WO2006121335A1

WO2006121335A1 - A method and a system for generating steam

Info

Publication number: WO2006121335A1
Application number: PCT/NO2005/000165
Authority: WO
Inventors: Terje Kaspersen
Original assignee: Kanfa Tec AS
Current assignee: Kanfa Tec AS
Priority date: 2005-05-13
Filing date: 2005-05-13
Publication date: 2006-11-16
Anticipated expiration: 2007-11-13

Abstract

A method for generating steam under pressure, primarily for injection into the combustion chamber of a gas turbine, comprises a system having a low pressure generator (1) and a high pressure generator (2) receiving heat from the exhaust gases (8) of a gas turbine. The low pressure generator (1) is supplied with sea water (9), which is converted to a mixture of steam and water in an evaporator (3) which may be operated at sub-atmospheric pressure. The steam is separated in a separator (4) and is condensed thereafter in a condenser (5). The condensed water is led via a feed pump (18) to the high pressure generator (2), where it is converted into high pressure steam in an evaporator (22), said steam being overhead in an overheater (23) before it is conducted further to the final user (25), especially the combustion chamber of the gas turbine.

Description

Λ method and a system for generating steam

The present invention relates to a method and a system for generating substantially pure steam as recited in the preamble of claim 1 and claim 8, respectively.

Background of the invention

In the North Sea there are many gas turbines having outdated combustion chamber technology and therefore emitting large amounts of NOx (about 200-300 ppm) . A recognized method for reducing the emissions is to inject steam at a pressure somewhat higher than the combustion chamber pressure into the combustion chamber of the gas turbine in order to reduce the combustion temperature and thereby the formation of thermal NOx. In this way, the emissions may be reduced substantially, from about 200-300 ppm down to below 25 ppm without loss of efficiency. On the contrary, one may obtain increased efficiency. This is regarded as one of the better measures for reducing NOx emissions from gas turbines on installations in the North Sea. It is relatively easy to perform, but the main problem is simply to provide the water necessary for producing the steam. The steam may quite simply be produced in a waste heat boiler. Since the steam consumption is high and the steam injected into the combustion chamber is lost, large amounts of water have to be produced. Today, the platforms rarely have extra water capacity. Furthermore, the available space for installing water production plants is limited.

Prior art

The common way of producing the steam for STIG is to install a waste heat boiler in the exhaust conduit (also called Heat Recovery Steam Generator, HRSG) . Pressurized water is fed to this generator, and heat is transferred to the water in the waste heat boiler so that the water evaporates and is overheated before the steam is conducted to the gas turbine. The water supplied to the waste heat boiler must be of a very high quality in order to limit corrosion and deposit formation in the waste heat boiler and because very strict purity requirements are imposed on the steam to be injected into the gas turbine. Due to the requirement imposed on the feedwater by current standards (e.g. EN 12952) , the water must be completely desalinated and deaerated. In order for this to be achieved, the water must be treated in a desalination plant. In addition, on an offshore installation the water has to be produced from sea water. Considering the plants presently found on the in- stallations, this means that the sea water first must be fed to a plant producing fresh water from sea water (typically by reverse osmosis or a distillation plant) , whereupon the water is fed to a total desalination plant before it can be fed to the waste heat boiler. Additionally, the system must have a buffer tank and deaerating tower (the deaeration depends on the choice of materials) .

From US 5329758 a gas turbine installation is known comprising a compressor which is driven by a first turbine, and a second power producing turbine. Between the compres- sor and the first turbine a combustion chamber is arranged operating at a relatively high pressure, whereas between the first turbine and the power producing turbine a second combustion chamber is arranged operating at a lower pressure. Both water and steam are fed to the first combustion chamber at said relatively high pressure, while steam at a lower pressure is fed to the second combustion chamber. The high pressure steam and low pressure steam are generated in a steam boiler of the HRSG type having two compartments . The boiler is furnished with fresh water from a desalina- tion plant based on reverse osmosis.

The production of fresh water by means of a common reverse osmosis plant is heavy and requires considerable space, and the same is true for common desalination plants. In addition, the process requires much energy. The amount of water which today is produced on offshore installations is lim- ited and can only to a small extent be used in a steam injection process where the water consumption is very high.

The major part of the water production on an offshore installation in the Norwegian sector takes place by the use of distillation plants. They have a very high heat requirement and use a heating medium as heat source. The heating medium is usually heated by high temperature energy (about 500⁰C) taken from the gas turbine exhaust.

Summary of the invention The basic idea of the invention is to build a waste heat boiler utilising sea water as feedwater. Furthermore, it is desirable to utilise low temperature energy present in exhaust gas from a gas turbine for distilling water and therefore obviating the installation of a large fresh water production plant. Common problems regarding deposit formation will thereby be avoided by lowering the evaporation pressure to a sub-atmospheric pressure. In addition, it is suggested to omit the common feedwater tank (buffer tank) and the common deaerating process and let these functions be integrated in the system. The result will be a plant having fewer components than usual, and the plant can therefore be very compact and relatively light .

The invention provides a steam plant using sea water from the sea water system of the platform as feedwater, and which integrates distilling and desalination in the steam plant . The residual heat in the exhaust after the exhaust having passed an ordinary part of the steam boiler may therefore be used as a source of energy for the distillation. This heat would otherwise* not be utilised. In order to utilise this residual heat the distillation must take place at a low pressure, and the plant must therefore have two pressure levels, where the high pressure part is as in an ordinary steam plant, while the low pressure level is used for distilling sea water. A variant of this scheme may also be used to recover steam having been injected into the gas turbine. By lowering the exhaust temperature sufficiently for the steam in the exhaust to condense (typically 40-60⁰C) , the condensed water may be drained off and recovered.

The invention is defined in claims 1 and 8.

Description of Figures

For better understanding of the invention it will be described more closely with reference to the exemplifying em- bodiments shown in the appended drawings, where

Figure 1 shows a process flow plan for a steam generating system according to the invention, and

Figure 2 shows a process flow plan for a modified embodiment of the steam generating system illustrated in Figure 1.

It is first referred to Figure 1, which schematically shows a first embodiment of a steam generating system according to the invention. The system comprises a low pressure generator generally designated 1 and a high pressure generator generally designated 2. Basically, the low pressure generator 1 comprises an evaporator 3 , a separator 4 and a condenser 5, with appurtenant pipes, pumps and valves. The evaporator 3 is arranged in an exhaust conduit 7 conducting exhaust gases 8 from a gas turbine (not shown) . The plant is located on an offshore installation.

Feedwater in the form of sea water 9 is taken from the sea water mains of the installation and is fed through the condenser 5, where it serves the purpose of a cooling medium while concurrently being preheated. The sea water continues via a level regulating valve 10 in through the bottom of the separator 4. From there, the water goes to a circulation pump 11 causing transportation of the water through the evaporator 3. In the evaporator, parts of the water will evaporate, resulting in a mixture of water and steam flowing into the separator 4 where steam is separated from the water. The water is mixed with the incoming feedwater and is circulated anew by means of the pump 11. The circulating water will have a higher salinity than the feedwater, and in order to avoid too strong a salt concentration, a part of the circulating water is drained off downstream of the pump 11 via the drainage valve 12. The drained water may be dumped overboard.

The steam in the separator 4 is led via a pressure regulating valve 13 to the condenser 5, where it is condensed through heat exchange with the sea water 9 flowing therethrough. If the flow rate of feedwater to the separator 4 is not sufficient to keep the desired temperature in the condenser 5, the flow rate through the condenser may be increased by opening the temperature regulating valve 14, which dumps a suitable amount of sea water overboard 6.

Oxygen and inert gases released from the water in the evaporator 3 will collect in the condenser 5 and may be removed therefrom by means of a vacuum pump 15. This is because a sub-atmospheric pressure may exist in the condenser 5 if one wishes to lower the temperature in the evaporator 3 sufficiently for water vapour in the exhaust gases 8 to be condensed out for recycling. For this purpose, a collecting device 16 for condensate is arranged in the exhaust duct 7 below the evaporator 3. The condensate may be sucked up into the separator 4 via a regulating valve 17. If necessary, a pump may also be arranged in the pipe between the collecting device 16 and the separator. In such a system one will not only recover steam having been injected into the gas turbine, but one will also take out more heat from the exhaust gases than usual for common HRSG plants.

The condensate formed in the condenser 5 is conducted via a feedwater pump 18 to the high pressure generator 2 of the system, more specifically via a level regulating valve 19 into the separator 20 of the generator. From this separator the feedwater is circulated via the pump 21 through a high pressure evaporator 22. In a manner similar to the one in the low pressure generator, parts of the water circulating in the high pressure evaporator 22 will evaporate and be conducted as steam out of the separator 20 to an overheater 23 , and further via a regulating valve 24 to the consumer 25 of the steam, in this case the combustion chamber of the gas turbine. If there is still some salt left in the water leaving the low pressure generator, it will be concentrated in the liquid in the separator 20. In order to keep the salt content low, some of the liquid in the separator may be drained off through the valve 26.

It is now referred to Figure 2, where a modified, somewhat simplified embodiment of the steam generating system according to the invention is shown. The same reference numerals are used for parts having like parts in Figure 1. Compared to Figure 1 the embodiment in Figure 2 is func- tionally different in that the sea water fed to the low pressure generator does not go into the separator, nor does a circulation loop go through the separator and the evaporator. Instead, the feedwater goes through a flow rate regulating valve 27 directly into the evaporator 3 and from there into a separator 28, which is arranged vertically and having a steam outlet at the upper end and a water outlet at the lower end. Water which is not evaporated in the evaporator 3 will be separated in the separator and pumped out by means of the pump 29.

In the high pressure generator also the feedwater goes directly into the evaporator 22 and on to a vertically oriented separator 30, from where the steam is led to the superheater 23 and further on to the consumer 25. For regulating the temperature of the steam a recycle line having a regulating valve 31 is arranged. The water separated in the separator 30 is drained via a valve 26. In this case a pump is not necessary due to the gauge pressure in the separator 30.

It will be understood that the invention is not limited to the exemplifying embodiments described above, but that it can be varied and modified by the skilled person within the scope of the appended patent claims and equivalents of the features recited therein. Thus, the separator and condenser may be built into the same container in order to save space, and a small extra volume in the condenser may be used as water buffer for the high pressure generator. In this case, the need for a separate feedwater tank is obviated. Furthermore, it will be understood that if the system according to the invention produces more steam than the primary user needs, the surplus steam may used for other purposes or condensed for use as fresh water. The invention may also be used for other contaminated feedwaters than sea water.

Claims

1. A method for generating substantially pure steam under pressure, wherein steam is generated in a low pressure generator (1) and a high pressure generator (2) by means of

5 heat from the exhaust gases of a gas turbine, and wherein sea water is used as the basic medium for the steam generation, c h a r a c t e r i s e d i n that the sea water (9) is supplied as feedwater directly to the low pressure genera- io tor (1) where it is partly evaporated, and that the resulting steam is condensed and supplied under increased pressure as feedwater to the high pressure generator (2) for conversion to said pure steam under pressure.

2. A method according to claim 1, wherein at least a part i5 of the pure steam is injected into the combustion chamber of said gas turbine.

3. A method according to claim 1 or 2 , wherein the steam temperature in the low pressure generator (1) is kept low enough for water vapour in the exhaust gases of the gas

20 turbine to condense out, the resulting condensate being collected and supplied to the low pressure generator (1) .

4. A method according to one of the preceding claims, wherein the low pressure generator (1) and the high pressure generator (2) receive heat from the same exhaust

25 gases .

5. A method according to one of the preceding claims, wherein the method comprises the steps of

- leading sea water into the low pressure generator (1) ,

- evaporating the sea water partly in order to create a 30 mixture of steam and liquid,

- separating the mixture of steam and liquid into a steam component and a liquid component,

- removing the entire or part of the liquid component from the low pressure generator,

- condensing the steam component and leading the resulting liquid under pressure into the high pressure generator (2) ,

- partly evaporating the liquid in the high pressure gen- erator in order to create a new mixture of steam and liquid at a higher pressure,

- separating the new mixture of steam and liquid into a steam component and a liquid component,

- removing the entire or part of the liquid component from the high pressure generator (2) ,

- superheating the steam component in the high pressure generator, and

- leading the overheated steam to a final user, preferably to said gas turbine for injection.

6. A method according to one of the preceding claims, wherein the condensation of the steam in the low pressure generator (2) is performed by heat exchange with at least the sea water supplied as feedwater.

7. A method according to one of the preceding claims, wherein the low pressure generator (1) is operated at a sub-atmospheric operating pressure.

8. A system for generating substantially pure steam under pressure, comprising a low pressure generator (1) and a high pressure generator (2) which is supplied with heat from exhaust gases (8) from a gas turbine in an exhaust conduit (7) for the gas turbine, c h a r a c t e r i s e d i n that the low pressure generator (1) is provided with means (3,4, 5) for converting sea water (9) into feedwater for the high pressure genera- tor (2) .

9. A system according to claim 8, wherein said means comprises an evaporator (3) arranged in the exhaust conduit

(7) of the gas turbine, a separator (4, 28) and a condenser (5) , said condenser using said sea water (9) as cooling medium.

10. A system according to claim 8 or 9, wherein the condenser (5) is provided with means (15) for maintaining a sub-atmospheric pressure therein, and wherein, below the evaporator (3) in the exhaust conduit (7) a collecting device (16) is arranged for collecting water condensated from the exhaust gases (8) in the evaporator (3) , said collecting device (16) communicating fluidly with the water supply to the evaporator (3) .