GAS TURBINE FUEL BLENDING USING INFERRED FUEL COMPOSITIONS

Abstract

A system, method, and computer-readable medium for blending a fuel for use in a gas turbine are disclosed. A measurement of a heating value of a process gas and a measurement of a molecular weight of the process gas is obtained. An estimate of a composition of the process gas is obtained using the obtained measurement of the heating value and the obtained measurement of the molecular weight. A blending ratio of the process gas and a natural gas is selected based on the estimate of the composition of the process gas. The process gas and the natural gas are then blended according to the selected blending ratio to obtain a fuel mixture for use in the gas turbine.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to fuel blending for using in a gas turbine, and more specifically, to determining a fuel mixture for use in a gas turbine.

[0002] Gas turbine fuel supply systems blend natural gas with a process gas flowing to obtain a fuel mixture that can be supplied to the gas turbine. The process gas generally includes various gas components in varying and unknown amounts. Blending the process gas with the natural gas in selected ratios controls the composition of the fuel mixture so that it is suitable for use at the gas turbine. However, in order to control the composition of the fuel mixture, the composition of the process gas needs to be determined. One method for determining the composition of the process gas involves gas chromatography. However, gas chromatography takes several minutes to perform. In addition, before proceeding based on gas chromatography measurements, several gas chromatography measurements are often made in order to validate the consistency of the measurements. Thus, using gas chromatography to determine process gas composition wastes process gas and delays the time for which a selected or desired fuel mixture may be provided to the gas turbine.

BRIEF DESCRIPTION OF THE INVENTION

[0003] According to one embodiment of the present invention, a method of blending fuel for use in a gas turbine includes: obtaining a measurement of a heating value of a process gas and a measurement of a molecular weight of the process gas; obtaining an estimate of a composition of the process gas using the obtained measurement of the heating value and the obtained measurement of the molecular weight; selecting a blending ratio of the process gas and a natural gas based on the estimate of the composition of the process gas; and blending the process gas and the natural gas according to the selected blending ratio to obtain a fuel mixture for use in the gas turbine.

[0004] According to another embodiment of the present invention, a system for blending fuel for use in a gas turbine includes: a device configured to obtain a measurement of a heating value of a process gas and a measurement of a molecular weight of the process gas; and a processor configured to: obtain an estimate of a composition of the process gas using the obtained measurement of the heating value and obtained measurement of the molecular weight; select a blending ratio of the process gas and a natural gas based on the estimate of the composition of the process gas; and blend the process gas and the natural gas according to the blending ratio to obtain a fuel mixture for use in the gas turbine.

[0005] According to another embodiment of the present invention, a non-transitory computer-readable medium includes a set of instructions stored thereon that when accessed by a processor enable the processor to perform a method for blending a fuel for use in a gas turbine, the method including: obtaining a measurement of a heating value of a process gas and a measurement of a molecular weight of the process gas from a measurement device; obtaining an estimate of a composition of the process gas using the obtained measurement of the heating value and the obtained measurement of the molecular weight; selecting a blending ratio of the process gas and a natural gas based on the estimate of the composition of the process gas; and blending the process gas and the natural gas according to the blending ratio to obtain a fuel mixture for use in the gas turbine.

[0006] Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

[0008] FIG. 1 shows a schematic diagram of a system for generating power and/or electricity using a gas turbine;

[0009] FIG. 2 shows a detailed view of a fuel supply system that provides fuel to the gas turbine of FIG. 1;

[0010] FIG. 3 shows a flowchart illustrating a method for performing a combustion operation according to the present invention; and

[0011] FIG. 4 shows a flowchart illustrating a method of obtaining a first fuel mixture according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0012] FIG. 1 shows a schematic diagram 100 of a system for generating power and/or electricity using a gas turbine 102. The diagram 100 shows gas turbine 102 including a compressor 104, a combustor 106 and a turbine stage 108. Ambient air 112 is received at the compressor 104 and compressed to a selected air pressure. The combustor 106 receives the compressed air from the compressor 104 and mixes the compressed air with a fuel mixture supplied to the combustor 106 by a fuel supply system 110. The combustor 106 ignites the air-fuel mixture to produce a working gas. The working gas is exhausted through the turbine stage 108 in order to produce a rotation of a rotor 114 of the turbine stage 108. The rotor 114 is coupled to a generator 116 so that rotation of the rotor 114 generates electricity at the generator 116.

[0013] FIG. 2 shows a detailed view of the fuel supply system 110 that provides fuel to the gas turbine 102 of FIG. 1. The fuel supply system 110 receives process gas 202 from a process gas line 203 and natural gas 204 from a natural gas line 205. The fuel supply system 110 further outputs blended mixtures of the natural gas and the process gas to one or more fuel circuits (206, 208) of the gas turbine 102. For illustrative purposes, the fuel supply system 110 is shown with a first fuel circuit 206 and a second fuel circuit 208. However, the fuel supply system 110 may include any number of fuel circuits in various embodiments. The natural gas 204 is coupled to the first fuel circuit 206 via gas control valve 210. The natural gas 204 is coupled to the second fuel circuit 208 via gas control valve 212. The process gas 202 is coupled to the first fuel circuit 206 via gas control valve 214. The process gas 202 is coupled to the second fuel circuit 208 via gas control valve 216. The gas control valves 210, 212, 214 and 216 are coupled to a control unit 220 that controls the states of the gas control valves 210, 212, 214 and 216. In various embodiments, the control unit 220 controls the states of the gas control valves 210, 212, 214 and 216 independently of each other. The gas control valves 210, 212, 214 and 216 may thus be operated to control a blending of the process gas 202 and the natural gas 204 in the fuel mixtures in the first fuel circuit 206 and/or in the second fuel circuit 208.

[0014] The control unit 220 includes a processor 222 and a memory storage device 224. The memory storage device 224 may include a suitable non-transitory computer-readable medium, such as a solid-state memory device, read-only memory device, etc. The memory storage device 224 may include programs 226 that may be accessed by the processor 222 to perform the various methods disclosed herein. In addition, the processor 222 may store various parameters and/or calculated values at the memory storage device 224.

[0015] Natural gas line 205 includes a natural gas Wobbe meter 230 that measures a quality of the natural gas 204. The natural gas Wobbe meter 230 measures a heating value and molecular weight of the natural gas 204 in the natural gas line 205. The natural gas Wobbe meter 230 can obtain measurements in a relatively short amount of time, such as within about 30 seconds.

[0016] Process gas line 203 includes a process gas Wobbe meter 232, a process gas analyzer 234 and a gas chromatograph 236. The process gas Wobbe meter 232 obtains measurements of heating value and molecular weight of the process gas 202. The process gas analyzer 234 also obtains measurements of heating value and a molecular weight of the process gas 202. The gas chromatograph 236 determines a composition of the process gas 202. The process gas Wobbe meter 232 obtains its measurements in a relatively short amount of time (about 30 seconds). The process gas analyzer 234 obtains its measurements within an intermediate amount of time (about 2 minutes) and the gas chromatograph 236 obtains composition in a relatively long amount of time (about 5 minutes). While the process gas Wobbe meter 232 can obtain measurements before either the process gas analyzer 234 or the gas chromatograph 236, such measurements may not be suitable for fully determining a composition of the process gas 202. Nonetheless, the process gas Wobbe meter 232 may be used to obtain a first estimate of the composition of the process gas 202, using the methods disclosed herein.

[0017] The natural gas Wobbe meter 230, process gas Wobbe meter 232, the process gas analyzer 234 and the gas chromatograph 236 are coupled to the control unit 220 and provide measurements to the control unit 220 for processing. The control unit 220 performs the methods disclosed herein for determining suitable blending ratio of the natural gas 204 and the process gas 202 in order to obtain a fuel mixture that has an estimated composition that is based on measurements of the heating values and molecular weights of at least the process gas 202. The control unit 220 further controls the gas control valves 210, 212, 214 and 216 to achieve the appropriate blending ratio of the process gas 202 and the natural gas 204 to obtain the estimated fuel mixture composition at either of the first fuel circuit 206 and the second fuel circuit 208. Methods for determining the suitable blending ratio for the fuel mixture are described below.

[0018] FIG. 3 shows a flowchart 300 illustrating a method for performing a combustion operation according to the present invention. In block 302, a first fuel mixture is supplied to the gas turbine. The first fuel mixture is obtained by blending process gas 202 and natural gas 204. The composition of the first fuel mixture is determined based on a first estimate of a composition of the process gas 202. The first estimate of the composition of the process gas 202 is obtained using the measurements of heating value and molecular weight from the process gas Wobbe meter 232. Details of the method for determining the first fuel mixture are discussed with respect to the Eqs. (1)-(5) below. In block 304, a second fuel mixture is supplied to the gas turbine. The second fuel mixture is obtained using by blending process gas 202 and natural gas 204 based on a second estimate of the composition of the process gas. The second estimate of the composition of the process gas 202 is based on the measurements of heating value and molecular weight of the process gas 202 obtained using the process gas analyzer 234. Details of the method for obtaining the second fuel mixture are discussed with respect to Eqns. (6)-(10) below. In block 306, a third (operating) fuel mixture is supplied to the gas turbine 102. The third fuel mixture is based on the composition of the process gas 202 obtained using the gas chromatograph 236.

[0019] FIG. 4 shows a flowchart 400 illustrating a method of obtaining the first fuel mixture according to one embodiment. In an exemplary embodiment, the method may be used at a startup of the gas turbine 102. In block 402, measurements of heating value and molecular weight for a process gas 202 are obtained using the process gas Wobbe meter 232. The measurements may be obtained several times in order to ensure consistent values of the measurements. In block 404, a first estimate of a composition of the process gas 202 is determined using the obtained measurement of the heating value and molecular weight of the process gas 202. The methods for obtaining the first estimate of the composition are discussed below with respect to equations (1)-(5). In block 406, a blending ratio for the process gas 202 and a natural gas 204 is determined using the determined first estimate of the composition of the process gas. In block 408, the process gas and the natural gas are blended (by controlling valves 210, 212, 214, 216) according to the selected blending ratio to obtain the first fuel mixture. In block 410, the first fuel mixture is supplied to the gas turbine.

[0020] The process gas 202 includes a composition of a plurality of gases (e.g. hydrogen, ethane, ethylene, propane, nitrogen, etc.) The molar fraction of these component gases in the process gas 202 are unknown and are represented as X₀, X₁, X₂, X₃ and X₄ and are constrained by Eqns. (1)-(5) below:

X 0 + X 1 + X 2 + X 3 + X 4 = 1 Eq . ( 1 ) X 0 M W 0 + X 1 M W 1 + X 2 M W 2 + X 3 M W 3 + X 4 M W 4 = M W mix Eq . ( 2 ) X 0 M W 0 LHV 0 + X 1 M W 1 LHV 1 + X 2 M W 2 LHV 2 + X 3 M W 3 LHV 3 + X 4 M W 4 LHV 4 = M W mix LHV mix Eq . ( 3 ) α = X 2 X 1 Eq . ( 4 ) β = X 3 X 1 Eq . ( 5 ) ##EQU00001##

wherein MW₀, MW₁, MW₂, MW₃ and MW₄ are molecular weights, LHV₀, LHV₁, LHV₂, LHV₃ and LHV₄ are heating values of the gas components of the process gas 202. The molecular weights (MW₀, MW₁, MW₂, MW₃, MW₄) and heating values (LHV₀, LHV₁, LHV₂, LHV₃, LHV₄) are known values. Parameters α and β are mixing parameters of the process gas 202 and can be tuned by an operator. The process gas Wobbe meter 232 obtains measurements of MW_mix and LHV_mix (i.e., the molecular weight and heating value, respectively, of the process gas 202) Eqns. (1)-(5) are a set of five linear equations with five unknowns (i.e., molar fractions). The fractional amounts X₀, X₁, X₂, X₃ and X₄ and thus the composition of the process gas 202 may be determined by solving the Eqns. (1)-(5). It is understood that the process gas 202 may include additional gases beyond the five gases included in equations (1)-(5). Thus, the composition of the process gas 202 determined using Eqns. (1)-(5) provides a first estimate of the composition of the process gas 202. Once the first estimate of the composition of the process gas 202 is determined, the process gas 202 and the natural gas 204 can be blended according to a selected blending ratio to obtain a first fuel mixture having a selected fuel composition at one or more of the first fuel circuit 206 and the second fuel circuit 208. Eqs. (1)-(5) may also be used with respect to measurements of the molecular weight and heating value of the natural gas to determine an estimate of the composition of the natural gas 204. The blending ratio of the first fuel mixture may be determined using the estimated composition of the process gas 202 and the estimated composition of the natural gas 204. Also, additional measurements obtained using the process gas Wobbe meter 232 and/or the natural gas Wobbe meter 230 may be used to adjust the blending ratio of the first fuel mixture.

[0021] After a selected amount of time, measurements from the process gas analyzer 234 are available to the control unit 220. Thus, the measurements from the process gas analyzer 234 can be used in place of the measurements from the process gas Wobbe meter 232. The process gas analyzer 234 provides measurements of heating value and molecular weight of the process gas 202. Such measurements may be taken several times in order to ensure that consistent values of the measurements are being obtained. The measured values may be used along with another set of linear equations (Eqns. (6)-(10)) which may be used to give a second estimate of the composition of the process gas 202. Equations (6)-(10) include molar fractions X_A and X_B of additional constituents of the process gas. Molar fractions X_A and X_B, and their molecular weight (MW_A, MW_B) and heating values (LHV_A, LHV_B), are known quantities. Therefore, Eqns. (6)-(10) are five equations of five unknown variables and can be solved to obtain the second estimate of the composition of the process gas 202.

X 0 + X 1 + X 2 + X 3 + X 4 + X A + X B = 1 Eq . ( 6 ) X 0 M W 0 + X 1 M W 1 + X 2 M W 2 + X 3 M W 3 + X 4 M W 4 + X A M W A + X B M W B = M W mix Eq . ( 7 ) X 0 M W 0 LHV 0 + X 1 M W 1 LHV 1 + X 2 M W 2 LHV 2 + X 3 M W 3 LHV 3 + X 4 M W 4 LHV 4 + X A M W A LHV A + X B M W B LHV B = M W mix LHV mix Eq . ( 8 ) α = X 2 X 1 Eq . ( 9 ) β = X 3 X 1 Eq . ( 10 ) ##EQU00002##

[0022] The second estimate of the process gas composition, obtained using Eqns. (6)-(10) provides a closer approximation of the process gas composition than the first estimate obtained using Eqns. (1)-(5). The second estimate of the composition of the process gas 202 may be used at the control unit 220 to control the blending ratio of the process gas 202 and the natural gas 204 to obtain a second fuel mixture at one or both of the first fuel circuit 206 and the second fuel circuit 208. While Eqns. (6)-(10) show two additional molar fractions X_A and X_B, any number of additional known molar fractions may be used in Eqns. (6)-(10).

[0023] Finally, once the composition of the process gas 202 has been determined using the gas chromatograph 236, this composition may be used to blend the process gas 202 and the natural gas 204 to obtain a third (operational) fuel mixture at one or both of the first fuel circuit 206 and the second fuel circuit 208. The measurements from the gas chromatograph 236 can be taken several times in order to ensure consistent values of the measurements.

[0024] While the invention is discussed with respect to determining a composition of the process gas 202, it is also possible to determine a composition of the natural gas 204 using the methods disclosed herein. For example, measurements from natural gas Wobbe meter 230 may be used along with the Eqns. (1)-(5) to obtain the composition of the natural gas 204. Knowledge of the composition of the natural gas 204 and the composition of the process gas 202 may be used to determine blending ratios for the first fuel mixture and the second fuel mixture.

[0025] While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A method of blending a fuel for use in a gas turbine, comprising: obtaining a measurement of a heating value of a process gas and a measurement of a molecular weight of the process gas; obtaining an estimate of a composition of the process gas using the obtained measurement of the heating value and the obtained measurement of the molecular weight; selecting a blending ratio of the process gas and a natural gas based on the estimate of the composition of the process gas; and blending the process gas and the natural gas according to the selected blending ratio to obtain a fuel mixture for use in the gas turbine.

2. The method of claim 1, further comprising obtaining the estimate of the composition of the process gas using a set of linear equations relating the composition of the process gas to the measurement of the heating value of the process gas and the measurement of the molecular weight of the process gas.

3. The method of claim 2, wherein the set of linear equations includes a set of five linear equations relating five unknown molar fractions representing component gases of the process gas.

4. The method of claim 3, wherein the set of linear equations further including at least one additional molar fraction representing an additional component gas of the process gas, wherein the at least one additional molar fraction is a known quantity.

5. The method of claim 1 further comprising obtaining the measurement of the heating value of the process gas and the measurement of the molecular weight of the process gas using at least one of: (i) a Wobbe meter; and (ii) a process gas analyzer.

6. The method of claim 1, further comprising controlling the blending ratio of the process gas and the natural gas based on additional measurements of the heating value and molecular weight of the process gas.

7. The method of claim 1, further comprising determining a composition of the natural gas using a set of linear equations relating the composition of the natural gas to a measurement of a heating value of the natural gas and a measurement of a molecular weight of the natural gas.

8. A system for blending fuel for use in a gas turbine, comprising: a device configured to obtain a measurement of a heating value of a process gas and a measurement of a molecular weight of the process gas; and a processor configured to: obtain an estimate of a composition of the process gas using the obtained measurement of the heating value and obtained measurement of the molecular weight; select a blending ratio of the process gas and a natural gas based on the estimate of the composition of the process gas; and blend the process gas and the natural gas according to the blending ratio to obtain a fuel mixture for use in the gas turbine.

9. The system of claim 8, wherein the processor is further configured to obtain the estimate of the composition of the process gas using a set of linear equations relating the composition of the process gas to the measurement of the heating value and the measurement of the molecular weight.

10. The system of claim 9, wherein the set of linear equations includes a set of five linear equations relating five unknown molar fractions representing component gases of the process gas.

11. The system of claim 10, wherein the set of linear equations further includes at least one additional molar fraction representing an additional component gas of the process gas, wherein the at least one additional molar fraction is a known quantity.

12. The system of claim 8, wherein the device further comprises at least one of: (i) a Wobbe meter; and (ii) a process gas analyzer.

13. The system of claim 8, wherein the processor is further configured to control the blending ratio of the process gas and the natural gas based on additional measurements of the heating value and molecular weight of the process gas.

14. The system of claim 8, wherein the processor is further configured to determine a composition of the natural gas using a set of linear equations relating the composition of the natural gas to a measurement of a heating value of the natural gas and a measurement of a molecular weight of the natural gas.

15. A non-transitory computer-readable medium including a set of instructions stored thereon that when accessed by a processor enable the processor to perform a method for blending fuel for use in a gas turbine, the method comprising: obtaining a measurement of a heating value of a process gas and a measurement of a molecular weight of the process gas from a measurement device; obtaining an estimate of a composition of the process gas using the obtained measurement of the heating value and the obtained measurement of the molecular weight; selecting a blending ratio of the process gas and a natural gas based on the estimate of the composition of the process gas; and blending the process gas and the natural gas according to the blending ratio to obtain a fuel mixture for use in the gas turbine.

16. The non-transitory computer-readable medium of claim 15, wherein the method further comprises obtaining the estimate of the composition of the process gas using a set of linear equations relating the composition of the process gas to the measurement of the heating value and the measurement of the molecular weight.

17. The non-transitory computer-readable medium of claim 16, wherein the set of linear equations includes a set of five linear equations relating five unknown molar fractions representing component gases of the process gas.

18. The non-transitory computer-readable medium of claim 17, wherein the set of linear equations further includes at least one additional molar fraction representing an additional component gas of the process gas, wherein the at least one additional molar fraction is a known quantity.

19. The non-transitory computer-readable medium of claim 15, wherein the method further comprises obtaining the measurement of the heating value of the process gas and the measurement of the molecular weight of the process gas from at least one of: (i) a Wobbe meter; and (ii) a process gas analyzer.

20. The non-transitory computer-readable medium of claim 15, wherein the method further comprises determining a composition of the natural gas using a set of linear equations relating the composition of the natural gas to a measurement of a heating value of the natural gas and a measurement of a molecular weight of the natural gas.

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