WO2006058542A2

WO2006058542A2 - A method of heating a boiler and a heating system

Info

Publication number: WO2006058542A2
Application number: PCT/DK2005/000769
Authority: WO
Inventors: Thomas Paarup Pedersen; Claus Stegmann STRØEH; Bodil Irene Mosekaer Nielsen; Esko Salo; Eero Pekkola
Original assignee: Aalborg Industries A/S; Oy, Oilon
Priority date: 2004-12-03
Filing date: 2005-12-02
Publication date: 2006-06-08
Also published as: WO2006058542A3

Abstract

A heating system and a method for heating a boiler comprising a combustion chambre (13) having an axis of symmetry (X), one or more chamber inlets (12, 12’) for introducing combustion air and fuel into the combustion chamber, heat exchanger means, pressurizing means (22) for pressurizing the combustion air, first fuel inlet means (33) and first burning means (31) for burning a first portion of the fuel in the combustion air for heating the combustion air and arranged at a first location upstream of the chamber inlets, second fuel inlet means (41) for introducing a second portion of the fuel in the flow and arranged at a second location downstream of the first location and upstream of the chamber inlets, the chamber inlets being arranged such that the flow of combustion air with the second portion of fuel flows along a generally helical path around the axis (X).

Description

A METHOD OF HEATING A BOILER AND A HEATING SYSTEM

The present invention relates to a method of heating a boiler and comprising the steps of providing a combustion chamber in said boiler, said combustion chamber having an axis of symmetry or central axis and one or more inlets for introducing combustion air and fuel into said combustion chamber.

In connection with such a method it is of great importance to burn the fuel efficiently and to do so in as small a boiler as possible. This is particularly important for marine boilers to be installed in seagoing vessels where reduced space requirements for a boiler and efficiency thereof are at a premium.

One of the factors influencing efficiency and size of a boiler of the type in reference is the efficiency of the mixture of combustion air and fuel because an increased admixture efficiency leads to more complete and quicker combustion of the fuel thus reducing the required size of the combustion chamber for a given heating output.

The efficiency of combustion and heat exchange in a boiler is very dependent on the turbulence and velocity of the mixture of combustion air and fuel and the heated flue gas. The size of the heat exchanger means in a boiler, for instance, as in the case of marine boilers, between heated flue gas and water is directly dependent on the flow velocity and turbulence of the flue gas in the heat exchanger means.

This velocity and turbulence gives rise to a pressure loss requiring pressurizing means for addressing this issue. Conventional boilers utilize fans or blowers to provide the necessary pressure of the combustion air. This requires relatively large space for the relatively large fans. There is also a limit to the economically feasible pressure levels attainable by means of a fan and thus a limit to the pressure available to generate the desired high velocities and turbulence.

The object of the present invention is to provide a method of the type in reference that allows reduction of the space requirements and fuel requirements of a boiler and increase in the efficiency of the boiler.

According to the invention, this object is achieved by the method comprising the following steps: - heating and pressurizing said combustion air to a first temperature and a first pressure by burning a first portion of fuel in said combustion air to form a flow of heated and pressurized combustion air to said one or more inlets,

- supplying a second portion of fuel to said flow of heated and pressurized combustion air in a region proximate to said one or more inlets, and

- causing said flow of heated and pressurized combustion air to enter said combustion chamber in a flow through said one or more inlets such that said combustion air and said second portion of fuel flows rotationally or along a generally helical path around said axis of symmetry.

Hereby, the need for a fan or blower for pressurising the combustion air is eliminated and it is possible to provide a much higher pressure for allowing large pressure losses owing to turbulence and high velocity of the combustion air and the flue gasses.

In the currently preferred embodiment of a method according to the invention, the method comprises the step of causing said flow to enter said combustion chamber through said one or more inlets in a direction that, when projected onto a plane at right angles to said axis of symmetry and extending through the center of the geometrical figure formed by the perimeter of said inlet on the interior surface of said combustion chamber, forms an angle between 20 and 80 degrees with a line in said plane extending from said center to said axis of symmetry, said angle preferably being between 20 and 70 degrees, more preferably between 20 and 60 degrees, even more preferably between 20 and 50 degrees, even more preferably between 20 and 40 degrees, even more preferably between 25 and 40 degrees and most preferably between 30 and 40 degrees.

Because of the heating and pressurizing of the combustion air, the tangential component of the flow causes a violently swirling vortex along the wall of the combustion chamber and causes a recirculation zone near the axis of symmetry which results in better combustion efficiency and reduced content of CO, NOX and soot in the flue gasses. The forceful swirl or vortex in the form of a "spiral sausage" results in a strong cooling of the flame which tends to give a very increased heat flux through the combustion chamber wall compared to prior art boilers because heat is transferred not mainly by radiation but also to a much greater extent by convection than in prior art boilers. Although the shape of the combustion chamber may be as wished, for instance having an elliptical, oval, rectangular, triangular or irregular cross section as well as having the shape of a truncated cone, in the currently preferred embodiment, said combustion chamber is circular cylindrical.

Although the axis of symmetry may form any angle with the horizontal plane, it is currently preferred that the axis of symmetry of the combustion chamber is substantially vertical or substantially horizontal.

In the currently preferred embodiment, said first pressure during maximum load of the boiler is in the range of approx. 0.02 bar to 0.40 bar, preferably approx. 0.03 bar to 0.35 bar, more preferably approx. 0.05 bar to 0.30 bar, even more preferably approx. 0.06 bar to 0.29 bar, even more preferably approx. 0.07 bar to 0.28 bar, even more preferably approx. 0.08 bar to 0.27 bar, even more preferably approx. 0.09 bar to 0.26 bar and most preferred approx. 0.10 bar to 0.25 bar.

In the currently preferred embodiment, the velocity of said flow out of said one or more inlets into the combustion chamber during maximum load of the boiler is in the range of approx. 100 to 200 m/s, preferably approx. 120 to 170 m/s, and said first temperature of said combustion air flow to said one or more inlets during maximum load of the boiler is approx. 400 to 650 °C, preferably approx. 400 to 550 °C and most preferably approx. 450 to 550 °C.

In the currently preferred embodiment, said boiler is a gas tube boiler, wherein the flue gas enters the gas tubes during maximum load of the boiler with an axial velocity in the range of approx. 50 to 200 m/s, preferably approx. 60 to 190 m/s, and most preferred approx. 80 to 150 m/s, and the temperature of the flue gas entering said gas tubes during maximum load of said boiler is in the range of approx. 900 to 1600 °C, preferably approx. 1000 to 1600 °C, more preferably approx. 1000 to 1550 °C, even more preferably approx. 1200 to 1500 °C and most preferably approx. 1400 to

1500 °C.

Although the use of other fuels such as natural gas, diesel oil or marine diesel is advantageous, in the currently preferred embodiment of the method according to the invention, particularly for marine boilers, said fuel is fuel oil, preferably heavy fuel oil, and the ratio of the amount of said first portion of fuel to the amount of said second portion of fuel during maximum load of the boiler is between approx. 0.15 and 0.50, preferably between approx. 0.18 and 0.40 and most preferably between approx. 0.20 and 0.30.

In the currently preferred embodiment, a vortex around said axis of symmetry is formed in said combustion chamber by said combustion air and the combustion products of said first and second fuel portions, the angular velocity of said vortex being such that a counterflow of said combustion air and combustion products is formed adjacent to said axis of symmetry in a direction away from the inlet to said gas tubes.

Hereby a particularly efficient combustion of the fuel is obtained.

In the currently preferred embodiment, said first portion of fuel is burned in one or more gas generators provided upstream of said one or more inlets, and combustion air for each of said one or more gas generators is pressurized by means of a compressor, preferably a compressor driven by a turbine powered by said gas generator.

The method according to the invention may be utilized in any type of boiler, including smoke tube or water tube boilers having the combustion chamber arranged in any suitable manner relative to the convection heat exchange section containing the smoke or water tubes.

Advantageously, however, for use of the method in a marine type boiler, said boiler is a generally circular cylindrical flue gas or smoke tube steam boiler with a second vertical axis being substantially parallel to said axis of symmetry, and provided with a water filled heat exchange compartment and flue gas or smoke tubes extending generally vertically through said heat exchange compartment.

In the currently preferred method according to the invention the gas tubes are arranged in an array around the second axis, wherein the area around the second axis is free of tubes such that a central first free space is formed at the centre of the array for allowing water to flow freely in said first free space, and preferably the central first free space communicates with the exterior of the array by means of at least one second free space free of tubes extending from the central first free space to the periphery of the array such that water can flow freely from the exterior of the array into the central first free space through the second free space. Hereby, water may flow freely into and in the centre of the array such that heat transmission from the tubes near the interior of the array is improved by increasing the temperature difference between the water and the tubes because of cooler water from the exterior of the array being allowed to flow freely to and in the central free space.

Advantageously, said combustion chamber is provided with two inlets for introducing combustion air and fuel in said combustion chamber, said inlets being located diametrically opposite one another with respect to said axis of symmetry.

Hereby irregularities or skewing of the vortex because of one-sided fuel/combustion air injection is avoided.

Both inlets may be provided with combustion air from one common gas generator, or each inlet may be provided with combustion air from each its own gas generator.

Advantageously, the combustion chamber is provided with two inlets for introducing combustion air and fuel in said combustion chamber, said inlets being located one above the other, only secondary combustion air being introduced into the combustion chamber through the upper one of said two inlets. Hereby better control of the combustion temperature is possible, and the residence time in the combustion chamber is prolonged.

Two pairs of inlets arranged one above the other may be located such that one pair is located diametrically opposite the other pair with respect to said axis of symmetry.

In a second aspect, the present invention relates to a heating system, particularly for carrying out a method according to any of the preceding claims, said heating system comprising:

- a boiler comprising a combustion chamber having an axis of symmetry or central axis and one or more chamber inlets for introducing combustion air and fuel into said combustion chamber, and heat exchanger means for exchanging heat generated by combustion of said fuel,

- pressurizing means for pressurizing said combustion air arranged upstream of said one or more inlets, - first fuel inlet means and first burning means for burning a first portion of said fuel in said combustion air for heating said combustion air and arranged at a first location upstream of said one or more chamber inlets,

- second fuel inlet means for introducing a second portion of said fuel in said flow and arranged at a second location downstream of said first location and upstream of said one or more chamber inlets, said one or more chamber inlets being adapted and arranged such that said flow of heated and pressurized combustion air with said second portion of fuel flows along a generally helical path around said axis of symmetry.

In the currently preferred embodiment, one or more inlet pipes are provided for directing said flow of heated and pressurized combustion air into said one or more chamber inlets in a direction that, when projected onto a plane at right angles to said axis of symmetry and extending through the center of the geometrical figure formed by the perimeter of said inlet on the interior surface of said combustion chamber, forms an angle between 20 and 80 degrees with a line in said plane extending from said center to said axis of symmetry, said angle preferably being between 20 and 70 degrees, more preferably between 20 and 60 degrees, even more preferably between 20 and 50 degrees, even more preferably between 20 and 40 degrees, even more preferably between 25 and 40 degrees and most preferably between 30 and 40 degrees.

In the currently preferred embodiment, said location of said second fuel inlet means is in said one or more inlet pipes at a distance upstream from said one or more chamber inlets of between approx. 50-300 mm, preferably between approx. 100-150 mm.

In the currently preferred embodiment, said one or more fuel inlet pipes are provided with a bend at a distance upstream from said one or more chamber inlets of between approx. 250-550 mm, preferably between approx. 350-450 mm. Hereby turbulence caused in the flow of heated and pressurized combustion air will entail good admixing of the second fuel portion into the flow of combustion air.

In the currently preferred embodiment, the ratio between length and diameter of the circular cylindrical combustion chamber is between approx. 1 :1 and 2:1 , preferably between approx. 1 :1 and 1.5:1. In the currently preferred embodiment, said combustion chamber is provided with two chamber inlets for introducing combustion air and fuel in said combustion chamber, said chamber inlets being located diametrically opposite one another with respect to said axis of symmetry, and both chamber inlets may be provided with combustion air from one common gas generator or each chamber inlet may be provided with combustion air from each its own gas generator.

In a third aspect, the present invention relates to a boiler for heating water and comprising a combustion chamber having an axis of symmetry or central axis and one or more chamber inlets for introducing combustion air and fuel into said combustion chamber, wherein said boiler is a generally circular cylindrical gas tube steam boiler with a second axis being substantially parallel to said axis of symmetry, and provided with a water filled heat exchange compartment and with generally mutually parallel gas tubes extending through said heat exchange compartment, said gas tubes being arranged in an array around said second axis, and wherein the area around said second axis is free of tubes such that a central first free space is formed at the centre of said array for allowing water to flow freely in said free space.

In the currently preferred embodiment of a boiler according to the invention, said central first free space communicates with the exterior of said array by means of at least one second free space free of tubes extending from said central first free space to the periphery of said array such that water can flow freely from the exterior of said array into said central first free space through said second free space.

Advantageously, the gas tube closest to said second axis is at a distance from said second vertical axis of at least approx. 5 cm, preferably at least approx. 6 cm and most preferred at least 7 cm.

The invention will be explained more in detail in the following in connection with specific embodiments of the invention shown solely by way of example in the accompanying drawings, in which:

Fig. 1 is a schematic, partly sectional view of a heating system according to the invention applied to a marine boiler installation,

Fig. 2 is a schematic cross sectional view taken along line A-A in Fig. 1, Fig. 3 is a diagrammatic elevational view of the boiler in Fig 1 illustrating the swirling combustion air, fuel and flue gas in the combustion chamber,

Fig. 4 is the arrangement of Fig. 1 with certain values of pressure and temperature etc. indicated for a specific embodiment of a heating system according to the invention,

Fig. 5 is very schematic cross sectional views corresponding to Fig. 2 illustrating the geometric factors determining the angle of the direction of flow of combustion air/fuel into the combustion chamber,

Fig. 6 is cross sectional views corresponding to Fig. 5 of currently preferred sizing and location of the inlet and inlet conduit relative to the combustion chamber, and

Fig. 7 is a schematic cross sectional view of the combustion chamber showing the disposition of flue gas tubes in a rectangular array,

A heating system 10 according to the invention is shown in the drawings, said system comprising a boiler 11 , a turbo charger 20 and a gas generator 30. The turbo charger 20 comprises a compressor 21 and a turbine 22 with a shaft 23 for driving the compressor 21. The turbine 22 is driven by the exhaust from the gas generator 30.

The gas generator 30 comprises a combustion chamber 31 and a burner 32, the chamber being connected to the compressor 21 of the turbo charger 20 by a conduit 24, and being connected to the turbine 22 of the turbo charger 20 by a conduit 25. A fuel pipe 33 supplies fuel from a fuel oil supply (not shown) to the burner 32.

The boiler 11 is a vertical, generally circular cylindrical structure with an axis Y and including an inlet opening 12 to a circular cylindrical combustion chamber 13 having an axis X, a boiler water compartment 14 with a free water surface 15 and separate outlets 17, 18 for flue gas E and steam S, respectively.

Fuel is fed through a fuel pipe 41 into an inlet pipe 26 extending from the turbine 22 and on through the fuel inlet 12 into the combustion chamber 13 to be ignited by an igniter 45. Furthermore, the boiler 11 contains a plurality of axially extending flue gas tubes 19 extending from a top wall 13A of the combustion chamber to a top wall 11A of the boiler structure.

The tubes 19 connect the combustion chamber 13 and the flue gas outlet 17 in the top wall 11 A of the boiler. The tubes are arranged in a substantially rectangular array (see Fig. 7) to form a pattern wherein, seen in a horizontal cross section through the array, central areas 11 B and 13B in the top walls 11A and 13A, respectively are void of tubes.

A heating system with a very compact design and a relatively small footprint is provided.

Process data has been recorded for a marine boiler installation and are shown in Table 1 below.

In operation of the boiler installation, the flow through the heating system begins with a flow of fresh combustion air entering the compressor 21 in the direction of the arrow L. The pressurized air having a temperature of 18O°C and a pressure of 2 barg flows through the conduit 24 into the combustion chamber 31 of the gas generator 30. A first portion of fuel is supplied from a fuel supply through the pipe 33 and injected into chamber 31 via the burner 32 and is mixed with the pressurized air for gas generation.

From the gas generator 30 the heated and pressurized combustion air having a temperature of approx. 700°C flows through the conduit 25 into the turbine 22 and further on through the inlet pipe 26 and inlet 12 into the combustion chamber 13.

The exhaust gas from the generator 30 drives the turbine 22 and the shaft 23 and thereby the compressor 21 for supplying pressurized combustion air to the gas generator 31. This function could also, but substantially less efficiently, be carried out by means of a fan or blower.

The flow of heated and pressurized combustion air and fuel from the conduit 26 enters the generally circular-cylindrical combustion chamber through the inlet 12 in the inner wall thereof in a direction D forming an angle A (see Figs. 5 and 6) with a line (radius) extending through the center 12a of the inlet 12 and intersecting the combustion compartment axis X of symmetry in the plane of the cross section A-A (at right angles to the axis X).

As illustrated in Figs. 5 and 6, the angle A depends on the diameter of the combustion chamber, the diameter of the inlet pipe 26 and the how close to tangential the location of the inlet pipe is relative the circumference of the combustion chamber 13.

Hereby the flow of combustion air and fuel has a substantial tangential component relative to the circular cross section of the combustion chamber 13, thereby causing a swirling helical air flow around the axis X (see Fig. 3).

The gas velocity at the mouth of the inlet pipe 26 ranges between 200 and 300 m/s.

The inlet opening 12 may optionally have a reduced sectional flow area in order to avoid the flame blowing back through the inlet to a not cooled region of the inlet pipe 26.

The velocity of the swirling flow of combustion air increases by passing through the inlet 12 and its optional flow deflecting means (not shown), and the swirling movement follows a helical path with a pitch of the helical flow path in the range between 15° and 25°.

A second portion of fuel is supplied from a fuel supply through the pipe 41 introduced into the inlet pipe 26 and is burned adjacent the inlet 12. The tip 41a of the fuel pipe 41 is positioned in the inlet conduit 26 at a distance b from the center 12a of the inlet 12, which distance preferably is 50-300 mm, more preferably between 100-150 mm.

In order to have as small fuel particles as possible, the fuel pressure is preferably selected from about 25 bar up to about 40 bar, but can also be lower or higher than this range.

For combustion chamber loads ranging from 4-8 MW/m3 the length/diameter ratio of the chamber preferably is in the interval of approx. 1 :1 to 2:1, more preferably approx. 1 :1 to 1.5:1 , making a very compact construction possible. Because of the relatively high pressure of the combustion air entering the combustion chamber (approx. 0.2 bar) there is sufficient pressure at hand to render a high pressure loss acceptable in forming the swirling high energy vortex that entails extremely efficient mixture of the fuel and combustion air.

In the combustion chamber the gas is ignited by igniter 45 at the inlet and the vortex creates a relatively high pressure along the inner wall of the combustion chamber and the pressure decreases radially towards the axis of symmetry X such that a counterflow indicated by arrows R1 in Fig. 3 along the axis X towards the inlet end of the combustion chamber 13, will be formed which further increases the combustion efficiency and allows for dramatically reducing the overall size of the combustion chamber for a given energy output and a reduction in the content of CO, NOX and soot in the flue gas.

The strong swirling entails a high heat flux through the walls of the combustion chamber due to the flux being comprised of as well convection heat exchange as radiant heat exchange. Measurements indicate a 50% increase relative to conventional boilers. For the boiler in the example, the heat flux is about 250-300 kW/m2 of combustion chamber.

Because of the counterflow and the low pressure causing the counterflow near the axis X it is important that the array or bundle of gas tubes has a free space 19a (see Fig. 7) in the middle corresponding to the areas 11 B and 13B such that flue gas exiting through the gas tubes in the array does not get sucked back into the combustion chamber by the relative underpressure in the central region of the combustion chamber.

This free space 19a inside the bundle of gas tubes also serves to enhance the circulation of the water in the water compartment as a downflow takes place in the free space. Hereby an integrated so-called downcomer is provided. Two elongate areas 19b and 19c that are likewise free of tubes 19 extend from the central tube free space 19a to the periphery of the tube array so as to allow water to flow into the central space 19a from the water surrounding the tube array whereby the circulation is further enhanced and consequently the heat transfer from the tubes in the interior of the bundle or array to the water is enhanced because the temperature difference is increased by cooler water flowing into the middle of the bundle from the periphery thereof. More than one interior free spaces and more than two free corridors can be employed in case of large arrays of tubes.

Although the array of tubes 19 is shown as having a rectangular cross section periphery in Fig. 7, other periphery shapes such as circular, oval, elliptical etc. are possible.

This creation of flow passages in and to the interior of the bundle or array of flue gas tubes is applicable to any water jacket boiler having a bundle or array of hot flue gas tubes extending though the water so as to improve the heat transmission efficiency of the boiler.

The relatively high pressure in the combustion chamber and the swirling movement of gas is propagated through gas tubes 19 even with the relatively small internal diameters hereof and provides a high gas velocity and turbulence which results in an efficient heat exchange with the boiler water surrounding the gas tubes. Hereby, the total heat exchange surface of the convection region is substantially reduced for a given heat output.

Internal helically extending swirler elements (not shown) may be installed inside the gas tubes 19 to impart a further swirling motion to the flue gasses flowing though said tubes 19.

As a consequence of the decreasing combustion chamber pressure radially inwards towards to the axis X, the diameter of each tube 19 may be selected such that increasing distance of the tube from the axis X entails a smaller inner diameter such that the gas velocity in the tubes is more or less the same.

The embodiment shown in Figs. 1-3 and described above is only one example of a heating system according to the invention. The axis of the combustion chamber need not be vertical but may be horizontal or any angle relative to horizontal, and the axis of the convection heat exchange chamber need not be parallel to the axis of the combustion chamber, but may for instance be at right angles to each other. The boiler may be of the type with water/steam tubes. The inlet means could be provided by a plurality of inlets, for instance diametrically opposed inlet pipes 26, 26' and inlets 12, 12' (see Fig. 2) to the combustion chamber in order to balance the swirling movement of combustion air and fuel as well as flue gas.

Two inlets could also be placed on the same side of the combustion chamber (not shown), one above the other. The upper inlet would then be only for introduction of air (secondary combustion air) whereby the combustion temperature can be controlled more precisely and the residence time in the combustion chamber will be prolonged. Two pairs of such inlets arranged above each other could be located at diametrically opposed locations in the combustion chamber wall to achieve the balance mentioned above.

The igniter 45 may be an oil burner or electric device preferably in a retracted position to the flow of combustion gas through the corresponding inlet opening 12, either inside the combustion chamber 13 or in the inlet pipe 26.

The combustion chamber may be of other shapes (for instance a truncated cone) in some of which cases the axis X merely is a central axis and not an axis of symmetry, and the chamber 13 may have a dome shaped upper wall 13A.

Claims

1. A method of heating a boiler, comprising the following steps:

- providing a combustion chamber in said boiler, said combustion chamber having an axis of symmetry or central axis and one or more inlets for introducing combustion air and fuel into said combustion chamber,

- heating and pressurizing said combustion air to a first temperature and a first pressure by burning a first portion of fuel in said combustion air to form a flow of heated and pressurized combustion air to said one or more inlets, - supplying a second portion of fuel to said flow of heated and pressurized combustion air in a region proximate to said one or more inlets, and

2. A method according to claim 1 , wherein the method comprises the step of causing said flow to enter said combustion chamber through said one or more inlets in a direction that, when projected onto a plane at right angles to said axis of symmetry and extending through the centre of the geometrical figure formed by the perimeter of said inlet on the interior surface of said combustion chamber, forms an angle between 20 and 80 degrees with a line in said plane extending from said centre to said axis of symmetry, said angle preferably being between 20 and 70 degrees, more preferably between 20 and 60 degrees, even more preferably between 20 and 50 degrees, even more preferably between 20 and 40 degrees, even more preferably between 25 and 40 degrees and most preferably between 30 and 40 degrees

3. A method according to claim 1 or 2, wherein said combustion chamber is circular cylindrical.

4. A method according to any of the preceding claims, wherein said axis of symmetry is substantially vertical.

5. A method according to any of the claims 1-3, wherein said axis of symmetry is substantially horizontal.

6. A method according to any of the preceding claims, wherein said first pressure during maximum load of said boiler is in the range of approx. 0.02 bar to 0.40 bar, preferably approx. 0.03 bar to 0.35 bar, more preferably approx. 0.05 bar to 0.30 bar, even more preferably approx. 0.06 bar to 0.29 bar, even more preferably approx. 0.07 bar to 0.28 bar, even more preferably approx. 0.08 bar to 0.27 bar, even more preferably approx. 0.09 bar to 0.26 bar and most preferred approx. 0.10 bar to 0,25 bar.

7. A method according to any of the preceding claims, wherein the velocity of said flow out of said one or more inlets into the combustion chamber during maximum load of said boiler is in the range of approx. 100 to 200 m/s, preferably approx. 120 to 170 m/s.

8. A method according to any of the proceeding claims, wherein said first temperature of said combustion air during maximum load of said boiler is in the range of approx.

400 to 650 °C, preferably approx. 400 to 550 °C and most preferably approx. 450 to 550°C.

9. A method according to any of the preceding claims, wherein said boiler is a gas tube boiler, wherein the flue gas enters the gas tubes during maximum load of said boiler with an axial velocity in the range of approx. 50 to 200 m/s, preferably approx. 60 to 190 m/s, and most preferred approx. 80 to 150 m/s.

10. A method according to claim 9, wherein the temperature of the flue gas entering said gas tubes during maximum load of said boiler is in the range of approx. 900 to

1600 °C, preferably approx. 1000 to 1600 °C, more preferably approx. 1000 to 1550 °C, even more preferably approx. 1200 to 1500 °C and most preferably approx. 1400 to 1500 °C.

11. A method according to any of the preceding claims, wherein said fuel is selected from the group comprising fuel oil, heavy fuel oil, diesel, marine diesel and gas.

12. A method according to any of the preceding claims, wherein the ratio of the amount of said first portion of fuel to the amount of said second portion of fuel during maximum load of said boiler is between approx. 0.15 and 0.50, preferably between approx. 0.18 and 0.40 and most preferably between approx. 0.20 and 0.30.

13. A method according to any of the claims 9-12, wherein a vortex around said axis of symmetry is formed in said combustion chamber by said combustion air and the combustion products of said first and second fuel portions, the angular velocity of said vortex being such that a counterflow of said combustion air and combustion products is formed adjacent said axis of symmetry in a direction away from the inlet to said gas tubes.

14. A method according to any of the preceding claims wherein said first portion of fuel is burned in one or more gas generators provided upstream of said one or more inlets.

15. A method according to claim 13, wherein combustion air for each of said one or more gas generators is pressurized by means of a compressor, preferably a compressor driven by a turbine powered by said gas generator, a so-called turbo charger.

16. A method according to any of the preceding claims, wherein said boiler is a generally circular cylindrical flue gas or smoke tube steam boiler with a second vertical axis being substantially parallel to said axis of symmetry, and provided with a water filled heat exchange compartment and flue gas or smoke tubes extending generally vertically through said heat exchange compartment.

17. A method according to claim 16, wherein said gas tubes are arranged in an array around said second axis, wherein the area around said second axis is free of tubes such that a central first free space is formed at the centre of said array for allowing water to flow freely in said free space.

18. A method according to claim 17, wherein said central first free space communicates with the exterior of said array by means of at least one second free space free of tubes extending from said central first free space to the periphery of said array such that water can flow freely from the exterior of said array into said central first free space through said second free space.

19. A method according to any of the preceding claims wherein said combustion chamber is provided with two inlets for introducing combustion air and fuel in said combustion chamber, said inlets being located diametrically opposite one another with respect to said axis of symmetry.

20. A method according to any of the preceding claims wherein said combustion chamber is provided with two inlets for introducing combustion air and fuel in said combustion chamber, said inlets being located one above the other, only secondary combustion air being introduced into the combustion chamber through the upper one of said two inlets.

21. A method according to claim 20, wherein two pairs of inlets arranged one above the other are located such that one pair is located diametrically opposite the other pair with respect to said axis of symmetry.

22. A method according to claim 19, wherein both inlets are provided with combustion air from one common gas generator.

23. A method according to claim 19, wherein each inlet is provided with combustion air from each its own gas generator.

24. A heating system, particularly for carrying out a method according to any of the preceding claims, said heating system comprising:

25. A system according to claim 24, wherein one or more inlet pipes are provided for directing said flow of heated and pressurized combustion air into said one or more chamber inlets in a direction that, when projected onto a plane at right angles to said axis of symmetry and extending through the centre of the geometrical figure formed by the perimeter of said inlet on the interior surface of said combustion chamber, forms an angle between 20 and 80 degrees with a line in said plane extending from said centre to said axis of symmetry, said angle preferably being between 20 and 70 degrees, more preferably between 20 and 60 degrees, even more preferably between 20 and 50 degrees, even more preferably between 20 and 40 degrees, even more preferably between 25 and 40 degrees and most preferably between 30 and 40 degrees.

26. A system according to claim 24 or 25, wherein said combustion chamber is circular cylindrical.

27. A system according to any of the claims 24-26, wherein said axis of symmetry is substantially vertical.

28. A system according to any of the claims 24-26, wherein said axis of symmetry is substantially horizontal.

29. A system according to any of the claims 24-28, wherein said first burning means comprises a gas generator.

30. A system according to any of the claims 24-29, wherein said pressurizing means comprises a compressor.

31. A system according to claim 30, wherein said pressurizing means further comprises a turbine adapted and arranged for being rotated by a gas generator and adapted and arranged for operating said compressor.

32. A system according to any of the claims 24-31 , wherein said second location of said second fuel inlet means is in said one or more inlet pipes at a distance upstream from said one or more chamber inlets in the range of approx. 50-300 mm, preferably approx. 100-300 mm.

33. A system according to any of the claims 34-32, wherein said one or more fuel inlet pipes are provided with a bend at a distance upstream from said one or more chamber inlets of between approx. 250-550 mm, preferably between approx. 350-450 mm.

34. A system according to any of the claims 26-33, wherein the ratio between length and diameter of the circular cylindrical combustion chamber is between approx. 1 :1 and 2:1 , preferably between approx. 1 :1 and 1.5:1.

35. A system according to any of the claims 24-34, wherein said boiler is a generally circular cylindrical gas tube steam boiler with a second axis being substantially parallel to said axis of symmetry, and provided with a water filled heat exchange compartment and with generally mutually parallel gas tubes extending through said heat exchange compartment.

36. A system according to claim 35, wherein said gas tubes are arranged in an array around said second axis with the gas tube closest to said second axis being at a distance from said second vertical axis of at least approx. 5 cm, preferably at least approx. 6 cm and most preferred at least 7 cm.

37. A system according to claim 35, wherein said gas tubes are arranged in an array around said second axis, wherein the area around said second axis is free of tubes such that a central first free space is formed at the centre of said array for allowing water to flow freely in said free space.

38. A system according to claim 37, wherein said central first free space communicates with the exterior of said array by means of at least one second free space free of tubes extending from said central first free space to the periphery of said array such that water can flow freely from the exterior of said array into said central first free space through said second free space.

39. A system according to any of the claims 35-38, wherein the internal diameter of the individual said gas tubes decreases with increasing distance between said individual gas tubes and said second vertical axis.

40. A system according to any of the claims 35-39, wherein the internal diameter of said gas tubes is less than approx. 20 mm, preferably less than approx. 19 mm, more preferably less than approx. 18 mm, even more preferably less than approx. 17 mm, even more preferably less than approx. 16 mm, even more preferably less than approx. 15 mm, even more preferably less than approx. 14 mm, even more preferably less than approx. 13 mm and most preferred less than approx. 12 mm.

41. A system according to any of the claims 24-40 wherein said combustion chamber is provided with two chamber inlets for introducing combustion air and fuel in said combustion chamber, said chamber inlets being located diametrically opposite one another with respect to said axis of symmetry.

42. A system according to any of the claims 24-41 wherein said combustion chamber is provided with two chamber inlets for introducing combustion air and fuel in said combustion chamber, said chamber inlets being located one above the other.

43. A system according to claim 42, wherein two pairs of inlets arranged one above the other are located such that one pair is located diametrically opposite the other pair with respect to said axis of symmetry.

44. A system according to claim 41, wherein both chamber inlets are provided with combustion air from one common gas generator.

45. A system according to claim 41, wherein each chamber inlet is provided with combustion air from each its own gas generator.

46. A System according to any of the claims 24-45, wherein said one or more combustion chamber inlets are provided with swirling means for swirling said flow of pressurized and heated combustion air.

47. A boiler for heating water and comprising a combustion chamber having an axis of symmetry or central axis and one or more chamber inlets for introducing combustion air and fuel into said combustion chamber, wherein said boiler is a generally circular cylindrical gas tube steam boiler with a second axis being substantially parallel to said axis of symmetry, and provided with a water filled heat exchange compartment and with generally mutually parallel gas tubes extending through said heat exchange compartment, said gas tubes being arranged in an array around said second axis, and wherein the area around said second axis is free of tubes such that a central first free space is formed at the centre of said array for allowing water to flow freely in said free space.

48. A boiler according to claim 47, wherein said central first free space communicates with the exterior of said array by means of at least one second free space free of tubes extending from said central first free space to the periphery of said array such that water can flow freely from the exterior of said array into said central first free space through said second free space.

49. A boiler according to claim 47 or 48, wherein the gas tube closest to said second axis is at a distance from said second vertical axis of at least approx. 5 cm, preferably at least approx. 6 cm and most preferred at least 7 cm.

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