CA2344033A1

CA2344033A1 - A novel gas-solid separator for fluidized bed boiler

Info

Publication number: CA2344033A1
Application number: CA 2344033
Authority: CA
Inventors: Prabir Basu
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-04-19
Filing date: 2001-04-18
Publication date: 2001-10-19

Abstract

An apparatus for separation of solids from a mixture of solid and fluid, which is supplied tangentially into a box shaped separation chamber, and leaves horizontally though opening (s) on side walls of the chamber. The invention also provides novel methods for converting a flame fired boiler into a circulating fluidized bed boiler and for building new compact circulating fluidized bed boiler.

Description

A NOVEL GAS-SOLID SEPARATOR FOR FLUDIZED BED BOILERS
This invention relates to process equipment and boilers particularly, a device for separating solids from a fluid-solid suspension. This invention also relates to a novel method for converting an existing fossil fuel fired (FFF) boiler into circulating fluidized bed {CFB) boiler.
Backgiround to the invention Gas-solid or liquid-solid separators are essential parts of many equipment including fluidized bed boilers. Circulating fluidized bed boilers and reactors in particular use gas solid separator to separate the solids from the gas-solid suspension and return the separated solids back to the reactor or boiler-furnace. Another important application of gas-solid separation in revamping of old fossil fuel fired (FFF) boilers is described in Canadian patent application no 2,159,949.
There are known methods of separating solids from the gas-solid suspension leaving a furnace. Khanna, in U.S patent No. 5,535,687 teaches the use of vertical upright un-cooled (hot) cylindrical cyclone to separate gas from the gas-solid mixture.
Such cyclones are bulky, requires a large amount of refractory and have high surface heat loss. Garcia-Mallol in Canadian patent 2,080,319 teaches the use of horizontal cylindrical uncooled cyclone in circulating fluidized bed boilers. Such hot cyclones suffers from the same problems of uncooled vertical cyclones. Gorzegno in Canadian patent, 2081401, teaches the use of plurality of above type of horizontal cylindrical cyclone for building new CFB boilers. This system is also complex owing to its cylindrical construction. U.S patent nos 4,891,052, and no 4, 992,085 and Canadian patent 2,160,650 teach use oif uncooled impact type particle separators in CFB
boilers. Such separators are geometric in shape and as such allow building compact CFB
boilers.
However, these separators work on the principle impact of solids on separator surfaces.
Impact separation results in wear of the impacting surfaces. Furthermore efficiency of such separators are lower than those which works on the principle of centrifugal separation. Weitzke and Ganesh (CFB- Technology V, Science Press, Beijing, p.
289,1997) describe the use of a rectangular water cooled vertical cyclone for use in circulating fluidized bed boilers. Such separators have collection efficiency higher than that impact types separators. These are not suitable for revamping existing boilers as it requires a head room above the exit of the cyclone, which is not easily available within the existing structure of an old boiler.
SUMMARY OF THE INVENTION
One aspect of i:he present invention is an arrangement for particle collection and separation which is more efficient, easily adaptable to existing boilers and cost effective than other known systems and arrangements. It also provides a novel method for converting an existing FFF boiler into CFB boiler.
In accordance with one aspect of the present invention there is provided separators for use in a CFB boilers, wherein at least one separator is formed as a rectangular box with an open slit facing upstream into the flow of gas/solid suspension, when in use, and wherein holes on side walls allows clean gas to leave the separator chamber. Flow axis of the entering gas solid suspension is generally perpendicular to that of the gas leaving the separator.
In accordance with another aspect of the invention a plurality of separation chambers are arranged in parallel with a common chamber for the clean gas.
In accordance with another aspect of the invention there is provided rectangular box shaped separators, wherein gas-solid mixture enters through a horizontal slit extending from the top of the front wall down to the outer periphery level of the circular exit of two vertical side walls. Two cylindrical pipes are placed horizontally on the two side walls. These tubes leave adequate room for the gas to enter from their open end inside the separator chamber. Clean gas leaves the separator chamber through these tubes. The solids are collected at the bottom of the separation chamber, from where they are removed appropriately.
In accordance with yet another aspect of the present invention, there is provided a method for adapting a fossil fuel fired boiler into a circulating fluidized bed boiler comprising the steps (a) installing a grid plate at lower section of a furnace; (b) installing fans under the grid plate to blow air up through the grid plate when in operation; (c) changing the shape of the water and steam carrying tubes to form a rectangular separation chambers, (d) installing circular exits on side walls of the separation chambers; (e) installing a solids return system at the bottom of the separation chamber to recycle the solids back to the furnace; (f) providing particulate fuel feeding devices; (g) providing secondary air openings; (h) providing a fluidized bed ignition system ; and (i) providing means for draining ash from the furnace.

In accordance with another aspect of the present invention, there is provided a compact CFB boiler comprising (a) a grid plate at lower section of a furnace;
(b) fans under the grid plate to blow air up through the grid plate when in operation;
(c) rectangular gas-solid separation chambers formed of water and steam carrying tubes, (d) circular exits on side walls of the separation chambers; (e) solids return systems at the bottom of the separation chamber to recycle the solids back to the furnace; (f) fuel feeding devices; (g) secondary air openings; (h) a fluidized bed ignition system ; and (i) means for draining ash from the furnace.
Advantages of the invention over prior devices:
The advantages and benefits of the present device for separating solids from a mixture of gas and solids include the followings:
1. The traditional means of separating solids from gas is a cylindrical vertical cyclone. Such a device with a tapered bottom requires a large cylindrical volume of space. It is very difficult to integrate that in the existing structure of a boiler. It would, therefore, require a large amount of space outside the existing boiler.
In most older or even newer generation boilers such space is unavailable. Thus this type of conventional device does not permit conversion of an existing flame fired boiler into circulating fluidized bed firing. The present device does not require that space. Its design allows it to be easily fitted inside existing walls of the fossil fuel fired boiler.
2. The vertical cyclone uses a tapered conical hopper for solid collection, construction of which is complex. Also, it is difficult to incorporate it within the generally rectangular configuration of conventional boilers. The present device does not use such a conical section. Thus the above difficulty is absent here.
3. The conical section makes it difficult to construct it out of boiler tubes.
A complex tube arrangement is required to do that. Present device uses more regular geometric shape, which makes it easily amenable to boiler tube construction.

4. One type of horizontal cylindrical cyclone is used which do not require the conical collection hopper. This device solves one problem, but its cylindrical shape does not allow it to blend in the rectangular geometric shape of conventional boilers.
So it is difficult to adapt it in the existing boiler configuration. The present device does not use such a cylindrical body. Thus it can be used for both new CFB

boiler of regular geometry and for adopting existing fossil fuel fired boilers to CFB
firing.

5. One type of square cyclone is used in some CFB boilers. Here the gas exits from the top. Thus a large space is required above the separator floor. It is very difficult to find such space in an existing boiler. The present device does not need this additional space because here the gas exits from the sides of the boiler.
Thus it can be easily used for converting FFF boiler into CFB firing.

6. The walls and the roof of the present device are straight. This allows easy manufacture if made of refractory or steel. Furthermore, it can be easily made of existing superheater panels or water wall simply through appropriate repositioning.

7. In conventional impact type separators the gas-solid mixture enters the device from an entry section at a high velocity. As a result the gas-solid mixture often hits the opposite wall with a high velocity resulting in erosion of this part of the separator. The present device incorporates a novel arrangement in this part of the wall which greatly reduces the chances of erosion.

8. Unlike circular cyclone the present device is modular in construction. One can increase the throughput of the device simply by increasing number of geometric shaped separation modules.
Use of the device 1. This device can be used for separating gas from a mixture of gas and solid or solids from a mixture of solid and gas.
2. This device can be used for separating solids from a mixture of solid-liquid or liquid from a mixture of liquid-solid 3. The device can be used for converting an existing fossil fuel fired boiler into circulating fluidized bed firing 4. The device can be used to enhance the combustion efficiency of a bubbling fluidized bed bailer by recycling unburnt carbon in the bed 5. The device can be used to design compact and economic circulating fluidized bed boilers 6. The device can be used for.designing and building high performance and compact circulating fluidized bed reactor.
7. The device can be used for heat exchange purposes 8. The device for can be used for generation of steam 9. The device can be used for superheating steam 10. The device can be used to reduce erosion of gas-solid separation device 11. The device can be used for moving solids after collection from one chamber to another predefined chamber.

12. The device can be used for pre-cleaning dust-laden gases entering a heat exchanger 13. The device obviates the need of external cyclone of a circulating fluidized bed boiler 14. The device can be used to eliminate the external stand pipe of a circulating fluidized bed boiler 15. The device can be used to cool the flue gas entering the back pass of a boiler 16. The device can be used to reduce the erosion potential of downstream heat exchanger surfaces of a boiler.

17. The device can be used to reduce the use of refractory in a circulating fluidized bed 18. The device can be used to reduce the start up time of a circulating fluidized bed boiler 19. The device can be used to improve the separation efficiency of impact type or vertical cavity separators where solids are separated through multiple changes in the flow direction.

20. The device could reduce the maintenance cost of equipment using gas-solid separation.
Brief description of the Drawings The present invention will be further understood from the following description by way of example with reference to the drawings in which:
Fig. 1 is an isometric view of a separator with single cylindrical exit Fig. 2 is a cross section view of a separator with single cylindrical exit and collection chute tapered on both sides Fig. 3 shows a separator with single taper collection chute. Fig 3a shows notch with shelves. Fig. 3b shows rear wall with shelves alone.
Fig. 4 shows the cross-section elevation taken through two parallel separators with cylinder exits. The vertical section is taken through the centre line of the gas exit.

Fig. 5 shows two locations of the exit section with reference to the separator. It also shows the erosion protecting layer on the rear wall.
Fig 6 shows three arrangements of gas exit from the separator. It also shows parallel arrangements, in which all separators would not necessarily have different exit design as shown here.
Fig. 7 shows an arrangement for conversion of a FFF boiler into circulating fluidized bed boiler by using the novel separators.
Fig. 8 shows an isometric view of use of multiple rows of novel separators Detail description of the preferred embodiment One embodiment of the separator is shown in figure 1. The box shaped separator comprises four vertical walls (21, 20, 18 & 19) and a horizontal roof (23).
The bottom of the separator (7) is a collection chamber for dust. One cylindrical exit (3) is located on the vertical side wall (20). A rectangular opening (1 ) is located at the top of the front vertical wall ('19). This opening is located such that its flow axis is perpendicular to axis of the cylindrical exit (3), also the suspension enters tangent to the an imaginary cylinder passing through the gas exit. The exit (3) is generally on the vertical centre line of the side wall (20). Dust laden gas or gas-solid suspension enters the separatar through opening (1). Relatively clean gas leaves the separator though exit (3) into the exit chamber (6) outside the side wall (20).
Separated solids leaves the separator through dust channel (7) which may exit through the front wall (19) or behind this wall.
The gas solid suspension is made to enter the separator through section (1 ).
The axis of this entry section (1) is tangent to the exit section. The upper edge of the exit pipe (3) is below but as close to the lowest edge of the entry section (1) as possible to maintain the tangent entry of the dust laden gas (Fig. 2). The shape of the separation chamber induces a flow pattern as a result of which the gas solid mixture try to travel straight: towards the rear wall (18), but the presence of the rear wall and the resulting pressure gradient make the mixture move around a near circular path around the axis of the exit section (3). The solids, by virtue of its higher momentum tend to move towards the rear wall continuing on its original travel path while the gas which has lower momentum follows the pressure gradient. While travelling in circular trajectory, centrifugal force drives the solids towards the wall where the gas velocity is too low to convey the solids further. Solids are, therefore, separated from the gas carrying them Then the solids drop under gravity. The collected solids drop into the hopper (5) from where it slides down into the solid collecting chute (7).
The dust collection section (5) can be made of vertical walls which are essentially continuation of side walls (20, 21 ) as shown in Fig. 1 and that of the front wall (19) as shown in Fig. 3. The rear wall (18) may also drop vertically without creating separate collecting chute. In another preferred arrangement shown in Figure (3), the rear wall (18) is bent at an angle exceeding the angle of repose of the solids and then drops vertically forming the collecting chute (7). This collection chute is made of uncovered metal for cold solids. For hot solids it is covered high temperature refractory. In another preferred arrangement it is made of heat absorbing surfaces carrying fluids being heated. The dust collection chute (7) is made circular in cross-section like a stand pipe in places where the neighbouring configuration demands it.
Another embodiment of the invention is shown in figure (4). Here two cylindrical exits are located on two vertical side walls (20, 21 ). Gas-solid suspension enters through the entry section (1 ) in a direction tangent to an imaginary cylinder passing through the circular exit sections (3, 3). The gas after it is separated from the suspension leaves the separator chamber (2) from both sides of the separator.
The solids drop into the collecting hopper (5). In a preferred arrangement, cylindrical sections (3,3) are replaced with the circular exits (9,9) as shown in Fig. 5.
In another preferred arrangement (Fig. 5) the exit cylinder (10) is flush with the inner face of the side walls (20, 21 ). In another arrangement (Fig. 5) the exit cylinder (8) is flush with the outer face of the side walls (21, 20).
The rear wall is subject to wear due to direct impact of solids. This wall is protected against the erosion potential by using one of three alternative designs. In the first embodiment shown in figure 5, the vertical rear wall (18) is covered with wear resistant materials like refractory (24). In another embodiment, shown in figure 3a, a number of horizontal shelves of varying size and shape and size as shown are attached to rear wall (18). Solids trapped between these shelves protect the target zone of the rear wall from erosion.
In another embodiment shown in figure 3b, a notch (4) is created in the upper part of the rear wall (18) directly opposite to the entry section (1 ). While in operation, a static layer of solids sits here. The solids abrade against these solids saving the wall. In a still improved design a number of horizontal shelves (17) are located inside this notch for better capture of the solids.

The exit hole (1) of the separator is generally located on the vertical centre line of the side wall as shown on figure 5(a). In a preferred design as shown in figure 5b, the exit hole is located on either side of the centre line according the flow field. Here the exit is located at the centre of the vortex created by the tangentially entering gas solid. Thus such location of the exit (fig. 5b) minimizes flow separation around the exit section (3). This would enhance solid separation efficiency of the separator.
In another embodiment of the separator, more than one separators are located parallel to each other. This arrangement is shown in figures 4, 6 and 8. In fig.4 two separators are placed side by side each sharing a common exit chamber (6). The front wall (19) is a continuous with discrete rectangular openings in front wall (19) of the separator chambers (2). The rear wall (18) is not necessarily continuous.
It covers only the individual separator chambers. In case of multiple parallel separators, the last separator could have only either one exit or only one exit chamber as shown in figure 4. Here all separators are shown to use cylindrical exits.
In Fig. 6, three alternative arrangement of separator's exit are shown. For the sake of explanation alternative designs of exit section each separator are shown to have different exit design. However, it is not a requirement for parallel arrangement. Fig. 8 shows isometric view of two rows of parallel separators set for optimum utilization of space. Here the second row receives gas-solid suspension from the top of exit section (6) of the first row. Such an arrangement makes it easy to adapt such separators within limited space available in an existing boiler, which is required for conversion of an existing FFF boiler into CFB boiler.
The entry sectian (1) generally covers the entire width and height of the front wall above the periphery line of the exit cylinder (3). In a preferred shape the height of the entry section is reduced from the top.
In a preferred embodiment all walls (21,20, 23, 18 & 19) are made of panels of heat exchanger tubes. The hopper (5) and chute (7) are also made of heat exchanger tubes obviating the need of refractory and saving additional cost and space required for heating surfaces required in boilers.
In another preferred shape the entry section (1) is narrower than the width of the separator.
In another embodiment this separator is used in existing boilers to convert it into circulating fluidized bed firing. Figure 7 shows one possible arrangement. The entrance (1 ) of the separator is located near the top of the rear wall of the furnace (28) of the existing boiler. Lower section of the furnace comprises a grid plate (29) for entry of primary air. The lower section (15) of the furnace is lined with refractory, while the upper section (28) is made of water wall tubes. Secondary air enters through a plurality of openings (16) located above the refractory lined lower section (15). The air velocity is controlled such that it creates the appropriate hydrodynamic condition in the furnace for circulating fluidized bed operation, where gas-solid suspension continuously leave the upper section (28) of the furnace and enters the separator through openings (1 ) and are recycled back through (7). The suspension enters one or multiple separators stacked in parallel. The hopper (5) is slanted towards the furnace (28). Solids after separation drops down rectangular channels (7). All sides of the channel are made of heat exchanger tubes as such they serve as heat exchanger. The solids drops into a loop seal (13) located at the bottom of the channel (7). The loop seal (13) comprises two chambers. The first chamber called supply chamber (7) is the bottom of the collection chute (7), which is closed at the bottom but opened on sides. Only a small amount of air may be passed through its bottom to facilitate solids transfer. The second chamber (12) of the loop seal is located adjacent to this. Depending upon the space available it can be located either in front as shown in figure 7 or on sides. This chamber, defined as recycle chamber (12) is fluidized by air which enters through a grid located at its base. The hot combustion gas, after cleaned of solids, leaves through exit (3) and enters the set of exit chambers (6) located in between adjacent separation chambers (2). The hot gas flows down the exit chamber (6) and then over heat exchangers tubes (11) located downstream of the separators and finally leaves the boiler through (17). These heat exchanger surfaces are modified within the existing confines of the boiler to absorb the required amount of heat from the combustion gas cooling it down and to contribute to the generation of steam as per the design parameters of the boiler. For initial ignition of the fuel a start up burner is located in the air box below the grid.
Preheated air and hot gas from the burner enters the furnace and heat the granular solids fluidized in the furnace.
In one particular application this type of separator is used to revamp old plant. In particular design the rear furnace wall tubes are bent to make room for entry of gas solid mixture from the furnace into the separation chamber. Panels of steam tubes form the separation chamber. These tubes leave one common gas exit passage between two separators forming multiple chambers. However, in the end chamber the cleaner gas passage is formed by between the vertical side wall of the existing boiler and last wall of the separator. The steam tubes bends at an acute angle to form the collection hopper. The same tube panel extends below down to the loop seal. On both sides of the dip leg just formed. On both sides of the collection chamber there are one fluidized bed that serves as the recycle chamber. Solids from these two fluidized bed overflow into the main furnace. These solids enter the main furnace through openings made in the water wall by bending the tubes away from the furnace.
Thus the bottom of the empty gas passage between two separators form the recycle chamber of the loop seal while the bottom of dip leg of the separator is the supply chamber of the loop seal.
In another embodiment a compact circulating fluidized bed boiler is built following the above description except that furnace which is sized and shaped to give optimum design.
In one embodiment of the design the entry velocity of the gas-solid through (1 ) is within 20-30 m/s to give the ideal separation efficiency.

Claims

1. A process for removing particles from a gas stream, comprising the step of providing a gas stream containing gas and particles;
separating said gas stream into a first and second portion;
increasing the concentration of the first portion;
removing the first portion from the separator through a different point than that from the second portion

2. A method of adapting a fossil fuel fired boiler into a circulating fluidized bed boiler comprising the steps of a) installing a grid plate at a lower section of a furnace;
b) installing fans under the grid plate to blow air up through the gird plate when in operation;
c) installing a solids return system to the furnace d) providing fuel feeding devices e) providing secondary air openings;
f) providing an ignition system;
g) providing the method of claim 1 wherein solids separated are sent to the solids return system

3. The method of claim 2 wherein the hopper communicates with the lower solid recycle system through a vertical channel

4. The method of claim 2 wherein the solids gas solid separators are placed at the exit of the furnace through which gas-solid suspension leaves the furnace.

5. The method of claim 4 comprising at least one separator formed as rectangular chamber with and open end facing upstream with respect to the gas flow, and closed at downstream a. gas-solid mixture is directed towards a the device and is made to travel at a high velocity b. The mixture enters the device through a rectangular opening c. The mixture travels in a near circular path d. The solids are separated during this travel e. The gas continues in the circular path into a spiral which leaves the chamber through a circular aperture located on the side wall of the chamber of the device f. Relatively clean gas leaves the chamber while the solids, separated from the gas, drops into the chamber from where it is collected in a preferred way.

6. Heat exchangers which also serve as gas-solid separator

7. Heat exchangers which generates superheated steam by absorbing heat from hot gas or hot gas-solid suspension

8. Heat exchangers which preheats water

9. Heat exchangers which generate steam by absorbing heat from hot gas or hot gas-solid suspension

10. A novel gas-solid separator with a geometry which can easily be adapted to the rectangular heating surface arrangements of existing boilers

11. A gas-solid separator comprising more than one separation chamber and collection chamber

12. A gas-solid separator with one circular exit on one of the vertical side wall

13. A gas solid separator with two circular exits on two vertical side walls of the separation chamber

14. A gas-solid separator with one circular exit located off axis on one vertical side wall

15. A gas-solid separator with two circular exit located off axis on two side walls of the separator

16. A gas-solid separator with recess wall in the rear wall of the separator chamber

17. A gas-solid separator as in claim 6- 15 with shelves in the rear wall

18. Gas -solid separator of claim 6-17 wherein the side walls are made of heat absorbing surfaces.

19. Gas solid separator of claim 6-18 wherein the roof, side, rear wall and front walls are made of heat absorbing surfaces

20. Gas solid separator of claim 19 wherein heat absorbing front, rear and roof walls are covered with thin refractory.

21. Gas-solid separator of claim 19 wherein the side walls are lined with refractory

22. Gas solid separator a hollow cylinder is inserted in the circular opening of the side walls

23. A gas-solid separator of claim 22 wherein the exit cylinder is flush with the inner face of the side wall

24. A gas-solid separator of claim 22 wherein the exit cylinder is flush with the outer face of the side walls

25. Gas-solid separators of claim 10-24 wherein the both front and rear wall tapers inward to form a discharge hopper.

26. Gas-solid separator claim 10-24 wherein the front wall is vertical but the rear wall tapered inward to form a hopper for the exit of the collected solids

27. A circulating fluidized bed boiler with gas-solid separator of claim 10-28 wherein a loop seal is connected to the discharge left of the separator

28. A circulating fluidized bed boiler wherein two discharge end of the loop seal are on two sides of the rectangular dip leg which is also the discharge ed of the separator

29. A CFB boiler with gas-solid separator described in 10-26 wherein solids collected in the rectangular dip leg enters one loop seal which is located towards the front wall of the separator

30. A CFB boiler with gas-solid separator described in claims 10-29 wherein the openings on front wall are made be bending vertical tubes sideways

31. A CFB boiler with gas-solid separator described in claims 10-29 wherein openings on the front walls are made by terminating tubes in horizontal headers.

32. A separator described in claim 5 where the gas solids enter with a velocity in the range of 20-30 m/s range