WO1994028298A1

WO1994028298A1 - Arrangement in combined-cycle power plant

Info

Publication number: WO1994028298A1
Application number: PCT/FI1994/000210
Authority: WO
Inventors: Eero Juho Ilmari Kurki-Suonio
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-05-31
Filing date: 1994-05-26
Publication date: 1994-12-08
Anticipated expiration: 1995-11-30
Also published as: FI932474L; FI94895C; FI94895B; FI932474A0

Abstract

An arrangement for increasing the overall efficieny by utilizing waste heat in a combined-cycle power plant in which a first electric generator (1) is powered by a supercharged internal combustion piston engine (3), the exhaust gases (5) whereof being conducted through a gas turbine driving the supercharger (6) for the intake air (8) of the engine and a steam boiler (10) for generating pressurized steam (15), said steam being conducted through a steam turbine (4) connected to a second electric generator (2) further into cycle. The exhaust gases of the internal combustion piston engines are conducted from the engine first (5a) into the steam boiler (10) at a pressure essentially higher than the atmospheric pressure and only thereafter (5b) into the gas turbine or turbines of the supercharger (6) for the intake air (8), where they expand into the atmospheric pressure in order to increase the efficiency of the elctricity production of the combined-cycle power plant.

Description

Arrangement in combined-cycle power plant

The present invention relates to an arrangement for in- creasing the overall efficiency by utilizing waste heat in a combined-cycle power plant, in which first electric generator or generators is / are powered by one or more supercharged internal combustion piston engine / engines, the exhaust gases whereof being first conducted at a pressure substantially higher than the atmospheric pressure into a steam boiler, comprising at least a steam drum and a superheater, for generating pressur¬ ized steam, said steam being carried through a steam turbine connected to a second electric generator further into the cycle, and the exhaust gases are first after the steam boiler conducted into the gas turbine or turbines of the supercharger of the intake air where they expand into the atmospheric pressure.

Utilisation of waste heat has as such been known for long in association with diverse power plants. Particularly waste heat at a low temperature can be made use of for various heating purposes independent of the type of power plant. An arrangement like that is described, for instance, in reference DIESEL & GAS TURBINE WORLDWIDE, January-February 1993: "Prize-Winning CHP Plant Exploits Solvent Vapors", p. 18 and 19, and "The Modigen Modular Power Plant Concept", p. 30 and 31. They are not, in fact, essentially so-called combined- cycle power plants, which refer to power plants in which electric energy is produced in a primary circuit by means of a generator, and waste heat is utilised in the secondary circuit for producing additional electric energy. Hereby, a problem typical of said combined-cycle power plants arises when the thermal energy of the secondary circuit prevails at a relatively low tem¬ perature, electricity being thus generated only at a fairly low efficiency. One way of increasing the efficiency of the use of waste heat in the secondary circuit of a combined-cycle power plant is to use additional heating wherewith the tem¬ perature of the exhaust gases emitted by the primary circuit is raised. Hereby, the secondary circuit generates greater quantities of relatively hot steam with the steam boiler, thus resulting in increased efficiency in the steam turbine and the generator. Now, the overall quantity of waste heat will grow, leading to reduced overall efficiency in the entire power plant.

Also such arrangements are known in which endeavours are made to increase the overall efficiency in combined- cycle power plants, in which the primary circuit comprises an internal combustion engine and an electric generator. In case a gas turbine is used as the internal combustion engine, the temperature of the exhaust gases whereof being relatively high, about 500°C, or clearly even higher, the waste heat of the exhaust gases can be used relatively effectively for steam generation, and further, for electricity generation. Such arrangements are introduced e.g. in articles DIESEL & GAS TURBINE WORLDWIDE, January-February 1993: "Industrial CHP Plant Meet Flexible Steam Requirements", p. 20-22, and JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER, OCTOBER 1991, VOL. 113, P. 475-481: "Combined Cycle Plants With Frame 9F Gas Turbines". Said publications introduce a number of different circulation processes for generating steam for use in steam turbine or turbines. No descriptions are therefore given in said references about how the exhaust gas of an internal combustion piston engine, such as supercharged diesel engines used in power plants, prevailing at substantially lower temperature, about 300 to 350°C, can be used effectively for increasing the efficiency, particularly in electricity production. With said exhaust gas of a diesel engine at a lower temperature and the steam generation cycles according to the above references, steam turbines cannot be used in the preferred operation range thereof.

Using the waste heat contained in the exhaust gases of the diesel engine in electricity production described in reference DIESEL ENGINEERING, Spring 1980: "MAN diesel based CHP schemes", p. 5-11. In the arrangement of the reference, the exhaust gases emitted from cylinders of a supercharged diesel engine, at about 500°C, are first conducted through the driving side of the supercharger with which supercharged intake air is supplied to the engine. For that purpose, the thermal energy of the ex¬ haust gases is sufficient. Thereafter, the exhaust gases are conducted at about 350°C into a steam boiler wherein steam is generated by means of a steam generator and a superheater into a steam turbine. The steam turbine has been connected to a second electric generator, whereas the diesel engine has been connected to the first electric generator for producing electricity. When using heavy fuel, the exhaust gases discharged from the steam boiler are required to be at about 160 to 180°C tem¬ perature, not to cause condensation of sulphuric compounds with any detrimental consequences thereof. The difference of temperature utilised in the steam boiler, about 170 °C, is thus very small, and above all, it occurs at the low temperature mentioned above, consequently, an increase in electric energy gained thereby is relatively small. Even though there were no temperature limitation as referred to above, caused by the sulphur contained in the heavy fuel, at lower temperatures it cannot be transformed substantially into energy of another form, i.e. steam, in a boiler plant.

In reference JP-58-143114, an inverse arrangement to the procedure described above is presented in which the exhaust gases are first conducted into a boiler comprising a steam generator and a superheater, and only thereafter, into the turbine of a supercharger. It has to be admitted that a hotter steam increases the power of the generator, though at the cost of reduced power in the supercharger, and therethrough, in the diesel engine. Consequently, the overall efficiency is no better than using the design described above. A lower power in the supercharger also causes difficulties in engine operation, particularly at partial load.

Reference DE-310 329 discloses an exhaust gas cycle in which the exhaust gases are first conducted into a water preheater, and thereafter, into the turbine of the supercharger. In the arrangement of the reference the exhaust gases do not drive the superheater, instead, in all embodiments of the reference, the superheating of the steam is performed in a separate part of the installation, in which the combustion of a second fuel is implemented irrespective of the diesel engine. In said reference, the aim is to reduce the operation temperature of the internal combustion engine, which operation, as well as the embodiments, result in lowered efficiency, which is entirely inappropriate. In addition, the compulsory superheating in a separate boiler installation makes the entire arrangement com¬ plicated, large in size and costs-involving.

The objective of the present invention is therefore to provide an arrangement in which the efficiency of the electricity production of an electric power plant operating primarily with an internal combustion piston engine, and particularly with a supercharged diesel engine, of a so-called combined-cycle power plant can be increased, in which power plant the waste heat contained in the exhaust gases of said engine is utilised with the aid of steam power, typically a steam boiler and a steam turbine. A second aim of the present invention is an arrangement in which the additional heating of the steam boiler is not necessary, or it is needed only to a very limited degree, or that waste heat at an extremely low temperature, i.e. below about 160 to 180°C, is produced minimally. The aim is therefore to produce energy of maximal refining degree, i.e. electricity, out of minimal overall fuel quantity. A further aim of the invention is gained through an arrangement with which a remarkable portion also from an exhaust gas at a tem¬ perature substantially below 160 to 180°C, of the thermal energy thereof, is recovered thus increasing the efficiency of the electricity production, this taking place typically when low-sulphuric fuel, or even sulphurless fuel, is used in the diesel engine, whereby no lower limit, caused by corrosion, etc., concerns the temperature of the discharging exhaust gases.

The drawbacks described above can be eliminated and the objectives stated above are achieved by means of an arrangement according to the present invention, which is characterized in what is stated in the characteristic features' part of claim 1.

The most essential advantage of the invention lies in that with an arrangement according thereto the tem¬ perature in the steam generating cycle can be raised compared with the state of art arrangements, so that the efficiency of the steam cycle in the electricity pro¬ duction increases remarkably.

This advantage is maintained also in the overall efficiency although part of the additional energy thus obtained may have to be used for supercharging. A second advantage of the invention lies, particularly in a situation in which there is no lower limit for the exhaust gas temperature due to corrosion or equivalent, in that the heat of exhaust gases can be recovered when approaching the temperatures equivalent to the ambient temperature, in order to increase the efficiency of electricity production and the overall efficiency. The invention is described below in detail referring to the accompanying drawings.

Fig. 1 presents schematically an arrangement according to the present invention for increasing the efficiency of electricity production in a combined-cycle power plant.

Fig. 2 presents schematically a second arrangement of the invention for increasing the efficiency of electricity production in a combined-cycle power plant.

Fig. 3 presents schematically a third arrangement according to the invention for increasing the efficiency of electricity production in a combined-cycle power plant.

Fig. 4 presents schematically a fourth arrangement according to the invention for increasing the efficiency of electricity production, illustra¬ ting merely part of the process.

Fig. 5 presents an arrangement of the invention implemented in a combined-cycle power plant containing a number of diesel engines.

Figs. 1-5 present a number of arrangements of the invention differing in details in a combined-cycle power plant. The power plants consist of supercharged internal combustion piston engines used as power sources to provide the net power, such as diesel engines, gas engines or dual fuel engines, of which there may be one in a power plant, as in Figs. 1 to 4, or several, as in Fig. 5. A diesel engine 3 drives a first electric generator 1 for the generation of electrical energy. The intake air 8 of the diesel engine is supercharged by means of an exhaust gas supercharger 6, through the driving part 6 of which the exhaust gases are conducted, for which a gas turbine in general serves. The super¬ charged and cooled air is supplied as combustion air 8' into the diesel engine. As the second source of energy in the combined-cycle power plant to supply net power, hot pressurized steam 15a,b,c is used, said steam^"being generated in the steam boiler 10 and conducted into the steam turbine 4. The steam turbine 4 drives a second electric generator 2 for producing electricity. Said hot steam 15a,b,c is in turn generated with the exhaust gases 5 of the diesel engine 3 by conducting them through the steam boiler 10. The steam boiler 10 includes at least a steam generator 18a with equivalent boiler pipings 20 and a superheater 19a for high- pressure steam 15a supplied into the steam turbine 4.

As taught by the invention, the hot exhaust gases 5a of the diesel engine 3 are conducted directly from the engine first into the steam boiler 10 and only there- after from there to the driving side 7 of the super¬ charger 6 of the intake air 8. By such arrangement, an about 260°C difference of temperature in the temperature range of about 500 to 240°C can typically be utilised in the steam boiler 10. With the use of such steam, a quite satisfactory efficiency in electricity production in the steam turbine can be obtained and consequently, in the second electric generator. Depending on the design of the diesel engine, the temperature of the exhaust gases thereof may be even higher, about 550°C, or as far as the development proceeds, even 700°C, whereby the efficiency of the use of steam rises even higher.

A consequence of the arrangement according to the invention is, however, that the temperature of the exhaust gases before the gas turbine of the supercharger is remarkably lower in an arrangement according to the invention than in a state of art arrangement. Since the power of a gas turbine operating at a given pressure ratio and mass flow of the gas is directly proportional to the absolute temperature, also the power of the supercharger is respectively lowered. Such situation can according to the invention be solved either (a) by supplying the additional power required in the super¬ charger 6 by means of a connection 13 from the steam turbine 4, (b) by dimensioning (by selecting) the gas turbine 7 so that the pressure of the gas before the gas turbine is higher than the state of art value, or (c) by raising the temperature of the combustion gases by burning in the steam boiler 10 an additional fuel. The alternatives (a) and (b) are more preferred alternatives in efficiency than (c) because the amount of the waste heat is not increased therein.

The above connection 13 of the steam turbine 4 to the driving shaft 9 of the supercharger 6 may, according to the invention, be performed in a number of ways. In the embodiment shown in Fig. 1 the steam turbine 4 includes a high pressure section 14a and a low-pressure section 14c, of which the low-pressure section 14c has been connected to the shaft 9 of the supercharger by the drive shaft 12. Such connection can be direct or it has been implemented by means of a gearbox 16 or equivalent. The high-pressure steam turbine section 14a has been connected with a shaft 17 only to the second generator 2, but not to the supercharger. In the implementation of Fig. 2 the steam turbine composed of a high-pressure section 14a and a low-pressure section 14c has been connected 13 by the shaft 17 thereof with the aid of the gearbox 16 or equivalent indirectly or alternatively, directly without a gearbox to the drive shaft 9 of the supercharger 6. Said embodiment is most advantageous according to the present concept because all power therein not needed by the supercharger (on partial engine power the supercharger needs abundantly additional power, while on the full power, the super¬ charger needs hardly any additional power) , drives the second electric generator. In the embodiment of Fig. 4 the medium-pressure section 14b of the steam turbine 4 has been connected 13 directly to the drive shaft 9 of the supercharger 6. In said embodiment the high pressure and low-pressure sections 14a of the steam turbine 4, respectively 14c, have in turn been connected by the shaft 17 thereof directly to the second generator 2.

As may be concluded from all presented above, in the arrangement according to the invention, in which relatively hot exhaust gases 5a can be used for steam generation, it is possible, and even preferred, to use steam on several pressure and temperature levels. There¬ fore, the steam cycle according to the invention includes high-pressure steam 15a and medium-pressure steam 15b and/or low-pressure steam 15c. Of these, at least the high-pressure steam 15a can in general be used in its entirety on the high-pressure side 14a of the steam turbine 4 for generating electricity with the second generator 2 because the additional energy required by the supercharger 6 in the arrangement of the invention is however relatively small. Typically, the additional energy needed by the supercharger 6 is extracted either from the section 14b of the steam turbine employing medium-pressure steam 15b, provided such arrangement has been arranged in the turbine, or from the low-pressure section 14c employing low-pressure steam 15c, or from both thereof, depending on the amount of the additional energy required and on the con- struction designed for the steam turbine.

Fig. 3 shows a different arrangement of the invention mentioned above, wherewith additional energy can be provided in the supercharger 6. Therein, extra heat 21 is conducted into the steam boiler 10 by burning appropriate fuel with e.g. a burner not shown in the figure, such as the fuel in the diesel engine functioning as the main power source. First, the pressure and temperature of the steam may in such case be raised to some extent as the temperature on the combustion chamber side 22 rises. In addition, the temperature of the exhaust gases 5b extracted from the combustion chamber 22 side of the steam boiler can be arranged to be sufficiently high, e.g. about 300°C, whereby the temperature and amount of the exhaust gases 5b suffice for driving the supercharger 6 on the power required. The amount of waste heat remaining after the supercharger somewhat exceeds the amount of the first design type of the present invention, but its significance is dependent on how said waste heat has been used or not used in a combined cycle power plant in each case, inter alia for heating purposes. A second possibility for increasing the efficiency of the super¬ charger in an equivalent manner is to take steam at a point of the steam cycle and to supply it together with the exhaust gases into the turbine of the supercharger.

All arrangements according to the present invention described above, wherein the gas turbine driving the supercharger is located behind the steam boiler, are characterized in the specific feature that on the combustion chamber 22 side of the steam boiler 10 or on the flue gas side, that is, on the side of the piping 20 of the steam boiler on which the hot exhaust gases 5 flow (the concept combustion chamber or flue gas side is used in the present application irrespective of whether fuel is burned in the boiler or not) , a substantially higher pressure p₂ is prevalent than the atmospheric pressure p_t. On the side of the combustion chamber 22 the overpressure Pj-p₂ = Δp over the atmospheric pressure thus prevails, which means that the combustion chamber is pressurized. Said overpressure Δp depends on resistance of flow of the operation side 7 of the super¬ charger 6, i.e. of the gas turbine 7 of the super¬ charger. The same pressure of the combustion chamber 22 of the steam boiler is as such directed also at the exhaust gases 5a emitted from the diesel engine in the form of backpressure of the engine. Provided that the operation conditions of the diesel engine 3 are maintained approximately conventional, whereby said backpressure is not raised to the extent that the operation principle of the engine essentially changes, the overpressure of said combustion chamber 22 is according to the invention preferredly in about the range of 1.0 to 3 bar, but it may rise up to 6 bar, whereby the power of the gas turbine substantially increases and that of the engine decreases. In the most conventional range the diesel engine operates when said overpressure Δp is of the order of magnitude of 2 bar. Said overpressure can be provided to be of desired magnitude by designing and providing such flow cross- section area in the exhaust gas turbine 7 driving the supercharger 6 that the flow resistance leads to the overpressure intended. Mainly the flow cross-section area of the exhaust gas turbine 7 can be dimensioned by the length of the turbine wings, though not described in detail in the present context, since it is a question of the state of art of its own.

In an instance in which a combined cycle-power plant comprises several internal combustion piston engines, such as supercharged diesel engines 3a,3b,3c, and respective electric generators 1, as shown in Fig. 5, it is in general preferred to use one steam boiler 10 only, to which the exhaust gases 5a from the engines 3a,3b,3c are together conducted. Similarly, it is in general preferred to use one supercharger 6 in common for said several engines, on the driving side 7 whereof all exhaust gases 5b from the steam boiler are conducted. Thus, the intake air 8 is conducted and distributed as supercharged and cooled combustion air 8' for all diesel engines. It is obvious that each diesel engine can be provided with a steam boiler or supercharger of its own, or a combined-cycle power plant may comprise one steam

RECTIFIED SHEET (RULE 91; boiler and a plurality of superchargers, or the numbers can be arranged differently. It is obvious that also in the present instance the additional energy required by the supercharger 6 or superchargers has been arranged according to the present invention, either by connecting the shaft 17 of the steam turbine 4 driving the second electric generator, or a shaft of a section 14a, b, c of the turbine to the shaft 9 of the supercharger, or by conducting additional thermal energy into the steam boiler 10, or by means of a combination of any of these.

It is obvious that the steam generating cycle according to the present invention includes a requisite number of, and as designed, also steam generators 18b and superheaters 19b for steam cycles 15b and 15c at lower pressures, and each time respective boiler pipings 20 in the the combustion chamber 22. Similarly, a steam generator cycle according to the invention may contain mixing preheaters 23, heat exchangers 24 and/or other potential components known themselves in the art. Figs. 4 and 5 present all said additional components in block 25 which may include any prior art components used in the steam processes.

A state of art combined-cycle power plant is compared below with a combined-cycle power plant according to the invention regarding the electric power values obtained therein, and consequently, the efficiency of the electricity production. A state of art combined-cycle power plant composed of a diesel engine and an electric generator is taken as a starting point in which the combined electric power of the generators is 100 MW. In the power plant, the temperature of the exhaust gases immediately after the cylinders is 495°C, and directly after a subsequent turbosupercharger it is 325°C. The lowest permitted temperature of the fuels is maintained at 160°C. The pressure of the condenser is 0.03 bar and the overall efficiency of the generators η^ = 0.95. In a state of art installation, the power of the steam turbine must also be calculated in addition to the power generated by the diesel engine. When a two-pressure process is in question, the pressures of the steam process after the supercharger are 20 bar and 6 bar, respective superheating temperatures, 300°C and 200 °C. The isenthropic efficiency of the steam turbine η_κ = 0.81. The preheating of the feed water is performed with the aid of a preheater using the cooling water of the engine, an economizer and a mixing preheater. The electric power generated by the steam turbine when η^ = 0.95 is 9.4 MW (without self-operation power).

In an installation according to the invention, it is to be considered in addition to the power generated by the steam turbine that an exhaust gas turbine positioned after the steam boiler generates less power in the supercharger than in a state of art supercharger, as described above, because the exhaust gases are cooled when arriving thereat. Here also, the steam process is a two-pressure process, the pressures thereof being 80 bar and 23.2 bar; respectively, the superheating temperatures 470°C and 270°C. The isenthropic efficiency of the turbine η_κ = 0.83. The preheating of the feed water consists of a preheater utilising the cooling water of the diesel engine and of three regenerative preheaters, the middlemost thereof being a mixing preheater.

Hereby, the mechanical power generated by the steam turbine is 23.9 MW. For the powers of the exhaust gas turbines, assuming the pressure ratio to be, i.e. for the counterpressure to the exhaust gas turbine, this being in the arrangement of the invention simultaneously the overpressure Δp on the side of the combustion chamber of the steam boiler and for the isenthropic efficiency η_sc = 0.85, the following values are obtained: state of art installation = 24.1 MW, installation according to the present invention = 15.3 MW, the difference thereof being 8.8 MW. In this embodiment of the invention, this power is derived directly from the shaft of the steam turbine, whereby the net power from the steam turbine of the installation of the invention is 15.1 MW.

All in all, the increase of net power provided by the arrangement of the present invention is 5.7 MW, i.e. about 6% compared with the state of art arrangement initially described.

Claims

1. An arrangement for increasing the overall efficiency by utilizing waste heat in a combined-cycle power plant where a first electric generator or generators (1) is or are powered by one or several supercharged internal combustion piston engine / engines (3) , the exhaust gases (5) whereof being first (5a) conducted at a pressure (p₂ ) essentially higher than the atmospheric pressure (p^ into a steam boiler (10) , comprising at least a steam drum (18a) and a superheater (19a) for generating pressurized steam (15a, 15b, ...), said steam being conducted through a steam turbine (4) connected to a second electric generator further into cycle, said exhaust gases being conducted first after the steam boiler (5b) into the gas turbine or turbines (7) of the supercharger (6) for the intake air (8) , in. which they expand into the atmospheric pressure, characterized in that in said arrangement the power of the supercharger (6) is increased with the aid of the part of the power of said steam turbine (4) mounted on the driving shaft

(9) thereof, in order to enhance the efficiency of the electricity production of said combined-cycle power plant.

2. Arrangement according to claim 1, characterized in that the part of the power of the steam turbine (4) connected on the driving shaft (9) of the supercharger (6) is derived from a low-pressure and/or medium pressure steam turbine (14c and/or 14b) , and in that at least the high-pressure steam turbine (14a) drives said second electric generator (2) .

3. Arrangement according to claim l or 2, characterized in that the shaft (17) of the high-pressure steam turbine (14a) is either entirely unconnected to the driving shaft (9) of the supercharger or alternatively, connected thereto by means of a coupling, transmission, such as gearbox (16) , or directly.

4. Arrangement according to claim 1, characterized in that on the flue gas side (22) of the steam boiler (10) on which the exhaust gases (5) of the engine flow, a pressure (p₂) substantially higher than the atmospheric pressure (p is prevalent, said overpressure (Δp) varying approximately in the range from 1.0 to 6 bar, that the driving side (7) of the supercharger comprises an exhaust gas turbine, and in that said overpressure

(Δp) is selected in advance to be appropriate with a view to the overall system of the combined-cycle power plant with the aid of the size of the flow cross-section area of the exhaust gas turbine (7) driving the supercharger (6) or in an equivalent manner.

5. Arrangement according to claim 1, characterized in that the internal combustion piston engine (3) is a diesel engine, a gas engine or a dual fuel engine, and in that the steam boiler (10) comprises furthermore a second or other steam drums (18b) and superheaters (19b) for lower pressure steam cycles (15b,15c), and respective steam pipings (20) and flue gas channels (22) , and condensers (11) .

6. Arrangement according to claim 1, characterized in that in addition to the flow of exhaust gases (5) also additional heating is used in the steam boiler (10) , such as additional heat (21) produced with burners, or alternatively, steam or equivalent, extracted from the steam cycle is fed into the gas turbine (6) , or alternatively, the pressure (p₂) in the flue gas channels (22) of the steam boiler (10) is raised for raising the temperature of the steam produced by means of dimensioning the flow channels of the gas turbine (6) and/or for increasing the power of the supercharger (6) .

7. Arrangement according to claim 1 in a combined-cycle power plant with two or more internal combustion engines

(3a,3b,...) driving first generators (1), characterized in that in the plant there is one supercharger (6) in common for two or more engines (3a and 3b and/or 3c) so that the hot exhaust gases (5a) of the engines are conducted together first into one steam boiler (10) , and thereafter, when cooled (5b) , into the gas turbine (6) , and the supercharged combustion air (8') thus produced is distributed to said engines.