Patent Description:
The invention also relates to a plant for implementing this process.

A process and plant for pre-heating a metal charge (generally scrap) are well known to skilled persons in the field, wherein said charge is fed in continuous to an electric melting furnace by means of a horizontal conveyor; said preheating process facilitates the subsequent melting process.

The pre-heating of the charge takes place during the passage inside a tunnel in which the sensible heat and combustion heat of the exhaust gases of the melting process are exploited (in some particular cases pre-heating could also be favoured by suitable auxiliary burners). The exhaust fumes are then evacuated from the preheating tunnel and sent to a suitable treatment system. The combustion heat that is exploited in the preheating process is essentially provided by the completion of the combustion of the CO (carbon monoxide) and H<NUM> (hydrogen) released by the process underway in the melting furnace, whereas the necessary oxygen is generally provided with the supply of environmental air.

A process and a plant such as those briefly described above are disclosed for example by US patent <CIT>, which describes the injection of the air needed for combustion uniformly along the preheating tunnel. This solution teaches the injection of air distributed along the preheating tunnel in such a quantity as to guarantee an excess of oxygen in the order of <NUM>-<NUM>% so as to ensure the complete combustion of the unburnt gases, assuming that the seal between furnace and tunnel is almost perfect. In plant-engineering practice, it has been seen that this situation cannot be achieved and there are always significant infiltrations of external air (in particular at the interface between the furnace and tunnel), often to an extent already more than sufficient for ensuring the complete combustion of the process gases exiting from the furnace; it has also been observed that these air infiltrations are not able to reach sufficient turbulence conditions, as this air tends to follow the internal walls of the heating tunnel, and mixing and combustion with the process gases take place slowly. An excessive infiltration of ambient air into the preheating tunnel must be absolutely avoided as it would overly lower the temperature of the gases, and if this temperature reaches a value very close to or even lower than the CO ignition limit, there is the risk of not completing its combustion with the consequent release of this toxic gas into the environment, in addition to the significant loss of efficiency of the pre-heating system of the charge.

Within the context of the technical solution described above, the injectable air from the hood is consequently extremely low if not zero; this fact worsens the problem related to the low turbulence, and prevents the best exploitation of the energy available inside the preheating tunnel.

The general objective of the present invention is to overcome the drawbacks of the known art and, in view of this objective, according to the invention, to improve the exploitation of the energy present in the fumes for heating the charge.

More specifically, the objective of the present invention is to increase the heat exchange between the hot process fumes and the metal charge.

The above objectives are achieved by a process and plant produced according to the enclosed independent claims and subordinate claims.

Thanks to the present invention, the heat exchange between the hot process fumes and the metal charge is improved by increasing the turbulence and the mixing of the gaseous stream inside the preheating tunnel, with a consequent acceleration of the combustion processes and an increase in the convective heat exchange coefficients between said combustion gas and charge material. This effect is obtained with high-speed jets of gas; the gas used is generally air, but the use of a different gas is not excluded, if needed to manage the chemical composition inside the preheating tunnel; this gas can also be advantageously preheated.

In particular, an object of the present invention relates to a process and plant for preheating a metal charge fed in continuous to an electric melting furnace through a preheating tunnel provided with a horizontal conveyor, wherein said metal charge is hit, in countercurrent, by the exhaust fumes or gases leaving said electric melting furnace and by jets of gas ejected through a plurality of nozzles positioned in the hood of said tunnel provided with side walls and said hood. Said nozzles are arranged in groups interspaced with respect to each other in a longitudinal direction with respect to the tunnel, and generate a small-scale turbulence or inject small fast gas jets that can penetrate the stream, and said nozzles simultaneously generate a "horseshoe vortex" structure, consisting of a descending central gas flow ("downwash"), and ascending flows ("upwash") close to the side walls of the preheating tunnel which allow the necessary circulation of the gases.

According to the present invention, said nozzles are arranged in groups, in each of which the nozzles are aligned in correspondence with certain transversal sections of the hood of the tunnel, suitably spaced apart. This allows a small-scale turbulence and simultaneously a large-scale vortex structure to be generated: the first one corresponds to the fact that the small fast jets of gas are able to penetrate the main gas stream passing through the tunnel, considerably accelerating the mixing and combustion of the gases; the large-scale vortex structure, which increases the heat exchange between fumes and charge, is commonly defined a "horseshoe vortex" and is characterized by a descending central flow ("downwash"), which increases the heat exchange in the centre of the preheating tunnel, and ascending flows ("upwash") close to the side walls of the tunnel which allow the necessary circulation of the gases, and which, after transferring part of their heat energy to the metal charge in the descending phase, limit the heat exchange with the side walls of the tunnel and horizontal conveyor. Contrary to what is disclosed in the known art, the above-mentioned gas jets are not arranged uniformly along the preheating tunnel but are rather arranged in groups, at least two, suitably interspaced; this is to avoid interference of a fluid-dynamic nature and to allow, first of all, a good mixing of the gases and a rapid development of the combustion (with a small-scale turbulence effect) and subsequently pushing them towards the metal charge (with the "horseshoe vortex" effect).

Contrary to what is present in the known art, the nozzles are not dimensioned so as to supply all the combustion air in a distributed and uniform mode, but, instead, they are dimensioned as small fast jets whose primary function is to supply kinetic energy and modify the field of motion according to what is described above; for this reason, the above-mentioned jets can be more accurately defined as "fluid-dynamic turbulence generators" or more simply, "fluid-dynamic turbulators".

The use of "fluid-dynamic turbulators" is much easier and cheaper than the alternative solution of increasing the turbulence inside the preheating tunnel by the insertion of deflector panels, or so-called "static turbulators"; these deflectors must operate within a gas flow characterized by high temperature and high content of dust and they are therefore normally built as water-cooled metal panels, which is not an efficient solution from a thermal point of view; independently from how these deflector panels are built, their use has been practically abandoned due to their rapid wear and frequent breakages.

The known art does not take into consideration the fact that in practice, there is always a significant infiltration of ambient air into the charge preheating tunnel through unavoidable openings, and that the quantity of combustion air is variable during the process, whereas the need for a good mixing is a substantially constant.

The advantage deriving from the present invention is therefore evident, whereby the operation of jets for controlling the turbulence inside the preheating tunnel is substantially decoupled by the control of the supply of possible combustion air.

The structural and functional characteristics of the invention and its advantages with respect to the known art will appear more evident from the following description, referring to the attached drawings, which illustrate a possible non-limiting embodiment of the invention itself applied to an electric arc furnace (EAF) for melting metal scrap charged in continuous.

With reference to the figures, <FIG> illustrate three plants produced according to the known art, in particular, <FIG> illustrates a traditional plant with a preheating tunnel without gas injectors; <FIG> illustrates a plant with air injectors arranged in the preheating tunnel according to the known art; <FIG> illustrates a plant with a heating and a preheating section, with burners, connected by a suction/evacuation section of the fumes.

In the <FIG> indicates as a whole a plant for continuously feeding a metal charge of scrap <NUM> to an Electric Arc Furnace (EAF) <NUM> in which a bath of molten metal is present in the liquid state.

In these configurations, the flow <NUM> of fumes coming from the furnace <NUM> follows a substantially linear path, which tends to be aligned with the walls of the preheating tunnel, thus moving away from the metal charge <NUM>. Also in the configuration of <FIG>, with the injectors of combustion air <NUM>, the flow <NUM> does not undergo significant deviations, as the combustion air introduced by the injectors <NUM> is normally extremely limited, due to the infiltrations of cold air <NUM> which are present at the interface between the furnace <NUM> and the tunnel and which are almost always sufficient for completing the combustion of the fumes <NUM> coming from the furnace <NUM>.

A plant <NUM> of this type is described for example in patent <CIT>.

The plant <NUM> comprises at least one horizontal conveyor <NUM> suitable for continuously moving the metal charge of scrap <NUM> towards the electric melting furnace EAF <NUM>, defining the respective continuous horizontal feeding line of the charge <NUM> to a charging area IV of the furnace itself <NUM>.

As can be clearly seen from the drawings, the horizontal conveyor <NUM> forms the base of a preheating tunnel <NUM> of the metal charge of scrap <NUM>.

More specifically, the plant <NUM> is composed of a preheating section III which brings the metal charge of scrap <NUM> into the electric melting furnace EAF <NUM>, of an evacuation section of the fumes II present in the plant <NUM>, located, considering the movement direction of the scrap <NUM>, upstream of said preheating section III, and of a section I which receives the metal charge of scrap <NUM> with a conventional receiving system of the scrap <NUM>.

The horizontal conveyor <NUM> conveys the metal charge of scrap <NUM> by oscillation and transfers it from the preheating section III to a movable terminal section, called "connecting car", which leads the scrap <NUM> into the electric melting furnace EAF <NUM>.

According to the present invention, nozzles are present on the hood of the tunnel of the preheating section III (preheating tunnel <NUM>), for the injection of gas <NUM>.

In particular, these are nozzles for the high-speed injection of gas <NUM>.

Said nozzles <NUM> are distributed so as to obtain a whirling turbulent motion inside the preheating tunnel <NUM> to improve the heat exchange between the off-gases <NUM> and metal charge of scrap <NUM>.

As illustrated in <FIG>, the nozzles <NUM> provided on the hood of the tunnel <NUM> of the preheating section III increase the turbulence of the off-gases <NUM> thus allowing the following to be obtained:.

In the plants of the known art devoid of said nozzles or "fluid-dynamic turbulators" <NUM>, the air entering the plant through the connection portions is uncontrolled, and with a limited turbulence and vorticity, without adequately mixing with the gases (<FIG>), and therefore causing a slow and often incomplete combustion inside the preheating tunnel <NUM>.

Thanks to the presence of the "fluid-dynamic turbulators" <NUM> inside the preheating tunnel <NUM>, on the contrary, a greater mixing of the gases is obtained together with a higher flame intensity, which also help to limit the cooling due to the entry of air from outside of the plant, in particular into the preheating tunnel <NUM>.

As illustrated in <FIG> and <FIG>, the arrangement of the nozzles or "fluid-dynamic turbulators" <NUM> allows the so-called downwash portion of the field of motion to be concentrated on the central portion of the horizontal conveyor <NUM>, where the maximum heat exchange is therefore obtained between the metal charge <NUM> and the gases/fumes <NUM> present in the preheating tunnel <NUM>.

In order to obtain the above-mentioned whirling-motion configuration, the nozzles <NUM>, and therefore the jets, are distributed transversely on the hood of the preheating tunnel <NUM> in a non-uniform manner with a greater concentration on the top of the hood of the tunnel <NUM>.

The arrangement of the gas jets is therefore such as to obtain a well-defined whirling structure (<FIG>) inside the preheating tunnel <NUM>, characterized by:.

This whirling structure of gases in the preheating tunnel <NUM> is commonly called a "horseshoe vortex" and is obtained, according to an embodiment of the present invention, by arranging the nozzles <NUM>, and therefore the jets, over about <NUM>/<NUM> of the cross section of the preheating tunnel <NUM>, leaving the two side walls, close to the side walls of the hood of said tunnel <NUM>, free.

The arrangement of the nozzles on the hood of the preheating tunnel <NUM> can vary in relation to the specific plant issues (see for example the embodiment solution shown in <FIG>), maintaining anyway the requirement that the high-speed jets always intercept the central portion of the flow of off-gases <NUM>, leaving the side portions free and thus favouring the establishment of an upward circulation of the gases and the formation of a horseshoe whirling motion in the entrainment flow.

The high-speed jets that act as "fluid-dynamic turbulators" are not uniformly distributed longitudinally on the hood of the preheating tunnel <NUM>, but according to "injection sections" adequately spaced apart from each other, so as to avoid fluid-dynamic interference phenomena; the distance between two adjacent injection sections should be <NUM>-<NUM> metres depending on the velocity of the gases passing through the preheating tunnel. The space between two adjacent injection sections is needed to allow the high-intensity flame produced by the section upstream to have time to develop before being pushed towards the charge by the injection section immediately downstream.

In order to best exploit the length of the preheating tunnel <NUM> for improving the heat recovery and completing the combustion of the CO and H<NUM> and possible pollutants in the process gases , the first injection section is positioned as close as possible to the electric melting furnace EAF <NUM>.

The first group of fast gas jets is located in proximity of the electric melting furnace <NUM>, within a distance of <NUM>-<NUM> metres from the same.

The invention provides for gas jets with velocities and/or flow-rates that increase among subsequent "gas injection sections". The number of injection sections varies from two to four, depending on the quantity of combustible gas produced by the melting process under consideration.

As illustrated in <FIG>, starting from the electric melting furnace EAF <NUM> and following the flow of gases towards the fume suction plant (gas flow opposite to the movement direction of the metal charge <NUM>), the plurality of nozzles <NUM> forming the first injection section can be located above the connecting car (first water-cooled hood) whereas the nozzles <NUM> forming the other injection sections can be arranged at the beginning of each segment of the refractory section of the preheating tunnel <NUM> (refractory lined hood).

In the example, three injection sections are used, each composed of four nozzles <NUM>.

The gas injected is generally air but another gas can also be adopted and the gas used can also be preheated.

Control means of the operating conditions of the nozzles <NUM> can be provided in each section.

The jets released from the nozzles <NUM> are small and fast as they must be capable of providing both a mixing and deviation action on the stream of gas passing through the preheating tunnel <NUM>, enabling the "downwash" motion of the hot gases <NUM> towards the metal charge <NUM>, with a velocity sufficient for penetrating the interstices of the material (the so-called "impingement" effect, as visible in <FIG>, where the flow of hot gases <NUM> is pushed downwards towards the metal charge <NUM>), thus improving the convective heat exchange.

This effect is obtained by assessing the flow-rate conditions and velocity of the gas flows involved in the process in question: defining impulse of a fluid stream as the product between mass flow-rate and velocity of the stream itself, each single jet shall be designed in such a way that the set of jets has an impulse similar to the impulse of the main stream of fumes passing along the tunnel <NUM> from the furnace <NUM> to the suction plant.

The design condition is therefore the following: <MAT> Wherein:.

For purely illustrative purposes, in the application described, considering the injection of air at room temperature, this condition is normally obtained with jets having a flow-rate of around <NUM>,<NUM><NUM>/h and a discharge velocity ranging from <NUM> to <NUM>/s.

Within the proposed technical solution, due to the gas injected, a progressive increase in the flow-rate of gas passing through the preheating tunnel <NUM>, is produced, therefore it may be necessary to consider jets with a greater impulse for the injection sections positioned further away from the furnace <NUM>.

Following the flow of gases leaving the melting furnace <NUM> and entering the preheating tunnel <NUM>, the first injection sections are designed to use lower flow-rates and velocities than the subsequent injection sections, due to the overall increasing flow-rate of the gas passing through the preheating tunnel.

Each injection section can be managed, controlled and regulated independently of the others, depending on the process status and the characteristics of the charge <NUM> present in the preheating tunnel <NUM> and gases <NUM> leaving the furnace.

In the simplest embodiment, the nozzles <NUM> are all the same and arranged on the top of the preheating tunnel <NUM> and their number basically depends on the width of the preheating tunnel <NUM> itself, considering an availability of about <NUM>/<NUM> of the central portion (where the "downwash" area is to be established) with a distance between each jet of about <NUM>-<NUM>. In order to obtain an effective impingement effect, the top of the preheating tunnel <NUM> must be positioned at a distance of around <NUM>-<NUM>,<NUM> from the charge present in the conveyor (if the present invention is applied to an existing plant, this may require a redesigning of the preheating tunnel). In case of particular configurations of the preheating tunnel <NUM>, for example in the presence of plant constraints that do not allow the nozzles <NUM> to remain at the same distance from each other, a different arrangement and dimensioning of the jets can be used for obtaining an equivalent fluid-dynamic effect.

Contrary to the known plants and processes wherein the air injection is linked to the control of the combustion process from a stoichiometric point of view, in the present invention the jets of air or other gas are mainly used for obtaining certain turbulence conditions inside the preheating tunnel <NUM>.

Even in the most common case of the use of air jets, the overall injection capacity of the system described is almost always lower than the flow-rate of air necessary for completing the combustion of the process gases <NUM> coming from the furnace <NUM>, as the primary objective of the system described is to stabilize the turbulence; the control of the supply of combustion air of the process gases inside the preheating tunnel <NUM> is basically delegated to the modulation of the suction depression and width of the gap between the entrance of the preheating tunnel and the furnace (that can never be completely eliminated from a plant-engineering point of view). In this way, the injection of air as turbulence generator is significantly decoupled by the supply of ambient air for completing the combustion of the process gases.

The final objective of the present invention is to increase the combustion intensity of the process gases coming from the furnace and the heat exchange between them and the charge, thus increasing the overall energy efficiency of the melting process.

The improved mixing and combustion of the process gases released from the furnace achieved with the present invention allows to obtain a better thermal destruction of the polluting substances (and related precursors) present therein.

The present invention can also be applied to plants such as those described, for example, in the document <CIT> provided with a preheating tunnel and a heating tunnel of the metal charge of scrap.

Thanks to the present invention, the scrap charge heating takes place through a turbulence created in the preheating tunnel <NUM>, unlike what happens in the known plants in which the introduction of air is carried out according only to the requirements of the chemical combustion process, without any link to the control of the motion field inside the preheating tunnel.

Thanks to the present invention, the use of deflectors inside the preheating tunnel <NUM> can also be avoided. These deflectors have disadvantages as they cause a significant pressure drop in the suction of the fumes, they require cooling and frequent maintenance operations as they operate within a flow of very hot and dusty gases (this not only represents a plant complication and a potential risk of leakages, but also causes a useless loss of thermal energy by the gases), they are difficult to regulate and manage from a practical point of view as it is not easy to change the incidence angle, and finally, their effect is limited when the velocity and therefore flow-rates to be treated are low.

The objectives of the invention mentioned in the preamble of the description have therefore been achieved.

Claim 1:
A process for preheating a metal charge (<NUM>) fed in continuous to an electric melting furnace (<NUM>) through a preheating tunnel (<NUM>) provided with a horizontal conveyor (<NUM>), wherein said metal charge (<NUM>) is hit, in countercurrent, by the exhaust fumes or gas (<NUM>) leaving said electric melting furnace (<NUM>) and by jets of gas ejected through a plurality of nozzles (<NUM>) positioned on the hood of said tunnel (<NUM>) provided with side walls and said hood, characterized in that said nozzles (<NUM>) are arranged in groups interspaced with respect to each other in a longitudinal direction with respect to the tunnel, and inject gas jets that can penetrate the main gas stream (<NUM>) passing through the preheating tunnel (<NUM>), and said nozzles (<NUM>) simultaneously generate a "horseshoe vortex" structure, consisting of a descending central gas flow and ascending flows close to the side walls of the preheating tunnel (<NUM>) which allow the necessary circulation of the gases and further characterized in that the gas jets released from said nozzles (<NUM>) are distributed transversely on the hood of the preheating tunnel (<NUM>) in a non-uniform manner, with a greater concentration on the top of the hood of said tunnel (<NUM>).