame temperatures can also be used simultaneously with Carbon Monoxide reducing algorithms, such as those described in U.S. Pat. Nos. 4,910,957 and 4,928,481. 
     It will be apparent to those skilled in the art that, while the invention has been illustrated and described herein in accordance with the patent statutes, modifications and changes may be made in the disclosed embodiments without departing from the true spirit and scope of the invention. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.