GB2100141A - Pressure-charged fluidized- bed combustion system - Google Patents

Abstract

A fluidized-bed combustion system comprises a fluidized-bed boiler 1 with a supply line for compressed air, means for compressing the air, a discharge line for flue gases and a circuit, for absorbing a portion of the heat generated in the fluidized bed, in which steam is the cooling medium. <IMAGE>

Description

SPECIFICATION Pressure-charged fluidized-bed combustion system This invention relates to a fluidized-bed combustion system comprising a fluidized-bed boiler with a supply line for compressed air, a discharge line for flue gases, and a circuit for absorbing a portion of the heat generated in the fluidized bed, and means for compressing the air.

Such a fluidized-bed combustion system is proposed in Dutch patent application 8000404. In it, an intermediate circuit is provided, which on the one hand is located in the fluidized bed and on the other hand is coupled, via a heat exchanger, with a conventional steam and water circuit. In the intermediate circuit, a cooling fluid circulates, which has no liquid/gas transition.

Fluidized-bed combustion systems command much interest, because these systems are capable of providing optimum combustion of coal, which combustion is also very friendly to the environment, because the sulphur liberated during the combustion can largely be bound by the limestone present in the bed.

In order that the sulphur may be properly bound, the temperature of the fluidized bed must be maintained within a limited temperature range, varying from about 700 C to 850,C. At 850"C, the sulphur is optimally bound; at temperatures in excess of 850"C, slagging of the ash contained in the coal may occur, whereas below 700"C the combustion efficiency and the extent to which the sulphur is combined are too low.

The fluidized bed furnace is a vessel mainly filled with ash and dolomite, in which coal is burnt. A compressor provided in the air supply line to the bed provides compressed combustion air at a temperature of approximately 250"C. The flue gases leave the furnace at a temperature of approximartely 850"C. In order to minimize flue gas losses, combustion is effected with a slight excess of air (20%). As this excess of air is insufficient to keep the temperature of the bed within the desired range, the bed must be cooled to avoid that its temperature increases to very high values (1 800 C). Direct cooling with a tube bank, through which water flows, which by the heat is converted into steam, is highly effective, but has the disadvantage of permitting only minor variation in power output.This will be clarified hereinafter.

The power transmitted between the fluidized bed and the cooling fluid is: Cl = O.F.i\T,n; in which Q = transmitted power (W) a = total heat transfer coefficient (W/m2K) F = installed heat transfer surface area (m2) AT1n = the logarithmic temperature difference between the two fluids (K).

In conventional boiler, power output is controlled by the simultaneous adaptation of l\T,n and a. This adaptaton is effected by varying the supply of fuel and combustion air.

A disadvantage of fluidized bed plants is that a is virtually invariable, and that the temperature of the bed can be varied in a limited range only. The adaptation of AT1n in water/steam-cooled fluidized bed plants by varying the water/steam temperature would cause very great variations in steam pressure, which may increase to 1 50 bars, which in connection with the demands to be made on the materials applied is impermissible.

In Dutch patent application 8000404, it is proposed to provide a partial-load control of the fluidized bed by means of an intermediate circuit, in which a cooling fluid circulates, w'nich in the temperature range being used has no phase transition, so that the problems inherent in water/steam-cooled plants are avoided. One disadvantage of such an intermediate circuit, in addition to the need of providing an extra tube bank for the circuit, is the fact that apparatus is required for pumping the coolant around in the circuit, which requires rather a great deal of power, and adversely affects the efficiency of he fluidized bed plant.

It is an object of the invention to provide a fluidized-bed combustion system which exhibits a good partial-load behaviour, without the need of using a closed intermediate circuit, and a further object is to minimize the power required for the circulation of the coolant.

According to the invention, there is provided a fluidized-bed combustion chamber of the above kind, in which the circuit capable of absorbing heat in the fluidized bed is formed as a circuit in which steam is the cooling fluid.

When switching from full load to partial load, the power output of the fluidized bed will have to be decreased. This can be effected by increasing the average temperature of the cooling fluid supplied to the bed, as in this way the value of liT,n can be varied. By slowing down a pump, the quantity of steam supplied to the bed for cooling purposes will decrease. As a consequence, owing to the fact that initially the power output of th fluidized bed is still the same as in the full-load situation, the circulating steam will leave the circuit in the bed at a temperature higher than the temperature under full-load conditions.In the stationary partial-load situation, the temperature of the steam leaving the bed is higher than in the full-load situation, while also the temperature of the steam supplied to the bed can be selected higher than in the full-load situation. Accordingly, the logarithmic average temperature difference between the fluidized bed and the coolant (steam) is less than in the full-load situation. The power output from the bed, and hence the quantity of process steam produced has decreased, whereas the temperature of the bed will not decrease below the lower limit of 700"C.

An advantage of the use of steam as the coolant in the circuit is, in the first place, that at temperatures not far from the satuation temperature, the specific heat of steam associated with a given operating pressure is very high. This effect is particularly noticeable at pressures higher than 20 bars, as shown by the steam table. A further advantage is that the full-load temperature-power diagram for the tube bank in the fluidized bed is almost invariably such that the inlet temperature of the steam is nearly the saturation temperature of the steam at the prevailing pressure.For the full-load situation this means that only a relatively small quantity of steam needs to be ciculated; that the pump capacity to be installed may be relatively low; and that at given, minimally required degree of loading of the fluidized bed boiler, the maximum temperature to be expected of the walls of the tubes for conducting steam is considerably lowered, by virtue of which less expensive materials may be used.

Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings. In said drawings: Figure 1 shows a fluidized-bed combustion system comprising a steam circuit according to the present invention; Figure 2 shows a first variant of the steam circuit; and Figure 3 shows a second variant of the steam circuit.

Fig. 1 shows a fluidized-bed boiler 1 comprising an inlet line 2 for air and a discharge line 3 for flue gases and a compressor 4 for compresing the air. Compressor 4 is connected to an expansion device 5, which is connected to line 3 and also to an A.C.

generator 6. The expansion device 5 together with compressor 4 forms a gas turbine, which is driven by the flue gases from the fluidizedbed circuit, and which, in addition to the power for compressing the air, also provides electrical power.

Formed in the fluidized bed within the boiler, by means of a tube bank, is a heat exchanger 7, which via an inlet line 8 and an outlet line 9 is coupled to a circuit located without the boiler. In this circuit, steam under pressure circulates, which serves for absorbing power in the fluidized bed, and hence for cooling the bed.

The circuit carrying the steam for cooling the fluidized bed may be built up in various ways. Figs. 1, 2 and 3 show some variants of the construction of the cooling circuit. Like parts are designated in the several figures by like reference numerals.

In Fig. 1, the circuit without the fluidized bed is built up from a mixing evaporator 10 and a pump or blower 11 for ciculating the steam. Connected to outlet line 9 is a conduit 12, which passes a small proportion of the steam, for example a one-seventh part, to a steam turbine serving to generate energy. This steam turbine is designed to work with steam having a specific temperature and pressure, e.g. 400"C at 40 bars.

When the fluidized bed is operated under full-load conditions, the temperature of the steam leaving the bed, in the present embodiment, is 400"C at 40 bars, so that this steam can be supplied direct to the steam turbine. If, however, the fluidized bed is operated under partial-load conditions, the temperature of the steam leaving the bed will rapidly rise, so that measures must be taken for cooling the steam conducted to the steam turbine. For this purpose a known per se injection cooler 1 3 is provided in the arrangement of Fig. 1, which is under the control of a valve 14. In this injection cooler, the superheated steam can be cooled to 400"C, and supplied to the steam turbine.

The remaining, major portion of the circulating steam is supplied via line 9 to the mixing evaporator 10, in which it is cooled to just superheated steam with a temperature of e.g.

255"C. The amount of water evaporated in the mixing evaporator is in principle equal to the amount withdrawn from the steam circuit via the steam turbine.

When the steam has passed the mixing evaporator, the (now cooled) steam is resupplied via blower 1 and line 8 to th cendu- its in the fluidzed bed. The partial load of the bed can now be adapted by controlling the rate of blower 11. When the blower displaces a smaller amount of steam, the residence time of the steam in the fluidized bed is longer, and it absorbs more heat, which leads to the desired effect, as described above.

Shown dotted in Fig. 1 is an embodiment of the circuit in which a portion of the' hot steam by-passes mixing evapoator 10 via a control valve 1 5 and a mixing unit 16, provided for mixing the cooled steam supplied via the mixing evaporator and the hot steam supplied via control valve 15, and suppling it to blower 11. By virtue of this construction of the circuit, it is possible to control the temperature of the steam supplied to the fluidized bed depending on the position of valve 1 5.

According as the temperature of the steam supplied is higher, so the iogarithmic temperature difference between the steam entering the heat exchanger in the bed and the steam leaving the same via line 9 is smaller, so that the power output is reduced, and partial-load control is obtained.

Fig. 2 shows an embodiment of the cooling circuit in which the injection cooler 1 3 of the embodiment of Fig. 1 has been omitted and replaced by a simpler mixing unit 17, which is less expensive to make. Mixing unit 17 is interposed in the circuit between line 9 and the branched line 12, and is connected via a control valve 15 to the delivery end of blower 11. Under partial-load conditions, if the steam in line 9 has a temperature higher than 400"C, it is now possible to supply a quantity of cooled steam from the blower via control valve 18 to the mixing unit, whereby the steam passed to the steam turbine via conduit 12 can be cooled to the desired temperature.

Naturally, under full-load conditions, the control valve 18 is fully closed, because in that case the steam in line 9 already has the correct temperature.

Fig. 3 shows a preferred embodiment of the circuit according to the inventon, in which the steam issuing from the heat exchanger in the bed is first passed through a heat exchanger 19. Heat exchanger 19 is coupled via a pair of conduits 20 and 21 to line 8, which resupplies the steam cooled by mixing evaporator 16 to the heat exchanger in the fluidized bed. Included in line 8, intermediate the locations where conduits 20 and 21 are connected to line 8, is a control valve 22. Furthermore, conduit 21 is coupled via a mixing unit 23 to line 8.

In the full-load situation, valve 22 is fully opened and no steam flows through conduits 20 and 21. If, however, the fluidized bed must be operated under partial-load conditions, the control valve is partially closed, so that a portion of the relatively cool steam is passed via conduit 21 through the heat exchanger, where it cools the steam in line 9 to the desired temperature of 400 C. The steam returned via conduit 21 and mixing unit 23 to line 8 has acquired a higher temperature than that of the steam issuing from the mixing evaporator, so that in combination with the increased outlet temperature of the steam from the fluidized bed, the value of AT1n is again decreased, and partial-load operation is possible.

A problem in the use of conventional gas turbines in pressure-operated fluidized-bed systems is that, as a result of the high pressure drop across the fluidized bed, and the small excess of air, the outlet pressure of the compressor is higher than the pressure for which it has been designed. As a consequence, the operating point of the compressor may be shifted to the so-called pumping limit, which gives the critical value for the ratio between the output pressure and the input pressure of the compressor. When the operating point is shifted past the pumping limit, the compressor strart "pumping" which of course is highly undesirable. This danger can be avoided by discharging a portion of the air supplied via, e.g., a throttle valve. This, however, gives a loss in efficiency for the fluidized-bed combustion system. Shown in dotted lines in Fig. 3 is a benficial solution, in which the excess pressure at the output end of compressor 4 can be used to advantage. For this purpose the output end of the compressor is coupled to an expansion device 24, which serves for driving blower 11. This has the advantage, on the one hand, that no loss in efficiency occurs from throttling the excess of air at the output end of the compressor, and on the other hand, that no extra power is required for controlling the blower. In addition, in connection with the requirements for reducing blower rate, it is more favourable to drive the blower by means of a fluid flow engine, rather than an electric motor.

Claims (8)

1. A fluidized-bed combustion system comprising a fluidized-bed boiler with a suppiy line for air under pressure, a discharge line for flue gases and a circuit for absorbing a porton of the heat generated in the fluidized bed, and means for compressing the air, char acterized in that the circuit capable of absorb ing heat in that the fluidized bed is formed as a circuit in which steam is the cooling fluid.

2. A fluidized-bed combustion system according to claim 1, characterized in that the circuit in the 1, fluidized bed is formed as a heat exchanger and, via an inlet line and an outlet line is coupled with a portion of the circuit located without the fluidized bed, there being provided a mixing evaporator and a blower, through which the outlet line is coupled to the inlet line, there being further provided a con duit line, for supplying a portion of the steam in the outlet line a turbine.

3. A fluidized-bed combustion system according to claim 2, characterized by the provi- sion of an injection cooler to be adjusted by means of a control valve, in the conduit for supplying steam to the turbine.

4. A fluidized-bed combustion system according to claim 2 or 3, characterized by a conduit with a control valve therein, said conduit being coupled at one end to the outlet line of the circuit in the fluidized bed and at the other end to a mixing unit interposed between the outlet line of the mixing unit evapora- tor an the input end of the blower, it being possible for a portion of the steam in the outlet line of the circuit in the fluidized bed to by-pas said mixing evaporator via said control valve.

5. A fluidized-bed combustion system according to claim 2, characterized by the provi- sion of a mixing unit connected with a the first supply line to the outlet line of the circuit in the fluidized bed, with an outlet line to the conduit to the mixing evaporator, and with a second input line, through a control valve, to the delivery end of said blower, whereby the steam in the outlet line can be cooled in the mixing unit before being supplied to the tur- bine.

6. A fluidized-bed combustion system according to claim 2, characterized by the provision of a heat exhchanger in the outlet line of the circuit in the fluidized bed, between the outlet from the fluidized bed and the branched conduit to the turbine, said hat exchanger being coupled through a pair of conduits to the supply line to the circuit in the fluidized bed, which supply line includes a control valve, the arrangement being such that the steam in the output line flowing through the heat exchanger can be cooled to a greater or lesser extent depending on the position of the control valve.

7. A fluidized-bed combustion system according to any one of the preceding claims, characterized in that said supply line for air is provided with a branch line, through which a portion of the compressed air can be supplied to an expansion device provided for driving the blower.

8. A fluidized-bed combustion system constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.