Improved burner for thermic generators

Abstract: A burner (4) with low NOx production, wherein the combustion air is subdivided in three streams with swirlers in each air stream. The stream of primary air is intercepted by a swirler comprising an array of radially fixed vanes (15) mounted in the corresponding outlet section of primary air duct (10) in the combustion chamber (3), while the secondary and tertiary air swirlers comprise vanes (18, 19) turnable around axes disposed perpendicular and parallel to the central axis of the burner, respectively. The primary and secondary air ducts (10,11) have a continuously adjustable inlet section. Furthermore, there are provided flow rate meters (24, 25) near the outlet section of the secondary and tertiary air ducts (11,12) substantially unaffected by the vorticity induced by said swirlers.

Description: The present invention relates to a burner for fuel oil and fuel gas with low NOx production according to the first part of claim 1. In the known burners of the above mentioned type it is known the technique to subdivide the total combustion air in more streams, expecially a primary, a secondary and a tertiary stream, in order to achieve an improved combustion control both from the thermal and stoichiometric point of view. The function of primary air is essentially to ensure the cleaning and the cooling of the central zone of the burner, whereas the function of a correct ratio between the secondary and tertiary air momentum is to provide the flame stoichiometric concentration suitable for the redox reactions to occur. The burners for thermic generators, expecially for thermoelectric generators, operating in this way, include a lance with relevant atomizer, when liquid fuel is used, or several lances when gaseous fuel is used, or a combination of them if the burner is designed to use both type of fuel. According to the above mentioned technique, in a coaxial relation to said fuel feeding means there are provided ducts for primary, secondary and tertiary air flowing to the combustion chamber together with the fuel and communicating with an air chamber. In one or more of these air ducts there are provided swirlers for the registration of the air vorticity for the aerodynamic control of combustion. In these burners the swirlers control both the air flowrate and the distribution of different air jets injected in the combustion chamber. The control of air distribution among the primary, secondary and tertiary air streams, also involves some modifications in the vorticity characteristics of the relevant turbulent jets. This results in a inadequate possibility of controlling the combustion process and expecially the NOx emission in different operating conditions of the burner, in relation both to the thermic load and the chemical-physic characteristics of the different fuels used. US Patent No. 3904349 discloses a low NOx burner for pulverized coal and fuel oil wherein the total air required for the combustion is supplied from a windbox and is distributed among three separate passageways, namely a central passageway (primary air), a first annular passageway (secondary air), a second annular passageway (tertiary air). Primary and secondary air are supplied through a common inlet port and controlled by an adjustable sleeve member, while the primary/secondary air ratio is controlled by a further sleeve member. The flow of tertiary air is controllred by inlet dampers. The initial burning of the fuel is conducted in a reducing zone by limiting the flow of primary air. Secondary air is recirculated about the outer periphery of the reducing zone so as to create a flame stabilizing zone. Tertiary air is controlled so as to envelop the reducing and stabilizing zones and eventually mix with the fuel to complete its combustion. With the above described system the control of the aerodynamics of the combustion is poor, it is difficult to regulate the quantity of primary and secondary air to be admitted and the NOx reduction is low. In the low NOx burner for pulverized coal, fuel oil, fuel gas disclosed in the European patent No.028056 the primary air is not supplied from the windbox but adducted through the coal duct from an external source and controlled by control means external to the air registers of the burner. Swirlers are provided on primary, secondary and tertiary air but the secondary air swirler is not adjustable and the primary air swirler must be weak and placed too far from the combustion chamber throat to produce a significant swirling action. As a consequence the control of the flame stability in the reducing zone close to the burner is very difficult and the primary air flowrate cannot be maximized. Furthermore in the prior art burners it is not possible to have accurate information about the flowrate of the different air streams due to the vortical motion of the air induced by the swirlers. This also limits the possibility of optimizing the control of the combustion. The object of the present invention is to provide an improved burner with low NOx emission, useful for fuel oil and fuel gas, fit to be installed in combustion units of both new steam generators or furnaces, and of existing plants. Another object of the present invention is to provide a burner, of the above mentioned type, capable of performing a multistage combustion with a suitable fluid dynamics of the combustion air subdivided in several streams each controlled as far as both flowrate and air distribution are concerned. A further object of the present invention is to provide a burner of the above mentioned type that allows a reduction of the maximum flame temperature while limiting the spatial intensity of heat release in order to reduce the rate of production of thermal NOx. Another object of the present invention is to provide a burner of the above mentioned type capable of ensuring a sufficient amount of combustion air for postcombustion in such a way to limit also the percentage of solid and gaseous unburnts in the smokes. These objects are achieved with the improved burner according to the present invention, which comprises primary, secondary and tertiary air ducts coaxially arranged about a longitudinal axis for feeding combustion air to a combustion chamber and having respective outlet ends connected to a inlet section of said combustion chamber, fuel ducts means coaxial to said air ducts, swirler means for controlling the vorticity of the combustion air arranged in said primary, secondary and tertiary ducts. The primary duct swirler comprises an array of radially fixed tabs in correspondence with said outlet section, the secondary and tertiary duct swirlers further comprising tabs turnable around axes disposed perpendicular to and, respectively, parallel to said longitudinal axis. The primary and secondary air ducts comprise air inlet means with continuously variable and independently adjustable cross sections, mass flowrate meters being further provided near the outlet ends of said secondary and tertiary ducts each in the form of a couple of annular pitot pipes coaxial to said longitudinal axis, thus being substantially unaffected by the vorticity induced by said swirler means. In a particularly preferred embodiment of the invention the streams of primary, secondary and tertiary air are collectec to the combustion chamber, in a coaxial relation to said fuel feeding means and there are provided among them separating means for deviating the streams of secondary and tertiary air in divergent directions. Further characteristics and advantages of the improved burner according to the present invention will become apparent from the following description of a not limiting and exemplifying embodiment thereof, with reference to the accompanying drawings, in which: - figure 1 is a schematic overall view in longitudinal section of the burner assembly according to the present invention. With reference to figure 1, the burner assembly according to the invention comprises an external wall 1 delimiting a combustion air chamber and an inlet section 2 of a combustion chamber 3 of a steam generator or of a furnace of known type or the like. A burner 4 according to the present invention is fixed to the wall 1 and to the inlet section 2. The burner 4 comprises a lance 5 for liquid fuel connected to a fuel injection header 6 external to the wall 1 and ending in the combustion chamber 3 with an atomizer 7, of known type and not described in detail, for nebulizing the fuel. A row of lances 8 for gaseous fuel with nozzles 9 inclined in relation to the central axis of the burner is arranged coaxialy to the lance 5, said lances extending from a toroidal header not shown, external to wall 1. The combustion air is divided in three air streams, i.e. primary, secondary and tertiary air and a primary duct 10, a secondary duct 11 and a tertiary duct 12 are respectively provided for feeding these streams to the combustion chamber 3. The primary air duct 10 extends coaxially to the lances 5 and 8, which are housed therein, and near the wall 1 is equipped with air feeding inlets 13 interceptable by movable air locks 14, while in proximity of its outlet section in the combustion chamber 3 tabs 15 radially fixed on a drum not shown are provided to ensure an adequate vorticity of the primary stream. The secondary air duct 11 is fixed externally and coaxially to the duct of primary air 10 and it also comprises air feeding inlets 16 interceptable by movable air locks 17. Furthermore, in order to give vorticity to the secondary air, inside the duct 11 there is provided an array of tabs 18 pivotally mounted on radial axes and therefore inclinable in relation to the flow direction. The tertiary air duct 12 has a radial inlet in which a plurality of tabs 19 pivotally mounted on a row of axes parallel to the longitudinal axis is arranged and, therefore, also the inclination of tabs 19 is adjustable with respect to the flow direction. For the control of the two swirlers (tabs 18 and 19) housed in the ducts of secondary and tertiary air, articulated control arms 20 and 21, respectively, are provided in a known way, connected to the tabs 18 and 19 and extended externally to the wall 1 for manual or motored operation. Likewise, for the air locks 14 and 17 installed on the inlets 13 and 16 of the primary and secondary air ducts, there are provided control arms 22 operable from the outside. The drum bearing the tabs 15 installed in the duct 10 of primary air can be axially slid by a rod 23 extending externally to the wall 1 for its operation. Near the outlet section to the combustion chamber 3 flowmeters 24 and 25 of the annular pitot type are provided in the ducts 11 and 12 of the secondary and tertiary air, each substantially comprising a couple of annular pipes coaxial to the longitudinal axis, which allow to detect a significant differential pressure in the operating range of the burner with a good measure of sensitivity. These flowmeters are widely insensitive to the orientation of tabs 18 and 19 of the swirlers placed upstream, and therefore unaffected by the vorticity induced by them in the secondary and tertiary streams. A flow divider 26 formed by a diverging frusto conic surface is provided between the duct of primary air 10 and secondary air 11, in correspondence with their outlet section to the combustion chamber. A similar flow divider 27 is provided in the same position between the duct of the secondary air 11 and that of tertiary air 12. In particular, the flow divider 27 and the truncated cone surface 2a of wall 2 delimits a throat 28 through which the stream of tertiary air is accelerated and further deviated with respect to the stream of secondary air, in such a way as to deviate it towards the part of the combustion chamber considered more suitable for the completion of the combustion itself. Advantageously, in order to obtain a further reduction of NOx production and also to limit the flame temperature in the combustion chamber the injection of recycled smokes or gases can be provided through a circuit fed by an independent fan, through an additional duct 29 confluent in the duct 10 of primary air, as shown in figure 1, or directly fed to the combustion chamber 3 by a coaxial independent duct, not shown. The burner according to the invention provides for the control of the combustion air flowrate by keeping constant the attitude of the tabs 18 and 19 of the swirlers placed in the stream of secondary and tertiary air, therefore without appreciable interferences on the vorticity range. This provides for the optimization of flame characteristics from the thermochemical point of view. In particular, the control of air distribution flowrate among the ducts associated with the vorticity control, allows the control of the mixing for the optimization of the characteristics of the combustion air jet. The combination of the above mentioned control capability with the availability of a reliable flowrate measurement in the secondary and tertiary ducts, is of help when balancing operation have to be carried out in the industrial, typically multiburner systems such as those provided for the steam generators. Furthermore, it has to be pointed out that the radial arrangement of the tertiary swirler with respect to the substantially axial arrangement of the swirler installed in the duct of secondary air, allows a more effective vorticity production and a pressure distribution of the tertiary air, behind the divider 27, which helps the penetration of the tertiary air into the combustion chamber.

Claims: Burner for fuel oil and fuel gas with low NOx production comprising primary, secondary and tertiary air ducts (10,11,12) coaxially arranged about a longitudinal axis for feeding combustion air to a combustion chamber (3) and having respective outlet ends connected to a inlet section of said combustion chamber (3), fuel duct means (5,9) coaxial to said air ducts, swirler means (15,18,19) for controlling the vorticity of the combustion air arranged in said primary, secondary and tertiary ducts, characterized in that the primary duct swirler comprises an array of radially fixed tabs (15) in correspondance with said outlet section, the secondary and tertiary duct swirlers further comprising tabs (18,19) turnable around axes disposed perpendicular to and, respectively, parallel to said longitudinal axis, said primary and secondary air ducts comprising air inlet means (13,16) with continuously variable and independently adjustable cross sections, mass flowrate meters (24,25) being further provided near the outlet ends of said secondary and tertiary ducts each in the form of a couple of annular pitot pipes coaxial to said longitudinal axis, thus being substantially unaffected by the vorticity induced by said swirler means. Burner according to claim 1, wherein flow separating means (26,27) are provided between primary, secondary and tertiary air ducts (10,11,12) downstream of their outlet end to said combustion chamber, for deviating the combustion air in divergent directions. Burner according to the previous claims, wherein said flow separating means comprise essentially a couple of coaxial baffles (26,27) with truncated cone shape and with different inclination arranged between said primary and secondary air ducts (10,11) and, respectively, between said secondary and tertiary air ducts (11,12), said baffles fixedly extending from the outlet end of said air ducts. Burner according to claim 3, wherein a throat (28) is delimited between said separating baffle (27) installed between the secondary and tertiary air ducts and a truncated cone divergent wall (2a) of the inlet section of said combustion chamber (3). Burner according to the previous claims, wherein the primary and secondary air is fed to respective primary (10) and secondary (11) air ducts through independent inlets (13,16) interceptable by movable air locks (14,17). Burner according to the previous claims, wherein said swirler of the primary air is axially sliding. Burner according to the previous claims further comprising an additional duct (29) for the injection of recycled smokes. Burner according to claim 7, wherein said additional duct (29) of recycled smokes flows into said primary air duct (10). Burner according to claim 7, wherein said additional duct (29) of recycled smokes is coaxial to said primary air duct (10) and flows directly into said combustion chamber (3).

Inventors: ANCONA GIUSEPPE, BENELLI GIANCARLO, DE MICHELE GENNARO, LIGASACCHI SERGIO, MASCALZI GIANNI, SCAVIZZI GIANCARLO, TREBBI GIULIANO, ZENNARO ALDO, ANCONA, GIUSEPPE, BENELLI, GIANCARLO, DE MICHELE, GENNARO, LIGASACCHI, SERGIO, MASCALZI, GIANNI, SCAVIZZI, GIANCARLO, TREBBI, GIULIANO, ZENNARO, ALDO
Applicants: ANSALDO GIE SRL, ENEL SPA, ANSALDO GIE S.R.L., ENEL S.P.A.
Application year: 1990
Main classification: F23C7
IPC codes: F23C7, F23D17, F23C9