EP0066337A1

EP0066337A1 - Pressure-charged fluidized-bed combustion system

Info

Publication number: EP0066337A1
Application number: EP82200623A
Authority: EP
Inventors: Willem Wiemer
Original assignee: Neratoom BV
Current assignee: Neratoom BV
Priority date: 1981-05-19
Filing date: 1982-05-18
Publication date: 1982-12-08
Also published as: JPS57204710A; NL8102454A

Abstract

A pressure-charged fluidized-bed combustion system, comprising a fluidized-bed boiler (1) having a supply line (2) for compressed air, a discharge line (3) for flue gases, as well as means for compressing (4) the air to be supplied and for expanding (5) the flue gases, there being provided a circuit (10, 11, 13) for recycling a part of the flue gases, said circuit being connected to the fluidized bed boiler above and underneath the area in the boiler where the fluidized bed is provided in operation, whereby in the circuit there is at least provided a blower (14) the speed of which can be controlled, furthermore means (12) being provided for absorbing heat from the flue gases.

Description

The invention relates to a pressure-charged fluidized-bed combustion system comprising a fluidized-bed boiler with a supply line for compressed air, a discharge line for flue gases as well as means for compressing the air to be supplied and for expanding the flue gases.
There is a strong interest in fluidized-bed combustion systems, since these systems may provide an optimal combustion of coal, which combustion is also very friendly to the environment, because the sulphur dioxide released during combustion can be substantially bonded by the limestone present in the fluidized-bed and the NO production is very low through the low combustion temperature.
The temperature of the fluidized-bed, for the sake of a proper sulphur bond, should remain within a limited temperature range, varying from about 800°C-850°C. At 850°C the bond of the sulphur is sufficiently effective; above 850°C, slagging of the ash contained in the coal may occur, while below 800°C the sulphur bond is strongly reduced.
The fluidized-bed furnace is a vessel partly filled with primarily ash and dolomite, in which coal is burned. A compressor provided in the air supply line to the bed provides combustion air at the required pressure and the associated compression end temperature. The flue gases leave the furnace at a temperature of about 850°C.
In order to minimize flue gas losses, combustion is normally effected with a slight excess of air of about 20%. Since the temperature of the bed is to he kept within the required limits, only about 1/3 of the developed quantity of heat can be discharged from the bed by means of this quantity of air. The remaining 2/3 is to be discharged differently.
Direct cooling with a tube bank which is traversed by water to be converted into steam by the heat is highly effective, but has the drawback that only a slight variation in power output is possible. This will be explained in the following.
The power transmitted between the fluidized-bed and the cooling fluid is:
wherein

Q = transmitted power (W);
K = total heat transfer coefficient (W/m²K);
F = installed heat transfer surface area (m^l);
ΔT_1n = the logarithmic temperature difference between the two fluids (K).

In a conventional boiler, the power output is controlled inter alia by the simultaneous adaptation of ΔT_1n and K. This adaptation is effected by varying the fuel and combustion air supply.
A drawback of fluidized-bed installations with water/steam cooling of the bed is that K can hardly be varied and that the bed temperature can only be varied in a limited range. Adaptation of A T_ln in water/steam-cooled fluidized-bed installations by varying the water/steam temperature is not possible, since a substantial portion of the cooling tubes in the bed is formed by the evaporator section wherein in general a constant pressure and hence temperature prevails.
Adaptation of the heat transfer surface area is possible by disconnecting parts of said surface area in combination with parts of the fluidized-bed.
The drawback of this arrangement is that the control is discontinuous.
Other possibilities for cooling the fluidized-bed are cooling by air which is conducted through a heat exchanging surface constituted by tubes in the fluidized-bed, or cooling by the flow of an air stream through the fluidized-bed itself, which then is about threetimes as large as the normal air stream required for the combustion.
The first mentioned possibility has the drawback over the water/steam-cooled fluidized-bed that

1) the temperature of the tube walls in the bed can increase to a value necessitating the use of highly expensive equipment,
2) the temperature of the gases for the gas turbine is relatively low as compared with the water/steam cooling system, which adversely affects the combustion system efficiency,
3) high flue losses occur, which likewise reduces the efficiency.

The second possibility mentioned lacks the above drawbacks mentioned under 1) and 2), but likewise has the drawback mentioned under 3).
Consequently, it is the object of the invention to provide a pressure-charged fluidized-bed combustion system which combines a proper controllability and an optimal efficiency, also in case of partial load, with a constructively simple build-up. The invention for this purpose provides a fluidized-bed combustion system of the above type wherein there is provided a circuit for recycling a part of the flue gases, which circuit, above and underneath the region in the boiler where in operation the fluidized-bed is present, is coupled to the fluidized-bed boiler, while there is likewise provided in the circuit at least one blower, the speed of which can be controlled, and there being provided means for taking up heat from the flue gases.
The use of the cooled flue gases for cooling the fluidized-bed by recycling said gases, has a number of advantages. The means destined for absorbing the heat from the waste gases, e.g. a water/steam circuit are coupled to a conventional steam turbine the partial-load efficiency of which in proportion to the full-load efficiency is appreciably better than with a gas turbine. It is now not necessary to operate the compressor/gas turbine combination in partial-load, which is favourable to the efficiency, since the partial-load efficiency of such an installation is relatively low.
By reducing the speed of the flue gas recycling blower, the quantity of recycling flue gas decreases and thereby the power output to the water/steam circuit. The power of the steam turbine can thus be properly controlled. Furthermore, it is not necessary for the cooling circuit according to the invention to apply in principle tubes for conducting a cooling fluid through the fluidized-bed, so that no material problems occur.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing, wherein:

Fig. 1 diagrammatically shows a first embodiment of a fluidized-bed combustion system with a circuit for recycling the flue gases;
Fig. 2 diagrammatically shows a second embodiment of the invention;
Fig. 3 diagrammatically shows a third embodiment of the invention; and
Fig. 4 diagrammatically shows a fourth embodiment of the invention.

The figures all show- a fluidized-bed combustion system which can be cooled by means of flue gas recirculation. In the figures identical parts are indicated by identical reference numerals.
Fig. 1 shows a fluidized-bed boiler having an inlet line 2 for air and a discharge line 3 for flue gases and a compressor 4 for compressing the air. In the boiler 1 reference numeral 7 diagrammatically indicates the area wherein the fluidized-bed is present in operation. The compressor 4 is connected to an expansion device 5 which is connected to line 3 and likewise to an A.C. generator 6. Compressor 4 and expansion device 5 together form a gas turbine which is driven by means of the flue gases and whereby a part of the energy contained in said gases is utilized for compression of the air and a part of the energy is converted into A.C. current. Air is supplied to compressor 4 through a line 8, while the outlet line 9 of the expansion device supplies the exhaust gases to a so-called spent gases cycle wherein a part of the remaining energy contents of the exhaust gases can be utilized. The exhaust gases, before being supplied to the expansion device, should be extensively dedusted, which can take place in a known manner in cyclones.
To the fluidized-bed boiler there is connected a further discharge line 10 via which a part of the exhaust gases can be supplied -to a steam boiler 11 wherein the heat of the discharge gases is utilized for converting into steam water tbatjs supplied to the boiler via a line 12. To the steam boiler 11 there is furthermore connected a line 13 wherein a blower 14 is received and which line again is coupled to the fluidized-bed boiler in such a way that the exhaust gases can be supplied to the bottom of the fluidized-bed. By controlling the speed of the blower and thereby controlling the mass flow of flue gases, the heat absorption from the bed can be controlled, as well as the heat emission to the water/steam cycle. Possibly, for controlling the temperature of the flue gases going to the steam/water cycle via a dotted parallel line 15 incorporating a controllable valve, a part of the flue gases from the blower 14 can again be supplied to line 10, without the gases flowing through said parallel line traversing the fluidized-bed.
Fig. 2 shows a different embodiment of the fluidized-bed combustion system with flue gas recirculation, in which the circuit 12, wherein water can be converted into steam, is received within the fluidized-bed boiler itself, so that no separate steam boiler is required. The discharge line 3 of the combustion gases 4 naturally is coupled in such a place to the fluidized-bed boiler that this is capable of discharging the combustion gases beE_brethese have passed the circuit 12 and hence have been cooled.
Fig. 3 shows another embodiment of the fluidized-bed combustion system with smoke recirculation, wherein, as in the embodiment according to Fig. 2, the circuit 12 is received within the fluidized-bed boiler, but wherein also this boiler is provided with an inner casing 16, which is open at the top and at the bottom, and which is positioned within the boiler proper. The fluidized-bed and the circuit 12 are disposed within the inner casing, while at the top of the inner casing there is provided a controllable blower 14 for transporting the flue gases. The flue gases can be resupplied via the space 17 between the inner and outer casing to the bottom of the fluidized bed. In this manner a highly compact structure of the fluidized-bed combustion system can be obtained, while the number of supply and discharge lines can be minimized and wherein likewise a sufficient cooling of the outer vessel is ensured.
A side effect when controlling the cooling of the fluidized-bed by means of the flue gases is that when recycling a substantial quantity of flue gases,there occurs a decrease of the partial O2 pressure and an increase in the partial CO₂ pressure in the gas supply to the fluidized-bed. This may have an adverse effect on the combustion process and on the sulphur bond. A solution for this problem is to increase the air excess which is supplied via the compressor.
A second side effect which may occur in flue gas recirculation is produced in that the fluidization speed, i.e. the speed at which a fluid flow should be transported through the fluidized-bed for maintaining said bed in the required floating condition, should remain within given limits. It is anticipated that between said limits a maximal ratio 1 : 2.5 may exist. As a result therefore limits are set to the speed at which and the degree to which flue..gases can be circulated.
Fig. 4 shows an embodiment wherein these problems are substantially eliminated. To this effect there is provided underneath the fluidized-bed a plate 18 containing apertures 19 on which apertures are disposed tubes 20 which terminate above the fluidized-bed. The supply linefcrtleflue gas 13 to be recycled is bifurcated downstream of the blower 14 into two lines 21 and 22. Line 21 terminates in the fluidized-bed boiler 1 in the space below the plate 18, while the line 22 terminates in the space above the plate 18 but underneath the fluidized-bed. In the line 21 there is received a controllable valve 23. When the combustion system is to be operated in partial load, first the flow of the flue gases through the line 21 is reduced, which flue gases have now no influence on the fluidization speed of the gases in the bed, since the gases are conducted from the line 21 via the tubes 20 to above the bed. When the flow in tubes 20 has been reduced to zero, subsequently the flow in the line 22 can bestill further reduced until the minimal fluidization speed for the fluidized-bed is attained. In this manner it is possible to control the partial load of the fluidized-bed over a larger area than is possible by means of a single line through which exhaust gases are returned to the bed. In the embodiment according to Fig. 4, the line 2 of the compressor for supplying the combustion air terminates in the space above plate 18, in order that said air will flow through the fluidized-bed and not through the tubes.
The embodiment with the plate provided with tubes is described in connection with the embodiment shown in Fig. 2; naturally, said plate with tubes can also be employed in the embodiments shown in Figs. 1 and 3.
It is also possible to combine the cooling by means of flue gas recirculation with known cooling methods, such as e.g. with a steam/ water circuit in the fluidized-bed. This may have the advantage that the dimensions of the boiler are smaller and that the efficiency of the boiler is slightly higher since the recycling blower requires less power.
It is observed that the principle of the cooling of a fluidized-bed by means of flue gas recirculation is not restricted to pressure-charged fluidized-beds, but is also applicable in principle to the atmospheric pressure-charged fluidized beds. The drawback going therewith, however, is that in such a fluidized-bed the surface area of the bed, which in an atmospheric fluidized-bed is already many times larger than that of a pressure-charged fluidized-bed, would have to be doubled at least, which would lead to inadmissible dimensions of the bed.
However, it is not excluded that it will become possible in future as a result of new technologies to apply flue gas recirculation also advantageously to an atmospheric pressure-charged fluidized-bed.
It is pointed out that the invention is described on the basis of a number of possible embodiments, but that it is naturally possible both to apply in the embodiments described various alterations obvious to one skilled in the art, and to apply the principle of flue gas recirculation to a fluidized-bed system of a different structure than the one described in the above.

Claims

1. A pressure-charged fluidized-bed combustion system, comprising a fluidized-bed boiler with a supply line for compressed air, a discharge line for flue gases as well as means for compressing the air to be supplied and for expanding the flue gases, characterized in that there is provided a circuit for recycling a part of the flue gases, which circuit is connected to the fluidized-bed boiler above and underneath the area in the boiler where the fluidized-bed is present in operation, while in the circuit there is at least provided a blower the speed of which can be controlled, while there are provided means for absorbing heat from the flue gases.

2. A fluidized-bed combustion system according to claim 1, characterized in that the circuit above. and underneath the area where the fluidized-bed is present, is always connected through a line to the fluidized-bed boiler and that said lines are connected to a steam boiler wherein the heat-absorbing surface of the water/steam circuit is incorporated.

3. A fluidized-bed combustion system according to claim 1, characterized in that the heat absorbing surface of the means for absorbing heat from the flue gases in the fluidized-bed boiler are incorporated in an area above the fluidized-bed, that the line for recycling the flue gases to the bottom of the fluidized-bed is connected to the boiler above the portion where the heat absorbing surface is provided, and that the discharge line for the flue gases is connected to the boiler in a place situated between the area where the fluidized-bed is present and the area where the heat absorbing surface is provided.

4. A fluidized-bed combustion system according to claim 3, characterized in that the fluidized-bed boiler is open at the top and at the bottom, that a second, closed boiler is provided spaced apart around said boiler, and that at the open top there are provided ventilating means for recycling the flue gases via the interspace between the outer boiler and the inner boiler.

5. A fluidized-bed combustion system according to any one of the preceding claims, characterized in that underneath the fluidized-bed there is provided an apertured plate, tubes being positioned on each of the apertures, which tubes terminate above the region where the fluidized-bed is provided, and that the supply line for flue gases at the bottom of the fluidized-bed comprises a first line containing a controllable valve and which terminates in the space of the boiler underneath the apertured plate, and a second line which terminates in the space between the bottom of the fluidized-bed and the apertured plate, to which space is likewise connected the line for supplying air.

6. A fluidized-bed combustion system according to any one of the preceding claims, characterized in that the means for absorbing heat from the flue gases comprise a water/steam circuit.