WO2024095189A1

WO2024095189A1 - Double fluid bed plant with pressure differential

Info

Publication number: WO2024095189A1
Application number: PCT/IB2023/061027
Authority: WO
Inventors: Matteo Carmelo ROMANO; Giulio GUANDALINI; Cristian Argiolas
Original assignee: REMOSA S.r.l.
Priority date: 2022-11-03
Filing date: 2023-11-02
Publication date: 2024-05-10

Abstract

Solid-gas reaction plant (100), comprising: - a first fluid bed reactor (10) and a second fluid bed reactor (20) operating at different pressures, where the pressure of the second reactor (20) is higher than the pressure of the first reactor (10), thereby defining a pressure differential between reactors, said first reactor (10) and said second reactor (20) extending between a respective tail (11, 21) and a respective head (12, 22) along an extension direction (X-X); - a first and second connection pipe (40) configured to place the reactors (10, 20) in solid material communication, said first connection pipe (30) being configured to transfer solid material from the first reactor (10) to the second reactor (20), said second connection pipe (40) being configured to transfer solid material from the second reactor (20) to the first reactor (10), wherein: - a first mechanical adjusting system (50) associated to the first connection pipe (30) and configured to transfer the flow of solids from the first lower pressure reactor (10) to the second higher pressure reactor (20) maintaining the aforementioned differential between the aforementioned reactors by accumulation of solids in the pipe (30) - a mechanical second pressure adjusting system (60) associated to the second pipe (40) and configured to transfer the flow of solids from the second reactor (20) to the first reactor (10) by pressure dissipation in the pipe (40)

Description

Title: “Double fluid bed plant with pressure differential”

DESCRIPTION

Field of the invention

The present invention concerns a plant for solid-gas reactions, preferably carried out in a fluid bed, by managing flows of solids among reactors, without excluding the movement of fluids as well, so as to maintain a pressure difference between such reactors. Such a plant can be used for the production of industrial heat (e.g. vapor) by means of a carbon-containing fuel and CO2 separation, for example, for processes of the chemical looping combustion (CLC) type, or for the conversion of carbon-containing fuels into synthesis gases containing CO, H2, CO2 and other gases, for example in chemical looping gasification (CLG), chemical looping reforming (CLR) and indirect gasification processes.

Background

In the literature, chemical looping technologies such as CLC, CLG and CLR are usually based on double fluid bed reactors at atmospheric pressure or pressurized, in which the pressure of the reactors is similar. It should be noted that even in the case of indirect gasification, the gasification reactor and the combustor are operated at similar pressures, generally atmospheric ones.

Specifically, the chemical looping processes use a solid material called ‘Oxygen Carrier’ (OC) which is cyclically exposed to contact with air in a reactor called ‘air reactor’ and in contact with a carbon-containing fuel in another reactor called ‘fuel reactor’, transferring oxygen from the air to the fuel. The process therefore allows to oxidize the fuel in the absence of nitrogen, obtaining an undiluted CO2 flow, suitable for transport and sequestration/use after water separation and after appropriate compression. CLC technology is known and widely described in the scientific technical literature [1,2]. In particular, the technology has already been tested in laboratories and pilot plants (up to a scale of about 1 MW, TRL6) with both reactors (AR / FR) at atmospheric pressure. Although some research is pursuing the application of the process in fixed bed reactors, most of the experience is with fluid bed reactors. In addition, several ‘Oxygen Carrier’ have been developed and tested, especially based on Fe, Ni, Cu, Mn and Perovskites. At the same time, several fuels, especially gaseous and solid ones, were also tested.

Instead, in indirect gasification a solid fuel (carbon or biomass) is fed to a first fluid bed reactor together with vapor and a stream of high temperature inert solids (e.g. olivine). The presence of vapor and high temperatures (700-900°C) activates the pyrolysis and gasification reactions, generating a flow of synthesis gases (mainly H2, CO, CO2, H2O and CH4, as well as tar and other trace gases) and a flow of residual solids, consisting of ashes, unconverted carbon and solids used as a thermal carrier. This flow of solids is recirculated to a second fluid bed where air is fed. Oxidation of the residual carbon and of any additional fuel provides the necessary heat to return the heat carrier to the necessary temperature, before feeding it to the gasifier. Compared to the conventional direct gasification, in which air is fed to the gasifier itself, the indirect solution allows a nitrogen- free synthesis gas to be obtained. Indirect gasification technology is available for commercial plants, for example with the technology offered by Repotec, used for the GoBiGas plant (Goteborg Energie AB, Sweden, 32 MWth) [3], and the MILENA technology offered by Royal Dahlman/TNO. Even in these technologies, the fluid beds are operated at a pressure close to the atmospheric pressure.

In the CLC plants and in the indirect gasification plants it would be advantageous if respectively CO2 and the synthesis gases were produced at a pressure higher than the atmospheric pressure in order to reduce energy consumption for the subsequent compression required in both cases.

For the above reasons, conceptual studies have been published in which the CLC system works with both pressurized reactors [4,5], The CLC system with pressurised reactors has the advantage of generating pressurized CO2 and therefore requiring lower CO2 compression consumptions for subsequent use or sequestration. On the other hand, it also requires adopting a turbo-air compression group, which entails electricity consumptions and high investment costs.

In fact, in the prior art the fluid bed circulation plants, for example described in US11090624 B2 and US2012148484A1, respectively use combinations of mechanical/non-mechanical valves to operate a reactor at a pressure greater than the pressure of the other reactor. The above solutions make managing the solids and the reactions complex as well as achieving high pressures difficult unless it is accompanied by a significant drop in performance. Summary of the invention

In order to overcome the aforementioned problems, a plant has been devised that allows maintaining a pressure differential among the reactors of a double fluid bed plant, using the solids circulating in the plant. The circulation of the solids from the lower pressure reactor to the higher pressure reactor is managed by controlling the formation of a charge of solids in the corresponding connection conduit.

Object of the present invention is therefore a plant for solid-gas reactions (100) comprising:

- a first fluid bed reactor (10) and a second fluid bed reactor (20) operating at different pressures, where the pressure of the second reactor (20) is higher than the pressure of the first reactor (10), thereby defining a pressure differential between reactors, by adjusting the pressure of the gases introduced into the reactors (10, 20), said first reactor (10) and said second reactor (20) extending from a respective tail (11, 21) to a respective head (12, 22) along an extension direction (X-X);

- a first connection pipe (30) and a second connection pipe (40) configured to place the reactors (10, 20) in solid material communication, said first connection pipe (30) being configured to transfer solid material from the first reactor (10) to the second reactor (20), said second connection pipe (40) being configured to transfer solid material from the second reactor (20) to the first reactor (10),

- a first mechanical adjusting system (50) associated to the first connection pipe (30) that connects the first reactor head (12) with the second reactor (20), said first adjusting system (50) being configured to transfer the flow of solids from the first reactor (10) operating at lower pressure to the second reactor (20) operating at higher pressure, while ensuring said pressure differential between said reactors (10) and (20) by means of accumulation of solid material in the first connection pipe (30) for a certain accumulation height as a function of the pressure of the first and second reactor (10, 20);

- a mechanical second pressure adjusting system (60) associated to the second connection pipe (40) that connects the tail of the second reactor (21) with the first reactor (10) and, said second pressure adjusting system (60) being configured to transfer the flow of solids from the second reactor (20) to the first reactor (10) by pressure dissipation in the second connection pipe (40) The plant according to the invention differs from the known technique in the possibility of operating the reactors with a pressure differential, managing the flow of solids with mechanical valves and a charge of solids between a low-pressure reactor and a high- pressure reactor.

Advantageously, the plant according to the present invention allows to use the charge of solids generated by means of mechanical valves in the connection between air reactor and fuel reactor, in the case of chemical looping combustion (CLC), in order to maintain the latter at a significantly higher pressure than the first one (1.5-20 bar, preferably 2-10 bar). Compared to the previous solutions, the configuration of the present invention allows to increase the yield of the process, recovering the heat deriving from the condensation of water from the pressurized gases exiting the fuel reactor, while reducing the compression consumptions of air (compared to the conventional pressurized solution) and of CO2 (compared to the conventional unpressurized solution). In other words, the plant according to the present invention therefore has significant advantages in terms of efficiency.

Advantageously, the plant, according to the present invention, allows, in the cases of indirect gasification, chemical looping gasification and chemical looping reforming, to maintain a significantly higher pressure in the gasification/reforming reactor, in a manner similar to what is described above, by means of a charge of solids and a mechanical valve system. In particular, pressurization allows to produce hydrogen-rich synthesis gases already at high pressure (1.5-20 bar, preferably 2-10 bar), reducing the need (and the costs) for a subsequent compression, generally required for the most common applications (synthesis of chemicals, fuels or hydrogen).

LIST OF FIGURES

Figure 1 : schematic representation of the plant according to an embodiment of the present invention;

Figure 2: represents the pressure profile for the plant according to the embodiment of Figure 1;

Figure 3: schematic representation of the plant according to an embodiment of the present invention when applied to a process of the chemical looping combustion (CLC) type;

Figure 4: block diagram representation of a plant according to the embodiment of Figure 3;

Figure 5: schematic representation of the plant according to the present invention when applied to an indirect gasification process;

Figure 6: represents a graph of comparison between the thermal recovery in a pressurized CLC plant according to the present invention and a CLC plant at atmospheric pressure.

DETAILED DESCRIPTION

The plant according to the present invention allows to use a charge of solids to allow the flow of solids from the first reactor, operating at lower pressure, and the second reactor, operating at higher pressure. Preferably, the plant 100 may for example be used to carry out chemical looping processes such as for example chemical looping combustion (CLC) or chemical looping reforming (CLR) or Chemical Looping Gasification (CLG) and indirect gasification processes.

For the purposes of the present invention, the definition comprising does not exclude the presence of further components other than those indicated after the aforementioned definition, such as for example further reactors along the pipe 40 that could use the heat transported by the solids for gasification or reforming.

For the purposes of the present invention, with the definition constituted by, consisting of the presence of further components in addition to those listed after these definitions is excluded.

For the purposes of the present invention, “pressure differential or gradient” means the absolute value of the pressure difference between the first lower pressure reactor and the second higher pressure reactor and vice versa.

For the purposes of the present invention by solid communication between two reactors and/or units it is meant that the interposed pipes allow the transfer of solid without however also excluding the transfer of fluids in terms of gases and liquids.

In other words, the pressure difference between the first lower pressure reactor and the second higher pressure reactor will take a negative value. Conversely, the pressure difference between the second higher pressure reactor and the first lower pressure reactor will take a positive value.

The plant 100 according to the present invention comprises the first fluid bed reactor 10 and the second fluid bed reactor 20 in solid material communication, between which solid material circulates to carry out reactions according to the process carried out therein.

In accordance with the present invention the first reactor 10 is maintained at an operating pressure lower than the operating pressure of the second reactor 20 thereby defining a pressure differential between the first reactor 10 and the second reactor 20. Preferably, the pressure for each reactor 10, 20 is a function of the pressure of the gases introduced into the respective reactors 10, 20.

Specifically, the pressure inside each reactor 10, 20 is adjusted by the introduction of gas into the relative reactor and possibly by the relative extraction of such gas. It is worth noting that the introduction of gas into one of the reactors 10, 20 increases the pressure inside it and the extraction of gas decreases it.

In accordance with a preferred embodiment, the plant 100 comprises one or more circuits in fluid communication with the first and second reactor 10, 20 for the introduction of gas into the relative reactor 10, 20. In this way, such circuits in fluid communication with a gas source adjust the pressure inside the reactors 10, 20.

The first 10 and second reactor 20 extend respectively between the respective tails 11, 21 and heads 12, 22 along an extension direction X-X. Specifically, the reactors 10, 20 can have any shape having, along the extension direction X-X, a height which is measured as the distance between the respective tails 11, 21 and heads 12, 22. This height can be measured as the difference between the dimensions that can be associated to the heads and to the tails in a reference system from the extension direction, preferably perpendicular to the ground, and a reference point, for example, the tail 11 of the first reactor. Compared to a reference system thus defined, for example, the tail of the first reactor 11 has a dimension equal to zero and the first reactor head 12 has a dimension equal to the height of the first reactor Hi, thus defining the height of the first reactor. The same reasoning can be applied for any point of the plant to define the relative height, such as, for example, the accumulation height H_acci in a pipe and the accumulation height of material in the second reactor H_acc2. It is therefore worth noting that the reactors 10, 20 have relative heights as a function of the heights of the tails 11, 21 and of the heads 12, 22 with respect to the extension direction X-X and relative dimensions with respect to the reference system. Thus, each point of the plant 100 has a relative reference height along the extension direction X- X and with respect to a reference point.

In accordance with a preferred embodiment, the first reactor 10 and the second reactor 20 can be staggered along the extension direction X-X, i.e. having different heights and dimensions referred to the tails with respect to the aforementioned reference system. It is also worth noting that the first reactor 10 and the second reactor 20 can also be spaced along a spacing direction Y-Y perpendicular to the extension direction by a distance D. Alternatively, the two reactors could develop along the same vertical axis or even be partially or entirely integrated one inside the other.

In accordance with the present invention, each reactor 10, 20 operates at a certain pressure as a function of the process carried out in the plant 100.

The circulation of the solid material between the reactors is carried out by means of pipes 30, 40 that place the first reactor 10 and the second reactor 20 in solid-fluid (gas) connection. The pipes 30, 40, according to the present invention, comprise first connection pipes 30 configured to transfer solid material from the first reactor 10 to the second reactor 20 and second connection pipes 40 configured to transfer solid material from the second reactor 20 to the first reactor 10.

Preferably, the first connection pipes 30 connect the first reactor head 12 with the second reactor 22 and the second connection pipes 40 connect the second reactor tail 21 with the first reactor 12.

More preferably, the first connection pipe 30 connects the first reactor head 12 with the second reactor 22 between the relative head and tail, while the second connection pipe 40 connects the second reactor tail 21 with the first reactor between the relative head and tail. In this way, the first reactor 10 and the second reactor 20 are in solid material connection so that the solid material can pass between the first reactor 10 and the second reactor 20.

In accordance with the embodiment of the present invention, the plant 100 comprises a first adjusting system 50 for adjusting the flow of solids, of the mechanical type. Said first adjusting system 50 is associated to the first connection pipes 30 so as to control the circulation of the solid material from the first low pressure reactor 10 to the second higher pressure reactor 20. Specifically, the first adjusting system 50 is configured to adjust the transfer of the flow of solids exiting the first reactor 10 and supplied to the second reactor 20 so as to ensure the pressure differential between the first and second reactor. In detail, the first adjusting system 50 is configured to generate an accumulation of solids such as to allow the transfer of the flow of solids between reactors having a pressure differential while maintaining the actual pressure difference between the first 10 and the second reactor 20.

Preferably, the first adjusting system 50 as a function of the pressure inside the second reactor 20 for the process to be carried out is configured to adjust the accumulation of solid material in the first connection pipe 30 for the transfer of solid from the first 10 to the second reactor 20. Such accumulation of material allows a flow of solids to be maintained between the lower pressure reactor 10 and the upper pressure reactor 20 and avoids or minimizes the flow of the gases between the upper pressure reactor 20 and the lower pressure reactor 10. The pressure differential that can be maintained between the two reactors is a function of the relative accumulation height and of the type of material used in the accumulation. Specifically, according to the known formulas of physics, the absolute pressure P is given by the sum between relative pressure given by the weight of the material and the pressure at a reference point lying on the material P0:

P ~Po+ p g H

Where: p is the average density of the material defining the solid bed, comprising the solid material and the gas between the solid particles; g is the acceleration of gravity

H is the height difference along a reference direction perpendicular to the ground between the reference points where the pressures P0 and P are considered

In the case of accumulation of material along a connection pipe 30, the pressure at the lower end of the pipe from where the accumulation is formed is substantially given by the sum of the pressure exerted by the weight of the accumulation and of the pressure lying on the upper end of the accumulation.

In light of what has been exposed, with reference to Figure 1 where they have been indicated with the pressures at the reference points of the plant Pl, P2, P3, P3a, P4, P5, P6, P7, P8, taking into account the relative heights with respect to the reference system previously considered:

- the pressure Pl at the tail 11 of the first reactor 10 is approximately equal to the sum between the pressure P2 at the head 12 of the first reactor 10 plus the weight exerted by the material present in the first reactor 10 between the head 12 and the tail 11 :

Pl ~Pi+ pi g Hi where: pl is the average density of the solid and of the gas in the first reactor, comprising the solid material and the gases; g is the acceleration of gravity

Hi is the difference in height between the reference points of Pi and P2 (see Figure 1)

- for the pressures P2, P3 and P3a, what has been above substantially applies. It is worth noting that for the purposes of the present invention the material present between these reference points and the height differences mean that the pressures are substantially the same. For the sake of completeness, the respective formulas are reported

P3 ~P2+ p2 g H2

P3a ~P3+ pl g H₃

Where:

P2 is the average density of the solid material exiting the reactor;

H2 is the difference in height between the reference points P2, and P3, respectively and H3 is the difference between the reference points P3 and P3a.

- the pressures P4, P5, P6, have substantially the same values in accordance with the present invention, specifically the pressure P4 is at least greater than or equal to the pressure P5. However, compared to the pressure P2, P3, P3a, the pressure in P4 is increased due to the presence of accumulation of solid material inside the connection pipe 30. Specifically: P4 >P5 ~ P6 ~P2 + P3 g Haccl where: p3 is the average density of the solid material exiting the first reactor and of the interstitial gas accumulated in the connection pipe 30 g is the acceleration of gravity

Hacci is the difference in height between the reference points of P4, P5, P6 and P2, P3, P3a (see Figure 1)

- the pressure P7 at the tail 21 to the second reactor is given by the accumulation of material present in the second reactor in addition to the pressure that is maintained in the second reactor itself:

P7 ~ P6 + p4 g H_acc2 where:

P4 is the average density of the solid material and of the gas in the second reactor g is the acceleration of gravity

H_acc2 is the difference in height between the reference points of P6 and P7 (see Figure 1)

- for the pressures P8 and Pl, according to what has been said above, if material is present, it will comply with the aforementioned laws as a function of the relative heights of the of the points of the accumulation of material.

For the purposes of the present invention we have approximated that the densities of the solid material and of the gas passing in the plant object of the present invention have similar values; therefore, in the following the density of the solid and of the gas will be generically indicated “p”.

It is therefore worth noting that in accordance with the present invention the accumulations of solid material guarantee to maintain the pressure inside the second reactor 20 greater than the pressure of the first reactor 10. The design of the height of the reactors 10, 20 allows the desired higher pressure to be maintained in the second reactor 20.

The graph of the pressures of Figure 2 highlights the trend of the pressures as a function of the position inside the plant. It is worth noting that this graph can be adapted to the different processes that can be carried out inside the plant depending on the material and the heights.

The first adjusting system 50 allows to maintain a flow of solids from the first reactor 10 at ambient pressure to the second reactor 20 at higher pressure, while avoiding or minimizing the flow of gas in the opposite direction, which would cause an undesired mixing between the flowing gas streams in the two reactors.

The pressure in the second reactor is preferably included in a range included between 1.5 and 20 bar, preferably between 2 and 10 bar.

In accordance with an embodiment, the system comprises a first circuit in fluid communication with the first reactor 10 and a second circuit in fluid communication with the second reactor 20. These first and second circuit are configured to adjust the pressure respectively inside the first reactor 10 and the second reactor 20 by introducing and extracting gas in the respective reactors 10, 20. It is worth noting that the first and second circuit draw the gas to be introduced from a gas source.

These circuits are provided with pressure adjusting systems so that the first reactor 10 is maintained at ambient pressure, while the second reactor 20 operates at a pressure included between 1.5 and 20 bar, preferably between 2 and 10 bar.

In accordance with the preferred embodiment of the present invention, the first reactor head 12 is arranged at a raised position relative to the second reactor head 22 along the extension direction X-X, according to the reference system, thereby defining an accumulation height Ha_cci along the first connection pipe 30. This accumulation height Hacci is given by the difference in the dimension according to the reference system between the extremes of the accumulation inside the connection pipe 30. Preferably, the first connection pipe 30 as a function of the first pressure adjusting system 50 is configured to accumulate solid material for a certain accumulation height as a function of which the pressure inside the second reactor 20 can be maintained in accordance with what has been said above, avoiding or minimizing the flow of gas from the high pressure reactor 20 to the low pressure reactor 10, along the same connection pipe 30.

According to a particularly preferred solution, it is possible to inject gas or vapor at the pressurized side of the valves (i.e. at P5 and P7) in order to completely avoid the flow of gas from the reactor under pressure to the atmospheric one, using for example seals of the loop seal type.

Preferably, the first adjusting system 50 comprises a first adjusting device 70 interposed between the first reactor 10 and the second reactor 20 along the first connection pipe 30. Said first adjusting device 70 is configured to act on the first connection pipe 30 to adjust the accumulation of material. It is worth noting that the accumulation of material is a function of the accumulation height, the density of accumulated material and the upstream pressure acting on the free end of the accumulation and allows to adjust the transfer of material from the first reactor to the second reactor while maintaining the pressure differential between the reactors. In other words, the first adjusting system 50 allows to generate an accumulation of material in the first connection pipe 30 such as to provide the first adjusting device 70 with a pressure at least equal to the pressure of the second reactor 20 so as to favour the entry of the material into the second reactor. Specifically, the first adjusting device 70 is configured to control the supply of the solid from the first reactor 10 to the second reactor 20 by adjusting the solid accumulation height along the first connection pipe 30. In this way, said first adjusting device 70 therefore allows to generate a pressure thanks to the accumulation equal to the pressure of the second reactor 20 so as to allow the transfer of the material from a starting reactor having a reduced pressure compared to the pressure of the target reactor. The pressure generated by the first adjusting device 70 in order to transfer the solid material is thus given by p g H_acci where p is the average density of solid material and interstitial gas accumulated in the pipe, g is the acceleration of gravity and H_acci is the height of the accumulation (see Figure 1), i.e. the difference between the ends of the accumulation along the first connection pipe 30.

The plant comprises a second mechanical adjusting system 60 associated to the second connection pipe 40 and configured to transfer the flow of solids from the second reactor 20 to the first reactor 10 by pressure dissipation in the second connection pipe 40. Specifically, the second adjusting system 60 comprises a second adjusting device 80 interposed between the reactors 10 and 20 along the second connection pipe 40. Specifically, the second adjusting device 80 is configured to act on the second connection pipe 40 so as to supply the solid material from the second reactor 20 to the first reactor 10. The second adjusting device 60 cooperates with the first adjusting device 50 so as to control the supply of solid material by restoring by dissipation the pressure of the first reactor 10 in the second connection pipe 40. Specifically, the adjusting devices are configured to adjust the accumulation height both along the first connection pipe 30 and inside the second reactor 20 so that the expulsion of solid material from the second reactor 20 is compensated by a replenishment inside the second reactor with a consequent supply of material to the first connection pipe 30 in order to maintain the accumulation height and the relative pressure difference between the two reactors.

In accordance with a preferred embodiment, the adjusting devices 70 and 80 comprise mechanical valves preferably of the same type used in the fluid bed catalytic cracking plants, the so-called catalyst slide valves. In the fluid bed catalytic cracking plants such valves are configured to operate at high temperatures and pressures as well as with highly abrasive fluids, and adjust the circulation of the catalyst between the reactor and the regenerator.

As anticipated above, the plant according to the present invention may comprise non-mechanical retaining systems (for example of the L-valve or J-valve type or preferably of the loop seal type) that assist the functions of the first and/or second adjusting system. In the case it assists the first adjusting system; these systems are preferably arranged between the adjusting devices comparable to the first adjusting system and the access to the second reactor and/or to a reactive unit, in the case of the second reactor comprising more than one reactive unit like in the embodiment described below. Such non-mechanical retaining systems reduce the mixing of the gases until it is avoided by inserting other gases such as water vapour in said systems.

In accordance with a preferred embodiment, the plant 100 comprises a cyclone 90 associated to the first reactor head 12, preferably alongside the first reactor head 12. This cyclone 90 is in solid communication with the first connection pipe 30. Specifically, the cyclone 90 is interposed along the first connection pipe 30 between the reactors. This cyclone 90 is configured to separate the solid to be supplied to the second reactor 20 from the gaseous components present during the execution of the reaction in the first reactor. It is worth noting that any solid-gas separation system may be used in place of the aforementioned cyclone 90.

In accordance with a preferred embodiment, the plant comprises one or more heat exchangers configured to use the residual heat extracted from the first and second reactor 10, 20 to produce vapor, to heat other process flows and/or to heat the flows themselves entering the reactors 10, 20. Preferably, the plant 100 may comprise a first heat exchanger 210 associated to the first reactor 10 and configured to use the residual heat of the gas flow separated at the head to the first reactor 12 from the solid material before the supply thereof to the second reactor 20. Such first exchanger 210, may be used for producing of vapor, heating water, vapor or other process fluids. Preferably, the first heat exchanger 210 may comprise sub heat exchanger units defining a relative group of exchangers.

It should be noted that the plant 100 may comprise a second heat exchanger 220 always associated to the first reactor 10 and arranged downstream of the first heat exchanger 210 to heat the air to be supplied to the first reactor 10. In this way, the first heat exchanger 210 is configured to use part of the residual heat of the gas flow coming from the first reactor 10 to produce vapor or to heat water, vapor or other process fluids and the second heat exchanger 220 to heat the flow entering the first reactor. Specifically, the second reactor 220 is configured to use the residual heat of the gas flow coming from the first reactor 10 and reduced by the percentage of heat transferred in the exchanger 210. Also in this case the second heat exchanger may comprise sub heat exchanger units defining a relative group of exchangers.

In accordance with a preferred embodiment, the plant 100 comprises a third heat exchanger 230 associated to the second reactor 20 and configured to use the heat generated in the second reactor 20 to produce vapor or to heat other fluids. Specifically, the third heat exchanger 230 provides for flowing a flow of water in relative pipes in contact by conduction and/or convection with the walls or with other surfaces inside and/or outside the second reactor 20. Also in this case the third heat exchanger may comprise sub units of heat exchangers defining a relative group of exchangers.

In accordance with a preferred embodiment, the plant may comprise a fourth heat exchanger 240 acting on the gas flow exiting from the second reactor 20 so as to recover further heat, preferably by using the condensation heat of the water contained in the flow exiting from the second reactor 20. Also in this case the fourth heat exchanger may comprise sub units of heat exchangers defining a relative group of exchangers.

In accordance with a further preferred embodiment, the plant may comprise a fifth heat exchanger 250 associated to the first reactor 10 and configured to use the heat of the first reactor 10 to produce vapor or to heat other fluids. Specifically, the fifth heat exchanger 250 provides for flowing a water flow in relative pipes in contact by conduction and/or convection with the walls or with other surfaces inside and/or outside the first reactor 10. Also in this case the fifth heat exchanger may comprise sub units of heat exchangers defining a relative group of exchangers.

It should be noted that all the possible combinations of the heat exchangers can be considered.

Thanks to the present invention, as illustrated in the graph of Figure 6, the condensation of water starts at higher temperatures for the second pressurized reactor (dashed line), compared to the case of a second atmospheric reactor (solid line). This allows to have a greater heat recovery at the same heat recovery temperature or to produce vapor or to heat other fluids to a higher temperature.

In accordance with a preferred embodiment for the production of vapor, shown in Figure 4, the plant may comprise a compressor 260 configured to compress the vapor produced by means of the heat exchangers. This vapor can therefore be used at a pressure and temperature higher than that of production in the heat exchanger 240, thanks to a “heat pump” system.

In accordance with preferred embodiments of the present invention, the plant 100 further comprises a plurality of connection channels configured to convey gases, fluids and material to and from the reactors 10, 20. Specifically, the plant comprises a channel for the entry of hot air to the first reactor and any discharge channels to adjust overpressures and/or dispose of excess material. Preferably, the plant further comprises further connection channels associated to the second reactor for introducing materials and extracting the products.

As already anticipated, the second reactor can comprise two or more reactive units arranged in series in fluid and solid communication with each other by means of relative third pipes comprising a relative adjusting system that can use the accumulation of solids in the pipe and/or the dissipation previously described for the first and second adjusting system of each reactor. The aforesaid two or more reactive units are arranged so that the first of said two or more units receives the solid material and gas coming from the first reactor and the last of said two or more units receives the solid material from the penultimate of said at least two units. In detail, the first of said at least two units receives the solid material coming from the first reactor head, while the second of said units receives the solid material from the first unit in accordance with the reaction carried out in the first unit and so on as a function of the number of reactive units until the last of said two or more reactive units supplies the reacted solid material to the first reactor tail. In this way, the pressure between the individual reactive units is managed so that, overall, each unit of said two or more reactive units can have a pressure equal to, greater than or less than the reactive units of the series, on the provision that: a) the first of said two or more reactive units receives the solid material from the first reactor via said first pipe comprising the aforementioned first adjusting system taking advantage of the accumulation of solids in said pipe, and b) the first reactor receives the solid material from the last of said two or more reactive units through the second pipe by means of the aforementioned second mechanical adjusting system by pressure dissipation.

A further object of the present invention is a process known in the literature as “Chemical Looping Combustion” carried out in the plant 100 described above, the performance of which is illustrated in Figure 3. The process comprises the step of oxidizing a solid material, preferably a metallic material such as for example Fe, Ni, Cu, Mn or perovskites or compounds comprising within the crystal lattice manganates and titanates of calcium and magnesium, in the first reactor 10 in a temperature range included between 800°C and 950°C by air fluidization. The solid material acts as a carrier for the oxygen and is called “oxygen carrier” (OC). The reactor 10, for the type of process, is called oxidation reactor or “Air Reactor”. The solid material is therefore oxidized in the oxidation reactor 10 and supplied to the second reactor 20, said reduction reactor or “Fuel Reactor”, in which the solid material is reduced by transferring one or more oxygen atoms to the fuel.

In accordance with a preferred embodiment, the process comprises a step of separation between air and solids by means of the cyclone 90, or other separation devices, prior to supplying the solid material from the first reactor 10 to the second reactor 20.

The process comprises a step of accumulating the oxidized solid material in the first connection pipe 30 by means of the first pressure adjusting system 50 to adjust the transfer of solid material from the first to the second reactor while maintaining the pressure difference between the first reactor 10 and the second reactor 20 included between 0.5-19 bar, preferably 1-9 bar. Specifically, the accumulation step provides for accumulating solid material along the first connection pipe 30 for an accumulation height h_acc, in Figure 3, such that the desired pressure in the second reactor 20 is maintained at the base of this column. Specifically, the first adjusting device 70 allows to control the flow of solids from the first 10 to the second reactor 20 so as to maintain the required charge of solids in the first connection pipe 30 so as to maintain the pressure difference between the reactors and at the same time transfer the solid material from the first reactor to the second one. The pressure difference between the reactors is also maintained thanks to the second adjusting system 60 with the relative second adjusting device 80 that dissipates the overpressure of the stream of solids coming from the second reactor 20 towards the reactor 10, maintained at a significantly lower pressure (0.5-19 bar, preferably 1-9 bar) than that of the second reactor.

The process comprises oxidizing under pressure in the second reactor 20 the fuel introduced in the second reactor 20 with the oxygen coming from the first reactor 10 conveyed by the oxygen carrier to produce carbon dioxide and water, while the now reduced oxygen carrier oxidizes again in the reactor 10. Preferably, the combustion takes place in a temperature range included between 800°C and 950°C. It is worth noting that the oxidant for the combustion is provided by reducing to the initial state the oxygen carrier coming from the first reactor 10 thanks to the reaction temperature and preferably to the oxygen-poor atmosphere.

The process comprises a step of recycling the reduced oxygen carrier during combustion from the second reactor 20 to the first reactor 10 by means of the second adjusting system 60 and relative second connection pipe 40. Specifically, the second pressure adjusting system 60 by means of the second adjusting device 80 cooperating with the first adjusting device 50 adjusts the expulsion of the reduced solid material from the second reactor to the first reactor while maintaining a pressure in the second reactor 20 greater than the pressure of the first reactor 10. Preferably, the second adjusting device 80 in addition to one or more mechanical valves may comprise static pressure heatsinks configured to maintain the reactor 10 at a significantly lower pressure (pressure gradient 0.5-19 bar, preferably 1-9 bar) than that of the second reactor.

It is worth noting that the process according to the present invention is overall exothermic and allows the recovery of heat (for example, as described above, producing vapor) from the oxygen-poor air exiting the first reactor 10, from the gas flow exiting the second reactor and directly from the reactors 10 and 20 through exchange surfaces arranged on the walls or inside the reactors themselves. In accordance with a preferred embodiment, the process comprises the step of i) recovering heat from the hot gas flows exiting the first reactor 10 in at least a heat exchanger 210 and possibly in a second heat exchanger 220 to heat air fed to the first reactor 10. Specifically, a flow of hot air exiting the first reactor 10 may be conveyed in sequence into the first heat exchanger 210 to produce vapor and into the second heat exchanger 220 to heat the air entering the first reactor 10. The process may also comprise a step ii) of recovering heat inside the first reactor 10 or the second reactor 20. Specifically, this step uses the previously described third heat exchanger 230 and possibly the fifth heat exchanger 250. Finally, the process comprises a step iii) of recovering heat with partial condensation of the pressurized water exiting the second reactor 20 which is thus separated from CO2, preferably inside or downstream of the fourth heat exchanger 240. It is worth noting that the process allows a separation of the water by condensation and the possibility of supplying to permanent storage the residual flow at high concentration of CO2, after compression and possible purification, consisting of the separation of any other gases (for example CO, H2, CH4, N2) not converted in the reactor 20.

The process, in case the heat recovery occurs through production of vapor, may comprise a possible step iv) of compressing the water vapor produced in at least one of the previous stages i-iii), and preferably that produced in stage iii), by means of condensation of pressurized water produced in the second reactor 20. In this way, it remains possible to produce vapor at a lower pressure (and temperature), maintaining a high heat recovery efficiency and to introduce a vapor compressor to increase its pressure, consequently increasing the temperature at which the heat is made available to the user, with a system equivalent to a heat pump.

A further object of the present invention is an indirect gasification process carried out in the plant 100 for gas-solid reactions previously described (Figure 5).

The process comprises a step of subjecting to gasification and pyrolysis in order to produce synthesis gases comprising H2, CO, CO2, H2O and CH4 and pressurized residual carbon-containing solids in the second fluid bed reactor 20 a solid fuel, preferably carbon and/or biomass, at high temperature (700-850°C) in the presence of vapor fed to the second reactor together with a higher temperature flow of solids from the first reactor 10. Specifically, the solid fuel fed to the second fluid bed reactor makes such reactor a gasifier. Vapor and a flow of solids from the first reactor 10 at a high temperature (800-950°C) are fed to this second reactor which favour the aforementioned reactions to generate the synthesis gases. The residual solids produced in the second reactor 20 comprise ashes, unconverted carbon, and solid material used as a heat carrier.

The process comprises a step of recycling in the first reactor 10 the residual solids coming from the second fluid bed reactor 20 by means of the second adjusting system 70 and relative second connection pipe 40. Like in the process described above, being carried out in the plant 100 described previously, the recycling step is controlled by the adjusting devices 60, 50 which allow the supply of material from the second reactor 20 to the first reactor while maintaining a pressure in the second reactor 20 greater than the pressure in the first reactor 10. The stable circulation of solids from the low pressure reactor 10 to the higher pressure reactor 20, without gas flow in the opposite direction is made possible by the material accumulation heights in the second reactor 20 h_gasif and in the first connection pipe 30 hacc, as indicated in Figure 5. In addition, the second pressure adjusting system 60 allows the pressure exerted on the solid from the second reactor to the first reactor to be lowered by dissipation to reach the pressure of the first reactor.

This recycling step comprises a step of oxidizing in the first reactor 10 the flow of residual solids comprising carbon by feeding air to the first reactor 10. The first reactor 10 acts as a combustor of the residual solid material possibly supplemented with further additional fuel.

In accordance with the present process the first reactor 10 is maintained at ambient pressure, while the second reactor 20 operates at a pressure included between 1.5 and 20 bar, preferably between 2 and 10 bar.

The pressure difference between the reactors is obtained as previously described. Specifically, the process comprises a step of accumulating solids in the first connection pipe 30. This accumulation is achieved by means of the first adjusting system of 50 to transfer the solid while maintaining the pressure difference between the first reactor 10 and the second reactor 20 included between 0.5 and 19 bar, preferably between 1 and 9 bar.

The steps described above for the “chemical looping combustion” process of separating hot gas from solid material near the first reactor head by means of a separation device such as a cyclone can also be applied for the present process. Furthermore, the process according to the present invention may comprise the steps i) -iii) relative to the heat recovery and possible step iv). It is worth noting that for the processes described in accordance with the present invention, the general operating principles relative to the plant 100 adapted as described for the processes themselves apply. The reference points along the plant for the evaluation of the pressures are also maintained.

A further object of the present invention is any “chemical looping” process such as for example the Chemical Looping Reforming or the Chemical Looping Gasification, comprising two or more reactors in which it is advantageous to use a charge of oxidized oxygen solid carrier to maintain the reactor in which the oxygen carrier is reduced, typically called fuel reactor, to a significantly higher pressure (0.5-19 bar, preferably 1-9 bar) than the pressure of the reactor, typically called air reactor, in which the carrier is oxidized.

It is worth noting that mechanical valves are known in the state of the art which, thanks to a combination of construction materials and surface coatings, are used and can be used in applications where the operating temperatures of the plant and of the valve itself are even higher than 850°C.

EXAMPLE

Below are reported the results of calculations of process simulations of a CLC system according to the invention, for the production of 100 MW of heat usable as low pressure vapor. The following table reports the properties of the main flows of the CLC system according to the invention, with the air reactor at atmospheric pressure and pressure of the fuel reactor equal to 6 bar. The solid used as oxygen carrier is perovskite C14 [6], while the fuel is natural gas (LHV 47.5 MJ/kg). In AR, an excess of air equal to 60% with respect to the stoichiometric value is assumed. The air is introduced into the AR after preheating to 100°C with the exhausted gases from the AR, after heat recovery for vapor production. The two reactors are maintained at the same temperature (950°C) by cooling the FR.

Flow T [°C] P [bar] ^kg/s ^ Composition

Ambient air 25 1.1 52.76 21 %_voi O₂, 79%_VO| N₂

The expected technical and economic performance of the CLC system according to the invention are summarized in the following table and compared with the following alternative technologies based on known techniques: - Conventional boiler without CO2 capture

- Conventional boiler with post-combustion CO2 capture with monoethanolamine (MEA)-based solvent, with a CO2 capture efficiency of 90%;

- Atmospheric CLC, in which both reactors are operated at ambient pressure. The economic performances of the system are reported in terms of total investment

(CAPEX) for the capture section (additional MEA system or complete CLC system), ‘Levelized Cost of Heat (LCOH)’ and ‘Cost of Carbon Avoided (CCA)’. The LCOH value is the price to be assigned to the production of heat that at the end of the useful life of the plant pays off exactly all the investment and operating costs incurred, taking into account write-downs and financial charges. The CCA value is the additional cost in order to avoid the emission of a given amount of CO2, compared to a reference case, here assumed to be the conventional boiler.

Convention Conventional al capture Atmospheri CLC According boiler boiler c CLC to the invention

In all cases, natural gas is oxidized (assumed composition 90% CH4, 7% C2H6, 1% N2, 2% CO2), in such an amount as to obtain a useful thermal power of 100 MW.

In the CLC cases, the combustion air is preheated up to 100°C, and an excess of 60% air is assumed with respect to the stoichiometric value, in order to guarantee a sufficient concentration of 02 in the air reactor for the complete oxidation of the oxygen carrier, similarly to what is reported in [6], Downstream of heat recovery and cooling, the CO2 flow exiting the fuel reactor is compressed up to 80 bars with a 4-stage intercooled compressor and then up to 150 bar with a pump. In the case of the CLC system according to the invention, the stream exiting the FR is cooled up to 115°C, recovering part of the condensation heat of the water.

The circulating amount of solids and the stock of solids present in the individual reactors are calculated from literature values [6,7], in order to completely convert the fuel. An advanced perovskite with composition CaMnO.9MgO.103-5, referred to as ‘C14’ in recent literature [Mayer], is considered as an ‘oxygen carrier’. The dimensions of the reactors are estimated starting from conventional fluidization speeds and consequent surfaces/heights required to manage the previously calculated amounts of solids.

For the case with post-combustion capture with MEA, a thermal consumption of 3.7 MJ/kgCO2, provided by an auxiliary boiler, and an additional electrical consumption of 122 MJ/tonCO2 are considered.

From an energy point of view, the system with post-combustion capture with MEA is the least efficient (71% thermal yield) and therefore requires a greater fuel input, because of the additional heat required for solvent regeneration. The CLC cases achieve a better thermal efficiency and therefore reduced gas consumptions, combined with a higher CO2 removal (potentially 100%, compared to 90% of the MEA case). The maximum thermal efficiency (104%) is obtained in the pressurized case, in which the condensation of the water present in the combustion fumes allows a greater heat recovery. Both in the case of MEA and in the case of CLC at ambient pressure, the CO2 produced is at low pressure, with significant electrical compression consumptions (3.02 and 2.51 MW, respectively). The pressurization of the fuel reactor allows, against a minimal increase in consumption of the other auxiliaries, an advantage over CO2 compression, with a reduction in the consumptions equal to about 40%.

In the economic analysis, the investment costs of the CLC systems have been estimated starting from literature values for similar systems (fluid bed boilers and atmospheric CLC systems), appropriately scaled [8,9], Lor the MEA case, the cost estimated by the literature is equal to 95.16 M€ for the capture system with a capacity of 82 tCO2/h [10] and a cost of 9.5 M€ for the auxiliary boiler with a nominal power of 150 MWth.

A price of 150 €/MWh for electricity and 0.37 €/Sm3 (36 €/MWh) for natural gas are considered. The discounted heat production cost (LCOH) is reported in the table, comprising the allocation of the discounted investment costs (10% discount rate with a useful life of 25 years) and operating costs (variable costs + 3% of the investment per year for O&M, considering an operation for 85% of the hours per year), considering a carbon tax of 0 and 100 €/tCO2. The CO2 transport and storage costs are similar for all the cases, equal to 30 €/tonCO2.

With a carbon tax of 100 €/t, the CLC plant according to the invention is significantly more competitive than in the other cases. The LCOH heat cost of 51.6 €/MWh is 20% lower than the conventional boiler without capture, 31% lower than the case with post-combustion with MEA and 10% lower than the atmospheric CLC plant, thanks to the higher thermal efficiency. The economic competitiveness of the CLC according to the invention increases in absolute terms with the increase in the cost of the fuel used. For example, with a fuel cost of 100 /MWh, the LCOH heat cost of the CLC according to the invention would be equal to 113.1 €/MWh against 125.0 €/MWh in the atmospheric case. With a natural gas cost equal to about 130 €/MWh, the pressurised solution is already convenient compared to the conventional boiler even without incentives thanks to the greater efficiency.

The economic competitiveness is also appreciable from the point of view of the cost of CO2 avoided, i.e. the breakeven carbon tax compared to the conventional solution without CO2 capture, which is 51.4 €/t, against 78.9 €/t of the atmospheric CLC case and 164 €/t for the case with post-combustion capture.

Bibliography;

[1] Juan Adanez, Alberto Abad, Francisco Garcia-Labiano, Pilar Gayan, Luis F. de Diego, Progress in Chemical-Looping Combustion and Reforming technologies, Progress in Energy and Combustion Science, Volume 38, Issue 2, 2012, Pages 215-282, ISSN 0360- 1285, https://doi.Org/10.1016/j.pecs.2011.09.001.

[2] Siddig Abuelgasim, Wenju Wang, Atif Abdalazeez, A brief review for chemical looping combustion as a promising CO2 capture technology: Fundamentals and progress, Science of The Total Environment, Volume 764, 2021, 142892, ISSN 0048-9697, https : //doi . org/ 10.1016/j . scitotenv.2020.142892.

[3] Thunman, H., Seemann, M., Berdugo Vilches, T., Marie, J., Pallares, D., Strom, H., et al. (2018). Advanced Biofuel Production via Gasification - Lessons Learned from 200 Man-Years of Research Activity with Chalmers’ Research Gasifier and the GoBiGas Demonstration Plant. Energy Sci. Eng. 6, 6-34. Doi: 10.1002/ese3.188

[4] Robert T. Symonds, Robin W. Hughes, Dennis Y. Lu, Philippe Navarri, Omid Ashrafi, Systems analysis of pressurized chemical looping combustion for SAGD applications, International Journal of Greenhouse Gas Control, Volume 73, 2018, Pages 111-123, ISSN 1750-5836, https://doi.Org/10.1016/j.ijggc.2018.03.008.

[5] Arturo Cabello, Robin W. Hughes, Robert T. Symonds, Scott Champagne, Dennis Y. Lu, Ehsan Mostafavi, Nader Mahinpey, Economic analysis of pressurized chemical looping combustion for steam assisted gravity drainage applications, International Journal of Greenhouse Gas Control, Volume 90, 2019, 102786, ISSN 1750- 5836, https://doi.Org/10.1016/i.iiggc.20I9.102786.

[6] Mayer K. et al., “Chemical Looping Combustion Using Two Different Perovskite Based Oxygen Carriers: A Pilot Study”, Energy Technology, 2018

[7] Mayer K. et al., “The different demands of oxygen carriers on the reactor plant - Results of oxygen carrier testing in a 120 kWth pilot plant”, Applied Energy, 2015

[8] Koornneef J. et al., “Development of fluidized bed combustion — An overview of trends, performance and cost”, Progress in Energy and Combustion Science, 2007 [9] Cabello A. et al., “Economic analysis of pressurized chemical looping combustion for steam assisted gravity drainage applications”, International Journal of Greenhouse Gas Control, 2019

[10] J. Gale Understanding the Cost of Retrofitting CO2 capture in an Integrated Oil Refinery, IEAGHG Technical Review, 2017

Claims

1. Solid-gas reaction plant (100), comprising:

- a first fluid bed reactor (10) and a second fluid bed reactor (20) operating at different pressures, where the pressure of the second reactor (20) is higher than the pressure of the first reactor (10), thereby defining a pressure differential between reactors by adjusting the pressure of the gases fed into the reactors (10, 20), said first reactor (10) and said second reactor (20) extending from a respective tail (11, 21) to a respective head (12, 22) along an extension direction (X-X);

- a first connection pipe (30) and second connection pipe (40) configured to place the reactors (10, 20) in solid material communication, said first connection pipe (30) being configured to transfer solid material from the first reactor (10) to the second reactor (20), said second connection pipe (30) being configured to transfer solid material from the second reactor (20) to the first reactor (10), characterized in that the plant comprises

- a mechanical first adjusting system (50) associated to the first connection pipe (30) which connects the first reactor head (12) with the second reactor (20), said mechanical first adjusting system (50) being configured to transfer the flow of solids from the first reactor (10) operating at lower pressure to the second reactor (20) operating at higher pressure, while ensuring said pressure differential between said reactors (10) and (20) by means of solid accumulation in the first connection pipe (30) for a certain accumulation height as a function of the pressure of the first and second reactor (10, 20),

- a mechanical second pressure adjusting system (60) associated to the second connection pipe (40) that connects the tail of the second reactor (21) with the first reactor (10) and, said second pressure adjusting system (60) being configured to transfer the flow of solids from the second reactor (20) to the first reactor (10) by pressure dissipation in the pipe (40).

2. Plant (100) according to claim 1, wherein:

- the first reactor head (12) is arranged at a raised position relative to the second reactor head (22) along the extension direction (X-X), thereby defining an accumulation height along the pipe (30) connecting both reactors (10, 20); - the first adjusting system (50) comprises:

- a first adjusting device (70) interposed between the reactors and acting on the first pipe (30), said first adjusting device (50) being configured to control the solid supply from the first reactor (10) to the second reactor (20) by adjusting the solid accumulation height along the pipe (30) to generate a pressure at the inlet of the second reactor (20) which is equal to or higher than the pressure of the second reactor, while maintaining the pressure differential between both reactors (10) and (20) during the transfer of solid material;

- the second adjusting system (60) comprises

- a second adjusting device (80) acting on the respective second pipe (40), said second adjusting device (60) being configured to control the solid supply from the second reactor (20) to the first reactor (10) restoring the pressure of the first reactor (10) by dissipation in the pipe (40).

3. Plant (100) according to claim 2, wherein the first and second adjusting devices (70, 80) comprise mechanical valves, preferably of the catalyst slide valve type.

4. Plant according to claim 3, wherein the first adjusting system (50) and/or the second adjusting system (60) comprises/comprise non-mechanical retaining systems, by insertion of other gases in said systems.

5. Plant (100) according to any one of claims 1-4, comprising a cyclone (90) associated to the first reactor head (12), preferably alongside the first reactor head (12), and being in solid communication with the first pipe (30).

6. Plant (100) according to any one of claims 1-5, further comprising a first circuit in fluid communication with the first reactor (10) and a second circuit in fluid communication with the second reactor (20), said first and second circuit being configured to adjust the pressure respectively inside the first reactor (10) and the second reactor (20) by introducing and extracting gas in the respective reactors (10, 20) so that the pressure of the first reactor (10) is maintained at a pressure close to ambient pressure and the pressure of the second reactor (20) is maintained in a range included between 1.5 and 20 bar, preferably between 2 and 10 bar.

7. Chemical looping combustion process implemented in plant (100) according to any one of claims 1-6, comprising the steps of:

- oxidizing a solid material in the first reactor (10) in a temperature range included between 800 °C and 950 °C by fluidization with air or other oxygen-containing gas mixture;

- transferring the oxidized solid material between said first and second reactors through the connection pipe (30) (while maintaining a pressure differential between said reactors included between 1 and 9 bar) by means of the first pressure adjusting system (50) associated to said pipe (30);

- pressure oxidizing the fuel filled in the second reactor (20) and at least partially reducing the oxidized solid material in the reactor (20) itself producing carbon dioxide and water vapor;

- transferring the at least partially reduced oxidized solid material from the second reactor (20) to the first reactor (10) through the respective pipe (40) and depressurizing the same by means of the second pressure adjusting system (60) associated to said pipe (40).

8. Process according to claim 7, comprising a step of:

- separating hot air and oxidized solid material by means of a cyclone (90) before supplying the solid material from the first reactor (10) to the second reactor (20).

9. Process according to any one of claims 7-9, comprising the steps of: i) recovering heat from hot gas flows exiting the first reactor (10) in at least a first heat exchanger (210) and possibly in at least a second heat exchanger (220) to heat air fed to said first reactor (10), ii) recovering heat inside the first (10) and/or second reactor (20); iii) recovering heat from the flow exiting the second reactor (20) by means of partial condensation of water vapor in a fourth heat exchanger (240) acting on gases exiting the second reactor; iv) in case the heat recovery occurs through production of vapor, possibly compressing the water vapor produced in at least one of the previous stages i-iii), and preferably that produced in stage iii), by means of condensation of pressurized water produced in the second reactor (20).

10. Indirect gasification process implemented in plant (100) for gas-solid reactions according to any one of claims 1-6, comprising the steps of:

- subjecting to gasification and pyrolysis, in order to produce synthesis gas comprising H2, CO, CO2, H2O and CH4 and pressurized residual solids, in the second reactor (20), a solid fuel, preferably carbon and/or biomass, at high temperature (700-850 °C) in the presence of vapor fed to the second reactor together with a flow of solids from the first reactor (10);

- transferring to the first reactor (10) the residual solids from the second reactor (20) with depressurization thereof by means of the second pressure adjusting system (60) associated to the connection pipe (40);

- oxidizing, in the first reactor (10), the flow of residual solids comprising carbon by feeding air to the first reactor (10) or combustor;

- transferring the solid material from the first reactor (10) to the second reactor by means of the first pressure adjusting system (50) associated to the connection pipe (30) between the first (10) and the second reactor (20), in order to maintain a pressure difference between the first (10) and the second reactor (20) included between 0.5 and 19 bar, preferably between 1 and 9 bar.

11. Solid-gas reaction plant, comprising:

- a first fluid bed reactor and a second fluid bed reactor operating at different pressures, where the pressure of the second reactor is higher than the pressure of the first reactor, said first reactor and said second reactor extending from a respective tail to a respective head along an extension direction (X-X);

- first and second connection pipes configured to place the reactors in solid communication, said first connection pipe being configured to transfer solid material from the first reactor to the second reactor and comprising in turn a first mechanical adjusting system configured to transfer the flow of solids from the first reactor operating at lower pressure to the second reactor operating at higher pressure, while ensuring said pressure differential between said first and second reactors by means of solid accumulation in the first pipe itself, and said second connection pipe being configured to transfer solid material from the second to the first reactor and comprising a second mechanical pressure adjusting system configured to transfer the flow of solids from the second reactor to the first reactor by pressure dissipation in the pipe, characterized in that: the second reactor comprises two or more reactive units arranged in series and in fluid and solid communication to each other via third pipes comprising respective mechanical adjusting systems, by means of solid accumulation and/or dissipation in said third pipes, in order to ensure equal, higher or lower pressure among said two or more reactive units; the first of said two or more reactive units receiving the solid material from the first reactor head, while the second of said two or more units receives the solid material from the first unit, and so on according to the number of reactive units, until the last of said two or more reactive units supplies the solid material to the first reactor tail and said third pipes on the proviso that a) the first of said two or more reactive units receives the solid material from the first reactor via said first pipe by means of the aforementioned first adjusting system taking advantage of the accumulation of solids in said first pipe, and b) the first reactor receives the solid material from the last of said two or more reactive units through the second pipe by means of the aforementioned second mechanical adjusting system by pressure dissipation in the second pipe itself.