# Patent

## Title
Method for operating a gas turbine and gas turbine system for carrying out the method

## Abstract

A method for operating a gas turbine uses a first fuel at full load. An improved emission behavior is achieved by operating the gas turbine (11) at partial load with a second fuel, which has a richer mix of higher-value hydrocarbons (C2+) with 2 and more carbon atoms per molecule like ethane (C2H6) and propane (C3H8).

## Background
This application claims priority under 35 U.S.C. § 119 to German application number 103 45 566.3, filed 29 Sep. 2003, the entirety of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the technical field of gas turbines. It concerns a method for operating a gas turbine, as well as a gas turbine system for carrying out the method.
2. Brief Description of the Related Art
Usually burners such as the so-called EV burner (double cone burner, ref., for example, U.S. Pat. No. 5,081,844) and the so-called SEV burner (secondary EV burner, ref., for example, an article by F. Joos et al. Development of the sequential combustion system for the GT24/GT26 gas turbine family, ABB Review 4, p. 4-16 (1998) or U.S. Pat. No. 5,454,220) used in gas turbines are designed for full load operation. Nonetheless it might be necessary to operate the gas turbine at partial load. This might be because the network cannot absorb the generated energy or the gas turbine is started up or shut down (repairs, etc.). If no special measures are taken, partial load operation results in an unfavorable emission behavior with regard to CO and NOx. Another problem with partial load operation of a gas turbine concerns the residue of the relatively cool exhaust gas in relatively short combustion chambers. Cold gases and short dwell times result in low NOx emissions, but unfortunately also in increased CO/UHC emissions.
Publication U.S. Pat. No. 5,081,844 mentioned in the introduction proposes addressing the issue of emissions with partial loads by alternating the arrangement of small and large premix burners and having the small premix burners function as pilot burners. Publication U.S. Pat. No. 5,454,220 mentioned in the introduction proposes this procedure for gas turbines with SEV burners.
The disadvantage with this kind of emissions optimization for partial load operation is the fact that the burners in the main combustion chamber must be specially configured in order to allow for the corresponding operation. This procedure is difficult to realize in retrospect in existing gas turbines without any special burner arrangement, and thus it is difficult to eliminate emissions issues during partial load operation.
SUMMARY OF THE INVENTION
One aspect of the present invention therefore is providing a method for operating a gas turbine that results in a material improvement of the emission behavior during partial load operation without any structural changes to the gas turbine itself, as well as to provide a gas turbine system for carrying out the method.
A principle of the invention is that the gas turbine that uses a first fuel at full load uses a second fuel at partial load to improve the emission behavior, with the second fuel having a richer mix of higher-value hydrocarbons (C2+) with 2 or more carbon atoms per molecule like ethane (C2H6) and propane (C3H8) compared to the first fuel (the higher-value carbons are hereinafter referred to as C2+).
In an exemplary manner gaseous fuels are used as a first and second fuel.
As a rule it is possible to use two separate gas sources for providing the first and the second fuel with the gas sources having different gas composition with regard to the higher-value hydrocarbons (C2+), whereby the two fuels are taken directly from the gas sources so that there is a switch between the gas source with the lower share of higher-value hydrocarbons (C2+) and the gas source with the higher share of high-value gas carbons (C2+) when there is a switch from full to partial load. As an alternative it is possible to generate the two fuels based on a different mixture of the gases in the two gas sources.
An alternative is to produce the first fuel from the second fuel by separating higher-value hydrocarbons (C2+). Especially favorable is a method in which the separated, higher-value hydrocarbons (C2+) are placed in intermediate storage and are added to the second fuel during partial load operation. Due to space issues it might be beneficial to liquefy the separated, higher-value hydrocarbons (C2+) prior to intermediate storage, to intermediately store them as a liquid gas and to evaporate them prior to adding.
Another alternative is characterized in that the second fuel is produced from the first fuel by adding higher-value hydrocarbons (C2+). The added, higher-value hydrocarbons can be taken from a local reservoir that is either a liquid gas storage tank, whereby the liquid gas taken from the liquid gas storage tank is evaporated in an evaporator prior to adding, or is a gas reservoir.
Furthermore it is possible to generate the first and second fuel based on a third fuel whose content of higher-value hydrocarbons (C2+) ranges between the first and the second fuel. The first and second fuel is generated from the third fuel by separating and adding higher-value hydrocarbons (C2+). The higher-value hydrocarbons (C2+) that are separated from the third fuel when generating the first fuel are, in an exemplary embodiment, placed in intermediate storage, are taken from storage when generating the second fuel and are added to the third fuel. Due to spatial restrictions it might again be beneficial to liquefy the separated, higher-value hydrocarbons (C2+) prior to placing them in intermediate storage, to store them as a liquid gas and to evaporate them prior to adding.
Exemplarily, the share of higher-value hydrocarbons (C2+) for the second fuel is approximately 10% to 30% higher than in the first fuel.
An exemplary embodiment of the gas turbine facility in accordance with the principles of the present invention is characterized in that the means for a controlled change of the composition of the fuel flowing through the main fuel feeding line comprise an auxiliary fuel feeding line that discharges into the main fuel feeding line, and that is connected to a reservoir that contains higher-value hydrocarbons (C2+). A controllable valve is arranged in the auxiliary fuel feeding line for controlling the gas stream that flows through the auxiliary fuel feeding line.
The reservoir that contains the higher-value hydrocarbons (C2+) can be a gas reservoir. It can also be a liquid gas reservoir, whereby an evaporator is arranged in the auxiliary fuel feeding line.
Another exemplary embodiment according to principles of the present invention includes a gas separating mechanism arranged in the main fuel feeding line that separates higher-value hydrocarbons (C2+) from the gas that flows through the main fuel feeding line and provides them to the reservoir that contains higher-value hydrocarbons (C2+). Much space is saved when the reservoir containing the higher-value hydrocarbons (C2+) is a liquid gas reservoir, and a gas liquefying mechanism is arranged between the gas separating mechanism and the liquid gas reservoir.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in more detail based on the exemplary embodiments in connection with the drawing. The figures show the following:
FIG. 1 shows a simplified scheme of a first exemplary embodiment of a gas turbine system in accordance with the invention that can be used to carry out the method in accordance with the invention;
FIG. 2 shows a simplified scheme of a second exemplary embodiment of a gas turbine system in accordance with the invention with the respective load-based control that can be used to carry out the method in accordance with the invention; and
FIG. 3 shows an enlarged section of the control blocks of the load-based control shown in FIG. 2.



DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
One principle of the present invention recognizes the fact that the emission behavior of a gas turbine can be positively influenced by adding C2H6 and higher molecular hydrocarbons (C2+). It is specifically CO emissions that can be kept below the required emission values across a much larger partial load range than is currently possible. Furthermore, this also has a positive influence on the extinguishing limit, which in turn results in lower NOx emissions.
A suitable gas turbine system is shown in a simplified scheme in FIG. 1. The gas turbine system 10 comprises a gas turbine 11 that is comprised, as is customary, of a compressor 12, a combustion chamber and a turbine 14. Compressor 12 and turbine 14 are arranged on a mutual shaft 15 that is used to drive a generator, not shown in FIG. 1, for power generation purposes. During operation the compressor 12 intakes the air, compresses it and feeds it into the combustion chamber 13, where it is mixed with a, for example, gaseous fuel, and the fuel is combusted. The hot combustion gases from the combustion chamber are expanded in the turbine with thermodynamic performance and then are reused (e.g., in Heat Recovery Steam Generator, HRSG) or are released to the outside. The fuel required in the combustion chamber 13 is taken from a fuel line 16 (e.g., a natural gas pipeline) and is supplied to the combustion chamber 13 via a main fuel feeding line 17. The supply can be controlled using a first controllable valve 18.
An auxiliary fuel feeding line 19 discharges into the main fuel feeding line 17. It is connected to a liquid gas storage tank 22 via a second controllable valve 20 and an evaporator 21. The liquid gas storage tank 22 contains liquid gas 23 in the form of liquefied, higher-value hydrocarbons (C2+). The liquid gas storage tank 22 can either be filled from the outside or—as is indicated by the dash-doted line in FIG. 1—can be connected to a gas separating mechanism 27 via a gas liquefying mechanism 28. The gas separating mechanism 27 is arranged in the main fuel feeding line 17, and its design is such that it can withdraw a set amount of the higher-value hydrocarbons (C2+) from the gaseous fuel that flows through the main fuel feeding line 17.
As an alternative to the liquid gas storage tank 22 it is possible to use a (e.g., underground) gas reservoir 24 as a reservoir for higher-value hydrocarbons. It can be filled via a filling port 26 and a liquid gas tanker, for example, with subsequent evaporation.
The operation of the gas turbine system 10 can be described as follows: The gas turbine 11 is started with the customarily available, usually gaseous fuel from the fuel feeding line and is connected to the network using the so-called pilot operation. As soon as the gas turbine 11 is coupled to the network and provides increasing power to the network, higher-value hydrocarbons (ethane C2H6, propane C3H8, etc.) are added to the fuel. This additive of so-called C2+ gas (with 2 and more C-atoms) is guided from the liquid gas storage tank 22 via the evaporator 21 and is added to the main fuel. A C2+ gas share of approximately 10 to 30% is added in a controlled manner. As an alternative it is possible to add the C2+ gas from the gas reservoir 24 to the fuel in a controlled manner.
If a flame temperature is reached that is accordingly high, operation is switched to the so-called premix operation. The so-called switching temperature at which operation can be switched to the premix operation strongly depends on the (C2+) gas content at which the gas turbine 11 is operated. The higher the (C2+) gas content, the lower the switching temperature can be set. The advantage of this operating procedure is that the load range with premix operation and thus with low NOx emission can be increased significantly by adding (C2+) gas. The result is that the operation of the gas turbine 11 significantly reduces the entire NOx emissions.
As already mentioned in the introduction, another issue with partial load operation of a gas turbine is the burn out of the relatively cool exhaust gases in relatively short combustion chambers. Cold gases and short dwell times result in low NOx emissions, but unfortunately also in increased CO/UHC emissions (UHC=Unburned Hydro Carbons).
Here, too, it is found that adding (C2+) gas to the fuel results in a faster burn out of the exhaust gases. Despite relatively short combustion chambers with correspondingly short dwell times, the toxic carbon monoxide is reduced to nontoxic CO2 over a considerably larger partial load range.
With increasing load the combustion temperatures approach full load operating temperatures. The combustion chamber 13 that is designed for full load operation now has sufficiently high temperatures, and the burnout from CO to CO2 occurs in a period of time that is shorter than the dwell time of the exhaust gases in the combustion chamber 13. The (C2+) gas content in the fuel can be accordingly reduced with increasing load until no more (C2+) gas from the liquid gas storage tank 22 or the gas reservoir 24 must be added.
The filling of a liquid gas storage tank 22 is the easiest. Customary infrastructure for filling liquid gas storage tanks is known and does not require any additional explanation at this point (supply of liquid gas with tanker/truck etc.). A gas reservoir 24 can be supplied with liquid gas as well. Before charging the gas reservoir 24 via the filling port 26, the liquid gas is returned to the gaseous state using an evaporator.
However, a different method is also feasible for a gas turbine system 10 according to FIG. 1: Natural gas as main fuel for gas turbines may have different compositions depending on the location where the natural gas is produced. Each gas displays a different combustion behavior that is based on its origin and composition. Not all “gas types” combust equally. However, there are regulations that require that emissions be within tight limits across the largest possible operating ranges.
This requirement cannot be met if different gases are combusted in the same manner. On the other hand it is possible to optimally design and operate the combustion system for a specifically defined gas. However, the gas to be combusted must correspond to the respective definition used for the design. This can be achieved by separating the fuel gas. To this end, a gas separating mechanism 27 (dashed line in FIG. 1) is integrated in the main fuel feeding line 17 to the gas turbine 11. It ensures that a fuel gas that meets the specification is added to the gas turbine 11. Any excess of (C2+) gas is withdrawn from the gas when the gas turbine 11 is operated at full load. The excess is liquefied in a gas liquefying mechanism 28 and is placed in intermediate storage in the liquid gas storage tank 22 via a filling line 29 until the gas turbine 111 is/must be operated in the partial load operation, and a lack of (C2+) must be offset instead of a surplus of (C2+). The withdrawn gas can also be sent to a suitable underground gas reservoir 24 via a feeding line 25. The gas reservoir 24 or liquid gas storage tank 22 is unloaded during the corresponding partial load operation. The size of the intermediate reservoir 22, 24 must be chosen based on the anticipated partial load and full load hours and the gas composition,
If different gas sources with different gas compositions are used for operating the gas turbine, it is furthermore possible to use smaller intermediate storage (liquid gas storage tank or gas reservoir). During partial load operation the gas turbine is operated with the (C2+) richer fuel gas with empty intermediate storage, and at full load (or correspondingly high partial load) operation is switched to the (C2+) poorer gas source, or the gas turbine is operated with the C2+ rich gas during full load and excess (C2+) gas is separated via a gas separating mechanism again and stored in intermediate storage. When operating with two gas sources, it might be advantageous from time to time to mix the two gases at a certain ratio and to add the mixture to the gas turbine according to the (C2+) content that is required at the time and in order to be able to optimally operate the respective operating point (low CO and low NOx emissions).
An example of a gas turbine system that works with two gas sources is shown in FIG. 2. The gas turbine system 30 of FIG. 2 comprises a gas turbine 31 with a compressor 32, two combustion chambers 35 and 36, a first turbine 33 arranged between them and a second turbine 34. The turbines 33, 34 and the compressor 32 are arranged on a mutual shaft 37 that drives a generator 38. The configuration corresponds to the one shown in U.S. Pat. No. 5,454,220. The combustion chambers 35, 36 are supplied with gaseous fuels via a main fuel feeding line 51, 52 from two gas sources in the form of two gas pipelines GPI and GPII. It is possible to produce a predefined mixture of both fuels in a preceding mixing device 46 with two control valves 47, 48, or it is possible to switch to a different fuel. (C2+) shares can be separated in a subsequent gas separating mechanism 45 and can be placed in a reservoir 43 via a compressor 42. The separated fuel is compressed in a compressor 41 and guided to the combustion chambers 35, 36 via the two main fuel feeding lines 51, 52. On each of the two main fuel feeding lines 51, 52 it is possible to add higher-value hydrocarbons (C2+) from the reservoir 43 via an auxiliary fuel feeding line 53, 54 and a system of nonreturn valves 40a, . . . ,d and control valves CV3, CV4. The fuel mass flow rate in the respective main fuel feeding line 51, 52 can be controlled with a control valve CV1 and/or CV2. The reservoir 43 can also be filled externally from a filling station 44.
The various fuel mass flow rates indicated in FIG. 2 and measured at the respective locations ({dot over (m)}x, x=I,II,1,2,CV1, . . . ,CV4;C2+ext,Res) are controlled with a load controller 39 via two control devices 49, 50 based on the power or load required from the network, with the devices on one hand controlling the entire fuel mass flow rate per combustion chamber 35, 36 via control valves CV1 and CV2, and on the other hand controlling the adding of (C2+) shares to the fuel via control valves CV3 and CV4. The respective exemplary control curves are indicated in the enlarged presentation of the control devices 49, 50 in FIG. 3. The following applies:

{dot over (m)}CV1=f(load)

{dot over (m)}CV2=f(load)

{dot over (m)}I+{dot over (m)}II−{dot over (m)}C2+ext={dot over (m)}CV1+{dot over (m)}CV2 

{dot over (m)}Res={dot over (m)}C2+ext−{dot over (m)}CV3−{dot over (m)}CV4 

mRes=∫{dot over (m)}res·dt. 

List of Reference Numerals
















10, 30
gas turbine system



11, 31
gas turbine



12, 32
compressor



13, 35, 36
combustion chamber



14, 33, 34
turbine



15, 37
shaft



16
fuel line



17, 51, 52
main fuel feeding line



18, 20
valve



19, 53, 54
auxiliary fuel feeding line



21
evaporator



22
liquid gas storage tank



23
liquid gas



24
gas reservoir



25
feeding line



26
filling port



27, 45
gas separating mechanism



28
gas liquefying mechanism



29
filling line



38
generator



39
load controller



40a, . . . , d
nonreturn valve



41, 42
compressor



43
reservoir



44
filling station



46
mixing device



47, 48
control valve



49, 50
control device



CV1, . . . , CV4
control valve



GPI, GPII
gas pipeline










While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents is incorporated by reference herein in its entirety.

## Claims

1. A method for operating a gas turbine using a first fuel at full load, the method comprising:
operating the gas turbine during partial load operation with a second fuel instead of the first fuel to achieve an improved emission behavior, with the second fuel having a richer mix of higher-value hydrocarbons with 2 or more carbon atoms per molecule than said first fuel.

2. A method according to claim 1, wherein the share of higher-value hydrocarbons is approximately 10% to 30% higher in the second fuel than in the first fuel.

3. A method according to claim 1, wherein the second fuel comprises a fuel selected from the group consisting of ethane (C2H6) and propane (C3H8).

4. A method for operating a gas turbine using a first fuel at full load, the method comprising:
operating the gas turbine with a second fuel instead of said first fuel to achieve an improved emission behavior, with the second fuel having a richer mix of higher-value hydrocarbons with 2 or more carbon atoms per molecule than said first fuel;
wherein the first fuel and second fuel each comprise gaseous fuel.

5. A method according to claim 4, comprising:
supplying the first fuel and the second fuel from two separate gas sources, the first fuel and the second fuel having different gas compositions with regard to said higher-value hydrocarbon.

6. A method according to claim 5, comprising:
supplying the two fuels directly from said gas sources; and
when switching from full load to partial load, switching between a gas source with a low share of higher-value hydrocarbon and a gas source with a higher share of high-value hydrocarbon.

7. A method according to claim 5, comprising:
generating the two fuels based on a different mixture of the gases from the two gas sources.

8. A method according to claim 4, comprising:
producing the first fuel from the second fuel, including separating the higher-value hydrocarbon.

9. A method according to claim 8, comprising:
placing the separated higher-value hydrocarbons in intermediate storage; and
adding said separated higher-value hydrocarbons to the second fuel during partial load operation.

10. A method according to claim 9, comprising:
liquefying the separated higher-value hydrocarbons prior to intermediate storage;
placing said liquefied separated higher-value hydrocarbons in intermediate storage as liquid gas; and
evaporating said liquefied separated higher-value hydrocarbons prior to said adding.

11. A method according to claim 4, comprising:
generating the second fuel from the first fuel, including adding higher-value hydrocarbons.

12. A method according to claim 11, comprising:
taking the added higher-value hydrocarbons from a local reservoir.

13. A method according to claim 12, wherein the local reservoir comprises a liquid gas storage tank; and comprising:
evaporating liquid gas from the liquid gas storage tank by an evaporator prior to said adding.

14. A method according to claim 12, wherein the local reservoir comprises a gas reservoir.

15. A method according to claim 4, comprising:
generating the first fuel and second fuel from a third fuel whose content of higher-value hydrocarbons ranges between those of the first fuel and the second fuel, including either separating the first fuel and the second fuel from the third fuel or adding higher-value hydrocarbons to the third fuel.

16. A method according to claim 15, comprising:
placing the higher-value hydrocarbons that are separated from the third fuel when generating the first fuel in intermediate storage;
removing higher-value hydrocarbons from said intermediate storage; and
generating the second fuel, including adding said removed higher-value hydrocarbons to the third fuel.

17. A method according to claim 16, comprising:
liquefying the separated higher-value hydrocarbons prior to said placing into intermediate storage;
wherein said placing comprises placing said liquefied separated higher-value hydrocarbons in intermediate storage as a liquid gas; and
evaporating liquefied separated higher-value hydrocarbons from intermediate storage prior to said adding.

18. A method according to claim 4, wherein the share of higher-value hydrocarbons is approximately 10% to 30% higher in the second fuel than in the first fuel.

19. A method according to claim 4, wherein the second fuel comprises a fuel selected from the group consisting of ethane (C2H6) and propane (C3H8).