WO2008105653A1

WO2008105653A1 - Method and burner for staged combustion and device provided with one or more burners of this type

Info

Publication number: WO2008105653A1
Application number: PCT/NL2008/000057
Authority: WO
Inventors: Marco Anton Frederik Derksen; Harm Geert Bos
Original assignee: Stork Thermeq B.V.
Priority date: 2007-02-27
Filing date: 2008-02-27
Publication date: 2008-09-04
Also published as: NL1033460C2

Abstract

With a method and burner (10) for staged combustion of a fuel in gaseous form, three distinct streams of, respectively, a premixed mixture of fuel and combustion air, pure combustion air and pure fuel are introduced into a combustion space (16) at different radial positions (70; 80; 90), viewed from the axis of the burner. As a result thereof, several combustion zones (O; C; D) and mixing zones (A; B; C) can be distinguished in the combustion space (16). The combustion conditions in these zones are such that in each case such a substoichiometric amount of either fuel or oxygen is present locally or such a dilution of the combustion gases with flue gases is present that combustion takes place at low temperature and thus the emission of harmful nitrogen oxides is limited.

Description

Method and burner for staged combustion and device provided with one or more burners of this type

The invention relates to a method and burner for the staged combustion of a fuel in gaseous form in a combustion space, and to a device provided with one or more burners of this type, for generating heat in order to heat a medium, for example water or steam, in a boiler, for example a steam boiler, furnace or kiln.

It is known that when a liquid or gaseous fuel is burned in stages in a first combustion zone at a lower concentration of air than is required for stoichiometric combustion, the flame temperature is lower than is the case with a stiochiometric combustion which, due to the reducing environment, leads to a suppression of the formation of NO_x compounds which are emitted into the atmosphere together with the flue gases from a chimney. The remaining amount of combustion air which is required in order to burn all of the fuel is introduced into a second zone downstream of the first combustion zone, where further combustion of the fuel from the first combustion zone takes place. This combustion method is known as combustion with air staging.

It is also known that when such a fuel is burned using a higher concentration of air than is required for stoichiometric combustion, the excess of air acts as a diluent and lowers the temperature of the resultant flue gases, thus reducing the formation of NO_x compounds. With this type of staged combustion, the residual amount of fuel is introduced into a second combustion zone downstream of the first combustion zone and burned there using the remaining quantity of oxygen which is present in the flue gases coming from the first combustion zone. This combustion method is known as combustion with fuel staging.

In the art, burners which are suitable for staged combustion in gas turbines are known, in which compressed oxygen-containing combustion air is used and in which the flue gases which are ultimately discharged via the discharge usually still contain approximately 15% oxygen. In addition, burners for use at atmospheric pressure are known, in which the residual percentage of oxygen in the chimney is much lower, typically in the range between 0.5 and 2%.

An example of a burner and combustion methods of the type using fuel staging and having a low NO_x emission is known from US patent no. 6,616,442. In this case, a first partial quantity of fuel and all the combustion air are mixed in order to obtain an initial premixed mixture of fuel and combustion air, which is therefore low in fuel. This premixed mixture is introduced into the combustion space - also referred to as hearth - and burned in a primary combustion zone, which leads to the formation of flue gases with a low NO_x concentration. A second partial quantity of the fuel is likewise introduced into the primary combustion zone and burned using the air which still remains in order to stabilize the flame in this primary combustion zone. The remaining quantity of fuel is introduced into the combustion space in a secondary combustion zone, and is mixed there with air and flue gases from the combustion space, so that a second mixture of fuel and combustion air is produced which is diluted with the flue gases. This second mixture is burned in the secondary combustion zone, with flue gas having a low NO_x content additionally being formed. If desired, the flame, which is produced in the primary combustion zone when the premixed lean mixture is burned, can be secured on a flame-stabilizing element. In the case of the burner described in this patent, the first mixture is formed in two parallel Venturi mixing tubes. At the outflow end, each of these tubes is provided with a nozzle which is arranged at such an angle to the front surface of the burner that the premixed mixture is directed towards the flame-stabilizing element situated on the underside of the primary combustion zone. For stabilizing purposes, a second partial quantity of pure fuel is supplied virtually at right angles to said front surface to a position below the nozzles of the Venturi mixing tubes in the direction of the flame-stabilizing element. The remaining quantity of fuel is introduced into the second combustion zone via an inlet arranged above the nozzles of the Venturi mixing tubes.

Another burner and combustion method of the type using staged fuel supply are known from EP-A2-0 565 196.

From US 2003/0148236 A1 a burner having a so-called " ultralow NO_x emission" is known for use in process heating among others. In this known burner a stream of (secondary) oxidant such as air, is introduced into the burner through a central pipe, which is surrounded by a fuel introduction tube extending beyond the exit of the central pipe, which tube on its turn is arranged within a surrounding outer pipe for supplying a (primary) oxidant, which outer pipe extends beyond the fuel supply. Together these 3 pipes are a so-called LSV ("Large Scale Vortex") device for flame stabilisation. Staged fuel lances are arranged around the LSV device. In this document it is stated referiring to Fig. 5A that good mixing occurs in the flame stabilisation zone (indicated by 1) as a result of the design of the LSV device and that the LSV has a very low peak temperature accompanied by very low NO_x emission.

A drawback of this known burner is that however local high flame temperatures occur having corresponding local significant NO_x emission.

EP-A2-1612481 discloses a burner design similar to US 2003/0148236 A1. It is an object of the present invention to provide a method and burner for staged combustion, in particular for use in a device for generating heat for heating a medium, such as steam or water, in which the peak temperature of the flame is reduced (further) whereby the formation of harmful emissions of nitrogen oxides (NO_x) is minimized (further), or at least to provide a usable alternative.

According to a first aspect, the invention provides a method for the staged combustion of a fuel in gasesous form in a combustion space, in particular of a device for generating heat for heating a medium, preferably a directly fired atmospheric steam boiler, or a furnace or kiln, which method comprises: a) introducing a stream of a premixed mixture of fuel in gaseous form and oxygen- containing combustion air at a first position into said combustion space, b) introducing a stream of oxygen-containing combustion air into the combustion space at an intermediate position which at least partially surrounds the first position, c) introducing a stream of fuel in gaseous form into the combustion space at a peripheral position which at least partially surrounds the intermediate position, d) burning the introduced stream of the premixed mixture of fuel in gaseous form and oxygen-containing combustion air in the form of a base flame in a first combustion zone (denoted by O in Fig. 3) adjacent to the first position in the combustion space, e) mixing the introduced stream of oxygen-containing combustion air in a mixing zone (denoted by B in Fig. 3) adjacent to the intermediate position, with flue gases coming from the first combustion zone O, f) mixing the introduced stream of fuel in gaseous form, in a mixing zone (denoted by A in Fig. 3) adjacent to the peripheral position, with flue gases circulating in the combustion space, g) mixing the mixed streams obtained in steps e) and f) with each other and partially burning them in an additional mixing and combustion zone (denoted by C in Fig. 3), adjacent to the mixing zones B and A of steps e) and f), h) essentially completely burning the partially burned mixture formed in the additional mixing and combustion zone C in a downstream combustion zone (denoted by D in Fig. 3).

With the method for staged combustion of a fuel in gaseous form according to the invention, three different streams are introduced into the combustion space at different positions. One stream of a fully or almost fully premixed mixture of fuel in gaseous form and oxygen- containing combustion air - referred to below as fuel and combustion air, respectively - is introduced into the combustion space at a first position. In other words, in the invention the mixing takes place upstream and fysically separated from the burning zone O (flame zone) such that a mixture having a homogeneous composition is obtained. Due to the preferred arrangement as explained below, this first position is also referred to as central position. The premixed mixture is burned as a base flame in an area adjacent to the first introduction position. In the specification, this area is called a first combustion zone O. Here it is emphasized that there is no premixing of secondary air and fuel in the burner according to US 2003/0148236 A1. Consequently, positions ("pockets") are present in zone 1 of this known burner - based on a statistic distribution of the streams supplied separately of secondary air and fuel respectively, -in which positions a stoichiometric or almost stoichiometric mixing ratio is prevalent. Such a mixing ratio results in a higher local temperature having a higher local NO_x emission. The emission of thermal NO_x increases exponentially as the temperature increases. Thus the overall NO_x emission of the LSV flame according to said US document is higher than the NO_x emission of the stabilisation flame in the invention, to which a homogeneous, fully or almost fully mixed in advance mixture of oxygen-containing combustion air and fuel is fed.

In an intermediate position which surrounds the first position, in other words at a radial distance to the first position, a stream of pure combustion air which does not contain fuel, is introduced into the combustion space. At a peripheral position which in turn surrounds the intermediate position, in other words at a radial distance to the first position which is further than the intermediate position, a stream of pure fuel without combustion air is introduced into the combustion space. While the method is being carried out, the introduced stream of pure combustion air mixes with the flue gases, which emanate from the first combustion zone O, in an area or mixing zone B adjacent to the intermediate position. As a result thereof, the flow of this combustion air is slowed down and the temperature of the combustion air increases. The stream of pure fuel introduced via the peripheral position mixes with the flue gases circulating in the combustion space in an area or mixing zone A, adjacent to the peripheral position. As a result of this mixing, the flow velocity of the fuel stream decreases and the fuel stream is diluted with the flue gases containing little oxygen which circulate in the combustion space. The temperature in this mixing zone A increases as the distance from the peripheral position, where the fuel was introduced, increases. The mixtures which are thus produced of combustion air and flue gases from the first combustion zone O, on the one hand, and fuel and circulating flue gases, on the other hand, then mix with each other in a subsequent or additional mixing and combustion zone C, which is adjacent to the above-described mixing zones B and A for combustion air and fuel, respectively. This additional mixing and combustion zone C is downstream of the first combustion zone O and the mixing zones B and A in which steps e) and f) take place. As a result, the concentration of oxidizing agent (oxygen) in this additional mixing and combustion zone C increases with respect to this concentration in the mixing zone A of pure fuel with circulating flue gases, due to being mixed with the mixture of pure combustion air and flue gases from the first combustion zone O, which usually still contain residual oxygen. The temperature of the mixture of fuel and circulating flue gases in this additional mixing and combustion zone increases further as the circulating flue gases often have a lower temperature than the flue gases from the first combustion space. The circulating flue gases have at least partially transferred their heat, for example to a heat exchanger in which steam is generated from water. Thus, it is only in the additional mixing and combustion zone C that conditions are created in which stable ignition of the fuel coming from the peripheral position takes place. After all, in the upstream mixing zone A of fuel, the concentration of oxygen is still too low for stable ignition. In addition, the velocity of the stream of pure fuel in said mixing zone A is too high for stable ignition. The base flame in the first combustion zone O serves as a constant ignition source for the main flame in this additional mixing and combustion zone C and the downstream combustion zone D which is still to be discussed. As a result of the introduction positions of the various streams, the combustion conditions in the additional mixing and combustion zone C are such that combustion is substoichiometric. In other words, fuel is burned using an undersize of oxygen. The mixing of the mixtures in the additional mixing and combustion zone C increases as the distance to the introduction positions increases. However, the oxygen is immediately consumed by the fuel which has been sufficiently heated by the flue gases. The situation where the additional mixing and combustion zone C contains a substoichiometric amount of oxygen thus remains. The resulting combustion products absorb the heat of combustion, so that the partial combustion in the additional mixing and combustion zone C will continue to take place at a relatively low temperature. Recirculation of flue gases in the combustion space thus assists combustion at low temperature. In addition, a so-called reburning effect occurs in this area. This means that any nitrogen oxides present in the flue gases from the first combustion zone O and/or circulating flue gases from the combustion space are reduced, as the oxygen present in the nitrogen oxides is used for burning the fuel and intermediate products which are formed during combustion. Complete combustion of the entire remaining fuel only takes place in combustion zone D which is downstream of the mixing zones B and A and the additional mixing and combustion zone C. Again at relatively low temperature since the flue gases which are present in large amounts absorb the heat of combustion. Thus, with the method according to the invention, the various streams are introduced into the combustion space separately from one another. As a result, one of the combustion reactants, be it fuel or be it oxygen, is locally always present in a substoichiometric amount. As a result of the locally substoichiometric amount, combustion takes place under circumstances in which the flame temperature is lower than the maximum temperature at stoichiometric combustion. This low temperature results in the formation of harmful nitrogen oxides being suppressed and thus in a low emission of these nitrogen oxides. With the method according to the invention, there are three physically separated zones in the flame area in the combustion space, which together result in a low production of nitrogen oxides, i.e. a lean (with respect to fuel) combustion zone O of the base flame of the premixed mixture, a rich mixing and combustion zone C in which partial combustion takes place and a further combustion zone D with a small excess of oxygen.

In this specification, the expression "fuel in gaseous form" comprises a gaseous fuel and vaporous fuel obtained from a liquid or solid fuel.

Preferably, the premixed mixture of fuel in gaseous form and oxygen-containing combustion air contains an excess of oxygen compared to a stoichiometric combustion reaction, preferably as large an excess of oxygen as possible. As a result of this measure, the flue gases produced in the first combustion zone O no longer contain fuel, which would otherwise still burn in the mixing zone B adjacent to the first combustion zone O and the intermediate position for the introduction of pure combustion air.

More preferably, the partial combustion takes place in the additional mixing and combustion zone C at a substoichiometric amount of oxygen compared to a stoichiometric combustion reaction. As has already been explained above, such a substoichiometric combustion results in a relatively low temperature and a correspondingly low formation of nitrogen oxides and in the reduction, through reburning, of any nitrogen oxides originating from the flue gases.

In a further preferred embodiment of the method according to the invention, the combustion is carried out in the final combustion zone D, where complete combustion takes place, using an excess of oxygen compared to a stoichiometric combustion reaction. Advantageously, this excess is in the range from 0.5 to 2 per cent. Thus, complete combustion of all the fuel introduced is ensured.

The method steps according to the invention are preferably carried out under atmospheric pressure in the combustion space. In that case, the method according to the invention can be used to generate steam, such as in a water heater or steam boiler, and for generating heat, such as in a furnace or kiln.

Advantageously, the first position, the intermediate position and the peripheral position are substantially concentrically located around a common centre, such as an axial burner axis of the burner used, or an axially arranged stabilizing element, the action of which will be explained below. Each position itself usually comprises a number of spray nozzles or ducts with apertures, which are situated at an equal radial distance from the centre. The various positions encircle one another.

Preferably, the first combustion zone O is partially shielded from the adjacent mixing zone B, where mixing of the flue gases originating from the first combustion zone O and the pure combustion air takes place. This shielding results in a certain degree of recirculation of flue gases in this first combustion zone O and in a stabilization of the base flame.

The total flow rate of fuel-containing combustion air and the total flow rate of oxygen- containing combustion air are preferably such that the fuel burns completely using a slight excess of oxygen based on the total of the combustion reactions. The flue gas usually still comprises 0.5-2 per cent oxygen.

Advantageously, the flow rate of the fuel introduced via the peripheral position is in the range from 80-90% of the total fuel flow rate. In other words, 10-20% of the total fuel flow rate is introduced via the first position and burned in the first combustion zone O.

The invention also relates to a burner for the staged combustion of a fuel in gaseous form in a combustion space, in particular of a device for generating heat for heating a medium such as for example water or steam, preferably a directly fired atmospheric steam boiler, comprising central introduction means for introducing a stream of a premixed mixture of fuel in gaseous form and oxygen-containing combustion air having at least one central supply opening ending in said combustion space, for feeding a central base flame in a first combustion zone O in the combustion space, - intermediate introduction means for introducing a stream of oxygen-containing combustion air into the combustion space, in which the intermediate introduction means comprise at least one combustion air supply opening ending in said combustion space, in which the combustion air supply opening is arranged in such a manner that it at least partially surrounds the central supply opening, - peripheral supply means for introducing a stream of fuel in gaseous form into the combustion space, in which the peripheral supply means comprise at least one fuel supply opening ending in said combustion space, in which the fuel supply opening is arranged in such a manner that it at least partially surrounds the combustion air supply opening, in such a manner that, during operation, in a first combustion zone O of the combustion space, the central base flame forms an igniting flame for the combustion of fuel in gaseous form which is introduced via the intermediate introduction means and the peripheral supply means and combustion air and the flue gases originating from the first combustion zone O in at least one further zone C, D of the combustion space, which further zone C, D is situated downstream of the first combustion zone O.

With the burner according to the invention, the first and intermediate introduction means and peripheral supply means are positioned in different positions with respect to one another, as has been explained above in great detail in the discussion of the method according to the invention. This also results in the effects and advantages described above. All introduction and supply means comprise one or more supply openings which end in the combustion space.

In a preferred embodiment of the burner according to the invention, the central introduction means and peripheral supply means comprise a common supply line for fuel which branches into a fuel line to the fuel supply opening of the peripheral supply means and a mixture line which opens out into at least one supply duct for combustion air, which supply duct is provided with a mixture discharge opening. This results in the supply of fuel in gaseous form via the respective introduction and supply means taking place at essentially the same pressure. The fuel and combustion air are mixed in the supply duct, so that, after leaving the mixture discharge opening, the premixed mixture obtained in this way can be burned directly in the first combustion zone O as a base flame.

This base flame is advantageously stabilized. Preferably, a stabilizing element is provided in the first combustion zone O for this purpose. More preferably, the central supply opening is situated around or at least surrounding a stabilizing element for stabilizing a base flame in the first combustion zone O. As a result of this construction, a central base flame can be obtained and kept in a predefined position (following initial ignition). In a further preferred embodiment thereof, the stabilizing element has a stabilizing surface which is arranged downstream of the mixture discharge opening in the flow direction of the premixed mixture of fuel in gaseous form and oxygen-containing combustion air and directly upstream of the first combustion zone O. In this preferred embodiment, the stabilizing surface protrudes beyond the mixture discharge opening in the downstream direction, thus preventing an undesired flashback of the base flame in a supply duct for the premixed mixture.

Preferably, the burner according to the invention is furthermore provided with constricting means for partially shielding the first combustion zone O by generating recirculation of flue gases in the first combustion zone O. Such constricting means delimit the first combustion zone O, so that the base flame bums in a defined area and cannot flash back into a mixture supply duct. The constricting means promote the recirculation of the flue gases in the first combustion zone O and thereby form a stabilizing vortex for the central base flame, on which the combustion of combustion air and fuel supplied via the intermediate introduction means and periferal supply means can stabilize and become secured.

The intermediate introduction means preferably comprise at least one combustion air supply duct, the combustion air supply opening of which is provided downstream of the first combustion zone O, viewed in the flow direction of the combustion air. Such a construction promotes the correct positioning of the mixing of flue gases from the first combustion zone O and the pure stream of combustion air. Thus, the first combustion zone O is advanrtageously delimited by the at least one central supply opening, the at least one combustion air supply duct and the constricting means. Furthermore, the combustion zone O borders the mixing zones B and A and the additional mixing and combustion zone C.

As mentioned above, the burner preferably is a symmetrical construction, in which the central introduction means, the intermediate introduction means and the peripheral supply means are arranged concentrically around a central burner axis.

More preferably, means for promoting recirculation of flue gases are provided between the intermediate introduction means and the peripheral supply means, which recirculation- promoting means extend beyond the combustion air supply opening in the flow direction of the oxygen-containing combustion air.

The above-described construction of the burner according to the invention promotes the targeted recirculation of flue gases in the combustion space and thus the combustion at low temperature. This construction prevents rotation of the flow pattern as will be described below in more detail with reference to Fig. 4, resulting in a relatively narrow and elongate flame. If, with a burner according to the invention, the distance of the wall to an opposite wall is small, a recirculation stream or vortex can easily develop. When several burners are arranged a small distance apart, an internal recirculation area between the burners may develop and become established. This effect is reinforced by said means for promoting recirculation of flue gases, which are positioned between the intermediate introduction means and peripheral supply means, and by the positioning thereof with respect to one another.

The invention also relates to a device for generating heat for heating a medium, such as, for example, water or steam or another medium, for example a directly fired atmospheric steam boiler, or a furnace or kiln, comprising a housing and a combustion space arranged in the housing, which combustion space is at least delimited by a wall with one or more burners according to the invention as described above. In the case of a steam boiler, the combustion space also borders a wall provided with flow ducts for water or steam. The medium itself does not take part in the combustion reaction.

Preferably, the method according to the invention is carried out on industrial scale. Preferably, the burner and device according to the invention are also designed for use in large-scale production. For industrial applications capacities of about 10 to about 100 MW per burner are usual.

The invention will be explained below with reference to the attached drawing, in which: Fig. 1 shows a diagrammatic cross section of an embodiment of a burner according to the invention;

Fig. 2 shows a view of the burner mouth of the burner illustrated in Fig. 1 ; Fig. 3 shows the various combustion zones and mixing zones during staged combustion of a fuel in such an embodiment in order to illustrate the method; and Fig. 4 illustrates the recirculation of flue gases occurring in the combustion space.

Figs. 1 and 2 are a diagrammatic cross section and view, respectively, of an embodiment of a burner according to the invention. In this embodiment, the burner 10 is positioned in an erect, vertical wall 12 of an atmospheric steam boiler. It will be understood that the burner may, in principle, also be placed in a horizontal wall or in an oblique wall at any suitable angular orientation or in a corner line where two walls of different angular orientation join one another. The atmospheric steam boiler furthermore comprises a wind box 14 for supplying the required combustion air. Reference numeral 16 denotes a combustion space in which the staged combustion takes place. This combustion space 16 is delimited, inter alia, by the wall 12 and an opposite wall (not illustrated in this figure) which is generally provided with a number of flow ducts for water. The burner 10 connects the wind box 14 to the combustion space 16.

The burner 10 comprises central introduction means 20 for supplying fuel in gaseous form, which are connected to a fuel source (not shown). In the illustrated case, the central introduction means 20 comprise a gas ring 22. Combustion air is supplied from the wind box 14 to a series of mixture supply ducts 24 via a valve 26, which ducts 24 are arranged in concentric circle positions. By means of valve 26, the amount of combustion air which is supplied to the ducts 24 can be adjusted. A mixture supply duct 24 comprises a so-called ejector duct 28. The gas ring 22 is provided with a series of gas spuds 30 (also referred to as ejectors) which are arranged in concentric circle positions. These gas spuds 30 inject fuel into the ends 32 of the corresponding ejector ducts 28. Each mixture supply duct 24 comprises an ejector duct 28 and an associated gas spud 30. In the ejector ducts 28, complete mixing of the supplied fuel and supplied combustion air takes place so that a premixed mixture of constant compositon flows into a chamber 38 via mixture discharge openings 34 on the opposite ends 36 of the ejector ducts 28. This chamber 38 is delimited by said mixture discharge openings 34 and a surrounding cylindrical wall or sleeve 40. On the inside, this sleeve 40 is provided with constricting means 42, in this case a constricting ring placed at an angle so as to leave clear a central supply opening 44 for supplying the premixed mixture of fuel in gaseous form and combustion air from the chamber 38 to a first combustion zone O in the combustion space 16. A stabilizing element 46 is arranged centrally in the middle of the burner 10, viewed in the axial direction, and has a stabilizing surface 48 which is positioned approximately at the level of the inner circumference of the constricting ring 42 and thus partially closes the central supply opening 44. In the combustion zone O, the premixed mixture and the resultant flue gases recirculate behind the constricting ring 42. This constricting ring 42 and a section 50 of the sleeve 40, which section protrudes beyond the constricting ring 42 in the flow direction, partially screen off the first combustion zone O against the effects of the other areas of the combustion space 16, the other streams occurring therein which are still to be discussed below, and the mixing and combustion processes occurring in the combustion space 16. Following ignition by an ignition burner (not shown), the premixed mixture will burn in a stable manner in the first combustion zone O and the base flame formed will stabilize on the stabilizing surface 48 and on the constricting ring 42. The base flame is low in fuel (excess of combustion air). Adjacent to the mixture supply duct 24, intermediate introduction means 51 for supplying a stream of combustion air to the combustion space 16 are provided. In this embodiment, these means 51 comprise an annular combustion air supply duct 52 to which pure combustion air is supplied from the wind box 14 by means of valve 54. A combustion air supply opening 56 of supply duct 52 opens out at a position which, viewed in the flow direction, is downstream of the central supply opening 44 in the combustion space 16. At a certain radial distance from the combustion air supply opening 56, peripheral supply means 60 are arranged for introducing pure fuel in gaseous form into the combustion space 16. These peripheral supply means 60 having peripheral fuel supply openings 62 are connected to the same source of fuel as the central introduction means 20, so that the pressure is the same. In order to promote targeted circulation of flue gases and thus mixing constricting means in the form of a constricting sleeve 64 are provided between the intermediate introduction means 51 and the peripheral supply means 60. The constricting sleeve 64 forms a cylindrical radial boundary for a central burner mouth 66, which extends for a certain depth from the wall 12 into the combustion space 16. The central burner mouth 66 comprises the central supply opening 44 and the combustion air supply opening 56 of the intermediate introduction means 51. The constricting sleeve 64 may be made of concrete, so that it forms a concrete burner mouth.

The view illustrated in Fig. 2 shows the concentric arrangement of the central introduction means 20 for a premixed mixture at a first position 70, of the intermediate introduction means 51 for pure combustion air at an intermediate position 80 and of the peripheral supply means 60 for pure fuel at a peripheral position 90.

The different positions of the various introduction and supply means and the composition of the differrent streams produce a number of zones in the combustion space 16 which will be described in more detail with reference to Fig. 3.

The gaseous fuel streams from the peripheral supply means 60 form gas jets and mix with flue gases circulating in the combustion space 16 in a mixing zone A which adjoins the peripheral supply means 60. These flue gases reduce the speed of the fuel streams. Due to the high temperature of the flue gases, the temperature of the mixture formed increases as the distance from the peripheral supply means 60 in the mixing zone A for fuel increases. In a mixing zone B for combustion air, adjacent to the intermediate position 80 of the intermediate introduction means 51 , the introduced pure combustion air is mixed with the flue gases originating from the first combustion zone O. Downstream, the mixing zones A and B meet in an additional mixing and combustion zone C, where the mixture of fuel and flue gases originating from mixing zone A and the mixture of combustion air and the flue gases produced in combustion zone O originating from mixing zone B are mixed together. In this zone C, the prevailing conditions satisfy the requirements for ignition of the mixture formed. Partial combustion at a substoichiometric amount of oxygen in this zone C results. Complete combustion of all remaining fuels using a slight excess of oxygen takes place in downstream combustion zone D. As a result of the large amounts of flue gases in zone D, which act as an energy-absorbing mass there, the combustion temperature in zone D is relatively low.

The embodiment of a burner according to the invention illustrated in Figs. 1 and 2 promotes circulation of flue gases and thus staged combustion at low temperature under local conditions of a substoichiometric amount of fuel (zone O), and/or a substoichiometric amount of oxygen (zone C), and/or a dilution by the mass of flue gases (zone D). Due to the relatively low combustion temperatures in the zones, the formation of nitrogen oxides is suppressed, so that the burner according to the invention according to the illustrated embodiment forms flue gases having a very low NO_x content.

Circulation in the combustion space 16 is further illustrated in Fig. 4, where the wall 12 and an opposite wall 13 (with flow ducts for water/steam which are not shown) of the atmospheric steam boiler from Fig. 1 are illustrated diagrammatically. The central burner mouth 66 is diagrammatically represented as a rectangle supported against the wall 12. Two thick arrows 68 indicate the introduction streams of fuel in gaseous form which flow from two of the peripheral fuel supply openings into the combustion space 16. Thinner arrows 69 indicate the local direction and speed (arrow length) of the circulation streams for the gases in the combustion space 16. During operation, a stable flame is produced in the combustion space 16 with a flow pattern indicated by arrows 69. The flow does not rotate, or hardly rotates, about its longitudinal axis, resulting in the flame being slim and elongate. As a result thereof, a recirculation vortex is produced via the opposite wall 13, even when the distance from the burner to that wall 13 is relatively small. Neighbouring burners in wall 12 can be positioned at a relatively small distance from one another while, at the same time, the creation of internal recirculation areas between the burners is retained. The depth of the central burner mouth 66 and the peripheral introduction streams of gaseous fuel (arrows 68) enhance the formation of the recirculation areas.

Claims

1. Method for the staged combustion of a fuel in gaseous form in a combustion space (16), in particular of a device for generating heat for heating a medium such as, for example, water or steam, preferably a directly fired atmospheric steam boiler, which method comprises: a) introducing a stream of a premixed mixture of fuel in gaseous form and oxygen- containing combustion air at a first position (70) into said combustion space (16), b) introducing a stream of oxygen-containing combustion air into the combustion space (16) at an intermediate position (80) which at least partially surrounds the first position (70), c) introducing a stream of fuel in gaseous form into the combustion space (16) at a peripheral position (90) which at least partially surrounds the intermediate position (80), d) burning the introduced stream of the premixed mixture of fuel in gaseous form and oxygen-containing combustion air in the form of a base flame in a first combustion zone (O) adjacent to the first position in the combustion space (16), e) mixing the introduced stream of oxygen-containing combustion air in a mixing zone (B) adjacent to the intermediate position (80), with flue gases coming from the first combustion zone (O), f) mixing the introduced stream of fuel in gaseous form, in a mixing zone (A) adjacent to the peripheral position (90), with flue gases circulating in the combustion space (16), g) mixing the mixed streams obtained in steps e) and f) with each other and partially burning them in an additional mixing and combustion zone (C), adjacent to the mixing zones (A) and (B), h) essentially completely burning the partially burned mixture formed in the additional mixing and combustion zone (C) in a downstream combustion zone (D).

2. Method according to claim 1 , wherein the premixed mixture of fuel in gaseous form and oxygen-containing combustion air contains an excess of oxygen compared to a stoichiometric combustion reaction.

3. Method according to one of the preceding claims, wherein the partial combustion takes place in the additional mixing zone (C) using a substoichiometric amount of oxygen compared to a stoichiometric combustion reaction.

4. Method according to one of the preceding claims, wherein the combustion is carried out in combustion zone (D) using an excess of oxygen compared to a stoichiometric combustion reaction.

5. Method according to one of the preceding claims, in which the method steps are preferably carried out under atmospheric pressure in the combustion space (16).

6. Method according to one of the preceding claims, in which the first position (70), the intermediate position (80) and the peripheral position (90) are substantially concentrically located.

7. Method according to one of the preceding claims, in which the first combustion zone (O) is partially shielded from the adjacent mixing zone (B).

8. Method according to one of the preceding claims, in which the total flow rate of fuel and the total flow rate of oxygen-containing combustion air are such that the fuel burns completely using a slight excess of oxygen compared to the total of the combustion reactions.

9. Method according to one of the preceding claims, in which the flow rate of the fuel introduced via the peripheral position (90) is in the range from 80-90% of the total fuel flow rate.

10. Burner (10) for the staged combustion of a fuel in gaseous form in a combustion space (16), in particular of a device for generating heat for heating a medium such as for example water or steam, preferably a directly fired atmospheric steam boiler, comprising central introduction means (20) for introducing a stream of a premixed mixture of fuel in gaseous form and oxygen-containing combustion air having at least one central supply opening (44) ending in said combustion space (16), for feeding a central base flame in a first combustion zone (O) in the combustion space (16), intermediate introduction means (51) for introducing a stream of oxygen-containing combustion air into the combustion space (16), in which the intermediate introduction means (51) comprise at least one combustion air supply opening (56) ending in said combustion space (16), in which the combustion air supply opening (56) is arranged in such a manner that it at least partially surrounds the central supply opening (44), peripheral supply means (60) for introducing a stream of fuel in gaseous form into the combustion space (16), in which the peripheral supply means (60) comprise at least one fuel supply opening (62) ending in said combustion space (16), in which the fuel supply opening (62) is arranged in such a manner that it at least partially surrounds the combustion air supply opening (56), in such a manner that, during operation, in a first combustion zone (O) of the combustion space (16), the central base flame forms an ignition flame for the combustion of fuel in gaseous form which is introduced via the intermediate introduction means (51) and the peripheral supply means (60) and combustion air and the flue gases originating from the first combustion zone (O) in at least one further zone (C, D) of the combustion space (16), which further zone (C, D) is situated downstream of the first combustion zone (O).

11. Burner according to claim 10, wherein the central introduction means (20) and peripheral supply means (60) have a common supply line for fuel which branches into a fuel line to the fuel supply opening and a mixture line (22, 30) which opens out into at least one supply duct (24) for combustion air, which supply duct (24) is provided with a mixture discharge opening (34).

12. Burner according to claim 10 or 11 , wherein the central supply opening (44) is situated around or at least surrounding a stabilizing element (46) for stabilizing a base flame in the first combustion zone (O).

13. Burner according to claim 12, wherein the stabilizing element (46) has a stabilizing surface (48) which is arranged downstream of the mixture discharge opening (34) in the flow direction of the premixed mixture of fuel in gaseous form and oxygen-containing combustion air and directly upstream of the first combustion zone (O).

14. Burner according to one of claims 10-13, which is furthermore provided with constricting means (42) for stabilizing the central base flame in the first combustion zone (O) by causing recirculation in the first combustion zone (O), which constricting means preferably surround and delimit the central supply opening (44).

15. Burner according to one of claims 10-14, wherein the intermediate introduction means (51) comprise at least one combustion air supply duct (52), the combustion air supply opening (56) of which is provided downstream of the first combustion zone (O), viewed in the flow direction of the combustion air.

16. Burner according to claim 15, wherein the first combustion zone (O) is delimited by the at least one central supply opening (44), the at least one combustion air supply duct (52) and the constricting means (42).

17. Burner according to one of claims 10-16, wherein the central introduction means (20), the intermediate introduction means (51) and the peripheral supply means (60) are arranged concentrically around a central burner axis.

18. Burner according to one of claims 10-17, wherein means (64) for promoting recirculation of flue gases are provided between the intermediate introduction means (51) and the peripheral supply means (60), which recirculation-promoting means (64) extend beyond the combustion air supply opening (56) in the flow direction of the oxygen-containing combustion air.

19. Burner according to claim 18, in which the means (64) for promoting the recirculation of flue gases comprise a constricting sleeve.

20. Device for generating heat for heating a medium, such as, for example, water or steam or another medium, for example a directly fired atmospheric steam boiler, a furnace or kiln, comprising a housing and a combustion space (16) arranged in the housing, which combustion space (16) is at least delimited by a wall (12) with one or more burners (10) according to one of the preceding claims 10-19.

21. Device according to claim 20, in particular a directly fired atmospheric steam boiler, wherein the combustion space (16) is furthermore delimited by at least one wall (13) provided with flow ducts for water and/or steam.