GB2254681A

GB2254681A - A method and apparatus for sox reduction in boilers

Info

Publication number: GB2254681A
Application number: GB9107911A
Authority: GB
Inventors: Robin George Holder; Robert William Mclaren
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1991-04-13
Filing date: 1991-04-13
Publication date: 1992-10-14
Also published as: GB9107911D0

Abstract

In a method for reducing SOx emissions from boilers, a sulphur suppressant material such as dolomite or limestone is supplied from a silo 4 having a rotating auger extractor 6 whence the material is fed to flow separator/feeder 16. The separate streams of material so produced are fed individually to venturis 20 for conveyance to injectors 26 in the combustion chamber 28 of a boiler 2 above the level at which combination takes place, the material injected being in the form of clouds for intimate mixing with the exhaust gases of combustion. <IMAGE>

Description

A METHOD AND APPARATUS FOR SOx REDUCTION IN BOILERS This invention concerns an improved method and apparatus for supplying a sulphur suppressant to boilers for reducing sulphur-oxide (SOx) emissions therefrom.

In particular the present invention has reference to the supply and introduction of a sulphur suppressant into the boiler combustion chamber in such manner as to effect a significant reduction in SOx emissions when compared with other methods.

It is of course known to add dolomite or limestone to a fuel either before or during its combustion and the manner of its introduction has been the subject of study for many years with the object of enhancing the efficacy of its suppressive role. In one example of previous proposals for improving sulphur suppression, limestone or dolomite is introduced in the form of a curtain across the full width and length of a fuel bed carried on a travelling grate. In such a scheme, the number of injection points for the suppressant has to be considerable as does the injection velocity of the material in order to generate the curtain, thus occasioning high capital and running costs.

It is also the case that to achieve at least a measure of success in suppressing the sulphur emission from a combustion process, the suppressant must be finely divided such that an intimate mixing of the exhaust gases and the limestone or dolomite can occur.

Indeed, it has been found that the suppressant should be in the form of a fine powder of less than 45 microns. In this size range the material displays difficult storage and flow characteristics. For example, bridging in the storage silo can occur and conveying the powder presents problems in terms of entrainment and pipe blockage.

An object of the present invention is therefore to provide an improved method and apparatus for the suppress ion of SOx that will avoid the problems associated with existing technology.

According to a first aspect of the present invention there is provided a method of supplying a sulphur suppressant material to a boiler for reducing the SOx emissions from the combustion chamber thereof in which fuel is combusted, the method comprising the steps of feeding a finely divided suppressant material to a storage vessel, extracting the material by means of a screw from the vessel, passing the extracted material to a flow separator in which the material is separated into at least two streams, feeding the streams of material into a conveying arrangement, pneumatically conveying the material to injectors mounted in the combustion chamber of the boiler, and injecting the material in the form of streams into the chamber above the level at which combustion takes place thereby to create a sulphur suppressant-laden atmosphere within the combustion chamber.

According to a second aspect of the invention there is provided apparatus for supplying a sulphur suppressant material to a boiler for reducing the SOx emissions from the combustion chamber thereof in which fuel is combusted, the apparatus including a storage vessel having an inlet for the material and an outlet therefor, an extractor screw mounted within the base region of the vessel, a conduit leading from the outlet to a material flow separator having discharge means registering with the inlet means of a pneumatic conveying arrangement for conveying the material to injectors mounted in the combustion chamber of the boiler.

Conveniently the storage vessel is a silo and the screw forms part of a silo extractor which sweeps the base of the silo to remove the material in a positive manner therefrom with a greatly reduced possibility of bridging.

The flow separator may conveniently be in the form of a vibratory feeder provided with a number of path dividers for the generation of a number of streams of material.

The paths defined by the dividers register individually with the inlet means of a corresponding number of pneumatic conveying elements of the arrangement. The pneumatic conveying arrangement includes a number of venturis with which the inlet means are in flow connection, a number of fans for supplying air to the venturis for the entrainment of material. The conveying arrangement further includes conduits for conveying the material to the injectors mounted in the boiler.

Advantageously, the injectors mounted in the boiler may be situated at the front of the boiler and may be evenly spaced laterally of the combustion chamber. Alternatively, the injectors mounted in the boiler may be situated at any other convenient location.

The velocity of the material injected into the combustion chamber is conveniently such that clouds of the fine suppressant material are formed above the level at which combustion takes place, thereby ensuring an intimate mixing of the combustion gases with the suppressant to enhance its effect. The velocity of the material as it is injected into the combustion chamber is relatively low compared with existing systems and is of the order of between 40 and 60 m/sec.

The boiler may be solid fuel fired, for example a coal fired boiler.

By way of example only, the method and apparatus of the present invention for supplying a sulphur suppressant material to a boiler for reducing the SOx emissions from the combustion chamber thereof in which fuel is combusted, is described below with reference to one embodiment of the apparatus which is shown in schematic form in the accompanying drawing.

Referring to the drawing, there is shown apparatus 1 for the supply of a sulphur suppressant material to a boiler 2 which in this example is coal fired. The apparatus comprises a storage vessel in the form of a silo 4 provided in its base region with a screw extractor 6 including a rotating auger 8 which itself sweeps the base region to extract material therefrom. The silo 4 has an inlet 10 through which the material is introduced, and an outlet 12 which registers with a conduit 14 for deposit of the material into a flow separator 16.

The flow separator 16 incorporates a vibratory feeder provided with a number of path dividers which in use generate four streams of material. Each path registers with the inlet means 18 of a venturi 20, the inlet of which connects with a fan 22 and the outlet of which is connected to a supply line 24 extending to an injector 26 mounted in the combustion chamber 28 of the boiler 2.

In operation, a sulphur suppressant material for example dolomite of limestone is introduced through the inlet 10 of the silo 4 for storage therein. When it is desired to supply the boiler 2 with the dolomite or limestone, the extractor 6 is actuated such that the auger 8 removes material in a positive manner from the mass of material within the silo. The material leaves the silo through the outlet 12 and passes into the conduit 14 whence it descends onto the flow separator 16 in which the material is separated into separate streams by the flow dividers and the action of the vibratory feeder to be discharged into the inlet means 18 of the venturis 20.

The air provided by the fans 22 entrains and accelerates the material entering the venturis 20 and conveys it along the lines 24 to the injectors 26 mounted in the combustion chamber 28 of the boiler 2. The dolomite or limestone is injected at a velocity of in the range of 40 m/sec to 60 m/s and produces clouds of the suppressant within the chamber above the level at which the fuel is being combusted.

In this example, the solid fuel forms a firebed on a travelling grate. The exhaust gases of combustion emerging from the firebed are thus intimately mixed with the suppressant and an enhanced suppression of SOx emission results. It has been found that for a Ca:S mole ratio of approximately 1:1 there is an abatement of up to 60% on the sulphur emission; for a Ca:S mole ratio of approximately 2:1 an abatement of up to 77% has been achieved.

The method and apparatus enable the suppressant material to be fed to the combustion chamber of the boiler without difficulty, in sharp contrast to conventional systems which tend to display storage and flow problems.

Claims

CLAIMS:

1. A method of supplying a sulphur suppressant material to a boiler for reducing the SOx emissions from the combustion chamber thereof in which fuel is combusted, the method comprising the steps of feeding a finely divided suppressant material to a storage vessel, extracting the material by means of a screw from the vessel, passing the extracted material to a flow separator in which the material is separated into at least two streams, feeding the streams of material into a conveying arrangement, pneumatically conveying the material therefrom to injectors mounted in the combustion chamber of the boiler, and injecting the material in the form of streams into the chamber above the level at which combustion takes place thereby to create a sulphur suppressant-laden atmosphere within the combustion chamber.

2. A method according to Claim 1 in which the velocity of the material injected into the combustion chamber is in the range of 40 to 60 m/sec.

3. A method according to Claim 1 or 2 in which the sulphur suppressant material has a size of less than 45 microns.

4. A method of supplying a sulphur suppressant material to a boiler for reducing the SOx emissions from the combustion chamber thereof in which fuel is combusted substantially as hereinbefore described with reference to the accompanying drawing.

5. Apparatus for supplying a sulphur suppressant material to a boiler for reducing the SOx emissions from the combustion chamber thereof in which fuel is combusted, the apparatus including a storage vessel having an inlet for the material and an outlet therefor, an extractor screw mounted within the base region of the vessel, a conduit leading from the outlet to a material flow separator having discharge. means registering with the inlet means of a pneumatic conveying arrangement for conveying the material to injectors mounted in the combustion chamber of the boiler.

6. Apparatus according to Claim 5 in which the storage vessel is a silo and the screw forms part of a silo extractor which sweeps the base of the silo to remove the material in a positive manner therefrom.

7. Apparatus according to Claim 5 or 6 in which the flow separator is in the form of a vibratory feeder provided with a number of path dividers for the generation of a number of streams of material.

8. Apparatus according to any one of the preceding claims 5 to 7 in which the pneumatic conveying arrangement includes a number of venturis with which the inlet means are in flow connection, and a number of fans for supplying air to the venturis for the entrainment of material.

9. Apparatus according to Claim 8 in which the pneumatic conveying arrangement further includes conduits for conveying the material to the injectors mounted in the boiler.

10. Apparatus for supplying a sulphur suppressant material to a boiler for reducing the SOx emissions from the combustion chamber thereof in which fuel is combusted substantially as hereinbefore described with reference to the accompanying drawing.