"ENERGY ACCUMULATION DEVICE AND CORRESPONDING PRODUCTION METHOD"

FIELD OF THE INVENTION

The present invention concerns an energy accumulation device to be applied in apparatuses for recovering the heat energy of gaseous wastes, also called off- gases, deriving, for example, but not exclusively, from melting furnaces.

In particular, the energy accumulation device is able to homogenize the temperature of the gaseous wastes coming, for example, from an apparatus installed upstream such as a melting furnace, to supply the recovered heat energy having a substantially constant development over time.

The present invention also concerns a method to produce said energy accumulation device.

BACKGROUND OF THE INVENTION

Melting plants for metal materials are known, which comprise a furnace for melting predominantly metal material, for example an electric arc furnace, a converter, or a blast furnace.

Devices are also connected to the melting furnace to discharge the fumes or gaseous wastes, which comprise an expansion chamber, also called the settling chamber, in which the fumes are expanded and the consequent precipitation of the heaviest particulate occurs.

It is also known to equip the melting plants with apparatuses for the recovery of heat energy from the gaseous wastes, in order to reduce the costs connected to the production of heat energy, to comply with the dictates of national and international energy recovery regulations and to promote the production of thermal and electrical energy from alternative sources.

It is known, however, that melting plants have a great discontinuity in functioning during a whole metal melting cycle, both in terms of temperature and duration.

Fig. 4 shows an example of the variation in the temperature of the fumes deriving from melting over time, during different cycles.

This discontinuity is due to the very nature of the melting process which is not stationary. In fact, a complete melting cycle comprises at least one step of loading the metal into the melting furnace, at least one step of heating the metal until it is melted and a step of removing the molten mass. This discontinuity affects the heat energy obtained from the gaseous wastes which, in turn, is equally discontinuous and negatively affects the efficiency of the apparatuses that recover energy from the fumes.

To overcome this disadvantage, energy recovery apparatuses are also equipped with energy accumulation devices which are installed, for example, in the fume expansion chamber and are hit by the fumes.

Energy accumulation devices generally comprise a container to contain phase change materials (PCM). One of the phase change materials particularly suitable for the recovery of heat energy from the fumes, due to its phase change temperature, is aluminum.

Phase change materials allow to damp the thermal profile of the gaseous wastes, thanks to the high latent heat during melting/solidification which they intrinsically have. In particular, phase change materials are able to accumulate the excess heat energy due to the heat energy peaks of the fumes and to return it when the latter have a lower heat energy.

One disadvantage of using phase change materials, in particular aluminum, is due to the corrosion phenomena to which the material of the container is subject, usually steel.

To overcome the problem of corrosion it is also known to use containers made of ceramic material, for example ceramic pipes made of silicon carbide or mullite, to contain the phase change material.

However, these materials are very fragile and not very suitable for an industrial application. Furthermore, these materials are also particularly expensive.

A heat accumulation device is also known from document US-A-4.512.388 comprising a phase change material, or PCM. The phase change material is supported in the pores of a support material. Both the phase change material and the support material can be contained in a container. A work fluid is made to circulate directly inside the container, that is, in direct contact with the support material and the phase change material. In this solution, the heat exchange with the energy vector is direct, that is, in this case the fumes pass through both the porous material and through the phase change material. This, however, can lead to a deterioration in the performance of the heat accumulation device if the work fluid used also carries polluting particles with it, such as for example in the case of the off-gases deriving from melting furnaces. Furthermore, this solution provides that the porous material and the phase change material are assembled together to define pellets or briquettes to be inserted into a container. The coupling of the porous material and the phase change material is obtained by pressing the particles of the two materials together and heating the whole to a temperature suitable for melting only the phase change material, that is, to a temperature lower than the melting temperature of the support material. When it is melted, the phase change material also incorporates the support or porous material inside it. However, this embodiment does not permit a reciprocal aggregation of the particles of the support material. During the phase change cycles of the phase change material, the support material can be subject to yielding, which can alter the thermal exchange efficiency of the device.

In document EP-A-0.299.903 a method is described to encapsulate a phase change material, or PCM, in a containing material. The containing material is a reaction product of the phase change material itself. This solution, however, has disadvantages connected to the difficulty of ensuring, over time, the encapsulation of the phase change material. The continuous process of melting and solidification of the phase change material can, in fact, cause dispersions of the latter.

In document US-A-2013/0056193 a phase change material PCM is described which is contained in containers which may or may not be coated with materials that are inert to the phase change material PCM in the molten state. In this solution, however, at least for some types of phase change materials, such as aluminum alloys, the anticorrosive coating of the container does not solve the problem of corrosion.

One purpose of the present invention is to provide an energy accumulation device which has a longer life and greater efficiency than known accumulation devices.

Another purpose of the present invention is to provide an energy accumulation device which reduces maintenance interventions. Another purpose of the present invention is to provide an energy accumulation device that is efficient.

Another purpose of the present invention is to perfect a method to produce an energy accumulation device which is simple and quick to implement.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

In accordance with the above purposes, the present invention concerns an energy accumulation device comprising a container and a heat accumulation body located in the container.

According to one aspect of the present invention, the heat accumulation body comprises a plurality of granules made of at least one energy accumulation material and powders made at least of a heat exchange material having a melting temperature higher than that of the energy accumulation material.

According to solutions of the invention, the container is provided with a cavity which is closed with respect to the outside, and the heat accumulation body is positioned inside the cavity.

Moreover, in accordance with possible solutions, the container is provided with a heat exchange surface facing toward the outside of the container, and which, during use, is lapped by gaseous wastes.

Moreover, the granules and the powders are sintered to define said heat accumulation body in a compact form, that is, without porosities, at least in the condition where the energy accumulation material is also in the solid state.

Here and hereafter in the description and the claims, by the term sintered we mean the heat and mechanical process intended to obtain compact materials from powdery substances. The sintering process entails compacting and heating both the granules and the powders to obtain the heat accumulation body with a compact conformation, that is, without internal porosities, as is provided on the contrary in known solutions. Sintering therefore also determines a compacting of the powders alone.

In this way it is possible to obtain an energy accumulation device which is extremely efficient and effective and allows to accumulate the heat energy for example of gaseous wastes, such as fumes exiting from a furnace, and to replace it when the latter have less heat energy.

Thanks to the fact that the granules and the powders are sintered it is possible to make the granules perform the function of heat energy accumulation, while the powders, which are sintered around the granules, exert a containing action on the energy accumulation material, preventing it from being dispersed in the container. This allows to limit, if not eliminate, the phenomenon of corrosion to which the container is subjected due to the action of the melted energy accumulation material.

Moreover, sintering eliminates the presence of air, or porosities, between the powders and the granules, thus increasing the heat exchange efficiency.

In accordance with another solution of the present invention, at least part of the granules is made of a phase change material (PCM).

By phase change material it is meant a material able to accumulate energy, for example a latent heat, when it changes phase, that is, when it passes from the solid state to the liquid state or vice versa.

Phase change materials allow to store energy 4-15 times more compared to other materials.

Moreover, in accordance with possible embodiments, the phase change material has a higher heat conductivity, for example comprised between 1 10W/(m K) and 410W/(m K), preferably between 200W/(m K) and 250W/(m K), which allows to speed up the heat exchanges with the fumes.

In accordance with another aspect of the present invention, the phase change material has a melting temperature that is in the range of the temperature of the gaseous wastes that are being processed. Merely by way of example, the melting temperature of the phase change material is comprised between 400°C and 700°C. In this way it is possible to guarantee the damping action on the temperature of the gaseous wastes.

In accordance with a possible advantageous solution the phase change material is aluminum or its alloys. According to a variant embodiment, the phase change material can be chosen from a group comprising at least one of either tin, copper, lead, zinc or their alloys.

In accordance with another variant embodiment, the phase change material can comprise inorganic salts or eutectic mixtures thereof.

The present invention also concerns an energy recovery apparatus to recover the heat from gaseous wastes, which comprises an expansion chamber for the gaseous wastes and at least one energy accumulation device installed in the expansion chamber.

The present invention also concerns a melting plant for metal comprising a melting furnace and an energy recovery apparatus as described above, connected to the melting furnace to receive from the latter gaseous wastes.

The present invention also concerns a method to manufacture the energy accumulation device, which provides to insert a heat accumulation body in a container.

According to one aspect of the present invention, the method also provides to sinter with each other granules made at least of an energy accumulation material and powders made of at least one heat exchange material having a melting temperature higher than the energy accumulation material so as to obtain the heat accumulation body in a compact form and without porosities.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a schematic view of a melting plant comprising an energy recovery apparatus according to the present invention;

- fig. 2 is a section view of an energy accumulation device according to the present invention;

- fig. 3 is a section view of a detail of the energy accumulation device;

- fig. 4 is a diagram of the development of the temperature of the gaseous wastes;

- fig. 5 shows a possible variant of fig. 1 ;

- fig. 6 shows a possible variant of fig. 2.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can conveniently be incorporated into other embodiments without further clarifications.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

We will now refer in detail to the various embodiments of the present invention, of which one or more examples are shown in the attached drawings. Each example is supplied by way of illustration of the invention and shall not be understood as a limitation thereof. For example, the characteristics shown or described insomuch as they are part of one embodiment can be adopted on, or in association with, other embodiments to produce another embodiment. It is understood that the present invention shall include all such modifications and variants.

An energy accumulation device 10 (fig. 2), according to the present invention, comprises a container 1 1 and a heat accumulation body 12 located in the container 1 1.

The container 1 1 can have a tubular elongated shape to increase its heat exchange surface and optimize the heat exchange efficiency.

By way of example only, the container 1 1 can have an external diameter comprised between 30mm and 90mm, a height comprised between 1000mm and 3500mm, and a thickness comprised between 2mm and 5mm.

In particular, the container 1 1 is provided with a cavity 13, closed with respect to the outside, in which the heat accumulation body 12 is positioned. The fact that the cavity 13 is closed, and therefore not affected by the passage inside it of the gaseous wastes, prevents the latter from coming into direct contact with the materials of which the heat accumulation body 12 consists, causing a possible oxidation thereof.

The container 1 1 has the function of supporting and protecting the heat accumulation body 12, for example from impacts.

The container 1 1 is also provided with a heat exchange surface facing toward the outside of the container, and which, during use, is lapped by gaseous wastes.

The container 1 1 can be provided with an aperture 14, made for example in correspondence with one end of the container 1 1 itself, and suitable to allow the introduction of the heat accumulation body 12. The aperture 14 can be closed by a closing element 15, for example a lid.

In accordance with possible embodiments, the container 1 1 can be defined by a tubular body at the ends of which respective closing elements are associated in order to contain the heat accumulation body 12.

The heat accumulation body 12 can be positioned in the container 11 so that the latter is substantially adherent, that is, in contact with the walls of the container 1 1, and possibly also with the closing element 15. The positioning of the heat accumulation body 12 in contact with the container 11 optimizes the action of heat exchange.

The container 1 1 can be made of a material having a heat conductivity equal to, or greater than, lOW/mK.

According to one embodiment, the container 1 1 can be made of a metal material. The use of a metal material guarantees a high heat exchange capacity with the heat accumulation body 12 contained therein.

By way of example only, it is possible to provide that the metal material is a steel, for example stainless steel, such as stainless steel with a low carbon content, for example AISI 304 steel.

The use of this type of material allows to increase the resistance of the container 1 1 to oxidation phenomena induced by the circulation of the gaseous wastes having high temperatures.

According to one aspect of the present invention, the heat accumulation body 12 comprises a plurality of granules 16 made of at least one energy accumulation material and powders 17 made of a heat exchange material having a melting temperature higher than the melting temperature of the energy accumulation material.

Moreover, the granules 16 and the powders 17 are sintered together to define the heat accumulation body 12 in compact form.

This configuration allows to obtain a heat accumulation body 12 which is particularly suitable for recovering the heat energy of gaseous wastes such as the fumes exiting from a melting plant.

The powders 17, once sintered, have the function of incorporating inside them the granules 16 which, during use, can melt to accumulate the heat energy. The molten material of the granules 16 remains confined and contained by the sintered powders 17.

The powders 17, in fact, having a melting temperature higher than that of the granules 16, can exert this containing action.

In accordance with a possible solution of the present invention, the energy accumulation material of which the granules 16 are made comprises a phase change material, or PCM, that is, a material able to store latent heat when it passes through a phase transition from solid to liquid.

The phase change material can be selected from a group comprising tin, copper, lead, zinc, or their alloys.

According to a possible variant embodiment, the phase change material can comprise molten salts such as a hydrated salt.

According to other embodiments, the phase change material can be selected from a group comprising organic materials, for example paraffins.

According to an advantageous embodiment, the phase change material is aluminum, or aluminum alloys. The choice of aluminum is particularly suitable for the recovery of the heat energy possessed by the gaseous wastes exiting from melting plants, which have a temperature variability field that is compatible with the phase transition temperatures of aluminum. Aluminum has a melting temperature comprised between 650°C and 700°C.

According to a possible solution, by way of example only, the phase change material comprises the aluminum alloy Al-Si 12%, which has a melting temperature of about 575°C and has a greater phase change enthalpy.

The latent heat of the aluminum provides, in fact, a high capacity of accumulation and release of heat during the heating and cooling cycles to which aluminum is subjected by the action of the flow of gaseous wastes.

The granules 16 can have a size equal to or less than 5mm. The size is estimated as the equivalent diameter of the granule 16.

According to a possible solution, the granules 16 can have a size comprised between 1mm and 5mm.

According to a possible embodiment (fig. 3), each granule 16 comprises, that is, is defined by a core 18 and an external coating 19 which completely covers the core 18, encapsulating the latter.

The core 18 is made of a first material and the external coating 19 is made of a second material having a melting temperature higher than the melting temperature of the first material.

The external coating 19, during use, acts as a container for the material of the core 18 which, due to the heat it receives, melts.

The external coating 19, in fact, is able to resist and remain solid even for higher temperatures than those of the material of which the core 18 is made.

By way of example only, the second material of the external coating 19 can have a melting temperature higher than or equal to 1500°C.

The material of which the core 18 consists is therefore contained both by the external coating 19 and by the sintered powders 17 which surround the granules 16. In this way, even if the external coating 19 yields, for example due to the occurrence of possible surface cracks, the sintered powders 17 are able to contain the molten material of the core 18.

According to a preferred solution, the first material of the core 18 is the phase change material PCM.

According to one embodiment of the invention, the second material of the external coating 19 is an oxide of the first material of the core 18.

In particular, by subjecting the core 18 to an oxidation process it is possible to make the layer of external coating 19 which encapsulates the core 18 inside it. According to possible embodiments, in the case of a core 18 made of aluminum, a possible oxidation process of the core 1 8 can comprise, by way of example only, the following steps:

- immersing the granules 16 in boiling water for about 3 - 6 hours in order to obtain, on the surface of the granules 16, a layer of aluminum hydroxide;

- drying the granules 16 for about 12 hours at a temperature of about 105°C;

- inserting the granules 16 of aluminum in a furnace, heating them up to about 1000°C with a gradient of about 10°C/min, and generating an air flow suitable to generate an oxidizing environment to convert the initial layer into alumina;

- maintaining the temperature for about 6 hours;

- free cooling in air to room temperature.

The oxide of the first material of the core 18 has a melting temperature higher than that of the material of the core 18.

According to a possible solution, the second material of the external coating 19 is aluminum oxide, or alumina.

According to another variant embodiment, the second material of the external coating 19 can be selected from a group comprising chrome or nickel.

The powders 17 of which the heat accumulation body 12 is made can have a thermal conductivity equal to, or greater than, 40 W/mK. This guarantees the transfer of heat from the gaseous wastes to the container 1 1 , from the container 11 to the heat exchange material defined by the powders 17, and from the heat exchange material to the energy accumulation material of the granules 16.

According to possible solutions of the present invention, the powders 17 have a melting temperature equal to or higher than 1500°C.

In accordance with possible solutions, the heat exchange material of which the powders 17 are made is a ceramic material.

Using ceramic material is particularly advantageous given its low reactivity with the material of which the granules 16 are made, and the material of which the container 1 1 is made.

By way of example only, the ceramic material can comprise at least one of either metal oxides, graphite, carbides or nitrides.

According to a possible solution, the ceramic material of which the powders 17 are made is silicon carbide. Using this material is particularly suitable due to its low reactivity with the aluminum of which the granules 16 can be made.

Moreover, silicon carbide is a good thermal conductor and, since it is inert to aluminum, even if the granules 16 break, the material of which the latter are made, it does not corrode the heat-exchange material of the powders 17.

According to possible variants of the present invention, the heat exchange material can be selected from a group comprising hafnium carbide, titanium carbide, tungsten carbide, silicon nitride, or suchlike.

According to one embodiment of the invention, the powders 17 can have sizes comprised between 30 nm and 100 nm, preferably comprised between 50 rjm and 80 nm.

Sintering allows to confine the granules 16 inside the sintered compact heat accumulation body 12, preventing the material of the granules 16 from dispersing in the course of the thermal cycles and, therefore, in the passage from the solid state to the liquid state, or vice versa. The energy accumulation device 10 according to the present invention is in fact subjected, during use, to a cyclical increase in temperature and a reduction in temperature which is variable in a similar way to the temperature of the gaseous wastes.

Furthermore, sintering also eliminates the so-called "open" porosities, that is, connected to the external environment, increasing the efficiency of heat exchange. The absence of porosities prevents the aluminum from being able to rise up due to capillarity along the porosities, and therefore dispersing.

In accordance with a possible solution (figs. 2 and 6), the energy accumulation device 10 comprises a heat exchanger 20 associated with the container 1 1 and in which a heat-carrier fluid is made to transit, in order to transfer the energy accumulated by the granules 16.

In accordance with possible solutions, the heat exchanger 20 can comprise a heat exchange circuit 21 in which a heat-carrier fluid is made to circulate, suitable to absorb the heat energy accumulated by the energy accumulation material and to supply the accumulated heat to the heat-carrier fluid.

The heat exchanger 20 (fig. 2) can be positioned at least partly in the container 1 1 and is put in contact with at least part of the heat accumulation body 12.

In particular, it can be provided that the heat exchanger 20 comprises at least one tube 22 in which a heat-carrier fluid is made to transit, and in which the tube is put in direct contact with the heat accumulation body 12.

The tube can be positioned passing through the container 1 1 and through the heat accumulation body 12.

In accordance with possible solutions (fig. 5), and if several accumulation devices 10 are provided, it is possible to provide that the tubes 22 of the various accumulation devices 10 are fluidically connected to each other so as to generate a passage circuit of the heat-carrier fluid through all the accumulation devices 10.

According to a variant embodiment (fig. 6), the heat exchanger 20 can be coupled to the closing element 15 of the container 1 1.

It is quite evident, however, that in possible solutions, the closing element 15 can only perform the function of closing the container 1 1.

In this case, the accumulation device 10 according to the present invention can be a passive element which can be positioned in a transit zone of the gaseous wastes in order to homogenize their temperature over time.

Embodiments described here using fig. 1 for example also concern a melting plant 100 configured to melt mainly metal materials and comprising at least one accumulation device 10 as described above.

The melting plant 100, shown by way of example in fig. 1, comprises a melting furnace 101 and a recovery apparatus 102 to recover the heat energy of the gaseous wastes generated by the melting plant 100.

The melting furnace 101 can be connected to the recovery apparatus 102 by at least one connection pipe 103 provided to convey the gaseous wastes from the melting furnace 101 to the recovery apparatus 102.

The recovery apparatus 102 according to the invention comprises at least one expansion chamber 104 in which the expansion of the gaseous wastes coming from the melting furnace 101 takes place.

The expansion of the gaseous wastes in the expansion chamber 104 allows to precipitate the heaviest powders present in the gaseous wastes onto the bottom of the expansion chamber 104.

The expansion chamber 104 is in fact connected to the melting furnace 101 by means of the connection pipe 103.

The expansion chamber 104 is provided with at least one discharge device 105 to discharge the gaseous wastes.

The gaseous wastes entering the expansion chamber 104 can have a temperature that varies over time in a range from about 300°C to about 1 100°C.

In the expansion chamber 104, at least one energy accumulation device 10 is installed, advantageously a plurality of energy accumulation devices 10.

The energy accumulation devices 10 can be attached, with at least one of their ends, to a wall of the expansion chamber 104.

In particular, the energy accumulation devices 10 are hit, during use, by the flow of fumes coming from the melting furnace 101 so as to absorb and/or give up the heat energy possessed by the fumes.

According to a possible solution, shown in fig. 1 for example, if the energy accumulation devices 10 are passive elements, that is, without an integrated heat exchanger, it can be provided that the energy accumulation devices 10 accumulate the heat energy of the gaseous wastes when these have high temperatures, and subsequently return it to the gaseous wastes, for example when these are supplied with lower temperatures.

In this way the gaseous wastes exiting from the expansion chamber 104 can have a temperature that is substantially constant, or homogenized, over time. This makes the gaseous wastes particularly suitable for subsequent use for energy recovery.

By way of example only, it can be provided that downstream of the expansion chamber 104 energy recovery means 107 are connected, configured to absorb the heat energy of the gaseous wastes discharged from the expansion chamber 104. According to a possible variant embodiment, the energy recovery means 107 can comprise, for example, an Organic Rankine circuit or ORC. In accordance with this variant embodiment, the energy recovery means 107 can comprise a heat exchanger 108 and a turbine 109 connected to the heat exchanger 108 used for example for the production of electrical energy.

Heat-carrier fluids can be made to circulate in the heat exchanger 108, such as, by way of example only, water, air, diathermic oil, supercritical carbon dioxide, emulsions, ionic liquids.

According to a possible variant embodiment (fig. 5), the energy accumulation devices 10 provided with the heat exchangers 20 are installed in the expansion chamber 104. The heat exchangers 20 can be fluidically connected to each other, for example in series or in parallel, to obtain an efficient heating of the gaseous wastes. In turn, the heat exchangers 20 can be connected to a user device 106 downstream, such as a turbine for the production of electric power.

It is clear that modifications and/or additions of parts can be made to the energy accumulation device 10 and the method to produce the device 10 as described heretofore, without departing from the field and scope of the present invention.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of energy accumulation device 10 and corresponding method, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

In the following claims, the sole purpose of the references in brackets is to facilitate reading: they must not be considered as restrictive factors with regard to the field of protection claimed in the specific claims.