EP0158629B1 - Steam cycle for a steam power plant - Google Patents

Description

The invention relates to a new steam cycle for improving the thermal efficiency of steam power plants. This cycle can be used for condensing power plants as well as for counter pressure plants. In the former, the absolute amount of the thermal efficiency that can be achieved is significantly improved, while in the latter, the proportion of electrical energy that can be generated is improved compared to the proportion of heat supply. Heat sources of all kinds are possible as heat sources, in particular the heat can be supplied to the steam cycle to be described below by an atmospherically fired steam boiler, but also by a pressure-fired boiler charged by means of compressors and gas turbines.

The state of the art can be determined. The main advantage of the steam cycle in its classic form according to Clausius and Rankine is that the compression takes place in the liquid phase of the circulation medium and therefore a comparatively very small amount of compressor work is required, which is also carried out in a machine that is easy to construct, namely the feed pump, which works at a relatively low temperature and is therefore simple and reliable to construct. Furthermore, that the subsequent evaporation of the circulating agent in the heating surface of the boiler brings about good cooling of the pipes and therefore provides relatively jealous thermal stress on the pipes. The real technical development problem was therefore only the development of a suitable expansion machine, in the current form of the steam turbine, a machine of ever increasing technical complexity, but in which the vast majority of electrical energy generation still takes place worldwide.

The advantage of being able to carry out the compression in the liquid phase also has the greatest disadvantage of the steam cycle in its current form. This is because, after the end of compression, the heat must begin to bring the working fluid to the maximum temperature of the circuit. However, this means that a large part of the heat in the steam cycle in its current form must be supplied to the cycle medium at relatively low temperatures for the purpose of preheating and evaporation. However, this means that, in terms of exergy, very significant losses in temperature difference and thus losses in possible efficiency gains already occur when heat is added to the process. This process can be easily explained by using the Carnot cycle for each differential step of supplying heat to the circulating medium.

Attempts at the simple water vapor process to improve the Rankine process were made very early. In addition to the constant increase in pressure and temperature, there was an improvement due to continually increasing preheating of the feed water and reheating. Attempts to change the process directly and to use a steam compressor instead of the feed pump were also made. (See A. Stodola, steam and gas turbines, Springer, 5th edition, 1922, page 1077). The methods of Thurston and Dolder try to do this; after partial condensation of the steam, it should be compressed to the original pressure level. The method of Lang GB-PS No. 1 470 527 also tries to compress the steam again directly after expansion into the turbine and to feed it to the boiler drum, but the intended reduction of the waste heat leads to an efficiency gain under otherwise identical conditions according to Carbot only if at the same time the temperature of the heat supply is increased.

According to FIGS. 1 and 2, US 4,433,545 Chang uses multiple reheating by heat exchangers which are connected between individual stage groups of the turbine. (Your designation 28, 36, 42, 28 ', 36', 42 '). In the latter scheme, the steam flow is cooled to saturated steam temperature and only then reheated to be fed to the next stage group of the turbine. French Patent No. 79 20276, No. de publication FR-A-2 435 600 also uses a heat exchanger (designated 6) interposed between the turbine stage groups, but which functions as a known reheater. Furthermore, a partial flow is branched off from the medium-pressure turbine to a condensation turbine which drives the feed pump. Apart from these two features of the use of heat exchangers and the branching off of partial streams, these patents have no further factual connection with the present invention described below.

The temperature increase occurs in a combined gas-steam process in that the heat is first fed to the gas process and its waste heat is processed in the steam process. See F. Pauker "Recent Proposals on the Gas-Vapor Process", Div. 111, Paper 28 G 1/6, 5th World Power Conference, Vienna 1956. As well as ÖP 172.202 and more recently DE-OS 26 37 924 (Stal-Laval). The methods mentioned relate to open gas turbine processes. However, methods that use closed gas turbine processes in a combined gas-steam process have also become known. Air, carbon dioxide and helium were used or suggested as working gases. See H. Bormann and J. Buxmann "Combined Power Plant Processes with Closed Gas and Steam Turbines", Brennst.-Heat-Kraft 1981.

The heat is supplied to the closed gas circuit, and the heat is removed from it partly to the downstream steam circuit and partly to the outer gas cooler in order to lower the temperature of the gas before compression as much as possible. It is the transfer Waste heat to the steam circuit only through a metallic heating surface and as a result of the water. server steaming only possible with considerable temperature differences.

Furthermore, closed circuits with carbon dioxide as a working medium have become known, which compress the medium gaseously after partial condensation for heat dissipation even in branches of the mass flows. See N. Gasparovic "Fluids and Cycle Processes for Thermal Power Plants with Large Unit Outputs", Brennst.-Wärmkraftkraft 1969. Furthermore, a proposal has been made to run a steam cycle entirely in the supercritical area, See JH Potter "The Totally Supercritical Steam Cycle ", Trans. ASME, Journal of Eng. for Power, 1969. This cycle consisting of compression, heat exchange, heat supply, expansion, heat exchange and heat dissipation is run through in a simple loop and lies in the T, s diagram of the water vapor above the limit curve and the compression in all cases to the left of the critical point, ie in the range of entropies smaller than the critical entropy. This means heat dissipation at a relatively high temperature and very high pressures in the entire circuit, thus the need for turbines and compressors with a very high wall thickness and a large number of stages.

The path of superimposing saturated steam processes of different fluids was also taken in order to raise the temperature of the heat supply and to avoid the high pressures of the water vapor. In his book, page 1090, Stodola describes the interconnection of saturated steam processes for mercury and water. More recently, the use of saturated steam processes with sodium or potassium and also with intermediate organic fluids has been proposed. In addition to the fact that the thermodynamic properties of these fluids are not always entirely favorable, the requirement to use several different media is disadvantageous for power plant operation. In addition, the heat must be conducted from one fluid to another through metallic heating surfaces.

Therefore, there is currently no known method that would only enable a significant improvement in the efficiency of thermal power plants using the extraordinarily tried and tested cycle medium water.

To remedy this is the purpose of the present invention. A steam cycle for a steam power plant to generate power from external heat is to be created, which maintains the advantage of heat dissipation at a low temperature in the condenser, but at the same time increases the temperature of the heat supply appreciably, so that the thermal efficiency in Carnot's sense experiences a significant increase.

This is done according to the features described in claim 1.

Another decisive advantage of this circuit is that the need for a high-pressure steam turbine, as is required in conventional systems for increasing efficiency, is no longer present. If the temperature of the heat supply is to be increased in a conventional process, this can only be done by raising the saturated steam temperature itself by increasing the pressure after extreme feedwater preheating, which is however limited to the approximation to the saturated steam temperature, but this only up to critical point is possible and this results in the highest possible overheating. However, this requires an extraordinarily high live steam pressure in order to obtain an expansion line of the steam turbine which ends in the moist area. However, these high live steam pressures, some of which are already supercritical and have reached the 300 bar range, result in extremely difficult, expensive and, in some cases, also unreliable designs of the high pressure steam turbines. In addition, this measure of supercritical pressure, as the course of the heating in the T, s diagram of the water vapor shows, does not raise the mean temperature of the heat supply in any way as in accordance with the present invention.

The design of the high-temperature steam turbine required here has the advantage that its operating pressure is comparatively low and roughly corresponds to the pressure of an intermediate superheating turbine of a conventional steam turbine, so that in this case the machine can be designed with a housing with a much smaller wall thickness. This, as well as the fact that the saturated steam temperature of the associated pressure of about 50 bar is very low, offers the possibility of designing such a machine with internal insulation in the sense of the construction of gas turbines. This requires insulation that works in the steam area and overcomes the temperature difference from the maximum temperature of the process to the saturated steam temperature of this highest pressure in the range of 50 bar. With the selection of suitable ceramic and mineral materials and the corresponding strengthening of an insulation body by means of metallic reinforcements, it is easily possible to construct such an internal insulation in a reliable manner. By tapping and draining the housing inside and with the full effect of the insulation, it is then achieved that the saturated steam state of the maximum temperature steam turbine is reached on the inside of the steam housing. However, this is a comparatively low temperature in the range of 200 to 250 ° C, so that such a housing can not only be made of low-alloy materials and welded construction, but also a very low temperature and the possible high conductivity of the steel to be used low heat distortion will show what benefits the operability and rub resistance of this turbine. Together with the fact that the volume flow is significantly increased compared to a conventional steam turbine, it follows that there can be a turbine with fewer stages, but larger blade lengths and fewer gaps. On the other hand, this means that the flow efficiency of such a machine will be significantly higher than that of a conventional high-pressure turbine. This fact also benefits the overall efficiency of the circuit.

The steam compressor works in the medium pressure and low temperature range and can therefore be designed as a conventional turbomachine. The envisaged water injection in front of the steam compressor is said to result in moisture in the range of 5 to 12%. After the operational experience with saturated steam turbines, which work perfectly in this humidity range, it does not seem to be a problem to offer a steam compressor, especially also e.g. to apply such a radial design with such entry moisture.

The desuperheater requires the warmed up condensate to be atomized by means of appropriate injection nozzles, which can be achieved in the necessary fineness by slightly increasing the pressure generated by the feed pump. The droplets injected into the vapor stream of the main stream are carried on by it and evaporated by supplying heat from the superheating area, so that a sufficiently fine distribution of the droplets in the saturated steam area arises in accordance with the selected size of the injection drops. As described above, these do not pose any risk of wear for the steam compressor.

The steam heat exchanger transfers heat from the steam of low pressure and higher temperature to the steam flow of higher pressure and lower temperature. Its temperature range can generally be selected so that only conventional boiler steels have to be used. It is therefore possible to build such a device with a sufficient cross-section and inexpensively so that there are low pressure losses on both sides.

Warming up in the steam heater (the term superheater is not quite appropriate here, since the preheater already does some of the overheating). This heating surface represents the only heat-absorbing heating surface of the boiler to be described below. In it, the steam must be heated to the maximum temperature of the process. At a pressure that is to be referred to as medium pressure according to today's conventional terms. If the temperature is high here and increased above the usual syringe value, higher quality materials have to be used for the corresponding pipes. However, this can be done in a suitable manner by selecting austenitic tube materials or also nickel-based tube materials.

The condensation steam turbine and the condenser, as well as the tap preheating system, are designed in a completely conventional manner. The same applies to the feed pump and its drive, only with the provision of having to apply the pressure difference to atomize the spray water.

Regarding the boiler, it should be noted that it can be designed as an atmospherically fired boiler with any fuel. It is only necessary to adapt the flue gases from the boiler and the air drawn in to one another in a heat exchanger in such a way that a temperature arises in the combustion chamber of the boiler which is above the temperature of the steam heater with a sufficient temperature difference. Taking into account the fact that the specific heat of the boiler exhaust gas and the intake air are different due to the combustion process and the fuel content, but also taking into account the fact that the air is brewed colder than the flue gas is released into the chimney, this is easily possible . The air must be preheated to approximately 500 ° C and the heat must be removed by exchanging it from the boiler flue gas. It is possible to supply a boiler in the manner described with only atmospheric firing with the air being supplied by a downwind fan or the flue gas being removed by a suction-draft fan. However, it is also possible to effect part of this temperature difference for heating the air not by means of the heat exchanger, but by means of a compressor, thus simultaneously increasing the pressure in the heat exchanger and in the combustion chamber of the boiler and increasing the heat transfer to the steam heater accordingly improve. This can go so far that the flue gas heat exchanger can be completely dispensed with and only the supercharging by the compressor and the relaxation by the corresponding gas turbine of the air or the flue gas in the boiler throughput takes place, as taught in DE-OS 22 62 305 (Brown Boveri) .

The present invention will be explained in more detail with reference to the figures, in which:

• Fig. 1 erfindungsgemässe Schaltung. Die einzelnen Aparate und Maschinen sind mit einfachen Ziffern bezeichnet, deren Bedeutung aus dem Text hervorgeht. Volle Linien bedeuten Leitungsverbindungen.
• Fig. 2: Temperatur-Entropie Diagramm der erfindungsgemässen Schaltung eines Dampfkreislaufes für ein Dampfkraftwerk. Es sind spezifische Werte der Entropie des Wasserdampfes eingetragen. Beziehungen zwischen verschiedenen Mengenströmen sind daher nicht ersichtlich. Lediglich Temperaturdifferenzen erscheinen in wahrer Grösse. Diefunktionelle Bedeutung der mit C1 bis C20 bezeichneten Zustandspunkte geht aus dem Beschreibungstext hervor.

Drawing description:

• Fig. 1 circuit according to the invention. The individual devices and machines are identified with simple numbers, the meaning of which is evident from the text. Solid lines mean line connections.
• Fig. 2: Temperature entropy diagram of the circuit according to the invention of a steam circuit for a steam power plant. Specific values of the entropy of water vapor are entered. Relationships between different mass flows are therefore not evident. Only temperature differences appear in true size. The functional meaning of the status points labeled C1 to C20 can be seen from the description text.

The external heat can be supplied by a bige heat source suitable high temperature. Here, for example, the external heat generation by burning fossil fuels in an atmospheric or pressure-fired steam boiler is described. 1 means the downwind blower, 2 its electric drive, 3 a suction fan, 4 a flue gas heat exchanger, 5 its air-side heating surface, and 6 its gas-side heating surface, 7 the fuel supply, and 8 the combustion chamber of the boiler with atmospheric combustion. If the boiler is charged, 1 designates the charging compressor, 3 the exhaust gas turbine, and 2 the electric drive and driven machine. In both cases, the heating surface 10 of the steam circuit in which the external heat supply takes place is located in the combustion chamber 8. This steam cycle is also described in accordance with FIG. 1. This is a double steam circuit for a steam power plant with heat supply to generate power from high-quality heat, the steam circuits being combined in the high temperature range - by injecting feed water in the injection coolers 23 and 24, and then compressing a partial steam quantity in the steam compressor 11, and the full one Circulation quantity in the steam compressor 12 is compressed to the maximum pressure of the circuit, and this full circuit quantity is heated on the high-pressure side of the heat exchanger 14, and is fed via line 9 to the heating surface of the steam heater 10, where it is heated to the maximum temperature of the process by external heat supply . In order to then subsequently run to the high-temperature steam turbine 13 in which the full circulation quantity is expanded. A small partial flow is fed from the high-pressure compressor 12 to the high-temperature turbine 13 for cooling the rotor via the line 13.1. After expansion in the high-temperature steam turbine 13, the exhaust steam is cooled on the low-pressure side of the heat exchanger 14, whereupon the steam flows are divided at the branching point 15, one partial flow flowing into the injection cooler 23 and consequently into the steam compressor 11, while the other part flows into the condensation turbine 16 expands further, releasing bleed steam, condenses in the condenser 17, and runs to the degasifier feed water tank 20 via the condensate pump 18 and the low-pressure preheater 19. From this, the injection water is injected into the injection cooler 23 and 24 via the feed pump 21 and further heating in the high pressure preheater 22, whereby the double circuit is closed. Separation of the volume flows of the two circuits thus takes place at the branching point 15, the combination of the volume flows to the full volume of the circuit takes place in the injection coolers 23 and 24. This circuit is characterized by the arrangement of a compressor or several stage groups of compressors and by the arrangement of a heat exchanger. This leads thermodynamically to the fact that the heat is supplied at preheated steam of moderate pressure at a very high average temperature. Furthermore, the process is characterized by an expansion of the steam in the high temperature range with subsequent cooling in the heat exchanger with an entry temperature into the condensation turbine, which results in a favorable condensation end point with regard to erosion and heat dissipation. This will be described with reference to FIG. 2 in the TS diagram for water vapor. Specific values of the entropy are entered in this, so that this diagram only shows the state of the medium of the medium and not the quantitative relationships of the circuit. These status points are therefore also designated C1 to C20. The maximum temperature of the steam after the external supply of heat is therefore designated as point C1.

The entry of these status points in FIG. 2 is to be regarded as an example of the steam cycle according to the invention. From a physical point of view, the position of the state points in relation to the limit curve is important with regard to the overheated state or wet steam state. In the following, the number of the component according to FIG. 1 and the state change according to FIG. 2 taking place therein are indicated by the hyphen between the state points notated with C. Thus, the heating of the full amount of the circuit in the heating surface 10 means the change of state-C14-C1, the expansion in the high-temperature turbine 13, the change of state-C1-C2, the cooling of the full amount of the circuit on the low-pressure side of the heat exchanger 14, the change of state C2-C3, the expansion of the partial flow branched off in the condensation turbine 16 at the branching point 15 corresponds to the change in state C3-C4-C5-C6-C7, the intermediate points removal of bleed steam and the end point C7 corresponding to the start of condensation of the heat transfer to the cooling water in the condenser 17. After the steam has been completely deposited, the condenser is pumped on by the condensate pump 18 in accordance with the change in state C15-C16. The warm-up in the low-pressure preheater 19 corresponds to the change in state (C16C17. The warm-up in the degasifier 2 corresponds to the change in state C17-C18. From the connected feed water tank 20, the feed pump flows to injection pressure in accordance with the change in state C18-C19, whereupon the remaining preheating in the high-pressure preheater 22 in accordance with the change in state C19-C20 takes place, so that the injection water is available for the desuperheaters 23 and 24 in the state C20.

The other partial flow of the steam undergoes the following change of state from the branching point 15: cooling in the injection cooler 23 C3-C8-C9 then compression in the first steam compressor part 11 in accordance with the change in state C9-C10. Then further cooling by water injection in the injection cooler 24 in accordance with the change in state C10 ― C11 ― C12. This is the full circle Running volume achieved by combining the partial flows. This is followed by the compression in the second steam compressor part 12 to the full circuit pressure corresponding to the change in state C12-C13 and then the heating of the full circulating flow on the high pressure side of the heat exchanger 14 in accordance with the change in state C13-C14, which describes the entire circuit.

According to the description of the invention and according to the claims, the detail of the change in state in the injection coolers 23 and 24 and in the steam compressors 11 and 12 can be carried out as described above with cooling down to the wet steam region, but can also be designed such that these state changes only in the region of overheated steam. The state points C8 and C9 are then identical, just like the state points C11 and C12 are identical - they then represent the states at the entry into the first steam compressor part 11 and in the second steam compressor part - and are located in the Ts diagram above the limit curve in the area of superheated steam. This avoids problems with wet steam flow at the inlet to the compressors. In addition, the evaporation of the water droplets which are formed in the injection cooler during the injection of the feed water takes place completely if the change in the state of the steam therein remains restricted to the superheated area of the steam.

Claims (2)

1. Double steam cycle for a steam power plant, with heat input for power production from high quality heat, in which case the two steam cycles are combined in the high temperature region and the full cycle mass flows are heated by external heat input, by guiding over heater surfaces, and said full mass flows are expanded in a high temperature steam turbine, providing thus useful power as driving power to an electric generator, at which part of the steam expanded in the high temperature turbine is further expanded in a condensing turbine from which steam is bled for feed heating, and the steam flow thus further expanded in the condensing turbine is condensed in a condenser and is thus liquified in giving up its latent heat to the cooling water or a heating wafer circle, in either case in the liquid condensate being brought to higher pressure by the condensate pump, now called feed water, and is heated by low pressure bled steam heaters to the higher temperature before it is fed to the deaerator for deaeration which serves also as a means for feed water storage and this deaerated feed water is pressurized by the feed pump to a specified high pressure and is further heated by bled steam from the condensing turbine in high pressure heaters characterized in that the fully cycle mass flows before being heated by external heat input are heated in a heat exchanger by heat exchange with the full cycle mass flows expanded in the high temperature steam turbine and flowing from the high temperature steam turbine to said heat exchanger where they are cooled, and that this part of the full cycle mass flows thus cooled in said heat exchanger which is not fed to the condensing turbine for further expansion, this other part is cooled by injection of feed water in an injection cooler, said feed water being pressurized by the feed pump and being feed heated by bled steam heater, said part of the full steam cycle mass flows being increased by the mass flow of the injected feed water is compressed in a steam compressor or a first stage group thereof, this steam compressor being driven by the high temperature steam turbine, after which a further injection and compression in a succeeding part of the steam compressor may follow, and that in consequence the full cycle mass flows being thus replenished and compressed are heated in the heat exchanger mentioned above.

2. Steam cycle for a steam power plant further characterized in that, instead of a condensing turbine a backpressure turbine is provided, the exhaust steam of which, after heat release from technological or room heating purposes is returned as condensate after being properly cleaned to the feed heating part of the plant and is fed via feed pump to the injection coolers as injection feed water, or in that, the newly purified fresh water is feed heated and fed via feed pump and injection coolers into the cycle in case the exhaust steam from the back pressure turbine is directly used for technological purposes.

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