Combustion gases from a furnace contain a certain amount of heat energy, which cannot be reclaimed in the combustion plant, or in conventional heat exchangers. There is also corrosive matter, such as sulphur compounds, dust particles and other contaminants, which are difficult to remove, and which may cause environmental trouble.

The object of the present invention is to propose a device, which permits an efficient utilization of the residual heat in combustion gases for heating air, and simultaneously makes it possible to purify the latter, comprising neutralization of corrosive/acid compunds and collecting gasentrained ahs ^' es, particle-bound heavy metals and certain gaseous mercury compounds. The rest product will be a chemically stable compound, which may be easily disposed of.

The invention includes a regenerative heat exchanger connected to a plant producing contaminated gas, said heat exchanger comprising at least two units for alternate use and each having a storage space holding heat storing bodies and being provided with connections permitting the passage of combustion gas and air, respectively. The invention is characterized in that the heat storing bodies include a certain quantity of basic granules, each unit being provided with connections at its lower end and further connections at its upper end, as well as switch-over valves guiding the flow through the storage spaces, so contamina¬ ted gas will pass in one direction and alternatively air will pass in the opposite direction, wherein each unit and the gas inlet connection thereto are arranged so the temperature within the storage space will be-reduced to below the dew point for moisture in the combustion gases. The moisture will bind fly ashes, particle-bound heavy

metals and certain gaseous mercury compounds, and will expedite the reaction between the sulphur dioxide and the basic granules.

The connections to the storage space are preferably arranged so the gas will pass the storage space from above, downwardly, whereas the air will pass from below, upwardly. Alternatively, the connections may be arranged so the gas will pass from the interior at the storage space and out¬ wardly, whereas the air will pass from the exterior and inwardly. The storage space is advantageously provided with internal, perforated portion walls, dividing the space into at least two chambers, permitting a basically transverse flow of the gas, and air, respectively.

The heat exchanger is preferably combined with an appa¬ ratus for cleaning the basic granules, as well as means for transporting the granules from the storage space to the appratus and back to the storage space, and for removing spent material therefrom. The cleaning apparatus preferably includes a rotating, heated drum. This may be provided with a connection for receiving part of ^" the heated air, so the chemical reaction continues. Hereby calcium sulphite bound to the granules will be further oxidized into calcium sul¬ phate. This is a stable and inert rest product, which may be deposed without noticeable inconvenience. The chemically spent surface layer of the granules will be ground off while the drum is rotated, so fresh material is available, when the granules are transferred back to the heat exchanger for re—use.

Part of the heated air may be transferred to the burner of a furnace, where the contaminated gas is produced.

During certain operating conditions an excess of heated air is produced in the heat exchanger. This may advan¬ tageously be transferred to the gas exhaust duct in order to increase the rising properties of the gases.

The heat exchanger is advantageously provided with means to measure the content of sulphur dioxide in the com¬ bustion gas effluent, and for govering the transfer of granules from and to the storage space to maintain a

predetermined value of reaction intense basic contact surface.

The invention will below be described with reference to the accompanying drawings, in which Figure 1 shows a cross section through a heat exchanger according to the invention connected to a furnace, and a regeneration apparatus, which are schemati¬ cally denoted, and Figures 2 - 4 schematically shows different arrangements of the storage space and the passages therein. In Figure 1 reference 10 denotes a furnace forming part of a boiler provided with a burner 11 for gaseous, liquid or solid fuel, which is supplied with heated air from a combustion gas purifier/heat exchanger through a conduit 12. The combustion gas conduit directly connected to the boiler/furnace is denoted by 13.

In order to reclaim residual heat in the combustion gases, and to render undesirable emissions therein harm¬ less there are heat exchangers/filters 14. These will alternatively be passed through by combustion gases, which give off part of their residual heat, and by air, which will be heated. The plant therefore includes at least two similar units, even if one only is shown in the drawing. Preferably a third unit is held as a reserve, which makes it possible to cut out each unit in turn for cleaning and overhaul.

The shape of the heat exchanger units may vary, and the size will of course depend upon the capacity of the combus¬ tion plant. A preferred embodiment comprises a tower-like structure, where the base measures are less than the height, and which encloses a storage space 15 containing basic granules 16. This advantageously contains crushed limestone, or similar matter, but may possibly consist of macadam or some other carrier, provided with a basic surface coating.

In the embodiment shown, the storage space is provided with a bottom 17, which from a central, longitudinally run¬ ning slot 18 is inclined downwardly to both sides, towards

exits 19. These may be closed by lids 20, and adjacent to the slot 18 there is a flap valve 21.

The storage space 15 is provided with internal, as well as external partition walls 22 and 23, respectively. The internal partition walls 22 are mounted above the slot 18, and from a wedge-shaped void above the latter. The external partition walls 23 are parallel to the internal walls, whereby wedge-shaped voids remain between the external walls and the side walls of the unit.

The ridge-shaped roof of the unit is provided with con¬ nections for the air conduit 12 to the burner and the com¬ bustion gas conduit 13, respectively, and within the roof there is a second flap valve 24. The final gas effluent is denoted by 25, and issues from the a space below the bottom 17.

The intention is that the storage space shall alterna¬ tively be passed by combustion gas and air, respectively, and with two units in operation in a plant one will be supplied with gases for heating the granules, while the other unit is supplied with air which is heated by pre¬ viously heated granules. After some time of operation the flows of fluids are reversed.

In the operating position shown in the drawing the heat exchanger 14 is passed by air, utilizing the heat absorbed by the granules in a previous stage.

Air enters .through an opening 26.to one side of the space below the bottom 17, which is located opposite to the connection for the combustion gas conduit 25. The flap valve 21 is swung (to the right in the drawing) so a con¬ nection is obtained from the opening 26, by way of slot 18, to the space between the internal partition walls 22.

Due to the design of the partition walls the air - and later the combustion gas - will pass substantially horizon¬ tally through the layers of granules, which have about uni¬ form thickness.

The upper flap valve 24 is turned so it closes the connection to the combustion ^" gas conduit 13, but maintains the connection with the air conduit open. When the flap

valves 24 and 21 are turned to their alternative positions connections for combustion gases in, and out, respectively are opened, while the air passage is cut off.

There is always some moisture in the combustion gases, and in conventional plants the gas exit temperature is maintained so high, that no risk for condensation upon the heating surfaces of the boiler occurs. This means that there is rather much residual heat in the effluent.

The storage space 15 and the connection to the combus¬ tion gas exit is according to the invention arranged so condensation of steam occurs within the storage space. The precipitated moisture greatly enhances the reaction between the sulphur dioxide content in the combustion gases and the basic granules into calcium sulphite. The moist and sticky granules prevalent in a large portion of the storage space has a high capacity for catching fly ash, as well as particle-bound heavy metals and certain gaseous mercury compounds.

Beside the air preheating a good purification of the gases will be obtained. When air later on passes through the granules and is heated the calcium sulphite, initally formed, will at least partly be further oxidized towards the more stable, final product calcium sulphate.

After some time of use the surface layer of the granu¬ les will be less reaction active. The mass of granules is made to slowly pass downwards in the unit, so spent material is removed at its bottom, while new, reaction- inclined material is supplied at the top.

In the drawing reference 27 denotes a rotatable drum enclosed in a casing (not shown), to which the granules are supplied by some suitable transport means 28. Transfer of granules from the opposite side of the unit will occur by some similar means - the transport part is indicated by the broken line 29.

A regeneration and a mechanical cleaning of the granules occurs in the drum 27. The surface layers of the granules are worn off and separated, for instance by the drum being perforated, wholly or in part. Hereby powdery

stuff and smaller pieces from the granules will be removed while the cleaned granules are fed out of the drum by suitable means (not shown) .

The cleaning apparatus is preferably common to the two heat exchanger units and contains an excess of granules, which is being treated while both storage spaces remain filled.

Transfer back to the storage spaces occurs by means of some suitable device 30, for instance a bucket or screw conveyor. A device 31 connected to the combustion gas effluent senses the content of sulphur dioxide in the combustion gases leaving the plant, and will govern the circulation of granules so the sulphur dioxide content does not exceed a predetermined value.

The heat content of the combustion gases is usually higher than what it is possible to utilize for heating the air to the burner. The surplus of air can be transferred to the drum 27 by way of conduit 32. The hot air promotes the oxidation of the calcium sulphite on the granules into calcium sulphate, which is a stable rest product, which can easily be separated from the granules by mechanical wear, or washing. The air leaving the drum may carry a certain amount of dust and is preferably transferred to the combus¬ tion gas conduit 13 or directly to the units 14, so the contaminated air must pass the granule store, where the dust will be collected.

On these occasions when the excess of air is not needed in the cleaning apparatus 27 part of the air may be trans¬ ferred to the combustion gas effluent 25 by way of a con¬ duit 33 terminating in a nozzle 34, in order to increase the rising properties of the combustion gases.

Figures 2 - 4 schematically show some alternative embodiments of granule stores, and the combustion gas and air conduits connected thereto. In all figures the air paths are shown in full lines, while the paths of combus¬ tion gases are shown in broken lines.

The embodiment according to Figure 2 largely corre¬ sponds to that of Figure 1, but the storage space 15a has a

form more like an inverted V. The partition walls 22a, 23a are inclined about corresponding to, or slightly exceeding the natural angle of repose of the granules.

Here comparatively small surfaces on the cold side are exposed towards the possibly still corrosive gases.

Figure 3 shows a modification where the walls 22b, 23b defining the storage space has the form of a regular V. Here the surfaces at the cold side will be correspondingly bigger than in the previous example, but on the other hand there are small surfaces only at the warm side, which must be insulated to prevent condensation.

As is evident from Figure 4, the partition walls 22c, 23c can be located vertically. In this manner two parallel part storage shafts 15c are obtained, which may be comple¬ tely separated, or may be interconnected by supply and feeding-out means for the granules. The combustion gases will pass from within and outwardly, while air passes from without and inwardly.

The embodiments above described and shown in the drawings are to be regarded as examples only, and the com¬ ponents thereof may be varied in many ways within the scope of the appended claims. The storage space may be formed as a single shaft - located between two partition walls, and the bottom 17 can be formed so its sides slope towards the centre. These will ensure a simpler feeding-out of the granules.

Instead of the rotating drum 27, a shaking screen or some similar device may be used, where the granules will be subjected to mechanical wear and the influence of air, while simultaneously smaller particles are removed.

The invention is described as used in connection with a furnace, but it is evident that it may also be used in con¬ nection with industrial processes, where hot, contaminated gas is produced.