The present invention relates to a novel low NOx burner for liquid and gaseous fuel. More precisely the invention relates to a low/medium capacity burner designed to equip steam or hot water generators both for civil purposes (for example remote location heating plants) and for industrial purposes (for example, boilers for steam generation for machinery, industrial laundries, etc.).
Low/medium capacity burners of the prior art generally comprise a duct for the combustion air, which can be whirled by means of an adjustable radial register or by means of a tangential inlet (volute inlet), and a large perforated and/or finned disk, placed at the outlet of the duct, for the stabilization of the flame. The burners of this kind are considerably simple in structure and construction, but have a low ability to reduce the nitrogen oxides emissions and very high aerodynamic pressure drops (in the range of several thousands of Pa). On the other side, for large size plants (such as thermoelectric power plants) there are known burners in which the combustion air is split into several streams, generally three streams, in such a way to control the reaction stoichiometry and the flame temperature. This system, named stage combustion, consists in suitably dosing the air and the fuel within the combustion system, in such a way to form a fuel-rich zone, in which the fuel pyrolysis processes take place and NOx reduction processes are activated, and a fuel-poor zone where the remaining portion of combustion air mixes with the fuel to complete the combustion. Burners of this type, however, are not suitable for low/medium capacity plants, both for cost reasons and the difficulties of reproducing their geometry on a lower scale.
EP 0536556 A1 discloses a burner for oil and gas fuels comprising a primary air duct and a secondary air duct coaxial to the primary air duct, oil/gas fuel supplying means extending in the primary duct, means for controlling the flowrate of the primary and the secondary air duct and means for creating vorticity in the combustion air comprising moving swirlers at the outlet side of the primary and secondary air ducts. US 3367385 A discloses a burner wherein the primary air duct is provided with an axially slidable radial swirler at its inlet side.
The object of the present invention is to provide a liquid and gaseous fuel burner suitable of performing a stage combustion typical of the "three airs" burners, with all the related advantages in term of reduced NOx production, but with a more simple structure with respect to this type of burners, thus being fit for installation in low/medium capacity steam/hot water generation plants.
Another object of the present invention is to provide a burner of the above-mentioned type, suitable to equip the combustion units of new steam and hot water generators, as well as to be mounted on existing plants.
The above objects are achieved by the burner of the invention, by virtue of the division of the combustion air into two streams, called primary and secondary air stream, as well as of a suitable geometry of the burner outlet and by means of the use of a turbulence generator (primary swirler) on the primary air with a radial flow divider and a controllable turbulence generator (secondary swirler) on the secondary air. Thanks to the above simplified structure of the burner it is possible to achieve a lower unit cost and lower air side pressure drop with the same capacity and pollutant emissions.
The features and advantages of the burner according to the present inventions will be clearer from the following description of an exemplifying and not limiting embodiment thereof with reference to the following drawings wherein:
With reference to Figure 1, it has been indicated at 1 a burner mounted in correspondence of a conical opening 2 of an inner wall 3 of a combustion chamber 4. Opening 2 is formed in the shape of a throat diverging toward combustion chamber 4. Combustion air is fed through a windbox 5 and split into two streams, called primary air stream A1 and secondary air stream A2 through respective primary air duct 11 and secondary air duct 17 coaxially arranged on longitudinal axis X-X of the burner.
The burner comprises a liquid fuel lance 6 connected to a flange 7 at the outer end and having an atomizer 8, of the known type, for converting the fuel into a fine spray at the inner end inside combustion chamber 4. Lance 6 is centrally arranged on the longitudinal axis X-X of the burner and a plurality of lances 9 for feeding gaseous fuels are arranged in parallel relation around it. Gas lances 9 extend from a toroidal header 10 located outside of flange 7 up to combustion chamber 4. Therefore, the burner can handle liquid fuel, gaseous fuel or both of them.
Primary air duct 11 extends coaxially to liquid fuel lance 6 and houses gas lances 9. It is connected to flange 7 at its outer end and communicates with combustion chamber 4 at the inner one. A plurality of radial windows 12 for feeding the air from windbox 5 are formed on primary air duct 11 close to flange 7 and a cylindrical damper 13 axially slidable by means of a pair of rods 14 (only one shown in figure 1) extending outside of flange 7 is provided inside primary duct 11 in correspondence to windows 12.
Secondary air duct 17 extends coaxially to primary air duct 11 from a flange 18 radially extending from duct 11 and outflows in combustion chamber 4. A plurality of radial windows 19 for feeding air from windbox 5 are formed on secondary air duct 17 close to flange 18. A cylindrical damper 20 axially slidable by means of a pair of rods 21 (only one shown in figure 1) is provided in correspondence to windows 19 on the outer side of secondary air duct 17. The axial sliding of cylindrical dampers 13 and 20 allows for a continuous adjustment of the respective inlet sections of primary and secondary air ducts and, by this way, the partitioning of the flowrate of the combustion air. The adjustment can be carried out both by manual operation of respective rods 14 and by motorized operation. The primary and secondary air ducts can be fed either by the same windbox 5, as shown, or by different windboxes.
A primary swirler 15 formed by a plurality of fixed blades 15a is arranged in primary air duct 11 in correspondence to the outlet end at combustion chamber 4. A radial flow divider 16, conically shaped and coaxial to axis X-X of the burner, is provided at the outlet of primary swirler 15. The blades 15a of primary swirler 15 have a fixed inclination defined as a function of the application. The blades can be flat (figures 2a, 2b), in the case in which the outlet angle is the same over the whole radius of the primary air duct, or they can have a variable section. The section can be varied in a continuous way so as to have an outlet angle of the primary air flow variable with the radius, as shown in figures 3a, 3b, or in a discontinuous way by making each blade of two differently inclined blade-portions connected to each other, as shown in figures 4a, 4b.
Radial flow divider 16, arranged on the side of primary swirler 15 facing toward combustion chamber 4, divides swirler 15 into two concentric annuli, of which the external one has flowing section 2-3 times greater than the inner one.
A movable swirler 22, formed by a plurality of inclined blades, is arranged at the inner side of secondary air duct 17 in correspondence to windows 19.
Swirler 22 is made axially slidable between two end positions, a first one completely downstream of windows 19 and a second one, shown with discontinuous line in figure 1, completely upstream of windows 19, the sliding of movable swirler 22 being controlled by rods 23 extending outside of flange 7. As also shown schematically in figures 5a, 5b, 5c, depending on the position of movable swirler 22 with respect to air inlet windows 19, the air vorticity can be controlled in such a way that it will be null when swirler 22 is in the backward position, i.e. the second position and the combustion air does not passes through it (fig.5a), medium when the swirler is in the intermediate position (fig. 5b), and maximum when the swirler 22 is in the forwardly extended position, i.e. the first position (fig. 5c) . The vorticity control is obtained by placing the swirler in one of the possible intermediate positions. A venturi 24 is placed downstream of movable swirler 22 in correspondence to the outlet of secondary air duct 24. Furthermore a radial flow divider 25 is arranged at the outlet of secondary air duct 17 in the combustion chamber 4.
Flow divider 25 has a frusto-conical shape and extends from the outlet cage of primary air duct 11 thus delimiting the primary air stream from the secondary air stream.
The primary swirler 15 placed on the primary air stream at the outlet of the burner allows to obtain a field of motion typical of a low-NOx burner with low pressure drop without the risk of moving backward the flame front, in any operating condition, and fouling or damaging burner parts. The blades are so shaped as to increase the amount of air passing through the central zone. If a structure with a double series of blades such as that shown in figures 4a, 4b is used, the same purpose is achieved and further the whirling, i.e. the vorticity of the part of primary air passing through the outer annulus, can be increased. An equivalent effect can be obtained by using, instead of a double-bladed swirler, a swirler with single blades shaped so as to have an outlet angle variable with the radius, such as that shown in figures 3a, 3b. Radial flow divider 16 on the primary air stream has the purpose of separating the two flow regimes created by the blade shaping and stabilizing the flame. If the flow divider is combined with a double-bladed swirler, the diameter of the flow divider at the outlet of the swirler is equal to the diameter of separation of the annuli defined by the different inclinations of the series of blades.