GB2088237A - Fluidised bed combustion apparatus - Google Patents

Abstract

In a fluidised bed shell boiler for producing steam or hot water or a fluidised bed hot gas generator or incinerator, the amount of bed material deposited downstream of the bed 52 in the furnace tube is reduced by providing a baffle 64, e.g. of fire brick which may have gas-flow passages 66 extending through the baffle, in the tube downstream of the bed. Particles of bed material strike the baffle and return back across the downstream boundary of the bed. The gas leaving the bed is caused to change direction, in a typical construction, away from the direction in which particles move towards the baffle. <IMAGE>

Description

SPECIFICATION Fluidised bed combustion apparatus The invention relates to fluidised bed combustion apparatus.

In this specification "fluidised bed combustion apparatus" includes boilers for heating water or other fluid, for producing steam, and apparatus for producing hot gas or for incineration.

In boilers, for example, in which fuel is burned in a fluidised bed, particles of bed material leave the bed and are conveyed by the gas leaving the bed along the path followed by the gas in its course to the smokestack. Differing amounts of bed material are deposited at different points along the path.

The material lost from the bed must be replaced and the deposition of material along the path reduces the efficiency of the boiler.

In the case of a typical boiler for example having a furnace tube which extends from a containment for the bed to a combustion chamber some material is deposited in the furnace tube between the containment and the combustion chamber, some is deposited in the latter, and some may be conveyed beyond the combustion chamber and be deposited in smoke-tubes or in a smoke-box beyond the latter.

Theoretical proposals have been made for example in British patent specification Nos.

1379578 and 1461411 for boilers having means above the bed which particles leaving the bed are supposed to strike so as to rebound back to the bed. Theoretical proposals have also been made for example in British Patent specification No.

1 506521 for a combustor in which gas leaving the bed is deflected by a cyclonic effect so as to cause any particles entrained in the gas to be deposited within a chamber downstream but relatively close to the bed containment.

The applicants have now found that particles of bed material which have left the bed and have travelled beyond the bed containment can be returned to the bed.

According to the invention, fluidised bed combustion apparatus comprises a horizontal furnace duct which extends from a containment for a fluidisable bed, the duct containing a baffle which is positioned so that particles of bed material which have passed beyond the downstream boundary of the containment strike and rebound from the baffle and return across said boundary.

Preferably, the baffle is arranged so as to cause gas leaving the containment to change direction away from the direction in which particles move towards the baffle.

A fluidised-bed shell boiler will now be described to illustrate the invention by way of example only with reference to the accompanying drawings, in which: Figure 1 is a vertical section through the boiler; Figure 2 is a cut-away perspective view of part of the furnace tube and the combustion chamber of the boiler; Figure 3 is a side elevation of a modified boiler; Figure 4 is a part vertical section at IV--IV in Figure 5 showing a baffle; and Figure 5 is a section on V-V in Figure 4.

The fluidised-bed shell boiler 10 (see Figures 1 and 2) has a shell formed by a horizontal cylindrical wall 12, a front wall 14 and a rear wall 16, the shell defining a water space 18. A horizontal furnace duct in the form of a tube 20 extends from the front wall 14 to a combustion chamber 22 which is located wholly within the water space 1 8 although in other constructions it could be outside, or only partly within, the water space 1 8. A first pass 24 of smoke-tube extends from the combustion chamber 22 to a front smoke-box 26 and a second pass 28 of smoketubes extends from the front smoke-box 26 to a rear smoke-box 30.

The rear smoke-box 30 communicates with a cyclone-type grit-arrestor 32 for removing ash particles from the gas stream which are removed from the arrestor 32 for disposal by a pneumatic conveyor 34. The grit-arrestor 32 has a stack connection 36 through an induced-draught (l.D.) fan 38.

A solid-fuel feed arrangement 40 is located at the front of the boiler 10. The arrangement 40 has a screw conveyor 42 for feeding coal from a bunker (not shown) to a hopper 44. Coal from the hopper 44 is fed to a spreader 46 by a rotary feeder 48. The arrangement shown is more particularly described in our co-pending concurrent UK patent Application.

A distributor plate 50 forms the base of a containment for a fluidisable bed 52 of particuiate material, for example, alumina, in the furnace tube 20. The plate 50 is supported at its front and rear by the front wall 14 of the shell and by a bedretaining wall 54 of refractory fire-bricks, respectively, and along its sides by the furnace tube 20, the walls 14 and 54 and the tube 20 completing the bed containment. The wall 54 represents a downstream boundary of the bed containment. The plate 50 has nozzles 56 through which fluidising air and fuel gas are introduced into the bed 52. The nozzles 56 that are at the periphery of the plate 50 and are adjacent the furnace tube wall are inclined to the plate 50 in a direction away from the centre of the plate 50.

Fluidising air is supplied to the furnace tube 20 below the plate 50 by a forced-draught (F.D.) fan 58 communicating with the tube 20 through a duct 60. Fuel gas, for example propane, is supplied through lines 62 (only one shown) which have side pipes for feeding fuel gas directly into the nozzles.

A baffle 64 is located in the furnace tube 20 between the bed-retaining wall 54 and the combustion chamber 22. The baffle 64 consists of a wall of refractory fire-bricks which are laid across the furnace tube 20. Some of the firebricks have been moulded with gas-flow passages 66 through them, which passages 66 taper towards the wall 54.

For example, in a boiler having a furnace tube 20 of 1.267m internal diameter, the following parameters have been used:- 1. area of flow path over wall 54 = 0.25m2; 2. separation of wall 54 and baffle 64 = 0.2m (the exact spacing to achieve equal areas = 0.1 84m); 3. area of flow path formed at 2. = 0.21 9m2; 4. the number of passages 66 in the wall 64 = 37; 5. the passages 66 were arranged in seven horizontal rows having a passage distribution (from top to bottom) of 4--55--66--77-6-5-4; 6. the pitch of the passage 66 = 0.1 8m; 7. the diameter of each passage 66 at the end adjacent the bed-retaining wall = 0.09m; 8. the included cone angle of each passage 66=10O;; 9. the total flow path area formed by the passages 66 (at their smaller ends) = 0.24m2; and 10. the thickness of the baffle 64 = 0.29m.

A chute 68 extends from the bottom of the combustion chamber 22 out through the cylindrical wall 12 of the shell. The chute contains a valve (not shown) which is normally closed but which is openable to allow elutriant to pass through.

The boiler 10 is started up by opening the fans 38 and 58 to fluidise the bed 52 slightly and by supplying fuel gas to the bed 52. The fuel gas is ignited at the bed surface and, as heat is imparted to the bed material, the flame boundary progressively approaches the plate 50. Once the temperature of the bed material is sufficiently high the feed arrangement 40 is operated to feed coal to the bed 52 and the fans 38 and 58 are operated to give the design through-flow of fluidising gases and relatively balanced conditions above the bed i.e. the pressure is slightly below atmospheric pressure above the bed. After a short period to allow the coal to ignite, the fuel gas supply is turned off. The feed arrangement 40 is then operated to supply coal to the bed 52 in response to the required steam demand from the boiler 10.

During running of the boiler 10, the velocity of gases passing from the bed 52 and over the bedretaining wall 54 is relatively high so that particles of bed material fuel and ash are elutriated from the bed 50. Ash and other particles that are small enough are carried right through the boiler to the grit-arrestor 32.

In tests using a full-size model of part of a boiler it has been found that some 40kg of bed material which had left the bed containment was collected in the furnace tube, the combustion chamber and further downstream of the gas path in twenty minutes where no baffle was used.

When the baffle was used the amount of bed material collected in twenty minutes was greatly reduced and in one test was as low as 10.5kg. The maximum amount collected in the 20 minute tests was 15.8kg.

In the tests the bed material was sand having a mean particle size of 755 micro-metres. The bed depth was 100mm, slumped and the bed area was 2m2. Air was used to fluidise the bed but no fuel was burned in the bed. The air leaving the bed therefore simulated products of combustion. The wall equivalent to the wail 54 extended 477mm above the distributor plate. The furnace tube in the rig had an internal diameter of 1.25mm and the area of the flow path over the equivalent to the wall 54 was 0.25m2. The air flowed over the wall at 14m/s in all tests.

The least material was collected in the tests when the baffle was spaced 200mm from the wall equivalent to the wall 54 of the boiler shown in Figure 1.

The amount collected in the tests varied whenthe pattern of gas-flow passages in the baffle was changed. In all the tests the top portion of the baffle adjacent the wall of the furnace tube 20 was devoid of gas-flow passages.

In some cases (not shown) the baffle may extend only part way across the furnace tube so that gas can pass around the baffle. In such modifications the baffle may be free of gas-flow passages; or gas-flow passages may be provided in such a baffle.

Whatever the form of the baffle, it is struck by particles of bed material which have passed beyond the wall equivalent to the wall 54 and some at least of the particles rebound from the baffle and return across the wall. This reduces the amount of bed material deposited downstream of the containment, as indicated by the results of the tests mentioned above.

A reduction in the amount of bed material deposited downstream of the bed containment has also been found in an actual boiler having a similar baffle. However, in the boiler particles of ash and fuel are also leaving the bed containment and the reduction in deposition of bed material such as alumina, or sand, has not yet been determined quantitively.

In the arrangement shown, the baffle causes the gas passing over the wall 54 or its equivalent to turn quite sharply. Particles of bed material apparently tend to continue in straight paths tangential to the gas flow path. Those with sufficient momentum moving in suitable directions rebound from the baffle with sufficient remaining momentum to travel back across the wall 54. The original speed and direction of the particles, which depend upon the conditions under which the bed is fluidised, and the position of the baffle are important factors in determining the proportion of material which is returned across the wall 54.

For alumina in air at 950C Centigrade, for particles of such size and having a particle density of 4000kg/m3, the terminal velocity has been calculated as 9.60m/s and the preferred maximum velocity downstream of the baffle 64 is then 6.40m/s. For alumina particles of mean particle size of 1000 micro-metres and the same particle density the calculated terminal velocity is 14m/s and the preferred maximum velocity downstream of the baffle 64 is 9.30m/s.

Figure 3 shows a boiler in which the fluidised bed occupies a relatively greater length of the furnace tube. The same reference numerals have been used for parts corresponding to those shown in Figures 1 and 2.

Typically, for example, the fluidised bed is 2.837m long and 1200mm wide in a furnace tube of 1265mm internal diameter.

The bed retaining wall 54 and the baffle 64 are shown diagrammatically and not necessarily to scale. However, the distance between them is 200mm.

The baffle 64 in this case is positioned very close to the rear end of the furnace tube 20.

The nozzles through which fluidising air is supplied to the bed are indicated schematically by an envelope shown by broken lines at 100.

Figures 4 and 5 show one form of construction of a baffle 64 in detail.

The baffle is built of nineteen hexagonal firebricks 102 cemented together by mortar arranged in this case to give a pattern of gas-flow passages which is different from those described above.

The baffle 64 includes make-up cement mortar shown at 104 which surrounds the array of bricks 102 and holds them in place in relation to the furnace tube 20.

In Figures 4 and 5 the baffle 64 is shown as built over a downwardly-extending chute 1 06. The chute 106 is closed by fire-bricks 108 and would provide for ash removal were a chain grate stoker fitted within the furnace tube instead of the fluidised bed.

A similar baffle 64 can be used in the boiler shown in Figure 3 but the array of bricks 102 would then be oriented in a position 300 anticlockwise from the position shown in Figures 4 and 5 so that horizontal rows of gas-flow passages would be present containing the following numbers of passages, beginning at the top row and proceeding downwardly; three; four; five; and three.

Each brick 102 has a central gas-flow passage having an entrance defined by a circular radiussed edge 110; a cylindrical section 112; and a frustoconical section 114 which terminates at a circular exit 116.

The baffle shown in Figures 4 and 5 produces a gas flow rate downstream of the baffle of not more than 6.40m/s where the rate over the bed retaining wall 54 is 32m/s. Furthermore, the gas flow rate downstream of the baffle is highly uniform across the furnace tube, with very little departure anywhere from the average flow rate.

Although the baffle 64 has been described as having equally-sized gas-flow passages through the wall, many different patterns of passages, sizes of passages and constructions of baffle can be devised to give the improvement referred to above. Gas-flow passages may be provided by gaps between bricks, for example.

In a modification, not shown, the gas flow path may be only partly defined by the baffle. For example, an annular flow path or several flow paths may be defined between the baffle and the furnace tube.

Passages through the baffle may be provided in addition to a passage or passages defined between the baffle and the furnace tube.

Whilst preferred gas flow velocities have been quoted above, other velocities may be used in some applications, within the scope of the invention.

Instead of fire-brick the material used for the baffle may be a heat resistant metal such as Incolloy or Hastelloy (Trade Names) steel; or ceramic material; or a composite metal and ceramic material.

Claims (12)

1. Fluidised bed combustion apparatus comprising a horizontal furnace duct which extends from a containment for a fluidisable bed, the duct containing a baffle which is positioned so that particles of bed material which have passed beyond the downstream boundary of the -containment strike and rebound from the baffle and return across said boundary.

2. Apparatus according to claim 1, in which the baffle is arranged so as to cause gas leaving the containment to change direction away from the direction in which particles move towards the baffle.

3. Apparatus according to claim 1 or claim 2, in which the baffle extends across the furnace duct and has gas-flow through-passages.

4. Apparatus according to claim 3, in which each through-passage tapers towards the bed containment.

5. Apparatus according to claim 3 or claim 4 in which the baffle comprises a wall of fire-bricks.

6. Apparatus according to claim 5, in which the through-passages are passages extending through respective bricks.

7. Apparatus according to any preceding claim, in which the baffle extends across both the upper and lower halves of the duct.

8. Apparatus according to any claim of claims 1 to 6, in which the baffle extends only part way across the duct.

9. Apparatus according to claim 8, in which the baffle is devoid of gas-fiow passages.

10. Apparatus according to any claim of claims 1 to 6, in which a gas flow path is defined only partly by the baffle.

11. Apparatus according to claim 10, in which a gas flow path is defined between the baffle and the furnace duct.

12. Apparatus according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.