Furnaces – Utilizing powdered fuel – Having rotary flow means
Reexamination Certificate
1999-07-01
2001-03-06
Ferensic, Denise L. (Department: 3749)
Furnaces
Utilizing powdered fuel
Having rotary flow means
C110S261000, C110S262000, C431S183000, C431S188000
Reexamination Certificate
active
06196142
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method (hereinafter referred to as of the kind described) for introducing solid, liquid or gaseous fuel into a burning zone of a kiln, such as a rotary kiln for manufacturing cement clinker or similar products, by which method fuel is conducted through a duct or ducts and primary air is conducted through at least two annular ducts arranged substantially concentrically with, and around, the fuel duct(s) , the portion of air in one of the air ducts flowing substantially axially, whereas the portion of air in the second of the air ducts comprises air which has a rotary component about the centre axis of the burner, and the amount of primary air in the two portions being independently controlled. The invention also relates to a burner for carrying out the method according to the invention.
Burners for this purpose are well-known. Originally, they merely consisted of one single pipe through which a mixture of pulverized coal meal and air was injected into the burning zone of the kiln. Over time, design improvements of the burners were implemented, with the incorporation of features such as additional ducts for introducing other types of liquid or gaseous fuel. Furthermore, most modern burners comprise one or several separate ducts for injection of air, so that only a small amount of the primary air is injected together with the pulverized coal. By imparting a rotary motion to some of the injected air it has, to a larger extent, been possible to control the flame shape in the kiln.
An example of a burner of the above mentioned kind is described in EP-B-0421903. This known burner comprises one or several ducts for introduction of fuel, being surrounded by two annular ducts for injecting primary air. In the annular nozzle opening of the innermost of these air ducts are provided oblique blades which impart a rotary motion to the air. In the outermost duct the air is conducted and injected in a substantially axial direction. An adjustment of the nozzle area of both ducts can be made by adjusting the ducts axially relative to one another, and the portions of primary air in the two ducts can also be independently controlled. Variability of flame shape is, therefore, possible with this burner given the possibility of adjusting the flow rate and velocity of the primary air as well as the amount of primary air which is subjected to rotation. However, the drawback of this burner is that the primary air is injected through two separate annular nozzles, resulting in a relatively high pressure loss and a less effective mixing of the primary air with the fuel in the burning zone.
A second example of a burner of the above mentioned kind is described in EP-A-0650012. This known burner also comprises one or several ducts for the introduction of fuel, surrounded by one single primary air duct which discharges into an annular nozzle. Immediately ahead of the nozzle the air is directed through a number of flexible tubes which, by means of a mechanism, can be bent sideways, thereby causing the air to rotate. The rotation of the air, and hence the flame shape, can thus be varied by changing the angle of bending of the tubes, and by changing the amount of primary air. The advantage of feeding all of the primary air through only one nozzle is that it will reduce the loss of pressure and ensure a more effective mixing of air and fuel, and hence a more steady flame. However, the drawback of this type of burner is that the system with the flexible tubes requires a relatively complex regulating mechanism which also appears to be vulnerable in its intended operating environment.
SUMMARY OF THE INVENTION
The purpose of the present invention is to provide a method as well as a burner by means of which an effective mixing of fuel and air can be ensured with minimum loss of pressure, and with which the flame shape may be varied, while, simultaneously, the construction has such degree of sturdiness that a reasonable service lifetime is ensured, taking into account the high thermal and mechanical loads imposed upon the burner in the burning zone of a rotary kiln.
This is achieved by means of a method of the kind described, characterized in that the two portions of primary air are mixed at a lower velocity in a collecting duct into a single mixed primary airstream which is subsequently accelerated up to a desired, higher discharge velocity.
The invention also includes a burner for introducing solid, liquid or gaseous fuel into a burning zone of a kiln, such as a rotary kiln for manufacturing cement clinker or similar products, which burner comprises a duct or ducts for conducting the fuel and at least two annular ducts arranged substantially concentrically with, and around, the fuel duct(s) for conducting primary air, the air ducts being arranged to cause the portion of air in one of these air ducts to flow axially and the portion of air in the second air duct to flow with a rotary component about the centre axis of the burner, and means for independently controlling the amount of primary air in the portions; characterized in that the primary air ducts discharge into a joint annular collecting duct for conducting the mixed primary air to an annular nozzle, and in that the flow area of the collecting duct decreases gradually in the axial direction of movement of the air.
There is thus obtained a method as well as a burner by means of which an effective mixing of fuel and air in an operationally reliable manner can be ensured with minimum loss of pressure, and with which the flame shape can be adapted to the optimum desired. This is due to the fact that the two substreams of the primary air, prior to being injected into the kiln, are mixed at a relatively low velocity into one airstream which is subsequently, at a relatively high velocity, injected via one nozzle, the fact that the degree of rotation of the primary air can be varied by changing the interrelated amounts of the two primary airstreams, and the fact that all necessary control means can be installed in a readily accessible manner outside the burning zone of the kiln. As a result, those parts of the burner which are subjected to thermal loads can be manufactured in a simple and sturdy design. It is preferred that the flow area of the collecting duct from the location where the primary airstreams are mixed to the annular nozzle decreases by a factor of between 5 and 12 so that the velocity of the mixed primary airstream is accelerated by an equivalent factor.
In a particularly preferred embodiment of the burner according to the invention which is particularly suitable for application in a rotary kiln for manufacturing cement, it is preferred that the primary air ducts and controlling means are arranged so that the axial velocity of the flow in the primary air ducts ranges between 20 and 25 m/s, and in that the collecting duct is arranged so that the mixed primary airstream is accelerated up to a flow velocity of between 160 and 200 m/s.
The collecting duct may be configured in any practicable manner which will give an acceleration corresponding to that mentioned above. However, it is preferred that the duct is made up of two concentric annular elements, of which the outermost is configured as a frustocone which is convergent in the direction of flow with an angle of inclination &agr; of between 30 and 60° relative to the centre axis of the burner, whereas the innermost annular element is substantially parallel to the centre axis of the burner. However, other configurations of the collecting duct are conceivable. Thus, the innermost annular element may also be configured as a frustocone which is convergent in the direction of flow. However, if this is the case, it must be formed with an angle of inclination which is considerably smaller than that of the outermost annular element.
The air in the second air duct can be made to rotate about the centre axis of the burner in different ways, inter alia by means of angularly turned tubes as previously noted. It is preferred, however, that the air is made
Baker & Botts L.L.P.
F. L. Smidth & Co. A/S
Ferensic Denise L.
Rinehart Ken
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