WO2008074099A1

WO2008074099A1 - A process for improving gas turbine power output in hot weather

Info

Publication number: WO2008074099A1
Application number: PCT/AU2007/002002
Authority: WO
Inventors: Donald Ray Cummings
Original assignee: Dut Pty Ltd
Priority date: 2006-12-21
Filing date: 2007-12-21
Publication date: 2008-06-26

Abstract

A process for improving the performance, power output and/or rating of a gas turbine which is mechanically coupled to and drives a compressor producing compressed air, the process including the following steps: (a) humidifying all or a part of the compressed air leaving the compressor by directly contacting the compressed air with water at a temperature of 50 to 160°C ; and, (b) partially de-humidifying the humidified compressed air by directly contacting the humidified compressed air with water at a temperature of 1 to 120°C; wherein the partially de-humidified compressed air is then mixed with a combustible gas and the resulting mixture combusted to drive the turbine.

Description

A PROCESS FOR IMPROVING GAS TURBINE POWER OUTPUT IN HOT

WEATHER

The present invention relates to a process for improving gas turbine power output in hot weather, and also a process which utilises waste water.

Background of the Invention

A typical gas turbine works by extracting energy from a flow of hot gas produced by combustion of a combustible gas in a stream of compressed air. An air compressor is normally located downstream and is mechanically driven by the turbine. There is also a combustion chamber in between.

Energy is released when compressed air is mixed with fuel and ignited in the combustion chamber. The resulting gases spin the turbine and thereby mechanically power the air compressor. Finally, the gases are passed through a nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure.

Energy is extracted in the form of shaft power from the turbine can then be used in many applications including the generation of electricity.

The power output or rating of a typical gas turbine varies with the ambient temperature. The ISO (International Standards Organisation) rating for a gas turbine is based on use of 60% saturated air at 15°C at sea level, and the power output drops below ISO rating as ambient temperatures increase. Conversely a gas turbine can operate above its ISO rating when the inlet air temperature is below 15°C. Consequently gas turbines have maximum de-rating during periods of hot weather which can also be a period of maximum power demand which is typically due to maximum air conditioning loads.

The humidification of air leaving the compression stage of a gas turbine can increase the power output from the gas turbine in the so called Humidified Air Turbine (HAT) cycle but there is the major problem that if a standard gas turbine is adapted for HAT operation the rating of the expansion stage can be significantly increased potentially doubling the net output of the turbine but grossly overloading the expander and generating system allied to the turbine.

Coal mines and Coal Seam Methane (CSM) production system can have severe problems associated with the co-production and disposal of waste water which typically has a high saline content, making it difficult to dispose of such water into inland and coastal waterways where high salt content is a recognised and serious problem.

In addition to having a high saline content such waste water can also have a high sodium bicarbonate content which with its lower solubility than sodium chloride, can cause precipitation and crusting problems in evaporation ponds making total evaporation of the CSM or coal mine waste water to dryness quite difficult.

Saline water may be used for humidification of air and also the evaporation of waste water but salt carry over in air passing to a turbine or in pressurised air leaving the compressor stage of a gas turbine can give rise to severe corrosion and blockage problems within any recuperator, and severe corrosion problems in the combustor and high temperature metal components of the gas turbine

Accordingly, the present invention seeks to provide a process for improving the power output or rating of a gas turbine and in particular during periods of hot weather and high electricity demand. The present invention also seeks to provide a process that utilises waste water.

Summary of the Invention

According to one aspect the present invention provides a process for improving the performance, power output and/or rating of a gas turbine which is mechanically coupled to and drives a compressor producing compressed air, the process including the following steps: a. humidifying all or a part of the compressed air leaving the compressor by directly contacting the compressed air with water at a temperature of 50 to 160⁰C ; and, b. partially de-humidifying the humidified compressed air by directly contacting the humidified compressed air with water at a temperature of 1 to 120°C; wherein the partially de-humidified compressed air is then mixed with a combustible gas and the resulting mixture combusted to drive the turbine.

According to one form the water directly contacted with the compressed air at step (a) is waste water with a high salt content which is then partially recovered as distillate with a reduced salt content at step (b) when the humidified compressed air is partially dehumidified. In this form, the liquid water that leaves step (a) after contacting the compressed air has an increased salts content which may then be disposed of through various methods, such as for example using an evaporation pond.

According to another form, the water directly contacted with the compressed air is heated prior to step (a) by indirectly contacting the water with hot gases leaving the turbine, and/or the partially de-humidified compressed air is also heated by indirectly contacting the partially de-humidified compressed air with hot gases leaving the turbine prior to being mixed with the combustible gas.

In a preferred form, at step (a) the compressed air is contacted directly with the water in a counter current configuration. In a further preferred form at step (b) the humidified compressed air passes through a demister to remove any liquid water from the humidified compressed air and/or the partially dehumidified compressed air leaving step (b) passes through a demister to remove any liquid water from the humidified compressed air. The use of the demister after the humidification, or partial dehumidification step reduces the amount of salts being carried by the humidified or partially dehumidified compressed air streams. Brief Description of the Drawings

The present invention will become better understood from the following detailed description of preferred but non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:

Figures 1 is a process diagram of one embodiment of the present invention.

Detailed Description of the Invention and Preferred Embodiments

According to an embodiment of the present invention there is provided a process where waste water is first pre-heated by the hot gases leaving a gas turbine and is then injected into the compressed air leaving the gas turbine compressor to humidify and increase the mass flow of the compressed air gas stream. This compressed and humidified air is then cooled to reduce the water content of the air stream and to recover the greater part of the evaporated waste water as high grade distillate. The increased mass flow of the compressed air gas stream provides that the output of the turbine at the prevailing air temperature is increased above the normal rating of that turbine at that air temperature.

The process of the present invention provides significant increases in performance, power output and/or rating when the gas turbine is run at temperatures above 15°C and in particular during temperatures above 25 °C when demand for electricity is typically increased due to air conditioning requirements.

In a preferred aspect of the present invention the gas turbine is a recuperated gas turbine and the waste water is heated by the exhaust gases leaving the recuperator and then evaporated to reduce the amount of saline waste water to be disposed of.

The process of the present invention is particularly suited to waste water generated by CSM production and Coal production and can deal with waste water including high salinity levels as well as high sodium bicarbonate. Where the waste water used in the process contains sodium bicarbonate, it is preferably to firstly heat the water to decompose the bicarbonate and particularly sodium bicarbonate present in the waste water prior to it being evaporated to dryness by solar pond or other means of evaporation. In a more preferred embodiment, the waste water is heated and flashed at low pressure to first decompose sodium bicarbonate and then pump and then heat the resultant saline solution under pressure.

In a preferred form the dehumidification of the humidified compressed air is undertaken by a counter-current continuous or stage-wise cooling and washing so as to enable the maximum possible removal of solids and salts from the air being de-humidified. This provides that the dehumidified air

In a further preferred form the cooling and/or partial dehumidification of the air leaving the humidification stage is used to boil bicarbonate containing waste water at low pressure to assist in bicarbonate decomposition.

Preferably the cooling and humidification device is a vertical or near vertical reflux condenser or series of condensers separated by demisting stages in which the gas passes up inside the tubes at below the flooding rate for the tubes and waste water is heated and boiled on the shell-side to both cool and de-humidify the air and to heat the waste water and also aid in the decomposition of bicarbonate in the waste water.

So as to not overload the expansion stage and generator system of an existing commercially available gas turbine, the air is partially de-humidified such that the on- going mass flow does not overload the expansion and generator components of the associated gas turbine. In this way, the process may be operated so as to increase the high ambient temperature rating of a conventional gas turbine, such that its efficiency may be increased, particularly, during periods of high temperature and increased energy consumption. In accordance with the present invention low grade waste water including dissolved salts can be used to humidify the compressed air flow and then the subsequently be partially dehumidified to produce a distillate quality water which can be used for special applications such as minimum blow-down cooling tower feedwater and other water using applications which require a high-grade, low dissolved salt content water.

It will be evident that the means used in this invention to recover waste water as high- grade distillate will also enable the maximum removal of salt and dust particles from the same air stream.

Unlike systems which humidify the hot exhaust gas leaving a gas turbine to recover high grade water from waste waters, the evaporation and subsequent condensation of surplus water vapour under pressure reduces the size of the de-humidification stages and also increases the required condensation temperature thus further reducing the size of the required cooling and de-humidification system.

Where gas turbines are used for embedded power generation and sewage disinfection, there can be a problem in that the treated and disinfected sewage would be inferior to normal towns water and whilst it could be used as feed to a cooling tower-based cooling water circuit, the disinfected sewage can have the problem of having a higher than normal dissolved salt content which can result in having to have a higher than normal rate of blown-down and consequent higher demand for make up water and related water treatment chemicals.

A preferred embodiment of the present invention will now be described with reference to Figure 1 in which a gas turbine 6 is mechanically coupled via a connecting shaft 4 to an air compressor 2. The hot gas expansion turbine 6 is also mechanically connected via a connecting shaft 8 to an electricity generator 10.

Air enters line 102 where it moves into the air compressor where the air is compressed and exits the compressor via line 104. The air then moves to the first humidification step (a) at 18 where it is directly contacted with waste water entering via line 110. The waste water has previously entered the process at line 124 where it has moved through a heat exchanger 16 where the water is indirectly contacted with the exhaust gasses exiting from the hot gas expansion turbine 6 via line 128.

As the compressed air entering the humidification step at 18 is directly contacted with the heated waste water at a temperature between 30 and 90°C the air is humidified and takes up a proportion of the waste water leaving behind a high salt content liquid water that exists the humidification step from line 108. Prior to proceeding to line 106, the humidified air passes through a de-misting device 20 which is in the form of an extended surface knitted alloy mesh which substantially removes much of the liquid water from the humidified compressed air stream. The now humidified compressed air moves along line 106 to the partial de-humidification step (b) where the humidified compressed air is directly contacted with cooling water at a temperature of between 1 and 30⁰C.

The partial de-humidification step at item 24 draws cooling water which is chilled in a cooler at item 30. The chilled water after direct contact with the humidified compressed air reduces the amount of water in the compressed air and removes this via line 112 producing a distillate with much reduced salt content than the original waste water stream entering the process at line 130. This distillate may be recycled into the chiller at 30 with an excess taken off at 132 that may be used in various other applications as hereinbefore described.

After passing through the de-humidification process at 24 the partially de-humidified compressed air passes through 26 which is another de-misting device placed to limit any water carrying salt being carried beyond the partial de-humidification step (b).

The partially de-humidified compressed air then travels along line 118 until it is indirectly contacted with the hot exhaust fumes from the hot gas expansion turbine 6 in the heat exchanger 14 after which the air is heated and moves along 120 prior to being mixed with a combustion gas such as for example methane in the combustor 12 before entering the hot gas expansion turbine used to drive the air compressor 2 and also to produce electricity via the electricity generator at 10.

Fuel gas for the turbine would be preferably chosen to consist of at least methane produced by the coal mining or Coal Seam extraction operations which also generates waste water that often includes high levels of salinity and/or sodium bicarbonate.

To those skilled in the art of air humidification and de-humidification, heat exchange and the like, other possible variations of the described invention will be apparent. Such variations could typically be (a) to recirculate concentrated waste water leaving in pipeline 108 back to item 18 to ensure optimum and continuous water and air contact in Item- 18, (b) to depressurise heated waste water leaving Item- 16 to promote bicarbonate decomposition and to follow this with pumping and further heating in item- 16, (c) to integrate item-24 and 18 in the form of a vertical reflux condenser for the humidified air with low pressure boiling saline water on the shell side of the cooler, (d) arrange Item-22 as a series of counter-current cooling and water wash stages to provide maximum assurance of total salts removal from air prior to it entering the gas turbine combustor and sequential expansion stage. Other examples other than these variations will be seen to be possible.

Some example calculations illustrating the process of the present invention are outlined in the table below and also with reference to Figure 1. These calculations are representative of a Solar Mercury type of gas turbine is modified for operation in accordance with the process of the present invention. The Solar Mercury turbine under ISO conditions has a nominal rating of 4,600 KW and would have an output rating of 3,690 KW at an ambient temperature of 35°C, excluding fuel and lubricant pumping, turbine ventilation and generator cooling fans, controls and the like.

The same turbine when modified in accordance with the process of the present invention would have the following mass and energy flows when referenced to items in Figure 1. Line/item Description Temp °C Kg/second Pressure Bar MW

102 inlet air 35 15.7 1.0

104 air leaving Item 2 354 15.7

106 air leaving Item20 121 18.4 8.4

108 water leaving Item 18 115 17.3 9.0

110 water leaving Item 16 126 20.0 20.0

118 de-humidified air 100 16.8 8.4

120 pre-heated air 390 16.8 8.0

122 methane 15 0.3 17 17.67

124 waste water feed 35 20.0 20.0

128 exhaust from Item 14 393 17.1

130 exhaust gas 55 17.1 1.0

132 exit waste water 115 17.3 8.0

Item 2 5.266

Item 6 9.983

Item 10 4.714

Item 12 exit combustor 1.093

The above example is for an application where the turbine, in addition to generating electricity, also heats and assists the evaporation of water from a waste water solar evaporation pond.

Use of the system as outlined in the above example may be confined to high temperature periods where maximum power output is a primary requirement and during other periods having the turbine operate normally and the waste water being heated and discharged direct from pipeline 110.

The outlined example is based on a Mercury turbine with its standard recuperator which is not designed to handle the increased mass flow and lower temperature air leaving the humidifϊcation and de-humidification stages. One way to improve the efficiency and also the power output from the turbine would be to pass some of the air in line 104 directly to the recuperator 14, and to humidify and de-humidify the remaining part of the air such that the increase in the air mass passing to the recuperator 14 would still be as shown in the above table but the temperature of the total air flow passing to the recuperator 14 would increase thus reducing the heat exchange load on the recuperator and thus increasing the efficiency of the overall turbine system as shown in the above table.

A further way to improve turbine efficiency and maintain the increase in power output would be to install an alternative recuperator capable of maintaining substantially the same recuperator exit air temperature as in the standard Mercury turbine and with such a system the output from the generator at 35°C would remain the same as in the above example and the turbine system efficiency would increase to 43% which is above the ISO rated efficiency of the Mercury turbine.

Finally, it can be understood that the inventive concept in any of its aspects can be incorporated in many different constructions so that generality of the preceding description is not superseded by the particularity of the attached drawings. Various alterations, modifications and/or additions may be incorporated into the various constructions and arrangements of parts without departing from the spirit or ambit of the present invention.

Claims

The Claims:

L A process for improving the performance, power output and/or rating of a gas turbine which is mechanically coupled to and drives a compressor producing compressed air, the process including the following steps: a. humidifying the compressed air leaving the compressor by directly contacting the compressed air with water at a temperature of 30 to 16O⁰C ; and, b. partially de-humidifying the humidified compressed air by directly contacting the humidified compressed air with water at a temperature of 1 to 120°C; wherein the partially de-humidified compressed air is then mixed with a combustible gas and the resulting mixture combusted to drive the turbine.

2. A process according to claim 1 wherein the water directly contacted with the compressed air at step (a) is waste water with a high salt content which is then partially recovered as distillate with a reduced salt content at step (b) when the humidified compressed air is partially de-humidified.

3. A process according to Claim 1 or claim 2 wherein the water directly contacted with the compressed air is heated prior to step (a) by indirectly contacting the water with hot gases leaving the turbine.

4. A process according to any one of claims 1 to 3 wherein the water directly contacted with the compressed air is pressurised prior to step (a).

5. A process according to any one of claims 1 to 3 wherein the partially de-humidified compressed air is heated by indirectly contacting the partially de-humidified compressed air with hot gases leaving the turbine prior to being mixed with the combustible gas.

6. A process according to any one of claims 1 to 4 wherein at step (a) the compressed air is contacted directly with the water in a counter current configuration.

7. A process according to any one of claims 1 to 5 wherein prior to step (b) the humidified compressed air passes through a demister to remove any liquid water from the humidified compressed air.

8. A process according to any one of claims 1 to 6 wherein the partially dehumidified compressed air leaving step (b) passes through a demister to remove any liquid water from the humidified compressed air.

9. A process according to any one of claims 1 to 8 wherein the ambient temperature is above 15⁰C, and preferably above 25°C.

10. A process according to any one of claims 1 to 9 wherein the water directly contacted with the compressed air at step (a) includes sodium chloride and sodium bicarbonate.

11. A process according to claim 10 wherein the water is initially heated to decompose the sodium bi-carbonate prior to step (a).

12. A process according to any one of claims 1 to 11 wherein step (a) is undertaken using a vertical reflux condenser wherein the compressed air passes through the vertical reflux condenser with low pressure boiling saline water on the shell side of the condenser.

13. A process according to any one of claims 1 to 12 wherein step (b) is undertaken using a series of counter-current cooling and water wash stages to ensure minimum salts carry through beyond step (b).

14. A process according to any one of claims 1 to 13 wherein only a first part of the compressed air leaving the compressor is humidified at step (a) and the remaining part of the compressed air is mixed with the first part after step (b).

15. A process according to claim 14 wherein after the first part of the compressed air and the remaining part of the compressed air are mixed, the resulting compressed air mixture is heated by indirectly contacting the compressed air mixture with hot gases leaving the turbine.

16. A process according to claim 15 wherein the indirect heating is performed in a recuperator.