AU8278787A - Process for the purification of flue gas - Google Patents

Description

Process for purification of flue gas.

The present invention relates to a process for purification of flue gas from waste combustion in a furnace under fluid-bed conditions.

Even after sorting, household waste is characterized by uneven composition and uneven size of the individual parts or particles. When burning such a waste material, the flue gas will therefore have a complex and practically unforeseeable composition. Irrespective of the fact that household waste represents a gigantic fuel potential, it has so far not been considered justified to utilize this energy source, a.o. because it has not been possible to clean the flue gas in a sufficiently efficient manner from an environmental point of view. Beyond the lacking exploitation of the fuel potential, it is evident that the alternative depositing presents a continusously growing problem in modern society because of the space it takes up in nature, the sight and smell inconve¬ niences thereof and because of the risk of seeping into the ground and pollution of the ground water. Another problematic waste material turns up in connection with the scrapping of old cars arid other sandwich structures containing iron and metal. In a mechanical fragmentation plant, normally called a shredder plant, the cars are smashed, whereafter a sorting of the produced mate¬ rial consisting of large and small parts or particles, takes place. Thus, for the industry that works up metal, various iron and metal frac¬ tions which are not allowed to contain non-metallic pollutions are sorted out. Another fraction from the sorting, called shredder waste, consists mainly of plastic, rubber, textile fibres, wood, glas, paint remainders, metals and metal oxides. Today this primarily non-metallic fraction cannot be reused and must be deposited on dumps, whereby it presents similar problems as discussed above in connection with house¬ hold waste. Typically, shredder waste represents energy of about 15000 kJ/kg, and today when Europe alone produces more than 1 million tons of shredder waste annually, it is easily understood that it would mean an enormous progress, if shredder waste could be eliminated in an environ¬ mentally defendable manner simultaneously with exploitation of the energy.

It is known that combustion under fluid-bed conditions may be used for many types of solids containing large, heavy and/or irregularly shaped foreign bodies and other non-inflammable components, such as metal, cf. US patent specification No. 4,576,102 and US patent specifi¬ cation No. 4,553,487. In particular, combustion under slow fluidization conditions in a so-called bubble bed will be suited for this kind of materials, because the relatively long residence time creates a possibi¬ lity for good contact between the combustion air and the particles to be combusted. However, known methods for fluid-bed combustion, such as the one mentioned above, may only be used^~ for combustion of the types of waste discussed above, if an efficient purification of the flue gas is provided.

The present invention is based on the observation that by combus¬ tion of the waste materials mentioned a considerable part of the pol¬ luting components of the flue gas may be deposited by a cooling of the flue gas, including oxides, in particular metal oxides pertaining typi¬ cally from combustion of light metals such as aluminum and magnesium. Other polluting components will be acid gas components, such as S0₂, SO,, HF and above all HC1. In its broadest aspect the invention aims at removing to the greatest possible extent the polluting components from the flue gas as solids and this in a manner where these solids are sepa¬ rated in fractions which may be individually deposited or, best, reused. Another essential aspect of the invention aims at utilizing the energy content of the waste by extracting as much energy as possible from the flue gas for the production of heat, electricity or combined power/heat. The process of the invention is characterized in that the hot flue gas generated by the combustion, in the furnace or in a flue gas channel connected therewith is brought into contact with a cooled inner surface in such a manner that coatings are formed thereon and that these coatings are removed from the surface. The coatings will consist of the components of the flue gas which may be deposited by the chosen cooling method. If lime or another alkaline absorption agent has not been added to the furnace, the above mentioned acid components will therefore not precipitate, but may be removed at a later time as will be explained in more detail below. Depending on the composition of the waste material, the combustion is preferably performed at a temperature of 750-1200°C, this being con¬ trolled by a suitable choice of the combustion parameters, in particular the velocity and pressure of the combustion air. When the combustion material contains metals, which is the case with the aforementioned shredder waste, a relatively low combustion temperature of 900-1000° C. is preferred in order to avoid to the greatest possible extent that the metal content melts, as would be the case by combustion in a traditional furnace. Components of the waste material, which are oxidizable or inflammable at the prevailing combustion parameters, are carried along by the flue gas and deposited on the cooled inner surface in question. When the waste material is mainly non-metallic shredder waste according to a preferred embodiment of the invention, the deposited coating will therefore consist of or contain metal oxides.

In order to provide an efficient fluidized bed it is moreover pre¬ ferred according to the invention that sand or a similar fluidization material be brought to the fluid bed of the furnace. Even though as men¬ tioned the fluidized bed is preferably of a slow or bubbling type, sand particles will to a certain extent be carried along by the flue gas, just as a certain amount of sand particles will be removed from the bot¬ tom of the combustion chamber together with non-inflammable waste compo¬ nents, e.g. pieces of metal.

The cooled inner surfaces mentioned may have a plane or cylindri- cal (not necessarily circular-cylindrical) shape and the coatings may then be removed by means of a scraping device having a contour corresponding to the shape of the soft curve of the cylindrical sur¬ faces, whereby the scraping device may be moved backwards and forwards in the generatrix direction of the cylindrical surface. The scraping device may also be in the form of a rotating brush or another rotating device.

The cooled inner surface may also form part of a upright or mainly vertically extending stack, which thus forms part of the flue gas chan¬ nel of the furnace. An automatically operating scraping device may then at pre-determined intervals be inserted in the stack through a closable opening at the top, and the loosened coatings may be collected at the bottom of the reaction chamber from where they may be removed at certain moments.

According to the most preferred embodiment of the invention, said surface forms part of a cooled cyclone. Hereby, it has been found that the coatings of e.g. metal oxides deposited on the inner surface of the cyclone are automatically and continuously worn off by the above mentioned entrained particles of sand or like fluidization material. In this embodiment the coating material may therefore together with sand and other solids, e.g. fly ash, at intervals or continuously be removed from the bottom of the cyclone and as desired directed to slag depots or to recirculation in the combustion zone to obtain a better burning out. The cyclone or the otherwise chosen inner surface may be cooled by any suitable medium, e.g. air, but is preferably water cooled in order to thereby create the possibility of the most efficient exploitation of the heat energy content of the flue gas.

Typically, the flue gas will arrive at the cooled surface at a temperature of the order of 1000°C. and a cooling of the flue gas to about 500°C. will be appropriate. Such a temperature drop may in prac¬ tice be used for heating of district heating water which is fed at a temperature of the order of 75°C.

From the cooling cyclone or the surface cooled in another way the flue gas is led on, freed from metal oxides and certain other solids, for removal of acid components. Based on a complete combustion in the furnace, the flue gas will at this point typically contain (XL, H^O, HC1, HF, SO.,, S0₂, N₂, C , various nitrogen oxides and fly ash. This flue gas may be purified according to a preferred embodiment of the invention described in more detail in the following.

This embodiment is based on the unexpected recognition that a par¬ ticularly efficient desulphurization may be obtained by direct addition to the flue gas of a dry, fresh, alkaline absorption agent, if prior to addition of the absorption agent the water dewpoint or temperature of the flue gas is adjusted in such a manner that the actual temperature of the flue gas is only slightly above the water dewpoint of the flue gas, in particular so that the distance between dewpoint temperature and flue gas temperature is 5-50°C, preferably 10-20°C.

By this adjustment a maximum relative water content of the flue gas is aimed at without the risk of condensing fluid water and under these conditions it has appeared that the SC content of the flue gas quickly and willingly reacts with the absorption agent to form the cor¬ responding sulphite which may be removed from the flue gas in a conven¬ tional manner. The alkaline absorption agent is chosen among oxides and hydroxides of calcium, magnesium and the alkali metals and is added in dry form to the flue gas, whereby is meant that it is not a question of a solution, a suspension or another fluid or semi-fluid form, though it is not necessarily an absolutely waterfree product. For economical and practical reasons, the preferred absorption agent is a powdered lime hydrate which is in practice about 90% pure Ca(0H)₂ with a small water content of maximum 5%. The required adjustment of the distance to the dewpoint may either be effected by decreasing the temperature of the flue gas or by adding water from the outside or by a combination of both. Since the flue gas as mentioned above typically leaves the cooling cyclone at a temperature of the order of 500°C, it is preferred to perform the dewpoint adjust- ment by passing the flue gas through a heat exchanger, where a cooling to about 55-60°C. typically has the effect that the desired adjustment of the distance to the dewpoint is obtained b the usual, absolute moi¬ sture content from a combustion process. In practice, this heat exchanger may be shaped as a flue cooler having a convection heat sur- face so that practically the entire content of thermal energy in the flue gas may be utilized.

When cooling the flue gas there is a risk of condensing acid gasses, e.g. SO-, HC1 and HF, whereby the heat exchanger may corrode. This may be avoided by making the heat exchanger from acid-resistent material or by performing a coating/enamelling of the heating surface. However, it is preferred to eliminate the corrosion problem by adding a quantity of a alkaline absorption agent in dry form before the heat exchanger or the flue cooler. Hereby the acid gasses are neutralized by a principle known per se from Swedish published application No. 437,123. After the dewpoint adjustment described above, a fresh alkaline absorption agent is as mentioned added in dry form, whereby a quick and willing reaction with the remaining content of S0₂, and possibly HC1 and HF, of the flue gas takes place. The reaction product formed may be separated from the flue gas in a known manner by filters, including electro-filters, or separation cyclones, in particular by means of bag filters of the type described in US patent specification No. 4,197,278. These bag filters may consist of woven cloth or steel web and their size and number may be dimensioned according to the demands for filtration. The bag filters may be emptied at intervals determined by the desire of the principally longest residence time in the filters against an undesired growing pressure drop with increasing quantities of deposited solid material. The pressure drop above the bag filters may in a known manner be counterbalanced by installing one or more suction draft blowers in the flue gas channel after the bag filters.

The solid product filtered off may be directed to a depot or to further processing, but it is preferred that part of the product be re¬ circulated for mixing with the dewpoint adjusted flue gas before addi- tion of a fresh alkaline absorption agent.

Moreover, it is preferred that in the flue gas channel after the above mentioned heat exchanger a separator is installed wherein part of the solid reaction mixture is separated from the flue gas so that the load on the bag filter is diminished. Preferably, the inserted separator is a cyclone or a cyclone battery, whereby the residence time is prolonged. By designing such a cyclone as a socalled ulti-cyclone fil¬ ter it is moreover possible to remove the fly ash selectively from the other solid particles. The solid particles separated from the inserted separator may be directed to a depot or to processing, but it is pre- ferred that at least part of the product be recirculated together with the recirculated mixture from the bag filter to the flue gas channel. When a separator has been inserted in the flue gas channel as mentioned above it is preferred that the main quantity of the fresh alkaline absorption agent is added thereafter, i.e. in the preferred embodiment between the cyclone and the bag filter. Hereby, the fresh absorption agent is injected at a site where the flue gas has a relatively low con¬ tent of S0₂ and it becomes possible to remove this residual amount and other acid components such as HC1 and HF, if any. Alternatively or sup- plementingly, the alkaline absorption agent may be added before the separator so that the desulphurization therein is increased.

The degree of purification obtainable by means of this embodiment of the process according to the invention depends substantially on the quantity of alkaline absorption agent added relative to the content of acid components and on the degree of recirculation. For further details and advantages of the desulphurization process reference is made to

Danish patent application No. 5487/86 which is incorporated by reference in the present application with its entire contents.

The invention also relates to a fluid-bed furnace for performing the process described above, and the furnace according to the invention is characterized in that an inner wall portion of the furnace and/or of a flue gas channel connected thereto is surrounded by a mantle for cool¬ ing agent. It is thereby obtained that the coating forming products entrained by the flue gas settle on this cooled inner wall portion which may be situated in such a way that the coatings can easily be removed. In particular, it is preferred thereby that the wall portion is a cooling cyclone whereby the advantages described above in connection with the corresponding process aspect is obtained. In the following, the invention will be illustrated in more detail with reference to the drawing, wherein

Figure 1 shows schematically a combustion plant for combustion and purifiation of flue gas waste in accordance with the process of the in¬ vention, Figure 2 shows in more detail a water cooled stack or a drop chamber forming part of the flue gas channel and which in a modified embodiment is situated immediately after the furnace, and

Figure 3 shows schematically a combustion plant for combustion of waste and purification of flue gas in accordance with the most preferred embodiment of the process of the invention.

The plant shown in Figure 1 for combustion of waste comprises a fluid-bed furnace 10, at the bottom having air distributing openings 11 in connection with a fluidization blower 12. Fuel in the form of more or less comminuted waste, e.g. shredder waste, is kept in a fuel silo 13 and led from the bottom thereof to the inlet end of a screw feeder 14 by means of a conveyor band 15. The screw feeder 14 leads to the furnace 10 somewhat above the air distributing openings 11, and above the inlet of the screw feeder 14 is the inlet of a silo 16 for a fluidization material, e.g. sand or lime. The furnace 10 has at its lower bottom a reception chamber 17 for solid combustion products, non-inflammable materials contained in the fuel, and fluidization material. These mate¬ rials may from the chamber 17 via a valve 18 be directed downwards to a vibrating screen 19 whereby the fluidization material may be separated from the coarser substances and returned to the silo 16 as indicated with a dotted line 20. The separated coarser substances may be collected in a waste container 21.

The flue gas generated by the combustion in the furnace 10 is led through a drop chamber 22 which at the bottom has an outlet provided with a valve 23. From the drop chamber 22 the fuel gas may as shown be led through a convection heat surface 24 having an outlet 25 provided with a valve, and through a bag filter 26 which is also provided with an outlet 27 comprising a valve. From the bag filter the cleaned fuel gas is blown to a chimney (not shown) by means of a flue suction device 28. Figure 2 shows in more detail the drop chamber 22 connected with the furnace 10. The upper portion of this drop chamber, having a mainly circular-cylindrical shape, is surrounded by a water mantle 29 to which e.g. district heating water may be directed via an inlet 30 and the water may leave the water mantle 29 via an outlet 31. The drop chamber 22 is at the top provided with a cover 32 which may be opened and through which an automatically operating brushing or scraping device (not shown) may be introduced, when the cover has been opened, for removal of coatings on the inner walls of the drop chamber. At the bot- torn, the drop chamber is provided with a reception chamber 33 for coating material removed by means of the cleaning device, and the loosened coating material may be removed through a gate 34. The entire drop chamber 22 is appropriately enclosed by a layer of insulating mate¬ rial 35, e.g. mineral wool. After the drop chamber 29 follows in the flue gas channel another flue gas cooler 36 which may have the shape of the convection surface 24 shown in Figure 1 or any other shape.

When the combustion plant described above operates, waste, e.g. shredder waste, is led continuously from the silo 13 to the furnace 10 by means of the conveyor band 15 and screw feeder 14. Moreover, appro- priate quantities of fluidization material in the form of lime or sand are supplied as required from the silo 16. By means of the fluidization blower 12 fluidization air is blown through the air distibuting openings 11 so that the fuel and the fluidization material form a fluid-bed 37, wherein a cool ng conduit 38 may be installed for obtaining the desired temperature of the fluid-bed. Uncombusted material, ash and excess flui¬ dization material are collected gradually in the chamber 17 from where it may be directed through the valve 18 to the vibrating screen 19 where the fluidization material may be separated and returned to the silo 16 as previously described. The flue gas passes through the drop chamber 22, the convection heat surface 24, the filter 26 an the flue suction device 28 to a chim¬ ney. When the flue gas meets the water cooled walls in the drop chamber 22, metal oxides and other substances in a more or less gaseous state entrained with the flue gas may settle on the inner cylindrical wall in the form of coatings. By means of an automatically operating scraping or brushing device (not shown) introduced through the upper opening closed with the cover 32, the coatings formed in the chamber 22 may be removed at appropriate interval and the scraped coatings may be removed through the valve 23 in Figure 1 and through the gate 34 in Figure 2.

The temperature of the fluid-bed 37 may appropriately be 750- 1000°C, whereas the temperature of the flue gas when leaving the furnace 10 may appropriately be 750-1100 °C. When the flue gas leaves the drop chamber 22 it may be cooled down to 600-900°C. and after pas¬ sing the convection heat surface 24 the temperature may have fallen to 150-200°C.

In the plant shown in Figure 3 the fluid-bed furnace 10 is of principally the same structure as the one shown at 10 in Figure 1, and analogous portions have the same reference numerals as in Figure 1. Thus, fluidization air is blown in by means of the blower 12, waste material is introduced by means of the screw feeder 14 and solid combus¬ tion products, non-inflammable waste products and fluidization material are removed via the valve 18. However, in this embodiment the flue gas generated at the combustion in the furnace 10 is directed to a cyclone 30 which analogously with the drop chamber 22 in Figure 2 may be enclosed by a water mantle having inlet and outlet of district heating water or another cooling agent. When the flue gas meets with the cooled walls of the cyclone, metal oxides and other substances entrained by the the flue gas may settle on the cyclone wall from where they are worn off by the fluidization particles entrained, preferably sand. The solid material deposited in the cyclone may then be removed through the valve 23 and directed to slag depot or recirculated to the furnace in order to obtain a better burning-out of e.g. fly ash. From the cyclone 30 the flue gas is directed via a flue gas chan¬ nel 32 to a flue cooler or heat exchanger 40. This may be in heat exchanging connection with an air preheater for preheating primary air for introduction into the combustion furnace 10 or it may be formed with a convection heat surface and subsequent heat spiral in order to exchange heat with a medium, such as district heating water, in a con¬ ventional manner. From the bottom of the flue cooler 40 solids may be removed via valve 42 and directed to depot via conduit 44. From the flue cooler 40 the flue gas channel leads to a separator 50 formed as a cyclone battery and from there to a battery of bag filters 60 formed as a separator. Finally, the flue gas channel leads via a suction draft blower 70 to a chimney. A silo 80 is intended for storing alkaline ab¬ sorption agent, e.g. lime hydrate, which by means of a blower (not shown), e.g. a high pressure blower, is injected via a conduit 82 directly into the flue gas channel 32 at a primary insert point between the cyclone 50 and the filter 60. By means of a control mechanism (not shown) part of the absorption agent may be directed via a branch 84 at a secondary insert point ending in the flue gas channel 32 before the flue cooler 40. The solids separated on the filter 60 are directed at the bottom by means of a valve/conveying mechanisms 62 via a conduit 66 to a receiver 68. A blower (not shown) from the receiver 68 may recirculate material via conduit 69 to an intermediary insert point on the flue gas channel 32 between the flue gas cooler 40 and the cyclone battery 50. From the cyclone battery 50 separated solids are directed via valves 52 and conduit 54 to the receiver 68 from where the collected material via conduit 65 may be directed to depot or further processing or recircu¬ lated via conduit 69.

By way of an example, shredder waste is stoked into the furnace 10 and the volatile metal oxides, e.g. MgO and A1₂0₃, are separated in the cyclone 30 as explained above. A small quantity of powdered lime hydrate, Ca(0H)₂, is injected via the conduit 84, whereas the remainder of the desired quantity of lime is injected in dry form via the conduit 82. At the point where the conduit 84 ends in the flue gas channel 32 the flue gas has a temperature of approx. 500°C. whereby the lime neu¬ tralizes the content of SO.,, HC1, HF and the l ke acid products of the flue gas. In the fl e cooler 40 the flue gas is cooled to a temperature ^■ of about 55-60°C, causing that the flue gas now has a temperature between 15-20°C. above the waterdew point subject to a usual water content in the combustion process. The lime addded via the conduit 82 reacts with S0₂, and HC1 and HF, if any, in the flue gas in accordance with the reactions.

Ca(0H)₂ + S0₂ ;^*• CaS0₃ + H₂0

Ca(0H)₂ + 2HC1 > CaCl₂ + 2H₂0

Ca(0H)₂ + 2HF CaF₂ + 2H₂0

Unreacted Ca(0H)₂ and the CaS0₃, CaCl₂ and CaF₂ formed are deposited on the bag filters 60 from where the mixture is removed at suitable intervals and led to the receiver 68. From here a large quantity (approx. 10 times the quantity of weight of fresh lime) is re- circulated and the unused recirculated Ca(OH)₂ reacts with S0₂, and HC1 and HF, if any, in the dewpoint adjusted flue gas directed to the cyclone 50 where CaS0₃, CaCl₂ and CaF₂ are precipitated together with unreacted Ca(0H)₂. Also this product is directed to the receiver 68 from where it is recirculated via conduit 69 or ^'directed to depot via conduit 65. Fly ash not separated in the cooling cyclone 30 is separated in the cyclone 50, if desired as an individual fraction.

Claims (10)

PATENT CLAIMS

1. A process for purification of flue gas from waste combustion in a furnace under fluid-bed conditions, in particular bubbling fluid-bed conditions, c h a r a c t e r !" z e d in that the hot flue gas generated by the combustion in the furnace or in a flue gas channel connected therewith is brought into contact with a cooled inner surface in such a manner that coatings are deposited thereon and that these coatings are removed from the surface.

2. The process of claim 1, c h a r a c t e r i z e d in that the waste material mainly is non-metall c shredder waste and that the coatings deposited consist of or comprise metal oxides.

3. The process of claim 2, c h a r a c t e r i z e d in that the coatings deposited are removed from the surface at intervals.

4. The process of claims 1-3, c h a r a c t e r i z e d in that also sand or a similar fluidization material is directed to the fluid- bed of the furnace.

5. The process of claims 1-4, c h a r a c t e r i z e d in that said surface to be cooled is plane or cylindrical.

6. The process of claims 1-5, c h a r a c t e r i z e d in that said surface forms part of a cooled cyclone.

7. The process of claims 1-6, c h a r ac t e r i z e d in that said surface is cooled by means of a surrounding water mantle.

8. The process of claims 1-7, c h a r a c t e r i z e d in that the dewpoint and/or temperature of the flue gas freed from metal oxides and other coating materials is adjusted in such a manner that the actual temperature of the flue gas is only slightly above the water dewpoint of the flue gas, in particular 5-50°C, and preferably 10-20°C, that a fresh, alkal ne absorption agent chosen among oxides and hydroxides of calcium, magnesium and alkali metals is added in dry form, and that the reaction product formed is separated from the flue gas.

9. The process of claim 8, c h a r a c t e r i z e d in that the dewpoint adjustment is performed by directing the flue gas through a heat exchanger, in particular a flue cooler.

10. The process of claim 8 or 9, c h a r a c t e r i z e d in that fresh, alkaline absorption agent, chosen among oxides and hydroxides of calcium, magnesium and alkali metals, is also added before said dewpoint adjustment.