GB2072040A - Fluidized bed combustors - Google Patents

Abstract

In a fluidized bed, e.g. a combustor, in which a perforated plate (20) within a housing supports a bed of particulate material, in order to protect the plate from the high temperatures generated in the bed (18), a plurality of hollow, elongated members (40) extend upwards, normally perpendicular to the plate (20) and respectively register with the perforations in the plate (20). Each elongated member (40) has an inlet at one end for receiving air and an outlet at its other end for discharging such air in an upward direction. The air discharging from the outlets fluidizes the particulate material above the outlets, and the lengths of the elongated members (40) are such that the plate (20) is insulated from the high temperatures occurring in the bed (18). <IMAGE>

Description

SPECIFICATION Air distribution system for a fluidized bed combustor This invention relates to a fluidized bed combustor and more particularly, to an improved air distribution system forfluidizing particulate material in such a combustor.

The use of fluidized beds has long been recognized as an attractive way 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 arragement include a relatively high heat transfer rate, 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 combustior; of the fuel, the bed temperature has to be maintained at a relatively high value (normally 1500 - 1550F). This requires the bed-supporting plateto be constructed of an expensive material, such as stainless steel, which will withstand these temperatures. However, even when a material such as stainless steel is used, the high operating temperatures and thermal gradients occurring across the surface of the plate reduce the life of the plate considerably. As a result, the plate has to be frequently replaced which is costly both from a material and a iabor standpoint, and which results in a considerable amount of down-time.

It is therefore an object of the present invention to provide a fluidized bed combustor in which the bed-supporting plate can be fabricated from a low alloy material and yet will enjoy an increased life when compared to plates of similar, or even more expensive, materials utilized in similar prior art arrangements.

It is a further object of the present invention to provide a fluidized bed combustorofthe above type which has an air distribution system that insulates the bed-supporting plate from the high operating temperatures of the bed.

It is further object of the present invention to provide a fluidized bed combustor of the above type in which a stagnant area of particulate materials or insulating material is provided immediately above the bed-supporting plate for insulating the latter against the relatively high operating temperatures in the bed.

It is a still further object of the present invention to provide a fluidized bed of the above type in which the fluidizing air for the bed is introduced at an area extending above the plane of the bed-supporting plate to provide for an insulating space immediately above the latter plate.

Toward the fulfillment of these and other objects, the system of the present invention comprises a plate adapted to support a bed of particulate material and having a plurality of holes extending therethrough for receiving air for fluidizing the material. A plurality of hollow elongated members extend per pendicularto the plate and respectively register with the holes. Each elongated member has an inlet at one end for receiving air and an outlet at its other end for discharging the air in an upward direction to fluidize the bed material. The elongated members extend from the plate for a length sufficient to insulate the plate from the relatively high temperatures in the bed.

The above brief description, as well as further objects, features, and advantages, of the present invention will be more fully appreciated by reference to the following detailed description of the presently preferred but nonetheless illustrative embodiment in accordance with the present invention, when taken in connection with the accompanying drawings in which: Figure lisa vertical sectional view of the fluidized bed combustor of the present invention; Figure 2 is a perspective view depicting a portion of the combustor of Figure 1; Figure 3 is a front elevational view of a portion of the structure of Figure 2; and Figure 4 is an enlarged elevational view of a component drawn in Figure 3.

Referring to Figure 1 of the drawings,, the reference numeral 10 refers in general to a fluidized bed combustor of the present invention which may be in the form of a boiler, a heat exchanger or any similar type device. The combustor 10 consists of a front wall 12, a rear wall 14, and two sidewalls, one of which is shown by the reference numeral 16. The upper portion of the combustor 10 is not shown for the convenience of presentation, it being understood that it consists of a convection section, a roof and an outlet for allowing the combustion gases to discharge, in a conventional manner.

A bed of particulate material, shown in general by the reference numeral 18, is disposed within the combustor 10 and rests on a perforated plate 20 extending horizontally in the lower portion of the boiler. The bed 18 can consist of a mixture of discrete particles of inert material, fuel material such as bituminous coal, and an adsorbentforthesulfur released by the combustion of the fuel material.

An air plenum chamber 22 is provided immediately below the plate 20 and an air inlet 24 is provided through the rear wall 14 in communication with the chamber 22 for distributing air from an external source (not shown) to the chamber. A pair of air dampers 26 are suitably mounted in the inlet 24 for pivotal movement about their centers in response to actuation of external controls (not shown) to vary the effective opening in the inlet and thus control the flow of air into the chamber 22. Since the dampers 26 are of a conventional design they will not be described in any further detail. A bed light-off burner 28 is mounted through the front wall 12 immediately above the plate 20 for initially lighting off the bed 18 during startup.

A plurality of overbed feeders 30,32 and 34 are provided which extend through the sidewall 16. The feeders 30,32 and 34 receive relatively fine particulate coal from inlet ducts or the like, and are adapted to feed the relatively fine coal particles onto the upper surface of the bed 18 in a conventional manner. It is understood that feeders identical to the feeders 30,32 and 34 can also be provided through one or more of the front wall 12, the rear wall 14 and the other sidewall.

A plurality of elongated tubular members 40 extend through the perforations in the plate 20 and are better described in connection with Figures 2-4.

In particular, each tubular member 40 has a head potion 42 formed integral therewith which abuts against the lower surface of the plate 20 as shown in Figure 3. The outer surface of each tubular member 40 is threaded so as to receive a nut 44 which can be tightened against the upper surface of the plate 20 in order to secure the tubular members 40 in the positions shown. An inlet opening 46 is provided in the head portion 42 which registers with the bore of its tubular member 30.

The end of each tubular member 40 opposite that from the head portion 42 is provided with an outlet opening 48 which has a reduced diameter when compared to that of the bore of the tubular member 40 for reasons to be described in detail later. A tubular member 40 is provided for each of the perforations in the plate 20 and, as can be appreciated from a view of the drawings, air from the air plenum chamber 22, passes upwardly into the inlet openings 46 in the head portion 42 and flows into and through the tubular members 40 before discharging from the outlet openings 48 into the bed 18 at a plane above the plane of the plate 20.

In operation, the bed is started up by opening the dampers 26 associated with the air inlet 24 and air is distributed up through the compartment 22 into the inlet opening 46 formed in the head portions 42. The air then flows upwardly through the lengths of the tubular members 40 before it discharges through the outlet openings 48 and into the bed 18 at a level above the level of the plate 20. Thus, that portion of the particulate material in the bed 18 extending immediately above the outlet openings 48 is fluidized while the portion extending between the latter openings and the upper surface of the plate 20 remains dormant, or stagnet.

The light-off burner 28 is then fired to heat the material in the bed until the temperature of the material reaches a predetermined level at which time particulate fuel is discharged from the feeders 30,32 and 34, and onto the upper surface of the bed 18 as needed to insure a continuous replenishing of the fuel material as it burns off.

Afterthe bed 18 has been fludized and has reached a predetermined elevated temperature, the light-off burner 28 is turned off while the feeders 30,32 and 34 continue to distribute particulate fuel to the upper surface of the bed 18 in accordance with predeter mined feed rates. As a result of the fluidization of the particulate material 18 occurring immediately above the outlet openings 48 of the tubular members 40, the area immediately below the plane of the latter openings and extending to the plate 20 remains dormant and thus acts as an insulatorforthe plate 20. The relatively narrow outlet openings 48 in each tubular member 40 minimizes any backflow of particulate material into the tubular members.

Thus, the plate can be fabricated of a relatively low-cost, low alloy material and yet will enjoy a relatively long life when compared to plates of more expensive, heat-resistant material that are not so insulated.

It is understood that several variations can be made in the foregoing without departing from the scope of the invention. For example, the overbed feeders 30,32 and 34 can be replaced by inbed feeders which are located and constructed to introduce coal into the bed at locations below the surface of the bed. Also, insulating or refractoring material may be provided in the area extending between a plane defined by the outlet openings of the tubular members 40 and the upper surface of the plate 20 instead of the dormant particulate material. In this case the tubular members would act as anchors for the insulation or refractory material.

Other modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

Claims (13)

1. An air distribution system for a fluidized bed, said system comprising a plate adapted to support a bed of particulate material and having a plurality of holes extending therethrough, a plurality of hollow elongated members extending perpendicular to said plate and respctively registering with said holes, each elongated member having an inlet at one end and an outlet at its other end, and means for introducing air into the inlets of said elongated members, said air passing through said elongated members and discharging from said outlets to fluidize the particulate material disposed above said outlets, the length of said elongated members being such that they extend from said plates for a length sufficient to insulate said plate from the relatively high temperatures in said bed.

2. The system of claim 1 wherein each elongated member extends within a hole for the entire thickness of said plate.

3. The system of claim 2 wherein each elongated member has a head portion formed on said one end and adapted to engage the surface of said plate opposite the surface thereof which supports said material.

4. The system of claim 3 wherein said inlet is formed through said head portion.

5. The system of claims 1 or 3 further comprising a nut in threaded engagement with said elongated member and in abutment with the upper surface of said plate to secure said elongated member to said plate.

6. The system of claim 5 wherein said nut is welded to said elongated member.

7. The system of claims 1 or 6 wherein said elongated member is resistance-welded to said plate.

8. The system of claim 1 wherein the hollow portion of each elongated member had a circular cross section, and wherein the diameter of said outlet is less than that of said hollow portion to reduce the possibility of said material entering said elongated member.

9. The system of claim 1 wherein the particulate material above said outlets is fluidized by said air and wherein the particulate material below said outlets insulates said plate from said high temperature.

10. The system of claim 1 wherein the particulate material above said outlets is fluidized by said bed and further comprises an insulation material extending between said outlets and said plate.

11. The system of claim 1 wherein said air introducing means comprises means defining a chamber immediately below said plate, said chamber having an inlet connected to a source of air.

12. The system of claim 11 further comprising damper means associated with said outlet for controlling the flow of air into said chamber.

13. An air distribution system for a fluidized bed combustor substantially as described herein with reference to the accompanying drawings.