GB2101497A - Combined scrubber and cyclone - Google Patents

Abstract

A cyclone 1 is fitted with liquid scrubbing sprays which improve the efficiency of collection of particles, and enable gaseous pollutants to be removed. Spray nozzles 15, 16 installed in the cyclone discharge tube 2 form sprays across the gas flow path, a descending liquid film on the tube wall, and a water curtain 10 falling from the tube. The effluent liquid is filtered to remove the solids and chemically treated before being recycled. Its pH value is controlled for efficient removal of sulphur oxides. <IMAGE>

Description

SPECIFICATION Combined scrubber and cyclone This invention relates to means for cleaning dirty gases and for controlling gaseous emissions.

For removing particles from gases, cyclones are widely used as they have many practical advantages including simplicity and small size.

However, the collection efficiency of cyclones deteriorates for particles below 10 y.

As emission-control standards are raised, efficient elimination of foreign particles becomes more important. Existing equipment for removing very fine particles tends to be expensive and/or bulky. Modification of cyclones to remove fine particles involves increasing gas velocities and therefore increasing pressure losses, thereby requiring larger or faster fans and increasing both the capital and running costs and the energy consumption.

An object of the present invention is to provide gas-cleaning apparatus comprising a cyclone, with a high collection efficiency for particles of sizes less than about 10 y.

According to the invention, a cyclone is provided with a liquid scrubbing system.

Preferably, the scrubber is in an axial discharge tube of the cyclone.

The invention is applicable to most types of cyclone including multi-cell cyclones, but is particularly suitable and advantageous in connection with a simple single cyclone, as single- cell cyclones are relatively cheap to manufacture and maintain.

The cyclone operates in the usual manner to collect entrained particles. Fine particles remaining in the gas in the cyclone are removed by the scrubbing liquid. By mounting the scrubber in the discharge tube, we ensure that the scrubbing liquid does not interfere with the normal operation of the cyclone, since it acts substantially only on the outgoing discharge gas stream.

Furthermore,-most parts of the cyclone normally subjected to heavy wear are scrubbed by the scrubbing liquid, thereby reducing wear.

By using a liquid scrubber to collect fine particles in a simple cyclone, the pressure drop across the complete gas cleaning system can be minimised. This saves energy and enables a cyclone plant to be made more efficient, for example to meet more severe emission control regulations, without the need to install higherpowered fans. An existing plant can be up-dated, instead of being replaced by a new and possibly larger and more expensive gas cleaning system.

In a preferred arrangement, water sprays are employed in a unique way to ensure optimum solid particle collection efficiency. Some of the methods are detailed below:~ In the discharge tube of the cyclone are three sprays (for other applications more or fewer sprays can be employed). These sprays can be designed to produce either a curtain of water wholly or partly across the gas flow path, preferably completely across the gas flow path to maximise particle collection, or an atomised spray of water droplets for particle agglomeration.

Atomised water sprays can be employed to increase the mass of solid particles and as such, the particles can be more easily collected and will be less likely to escape collection by other means.

In one very efficient arrangement, the two sprays nearest to the discharge of the cyclone provide a curtain of water across the discharge tube of the cyclone. These sprays collect solid particles from the flue gas. The water from these sprays falls down the discharge tube and across the gas flow path, and also flows down the walls of the discharge tube. It is known that the swirling gas flow in the discharge tube induces some solid particles to travel along the wall of the discharge tube. Water from the sprays, which coats the walls of this tube, travels downwards collecting these solid particles to increase collection efficiency. it must be noted that solid material flowing up the wall of the tube is the most difficult to collect in the region of the water sprays.By spacing the two sprays apart, sufficient time is given for water falling down from the last spray to agglomerate onto the particles across the duct for optimum collection. Similarly, a long length of discharge tube duct is wetted to collect solid material travelling up the walls. Water flowing off the bottom of the discharge tube and travelling down through the tube against the flow of flue gas, falls into the conical base of the cyclone where the water with suspended solids may be collected ready for treating. The water falling down off this tube and from within the tube provides a water curtain through which the gases have to pass and where particles can be washed out of the gas stream and agglomeration can take place. To aid these processes, the first spray provides water in the region just upstream of the tube entry.

It is important to note that the combination of sprays described above is only one possible combination of spray systems.

Sprays can also be employed in any region of the cyclone to assist particle collection by means of mass increase by water/particle agglomeration and directly by washing out particles either in the gas stream or by water flowing down the walls of the cyclone system.

Although primarily designed for gas-borne particulates removal, the present scrubber is also effective in removing certain gaseous pollutants.

For example by controlling the pH value of the recycled water a very high proportion of the SOx in the gas stream can be scrubbed out. The scrubber design can as a consequence be utilised as a flue gas desulphuriser, whether particulates are present or not.

To treat the water for suspended and dissolved solids in the water removed from the scrubber a complete treatment system comprising three basic units has been developed. The basic units are the cyclone gas scrubber, a suspended-solids water filter, and a chemical treatment vessel.

In the accompanying drawings:~ Figure 1 shows, in axial section, a combined cyclone and scrubber embodying the invention, Figure 2 shows, in axial section, a preferred embodiment of the invention, Figure 3 shows details of a water spray nozzle used in the combined cyclone and scrubber shown in Figure 2, and Figure 4 is a schematic diagram of plant for cleaning flue gases, incorporating the present invention.

A A simple embodiment of the invention is shown in Fig. 1 of the accompanying drawings. A cyclone 1 has an axial discharge tube 2, a gas inlet 3, a gas outlet 4 and a discharge outlet 5. Broken-line arrows show the paths of the gas and entrained particles through the cyclone.

The cyclone discharge tube is constructed as a scrubber, with scrubbing spray nozzles 6 for spraying a scrubbing liquid, usually water. The liquid passes down the discharge tube as a spray 7 and as a film 8 on the tube wall, and falls from the bottom aperture 9 of the tube as a curtain 10.

The scrubbing liquid and entrained material drops directly to the conventional conical bottom section 11 of the cyclone leading to the outlet 5, comprising a pipe connection, so that the scrubbing liquid can if desired be treated to remove the suspended solids and other entrained material.

The scrubber removes particles in three ways.

The gases are scrubbed as they pass through the liquid curtain 10 towards the bottom of the discharge tube 2. As the gases pass up through the tube the swirling gas flow throws particles outwards towards the tube wall and these particles are removed by the descending liquid droplets and the liquid film on the tube wall.

Finally, the gases are scrubbed as they pass through the liquid sprays, the particles thus collected being flushed downwardswith the liquid stream.

Figure 2 shows a preferred embodiment of combined scrubber and cyclone. Elements corresponding to those shown in Figure 1 are indicated by the same reference numerals. The cyclone and scrubber shown in Figure 2 differs from that shown in Figure 1 in that it has three water sprays, formed at the open lower ends of three coaxial tubes 12, 13, 14 mounted coaxially in the cyclone discharge tube and supplied with water through separate flow control systems. The innermost spray tube 12 produces a fine annular spray of water just below the intake end of the cyclone discharge tube. The nozzle 15 at the bottom of the second water tube 1 3 produces a medium-velocity annular spray extending completely across the discharge tube. The nozzle 1 6 at the bottom of the outermost water tube 14 produces an annular spray of coarse droplets across the entire width of the discharge tube.The sprays from the nozzles 15, 16 produce a spray 7 of water descending inside the cyclone discharge tube, a water film 8 on the tube wall, and a water curtain 10 falling from the bottom of the discharge tube. The described arrangement and combination of water sprays provides extremely efficient removal of solid particles from the incoming gases, as described herein above.

The described apparatus is intended primarily for reducing the emission of solid particles from a plant, for example combustion plant. It is particularly advantageous for cleaning the combustion gases of a pulverised-fuel combustion plant, because such plant tends to produce a high proportion of fine ash particles in its exhaust. Such combustion plant is described for example in our British Patent Applications Nos. 8114979 and 8114986. For such use, the scrubbing liquid can be water.

When a cyclone/scrubber as shown in Figure 2 was used to clean the flue gases from a pulverised coal-fired boiler plant, it removed 98% of the particulate solids down to at least one micron. The effluent from the cyclone had a suspended solids concentration of, typically, 0.5%.

The described scrubber can also be used for many industrial applications where solid particles have to be removed from a stream of gas, for example in the metallurgical industry.

Furthermore, many gaseous pollutants can be effectively scrubbed in the described cyclone/scrubber by means of a suitable scrubbing liquid. Using water as the scrubbing liquid, up to 90% of sulphur oxides can be absorbed, if the pH of the water is carefully controlled. A mixture of lime and water can be used for removing sulphur trioxide.

The cyclone/scrubber can of course remove both particulate solids and gaseous pollutants simultaneously.

Figure 3 shows one possible form of spray nozzle for the cyclone/scrubber of Figure 2. The intermediate nozzle 15 is illustrated. The inner water tube 12 and intermediate tube 13 form an annular water passage 1 7 open at the bottom, but facing a vertically adjustable nozzle ring 18 mounted on the tube 12. The nozzle ring and the lower end of the tube 13 form an annular nozzle 19 of adjustable width, and this produces a water spray of adjustable velocity dependant on the position of the nozzle ring 1 8. The upper nozzle 1 6 can be of similar construction. The lowest nozzle, on the tube 12, can be of any desired construction for producing a fine annular spray.

The present cyclone/scrubber will usually be installed in a cleaning plant which includes equipment for recycling the scrubbing liquid, and for removing the scrubbed particles and gases.

The nature of this equipment will of course depend on the nature of the gases and particles being cleaned by the scrubber. By way of example only, Figure 4 shows schematically a plant for cleaning flue gases from a pulverized-coal burner.

The outlet 5 of the cyclone and scrubber 1 is connected to a filter 20 for separating the suspended solids. The filter may for example consist of a vessel containing an array of sintered stainless steel tubes, with a high collection efficiency at least down to particles one micron in size. A cake of particles is formed on these tubes, which at intervals is dislodged for example by compressed air blown backwards through the filter.

The sedimented sludge cake is discharged, at intervals, from the filter vessel into a suitable container. The filter cake will typically contain 20% to 30% solids by weight and be slightly alkaline. The water issuing from the filter is passed through a chemical treatment vessel or filter bed 21. This contains a mixture of magnesium and calcium oxides. It serves to remove some of the dissolved solids from the water, and to raise the pH to between eight and nine, for efficient absorption of sulphur oxides in the cyclone/scrubber. Make-up water is supplied to the chemical treatment vessel. Calcium and magnesium sulphates are formed in the latter and are removed continuously or intermittently.

With suitable control of the pH value, the scrubbed flue gas leaving the cyclone may contain 50 ppm or less sulphur dioxide.

Such a cleaning plant, applied to a pulverized fuel steam boiler burning 6% ash coal, had the following performance: Exhaust Gas Data: Flow Rate~3090 NM3/Hr Cyclone Scrubber Pressure Drop~70 mm WG Exhaust Temp. Drop - 250C to 900C Solids Emission~160 mg/N Ma SOz~Approx. 50 ppm Scrubbing Water: Circulating Flow Rate~77 litre/min Circulating Temp.- 500C Make up - 7 litre/min It has been found that the stack solids emission is not very sensitive to the flow rate of water through the scrubber. Consequently a satisfactory reduction in the amount of solids in the flue gas can be obtained despite wide variations in the water flow rate. This greatly simplifies the design and operation of the cyclone/scrubber.

The present cyclone scrubber has a demonstrated efficiency far in excess of that achievable with dry multi-cell cyclone systems.

Alternative systems such as electro-static precipitators and bag-house filters have claimed collection efficiencies higher than the present system but these have the major disadvantages of: 1. No sulphur oxide absorption 2. Very large installation space required 3. Very expensive 4. High maintenance costs Alternative water scrubbing systems have much higher pressure drops on both the gas and water sides.

Claims (14)

1. Apparatus for cleaning gases, comprising a cyclone, and liquid scrubbing means in the cyclone.

2. Apparatus as claimed in claim 1 in which the liquid scrubbing means comprise means for producing an atomised liquid spray in the gas flow path.

3. Apparatus as claimed in claim 1 or 2 in which the liquid scrubbing means comprise means for producing a liquid container across the gas flow path.

4. Apparatus as claimed in claim 1, 2 or 3 in which the cyclone has an axial discharge tube and the liquid scrubbing means is at least partly in the discharge tube.

5. Apparatus as claimed in claim 4 in which the liquid scrubbing means comprise at least one spray nozzle in the discharge tube.

6. Apparatus as claimed in claim 4 or 5 in which the liquid scrubbing means is arranged to produce a liquid film in the interior wall of the discharge tube.

7. Apparatus as claimed in claim 4, 5 or 6 in which the liquid scrubbing means comprise at least one spray nozzle upstream of the discharge tube, relative to the gas flow.

8. Apparatus as claimed in claim 4 in which the scrubbing means comprise a first spray nozzle just before the inlet end of the discharge tube, a second spray nozzle in the discharge tube arranged to form a liquid spray across the tube, and a third spray nozzle in the discharge tube spaced downstream of the second spray nozzle relative to the gas flow and arranged to form a liquid spray across the discharge tube, at least one of the second and third nozzles being arranged to throw liquid onto the interior wall of the discharge tube to form a descending liquid film on the said wall and a liquid curtain falling from the inlet end of the tube.

9. Plant for removing gaseous and/or particulate material from gases, comprising at least one cleaning apparatus as claimed in any of claims 1 to 8, a filter connected to receive the liquid effluent from the cleaning apparatus and to separate the scrubbing liquid from particulate solids entrained in the liquid, chemical treatment means for treating the filtered scrubbing liquid, and means for circulating the scrubbing liquid in a closed circuit through the cleaning apparatus, the filter and the chemical treatment means.

10. A method of removing pollutants from a gas, comprising passing the polluted gas through a cyclone, and scrubbing the gas with a liquid as the gas passes through the cyclone.

11. A method as claimed in claim 10 in which the liquid forms a curtain extending at least partly across the path of the gas.

12. A method as claimed in claim 10 or 1 1 in which the liquid forms an atomised spray.

13. A method of removing pollutants from a gas, substantially as herein described with reference to the accompanying drawings.

14. Apparatus for cleaning gases, substantially as herein described with reference to Fig. 1 or 5 Fig. 2 of the accompanying drawings.