Method for operating a gas turbine plant with gaseous fuel

Abstract

A method for operating a gas turbine plant with gaseous fuel, which is conveyed to the gas turbine plant through a gas line, burnt in a combustion chamber and then supplied to a gas turbine. At least one valve for controlling the flow of the fuel to the combustion chamber is installed in the gas line, a critical opening position being defined for the valve. To provide an improved method for operating a gas turbine plant in which the power of the gas turbine plant is kept at a maximum for as long as possible in the event of insufficient pressure in the gas line, a temperature of the fuel in the gas line is reduced when the valve is about to exceed the critical opening position.

Claims

1. A method for operating a gas turbine plant with gaseous fuel, which is transported through a gas line to the gas turbine plant, is burned in a combustion chamber and is subsequently supplied to a gas turbine, wherein at least one valve for regulating the throughflow of the gaseous fuel to the combustion chamber is installed in the gas line, the method comprising: defining a critical opening position (S.sub.crit) for the at least one valve; operating the gas turbine plant such that a power of the gas turbine plant is substantially constant at a target value; opening the at least one valve to the critical opening position (S.sub.crit); and reducing a temperature (T.sub.B) of the gaseous fuel in the gas line, based on the at least one valve having reached the critical opening position (S.sub.crit), while maintaining operation of the gas turbine plant such that the power of the gas turbine plant is substantially constant at the target value over a first time period.

2. The method as claimed in claim 1, wherein the critical opening position (S.sub.crit) of the at least one valve is above 70% of a maximum opening position.

3. The method as claimed in claim 1, wherein the gas turbine plant has a preheating system for the fuel, and the temperature (T.sub.B) of the gaseous fuel in the gas line is reduced by reducing an amount of heat supplied to the gaseous fuel by the preheating system.

4. The method as claimed in claim 1, wherein a threshold value (T.sub.min) for a minimum temperature (T.sub.B) of the gaseous fuel is determined with account taken of a present value for an operating parameter of the gas turbine plant, and the reduction of the temperature (T.sub.B) of the gaseous fuel in the gas line is stopped if the threshold value has been reached.

5. The method as claimed in claim 1, wherein the at least one valve comprises multiple valves.

6. The method as claimed in claim 1, wherein subsequent to maintaining operation of the gas turbine plant such that the power of the gas turbine plant is substantially constant at the target value over the first time period, the power of the gas turbine plant is reduced.

7. A control device for operating a gas turbine plant with gaseous fuel, which is transported through a gas line to the gas turbine plant, is burned in a combustion chamber and is subsequently supplied to a gas turbine, wherein at least one valve for regulating the throughflow of the gaseous fuel to the combustion chamber is installed in the gas line, the control device adapted to: store a predefined critical opening position (S.sub.crit) for the at least one valve; operate the gas turbine plant such that a power of the gas turbine plant is substantially constant at a target value; open the at least one valve to the predefined critical opening position (S.sub.crit); control a heat exchanger to reduce a temperature (T.sub.B) of the gaseous fuel if the at least one valve reaches the predefined critical opening position (S.sub.crit); and maintain operation of the gas turbine plant such that the power of the gas turbine plant is substantially constant at the target value while controlling the heat exchanger to reduce the temperature (T.sub.B) of the gaseous fuel in the gas line.

8. A gas turbine plant comprising the control device of claim 7.

9. The method as claimed in claim 1, wherein the reducing step is performed based on a disturbance in a pressure of the gaseous fuel in the gas line.

10. The method as claimed in claim 1, wherein the critical opening position is a fixed position and is less than a maximum open position of the at least one valve.

11. The method as claimed in claim 1, wherein the reducing step is performed over the first time period based on a first disturbance in a pressure of the gaseous fuel in the gas line over the first time period; and wherein the power of the gas turbine plant is reduced from the target value over a second time period after the first time period based on a continued disturbance in the pressure of the gaseous fuel in the gas line over the second time period to maintain the at least one valve at the critical opening position.

12. The method as claimed in claim 11, wherein the power of the gas turbine plant is increased over a third time period after the second time period based on the pressure of the gaseous fuel in the gas line stabilizing over the third time period.

13. The method as claimed in claim 12, wherein the at least one valve is maintained at the critical opening position (S.sub.crit) during the first time period, the second time period and the third time period.

14. The method as claimed in claim 1, wherein the at least one valve is maintained at the critical opening position (S.sub.crit) during the reducing step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An exemplary embodiment of the invention will be discussed in more detail on the basis of a drawing. In the figures:

(2) FIG. 1 shows, schematically and in a highly simplified form, a fuel system of a gas turbine plant, and

(3) FIG. 2 shows, in a diagram, the temporal profile of various parameters of the gas turbine plant.

DETAILED DESCRIPTION OF INVENTION

(4) Identical reference signs have identical meanings in the figures.

(5) FIG. 1 schematically shows the structure of a fuel system 2 which is part of a gas turbine plant (not shown in more detail) in which natural gas is used as fuel. The gas turbine plant generally comprises a compressor, a combustion chamber 4 and a gas turbine, coupled to which, for example, is a generator for generating electricity.

(6) The fuel system 2 comprises a gas line 6 via which gaseous fuel is supplied to the combustion chamber 4. In the combustion chamber 4, there are arranged in particular multiple burners, which, in the exemplary embodiment shown, are of a multi-stage design and which, in the figure, are illustrated symbolically by a main burner 8 and a pilot burner 10. A sub-line 6a, 6b, in which a respective regulating valve 12a, 12b is installed, is branched off to each of the burner stages 8, 10. The gas line 6 moreover contains an emergency valve 14. Upstream of the emergency valve 14, there is also arranged on the gas line 6 a heat exchanger 16, which is part of a preheating system, which serves for preheating the fuel in the gas line 6.

(7) The gas turbine plant furthermore comprises a control or regulating device 18, which regulates inter alia the position of the regulating valves 12a, 12b. Here, there is stored in the regulating device 18 a critical opening position for the regulating valves 12a, 12b, which is for example 80% of a maximum opening position of the regulating valves 12a, 12b. The critical opening position may in this case for example also be 70%, 75%, 85%, 90%, 95% of the maximum opening position of the regulating valves 12a, 12b or correspond to the maximum opening position.

(8) The course of the method according to the invention can be seen from FIG. 2. It is generally the case that the position of the respective regulating valve 12a, 12b is, via a regulating loop in the control or regulating device 18, always set in such a way that the gas turbine operates according to a predefined power or combustion temperature. It thus reacts indirectly automatically to variations in the following variables: natural gas supply pressure, natural gas temperature, natural gas quality, ambient conditions, pressure loss via the natural gas supply system and the burner (fouling/wear) and/or efficiency of the gas turbine (wear). All of these parameters constitute possible disturbance factors on the basis of which the position of the regulating valves 12a, 12b is regulated in order, in this way, to set in particular a turbine power P.

(9) A decreasing fuel supply pressure BD (natural gas supply pressure in this case) is considered as a disturbance variable in FIG. 2. Instead of a decreasing natural gas pressure, it would alternatively be possible for use to be made of a deteriorating gas quality, a decreasing ambient temperature, an increasing ambient humidity, an increasing ambient pressure, fouling of a burner or natural gas system components, a reduced gas turbine efficiency, etc.

(10) According to FIG. 2, from time t.sub.0 to time t.sub.1, the fuel supply pressure BD is constant and a regulating valve position RV is below a critical opening position S.sub.crit. The fuel temperature T.sub.B and the turbine power P remain stable at their target value (P.sub.S for the turbine power).

(11) From t.sub.1 to t.sub.2, the fuel supply pressure BD in the gas line 6 decreases. In order to keep the turbine power P (or the fuel mass flow) constant, the corresponding regulating valve or both regulating valves 12a, 12b are opened further until a critical opening position S.sub.crit has been reached.

(12) In the time period from t.sub.2 to t.sub.3, the fuel supply pressure BD decreases further. The regulating valves 12a, 12b have reached their predefined critical opening position S.sub.crit, and for this reason, from t.sub.3, they are not opened further but remain at S.sub.crit. In order to keep the power P (or the fuel mass flow) constant, the gas temperature T.sub.B is decreased.

(13) From t.sub.3, the fuel supply pressure BD decreases, with an even steeper gradient, further. The gradient is too large to be compensated by the slow changing of the fuel temperature T.sub.B. The fuel temperature T.sub.B decreases further with its maximum gradient, the gas turbine power P additionally also being slightly reduced in order that the regulating valves 12a, 12b continue to be held in the critical opening position S.sub.crit.

(14) Between t.sub.4 and t.sub.5, the fuel supply pressure BD is stabilized to a lower level than the original one. The regulating valves 12a, 12b are still in the critical opening position S.sub.crit. Since t.sub.3, the turbine power P has constantly been below the power target valve P.sub.S, but, in parallel to the progressive reduction of the fuel temperature T.sub.B, the gas turbine power P is raised slowly to the target value P.sub.S again. Here, it needs to be ensured that the fuel temperature T.sub.B remains above a minimum threshold value T.sub.min, wherein the threshold value T.sub.min correlates for example with the NOx emissions or some other operating parameter of the gas turbine plant.

(15) From t.sub.5, stable operation is achieved once again. The gas turbine power P has reached its target value P.sub.S again, and the gas turbine is operated further at reduced fuel temperature T.sub.B. The fuel temperature T.sub.B is raised again only when the regulating valves 12a, 12b assume a position RV below the critical opening position S.sub.crit (this case not being shown).