"CONTROL DEVICE AND METHOD OF A TURBINE ELECTRIC POWER PLANT SUPPLIED WITH LOW CALORIFIC VALUE GAS""

TECHNICAL FIELD The present invention relates to a control device and method of a turbine electric power plant supplied with low calorific value gas.

BACKGROUND ART As is known, gas-turbine power plants, or turbogas plants, normally comprise a motor assembly (turbo-assembly) , forming part of which are a compressor having a variable-geometry stage, a combustion chamber, a gas turbine, and a generator, mechanically connected to a same shaft of the turbine and of the compressor and connected to an electric-power distribution network through a main switch. Turbogas plants are moreover equipped with control devices, which implement different functions necessary for proper operation of the plant and for meeting the increasingly stringent normative requirements corresponding to the performance of the plants in terms of safety, stability and capacity for responding to variations of the requirement of power by the distribution network.

In the last few years, the scarcity of energy resources has determined an acceleration of the increase in the cost of conventional fuels, such as natural gas and distilled oil. For this reason, the current tendency is to revalorize the use of more economical fuels, such as, for example, coal, orimulsion, heavy oils, and, in particular, gas with low calorific value, such as synthetic gases (syngases) , derived from the gasification of coal, asphalts or gases that are by-products of siderurgical and refinery processes. These gases are characterized by a calorific value comprised between 4 and 10 MJ/kg.

Currently, in conventional gas-turbines natural gases are

used, having a calorific value comprised between 45 and 50 MJ/kg.

In a turbogas plant, normally used with gases with high calorific value, the use of a gas with low calorific value entails the need to increase the flowrate of gas to the combustion chamber in order to maintain the production of mechanical power and, hence, electric power, unvaried.

The overall flowrate evolving in the turbine, however, must range around an optimal value and, above all, must absolutely not exceed a limit value established in the design stage, in order to prevent damage to the structure of the turbine and malfunctioning thereof.

Consequently, it is necessary for the air flow rate supplied by the compressor to be reduced proportionally to the increase in the flowrate of fuel, even though this can jeopardize the achievement of the power requirement.

Above all, it should be considered that, in particular operating conditions, such as, for example, variations in ambient temperature during operation of the plant, the air flow rate supplied by the compressor varies; in particular, the air flow rate at low temperatures increases on account of the increase in the density of the air and hence determines an increase in the risk of reaching the limit value of the flowrate evolving in the turbine, above all in the case where, gases with low calorific value are used, such as syngases.

Control devices are known for turbogas plants supplied with gas with low calorific value. The known control devices intervene so as to prevent the electric power generated by the plant from overstepping a pre-set threshold; in particular, a critical value of electric power is defined, also referred to as "critical load", for which,, in optimal operating conditions, the limit of flowrate evolving in the turbine is

reached. When the critical load is exceeded, the control devices block the supply of the gas to the combustion chamber. However, control devices of this type suffer from the main drawback of not taking into consideration the thermodynamic evolution of the plant; for example, the use of just the generated electric power as main control parameter does not enable consideration of factors such as the wear of the components or the soiling of the compressor, which affect the thermodynamic variables of the plant and can jeopardize safety or degrade the performance of the plant.

DISCLOSURE OF INVENTION

An aim of the present invention is to provide a control device and method that will be free from the drawbacks of the known art highlighted herein. In particular, an aim of the invention is to provide a control device and method capable of limiting, in a reliable and safe way, the flowrate evolving in the turbine in a gas-turbine plant for the production of energy supplied with gas with low calorific value.

In accordance with the above aims, the present invention relates to a control device and method of a turbine electric power plant supplied with low calorific value gas, as claimed in Claims 1 and 12, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will appear clearly from the ensuing description of a non-limiting example of embodiment thereof, with reference to the figures of the annexed plate of drawings, wherein: Figure 1 is a simplified block diagram of a power plant; Figure 2 is a more detailed block diagram of a part of the plant in Figure 1, obtained according to the present invention; and Figures 3 and 4 are graphs corresponding to parameters of the plant of Figure 1.

BEST MODE FOR CARRYING OUT TR£ INVENTION

In Figure 1 designated by the reference number 1 is a gas- turbine plant, in particular for the production of electrical energy. The plant 1 can be selectively supplied with a conventional gas with high calorific value (i.e., having nominal calorific value comprised in a nominal range of between 45 and 50 MJ/kg) , in a first operating condition, or else with gas with low calorific value (i.e., having a calorific value lower than the nominal calorific value and comprised between 4 and 10 MJ/kg) , in a second operating condition. The plant 1 can be selectively connected to a distribution network 2 through a main switch 3 and' comprises a turbo-assembly 5, a generator 6, a sensor module 7, a control device 8, and a first actuator 9 and a second actuator 10. The plant moreover comprises a detection device 11, for detecting the composition of the gas used by the turbo-assembly 5.

The turbo-assembly 5 is of a conventional type and comprises a compressor 12, equipped with a variable-geometry inlet stage 13, a combustion chamber 14, and a gas turbine 15; in particular, the inlet stage ^' 13 of the compressor 12 is provided with a plurality of vanes (not illustrated) , referred to as IGVs (inlet guide vanes) , the inclination of which can be modified by means of the first actuator 9, for regulating an air flow rate Q _A taken in by the compressor 12 itself. The combustion chamber 14 receives the fuel through a supply valve, of a known type and not illustrated, which is actuated by the second actuator 10, for supplying a flowrate of fuel Q _G to the combustion chamber 14. The generator 6, here a synchronous alternator, is mechanically connected on the same shaft of the turbine 15 and of the compressor 12, for being driven in rotation at the same angular velocity ω. The generator 6 converts the mechanical power supplied by the turbine 15 into active electric power P _E and makes it available for the distribution network 2.

The sensor module 7 comprises a plurality of sensors (known and not illustrated in detail) , which detect a plurality of parameters regarding the plant 1; in particular, the sensor module 7 detects the ambient temperature T _a , the inlet and outlet pressures Pi, P ₀ of the compressor 12, the current position POS _IGV of the IGV blades of the inlet stage 13 of the compressor 12, and other parameters, such as, for example, the angular velocity ω, the electric power P _E , the discharge temperature T _E of the gases at the discharge of the turbine 15 and a position signal MAIN corresponding to the position of the main switch 3.

The detection device 11, for example based upon a gas chromatography detects the composition of the gas supplied to the turbine 15 and determines the calorific value P _CAL and/or the carbon/hydrogen ratio C/H thereof.

The sensor module 7 and the detection device 11 supply the detected parameters to the control device 8, which uses them for generating a first control signal ϋi _G v and a second control signal Upv

In turn, the first and second control signals U _IGV * U _FV/ are supplied, respectively, to the first and to the second actuators 9, 10.

With reference to Figure 2, the control device 8 comprises a regulator 18, a limiter 19, and an analysis module 20.

The regulator 18, in itself known, receives the parameters detected by the sensor module 7 and generates a regulation signal U _FVREG and the first control signal Ui _GV , for example using a control algorithm of the "sliding mode" type. In particular, the regulator 18 operates so as to maintain at pre-set values a plurality of quantities of the plant 1, both in the first operating condition, in which a gas with high

calorific value is used, and in the second operating condition, in which a gas with low calorific value is used. In the second operating condition, however, the regulator 18 concurs with the limiter 19 in controlling the plant 1. Normally comprised between the quantities controlled by the regulator 18 is also the compression ratio β, which has a domain of admissable values Rp for conventional gases with high calorific value.

The regulator 18 moreover regulates the discharge temperature T _E of the exhaust gases of the turbine 15. For this purpose, the regulator 18 uses the first control signal U _IGV * which enables setting of the openings of the IGVs and consequently control of the air flow rate Q _A taken in by the compressor 12.

The limiter 19, which intervenes in the second operating condition, is supplied with the parameters: ambient temperature T _A , inlet pressure Pi and outlet pressure P ₀ of the compressor 12, calorific value P _CALλ and/or carbon/hydrogen ratio C/H of the gas. The limiter 19 issues a limitation signal S _LIM for blocking the second actuator 10, preventing an increase in the flowrate of fuel Q ₅ to the combustion chamber 14 so as to prevent a flowrate Q _τ evolving in the turbine 15 from overstepping a pre-set limit flowrate Q _TMAX - The value of the limit flowrate Q _TMAX is defined in the design stage and represents a threshold beyond which damages to the turbine 15 may occur.

In particular, the limiter 19 calculates the current compression ratio β of the compressor 12 and compares it, instant by instant, with a reference value β _SE τ depending upon the ambient temperature T _a and correlated to the limit flowrate Q _TMAX of the turbine 15. The reference value' β _SE τ moreover depends upon the type of syngas used (in particular upon its calorific value P _CAL ) and takes into account also the density of the syngas itself and of the air/fuel ratio

required. If the compression ratio β of the compressor 12 reaches or exceeds the reference value βsET/ the limiter 19 generates the limitation signal S _L i _M of the flow rate of fuel Q _G to the combustion chamber 14. If, instead, the current compression ratio β of the compressor 12 does not exceed the reference value βs _E T _/ the limiter 19 does not generate any signal. Given the composition of the syngases, in fact, the reference value β _S Eτ is such that the flowrate Q _T evolving in the turbine 15 remains below the limit flowrate Q _TMAX only if the current compression ratio β is lower than the reference value βs _ET - The evolving flowrate Q _τ would overstep, instead, the limit flowrate Q _TMAX in the presence of values of the compression ratio β comprised in the domain of admissable values Rp, but higher than the reference value βs _E T- The limitation signal S _L i _M generated by the limiter 19 and the regulation signal U _FVREG generated by the regulator 18, are supplied to the analysis module 20, which generates the second control signal U _F v In the case in point, in the absence of the limitation signal S _LIM/ the analysis module 20 supplies the regulation signal U _FVREG to the second actuator 10. Instead, if the analysis module 20 receives at input the limitation signal S _LIM/ which has maximum priority, it supplies it to the second actuator 10, neglecting the regulation signal U _FVREG -

The limiter 19 comprises a calculation module 22, a library module 23, and a comparison module 24.

The calculation module 22 receives at input the outlet pressure P ₀ and the inlet pressure Pi of the compressor 12 and uses them for calculating the current compression ratio β of the compressor 12 (β = Po/Pi) • The compression ratio β is supplied to the comparison module 24.

As illustrated in Figure 3, the library module 23 contains a plurality of reference curves βs _E τ(T _a ) _/ which supply values of the reference compression ratio β _SET as a function the ambient

temperature T _A and of the type of syngas used in the plant 1. Each reference curve βs _E τ(TA) stored in the library module 23 regards a different type of syngas . In the embodiment described herein, the type of syngas is determined on the basis of the value of the calorific value P _CAL - Alternatively, it is possible to use the carbon/hydrogen ratio C/H of the gas or also a combination between the calorific value P _CAL and the carbon/hydrogen ratio C/H.

With reference to Figure 2, the library module 23 receives at input the value of the calorific value P _CAL of the gas in circulation in the plant 1 and the ambient temperature T _A , selects a 'reference curve βsEτ(T _a ) corresponding to the value of calorific value P _CAL received, and supplies to the comparison module 24 the reference compression ratio β _SE τ at the current ambient temperature T _a .

As illustrated in Figure 4, the library module 23 considers an appropriate hysteresis in the passage from a limit curve P _SET (T _A ) of one gas to the limit curve βsEτ(Iα) corresponding to another gas to prevent continuous oscillations from one curve to another even for minor fluctuations of the calorific value P _C AL of the gas .

If the current value of the compression ratio β of the compressor 12 reaches or exceeds the reference compression ratio value βs _ET* the comparison module 24 generates the limitation signal S _LIM of the flowrate of fuel Q _G to the combustion chamber 14. The limitation of the flowrate of fuel Q _G to the combustion chamber 14 causes the regulator 18, by virtue of the regulation of the discharge temperature T _E , to generate the first control signal U _IGV of intake of air flow rate Q _a from the compressor 12 to obtain arrest of the movement of the IGVs.

If, instead, the current value of the compression ratio β of

the compressor 12 does not exceed the reference compression ratio value βs _ETr the comparison module 24 does not generate any signal.

In practice, the regulator 18 operates so as to maintain at pre-set values a plurality of quantities of the plant 1 in various steps of the first operating condition, in which a gas with high calorific value is used (said values may be different in the different steps, for example at start-up, in conditions of nominal charge, in the presence of variations of charge or of disturbance by the network 2, etc.) .

In the second operating condition, in which a gas with low calorific value is used, the regulator 18 is in any case active, but concurs with the limiter 19 in controlling the plant 1. In particular, the limiter 19 limits the domain of admissable values Rp for the compression ratio β below the reference value β _SE τ determined for the type of syngas in use and for the current ambient temperature T _A . When the plant 1 is supplied with syngas, in fact, the flowrate Q _τ evolving in the turbine 15 reaches the limit flowrate Q _MAX already at the reference value β _SET of the compression ratio β. The evolving flowrate Q _τ is excessive and dangerous for values higher than the compression ratio β, which would instead be admissible in the presence of a gas with higher calorific value.

The invention presents the advantages outlined in what follows.

First of all, the introduction of the limiter 19 within the control device 8 enables optimal performance of the plant 1 to be obtained, preserving the turbine 15 from any possible damage on account of the circulation of an evolving flowrate Q _T higher than the limit one Q _TMSX defined in the design stage; in fact, the value of the compression ratio β provides a reliable estimation of the air flow rate effectively supplied

by the compressor 12 and, hence, enables precise determination also of the overall flowrate of fluid evolving in the turbine 15. On the one hand, then, the limiter 19 intervenes only when the overall flowrate reaches the limit values set in the design stage, preventing degradation of the performance of the plant. On the other hand, the action of the limiter 19 is not vitiated by the variations of the thermodynamic parameters that inevitably arise with ageing of the plant; consequently, the plant is constantly able to supply the maximum electric power compatible with the safety specifications.

In the second place, the limiter 19 of the control device 8 also considers the frequent oscillations of the composition of the syngases that can shift the point of optimal operation of the plant 1; the syngases, in fact, are generally obtained by gasification of oil residue or by combustion of residual products in steelworks and, consequently, are characterized by fluctuations of the carbon/hydrogen ratio C/H and of the calorific value P _CAL - This aspect is very important, because, as the combustible gas varies, it is possible to control operation of the turbogas plant 1 in a precise and safe way.

Finally, it is evident that modifications and variations can be made to the control device described, without thereby departing from the scope of the present invention, as defined in the annexed claims. In particular, the selection of the reference curve for the reference value βsEτ(TA) can be performed even manually by an operator, since the type of gas supplied to the turbine is normally known. Possibly, the limiter could also use just one reference curve for the reference value P _SET (T _A ), if it is envisaged that the plant uses just one type of syngas or types of syngas with very similar characteristics.