TECHNICAL FIELD

The present invention relates to a process for recovering energy in metallurgical processes compris¬ ing preheating of scrap, or other processes in asso- ciation with metallurgical industry where combustible gases, such as hydrocarbons, are generated. The method is especially advantageous when hot gases, e.g. from an electric arc furnace, are conducted through a basket for direct heating of scrap, such as iron, which is polluted with lacquer or oil, these pollutants being entrained in the form of droplets or vapour with the emitted gases.

The present invention also relates to an installa¬ tion for carrying out the method. STATE OF THE ART

In many metallurgical processes, there are emis¬ sions of hot gases which are often heavily polluted with dust. These hot exhaust gases represent a major part of the energy losses in such processes. Many methods have therefore been suggested in order to re¬ duce the losses of energy, e.g. by transmitting ener¬ gy from the exhaust gases to the raw material supplied in the process. In certain case, the heat in the ex¬ haust gases may also be used for preheating combustion air or other process gases by means of different types of heat exchangers.

It has long been customary to use regenerative heat exchangers for transmitting energy from an emitted polluted gas to gas being supplied. Recuperative heat exchangers, in which heat is transmitted through a par¬ tition, become much more expensive and require far more maintenance. Also, they often provide lower energy saving.

The regenerative heat exchangers may be either- fixed or movable. In the former case., the. direction of gas flow is periodically changed so that one and: the same heat exchanger will be alternately traversed- ^■ by incoming and outflowing gas. In the latter case, . the heat exchanger material is shifted between the ducts for incoming and outflowing gas, e.g. by a ro¬ tary movement.

Regenerative heat exchange thus effected is use- ful also in other processes involving e.g. thermal oxidation (combustion). One example of this is given in US 3,895,918. Of principle interest also are, inter alia, US 2,121,733 and 3,870,474.

Cooling of process gases leaving e.g. an electro- steel furnace, with the scrap iron which is part of the next charge in the furnace is disclosed e.g. in US 4,666,402.

The heating of scrap with hot gases generally involves the emission of considerable amounts of pol- lutants. The scrap often contains metal parts which are covered with oil, lacquered or otherwise contami¬ nated with substances which evaporate when heated. The gases leaving the scrap preheating process may thus be a considerable strain on the environment. Many methods are known for cleaning gases. The technique which generally is best suited for the de¬ struction of e.g. hydrocarbons is thermal combustion, which means heating the polluted gas to such a high temperature that residues of oil and lacquer are com- busted and substantially leave only harmless remainders, carbon dioxide and water. One example of this given in EP-A-82304463.1, Publ. No 74214.

To recover energy and, thus, reduce the cost for . the destruction of combustible pollutants, it is cus- to ary. practice to perform heat exchange between the gas supplied to the combustion chamber and the gas leaving the combustion chamber. Examples of this are

given in earlier cited US 3,895,918, US 2,121,733 and US 3,870,474. BRIEF ACCOUNT OF THE PRESENT INVENTION

Technical problem Heat exchangers, both recuperative and regenera¬ tive ones, contain relatively narrow ducts through which e.g. a gas flows. If the gas is polluted, pollu¬ tants deposit in these ducts which are successively blocked. In recuperative heat exchangers, this very soon means impaired performance of the heat exchangers. While not being equally sensitive, regenerative heat exchangers, too, suffer from impaired heat transfer capacity and increased flow resistance with increasing soiling. Above-mentioned US 2,121,733 and US 3,870,474 thus disclose means for continuously removing pol¬ luted material from the heat exchangers and supply¬ ing them with new or cleaned material. If the removed pollutants can be reused in the process from which they have been removed, they cannot generally be reco¬ vered without using complicated washing and purifying apparatuses. If the pollutants are highly adhesive to the heat exchanger bodies or even chemically combine with substances in these bodies, cleaning may prove im- possible or be so costly that deposition is preferred. This is a waste of natural resources and should of course be avoided.

Solution of the problem In order, with improved economy of operation and reduced clogging problems, to recover energy from hot process gases and transmit this energy to cold gases supplied in the same process, the present invention proposes a method relying on regenerative heat ex ^¬ change. In this method, hot and cold gases are alternately conducted through two or more beds of packing material. The packing consists of a material supplied in connec-

tion with charging in the process from which the' hot- gases originate or in a suitable nearby process. When the heat exchangers have been soiled to such an extent that they do not operate satisfactorily any more, the packing is removed and charged into the process, and a new packing is supplied to the heat exchangers.

It is of course not necessary that both heat ex¬ changers are emptied simultaneously and, optionally, they are not always completely emptied. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a flow diagram of an installation appro'- priate for carrying out the method according to the in¬ vention.

Fig. 2 is an example of an embodiment of the cha- racterising portion of the invention. DESCRIPTION OF A PROPOSED EMBODIMENT

The invention will now be described in more detail with reference to the accompanying drawings.

Hot gases produced in an electric arc furnace 1 pass through a duct 2 with a regulating throttle 3 to a combustion chamber 4 where they are mixed with gases supplied through a duct 13 from the scrap preheating process and charged with combustible pollutants. In the combustion chamber, these pollutants are burnt and the resulting exhaust gases are conducted, partly through a duct 5 to the scrap preheating process, part¬ ly through a duct 14 for cooling and cleaning prior to emission into the atmosphere.

The part of the gases led through the duct 5 passes through a first scrap basket 6, a duct 7 and a second scrap basket 8, and leaves the scrap preheating process charged with combustible pollutants, through a duct 9, a fan 10 and a duct 11. If so desired, it is of course possible to make such a connection that only one of the scrap baskets 6 and 8 is traversed by the gases.

The cooled and polluted gases pass through the duct 11 to a first additional scrap basket 12 with a previously heated packing for heating the gases. The gases are then led through a duct 13 to the combus- tion chamber 4 where they are mixed with the hot gases from the electric arc furnace, and the major portion of the pollutants entrained from the scrap preheating process is combused. Oxygen or air and, if required, additional fuel are supplied to the combustion chamber 4 through suitable means (not shown).

The part of the gases passing from the combustion chamber 4 through the duct 14 flows through a second additional scrap basket 15 containing a previously cooled packing which is heated by the gases, and there- after passes through a duct 16, a gas cooler 17, a duct 18, a dust filter 19, a fan 20 and a duct 21 and out into the atmosphere.

The amount of gas passing through the scrap baskets 6 and 8 is adjusted, as required, by means of the fan 10. If, in operation, one of these scrap baskets is emptied, the gas is passed only through the other scrap basket.

When the contents of the first additional scrap basket 12 have been cooled to a predetermined tempera- ture and/or the contents of the second additional scrap basket 15 have been heated to another predetermined tem¬ perature, there is a switch of the function of these two additional scrap baskets, such that the gases from the duct 11 are conducted to the scrap basket 15. The gas from the duct 14 is then instead conducted to the scrap basket 12.

In the former case of operation, the valves 11a, 13a, 14b and 16b are thus open, while the valves lib, 13b, 14a and 16a are closed. In the latter case of ope- ration, it is the other way round.

In operation, the pressure drop increases succes¬ sively in the additional scrap baskets 12, 15 because

of the deposition of dust and possibly condensed pol¬ lutants and also because of oxidation of the packing material. When it is deemed appropriate,, the baskets and/or the packing therein are exchanged, and the used packing is charged into the arc furnace 1.

If required, it is possible, by means of valves 31-42, to perform reconnections, e.g. changing the relationship between the scrap baskets 6 and 8 or by¬ passing one or more of the scrap baskets 6, 8, 12 and 15, when this is justified.

Fig. 2 shows in more detail the additional scrap baskets 12 and 15 with the associated duct system. A detachable top 22 is disposed on top of the scrap basket 12 which is placed on a suction box 24 and a truck 23. The position of the valves corresponds to latter state of operation as described above, when the scrap basket 15 is upstream of the combustion chamber 4 and the scrap basket 12 downstream thereof.

The invention is of course not restricted to the foregoing description thereof which is given by way of example only, but may be varied in many different ways within the scope of the accompanying claims.