COMBINED CYCLE ELECTRIC POWER PLANT AND RELATING OPERATING METHOD

TECHNICAL FIELD The present invention relates to a combination-cycle electric power plant.

More specifically, the present invention relates to a combination-cycle electric power plant featuring a post-firing, heat-recovery steam generator. BACKGROUND ART

A combination-cycle electric power plant normally comprises a gas turbine unit; a steam turbine unit; a steam circuit extending through a number of heat exchange modules arranged in line; and a post-firing, heat- recovery steam generator, which transfers the residual energy of the exhaust gases from the gas turbine unit to the steam, and is equipped with a power-adjustable first burner for heating the heat ^' exchange modules .

In post-firing, heat-recovery steam generators of the type described above, the single post-firing burner is located at the gas turbine unit, i.e. upstream from the heat exchange modules with reference to the gas flow direction.

This configuration of the post-firing, heat-recovery steam generator results, on the one hand, in increased electric energy production, and, on the other, in reduced efficiency of the combination-cycle plant. The reduction in efficiency is due to the heat- recovery steam generator being designed to operate in both post-firing and non-post-firing mode, and to the heat exchange modules of the heat-recovery steam generator being designed to maximize efficiency in non- post-firing operating mode. When the heat-recovery steam generator switches from non-post-firing to post-firing mode, an excessive increase in steam temperature occurs, to the extent that the steam must be systematically cooled before it is supplied to the steam turbine unit. The amount of cooling required is in direct proportion to the amount of gas burned in post-firing mode, thus seriously impairing efficiency of the combination-cycle plant.

Another critical aspect of post-firing, heat- recovery steam generators is the increase in fume temperature, which calls for replacing the conventional component parts of the heat-recovery steam generator with more expensive parts, even to the extent of having to replace the entire heat-recovery steam generator with a much more expensive conventional steam generator.

In view of the above drawbacks, combination-cycle plants are frequently equipped with non-post-firing, heat-recovery steam generators .

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a combination-cycle plant designed to eliminate the drawbacks of the known art, and which, in particular, produces more power simply and cheaply, with no serious impairment in efficiency.

According to the present invention, there is provided a combination-cycle electric power plant comprising a gas turbine unit; a steam turbine unit; a steam circuit extending through a number of heat exchange modules arranged in line; and a post-firing, heat- recovery steam generator, which transfers the residual energy of the exhaust gases from the gas turbine unit to the steam, and is equipped with a power-adjustable first burner for heating the number of heat exchange modules; the plant being characterized in that the heat-recovery steam generator comprises a power-adjustable second burner arranged to only heat some of the heat exchange modules in said number of heat exchange modules . As such, the present invention provides for improving efficiency of the plant in all operating conditions, while greatly reducing steam cooling requirements .

Simulation tests conducted by the applicant show the present invention provides for a slight increase in electric power output by the plant as a whole, as compared with the known art and for a given amount of fuel burned in post-firing mode; for a considerable

improvement in the efficiency of the plant; and for a considerable reduction in post-firing exhaust fume temperature, thus allowing significant freedom in the choice of materials from which the heat exchange modules of the heat-recovery steam generator are made.

The present invention also relates to a combination- cycle plant operating method.

According to the present invention, there is provided a method of operating a combination-cycle electric power plant, wherein the combination-cycle plant comprises a gas turbine unit; a steam turbine unit; a steam circuit extending through a number of heat exchange modules arranged in line; and a post-firing, heat- recovery steam generator, which transfers the residual energy of the exhaust gases from the gas turbine unit to the steam; the method heating the number of heat exchange modules of the heat-recovery steam generator by means of a power-adjustable first burner; and the method being characterized by only heating some of the heat exchange modules in said number of heat exchange modules by means of a power-adjustable second burner.

BRIEF DESCRIPTION OF THE DRAWINGS

A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a diagram, with parts removed for clarity, of a combination-cycle plant in accordance with the present invention;

Figure 2 shows a block diagram, with parts removed for clarity, of the control unit of the Figure 1 plant. BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figure 1 indicates as a whole a combination-cycle plant comprising a gas turbine unit 2; and a steam turbine unit 3. Gas turbine unit 2 is the first motor of combination-cycle plant 1, may be fuelled with natural gas or gas oil, is connected to an alternator 4, and comprises a shaft 5, a compressor 6, a combustion chamber 7, and a gas turbine 8.

Steam turbine unit 3 is connected to an alternator 9, and comprises a shaft 10; a high-pressure steam turbine 11; a intermediate-pressure steam turbine 12; a low-pressure steam turbine 13; a condenser 14; and a heat exchange module 15 - in the example shown, an evaporator/deaerator .

Combination-cycle plant 1 comprises a heat-recovery steam generator 16 for recovering the residual heat generated by gas turbine unit 2; and a steam circuit 17, which supplies steam turbine unit 3 and extends through heat-recovery steam generator 16.

Heat-recovery steam generator 16 comprises a combustion chamber 18 extending from the outlet of gas turbine 8 to the mouth 19 of a stack (not shown) and housing part of steam circuit 17. In other words, combustion and post-firing fumes are substantially fed in a direction Dl inside combustion chamber 18, whereas steam is substantially fed in a direction D2 opposite

direction Dl. Heat-recovery steam generator 16 comprises a duct burner type power-adjustable burner 20 housed inside combustion chamber 18, at the outlet of gas turbine 8; and a duct burner type power-adjustable burner 21 housed inside combustion chamber 18, downstream from burner 20 in direction Dl.

Steam circuit 17 comprises a branch 22 connecting condenser 14 to heat-recovery steam generator 16, and along which is fitted a condensate extraction pump 23 from condenser 14. Inside combustion chamber 18, branch 22 supplies a heat exchange module 24 located at the mouth 19 of the stack to heat the condensate. Downstream from heat exchange module 24 in direction D2, branch 22 supplies evaporator/deaerator 15 housed inside combustion chamber 18, and a recirculation branch 25 extending outside combustion chamber 18 and along which is fitted a recirculation pump 26. Branch 25 provides for heating the condensate, before it is fed to heat-recovery steam generator 16, to prevent condensation phenomena. From heat exchange module (evaporator/deaerator) 15, steam circuit 17 comprises a low-pressure steam branch 27 supplying low-pressure steam turbine 13; a intermediate- pressure steam branch 28- supplying intermediate-pressure steam turbine 12; and a high-pressure steam branch 29 supplying high-pressure steam turbine 11.

Branch 27 supplies a heat exchange module 30 - in the example shown, a low-pressure superheater housed inside combustion chamber 18 - and is connected to a

connecting branch 31 from the outlet of intermediate- pressure steam turbine 12, before supplying low-pressure steam turbine 13.

Branch 28 has a feed pump 32, and supplies in succession a heat exchange module 33 - in. the example shown, a intermediate-pressure economizer; and a heat exchange module 34 - in the example shown, a intermediate-pressure evaporator housed inside combustion chamber 18. Upstream from intermediate-pressure evaporator 34 in direction Dl, branch 28 supplies a heat exchange module 35 - in the example shown, a intermediate-pressure superheater - upstream from which it is connected to a branch 36 from high-pressure steam turbine 11 before reentering combustion chamber 18. Next, branch 28 supplies in succession two heat exchange modules 37 and 38 - in-, the example shown, two series intermediate-pressure re-superheaters - and intermediate- pressure steam turbine 12.

A de-superheater Fl is located along branch 28, between heat exchange modules 37 and 38, to cool the steam when it exceeds a given threshold temperature.

Branch 29 has a feed pump 39, and supplies a heat exchange module 40 - in the example shown, a high- pressure economizer housed inside combustion chamber 18, between the evaporator/deareator (heat exchange module 15) and the intermediate-pressure evaporator (heat exchange module 34) ; and a heat exchange module 41 - in the example shown, a further high-pressure economizer

downstream from intermediate-pressure evaporator 34 in direction D2. Next, branch 29 supplies a heat exchange module 42 - in the example shown, a high-pressure evaporator - downstream from which it supplies in succession two heat exchange modules 43 and 44 - in the example shown, two high-pressure superheaters - and high- pressure steam turbine 11.

A de-superheater F2 is located along branch 29, between heat exchange mo.dules 43 and 44, to cool the steam when it exceeds a given threshold temperature.

Combination-cycle plant 1 comprises a line 45 supplying fuel, normally gas or gas oil, to combustion chamber 7; and a line 46 supplying fuel, normally gas, to burners 20 and 21, and which forks into a line 47 supplying burner 20, and a line 48 supplying burner 21. The plant comprises a powered valve 49 located along line 46; and a powered valve 50 located along line 47.

Combination-cycle plant 1 comprises a control unit 51 for selectively activating post-firing operating mode and non-post-firing operating mode of heat-recovery steam generator 16.

Plant 1 comprises a temperature sensor 52 located along branch 28, and which emits a signal TM related to the intermediate-pressure steam temperature along the section between heat-recovery steam generator 16 and the inlet of intermediate-pressure steam turbine 12; a temperature sensor 53 located along branch 29, and which emits a signal TH related to the high-pressure steam

temperature along the section between heat-recovery steam generator 16 and the inlet of high-pressure steam turbine 11; a power sensor 54 located at alternator 4, and which emits a signal Pl related to the power emitted by alternator 4; and a power sensor 55 located at alternator 9, and which emits a signal P2 related to the power emitted by alternator 9.

Control unit 51 receives and processes signals TM, TH, Pl and P2; a signal Pset related to the total power demanded of combination-cycle plant 1; a signal TMmax related to the maximum temperature permitted along intermediate-pressure steam branch 28; a signal TMmin related to the minimum temperature permitted along intermediate-pressure steam branch 28; a signal THmax related to the maximum temperature permitted along high- pressure steam branch 29; and a signal THmin related to the minimum temperature permitted along high-pressure steam branch 29.

Control unit 51 emits a power signal Vl for adjusting the opening of valve 49; a dividing signal V2 for adjusting the opening of valve 50; a signal Rl for activating and regulating de-superheater Fl; a signal R2 for activating and regulating de-superheater F2; and a warning signal C indicating a malfunction of control unit 51 or sensors 52 and 53.

With reference to Figure 2, the control unit comprises a computing block 56, which calculates the difference between signal Pset related to the power

demanded of plant 1, and the sura of signals Pl and P2 related to the total power output of plant 1, and accordingly supplies power signal Vl to adjust valve 49 to increase or reduce total gas supply to burners 20 and 21.

Control unit 51 comprises computing blocks 57-60, which calculate the difference between signal THmax and signal TH; between signal TH and signal THmin; between signal TMmax and signal TM; and between signal TM and signal TMmin. Control unit 51 comprises comparing blocks

61-64, which determine whether the calculated differences are greater than zero, emit respective 0 signals when the conditions set in respective comparing blocks 61-64 are satisfied, and otherwise ^' emit respective 1 signals. Control unit 51 comprises a logic block 65, which compares and determines any incongruity in the signals emitted by comparing blocks 61-64, e.g. whether signal TH is simultaneously greater than THmax and less than THmin.

In the event of this occurring, logic block 65 emits warning signal C indicating a malfunction of control unit

51 or temperature sensor 52. Similarly, incongruity is indicated if signal TM is simultaneously greater than

TMmax and less than TMmin. In control unit 51 in the example shown, there are a total of seven possible incongruous combinations resulting in the emission of signal C.

When all the comparing blocks emit 0 signals, this means signals TH and TM are within the respective

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predetermined operating ranges, and no signals are emitted by logic block 65.

When signal TH is outside the predetermined range, and signal TM is within the respective predetermined range or outside the respective range and on the same side as signal TH, logic block 65 transmits a consent signal Sl to a control _. block 66, authorizing control block 66 to emit signal V2 to adjust valve 50. The amount of adjustment is defined by one of computing blocks 57-60 connected to control block 66.

When signal TH is above the set maximum value THmax and signal TM is below the set minimum value TMmin, or when signal TM is above the set maximum value TMmax and signal TH is below the set minimum value THmin, working on burner 20 alone is not enough to reestablish correct operating temperatures, so the control block emits signal Rl or R2 to activate de-superheater Fl or F2.

Operation of de-superheaters Fl and F2 may also be authorized by a timer (not shown) , which determines continuation of an abnormal operating condition even after the power division between first and second burner 20 and 21.

Operation of de-superheaters Fl and F2, which withdraw thermal power from combination-cycle plant 1, is therefore only required, in two operating conditions with respect to the sixteen columns in the logic block 65 matrix. This is made possible by heat-recovery steam generator 16 comprising two burners 20, 21 located at

different parts of heat-recovery steam generator 16, and by the possibility of adjusting fuel supply to burners 21, 22, while maintaining total fuel supply to burners 21, 22 constant. Heat-recovery steam generator 16 extends from the outlet of gas turbine 8, and heat exchange modules 15, 24, 30, 33, 34, 35, 37, 38, 40, 41, 42, 43, 44 are preferably distributed as shown in the accompanying drawing. That is, from the outlet of gas turbine 8 to mouth 19, intermediate-pressure heat exchange module 38, high-pressure heat exchange module 44, intermediate- pressure heat exchange module 37, high-pressure heat exchange module 43, and heat exchange module (high- pressure evaporator) 42 are arranged successively in direction Dl inside combustion chamber 18. Burner 20 is located between the outlet of gas turbine 8 and intermediate-pressure heat exchange module 38, while burner 21 is located between high-pressure heat exchange module 43 and heat exchange module (high-pressure evaporator) 42. Burners 20 and 21 are preferably oriented in direction Dl. That is, the thermal power emitted by burner 20 is directed towards high-pressure heat exchange module 38, and the power emitted by burner 21 is directed towards high-pressure evaporator 42. Downstream from heat exchange module (high-pressure evaporator) 42, are arranged successively in direction Dl: intermediate-pressure heat exchange module 35, high- pressure heat exchange module 41, heat exchange module

(intermediate-pressure evaporator) 34, intermediate- pressure heat exchange module 30, high-pressure heat exchange module 40, intermediate-pressure heat exchange module 33, heat exchange module (evaporator/deaerator) 15, and heat exchange module 24 of branch 22 from condenser 14.

In a variation not shown, heat exchange module (evaporator/deaerator) 15 is replaced by an evaporator housed inside combustion chamber 18, and by a deaerator outside combustion chamber 18.

Particularly noteworthy is the location of burners 20 and 21 between gas turbine 8 and heat exchange module (high-pressure evaporator) 42, which is the area of heat- recovery steam generator 16 in which the highest temperatures most critical for both the steam and the structure of heat-recovery steam generator 16 are generated in post-firing mode.

As will be clear from the foregoing description, the advantages of the present invention are numerous, both in terms of improved efficiency and output of the combination-cycle plant, ^• ■ and the reduction in cost of the materials from which the heat exchange modules of heat- recovery steam generator 16 are made.