GB2299377A - Gas turbine powere generation system - Google Patents

Abstract

In a gas turbine power generation system a multiple stage air compressor 8 delivers air at 15 to 30 atmospheres and 100 C, intercooling 7 maintaining stage temperatures at about 100 C. and a stage compression ratio of around two. A minor portion 31 of said air is cooled (cooler 6) and used for turbine (5) blade cooling and the remaining air stream 30 is saturated with water (saturator 15) and used for combustion (combustor 2) after passing through a recuperator 10, 12, 14 in an exhaust gas heat recovery duct 9 of the turbine 5. The combustion chamber 2, produces gases within a temperature range 1300 C. to 1600 C. (firing temperature of 1600-1800{C. are also mentioned) to drive the turbine and generate electricity. Fuel gas for the combustor 2 is utilised in the intercooler 7 prior to passing to a saturator 19, heat exchange coils 20, 21, 23, 25, quench unit 22 and saturator 24.

Description

This invention relates to gas turbine power plants and to methods of improving the operating efficiencies of such plants.

In a conventional power plant the gas turbine and the air compressor operate on the same shaft, the gas expander providing power to drive the compressor and provide power generation or mechanical drives. In certain aeroderivative versions a first hot gas expander drives the air compressor only and a second high pressure gas stream is utilised to drive a second expander to provide power generation or mechanical drives.

The following are common features of both types of turbine plant. The air compressor is uncooled, that is it has no intercoolers. This means that even with the most efficient designs of air compressors the air exit temperatures are very high and in the region 350-550 C dependent on the compression ratio adopted. Turbine exhaust temperatures are in the range 450-600 C. and the use of recuperation to recycle exhaust heat back to the combustor is not possible.

Because recuperation is not possible the exhaust is used to raise and superheat steam. This steam is passed through a steam turbine to generate additional power.

This system of gas turbine/steam turbine is known as combined cycle. In combined cycle power generation, in approximate terms, for every two units of power produced by the gas turbine, one additional unit is generated by the steam turbine.

Special combustor burners are used to suppress NOX formation. These are known as dry low-NOX burners.

They achieve low levels of exhaust gas NOX in spite of combustion air temperatures in the range 350-550 C.

In order to allow high temperature combustor exhaust gases to flow over the first few rows of expander blades, the blades are constructed of special heat resistant materials. The blades are hollow with specially formed passages and exit slots. Air from the air compressor is led through passage ways in the shaft and rotor discs to feed such air to the base of each hot blade. This air flows through the blade internal passages and exits through specially positioned exit slots or holes. By such air cooling the blade temperature is maintained cooler than the hot gas working fluid temperature, enabling the blade materials to withstand the severe stresses imposed on them by the operating conditions.

Because of the above constraining factors combined cycle power plants are limited to generation efficiencies of around 55% and because of the use of steam turbines significant heat rejection is required, calling for the use of either cooling towers or access to sea or river water.

It is an object of the present invention to significantly reduce the effect of the above constraints and produce power at an increased efficiency over conventional combined cycle plant.

To enable the invention to be understood, a method of operating a gas turbine power plant will now be described, solely by way of example and with reference to the accompanying drawing.

In the drawing, a gas turbine power plant comprises a generator 1, a fuel gas combuster 2 and a hot gas expander 5. Exhaust gases exit the expander 5 via exhaust duct 9. Combustion air is supplied by an independantly driven air compressor 8. Air is drawn via line 28 to the suction side of compressor 8. This machine is operated in efficient conditions using successive stages of intercooling (shown generally as an intercooler 7) to maintain stage delivery temperatures at about 100 C .A plurality of transfer lines 60 and 70 carry the hot air from a given stage via the intercooler 7 to the suction inlet of the next stage. Heat is rejected into cooling water line (not shown). A portion of this heat is recovered by heating up gas fuel from ambient temperature up to about 90 C This recovers heat for recycle to the gas turbine combustor and reduces requirements for cooling water.Hot air from the air compressor delivery line 29 is split into a minor stream 31 and a major stream 30.

Air in line 31 is cooled to approaching ambient temperature in cooler 6. and passes via line 32 to a compartment 4. Compartment 4 is fixed to the front casing of expander 5 and is provided with sealing means at each end to permit a generator shaft 3 to rotate within 4 while delivering power to generator 1. This cooled air passes into that portion of the shaft which is enclosed between the ends of compartment 4 and along the shaft before passing into the rotor disc passage ways previously referred to in describing the known features of existing gas turbines, and flows through the "hot" expander blades also as previously described.Because the cooling air is at only 30-50 C. (it could of course be specially chilled down further if required) the blades can withstand much hotter combustor exit gases such that firing temperatures in the range 1600-1800 C. are entirely achievable using known blade materials and cooling designs.

Air in line 30 may be optionally augmented, via a line 80, by outside working fluids, for example (a) Nitrogen or Carbon Dioxide (suitably compressed) from adjacent factories or process units such as gasification or ammonia production plants or (b) Low oxygen content flue gases (suitably cooled, demoisturized and compressed).

Flue gas entering stack 18 would be quite suitable. Its oxygen content is about 7-8% dependent on the combustor firing temperature.

No aftercooling would be required for any such fluid after compression. It is particularly advantageous if the nitrogen or carbon dioxide are available at source under some pressure to reduce compression requirements.

Nitrogen, carbon dioxide and low oxygen content flue gases will also aid in NOX suppression in the combustor 2. Similar fluids can also be injected by via line 44 into fuel gas line 43 to dilute the fuel gas and further aid NOX suppression in combustor 2. Air from line 30 enters saturator 15 and is moisturized to about equilibrium vapour content at around 100 C. Water circulation via lines 34 and 35 enables the necessary heat to be taken from duct 9. Line 33 supplies make-up water to replace that transferred into the air. Air flows from saturator 15 via line 46 to coil 14 in duct 9. The water stream after point of juncture with line 33 may be routed through 7 to be heated up by intercooling of hot air prior to flowing to coil 17. Coil 14 heats the air up moderately (by about 30-50 C). The air then passes via line 47 to a quench unit 13, in which a water spray reduces the air temperature by about 30 C. The quench cooling enables a wider temperature range to operate on coils 10 and 12, and also provides extra working fluid with some benefit in NOX suppression. The air is then further heated in coil 12 via line 48 and then optionally passes to a further saturator 11. Low grade heat in the range 130-180 C from adjacent process Units such as gasification plants, is used in saturator 11 to significantly increase the air moisture content, providing additional working fluid and extra assistance in combustor NOX suppression. From saturator 11 the air passes via line 50 to coil 10 and is heated up to about 550 C by the hot expander exhaust gases.The air then passes to combustor 2 where its temperature is raised to a temperature in the range 1600-1800 C before it passes via line 16 to expander 5. The hot combustor gases are expanded to achieve an expander exhaust temperature of about 600 C. This exhaust gas temperature is achieved by suitable choice of compression ratio in air compressor 8.

The hot gases then pass through duct 9 to be successively cooled by the various heat exchange coils 10, 25, 12, 23, 14, 21, 17 and 20 and exit to atmosphere via stack 18 at around 100 C.

Fuel gas fuel passes via line 51 to intercooler system 7 in which as previously described it is heated to about 90 C, it then passes into line 43. Additional fluid such as nitrogen, carbon dioxide or demoisturized flue gas may optionally be added via line 44, in like manner to the addition of such fluids via line 80 to the major air stream flowing in line 30 and with the same benefical results. The fuel gas then flows into a saturator 19 which abstracts heat from the low temperature end of the duct using water circulation via line 36, coil 20, and line 37. The line 45 supplies make-up water to replace that transfered into the fuel gas.The fuel gas is then passed via line 38 to coil 21 to be heated moderately by 30-50 C before passing via line 39 to quench unit 22 where water is sprayed into the gas to reduce its tempeature by around 30 C to benefit subsequent coils temperature driving forces.The quenched fuel gas then passes via line 40 to coil 23 in which it is further heated before optionally passing into a saturator 24.

This saturator employs process waste heat from adjacent units in the range 130-180 C to saturate the fuel gas with water. Saturation confers benefits by providing extra working fluid and additional NOX suppression in combustor 2. The hot moisturized fuel gas then passes via line 42 and coil 25, emerging at about 500 C and then via line 26 to combustor 2 to provide heat to raise working fluid temperatures such that combustion gases in the range 1600 -1800 C flow to the hot gas expander 5.

It will be noted by those skilled in the art that fuel gas should not be directly heated by hot flue gas for safety reasons.

The use of direct heat exchange between gas fuel and hot flue gases is readily avoided by known means such as a secondary heat transfer loop employing circulating inert gas to recover duct heat and then to exchange this with gas fuel. This secondary loop is not shown but its effect is allowed for in that the gas is only envisaged as being heated to 500 C as compared to 550 C. for the air stream.

It will also be apparent that the various coils (20,21,23, 25) which heat up the fuel gas are arranged in parallel with the coils (17,14,12,10) which heat up the combustion air stream. The parallel arrangement of coils is used so that the combined heat absorption capacities of combustion air and fuel gas can be deployed to cool the flue gas in the duct 9. Their combined heat capacity is only about two percent less than that of the flue gas and this together with the use of water quenching in quench units 13 and 22 ensures that good temperature differences for heat exchange are achieved and maintained.

For an improved gas turbine system as above described, using Methane as the fuel and operating without any of the optional items mentioned viz input lines 80 and 44 and saturators 11 and 24 the efficiency of power generation lies in the range 60-70 % based on lower heating value, dependent on the combustor temperature, ambient conditions, compressor and hot gas expander efficiencies and approaches used on the various heat exchange coils.

Claims (10)

1 A gas turbine power generation system designed for continuous operation in which a multiple stage air compressor is employed to deliver air at a pressure of 15 to 30 atmospheres and a temperature around 100 C, said compressor, having stage intercooling to maintain stage temperatures at about 100 C.and a stage compression ratio of around two, a minor portion of said air is used for turbine expander blade cooling and the remaining major air stream is used for combustion after passing through a recuperator located in an exhaust gas heat recovery duct of the turbine expander to recover most of the heat contained in the exhaust gases from the turbine expander before entering a combustion chamber to be mixed and fired with a fluid fuel to produce hot high pressure combustion gases within a temperature range 1300 C. to 1600 C. to drive the turbine expander and generate electrial power, the exhaust gases leaving the turbine expander via the exhaust duct at a pressure slightly above atmospheric pressure.

2 A power generation system as claimed in claim 1, wherein the minor air stream is cooled before delivery to the turbine expander blades.

3 A power generation system as claimed in claim 1 or claim 2 wherein the major air stream is saturated with water prior to entering the recuperator.

4 A power generation system as claimed in claim 3 wherein the major air stream is passed upwardly through a saturator column and becomes saturated with water at temperatures around 100 degrees Celsius before entering said recuperator, said saturator column being provided with mass transfer packing and a circulating water system which pumps water from the base of said saturator column through a heating coil placed in said exhaust flue gas heat recovery duct between said recuperator and a an exhaust gas discharge stack to re-enter said saturator column and flow downwardly over said mass transfer packing and back into the base of said saturator column.

5 A power generation system as claimed in claim 3 or claim 4 wherein water supplied to saturate the major air stream is preheated by heat exchange with the inter stage cooling arrangents of the compresser.

6 A power generation system as claimed in claim 3 wherein the fuel supplied to the combuster is preheated by heat exchange with the inter stage cooling arrangents of the compresser.

7 A power generation system as claimed in claim 3 or claim 6 wherein the fuel supplied to the combuster is preheated in association with a coil located adjacent the recuperator in the exhaust gas heat recovery duct.

8 A power generation system as claimed in claim 7 wherein the fuel is pre heated in a secondary heat exchanger in which an inert gas acts as a heat exchange medium between the coil and the heat exchanger.

9 A power generation system as claimed in claim 7 or claim 8 wherein the a gaseous fuel is saturated with water before said pre heating.

10 A power generation system substantially as described herein with reference to the accompanying drawing.