Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
A BOILER, A METHOD OF CONTROLLING THE COMBUSTION IN A BOILER AND A HEAT EXCHANGER TUBE FOR USE IN A BOILER
Document Type and Number:
WIPO Patent Application WO/2004/048850
Kind Code:
A2
Abstract:
A boiler comprising a furnace chamber, a passage extending through an aperture 9 in a bottom wall of the chamber and defined by an exterior truncated cone 18 tapering towards a boundary interface 19 with an interior truncated cone 17 also tapering towards the interface, the exterior and interior truncated cones having a substantially common axis, a burner lance 21 extending along the axis through the passage, a burner nozzle 23 mounted at the interior end of said lance, and a substantially circular flame stabilizing disc 24 mounted on the interior end of the lance coaxially with the truncated cones and interiorly of the nozzle and having a central aperture 34 in register with the nozzle for allowing combustion fuel from the nozzle to enter the chamber, the lance being arranged axially displaceable between a first , innermost, position and a second, outermost, position such that the stabilizing disc is located at a distance B from the boundary 19 in a position between the largest and the smallest diameter of the interior truncated cone when the lance is displaced axially to an intermediate position between the innermost position and the outermost position.

Inventors:
HOLM PIL (DK)
ANDERSEN THOMAS PERTOU (DK)
Application Number:
PCT/DK2003/000798
Publication Date:
June 10, 2004
Filing Date:
November 20, 2003
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
AALBORG IND AS (DK)
HOLM PIL (DK)
ANDERSEN THOMAS PERTOU (DK)
International Classes:
F23C7/00; F23D11/40; F23L5/02; F23M9/00; F23M9/08; F23N5/00; F23N5/02; F24H9/00; F28F1/08; F28F13/06; (IPC1-7): F23C7/00
Foreign References:
US5259342A1993-11-09
EP0655580A21995-05-31
DE2233754A11974-01-24
DE10164217A12003-07-17
US5470224A1995-11-28
Attorney, Agent or Firm:
BUDDE, SCHOU & OSTENFELD A/S (KĂžbenhaven V, DK)
Download PDF:
Claims:
CLAIMS
1. A boiler comprising: a furnace chamber a passage extending through a wall of said furnace chamber from the exterior of said furnace chamber to the interior thereof and defined by an exterior truncated cone tapering towards a boundary interface with an interior truncated cone also tapering towards said boundary interface, said exterior and interior truncated cones having a substantially common axis, a lance extending along said axis through said passage, a burner nozzle mounted at the interior end of said lance a substantially circular flame stabilizing disc mounted on said interior end of said lance coaxially with said truncated cones and interiorly of said nozzle and having a central aperture in register with said nozzle for allowing combustion fuel from said nozzle to enter said furnace chamber, said lance being arranged axially displaceable between a first, innermost, position and a second, outermost, position such that said stabilizing disc is located at a position between the largest and the smallest diameter of said interior truncated cone when said lance is displaced axially to an intermediate position between said innermost position and said outermost position.
2. A boiler according to claim 1, wherein said furnace chamber is circular cylindrical with a substantially vertical axis and said passage is located in the bottom wall of said furnace chamber with said axis of said truncated cones substantially coinciding with said axis of the furnace chamber.
3. A boiler according to claim 1 or 2, wherein said stabilizing disc is provided with radial slits for allowing combustion air to flow through said disc, said slits preferably extending from said central aperture.
4. A boiler according to claim 3, wherein air deflection vanes extend along said slits for imparting a swirling motion to the combustion air flowing through said slits, the angle of said vanes relative to the plane of said disc preferably being between 30 and 60 degrees.
5. A boiler according to any of the preceding claims, wherein the ratio between the flow of combustion air in m3/sec through said passage to the cross sectional area in m2 of said interior truncated cone at the smallest diameter thereof is between approx. 12 m/sec and 23 m/sec, preferably between 15 m/sec and 22 m/sec, more preferably between 18 m/sec and 22 m/sec and most preferably between 21 m/sec and 22 m/sec.
6. A boiler according to any of the preceding claims, wherein the ratio between said smallest diameter of said interior truncated cone and the diameter of said stabilizing disc is between approximately 1.3 and 1.6, preferably between 1.35 and 1.55 and most preferably between 1.4 and 1.5.
7. A boiler according to any of the claims 26, wherein the ratio between the diameter of said furnace chamber and said largest diameter of said interior truncated cone is between approximately 3 and 6, preferably between 3.5 and 5.5 most preferably between 4 and 5.
8. A boiler according to any of the preceding claims, wherein an annular swirler surrounding said lance is located in said exterior truncated cone adjacent said boundary interface for imparting a swirling motion to the combustion air, said swirler having vanes inclined at angle with said common axis of between approximately 15 degrees and 45 degrees, preferably between 25 degrees and 35 degrees.
9. A boiler according to any of the preceding claims, wherein the axial gap between said boundary interface and said stabilizing disc at full load operation of the boiler is between approximately 0 mm and 30 mm, preferably between 10 mm and 25 mm and more preferably between 15 mm and 20 mm.
10. A boiler according to any of the preceding claims and further comprising: a water jacket surrounding said circular cylindrical furnace chamber, a circular cylindrical convection chamber located directly above said furnace chamber communicating with said water jacket and having a vertical axis substantially coinciding with said vertical axis of said furnace chamber, a smoke gas outlet at the top of said convection chamber, an array of heat exchanger tubes for exchanging heat between flue gas flowing through said tubes and water in said convection chamber, said tubes extending through said convection chamber for communicating said furnace chamber with said smoke gas outlet.
11. A boiler according to claim 10, wherein the interior surface of each of said heat exchanger tubes is provided with an axially extending helical ridge with a first helical pitch, an axially extending helical insert with a second helical pitch being arranged inside said tube, said first and second helical pitches preferably both having a clockwise orientation or both having a counter clockwise orientation such that the swirling motion imparted to flue gas flowing through said tube by said helical ridge is enhanced by said helical insert.
12. A boiler according to claim 11, wherein said helical ridge is a helical indentation of the tube wall such that said helical ridge of said interior surface corresponds to a helical trough of the outer surface of said tube.
13. A boiler according to claim 11 or 12, wherein said helical insert is a generally planar strip of heat resistant material such as steel twisted into a helical shape around the longitudinal axis of the strip.
14. A boiler according to any of the claims 1113, wherein said second helical pitch is larger than said first helical pitch.
15. A boiler according to any of the claims 1114, wherein the ratio of said first helical pitch to the maximum exterior diameter of said tube is between approximately 0.4. and 0.8, for instance between 0.45 and 0.75, between 0.5 and 0.7 or between 0.55 and 0.65, and wherein the ratio of said second helical pitch to said maximum exterior diameter of said tube is between approximately 2.0. and 3.0, for instance between 2.2 and 2.8, between 2.3 and 2.7 or between 2.4 and 2.6.
16. A boiler according to any of the claims 1115, wherein said helical insert extends along substantially the entire length of said heat exchanger tube.
17. A boiler according to any of the claims 1016, wherein said heat exchanger tubes are substantially rectilinear and extend substantially parallel to said axis of said convection chamber.
18. A boiler according to any of the preceding claims, wherein at least said interior truncated cone is arranged axially displaceable and preferably both said interior and said exterior truncated cones are arranged axially displaceable as a unit.
19. A method of controlling the combustion in a boiler according to any of the claims 118 and comprising the following steps: axially displacing said lance to a first operative position of said flame stabilizing disc relative to said interior truncated cone, operating said boiler at an interpolation load being a selected percentage of full design load. observing and registering operating conditions with respect to content of nitrogen oxides in the exhaust gas and/or flame stability and/or excess air ratio and/or thermal output and/or fuel consumption and/or content of carbon monoxide in the exhaust gas, axially displacing said lance to at least one second operative position, for each said second operative position operating said boiler at said interpolation load observing and registering operating conditions with respect to content of nitrogen oxides in the exhaust gas and/or flame stability and/or excess air ratio and/or thermal output and/or fuel consumption and/or content of carbon monoxide in the exhaust gas" selecting a selected operative position of said lance from among said first and second operative positions such that the formation of nitrogen oxides and flame stability during combustion and other operative parameters comply substantially as well as possible with predetermined criteria when said lance is in said selected position and the boiler is operating at said interpolation load, displacing said lance to said selected operative position, and operating said boiler at said interpolation load.
20. A method according to claim 19, wherein said interpolation load is between 10% and 110% of the full design load of said boiler.
21. A method according to claim 19 or 20 and comprising the further steps of: performing intermittent or continuous measurements with respect to content of nitrogen oxides in the exhaust gas and/or flame stability and/or excess air ratio and/or thermal output and/or fuel consumption and/or content of carbon monoxide in the exhaust gas, displacing said lance to a different operative position determined by differences between the values of said measurements and predetermined desired values of said measurements.
22. A boiler comprising: a circular cylindrical furnace chamber with a vertical axis surrounded by a water jacket, a circular cylindrical convection chamber located directly above said furnace chamber communicating with said water jacket and having a vertical axis substantially coinciding with said vertical axis of said furnace chamber, a smoke gas outlet at the top of said convection chamber, an array of heat exchanger tubes for exchanging heat between flue gas flowing through said tubes and water in said convection chamber, said tubes extending through said convection chamber for communicating said furnace chamber with said smoke gas outlet, a burner body mounted in an aperture of the bottom wall of said furnace chamber and having a passage extending from the exterior of said furnace chamber to the interior thereof, a lance extending through said passage along said vertical axis of said furnace chamber, and a burner nozzle mounted at the interior end of said lance.
23. A boiler according to claim 22, wherein said heat exchanger tubes are substantially rectilinear and extend substantially parallel to said axis of said convection chamber.
24. A boiler according to claim 22 or 23, wherein the interior surface of each of said heat exchanger tubes is provided with an axially extending helical ridge with a first helical pitch, an axially extending helical insert with a second helical pitch being arranged inside said tube, said first and second helical pitches preferably both having a clockwise orientation or both having a counter clock wise orientation such that the swirling motion imparted to flue gas flowing through said tube by said helical ridge is enhanced by said helical insert.
25. A boiler according to claim 24, wherein said helical ridge is a helical indentation of the tube wall such that said helical ridge of said interior surface corresponds to a helical trough of the outer surface of said tube.
26. A boiler according to claim 24 or 25, wherein said helical insert is a generally planar strip of heat resistant material such as steel twisted into a helical shape around the longitudinal axis of the strip.
27. A boiler according to any of the claims 2426, wherein said second helical pitch is larger than said first helical pitch.
28. A boiler according to any of the claims 2427, wherein the ratio of said first helical pitch to the maximum exterior diameter of said tube is between approximately 0.4. and 0.8, for instance between 0.45 and 0.75, between 0.5 and 0.7 or between 0.55 and 0.65, and wherein the ratio of said second helical pitch to said maximum exterior diameter of said tube is between approximately 2.0. and 3.0, for instance between 2.2 and 2.8, between 2.3 and 2.7 or between 2.4 and 2.6.
29. A boiler according to any of the claims 2428, wherein said helical insert extends along substantially the entire length of said heat exchanger tube.
30. A heat exchanger tube for use in a boiler for exchanging heat between flue gas flowing through said tube and a fluid such as water surrounding said tube, the interior surface of said tube being provided with an axially extending helical ridge with a first helical pitch, an axially extending helical insert with a second helical pitch being arranged inside said tube, said first and second helical pitches preferably both having a clockwise orientation or both having a counter clockwise orientation such that the swirling motion imparted to flue gas flowing through said tube by said helical ridge is enhanced by said helical insert.
31. A heat exchanger tube according to claim 30, wherein said helical ridge is a helical indentation of the tube wall such that said helical ridge of said interior surface corresponds to a helical trough of the outer surface of said tube. <BR> <BR> <BR> <BR> <BR> <BR> <P> .
32. _A heat exchanger tube_according_to_claim_30 or 31, wherein said helical insert is a generally planar strip of heat resistant material such as steel twisted into a helical shape around the longitudinal axis of the strip.
33. A heat exchanger tube according to any of the preceding claims, wherein said second helical pitch is larger than said first helical pitch.
34. A heat exchanger tube according to any of the preceding claims, wherein the ratio of said first helical pitch to the maximum exterior diameter of said tube is between approximately 0.4. and 0.8, for instance between 0.45 and 0.75, between 0.5 and 0.7 or between 0.55 and 0.65, and wherein the ratio of said second helical pitch to said maximum exterior diameter of said tube is between approximately 2.0. and 3.0, for instance between 2.2 and 2.8, between 2.3 and 2.7 or between 2.4 and 2.6.
35. A heat exchanger tube according to any of the preceding claims, wherein said helical insert extends along substantially the entire length of said heat exchanger tube.
Description:
A BOILER, A METHOD OF CONTROLLING THE COMBUSTION IN A BOILER AND A HEAT EXCHANGER TUBE FOR USE IN A BOILER The present invention relates to a boiler comprising a furnace chamber with a passage extending trough a wall of said furnace chamber from the exterior of said furnace chamber to the interior thereof and a lance extending through said passage with a burner nozzle mounted at the interior end of said lance.

Formation of NOx increases with increasing flame temperature which is influenced by the flame shape, the ratio of primary combustion air to secondary combustion air, the manner in which the fuel and primary air and secondary air is supplied to the flame and a number of other factors.

Furthermore, the possibility of maintaining a stable flame under widely varying operational conditions is of importance for the possible turndown of the boiler, i. e. the flexibility of the boiler with respect to operating at a thermal load substantially below the full design load of the boiler.

Much effort has been invested in designing boilers and burners therefor with the objective of reducing NOx emissions. Reference is made to US patents No.

5, 407, 347, No. 5, 470, 224, No. 5,411, 394 and No. 4,085, 708 hereby incorporated herein by reference.

The prior art boilers are relatively complex and are not suitable for relatively simple and straightforward regulation of the flame and combustion to take into account variations in boiler load and other variable factors having influence on the formation of NOx and CO and on the flame stability.

The object of the present invention is to provide a boiler of the type in reference which allows relatively simple and reliable regulation of the flame and combustion with a view to reduce emission of NOx and C as well as to stabilize the flame.

This object is achieved by said passage being defined by an exterior truncated cone tapering towards a boundary interface with an interior truncated cone also tapering towards said boundary interface, said exterior and interior truncated cones having a substantially common axis, and said boiler furthermore comprising: - a lance extending along said axis through said passage, - a burner nozzle mounted at the interior end of said lance - a substantially circular flame stabilizing disc mounted on said interior end of said lance co-axially with said truncated cones and interiorly of said nozzle and having a central aperture in register with said nozzle for allowing combustion fuel from said nozzle to enter said furnace chamber, said lance being arranged axially displaceable between a first, innermost, position and a second, outermost, position such that said stabilizing disc is located at a position between the largest and the smallest diameter of said interior truncated cone when said lance is displaced axially to an intermediate position between said innermost position and said outermost position.

The position of the flame stabilizing disc or primary swirler with respect to the outer truncated cone determines the ratio between the primary combustion air flowing through the disc or swirler and the secondary combustion air flowing through the annular gap between the edge of the disc and through the truncated cone. This ratio and the flow and swirl characteristics of the primary and secondary combustion air flows determines the flame stability and the temperature in and around the flame such that the high temperature zones giving rise to NOx can be minimised and cleaner combustion may be obtained.

Axial displacement of the lance is the sole regulating action and is therefore very simple and straightforward to carry out.

Advantageously, said furnace chamber is circular cylindrical with a substantially vertical axis and said passage is located in the bottom wall of said furnace chamber with said axis of said truncated cones substantially coinciding with said axis of the furnace chamber. Hereby a particularly simple boiler is obtained further simplifying the regulation of the flame and combustion.

In the currently preferred embodiment of a boiler according to the invention, wherein said stabilizing disc is provided with radial slits for allowing combustion air to flow through said disc, said slits preferably extending from said central aperture, and air deflection vanes extend along said slits for imparting a swirling motion to the combustion air flowing through said slits, the angle of said vanes relative to the plane of said disc preferably being between 30 and 60 degrees. Hereby further flame stabilizing primary combustion air may be supplied to the flame with a swirling, combustion temperature equalizing motion imparted to said primary combustion air.

Preferably, the ratio between the flow of combustion air in m3/sec through said passage to the cross-sectional area in m2 of said interior truncated cone at the smallest diameter thereof is between approx. 12 m/sec and 23 m/sec, preferably between 15 m/sec and 22 m/sec, more preferably between 18 m/sec and 22 m/sec and most preferably between 21 m/sec and 22 m/sec, and the ratio between said smallest diameter of said interior truncated cone and the diameter of said stabilizing disc is between approximately 1.3 and 1.6, preferably between 1.35 and 1.55 and most preferably between 1.4 and 1.5, and the ratio between the diameter of said furnace chamber and said largest diameter of said interior truncated cone is between approximately 3 and 6, preferably between 3.5 and 5.5 most preferably between 4 and 5.

In the currently preferred embodiment of a boiler according to the invention, an annular swirler surrounding said lance is located in said exterior truncated cone adjacent said boundary interface for imparting a swirling motion to the combustion air, said swirler having vanes inclined at angle with said common axis of between approximately 15 degrees and 45 degrees, preferably between 25 degrees and 35 degrees. Hereby a swirling motion is imparted to the secondary combustion air for further equalizing the temperature in the flame and combustion zone.

Preferably, the axial gap between said boundary interface and said stabilizing disc at full load operation of the boiler is between approximately 0 mm and 30

mm, preferably between 10 mm and 25 mm and more preferably between 15 mm and 20 mm.

The currently preferred embodiment of a boiler according to the invention further comprises: - a water jacket surrounding said circular cylindrical furnace chamber, - a circular cylindrical convection chamber located directly above said furnace chamber communicating with said water jacket and having a vertical axis substantially coinciding with said vertical axis of said furnace chamber, - a smoke gas outlet at the top of said convection chamber, - an array of heat exchanger tubes for exchanging heat between flue gas flowing through said tubes and water in said convection chamber, said tubes extending through said convection chamber for communicating said furnace chamber with said smoke gas outlet, wherein the interior surface of each of said heat exchanger tubes is provided with an axially extending helical ridge with a first helical pitch, an axially extending helical insert with a second helical pitch being arranged inside said tube, said first and second helical pitches preferably both having a clock-wise orientation or both having a counter clock- wise orientation such that the swirling motion imparted to flue gas flowing through said tube by said helical ridge is enhanced by said helical insert.

Hereby increased turbulence is provided in the smoke or flue gas flowing through the heat exchanger tubes because of the barrier constituted by the ridge. Furthermore, the rotative motion imparted to the flue gas by the insert increases the contact area between the flue gas and the heat exchanger tube inner surface thereby enhancing the heat transfer from the flue gas to the water surrounding the heat exchanger tube in the convection chamber.

In a currently preferred embodiment of a boiler according to the invention said helical ridge is a helical indentation of the tube wall such that said helical ridge of said interior surface corresponds to a helical trough of the outer surface of said tube, and said helical insert is a generally planar strip of heat resistant

material such as steel twisted into a helical shape around the longitudinal axis of the strip, and said second helical pitch is larger than said first helical pitch.

Preferably, the ratio of said first helical pitch to the maximum exterior diameter of said tube is between approximately 0.4. and 0.8, for instance between 0.45 and 0.75, between 0.5 and 0.7 or between 0.55 and 0.65, and wherein the ratio of said second helical pitch to said maximum exterior diameter of said tube is between approximately 2.0. and 3.0, for instance between 2.2 and 2.8, between 2.3 and 2.7 or between 2.4 and 2.6.

In the currently preferred embodiment of a boiler according to the invention said helical insert extends along substantially the entire length of said heat exchanger tube and said heat exchanger tubes are substantially rectilinear and extend substantially parallel to said axis of said convection chamber.

In an alternative embodiment of a boiler according to the invention, at least said interior truncated cone is arranged axially displaceable and preferably both said interior and said exterior truncated cones are arranged axially displaceable as a unit. injan) iheLaspect. the invention relates to a method of controlling the combustion in a boiler according to the invention as described above and comprising the following steps: - axially displacing said lance to a first operative position of said flame stabilizing disc relative to said interior truncated cone, - operating said boiler at an interpolation load being a selected percentage of full design load, - observing and registering operating conditions with respect to content of nitrogen oxides in the exhaust gas and/or flame stability and/or excess air ratio and/or thermal output and/or fuel consumption and/or content of carbon monoxide in the exhaust gas, - axially displacing said lance to at least one second operative position,

- for each said second operative position operating said boiler at said interpolation load observing and registering operating conditions with respect to content of nitrogen oxides in the exhaust gas and/or flame stability and/or excess air ratio and/or thermal output and/or fuel consumption and/or content of carbon monoxide in the exhaust gas, - selecting a selected operative position of said lance from among said first and second operative positions such that the formation of nitrogen oxides and flame stability during combustion and other operative parameters comply substantially as well as possible with pre-determined criteria when said lance is in said selected position and the boiler is operating at said interpolation load, - displacing said lance to said selected operative position, and - operating said boiler at said interpolation load.

Preferably, said interpolation load is between 25% and 110% of the full design load of said boiler.

In a further embodiment of a method according to the invention, said method comprises the further steps of: - performing intermittent or continuous measurements with respect to content of nitrogen oxides in the exhaust gas and/or flame stability and/or excess air ratio and/or thermal output and/or fuel consumption and/or content of carbon monoxide in the exhaust gas, - displacing said lance to a different operative position determined by differences between the values of said measurements and pre-determined desired values of said measurements.

In a yet further aspect, the invention relates to a boiler comprising: - a circular cylindrical furnace chamber with a vertical axis surrounded by a water jacket, - a circular cylindrical convection chamber located directly above said furnace chamber communicating with said water jacket and having a vertical axis substantially coinciding with said vertical axis of said furnace chamber, - a smoke gas outlet at the top of said convection chamber,

an array of heat exchanger tubes for exchanging heat between flue gas flowing through said tubes and water in said convection chamber, said tubes extending through said convection chamber for communicating said furnace chamber with said smoke gas outlet, - a burner body mounted in an aperture of the bottom wall of said furnace chamber and having a passage extending from the exterior of said furnace chamber to the interior thereof, - a lance extending through said passage along said vertical axis of said furnace chamber, and - a burner nozzle mounted at the interior end of said lance.

Hereby, a boiler having a small foot print is obtained allowing the boiler to be mounted in a corner of a room or building and below or inside a chimney for the boiler. A boiler according to this aspect of the invention is particularly well suited for use in a marine vessel where floor space is at a premium.

In a final aspect the invention relates to a heat exchanger tube for use in a boiler for exchanging heat between flue gas flowing through said tube and a fluid such as water surrounding said tube, the interior surface of said tube being provided with an axially extending helical ridge with a first helical pitch, an axially extending helical insert with a second helical pitch being arranged inside said tube, said first and second helical pitches preferably both having a clock- wise orientation or both having a counter clock-wise orientation such that the swirling motion imparted to flue gas flowing through said tube by said helical ridge is enhanced by said helical insert.

In the following the invention will be described and explained more in detail with reference to an embodiment of a boiler according to the invention shown solely by way of example in the accompanying drawings, where Fig. 1 is a schematic partly sectional elevational view of a boiler according to the invention,

Fig. 2 is a top plan view of the boiler of Fig. 1 arranged in a corner, Figs. 3-5 are schematic isometric views from different angles of the burner assembly according to the invention of the boiler of Figs. 1-2, Fig. 6 is a schematic isometric view of a flame stabilizing disc of the burner assembly of Figs. 3-5, Figs. 7-8 are schematic partly sectional views of a burner according to the invention for a boiler according to Figs. 1-2 shown in two different flame regulating positions of the burner lance with mounted burner nozzle and flame stabilizing disc, Figs. 9-11 are schematic cut away views of a smoke tube and corresponding helix insert according to the invention, and Fig 12 is a schematic sectional view of a cut out wall portion of a smoke tube according to the invention.

Referring now to Figs. 1-5 and 7-8, a single-pass smoke gas boiler generally referred to by the numeral 1 comprises a cylindrical casing 2, a cylindrical furnace chamber 3, a water jacket 4 between the wall of said furnace chamber 3 and said casing 2, a water-filled upper convection chamber in said casing 2 and a plurality of smoke gas or flue gas heat exchanger tubes 6 communicating said furnace chamber 3 with a not shown chimney through a top wall 7 of the upper convection water chamber 5. Various fittings such as safety valves, feed water inlets and steam outlets are mounted in the top wall 7. A flange 7a is provided for connection to a not shown flue gas outlet leading to a not shown chimney.

An aperture 9 is provided in the bottom wall 10 of the boiler, a burner body 11 being inserted in said aperture 9, the burner body 11, the furnace chamber 3 and the casing 2 having a common axis A. The burner body 11 communicates

with a wind box 12 communicating with a combustion air fan 13 via an air damper 14 regulated by a servo motor 15. The boiler is supported by columns 16.

The burner body 11 is composed of two truncated cones 17 and 18, the exterior truncated cone 17 tapering upwards in a direction inwards into the furnace chamber 3, while the interior truncated cone 18 tapers downwards in a direction outwards from the furnace chamber, the two truncated cones abutting each other at a boundary interface 19.

Guide vanes 20 are provided in the wind box 12 to distribute the combustion air evenly and to impart a counter clock-wise rotative swirling motion to the combustion air. A burner lance 21 (see also Figs. 7-8) is mounted axially displaceable in a sleeve 22 of the wind box 12. The burner lance 21 is at the interior end thereof provided with a burner nozzle 23 for atomizing the fuel for the combustion in the furnace chamber 3. A flame stabilizing disc 24 is mounted on the burner lance 21 above the nozzle 23 by means of arms 25 and 26. The burner lance 21 may be displaced axially by means of a spindle 27 attached to the burner lance 21 by means of a bracket 28. An increase of the distance A between the bracket 28 and the bottom surface of the boiler 1 entails a corresponding reduction of the distance B between the disc 24 and the boundary interface 19 Fuel is supplied to the nozzle through fuel inlet 29, and fuel overflow is discharged through fuel outlet 30. An igniter electrode 31 for igniting the fuel is provided together with flame scanners 32 for monitoring the flame.

An annular swirler 33 is provided in the exterior truncated cone 17 adjacent to the boundary 19 for imparting rotative motion to the combustion air immediately before said air enters the furnace chamber 3.

Referring now to Fig. 6, the flame stabilizing disc 24 is provided with a central aperture 34 for allowing atomized fuel from the nozzle 23 to enter the furnace

chamber 3 and with radially extending slits 35 provided with vanes 36 for allowing combustion air to pass through the flame stabilizing disc and at the same time impart a rotative swirl motion to this combustion air. The angle of the vanes 36 relative to the plane of the disc 24 determines the intensity of the swirling motion imparted to the combustion air and may vary according to the desired flame characteristics, but is typically between 30 and 60 degrees.

Referring now to Figs 1 and 8, in the currently preferred embodiment of a boiler according to the invention, the following geometrical conditions have turned out to be advantageous: 1. The ratio between the flow of combustion air in m3/sec through the truncated cones 17 and 18 to the cross-sectional area in m2 of the interior truncated cone at the smallest diameter thereof is between approx. 21 m/sec and 22 m/sec.

The ratio may advantageously be in the range between 12 m/sec and 23 m/sec depending on the other geometric and operative conditions of the individual boiler design.

2. The ratio between the smallest diameter dmin of the interior truncated cone 18 (i. e. the diameter of the boundary interface 19) and the diameter dd of the stabilizing disc 24 is between approx. 1.4 and 1.5. The ratio may advantageously be in the range between approximately 1.3 and 1.6 depending on the other geometric and operative conditions of the individual boiler design.

3. The ratio between the diameter df of the furnace chamber and the largest diameter dmax of the interior truncated cone 18 is between approximately 4 and 5. The ratio may advantageously be in the range between approximately 3 and 6 depending on the other geometric and operative conditions of the individual boiler design.

4. The axial gap B between the boundary interface 19 and the stabilizing disc 24 at full load operation of the boiler is between approximately 15 mm and 20 mm. The ratio may advantageously be in the range between approximately 0

mm and 30 mm depending on the other geometric and operative conditions of the individual boiler design.

5. The burner nozzle 23 spreads fuel in a cone with an apex angle of approximately 60 degrees. The angle may advantageously be approximately 80 degrees or 90 degrees depending on the other geometric and operative conditions of the individual boiler design.

6. The swirler 33 has vanes oriented at an angle of approximately 30 degrees relative to the axis of the swirler. The angle may advantageously be in the range of approximately 15 degrees to 45 degrees depending on the other geometric and operative conditions of the individual boiler design.

For determining the characteristics of the individual boilers as regards operation with different positions of the burner lance 21, the lance is displaced axially to a position wherein the stabilizing disc 24 is located at a first axial distance from the boundary interface 19 whereafter the boiler 1 is operated at an intermediate or interpolation load between 10% and 100% of full design load. During such interpolation load operation observations and measurements of various operational parameters are carried out, particularly as regards content of nitrogen oxides in the exhaust gas and/or flame stability and/or excess air ratio and/or thermal output and/or fuel consumption and/or content of carbon monoxide in the exhaust gas and other factors of importance for the optimal operation of the boiler 1.

Thereafter the lance 21 is displaced axially at least one second time to a second position where the stabilizing disc 24 is located at a second axial distance from the boundary interface 19 whereafter the boiler 1 is again operated at said interpolation load and observations and measurements of said operational parameters are carried out.

Finally, the optimal operative position of the lance 21 for the interpolation load in question is selected according to pre-determined criteria.

This sequence is repeated for a number of other interpolation loads and the full design load such that a look-up table is generated for manually or automatically selecting the optimum lance position for a given boiler load.

The position of the flame stabilizing disc or primary swirler 21 with respect to the outer truncated cone 18 determines the ratio between the primary combustion air flowing through the disc or swirler 21 and the secondary combustion air flowing through the annular gap between the edge of the disc 21 and the truncated cone 18. This ratio and the flow and swirl characteristics of the primary and secondary combustion air flows determines the flame stability and the temperature in and around the flame such that the high temperature zones giving rise to NOx can be minimised, and cleaner combustion may be obtained.

The position of the lance 21 may in a further embodiment of the boiler according to the invention be determined automatically based on actual conditions regarding NOx and/or CO in the exhaust gas and/or the excess air ratio and/or flame stability. Not shown sensors continuously or intermittently perform the measurements pertinent to these parameters and said measurements are transmitted to a not shown computing means that controls a notshown servo motor for disptacing the lance 21 according_to pre- programmed algorithms based inter alia on the look-up table described above.

In general it can be said that the larger the distance between the disc 21 and the boundary interface 19, i. e. the larger the gap between the edge of the disc 21 and the truncated coned 18, the larger the amount of secondary combustion air relative to the amount primary combustion air. This results in reduced combustion intensity and thereby reduced formation of NOx.

Furthermore, generally speaking, when the load of the boiler is decreased, decreasing said gap results in a higher combustion intensity because of the higher proportion of primary air that furthermore is supplied to the root of the

flame whereby a more stable flame is achieved. This increases the turndown capacity of the boiler.

Referring now to Fig. 2, the boiler is shown located in a corner of a building with all elements requiring attention, access or maintenance being accessible. This illustrates the advantage of the vertical in-line structure of the boiler with the burner oriented vertically in the bottom wall of the boiler and the furnace chamber 3 and convection chamber 5 arranged coaxially above one another.

The height of the support columns 16 is sufficient for allowing removal of the lance 21 for maintenance or replacement thereof.

Hereby, the foot print of the boiler is minimised such that the boiler may be located directly under or even inside the chimney of the boiler. The combination of corner location and small foot print is advantageous in many applications, for instance when installing the boiler aboard a marine vessel or in commercial or residential buildings.

Referring now to Figs. 9-12, a smoke gas tube 6 having a wall thickness tt of 3 mm is provided with a helical indentation resulting in a helical trough 40 corresponding to a helical ridge 41 with a height hr on the interior surface of the tube 6. The tube 6 is provided with a helical insert 42 extending along the interior of the tube 6. The pitch of the helical indentation is Pt while the helical pitch of the helical swirler insert 42 is Ps.

In the currently preferred embodiment of the boiler according to the invention, the dimensions of the smoke gas tubes are as follows : Dint = 42. 30 mm Dout = 48. 30 mm t = 3. 00 mm hr = 3. 00 mm Pt = 29. 00 mm Ps 120. 75 mm

For other exterior diameters of the smoke gas tubes the ratio Pt/Dout should be between 0.4 and 0.8 and the ratio Ps/Dout should be between 2.0 and 3.0.

This combination of an interior helical ridge 41 and a helical insert or swirler 42 substantially increases the heat transfer from the smoke gas flowing through the smoke gas tubes 6 to the surrounding boiler water in the convection chamber 5.

This is owing to increased turbulence of the smoke gas and increased contact area between the smoke gas and the interior surface of the smoke gas tube 6.

The turbulence is increased because the smoke gas is forced to flow transversely over the ridge 41 so that the smoke gas"trips"over the ridge.

The contact area is increased because the insert or swirler 42 forces the smoke gas to rotate up through the tube 6 thereby increasing the travel distance and thus the contact area.

The interior ridge 41 may be provided in other ways, for instance by inserting a helical wire with a diameter slightly larger than the interior diameter of the tube 6 while stretching the helical wire in the axial direction, thereby reducing the diameter of the helix, and thereafter relieving the stretching force after insertion of the helical wire in the tube whereby the diameter of the helical wire expands and the wire lodges against the inner surface of the tube 6.

Although the flame stabilizing disc has been described as a disc with radial slits, it will be obvious to those skilled in the art that the disc may be composed solely of inclined radially extending vanes or a disc perforated by discrete apertures instead of slits. Furthermore, the disc may have such an axial extension that it may more properly be designated as a cylinder.

The functionality of the invention may also be achieved or enhanced by arranging the two truncated cones 17 and 18 axially displaceable such that the gap between the stabilizing disc 24 and the interior truncated cone 18 may be varied by axially displacing the truncated cones instead of or in addition to axially displacing the lance 21.

The apex angles of the truncated cones 17 and 18 may vary over a wide range and the outermost truncated cone 17 may have a very small apex angle and may as a borderline case be a circular cylinder instead of a truncated cone. In such case the advantage of an exterior truncated cone 17 with regard to reduction of pressure loss will of course not be to hand.