"Iπprovaπents Relating to Power Generation Plant" ,

This invention relates to fluidised bed furnaces and to power generating plant including a fluidised bed furnace.

5. According to one aspect of the invention there is provided a fluidised bed furnace including, connected in a circulatory arrangement, a combustion chanfoer section, a separating section and a heat- transfer bed space section, the cαrbustion chamber section being arranged to be supplied with fuel particles and 10. fluidi≤ing gases at a relatively high velocity and discharge coπi-iistion products to the separating section, the separating section being arranged to effect separation of solids particles frcm combustion gases in the coπioustion products and discharge the solids particles to the heat transfer bed space section and the 15. corrbustion gases from the furnace and the heat transfer bed space section being arranged to be supplied with fluidi.si.ng gases at a relatively low velocity to ef ect flow of the solids particles around heat transfer sur aces and discharge to the combustion chamber section.

20. According to another aspect of the invention there is provided power generation plant including the fluidised bed furnace and a coal devolatil i ation unit, the coal devolatilisation unit being connected to receive air frcm an air heater arranged to derive heat frcm the fluidised bed furnace and to discharge ccm-

25. bustible gases to burner means connected to a gas turbine and the fluidised bed furnace being connected to receive char frcm the coal devolatil i sation unit and exhaust gas from the gas turbine, and being provided with vapour generating and vapour heating surfaces

in a heat transfer bed space of the fluidised bed furnace and in a combustion gas pass connected to discharge vapour to a vapour turbine.

The invention will now be described, by way of example, with reference to the acccmpanying diagraiimatic drawings, in which:- ^'

5. KLgure 1 is a representation of a fluidised bed coirbustor ^' together with a steam generating and heating unit; ϊlgure 2 is an isometric representation of a form of fluidised bed cc_±) ^* ustor; and iigure 3 is a representation of the cαrbustor in conjunction iO- with gas turbine and coal devolatilisation plants.

As shown in ITlgure 1, the fluidised bed corobustor 2 includes an upright, refractory lined, combustion chamber 4 discharging through a lateral duct 6 frcm an upper region 8 to a separation region 10. A partiallate solids return duct 12 extends downwardly from the

15. separation region 10 to a weir chamber 14 having a weir plate 16 and, adjacent the weir plate, spaced fluidising air nozzles 18. The weir chamber 14 discharges, over the weir plate 16, to a heat transfer bed space 20 formed as parallel extending cxxipartments by vertical partitions each provided with spaced fluidising air nozzles 22 and heat

20. exchange tube banks 24. Particle _recircul__.tion ducts 26 lead from the bed space 20 to the combustion chamber 4.

The heat exchange tube banks 24 in the bed space 20 foim a part of the flow circuit of a forced flow steam generating and superheating unit, the ra_πaining tube banks 30, 32, 34 and 36 of which

25. are positioned in a combustion gas pass 38 leading from the separation region 10. The flow circuit of the unit also includes tube lengths (not shown) lining the walls of the bed space 20 and the combustion gas pass 38. An airheater 40 is positioned in the combustion gas pass 38 downstream, in the gas flow path, of the tube

30. bank 30 and the pass is connected to discharge, through a bag filter and induced draught fan, to a stack (all not shown).

The combustion chamber 4 is foπned with a convergent base 42 provided with primary fluidising air nozzles 44, an inlet 46 for dust particles collected frcm the combustion gas pass 38 and the bag filter and an outlet 48 for ash particles. 5. A screw feeder 50 for coal particles is positioned adjacent the level of the particle recirculation ducts 26 whilst secondary fluidising air nozzles 52 extend through the convergent base wall frcm a windbox 54 super acent the screw feeder 50.

In operation, combustion is initiated in the combustion 10. chamber 4 by utilising an oil burner (not shown) to heat up material in the base of the combustion chamber to about 700 C, fluidising air to achieve a fluidisation velocity of about 0.5 metres per second being supplied through the primary nozzles 44. Upon coal ignition tenperature being reached in the fluidised 15. material, coal particles are added through, the screw feeder 50 at a . rate suf icient to establish self-sustaining combustion in the bed, at which stage the use of the oil burner is discontinued. As the teπperature of the lu±dised material rises so the supply of coal particles and fluidising air is increased until a 20. temperature of about 850°C is achieved, at which stage secondary fluidising air is supplied through the windbox 54 and secondary air nozzles 52 to achieve a fluidisation velocity of about 3 metres per second. A stream of combustion gases, ash, and unburnt particles frcm the cαrbustion chamber 4 is discha ged through the lateral 5- duct 6 to the separation region 10 where a substantial fraction of the ash and unburat particles separate out from the stream tα fall into the particulate solids return duct 12, and the combustion gases are discharged through the combustion gas pass 38. The ash and unburnt particles gravitate to the base of the return duct 12

and into the weir chamber 14. upon the rate of deposition of particles in the return duct 12 reaching a rate su ficient for recirculation to be initiated, fluidising air is supplied to those of the nozzles 18 associated with a selected compartment

5. of the bed space 20 to cause the particles to flow over the associated portion of the weir plate 16 into the ccπ artment, and thence through the return duct 26 to the combustion chamber 4. As the rate of flow and teσperature of the particles increases so those of the fluidising air supply nozzles 22 associated with

1 ^Q .the selected compartment are brought into action to produce a fluidised heat transfer bed in the compartment to enhance transfer of heat from the particles to evaporator tube lengths extending through the compartment. The rates of supply of coal, fluidising air and water to the tube banks are then progressively

15.increased to full load conditions at which fluidising velocities of between 9 and 13 metres per second obtain at the upper end of the combustion chamber and of between about 0.5 and 1.0 metres per second obtain at the bed space 20. Limestone sorbent is supplied, as appropriate, through inlets 52 discharging to the bed space 20.

20. The combustion gases are discharged frcm the separation region 1 to the combustion gas pass sequentially to flow over the evaporator tube banks 36, 34, 32 and the econcmiser tube bank 30 to a turning space 39, where further ash particles - carried over from the separation region - are deposited. The combustion gases then flow,

25.over the airheater 40, to the bag filter and induced draft fan for discharge to the stack. Ash particles ^' frcm the turning space 39 and the bag filter are returned through ducting to the combustion chamber 4 through the ash return nozzles 46.

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Air is supplied through a forced draft fan 56 to the airheater. Air from the airheater is supplied to the windbox 54 and, through a booster fan 58, to the fluidising air nozzles 18, 22 and 44. Spent ash is discharged frαn the combustio 5. chamber 4 through the outlet 48.

By combining the combustion chamber 4 operating with a relatively high fluidisation velocity with the coπ artmented bed space 20 operating at relatively low fluidisation velocity a very flexible system is achieved with ^' good combustion conditions

10. in the combustion chamber 4 and good heat transfer conditions in the bed space 20. To operate at low loads, or without superheating, the supply of luidising air to appropriate com¬ partments in the bed space is discontinued, allowing the bed to slump, thereby restricting heat transfer. At loads at which

15. combustion will not be sustained by the input of coal particles, the oil burner may be utilised as a supplementary heat supply to the circulating particles.

As shown in Figure 2, separation regions 10 and particulate solids return ducts 12 may be positioned to two sides of the

20. combustion chamber 4 to discharge cαrbustion gases through outlets 37 to the combustion gas pass 38. The ducts 12 deliver particulate material to cαπpartmented weir chaπ±jers 14 and bed spaces 20 discharging to the base of the combustion chamber 4. This achieves a very compact arrangeπent, with the space between

25. the combustion chamber 4 and the return ducts 12 serving as the wind box 54.

Referring to Figure 3, the ccmbustor 2 is utilised in conjunction with a devolatiliser 60 and a gas turbine unit 62. The devolatiliser is connected to receive coal through an inlet 64 and discharges hot combustible gases through an outlet 66

5. and burner 68 to a gas turbine 70 coupled to a compressor 72. The compressor is connected to discharge compressed air at a relatively high pressure to an air heater, tube bank 74 positioned in the bed space 20 of the cocobustor 2 and, at a relatively lower pressure to the fluidising nozzles 22. The air heater

10. tube bank 74 is connected, through valves (not shown) both to an air inlet 76 to the devolatiliser 60 ^' and to the burner 68.

The gas turbine 70 discbarges to the base of the combustion chamber 4 through the fluidising nozzles 44 whilst char discharged frcm the devolatiliser 60 is supplied to the chamber through an 15. inlet 78 subjacent the coal screw feeder 50.

The steam generating,and superheating unit associated with the ccmbustor 2 is connected to deliver steam to a steam turbine 80 driving an electric generator 82. A further electric generator 84 is connected to be driven by the gas turbine 70.

20. in operation, the devolatiliser is supplied through the ^• inlet 64 and a lock hopper (not shown) with coal having a sufficiently high volatile content (that is above Ϊ0%-15% volatiles) and, through the inlet 76 with a stream of compressed hot air at 500 to 850°C from the air heater tube bank 74. The combustible

25. gases which result frαn the heating of the coal by the compressed hot air are discharged, through the outlet 66 and dust rβrDval equipment (not shown), to the burner 68. In the burner 68 the combustible gases, at about 500°C, are mixed with a further stream of compressed hot air frcm the air heater tube bank 74 and burnt

to produce combustion gases at about 800 C to 1200 C which pass through and drive the gas turbine 70. The exhaust gases frαn the gas turbine are discharged through the fluidising nozzles 44 at the base of the combustion chamber 4. Char from

5. the devolatiliser 60 is discharged to the cαribustion chamber 4 through the inlet 76 together with a further supply of coal, if required to attain a desired heat output. Exhaust gases frcm the gas turbine 70 are supplied through the fluidising nozzles 44 and 52 to achieve a fluidisation velocity of about

10. 10 metres per second with a rapid circulation and mixing effect enhancing combustion with n the charrfcer.

The combustion gases at a temperature of up to 950 C pass frcm the chamber, through the separation region 10, to the combustion gas pass 38 and over the evaporator and economiser tube 15. banks 36, 34, 32 and 30 and then through a filter 90 prior to discharge to atmosphere through a stack 92.

The ^' hot particles, at a teπperature of up to 950 C, separated frαn the combustion gases at the separation region 10 are passed to the cctπpartmented heat transfer bed space 20 20. through the weir chambers 14 and fluidised by air frαn the gas turbine driven cαnpressor 72 to achieve a fluidising velocity of about 0.5 metres per second to circulate the hot particles around the tube banks.

The hot particles having given up heat to the tube banks 25. i the heat transfer bed space are discharged with the fluidising 'air and recirculated to the combustion chamber 4. Spent limestone and ash particles are discharged rαn the base of the heat transfer bed .space, through the ash disposal outlet 46.

The coal devolatiliser 60 normally operates in the teπιperatι_re range of between 450°C and 700°C for the combustible gases discharged frcm the devolatiliser. Ibllσwing combustion of the combustible gases from the devolatiliser in the burner 68

5. the temperature of the gases discharged to the gas turbine after tempering with cool air, if necessary, will be up to about

1200°C - which is within the normal operating limit of cαπnercially available gas -turbines - and is likely to give rise to lower concentrations of alkal metals in the gases coπpared to gases

10 resulting frαn complete cαrbustion or gasification of the coal. Furtheimore, since the devolatiliser only produces volatile gases and char (and not combustion gases), the gaseous discharge frcm the devolatiliser is relatively small in volume compared with the gaseous discharge fran the complete plant and accord gly 15. any deleterious small particles in the gaseous discharge frcm the devolatiliser may be removed without incurring large penalties in operating costs.

Since the gas turbine 70 is upstream, in the gas flow path, of the various water heating and steam generating and heating tube 20.banks any failures of tubes in those banks will not affect operation of the gas turbine.

G_.ntrol of the plant is achieved ^" by regulating the supply of coal to the devolatiliser and to the combustion chamber.

As the gas turbine output falls, coal is supplied to the 25.combustion chamber to supplement the reduced flow of char in order to maintain combustion conditions in the chamber. The temperature in. the chamber can be lowered to 750°C, provided that the excess air level is maintained above 20%. The heat transfer bed spaces are cαnpartmented in order that the fluidising 30.control air may be adjusted between compartments. This controls

the flow of solids through each compartment, which in turn alters the heat absorbed by the tube banks. In this manner the steam cycle and air heater are independently controlled, while maintaining the minimum solids recirculation rate to 5. the ccmbustion chamber.

The supply of coπbustible gases frαn the devolatiliser 60 may be supplemented, or temporarily replaced, by oil or gas firing of the burner 68.

Cαrbustion gases frcm the burner 68 may be tempered with 10. air frαn the compressor 72 in order to maintain the ccmbustion gas temperature within the operating limits of the gas turbine 70.