WO2000032988A1

WO2000032988A1 - Reactor for afterburning of gases resulting from combustion

Info

Publication number: WO2000032988A1
Application number: PCT/SE1999/002111
Authority: WO
Inventors: Anders Eriksson
Original assignee: Bernardini, Mario; Gill, Nils, Gunnar; Nordström, Curt; THYR, Sören
Priority date: 1998-11-18
Filing date: 1999-11-18
Publication date: 2000-06-08
Also published as: SE9803933L; SE513303C2; AU1902500A; SE9803933D0

Abstract

A reactor for aftercombustion of combustion gases resulting from a combustion of a fuel comprising an aftercombustion chamber (1) with an inlet (4) for receiving hot combustion gases and an outlet (28) for gases resulting from the aftercombustion in the chamber. The reactor has an arrangement adapted to prolong the residence time of the combustion gases in the aftercombustion chamber, and this arrangement has members (16-19) arranged to influence the combustion gases to form centripetal vortexes for entering the chamber through the inlet in the form of such vortexes for prolonging said residence time.

Description

REACTOR FOR AFTERBURNING OF GASES RESULΗNG FROM COMBUSΗON

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a reactor for aftercombustion of combustion gases resulting from a combustion of a fuel com- prising an aftercombustion chamber with an inlet for receiving hot combustion gases and an outlet for gases resulting from the aftercombustion in said chamber, the reactor comprising an arrangement adapted to prolong the residence time of the combustion gases in the aftercombustion chamber.

The invention is in connection with that related to a reactor for aftercombustion of flue gases resulting from a combustion of any fuel at all, but the invention will hereinafter be discussed for illustrative, but thus not at all limitating purpose in the case with combustion of environmentally hazardous waste in the form of molecularly stable hydrocarbon compounds, i.e. destruction of environmentally hazardous waste.

In particular at combustion of environmentally hazardous waste it is important for the combustion to be as complete as possible, so that the molecules of the environmentally hazardous compounds in the waste are split to such a large extent as possible for forming of non-hazardous residual products. It is of course also desirable to, if possible, achieve a complete combustion of the fuel from an energy recovery point of view, when this means that as much energy as possible can be obtained from the waste.

In order to achieve such a complete combustion, reactors of the initially defined kind have been provided. By keeping the combustion gases resulting from the combustion of the fuel via a said arrangement a long time in an aftercombustion chamber, therein occurring incompletely burnt parts can be burnt further and preferably the contents of environmentally hazardous nitro- gen and sulphuric oxides therein can be reduced. For the combustion to be complete it is, however, not sufficient in itself to make to resident time of said combustion gases in the aftercombustion chamber long, it is instead also of utmost importance that the temperature of the combustion gases are kept at a suf- ficiently high level long enough in the aftercombustion chamber for the combustion to be complete. Reactors known until now are not capable of that. At earlier known plants for combustion of environmentally hazardous waste, one has tried to solve this problem with large additions of extra fuel to achieve the high temperatures required for destroying environmentally hazardous substances, such as PCB and dioxins. This has resulted in that the profitability for such a plant becomes very poor. Moreover, the residence time in a boiler of such a plant decreases concurrently with the increase of the combustion velocity, which occurs when additional fuel is supplied, so that the residence time in a possible aftercombustion chamber becomes shorter. Consequently, the combustion efficiency becomes very poor. To act this way results mainly only in a dilution of molecularly stable hydrocarbon compounds, such as dioxin, in the flue gases.

To prolong the residence time in the aftercombustion chamber, SE 460 220 proposes use of a conic shielding wall, so that the combustion gases not directly are conveyed to the outlet of the aftercombustion chamber but makes a few detours before reaching the outlet and thereby the content of nitrogen and sulphuric oxides in the gases leaving the aftercombustion chamber through the outlet is reduced. This plant is not, however, capable of achieving such a long residence time and such temperatures in the aftercombustion chamber that a complete combustion of the fuel can take place.

Furthermore, it is known from for instance EP 0 478 789 to supply the combustion gases in vortexes to the aftercombustion chamber, so that they shall whirl around in it and reside there longer than otherwise. However, reactors of this earlier known type are not capable of achieving substantially complete combustion of the fuel either.

SUMMARY OF THE INVENTION

The object with the present invention is to provide a reactor of the initially defined kind, which enables achieving of a more far reaching combustion of the fuel of a combustion plant that it is part of than in such previously known reactors.

This object is achieved according to the invention in that the arrangement has members arranged to influence the combustion gases to form vortexes for entering the chamber through the inlet in the form of such vortexes for prolonging said residence time. By that the combustion flue gases entering the combustion chamber perform a motion in the form of a centripetal vortex, the residence time thereof in the aftercombustion chamber can be prolonged substantially, primarily by that these tend to better be maintained than tangential vortexes, which easier breaks up gradually. By the stability of the centripetal vortexes and their ability to maintain an "ordered" vortex motion in the aftercombustion chamber, said residence time can be prolonged substantially and thereby the temperature can be increased substantially in the aftercombustion chamber, so that the combustion of the fuel becomes almost complete. It may in this manner become possible to get a combustion of almost 100 percent of all hydrocarbons in the fuel, so that in combustion of environ- mentally hazardous waste for instance dioxins can be split to more harmless products.

According to a preferred embodiment of the invention, said members are arranged to convey gases in a conduit in the form of a ring arranged to open into its central part in said inlet or into a thereto connected part in the way of the combustion gases upstreams thereof. By arranging a ring of this kind, the desired centripetal vortexes can be achieved, in an effective manner, for the combustion gases, resulting from the combustion of the fuel, which enters the aftercombustion chamber through the inlet and thereby the advantages with such centripetal vortexes can be utilised.

According to another preferred embodiment, said members are arranged to supply air required for the combustion of the fuel to a conduit for conveying the fuel in the form of centripetal vortexes to set the fuel and also the combustion gases resulting thereof in such a vortex motion. It is advantageous to supply air required for the combustion in this manner to achieve the desired centripetal vortexes of the combustion gases in the aftercombustion chamber, at which this air then, in the aftercombustion chamber in said centripetal vortexes, can support the final combustion in an advantageous way and with that the splitting of the unburnt parts of the combustion gases.

According to another preferred embodiment of the invention, the reactor comprises a pipe, which is arranged centrally in relation to said ring and designed to convey the fuel for the combustion and/or the combustion gases resulting from the combustion, and this central pipe has openings for said air entering thereto for admixture in the fuel or in the combustion gases resulting from the combustion. This constitutes a very advantageous and simple way to achieve said centripetal vortexes and transfer the centripetal motion from said gases to the fuel or the combustion gases. According to another preferred embodiment of the invention, the arrangement comprises members arranged to. under pressure, introduce said gases/air towards the inlet of the aftercombustion chamber, and said member for supplying gases/air under pres- sure is preferably a fan. In this manner, intense centripetal vortexes hard to dissolve can be produced of the combustion gases entering the aftercombustion chamber through the inlet.

According to another preferred embodiment of the invention, the arrangement comprises members arranged in the aftercombustion chamber, which members are arranged to guide therein entering combustion gases for increasing the residence time thereof in the chamber, the guiding member preferably comprising a cone-shaped element, substantially aligned with the inlet, the element having the base towards the inlet and being arranged at a substantial distance from the inlet for deviating thereon hitting centripetal vortexes of combustion gases in direction back towards the inlet. In this way, the residence time of the combustion gases in the aftercombustion chamber can be prolonged substantially. In that connection is it also advantageous to provide the aftercombustion chamber with two consecutively, at a distance from each other with respect to the alignment of the inlet, arranged said cone-shaped elements. By such a series coupling of cone-shaped elements, the residence time of the combustion gases in the aftercombustion chamber can be prolonged and thereby the therein present temperature can be increased, so that the combustion of the fuel can become more complete.

According to another preferred embodiment of the invention, the aftercombustion chamber has walls arranged to enable the combustion gases to flow past the cone-shaped element, which is situated closest to the inlet, but not past the other cone-shaped element and be diverted to move downwards relative to this cone-shaped element. Thereby a prolonged residence time and an increased temperature in the aftercombustion chamber are achieved.

According to another preferred embodiment of the invention, the reactor comprises a roof, which is situated downstreams of said cone-shaped element with respect to the inlet, of an inner space of the aftercombustion chamber, and this roof has an opening for flowing of combustion gases out of the space and downwards bended roof sections defining the opening which are arranged to guide combustion gases passing said cone-shaped element downwards towards the cone-shaped element again. These downwards bended sections further prolong the residence time of the combustion gases in the aftercombustion chamber and support the maintaining of the centripetal vortexes.

According to another very preferred embodiment of the invention, the aftercombustion chamber has a plurality of, outside each other, arranged spaces defined by separating walls for guiding combustion gases from within these in a path, which leads back and forth, towards the periphery of the aftercombustion chamber for prolonging the residence time of the combustion gases in the aftercombustion chamber. In this way, said combustion gases will remain in the aftercombustion chamber a long time and thereby the fuel will be burnt almost completely in the aftercombustion chamber.

According to another preferred embodiment of the invention, which constitutes a further development of the last mentioned embodiment, said separating walls are made of plates with low specific heat, and in connection with that the plates are preferably designed to be heated, by the hot combustion gases, to very high temperatures, with advantage above 600°C, advantageously above 700°C and preferably so that they are annealed. By the plates requiring so small amount of heat to achieve very high temperatures, they will also have poor cooling effect on the hot combustion gases, which thereby can have a temperature sufficiently high for final combustion of the fuel during a long time inside the aftercombustion chamber.

According to another preferred embodiment of the invention, the reactor comprises means for returning gases coming from the aftercombustion chamber into the aftercombustion chamber again for further combustion thereof. By such a returning fuel fractions despite the combustion possibly still existing in the gases flowing out through the outlet can be completely burnt when they once more are conveyed into the aftercombustion chamber. In that connection, said means can with advantage be arranged to convey the returned gases together with the air required for the aftercombustion in the aftercombustion chamber into the combustion gases. In this way, accordingly, also these returned gases can be utilised for achieving said centripetal vortexes and for themselves to be put into the advantageous centripetal motion.

The invention is also related to the advantageous use of a plant according to any of patent claims 1 -17, for destruction of environmentally hazardous waste, at which it in an advantageous way is adapted to occur for combustion of PCB and/or compounds containing dioxin. At such a use, the advantages that a reactor according to the invention has with respect to achieving long residence time and high temperatures in the aftercombustion chamber are specially useful.

Further advantages with and advantageous characteristics of the invention will be apparent from the following description and other dependent patent claims.

BRI EF DESCRIPTION OF THE DRAWINGS

Hereinafter preferred embodiments of the invention will be de- scribed as examples with reference to enclosed drawings, in which: ig 1 is a schematic view of a plant for combustion of environmentally hazardous waste, which a reactor according to a preferred embodiment of the invention is part of,

ig 2 is a sectional view of a reactor according to a first preferred embodiment of the invention,

ig 3 is a simplified view from above of a ring, arranged in the bottom of the reactor in Fig 2, for producing centripetal vortexes of thereto with pressure introduced gases,

ig 4 is a partly sectional view illustrating a crematory fur- nace, which a reactor according to a second preferred embodiment of the invention is part of,

ig 5 is a detail view of the bottom section of a reactor according to the second preferred embodiment of the in- vention with thereunder arranged attachment for use of primarily combustion gases originating from a combustion of solid fuels, and

Fig 6 is a detail view from above of the attachment according to Fig 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In Fig 1 is illustrated how a reactor 1 , in the form of a steam boiler, according to the invention could be built in in a plant for destruction of environmentally hazardous waste, especially PCB and oils containing dioxin. The environmentally hazardous waste is ignited -in an oil burner 2, the waste being gasified in a gas- ifying member before reaching the oil burner, as implied at 3. The combustion gases resulting from the combustion of the waste in the burner are supplied via an inlet 4 to the aftercombustion chamber 1 in a way specific for the invention, which will be explained later with reference to Fig 2. There is an arrangement in the aftercombustion chamber arranged to try to keep the combustion gases as long a time as possible in the aftercombustion chamber, so that the temperature in the aftercombustion chamber becomes high and possibly not burnt hydrocarbons of the waste fuel are burnt there. The resulting gases leave the aftercombustion chamber via an outlet 5 and then pass through the tube pipe system of the boiler (not shown) where they are cooled down to a temperature of for instance approximately 400°C, while the temperature in the aftercombustion chamber could have been as high as 800°C. Subsequently, the gases are conveyed further to the two convectors 6, 7, having the same function as the tube pipes in the boiler, i.e. cooling down the gases, so that condensing water can be discharged at 8. After the convectors, the gas flows into a separator 9 for sulphur, heavy metals and other minerals which not can be burnt. When the gas flows into the separator it has a temperature of ap- proximately 120°C. Subsequently, remaining air is conveyed, after cooling in a cooler 10 for exhaust gases, in a return conduit 1 1 back to an air intake 12 of the steam boiler, to there, via a mixing member 1 3, be mixed into the air being conveyed into the boiler. This return conveying gas has a temperature of about 45°C and a chlorine content originating from the polychlorinated hydrocarbons that PCB and dioxins are, and the chlorine is not possible to be separated earlier in the system. Contrary hereto, it has become apparent that this chlorine, now liberated after return conveying and introducing to the process again, can be removed, when this for instance can be done by admixture of some sodium compound somewhere in the just described cycle for forming of common salt.

What is characterising for the invention, namely the design of the aftercombustion chamber and how the gases, generated by the combustion of the waste, are supplied to the aftercombus- tion chamber, will now be described with simultaneous reference to Fig 2 and 3. Under the bottom 14 of the combustion space 15 of the aftercombustion chamber, a ring-shaped inlet part 16 (see Fig 3) is arranged, having a peripheral opening 17 through which secondary air is introduced tangentially with a substantial pressure by a not shown fan, so that centripetal vortexes are formed in the plane of the drawing in Fig 3, a centripetal vortex being shown at the lowermost right part of Fig 3. "Centripetal vortex" is consequently here defined as a flow of a medium run- ning in a vortex-shaped path towards a vortex centre. There is, centrally in the ring 16, arranged a pipe 18 which has openings 19, through which the air is sucked in thereto and being mixed with the hot combustion gases, resulting from the combustion in the flame of the boiler, and primary air and make them following its centripetal motion. Consequently, the hot combustion gases are supplied to the inner space 15 of the aftercombustion chamber under pressure, while the gases all the time perform a centripetal motion in a plane substantially perpendicular to the drawing plane in Fig 2, at the same time as the gases move in- wards in the space in the direction of the arrows 20. Once inside the space 15 they hit a cone-shaped element 21 , substantially aligned with the inlet, the element having the base towards the inlet and being arranged at a substantial distance from the inlet for deviating thereon hitting centripetal vortexes of combustion gases in direction back towards the inlet. The cone-shaped element 21 has a cone angle of about 45° and guides the gases back, so that shown vortexes are formed, which superimposes the centripetal motion performed by the gases in the plane perpendicular to the extension of the vortexes. The aftercombustion chamber has also walls 22 arranged to enable the combustion gases to flow past the cone-shaped element and either move upwards to an upper second cone-shaped element 23, which is series coupled to the first mentioned cone-shaped element 21 , for deviation thereof downwards or be caught by an opening 24 of a roof 25 of a inner space of the aftercombustion chamber with downwards bent roof sections 26 defining the opening and which are arranged to guide combustion gases passing said cone-shaped element 21 downwards towards the cone-shaped element again.

The gases will in this manner whirl around in the space 15 and the residence time there becomes long, so that a total combustion of hydrocarbon compounds will occur. In that connection a part of the gases will leave the innermost space 15 via the upper opening 24 and reach the upper cone-shaped element 23 and be diverted by that in such a manner downwards that they can reach the outside of the inner walls 21 and move downwards, where they reach guiding plates 27, guiding a part of the gases back to the space 15 and other further outwards towards the periphery of the aftercombustion chamber in a path, which leads back and forth, for prolonging of the residence time of the combustion gases in the aftercombustion chamber, after which they finally leave the aftercombustion chamber via upper openings 28. The different separating walls 22, 29-31 are preferably made of plates with low specific heat, so that these not have to absorb any large heat amounts from the hot combustion gases so as to be heated to very high temperatures and thereby the gases can be kept at a very high temperature inside the aftercombustion chamber. When said plates become annealed red, the combustion of the fuel inside the aftercombustion chamber becomes very effective.

In Fig 4 another possible use of the reactor according to the invention is shown, wherein this is applied to a crematory furnace, at which a coffin 32 is burned inside a combustion chamber 33, and subsequently the combustion gases resulting thereof are conveyed further according to the arrows 34 towards the aftercombustion chamber 35. Here these enter through slits in the periphery of a ring-shaped member 36 and are drawn downwards to the lower mouth of a central pipe 37, leading into the aftercombustion chamber space 38. At the same time secondary air is conveyed to a ring 39, designed in a similar way as shown in Fig 3, for forming of a centripetal vortex thereof, which enters the pipe 37 in the manner shown in Fig 2. The secondary air influences said combustion gases to enter the aftercombustion chamber 35 while describing a centripetal motion with the above described advantages as a result. The interior of the aftercombustion chamber can be designed in the way shown in Fig 2. Here is schematically illustrated how the gases are conveyed to a cleaning arrangement 41 , when they once have left the aftercombustion chamber and passed through a not shown tube pipe system for cooling thereof, the heavy metals being separated in the bottom 42 of the cleaning arrangement 41 , after which the air is conveyed further to a cooler 43 and then via a return conduit 44 can be conveyed back to the inner space 38 of the aftercombustion chamber.

In Fig 5 and 6 is illustrated more in detail how an attachment 45, arranged under the bottom section of the reactor, can be designed to reliably achieve forming of centripetal vortexes of the combustion gases resulting from the combustion at entering the aftercombustion chamber also in the case of combustion of solid fuels. The gas flow can, namely, not at all be guided as easily at combustion of solid fuels as at combustion of for instance an oil, and because of that special arrangements are required to give the combustion gases a centripetal vortex motion. More specifi- caily, the attachment comprises members in the form of guide rails 46 designed to convey the combustion gases, produced by the combustion of the solid fuel, obliquely tangentially/radially inwards towards the central pipe 37 while producing centripetal vortexes in the manner shown in Fig 6. The centripetal vortexes will then move downwards in the bowl 47 and reach the mouth 40 of the pipe and then move upwards in the pipe 37 for together with the secondary air, which enters the pipe via the openings 19, reaching the aftercombustion chamber space in the form of centripetal vortexes. It is apparent from Fig 6 that the guide rails 46, forming guiding members, extend from the periphery and inwards at first substantially radially but later deflecting in the same direction to come to an end with a section aligned with an angle of between 45° and 90° relative to the opposite end of the conveying bar.

A plurality of other different applications for a reactor according to the invention are also possible. Then the reactor can be arranged in a plant for combustion of as well gases as liquid and solid fuel, at which, however, in both the latter cases it is advantageous if the fuel is gasified before the combustion,

The invention is consequently not in any way restricted to the preferred embodiments described above, but many possibilities to modification thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the in- vention.

In Fig 1 and 4 two different ways of realising the ring for achieving a centripetal vortex have been illustrated, and in the latter case the ring has a helix-shaped extension, and this is intended to also be included in the patent claim definition "ring". Also other ways of designing this ring are conceivable, such as for instance conveying the gases in a path running in several revolutions with every revolution lying inside the previous one, until the inner revolution opens into a centrally arranged pipe or opening to the inner space of the aftercombustion chamber.

It is not absolutely necessary for the inlet of the aftercombustion chamber to be situated in the bottom thereof and the outlet to be situated in its upper part, although this is advantageous.

The angle of the cone-shaped element could be another and is between 30-60°, preferably 40-50°.

"Plates" is here and in the patent claims intended to have a wide meaning and shall be interpreted as thin, large surface plates, which can be of any material, not necessarily metal.

Claims

CLAIMS:

1 . A reactor for aftercombustion of combustion gases resulting from a combustion of a fuel comprising an aftercombustion chamber (1 ) with an inlet (4) for receiving hot combustion gases and an outlet (28) for gases resulting from the aftercombustion in said chamber, the reactor comprising an arrangement adapted to prolong the residence time of the combustion gases in the aftercombustion chamber,, charac- terized in that the arrangement has members (16-19) arranged to influence the combustion gases to form vortexes for entering the chamber through the inlet in the form of such vortexes for prolonging said residence time.

2. A reactor according to claim 1 , characterized in that said members are arranged to convey air in a conduit in the form of a ring (16) arranged to open into a pipe (18), which is arranged centrally in relation to said ring (16) and designed to convey the combustion gases, and in that this central pipe has openings (19) for said air entering thereto for admixture of said air in the combustion gases.

3. A reactor according to claim 1 or 2, characterized in that said members (16-19) are arranged to supply air required for the aftercombustion of the combustion gases to a conduit for conveying the combustion gases in the form of vortexes to set the combustion gases and also the air in such a vortex motion.

4. A reactor according to claim 2, characterized in that it comprises members (46) designed to convey said combustion gases into an inlet (40) of the central pipe (37) while forming centripetal vortexes.

5. A reactor according to claim 4, characterized in that the conveying members are formed by guiding members (46) ar- ranged to guide the combustion gases obliquely tangentially- radially inwards towards the central pipe (37) while producing a centripetal motion.

6. A reactor according to any of claims 1 -5, characterized in that the arrangement comprises members arranged to, under pressure, introduce the combustion gases towards the inlet of the aftercombustion chamber.

7. A reactor according to claim 6, characterized in that said member for supplying the combustion gases under pressure is a fan.

8. A reactor according to any of claims 1 -7, characterized in that said arrangement comprises members (21 , 23, 27, 29-

31 ) arranged in the aftercombustion chamber, which members are arranged to guide therein entering combustion gases for increasing the residence time thereof in the chamber, that said guiding members comprise a cone-shaped element (21 ) substantially aligned with the inlet (4), the element having the base towards the inlet and being arranged at a substantial distance from the inlet for deviating thereon hitting vortexes of combustion gases in direction back towards the inlet.

9. A reactor according to any of claims 1 -8, characterized in that the inlet (4) is arranged in the bottom (14) of the aftercombustion chamber.

10. A reactor according to claim 8, characterized in that it comprises two consecutively, at a distance from each other with respect to the alignment of the inlet, arranged said cone- shaped elements (21 , 23).

1 1 . A reactor according to claim 10, characterized in that the aftercombustion chamber has walls (22) arranged to enable the combustion gases to flow past the cone-shaped element (21 ), which is situated closest to the inlet (23), but not past the other cone-shaped element and instead be diverted to flow downwards relative to this cone-shaped element.

12. A reactor according to claim 8, characterized in that said cone-shaped elements (21 , 23) have a cone angle of 30-60°, preferably 40-50°.

13. A reactor according to claim 8, characterized in that it comprises a roof (25), which is situated downstreams of said cone-shaped element (21 ) with respect to the inlet (4), of an inner space (15) of the aftercombustion chamber, and in that this roof has an opening (24) for flowing of combustion gases out of the space and downwards bended roof sections (26) defining the opening which are arranged to guide combustion gases passing said cone-shaped element (21 ) downwards, towards the cone-shaped element again.

14. A reactor according to any of claims 1 -13, characterized in that the aftercombustion chamber has a plurality of, outside each other, arranged spaces defined by separating walls (22, 29-31 ) for guiding combustion gases from within these in a path, which leads back and forth, towards the periphery of the aftercombustion chamber for prolonging the residence time of the combustion gases in the aftercombustion chamber.

15. A reactor according to claim 14, characterized in that said separating walls (22, 29-31 ) are made of plates with low specific heat.

16. A reactor according to claim 15, characterized in that the plates (22, 29-31 ) are designed to be heated, by the hot combustion gases, to very high temperatures, with advantage above 600°C, advantageously above 700°C and preferably so that they are annealed.

17. A reactor according to any of claims 1 -16, characterized in that it is designed to receive, in the aftercombustion chamber, combustion gases with a temperature exceeding 600°C, preferably 700°C.