Patent Application
20120167581
As is known, for a gas turbine to operate efficiently it is necessary for different parameters to be set in an optimal way as a function of the load, i.e., of the power that the gas turbine is effectively supplying, and of the ambient conditions. A wrong setting of the parameters leads in fact to a deterioration in the conditions of combustion, and this causes, on the one hand, a reduced efficiency and, on the other hand, an increase in pollutant emissions.
It has been noted that in modern gas turbines, which use burners with low NOx emissions, the flowrate of fuel fed to the pilot burners and the temperature of the exhaust gas are particularly critical, and setting of reference values that are not adequate as a function of the operating conditions has severe consequences on the efficiency of the machine.
The identification of the operating conditions does not in general pose problems in autonomous gas-turbine systems and in combined-cycle systems of a “multishaft” type (i.e., in which each gas turbine is mounted on a respective shaft that is independent with respect to the shaft of the steam turbine and is coupled to a respective electric generator). In this case, in fact, the power supplied by the gas turbine can be easily estimated from the electric power supplied by the generator coupled to the gas turbine itself. The measurement of the electric power is practically always available.
Difficulties arise, instead, in the case of combined-cycle systems of a “single-shaft” type, where a gas turbine, a steam turbine, and an electric generator are coupled to the same shaft. The power supplied by the gas turbine can hence not be estimated from the electric power supplied by the generator, which also contains a contribution of the steam turbine. In addition, it should be considered that, in many operating conditions, where correct setting of the parameters is particularly important, the load associated, respectively, to the gas turbine and to the steam turbine can depart sensibly from the power references provided by the system controller. In some transients, for instance, the system may be required to deliver a supplementary power rapidly, and the system controller consequently modifies the power reference for the gas turbine and for the steam turbine. The response of the steam turbine is, however, very slow with respect to that of the gas turbine, which in the initial steps of the transient supplies practically all the supplementary power required. The power references are hence unreliable at least during the transients.
The operating conditions of the steam turbine may vary in an unforeseeable way also for other reasons. For instance, a fraction of the steam can be drawn off to be used for district-heating systems.
The aim of the present invention is hence to provide a method of controlling a combined-cycle system and a combined-cycle system that is free from the limitations described and, in particular, enables the operating parameters of the gas turbine to be set correctly as a function of the load.
According to the present invention a method of controlling a combined-cycle system and a combined-cycle system are provided as defined, respectively, in Claims 1 and 10.
The present invention will now be described with reference to the annexed drawings, which illustrate a non-limiting example of embodiment thereof and in which:
With reference to
A control device 7 controls the gas-turbine assembly 2, the steam turbine 3, and the electric generator 4 on the basis of measurement signals supplied by measuring devices 6 in such a way that the operating conditions of the system 1 are optimised for supplying an electric power PE required by the loads.
The gas-turbine assembly 2 comprises a compressor 8, a combustion chamber 9, and a gas turbine 10. The compressor 8 and the gas turbine 10 are mounted, on the shaft 5.
The compressor 10 is provided with an anti-icing device 11 that comprises a recirculation line 12 and a regulating valve 13. The recirculation line 12 connects the outlet and the inlet of the compressor 8, and the regulating valve 13 enables an anti-icing air flowrate QAI (at approximately 400° C.) to be taken from the outlet of the compressor 8 and to be fed back at inlet to prevent the formation of ice.
An input stage of the compressor 8 is provided with a stage 8a of inlet guide vanes (IGVs), which are controlled by the control device 7 through an IGV actuator 15 for regulating an air flowrate QA taken in by the compressor 8. The air flowrate QA taken in by the compressor 8 in turn enables regulation of the exhaust gas temperature TE.
The combustion chamber 9 is provided with pilot burners 9a and premixing burners 9b (see
The system 1 further comprises a recovery boiler 17, which uses hot exhaust gas from the gas turbine 10 to generate steam for the steam turbine 3, and a condenser 18, which receives the steam processed by the steam turbine 3.
The measuring devices 6 supply measurement signals indicative of operating quantities of the system 1. In particular, the measurement signals comprise:
a signal Sβ indicative of a current compression ratio βC of the compressor 8;
a signal ST indicative of the ambient temperature;
a signal STE indicative of the exhaust gas temperature TE;
a signal SN indicative of the angular speed of the shaft 5; and
a signal SB indicative of the position of the regulating valve 13.
The control device 7 shares the load requested to the system between the gas turbine 10 and the steam turbine 3, detects the current operating conditions at least of the gas turbine 10 and selects optimal reference values (setpoints) of operating quantities of the system 1 as a function of the detected operating conditions. Moreover, the control device 7 applies regulating signals to the actuators of the system 1, in particular to the IGV actuator 15 and to the fuel valve 16 in such a way that the operating quantities tend to reach the respective setpoints. The control device 8 also performs a function of supervision of the accessory apparatuses, such as, for example, the anti-icing device 11. In particular, the control device 7 acts on the regulating valve 13 so as to activate the anti-icing device 11 and regulate the anti-icing air flowrate QAI when the ambient temperature drops below a threshold.
In order to determine the operating conditions of the gas turbine 10, the control device 7 uses the current compression ratio βC of the compressor 8, which is obtained from the signal Sβ supplied by the measuring devices 6.
The current compression ratio pc is normalized with respect to reference operating conditions and corrected to take into account the effect of factors such as the ambient conditions, the rotation speed of the shaft 5, and the action of the anti-icing device 11.
The reference conditions may be ISO conditions (temperature T=15° C.; pressure P=1.013 bar).
The normalized compression ratio βN is given by
where βRIF is the compression ratio in the reference conditions, and CT, CN, CQ are, respectively, a corrective temperature coefficient, a corrective speed coefficient, and a corrective flowrate coefficient.
The corrective temperature coefficient CT takes into account the effect of the ambient temperature, which is detected by the measuring devices 6 (signal ST).
The corrective speed coefficient C3 depends upon the rotation speed, of the gas turbine 10, which can also be detected by the measuring devices 6 (signal SN).
The corrective flowrate coefficient C3 depends upon an estimate of the anti-icing air flowrate QAI that is taken at output from the compressor 8 and fed back at input to the compressor 8 itself. In practice, the third corrective coefficient C3 takes into account that not the entire air flowrate QA taken in by the compressor 8 is introduced into the combustion chamber 9 and, moreover, the recirculation of air from the outlet to the inlet of the compressor 8 modifies the conditions of temperature. In one embodiment, estimation of the anti-icing air flowrate QAI is determined on the basis of the position of the regulating valve 13, which is set by the control device 7. In a different embodiment, the anti-icing air flowrate QAI is measured, for instance with a flowmeter.
It has been found that the normalized compression ratio βN defined above represents the power supplied by the gas turbine 10, normalized with respect to the same conditions (for example ISO or standard conditions). Hence, in practice, it is possible to determine the load conditions of the gas turbine 10 starting from the calculation of the normalized compression ratio βN.
The control device 7 operates as described hereinafter, with reference to
The control device 7 first of all acquires the measurement signals supplied by the measuring devices 6, amongst which, in particular (block 100):
the signal Sβ indicative of the current compression ratio βC of the compressor 8;
the signal ST indicative of the ambient temperature;
the signal STE indicative of the exhaust gas temperature TE;
the signal SN indicative of the angular speed of the shaft 5; and
the signal SR indicative of the position of the regulating valve 13.
Once the measurement signals have been acquired, the control device calculates the current compression ratio βC from the signal Sβ (block 110) and determines the values of the corrective temperature coefficient CT, of the corrective speed coefficient CN, and of the corrective flowrate coefficient CQ (block 120) using functions determined experimentally and stored, for example, in the form of tables.
Once the current compression ratio βC and the current values of the corrective temperature coefficient CT, of the corrective speed coefficient CN, and of the corrective flowrate coefficient CQ are available, the control device 7 calculates the normalized compression ratio βN applying Eq. (1) (block 130).
Next (block 140), the control device 7 determines the load conditions of the gas turbine 10 on the basis of the value of the normalized compression ratio βN. For this purpose, a function is used, stored for instance in the form, of a table in the control device 7. The function may be defined experimentally, starting from historic series, or else using a model of the system 1, which can be described with sufficient precision to yield reliable results.
In an alternative embodiment, instead of determining the load conditions of the gas turbine 10 directly from the normalized compression ratio βN, the control device 7 calculates an estimate of the power supplied by the gas turbine 10 on the basis of the normalized, compression ratio βN. The load, conditions of the gas turbine 10 are then determined as a function of the estimate of the power delivered.
After determining the current load conditions of the gas turbine 10, the control device 7 selects respective setpoints for the critical quantities that significantly affect, the efficiency of the gas turbine 10 (block 150). In particular, the control device 7 defines a first setpoint SPTE, indicative of a target temperature of the exhaust gas, and a second setpoint SPP, indicative of a target pilot fuel flowrate to be fed to the pilot burners of the combustion chamber 9.
Finally (block 160), the control device 7 applies a first regulating signal SIGV to the IGV actuator 15 and a second regulating signal SFV to the fuel valve 16 in such a way that the temperature exhaust gas TE and the pilot fuel flowrate QFP supplied to the pilot burners 9a tend to reach the first setpoint SPTE and the second setpoint SPP, respectively.
Thanks to the method described, the power supplied by the gas turbine of a single-shaft combined-cycle system can be easily estimated with good precision and in a reliable way, also considering the fact that the measurement of the current compression ratio βC is normally available in the systems. The parameters of the gas turbine can thus be correctly set as a function of the load and of the ambient conditions, and it is possible to maintain optimal conditions of combustion with high efficiency and low emissions of pollutant substances, in particular NOx.
Finally, it is evident that modifications and variations may be made to the method and to the system described herein, without departing from the scope of the present invention, as defined in the annexed claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| TO2010A001077 | Dec 2010 | IT | national |
