EP0724683B1

EP0724683B1 - Integration construction between a steam boiler and a steam turbine and method in preheating of the supply water for a steam turbine

Info

Publication number: EP0724683B1
Application number: EP94928907A
Authority: EP
Inventors: Ilmari MÄKILÄ; Markku Raiko; Sasu Valkamo; Jarmo Tuominen
Original assignee: Fortum Power and Heat Oy
Current assignee: Fortum Power and Heat Oy
Priority date: 1993-10-19
Filing date: 1994-10-11
Publication date: 2000-06-28
Anticipated expiration: 2014-10-11
Also published as: DE69425064D1; EP0724683A1; DE69425064T2; WO1995011370A1; PT724683E; ES2148346T3; FI934603A0; ATE194208T1; FI101163B; DK0724683T3; GR3034073T3; AU7814694A; FI934603A7; EE03219B1

Abstract

The invention concerns an integration construction between a steam boiler and a steam turbine and a method for preheating of the supply water. In the integration construction the steam is passed from the steam boiler (10) along the duct (24a) into the steam turbine (11) so as to rotate the electric generator (12) which generates electricity. The economizer (23) consists of at least two parts, comprising at least one first economizer part (23') and at least one second economizer part (23''). The supply water is passed from the cold economizer part (23') to a supply-water preheater, which consists of a heat exchanger (24) in which thermal energy is transferred from bled steams of the steam turbine, either directly or through a medium, preferably water, into the supply water. After this, the supply water, which had been preheated by means of bled steams from the steam turbine, is passed in the steam boiler (10) into the hot economizer part (23'') and further to the vaporizer (240) and into the superheater (24) and through the superheater into the steam turbine.

Description

The invention concerns a powerplant according to the preamble of the patent claim 1 and a method in preheating of the supply water for a steam turbine according to the preamble of the patent claim 5.

The last heat face of a steam boiler before the smokestack is either a flue-gas/air heat exchanger or an economizer. In the present application, a flue-gas/air heat exchanger is understood as a heat exchanger between flue gas and combustion air, in' which the heat is transferred from flue gas to combustion air to preheat the combustion air. In the present application, an economizer is understood as a heat exchanger in which thermal energy is transferred from the flue gases to the supply water.

When a flue-gas/air heat exchanger is used, the supply water for the boiler can be preheated by means of bled steam from the steam turbine, whereby the efficiency of the steam turbine process is improved. A flue-gas/air heat exchanger, i.e. a heat exchanger in which thermal energy is transferred from the flue gases directly into the combustion air, is usually not used in small steam power plants because of its high cost.

When a flue-gas/air heat exchanger is not used, the flue gases of the steam boiler are cooled, before they are passed into the smokestack, by means of an economizer. In such a case, the supply water cannot be preheated by means of bled steam from the steam turbine, because the preheating would raise the ultimate temperature of the flue gases and would thereby lower the efficiency of the boiler.

From the prior art, also the mode described in the patent DE 2 243 380 is known for integration of a steam boiler and a steam turbine. In the method, the supply water is divided into two parts, i.e. into a duct passing to the boiler and into a duct passing to the high-pressure preheating plant. It is an advantage of said invention that the flue-gas/air heat exchanger can be substituted for by means of this solution. On the other hand, the complexity of the system is a drawback.

From the prior art, also the solution described in DE 3 111 011 A1 is known, in which the supply water is preheated, out of control-technical reasons, both by means of bled steam and by means of flue gases. Further, the solution also includes a flue-gas/air heat exchanger. A drawback is the complexity and the high cost of the system of integration.

In the economizer of a steam boiler, heat is transferred from the flue gases to the supply water. The change in the temperature of the supply water in the economizer is lower than the change in temperature at the flue gas side. The increase in the temperature of the supply water is, as a rule, 40...50 % of the corresponding lowering of the temperature at the flue gas side. Therefore, the difference in temperature at the hot end of the economizer is considerably higher than at the cold end. It follows from this observation that, besides the heat obtained from the flue gases, also other heat can be transferred to the supply water. In a steam turbine process, it is advantageous to utilize bled steam for preheating of the supply water.

In an arrangement of US 3,913,330, a flue gas-air heater is accomplished by means of an intermediate circuit. The power of the heat exchanger is transferred in its entirety to preheating of air and thus it does not constitute a part of the preheating circuit of supply water. The supply water serves as a fluid of the intermediate circuit. The use of an intermediate circuit always involves exergy losses caused by temperature differences in heat transfer. In this case, preheating of air takes place by means of high-temperature supply water, and the energy losses associated therewith are considerable. The ratio of the heat capacity flow of combustion air to that of supply water is 2 to 2.5. The advantages of this coupling arrangement relate to the fact that it is not desirable to build hot air ducts associated with a flue-gas/air heater in a boiler construction, inter alia, in ships.

The US patent 3,913,330, therefore, describes a conventional preheating system where a preheater of air (heat exchanger) is replaced by an indirect system. The preheating power to be transferred from flue gases to combustion air is in this construction first transferred to the supply water, which transfers the heat to the combustion air, after which the supply water has cooled back to its starting temperature. The supply water is only a fluid in said process, and thus it does not participate in the preheating process in the thermodynamical sense. Instead of supply water, there could also be a closed water circulation commonly used in heating, plumbing and air conditioning systems.

In view of this state of the art it is an object of the present invention to provide a power plant and a method in preheating of the supply water for a steam turbine with which an improved efficiency of the entire power plant process can be achieved.

By means of the invention, integration of the steam boiler and of the steam turbine process is made more efficient. By means of the arrangement in accordance with the invention, the flue gases of the steam boiler can be cooled efficiently at the same time as the efficiency of the steam turbine process is improved.

The investment cost is lower than that of an alternative provided with a flue-gas/air heat exchanger:

improved controllability and boiler efficiency
smaller boiler building
lower cost of the boiler.

When a flue-gas/air heat-exchanger solution is unprofitable, by means of the present invention it is possible to achieve an improved process when the use of bled steam can be increased.

The invention is advantageous especially when the combustion air of the steam boiler is heated in one or several steam/air heat exchangers that are connected in series and that utilize bled steam.

The invention will be described in the following with reference to some preferred embodiments of the invention illustrated in the accompanying figures, the invention being, yet, not supposed to be confined to said embodiments alone.

Figure 1 shows an embodiment of the integration construction which is not part of the present invention, in which the boiler construction comprises an economizer for preheating the supply water by means of thermal energy taken out of the flue gases, which economizer is divided into two parts, the supply water duct between said economizer parts being provided with a supply-water preheater, which consists of a heat exchanger in which heat is transferred from bled steams to preheating of the supply water.

Figure 2 shows a first preferred embodiment of the invention in which the economizer is divided into two parts, between which there are two preheaters of supply water, which consist of heat exchangers in which the supply water is preheated in two steps by means of bled steams from the steam turbine.

Figure 3 is a temperature/thermal-capacity graph of an economizer operation as per the embodiment of Fig. 1 and which is also not part of the invention.

Figure 4 is a temperature/thermal-capacity graph of the economizer of a steam boiler as per Fig. 2.

Figure 5 is a temperature/thermal-capacity graph of an economizer operation in which the economizer consists of three parts and in which preheating of the supply water is carried out between the parts of the economizer by means of bled steams.

According to Fig. 1, the steam boiler is denoted with the reference numeral 10, the steam turbine with the reference numeral 11, and the electric generator that is rotated by the turbine and that generates electricity with the reference numeral 12. In the illustrated embodiment of Fig. 1, the combustion air is introduced (arrow L₁) by means of the fresh-air blower 14 of the boiler 10 along the duct 13 into the furnace M of the boiler 10. The fuel is introduced along the duct 15 as is indicated by the arrow L₂.

The frame constructions of the boiler 10 are denoted with the reference R. The flue gases are passed from the boiler 10 into the smokestack 16.

In the embodiment of Fig. 1 which is only for a better understanding of usual constructions, the condenser is denoted with the reference numeral 17 and the supply water tank with the reference numeral 18. From the condenser 17, which is a heat exchanger, there is, for example, a district heating duct 17a for utilization of the condensing heat. The condensate pump is denoted with the reference numeral 19.

To the supply-water tank 18, there is a bled-steam duct 20 from the steam turbine. The steam duct 21a from the steam turbine 11 communicates with the inlet side of the condenser 17, and the condensate-water duct 21b communicates with the outlet side of the condenser 17, while the condensate-water pump 19 circulates the condensate water into the supply water tank 18.

From the supply water tank 18 there is a supply water duct 22a to the economizer 23', i.e. to the heat exchanger, which is placed inside the frame construction R of the boiler 10 as one heat face in connection with the flue gas duct D. In this connection, heat is transferred from the flue gas S before the smokestack 16, by means of the heat exchanger 23, to the supply water. The heated supply water is made to flow by means of the pump 190 along the duct 22b to the supply-water preheater, i.e. the heat exchanger 26, to which a bled-steam duct 27a passes and from which heat exchanger 26 there is a duct 27b for condensate water to the supply water tank 18. Thus, in the heat exchanger 26, preheating of the supply water that flows in the duct 22b is carried out by means of the thermal energy obtained from bled steams. The supply water is passed further along the duct 22b, after it has been brought to a higher temperature, into the second part 23'' of the economizer 23, i.e. of the flue-gas/supply-water heat exchanger, and further from the economizer 23'' through the vaporizer 240 to the superheater 24 and, in the form of steam, along the duct 24a, to the steam turbine 11.

Fig. 2 shows a first preferred embodiment of the invention which is in the other respects similar to that shown in Fig. 1, except that combustion-

air preheaters

25a,25b, i.e. steam/air heat exchangers, are placed in the duct 13. They are heat exchangers in which bled-steam heat is transferred to the combustion air. It is a further difference in comparison with the embodiment shown in Fig. 1 that, between the first part 23' and the second part 23'' of the economizer of the boiler, the supply water is heated in two stages by means of thermal energy recovered from bled steams.

In Fig. 2, as is also the case in the conventional configuration of Fig. 1, the steam boiler is denoted with the reference numeral 10, the steam turbine with the reference numeral 11, and the electric generator that generates electricity and that is rotated by the turbine with the reference numeral 12. The combustion air is introduced (arrow L₁) by means of the fresh-air blower 14 of the boiler 10 along the duct 13 into the furnace M of the boiler 10. The fuel is supplied along the duct 15 in the way indicated by the arrow L₂.

The frame constructions of the boiler 10 are denoted with the letter R. The flue gases are passed from the boiler 10 into the smokestack 16.

The condenser is denoted with the reference numeral 17, and the supply water tank with the reference numeral 18. The condenser 17 is a heat exchanger. It comprises a cooling-water duct 17a for removal of the condensate heat. Thus, condensate heat is transferred from the exhaust steam of the turbine to the cooling water. The condensate pump is denoted with the reference numeral 19.

From the steam turbine 11 there is a bled-steam duct 20 to the supply water tank 18. The exhaust-steam duct 21a from the steam turbine 11 communicates with the inlet side of the condenser 17, and the condensate-water duct 21b communicates with the outlet side of the condenser 17 while the condensate-water pump 19 circulates the condensate water into the supply water tank 18. From the supply water tank 18 there is a supply-water duct 22a to the heat exchanger 23.

In the inventive embodiment of Fig. 2, the supply water is made to flow by means of the pump 190 along the supply-water duct 22a to the economizer 23', i.e. to the flue-gas/supply-water heat exchanger, in which thermal energy of the flue gas is transferred into the supply water through tubular heat faces of equivalent placed in the heat exchanger 23' in the flue-gas duct D. Along the duct 22b, the supply water which has been preheated in accordance with the invention is passed further into a first heat exchanger 26a, to which there is a bled-steam duct 27a from the steam turbine 11 and from which there is an outlet duct 27b for condensate/steam into the supply water tank 18.

After this, the supply water that was preheated by means of bled steam from the steam turbine in the first heat exchanger 26a is transferred into a second heat exchanger 26b, to which there is a bled-steam duct 28a from the higher-pressure steam turbine side and from which there is an outlet duct 28b. The duct 28b passes to the heat exchanger 26a, so that the condensate is transferred further through the outlet duct 27b into the supply water tank 18. Thus, the supply water that was preheated in two stages by means of bled steams is transferred into the second part 23'' of the two-part economizer of the boiler 10, from which part 23'' the supply water is passed further into the vaporizer 240 placed next to the furnace of the boiler and into the superheater, and through its heat exchanger constructions along the duct 24a, in the form of steam into the steam turbine 11.

In the inventive embodiment of Fig. 2, it is preferable that the bled-steam duct 27a includes a branch point C₂ for passing a bled-steam duct 29a to the heat exchanger 25a for preheating of the combustion air, and that the bled-steam duct 28a includes a branch point C₃ for passing a bled-steam duct 30a to the heat exchanger 25b. From the

heat exchangers

25a,25b there are

ducts

29b,30b to the branch point C₄ for passing the condensate to the duct 27b and further into the supply water tank 18.

Within the scope of the invention, an embodiment is also possible in which bled steam of the same pressure level is passed into one or several supply-water preheaters and/or into one or several combustion-air preheaters.

Thus, in the construction illustrated in Fig. 2, the preheating of the supply water is carried out by means of bled steams from the steam turbine between the two parts of the two-part economizer. Further, in the construction, by means of the thermal energy recovered from the bled steams, combustion air is also heated, which is passed along the duct 13 into the furnace M of the boiler 10.

Fig. 2 illustrates a preferred mode of carrying out the invention. The supply water is passed into the first economizer package 23' of the steam boiler at a temperature of about 100°C from the supply water tank. The supply water is heated in the first economizer package to about 150°C. After this the supply water is passed to the high-pressure side bled-steam preheater, i.e. to the heat exchanger 26a, in which the supply water is preheated by means of bled steam to about 175°C. From the high-pressure side supply-water preheater, the supply water is passed to the second preheater, to the heat exchanger 26b, where the supply water is heated to about 200°C, and further into the second economizer package 23'', where the supply water is heated by further 50...100°C. At the same time, the combustion air is preheated, likewise by means of bled steam, in one or several steps, preferably to a temperature of about 200°C. In the preheaters of air and supply water, it is preferable to use the same bleeding points C₂ and C₃ of the steam turbine.

Fig. 3 shows a temperature/thermal-capacity graph of an economizer which corresponds to the conventional construction of Fig. 1. The temperature is indicated in the vertical system of coordinates, and the thermal capacity in the horizontal system of coordinates. In the figure, the temperature increase line 1-2 illustrates a conventional prior-art solution, in which the supply water is heated from the state 1 to the state 2 and the flue gases are cooled from the state 3 to the state 4. Further, in the figure, the solution of Fig. 1 is illustrated, in which, from the status point 5 to the status point 6, preheating of the supply water is carried out by means of bled steams, after which the supply water is heated further in the latter part 23'' of the economizer from the status point 6 to the status point 7.

Fig. 4 is a temperature/thermal-capacity graph of an economizer corresponding to the inventive embodiment of Fig. 2. The embodiment of the figure is in the other respects similar to the illustration in Fig. 2 except that from the status point 5 to the status point 6 the preheating of the supply water is carried out in two steps, first from the point 5 to the point 5a and from the point 5a to the point 6. From the point 5 to the point 5a, the preheating of the supply water is carried out by means of bled steams at a lower pressure of steam, and from the status point 5a to the status point 6 the preheating of the supply water is carried out by means of bled steams at a higher pressure of steam. Thus, in the illustration in Fig. 2, the preheating of the supply water takes place in four steps: from the status point 1 to the status point 5 by means of the first part 23' of the economizer; from the status point 5 to the status point 5a and from the status point 5a to the status point 6 by means of bled steams by means of the

heat exchangers

26a,26b, and from the status point 6 to the status point 7 by means of the second part 23'' of the economizer.

Within the scope of the invention, of course, an embodiment is possible in which the economizer comprises more than two parts, between which parts preheating of the supply water is carried out separately by means of bled steams. An operation of an economizer in three parts is illustrated by the temperature/thermal-capacity graph of an economizer in Fig. 5. Preheating of the supply water by means of bled steams takes place between the economizer parts 23',23'' from the status point 5 to the status point 5a, and between the economizer parts 23'' and 23''' from the status point 5b to the status point 6.

Claims

A power plant formed by a steam boiler, which is provided with a furnace, and by a steam turbine, in which

steam is passed from the steam boiler (10) along a duct (24a) into the steam turbine (11) so as to rotate an electric generator (12) which generates electricity,

the supply water that has been circulated through the steam boiler (10) is vaporized in a vaporizer (240) placed in the steam boiler (10) and superheated in a superheater (24),

the supply water is passed into the boiler through an economizer (23) serving as a heat exchanger, in which heat is transferred from the flue gases of the boiler into the supply water,

the economizer (23) consists of at least two parts, comprising at least one first economizer part (23') and at least one second economizer part (23''),

the supply water is passed from the first economizer part (23') to a supply-water preheater, which consists of a heat exchanger (26), in which thermal energy is transferred from bled steams of the steam turbine into the supply water,

the supply water preheated by means of bled steams from the steam turbine is passed into the second economizer part (23'') of the steam boiler (10) and further to the vaporizer (240) and into the superheater (24) and through the superheater into the steam turbine,
characterized in that,
in the power plant, the temperature of the supply water is continuously raised while the supply water flows in the first economizer part (23') and from the first economizer part (23') to the preheater (26) of the supply water and through it to the second economizer part (23''), and that, besides for preheating of supply water, bled steams are also used for preheating of combustion air (L₁), the power plant comprising a bled-steam duct to a combustion-air preheater (25a, 25b), wherein the combustion air is preheated by means of bled steams in at least two steps, bled steam at a lower steam pressure being passed into the heat exchanger (25a) of the first step, and bled steam at a higher steam pressure being passed into the heat exchanger (25b) of the latter step (seen in the air flow direction L₁), and that a bled-steam duct (29a) is passed to the heat exchanger of the first combustion-air preheater (25a) from a branch point (C₂) of a bled-steam duct (27a) passing to a first preheater (26a) of supply water, and that a bled-steam duct (30a) is passed, through a branch point (C₃), to the heat exchanger of the second combustion-air preheater (25b) from a second bled-steam duct (28a) passing to a second preheater (26b) of supply water.
A power plant as claimed in claim 1,
characterized in that
the preheater consists of heat exchangers through which bled steams are passed, which bled steams, after they have delivered their heat to the combustion air, are passed as condensate water into a supply water tank (18).
A power plant as claimed in claim 1 or 2,
characterized in that
the heat exchanger faces of the economizer (23) are placed in a flue-gas duct (D) of the steam boiler (10), and that the supply water is first made to flow in the economizer part (23') that is placed at the trailing side of the flue-gas flow, seen in the flow direction (S) of the flue gases, and only thereafter, through bled-steam preheating, into the second economizer part (23''), which is placed, in relation to the first-mentioned economizer part (23'), at the inlet side of the flue-gas flow, seen in the flow direction (S) of the flue gases.
A power plant as claimed in claim 1, 2 or 3,
characterized in that
the supply-water preheater consists of two heat exchanger parts, into the first one of which bled steams are passed at a lower pressure, and into the latter one of which, seen in the flow direction of the supply water, bled steams are passed at a higher pressure, the preheating of the supply water passed to the steam boiler (10) taking place in two steps.
A method in preheating of the supply water for a steam turbine, in which

the supply water is passed into an economizer (23) of a steam boiler (10) provided with a furnace, in which economizer heat is transferred in a heat exchanger from flue gases into the supply water,

the economizer (23) is fitted so that its heat faces are at least partly placed in a flue-gas duct (D) of the steam boiler (10),

an economizer comprising at least two parts (23', 23'') is used for heating of supply water,

the supply water is, before a vaporizer (240), preheated in at least three steps, the preheating involving at least two economizer parts (23', 23''),

the first preheating of the supply water takes place in the first economizer part (23') by means of the thermal energy taken out of the flue gases of the boiler,

the second preheating step (26) takes place between the economizer parts (23', 23''), in which step the preheating of the supply water is carried out by means of thermal energy taken out directly or indirectly from bled steams,

the supply water preheated by means of bled steams is passed into the second economizer part (23'') and further into the vaporizer (240) and into a superheater (24) and further, in the form of steam, into the steam turbine (11) so as to rotate an electric generator (12) and to generate electricity,
characterized in that

in the method, the temperature of the supply water is continuously raised while it flows in the first economizer part (23') and from the first economizer part (23') to the preheater (26) and from said preheater part (26) to the second economizer part (23'') containing hotter supply water, and that,

in the method, also combustion air is preheated by means of energy taken out of bled steams, wherein in the method, bled steam is passed, at the same pressure level, besides to preheating of supply water, also to preheating of combustion air, and that bled steam is passed at the same pressure from the same duct both to preheating of combustion air and to preheating of supply water.