GB2203672A

GB2203672A - Recycling fines to fluidised beds

Info

Publication number: GB2203672A
Application number: GB08709256A
Authority: GB
Inventors: Michael Anthony Smith
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1987-04-16
Filing date: 1987-04-16
Publication date: 1988-10-26
Anticipated expiration: 2007-04-16
Also published as: GB8709256D0; GB2203672B

Abstract

A system 1 for recycling fines carried by the product gas from a fluidised bed gasifier incorporates an L-valve 2 registering with a nozzle 15 associated with an ejector 14 which leads to a tube 16. The tube extends into a venturi 20 to which is supplied compressed air, the outlet part 26 of the venturi being flared to expand, in use, the lean phase for injection in the form of a spout into the fluidised bed. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO FLUIDISED BEDS This invention concerns improvements in or relating to fluidised beds.

Particularly, although not exclusively, the present invention has reference to fluidised beds employed for the gasification or combustion of coal.

One of the problems attendant upon the operation of fluld;sed beds is that of elutriation of material therefrom, fine particles being carried by the off gases into the exhaust. Such particles contain a mixture of elements some of which are useful and for the sake of efficiency, need to be returned to the bed for further treatment.

It is, therefore, necessary to recycle the material to the fluidised bed, but the difficulty arises in reinjection into a high pressure zone, which is usually characterlstic of the gasification process.

Mechanical means for achieving such recycling are known but suffer fran the disadvantage of being unable to seal effectively against the high pressures prevailing in gasifiers; furthermore, the high temperatures associated with material emanating from a gaslfier bed, typically 1000 C, , also present problems for mechanically operated systems.

It is also known to utilise pneumatic handling systems for the purpose of transporting material from a fluidised bed to a cleaning zone wherein the material is regraded before being returned to the bed.

It is an object of the present invention to provide an improved system for recycling fines derived from the off-gases of a fluidised bed Accordingly the invention provides a system for recycling fines issuing with the off-gases from a fluidised bed reactor, the system including a columnar receptacle for the fines material leading to a horizontal or substantially horizontal outlet conduit with which is associated a gas nozzle, an outlet from the conduit feeding into an ejector provided with a fluid energy source, the ejector registering with a tube which extends substantially concentrically into the throat of a venturi having a gas inlet, the outlet fran the venturi being of diverging form and extending in use into the reactor.

The fluidised bed reactor may be a gasifier or a combustor.

The columnar receptacle together with the outlet conduit and the gas nozzle provides a combination commonly known as an L-valve, the gas nozzle in operation effecting aeration of the material in use within the conduit to cause movement towards the outlet.

The fluid energy source associated with the ejector is preferably a supply of steam which effectively conveys the fines material through the tube and thence into the venturi. The ejector also acts as a valve during start-up of the reactor , since steam at high pressure prevents any back flow into the L-valve.

The venturi essentially consists of two parts comprising concentric tubes, one of which comprises the tube referred to above and the other which consists of an outer tube to which the gas inlet is connected and in use is supplied with air.

By way of example only, one embodiment of the system for recycling fines according to the present invention is described below with reference to the accompanying drawing which depicts a diagrammatic layout.

Referring to the drawing, there is illustrated in diagrammatic form a system 1 for recycling the fines derived from a fluidised bed gasifier (not shown), the system including an Valve 2 comprising a columnar receptacle 4 having an inlet 6 and an outlet 8 formed at the end of a horizontal pipe 10 into in use a gas such as nitrogen is injected at a point 12.

The outlet from the pipe 10 registers with the inlet of an ejector 14 with which is associated a steam supply nozzle 15. The ejector is in turn connected to a tube 16 which extends horizontally and then vertically into the throat 18 of a venturi 20 provided with an inlet 22 for air. An outer tube 24 extends concentrically around the tube 16 and the venturi 20 which has a diverging outlet portion 26. The outer tube 24 has an outlet which in use extends into the gaslfier to provide a spout.

In operation, the system is used to recycle to the gasifler the fines separated from the exhausting gas. The fines are passed to the L-valve 2 and are deposited through inlet 6 into the columnar receptacle 4 whence they are transported initially by the injection of nitrogen at 12 into the vertical leg thereof and to the outlet 8 in the horizontal pipe 10. The low pressure steam from nozzle 15 carries the particles into the ejector 14 and thence in a lean phase into the tube 16, wherein the particles are transported into the venturi 20 and more particularly into the throat 18 thereof. The air injected through the inlet 22 serves to provide the impulsion to transport the particles further and to form a spout in the outlet portion 26 of the venturi 20.The outlet portion being located in use within the lower part of the fluidised bed gaslfier, creates a spout of air, steam and fine particles and in so doling effectively forms a closed loop system, with the fluidised bed constituting part of the loop.

During the start-up of the fluidlsed bed, there will be no fines or very few in the columnar receptacle 4 and thus in the absence of the steam injected through the nozzle 15, there would be a tendency for a reverse flow to occur through the venturi 20 and into the L-valve.

High pressure steam, however, injected through the nozzle 15 serves to seal off the outlet 6. This is an effective sealing valve without moving parts. Once the gasification process is under way, fines will begin to build up in the vertical leg of the L-valve and when necessary the high pressure steam can be discontinued and low pressure transport steam can be introduced through the nozzle to afford the requisite transport medium for the fines to be recycled into the fluldlsed bed.

The venturi produces a reduced pressure zone around the Inner pipe by utllising the pressure energy in the main spout air introduced through the inlet 22 generated by the main compressors on the gasification plant. The pressure exerted by the fluidised bed is thus much reduced at this point and therefore also at the L-valve.

The height required by the L-valve can be reduced by a factor of 3 in this way compared with the conventional arrangements, but still using low pressure air.

The advantages of the present invention are therefore the use of a combination of elements which contain- no moving or mechanical parts, thereby removing the maintenance problems usually associated with such equipment, and also the ability of the L-valve to be sealed during start-up when few if any fines are in the vertical leg, without the need for mechanical sealing. The L-valve can also be smaller than is usually the case and this makes for compactness.

The system is capable of handling materials at high temperatures, for example in the range of 1000 C.

Claims

1. A system for recycling fines issuing with the off-gases from a fluidised bed reactor, the system including a columnar receptacle for the fines material leading to a horizontal or substantially horizontal outlet conduit with which is associated a gas nozzle, an outlet from the conduit feeding into an ejector provided with a fluid energy source, the ejector registering with a tube which extends substantially concentrically into the throat of a venturi having a gas inlet, the outlet from the venturi being of diverging form and extendIng In use into the reactor.

2. A system according to claim 1 in which the fluidlsed bed reactor is a gasifier or a combustor.

3. A system according to claim 1 or 2 in which the gas nozzle in operation effects aeration of the material In use within the conduit to cause movement towards the outlet.

4. A system according to any one of the preceding claims in which the fluid energy source associated with the ejector is a supply of steam which In use conveys the fines material through the tube and thence into the venturi.

5. A system according to any one of the preceding claims in which the venturi consists of two parts comprising concentric tubes, one of which comprises the said tube and the other of which consists of an outer tube to which the gas inlet is connected and in use is supplied with air.

6. A system substantially as herein before described with reference to the accompanying drawing.