The invention is directed to a burner. The invention is also directed to a burner for use in a boiler, a furnace or in a combustor of a gas turbine engine.

Burners for boiler applications are known and for example described in GB2259567.

GB2259567 described a burner wherein a gaseous fuel is fed via a central channel and radially dispersed into a flow of air. Emission of NOx in the exhaust gas is reduced by recirculating part of the exhaust gas and mix this exhaust gas with fresh air before feeding this mixture to the burner.

US5350293 describes a burner wherein fuel and primary air are dispersed in an inner tubular part. To said inner tube also a recycle gas is fed. The inner tube is placed in a larger outer tube to which also secondary air is supplied. Combustion takes place within the outer tube in a space downstream the inner tube.

US5411394 describes a burner wherein the fuel is introduced via a central channel. Air and recycle flue gas are supplied via three annular channels surrounding said central channel. Swirl imparting means are present to swirl the air. Low NOx emissions are claimed because of the recycle of flue gas.

US5772421 describes a burner wherein the discharge ports for fuel and oxidant diverge. This results in that combustion will take place at a position wherein dilution of fuel and an oxidant has taken place with inert product gasses. This would reduce the flame temperature and thus the formation of NOx.

US6027330 described a burner wherein fuel is supplied to the burner front via a central supply tube. Air is supplied via an annular channel and provided with a diverting outlet opening. The diverting opening results in an increased kinetic energy to result in an uniform mixture.

US2003054303 describes a low NOx burner with a secondary combustion stage. Cooling jackets are required to cool the second stage.

US2004234912 describes a burner having a central fuel supply line to the burner front and an annular channel to provide air to the burner front. The annular channel is provided with vanes to provide a swirl to the air such to reduce flame length and reduce CO and NOx emissions.

US2008193886 describes a burner having a central fuel supply line to a burner front and two annular supply channels for air. The outer supply channel is provided with a swirler. The inner supply channel has a somewhat radial outward outlet opening such that the air of both annual channels mix. In another embodiment recirculating oxygen comprising gas is introduced in a further outward annular channel which is also provided with a swirler.

US5407347 describes a low NOx natural gas burner provided with an annular channel in which natural gas is mixed with a flow of air. The annular channel is provided with vanes as swirl imparting means. The burner is provided with a divergent quarl to define a combustion zone. In one of the illustrated burners a central oil gun is present in an open central space. Along the annular space between the central oil gun and the annular channel air is passed to avoid coke and ash particles to deposit on the oil gun.

A problem with the low NOx prior art burner of US5407347 is that is that they are complex. The complexity is found in the presence of the divergent quarl. Such a quarl is used to define a combustion zone and avoid recirculation of hot combustion gasses towards the burner. This would then protect the burner from the high flame temperatures. A problem of such a quarl is that the burner cannot be used to replace burners not having such a quarl or that the space in which the combustion gasses are expelled into need to be specially designed to accommodate such a quarl.

The present inventions aims to avoid the aforementioned problems. The invention is directed to the following burner.

Burner comprising of a burner axis, wherein along the axis and in a downstream direction the burner is provided with a fuel and an oxygen comprising gas supply section at an upstream end and a burner front section at a downstream end and,

wherein the burner front section is comprised of:

an open central tubular space positioned along the burner axis defined by a co-axial inner tubular wall extending, in an upstream direction, from an open end at the burner downstream end to a closed end, and of a co-axial outer tubular wall positioned around burner axis and around the inner tubular wall defining an annular channel between said outer and inner tubular walls which annular channel is provided with an open end at its downstream end, and

wherein the fuel and oxygen comprising gas supply section is comprised of an inlet for an oxygen comprising gas fluidly connected to a contacting zone, an inlet for the fuel fluidly connected to the contacting zone, wherein the contacting zone is fluidly connected to upstream end of the annular channel and wherein open end of the annular channel and the open end of open central tubular space are in one plane perpendicular with the burner axis, and

wherein the burner is comprised of swirl imparting means to impart a swirling motion around the burner axis to the oxygen comprising gas.

Applicants found that the burner according to the invention does not require a quarl as in US5407347. As will be explained in more detail below the spiral flow in the annular channel will provide sufficient cooling of both the inner and outer tubular wall to avoid that the tips of the tubular walls erode. This further allows that the burner front section of this burner may protrude into a combustion space and does not necessarily have to be flush with the opening in the, for example refractory, wall of said combustion space.

Applicants found that with the simple burner according to the invention a low NOx emission may be achieved. Without wanting to be bound to the following theory it is believed that this is achieved by the fact that in this burner fuel and oxygen comprising gas are first sufficiently mixed in the annular channel before being discharged from said channel with a certain minimum velocity and swirl. Because the mixture of discharged fuel and oxygen comprising gas will have a swirling motion, the discharged mixture and the resulting flame will have a frusto conical form. The flue gas will subsequently flow away, transfer part of its heat by means of convection and/or radiation and a cooled flue gas recycle flow will return to the inner side of this flame cone. Because this inner area of the conical flame is relatively large an efficient cooling of the flame results. This in turn results in a low flame temperature and thus a lower NOx formation. If the burner is placed in a combustion space wherein the expanding flue gas relatively quickly contacts the wall of the combustion space the NOx formation is reduced further. This because the temperature of the flame and flue gas is quickly reduced by the colder combustor wall. This temperature reduction and the direction of the wall will further reduce the temperature of the recycle flow and may enhance the desired direction of the recycle flow. Thus no external recycle of flue gas is required to achieve the low NOx emission thereby simplifying the design.

The return flow of relatively colder flue gas to the inner side of the flame cone is different from prior art burners. In prior art burners such a recycle typically takes place in a space around the flame. Such a recycle will not reduce the temperature as in the burner according the invention because the flame area which is contacted with the recycle stream in the prior art burner is smaller.

The burner according to the invention is provided with an outer tubular wall and an inner tubular wall positioned around a burner axis and defining an annular channel between said walls, wherein the inner tubular wall and outer tubular wall are open at its downstream end and wherein the inner tubular wall is closed at its upstream end to form the open central tubular space. In said space additional inspection means and start-up means may be present. Because the upstream end is closed no additional gas can be provided to said space. Additional gas being for example air, oxygen, fuel or any inert gas, such as nitrogen. Thus only flue gas as generated by the burner will in use enter the open central tubular space via the open end at its downstream end. The distance x between the closed end and the open end in the open central tubular space is suitably more than 0.5 times and preferably more than 1 times the inner diameter of the inner tubular wall.

The radial distance between the inner and outer wall should be chosen such that suitably the spirally flowing fuel and oxygen comprising gas remains attached to the outer surface of the inner wall. This is preferred to cool the inner wall and prevent the upstream flow of hot flue gas. The length of the annular channel, to achieve sufficient mixing of the oxygen comprising gas and fuel, is suitably more than 3 times and preferably more than 5 times this radial distance of the channel. Radial distance is the height of the channel, i.e. the distance between the inner and outer tubular wall. The length is here defined as the axial distance from the contacting zone to the downstream end of the burner. To achieve a sufficient mixing length it is preferred to locate the fuel inlet as far upstream as possible. The inner and outer tube wall are suitably flush at the downstream end to achieve that the opening of the annular channel and the opening of the open space are in one plane.

The burner front section will be comprised of an open central tubular space extending from the burner downstream end in an upstream direction to end at the closed end of the inner tubular wall. This open space and especially the inner tubular wall will be exposed to the hot oxygen comprising gasses as generated by the burner. As explained above the burner will generate a return flow of relatively colder flue gas along the tubular axis towards this open space. Applicants found that the combination of this relatively colder return gas and the cooling action of the spiral flow in the annular channel can keep the temperature of the inner tubular wall below the temperature at which its mechanical integrity would be at stake.

The burner has means to impart a swirling motion around the burner axis to the oxygen comprising gas. These swirl inducing means may be located at the inlet for the oxygen comprising gas or further downstream in the burner passage. These swirl imparting means are designed such that the oxygen comprising gas and the mixture of oxygen comprising gas is brought into a spiral flow around the burner axis in the direction of the downstream end of the burner. These means to impart a swirl may be vanes as present in the flow path of oxygen comprising gas and optionally also in the flow path of the mixture of oxygen comprising gas and fuel. Such a flow path is preferably an annular channel provided with vanes positioned along the burner axis. Such means may also be a spiral channel positioned around and perpendicular with respect to the burner axis, which spiral channel is provided with a tangentially positioned inlet opening at its radial outer end and an outlet opening at a more radially inner position. The outlet opening is fluidly connected to the contacting zone. Such a spiral channel, also referred to as a volute, preferably has a decreasing cross-sectional area in the flow direction such to increase the velocity of the gas and the intensity of the swirl.

The contacting zone suitably has a design wherein the spiral flow as generated by the inlet is not or not significantly affected. Suitably the contacting zone is an annular space and more suitably the annular space extends upstream from the annular channel. This may be achieved by a burner wherein the outer tube protrudes further upstream than the inner tube and wherein the wall of the protruding part of the outer tube is the outer wall of the contacting zone.

The contacting zone will be fluidly connected to the inlet for oxygen comprising gas and to the inlet for fuel. The inlet for fuel may be positioned such that the spiral flow of the oxygen comprising gas is not or not significantly affected by the fuel as it contacts the oxygen comprising gas. This may be achieved by semi-tangentially positioned inlets at the radial outer end of a annular contacting zone. By semi-tangentially is here meant that the inlets are tangential with respect to the annular contacting zone and positioned somewhat in the direction of the spiral flow. Suitably the inlet for the fuel comprises of numerous radial extending flow paths fluidly connected at a central position to a fuel supply conduit and fluidly connected at their radial outer ends with the contacting zone. Preferably the radial extending flow paths are present as channels in a cylinder part positioned upstream the closed end of the inner tube. In order to not or not significantly affect the spiral flow it is preferred that the angle of the radial extending flow paths for fuel and the burner axis is smaller than 90° such that the fuel as it will exit the flow path partly has a downstream direction component.

The fuel may also be provided to the flow of oxygen from a radially more distant position relative to the contacting zone. In such a burner the inlet for the fuel suitably comprises of numerous openings in the outer tubular wall which openings fluidly connect the contacting zone with a fuel supply zone as present at the radial exterior of the outer tubular wall. This may be advantageous when the fuel has a lower density than the oxygen comprising gas as will be explained below in more detail. In such a burner it is preferred to create an as long as possible flow path for the mixed fuel and flow of oxygen. Thus it is preferred to position such openings more upstream in the burner. For example a burner having vanes as present in the annular channel as the swirl imparting means may have such openings for fuel located upstream relative to the vanes.

Figure 1 illustrates a possible burner according to the invention. The burner 1 has a burner front section 2 at its downstream end 3 and a fuel and an oxygen comprising gas supply section 4 at its upstream end 5. The burner front section 2 has outer tubular wall 6 and an inner tubular wall 7 positioned around a burner axis 8. Inner tubular wall 7 defines an open central tubular space 22 along the burner axis 8 extending, in an upstream direction, from an open end 23 at the burner downstream end 3 to a closed end 10. Annular channel 9 is formed between said walls 6 and 7. As shown the inner tubular wall 7 and outer tubular wall 6 are open at its downstream end 3 forming an open end 24 of the annular channel 9. The inner tubular wall 7 is closed at its upstream end 10. The fuel and oxygen comprising gas supply section 4 has an inlet 11 for an oxygen comprising gas fluidly connected to a contacting zone 12. As means to impart a swirling motion around the burner axis to the oxygen comprising gas a spiral channel 13 is positioned around and perpendicular with respect to the burner axis 8. This channel has a relatively large inlet 14 and a smaller outlet 15 which communicates fluidly with the contacting zone 12. The distance between the open end 23 and the closed end 10 is shown as x in Figure 1.

Figure 1 also shows an inlet 16 for the fuel fluidly connected to the contacting zone 12. The inlet 16 for the fuel comprises of numerous radial extending channels 17 in a cylinder 18 positioned upstream of closed end 10. The channels 17 are connected to a centrally positioned fuel supply conduit 19. Figure 1 shows an annular contacting zone 12 which extends upstream from the annular channel 9. Figure 1 also shows an igniter 20 and a flame detector 21. These elements are advantageously positioned in the open space formed by the inner tubular wall 7. Thus the burner allows these elements to be positioned centrally as part of the burner. Many of the prior art burners do not allow such positioning of igniters and flame detection and separate arrangements are required. This is however disadvantageous because it may require separate protrusions in the vessel walls. No means to add additional gas to the open central tubular space 22 are present in this burner.

The invention is also directed to the following process for combusting a fuel. Such a process may advantageously be performed in the burner according to the invention. Process for combusting a fuel by performing the following steps:

(i) adding the fuel to a flow of an oxygen comprising gas, wherein the oxygen comprising gas has a spiral flow direction around an axis resulting in a combined flow of fuel and oxygen comprising gas, which combined flow has a spiral flow direction around the axis with a swirl number of above 0.5, (ii) maintain the combined flow for a time sufficient to achieve mixing between fuel and oxygen comprising gas in an annular channel,

(iii) discharge the combined flow from the annular channel via a circular split opening, wherein the axial velocity component of the discharged flow as discharged is at least 5 times larger than the laminar flame speed of the combined flow and

(iv) ignite and combust the combined flow as discharged from the circular split.

In step (i) it is preferred to add the fuel to a radial end of the annular channel such that the fuel and air will sufficiently mix in said annular channel due to the spiral flow when it flows towards the circular split opening. In case the fuel gas has a molecular weight less than the oxygen containing gas, it is injected in radial direction into the oxygen comprising gas with such momentum that it settles initially evenly distributed near the outer wall of the annulus.

Preferably the fuel will then be injected from a more radially distant position towards the flow of oxygen comprising gas as illustrated in Figure 4. Thanks to the action of centrifugal forces induced by the downstream located swirl imparting means as in the burner of Figure 4, the fuel will be distributed evenly over the annular channel 9 as it travels through the annular channel 9 towards the open end 24. In case of a fuel having a molecular weight which is higher than the oxygen containing gas, it is preferred to inject the fuel in a substantially radial direction into the oxygen containing gas with such momentum that it settles initially evenly distributed near the inner wall of the annulus. Thanks to the action of centrifugal forces induced by the spiralling motion, the fuel will be distributed evenly over the annular channel 9 as it travels through the annular channel 9 towards the open end 24. Preferably the fuel will then be injected from a more radially inward position towards the flow of oxygen comprising gas as illustrated in Figure 1.

The swirl number is defined as the ratio of tangential and axial velocity component of the combustible gas mixture at the point of discharge from the burner. The laminar flame speed is defined as the speed of a freely propagating flame through a combustible gas mixture. Suitably the Reynolds number of the combined flow in the annular channel is above 20000. The characteristic velocity of the Reynolds number is the cross sectional area averaged axial velocity component of the combined flow in the annular channel. As explained above, applicants found that by igniting and combustion of a swirling mixture of oxygen comprising gas and fuel, a flow pattern will result wherein the flue gas is expanded away from the burner axis as a cone and a relatively colder return flow results towards the centre of the cone along the burner axis. This relatively colder return flow suitably has a temperature of at least the minimum ignition temperature of the combined flow and lower than the adiabatic flame temperature of the combined flow. Such a return flow will reduce the flame temperature such that less NOx will be formed and a more stable flame is obtained. In order to achieve the desired temperature of the return flow some heat exchange of the flue gas preferably takes place with the walls of a combustion chamber in which the burners fires such to lower the temperature. Thus the process is preferably performed such that the flue gas as produced in a radially and axially extending flame in step (iv) is reduced in temperature by indirect heat exchange against a cooler tubular wall positioned downstream and axial with the annular channel and wherein part of the thus cooled flue gas recirculates along a more central flow path to the radially inner side of the flame, for example in an apparatus according to Figure 3.

In the above description oxygen comprising gas is any gas comprising oxygen, such as for example air, enriched air, oxygen. Fuel may be any fuel, suitably a gaseous or liquid fuel and more preferably a gaseous fuel. Examples of suitable fuels are gasoline, naphtha, methane, ethane, propane, natural gas, LPG, hydrogen and synthesis gas.

The flue gas as generated in the burner may have a pressure of between 10 kPa above ambient to 1 MPa. In the lower pressure range the combustion gas as obtained in step (iv) is suitably used to directly or indirectly to heat a liquid or gaseous medium or object by heat exchange . Examples of such applications are boilers wherein water is indirectly heated to produce steam, dryers, wherein objects are directly contacted with the heated gas and evaporators such as for example distillation column reboilers and distillation column feed furnaces. The boilers may be fire-tube boilers, such as for example a two-pass or three-pass boiler, or water-tube boilers. The invention is also directed to a process to prepare steam in a boiler wherein a heated gas is used as prepared by the method according to the invention. The burner is preferably used in a fire boiler or water-tube boiler, wherein a combustion space is defined by the walls or surfaces present within said boiler and wherein the burner is positioned in an opening in a wall of the boiler and the burner front section protrudes into the combustion space. The outer wall of the burner which protrudes the space will then be exposed to the hot recirculating combustion gasses. However as explained above no damage to this wall part due to overheating of the walls is expected because of the cooling capacity of the gasses which flow in the annular channel. The fact that the burner may protrude this combustion space is advantageous because it can then fire the heat exchanging surfaces of the boiler more efficiently.

This flow pattern is illustrated in Figure 2. Figure 2 schematically shows the flow pattern of the burner 30 according to the invention as it fires into a tubular space 31 from one upstream end 32 of the tubular space with an outlet for combustion gasses at its opposite downstream end 34. From the outlet 35 of annular channel 36 circular a cone like flame 37 discharges flue gas along tubular wall 38 of tubular space 31. The flue gas will cool due to heat exchange with the relatively colder wall 38 and part of this colder flue gas recirculates via a more central pathway 39 towards the radially inner side 40 of flame 37. flow will thus lower the flame temperature and thus reduce the NOx formation. This flow of colder flue gas will reduce the flame temperature and thus enhance the reduction in NOx formation and the flame stability. The diameter of such a tubular space 31 is preferably between 2 to 6 times the diameter of the outer tubular wall of the burner. The length of such a tubular space as calculated from the downstream end of the burner is preferably at least 1 times and preferably 2 times the diameter of the outer tubular wall of the burner. By diameter is here meant the outer diameter of the outer tubular wall. There is no maximum length, but for practical reasons this length will typically be below 10 times this diameter. The diameter of the outer tubular wall in the context of the present invention is defined as its inner diameter.

The invention is also directed to an apparatus for expanding a compressed combustible gas comprising a combustor and a turbine rotor wherein the combustor is comprised of an axially extending space around an axially extending combustor axis provided with a burner according to the present invention at one upstream end and a circular slit as discharge opening at its opposite downstream end,

the turbine rotor provided with an inlet for compressed flue gas and fluidly connected to the discharge opening of the combustor, wherein the turbine rotor is fixed to an rotatable shaft, which shaft is positioned axial with the combustor axis.

Figure 3 illustrates such an apparatus having a combustor 41 and a turbine rotor 42. The combustor 41 is comprised of an axially extending space 43 around an axially extending combustor axis 44 provided with a burner 45 according to the present invention at one end 46 and an circular slit 47 as discharge opening at its opposite downstream end 48. The dimensions of the axial extending space 43 are preferably as described for the tubular space 31 of Figure 2. The turbine rotor 42 is provided with an inlet for compressed flue gas and connected to the discharge opening 47 to discharge flue gas. The turbine rotor 42 is fixed to an rotatable shaft 49, which shaft is positioned axial with the combustor axis 44.

The discharge outlet of the turbine rotor 42 is fluidly connected to a circular inlet opening 50 of a diffusor 51. The diffusor 51 is comprised of a flow path 52 for gas which extends axially away from the combustor 41 and extends radially outward towards an circular outlet 53. The circular outlet 53 is fluidly connected along its circumference to a volute 54 which functions as a receiving space for expanded combustion gas.

Figure 3 also shows at the downstream end 46 of the axially extending space 43 of the combustor a gas diverting body 55 is present and is positioned along the combustor axis 44. This defines a combustor gas pathway 56 between said body 55 and the wall 56 of the axially extending space and terminating at the circular slit 47 at the downstream end 46 of the combustor 41. Because of the aforementioned flow pattern of the hot gas no overheating of the diverting body 55 will occur.

Around the wall 56 of the axially extending space 43 a second shell 57 is placed defining a cooling zone 58. The cooling zone 58 is provided with an inlet 59 for compressed oxygen comprising gas and one or more outlets for compressed oxygen comprising gas. At least one of these outlets is fluidly connected at 60 to the inlet for oxygen comprising gas of the burner 45. Further a fuel supply 65 is shown. Because hot flue gas will be directed along the interior side of wall 56 an efficient heat exchange will take place between the compressed oxygen comprising gas flowing in the cooling zone 58 and the combustion gas flowing in the axial extending space 43. The temperature increase of the compressed oxygen comprising gas is advantageous for better combustion in the burner. Figure 3 also shows openings 61 in the wall 56 of the axially extending space. Through these openings some of the compressed oxygen comprising gas will dilute the compressed combustion gas in the axially extending space and thus further reduce its temperature and thus protect the gas diverting body 55 and turbine rotor 42 against overheating.

Figure 3 also shows that to the rotatable shaft 49 a generator 62 and a compressor 63 is further directly fixed in a sequence along the shaft 49 of turbine rotor 48, generator 62 and compressor 63. The compressor 63 as shown in a centrifugal compressor and is provided with an outlet 64 for compressed oxygen comprising gas which is fluidly connected to the combustor 41. Preferably no heat exchange takes place between the expanded gas as it leaves the apparatus via 52 and the compressed oxygen comprising gas as it is supplied to the combustor.

Figure 4 shows a burner as in Figure 1. The difference is that the swirl imparting means are vanes 70 as present in a co-axial annular channel 9. The inlet for the fuel comprises of numerous openings 71 in the outer tubular wall 6 which openings 71 fluidly connect the contacting zone 12 with a fuel supply zone as present at the radial exterior of the outer tubular wall 6 as an annular space 72 provided with a fuel inlet 73. The fuel openings 71 and contacting zone 12 are located upstream the swirl vanes 70. Flange 74 as fixed to the burner may be used to fix the burner in a furnace and the like. The position of the flange 74 illustrates that the burner 1 protrudes the combustion space of such a furnace. Figure 4 further shows that the burner does not have a quarl.

The invention shall be illustrated by the following non-limiting example.

Example

To a burner according to figure 1 a 0.02 kg/s methane fuel and 0.32 kg/s air was supplied. The capacity of the flame was 800 kW. The inner diameter of the outer wall was 162 mm, the inner diameter of the inner wall was 114, the radial height of the annular channel was 24 mm, the depth x as in Figure 1 was 191 mm. The swirl number was 1.

The burner was operated for 1400 hours and the average NOx was measured to be 30 ppm. The NOx in combustion gasses of comparable natural gas burners having the same capacity was measured or compared and no burner was found which had a lower NOx content in the combustion gasses than 50 ppm. The burner tips showed no visible damage or aging.