Pulverized coal injection nozzle

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 burner, 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 sideways 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 delimited by the stationary annular part at the outlet end. Air ducts for the coal-dust-poor air stream are formed between each pair of adjacent coal ducts in the stationary annular part communicating with the central duct and having a circumferential width increasing toward the outlet end. Deflectors 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.

FIELD OF THE INVENTION 
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. 
BACKGROUND ART 
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 gases 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 operating 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 solid fuel nozzle of a known type 
manufactured by Foster Wheeler Energy Corporation (see U.S. Pat. No. 
5,347,937), 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 two adjacent mixture ducts there is a duct 
with a diverging cross-section, 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. 
SUMMARY OF THE INVENTION 
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 claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
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, as described for instance in U.S. Pat. No. 
5,823,764, filed on the same date as the present application by the same 
inventors. 
With reference to the FIGS. 1 through 4, the nozzle according to the 
invention comprises a stationary part l 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 stationary 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 FIG. 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 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 connected to central tube 4 and is moved 
by means of a 11, shown for schematic purposes only, 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 center. The coal dust rich stream enters 
ducts 3 of stationary part 1 of the nozzle, as shown by dark arrows in 
FIG. 3, 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, as shown by the 
light arrows in FIG. 3, 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 radially deflecting the coal-dust-poor 
primary air away from the center to it with the secondary air, and have an 
inclination of between 45 and 90.degree. with respect to a longitudinal 
axis of the central duct, optionally adjustable in a known manner by means 
of a control rod, not shown. 
In an embodiment shown in FIG. 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 8a, 8b 
connected by radial arms not shown, while at their outlet ends ducts 3 are 
connected by means of circumferential tie rods 9 in the shape of rings 
which give adequate allowance for thermal dilatation. 
The composition of the two-phase mixture at the burner outlet depends on 
how the process of mixing the coal-dust-poor and the coal-dust-rich air 
streams is performed, this composition being the basic parameter for the 
control of NOx emission, 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 
according to our copending U.S. application, now U.S. Pat. No. 5,823,764. 
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 a 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.