Pulverized coal injection nozzle

Abstract: A nozzle for the injection of solid fuel in dust form into a combustion chamber, to be installed in correspondence with the outlet of a burner coaxially to the longitudinal axis of the latter, comprising a stationary annular part divided into coal ducts of a circumferential width that decreases towards outlet section to form coal dust rich air streams, a central duct for a coal dust poor air stream delimited by the coal ducts and having an outlet end, a central member that is axially mobile within the central duct, for varying the cross section of a central passage (37) delimited by the stationary annular part at the outlet end. Air ducts for the coal dust poor air stream are formed between two adjacent ducts in said stationary annular part communicating with said central duct and having a circumferential width increasing toward said outlet end. Deflector means for the coal dust poor stream are provided at the outlet from the ducts to direct the stream from each air duct in a direction diverging radially from the central duct.

Description: The present invention relates to a nozzle for injecting a solid fuel, such as pulverized coal, into a combustion chamber particularly suitable for being mounted on a burner providing a three-phase combustion to limit the production of nitrogen oxides. The nozzle of the present invention is especially designed for burners operating with either liquid or solid fuels and with either gas or liquid or solid fuels. It is a known fact that nitrogen oxides are one of the most important pollutants produced during combustion processes, and are found in exhaust fumes released into the environment by steam generators, thermoelectric power stations and other industrial installations in which fossil fuels are burned. The formation of nitrogen oxides is due in part to the presence of N-compounds in the fuel (chemical NOx) and in part to the atmospheric nitrogen in the combustion air (thermal NOx). The NOx formation during the combustion process is dependent on a number of parameters, among which the main ones are the flame temperature, the time the combustion gasses remain in the high - temperature zone and the excess air. More specifically, formation of nitrogen oxides increases as the flame temperature increases, and can be minimised by keeping the peak values under control. A similar effect is produced when the fuel remains in the high-temperature zone for a short time and by a reducing atmosphere or an excess of fuel in the ignition area. From what has been stated above it is clear that a combined control of the above mentioned working parameters will allow the formation of nitrogen oxides during combustion to be limited. In burners in which it is possible to burn solid fuel in powder form, for example coal dust, the geometry of the air-coal mixture injector nozzle is of primary importance for the control of combustion and therefore for the production of NOx. In particular .it is necessary to create areas that are poor in oxygen and rich in coal in the ignition area, and furthermore the time that the fuel remains therein must be such as to minimise the production of NOx and unburned substances. Finally, the recirculation streams around the end of the nozzle must be controlled in order to avoid excessively high temperatures and local fusion phenomena, which would damage the nozzle irreversibly. In a nozzle for solid fuels of a known type manufactured by Foster Wheeler Energy Corporation, (see EP-A-0 554 014), a tangential inlet for the air-coal mixture is provided to convey said mixture towards four or more ducts, whose cross-sections converge towards the combustion chamber and which are arranged in correspondence with the outlet into said chamber along an annulus coaxial with the primary air duct. In this way jets of air-coal mixture rich in coal dust are formed. Between each mixture duct and the adjacent one there is a duct with a diverging crosssection, through which air is injected into the ignition area, said air being taken up through slots formed on the external shell of the nozzle. Injection of the air through these ducts is necessary in order to prevent the temperature of the nozzle from reaching excessively high values, but it also increases the level of oxygen in the ignition area, with an adverse effect on the reduction of NOx and therefore on the performance of the nozzle. EP-A-0163423 discloses a nozzle formed by an outer tubular member and an inner tubular member defining an annular channel therebetween. A divider cone extends between inner and outer tubular members. A plurality of V-shaped splitters are circumferentially spaced between the outer tubular member and the divider cone at the outlet portion of the nozzle, defining conduits for coal dust-rich air stream. Pie-shaped openings are formed through the outer tubular member over the splitters to admit secondary air in the annular space between the divider cone and the outer tubular member. An axially movable tip is formed at the end of the inner tubular member to regulate the amount of air flowing through the opening defined between the tip and the cone. As in EP-A-0554014, the coal dust-poor air stream, which still contain coal, is fed to the combination chamber within the flame region, and a central flame is additionally generated, which increases not only the flame stability, but also the risk of burning the nozzle end. To avoid this inconvenience secondary air is sucked through pie-shaped openings to reduce the temperature, but this also increases the amount of available combustion air, resulting in a higher NOx production. Axially movable tip provides for a partitioning of the coal dust-rich and coal dust-poor air streams and a control of the amount of coal dust present in the coal dust-rich air stream, but anyway all these streams are admitted into the flame region. The object of the present invention is to provide a nozzle for injection of solid fuel-air mixtures, the solid fuel being coal dust or the like, capable of giving improved combustion conditions with respect to known devices of this kind. The above object is achieved with the nozzle according to the present invention, the main features of which are defined in the characterising portion of claim 1. Further characteristics and advantages of the pulverized coal injection nozzle according to the present invention will become clear from the following description of a preferred embodiment thereof, given merely as an example and with reference to the enclosed drawings, in which: Figure 1 is a perspective view of a nozzle for coal/air mixtures according to the present invention; and Figure 2 is a partial cross-sectional view of the nozzle of figure 1 taken along line II-II. The nozzle according to the invention is designed to be installed at the outlet end of a burner in a coaxial relationship to the primary air duct in a conventional way. With reference to the figures 1 and 2, the nozzle according to the invention comprises a fixed part 1 which serves the purpose of concentrating the coal dust into physically separated streams, and a mobile member 2 which serves the purpose of registering the outflow of primary and combustion air according to the manner in which the desired combustion process is to be carried out. The fixed part 1 consists of a group of at least three ducts 3 with the shape of a truncated cone converging towards a combustion chamber, not shown, suitable to be arranged according to an annulus coaxial with the primary air duct of the burner and an oil lance within a central tube 4, so as to transform 100% of the annulus section at the inlet into 40 - 60% of the section at the outlet. As an example, in figure 1 an embodiment with four ducts 3 is shown. In this way the coal dust, which is already concentrated toward the wall of the primary air duct by a swirler not shown, installed in said duct upstream from the nozzle, is divided into various streams characterized by a low air/coal ratio. Between coal ducts 3 air ducts 5 are formed each having a section which increases towards the combustion chamber and communicating with an inner duct 6 delimited by ducts 3 therearound. Therefore, at the nozzle outlet the outlet ports of air duct 5 alternate with those of air ducts 3. The mobile member 2, coaxial with the oil lance, has a conical cup shape and can be displaced between two end positions and a plurality of intermediate positions, as it is slidingly mounted on central tube 4 and is moved by means of a rod, not shown, along inner duct 6. The primary air transporting the coal dust reaches the nozzle, as stated above, with a coal dust-rich stream at the periphery of the air duct and a coal dust-poor stream at the centre. The coal dust-rich stream enters coal ducts 3 of fixed part 1 of the nozzle, while the coal dust-poor stream, flowing through the central section of primary air duct of the burner, enter inner duct 6 and is divided in two sub-streams depending on the position of mobile part 2 of the nozzle. A first sub-stream flows through air duct 5 and the second one is directed toward the outlet of inner duct 6 and fed to the combustion chamber through the passage defined by the outlet end of inner duct 6 and mobile member 2 of the nozzle coaxial thereon. Inclined plates 7 are positioned at the nozzle outlet in correspondence with the outlets from the air ducts 5. Inclined plates 7 have the aim of deflecting radially away from the centre the coal dust-poor air stream of primary air, for mixing it with the secondary air, and have an inclination of between 0 and 45°, optionally adjustable in a known manner by means of a control rod, not shown. In an embodiment shown in figure 2, given as an example, ducts 3 for the air-coal mixture are tubular sectors of an annulus with a cross section that decreases from the inlet to the outlet, which are welded at their inlet end to a support 8 made up of two concentric annular members connected by radial arms not shown, while at their outlet ends ducts 3 are connected by means of circumferential tie rods 9 which give adequate allowance for thermal dilatation. The composition of the two-phase mixture at the burner outlet depends upon performance of the process of mixing the coal dust poor and the coal dust rich air streams, this composition being the basic parameter for control of emission of NOx, unburned fuel and flame stability. According to the present invention, by suitably combining the position of the mobile member 2 with the effect of the stationary part 1 it is possible to define an optimum solution to respond to the various requirements of different installations. With mobile member 2 a long way back, the flame is longer, the combustion process is slow, so that NOx is minimum, but CO and unburned fuel are maximum. With mobile member 2 a long way forward, combustion is intense and localised, due to the high level of coal dust/air mixing, NOx are maximum, CO and unburned products minimum. The nozzle according to the invention has been installed on a burner. The burner has been tested at full scale underlining the following performances: a NOx reduction in the order of 50%, when oil and coal are burnt, as compared with traditional "CIRCULAR" type burners; during coal fuelling a reduction in the level of unburned products in the order of 50%, as compared with first generation "Low NOx" burner. Variations and/or modifications can be brought to the pulverized coal injection nozzle fuels according to the present invention, without departing from the scope of the invention itself, as defined in the appended claims.

Claims: A nozzle for the injection of solid fuel in dust form into a combustion chamber, to be installed at the outlet of a burner coaxially to the longitudinal axis of the latter, comprising a fixed annular part (1) divided into coal ducts (3) of a circumferential width that decreases towards said outlet section to form coal dust-rich air streams, a central duct (6) for a coal dust-poor air stream delimited by said coal ducts and having an outlet end, a central member (2) that is axially mobile within said central duct (6), for varying the cross section of the central duct delimited by said fixed annular part at said outlet end of said central duct (6), characterized in that air ducts (5) for the coal dust-poor air stream are formed between two adjacent coal ducts (3) in said fixed annular part (1) communicating with said central duct (6) and having a circumferential width increasing toward said outlet end, deflector means (7) for said coal dust-poor air stream being provided at the outlet from said air ducts (5) to direct the stream from each air duct in a direction diverging radially from said central duct. The nozzle according to claim 1, in which the inclination of said deflector means (7) is adjustable within a range of between 45° and 90° with respect to a longitudinal axis of said central duct (6). The nozzle according to one of the previous claims, in which the overall cross section of said ducts (3) for the coal dust-rich air stream at the inlet thereof is substantially equivalent to 100% of the section at the inlet of said fixed annular part (1), whereas the overall cross section of said coal ducts (3) at the outlet of said fixed annular part (1) is equal to 40-60% of the outlet section thereof. The nozzle according to one of the previous claims, in which said coal ducts (3) are axially fixed at their inlet ends to a support (8) formed by two concentric ring elements (8a, 8b) connected to each other and radially spaced at a distance substantially equal to the radial width of said coal ducts, said coal ducts being connected to one another by means of circumferential tie rods (9) at their outlet ends. The nozzle according to one of the previous claims, in which said mobile member (2) is a cup-shaped cone with a hollow part facing towards the combustion chamber. The nozzle according to one of the previous claims, in which a tubular housing (4) is provided in the central duct (6) to house an oil lance.

Inventors: ALBERTI MARCO, LIGASACCHI SERGIO, MARTANO MARCELLO, MICHELE GENNARO DE, RIZZI ROBERTO, ALBERTI, MARCO, DE MICHELE, GENNARO, LIGASACCHI, SERGIO, MARTANO, MARCELLO, RIZZI, ROBERTO
Applicants: ANSALDO CALDAIE SPA, ENEL SPA, ANSALDO CALDAIE S.P.A., ENEL S.P.A.
Application year: 1996
Main classification: F23C7
IPC codes: F23D17, F23D1, F23C7