Method for operating a gas turbine below the nominal power thereof

Abstract

A method for operating a gas turbine below the nominal power includes: determining a lower power threshold value of the gas turbine which causes the gas turbine to leave a CO-emission-compliant partial load range of the gas turbine; providing a specified threshold value for output gas turbine power, wherein the specified threshold value is less than the nominal power of the gas turbine; and operating the gas turbine at an output gas turbine power above the specified threshold value at a constant exhaust gas temperature, wherein the inlet guide blades of a compressor of the gas turbine are closed further in order to reduce the output gas turbine power. A sufficiently large valve is selected for the specified threshold value so that increases of the primary zone temperature, combustion temperature, and exhaust temperature extend over a CO-emission-compliant partial load range of the gas turbine that is as large as possible.

Claims

1. A method for operating a gas turbine during partial load, the gas turbine including a compressor with inlet guide vanes, the method comprising: choosing a lower power limit value (p.sub.1) for an output gas turbine power as a lower limit of a partial load range that corresponds with a carbon monoxide (CO) emissions limit when an exhaust gas temperature is controlled at a constant value, choosing an upper power limit value (p.sub.2) for the output gas turbine power between the lower power limit value (p.sub.1) and a rated load of the gas turbine, operating the gas turbine with the output gas turbine power above the upper power limit value (p.sub.2) with the exhaust gas temperature controlled at the constant value, and lowering the output gas turbine power by decreasing an opening angle of the inlet guide vanes, wherein, when the output gas turbine power drops below the upper power limit value (p.sub.2), raising the exhaust gas temperature of the gas turbine as the output gas turbine power is lowered, up to a maximum exhaust gas temperature, wherein the rise in exhaust gas temperature causes a downward shift in the lower limit of the partial load range resulting in a second lower power limit value (p.sub.1@TAT+), thereby extending the partial load range, and wherein the upper power limit value (p.sub.2) is chosen to be closer to the rated load than the lower limit of the partial load range.

2. The method as claimed in claim 1, wherein the upper power limit value (p.sub.2) is greater than 70% of the rated load.

3. The method as claimed in claim 1, wherein a last exhaust gas temperature rise up to the maximum exhaust gas temperature takes place before the second lower power limit value (p.sub.1@TAT+) is reached.

4. The method as claimed in claim 1, wherein, as the output gas turbine power is lowered, said exhaust gas temperature is raised stepwise or continuously.

5. The method as claimed in claim 4, wherein, in order to raise said exhaust gas temperature, a quantity of combustion air supplied to a combustion chamber of the gas turbine is reduced by further decreasing the opening angle of the inlet guide vanes.

6. The method as claimed in claim 1, wherein said exhaust gas temperature of the gas turbine is regulated, and a set point value for the exhaust gas temperature is raised, when the upper power limit value (p.sub.2) is undershot and as the output gas turbine power is further lowered, until a maximum exhaust gas temperature is reached.

7. The method as claimed in claim 1, wherein the upper power limit value (p.sub.2) is greater than 80% of the rated load.

8. The method as claimed in claim 1, wherein the upper power limit value (p.sub.2) is greater than 90% of the rated load.

9. The method as claimed in claim 1, wherein the upper power limit value (p.sub.2) is greater than 95% of the rated load.

10. The method as claimed in claim 1, wherein the upper power limit value (p.sub.2) approximately corresponds to the rated load of the gas turbine.

11. The method as claimed in claim 1, wherein the exhaust gas temperature is raised by closing the inlet guide vanes and lowering a compressor mass flow rate.

12. A method for operating a gas turbine during partial load, the gas turbine including a compressor with inlet guide vanes, the method comprising: choosing a lower power limit value (p.sub.1) for an output gas turbine power as a lower limit of a partial load range that corresponds with a carbon monoxide (CO) emissions limit when an exhaust gas temperature is controlled at a constant value, choosing an upper power limit value (p.sub.2) for the output gas turbine power between the lower power limit value (p.sub.1) and a rated load of the gas turbine, operating the gas turbine with the output gas turbine power above the upper power limit value (p.sub.2) with the exhaust gas temperature controlled at the constant value, and lowering the output gas turbine power by decreasing an opening angle of the inlet guide vanes, wherein, when the output gas turbine power drops below the upper power limit value (p.sub.2), raising the exhaust gas temperature of the gas turbine as the output gas turbine power is lowered, up to a maximum exhaust gas temperature, wherein the rise in exhaust gas temperature causes a downward shift in the lower limit of the partial load range resulting in a second lower power limit value (p.sub.1@TAT+), wherein the exhaust gas temperature rises continuously from when the output gas turbine power drops below the upper power limit value (p.sub.2) up to the maximum exhaust gas temperature, then levels off so as to not exceed the maximum exhaust gas temperature before reaching the second lower power limit value (p.sub.1@TAT+).

13. The method as claimed in claim 12, wherein the exhaust gas temperature begins to fail after reaching the maximum exhaust gas temperature before reaching second lower power limit value (p.sub.1@TAT+).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and features of the invention are explained in more detail with reference to an exemplary embodiment.

(2) In the drawings:

(3) FIG. 1 shows the schematic structure of a gas turbine;

(4) FIG. 2 is a diagram showing the position of the inlet guide vanes, the exhaust gas temperature and the CO emissions occurring in the exhaust gas over the generator terminal power.

DETAILED DESCRIPTION OF INVENTION

(5) FIG. 1 shows, schematically, a static gas turbine 10 with a compressor 12 and a turbine unit 14 whose rotors are rigidly coupled to one another. A combustion chamber 16 is provided between the compressor outlet and the inlet section of the turbine unit 14. This can be configured as a silo combustion chamber, a tubular combustion chamber or as an annular combustion chamber. In the case of tubular combustion chambers, generally ten, twelve or even more tubular combustion chambers are provided. Also coupled to the compressor rotor is a generator 11 for generating current. At the air inlet of the compressor 12, there are provided compressor inlet guide vanes 13 which can be pivoted about their longitudinal axis and by means of which it is possible to set the compressor mass flow rate m.sub.v. These guide vanes 13 are represented merely schematically. The turbine unit 14 comprises, according to the exemplary embodiment, four successive turbine stages 14a, 14b, 14c and 14d, which in the single figure are also represented merely schematically.

(6) In operation, the compressor 12 sucks in ambient air, compresses it and supplies it to the combustion chamber 16. There, the compressed air is mixed with a fuel B and is combusted in a flame to give a hot gas HG. The hot gas HG flows into the inlet of the turbine unit 14 and expands, performing work, at the turbine blades (not shown in more detail) of the turbine unit 14. At the outlet of the turbine unit 14, the resulting exhaust gas RG flows away via an exhaust gas diffuser (not shown). Then, either the exhaust gas RG is discharged to the environment via a chimney, or the exhaust gas RG is used in what is termed a boiler, which is known as a waste heat steam generator, for generating steam. The steam generated in the waste heat steam generator then serves for driving steam turbines (not shown in more detail) or also as process steam. With the aid of the fuel mass flow rate m.sub.B and of the compressor mass flow rate m.sub.v, it is possible to set the power to be delivered by the gas turbine 10.

(7) Insofar as the gas turbine 10 is operated below its rated power, it is provided that, even in the case of high partial load, the exhaust gas temperature of the gas turbine is raised slightly, although such an exhaust gas temperature rise is not yet necessary to avoid CO emissions.

(8) This method is explained in detail with reference to the diagram shown in FIG. 2. The abscissa of the diagram shows the relative generator terminal power PKL, which represents the output gas turbine power. The maximum value of the relative gas turbine power is 100% (the rated power). Three parameters are plotted on the ordinate. The first parameter is the relative opening angle of the adjustable guide vanes, wherein a value of 100% represents the compressor inlet being fully open. As the percentage for the position of the inlet guide vanes decreases, the inlet of the compressor is further closed, causing the compressor mass flow rate m.sub.v to decrease. Second, the CO emissions contained in the exhaust gas are plotted on the ordinate with the unit [ppm], i.e. parts per million. The third parameter plotted in the diagram on the ordinate is the exhaust gas temperature.

(9) The diagram shows one characteristic curve for each of the three parameters. The characteristic curve 22 shows the degree of opening of the adjustable inlet guide vane depending on the output gas turbine power. The characteristic curve 20 shows the CO emissions depending on the output gas turbine power for a constant exhaust gas temperature. The characteristic curve 21 shows the CO emissions depending on the output gas turbine power for an exhaust gas temperature raised in accordance with the invention. The characteristic curve 24 shows the temperature of the exhaust gas of the gas turbine depending on the output gas turbine power in accordance with the method according to the invention.

(10) The characteristic curve 20 shows the dependence of the CO emissions in the exhaust gas of the gas turbine 10, as they occur during operation of a conventional gas turbine 10that is to say without the method according to the invention. In the following explanation of the method according to the invention, a legal limit value of for example 10 ppm of carbon monoxide is assumed. With the aid of the legal limit value, and in conjunction with the characteristic curve 20, it is possible to determine a lower power limit value p.sub.1@TATK=const. as the lower limit of the partial load range, in accordance with CO emissions limits, of the gas turbine 10: the power range, in accordance with CO emissions limits, of the gas turbine 10 lies in the interval between p.sub.1@TATK=const. and below 100% of the rated power of the gas turbine 10, if the gas turbine 10 is operated in partial load as hitherto with constant exhaust gas temperature.

(11) In addition, a predefined limit value p.sub.2 is to be determined. this limit value could also be termed the upper power limit value and lies comparatively close to the rated load of the gas turbine 10, for example at a value of 95% of the rated power. It is however also possible to choose a somewhat lower figure for this value. All that is important is that p.sub.2 lies closer to the rated power than to a lower, reduced power limit value p.sub.1@TAT+. The lower, reduced power limit value p.sub.1@TAT+ results from that operating point of the gas turbine 10 at which, despite raised primary zone temperature and raised exhaust gas temperature, there occur in the exhaust gas CO emissions which are above the legal limit value for CO emissions.

(12) However, the lower power limit values p.sub.1@TATK=const., p.sub.1@TAT+of the gas turbine 10 can also be determined in the case of emissions which deviate slightly from the legally prescribed CO emissions limit, in order to ensure that this CO emissions limit is always reliably observed.

(13) The predefined limit value p.sub.2 is used to determined that operating point of the gas turbine 10 at which, in the event of the predefined limit value p.sub.2 being further undershot, the exhaust gas temperature is raised by closing the inlet guide vanes 13. By closing the inlet guide vanes 13 in the case of an only slightly reduced fuel mass flow rate m.sub.B, it is possible to raise slightly the combustion temperature and thereby the primary zone temperature, which moreover allows the CO emissions to remain at a low level. As the gas turbine power is further lowered, the compressor inlet guide vanes close further, thus further lowering the compressor mass flow rate m.sub.v. At the same time, the gas turbine 10 is operated in such a fashion that the combustion temperatures and thus also the exhaust gas temperatures rise further. This shifts the characteristic curve for the CO emissions from higher partial load values to lower partial load values. On account of the fact that the predefined limit value p.sub.2 is significantly greater than the lower reduced power limit value p.sub.1@TAT+ and the predefined limit value p.sub.2 approximately corresponds to the rated load of the gas turbine, the rise in the exhaust gas temperature and also the rise in the primary zone temperature is rather flat in comparison with a mode of operation of the gas turbine in which the predefined limit value p.sub.2 is comparatively close to the lower, reduced power limit value p.sub.1@TAT+ of the gas turbine. It is thus possible to ensure operation of the gas turbine 10 which is comparatively protective of components.

(14) Overall, the invention thus relates to a method for operating a gas turbine 10 below its rated power, having the steps of: determining a lower power limit value p.sub.1@TATK=const. for the gas turbine 10 as that power limit value beyond which further lowering of the output gas turbine power leads to departure from a partial load range of the gas turbine 10 in accordance with CO emissions limits, preparing a predefined limit value p.sub.2 for an output gas turbine power, wherein the predefined limit value p.sub.2 is smaller than the rated power of the gas turbine 10, operating the gas turbine 10 with an output gas turbine power above the predefined limit value p.sub.2 with a constant exhaust gas temperature, wherein in order to lower the output gas turbine power the inlet guide vanes 13 of a compressor 12 of the gas turbine 10 are closed further,

(15) In order, when preparing a particularly low partial load range of the gas turbine 10, to avoid relatively large temperature gradients and consequently to protect gas turbine components, it is provided that the predefined limit value p.sub.2 is chosen such that the temperature rises of the primary zone temperature, of the combustion temperature and/or of the exhaust gas temperature extend over as large as possible a partial load range, in accordance with CO emissions limits, of the gas turbine 10.