IE921308A1

IE921308A1 - A method and apparatus for reducing the temperature of a¹flue gas stream

Info

Publication number: IE921308A1
Application number: IE130892A
Authority: IE
Inventors: Anthony William Stapleton; Gerard Christopher Slattery; Martin Anthony Redington; Martin Laurence Redington
Original assignee: Schwan Ltd
Priority date: 1992-04-23
Filing date: 1992-04-23
Publication date: 1993-11-03
Also published as: GB2266362B; GB2266362A; GB9308441D0; BE1004181A7

Abstract

Apparatus (1) for reducing the temperature of flue gases adiabatically in a flue gas stream comprises a duct (3) for the flue gas stream having an upstream flue gas inlet (20) and a downstream flue gas outlet (23). Nozzles (28) in the duct (3) adjacent the inlet (20) deliver a cooling water spray of droplet size which is not greater than 50 microns into the flue gas stream. An upstream baffle (35) formed by parallel spaced apart tubular baffle members (40) causes the water droplets to merge with the flue gases of the flue gas stream. The baffle (35) may comprise six baffle members (40) which form a baffle with a porosity of 50% and which occludes approximately 60% of the area of the flue gas inlet (20). Further baffle portions (36, 37) may be provided.

Description

The present invention relates to a method and apparatus for reducing the temperature of flue gases adiabatically in a flue gas stream.

It is essential in the treatment of a flue gas stream 5 that all noxious gases, vapours and particles should be removed from the flue gas stream prior to being discharged into the atmosphere. To remove some vapours and particles, for example, hydrochloric acid vapour from a flue gas stream, it is common to pass the flue gas through a counter current lime stream to react hydrochloric acid vapour in the flue gas stream with the lime to produce calcium chloride. However, in order to obtain a reaction with lime, it is essential that the hydrochloric acid vapour be at the dew point of hydrochloric acid. This requires reducing the temperature of the flue gas stream to the dew point of hydrochloric acid just before the flue gas stream is to be passed through the lime stream.

It is also essential that the temperature of the flue gas stream should not be reduced below the dew point of hydrochloric acid, and preferably, should be maintained just above the dew point of hydrochloric acid prior to reaching the lime stream in order to avoid the hydrochloric acid and other vapours condensing onto a duct through which the flue gas stream is passing. Condensing of such vapours on the duct carrying the flue gas stream causes undesirable corrosion of the duct. Furthermore, it is preferable that the temperature of the flue gas stream should be reduced with the minimum pressure drop in order to minimize any disturbance of the flow of flue gases through the flue gas duct.

Attempts to achieve such cooling of a flue gas stream have so far been unsuccessful without a relatively large related pressure drop occurring along the flue gas stream, generally of the order of 1,000 Pascals to 5,000 Pascals.

There is therefore a need for a method and apparatus for reducing the temperature of flue gases of a flue gas stream which substantially minimizes the pressure drop across the flue gas stream.

The present invention is directed toward providing such a method and apparatus.

According to the invention, there is provided a method for reducing the temperature of flue gases adiabatically in a flue gas stream, the method comprising the steps of: passing the flue gas stream through a duct means having a flue gas inlet and a flue gas outlet, introducing a cooling liquid spray of mean droplet size not greater than 50 microns into the flue gas stream in the duct means for reducing the temperature of the flue gas stream, the cooling liquid spray being introduced through a spray means adjacent the flue gas inlet, and merging the liquid spray with the flue gas stream by passing the flue gas stream carrying the liquid spray through a baffle means in the duct means without substantially affecting the level of turbulence of the flow of the flue gas stream for evaporating the liquid spray for reducing the temperature of the flue gas stream.

In one embodiment of the invention, the mean droplet size of the liquid spray is not greater than 40 microns. Preferably, the mean droplet size of the liquid spray is not greater than 30 microns. Advantageously, the maximum deviation of droplet size from the mean droplet size is not more than 10% of the mean droplet size.

In one embodiment of the invention, the cooling liquid spray is a water spray. Preferably, the cooling liquid spray is at a temperature not greater than 90°C. Advantageously, the cooling liquid spray temperature is in the range of 1°C to 60°C, and preferably, the cooling liquid spray temperature is approximately 10°C.

In one embodiment of the invention, the quantity of cooling liquid introduced into the flue gas stream is in the range of 10 χ IO6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required to 30 χ 10'6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required.

Preferably, the quantity of cooling liquid introduced into the flue gas stream is in the range of 15 χ 10*6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required to 25 x 10-6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required.

Advantageously, the quantity of cooling liquid introduced into the flue gas stream is approximately .9 χ 10-6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required.

In one embodiment of the invention, the temperature of the flue gas stream at the flue gas inlet of the duct means is in the range of 230°C to 270°C. Preferably, the temperature of the flue gas stream at the flue gas inlet of the duct means is in the range of 240°C to 260°C. Advantageously, the temperature of the flue gas stream at the flue gas inlet of the duct means is approximately 260°C.

In one embodiment of the invention, the temperature of the flue gas stream at the flue gas outlet of the duct means is in the range of 140°C to 205°C. Preferably, the temperature of the flue gas stream at the flue gas outlet of the duct means is in the range of 180°C to 205°C.

In another embodiment of the invention, the velocity of the flue gas stream through the duct means is in the range of 5 metres per second to 25 metres per second. Preferably, the velocity of the flue gas stream through the duct means is in the range of 10 metres per second to 20 metres per second.

In one embodiment of the invention, the moisture content of the flue gas stream just prior to entering the duct means is not greater than 10% by weight of the flue gas stream.

In another embodiment of the invention, the pressure drop across the flue gas stream along the duct means in the direction of flow from the inlet to the outlet is not greater than 50 pascals per M of length along the duct means. Advantageously, the pressure drop across the flue gas stream along the duct means in the direction of flow from the inlet to the outlet is not greater than 33 pascals.

In a further embodiment of the invention, the baffle means is a porous baffle means, and an upstream portion of the baffle means is provided adjacent the flue gas inlet, downstream of the spray means.

Preferably, the upstream portion of the baffle means occludes not more than 80% of the transverse area of the flue gas inlet when the flue gas inlet is viewed in an upstream direction through the upstream portion of the baffle means along a centre line of the flue gas inlet. Advantageously, the upstream portion of the baffle means occludes not more than 60% of the transverse area of the flue gas inlet when the flue gas inlet is viewed in an upstream direction through the upstream portion of the baffle means along a centre line of the flue gas inlet.

In one embodiment of the invention, the porosity of the baffle means does not exceed 50%.

Additionally, the invention provides apparatus for Ί carrying out the method according to the invention for reducing the temperature of flue gases adiabatically in a flue gas stream, the apparatus comprising: a duct means having a flue gas inlet and a flue 5 gas outlet for accommodating a flow of the flue gas stream therethrough, spray means mounted adjacent the flue gas inlet for introducing a cooling liquid spray of mean droplet size not greater than 50 microns into the flue gas stream in the duct means for reducing the temperature of the flue gas stream, and baffle means in the duct means extending transversely of the flow of flue gases for merging the liquid spray with the flue gas stream in the duct means without substantially affecting the level of turbulence of the flow of the flue gas stream for evaporating the liquid spray for reducing the temperature of the flue gas stream.

In one embodiment of the invention, the baffle means comprises a porous baffle means extending transversely in the duct means, an upstream portion of the baffle means being provided adjacent the flue gas inlet downstream of the spray means. Advantageously, the upstream portion of the baffle means occludes not more than 80% of the transverse area of the flue gas inlet when the flue gas inlet is viewed in an upstream direction through the upstream portion of the baffle means along a centre line of the flue gas inlet. Preferably, the upstream portion of the baffle means occludes not more than 60% of the transverse area of the flue gas inlet when the flue gas inlet is viewed in an upstream direction through the upstream portion of the baffle means along a centre line of the flue gas inlet.

In one embodiment of the invention, the porosity of 10 the baffle means does not exceed 50%.

In a further embodiment of the invention, the baffle means comprises a plurality of spaced apart elongated parallel baffle members extending transversely of the duct means. Advantageously, the baffle members are of circular cross section. Preferably, the baffle members are spaced apart a distance in the range of 75 mm to 250 mm centre to centre. Advantageously, the baffle members are spaced apart a distance in the range of 100 mm to 175 mm centre to centre, and preferably, the baffle members are spaced apart a distance of approximately 141 mm centre to centre.

In one embodiment of the invention, the duct means comprises an elongated duct defining a passageway extending longitudinally therethrough for accommodating the flue gas stream. Preferably, the flue gas inlet is formed by an opening extending in a plane transversely of the duct means. Advantageously, the passageway defined by the duct means is of rectangular, square or circular cross section.

The invention will be more clearly understood from the following description of a preferred embodiment thereof given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a cut-away perspective view of apparatus according to the invention for reducing the temperature of a flue gas stream, Fig. 2 is a plan view of the apparatus of Fig. 1, Fig. 3 is an end elevational view of the apparatus of Fig. 1, Fig. 4 is a sectional side elevational view of the apparatus of Fig. 1 on the line IV-IV of Fig. 2, Fig. 5 is a sectional plan view on the line V-V of Fig. 4, ίο Fig. 6 is a sectional end elevational view of a portion of the apparatus of Fig. 1, and Fig. 7 is a schematic representation of selected particles in a flue gas stream flowing through the apparatus of Fig. 1.

Referring to the drawings, there is illustrated apparatus according to the invention indicated generally by the reference numeral 1 for reducing the temperature of flue gases of a flue gas stream close to the dew point temperature of hydrochloric acid while at the same time maintaining the pressure drop across the flue gas stream to a minimum. The apparatus 1 reduces the temperature of the flue gas stream adiabatically. The apparatus 1 comprises a duct means, in this case, an elongated duct 3 defining a passageway 5 of rectangular cross section extending therethrough from an upstream end 6 to a downstream end 7 for accommodating the flow of the flue gas stream therethrough. The duct 3 is mounted on a framework 8 and comprises a lower wall 10 and an upper wall 11 joined by side walls 14 and 15. The upper wall 11 is formed by a pair of wall portions 12 and 13 joined by a connecting wall 16. The upstream end 6 of the passageway 5 is closed by a transverse front end wall 17 while the downstream end 7 of the passageway 5 is closed by a transverse rear end wall 18.

A flue gas inlet to the passageway 5 is provided by an inlet opening 20 in the front wall 17. The inlet opening 20 is of circular cross section and extends in a plane transversely of the passageway 5 and to the flow of the flue gas stream through the passageway 5 at the upstream end. A flange 21 extending around the inlet opening 20 from the front end wall 17 facilitates connection of the duct 3 to a flue gas duct (not shown) for delivering the flue gas stream into the passageway 5.

A flue gas outlet from the passageway 5 is provided by an outlet opening 23 in the wall portion 12 of the upper wall 11 adjacent the downstream end 7 of the passageway 5. The outlet opening 23 is of circular cross section and extends in the plane of the wall portion 12 of the upper wall 11 which, in use, is substantially transversely of the flow of the flue gas stream through the outlet opening 23. A flange 24 extends around the outlet opening 23 from the wall portion 12 for connecting the duct 3 to a downstream flue gas duct (not shown) for delivering the flue gas stream from the passageway 5 through a counter current lime stream (not shown) for reacting hydrochloric acid vapour in the flue gas stream with the lime. A •Ε 921308 secondary flue gas outlet provided by a secondary outlet opening 25 from the passageway 5 is provided in the upper wall 11 adjacent the outlet opening 23 for returning some of the flue gas for blending with the flue gas stream upstream of the duct 3. A flange 26 extends around the outlet opening 25 for connecting a duct (not shown) to the outlet opening 25 for returning portion of the flue gas stream.

In this embodiment of the invention, the internal 10 height of the side walls 14 and 15 adjacent the upstream end of the passageway 5 is 2 M. The internal width of the upper and lower walls 10 and 11 adjacent the upstream end of the passageway 5 is 2.5 M. Accordingly, the transverse cross sectional area of the passageway 5 adjacent the upstream end 6, in other words, adjacent the front end wall 17 is 5 sqM. The area of the inlet opening 20 transversely of the passageway 5 is 0.655 sqM. Accordingly, the area of the inlet opening 20 is approximately 0.131 times the transverse area of the passageway 5 adjacent the upstream end 6. The area of the outlet opening 23 transversely of the flow of the flue gas stream is 0.442 sqM. The area of the secondary outlet opening 25 transversely of the flow of the flue gas stream is 0.213 sqM. Accordingly, the sum of the areas of the outlet openings 23 and 25 is 0.655 sqM, namely, similar to the area of the inlet opening 20. The internal length of the side walls 14 and 15 and in turn the passageway 5 is 3.56 M.

Spray means for introducing a cooling liquid spray, namely, an atomised water spray into the passageway 5 for reducing the temperature of the flue gas stream as droplets of the spray evaporate comprises a pair of spray nozzles 28 mounted at the upstream end 6 of the passageway 5 adjacent the inlet opening 20 and downstream thereof. Each spray nozzle 28 is mounted on and carried on the end of a pair of delivery pipes 29 which extend through and from a corresponding mounting plate 30 secured to the upper wall 11. Each mounting plate 30 closes a corresponding opening 32 in the portion 13 of the upper wall 11. The openings 32 accommodate the delivery pipes 29 and spray nozzles 28 through the upper wall 11 for removal and replacement for cleaning, maintenance and the like. Screws 33 secure the mounting plate 30 to the upper wall 11.

The spray nozzles 28 are positioned substantially on a central horizontal plane extending through the inlet opening 20 and longitudinally of the passageway 5 at the upstream end, and are equi-spaced on opposite sides of a central vertical plane which also extends through the inlet opening 20 in the direction of the passageway 5 at the upstream end 6. In this embodiment of the invention, the spray nozzles 28 are spaced apart 400 nun centre to centre. Connectors (not shown) on the delivery pipes 29 adjacent the mounting plates 30 exteriorly of the duct 3 connect a pressurised water supply to the delivery pipes 29 for forming the atomised spray through the spray nozzles 28.

Each spray nozzle 28 comprises an outlet jet 31 arranged for directing the atomised spray of water in a generally downstream direction into the flue gas stream in the passageway 5. The outlet jets 31 form respective atomised sprays each with an internal cone angle of approximately 90°. The mean droplet size of the spray is approximately 40 microns with a maximum deviation of droplet size not greater than 10% of the mean droplet size.

Baffle means for merging the water droplets of the atomised spray into the flue gas stream comprises three porous baffles, namely, an upstream baffle 35, a downstream baffle 36 and an intermediate baffle 37 which are mounted in the duct 3 and extend transversely of the passageway 5 and transversely of the flow of the flue gas stream. The upstream baffle 35 is mounted towards the upstream end 6 of the passageway 5 adjacent the inlet opening 20 and downstream of the spray nozzles 28. The downstream •Ε 921308 baffle 36 is mounted towards the downstream end 7 of the passageway 5, while the intermediate baffle 37 is mounted intermediate the upstream end 6 and downstream end 7. The baffles 35, 36 and 37 are porous and each comprise elongated spaced apart parallel baffle members 40 which extend transversely across the passageway 5 between the side walls 14 and 15 and are mounted on the side walls 14 and 15. Each baffle member 40 is of circular cross section of 48 mm outside diameter. The baffle members 40 of the baffles 35, 36 and 37 are spaced apart 141 mm centre to centre.

The upstream baffle 35 comprises six baffle members 40, and is arranged at an angle of approximately 45° to the horizontal. The porosity of the upstream baffle 35 is approximately 50%. The lowest of the baffle members 40 of the upstream baffle 35 is positioned just below the centre line 39 of the flue gas inlet 20, while the uppermost baffle member 40 of the upstream baffle 35 is at a level substantially coinciding with the uppermost edge of the flue gas inlet . Accordingly, when the flue gas inlet 20 is viewed through the upstream baffle 35 along the centre line 39 of the flue gas inlet in the direction of the arrow A, namely, in an upstream direction, the upstream baffle 35 occludes approximately 60% of the transverse area of the flue gas inlet 20.

The baffle members 40 are of tubular construction having bores 41 extending therethrough for engaging mounting spigots 42 and 43 extending from the side walls 14 and 15, respectively. The length of the mounting spigots 42 and 43 is chosen to facilitate easy removal of the baffle members 40 for cleaning, see Fig. 6. Releasable retaining collars 46 releasably engage the spigots 43 for securing the baffle members 40 in position on the corresponding spigots 42 and 43. A door 48 closes an access opening 49 in the lower wall 10 to the passageway 5 to facilitate removal and replacement of the baffle member 40 for cleaning and the like. The door 48 is secured to the lower wall 10 by screws 50.

In use, the flue gas stream to be cooled is passed into the passageway 5 through the inlet opening 20 at a temperature of approximately 260°C. The atomised water spray at a water temperature of approximately °C is delivered into the flue gas stream from the spray nozzles 28 and is merged with the flue gas stream in the passageway 5 by the baffles 35, 36 and 37 without substantially affecting the level of the turbulence of the flow of the flue gas stream. As the water droplets are merged with and throughout the flue gas stream, the water droplets evaporate, thereby reducing the temperature of the flue gas stream.

In this embodiment of the invention, the flue gas stream passes through the passageway 5 at the rate of 11.2 cuM per second of actual volume. The velocity of the flue gas stream flowing through the passageway 5 is approximately 17 M per second. The moisture content of the flue gas stream being delivered into the passageway 5 is of the order of 10% moisture by weight of the flue gas stream. The temperature of the flue gas stream exiting through the outlet opening 23 is 204°C, namely, just above the dew point of hydrochloric acid. Accordingly, the temperature drop of the flue gas stream from the inlet opening 20 to the outlet opening 23 is 56°C. The pressure drop in the flue gas stream along the passageway 5 between the inlet opening 20 and the outlet opening 23 in this case is less than 100 Pascals. This is equivalent to a pressure drop of approximately 28 Pascals per M length of the flue gas stream. This is achieved by delivering water through the spray nozzles 28 at the rate of 0.01264 kg per second. This is the equivalent of 20.9 x 10'6 kgs of water per cuM of gas (actual volume) per 1°C drop in temperature of the flue gas stream required.

Referring now to Fig. 7 the passage of twelve sample water droplets introduced by the nozzles 28 into the flue gas stream in the passageway 5 of the duct 3 is illustrated. The positions 60 indicated by the crosses are the positions at which the sample water droplets completely evaporate. As can be seen, the upstream baffle 35 significantly slows down the water droplets 60 in the centre of the flue gas stream, which without the upstream baffle 35 would normally pass through the passageway 5 of the duct 3 at a considerably more rapid rate than the water droplets at the outer extremities of the flue gas stream. Needless to say, the upstream baffle 35 as well as slowing down the water droplets 60 in the centre of the flue gas stream also slows down the centre portion of the flue gas stream. Since a greater proportion of the water droplets are carried by the flue gas stream towards the centre of the flue gas stream, it is believed that by slowing the centre portion of the flue gas stream as well as the water particles in the centre portion, greater merging of the droplets with the flue gas stream on the outer extremities of the flue gas stream is achieved. Further, it is believed that by retarding the flow of the flue gas stream towards the centre of the flue gas stream, the effect on the level of turbulence of the flue gas stream by introducing and merging the water droplets with the IE 92130® flue gas stream is minimized. It is believed that this is largely achieved by the use of the porous baffles, and in particular by the upstream baffle 35. It is believed that the desired degree of slowing down of the water droplets and the flue gases in the flue gas stream adjacent the centre thereof is achieved when the porosity of the upstream baffle is of the order of 50%, and the upstream baffle occludes approximately 60% of the flue gas inlet opening when the flue gas inlet opening is viewed through the upstream baffle in an upstream direction along the centre line of the flue gas inlet opening. It is believed that provided the area of the inlet opening occluded by the upstream baffle does not exceed 80% of the flue gas inlet opening, adequate results are achieved. It is also believed that the area of the flue gas inlet opening occluded by the upstream baffle should not be less than 20% of the area of the flue gas inlet opening. It is also believed that adequate results would be achieved with a baffle with a porosity in the range of 40% to 60%.

It will be appreciated that while the duct has been described as being of a specific shape and size, ducts of other shapes and sizes may be used. Furthermore, it will be appreciated that in many cases the duct may be provided with an inlet in the front wall and an outlet in the rear wall to provide a substantially straight through passageway, rather than in the case of the duct of the embodiment just described, where the passageway turns the flue gas stream through 90°.

Ducts defining passageways of other cross sections besides rectangular cross sections may be used, and needless to say, inlet openings and outlet openings of other shapes besides circular may be provided.

While the apparatus has been described as comprising 10 two spray nozzles, a single spray nozzle may be used, and indeed, in certain cases, more than two spray nozzles may be used. It will also be appreciated that other numbers and arrangements and shapes of baffles and baffle members may be used, and needless to say, other suitable nozzle means may be provided.

While the flue gas stream has been described as having a moisture content of approximately 10%, the moisture content of the flue gas stream may vary. Needless to say, the inlet and outlet temperatures of the flue gas stream may also vary, as may the volume and flow rate of the flue gas stream.

Claims

1. A method for reducing the temperature of flue gases adiabatically in a flue gas stream, the method comprising the steps of: 5 passing the flue gas stream through a duct means having a flue gas inlet and a flue gas outlet, introducing a cooling liquid spray of mean droplet size not greater than 50 microns into the flue gas stream in the duct means for reducing the 10 temperature of the flue gas stream, the cooling liquid spray being introduced through a spray means adjacent the flue gas inlet, and merging the liquid spray with the flue gas stream by passing the flue gas stream carrying the liquid 15 spray through a baffle means in the duct means without substantially affecting the level of turbulence of the flow of the flue gas stream for evaporating the liquid spray for reducing the temperature of the flue gas stream. 20

2. A method as claimed in Claim 1 in which the mean droplet size of the liquid spray is not greater than 40 microns.

3. A method as claimed in Claim 2 in which the mean droplet size of the liquid spray is not greater than 25 30 microns.

4. A method as claimed in any preceding claim in which the maximum deviation of droplet size from the mean droplet size is not more than 10% of the mean droplet size.

5. 5. A method as claimed in any preceding claim in which the cooling liquid spray is a water spray.

6. A method as claimed in any preceding claim in which the cooling liquid spray is at a temperature not greater than 90°C. 10

7. A method as claimed in Claim 6 in which the cooling liquid spray temperature is in the range of 1°C to 60°C.

8. A method as claimed in Claim 7 in which the cooling liquid spray temperature is approximately 15 10°C.

9. A method as claimed in any preceding claim in which the quantity of cooling liquid introduced into the flue gas stream is in the range of 10 x 10‘ 6 kg of cooling liquid per cuM of flue gas per 1°C drop in 20 temperature of the flue gas stream required to 30 x 10 -6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required.

10. A method as claimed in Claim 9 in which the quantity of cooling liquid introduced into the flue gas stream is in the range of 15 x 10‘ 6 kg of cooling 5 liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required to 25 χ 10' 6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required.

11. A method as claimed in Claim 10 in which the 10 quantity of cooling liquid introduced into the flue gas stream is approximately 20.9 x IO 6 kg of cooling liquid per cuM of flue gas per 1°C drop in temperature of the flue gas stream required.

12. A method as claimed in any preceding claim in 15 which the temperature of the flue gas stream at the flue gas inlet of the duct means is in the range of 230°C to 270°C.

13. A method as claimed in Claim 12 in which the temperature of the flue gas stream at the flue gas 20 inlet of the duct means is in the range of 240°C to 260°C.

14. A method as claimed in Claim 13 in which the ,ε 921308 temperature of the flue gas stream at the flue gas inlet of the duct means is approximately 260°C.

15. A method as claimed in any preceding claim in which the temperature of the flue gas stream at the 5 flue gas outlet of the duct means is in the range of 140°C to 205°C.

16. A method as claimed in Claim 15 in which the temperature of the flue gas stream at the flue gas outlet of the duct means is in the range of 180°C to 10 205°C.

17. A method as claimed in any preceding claim in which the velocity of the flue gas stream through the duct means is in the range of 5 metres per second to 25 metres per second. 15

18. A method as claimed in Claim 17 in which the velocity of the flue gas stream through the duct means is in the range of 10 metres per second to 20 metres per second.

19. A method as claimed in any preceding claim in 20. Which the moisture content of the flue gas stream just prior to entering the duct means is not greater than 10% by weight of the flue gas stream.

20. A method as claimed in any preceding claim in which the pressure drop across the flue gas stream along the duct means in the direction of flow from the inlet to the outlet is not greater than 50 pascals per 5 M of length along the duct means.

21. A method as claimed in Claim 20 in which the pressure drop across the flue gas stream along the duct means in the direction of flow from the inlet to the outlet is not greater than 33 pascals. 10

22. A method as claimed in any preceding claim in which the baffle means is a porous baffle means, and an upstream portion of the baffle means is provided adjacent the flue gas inlet, downstream of the spray means . 15

23. A method as claimed in Claim 22 in which the upstream portion of the baffle means occludes not more than 80% of the transverse area of the flue gas inlet when the flue gas inlet is viewed in an upstream direction through the upstream portion of the baffle 20 means along a centre line of the flue gas inlet.

24. A method as claimed in Claim 23 in which the upstream portion of the baffle means occludes not more than 60% of the transverse area of the flue gas inlet when the flue gas inlet is viewed in an upstream direction through the upstream portion of the baffle means along a centre line of the flue gas inlet.

25. A method as claimed in any of Claims 22 to 24 in 5 which the porosity of the baffle means does not exceed 50%.

26. A method for reducing the temperature of flue gases adiabatically in a flue gas steam, the method being substantially as described herein with reference 10 to the accompanying drawings.

27. Apparatus for carrying out the method of any preceding claim for reducing the temperature of flue gases adiabatically in a flue gas stream, the apparatus comprising: 15 a duct means having a flue gas inlet and a flue gas outlet for accommodating a flow of the flue gas stream therethrough, spray means mounted adjacent the flue gas inlet for introducing a cooling liquid spray of mean droplet 20 size not greater than 50 microns into the flue gas stream in the duct means for reducing the temperature of the flue gas stream, and baffle means in the duct means extending transversely of the flow of flue gases for merging the liquid spray with the flue gas stream in the duct means without substantially affecting the level of turbulence of the flow of the flue gas stream for evaporating the liquid spray for reducing the 5 temperature of the flue gas stream.

28. Apparatus as claimed in Claim 27 in which the baffle means comprises a porous baffle means extending transversely in the duct means, an upstream portion of the baffle means being provided adjacent the flue gas 10 inlet downstream of the spray means.

29. Apparatus as claimed in Claim 27 or 28 in which the upstream portion of the baffle means occludes not more than 80% of the transverse area of the flue gas inlet when the flue gas inlet is viewed in an upstream 15 direction through the upstream portion of the baffle means along a centre line of the flue gas inlet.

30. Apparatus as claimed in Claim 29 in which the upstream portion of the baffle means occludes not more than 60% of the transverse area of the flue gas inlet 20 when the flue gas inlet is viewed in an upstream direction through the upstream portion of the baffle means along a centre line of the flue gas inlet.

31. Apparatus as claimed in any of Claims 28 to 30 in which the porosity of the baffle means does not exceed 50%.

32. Apparatus as claimed in any of Claims 28 to 31 in 5 which the baffle means comprises a plurality of spaced apart elongated parallel baffle members extending transversely of the duct means.

33. Apparatus as claimed in Claim 32 in which the baffle members are of circular cross section. 10

34. Apparatus as claimed in Claim 32 or 33 in which the baffle members are spaced apart a distance in the range of 75 mm to 250 mm centre to centre.

35. Apparatus as claimed in Claim 34 in which the baffle members are spaced apart a distance in the 15 range of 100 mm to 175 mm centre to centre.

36. Apparatus as claimed in Claim 34 in which the baffle members are spaced apart a distance of approximately 141 mm centre to centre.

37. Apparatus as claimed in any of Claims 27 to 36 in 20 which the duct means comprises an elongated duct defining a passageway extending longitudinally therethrough for accommodating the flue gas stream.

38. Apparatus as claimed in any of Claims 27 to 37 in which the flue gas inlet is formed by an opening extending in a plane transversely of the duct means.

39. Apparatus as claimed in Claim 37 or 38 in which the passageway defined by the duct means is of rectangular, square or circular cross section.

40. Apparatus for reducing the temperature of flue gases adiabatically of a flue gas stream, the apparatus being substantially as described herein with reference to and as illustrated in the accompanying