GB2101723A - Fluidised bed boiler installations and methods of constructing such installations - Google Patents

Abstract

An existing boiler (10) is converted to fluidised bed operation by installing a water-cooled fluidised bed furnace (40) adjacent the boiler, connecting an exhaust combustion gas outlet (70) of the furnace (40) to a hot gas flow path (14,22,24) of the boiler so that the furnace exhaust gas flows through the hot gas flow path of the boiler, and connecting a water space (52) of the furnace (40) to a water space (28) of the boiler by means of flow and return headers (56,62). Existing firing means of the boiler may be removed, or it may be left in place so that firing can be effected in the furnace or in the boiler. <IMAGE>

Description

SPECIFICATION Fluidised bed boiler installations and methods of constructing such installations This invention relates to fluidised bed boiler installations, that is to say, to steam or hot water boiler installations designed to burn solid fuel (e.g. coal, wood, peat or waste material) and/or liquid fuel and/or gaseous fuel using the known fluidised bed combustion process, and to methods of constructing such installations, The burning of coal, other solid fuels, liquid or gaseous fuels, or a combination of such fuels using the fluidised bed combustion process has certain significant advantages over conventional combustion techniques. The advantages are set forth below.

(A) When water or some other fluid to be heated is disposed within a heating surface arranged so as to be immersed in a fluidised bed in which combustion is taking place, high and consistent heat transfer rates can be obtained. This enables the absorption of heat within apparatus of smaller size than apparatus absorbing the same amount of heat from a conventional firing process.

(B) The absorption of heat from within a fluidised bed as described in (A) above means that the combustion temperature of the fluidised bed can be maintained below the fusion temperature of the coal ash, in the case when the fuel is coal. A typical operating temperature of a fluidised bed having immersed therein a heating surface as mentioned above is 900"C; and at this temperature the ash will not fuse or form clinker. Instead, it remains soft and of small particle size and will therefore readily mix with the bed meterial and can easily be drained off for disposal.

(C) By adding dolomite or limestone to the bed the sulphur content (if any) of the fuel will be retained in the bed and not discharged in the exhaust combustion gases to pollute the atmosphere.

Because of the increasing shortage of oil and gas fuels and the need to replace these fuels with coal, and because ofthe advantages of burning coal or other fuels using the fluidised bed combustion process, much effort has been expended on designing new boilers having combustion or furnace chambers designed expressly to suit the fluidised bed combustion process. In general, these new designs of boiler are different from traditional boiler designs. In particular, the new designs differ from the traditional horizontal shell type boiler which has a horizontally extending cylindrical furnace chamber or section.

The cylindrical furnace chamber is not particularly suitable for the fluidised bed combustion process.

Neither is it particularly suitable to be modified or adapted to use the fluidised bed combustion process.

Problems involved in attempting to use the fluidised bed combustion process in a cylindrical combustion chamber are as follows.

(i) It is difficult to provide sufficient heating surface within the bed to absorb the required quantity of heat as described in (A) above.

(ii) It is difficult to provide a fluidised bed grate of sufficient area along the bottom portion of the cylin drical furnace.

(iii) The limited height above the fluidisation level and the limited volume along the length of the bed adversely effect the flow of combustion gases away from the surface of the bed.

Thus, for the reasons outlined at (i), (ii) and (iii) above, it is not desirable to modify existing horizon tai shell type boilers by adapting the furnace chamber or section and arranging for the fluidised bed combustion process to take place within the fur nace chamber. Thus, hitherto, the owner or user of an existing horizontal shell type boiler who wished to change from the conventional firing of oil, gas or coal to the firing of solid or other fuel using the fluid ised bed combustion process could do so only by replacing the existing boiler with a new, purpose designed boiler.

According to the invention there is provided a method of constructing a fluidised bed boiler instal lation, the method comprising installing an at least partially water-cooled fluidised bed furnace, having heating surface within its bed, adjacent an existing boiler, connecting an exhaust combustion gas outlet of the fluidised bed furnace to a hot gas flow path of the boiler whereby exhaust gas from the fluidised bed furnace will, in use, traverse at least part of said path, and connecting a water space ofthefluidised bed furnace to a water space of the boiler by means of flow and return headers.

The invention thus enables the use of a fluidised bed combustion process in association with a boiler which may not have been designed to use such process and which may well be unsuitable for usin such process.

The method ofthe invention may be applied to a variety of types of boiler, though the method is particularly applicable to a conventional horizontal shell type boiler, for instance a multi-pass horizontal shell type boiler. The exhaust gas outlet of the fluidised bed furnace may be connected to a furnace chamber of the boiler, whereby gas from the fluidised bed furnace will traverse substantially the same path as that intended to be taken by gas resulting from com bustion within the furnace chamber, or at least part of such path.

The method may comprise the removal of existing firing means of the boiler whereby firing can take place solely within the fluidised bed furnace. Alter natively, the fluidised bed furnace and boiler may be so united to form the installation that firing may take place within the fluidised bed furnace or within the boiler. For example, in an existing boiler having a horizontally-extending furnace chamber, the fluid ised bed furnace exhaust outlet may be connected to one end of the furnace chamber in such a manner as not to interfere with the operation of existing firing means (e.g. a gas burner, oil burner or grate) within the furnace chamber.

The fluidised bed furnace may comprise a combustion chamber, a water space surrounding the combustion chamber, and thermal syphon tubes each extending through the combustion chamber from a relatively low to a relatively high part of the water space to promote the movement of water upwardly of the water space. Each thermal syphon tube preferably comprises a first portion which is inclined to the horizontal and positioned to be disposed at least partly in the fluidised bed when the furnace is in use, and a second portion extending substantially vertically upwardly from the first portion.

The invention also provides a fluidised bed boiler installation comprising a boiler and an at least partially water-cooled fluidised bed furnace installed adjacent the boiler, wherein an exhaust combustion gas outlet of the fluidised bed furnace is connected to a hot gas flow path of the boiler whereby exhaust gas from the fluidised bed furnace will, in use, traverse at least part of said path, and wherein a water space of the fluidised bed furnace is connected to a water space of the boiler by means of flow and return headers.

In embodiments of the invention described below the fluidised bed furnace is fired by solid fuel. However, it should be noted that it is within the scope of the invention for the furnace to be fired by solid liquid fuel or gaseous fuel or any combination of same.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying somewhat schematic drawings, in which the same references designate the same or similar items throughout, and in which: Figure 1 is an axial sectional view through a known horizontal shell type boiler; Figure 2 is a transverse sectional view ofthe boiler of Figure 1; Figure 3 is a sectional view through a fluidised bed furnace; Figure 4 is a side view of the furnace of Figure 3 as viewed from the right in Figure 3; Figure 5 is a sectional view of the furnace of Figure 3 taken along the line V-V in Figure 3; Figure 6 is a scrap view corresponding to the left hand portion of Figure 5 and showing a modification that can be made to the furnace of Figures 3 to 5;; Figure 7 is an end view of the portion of the modified furnace shown in Figure 6; Figure 8 shows a first boiler installation formed by operatively connecting an existing horizontal shell boiler and the fluidised bed furnace; Figure 9 is a section through the boiler installation of Figure 8, taken along the line IX-IX in Figure 8; Figure 10 shows a second boiler installation formed by operatively connecting an existing hori zontal shell boiler and the fluidised bed furnace; Figures 11 to 15 schematically shows different arrangements for connecting an existing boiler to a fluidised bed furnace; Figure 16 shows part of the arrangement of Figure 15 in more detail; Figure 17 is a perspective view of another boiler installation embodying the invention; and Figure 18 is a sectional view of another embodiment of the invention, generally similar to that shown in Figure 17.

Figures 1 and 2 show a typicai horizontal shell type boiler. The boiler comprises a horizontally extending cylindrical shell 10 having therein a cylindrical furnace 12 defining a cylindrical furnace chamber or section 14. An oil or gas burner shown generally at 16 is arranged at one end of the furnace chamber 14.

Front and rear refractory-lined gas transfer chambers 18,20, respectively, are arranged at front and rear ends of the shell 10, respectively. The rear gas transfer chamber 20 is arranged to direct exhaust gas emerging from the rear end ofthe chamber 14 into a first bank of small bore convection tubes 22 that extend parallel to one another through the shell 10, parallel to the axis thereof. At their opposite ends the tubes 22 debouch into the front gas transfer chamber 18, which redirects the gas into a second bank of small bore convection tubes 24 extending parallel to the bank oftubes 22 at a higher position within the shell 10. The opposite ends of the tubes 24, i.e. the right hand ends in Figure 1, debouch into an upper portion of the rear gas transfer chamber 20 from which they are directed to a gas exit 26.

In use, the shell 10 contains water, the inside of the shell 10 comprising a water space 28 surmounted by a steam space 30, the water level being shown at 32.

A steam offtake 34 is disposed at the top of the steam space 30 where shown. The furnace chamber 12 and the tubes 22 and 24, which cooperate to form a hot combustion gas flow path, are sealed from and in heat exchange relationship with the water within the water space 28.

The illustrated boiler is of three pass design. That is to say, heat exchange takes place between the hot combustion gases and the water in three passes. The first pass takes place within the furnace chamber 14, the second pass takes place within the tubes 22, and the third pass takes place within the tubes 24.

In embodiments of the invention described below, the horizontal shell type boiler described with referpence to Figures 1 and 2 is modified to form a fluidised bed boiler installation. It should be clearly appreciated, however, that the invention may be applied to other types of horizontal shell type boiler and indeed to completely different types of boiler.

Various modifications with which the invention can be employed will now be outlined briefly, though it will be appreciated by those skilled in the art, in the light of the following disclosure, that other types of boiler can be used.

One other design that can be employed comprises a modification of the illustrated boiler which burns solid fuel, (e.g. coal), the oil or gas burner 16 in this case being replaced by a horizontally-extending grate 36 disposed where shown by the dotted line in Figure 2. The grate would be located as shown in the bottom part of the furnace chamber 14 and would extend along the furnace chamber for about twothirds to three-quarters of the length of the chamber.

Coal or other solid fuel is burnt on the grate in a conventional manner, the combustion gas flow through the boiler being as described above.

Other types of horizontal shell boiler with which the invention can be employed comprise a single pass design (furnace chamber only), a two pass design wherein the furnace chamber is followed by a single bank of convection tubes, in which case the gas exit 26 will be located at the left-hand end of the boiler as shown in Figure 1, or a four pass design in which the furnace chamber is followed by three convection tube banks, in which case the gas exit would again be located at the left-hand end of the boiler. In another modification, the rear gas transfer chamber 20 may be contained within the boiler shell 10 and therefore be water cooled.

In a manner described below, a fluidised bed furnace can be installed to cooperate with the boiler of Figures 1 and 2 to form a fluidised bed boiler installation. The way in which these two components are conjoined to form the installation will be described below. Firstly, however, the furnace itseif will be described with reference to Figures 3 to 7.

The fluidised bed furnace, generally designated by the reference numeral 40, comprises a combustion chamber 42 having at the bottom thereof a fluidised bed grate 44 supplied with a combustion and fluidising air supply via a duct 46. A fluidised bed 48 is disposed above the grate 44 and is fed with solid fuel (e.g. coal) via a vertical fuel feed tube 50 which extends through a water space 52 that surrounds the combustion chamber 42.

Water is fed to the bottom of the water space 52 by means of a pair of water distribution headers 54 arranged on opposite sides of the combustion chamber 42 and connected to a large bore main water feed header 56. The headers 54 are connected to the water space 52 by spaced arrays of tubes 58.

Thermal syphon tubes 60 extend through the combustion chamber 42 from relatively low to relatively high parts of the water space 52. As can best be seen from Figure 3, each thermal syphon tube 60 comprises a lower portion disposed substantially within the fluidised bed 48 and inclined slightly to the horizontal, such portion being followed by a substantially veritcally upwardly directed portion.

A water outlet header 62 is connected to the top of the water space 52 by a spaced array of tubes 64.

At its opposite ends, the combustion chamber 42 is provided with respective refractory lined doors 66.

Exhaust tubes 67 extend up from positions near the top of the combustion chamber to direct the hot combustion gases to a refractory lined transfer duct 68 which extends as shown from adjacent the outlet ends of the exhaust tubes 67 to conduct exhaust combustion gas out of the furnace 40, the transfer duct 68 having a cylindrical outlet portion 70. An isoiation damper 72 is arranged within the transfer duct 68.

The mode of operation ofthefluidised bed furnace 40 as described with reference to Figures 3 to 5 will now be explained. Water supplied via the header 56 and the water distribution headers 54 flows upwardly through the water space 52, where it is heated, and out into the header 62. Thus, heating of the water takes place around the fluidised bed 48; in fact around substantially the whole of the exterior of the furnace chamber 42. Heat exchange also takes place, i.e. heat absorption surface is also provided, within the fluidised bed 48. The thermal syphon tubes 60 provide heat absorption surface and also act as means to promote a strong flow of water upwardly within the water space 52.The induced circulation provided by means of the thermal syphon tubes 60 is augmented by supplying cool water to the bottom of the water space 52 by means of the water distribution headers 54 and positioning the heated water supply outlet header 62 at the extreme top of the water space 52.

The exhaust combustion gases leave the chamber 42 via the bank of small bore exhaust tubes 67, enter the transfer duct 68 and leave the transfer duct 68 via the cylindrical outlet portion 70 thereof.

Figures 6 and 7 illustrate a modification of the furnace of Figures 3 to 5. In the modification, the external water distribution headers 54 are replaced by water distribution headers 54' disposed within the bottom of the water space 52.

Figures 8 and 9 show one way by which a boiler as shown in Figures 1 and 2 may be adapted by the association therewith of a fluidised bed furnace 40 as described with reference to Figures 3 to 7 to constitute a fluidised bed boiler installation. The original firing means of the boiler, namely the oil or gas burner 16 or the grate 36, is removed. The furnace 40 is installed adjacent the boiler, such that the outlet 70 of th exhaust gas transfer duct 68 is connected directly to the cylindrical furnace chamber 14. The water space 28 of the boiler and the furnace 40 are connected by two large bore headers, namely the header 56 and a further header 74, the header 56 being disposed at a low level and the header 74 being disposed at a high level. At the furnace 40, the header 74 is connected to the header 62.In a manner shown only in detail for the header 74 - see Figure 8 - both the headers 74 and 56 are connected to the boiler water space 28 by a spaced array of pipes 76.

In use of the installation of Figures 8 and 9, hot combustion gas from the furnace 40 passes along the existing cylindrical furnace chamber 14 of the boiler and then through the small bore convection tubes 22, 24 so as to follow a like path to that followed by exhaust gases when the boiler was fired by a conventional technique as described with reference to Figures 1 and 2. Due to the difference in water density in the boiler between the top and bottom of the water within the water space 28, and the positive water circulation provided by the heating surface in the furnace 40, boiler water will flow along the large bore water header 56 connected to the boiler at low level and from there into the water distribution headers 54 at the bottom of the furnace 40.

The water then flows upwardly through the water space 52 of the furnace 40 and flows from the water outlet header 62 at the top of the furnace 40 into the large bore header 74 connected to the boiler at high level.

In Figure 9, two further large bore headers 74' and 56' are shown by dotted lines. The headers 74' and 56' represent opposite handing and may be emp loyed in place of the header 74 and 56 if required.

That is, in both cases, the two headers employed will be disposed on opposite sides of the centre line of the boiler. In some instances, it may be appropriate to employ all four of the headers 56, 56', 74 and 74', in which case the headers 56 and 56' both extend from a position towards the bottom of the water space 28 to supply water to the bottom of the fur nace 40 and both of the headers 74,74' extend from a position towards the top of the water space 28to receive heated water from the top of the furnace 40.

Figure 10 shows an installation similar to that shown in Figures 8 and 9. To avoid unnecessary repetition, the installation of Figure 10 will be described only in so far as it differs from that of Figures 8 and 9. The main diff( rence between the insta4- lation of Figure 10 and that of Figures 8 and 9 is that, in Figure 10, the oil or gas burner 16 is left in place and the outlet portion 70 of the gas transfer duct 68 of the furnace 40 is connected to the opposite end of the furnace chamber 14 to the burner 16, via the rear gas transfer chamber 20. In this case, therefore, hot gas from the furnace 40 flows directly into the first bank of small bore convection tubes 22 and thereafter follows the remainder of the hot gas flow path (i.e. the tubes 22 and 24) described with reference to Figure 1.

The installation of Figure 10 permits the existing gas or oil burner 16 (or, in an alternative arrange ment, the grate 36) to remain in position at the front end of the boiler and to be used when required. By means of a shut-off damper 78 in a duct 80 supplying air to the burner 16, and the shut-off or isolation damper 72 in the exhaust gas transfer duct 68, the operation of the installation can be selectively controlled. Thus, when the furnace 40 is fired ad the air damper 78 tightly closes offthe duct 80, hot gas flow from the furnace 40 will not travel down the furnace chamber 14 toward the burner 16. Conversely, when the boiler is fired and the damper 72 tightly shuts off the exhaust gas transfer duct 68, hot gas from the burner 16 will not flow into the furnace 40.In this way, an existing boiler can be converted to fire coal or other solid fuel using the fluidised bed combustion process and retain the original firing means, thus providing complete fuel flexibility and the possibility of continuing operation in the event of partial breakdown of the installation. The fact that there is no water flow through the furnace 40 when firing employing a conventional technique is employed is of no consequence.

Various arrangements for connecting together the water spaces 52 and 28 of the furnace 40 and the boiler will now be described with reference to Fig ures 11 to 16.

Figure 11 shows an arrangement in which a boiler water circulating pump 84 is connected in the header 56. Such an arrangement might be necessary or at least desirable in the case of small, highly rated existing boilers andxor where space precludes the use of header sizes sufficient to obtain natural circulation.

Figure 12 shows a natural circulation arrangement comprising four large bore headers 56a, 56b, 74a and 74b.

Figure 13 shows a water circulation arrangement that can be employed when the existing boiler is a fully flooded hot water unit. The furnace 40 is con nected to the boiler via a hot water header 74. A hot water system circulation pump 84 supplies cool sys tem return water from the boiler, via the system heating load 86, to the furnace 40 through the header 56 and distribution headers 54. and heated water is fed through the header 74to the boiler shell 10 where it is further heated by the combustion gases from the furnace 40, passing through the boiler heating surface as described previously.

Figure 14 shows how steam can be taken from the boiler and passed through a superheater coil or coils 88 located in the furnace chamber 42 of the furnace 40 either above or in the fluidised bed 48 to provide superheated steam in a pipe 90.

Figures 15 and 16 shows an arrangement in which the natural circulation between the boiler and the furnace 40 can be augmented by means of a boiler feed water pump 92 arranged as shown to pump feed water into the large bore supply header 56 via a nozzle 94 instead of directly into the boiler shell, as is usual. Such a technique is particularly advantageous when the feed water is cold, since the furnace 40 would act as a feedwater heater thereby avoiding thermal stresses within the existing boiler.

The invention can, of course, be embodied in many other ways than those described above by way of example. Various forms of modification have been indicated above in outline. Also, for example, the furnace 40 can be modified essentially by rotating the combustion chamber 42 through 90" about a vertical axis. Such an arrangement may, for example, be desirable when it is wished to have the combustion chamber 42 in line with the longitudinal centreline of the existing boiler instead of being at right angles thereto, as in the arrangement described above, for reasons of bed withdrawal.

Two modified embodiments will now be described with reference to Figures 17 and 18, which show respective boiler installations similar to those described above, except that the fluidised bed furnace 40 in each case is a water tube furnace. The furnaces 40 of the installations 17 and 18 are in fact substantially the same, execpt as regards the arrangement of the thermal syphon tubes 60. A water combustion chamber 42 is defined by an enclosure of water tubes, the enclosure being of membrane or tangent wall construction and having an end wall and roof portion 95, side walls 96 and another end wall 97. The end wall 97 is so constructed as to have gaps between the water tubes thereof at the upper part of the wall only so as to permit combustion gases from the chamber 42 to pass through the wall into the exhaust gas transfer duct 68.

In Figure 17 the thermal syphon tubes 60 are arranged in alternate rows extending from the bottom headers 56 to each of the top headers. In Figure 18, the thermal syphon tubes extend from one of the headers 54 to the header 62 as shown.

Claims (22)

1. A fluidised bed boiler installation comprising a boiler and an at least partially water-cooled fluidised bed furnace installed adjacent the boiler, wherein an exhaust combustion gas outlet ofthe fluidised bed furnace is connected to a hot gas flow path of the boiler whereby exhaust gas from the furnace will, in use, traverse at least part of said path, and wherein a water space of the fluidised bed furnace is connected to a water space of the boiler by means of flow and return headers.

2. An installation according to claim 1, wherein firing means has been removed from the boiler and the exhaust combustion gas outlet of the fluidised bed furnace is connected to the boiler in the vicinity of the position where the firing means was located.

3. An installation according to claim 1, wherein the boiler comprises a horizontally-extending furnace chamber having firing means at one of two ends thereof, the exhaust combination gas outlet of the fluidised bed furnace being connected to the other of said two opposite ends.

4. An installation according to claim 1, claim 2 or claim 3, wherein the water space ofthe fluidised bed furnace surrounds a combustion chamber of the furnace and a plurality ofthermal syphon tubes each extend through the combustion chamber from a relatively low to a relatively high part of the water space.

5. An installation according to claim 1, claim 2 or claim 3, wherein a combustion chamber of the fluidised bed furnace has walls defined by water tubes and a plurality of thermal syphon tubes each extend generally upwardly through the combustion chamber between said flow and return headers.

6. An installation according to claim 4 or claim 5, wherein lower parts of the thermal syphon tubes are positioned to extend through the fluidised bed when the installation is in use.

7. An installation according to claim 6, wherein the thermal syphon tubes each comprise a first portion which is inclined to the horizontal and positioned to be disposed at least partly in the fluidised bed when the furnace is in use, and a second portion extending substantially vertically upwardly from the first portion.

8. An installation according to any one of the preceding claims, wherein water circulation occurring, in use, between the boiler water space and the furnace water space is caused at least partially by convection engendered by heat absorption in the furnace.

9. An installation according to any one of the preceding claims, wherein water circulation occurring, in use, between the boiler water space and the furnace water space is caused at least partially by a water circulation pump.

10. An installation according to any one of the preceding claims, wherein the boiler is a shell boiler.

11. An installation according to claim 10, wherein the shell boiler is a multi-pass horizontal shell boiler.

12. An installation according to claim 10 or claim 11, wherein the boiler is a steam boiler.

13. An installation according to claim 10 or claim 11, wherein the boiler is a fully flooded hot water boiler.

14. A fluidised bed boiler installation comprising a boiler and an at least partially water-cooled fluidised bed furnace installed adjacent the boiler, the installation being substantially in accordance with any of those installations particularly described herein with reference to the accompanying drawings.

15. A method of constructing a fluidised bed boiler installation, the method comprising installing an at least partially water-cooled fluidised bed furnace adjacent an existing boiler, connecting an exhaust combustion gas outlet of the fluidised bed furnace to a hot gas flow path of the boiler whereby exhaust gas from the furnace will, in use, traverse at least part of said path, and connecting a water space of the fluidised bed furnace to a water space of the boiler by means of flow and return headers.

16. A method according to claim 15, wherein a firing means of the boiler is removed and the exhaust combustion gas outlet of the fluidised bed furnace is connected to the boiler in the vicinity of the position where the firing means was located.

17. A method according to claim 15, wherein the boiler comprises a horizontally-extending furnace chamber having firing means at one of two ends thereof and the exhaust combustion gas outlet of the fluidised bed furnace is connected to the other of said two opposite ends.

18. A method according to claim 15, claim 16 or claim 17, wherein the boiler is a shell boiler.

19. A method according to claim 18, wherein the shell boiler is a multi-pass horizontal shell boiler.

20. A method according to claim 18 or claim 19, wherein the boiler is a steam boiler.

21. A method according to claim 18 or claim 19, wherein the boiler is a fully flooded hot water boiler.

22. A method of constructing a fluidised bed boiler installation, the method being substantially in accordance with any one of those methods particularly described herein with reference to the accompanying drawings.