CERAMIC TREATMENT PLANT AND METHOD FOR SUBJECTING CERAMIC MATERIALS TO A HEAT TREATMENT

Abstract

 A ceramic plant is described comprising an apparatus (15) for the generation of hot air (22), said apparatus comprising: a combustion system (1) adapted to generate hot combustion fumes (11) from air (12) and an incoming fuel (13), a heat exchanger (2) having air (5) at the inlet and adapted to receive said hot combustion fumes for heating said incoming air so as to emit hot air (22), said ceramic plant comprising at least one unit (10, 30) adapted to receive the hot air leaving said heat exchanger to heat ceramic material (50, 51).

Description

TECHNICAL FIELD

[0001] The present invention concerns a ceramic plant and a method for subjecting ceramic materials to a heat treatment. In particular, the present invention concerns a ceramic plant comprising an innovative apparatus for the generation of hot air that can be used by one or more heat treatment units of the ceramic plant.

PRIOR ART

[0002] Currently, the production process of ceramic tiles and/or slabs of the ceramic plants involves numerous steps, including a step of grinding the raw materials to obtain a slip, a step of spray drying (or atomizing) the slip to obtain ceramic powders, a step of pressing the ceramic powders to obtain a raw ceramic substrate, a step of drying the raw substrate, a step of glazing and/or decorating the raw substrate, and finally a firing step.

[0003] These steps are carried out industrially by means of a sequence of units or machines individually dedicated to each step, which together form a ceramic plant.

[0004] To perform their function, some of these machines, in particular the "heat treatment machines or units" such as the atomisers, dryers and firing kilns, must be able to generate large amounts of heat.

[0005] This entails a huge energy expenditure and a considerable increase in the carbon dioxide (CO₂) emissions into the atmosphere, which is completely at odds with not only European, but world-wide indications.

[0006] In order to abate these emissions, it has so far been thought to act through appropriate flue gas treatments downstream of the thermal machines or through the use of forms of "clean" energy.

[0007] However, in the thermal machines used in the ceramic sector, the volume % of CO₂ leaving the chimneys is too low to be abated with sustainable techniques, so downstream decarbonisation is not currently applicable.

[0008] In contrast, upstream CO₂ abatement, by replacing fossil fuels with other forms of clean energy (for example green hydrogen or electricity from renewables), has considerable technical and economic drawbacks.

[0009] Consequently, for economic, technical and environmental reasons, the only fuels that can currently be used to generate heat are natural gas/methane and LPG.

[0010] In particular, the atomisers use hot gases produced through the use of air flow burners which, in most cases, are precisely supplied by fossil fuels (methane or LPG).

[0011] The heat required to make the ceramic atomised matter is introduced in the form of combustion gas and causes the evaporation of the liquid contained in the slip by convection or conduction.

[0012] Regardless of the type of fuel used, the fume leaving the atomiser has very low volume percentages of CO₂, making it impossible to abate it with known techniques (one example is the technique of capture, i.e. sequestration, in other words abatement, of CO₂ in the treatment of biogas that exploits the reaction between Potassium Carbonate and Potassium Bicarbonate).

[0013] The low percentage of CO₂ (< 6 %) in output is not to be understood in absolute value but in terms of the ratio with the other gases present, the main ones being nitrogen, oxygen and water.

[0014] The drying mechanism of the atomisers, in fact, involves using huge volumes of gas at a temperature comprised between 550 and 650 °C, which are produced from a flame generated by methane gas in stoichiometric ratio with air.

[0015] This flame reaches temperatures around 1900 °C and it is therefore obvious that it cannot directly hit the material to be dried, consequently a high amount of cold or slightly preheated air must hit the flame and then reach the material to be dried in the correct amounts and at the correct temperatures.

[0016] For these reasons, currently, the volume percentage of CO₂ in the fumes leaving the atomiser is too low to be abated.

[0017] Furthermore, the current configuration of the atomisers would not allow the use of fuels other than methane gas or LPG for environmental, economic and technical issues. From an environmental point of view, the use of combustion gas generated by naphtha or diesel would produce decidedly "dirtier" fumes characterized by high amounts of polluting volatile organic compounds (VOCs).

[0018] From an economic point of view, there are two main aspects to take into account.

[0019] The first is closely linked to the environmental factor since, in order to abate the organic pollutants present in the fumes, filtering systems and/or post-combustion systems capable of treating huge volumes of fume would be necessary, which would therefore be very costly both in terms of installation and maintenance costs.

[0020] The second economic aspect is linked to the impossibility for the ceramic producer, the so-called hard-to-abate sector, to be able to use the most advantageous fuel depending on the geopolitical situation and market fluctuations.

[0021] While, on the one hand, multi-fuel burners have been a well-known reality for years, which would make it possible to use liquid and/or gaseous fuel at their own discretion and according to company strategies, on the other hand, for the reasons described below, currently the ceramic producer can only choose between methane and LPG.

[0022] Finally, the technical aspect linked to the use of these liquid fuels must be considered.

[0023] During the combustion step they are not completely transformed into CO₂ but the long organic chains present break up into smaller fragments (VOCs) some of which can be adsorbed by the particles of atomized material increasing the percentage of unwanted organic compounds.

[0024] Firing kilns, of the roller type, also use hot gases produced through the use of burners which, in most cases, are supplied by gaseous fuels (methane or LPG).

[0025] Also in this case the heat necessary for firing the ceramic product is introduced in the form of combustion gas and causes a series of reactions, including elimination of hygroscopic, crystallization and constitution water, elimination of organic substances, decarbonation of limestones and formation of new crystalline phases.

[0026] Regardless of the type of fuel used, the fume leaving the chimney has very low CO₂ volume percentages, approximately of the order of 2% by volume, making it impossible to abate it with the known techniques.

[0027] Also in this case, the low percentage of CO₂ in output is not to be understood as absolute value but in terms of the ratio with the other gases present, the main ones being nitrogen, oxygen and water.

[0028] The firing mechanism of the roller kilns involves using multiple burners whose flames are directly introduced into the firing tunnel and appropriately regulated.

[0029] The air/fuel ratio is maintained as close as possible to the stoichiometric, which would result in a theoretical percentage of CO₂ to the chimneys suitable for abatement, but, as it is an open system in which additional volumes of air are suitably introduced for the proper management of the thermal gradient, currently all roller kilns for the ceramic industry have insufficient volume percentages of CO₂ to the chimneys for chemical sequestration.

[0030] Other carbon dioxide capture techniques that require lower volume percentages of carbon dioxide are currently being investigated.

[0031] However, as well as not yet being industrially available, these techniques would still involve very high costs, as they would involve treating extremely high volumes of air and fumes containing not only the combustion products, but also other elements from the treated ceramic material, such as metals and other inorganic components, chlorine, fluorine, and sulphur.

[0032] Furthermore, the current configuration of the kilns would not allow the use of fuels other than methane gas or LPG for environmental, economic and technical issues completely similar to those previously outlined in relation to the atomisers.

[0033] With particular reference to the ceramic kiln, other possibilities for generating heat without emitting CO₂, namely the use of hydrogen as a fuel in burners and direct electricity (for example in the form of heating resistors), are currently being developed and/or partially available.

[0034] However, with regard to hydrogen, the production via electrolysis of 1 m³ of H₂ requires about 5kWh of electricity and has about a third of the calorific value of methane. Considering the prices discounted to the first quarter of 2023, this would result in a cost for purchasing electricity that is 5.5 times higher than the cost of natural gas.

[0035] On the other hand, if the electricity needed to produce H₂ came from photovoltaic panels, considering their efficiency, photovoltaic fields of several hectares would be needed to replace 100% of the methane currently used by the ceramic kiln.

[0036] Specifically, in order to replace 100% natural gas from a modern kiln producing 9000 m² day of ceramic tiles, which consumes 450 Nm³/h of natural gas, a photovoltaic park of about 28 hectares (equal to about 40 football pitches) would be necessary.

[0037] In the event that, to generate heat in the kiln, electrical resistors were directly used, the cost would be about 3.7 times higher than natural gas and, for the production kiln described earlier, a photovoltaic park of about 20 hectares (equal to about 28 football pitches) would be necessary.

[0038] In addition to the undeniable economic disadvantage linked to the replacement of natural gas with hydrogen or direct electricity, some aspects related to the technical feasibility of these operations must be added.

[0039] With regard to hydrogen (see Italian patent application no. 102021000006797), currently only a partial replacement of fossil gas is possible due to issues related to the delivery of heat to the ceramic artefact in the form of hot gases.

[0040] As far as the total electrification of the ceramic kiln is concerned, the heat delivery mechanism would totally change with considerable inefficiencies in the firing process.

[0041] In summary, the production of ceramic articles requires high energy consumption.

[0042] The production steps that involve the greatest energy burden are firing, an indispensable and irreplaceable process in order to obtain ceramic products characterized by the technical and aesthetic properties that we all know, and atomization (or spray drying), a fundamental process for making porcelain stoneware.

[0043] Currently the most used energy carrier in Italy and Europe is natural gas or methane gas, mainly for the following reasons:

• with the exception of the recent past, it has been, for a very long time, the cheapest fossil fuel,
• compared to other fuels, it has high efficiency, reliability and controllability,
• combustion systems using natural gas are simple,
• with the same combustion system, it generates cleaner fumes than other fossil fuels. The current alternatives to methane gas present difficulties in economic and/or technological feasibility, summarized in the following table:

	Economic feasibility	Technological feasibility
Fuels and fossil fuels other than methane (Diesel, fluid BTZ, LPG, Petrol, etc.)	Today, replacing methane with the cheapest fuel (fluid BTZ fuel oil) would not lead to an appreciable reduction in the energy costs.	The replacement of methane with fuel oil in ceramic kilns is possible for every sector.
		Ready Technology
Green Hydrogen	Currently, replacing methane with hydrogen would lead to a cost increase by about 5.5 times.	The replacement of 30% of methane with hydrogen is now available with the patented Italforni HECO2® technology.

		Replacing 100% of the methane is not yet possible.
Renewable electricity	Currently, the replacement of methane with the heating by means of electrical resistors would lead to an increase in cost by about 3.7 times.	Replacement of 100% of the methane with electrical resistors is only possible in limited cases.
		In most applications the technology is not yet available.

[0044] An object of the present invention is therefore to overcome the drawbacks outlined above. A further object is to achieve such objective in the context of a rational, effective and affordable solution.

[0045] These objects are achieved by the features of the invention set forth in the independent claim. The dependent claims outline preferred and/or particularly advantageous aspects of the invention.

DISCLOSURE OF THE INVENTION

[0046] In particular, the invention makes available a ceramic plant comprising an apparatus for generating hot air, said apparatus comprising:

• a combustion system adapted to generate hot combustion fumes from at least air and an incoming fuel,
• a heat exchanger having air at the inlet and adapted to receive said hot combustion fumes for heating said incoming air so as to emit hot air,

said ceramic plant further comprising at least one heat treatment unit adapted to receive the hot air leaving said heat exchanger to heat ceramic material (e.g. slip or raw pressed slabs).

[0047] Thanks to this solution, the ceramic material contained in the heat treatment unit is heated by clean hot air substantially free of CO₂ (except for the one already present in the air entering the heat exchanger), thus eliminating the need to equip said heat treatment unit with CO₂ abatement systems.

[0048] Conversely, the CO₂ produced with the combustion used for the generation of the hot air will generally be present, within the combustion fumes leaving the heat exchanger, in rather high percentages, preferably higher than 6% (with respect to the total volume of the fumes), and therefore suitable to be treated by means of a CO₂ scrubber.

[0049] In this regard, it is a preferred aspect of the present invention the fact that the ceramic plant may comprise a carbon dioxide scrubber adapted to receive and treat the combustion fumes leaving said heat exchanger.

[0050] This carbon dioxide scrubber may be of the type comprising an aqueous solution of potassium carbonate adapted to be passed through by the combustion fumes leaving the heat exchanger.

[0051] To fully exploit the scrubber, the plant can also comprise a post-combustion system able to convert the organic compounds present in the combustion fumes leaving said heat exchanger into carbon dioxide, thus increasing its percentage.

[0052] Regardless of that, another advantage of the invention consists in that the combustion fumes generated in the combustion system only serve to heat the air in the exchanger, so their overall volume is much smaller than the volumes of fumes currently produced in the traditional heat treatment units (e.g. atomisers or roller kilns), which makes it easier and cheaper to adopt any type of CO₂ and/or other pollutant abatement system.

[0053] Which abatement systems can be further simplified and made cheaper because the combustion fumes generated in the combustion system do not contain elements from the treated ceramic material, such as metals and other inorganic components, chlorine, fluorine and sulphur.

[0054] According to another aspect of the invention, the heat exchanger may be configured so that the hot air introduced into the heat treatment unit has a temperature greater than or equal to 500 °C, for example substantially equal to 600 °C (e.g. for an atomiser), or possibly greater than or equal to 600 °C, for example substantially equal to 650 °C (e.g. for a kiln).

[0055] In this way, the hot air that is introduced into the heat treatment unit is able to provide a sufficiently high amount of heat to obtain an effective heating of the ceramic material.

[0056] In some embodiments (e.g., atomiser), the hot air emitted by the heat exchanger may be the only heat source that is used to heat the ceramic material in the heat treatment unit. However, in other embodiments (e.g. kiln), it may not be sufficient to complete the treatment.

[0057] Therefore, one aspect of the invention provides that the heat treatment unit may also comprise one or more burners adapted to generate hot combustion fumes from at least air and a fuel, which may be liquid or more preferably gaseous, and to introduce said hot combustion fumes directly into the heat treatment unit.

[0058] In this way, a hybrid system capable of reaching very high temperatures is obtained but which, with a substantially traditional technology (that of the burners), combines the hot air produced by the heat exchanger, thus reducing the fuel consumption by the burners and consequently the CO₂ emissions of the heat treatment unit.

[0059] The fuel used for the above burners may be hydrocarbon-based, e.g. comprise or consist of methane or LPG.

[0060] More preferably, however, said fuel may be hydrogen-based, e.g. comprise or consist of oxyhydrogen, i.e. a gaseous mixture of hydrogen and oxygen, such as the so-called HHO, or of molecular hydrogen (H₂).

[0061] In both cases, the hydrogen-based fuel can be obtained by means of an electrolytic cell, preferably powered by electricity obtained from renewable sources (e.g. from photovoltaic panels), in which a process of electrolytic dissociation of water takes place.

[0062] In this way it is advantageously possible to obtain a plant with a very low environmental impact since, on the one hand, the carbon dioxide produced for the generation of hot air can be effectively abated by means of the scrubber outlined above, while, on the other hand, the combustion of hydrogen in the burners would not produce carbon dioxide or other pollutants but only water.

[0063] Of course, it is not excluded that the burners can be adapted to generate the hot combustion fumes from air and from two fuels, for example from both a hydrocarbon-based fuel and a hydrogen-based fuel, possibly in a mixture with each other, as described for example in the aforementioned Italian patent application no. 102021000006797.

[0064] In addition to or as an alternative to the aforementioned burners, the heat treatment unit could also comprise one or more thermoelectric heaters, e.g. heating resistors, preferably powered by electricity obtained from renewable sources (e.g. from photovoltaic panels), adapted to heat the interior of the heat treatment unit itself.

[0065] This solution also makes it possible to obtain a hybrid heating system capable of obtaining adequately high temperature levels with a reduced environmental impact.

[0066] Compared to a hypothetical purely thermoelectric heating solution, this hybrid system, thanks to the presence of hot air flows that continue to lap the ceramic material, achieves the advantage of not completely changing the heat delivery mechanism, making the plant more reliable.

[0067] Preferably, the heat treatment unit adapted to receive the hot air leaving the exchanger is an atomiser having a ceramic slip at its inlet and configured to dry it by means of the hot air received from said heat exchanger to produce ceramic powders.

[0068] In addition or alternatively, the unit may be a roller kiln having a raw ceramic substrate at its inlet (i.e. a layer of pressed but not yet sintered/ceramic/thermally consolidated ceramic powders) and configured to fire it (also) by means of the hot air received from said heat exchanger to produce a ceramic tile or slab.

[0069] As anticipated, the heating of the roller kiln can be assisted by fuel burners (based on hydrocarbons and/or hydrogen) and/or by thermoelectric heaters.

[0070] According to one embodiment, the hot air leaving the heat exchanger can be conveyed both to the atomiser and to the roller kiln.

[0071] The fuel used in the combustion system of the apparatus for the generation of hot air can be of any type, whether solid, liquid or gaseous.

[0072] For example, it may comprise or consist of methane, LPG, naphtha, diesel, gasoline or BTZ fuel oil.

[0073] In all cases, in fact, hot combustion fumes, although "dirty", never come into direct contact with the ceramic material, and combustion can always be managed in an optimal way so that the pollutants contained in them can be abated/eliminated by means of appropriate filtration and/or post-combustion treatments.

[0074] In particular, a preferred aspect of the invention provides that said fuel (i.e. the one used in the combustion system of the apparatus for the generation of hot air) may be pyrolysis oil.

[0075] The use of pyrolysis oil (i.e. fuel oil from thermal depolymerization) as a fuel represents a valid alternative to methane or LPG gas normally used in the combustion systems for firing ceramic products and for the production of ceramic powders.

[0076] Moreover, since this oil is generated from non-recyclable waste plastic material, therefore destined for landfills or, to a small extent, to the incinerators, it represents a completely free source of energy.

[0077] The use of pyrolysis oil also provides an environmental benefit as it prevents the accumulation of plastic material in landfills.

[0078] In fact, the world currently consumes 420 million tons of polymers per year (9 million in Italy alone), with the forecast to reach 1.2 billion tons by 2050.

[0079] To understand this forecast, it is obviously necessary to consider which are the indispensable polymer products that are used in daily life (packaging, consumer goods, fibres, pipes, cars, the medical sector, etc.). According to the 2016 report "The New Plastics Economy: Rethinking the Future of Plastics", the weight ratio between plastic and fish in the seas in 2014 was 1:5 while in 2050 it will be 1:1.

[0080] These raw materials are indisputably abundant and their quantity is directly proportional to their use in society. Polyethylene (PE), polystyrene (PS) and polypropylene (PP) are the most abundant polymers in municipal waste plastic.

[0081] Most of the plastic waste produced is landfilled or incinerated.

[0082] Hence the main problem of waste disposal and recycling in the post-use phase.

[0083] The main recycling methods are as follows.

[0084] Mechanical recycling: consists of the collection of plastic waste, subsequent grinding, washing, drying and melting into new objects. It is an uneconomical process that requires an extremely homogeneous starting material in order to obtain a finished product with adequate properties.

[0085] Chemical recycling: consists of recovering from polymers, through controlled chemical reactions, chain fragments that can then be reused to build new polymers or other uses. It is a simple process for some types of polymers such as PET, polycarbonates, polyurethanes and PMMA but which is not applicable for Polyethylene, Polypropylene, Polystyrene and PVC (which alone account for more than 65% of the total plastic). Also in this case, the process is often uneconomical and requires high input homogeneity. Thermal recycling (Pyrolysis): consists of heating to temperatures between 350 and 650 °C, in the absence of oxygen. The advantages of this process are countless as it allows to work with mixed plastics, thermosetting resins, rubbers and composites without costly separations. In addition, the resulting liquid has characteristics very similar to diesel, the gaseous fraction is used to self-feed the process itself and the solid residue can be sold as carbon black for industrial uses.

[0086] In other words, pyrolysis (or thermal depolymerization) is a thermal degradation process that consists of subjecting long-chain hydrocarbon materials to heat in inert atmospheres. Specifically, within a closed and inert system, a flow of gas originates which is subsequently partly condensed (liquid fraction) and partly used to feed the pyrolysis kiln (gaseous fraction), and from a solid residue.

• Liquid fraction: also called pyrolytic oil, it is a very complex mixture of hydrocarbons with characteristics completely comparable to commercial fuel oil. The percentage of this fraction strongly depends on the type of plastic in input but is generally never less than 50% and is on average between 50 and 80%. Its calorific value is about 10,000 kcal/kg.
• Gas fraction: it is the so-called syngas or a mixture of hydrogen, methane, ethanol, etc. extremely variable that in pyrolysis plants is reused in the process to generate heat.
• Solid fraction: if the raw material is characterized only by non-recycled plastic, it is carbon black and, generally, it is present in a percentage of less than 10%. This type of solid can be recycled as activated carbon in different industrial processes.

[0087] Generally, the treatment mechanism includes a first feeding section where the entry of plastic material is managed. The incoming plastics can undergo a pre-treatment consisting of shredding and subsequent washing, or they can be used as they are. Subsequently, the process involves the application of heat to the raw material, at temperatures generally between 350 and 650 °C. The heat is applied indirectly and the kiln is suitably designed to provide the necessary amount of heat to the material without the latter coming into contact with oxygen. To prevent any incorporation of oxygen, in fact, the reaction chamber is kept under a nitrogen atmosphere during the entire process. During the thermal degradation step the temperatures could increase up to 700-800 °C. According to the following invention, therefore, pyrolysis oil is one of the most appropriate fuels to heat the fumes entering the heat exchanger and, thanks to the presence of the heat exchanger itself, has the sole function of heating the elements suitable for heat exchange. In other words, it can be burned without excess air thus ensuring a final CO₂ percentage suitable for chemical sequestration. The presence of the exchanger also allows the use of clean hot air in the ceramic thermal machines.

[0088] In addition to the plant outlined above, the present invention also makes available a corresponding method for subjecting ceramic material to a heat treatment, comprising the steps of:

• generating by combustion hot fumes from at least air and a fuel,
• introducing said hot combustion fumes into a heat exchanger to heat incoming air and emit it at the outlet as hot air,
• introducing the hot air leaving the heat exchanger into at least one heat treatment unit into which said ceramic material is also introduced.

[0089] Of course, all the advantages and technical effects mentioned above with reference to the plant also apply identically to the corresponding method, as do the related accessory aspects.

[0090] Thus, for example, it is preferable that the combustion fumes leaving the exchanger have a CO₂ concentration greater than 6% (with respect to the total volume of the fumes).

[0091] It is further preferable that the method comprises the step of introducing the combustion fumes leaving the heat exchanger into a carbon dioxide scrubber, possibly after having introduced them into a post-combustion system capable of converting the organic compounds present in the combustion fumes into carbon dioxide.

[0092] According to another aspect of the invention, the hot air introduced into said heat treatment unit may have a temperature greater than or equal to 500 °C, for example substantially equal to 600 °C (e.g. per atomiser), or possibly greater than or equal to 600 °C, for example substantially equal to 650 °C (e.g. per kiln).

[0093] The method may further comprise the steps of generating by combustion hot combustion fumes from at least air and a fuel, liquid or gaseous, based on hydrocarbons and/or hydrogen, and of introducing said hot combustion fumes directly into the heat treatment unit.

[0094] In addition or alternatively, the method may comprise the step of heating the interior of said heat treatment unit by one or more thermoelectric devices.

[0095] The heat treatment unit may be an atomiser into which ceramic material in the form of a ceramic slip is introduced or a roller kiln into which ceramic material in the form of raw ceramic substrates is introduced.

[0096] The fuel used in the combustion system of the apparatus for the generation of hot air can be of any type, whether solid, liquid or gaseous, for example but not exclusively pyrolysis oil.

BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Further features and advantages of the invention will be more apparent after reading the following description provided by way of a non-limiting example, with the aid of the accompanying drawings.

Figure 1 is a diagram of a ceramic plant in accordance with the present invention.

Figure 2 is a diagram of a ceramic atomiser that may be used in the plant of Figure 1.

Figure 3 is a schematic longitudinal section of a roller kiln that can be used in the plant of Figure 1.

Figure 4 is section IV-IV of Figure 3 shown schematically and on an enlarged scale.

Figure 5 is a flowchart of a pyrolysis plant.

BEST MODE TO IMPLEMENT THE INVENTION

[0098] With reference to the aforementioned figures, a ceramic plant comprising an apparatus 15 for the production of hot air and at least one heat treatment unit 10 and/or 30, for example an atomiser 10 and/or a roller kiln 30, adapted to receive the hot air produced by the apparatus 15 for heating a ceramic artefact, for example a ceramic slip 50 or a raw ceramic substrate 51, is indicated globally with reference 100.

[0099] The apparatus 15 comprises an energy source 1, in particular a combustion system comprising a burner fed by a fuel 13 and an oxidizing gas 12, for example air.

[0100] The fuel 13 can be of any type, whether solid, liquid or gaseous.

[0101] For example, it may comprise or consist of methane, LPG, naphtha, diesel, gasoline or BTZ fuel oil.

[0102] In particular, a preferred but not essential aspect of the invention provides that said fuel 13 may be pyrolysis oil.

[0103] By pyrolysis oil 13 is meant both oil from plastic waste and oil obtained from biomass.

[0104] It is known that plastics are widely used in the world; said raw materials are indisputably abundant and their quantity is directly proportional to their use in society. Polyethylene (PE), polystyrene (PS) and polypropylene (PP) are the most abundant polymers in municipal waste plastic.

[0105] One way to recycle plastic from municipal waste is thermal recycling (Pyrolysis): it consists of heating plastics (e.g. from municipal waste) to temperatures between 350 and 650 °C, in the absence of oxygen. The decomposition of the polymers gives rise to a flow of gas, which is subsequently partly condensed (liquid products) and partly used to feed the pyrolysis kiln (gaseous fraction), and from a carbonaceous residue. The advantages of this process are countless as it allows to work with mixed plastics, thermosetting resins, rubbers and composites without costly separations. In addition, the resulting liquid, i.e. pyrolysis oil, has characteristics very similar to diesel, the gaseous fraction is used to self-feed the process itself and the solid residue can be sold as carbon black for industrial uses.

[0106] Pyrolysis oil has characteristics quite comparable to a commercial fuel oil, so it can be easily used in firing ceramic products. From an economic point of view, it is a completely free energy source as it is generated from non-recyclable waste plastic material and therefore destined for landfills or, to a lesser extent, to incinerators.

[0107] In addition to the incredible economic benefit there is the environmental benefit as currently all this plastic waste is given to landfills and incinerators, often located outside the national territory. We are also well aware of the problem related to the accumulation of this plastic waste, which from year to year adds up since the receiving capacity, understood as reception thereof in the landfills and reception thereof in the incinerators, is not sufficient.

[0108] A flowchart of a plant for the production of pyrolysis oil from plastic waste 401 is described in Figure 5.

[0109] Plastic waste 401 normally comes from a landfill and is generally shredded 402 both because it is not possible for a company to purchase waste as such but only "secondary raw materials", so only an already treated product can be supplied, typically selected and shredded and even in case the pyrolytic kiln was of the continuous type; however, if the pyrolytic kiln was of the discontinuous type (static chamber) it could also not be shredded. The shredded material is then introduced into the pyrolytic kiln 404, possibly through a hopper 403; in the pyrolytic kiln 404, preferably by effect of combustion in the absence/lack of oxygen, the organic component of the waste vaporizes, producing a fume. This fume enters a tube bundle heat exchanger 405, typically air/air or sometimes air/water (for example to use water in swimming pools or miscellaneous utilities), where it cools. By cooling, the fume condenses and the condensate constitutes the pyrolysis oil (fuel) 406. The pyrolysis oil is then filtered through a filter 408, primarily to remove very waxy components such as paraffins, and finally stored in a tank 409 for further use. In the exchanger 405, not all fume condenses. The non-condensing part that remains in the gaseous state is called syngas 406. This syngas 406 contains very volatile compounds, such as methane, ethanol, methanol, hydrogen, etc. with such a variable composition that it cannot be used in industrial processes. Thus, after being passed into a Jet scrubber 410, which is a device primarily adapted to remove any chlorine that may be present or sulphur (although sulphur is generally absent), the syngas 406 is sent to a generator set 412. The generator set 412 is of the bifuel type because it works both with the syngas and (precisely due to the variability of the latter) with another fuel, typically with a part of the pyrolysis oil obtained. The electricity produced by the generator set can finally be used to power the kiln. A torch 411 is provided between the jet scrubber 410 and the generator set 412 to burn off excess syngas. Combustion fumes are naturally produced at the generator set exhaust and it is advisable to treat them with a post-burner. Preferably, with the same plant of Figure 5 and with the same process it is possible to treat the biomass instead of the plastic waste 401, obtaining, instead of the pyrolysis oil 406, a pyrolytic bio-oil (i.e. a pyrolytic oil from biomass). Returning to the plant 100, the combustion system 1 generates many hot gases or combustion fumes 11 that feed a heat exchanger 2. The heat exchanger 2 is fed by air 5, preferably at room temperature but can also be preheated air; in particular the air 5 is forced into the heat exchanger 2 by a fan, not visible in the figures.

[0110] The air 5 has an inlet separate from that of the combustion fumes 11 of the combustion system 1; the air 5 travels along a path inside the heat exchanger 2 so that it is heated by the heat released by the combustion fumes 11 leaving the combustion system 1.

[0111] For example, the heat exchanger 2 may comprise an outer casing 200 provided with a first inlet 205 for the air 5 to be heated, a first outlet 210 for the heated air, a second inlet 215 for the combustion gases or fumes 11 coming from the combustion system 1 and a second outlet 220 for the same combustion gases or fumes 11.

[0112] All these inlets 205, 215 and outlets 210, 220 are preferably separate and independent from each other.

[0113] Inside the outer casing 20 there may also be (or defined) a first channeling adapted to put the first inlet 205 in communication with the first outlet 210, and a second channeling adapted to put the second inlet 210 in communication with the second outlet 220. These channelings, not illustrated since in themselves conventional for a technician in the field, are distinct, so that the air remains physically separated from the combustion gases or fumes, but they are also structured such that the air and the combustion gases are put in a heat exchange relationship, so that in the simultaneous crossing of the heat exchanger 2, the combustion gases or fumes 11 can transfer heat to the air 5, heating it. For example, the first channeling may comprise at least one tube or tube bundle contained within the outer casing 200 and adapted to connect the first inlet 205 with the first outlet 210, while the second channeling may be defined by the remaining internal volume of the casing 200 that laps said tube or tube bundle and into which both the second inlet 210 and the second outlet 220 open.

[0114] Or, conversely, the second channeling may comprise at least one tube or tube bundle contained within the outer casing 200 and adapted to connect the second inlet 215 with the second outlet 220, while the first channeling may be defined by the remaining internal volume of the casing 200 that laps said tube or tube bundle and into which both the first inlet 205 and the first outlet 210 open.

[0115] After having passed through and cooled in the heat exchanger 2, the combustion fumes 11 can be introduced into a carbon dioxide or CO₂ 4 scrubber adapted to reduce the concentration of carbon dioxide present therein, before being released into the environment.

[0116] Preferably, the scrubber 4 uses a technology that provides for the combustion fumes 11 to pass through an aqueous solution of potassium carbonate (K₂CO₃) that absorbs the carbon dioxide present by transforming into potassium bicarbonate (KHCOs), according to the following reaction equilibrium:

        CO₂ + H₂O + K₂CO₃ ⇆ 2 KHCOs

[0117] Generally pH > 8, hence the mechanism is as follows:

        CO₂ + OH^- ⇆ HCO₃^-

        HCO₃^- + OH- ⇆ CO₃^- + H₂O

[0118] Subsequently, the carbon dioxide absorbed in the so-called absorption column is released and suitably stored (Off-Gas with CO₂ >99.9%). The bicarbonate returns carbonate in a closed cycle.

[0119] The main advantages of capture solutions using potassium carbonate solutions are:

• high CO₂ solubility;
• easy regeneration of the solution (more efficient and economical);
• low cost of solvent;
• non-toxic solvent;
• stable solvent.

[0120] In contrast, this technology is usually efficient only if the combustion fumes 11 contain volume percentages of carbon dioxide greater than 6-7% with respect to the total volume of the fumes.

[0121] By means of the proposed solution, this constraint does not represent a problem, as the combustion of the fuel in the combustion system 1 can be controlled so as to obtain combustion fumes 11 with percentages of carbon dioxide greater than or equal to 6-7%, therefore perfectly suitable for being abated.

[0122] In particular, to achieve this effect, the combustion of the fuel in the combustion system 1 may be controlled to be stoichiometric with air.

[0123] As proof of this, it is possible to hypothesize as a fuel a pyrolysis oil that has characteristics completely comparable to a fuel oil or diesel oil, i.e. a complex mixture of aliphatic (about 75%) and aromatic hydrocarbons (about 25%) whose chemical composition can be simplified into C 85% and H 15% i.e., indicatively, C₇H₁₅.

[0124] This pyrolysis oil reacts stoichiometrically with air in a ratio of 1:15 m/m which, translated into a percentage by final volume of carbon dioxide, means about 10%, or a sufficient percentage for abatement.

[0125] Regardless of the combustion control, in order to increase the concentration of carbon dioxide in the combustion fumes 11 leaving the heat exchanger 2, the latter can be treated with a post-combustion system 3, before reaching the scrubber 4.

[0126] In fact, the combustion fumes 11 generated by the combustion of pyrolysis oil or other fuels can contain various volatile organic compounds, which can be converted into carbon dioxide by means of the aforementioned post-combustion system 3.

[0127] Regardless of the presence or not of the post-combustion system 3, thanks to the solution outlined above, the combustion fumes 11 may contain a percentage of carbon dioxide greater than 6-7%, therefore suitable for being abated.

[0128] However, it is not excluded that the scrubber 4 may use other carbon dioxide capture techniques that require lower volume percentages of carbon dioxide, which techniques are currently still being investigated.

[0129] Whatever the abatement technique used, an advantage connected with the apparatus in accordance with the invention consists in the fact that the combustion fumes 11, whose sole function being heating in the heat exchanger 2, have relatively low volumes, certainly reducing the overall costs related to the process.

[0130] A further advantage linked to the invention is that the CO₂ scrubber must treat less contaminated fumes, as they do not come into contact with ceramic materials but they come from the combustion of pyrolysis oil or other fuels only, so they contain almost exclusively CO₂ (all organics have been transformed into CO₂ by the post-burner), N₂, NOx, O₂ and H₂O, making the process more efficient and economically advantageous.

[0131] Regardless of all these considerations, the hot air 22 leaving the heat exchanger 2 can be supplied in input to an atomiser 10 adapted to exploit it for eliminating the aqueous component of ceramic slips 50, i.e. to heat the ceramic slip 50, in particular to dry it, so as to produce ceramic powders 60.

[0132] The ceramic slip 50 is an aqueous suspension of clays and selected raw materials that are subjected to a wet grinding process inside mills, generally with the addition of fluidifiers.

[0133] To carry out the drying process of the slip 50 adequately, the atomiser 10 generally requires very large air volumes, for example with a flow rate of about 90000 Nm³/h, but at relatively moderate temperatures, approximately between 580 and 600 °C.

[0134] In the known atomisers, these air volumes are generated by a fan and are heated by a burner.

[0135] By means of the proposed solution, this burner may no longer be necessary, as the temperature and volumes of the air leaving the exchanger 2 may be such as to fully meet the needs of the atomiser 100, for example 90000 Nm³ at 600 °C.

[0136] In the illustrated example, the atomiser 10, better visible in Figure 2, performs a spray drying of the ceramic slip 50. The hot air 22 in contact with the liquid particles of the ceramic slip 50 causes them to evaporate by convection. Some components of the atomiser 10 will be described in more detail below.

[0137] The atomiser 10 comprises a drying tower 305 defining therein an operating chamber. The ceramic slip 50 is sprayed inside the drying tower 305 of the atomiser 10 by means of the so-called pressure centrifuge nozzle system.

[0138] The nozzle system allows to obtain ceramic powders with controlled granulometry and humidity; generally, once the flow rate and characteristics of the ceramic slip 50 have been set, it is possible to change the granulometry simply by using nozzles having different characteristics.

[0139] The atomiser 10 is preferably of the mixed current type, i.e. with descending hot air flow 22 and nebulized of ceramic slip 50 directed upwards; in this way the particles of the nebulized meet the hot air 22 first in countercurrent and then in equi-current, increasing the heat exchange efficiency.

[0140] Generally, the atomiser 10 comprises at least one pump 310 to transfer the ceramic slip 50 at a certain pressure, for example between 20 and 30 atmospheres, towards the nozzles 315 normally arranged in a ring; filters 320 are interposed between nozzles 315 and pump 310 to eliminate impurities or foreign bodies present in the slip 50. Preferably the nebulized particles have a speed of about 30 m/s so as to overcome the low viscosity of the fluid and obtain tiny drops. The spray has a conical shape and rises upwards in the drying tower 305; here it is hit by the hot air 22, preferably conveyed inside the drying tower by a fan 335, for the drying step. It is possible that the nozzles 315 are of the spiral type (not visible in the figure) to allow a rotary motion of the particles at the exit of the nozzle which results in a spiral-shaped spray.

[0141] The ceramic powders 60 produced fall onto the bottom of the drying tower 305, for example in the conical lower part, from which they can be discharged onto a conveyor belt 61.

[0142] The flow of hot air 22 and the powders remained in suspension can be sucked by a fan 345, which is in communication with the operating chamber via a suction duct 350.

[0143] A cyclone separator 355 can be arranged along this suction duct 350 and which is adapted to carry out a first stage of separation of the ceramic powders that remained in suspension.

[0144] The flow of hot air 22 that reaches the fan 345 is then conveyed to an exhaust chimney 360, possibly after passing through a centrifugal wet scrubber 365 adapted to carry out a second stage of separation of the residual powders.

[0145] In the production of ceramic artefacts, such as ceramic tiles or slabs, ceramic powders such as those obtained from the atomiser 10 or other traditional atomisers are then subjected to a pressing step, so as to obtain a mass of pressed ceramic powder, having for example the shape of a slab or a tile, which is conventionally called raw ceramic substrate 51.

[0146] After pressing, the raw ceramic substrate 51 can be cut, for example to trim it along the edges and/or to obtain, from a large-sized raw substrate, individual smaller portions that will constitute the individual ceramic tiles or slabs.

[0147] The raw ceramic substrate 51 can be subjected to a drying step, during which it is heated to moderate temperatures in order to reduce the humidity of the pressed ceramic powders.

[0148] In some embodiments of the invention, the hot air 22 could also (or alternatively) be used within a dryer in which raw (not subjected to firing) pressed ceramic artefacts are dried. Independently of this, subsequently, the raw ceramic substrate can be decorated, for example by glazing and/or application of ceramic inks.

[0149] Finally, the raw ceramic substrate 51, possibly dried and decorated, is subjected to a firing step in a roller kiln 30 in order to ceramicate and/or thermally consolidate the pressed ceramic powders and all the solid parts present in any decorative layers, so as to finally provide the ceramic tile or slab 62.

[0150] According to a preferred aspect of the present invention, the hot air 22 leaving the heat exchanger 2 can be conveyed, in addition to the atomiser 10 and/or the dryer, also towards the roller kiln 30. Alternatively, the hot air 22 leaving the heat exchanger 2 can be conveyed only towards the roller kiln 30.

[0151] An example of the roller kiln 30 is shown in Figures 3 and 4. It comprises an outer casing 505 adapted to define a duct/tunnel, normally with a rectangular section, inside which one or more raw ceramic substrates 51 can advance on a motorised roller conveyor 510. To carry out an effective firing step, a temperature diagram is established in the direction of the longitudinal axis of the kiln 30 which, as a function of the advancement speed of the raw ceramic substrates 51, in turn defines the desired firing diagram thereof.

[0152] In particular, the temperature diagram envisages a gradual increase of the temperatures starting from the inlet of the kiln 30 towards a central zone of the tunnel, in which the raw substrates reach the maximum temperatures (even beyond 800 °C and up to 1200-1300 °C), and then gradually decrease downstream of the central zone until the outlet. To heat the raw ceramic substrate 51, in particular to fire it, the flow of hot air 22 coming from the heat exchanger 2 can be introduced inside the roller kiln 30.

[0153] In this case, however, unlike the atomiser 10, the heat provided by the hot air 22 may not be sufficient to achieve complete firing of the raw ceramic substrate 51.

[0154] Therefore, the kiln 30 can be made as a mixed or hybrid kiln, of the hot air type but with an additional thermal contribution, to reach the necessary firing temperatures.

[0155] This additional thermal contribution can be provided by a plurality of open flame burners 515, which are installed on the vertical and opposite longitudinal walls of the outer casing, so as to generate the heat necessary for firing the raw ceramic substrates 51.

[0156] Each burner 515 generally comprises an inlet 520 for a fuel and an inlet 525 for the oxidising gas, typically air, so as to cause combustion of the fuel and obtain a combustion gas at higher temperature that is introduced directly into the kiln 30, for example through an outlet mouth 530.

[0157] In this way, the combustion gas laps and heats the raw ceramic substrates transiting inside the kiln 30.

[0158] The burners 515 generally orient the flame towards the opposite wall of the tunnel and are adjusted so as to ensure temperature uniformity in the cross sections of the kiln 30. Hot fumes or combustion gases altogether generated by the burners 515 can be conveyed countercurrently with respect to the direction of advancement of the raw substrates 51, for example until they exit near the inlet of the kiln 30 through a special chimney (not illustrated), the draught of which is favoured by the presence of suitable pneumatic means, such as centrifugal fans or the like, or also valves or other mechanical means.

[0159] In this way, the raw ceramic substrate 51 enters the kiln and is countercurrently hit by the hot fumes coming from the firing chamber. Especially in the early firing steps, heat is transmitted not by irradiation but by convection and all substances released by the product (water, organic products, carbonates, etc.) are evacuated.

[0160] In some embodiments, the fuel used for the aforementioned burners 515 may be hydrocarbon-based, for example comprise or consist of methane or LPG.

[0161] The hot air 22 coming from the heat exchanger 2 can be introduced into the roller kiln 30 at a temperature greater than or equal to 600 °C, for example at 650 °C, preferably but not necessarily in the initial zones of the kiln 30, upstream of the highest temperature firing zone.

[0162] In this way, compared to the traditional roller kilns, it is advantageously possible to reduce the number of burners or, in any case, it is possible to reduce their heat input and therefore fuel consumption.

[0163] For example, it is considered that, by introducing adequate volumes of hot air 22 at 650 °C into the kiln 30, it is possible to reduce methane gas consumption by about 50%. Moreover, by exploiting the mixed heating solution outlined above, it is possible to realize a kiln 30 capable of completely eliminating the carbon dioxide emission.

[0164] This can be achieved for example by using burners 515 which are fed with a hydrogen-based fuel, for example comprising or consisting of oxyhydrogen, i.e. a gaseous mixture of hydrogen and oxygen, such as the so-called HHO, or comprising or consisting of molecular hydrogen (H₂).

[0165] In both cases, the hydrogen-based fuel can be obtained by means of an electrolytic cell, preferably powered by electricity obtained from renewable sources (e.g. from photovoltaic panels), in which a process of electrolytic dissociation of water takes place.

[0166] In this way, it is advantageously possible to obtain a plant 100 with a very low environmental impact since, on the one hand, the carbon dioxide produced for the generation of the hot air 22 can be effectively abated by means of the scrubber 4 outlined above, while, on the other hand, the combustion of hydrogen in the burners 515 produces neither carbon dioxide nor other pollutants but only water.

[0167] Of course, it is not excluded that the burners can be adapted to generate the hot combustion fumes from air and from two fuels, for example from both a hydrocarbon-based fuel and a hydrogen-based fuel, possibly in a mixture with each other, as described for example in the aforementioned Italian patent application no. 102021000006797.

[0168] The same decarbonization result can be advantageously obtained by using, in place of or in addition to the hydrogen burners 515, the thermoelectric heaters 600, e.g. electric heating resistors, preferably powered by electricity obtained from renewable sources (e.g. from photovoltaic panels), which are adapted to heat the interior of the kiln 300, for example the central zone to a maximum temperature.

[0169] It is to be noted that by using the hot air 22 coming from the exchanger 2, the thermal contribution to be provided by the hydrogen burners 515 and/or by the thermoelectric heaters 600 can be relatively contained and, certainly, much lower than the case in which the kiln 30 is heated solely with said hydrogen burners 515 and/or with said thermoelectric heaters 600.

[0170] Therefore, thanks to the combined use of hot air 22, solutions that, if used alone, would be economically and/or technically inadequate as explained in the introduction, now become very interesting.

[0171] For example, in the case of a kiln 30 having hot air at 650 °C available, i.e. not less than 50% of the necessary thermal energy (most ceramic artefacts are fired at temperatures below 1300 °C, for example tiles at 1200 °C), a thermal contribution from thermoelectric heaters and/or hydrogen burners, technically and economically feasible, would be sufficient to completely decarbonise it.

[0172] With particular reference to the solution of the thermoelectric heaters 600, the combined use of hot air 22 also has the advantage of not completely disrupting the method of delivery of the heat to the ceramic product, so that the firing cycle can be carried out according to process variables that are already widely standardized and efficient today, starting from the chemistry of the product.

[0173] The invention thus conceived is susceptible to many modifications and variants, all falling within the same inventive concept.

[0174] Moreover, all the details can be replaced by other technically equivalent elements.

[0175] In practice, the materials used, as well as the contingent shapes and sizes, can be whatever according to the requirements without for this reason departing from the scope of protection of the following claims.

Claims

1. Ceramic plant comprising an apparatus (15) for generating hot air (22), said apparatus comprising:

- a combustion system (1) adapted to generate hot combustion fumes (11) from at least air (12) and an incoming fuel (13),

- a heat exchanger (2) having air (5) at the inlet and adapted to receive said hot combustion fumes for heating said incoming air so as to emit hot air (22),

said ceramic plant comprising at least one heat treatment unit (10, 30) adapted to receive the hot air leaving said heat exchanger to heat ceramic material (50, 51).

2. Plant according to claim 1, characterised in that it comprises a carbon dioxide scrubber (4) adapted to receive and treat the combustion fumes (11) leaving said heat exchanger (2).

3. Plant according to claim 1 or 2, characterised in that it comprises a post-combustion system (3) able to convert the organic compounds present in the combustion fumes (11) leaving said heat exchanger (2) into carbon dioxide.

4. Plant according to any one of the preceding claims, characterised in that said heat treatment unit (30) further comprises one or more burners (515) adapted to generate hot combustion fumes from at least air and a fuel and to introduce said hot combustion fumes directly into the heat treatment unit (30).

5. Plant according to claim 4, characterised in that the fuel used by said burners (515) is hydrogen-based.

6. Plant according to any one of the preceding claims, characterised in that said heat treatment unit (30) further comprises one or more thermoelectric heaters (600) adapted to heat the inside of the heat treatment unit (30).

7. Plant according to any one of the preceding claims, characterised in that said at least one heat treatment unit is an atomiser (10) adapted to receive the hot air leaving said heat exchanger (2) for drying ceramic slip (50) and producing ceramic powders (60).

8. Plant according to any one of claims 1 to 7, characterised in that said at least one heat treatment unit is a roller kiln (30) adapted to receive hot air leaving said exchanger (2) for firing raw ceramic substrates (51) and producing ceramic tiles or slabs (62).

9. Plant according to claim 1, characterised in that it comprises at least two heat treatment units (10, 30) adapted to receive the hot air leaving said heat exchanger (2), including at least one atomiser (10) adapted to receive the hot air leaving said heat exchanger for drying ceramic slip (50) and producing ceramic powders (60), and a roller kiln (30) adapted to receive the hot air leaving said heat exchanger for firing raw ceramic substrates (51) and producing ceramic tiles or slabs (62).

10. Plant according to any one of the preceding claims, characterised in that said heat exchanger (2) is configured so that the hot air (22) introduced into the heat treatment unit (10, 30) has a temperature greater than or equal to 500 °C.

11. Plant according to any one of the preceding claims, characterised in that the fuel used in the combustion system (1) of the hot air generation apparatus (22) is a hydrocarbon-based fuel.

12. Plant according to any one of the preceding claims, characterised in that the fuel used in the combustion system (1) of the hot air generation apparatus (22) is a pyrolysis oil.

13. Plant according to any one of the preceding claims, characterized in that said heat exchanger (2) comprises an outer casing (200) provided with a first inlet (205) for the air (5) to be heated, a first outlet (210) for the heated air, a second inlet (215) for the combustion fumes (11) coming from the combustion system (1) and a second outlet (220) for the same combustion fumes (11).

14. Plant according to claim 13, wherein in the outer casing (200) of the heat exchanger there are two distinct channelings, of which a first channeling adapted to put the first inlet (205) in communication with the first outlet (210), and a second channeling adapted to put the second inlet (210) in communication with the second outlet (220), said channelings being structured such that the air and the combustion fumes are put in a heat exchange relationship keeping them physically separated.

15. Method for subjecting ceramic material to heat treatment, comprising the steps of:

- generating by combustion hot fumes from at least air and a fuel,

- introducing said hot combustion fumes into a heat exchanger (2) to heat incoming air and emit it at the outlet as hot air,

- introducing the hot air leaving the heat exchanger (2) into at least one unit (10, 30) into which said ceramic material is also introduced.

Drawing