Classifying – separating – and assorting solids – Fluid suspension – Gaseous
Reexamination Certificate
2001-08-16
2003-08-19
Nguyen, Tuan N. (Department: 3653)
Classifying, separating, and assorting solids
Fluid suspension
Gaseous
C209S139100, C209S714000, C209S721000
Reexamination Certificate
active
06607079
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to improvements in particle classifiers, which separate coarse particles from a stream of gas entrained with a mixture of coarse and fine particles, having several outlet conduits for discharging gas and fine particles. This invention relates particularly to a fuel classifier and a method for separating coarse fuel particles from a mixture of fine and coarse fuel particles entrained in an air stream and returning larger fuel particles to a pulverizer for further size reduction. Meanwhile, the air stream carrying the fine fuel particles can be used for firing a boiler.
Industrial and utility-sized coal boilers may be equipped with two to several dozens of coal burners to deliver fuel to a combustion furnace. The number of burners depends on the size of the boiler and the configuration of the furnace. Commonly-used burner configurations include single wall firing, opposed wall firing, and tangential firing. Coal is typically pulverized in a mill, e.g., a spindle mill or a ball mill, to a fineness that is suitable for combustion in the furnace. Large boilers include several pulverizers, each of which delivers fuel and primary air to a set of burners. One requirement for efficient combustion and low emission levels is that equal or controlled quantities of fuel be delivered to each of the separate burners.
A pulverizer is usually combined with an aerodynamical classifier, which imparts a swirling motion to a coal-air mixture discharged from the mill and centrifugally separates the coal fines from the coarse product. Classifiers operated in a positive air pressure usually have multiple outlet conduits through which coal fines are transported by a flow of primary air to multiple burners in the boiler. The coarse material is returned to the mill and re-ground.
In order to observe stringent environmental regulations and to achieve efficient boiler operation, the flows of coal and air from the classifier to each burner must be precisely controlled. Generally, the air distribution can be balanced quite easily by adjusting the flow impedances of the various coal-air lines. The coal flow, on the other hand, is more difficult to control since it is dependent, in a complicated way, on the conditions within the pulverizer, classifier, and fuel lines, including the burners.
The distribution of the coal flow between the various outlet conduits in a classifier can be improved by increasing the homogeneity of the pulverized coal in the vicinity of the classifier outlets. This can be achieved by improving the pulverizer performance or by optimizing the geometry of the classifying blades. Japanese Patent Publication JP 63259316 A2, for example, discloses a coal distributor wherein a swirling solid-gas flow is transformed by radial vanes into a vertically uprising flow which collides with a horizontal plate so as to achieve a uniform particle concentration. Despite this and other like measures to provide a homogeneous coal distribution, the coal tends to turn stratified in the classifier, resulting in flow variations as large as 20% among the various outlets.
U.S. Pat. No. 4,540,129 discloses a pulverizer wherein each of the multiple lines between the pulverizer and a set of coal burners includes a valve to control the flow rates of coal and primary air. This commonly-used method controls the coal and air streams simultaneously, but does not make it possible to affect the coal flow irrespective of the air flow. Due to the different characteristics of air and coal streams, there are often situations where a coal stream needs to be adjusted independently of the air stream.
It is also known to arrange adjustable guide vanes at the outlets of each coal line in a classifier. The guide vanes either capture the pulverized coal or divert it from the outlet. These vanes, however, also impede the flow of primary air, and thus affect both the air and the coal flow. Therefore, such vanes have only limited potential for balancing an asymmetrical coal distribution within a classifier.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus and method for achieving efficient and environmentally advantageous operation of a pulverized fuel fired boiler.
More particularly, an object of the present invention is to provide a particle classifier and a method for utilizing the particle classifier in order to balance the distribution of pulverized fuel among multiple outlet conduits of the particle classifier.
A further object of the present invention is to provide a new particle classifier and a method for utilizing the particle classifier in order to balance the distribution of pulverized fuel among the multiple outlet conduits of the particle classifier while minimizing the effect on the primary air flow distribution.
Another object of the present invention is to provide a new particle classifier and a method for utilizing the particle classifier in order to maintain a balanced distribution of pulverized coal among the multiple outlet conduits of the particle classifier in various process conditions.
In one aspect, the present invention relates to a classifier for separating coarse particles from a stream of gas and a mixture of coarse and fine particles. The classifier includes a generally cylindrical outer casing including a vertically-oriented side wall and an upper head, a generally conical inner casing provided within the outer casing and configured so as to provide an annular passageway between the inner casing and the side wall of the outer casing through which the stream of gas and particles can flow upwardly, a plurality of angled circumferentially-spaced vanes supported by the upper head of the outer casing for imparting rotational motion to the stream of gas and particles so as to separate coarse particles from the mixture of coarse and fine particles, and an outlet chamber at an upper portion of the inner casing. The outlet chamber includes (i) a top plate with a plurality of outlet openings for discharging streams of gas and fine particles from the classifier, and (ii) at least one pivotable distribution vane for controlling the distribution of the fine particles between each of the outlet openings by affecting the rotational movement of the stream of gas and particles.
In another aspect, the present invention relates to a method for separating coarse particles from a stream of gas and a mixture of coarse and fine particles in a classifier. The method includes (a) passing the stream of gas and particles upward through an annular passageway between a side wall of a generally cylindrical outer casing and a generally conical inner casing, (b) imparting rotational motion to the stream of gas and particles so as to separate coarse particles from the mixture of coarse and fine particles by passing the stream of gas and particles through a plurality of angled circumferentially-spaced vanes attached between an upper edge of the inner casing and an upper head of the outer casing, and (c) discharging streams of gas and fine particles through a plurality of outlet openings in a top plate of an outlet chamber at an upper portion of the inner casing. In step (c) the rotational movement of the stream of gas and particles in the outlet chamber is affected by adjusting the pivot angle of at least one pivotable distribution vane arranged in the outlet chamber so as to control the distribution of fine particles between each of the outlet openings.
In still another aspect, the present invention relates to an apparatus for separating coarse particles from a stream of gas entrained with a mixture of coarse and fine particles. The apparatus includes an outer casing, an inner casing disposed within the outer casing and configured to define a passageway between the outer casing and the inner casing through which the stream of gas and mixture of coarse and fine particles can flow substantially upwardly, a plurality of angled vanes for imparting a rotational flow to the stream of gas and particles as the stream passes from the passageway to w
Foster Wheeler Energy Corporation
Nguyen Tuan N.
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