GB1605000A

GB1605000A - Boilers

Info

Publication number: GB1605000A
Application number: GB19685/79A
Authority: GB
Original assignee: Deborah Fluidised Combustion Ltd
Current assignee: Deborah Fluidised Combustion Ltd
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-12-16

Description

(54) IMPROVEMENTS IN OR RELATING TO BOILERS (71) We, DEBORAH FLUIDISED COM BUSTION LIMITED, of 6, Davy Drive, North West Industrial Estate, Peterlee, County Durham, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method bv which it is to be performed, to be particularly described in and by the following statement: This invention relates to boilers.

Solid fuel boilers require stoking of the fuel in order to achieve efficient combustion thereof. Automatic mechanical stoking has proved more efficient than manual stoking, due to better control of fuel/air ratios. The use of a fluidised bed has led to a further improvement in that the amount of excess air required for efficient combustion is much lower than with automatic mechanical stoking.

Boilers utilising a conventional fluidised bed are such that a substantially vertical turbulent motion is produced within the bed, due to the flow of gases upwardly through the bed. The intimate mixing of fuel and air within the bed allows higher combustion rates per unit area of grate than could be achieved by mechanical stoking. However, a plurality of fuel inlets, or at least a single fuel inlet with a mechanical device for distributing or scattering the fuel over the surface of the bed, is required in order to give a good distribution of fuel over the grate area and the feeding of the fuel through the various inlets has to be carefully balanced in order to give most efficient combustion by the bed as a whole. Furthermore, volatile or finely divided fuels cannot readily be burned in an efficient manner as the vertical passage of gases through the bed allows insufficient time for complete combustion. In addition the spent fuel, such as heavy ash, can be difficult to remove from the bed and can cause blockage of the air inlets unless a plurality of ash extraction outlets are provided so as to cover the grate area.

According to the present invention there is provided a boiler comprising fuel burning means and means allowing heat exchange between the products of combustion of the fuel and a working fluid, the fuel burning means including a substantially cylindrical housing for accommodating a bed of particulate material, air distributors located within the housing so as to be surrounded by bed material, said air distributors effecting fluidisation of the bed material, said air distributors extending longitudinally of and being spaced from the housing and comprising first and second air distributors arranged on opposite sides of a vertical longitudinal plane through the bed and on each side extending substantially circumferentially with respect to the axis of the housing and means for feeding air to the air distributors to effect circulation of the bed material in two zones on said opposite sides of said longitudinal plane.

Preferably the heat exchange means includes a plurality of tubes for transferring heat from the combustion gases to the working fluid. More preferably these passages are arranged substantially horizontally above said bed. The arrangement may be a socalled single pass system in which the gas flow through the passages is in one direction only before leaving the boiler. Alternatively, the arrangement may be a so-called double pass system whereby gases flow in one direction through passages and are redirected through further passages before leaving the boiler.

Preferably the working fluid is, in use, passed through the space defined by the exterior of the cylindrical housing and the interior of the boiler wall and the bed material extends so as to contact the interior of the housing over at least a portion thereof. In this way a substantial heat exchange can take place between the bed and the water outside the chamber and this increases the over-all efficiency of heat exchange and ensures that the fluidised bed does not become too hot.

Preferably the bed fluidizing means in dude means for feeding air into the bed and the bed includes a spent fuel outlet, the ar rangement being such that air may be preheated by spent fuel before the air is fed to the bed. For instance the spent fuel may be ash, which contains a great deal of heat and this hot ash may be used to pre-heat the air, the pre-heated air then being mixed with the fuel so that it is used to pump the fuel on to the fluldised bed.

Preferably the boiler includes a single fuel inlet to the bed. With known boilers utilising fluidised beds, it is necessary to provide a plurality of fuel inlets so as to give a good distribution of fuel over the grate area, as mentioned above. However, in the case of the boiler of the present invention only a single fuel inlet is necessary because the bed is fluidised in such a way that there is a circulation of the bed material thereby distributing the fuel throughout the bed.

Preferably a longitudinally arranged screw device is provided for removing spent fuel from the boiler.

Included within the scope of the present invention is fuel combustion means adapted for fitting to a boiler, said fuel combustion means comprising a substantially cylindrical housing for accommodating a bed of particulate material, air distributors located within the housing so as to be surrounded by bed material, said air distributors effecting fluidisation of the bed material, said air distributors extending longitudinally of and being spaced from the housing and comprising first and second air distributors arranged on opposite sides of a vertical longitudinal plane through the bed and on each side extending substantially circumferentially with respect to the axis of the housing, and means for feeding air to the air distributors to effect circulation of the bed material in two zones on said opposite sides of said vertical longitudinal plane.

A boiler in accordance with the present invention may use any fuel and it is a particular advantage that the boiler may operate efficiently with fuel having a low calorific value, for instance, low grade coal or municipal refuse. The fuel may be solid, liquid or gaseous although only solid fuels give rise to a large amount of ash. Accordingly, it is a particular advantage that a boiler in accordance with the present invention can utilise fuels with high ash content. These are not normally burnt because the amount of heat lost in the ash is very high. Thus, such fuels have the same transportation costs as high grade fuels, but a large percentage of the potential heat output has hitherto been discarded with the ash.

Because the material in the bed is caused to circulate and the floor of the bed is in dined, the removal of defluidised ash is relatively simple due to the gravity flow of ash to the lowest end of the bed keeping the distributor clear. In known boilers, the heavy ash tends to build up within the bed and eventually the bed becomes defluidised blocking the distributor and is not then capable of supporting combustion.

The particulate bed material may be inert, for instance, a silica sand or it may be reactive, for instance calcium carbonate which will react with sulphur in the fuel to form calcium sulphate, thereby reducing the emission of sulphur into the atmosphere.

An embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a boiler in accordance with the present invention; and Figure 2 is a transverse section of the boiler of Figure 1.

Referring to the drawings, a boiler in accordance with the present invention comprises a shell 101 within which is located a combustion chamber 103, the space between combustion chamber 103 and the wall of shell 101 being provided with tubes 105 around which water may be circulated.

Gases from the combustion chamber 103 pass into a conduit 107 through ports 109 and thence to the front of boiler 101 where they enter tubes 105. The gases pass through tubes 105 from the front to the back of the boiler and enter outlet 111 controlled by butterfly valve 113. Thus the gas makes only a single pass through the boiler although in another similar embodiment, the arrangement may be a double pass system.

The boiler shown in the drawings is a wet back boiler, the water passing into space 115 at the back of the combustion chamber and also circulating under the combustion chamber within space 117.

Within combustion chamber 103 there is a fluidised bed, the wall of which is provided entirely by the wall of chamber 103.

Within the bed there is provided particulate material 119 which may be inert material such as silica sand or catalytic material such as calcium carbonate. Located within the bed material is a plurality of tubes 121 which extend longitudinally of the boiler.

These tubes 121 are arranged in two arcs, each arc of tubes being suspended some distance away from the wall of combustion chamber 103. Air may be supplied to the tubes through inlet 123 and tubes 121 are perforated so that the air passes from the tubes into the bed of material thereby to fluidise it. The arrangement is such that the air causes fluidisation of the bed in a circulating manner as indicated by the arrows in Figure 2. It will be seen that the circulation of the bed material is in planes perpendicular to the lontudinal axis of the boiler. By arranging the air distribution tubes within the bed material itself, there is contact of the bed material over a much greater area of the wall of the combustion chamber, that is to say there is no "dead space" at the bottom of the fluidised bed and it is possible thereby to achieve much greater heat exchange through the wall of the combustion chamber.

Since the movement of the bed material is in planes perpendicular to the longitudinal axis of the boiler a screw device 125 is provided between the two arcs of tubes. This device is rotated during use of the boiler and acts to remove ash from the bottom of bed through outlet 127. The removed ash, together with some bed material, is then mixed with air to preheat the latter. The ash is separated in separator device 129 and the resultant pre-heated air and bed material is then mixed with fuel and fed to the bed through inlet 131.

WHAT WE CLAIM IS:- 1. A boiler comprising fuel burning means and means allowing heat exchange between the products of combustion of the fuel and a working fluid, the fuel burning means including a substantially cylindrical housing for accommodating a bed of particulate material, air distributors located within the housing so as to be surrounded by bed material, said air distributors effecting fluidisation of the bed material, said air distributors extending longitudinally of and being spaced from the housing and comprising first and second air distributors arranged on opposite sides of a vertical longitudinal plane through the bed and on each side extending substantially circumferentially with respect to the axis of the housing and means for feeding air to the air distributors to effect circulation of the bed material in two zones on said opposite sides of said longitudinal plane.

2. A boiler according to claim 1 wherein the heat exchange means includes a plurality of tubes for transferring heat from the combustion gases to the working fluid.

3. A boiler according to claim 2 wherein the tubes are arranged substantially horizontally above said bed.

4. A boiler according to claim 2 or claim 3 wherein the tubes are arranged as a single pass system in which the gas within the tubes passes the length of the boiler only once before leaving the boiler.

5. A boiler according to claim 2 or claim 3 wherein the tubes are arranged in a double pass system whereby the gas passes within the tubes and twice across the length of the boiler before leaving the boiler.

6. A boiler according to any of the preceding claims wherein the working fluid is, in use, passed through the space defined by the exterior of the cylindrical housing and the interior of the boiler wall and the bed material extends so as to contact the interior of the housing over at least a portion thereof.

7. A boiler according to any of the preceding claims wherein the bed includes a spent fuel outlet, means whereby heat may be passed from the spent fuel to air mixed therewith, and means for feeding the air heated by the spent fuel back to the boiler.

8. A boiler according to any of the preceding claims wherein a longitudinally arranged screw device is provided for removing the spent fuel from the boiler.

9. A boiler according to any of the preceding claims wherein there is a single fuel inlet to the boiler.

10. A boiler according to claim 1 and substantially as described herein.

11. A boiler substantially as described herein with reference to the accompanying drawings.

12. Fuel combustion means adapted for fitting to a boiler, said fuel combustion means comprising a substantially cylindrical housing for accommodating a bed of particulate material, air distributors located within the housing so as to be surrounded by bed material, said air distributors effecting fluidisation of. the bed material, said air distributors extending longitudinally of and being spaced from the housing and comprising first and second air distributors arranged on opposite sides of a vertical longitudinal plane through the bed and on each side extending substantially circumferentially with respect to the axis of the housing, and means for feeding air to the air distributors to effect circulation of the bed material in two zones on said opposite sides of said vertical longitudinal plane.

13. Fuel combustion means according to claim 12 substantially as described herein.

14. Fuel combustion means substantially as described herein with reference to the accompanying drawings.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. tubes within the bed material itself, there is contact of the bed material over a much greater area of the wall of the combustion chamber, that is to say there is no "dead space" at the bottom of the fluidised bed and it is possible thereby to achieve much greater heat exchange through the wall of the combustion chamber. Since the movement of the bed material is in planes perpendicular to the longitudinal axis of the boiler a screw device 125 is provided between the two arcs of tubes. This device is rotated during use of the boiler and acts to remove ash from the bottom of bed through outlet 127. The removed ash, together with some bed material, is then mixed with air to preheat the latter. The ash is separated in separator device 129 and the resultant pre-heated air and bed material is then mixed with fuel and fed to the bed through inlet 131. WHAT WE CLAIM IS:-

1. A boiler comprising fuel burning means and means allowing heat exchange between the products of combustion of the fuel and a working fluid, the fuel burning means including a substantially cylindrical housing for accommodating a bed of particulate material, air distributors located within the housing so as to be surrounded by bed material, said air distributors effecting fluidisation of the bed material, said air distributors extending longitudinally of and being spaced from the housing and comprising first and second air distributors arranged on opposite sides of a vertical longitudinal plane through the bed and on each side extending substantially circumferentially with respect to the axis of the housing and means for feeding air to the air distributors to effect circulation of the bed material in two zones on said opposite sides of said longitudinal plane.

10. A boiler according to claim 1 and substantially as described herein.