GB2315432A

GB2315432A - Fluidised bed furnace

Info

Publication number: GB2315432A
Application number: GB9615425A
Authority: GB
Inventors: Jacek Antoni Kostuch
Original assignee: Mortimer Technology Holdings Ltd; ECC International Ltd
Current assignee: Imerys Minerals Ltd; Mortimer Technology Holdings Ltd
Priority date: 1996-07-23
Filing date: 1996-07-23
Publication date: 1998-02-04
Anticipated expiration: 2016-07-23
Also published as: GB2315432B; GB9615425D0

Abstract

Hot fluidising gas is introduced into a bed of material to be treated along the line of arrow "X". Below the bed it passes through a ring of blades 21, which are at an angle to the vertical in order to introduce a toroidal, circular flow to the gas and particles. More particles can be added through the radial tubes 23. In order to avoid the build-up of fused solid on the blades 21, the zone of greatest heating is caused to be well above them. To this effect, fuel is introduced via pipe 49 and radial pipes 45, which are angled upwards at their outer ends, so clearing the blades 21. Treated particles are led up cone 35 to cyclone 41. To avoid over-heating of blocks 13, 45, coolant gas may be passed down pipe 33.

Description

FURNACES The present invention relates to furnaces. In particular, it relates to furnaces of the kind in which a toroidal fluid flow heating zone is established. Such furnaces are described for example in USP 4,479,920.

Generally, a hot gas is passed through gaps between angled blades or vanes in a ring of blades or vanes provided in the operational chamber of the furnace. The blade ring is formed in an annular gap between the wall of the chamber and a central block, eg. an upwardly pointing conical portion, located on the axis of the chamber. Gas flow is caused to follow a rotary path in a doughnut shaped region around the block and in individual swirls within the rotary path. This ensures efficient residence of and heat transfer to material, eg.

particulate material, to be heated in the gas flow.

Furnaces of the said kind may be used for the heat treatment of particulate material. However, we have found that where the feed particulate material comprises material which fluxes at temperatures at which it is to be heat treated unwanted build up of the fluxed material occurs in various parts of the furnace, especially on or around the central block. As illustrated hereinafter, such build up can cause a back pressure to occur which impedes the particulate feed system and/or extinguishes the burner employed to provide hot gas. Such build-up of material requires removal before the furnace can be suitably operated again. This necessitates termination of the process of use of the furnace and undesirably limits the duration of the process of use. Such limitation makes prior art furnaces of the said kind unsuitable for some continuous heat treatment processes.

According to the present invention there is provided a furnace of the kind in which a toroidal fluid flow heating zone may be established, the furnace including a chamber in which there is provided an inner block, and a ring of angled blades between the inner block and the inner wall of the chamber, means for delivering fluid into the chamber whereby the fluid passes through gaps between the said blades and establishes a toroidal fluid flow heating zone in the chamber above the ring of angled blades and means for injecting into the chamber in the region where the toroidal fluid flow is to be established feed particulate material and characterised in that means for cooling the inner block is also provided in the chamber.

The means for cooling the said block may conveniently comprise a radiation heat shield in the form of a shroud provided over the inner block. Conveniently, there may also be means for delivering coolant gas to the shroud to be applied over the surface of the block and to cool the shroud. The means for delivering coolant gas may comprise a gas inlet pipe or pipes extending from a region outside the furnace to the shroud. The gas inlet pipe(s) may extend from a region above or below or to the side of the shroud. The coolant gas may for example comprise air.

The inner block may, as in the prior art, comprise a portion whose cross-sectional area decreases along its axis in a direction away from the angled blade ring, eg.

the block may comprise an upwardly pointing conical or frusto-conical portion. The portion may be made of refractory material. The shroud may comprise a cover of similar shape, eg. an upwardly pointing frusto-conical portion spaced from the block. The means for delivering coolant gas to the shroud may include a coolant gas delivery pipe extending downwardly to the top of the shroud, eg. from the top of the furnace or extending upwardly, eg from the bottom of the furnace.

In the region above the ring of angled blades, the said chamber may have a wall having a frusto-conical portion which is connected at its narrower end to a product outlet pipe. Such an arrangement allows the use of steps or ledges on which accumulation of solid particulate material, or fluxed material formed therefrom, to be avoided.

In a preferred form of the present invention, the furnace according to the present invention may include means for injecting a fuel into the said chamber above the ring of angled blades. Such means per se is the subject of a copending UK patent application of even date by the present Applicants.

The fuel injected by such means may be reacted with a reacting fluid delivered into the said chamber in the usual manner through the gaps between the angled blades.

For example, the fuel may be a combustible fuel and the reacting fluid may be air or an oxygen containing fluid.

The advantage of this form of the present invention is that the delivery of excessively hot gas to the operational chamber via the ring of angled blades can be avoided. Where a rapid throughput of material is required to be processed the energy demands of the furnace are raised and hot gas at a temperature as high as 16000C to 17000C might be required. Such a temperature could cause damage to the ring of angled blades and neighbouring components and, in any case, will cause unwanted accretion of material being processed on the hot surfaces in the manner described hereinafter.

The said reacting fluid may be delivered in the preferred form of the invention at a temperature in the range 7000C to 9000C, especially 7000C to 8000C to provide a heating zone temperature of from 7500C to 10500C, eg. 9200C to 10200C.

The means for injecting fuel in the said preferred form of the invention may comprise a ring of fuel inlet tubes extending from a common joint or housing t which input fuel is applied via an inlet pipe, the inlet tubes ending in the operational chamber. Preferably, the tubes are upwardly pointing at their ends in the chamber providing jets of fuel which are injected into the main fluid flow to provide reaction to form a plasma which provides the heating zone in the toroidal flow..

The main fluid flow may comprise air which is preheated before delivery into the operational chamber through the gaps in the ring of angled blades. The heated air flow may be provided by combining with an excess supply of air a burning pre-heating fuel and exhaust gases produced by burning the pre-heating fuel.

The fuel which may be employed to provide preheating of the air flow and the fuel which may be injected directly into the operational chamber in the furnace according to the present invention may be the same or different fuels. Preferably, the two fuels are the same.

The fuel employed in the said preferred form of the invention may be natural gas. It could alternatively comprise fuel oil, pulverised coal, or combustibles obtained from lignitic materials.

In a method of use of the furnace according to the present invention the feed material may comprise a particulate material of a kind which fluxes at temperatures above about 8000C. For example, the feed material may comprise mineral particles, eg. clay such as kaolin, calcium carbonate or mica to be flash calcined using the furnace. Heat treatment of such materials in a furnace of the kind in which a toroidal fluid flow heating zone is established is the subject of a further copending UK Patent Application of even date by the present Applicants. The furnace may be adjusted so that the temperature of calcination is in the range 7500C to 10500C, eg. 9200C to 10200C.

By carrying out heat treatment of such a material in the furnace according to the present it is possible to avoid the problem of build-up of feed material and material produced therefrom and the need for consequent stoppage of the process and cleaning of the operational chamber of the furnace.

We have found that in the heat treatment of particulate materials of a kind which flux when heated, the above described problem of the build up of feed material and material produced by heating such material which occurs in prior art furnaces of the kind producing a toroidal fluid flow heating zone is caused by the following effect. Various critical internal surfaces in the furnace tend to overheat and thereby cause feed particulate material to adhere by fluxing/sintering at such surfaces causing unwanted accretion of material which has the deleterious effects described hereinbefore.

The furnace according to the present invention beneficially prevents such overheating occurring, especially at the said inner block. The said shroud provides a radiation heat shield for the said inner block and also allows cooling of the inner block by delivery of coolant gas for application over the surface of the inner block.

The coolant gas, eg. air, provides an insulating layer between the shroud, which may be made for example of stainless steel, and the inner block, which may be made for example of stainless steel or of refractory material. The coolant gas also serves to remove any conducted heat from the shroud thus improving its function as a radiation shield.

Also, the conical shape of the operational chamber enclosure leading to the product exit chamber may have a smooth profile with no internal ledges or low velocity zones which facilitates transfer of product material after heat treatment in the operational chamber of the furnace without significant accretion on the walls of the chambers. The application of fuel directly into the operational chamber through the said inlet means allows the required operational temperature in the chamber to be achieved without overheating of the ring of blades and the regions of the chamber wall and the inner block immediately above the ring of blades.

These measures ensure that the temperature of the critical surfaces inside the operational chamber of the furnace are kept below the sintering temperature of the feed particulate material to be treated in the furnace thereby avoiding significant accretion of material on such surfaces.

Embodiments of the present invention will now be described by way of example with reference to the Rccompanying drawings in which: Figure 1 is a cross-sectional side elevation of a prior art furnace of the kind producing a toroidal fluid flow heating zone.

Figure 2 is a cross-sectional side elevation of a furnace of the kind producing a toroidal fluid flow heating zone, the furnace embodying the present invention.

A prior art furnace producing a fluid flow heating zone of the toroidal kind is shown in Figure 1. An inclosure 1 has a top 2, a base 3 and a side wall 4. A structure 5 made of refractory material comprising stacked annular portions 5a, 5b, 5c, 5d and 5e is support by the base 3. An annular refractory portion 6 is provided between the portions 5a and 5b and an annular refractory portion 8 is provided between the portions Sa, 5b and Sc and covers an opening 4a in the side wall 4 hereby a passage 7 is provided inside the structure 5, the passage 7 communicating with a pipe 9 fitted to the side wall 4 at the opening 4a.

A tubular support 11 extends upwardly from the portion 6 through the passage 7. The support 11 carries R frusto-conical refractory portion 13 having an internal axial bore 14 and a refractory portion 15 located on top Df the portion 13 by a portion 15a which engages within the top of the bore 14. A support 17 is attached to the tubular support 11 near its upper end and to a flange 19 extending into the passage 7 from the portion 5c. A ring 21 of angled blades is provided in the narrow gap between the lower end of the portion 13 and the outer portion 5d.

The blades are of the form described in USP 4,479,920.

The ring 21 is supported between the portion 5d and the support 17.

The uppermost refractory portion 5e in the structure 5 is fitted to an outlet chamber 25 whereby the chamber 7a communicates with the outlet chamber 25. An outlet pipe 28 extends from the outlet chamber 25. An inlet pipe 27 extends from the top 2 of the enclosure 1 through the chamber 25 and extends into the chamber 7a.

In use of the furnace shown in Figure 1 hot gas from a burner (not shown) at the temperature required to establish heating in the chamber 7a is delivered into the passage 7 via the pipe 9. The gas passes through the gaps between the blades of the ring 21. A toroidal gas flow is thereby established near the ring 21 in the chamber 7a. Material to be heat treated in the furnace is introduced via the inlet pipe 27 into the heating zone provided by the toroidal flow. The powdered product formed by this process is eventually (after a residence time typically of a few seconds in the heating zone) transferred into the chamber 25 from the chamber 7a and is extracted by a cyclone (not shown) attached to the outlet pipe 28 where product solid material is separated from output gases.

In use of the furnace shown in Figure 1 for the calcining of kaolin powder at a temperature of above 8000C, eg. at 9500C, we found that unwanted deposits of material formed from the feed kaolin built up in various regions of the furnace, especially in the regions labelled R1, R2, R3, R4, R5 and R6 shown in Figure 1.

This problem has been solved as follows.

Figure 2 shows a furnace embodying the present invention for producing a fluid flow heating zone of the toroidal kind. In Figure 2 items which are similar to items in the furnace shown in Figure 1 have like reference numerals.

In Figure 2, an inverted conical shroud 31 is ?rovided to cover the conical refractory portion 15 and Part of the surface of the portion 13 below it. An inlet ube 33 extends downwardly along the axis of the inclosure 1 and enters the shroud 31. The uppermost portion 5e is shortened and a conical enclosure wall 35 extending from the top of the portion 5e is formed around he upper chamber 7a.

The upper end of the enclosure wall 35 has a tubular ieck 35a which is fitted through the top 2 of the inclosure 1 into a tubular connector 37 outside the enclosure. 1. An outlet pipe 39 is also fitted into the connector 37. The pipe 39 leads to a cyclone 41.

The upper part of the inlet tube 33 extends through he interior of part of the pipe 39, and the connector 37 nd the neck 35a into the conical enclosure 35.

The portion 13 in Figure 2 has no bore and is support by blocks 42, 43 which are stainless steel 3upport rings provided between the support 17 and portion L3. A ring of fuel inlet tubes 45 (two only shown in igure 2) is provided beneath the blocks 42, 43. The ubes 45 project upwardly at their inner ends into the chamber 7a above the ring 21. The tubes 45 are connected it a central joint 47 to which in turn is connected to a tingle inlet pipe 49 extending through the base 3, portion 6c and tubular support 11.

A series of feed material inlet tubes 23 (one only shown) is provided. The tubes 23 are spaced ircumferentially around the wall 4 and are fitted trough the wall 4 and the portion 5d to enter the chamber 7a.

In use of the furnace shown in Figure 2, hot air at i temperature below that required to provide a heating zone in the chamber 7a, eg. at a temperature of 7500C to 3000C, is delivered into the passage 7 in the direction Df the arrow X. The air is heated by a burning fuel in a turner (not shown) and allowing the burning fuel and hot exhaust gases produced thereby to be combined with the air flow to be heated. The hot air passes through the gaps between blades of the ring 21 and thereby forms a toroidal flow above the ring 21. Fuel is delivered along the inlet pipe 49 and is injected as jets into the chamber 7a via the connector 47 and tubes 45. The fuel undergoes a spontaneous reaction in the air flow in the chamber 7a thereby providing a plasma heating zone in the toroidal flow. This causes the location in the chamber 7a at which the heating zone is established to be elevated to a region where the fuel is burned, ie. clear of the narrow gap between the portion Sd and the base of the portion 13 and the blocks 42 and 43 thereby avoiding overheating of the blades of the ring 21 and the surfaces adjacent to the ring 21. Overheating of the ring 21 and adjacent surfaces is also avoided because the air is delivered at a temperature eg. 7500C to 8000C less than that, eg. 9200C to 10200C inside the chamber 7a.

Particulate material is injected via the inlet tubes 23 into the plasma heating zone established in the toroidal flow. Coolant gas is delivered down the inlet pipe 33 to the shroud 31 and is applied over the surface of the portion 15 and part of the portion 13. The coolant gas prevents overheating of the portions 13 and 15 and also of the shroud 31 and thereby eliminates a local hot surface on the block 15 on which accretion of particulate material can occur.

The treated particulate material, after suitable residence in the chamber 7a, eg. typically less than 0.1 seconds, is drawn upwardly through the conical enclosure 35 and into the outlet pipe 35. This product is extracted and separated from the gas stream containing it by the cyclone 41.

Significant build up of solid material from the particulate material being treated in the furnace shown in Figure 2 does not occur because of the differences in construction and operation applied in the case of Figure 2. The avoidance of excessively hot surfaces and internal steps and ledges in the operational chamber 7a and the product extraction arrangement avoids accretion of material on such surfaces and collection of deposits on such ledges as in the chamber 25 in the prior art furnace of figure 1.

Heat treatment processes may be operated in the furnace shown in Figure 2 in a continuous manner without the need for frequent cleaning of deposited material from inside the chambers of the furnace as with the prior art furnace shown in Figure 1.

The temperature in the chamber 7a enclosed by the enclosure wall 35 may be monitored, eg. by a thermocouple (not shown), fitted to the inside of the wall 35, and variations of the temperature from a required means may be employed as control signals to adjust the rate of delivery of fuel to the chamber via the fuel inlet tubes 45.

Claims

1. A furnace of the kind in which a toroidal fluid flow heating zone may be established, the furnace including a chamber in which there is provided an inner block, and a ring of angled blades between the inner block and the inner wall of the chamber, means for delivering hot fluid into the chamber whereby the hot fluid passes through gaps between the said blades and establishes a toroidal fluid flow heating zone in the chamber above the ring of angled blades and means for injecting into the chamber in the region where the toroidal fluid flow heating zone is to be established feed particulate material and characterised in that means for cooling the inner block is also provided in the chamber.

2. A furnace as claimed in claim 1 and wherein the means for cooling the said block comprises a radiation heat shield in the form of a shroud provided over the inner block.

3. A furnace as in claim 2 and wherein the furnace also includes means for delivering coolant gas to the shroud to be applied over the surface of the block.

4. A furnace as in claim 3 and wherein the means for delivering coolant gas comprises a coolant gas delivery pipe extending to the shroud from a region outside the furnace.

5. A furnace as in claim 1, 2, 3 or 4 and wherein the inner block comprises refractory material and has a cross-sectional area which decreases along its axis in a direction away from the ring of angled blades.

6. A furnace as in claim 5 and wherein the inner block comprises a conical or frusto-conical portion and the shroud comprises a cover of similar shape.

7. A furnace as in any one of the preceding claims and wherein the said chamber has a wall having a frustoconical portion leading without internal steps or ledges to a product outlet pipe.

8. A furnace as in any one of the preceding claims and wherein the furnace further includes means for injecting fuel into the said chamber in the region above the ring of angled blades.

9. A furnace as in claim 8 and wherein the said means for injecting includes a ring of circumferentially spaced fuel inlet tubes for injecting fuel into the chamber.

10. A furnace as in claim 9 and wherein the inlet tubes are connected to a common joint or housing connected to a common inlet fluid delivery pipe.

11. A furnace as in claim 8, 9 or 10 and wherein the furnace includes a source of hot fluid which in the chamber reacts with the said fuel, which source is arranged so as to apply the said hot fluid to the chamber via the ring of angled blades.

12. A furnace as in claim 11 and wherein the said source includes a heater for heating a fluid before delivery to the said chamber.

13. A furnace as in claim 12 and wherein the reacting fluid comprises air and the said heater includes means for burning fuel to pre-heat the air to be delivered to the chamber.

14. A furnace as in claim 11 or claim 12 or claim 13 and wherein in operation the fuel delivered to the chamber via the means for injecting fuel comprises a combustible fuel which spontaneously reacts with air in the chamber.

15. A furnace as in claim 11, 12, 13, or 14 and wherein the hot fluid be for reaction with the injected fuel is delivered at a temperature lower than the heating zone required to be established inside the chamber.

16. A furnace as in claim 15 and wherein the temperature of the chamber is monitored and the rate of delivery of the fuel and/or the reacting fluid is adjusted according to temperature variations from a required mean temperature.

17. A furnace as claimed in any one of claims 11 to 16 and wherein the fuel comprises natural gas.

18. A furnace as in claim 1 and substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.