GB2064362A

GB2064362A - Gas filter

Info

Publication number: GB2064362A
Application number: GB8038330A
Authority: GB
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1979-12-03
Filing date: 1980-11-28
Publication date: 1981-06-17
Also published as: GB2064362B

Abstract

A gas filter comprises a perforate drum 24 which is rotated to maintain a bed of granular filter medium against its periphery by centrifugal force, the gas flowing inwards through the bed and fluidising it. The medium may be continuously circulated from outlet 32 through a vibrating device which removes dust and back to inlet 27. The figure shows a fluidised bed combustor 2 constructed similarly to the filter and rotating about the same vertical axis. Coal is crushed at 16 and fed to the combustor with transporting air, and more air enters by duct 8. The combustion gas is partly cleaned by cyclones 21 and then passes through the filter to a gas turbine. The filter bed material may be inert sand or alumina or chemically active dolomite. <IMAGE>

Description

SPECIFICATION Dust filter This invention relates to dust filters and particularly but not exclusively to a dust filter for use in cleaning a stream of dust laden combustion gas such as may be provided by a fluidised bed combustor. Such combustors, particularly of the rotating kind, are highly effective as gas generators for gas turbines but have a disadvantage in that they tend to produce a substantial amount of ash and other dust in the output gas. There are, of course, many forms of dust filter available but the difficulty is in obtaining one which is highly efficient with regard to the proportion of dust removed and is at the same time compact.

According to the present invention therefore, a dust filter comprises a perforated wall rotatable about an axis, one side of said wall in operation supporting a bed of filter medium against centrifugal force on the bed, means for rotating said wall about said axis, means for supplying a particle-laden gaseous fluid to the other side of said wall at a pressure such as to cause said gaseous fluid to pass through the wall, fluidise the bed and be filtered by it, and means for supplying the filtered fluid to an output.

According to a second aspect of the invention a dust filter comprises a perforated wall rotatable about an axis, one side of the wall in operation supporting a bed of filter medium against centrifugal force on the bed, means for rotating said wall about said axis, means for supplying a particle-laden gaseous fluid to the other side of said wall at a pressure such as to cause said gaseous fluid to pass through the wall, fluidise the bed and be filtered by it without any significant increase in temperature, and means for supplying the filtered fluid to an output.

The dust filter may include means for extracting used filter medium from the wall. There may be included means for cleaning the extracted filter medium and recirculating it to the filter in a continuous operation. Alternatively the filter may include means for supplying a fresh filter medium.

The filter medium may comprise inert or chemically active particles, or a combination of these.

According to a third aspect of the invention, a gas generator comprises a rotating fluidised bed combustor providing, in operation, dust laden combustion gases, and a dust filter as aforesaid coupled into the output path of the combustor.

The gas generator may include a cyclone dust extractor coupled into the output path of the combustor upstream of the dust filter. The dust filter and the combustor are preferably mounted in adjoining manner, their rotary axes being coincident.

A dust filter in accordance with the invention, and a gas generator employing such a dust filter, will now be described, by way of example, with reference to the accompanying drawing showing a partly broken away perspective view of a gas generator plant.

The dust filter 1 is mounted on top of a rotating-fluidised-bed combustor 2 with their axes aligned. The combustor comprises a rotating drum having a base 3 and an outwardly inclined perforated wall 4, the drum being driven to rotate about a vertical axis by a motor 5. The drum is mounted in a housing 6 through which a coal feed pipe 7 projects, passing over the drum and opening into it. A supply of cold compressed air is fed through a duct 8 to pass up the driving shaft 11 of the drum to the exterior of the perforated wall 4. From there it passes through the holes in the wall to 'fluidise' the granulated coal burning bed 12 which is held against the wall 4 by centrifugal force. Combustion of the coal is commenced by ignition means not shown and once started it continues at a very great rate. Such operation of a rotating-fluidised-bed is already known.

The coal granules are supplied by equipment including a hopper 15, a coal grinder 16 and a pressurised supply of .ransport air 17. As the coal is burnt the ash is carried out of the drum with the combustion gases and to a large extent is collected at a dust outlet 18. However, a significant quantity of ash dust passes out of the combustor in the main stream of combustion gas.

A conventional cyclone dust extractor 21 is used to remove the larger dust particles but smaller particles in the range 0.3 to 15 microns tend to escape. In some applications dust of this magnitude may be of little significance but where the gas is generated to power a turbine it is necessary to remove as much as possible of any sold particles to avoid erosion damage.

The outlet of the cyclone 21 is therefore applied to the dust filter 1 for the removal of the smaller particles. The construction of the dust filter is very similar to that of the rotating-fluidised-bed combustor on which it is mounted. It has a drum 24 mounted for rotation on a hollow shaft 25 driven by a motor 26. Through the shaft 25 extends a feed pipe 27 which opens into the drum 24 to supply a filter medium 28 which may be inert, chemically active, of a combination of the two, according to the application. Suitable inert materials are silica sand and alumina, and a suitable active material is dolomite. The size range of the particles depends upon such parameters as the fluid flow rate and the rotational velocity of the drum 24. Typical sizes of the filter particles are in the range 500-600 microns.

The wall of the drum acts as a distributor for the transfer of gas from the plenum chamber outside the drum, it having perforations which produce high velocity gas jets into the filter medium. Filtration occurs by the process of dust particle inertial impaction on the fluidised collector medium.

In order to achieve the desired capture efficiency of the dust particles with the minimum pressure loss across the drum wall and filter bed, the velocity of the particle-laden gas through the perforations is carefully controlled according to the size of particulate to be captured and the size of particle in the bed. The holes are therefore chosen in size and density, to occupy 5~30% of the perforated area of the drum wall, depending primarily on the size of the dust fines and ot the bed particles as well as on the operating conditions of the filter. The holes in the drum (i.e.

the perforations) typically have sizes, i.e.

diameters of the order of 2 mm, say between X and 4 mm, and are contoured or countersunk on the input side so as to prevent build-up of fines and the subsequent blocking of the holes. The shape of the holes, assumed circular above, is not critical.

In contrast, for a combustor good fluidisation is achieved by creating an adequate pressure drop across the bed by making the holes larger (of the order of 1 mm) than for the filter, and about 10-1 5% of the drum wall open to the gas flow.

There is, of course, no ignition arrangement in the dust filter and no significant increase in temperature of the working fluid on passing through the filter bed.

The filtered gas passes from the drum up the hollow shaft around the feed pipe 27 and through vents 29 into an outlet duct 31. The duct 31. is then coupled to the inlet of a gas turbine in the particular example.

The filter medium emerges slowly from the filter, by way of an outlet 32 and is then cleaned, by a vibratory process not shown, for recirculation to the filter by way of the feed pipe 27.

The advantages of the dust filter described stem from the increased fluidising velocities that are possible with a rotating bed and the resulting increased gravitational field. A comparable flow of fluid through a static filter bed would require a much larger bed area to prevent the bed particles being carried away with the working fluid. The rotary bed is thus very much more compact.

Inertial impaction of the dust particles on the 'collector' particles of the filter medium is improved with the increased jet velocity.

It will be clear that the dust filter will find application in other than gas generators but in this context its advantages are particularly valuable.

Claims

1. A dust filter comprising a perforated wall rotatable about an axis, one side of said wall in operation supporting a bed of filter medium against centrifugal force on the bed, means for rotating said wall about said axis, means for supplying a particle-laden gaseous fluid to the other side of said wall at a pressure such as to cause said gaseous fluid to pass through the perforated wall, fluidise the bed and be filtered by it, and means for supplying the filtered fluid to an output.

2. A dust filter comprising a perforated wall rotatable about an axis, one side of the wall in operation supporting a bed of filter medium against centrifugal force on the bed, means for rotating said wall about said axis, means for supplying a particle-laden gaseous fluid to the other side of said wall at a pressure such as to cause said gaseous fluid to pass through the perforated wall, fluidise the bed and be filtered by it without any significant increase in temperature, and means for supplying the filtered fluid to an output.

3. A dust filter according to Claim 1 or Claim 2 wherein, in said wall the holes have sizes between 4 and T mms, and occupy 5~30% of the perforated area of the wall.

4. A dust filter according to Claim 1 or Claim 2, including means for extracting used filter medium from said wall.

5. A dust filter according to Claim 4, including means for cleaning the extracted filter medium and recirculating it to the filter.

6. A dust filter according to Claim 5, including means arranged to recirculate said filter medium continuously.

7. A dust filter according to any preceding claim, including means arranged to supply fresh filter medium.

8. A dust filter according to any preceding claim, wherein said filter medium comprises inert or chemically active particles, or a combination of these.

9. A gas generator comprising a rotating fluidised bed combustor providing, in operation, dust laden combustion gases, and a dust filter according to any preceding claim coupled into the output path of the combustor.

10. A gas generator according to Claim 9, and including a cyclone dust extractor coupled into the output path of the combustor upstream of the dust filter.

11. A gas generator according to Claim 9 or Claim 10, wherein said dust filter and said combustor are mounted in adjoining manner, their rotary axes being coincident.

12. A dust filter substantially as hereinbefore described with reference to the accompanying drawing.

13. A gas generator substantially as hereinbefore described with reference to the accompanying drawing.