GB1592847A

GB1592847A - Fluid-bed furnaces

Info

Publication number: GB1592847A
Application number: GB3816476A
Authority: GB
Original assignee: Worsley & Co Ltd G P
Current assignee: Worsley & Co Ltd G P
Priority date: 1977-12-14
Filing date: 1977-12-14
Publication date: 1981-07-08

Description

(54) FLUID-BED FURNACES (71) We, G. P. WORSLEY & COMPANY LIMITED, a British Company of Haydock Lane Works, Haydock, St.

Helens, Merseyside WAll 0UU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The invention relates to fluid-bed furnaces where combustion promoting gas, normally air, is forced through and "ííuidises", that is maintains in motion, a bed of particulate material at least a proportion of which is combustible normally coal, or even a liquid fuel.

A requirement of fluid-bed furnaces is to maximise heat output and thus the rate at which combustible material is burnt in the bed. To do this, the bed should operate to burn the combustible material at a high temperature but that temperature should not rise so high that the bed material fuses, either generally or locally, to destroy the fluidised nature of the gas/material mixture.

Such fusing is most often due to foreign matter in the combustible material as basic incombustible bed material that is fluidised and acts as a carrier/diluent for the combustible material will normally be carefully chosen as to its constituents and particle size. We prefer to use a combustible material bed area that is large compared with the thickness or depth thereof, with gas release spread throughout the bed as by spaced apertured pipes or ducts passing into the bed.

Problems can arise, particularly but not exclusively, for shallow bed furnaces in relation to low grade fuels, where the particulate combustible material has a signficant, often high, inclusion of foreign incombustible matter, such as ash, stone or dirt. It is an object of this invention to facilitate the use of such feeds having a significant inclusion of foreign material.

According to the invention there is provided a fluid-bed furnace including fluidising means operative above a predetermined bed depth, means operable for removing material from below that depth of the bed, means for separating out relatively large size inclusions from removed material, and means for returning the remainder to the furnace. Removal of material may be continuous or intermittent and serves to remove foreign material, such as ash, clinker or dirt, that is dense enough to sink through the burning fluidised bed, preferably during operation thereof or during a shut-down period if desired.

Where, as is usual, the combustible particulate material is fed into a fluidised layer of incombustible particulate material, such as sand, it is often convenient for the material removal means to be below the highly fluidised upper layer of incombustible particulate material or at the bottom of such material or even positioned below that material with a feed or feeds thereto. Such removed material will obviously include a substantial, often major, proportion of the incombustible particulate material forming the basis of the bed and this is, as far as is practicable, removed from the unwanted foreign material, say by screening, and returned to the furnace proper, preferably via the fuel feed path or system for the combustible particulate material and may accompany such fuel feed to the furnace.

The removal means may be external of the combustion chamber proper of the furnace with communication thereto via apertures in the base of that chamber, which apertures may be in the form of selectively openable channels. These apertures may be relatively peripherally disposed compared w.:th an inlet or inlets for fuel and/or recycled bed material.

It is also preferred that carry-over of incombustible materials, such as grit, in the furnace exhaust or flue be trapped therein, say by a suitable enforced flow path, and returned to the material removal and/or recirculation means.

A shallow-bed furnace embodying this in vention is particularly well suited to ignition by over-firing using an oil or gas burner extending through a wall of the combustion chamber and which, if desired, may serve as an alternative or auxiliary furnace heater system, say in providing a multi-fuel capability to the furnace.

Whether or not the furnace is of the shallow bed form referred to above, it is further preferred that (as described and claimed in our copending application No.

27767/76), (Serial No. 1,592,846) a fluidbed furnace that is overfired at least for ignition purposes be traversed substantially evenly by fluidising gas release pipes or ducts, referred to herein as sparge pipes, and for a single gas supply to be used both for the auxiliary or ignition overfiring burner or burner system and for the sparge pipes with provision for alternative or selectively preferential routing of such flow, so that, at least on ignition there is sufficient flow to the overfiring burner system to generate bed ignition conditions, and that, subsequently, there is only as much flow as is required to keep the burner system cooled to prevent damage. In a multi-fuel arrangement, it may be necessary to have substantially the full flow directed to the overfiring burner system and little or none to the sparge pipes, though a combination of such nows may be preferred. For fluidbed operation similar flow conditions may be applied for ignition with an initial flow to the sparge pipes sufficient to agitate at least upper parts of the incombustible bed material to promote heat distribution therein, and temperature sensing to control maximum desired diversion to the sparge pipes when ignition temperatures are detected.

Preferably the gas supply system is in the form of a forced draught fan at one side of the furnace with adjustable flow control ducting (or valve means therein) to the burner system and valved or otherwise adjustable flow to the sparge pipes.

Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view through a fluid-bed furnace showing both the combustion chamber and the exhaust arrangement; Figure 2 is a cross-section through the combustion chamber at the position of fluid flow supply and particulate material removal recirculation; Figure 3 is a sectional view through the combustion chamber of another fluid-bed furnace with gas ignition; and Figure 4 is a cross-section through a preferred form of sparge pipe.

In Figures 1 and 2, the fluid-bed furnace has a combustion chamber 10 of elongated, substantially circular cyindrical form, though other sectional shapes could be employed, and will normally be a refractory lined metal casing. A lower sector or part of this combustion chamber is shown at 11 as being of relatively reduced thickness and supports a bed or layer 12 of incombustible particulate material, specifically sand. This sectoral part 11 could be replaced by a differently shaped holder of any suitable design, for example a U-shaped well.

At each end of the lowermost part of the combustion chamber, apertures 13 and 14 are shown communicating with a chamber or channel 15 that is partitioned at 16 and carries a rotatable shaft 17 bearing two oppositely directed auger blades 18 and 19 located on opposite sides of the partition 16. On each side of the partition 16, the chamber or channel 15 is apertured at its lowermost part and joined by ducting 21 discharging via a common duct 22 onto a conveyor system 23. Drive means for the shaft 17, and thus the auger blades 18 and 19, is indicated generally at 25, and may take any suitable form. In operation, the auger blades will serve to translate material towards the ducting 21 from both sides of the partition 16.

Within the combustion chamber 10 the sand bed 12 is indicated as having an upper layer 26 to be highly fluidised and carry combustible material typically coal and which, in operation of the furnace, may be fluidised with or without further substantial disturbance of the lower parts of the sand bed 12. To achieve this, four fluidisation gas release ducts or sparge pipes 27 are shown at the bottom of the layer 26 and extend generally longitudinally of the combustion chamber 10 from one end wall 29 thereof via apertures 30 therein. These sparge pipes are shown as being at least partially within the layer 26 of which the sand at rest will preferably extend above the sparge pipes to a depth of at least equal to half the sparge pipe depth. The sparge pipes are apertured in any desired way, or otherwise formed, so as to release pressurised gas, typically air, into the layer 26 substantially evenly throughout the bed so as to agitate and maintain fluidlike movement of the material thereof. In order to promote sinking of relatively dense incombustible fuel inclusions, the sparge pipes may be additionally apertured or disposed to provide some degree of fluidisation or movement of the lower parts of the sand bed, thereby usually increasing the burning depth of the furnace. In the embodiment being described, see Figure 2, two lower sparge pipes are indicated to serve in promoting movement of the particles of that layer at a level below the layer 26 together with sinking foreign material associated with the fuel particles, so as to assist outflow of the sand and foreign material towards the outlets 13 and 14. It will, of course, be appreciated that the bottom of the combustion layer 26 may not be well defined by a sharp transition, particularly using the lower sparge pipes.

Sparge pipes with simple bored apertures have been found to be subject to blockage, particularly when using dirty or low grade fuels. It is therefore proposed that sparge pipes be slotted either transversely or longitudinally of their length at intervals and for distances suitable to avoid or reduce the incidence of blockage. A particular preferred construction for a sparge pipe is indicated in cross-section in Figure 4 and can be seen as comprising lower and upper flanged channel-like parts 33 and 34, respectively, that are selectively spaced one from another, as at 35, to release pressurised gas from within the sparge pipe to a position outside and generally upwardly of that pipe.

Specifically, the lower duct part 33 is shown as being for the most part in the at least relatively static sand and as having its outwardly directed flange 36 turn upwardly into the combustion layer 12 and then return with an acute angle towards an outwardly directed flange 37 of the upper part, to which it is connected, or from which it is spaced, at least at intervals suitable to provide the desired apertures 35. The flange 37 of the upper member 34 is also shown as being turned upwardly within and parallel to the downturn of the flange of the lower part so as to provide for air flow out of the sparge pipe substantially as arrowed. It will, of course, be realised that slotted or flanged sparge pipes as just mentioned are applicable generally to fluid bed furnaces, whether or not they incorporate other particular mentioned aspects of this invention. Such sparge pipes provide spaced distribution parts with a low likelihood of blockage and assist in obtaining good diffusion over a wide area.

Returning now to Figures 1 and 2, the side wall 29 of the combustion chamber 10 is further shown as being apertured at 51 to accommodate an oil burner 52 and associated air supply ducting 53 at a level above that of the combustible material 26.

At its upper part the combustion chamber 10 is shown with a fuel supply inlet 55 by which particulate fuel is supplied for fluidising and burning within the combustion chamber 10. As can be seen particularly from Figure 2, the fuel supply inlet 55 is supplied via a substantially sealing rotary feed device 56 from an entry serving both for fuel and recycled sand. To this end, the conveyor system 23 is shown as being taken past a fuel feed 58 so that fuel may be added by way of the conveyor system if desired in admixture. Preferably, extracts via the auger and ducting 21, 22 are screened for fuel debris or foreign matter associated therewith prior to its vertical runpast the fuel feed 58. If desired, however, such screening may be incorporated at the upper part of the conveyor system 23 prior to its discharging into the aperture 57.

Near its upper end the combustion chamber 10 is formed with an upwardly extending part 61 forming an exhaust flue taking hot exhaust gases up over an end wall 62 and to the other side thereof where a depending low wall or division 63 serves to direct such waste gases downwardly past the free edge thereof and then upwardly to a resumed direction of flow via outlet passageway 64 that is generally longitudinal of the combustion chamber and the furnace as a whole.

The chamber formed below the lower edge of the wall 63 is effectively a grit trap and has a lower wall substantially parallel and aligned with that of the combustion chamber and communicates via an outlet 66 with the right hand side of the auger housing chamber or channel 15.

Air supply to the furnace is shown to be by way of inlet 68 under the action of a fan 69 serving to force air into a plenum cavity 70 and through adjustable ducting 53 to the oil burner 52, and/or by way of a control valve arrangement 71 to the sparge pipes 27. If desired there may be separate air control valves 71 for each sparge pipe, or one for each of groups of the sparge pipes, or one for the entire sparge pipe system to enable even gas dispersal control and/or sectional working of the furnace as mentioned below.

In operation, air supplied to the sparge pipes 27 to fluidise the bed of combustible material is effectively heated and discharged via the exhaust system to carry away the energy gencrated by the furnace. With openings only at their upper parts or producing flow relatively upwardly thereof, the sparge pipes serve largely to fluidise the combustion layer and to leave the immediately under- lying incombustible layer of sand relatively unmoved, though some flow may be promoted at least at lower levels and towards the ends of the combustion chamber and the outlets 13 and 14. The intended conditions within the combustion chamber are such that there is a high maintained fluidity of a sand combustible material mixture and, at most, a relatively lower fluidity of the underlaying part of the sand bed. This type of furnace is particularly useful in supplying air for drying shredded or other material which can be entrained into the outflowing mass via a feed to the overall gas outlet 64.

The combustion chamber outlets 13 and 14 and grit chamber outlet 66 are shown as being gated to control supply to the chamber 15. When these outlets are open the augers 18 and 19 will ensure that there is a suitable flow of material both from the collected grit in the grit trap 65 and the sand bed 12.

It will be appreciated that the sparge pipes could run from side to side within the combustion chamber rather from end to end, and this would facilitate sectional working of the bed, i.e. with some of the sparge pipes cut off and, if desired, with optional positions for combustible fuel material to be fed to the bed.

On ignition, the oil burner is used to heat up the refractories of the boiler, including the sand bed which is subjected to at least a minimum sparge pipe gas supply sufficient to agitate the upper part of the sand bed and assist heat distribution.

Sand can, of course, be withdrawn and/or recycled at this time if required. The sand turbulence induced is sufficient to cause flowing of the upper sand layers which will thus maintain a level so that recycling will assist heat transfer to the sand bed. When a predetermined temperature is reached, the oil burner supply is cut off and the solid fuel introduced to be burnt, with the gas flow via the sparge pipes automatically increased to expand the fluidised layer of the bed and more vigorously agitate it to a full fluidlike state so that the solid fuel particles will be immediately dispersed from their admission position and distributed throughout the area and depth of the fluidised bed, so as to burn on contact with the preheated sand in the presence of the gas from the sparge pipes.

This latter gas flow should be appropriate to maintain fluidity and restrict temperature to avoid fusion of bed particles. It has been found satisfactory to operate the furnace chamber at a slight positive relative pressure up to the exhaust part beyond the grit trap with induction from that point due to extraction fans for the desired hot gas output.

It has been mentioned that the furnace chamber may take other forms and another particular form is shown in Figure 3, where a bed of incombustible material 70, is shown as being in a hopper-like container of substantially conical overall form with a lowermost outlet 71 for discharge of ash and any other foreign material that sinks through the sand. As with the previously described furnace, such discharge and/or carrying away of foreign material could be mechanised using an auger or other drive means associated with a recirculation system whereby material is added back to the furnace. Specifically, Figure 3 shows a discharge via a side inlet vent 73 that is selectively openable at 74 to admit pressurised gas, say from a plenium chamber, in order to at least assist removal of material via the pipe 71, when valve 75 is opened.

It will be noted that the furnace of Figure 3 is equipped specifically for gas ignition preheating using an electro-igniter 75 a fuel gas/air mixture supply 76 to an upper layer of sparge pipes. Control valving 77 is also shown for supply of fluidising air to a lower layer of sparge pipes 78 after ignition temperature is reached so as to provide sufficient flow of solid fuel combustion at the desired temperature and with a desired depth of burning bed.

If desired, of course, supply pipes to the sparge pipes could traverse the bed for preheating purposes as in our copending Application No. 27767/76 (Serial No.

1,592,846).

For ignition using the igniter 75, it will, of course, also be desired to supply a controlled flow of air via the valve 77.

The combustion chamber of the furnace of Figure 3 may have any desired crosssectional shape through the plane of the drawing and may, specifically, be equipped with a grit trap in its exhaust system. The furnace as a whole may resemble that of our above-mentioned copending application.

WHAT WE CLAIM IS:- 1. A fluid-bed furnace including, fluidising means opelative above a predetermined bed depth, means operable for removing material from below that depth of the bed, means for separating out relatively large size inclusions from removed material, and means for returning the remainder to the furnace.

2. A fluid-bed furnace according to claim 1, wherein said means is operative te remove material from the bottom of the bed.

3. A fluid-bed furnace according to claim 1 or claim 2, wherein said means includes a drivable element or system to transport material substantially parallel to the bed surface.

4. A fluid-bed furnace according to claim 3, comprising a worm screw as said drivable element.

5. A fluid-bed furnace according to claim 4, comprising two worm-screws each operative to take material towards an end or side of the furnace.

6. A fluid-bed furnace according to any preceding claim, wherein said means is disposed below a support for the bed material but with access thereto.

7. A fluid-bed furnace according to claim 6, wherein said access is selective via one or more openable or adjustable passages or vents.

8. A fluid-bed furnace according to claim 6 or claim 7, wherein the bed support has inclined surfaces chanelling material towards said access.

9. A fluid-bed furnace according to claim 8, wherein the bed support is of elongate generally U-section form.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 18 and 19 will ensure that there is a suitable flow of material both from the collected grit in the grit trap 65 and the sand bed 12. It will be appreciated that the sparge pipes could run from side to side within the combustion chamber rather from end to end, and this would facilitate sectional working of the bed, i.e. with some of the sparge pipes cut off and, if desired, with optional positions for combustible fuel material to be fed to the bed. On ignition, the oil burner is used to heat up the refractories of the boiler, including the sand bed which is subjected to at least a minimum sparge pipe gas supply sufficient to agitate the upper part of the sand bed and assist heat distribution. Sand can, of course, be withdrawn and/or recycled at this time if required. The sand turbulence induced is sufficient to cause flowing of the upper sand layers which will thus maintain a level so that recycling will assist heat transfer to the sand bed. When a predetermined temperature is reached, the oil burner supply is cut off and the solid fuel introduced to be burnt, with the gas flow via the sparge pipes automatically increased to expand the fluidised layer of the bed and more vigorously agitate it to a full fluidlike state so that the solid fuel particles will be immediately dispersed from their admission position and distributed throughout the area and depth of the fluidised bed, so as to burn on contact with the preheated sand in the presence of the gas from the sparge pipes. This latter gas flow should be appropriate to maintain fluidity and restrict temperature to avoid fusion of bed particles. It has been found satisfactory to operate the furnace chamber at a slight positive relative pressure up to the exhaust part beyond the grit trap with induction from that point due to extraction fans for the desired hot gas output. It has been mentioned that the furnace chamber may take other forms and another particular form is shown in Figure 3, where a bed of incombustible material 70, is shown as being in a hopper-like container of substantially conical overall form with a lowermost outlet 71 for discharge of ash and any other foreign material that sinks through the sand. As with the previously described furnace, such discharge and/or carrying away of foreign material could be mechanised using an auger or other drive means associated with a recirculation system whereby material is added back to the furnace. Specifically, Figure 3 shows a discharge via a side inlet vent 73 that is selectively openable at 74 to admit pressurised gas, say from a plenium chamber, in order to at least assist removal of material via the pipe 71, when valve 75 is opened. It will be noted that the furnace of Figure 3 is equipped specifically for gas ignition preheating using an electro-igniter 75 a fuel gas/air mixture supply 76 to an upper layer of sparge pipes. Control valving 77 is also shown for supply of fluidising air to a lower layer of sparge pipes 78 after ignition temperature is reached so as to provide sufficient flow of solid fuel combustion at the desired temperature and with a desired depth of burning bed. If desired, of course, supply pipes to the sparge pipes could traverse the bed for preheating purposes as in our copending Application No. 27767/76 (Serial No.

1,592,846).

10. A fluid-bed furnace according to claim

8, wherein the bed support is of generally conical form.

11. A fluid-bed furnace according to any one of claims 1 to 10, wherein the means for separating out is a screening apparatus.

12. A fluid-bed furnace according to claim 11 wherein the means for returning includes a conveyor system to the main fuel inlet of the furnace.

13. A fluid-bed furnace arranged and adapted to operate substantially as herein described with reference to and as shown in Figures 1 and 2 or Figure 3 of the drawings.