GB2049816A - A Gas Turbine Power Plant having an Air-Cooled Pressurized Fluidized Bed Combustor - Google Patents

Abstract

A gas turbine power generation plant having an air-cooled pressurized fluidized bed combustor 12 for generating combustion gases from the burning of pulverized fuel 18 in the presence of crushed dolomite 20, the plant having a gas turbine engine 32 connected to be driven by the combustion gases and for driving an air compressor 42 which provides compressed air for cooling, fluidizing the bed and supporting combustion of the fuel in the fluidized bed combustor 12, the plant also being provided with a free power turbine 56 which is connected to drive a load 58, such as an electrical generator and to be driven by gases exhausted from the gas turbine engine 32. The control system has, for start-up operation, various valve-controlled by-pass conduits 40, 44, 84, 92 for preheating compressed air before entry into the fluidized bed combustor 12, diverting compressed air for cooling from the fluidized bed combustor 12 and diverting exhaust gases around the free power turbine 56. The valve- controlled by-pass 92 for diverting exhaust gas around the free power turbine 56 also functions to quickly adjust the output of the free power turbine 56 as a result of reductions in load thereon and, upon sudden loss of load, prevents destructive overspeeding of the free power turbine 56 without disruption of the function and thermodynamic balance of the fluidized bed combustor 12 and gas turbine engine 32. <IMAGE>

Description

SPECIFICATION A Gas Turbine Power Plant having an Aircooled Pressurized Fluidized Bed Combustor This invention relates to a gas turbine power generation plant having a pressurized fluidized bed combustor and to a control system and method of start-up for such a power plant.

In gas turbine power generating plants having a free power turbine for driving a load and driven by gases generated in a pressurized fluidized bed combustor wherein pulverized fuel is burned, as exemplified in U.S.A. Patents 3791137, 3924402 and 4028883, operational control is most difficult. This control difficulty in commercial size plants is attributable to the large volume of heated pressurized air and large amount of fuel in the fluidized bed combustor and, in plants where the fuel is burned in the presence of crushed dolomite, the many tons of heated dolomite, which factors render the plant slow to respond to changes in load demand.

In start-up operation of such a plant, this large volume and quantity of material must be brought into thermodynamic balance relatively slowly before a load (such as an electric generator) can be effectively driven. As, for example, it may take as much as three or four hours to achieve proper operation of the fluidized bed combustor. Where a free power turbine, as distinguished from the expander of a gas turbine engine, is employed to drive an electrical generator, a sudden substantial decrease in the electrical generator load can result in overspeed and damage to the free power turbine since the gas turbine engine and fluidized bed combustor operation cannot be quickly altered to meet speed and/or load demand changes on the free power turbine.

There is therefore a need to provide, in a gas turbine power plant having a pressurized fluidized bed combustor and a free power turbine for driving a load, a control system which is capable of responding quickly to speed and/or load demand changes imposed on the free power turbine without upsetting the thermodynamic operation of the fluidized bed combustor and the gas turbine engine.

According to the present invention there is provided a gas turbine power plant comprising: a) a pressurized fluidized bed combustor having means for receiving therein pulverized fuel for burning: b) a heat exchanger in said fluidized bed combustor for controlling the reaction temperature in the latter; c) an air compressing means; d) first conduit means connecting the compressing means to the heat exchanger and the fluidized bed combustor for passing one portion of the compressed air discharged from the air compressing means to the heat exchanger and another portion to the fluidized bed combustor for suspending the fuel and supporting combustion of the fuel; e) a gas turbine engine having an expander connected to drive the air compressing means and having a fuel combustion zone to provide exhaust gas to drive the expander;; f) a second conduit means for conducting combustion gas from the fluidized bed combustor into admixture with the combustion gas from the gas turbine engine combustion zone and thence into the expander; g) a free power turbine connected to drive a load and to receive exhaust gas from the expander to be thereby driven; and h) a control system comprising: a first valve means in said first conduit means operative between open and closed positions for independently controlling flow of compressed air into the heat exchanger and into the fluidized bed combustor; a valve controlled by-pass conduit means for conducting exhaust gas from the expander around the free power turbine during start up operation when said first valve means is in a closed position and to prevent overspeeding of the free power turbine upon a sudden substantial decrease in the amount of the load thereon.

The gas turbine power plant preferably has a pressurized fluidized bed combustor in which pulverized solid fuel, such as coal, is burned in the presence of crushed dolomite (limestone) to remove sulphur dioxide from the combustion gas.

A heat exchanger is provided in the fluidized bed combustor for controlling the reaction temperature, as for example between 7000C and 900 C, below the fusion point of the fuel ash. An air compressing means is provided to supply compressed air, via a first conduit means, to the heat exchanger and the fluidized bed combustor in a ratio of about 2:1. The compressed air conducted to the fluidized bed combustor is for fluidizing the bed and to support combustion of the fuel. A gas turbine engine has an expander connected to drive the air compressing means and has a fuel combustion zone to provide exhaust gas to drive the expander during start-up.

A second conduit means is provided for conducting combustion gases from the fluidized bed combustor into admixture with the combustion gases, if any, from the combustion zone of the gas turbine engine, and thence into the expander. A free power turbine is connected to drive a load, such as an electrical generator, and a third conduit means is provided for conducting exhaust gases from the expander to drive the free power turbine.

The control system preferably comprises a first valve means in the first conduit means for independently controlling flow of compressed air into the heat exchanger and into the fluidized bed.

A valve-controlled by-pass conduit means is provided for conducting exhaust gases from the expander around the free power turbine during start-up operation and for regulating the free power turbine in response to load demand on the free power turbine without upsetting the thermodynamic balance of the fluidized bed combustor and gas turbine engine. Also, the valve-controlled by-pass conduit means functions to prevent destructive overspeeding of the free power turbine upon a sudden and substantial decrease in load on the free power turbine. A third valve means may be provided in the third conduit means to control flow of exhaust gases into the free power turbine and to ensure, in the event of sudden loss of load on the free power turbine and in co-operation with the valve-controlled by-pass conduit means, that no gases flow through the free power turbine and it stops rotation.

The start-up method, according to this invention, comprises the following steps. Initially, start-up requires the gas turbine engine to be driven through an independent source of rotative power similar to conventional gas turbine practice and the fuel delivered to the combustion zone of the engine ignited so that compressed air is generated by the air compressing means.

Simultaneously, compressed air is prevented from flowing to the fluidized bed combustor and the heat exchanger. After the gas turbine engine has attained sustained self-operation, the independent source of rotative power is stopped.

Part of the compressed air is heated and directed to the fluidized bed combustor while part supports combustion of a fuel in the turbine combustion zone. Heated compressed air is flowed into the fluidized bed combustor and, when the volume, temperature and pressure of this air reaches the levels sufficient to suspend and ignite the fuel, pulverized fuel and dolomite are admitted into the fluidized bed combustor and the fuel is ignited. Within a first predetermined temperature range in the fluidized combustor, the compressed air is allowed to flow in progressively increasing amounts into the heat exchanger. The exhaust gases are directed to by-pass the free power turbine.Within a second predetermined temperature range in the fluidized bed combustor higher than said first temperature range, the preheating of the compressed air and firing of the gas turbine combustion zone are ceased and bypassing exhaust gases around the free power turbine is stopped and such gases are then directed to the free power turbine to drive the latter and thereby drive a load.

An embodiment of the invention will now be described, by way of an example, with reference to the accompanying drawing, in which the single figure diagrammatically illustrates a control system according to the present invention.

The reference number 10 generally indicates a gas turbine power plant of the type having a pressurized fluidized bed combustor (hereinafter referred to as the "power plant"), and having a control system therefor, according to this invention.

The power plant 10 and control system therefor comprises a pressurized fluidized bed combustor 12 which is connected through supply means, such as conduits 14 and 16, to a source of pulverized solid fuel 18 and sulphur dioxide absorbing material 20 as, for example, respectively, coal and crushed dolomite. The combustion gases generated by the burning of fuel in the fluidized bed combustor 12 is conducted from the latter, via conduit 22, to separators 24 and 26, such as cyclone separators for two-stage separation, and, by way of conduit 28 to the expander 30 of a gas turbine engine 32.

The pressurized fluidized bed combustor 12 is provided with an air cooling system for controlling the reaction temperature in the fluidized bed 34 within the range of about 7000C and about 9250C.

The air cooling system has a heat exchanger 36 of any suitable type in a fluidized bed 34, which heat exchanger 36 is connected, through conduits 38 and 40, to an air compressor 42 for receiving compressed air from the latter. The heat exchanger 36 is also connected through outlet conduit 44 to conduit 28 so that heated compressed air is conducted into admixture with the cleaned combustion gases discharged from the fluidized bed combustor 1 2 and flowing through conduit 28.

The fluidized bed combustor 1 2 is connected via conduit 38 to receive a portion of the compressed air discharged from the compressor42. This compressed air delivered into the fluidized bed combustor 12 is distributed to the fluidized bed 34 by suitable distribution means such as a perforated distribution baffle 46. The compressed air serves to maintain fuel and other particulate material, such as dolomite, in a suspended, fluid state and to provide the oxygen for supporting the combustion of the fuel.

The air compressor 42 is connected to be driven by the expander 30 of the gas turbine engine 32 and may be part of the gas turbine engine assembly or may-be a separate unit suitably connected to be rotated by the expander 30. The combustion zone or combustor 48 may also be an integral part of or a separate unit of the gas turbine assembly. A combustor 48 is connected to receive compressed air from the compressor 42, via conduit 50, and fuel is supplied thereto from a suitable supply thereof to generate combustion gases which are discharged, via conduit 52, into admixture with combustion gas in conduit 28. Compressed air flow through conduit 52 is controlled by a valve 51. The combustion gas from the combustor 48 functions alone or in conjunction with combustion gas and heated compressed air to drive expander 30.

The expander or exhaust gas from the expander 30 is conducted by passageway or conduit 54 to a free power turbine 56. The free power turbine 56 is connected to drive a load such as an electrical generator 58. The exhaust gas from the free power turbine 56 is discharged by way of an exhaust conduit 60 to a steam and power generating system 61.

The steam and power generating system 61 comprises a waste heat boiler 62 which receives the exhaust gas from the free turbine 56 and passes the gas in indirect heat exchange relationship with water from a supply conduit 64 to convert the water to steam. A steam turbine 66 is connected to drive an electric generator 68 and to receive, via outlet conduit 70, steam. The spent steam is passed from steam turbine 66 to a steam condenser 72 where it is converted back to water and discharged through conduit 74 for recirculation to waste heat boiler 62. The water from the condenser 72 and make-up water are passed to a feedwater heater 76 and thence, through supply conduit 64, into the waste heat boiler 62.

The control system, according to the present invention, comprises several valve-controlled bypass conduits which serve to place the power plant 10 in service and to permit quick adjustment of the free power turbine 56 to changes in load demand on the electrical generator 58 as well as to protect the free power turbine 56 against overspeeding upon sudden loss of load demand on electrical generator 58.

The control system, more specifically, comprises a by-pass conduit 78 which is arranged to interconnect pipe 40, which conducts compressed air to heat exchanger, 36 with an outlet conduit 44 to thereby bypass air around the heat exchanger 36. A valve 80 is disposed in the bypass conduit 78 to control flow therethrough, while a valve 82 is disposed in conduit 40 for control of compressed air flow therethrough. The valves 80 and 82 are operable to totally bypass compressed air around the heat exchanger 36 as is done during part of the start-up period or to modulate flows in accordance with the temperature of fluidized bed 34 to maintain the bed temperature within the desired temperature range of 7000C to 9250C during operation. The valves 80 and 82 also co-operatively function to maintain the fluidizing air flow velocity in the fluidized bed 34 at a constant actual value.This latter function is accomplished by sensing air flow velocity in conduit 40 downstream of valve 82, e.g. at 35, and correlating that measurement with air velocity in the fluidized bed by sensing as a function of velocity the temperature and pressure therein.

A second bypass conduit 84 in the cooling air system is provided to bypass the compressed air discharged from air compressor 42 into conduit 38. A suitable heater 86 is disposed in the bypass conduit 84 to heat compressed air before it is passed into the fluidized bed combustor 12. The heater 86 may be of any suitable type for heating the compressed air and may be, as shown, a fuel fired combustor. To control the flow of compressed air through the bypass conduit 84 and conduit 38, valves 88 and 90 are provided in the respective conduits 84 and 38.

Another bypass conduit 92 is connected, at one end, to conduit 54 and, at the opposite end, to exhaust conduit 60 to provide for bypassing exhaust gas from the expander 30 around the free power turbine 56. A valve 94, which is operative in response to a signal generated by a speed switch or other suitable load sensing device 96, is disposed in the bypass conduit 92 to control flow through the latter. For operation under substantially constant load on free power turbine 56, valve 94 is in a closed position. However, for start-up operation of the power plant 10, valve 94 is fully open. Also valve 94 functions to adjust the torque developed by the free power turbine 56 to substantial changes in the load demand on generator 58 and ence the free power turbine 56.

Furthermore, in the event of a sudden loss of load demand, valve 94 opens to drop the pressure differential across the free power turbine 56 to substantially zero and thus prevent the free power turbine 56 from overspeeding and the damage resulting therefrom. This bypass conduit 92 and valve 94 therein provides a rapid and precise control of the free power turbine output without upsetting the thermodynamic balance of the fluidized bed combustor 12. To further ensure protection of the free power turbine 56, it is preferred that a valve means 98 be provided in conduit 54.

The valve means 98 is normally in an open position and functions to shut off flow of gas to the free power turbine 56 where there occurs a sudden reduction of output load on the free power turbine 56. This sudden loss of demand load may arise when there is an electrical failure in generator 58, circuit interruption, or a drive coupling failure between the free power turbine 56 and the generator 58 which is driven by the free power turbine.This valve means 98 may take any suitable form, as for example, a set of noncambered vanes or louvres in the entrance annulus of the free power turbine 56, which vanes or louvres can be arranged in alignment with the direction of gas flow for a normally open position and rotated through suitable linkage and unison ring assembly to where the vanes or louvres are in a substantially perpendicuiar orientation with respect to the direction of gas flow to form a fully closed position; a set of iris or guillotine plates can be arranged and actuated to form an annular gate valve; or, in new power turbine designs, the first stage stator vanes can be designed to be rotatable to a closed position to accomplish shutting off exhaust gas flow.

It is desirable that the valve means 98 be provided in the control system and to cooperatively function with valve 94 in bypass conduit 92 because it has been found that, in some situations, not enough exhaust gas can be bypassed through conduit 92 to reduce power output of the free power turbine 56 to zero and prevent overspeeding of the free power turbine.

The valve means 98, similar to valve 94, is connected to respond to load sensing device 96 and to close while valve 94 opens. The valve means 98 also functions, when in a closed position and in conjunction with closed valve 94, to maintain back pressure on the gas turbine engine 32 and thus prevent its overspeeding.

Start-up of the power plant 10 is achieved by first closing valves 80, 82, 88, 90 and 98 while opening valves 51 and 94. This positioning of the aforesaid valves blocks flow to compressed air to the fluidized bed combustor 1 2 and opens bypass pipe 92 so that exhaust gas from the expander 30 does not enter the free power turbine 56. With valve 51 open, a suitable starter mechanism 100, such as an internal combustion engine, is operated to drive the air compressor 42. All of the compressed air discharged from the air compressor 42 flows through conduit 50 into the combustor 48 where fuel is injected and the fuel/air mixture ignited to produce hot combustion gas. This combustion gas is conducted, via conduit 52, to a conduit 28 and, thence, to the expander 30 to drive the latter.A check valve 102, or similar shut-off device in conduit 28, prevents flow of combustion gas in a direction towards the fluidized bed combustor 12. The exhaust gas from the expander 30 is conducted to the steam and power generating system 61 by way of conduit 54, bypass conduit 92 and exhaust conduit 60. A check valve 104 prevents reverse flow of exhaust gas in exhaust conduit 60.

Once the expander 30 is being driven, self-starter mechanism 100 is stopped and combustor 48 is operated so that the gas turbine engine is heated.

When the gas turbine engine 32 operatively stabilizes, valve 88 in bypass conduit 84 is opened to permit flow of compressed air to the heater 86 in which part of the compressed air supports the burning fuel, the mixture of combustion gas and heated compressed air flow into the fluidized bed combustor 12, via conduit 38. After the heated compressed air and combustion gas introduced into the fluidized bed combustor 12 attains the volume and pressure sufficient to support fuel and particulate material in suspension, fuel and particulate material are fed into the fluidized bed combustor 12 through the conduits 14 and 16. The mixture of hot combustion gases and compressed air also provides the heat and the oxygen for causing the fuel in the fluidized bed 34 to ignite.When the fluidized bed temperature reaches about 7500C to about 8000C, as sensed by a temperature sensing and signailing device 106, valve 82 is gradually opened to permit compressed air to flow via conduit 40 into the heat exchanger 36.

By gradually increasing compressed air flow into and through the heat exchanger 36, excessive thermal shock to the heat exchanger 36 is prevented as the relatively cool compressed air begins to flow through the heat exchanger 36.

When the fluidized bed 34 reaches a temperature within the range of about 8700C to about 9250C, valve 80 opens so that the amount of compressed air flow through heat exchanger 36 is controlled to maintain the fluidized bed 34 at the desired temperature range of about 8700C to about 9250C by modulating flow through the heat exchanger and bypass conduit 78. The valves 80 and 82 co-operate by one moving to a closed position while the other moves to an open position thereby splitting the air flow to maintain the aforesaid desired fluidized bed temperature.

These valves also coact to maintain the fluidizing air velocity at a constant actual velocity.

The gas generated in the fluidized combustor 34 passes, via conduits 22 and 28, into the expander 30 of the gas turbine engine 32. The exhaust gas from the expander 30 is, at this time, bypassing the free power turbine 56 through bypass conduit 92. Also, when the fluidized bed 34 attains a temperature in the range of about 7500C to 8000C and with the gas turbine engine 32 achieving synchronized idle point, valve 88 is closed and fuel is cut off to the heater 86.

Substantially simultaneously with the closing of the valve 88, valve 90 is opened so that compressed air now flows directly to the fluidized combustor 12 where the fluidized bed 34 has attained thermodynamic equilibrium. At this time, the bypass valve 94 is closed and valve 98 opened to admit the exhaust gas from the expander 30 into the free power turbine 56 to thereby drive the latter. Also at this time, valve 51 is closed and the fuel is shut off to the combustor 48 so that the expander 30 is driven only by a mixture of combustion gas and heated compressed air delivered to the expander 30 via conduits 28 and 44.The power plant 10 is now operating at full load demand on generator 58 with valves 94 and 98 modulating flow through conduit 54 and bypass conduit 92 to compensate for fluctuations in load demand and, upon a sudden and substantial loss of load demand, opening and closing, respectively, to prevent overspeeding of free power turbine 56 and the expander 30.

It will be now readily apparent that the present invention provides a control system for a gas turbine power plant which has a fluidized bed combustor capable of providing improved and simplified start-up operation, adjustment for changes in load demand without upsetting the thermodynamic balance of the fluidized bed combustor, and protection of the free power turbine against overspeeding in the event of a sudden and substantial loss of load demand. It is a control system in which the valves thereof are located to control gaseous flow when such gases are at relatively low temperatures and therefore need not be of special design.

Although but one embodiment of the invention has been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto except by the scope of the

Claims (14)

appended claims. Claims

1. A gas turbine power plant comprising: a) a pressurized fluidized bed combustor having means for receiving therein pulverized fuel for burning; b) a heat exchanger in said fluidized bed combustor for controlling the reaction temperature in the latter; c) an air compressing means; d) first conduit means connecting the compressing means to the heat exchanger and the fluidized bed combustor for passing one portion of the compressed air discharged from the air compressing means to the heat exchanger and another portion to the fluidized bed combustor for suspending the fuel and supporting combustion of the fuel; e) a gas turbine engine having an expander connected to drive the air compressing means and having a fuel combustion zone to provide exhaust gas to drive the expander;; f) a second conduit means for conducting combustion gas from the fluidized bed combustor into admixture with the combustion gas from the gas turbine engine combustion zone and thence into the expander; g) a free power turbine connected to drive a load and to receive exhaust gas from the expander to be thereby driven; and h) a control system comprising: a first valve means in said first conduit means operative between open and closed positions for independently controlling flow of compressed air into the heat exchanger and into the fluidized bed combustor; a valve controlled bypass conduit means for conducting exhaust gas from the expander around the free power turbine during start up operation when said first valve means is in a closed position and td prevent overspeeding of the free power turbine upon a sudden substantial decrease in the amount of the load thereon.

2. A gas turbine power plant as claimed in claim 1, in which a heater is located in parallel with said first conduit means and valve means is provided for coacting with said first valve means to pass compressed air to said heater before entry into said fluidized bed combustor during start-up operation of the gas turbine power plant.

3. A gas turbine power plant as claimed in claim 1 or claim 2, in which a third conduit means is provided for conducting exhaust gas from the expander to the free power turbine, and wherein third valve means is provided in said third conduit means to control flow of exhaust gas into the free power turbine in inverse relation to the amount of exhaust gas permitted to flow through said valve controlled bypass conduit means.

4. A gas turbine power plant as claimed in claim 3, in which said valve-controlled bypass conduit means includes a valve operative in response to free power turbine speed so that at speeds below a predetermined maximum, flow is prevented through the valve-controlled bypass conduit means and wherein said third valve means normally functions to allow exhaust gas flow into the free power turbine at speeds below said predetermined maximum speed.

5. A gas turbine power plant as claimed in any preceding claim, in which means is provided for isolating said combustion zone of the gas turbine engine from the compressing means and expander so that the expander is operative only on combustion gas from the fluidized bed combustor.

6. A gas turbine power plant as claimed in any preceding claim, in which said gas turbine engine has conduit means for conducting compressed air from said compressing means to the combustion zone and a third valve means for controlling compressed air-flow to said combustion zone.

7. In a gas turbine power plant comprising a pressurized fluidized bed combustor in which pulverized solid fuel is burned therein under contro!led conditions to produce a combustion gas, an air compressor connected to deliver at least part of the discharged compressed air to the combustor for entraining and supporting the fuel for combustion, a gas turbine engine connected to receive combustion gas from the combustor, a free power turbine connected to drive a load and to receive exhaust gas from, the gas turbine engine to be driven thereby, a control system comprising: a) bypass means for conducting the exhaust gas around the free power turbine to prevent overspeeding of the free power turbine upon decrease in load demand in said free power turbine; b) a second valve means is for controlling exhaust gas flow into the free power turbine; and c) sensing and signal generating means for sensing the load on the free power turbine and effect actuation of the second valve means to reduce exhaust gas flow through the free power turbine and said bypass valve to increase flow of exhaust gas through the bypass conduit and thereby prevent overspeeding of the free power turbine when a decrease in the amount of the load on the free power turbine occurs.

8. A method of controlling a gas turbine power plant comprising a gas turbine engine having a combustion zone for burning fuel therein and an expander to be driven by exhaust gases from said zone for driving an air compressing means, a pressurized fluidized bed combustor connected to a source of pulverized solid fuel and dolomite and to the air compressing means to receive at least part of the compressed air discharge to thereby provide a bed of suspended fuel and dolomite for burning the fuel, and a heat exchanger in the combustor connected to receive another portion of the compressed air discharged from said air compressing means for controlling the temperature of said bed and connected to conduct air heated in said heat exchanger to the expander of said gas turbine engine, and a free power turbine connected to receive exhaust gases from said expander of the gas turbine engine, the method comprising the following steps: a) driving said gas turbine engine by an outside source of rotative power and igniting fuel delivered to the combustion zone of the engine to start said gas turbine engine and thereby drive the air compressing means; b) simultaneously preventing flow of compressed air to said fluidized bed combustor and said heat exchanger; c) after the gas turbine engine has attained sustained self-operation stopping the outside source of rotative power and the compressed air is preheated and conducted simultaneously with flow of pulverized fuel and dolomite, into the fluidized bed combustor so that the fuel and dolomite are fluidized and the fuel burned;; d) within a first predetermined temperature range in the fluidized bed combustor the compressed air is commenced to flow into the heat exchanger and in gradually increasing amounts to maintain the fluidized bed; e) bypassing the exhaust gases from the expander around said free power turbine; and f) within a second predetermined temperature range higher than said first predetermined temperature range in said fluidized bed the preheating of compressed air and the generation of combustion gases in the gas turbine engine combustion zone are ceased and the bypassing of said exhaust gas is stopped and such gas is directed to the free power turbine to drive the latter.

9. The method claimed in claim 8, wherein said first predetermined temperature range is about 7500C to about 8O00C.

10. The method claimed in claim 8, wherein said second predetermined temperature range is about 8700C to about 9250C.

11. The method claimed in claim 8, wherein said first and second predetermined temperatures are respectively about 1 3O00F and 1 65O0F.

12. The method claimed in claim 8, wherein there is provided the step of permitting the bypassing of some of the compressed air around the heat exchanger at temperatures at or above said second predetermined temperature range.

13. The method claimed in claim 8, wherein said second predetermined temperature range is sensed within the fluidized bed combustor to generate a signal which automatically functions to effect flow of compressed air into the heat exchanger, shut-down of the heating means and ceasing of the flow of exhaust gases around the free power turbine.

14. The method claimed in claim 8, wherein said fluidizing air velocity flow into the fluidized bed combustor is maintained to a constant predetermined value.

1 5. A gas turbine power plant substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

1 6. A method of controlling a gas turbine power plant substantially as hereinbefore described with reference to the accompanying drawing.

1 7. A control system for controlling a gas turbine power plant substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.