CA1123210A - Control system for a gas turbine power plant having an air cooled pressurized fluidized bed combustor - Google Patents

Abstract

ABSTRACT

The control system and method of start-up are for a gas turbine power plant having an air-cooled pressurized fluidized bed combustor for generating combustion gases from the burning of pulverized fuel in the presence of crushed dolomite. The plant also has a gas turbine engine connected to be driven by the combustion gases and for driving an air compressor which provides compressed air for cooling, fluid-izing the bed and supporting combustion of the fuel in the fluidized bed combustor. In addition, the plant is provided with a free power turbine which is connected to drive an alternator and to be driven by gases exhausted from the gas turbine engine. The control system has, for start-up opera-tion, various valve-controlled by-pass conduits for preheat-ing compressed air before entry into the fluidized bed com-bustor, diverting compressed air for cooling from the fluid-ized bed combustor and diverting exhaust gases around the free power turbine. The valve-controlled by-pass for divert-ing exhaust gas around the free power turbine also functions to quickly adjust the output of the free power turbine to reductions in load thereon and, upon sudden loss of load, prevents destructive overspeeding of the free power turbine without disruption of the function and thermo-dynamic balance of the fluidized bed combustor and gas turbine engine.

Description

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This invention relates to gas turbine power genera-tion plants having a pressurized fluidized bed combustor and, more particularly, a control system and method of start-up for such power plant.
In gas turbine power generating plants having a free power turbine :Eor driving a load and driven by gases generated in a pressurized fluidized bed combustor wherein pulverized fuel is burned, as exemplified in the U.S. Patents to Jubb et al, No. 3,791,137 dated February 12, 197~; Harboe, No.3,924,402 dated December 9, 1975; and Meyer-Kahrweg, No. 4,028,883 dated June 1'., 1977, operational control is most difficult. This control difficulty in co~mercial size plants is attributable to the large volume o~ 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 o such a plant, this large volume and quantity of material must be brought into ~hermo-dynamic balance relativ~ly slowly before a load (such as anelectric generator2 can be effectively driven. ~s, for example, it may take as much as three to 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 ~ree power ~urbine since thQ gas turbine engine and fluidized bed combustor opera-tion cannot be quickly altered to meet speed and/or load demand changes on the free power turbine.

It is, therefore~ an object of this invention to provide, in a gas turbine power plant having a pressurized fluidized bed combustor and a free power turbine ~or driving a load, a control system which is capable of responding quickly to speed and/or load demand changes imposed on the free power ~urbine without upsetting the thermodynamic opera-tion of the fluidized bed combustor and the gas turbine engine.
It is another object of the present invention 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 wherein valves are not subject to very high temperatures and, therefore, special valves ar~ not required.
It is a further object of this invention to provide in a gas turbine power plant having a pressurized fluidized bed combustor and a free power turbine for driving a load, a start-up control system and method which obviates the need for separate startin~ and primary air compressors, It is therefore contemplated by the present inven-tion to provide a novèl control system and method of start-up for a gas turbine power plant.
The gas turbine power plant comprises a pressurized fluidized bed combustor in which pulverized solid fuel, such as coal, is burned in the presence of crushed dolomite (lime-stone) to remove sulfur dioxide from the combustion gas. A
heat exchanger is provided in the fluidized bed combustor for controlling the reaction temperature, as for example between 7Q0C and 900C, 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 exc~anger and the ~ 23~

fluidized bed combustor in a ratio of about 2:1. The com-pressed 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 durin~ 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 elec-trical generator, and a third conduit means is provided for conducting exhaust gases from the expander to drive the free power turbine.
The control system comprises a first valve means in the first conduit means for independently controlling flow of compressed air inLo the heat exchanger and into the fluid-ized bed. A valve-controlled by-pass conduit means is pro-vided 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 f~ee 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 insure~ in the event of sudden loss of load on the free power turbine and in cooperation with the valve-controlled by-pass conduit means, that no gases ~low through the free power turbine and it stops rotation.
The st~rt-up method, according to ~his invention, comprises the following steps. Initially, start-up re~uires 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 pre-vented from flowing to the fluidized bed combustor and theheat 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 1uidized 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, temper&ture and pressure of this air reaches the levels sufficient to suspend and ignite the fuel, pulver-ized fuel and dolomite are admitted into the fluidized bed combustor and the fuel is ignited, Within a first predeter-mined temperature range in the fluidized combustar, the com-pressed 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 com-bustor higher than said first temperature range, the preheat-ing of the compressed air and firing o. the gas turbine com-bustion zone are ceased and by-passing exhaust gases around the free power turbine is stopped and such gases are then 3Q directed to the free power turbine to drive the latter and ~'~f~ ~ 2 thereby drive a load.
The invention will be more fully understood from the following description when considered in connection with the accompanying drawing in which the control system, accord-ing to this invention, is schematically shown.
Now, referring to the drawingt the reference num-ber 10 generally refers to a gas turbine power plant of the type having a pressurized fluidized bed combustor (herein-after referred to as the "power plant'l~, and having a control system therefor, according to this invention.
The power plant 10 and control system therefor com-prises a pressurized fluidized bed combustor 12 which is con-nected through supply means, such as conduits 14 and 16, to a source of pulverized solid fuel 18 and sulfur dioxide absorb-ing material 20 as, for example, respectively, coal and crushed dolomite. The combustion gases generated by the burning of fuel in fluidized bed combustor 12 is conducted from the latter, via pipe 22, to separators 24 and 26, such as cyclone separators for two-stage separation, and, by way of pipe 28, to the ex-pander 30 of a gas turbine engine 32. The pressurized-fluid-ized 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 700C and about 925C.
The air cooling system has a heat exchanger 36 of -any suitable type in fluidized bed 34, which heat exchanger is ~onnected, through pipes 38 and 40, to an air compressor 42 for receiving compressed air from the latter. The heat ex-changer 3~ is also connected through outlet pipe 44 to pipe 28 so that heated compressed air is conducted into admixture with the cleaned combustion gases discharged from fluidized bed ~2 3 ~ ~ ~

combustor 12 and flowing through pipe 28.
The fluidized bed combustor 12 is connected via pipe 38 to receive a portion of the compressed air disc~arged from compressor 42. This compressed air delivered into fluidized bed combustor 12 is distributed to 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 fuel.
The air compressor 42 is connected to be driven by expander 30 of gas turbine engine 32 and may be part of the gas turbine er.gine assembly or may be a separate unit suitably con-nected to be rotated by expander 30. The combustion zone or combustor 48 may also be an integral part of or a separate unit o~ the gas tur~ine assembly. The combustor 48 is connected to receive compressed air from compressor 42, via passageway 50, and fuel from a suitable supply thereof to generate combustion gases which are discharged, via passageway 52, into admixture with combustion gas in pipe 28. Compressed air flow through passageway 52 is controlled by a valve 51. The combustion gas from combustor 48 functions alone or in conjunction with combustion gas and heated compressed air to drive ex2ander 30.
The expander or exhaust gas from expander 30 is con-ducted by passageway or pipe 54 to a free power turbine 56. The free power turbine is connected to drive a load such as an elec-trical generator 58. The exhaust gas from free power turbine 56 is discharged by way of an exhaust pipe 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 fr~e ~3~fll:~

turbine 56 and passes the gas -n indirect heat exchange ralation-ship with water from supply pipe 64 to convert the water to steam.
A steàm turbine 66 is conrlected to drive an electric genera-tor 68 and to receive, via outlet pipe 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 pipe 74 for recir-culation to waste heat boiler 62. The water from condenser 72 and make-up water are passed to a feedwater heater 76 and thence, through supply pipe 64, into waste h~at boiler 62.
The control system, according to this invention, com-prises several valve-controlled bypass pipes or lines which serve to place the power plant in service and to permit quick adjust-ment of free power turbine 56 to changes in load demand on elec-trical generator 58 as well as protect free power turbine 56 against overspeeding upon sudden loss of load demand on electri-cal generator 58.
The control system, more specifically, comprises a bypass pipe 78 which is arranged to interconnect pipe 40~ which conducts compressed air to heat exchanger 36, with outlet pipe 44 to thereby bypass air around heat exchanger 36. A valve 80 is disposed in bypass pipe 78 to control flow therethrough, while a valve 82 is disposed in pipe 40 for control of compressed air flow therethrough. The valves 80 and 82 are adjustable to to-tally bypass compressed air around heat exchanger 36 as is done during part of the start-up period or modulate flows in accord-ance with the temperature of fluidized bed 34 to maintain the bed temperature within the desired temperature range of 700C
to 925C during operation. The valves 8Q and 82 also coopera-tively f~mction to maintain fluidizing air flow velocity in f'uidized bed 34 at a constant actual value. This latte~

function is accomplished by sensing air flow velocity in pipe 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 pipe 84 in the cooling air system is provided to bypass the compressed air discharged from air com-pressor 42 into pipe 38. A suitable heater 86 is disposed in bypass pipe 84 to heat compressed air before it is passed into fluidized bed combustor 12. The heater 86 may be of any suit-able type for heating the compressed air and may be) as shown,a fuel fired combustor. To control flow of compressed air through bypass pipe 84 and pipe 38) valves 88 and 90 are pro-vided in the respective pipes 8L and 38.
Another bypass pipe 92 is connected~ at one end, to pipe 54 and, at the opposite end, to exhaust pipe 60 to provide for bypassing e~haust gas from expander 30 around free power turbine 56. A valve 94, which is operative in response to a signal generated by a speed switch or other suitable load sens ing device 96, is disposed in bypass pipe 92 to control flo~7 through the latter. For operation under substantially constant load on free power turbine 5~, valve 94 is in a closed position.
However, for start-up operation of power plant 10, valve 94 is fully open. Also valve 94 functions to adjust the torque de-veloped by free power turbine 56 to substantial changes in the load demand on generator 58 and hence ~he free power turbine.
Furthermore, in the event of a sudden loss of load demand, valve 94 opens to drop the pressure differential across free power turbine 56 to substantially zero and thus prevent free power turbine 56 from overspeeding and the damage resulting therefrom~
This bypass 92 and valve 94 therein provides a rapid and precise _g_ ~ ~3~

control of free power turbine output ~i~hout upsetting the thermodynamic balance of fluidized bed combustor 12. To further insure protection of free power turbine 56 ? it is preferred that a valve means 98 be provided in pipe 54.
The valve means 98 is normally in an open position and functions to shut off flow of gas to free power turbine 56 where there occurs a sudden rejection of output load on the free power turbine. 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 free power turbine 56 and generator 58 which i3 driven by the free power turbine. This valve means 98 may take any suitable form; as for example, a set of non-cambered vanes or lou~ers in the entrance annulus of the free power turbine which vanes or louvers 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 louvers are in a substantially perpendicular 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 of-f exhaust gas flow.
It is desirable that Yalve means 98 be provided in the control system and to cooperatively function with valve ~4 in bypass pipe 92 because it has been folmd that ? in some situa-tions, not enough exhaust gas can be bypassed through pipe 92 to reduce power output of free power turbine 56 to zero and prevent overspeeding of the free power turbin~. The valve means 98, similar to valve 94 ? iS connected to respond to load sensing 3~

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 gas turbine engine 32 and thus prevent its overspeeding.
Start-up of the power plant is achieved by first clos-ing valves 80, 82, 88, 90 and 98 while opening valves 51 and 94.
This positioning of the aforesaid valves blocks flow of compres-sed air to fluidized bed combustor 12 and opens bypass pipe 92 so that exhaust gas from expander 30 does not enter the free power turbine 56. With valve 51 open, a suitable starter mecha-nism 100, such as an internal combustion engine, is operated to drive air compressor 42. All of the compressor air discharged from air compressor 42 is flowed through passageway 50 into com-bustor 48 where fuel is injected and ignited to produce combustion gas . This combustion gas is conducted, via passageway 52 ? to pipe 28 and, thence, to expander 30 to drive the latter. A check valve 102, or similar shut-off device in pipe 28, prevents flow of com-bustion gas in a direction to~ard fluidized bed combustor 12. The exhaust gas from expander 30 is cond~tcted to the steam and power generating system 61 by way of pipe 54, bypass pipe 92 and exhaust pipe 60. A check valve 104 prevents reverse flow of e~haust gas in exhaust pipe 60. Once expander 30 is being driven ? self-starter mechanism 100 is stopped and combustor 48 is operated so that the gas turbine engine is heated. ~Jhen the gas tur~ine engine 32 operatively stabiliz~s ? valve 88 în bypass pipe 84 is opened to permit flow of compressed air to heater 86 in which part of the compressed air supports ~he burning fuel, ~he mix-ture of combustion gas and heated com~ressed air flowing into fluidized bed combustor 12? via pipe 38. After the heated com-pressed air and com~ustion gas introduced into fluidized bed combustor 12 attains the volume and pressure sufficient to sup-port ~uel and particulate material in suspension~ ~uel and parti-culate material are fed into the fluidized bed combustor 12 through conduits 14 and 16. The mixture o~ hot combus-tion gases and compressed air also provides the heat and the oxygen for causing the fuel in fluidized bed 34 to ignite. ~en the fluid-ized bed temperature reaches about 750C to about 800C, as sensed by a temperature sensing and signaling device 1O6J valve 82 is gradually opened to permit compre-ssed air to flow via pipe 40 into heat exchanger 36. By gradually increasing compressed air flow into and through heat exchanger 36, excessive thermal shock to the heat exchanger is prevented as the relatively cool compressed air begins to flow through the heat exchanger. ~1hen fluidized bed 34 reaches a temperature within the range of about 870C to about 925C, valve 80 opens so that the amount of co~-pressed air flow through heat exchanger 36 is controlled to maintain fluidized bed 3~ at the desired temperature range of about 870C toabout 925C by modulating ~low through the heat exchanger and bypass pipe 78. The valves 80 and 82 cooperate 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 tem~erature. These valves also coact to maintain the fluidizing air velocity at a constant actual velocity.
The gas generated in fluidized combu~-tor 34 passes, via pipes 22 and 28, into expander 30 of gas turbine engine 32. The exhaust gas from expander 30 is, at this time, bypassing free power tur-bine 56 through bypass pipe 92. Also, ~Jhen fluidized bed 34 attains a temperature in the range of about 750C and about 80QC
and with gas turbine 32 achieving synchronized idle point, valve 88 is closed and fuel is cut off to heater 86. Substantially simultaneously with the closing of valve 88, valve 90 is opened ~;3~

so that compressed air now flows directly to fluidized combustor 12 where the fluidized bed has attained thermodynamic equili~riu~.
At this time, the bypass valve 9~ is closed and valve 98 opened to admit the exhaust gas from 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 combustor 48 so that expander 30 is driven only by a mixture of combustion gas and heated compressed air delivered to the expander via pipes 28 and 44. The power plan~ 10 is now operating at full load demand on generator 58 with valves 94 and 98 modulating flow through pass-ageway 54 and bypass pipe 92 to compensate for fluctuations in load demand and, upon a sudden and substantial loss of load de-mand, opening and closing, respectively, to prevent overspeed-ing of free power turbine 56 and expander 30.
It is believed 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 fluid-ized bed combustor, and protection o~ the free power turbineagainst overspeeding in the event o~ 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 de~ail, it is to be expressly understood that the invention is not limited excep~ by the appended claims.

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The 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 con-nected to receive another portion of the compressed air dis-charged 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 compris-ing the following steps:
a) drive said gas turbine engine by an outside source of rotative power and ignite fuel de-livered to the combustion zone of the engine to start said gas turbine engine and thereby drive the air compressing means;
b) simultaneously prevent flow of compressed air to said fluidized bed combustor and said heat exchanger;
c) after the gas turbine engine has attained sustained self-operation, the outside source of rotative power is stopped 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 has is directed to the free power turbine to drive the latter.

2 The method of claim 1 wherein said first predetermined temperature range is about 750°C to about 800°C.

3. The method of claim 1 wherein said second predeter-mined temperature range is about 870°C to about 925°C.

4. The method of claim 1 wherein said first and second predetermined temperatures are respectively about 1300°F and 1650°F.

5. The method of claim 1 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.

6. The method of claim 1 wherein said second predeter-mined 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 round the free power turbine.

7. The method of claim 1 wherein said fluidizing air velocity flow into the fluidized bed combustor is maintained to a constant predetermined value.