IE50817B1 - Multi-zone fluidized bed combustor and method of operating same - Google Patents

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).

Description

This invention relates to a fluidised bed combustor and a method of operating sama 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 bituminous 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 includes 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 15OO°-155O°F). At this temperature, the bed material will contain guanti25 ties of unburned fuel material, especially in the case when low reactive fuels and/or fuels of a relatively large si2e are used. However, since the bed must be continuously drained to discharge the spent bed material, a good portion - 3 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.

The present invention seeks to provide a fluidized bed combustor and a method of operating same which enables an increased percentage of fuel material to be combusted. To this end a combustor according to the invention comprises a housing; a plurality of substantially vertical partitions disposed in the housing and defining a first, a second and a third chamber therein, the first chamber having heat exchange means disposed therein with means for passing a relatively cool heat exchange medium therethrough; means for establishing a bed of particulate material containing fuel in the first and second chambers; means associated with said partitions for permitting flow of such material from the first to the second chamber and from the second to the third chamber; means respectively associated with each chamber for passing air therethrough to fluidize particulate material therein, promote the combustion of fuel in the 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; means associated with said third chamber to permit the continuous discharge of cooled particulate material from said third chamber; and means associated with the first and second chambers for controlling the combustion of fuel therein such that the operating temperature in the second is greater than that in the first chamber. Thus, the first chamber or at leastapart thereof can be maintained at a temperature compatible with the recovery of sulphur by the adsorbent material in the bed, and the second chamber maintained at a temperature sufficient to promote complete combustion of the fuel - 4 material. A higher temperature can be maintained in the second chamber by for example, maintaining a higher air to fuel ratio in the second chamber than in the first.

The first chamber in a combustor according to the invention can be divided into a plurality of sub-chambers between which the bed material flows prior to passing to the second chamber. In one such embodiment, the sub-chambers are arranged such that material flows successively from one to the next before passing to the second chamber. In all embodiments of the invention it is preferred that the flow of fluidizing and combustion air to each chamber or subchamber, is individually regulated enabling close control of the combustion in the individual zones of the combustor. The flow of bed material between the respective chambers or sub15 chambers is most conveniently through openings formed in the partitions.

A method of operating a combustor according to the invention to burn fuel comprises the steps of establishing a bed of particulate material containing fuel in a first and a second chamber; passing air through each bed to fluidize the particulate material and promote the combustion of fuel contained therein; passing a relatively cool heat exchange medium through heat exchange tubes in the first chamber; maintaining the temperature in the second chamber at a value higher than that in the first chamber; permitting flow of the particulate material from the first to the second chamber and from the second chamber to a third chamber; passing air through said third chamber to cool the material in the third chamber, and permitting the cooled material to discharge from the third chamber.

The invention will now be described by way of example and with reference to the accompanying drawings wherein:50817 - 5 Figure 1 is a partial perspective view of the combustor of the present invention; Figure 2 is a reduced plan view of the combustor 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 by a plurality of interconnected tubes in a conventional manner, it being understood that the ends of the tubes can be connected between 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 rear wall 14 and extends from the side wall 16 to 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 parti50817 - 6 30 tions 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 space 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 chamber 28 to 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 - 7 the height of the partitions 20, 22 and 23, as indicated by the reference numeral 62 in Figure 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 side wall 16 in communication with the chamber 26 to receive an igniter or the 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 - 8 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 particu5 late 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 conduit 40 into the air plenum 32 which fluidises 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. 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 sulphur formed as a result of the fuel combustion. In order to ensure optimum adsorption of the sulphur 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 15OO°-155O°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 combus30 tion 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 - 9 the amount of air flow through the chamber, the bed height in the chamber, the amount of 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 passesthrough the opening 54 in the partition 22 and into the chamber 28. The air passing upwardly through 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 start off chamber, i.e., during start-up of the 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 so that the ignited fuel spills over 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 combustor would operate in the manner described above.

Figures 5 and 6 depict alternate bed arrangements for the combustor shown in Figures 1-4. In particular, accordSO®!? - 10 ing 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 sub-chambers 24a, 24b and 24c. A pair of openings 94 are provided in each partition 90 and 92 to permit communication between the sub-chambers 24a and 24b and between the sub-chambers 24b and 24c. Therefore, the particulate material, including fuel material continuously introduced into each of the sub-chambers 24a, 24b and 24c, would pass from the sub-chamber 24a to the sub-chamber 24b and from the sub-chambers 24b into the sub-chamber 24c before it passes into the chamber 26 and 28 in the manner described in connection with the previous embodiment. The sub-chambers 24a, 24b and 24c are main15 tained at a temperature compatible with sulphur adsorption as discussed above in connection with the previous embodiment and chambers 26 and 28 can also operate in the same manner as discussed above. In addition to ensuring high sulphur capture and maximum fuel combustion as in the previous embodiment, this arrangement permits additional load turndown capability, i.e., the sub-chamber 24a (and 24b, as necessary) may not be fluidized at relatively low loads, but can be fluidized as necessary to accommodate 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 sub-chambers 24d, 24e, 24f and 24g. 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 subchambers 24d, 24e, 24f and 24g. Two openings 100 are pro35 vided in the partition 96 to permit flow of particulate material from the sub-chambers 24d and 24e into the sub50817 - 11 chambers 24f and 24g, respectively. An opening 102 is provided in the partition 98 to permit the flow of particulate material from the sub-chamber 24g into the sub-chamber 24f before it flows into the chamber 26 and chamber 28 as described in connection with the previous embodiment. The sub-chambers 24d, 24e, 24f and 24g operate in a manner similar to 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 ensuring high sulphur capture and maximum fuel combustion.

Claims (20)

1. CLAIMS :1. A fluidized bed combustor comprising a housing; a plurality of substantially vertical partitions disposed in the housing and defining a first, a second and third chamber 5 therein, the first chamber having heat exchange means disposed therein with means for passing a relatively cool heat exchange medium therethrough; means for establishing a bed of particulate material containing fuel in the first and second chambers; means associated with said partitions for permitting 10 flow of such material from the first to the second chamber and from the second to the third chamber; means respectively associated with each chamber for passing air therethrough to fluidize particulate material therein, promote the combustion of fuel in the first and second chambers, and to cool the 15 material in said third chamber; means for adding fuel to the bed in said first chamber to maintain continuous combustion in said first chamber; means associated with said third chamber to permit the continuous discharge of cooled particulate material from said third chamber; and means associated 20 with the first and second chambers for controlling the combustion of fuel therein such that the operating temperature in the second is greater than that in the first chamber.

2. A combustor according to Claim 1 wherein said controlling means comprises means associated with each of said 25 chambers for individually controlling the flow of air to and through each chamber.

3. A combustor according to Claim 2 wherein said controlling means is operable to maintain a higher air-tofuel ratio in the second chamber than in the first chamber. 30

4. A combustor according to any preceding Claim wherein the first chamber is divided into a plurality of sub-chambers, the controlling means being operable to vary the fluidization - 13 of material in the respective sub-chambers.

5. A combustor according to Claim 4 wherein the subchambers are arranged such that material flows successively from one to the next before passing to the second chamber.

6. A combustor according to any preceding Claim including means for igniting fuel in the second chamber and means for passing such ignited fuel from the second chamber to the first chamber during start-up of the combustor.

7. A combustor according to any preceding Claim including a single gas outlet associated with the housing for air and combustion gases from the chambers.

8. A conibustor according to Claim 7 including means for separating particulate material entrained in the mixture of air and combustion gases? and means for injecting into the second chamber any such particulate material.

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

10. A method of burning fuel in a fluidized bed combustor comprising the steps of establishing a bed of particulate material containing fuel in a first and a second chamber; passing air through each bed to fluidize the particulate material and promote the combustion of fuel contained therein; passing a relatively cool heat exchange medium through heat exchange tubes in the first chamber; maintaining the temperature in the second chamber at a value higher than that in the first chamber; permitting flow of the particulate material from the first to the second chamber and from the second chamber to a third chamber; passing air through said third chamber to cool the material in the third chamber, and permitting the cooled material to discharge from the third chamber. - 14

11. A method according to Claim 10 wherein no fuel is added to the second chamber other than that received from the first chamber so that the air passed through the bed in the second chamber promotes combustion of the fuel in the 5 second chamber to the extent that more complete fuel combustion is attained.

12. A method according to Claim 10 or Claim 11 wherein the maintaining step comprises individually controlling the flow of air to and through each of the first and second 10 chambers.

13. A method according to any of Claims 10 and 12 wherein an amount of air is passed to the second chamber sufficient to promote complete fuel combustion therein.

14. A method according to any of Claims 10 to 13 15. Including the steps of igniting fuel in the second chamber and passing such ignited fuel to the first chamber during start-up of the combustor.

15. A method according to any of Claims 10 to 14 including the step of discharging the mixture of air and combustion 20 gases from each chamber through a single outlet in the combustor housing.

16. A method according to Claim 15 including the step of separating particulate material entrained in said mixture and injecting such separated particulate material into the 25 second chamber.

17. A method according to any of Claims 10 to 16 wherein the bed material contains an adsorbent for the sulphur contained in the fuel; and wherein the first chamber is maintained at a temperature compatible with the adsorption of 30 said sulphur. 80817 - 15

18. A method according to any of Claims 10 to 17 wherein the first chamber comprises a plurality of subchambers arranged such that bed material flows successively from one to the next prior to passage thereof to the second 5 chamber.

19. A method according to Claim 18 wherein the passage of fluidizing air through the beds in respective sub-chambers is varied to accommodate different load requirements.

20. A method of operating a fluidized bed combustor 10 substantially as described herein with reference to the accompanying drawings.