EP2933556A1 - Condensate preheating - Google Patents

Abstract

The invention relates to a heat recovery steam generator (1) comprising an exhaust path (2) with a first heat transfer surface (3) arranged in the exhaust path (2), the first heat transfer surface (3) being a first part (4) of a closed working medium circuit (5) and a first second heat transfer surface (6) is arranged as a second part (7) of the working medium circuit (5) separate from the exhaust path (2) and the first and the second heat transfer surface are part of a thermosyphon, wherein first (4) and second part (7) of the working medium circuit ( 5) are connected to one another such that when the first part (4) is exposed to heat from the exhaust gas (9), an evaporating working medium (10) of the working medium circuit (5) flows from the first (4) into the second part (7) Heat via the second heat transfer surface (6) gives off to a condensate and condenses and by gravity back into the first part (4) flows back while the heated condensate is discharged via a line (8). The invention further relates to a method for preheating a condensate (12) with exhaust gas (9).

Description

The invention relates to a heat recovery steam generator. The invention further relates to a method for preheating a condensate in a heat recovery steam generator.

The fuels used in gas and steam power plants (GUD) have, inter alia, more or less high sulfur contents. Together with the resulting in the combustion of gas or oil content of water, there is a risk that precipitate falls below the corresponding Abgastaupunkte sulfuric acid, sulfurous acid and water in the "cold" part of the heat recovery steam generator (especially condensate preheater) and corrosion and end to component failure to lead.

To avoid falling below the Abgastaupunktes and the associated accumulation of water / acid (especially sulfuric acid), the condensate temperature must be raised at a gas and steam turbine power plant before the occurrence of the condensate in the condensate preheater of the heat recovery steam generator to a corresponding minimum temperature, because the heat transfer in the heat exchanger surfaces in Heat recovery steam generator is determined from the water side, ie the exhaust gas pipe wall temperature corresponds approximately to the inside applied condensate temperature. The condensate temperature is in turn set by the defined recooling conditions (type of cooling, design of the cooling system, ambient conditions, etc.).

The minimum inlet temperature of the condensate to avoid the Abgastaupunktunterschreitung has hitherto mostly ensured by recirculation of hot water from Kondensatvorwärmeraustritt to Kondensatvorwärmereintritt by means of recirculation pumps or feed water pump tap.

Another possibility is to heat the condensate by an external steam or water-heated preheater before entering the heat recovery steam generator. For this purpose, an external hot water or steam source can supply the necessary heat, or a steam heating of this preheater by steam tapped in the steam turbine or heating steam generation takes place in the waste heat steam generator at a temperature level above the dew point.

Another option hardly used is the design of the condensate preheater in (largely) acid-resistant material.

These previous solutions have in common that they mean a relatively high amount of components (comparatively large heat exchanger surface in the heat recovery steam generator, recirculation pumps) and deteriorate performance or efficiency of the power plant (by increased pressure losses in the exhaust stream, the electrical intrinsic demand of the components used).

The object of the invention is therefore to further develop said device and said method, so that the simplest possible, cost-effective and reliable condensate preheating is possible.

According to the invention, this object is achieved by the device according to claim 1 and the method according to claim 9. Advantageous developments of the invention are defined in the respective dependent claims. The heat recovery steam generator according to the invention comprises an exhaust path having a first heat transfer surface arranged in the exhaust path, wherein the first heat transfer surface is a first part of a closed working medium circuit and a second heat transfer surface is arranged as a second part of the working medium circuit separate from the exhaust path and the first and second heat transfer surfaces are part of a thermosyphon , Wherein the first and second parts of the working medium circuit are connected to one another in such a way that upon application of the first Part with heat from the exhaust gas, an evaporating working medium of the working medium circuit flows from the first to the second part where it gives off heat via the second heat transfer surface to a condensate and condenses and flows back by gravity into the first part, while the heated condensate is discharged via a line ,

In the heat recovery steam generator according to the invention, therefore, heat from the exhaust gas is to be used by one or more thermosyphon arranged in the heat recovery steam generator. Thermosyphons basically contain a hermetically encapsulated volume. The thermosyphon is filled with a working medium, which fills the volume to a small extent in the liquid state, to the larger in the vapor state (saturated conditions). Each contains a heat transfer surface for the heat source (= evaporator) and sink (= condenser).

With the use of the thermosyphon can be achieved that takes place in a cost effective manner in the exhaust stream no dew point on Kondensatvorwärmerheizflächen.

For this purpose, it is advantageous if the condensate-carrying line opens after flowing through the second heat transfer surface arranged in the exhaust path Kondensatvorwärmerheizflächen and when downstream of the Kondensatvorwärmerheizflächen in the flow direction of the exhaust gas, the first heat transfer surfaces.

The evaporator of the thermosyphon is disposed within the exhaust path in the heat recovery steam generator and is heated by the passing exhaust gas, the condenser is located above the evaporator outside the exhaust path and is cooled by circulation condensate. The circulation condensate is thereby heated and in fact to such an extent that it can enter the heating surfaces of the condensate preheater without or at least greatly minimized further preheating (for example by admixing recirculated condensate from the condensate preheater outlet) without the dew point being undershot.

If it could be determined by the design of the thermosyphon there could be a risk of being below the exhaust dewpoint at the evaporator of the thermosyphon itself by operating conditions deviating from the design point of the thermosyphon, this can be prevented by simple measures. By way of example, a bypass circuit may be mentioned here. In this case, a part of the cold condensate would pass the condenser of the thermosyphon (thus increasing the temperature / pressure level in the evaporator of the thermosyphon) due to the inferior cooling of the condenser, to be remixed at the outlet of the condenser.

In comparison to previous solutions, the heat exchanger surface required for preheating the circulation condensate in the waste heat steam generator decreases because the thermodynamic temperature difference at the evaporator surface of the thermosyphon increases compared to previous solutions (the thermosyphon is operated just above the exhaust dew point and, for example, the condensate preheater does not have to be interpreted as large, since it does not have to be recirculated to the previous extent). Overall, the solution thus leads to improved performance through lower pressure losses in the heat recovery steam generator and lower costs for heat exchanger surfaces.

In an advantageous embodiment, the first heat transfer surface consists at least partially of acid-resistant material. Because of the thermosyphon, which is connected only indirectly to the steam circuit, it is possible to use cheaper and, above all, acid-resistant materials (for example plastic with graphite inclusions) since the thermosyphon does not have the same high pressure and chemical properties as components in the condensate system of a power plant.

It is particularly advantageous if at least two first heat transfer surfaces of closed working fluid circuits are provided in the heat recovery steam generator, of which at least a first heat transfer surface is at least partially made of acid-resistant material.

In such cases, it is expedient for at least one first heat transfer surface to have an evaporator temperature below an exhaust gas drop point. Namely, if acid-resistant material for the evaporator section of the thermosyphon is used, the heat exchanger surface in the heat recovery steam generator can be further reduced, since then the application of several thermosyphon offers, of which at least one has an evaporator temperature below the Abgastaupunktes. Thus, the thermodynamic temperature difference can be further increased and the necessary Kesselheizfläche further reduced.

Appropriately, the working fluid of the working medium circuit differs from the condensate in terms of a chemical composition.

It is also useful if working media differ from one another at least two working medium cycles.

Examples of suitable working media are e.g. Ethanol or ammonia. But water is of course a conceivable working medium, which would then often have a pressure below the ambient pressure.

In the inventive method for preheating a condensate with exhaust gas, which flows through an exhaust path of a heat recovery steam generator, heat is transferred from the exhaust gas to a working fluid in a closed working medium circuit in the exhaust path, wherein working fluid evaporates and flows from the exhaust path and wherein the vaporized working fluid outside the exhaust path in Heat exchange with condensate is liquefied again and flows back into the exhaust path.

It is expedient if the condensate is heated to above a dew point of the exhaust gas before it is further heated in the heat exchange with the exhaust gas in the heat recovery steam generator.

Advantageously, a first heat transfer surface of the closed working medium circuit at least partially made of acid-resistant material and it is set an evaporator temperature in the closed working medium circuit below a Abgastaupunktes.

With the invention, recirculation pumps, piping, valves and other equipment, which was necessary for the previous solutions, omitted or at least greatly reduced, and thus the internal electrical requirements and costs are reduced accordingly. In addition, availability increases as active components (pumps, valves) are replaced by a passive thermosyphon.

Figur 1

eine Ansicht eines Abhitzedampferzeugers mit bekanntem Kondensatvorwärmer mit Speisewasserentnahme,

Figur 2

eine Teilansicht eines weiteren Abhitzedampferzeugers mit bekannten Kondensatvorwärmer mit zusätzlicher Kondensatrezirkulationspumpe,

Figur 3

eine Ansicht einer Ausführungsform eines Abhitzedampferzeugers nach der Erfindung.

The invention will be explained in more detail by way of example with reference to the drawings. Shown schematically and not to scale:

FIG. 1

a view of a heat recovery steam generator with a known condensate preheater with feed water extraction,

FIG. 2

a partial view of another heat recovery steam generator with known condensate preheater with additional condensate recirculation pump,

FIG. 3

a view of an embodiment of a heat recovery steam generator according to the invention.

The FIG. 1 shows schematically and by way of example the view of a heat recovery steam generator 13 according to the prior art with a known condensate preheater 14 in the so-called cold part 15 of the heat recovery steam generator 13 with feed water removal. Condensate is fed via the condensate line 16 to the condensate preheater 14 and there heated in heat exchange with the exhaust heat 9 flowing through the waste heat steam generator. The condensate line 16 branches into the lines 17 and 18. The line 17 passes the condensate to the low-pressure drum 19. The condensed into the line 18 condensate is fed through a feedwater pump 24 to the middle 20 and high-pressure drums 21.

The FIG. 1 also shows a cold bypass of the condensate preheater via a bypass line 22 and a line 23 for a minimum return of the feed water in the condensate line 16, because the feedwater pump 24 is in continuous operation and even if medium and high-pressure drum 21 is not supplied with feed water to be, a low flow of feed water through the feedwater pump 24 must be ensured.

The minimum inlet temperature of the condensate to avoid the exhaust dew point is below, as in FIG. 1 shown, mostly ensured by recirculation of feed water from a feedwater pump tap 25. With this comparatively few components comprising a low complexity and therefore also cost-effective system typically a condensate preheater inlet temperature of about 55 ° C can be ensured. This temperature would be sufficient to avoid falling below the Wasserdampftaupunkts usually.

With a view to avoiding falling below the sulfuric acid dew point may additionally, as FIG. 2 shows hot water from the condensate preheater outlet 26 to the condensate preheater inlet 27 to be recycled. However, this is in addition to the feedwater pump 24 of the FIG. 1 a recirculation pump 28 is needed.

FIG. 3 shows the heat recovery steam generator 1 according to the invention. The heat recovery steam generator 1 has an exhaust gas path 2. In it is next to in the FIG. 3 Not shown superheater and evaporator heating a first heat transfer surface 3 is arranged. The first heat transfer surface 3 is a first part 4 of a closed working medium circuit 5. A second heat transfer surface 6 is the second part 7 of the Working medium circuit 5 is arranged separately from the exhaust path 2. A line 8 opening into the condensate preheater heating surface 11 arranged in the exhaust gas path 2 is thermally connected to the second heat transfer surface 6. The first 4 and second part 7 of the working medium circuit 5 are connected to each other so that upon exposure of the first part 4 with heat from the exhaust gas 9, an evaporating working medium 10 of the working medium circuit 5 from the first 4 flows into the second part 7, there heat through the second heat transfer surface 6 gives off to the condensate of the steam cycle and condenses and flows back by gravity back into the first part 4 in the exhaust path 2 of the heat recovery steam generator 1 while the heated condensate is then discharged via the line 8.

Claims (11)

A heat recovery steam generator (1) comprising an exhaust path (2) with a first heat transfer surface (3) arranged in the exhaust path (2), characterized in that the first heat transfer surface (3) is a first part (4) of a closed working medium circuit (5) and a second Heat transfer surface (6) as a second part (7) of the working medium circuit (5) separated from the exhaust path (2) is arranged and the first and the second heat transfer surface part of a thermosyphon, wherein the first (4) and second part (7) of the working medium circuit (5 ) are so connected to each other, that upon exposure of the first part (4) with heat from the exhaust gas (9) an evaporating working medium (10) of the working medium circuit (5) from the first (4) into the second part (7) flows there heat via the second heat transfer surface (6) to a condensate and condenses and by gravity back into the first part (4) flows back while the heated condensate over a line (8) is discharged. Heat recovery steam generator (1) according to claim 1, wherein the line (8) in the exhaust path (2) arranged condensate preheater heating surfaces (11) opens. Heat recovery steam generator (1) according to claim 2, wherein in the flow direction of the exhaust gas (9) the first heat transfer surfaces (3) are connected downstream of the condensate preheater heating surfaces (11). Heat recovery steam generator (1) according to one of the preceding claims, wherein the first heat transfer surface (3) consists at least partially of acid-resistant material. Heat recovery steam generator (1) according to claim 4, wherein at least two first heat transfer surfaces (4) closed working fluid circuits (5) in the heat recovery steam generator (1) are provided, of which at least one first heat transfer surface (4) consists at least partially of acid-resistant material. A heat recovery steam generator (1) according to any one of claims 4 or 5, wherein at least one first heat transfer surface (4) has an evaporator temperature below an exhaust dew point. Heat recovery steam generator (1) according to one of the preceding claims, wherein the working medium of the working medium circuit (5) differs from the condensate with respect to a chemical composition. Heat recovery steam generator (1) according to one of the preceding claims, wherein working media of at least two working fluid circuits differ from each other. A method for preheating a condensate (12) with exhaust gas (9), which flows through an exhaust path (2) of a heat recovery steam generator (1), characterized in that in the exhaust path (2) heat from the exhaust gas (9) to a working medium (10) in a closed working medium circuit (5) is transferred and the working medium (10) is thereby evaporated and flows from the exhaust path (2), the evaporated working medium (10) outside the exhaust path (2) in the heat exchange with condensate (12) is liquefied again and in the exhaust path (2) flows back. A method according to claim 9, wherein the condensate (12) is warmed up to above a dew point of the exhaust gas (9) before it is further heated in heat exchange with the exhaust gas (9) in the heat recovery steam generator (1). Method according to one of claims 9 or 10, wherein a first heat transfer surface (4) of the closed working medium circuit (5) consists at least partially of acid-resistant material and an evaporator temperature in the closed working medium circuit (5) is set below a Abgastaupunktes.

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