Heat carrier for heating raw material in a reactor, plant for the pyrolysis of raw material using said heat carrier and method for the pyrolysis of raw material

The present invention refers to the technical field of heat treating raw materials by a pyrolysis reaction .

Specifically, but not restrict!vely, the invention refers to the technical field of heat treating raw material in the form of biomass or mass of waste materials by pyrolysis reaction in order to produce pyrolysis gases as direct or indirect sources of energy.

The treatment processes and plants of raw materials also in the form of biomasses or masses of waste materials are known in the state of the art.

By using temperatures ranging from 400 to 950°C and in the absence of oxygen, the material is converted from the solid state to liquid products (so-called tar or pyrolysis oil) and/or gaseous products (syngas) . These materials can be used as fuels or raw materials intended for subsequent chemical processes. The obtained carbonaceous solid residue can be further refined thus providing products such as for example activated carbon. The pyrolysis products are either gaseous, or liquid, or solid, in proportions that depend on the pyrolysis methods (quick, slow, or conventional pyrolysis) and the reaction parameters. The heating of the mass of material under anoxic conditions (total absence of oxygen) causes the disruption of the original chemical bonds with formation of simpler molecules. The heat provided in the pyrolysis process is thus used to disrupt the chemical bonds, implementing what is defined as thermally induced homolysis.

The pyrolysis products, be they gaseous or liquid or solid, find different uses among which the use as fuels for turbines, boilers, engines or even fuel cells, the use in chemical processes as reagents , hydrogen sources and other uses .

The efficiency of the pyrolysis process for what concerns the energy balance related to the energy recovered by the masses of raw material very much depends on the treatment techniques of the raw material mostly related to the heat transfer to the latter .

For the anaerobic heating of the raw material different methods are known with the related heat carriers and the corresponding implementation plants .

Among the different methods of heating the raw material, in order to generate a pyrolytic reaction, mixing a raw material mass in the granular form with heat carriers constituted by particulate elements is known, particularly spherical elements that have been previously heated to predetermined temperatures .

The steps of these processes are mainly the following ones:

a) heating a charge of particulate carriers to a predetermined temperature;

b) transferring the heat stored in the particulate carriers to a raw material mass in granular form and under anaerobic or at least anoxic condition ;

c) collecting pyrolysis products;

d) separating the residual solid mass of the pyrolysis reaction and drawing the particulate carriers .

In the present description and in the claims , by the term particulate carrier is meant a unitary body in the form of solid particle having predetermined dimensions , a predetermined shape and a predetermined composition of the material or materials constituting thereof, as it will clearly appear also from the following description. The term particulate can also mean granular.

The process can be carried out in a continuous cycle by continuously repeating the steps a) to d) , for example until the provided raw material is exhausted.

According to a characteristic, the charge of particulate carriers comprises a predetermined number of said particulate carriers. Depending on the type of material the carriers are made of, the number varies in order to provide a predetermined mass of material, i.e. in order to ensure a predetermined thermal capacity storable in the charge of particulate carriers.

According to a characteristic, also the raw material mass is determined in accordance with the mass of the charge of particulate carriers and so as to ensure, inside the reactor, the heating effect needed to produce the pyrolytic reaction.

The granular form of the raw material can be controlled by different processes, for example mincing and also screening processes, so as to obtain a predetermined distribution of granulometry.

According to an embodiment, the heat transfer from the charge of particulate carriers takes place by thermal contact, for example by mixing said charge of particulate carriers to said raw material mass and for a predetermined period of time.

Still according to an embodiment, at the end of the heat transfer process between the charge of particulate carriers and the charge of raw material, the particulate carriers are separated from the residues of the pyrolysis reaction and are subjected to a cleaning treatment prior to be subjected to a further heating cycle of the carriers themselves , mixing to the charge of raw material in the reactor, transferring heat to the raw material and again separating from the solid residues of the pyrolysis reaction .

Currently different types of heating methods of the particulate carriers, different methods for transferring heat from these carriers to the raw material to be subjected to the pyrolysis reaction and different methods of recirculation, i.e. separation from the residues of the pyrolysis reaction, possible cleaning and heating again the particulate carriers for a further pyrolysis cycle, are known .

It has to be noted that both the feeding of the raw material to be subjected to the pyrolysis reaction and the recirculation of the particulate carriers for heating the raw material can occur in a cycle with subsequent charges, which charges comprise pre-established amounts in the form of mass or volume of raw material and/or particulate carriers, or else continuous charges .

Still according to an embodiment variation, the succession of charges can also be parted in such a way so as to have an almost-continuous cycle.

Currently different types of particulate carriers are known that differentiate one another for the shape, dimensions and material they are made of.

The different known shapes are typically irregular, spherical or toroidal.

The currently more widespread materials are iron, steel, stainless steel, cast iron, ceramics, aluminum, aluminum oxide, ceramics, titanium, other metal oxides , earths and thermal sands .

The dimensions vary depending on the desired pyrolysis processes from some microns to about 100 mm .

The purpose of the invention is to refine the pyrolysis processes in order to improve their performance mostly for what concerns the energy balance of the reaction, the amount of products obtained from the pyrolytic reaction of a predetermined mass of material, without increasing the complexity of the process and means, as well as the plants needed for implementing the pyrolysis process .

From the above it is evident that one of the most critical aspects is the heating process of the raw material and, in this case, the steps of heating the particulate carriers and transferring the stored heat from said carriers to the raw material mass.

According to a first aspect, object of the present invention is a particulate heat carrier, i.e. in the form of particle, for the heat transfer to a raw material mass by mixing with said raw material mass, the heat carrier having an ellipsoidal or ovoid shape .

By the term ellipsoidal it is defined a solid having an external surface defined by the following equation :

.:,·' y ² s ²

- :;- -f- - ==== 1

/> ^<■ &

wherein x, y, z are Cartesian coordinates;

a, b, c are real numbers representing the three semi-axes of an ellipsoid.

Preferably for the parameters a, b, and c it is true that they are higher than zero and at least two of the parameters a, b and c are different from each other.

According to an embodiment the three parameters a, b and c satisfy the following equation:

a>b>c

According to a further embodiment variation, the parameters a, b and c satisfy the equation

a>b=c or else a=b>c.

Regarding the use of a heat carrier of elliptical shape, contrary to what could theoretically be hypothesized, the non-symmetric shape in the three dimensions, i.e. the non-spherical shape has a better heat balance between the heat energy absorbed in the heating step and the heat energy released in the step of transferring the heat to the mass to be treated in the reactor. Such an effect is surprising as the contact sections of an element perfectly spherical are identical for any orientation, whereas in the presence of an ellipsoidal shape the contact surface for the heat transfer is strongly depending on the orientation of the ellipsoid in the raw material mass to be treated, i.e. to which the heat energy must be released. Thus, in a purely theoretical way, we would expect a heat balance more unfavorable for the non-spherical carriers than spherical ones , whereas the experiments clearly demonstrated that this presumption is not valid, (see fig. 4)

Preferably said heat carrier has a regular elliptical shape.

According to a preferred, but not restrictive, embodiment, said carrier is made of steel.

According to a further characteristic, the dimensions of said heat carrier can be inscribed in a spherical shape having diameter ranging from few microns to 100 mm, preferably from 5 mm to 100 mm.

For what concerns the ellipticity, the ratio of the minor axis to the major axis is advantageously ranging from 0.20 to 0.80, preferably from 0.30 to 0.70.

An embodiment provides a plurality of heat carriers according to one or more of the combination of characteristics described above, the plurality of heat carriers comprising, for at least a certain part of the total number of said heat carriers , heat carriers having a first dimension and, for at least a further part of the total number of said heat carriers, heat carriers having at least one second dimension different from the first dimension.

An embodiment variation of the aforesaid embodiment may provide, alternatively or in combination, that at least a certain part of the total number of said heat carriers is made of a first material, while at least a further part of the total number of said heat carriers is made of at least one second material different from the said first material .

As already stated above with reference to the discussion on the known art, the plurality of heat carriers depends on the thermal capacity of said plurality of carriers and, in first approximation, this is depending on the mass and the material, therefore the number of heat carriers of said plurality of heat carriers varies depending on the material they are made of and on the thermal capacity, as a predetermined heat amount must be ensured to heat a certain amount of raw material to the temperature needed for the pyrolytic reaction to happen .

Still according to a further characteristic that can be provided in any combination or subcombination with the previously described characteristics, said heat carriers can have a three- dimensional surface structure, i.e. it is not smooth, but rough, knurled or has ribs or fins or protrusions distributed according to different patterns on the mantle surface of said carriers.

According to a further aspect, the invention provides a plant for the pyrolysis of raw materials, such as particularly biomasses, which plant comprises :

a pyrolysis reactor having

an inlet for the raw material and at least one outlet for the products of the pyrolysis reaction; an inlet for a plurality of heat carriers prior to the heat transfer to the raw material mass and an outlet of said heat carriers after the heat transfer to the raw material mass;

mixing means to mix said plurality of heat carriers with said raw material mass for a predetermined period of time needed for the heat transfer from said heat carriers to the raw material mass ;

heating means to heat the heat carriers of said plurality of heat carriers and wherein the heat carriers have one or more of the combination of characteristics mentioned above and described according to the various combinations provided.

According to an embodiment, the plant further has recovering means to recover the plurality of heat carriers at the outlet of the pyrolysis reactor, transferring means to transfer said heat carriers to the heating means to heat said carriers and their new use in a further heating cycle of a further raw material mass to be subjected to pyrolysis reaction.

Said recovering means advantageously comprise a unit separating the heat carriers from solid residues of the pyrolysis reaction and means cleaning said heat carriers from pyrolysis reaction residues that remain adherent against them.

Still according to a further characteristic, the inlets for the feeding of the raw material and the plurality of heat carriers to the reactor and/or the outlets for the residues of the pyrolysis reaction and for recovering the heat carriers, can be the same .

According to a further characteristic, the mixing means of the plurality of heat carriers with the raw material mass consists of a screw mixer having a predetermined axial length and housed in a reaction chamber coaxial or having axis parallel to the screw of the screw mixer, and which chamber and/or which screw mixer extend between said inlets for the raw material mass and for the plurality of heat carriers and said outlets for the pyrolysis reaction products and heat carriers .

An embodiment variation can provide at least one further inlet and/or outlet or possibly different further inlets and/or outlets for one or more further charges of raw material and/or heat carriers with a predetermined number, said carriers being made according to one or more of the alternative variations described above, the further inlets and/or outlets being distributed along the length of the screw mixer.

Still according to a further variation, the screw mixer consists of a stationary screw or auger about which a tubular reaction chamber, or a tubular chamber housed together with the screw in the reaction chamber, coaxially rotates.

The screw mixer has any orientation, also a vertical one. According to an embodiment it particularly has an orientation, with the axis of the screw and/or the reaction chamber, that is horizontal or inclined with respect to the horizontal line.

The mixing screw can be made according to different geometries corresponding to the heat transfer process, in order to ensure the maximization of the heat transfer between the heat carriers and the mass of matter to be treated.

Also the feeding of the heat carriers can occur according to different variations. A variation can provide for feeding the heat carriers by gravity or free fall along a vertical path or an inclined plane.

The heat carriers can be directed centrally or eccentrically with respect to the axis of the screw of the screw mixer.

A further variation can provide a feeding that is tangential and/or radial with respect to the axis of the screw mixer.

Object of the invention is also a method for the pyrolysis of a raw material mass, which method comprises the steps of heating the raw material mass to the activation temperature of the pyrolysis reaction by transfer of heat energy from a plurality of heat carriers. According to the invention, said heat carriers are made according to one or more of the preceding characteristics and embodiments described above and in any combination and subcombination previously described.

According to an embodiment, the method for the pyrolysis according to the present invention provides the following steps:

a) heating a charge of particulate carriers to a predetermined temperature;

b) transferring the heat stored in the particulate carriers to a raw material mass in granular form and under anaerobic or at least anoxic condition ;

c) collecting pyrolysis products;

d) separating the residual solid mass of the pyrolysis reaction and drawing the particulate carriers ; The process can be carried out in a continuous cycle by continuously repeating the steps a) to d) , for example until the provided raw material is exhausted.

According to a characteristic, the charge of particulate carriers comprises a predetermined number of said particulate carriers. Depending on the type of material the carriers are made of, the number varies in order to provide a predetermined mass of material, i.e. in order to ensure a predetermined thermal capacity storable in the charge of particulate carriers.

According to a characteristic, also the raw material mass is determined in accordance with the mass of the charge of particulate carriers and so as to ensure, inside the reactor, the heating effect needed to produce the pyrolytic reaction.

The granular form of the raw material can be controlled by different processes, for example mincing and also screening processes, so as to obtain a predetermined distribution of granulometry .

According to an embodiment, the heat transfer from the charge of particulate carriers takes place by thermal contact, for example by mixing said charge of particulate carriers to said raw material mass and for a predetermined period of time.

Still according to an embodiment, at the end of the heat transfer process between the charge of particulate carriers and the charge of raw material, the particulate carriers are separated from the residues of the pyrolysis reaction and are subjected to a cleaning treatment prior to be subjected to a further heating cycle of the carriers themselves , mixing to the charge of raw material in the reactor, transferring heat to the raw material and again separating from the solid residues of the pyrolysis reaction .

It has to be noted that both the feeding of the raw material to be subjected to the pyrolysis reaction and the recirculation of the particulate carriers for heating the raw material can occur in a cycle with subsequent charges, which charges comprise pre-established amounts in the form of mass or volume of raw material and/or particulate carriers, or else continuous charges. The succession of charges can also be parted in such a way so as to have an almost- continuous cycle.

An embodiment variation of the method provides using a charge of particulate heat carriers which alternatively or in combination have:

for at least a certain part of the total number of said heat carriers, heat carriers having a first dimension and for at least a further part of the total number of said heat carriers, heat carriers having at least one second dimension different from the first dimension;

a first material for at least a certain part of the total number of said heat carriers , whereas at least one second material for at least a further part of the total number of said heat carriers, which is different from the said first material.

Still according to a further characteristic, the method according to the present invention can provide the displacement of the mixture of raw material mass and heat carriers along a predetermined path, from a separated or shared feeding inlet for the raw material mass and/or for a charge of a plurality of heat carriers, to a separated or shared outlet for pyrolysis reaction residues and/or for the heat carriers .

An embodiment variation provides one or more further charges of heat carriers identical or different from the first charge to be fed and/or discharged from the mixture of raw material mass and heat carriers and such further charge or charges to be fed and/or discharged in predetermined points in the path of the mixture of raw material mass and heat carriers from the start point to the end point of said path, the start point of said path corresponding substantially with the shared inlet or separated inlets of the reaction chamber of the raw material mass and a first charge of heat carriers, and the end point of said path corresponding substantially with a shared outlet or the separated outlets for the pyrolysis reaction residues and heat carriers.

Further characteristics of the invention are the object of the dependent claims.

Further characteristics and advantages of the present invention will be better apparent from the following description of some exemplary embodiments depicted in the attached drawings wherein:

Fig. 1 shows a schematic view of a plant according to the present invention.

Fig. 2 shows a schematic example of the detail related to the screw mixer provided in the reaction chamber .

Figure 3 shows a particulate heat carrier in the form of an elliptical carrier and the dimensions of the radiuses of the major axis and minor axis. Figure 4 compares different solutions of particulate heat carriers with reference to the heat power released and absorbed by the same.

Fig. 5 and fig. 6 show two variations of the feeding mode of the heat carriers and/or raw material .

Fig. 7 schematically shows the chance to provide several different feeding inlets of the heat carriers, which are distributed along the path of the raw material mass in the reactor and along the mixing screw.

Fig. 8 schematically shows a variation wherein at least two discharge outlets of the heat carriers are provided, which are distributed along the path of the raw material mass in the reaction chamber and along the mixing screw.

Figure 9 shows two alternative feeding modes of particulate heat carriers having different dimensions one to another and wherein one is bigger than the other one .

With reference to the figures, these are purely exemplary and do not only constitute an exemplary, but not restrictive embodiment of the different operating units and/or means in the plant. These can be made according to any of the possible choices available to the technician of the art in his basic cultural technical skill.

A treatment plant for the raw material, such as biomasses, wastes or the like by pyrolysis in an anaerobic or anoxic reactor, comprises a feeding station 1 of the raw material in the minced form in pieces having a predetermined granulometry or a predetermined distribution of granulometry. In the station 1 a hopper 101, in case combined with a mincing/granulating unit, communicates with an inlet 102 of a reaction chamber 2. In this case this is formed by a screw mixer.

In the specific case and according to a particular characteristic, the screw mixer comprises a cylindrical tubular chamber 202 rotatingly supported coaxially to a mixing screw or auger 302 that is stationary instead.

In the example, the cylindrical rotating chamber

202 and the screw or auger 302 are coaxial one to another. However, such a solution is not intended to be limited, but constitutes a specific implementation example .

A further inlet 402 communicates with a feeding duct 3 of particulate heat carriers depicted with V.

The reaction chamber 2 further has at least one outlet 502 for drawing the product (s) of the pyrolytic reaction, for example and not restrict!vely depicted the so-called syngas, at least one discharge outlet of the solid residues of the pyrolytic reaction depicted with 602 and at least one outlet 702 for the particulate heat carriers V.

Inside the reaction chamber, a certain number of particulate carriers heated to a predetermined temperature is mixed with a raw material mass corresponding to a predetermined amount of said raw material, in order to transfer the heat energy from said hot carriers V to the raw material in the path between the inlets 102, 402 and the outlets 502, 602, 702.

The exiting carriers V, having released the heat to the raw material mass and originated the pyrolytic reaction, are separated from the reaction residues and, thanks to one or more carriers symbolically depicted by the carriers 4 and 5, are fed to a heating unit, such as an oven 6 or the like, from which they are then fed to the reaction chamber through the inlet 402.

It has to be noted that in the depicted plant, the circulation of the heat carriers is by fall from the oven 6 to the inlet 402 of the reaction chamber and the path of the carriers V provides the same to be drawn from the corresponding outlet 702 of the reaction chamber 2 and lifted by a conveyor to a level higher than that of the oven 6, where they are transferred by gravity thanks to a descending feeder and/or thanks to the shape of the carriers that allows their rolling. The inlet of the oven 6 is obviously placed at a lower level than the discharge end of the carriers V from the lifting conveyor 4.

The pyrolysis products, such as for example the syngas, are transferred to a exploitation and/or storage unit, such as for example energy converters 7 of the type called ORC (Organic Ranking Cycle) or else to the ICE unit 8 or to steam generators 9 or other exploitation and/or storage units.

With reference to the figures, the plant has the following operation corresponding to a cycle of the heat carriers V.

STEP 1 : Heating the heat carriers V in the oven

6

The function of the oven 6 is to heat the heat carriers. The oven can perform the heating according to one or more of the convection, conduction or irradiation mechanisms, i.e. possibly by a combination of these mechanisms.

- The convective heat exchange is carried out by the hot fumes that are in the oven, lapping against the surface of the heat carriers.

- The conductive heat exchange is carried out by the hot walls of the oven in contact with the heat carriers and by the hot heat carriers in contact with the colder ones .

- The radiant heat exchange is carried out by the hot walls of the oven and by the flames of the burners, the heat carriers not contacting them.

STEP 2 : Heat transfer of the heat carriers in the reactor

The hot heat carriers leaving the oven are entered into the reactor at the first stages of the screw mixer.

The function of the heat carriers in the reactor is to release the heat stored in the raw material so that the latter can reach the activation temperature of the pyrolysis reaction, in a sufficiently short time .

To do this, the heat carriers directly come in contact with the raw material to which they release the heat by conduction.

- In the first stages of the reactor, the heat carriers meet a great amount of raw material . The heat carriers cannot come in contact directly with the whole amount of raw material . The heat carriers have the maximum temperature .

- In the following stages there is less and less raw material that has to react, which has not yet come in contact with the heat carriers. The heat carriers that are "cold" and dirty with ash exit the reactor at the last stages in one outlet only.

STEP 3: Cleaning the heat carriers

The heat carriers are cleaned from the ashes being on their surface by a mechanical action.

STEP 4 : Transporting the heat carriers

The heat carriers cleaned up and still warm (the temperature is higher than the ambient temperature) are brought back to the oven to be heated again and thus to start a new heating cycle of the raw material mass in the reactor.

With reference to figure 3, a heat carrier V according to the present invention is depicted.

The depicted shape is a specific, non- restrictive shape since it is possible to provide elliptical shapes having a ratio of the minor axis 2a or 2c to the major axis 2b advantageously ranging from 0.20 to 0.80, preferably from 0.30 to 0.70.

For what concerns the shape, the carrier V can also have an irregular ellipsoid shape or an ovoid shape .

By the thermal analysis on the heat carriers, different possibilities related to the geometry and materials of the same have been investigated.

In general, with the same material, heat carriers having great mass and low surface retain heat longer; heat carriers with great surface and lower mass are quicker in storing and giving the heat .

Figure 4 depicts a graph comparing the absorbed power to the power that can be released from the single heat carriers having different shapes, i.e. spherical, spherical with diametrical hole with a port having a first measure, spherical with a diametrical hole with a light having a second measure bigger than the first measure and elliptical.

The graph depicts the following results:

The elliptical heat carriers have higher absorption of heat power and a value of the heat power that can be released to the raw material mass itself also higher than the spherical ones with and without diametrical hole having a port with a first dimension .

Also, in said two other cases the difference between the released heat power and the heat power that can be released is lower.

The elliptical heat carriers are second only to the spherical ones having diametrical hole with a port according to a second dimension bigger than the first one.

However this type of heat carriers has a considerable drawback related to their cleaning, as in the hole incrustations of ash or other materials are deposited and require more complex and long cleaning processes than the elliptical ones.

The functional comparisons of the different types of heat carriers depict what follows:

- Spherical heat carriers: In the reactor they release better in the last stages with the not yet reacted raw material; they can be more easily moved in the feeding and recirculation circuit as they can roll; they can be more easily cleaned from the ashes.

- Ellipses (Big surface) : They release better the heat in the first stages. - Heat carriers with holes (Big surface) : They release better the heat in the first stages; they are difficult to be cleaned in the cavity with traditional scraping methods that are difficult to be carried out in the cavity.

Concerning the considerations on the material of the heat carriers, different types of materials have also been studied:

- Aluminum: it is the lightest but it has low surface hardness and stability and resistance to high temperatures .

- Steel: it has good properties either in terms of surface hardness, resistance to high temperatures and heat transmission.

- Ceramics: they have a wide range of properties. Some of them are better than steel for what concerns the hardness but not for the conductivity, others do the opposite. It is substantially difficult to be able to find a ceramic material for the heat carriers that is better than steel in all the characteristics of interest. Furthermore they are on average more expensive than the heat carriers made of steel .

Other materials can be stainless steel, cast iron, aluminum oxide, titanium, other metal oxides, earths and thermal sands .

From the above it is clear that the elliptical heat carriers have superior properties both in terms of heat stored and heat released and furthermore they do not have cleaning problems from the ashes.

The analysis of the different types of materials demonstrated the steel is the preferred material, however also the other materials provided good performance and it is also possible to provide combinations of materials also arranged in more than one layer.

For what concerns the dimensions of the heat carriers, depending on the type of raw material to be treated and on the reaction parameter setting, as well as depending on the reaction products we want to obtain, it is possible to provide heat carriers having different dimensions ranging from few microns to 100 mm. In the case of elliptical heat carriers such dimensions are referring to a sphere inscribing the elliptical shape.

For what concerns the heating temperature of the heat carriers, generally two ranges of operating temperature 400-950°C or 1400 - 1800°C are provided.

However the heating temperature of the heat carriers depends on the type of the pyrolysis reaction carried out, which can be: slow (with lower temperatures) or quick (or flash) with much higher temperatures .

The cleaning of the heat carriers can be carried out according to different techniques such as for example :

- mechanical cleaning: by scraping

- magnetic cleaning: by removing the ashes by using magnetic fields

- thermal cleaning: by burning the ashes.

Figures 5 and 6 depict two alternative feeding variations of the heat carriers in the reaction chamber .

Figure 5 depicts a feeding of the heat carriers by rolling on an inclined plane and having an inlet according to a direction tangential to the screw or the auger in the reaction chamber 2.

Figure 6 depicts instead a feeding by fall of the heat carriers in the reaction chamber according to a path centered with respect to the axis of the screw or auger.

With reference to figure 7, a variation is therein schematically depicted according to which, in addition to the inlet 402 for the heat carriers provided at or immediately downstream of the inlet for the raw material with reference to the path of the same in the reaction chamber 2 , along said path i.e. along the extent of the reaction chamber 2 (in the direction of said path of the raw material) , one or more further inlets distributed along said path of the raw material mass in the reaction chamber can be provided, that in the example depicted substantially coincides with the length extent of said reaction chamber 2.

In figure 7 two further inlets 402', 402'' for the hot heat carriers are depicted.

It has to be noted that still according to a further embodiment variation, since the different inlets 402, 402', 402'' are provided in points corresponding to different stages of the pyrolysis process, it is possible to provide the heat carriers V fed through the different inlets for being different concerning one or more of the following characteristics: shape, size, heating temperature, material, and this so as to adapt the heat transfer determined by these carriers to the conditions of the pyrolysis reaction corresponding to the position of the corresponding inlet. With reference to figure 8 instead, the same concept is applied to the outlets for the heat carriers V that have undergone cooling during the transfer process of the heat energy to the raw material mass. In this case, in addition to the outlet 702 at the terminal end of the reaction chamber 2, it is possible to provide one or more further outlets distributed along the path of the raw material mass in the reaction chamber 2, i.e. in the present example along the extent of said camera in the direction of said path.

With reference to the example of figure 8, the drawing of the cold carriers can take place thanks to magnetic pickers as schematically depicted by 9.

Obviously the characteristics of figure 8 can also be provided in combination with those of figure 7.

Still according to a further embodiment variation, it is possible to provide at least two different types of heat carriers V differentiating one another by to at least one parameter or a combination of parameters included in the following ones: shape, size, material, heating temperature.

These at least two types of heat carriers can be fed together in one or more of the openings 402, 402' , 402' ' , or else can be fed separately through one or more of said openings and/or at different times for each type of carrier.

Figure 9 schematizes the two alternative possibilities when there are two types of carriers V having two different dimensions, i.e. one type of heat carrier having a major or minor diameter bigger than that of the other type of heat carriers . As an alternative or in combination, the heat carriers of the two different types can have a different shape for each type, for example spherical and elliptical .

It is also possible that under the set of heat carriers V there is a certain distribution of more than one different type of carriers concerning one or a combination of two or more of the above mentioned parameters, which distribution provides values included in a predetermined range of variation of said one or more parameters, such as for example a dimensional range, a heating temperature range, and others .

For example it is also possible that in the set of heat carriers circulating in the plant, according to certain respective quantitative ratios, elliptical heat carriers are provided in combination with heat carriers having a shape according to one or more of the variations as per figure .

Still according to a further variation not depicted in detail, the heat carriers V can have a non-smooth surface, but provided with a three- dimensional shaping, such as for example a porous and/or wrinkled surface or a knurled surface or else a surface with projections or fins according to one or more different patterns or combination thereof.

From the above, it is evident how the heat carrier in the shape of elliptical or ovoid element constitutes the best comprise among storage and transfer capacity of heat energy, convenience of movement in the recirculation path of the heat carriers, ease of separation, transport and mostly also ease of cleaning from the slag and residues of the pyrolytic reaction.

The heat carrier being the base of the energy transfer to the raw material mass to give free rein to the pyrolytic reaction is one of the key parameters in the optimization of the process efficiency from the energy and the reaction productivity point of view.

Further variations are refinements improving the processes for the heat transfer to trigger and control the pyrolysis reaction, avoiding energy losses and adapting the plant with higher precision to the process conditions.

Still according to an advantageous effect of the present invention, the heat carriers during the mixing/stirring process together with the raw material mass to be subjected to the pyrolysis reaction, exert a mechanical action on the solid residue analogous to a ball-mill. Such an action, obtained thanks to the continuous shuffling of the heat carriers with the raw material, leads to the mincing of the solid residue of the reaction thus easing the cleaning of the heat carriers themselves and the use of the solid residue obtained from the pyrolysis cycle.