Gas separation: processes – Deflecting – Centrifugal force
Reexamination Certificate
2000-12-13
2002-11-05
Hopkins, Robert A. (Department: 1724)
Gas separation: processes
Deflecting
Centrifugal force
C055S396000, C055S459100, C055S466000, C110S229000, C110S264000
Reexamination Certificate
active
06475267
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a system and method for removing gas from a stream of a mixture of gas and particulate solids. The invention is especially applicable for removing air from a stream of a mixture of pulverized coal and air to be injected to a pulverized coal furnace.
For use in pulverized coal furnaces, coal is typically pulverized in a mill into a particulate state and then delivered to the furnace, suspended in air. It is conventional to use the same air for grinding the coal, drying the coal, transporting the coal to a burner, and finally, for injecting the coal into the combustion chamber of the furnace. This air is commonly referred to as “primary air”. The amount of primary air used for injecting the coal into the combustion chamber is an important variable with respect to the ignition and combustion efficiency of the coal. The amount of air is not, however, generally variable because of the requirements of adequate grinding, drying and transportation of the coal. Therefore, the ratio of primary air to coal resulting in optimal ignition and combustion efficiency is generally not achieved unless the amount of air is controlled separately, prior to the burner.
When burning low volatile fuels, such as anthracite, which are not easily ignitable, a decrease in the primary air-to-coal ratio may be required for efficient ignition and burning. Especially, there is a need to arrange means to remove excess primary air when the fuel is changed to a harder-to-burn coal. Also, low load burning may require a decrease in the amount of primary air injected into the combustion chamber to offset the decrease in fuel.
The idea of arranging a centrifugal separator above a vertical burner in order to control the concentration of fuel delivered to the burner is disclosed in U.S. Pat. No. 2,118,600. U.S. Pat. No. 4,412,496 shows an external cyclone to be used at low loads for separating a large quantity of air from a coal-air mixture, and producing a coal-rich stream to be delivered to the furnace through an inner nozzle of a burner and a coal-lean stream delivered to the furnace through an annular passage in a coaxial relationship to the inner nozzle. The cyclone separators are complex and expensive constructions which, especially when used with horizontal burners, may lead to problems related to the transport of the dense stream to the burner.
Also, many other solutions have been suggested for removing air from a coal-air stream. U.S. Pat. No. 4,497,263 discloses a burner with a louvered conical inner part to increase the coal concentration of the inner stream. U.S. Pat. No. 4,448,135 suggests a take-off conduit to be arranged downstream of an elbow section in a burner to remove a coal-lean portion from the coal-air stream. U.S. Pat. No. 5,090,339 shows a throat nozzle immediately upstream of a sleeve part, thus directing a coal-rich stream into the central section of a burner. These types of constructions may suffer high pressure losses and/or have a low separation efficiency.
Japanese patent publication No. 60-194208 discloses an arrangement in which a coal-air mixture is introduced tangentially into an annular space of a horizontal burner, whereby the coal-rich and coal-lean portions are divided by centrifugal force into radially separate partitions. The coal-lean portion is then directed in a switch-over part of the burner, located within the windbox of a burner system, through narrow channels from the inside of the coal-rich portion to outside of it, to be mixed with secondary air. In this construction, both portions of the coal-air mixture are layered in the same combustion zone. Moreover, when using this construction, the coal-rich stream may be disrupted by the channels of the coal-lean stream, and there is a considerable and non-controllable pressure loss of the coal-lean stream, which limits the amount of air which is separated from the coal-rich stream.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and method for removing gas from a stream of a mixture of gas and particulate solids.
Especially, it is an object of the present invention to provide a system and method for removing gas from a stream of a mixture of gas and particulate solids without causing stagnation of the solids or any major pressure drops in the stream.
Also, it is an object of the present invention to provide a system and method for a pulverized coal fired furnace in which the coal concentration of a coal-air stream to be injected into the furnace can be effectively increased.
Additionally, it is an object of the present invention to provide a system and method for a pulverized fuel fired furnace in which a controllable amount of excess air can be removed from a fuel-air stream, upstream of a burner, using a simple device.
In order to achieve these and other objects, the present invention provides a system for removing gas from a stream of a mixture of gas and particulate solids. The system comprises a separation vessel having an upstream end portion and a downstream cowl section, the upstream end portion having an inlet for introducing the stream tangentially into the vessel so as to separate centrifugally from the stream (i) a rich portion comprising a solids-rich mixture, proceeding along a helical path at an outer annular portion of the separation vessel to the cowl section, and (ii) a lean portion, comprising a solids-lean mixture, proceeding along a helical path at an inner portion of the separation vessel to the cowl section; multiple helical ports through which the rich portion proceeds axially through the cowl section, without significantly losing its momentum, to be discharged axially from the separation vessel; and multiple passages between the helical ports through which the lean portion proceeds radially outwards to be discharged from the separation vessel.
In another aspect, the present invention provides a method for removing gas from a stream of a mixture of gas and particulate solids, comprising the steps of introducing the stream tangentially into a separation vessel through an inlet at an upstream end portion of the vessel; separating centrifugally from the stream a rich portion, comprising a solids-rich mixture, and a lean portion, comprising a solids-lean mixture; allowing the rich portion to proceed along a helical path at an outer annular portion of the separation vessel to a cowl section at a downstream end portion of the separating vessel; allowing the lean portion to proceed along a helical path at an inner portion of the separation vessel to the cowl section; allowing the rich portion to proceed axially through the cowl section through at least two helical ports, without significantly losing its momentum, and discharging the rich portion axially from the separation vessel; allowing the lean portion to proceed radially outwards through multiple passages between the helical ports and discharging the lean portion from the separation vessel.
According to a typical application of the present invention, air is removed from a coal-air stream to be injected to a low volatile coal burner located on a wall of a pulverized coal fired furnace. Typically, a coal mill provides the original fuel-air stream, which is pneumatically transported to a set of burners. The burners are preferably arranged on the vertical sidewalls of the furnace, thus having a horizontal axis, but they may also be in another direction. According to a preferred embodiment of the present invention, upstream of each burner is connected a primary air separator. According to another preferred embodiment of the present invention, upstream of more than one burner is connected a common air separator.
One of the advantages of the air separation system according to the present invention is that it functions horizontally and can thus be connected to burners having a horizontal axis, without any constructional or operational difficulties. On the other hand, the air separation system according to the present invention is not designed as an integral part of
Fitzpatrick ,Cella, Harper & Scinto
Foster Wheeler Energy Corporation
Hopkins Robert A.
LandOfFree
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