GB2069862A

GB2069862A - Fluidised bed combustor

Info

Publication number: GB2069862A
Application number: GB8104457A
Authority: GB
Original assignee: Foster Wheeler (London) Ltd
Current assignee: Foster Wheeler (London) Ltd
Priority date: 1980-02-19
Filing date: 1981-02-12
Publication date: 1981-09-03
Also published as: IE50817B1; JPS56121903A; FR2476273A1; ZA811061B; HU183338B; IE810337L; IT1135526B; DE3106840A1; JPS6022242B2; NL8100818A; FR2476273B1; GB2069862B; CA1149234A; IN155322B; IT8119837A0

Abstract

The invention relates to a fluidized bed combustor. Such combustors have many advantages but, because there is a relatively continuous flow of fuel through the bed, unburnt fuel is frequently discharged. In a combustor according to the invention, a plurality of substantially vertical partitions (20, 22, 23) are disposed and arranged in a housing to divide the housing into three or more zones, or chambers (24, 26, 28). A bed of particulate material containing fuel is established in at least two of the chambers (24, 26) and openings (50, 52) are provided in the partitions (20, 22) to permit flow of the material from one of the two chambers to the other and from the latter to a third chamber (28). Air is passed through the beds to fluidize the particulate material and promote the combustion of the fuel in the two chambers (24, 26) and to cool the material in the third chamber (28). <IMAGE>

Description

SPECIFICATION Multi-zone fluidized bed combustor and method of operating same This invention relates to a fluidized bed combustor and a method of operating same and, more particularly, to such a combustor and method in which three or more separate beds of particulate material are established within a single housing.

The use of fluidized beds has long been recognized as an attractive means of generating heat. In a normal fluidized bed arrangement, air is passed through a bed of particulate material, which usually includes a mixture of inert material, a fuel material such as high sulfur bitaminous coal, and an adsorbent material for the sulfur released as a result of the combustion of the fuel material. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. The basic advantages of such an arrangement include a relatively high heat transfer rate, a substantially uniform bed temperature, combustion at relatively low temperatures, ease of handling the fuel materials, a reduction in corrosion and boiler fouling and a reduction in boiler size.

In the fluidized bed combustion process, in order to insure capture by the adsorbent of the sulfur released during combustion of the fuel, the bed temperature has to be kept close to a predetermined value (normally 1 500s 550 F). At this temperature, the bed material will contain quantities of unburned fuel material, especially in the case when low reactive fuels and/or fuels of a relatively large size are used. However, since the bed must be continuously drained to discharge the spent bed material, a good portion of the unburned fuel material will also be discharged. This, of course, results in a reduction in the recovery of sensible heat in the bed and in the overall efficiency of the system.

It is, therefore, an object of the present invention to provide a fluidized bed combustor and a method of operating same which enables a relatively high percentage of fuel materials to be combusted.

It is a further object of the present invention, to provide a combustor and method of the above type in which a plurality of zones, or chambers, are established within the combustor housing with one or more of the chambers being maintained at a temperature compatible with the recovery of sulfur by the adsorbent material in the bed, and another chamber being maintained at a temperature sufficient to promote complete combustion of the fuel material.

It is still a further object of the present invention to provide a combustor and method of the above type in which a fluidized bed is established in a main chamber and the particulate material is passed to a second chamberforfurther burning and to a third chamber for cooling before it is discharged from the combustor housing.

It is a further object of the present invention to providea combustor and method of the above type in which the flow of fluidizing and combustion air to each zone, or chamber, is individually regulated.

Toward the fulfillment of these and other objects, the system of the present invention comprises a housing, a plurality of substantially vertical partitions disposed in the housing and dividing the housing into a first, second and third chamber which are placed in communication by openings formed in the partitions. A bed of particulate material containing fuel is formed in at least the first and second chambers, and air is passed through each of the beds to fluidize the particulate material in the first and second chambers and to cool the material in the third chamber. Fuel is added to the first chamber to maintain continuous combustion and the temperature in the first chamber is maintained at a value consistent with optimum capture of the sulfur formed as a result of the fuel combustion.The temperature in the second chamber is maintained at a value consistent with the combustion of substantially all of the unburned material in the second chamber, and the material is passed to the third chamber where it is cooled before being discharged.

Figure 1 is a partial perspective view of the combustor of the present invention; Figure 2 is a reduced plan view of the combustion of Figure 1; Figures 3 and 4 are cross-sectional views taken along the line 3-3 and 4-4, respectively, of Figure 2; and Figures 5 and 6 are views similar to Figure 2, but depicting alternative embodiments of the present invention.

Referring specifically to the drawings, which depict a fluidized bed combustor of the present invention, the reference numeral 10 refers, in general, to a housing having a front wall 12, a rear wall 14 and two side walls 16 and 18. The walls 12,14,16 and 18 are formed buy a plurality of interconnected tubes in a conventional manner, it being understood that the ends of the tubes can be connected between the inlet and outlet headers to permit the flow of water through the tubes to add heat from the combustor to the water, The housing 10 is also provided with a roof (not shown) and a floor which will be described in detail later.

A partition 20 is provided within the housing 10 in a spaced relationship to the rearwall 14 and extends from the side wall 1 6to the side wall 18. A second partition 22 extends between the partition 20 and the rear wall 14 and parallel to the side walls 16 and 18 in a spaced relationship thereto. A third partition 23 extends immediately adjacent the rear wall 14 and parallel thereto, and between the side wall 16 and the partition 22. As a result of the foregoing, a main chamber 24 is defined by the walls 12, 16 and 18 and the partition 20; a second chamber 26 is defined by the wall 16 and the partitions 20,22 and 23; and a third chamber 28 is defined by the walls 14 and 18 and the partitions 20 and 22.

The floors of the chambers 24,26 and 28 are formed by a perforated plate, or grate, 30 extending between the walls 12,14, 16 and 18. The partitions 20,22 and 23 extend vertically from the grate 30 to a plane below the plane of the roof of the housing to define a free board spaced between the upper portions of the partitions and the roof.

An air plenum 32 extends immediately below the chamber 24, an air plenum 34 extends immediately below the chamber 26, and an air plenum 36 extends immediately below the chamber 28. An inlet duct, shown schematically by the reference numeral 40, receives air from an external source (not shown) and passes same into the air plenum 32. A duct 42 branches from the duct 40 and connects with two branch ducts 44 and 46 which pass air from the duct 40 into the air plenums 34 and 36, respectively. A valve 48 is associated with each of the ducts 40,44 and 46 to control the flow of air into the plenums 32, 34 and 36, respectively. As a result, the flow of air into the chambers 24,26 and 28, respectively, can be individually controlled.

A pair of openings 50 and 52 are provided in the lower portion of the partition 20 to permit the particulate material from the chamber 24 to pass into the chamber 26. An opening 54 is provided in the partition 22 to permit the particulate material from the chamber 26 to pass into the chamber 28. A discharge opening 56 is provided in the wall 18 in communication with the chamber28to permit the particulate material from the chamber 28 to discharge from the housing 10 into a conduit 58 under the control of a valve 60.

A bed of particulate material is established in each of the chambers 24 and 26 with the height of the material extending approximately to a level which is slightly below the height of the partitions 20,22 and 23, as indicated by the reference numeral 62 in Figure 1. Each bed of particulate material includes a mixture of crushed coal and an inert material such as sand or a commercial grade hematite iron ore. Also a fine limestone or dolomite is included for use as an adsorbent for the sulfur released during the combustion of the fuel. A pair of inlet openings 64 and 66 are provided in the side wall 18 for permitting the continuous introduction of the particulate fuel material and adsorbent, respectively, into the upper portion of the chamber 24.

Although not shown in the drawings, it is understood that a roof is provided over the walls 12, 14, 16 and 18 and contains a single outlet for the discharge of the mixture of air and gaseous products of combustion from the chambers 24,26 and 28. A pair of injectors 70 and 72 are provided for reinjecting into the chamber 26 the particulate material entrained in the mixture of air and gaseous products of combustion exiting from the housing 10 and separated from the latter mixture by external equipment (not shown).

A notch 74 is provided in the upper portion of the partition 22 to permit the flow of the particulate material from the chamber 26 to the chamber 28 in the event the height of the material in the chamber 26 exceeds the level represented by the line 62.

An opening 76 is provided in the sidewall 16 in communication with the chamber 26to receive an igniterorthe like to ignite the bed material in the chamber 26 for reasons also to be described in detail later.

A plurality of heat exchange tubes 80 are provided in the chamber 24, with one of the tubes being shown by the reference numeral 80. Each tube extends between an inlet header 82 and an outlet header 84 disposed externally of the housing 10 and penetrates the front wall 12 before extending in a serpentine relationship in the bed of particulate material disposed in chamber 24. Water can then be passed through the tubes 80 to raise the temperature of the water and lower the temperature of the bed.

In operation of the combustor 10, air is introduced through the conduit40 into the air plenum 32 which fluidizes the bed of particulate material in the chamber 24 and, after pre-ignition, the fuel material burns and releases its sensible heat to the water passing through the tubes 80. Additional fuel and adsorbent is introduced into the upper level of the bed in the chamber 24 through the inlets 64 and 66, respectively.In order to insure optimum adsorption ofthe sulfur contained in the particulate fuel material by the adsorbent, the temperature of the bed in chamber 24 is maintained close to a predetermined value, normally 150091550"F. The maximum bed temperature is maintained by the presence of the cooling tubes 80, the regulation of the flow of air into the chamber by the valve 48, the bed height and the amount of fuel material introduced into the bed through the inlet 64.

A portion of the particulate material in the chamber 24 passes through the openings 50 and 52 into the chamber 26 which is kept at a temperature consistent with optimum combustion of the unburned fuel particles received from the chamber 24. This temperature, of course, depends on the various operating parameters associated with the chamber 26, such as the amount of air flow through the chamber, the bed height in the chamber, the amountof fuel in the chamber, etc., and for the purpose of example, could be approximately 1800"F.

As a result of the foregoing, a great percentage of the fuel material in the chamber 26 is combusted and the spent material along with the inert material and the adsorbent in the chamber 26 passes through the opening 54 in the partition 22 and into the chamber 28. The air passing upwardlythrough the chamber 28 from the duct 46 under the control of the valve 48 associated therewith, cools this material before it is discharged from the chamber 28 through the opening 56 and into the discharge duct 58. As a result of the-foregoing, an unprecedented high percentage of the fuel material introduced into the housing 10 is combusted while maintaining optimum sulfur adsorption, thus increasing the sensible heat given off and the overall efficiency of the process.

Another advantage of the present invention is that the chamber 26 can be used as a startoff chamber, i.e., during start-upofthe entire process, the fuel material in the chamber 26 can be ignited by an igniter or the like extending through the opening 76 in the wall 16 in communication with the chamber 26.

The level of the fuel in the bed in the chamber 26 can be increased sothat the ignited fuel spills into the chamber 24 and ignites the fuel material in this chamber. Alternatively, an air jet, or the like, can be provided at the openings 50 and 52 into the chamber 24. Of course, after start-up in the foregoing manner, the combustorwould operate in the manner described above.

Figures 5 and 6 depict alternate bed arrangements for the combustor shown in Figures 14. In particular, according to the embodiment of Figure 5, a pair of additional partitions 90 and 92 are provided in the housing 10 and extend parallel to the partition 20 to divide the chamber 24 into three chambers 24a, 24b and 24c. A pair of openings 94 are provided in each partition 90 and 92 to permit communication between the chambers 24a and 24b and between the chambers 24b and 24c. Therefore, the particulate material, including fuel material continuously introduced into each of the chambers 24a, 24b and 24c, would pass from the chamber 24a to the chamber 24b and from the chambers 24b into the chamber 24c before it passes into the chamber 26 and 28 in the manner described in connection with the previous embodiment.The chambers 24a, 24b and 24c are maintained at a temperature compatible with sulfur adsorption as discussed above in connection with the previous embodiment and chambers 26 and 28 also operate in the same manner as discussed above. In addition to insuring high sulfur capture and maximum fuel combustion as in the previous embodiment, this arrangement permits additional load turndown capability, i.e., the chamber 24a (and 24b, as necessary) may not befluidized at relatively low loads, but can be fluidized as necessary to accomodate higher loads.

The embodiment of Figure 6 is similar to that of Figure 5 with the exception that a first partition 96 is provided and extends between the partition 20 and the front wall 12, and a second partition 98 extends between the side walls 16 and 18 and intersects with the partition 96 to form chambers 24d, 24e, 24f and 249. It is understood that inlet openings for the introduction of fuel and particulate material are provided in the wall 16: 18 as necessary to supply the fuel and adsorbent material to the chambers 24d, 24e, 24f and 249. Two openings 100 are provided in the partition 96 to permit flow of particulate material from the chambers 24d and 24e into the chambers 24f and 24g, respectively.An opening 102 is provided in the partition 98 to permit the flow of particulate material from the chamber 249 into the chamber 24f before it flows into the chamber 26 and chamber 28 as described in connection with the previous embodiment. The chambers 24d, 24e, 24f and 249 operate in a manner similarto chamber 24 in the first embodiment, while chambers 26 and 28 operate in the same manner as discussed above.

Thus the arrangement of Figure 6 offers the same load turndown capability as the embodiment of Figure 5 while insuring high sulfur capture and maximum fuel combustion.

It is also understood that other variations of the specific construction and arrangement of the system and method disclosed above can be made by those skilled in the art without departing from the invention as defined in the appended claims.

Claims

1. Afluidized bed combustorcomprising a housing; a plurality of substantially vertical partitions disposed in said housing and dividing said housing into a first chamber, a second chamber and a third chamber; means for establishing a bed of particulate material containing fuel in said first and second chambers; means associated with said partitions for permitting flow of said material from said first chamber to said second chamber and from said second chamber to said third chamber; means respectively associated with each chamber for passing air through each of said beds to fluidize the particulate material and promote the combustion of said fuel in said first and second chambers and to cool the material in said third chamber; means for adding fuel to the bed in said first chamber to maintain continuous combustion in said first chamber; and means associated with said third chamber to permit the continuous discharge of cooled particulate material from said third chamber.

2. The combustor of claim 1 wherein no fuel is added to said second chamber other than that received from said first chamber so that the air passed through the bed in said second chamber promotes combustion ofthefuel in said latterchamberto the extent that more complete fuel combustion is attained in said second chamber.

3. The combustor of claim 1 further comprising means associated with each of said chambers for individually controlling the flow of air to and through each chamber.

4. The combustor of claim 3 wherein said controlling means passes an amount of air to said second chamber sufficient to maintain a higher air-tofuel ratio in said second chamber than in said first chamber.

5. The combustor of claim 1 further comprising heat exchange means disposed in said first chamber and means for passing a relatively cool heat exchange medium through said heat exchange means.

6. The combustor of claim 1 further comprising means for igniting fuel in said second chamber and means for passing said ignited fuel from said second chamber to said first chamber during start-up of said combustor.

7. The combustorof claim 1 further comprising a single gas outlet associated with said housing and wherein the mixture of air and combustion gases from each of said chambers exit from said housing through said outlet.

8. The combustor of claim 7 further comprising means for injecting into said second chamber any particulate material entrained in said mixture of air and combustion gases.

9. A method of operating a fluidized bed combustor comprising the steps of establishing a bed of particulate material containing fuel in at least a first and second chamber, passing air through each of said beds to fluidize said particulate material and promote the combustion of said fuel, maintaining the temperature in said second chamber at a value higher than that in said first chamber; permitting the flow of said material from said first chamber to said second chamber and from said second chamber to a third chamber, passing air through said third chamber to cool the material in said third chamber, and permitting the cooled material to discharge from said third chamber.

10. The method of claim 9 wherein no fuel is added to said second chamber other than that received from said first chamber so that the air passed through the bed in said second chamber promotes combustion of the fuel in said latter chamber to the extent that more complete fuel combustion is attained in said second chamber.

11. The method of claim 9 further comprising the steps of individually controlling the flow of air to and through each chamber.

12. The method of claim 9 wherein an amount of air is passed to said second chamber sufficient to promote complete fuel combustion in said second chambers

13. The method of claim 9 further comprising the step of passing a relatively cool heat exchange medium through said first chamber.

14. The method of claim 9furthercomprisingthe steps of igniting fuel in said second chamber and passing said ignited fuel from said second chamber to said first chamber during start-up of said combustor.

15. The method of claim 9 further comprising the steps of discharging the mixture of air and combustion gases from each of said chambers through a single outlet.

16. The method of claim 15 further comprising the step of separating from said mixture of air and gases all particulate material entrained therein and injecting said separated particulate material into said second chamber.

17. The method of claim 9 wherein said bed material contains an adsorbentforthe sulfur contained in said fuel material and wherein said first chamber is maintained at a temperature compatible with the adsorption of said sulfur.

18. A fluidized bed combustor substantially as described herein with reference to Figures 1 to 4, Figure 5 or Figure 6 of the accompanying drawings.

19. A method of operating a fluidized bed combustor substantially as described herein with reference to the accompanying drawings.