Patent Description:
It is well known that one of the most limiting problems in the efficient use of energy converted from primary renewable energy sources, such as wind, tides, currents or solar, is the time lag between the availability of these sources and the demand for energy. This problem forces, during peaks in demand, the use of peak energy generators with fast start-up times, typically consisting of power generating sets with spark ignition engines or gas turbines (which of course generate carbon dioxide in the exhaust which is dispersed into the atmosphere); in addition, this problem forces the use of energy storage systems during peaks of excess energy availability compared to demand, such as hydroelectric pumping plants (only possible in countries with suitable orographic characteristics) or electric accumulators, which are however a very expensive solution.

The development of sustainable industry in both developed and less developed or emerging countries requires technologies and technical solutions to limit and reverse climate change, which is mainly induced by the increasing concentration of greenhouse gases (including carbon dioxide) in the atmosphere. This objective requires effective methods to reduce carbon dioxide emissions into the environment, including through capture, i.e. its separation from the atmosphere, and its containment, also known as sequestration or storage, either permanently in the gaseous phase (e.g. in depleted hydrocarbon deposits) or in the solid phase (e.g. in carbonate-based construction materials). Of course, carbon dioxide also has industrial uses for which it is specifically produced, which could be limited through a more integrated cycle of its use in order to limit its cumulative release into the atmosphere.

The capture of carbon dioxide from the atmospheric mixture, in which it appears in any case in a low molar percentage (<NUM> ppm), requires high energy and large exchange surfaces, for example with molecular sieves or semipermeable membranes. Being able to intercept and separate this combustion product at the source (i.e. extracting it from a mixture in which it prevails as a concentration) has the advantage of limiting the energy spent to separate the unit of carbon dioxide produced by combustion.

At the same time, a more flexible use of renewable energies can be achieved by transforming the electrical energy they produce, for the part that cannot be immediately absorbed and used through the distribution network, into chemical energy, by breaking the bond between hydrogen and oxygen in the water molecule through the well-known process of electrolysis, which produces the two substances, separated, in a gaseous state. Of the two gases, however, the true energy 'carrier' is hydrogen, which is not found free in nature, while oxygen is a valuable gas, as it is obtained pure by this process, only if it is intended for uses in which it is required as such, particularly in the iron and steel industry for the production of steel. Otherwise, e.g. to feed combustion, it is already available in the atmosphere and for such a use must normally be diluted anyway to limit combustion temperatures. In other words, to power vehicles propelled by hydrogen-powered fuel cells, it is neither necessary nor convenient (except in manned submarine vehicles) to store pure oxygen, as these fuel cells can directly exploit the oxygen in the air in an open-cycle system (i.e. a system that exchanges gas with the external environment).

Extensive use of electrolysis, as a method of efficient hydrogen production with no or limited carbon dioxide production, is closely linked to the use of oxygen, as well as the sending of hydrogen into pipelines or hydrogen storage tanks and cylinders.

Patent <CIT> describes a power conversion system for storing energy through a hydrogenation reaction of an aromatic hydrocarbon, wherein an internal combustion engine of the compression ignition type is used as the heat source in the hydrogenation reaction. This conversion system uses only the heat released by the internal combustion engine to promote the hydrogenation reaction of the aromatic hydrocarbon and thereby absorb the hydrogen produced by an electrolytic process; in addition, in this conversion system the internal combustion engine emits carbon dioxide into the atmosphere.

Patent application <CIT> describes an energy conversion device for converting electrical energy into chemical energy. The device includes: an electrolysis device that can be connected to an electrical grid and is designed to separate water into hydrogen and oxygen using electrical energy; a fuel synthesis device that is connected to the electrolysis device to be supplied with hydrogen as a reactant to synthesize a fuel from hydrogen and carbon dioxide; and an internal combustion engine that is connected to the electrolysis device to receive oxygen generated in the electrolysis device.

Patent application <CIT> describes an internal combustion engine that is powered by hydrogen-oxygen and uses an inert gas, such as argon, as a working fluid to increase engine efficiency, eliminate pollution and facilitate the operation of a closed-loop energy system.

Patent application <CIT> describes a plant for generating methanol and supplying heat and/or electricity, and a water electrolysis plant that can be powered by electricity and water and is designed to produce hydrogen and oxygen gas.

Patent application <CIT> describes the production of hydrogen and/or electricity.

Patent application <CIT> describes an energy storage system comprising: a primary electrical energy source, an electrolysis unit for the production of hydrogen and oxygen, a hydrocarbon reactor for the production of hydrocarbons and water, and a secondary electrical energy generator for the production of secondary electrical energy and carbon monoxide.

Patent application <CIT> describes a power generation system comprising a rotating electric generator with permanent magnets, an electrolyser that produces oxygen and hydrogen from electricity and water, and a thermal engine that drives the electric generator.

Patent application <CIT> describes a method and apparatus for generating oxygen by decomposing water via electrolysis into oxygen and hydrogen fractions; the hydrogen may be combusted with air, and the resulting water may be recycled for subsequent electrolysis.

The aim of the present invention is to propose a combined system for the production of hydrogen, oxygen and segregated and sequestered carbon dioxide equipped with a closed-cycle thermal engine, which combined system allows maximising energy efficiency (i.e. minimising energy consumption for hydrogen production) while finding an immediate useful use for the oxygen and avoiding the release of carbon dioxide into the atmosphere.

According to the present invention, a combined system for the production of hydrogen, oxygen and segregated and sequestered carbon dioxide is provided which is equipped with a closed-cycle thermal engine according to the appended claims.

The claims illustrate a preferred embodiment of the present invention forming part of this description.

The present invention will now be described with reference to the accompanying drawings, which illustrate certain non-limiting examples of implementation, wherein:.

In <FIG>, a combined system for the production of hydrogen, oxygen and segregated and sequestered carbon dioxide is shown as a whole.

Combined system <NUM> carries out an electrolysis process in an electrolysis plant <NUM> of known type. The electrolysis plant <NUM> receives as an input water (indicated by the chemical formula H<NUM>O) and electrical energy (from two different sources and indicated by E<NUM> and E<NUM>) and outputs hydrogen and oxygen in gaseous form (indicated by the respective chemical formulae H<NUM> and O<NUM>). Non-exhaustive examples of high-pressure water electrolysers are described in patent documents <CIT>, <CIT>, and <CIT>, in which the electrical energy supplied is converted into the chemical and pneumatic energy of the two gases, either both or only one of which, most conveniently hydrogen, under pressure.

According to a preferred form of implementation, the electrolysis plant <NUM> is connected to an electricity generation plant <NUM> using renewable sources of energy (such as wind or solar) and is also connected to an electricity generation plant <NUM> equipped with a closed cycle thermal engine <NUM> (i.e. not exchanging gas with the external environment), preferably but not exclusively of the self-ignition or compression ignition or Diesel cycle type (i.e. it is a reciprocating piston engine). Alternatively, the thermal engine <NUM> could be a turbine engine. According to a different form of implementation not illustrated, the electrical energy could be exclusively provided by the electricity generation plant <NUM>, i.e. without the contribution of renewable sources (and in this case the thermal engine <NUM> of the electricity generation plant <NUM> could operate by an only partially closed cycle); alternatively the electricity generation plant <NUM> using renewable sources could be replaced by the normal electrical network.

In the plant <NUM> for the generation of electric energy, there is an electrical generator <NUM> which is brought into rotation by the thermal engine <NUM>; in turn, the thermal engine <NUM> is fed by a liquid or gaseous fuel <NUM> consisting of hydrocarbons or derivatives (and indicated with the generic chemical formula CnHm), for example natural gas or biofuels obtained from biological processes or waste treatment. As mentioned above, the thermal engine <NUM> operates in closed cycle and therefore does not exchange gases with the external environment (i.e. it does not intake fresh air from the atmosphere and does not exhaust carbon dioxide and other gases into the atmosphere). In particular, in the thermal engine <NUM> the exhaust gases generated by the thermal engine <NUM> are recycled at the intake after having extracted the aqueous fraction produced by combustion (i.e. after having dehumidified the exhaust gases through condensation of water vapour, at room temperature), so that the carbon dioxide contained in the exhaust gases is sequestered immediately downstream of the thermal engine <NUM> due to its high concentration.

The concentration of carbon dioxide in the exhaust gases of the thermal engine <NUM> is maintained at a high level both as a result of the initial gas charge and as a consequence of the total recirculation of the dehumidified exhaust gases at the intake, after their comburent power has been restored by injecting the oxygen produced during the electrolysis, which is added to the residual oxygen in the combustion products, the latter being dependent on the excess of comburent in relation to the stoichiometric mass of combustion.

The production of hydrogen by the combined system <NUM> is ultimately a way of storing and transporting energy remotely, for its distribution and use both for power generation, as a fuel, and as a process gas. Hydrogen is produced by means of a flexible process, in which the share of energy sent to the electrolysis plant <NUM> can be modulated according to the availability of the primary renewable source (solar or wind) of the electricity generation plant <NUM> and of the fuel source (the fuel <NUM>) of the electricity generation plant <NUM>, while on average respecting the proportions of the products of the respective reactions, which in turn also depend on the atomic carbon/hydrogen ratio in the fuel <NUM> feeding the thermal engine <NUM> and on the efficiencies of the electricity generation plants <NUM> and <NUM>.

The chemical reaction occurring in the electrolysis plant <NUM> (balanced stoichiometrically with respect to the chemical reaction occurring in the thermal engine <NUM>) is as follows:.

(2n + m/<NUM>)H<NUM>O + E<NUM> + E<NUM> -> (2n + m/<NUM>)H<NUM> + (n + m/<NUM>)O<NUM>.

The chemical reaction occurring in the thermal engine <NUM> (balanced stoichiometrically with respect to the chemical reaction occurring in the electrolysis plant <NUM>) is as follows:.

CnHm + (n + m/<NUM>)O<NUM> -> (m/<NUM>)H<NUM>O + (n)CO<NUM> + E<NUM>.

The electrolysis plant <NUM> is of a known type and preferably (but not necessarily) is of the polymer membrane type (so-called "PEM"), which has the advantage of allowing the production of hydrogen at high pressure starting from water pumped at the pressure of the electrolysis process, therefore with much less energy expenditure than that possibly required for the compression of permeated hydrogen from atmospheric pressure to the high pressure of storage and transport, for example in cylinders, typically between <NUM> and <NUM> bar.

The thermal engine <NUM> is preferably of positive displacement and compression ignition type; however, it is also possible to use positive displacement and spark ignition engines or gas turbines, the latter especially for high power.

The plant <NUM> for generating electric energy provides for an extraction of the carbon dioxide produced by the combustion in the thermal engine <NUM> and also provides for the recovery of the incondensable fraction of the exhaust gases of the thermal engine <NUM>, consisting practically only of oxygen exceeding the stoichiometric quantity. The closed cycle of the thermal engine <NUM> can preferably be realized according to one of the solutions known from the state of the art of closed cycle engines (for example as described in patent application <CIT>).

Closed-cycle thermal engines, in particular using compression-ignition, have already been studied, developed and applied for the propulsion of manned and unmanned submarines, in so-called "air independent" long endurance missions underwater. Among the various systems that allow the recirculation of exhaust gases to the intake, after separation of excess combustion gases produced by combustion, the technology of particular interest for the present application is the one that provides for partial compression of the exhaust gases and liquefaction of carbon dioxide (as described in patent application <CIT>). For particularly long missions, such as those required for autonomous underwater vehicles (AUVs), an improvement of the system described in patent application <CIT> has also been devised, whereby liquid oxygen and/or liquefied natural gas are used as reactants, in which the liquefaction of carbon dioxide takes place at a lower pressure and temperature, and thus with less energy consumption for compression, due to the cooling achieved by the evaporation of the liquefied oxygen and/or natural gas during engine operation (as described in patent <CIT>).

As regards the closed-cycle operation of a thermal engine, it is known that the reduction in thermodynamic efficiency resulting from the prevalence of a triatomic gas (with a lower ratio γ = cp/cv, i.e. with a lower adiabatic expansion coefficient or adiabatic index commonly denoted by the letter γ) can be compensated for by increasing the volumetric compression ratio of the reciprocating engine or by adding a monoatomic gas (typically Argon) in suitable proportions to the initial charge of the inert gases recycled in the engine; while the first solution can be applied independently of the process for managing the carbon dioxide produced by the thermal engine, the second solution is only conveniently applicable if the thermal engine also includes a process for liquefying the carbon dioxide, in order to avoid the need for further storage of the monoatomic gas and its costly dispersal in the environment.

According to what is illustrated in <FIG> and <FIG> (which differ only for the different cooling method used in the carbon dioxide liquefaction process), the thermal engine <NUM> includes an intake duct <NUM> through which the cycle gas containing inert gas is fed to the combustion chambers (for example, obtained in cylinders inside which pistons slide), said cycle gas consisting of carbon dioxide, preferably pure or mixed with inert monoatomic or diatomic gases which do not participate in the combustion but limit the temperature to values compatible with the materials constituting the combustion chamber, and a sufficient quantity of oxygen (i.e. comburent) to feed the combustion reaction. The thermal engine <NUM> includes an exhaust duct <NUM> through which the combustion chambers eject the exhaust gases generated by the combustion. The thermal engine <NUM> includes a recirculation duct <NUM> connecting the exhaust duct <NUM> to the intake duct <NUM> so that the exhaust gases are recirculated through the intake duct <NUM> back to the combustion chambers.

A heat exchanger <NUM> cooled by a refrigerant (e.g. air or water at substantially ambient temperature) is arranged along the exhaust duct <NUM> to cool the exhaust gases coming from the combustion chambers to a temperature low enough to condense and thus separate the water vapour produced by combustion.

Preferably, the exhaust gases are cooled while maintained at atmospheric pressure or at a pressure value above atmospheric, which latter allows subsequent turbocharging of the engine without the need for turbochargers when the cooled exhaust gases are recirculated in the intake duct <NUM> through the recirculation duct <NUM>.

A separator <NUM> is arranged along the exhaust duct <NUM> and downstream of the heat exchanger <NUM>, which separates from the exhaust gases the water that has been produced by combustion and has been condensed in the heat exchanger <NUM>; in particular, the exhaust duct <NUM> terminates in the separator <NUM>. The water separated by the separator <NUM> is fed to a tank <NUM> from which the water is taken to feed the electrolysis plant.

Obviously, in order to maintain a constant working pressure in the exhaust duct <NUM>, in the recirculation duct <NUM>, and in the intake duct <NUM>, it is necessary to continuously or cyclically remove a part of the exhaust gases, which therefore are not fully recirculated; consequently, along the exhaust duct <NUM> there is also arranged an extraction duct <NUM>, which takes out (pushes out) from the exhaust duct <NUM> a part of the exhaust gases, removing them from the recirculation duct <NUM>. In particular, the extraction duct <NUM> originates from the separator <NUM> which forms the end of the exhaust duct <NUM>. Preferably, the recirculation duct <NUM> originates from the separator <NUM> and the extraction duct <NUM> originates from the separator <NUM> upstream of the recirculation duct <NUM>. A dehumidifier filter <NUM> is arranged along the extraction duct <NUM>, which retains the residual water and steam entrained in the exhaust gases and sends them to the separator <NUM>, and a dehydrating filter <NUM> is also arranged downstream of the dehumidifier filter <NUM>.

A compressor <NUM> is arranged along the extraction duct <NUM> and downstream of the filters <NUM> and <NUM>, which is driven directly by the thermal engine <NUM> or is driven by a dedicated electric motor and is designed to compress the excess exhaust gas (i.e. flowing along the extraction duct <NUM>). The flow rate of the compressor <NUM> is regulated by means of a pressure sensor (i.e. a pressure controller) located in the condensed water separator <NUM> in such a way that the pressure in the separator <NUM> is always equal to a desired value within assigned tolerances with respect to the reference value. In particular, it is provided a control unit configured to regulate the flow of the compressor <NUM> according to the pressure measured in the separator <NUM> and in such a way that the pressure measured in the separator <NUM> is always equal to a desired value.

A heat exchanger <NUM> (present in the embodiment illustrated in <FIG> and not present in the embodiment illustrated in <FIG>) may also be arranged along the extraction duct <NUM> and downstream of the compressor <NUM>, which cools (desuperheats) the dehumidified and compressed exhaust gases of the compressor <NUM>; that is, the heat exchanger <NUM> removes from the dehumidified and compressed exhaust gases the heat absorbed during compression in the compressor <NUM>.

A non-return valve <NUM> (i.e. a one-way valve <NUM>) is arranged along the extraction duct <NUM> and downstream of the heat exchanger <NUM> (or downstream of the compressor <NUM> if the heat exchanger <NUM> is not present), which only allows an exhaust gas flow away from the compressor <NUM>.

Along the extraction duct <NUM> and downstream of the non-return valve <NUM> there is a first heat exchanger <NUM> (which uses as a refrigerant for cooling either air in the actuation form illustrated in <FIG> or water in the actuation form illustrated in <FIG>) which condenses the carbon dioxide; in fact, the refrigerant (air or water) is made to enter the heat exchanger <NUM> in counterflow with respect to the compressed exhaust gases and above all at a temperature lower than the dew point of the exhaust gases; in this context it is important to note that the exhaust gases are composed of a mixture of carbon dioxide and other non-condensable gases (residual oxygen and any inert gases other than carbon dioxide, for example preferably a monoatomic gas, Argon, introduced specifically and as explained above to maintain the γ ratio close to the γ ratio of atmospheric air).

The efficiency of the carbon dioxide liquefaction system depends on the temperature of the refrigerant used in the heat exchanger. In cold climates, the outside air temperature below <NUM> may allow carbon dioxide to be liquefied at relatively low pressure, thereby resulting in a modest compression power of the excess exhaust gases (i.e., a modest power input to compressor <NUM>), relative to the power generated by the thermal engine <NUM>. Furthermore, if methane stored in a liquid state (LNG) is used as fuel for the thermal engine <NUM>, the carbon dioxide liquefaction system can be even more efficiently achieved by using the evaporation of the liquid methane to condense the carbon dioxide at a very low temperature, between -<NUM> and -<NUM>, as also described in patent <CIT>.

Finally, the extraction duct <NUM> ends in a first separator <NUM>, i.e. the separator <NUM> is arranged along the extraction duct <NUM> downstream of the heat exchanger <NUM>; in the separator <NUM> the liquefied carbon dioxide is separated from the non-condensable gases present in the exhaust gas and then the liquefied (and therefore segregated) carbon dioxide is sent to an external tank (not illustrated). The non-condensable gases resulting from the separation of the liquefied carbon dioxide from the exhaust gas are sent to a recovery duct <NUM>.

The electricity generation plant <NUM> comprises a feed duct <NUM> which feeds almost pure oxygen and terminates (i.e. flows into) a mixing device <NUM> in which the recirculation duct <NUM> also terminates (i.e. flows into). The mixing device <NUM> mixes together the nearly pure oxygen fed by the feed duct <NUM> with the oxygen-poor exhaust gases fed by the recirculation duct <NUM> (and originating from the separator <NUM>) to obtain a new mixture having a quantity of oxygen sufficient for combustion in the combustion chambers of the thermal engine <NUM>, said quantity of oxygen also possibly differing from the fraction of oxygen present in the atmospheric air, in relation to the total mass of the engine feed gas.

The feed duct <NUM> starts from a storage vessel <NUM> in which pure oxygen produced by electrolysis is stored for reoxygenation of the recirculated exhaust gases from the recirculation duct <NUM>. A mixing device <NUM> is arranged along the feed duct <NUM>, in which the pure oxygen from storage vessel <NUM> is mixed with the oxygen-rich recirculated noncondensable gases fed from the recovery duct <NUM> and from the separator <NUM>; therefore, downstream of the mixing device <NUM> flows nearly pure oxygen (i.e. a mixture in which oxygen is by far the predominant component). Two isolation (shutoff) valves <NUM> and <NUM> are arranged along the feed duct <NUM> and upstream and downstream of the mixing device <NUM>; they are opened during operation to allow respectively the inflow of pure oxygen to the mixing device <NUM> and the inflow of almost pure oxygen to the mixing device <NUM>; i.e. the two isolation valves <NUM> and <NUM> are "ON/OFF" valves which can assume only two positions (fully open or fully closed).

A regulating valve <NUM> is arranged along the feed duct <NUM> and upstream of the mixing device <NUM>, which is controlled by a control unit <NUM> according to the signal provided by an oxygen concentration measuring sensor <NUM> coupled to the mixing device <NUM>.

Along the duct <NUM> for recovering oxygen and any other non-condensable inert gases from the separator <NUM>, an isolation valve <NUM> (which can only assume a fully open position or a fully closed position) and a regulating valve <NUM> are arranged in series. The regulating valve <NUM> is used (controlled) to regulate the pressure in the separator <NUM> of the liquefied carbon dioxide; this pressure, in fact, increases as residual oxygen and non-condensable inert gases accumulate in the separator <NUM> and must be regulated to remain equal to a desired and predetermined value. Keeping the pressure in the separator <NUM> relatively low makes it possible to limit the compression ratio and the power required by the compressor <NUM> to bring the mixture extracted from the separator <NUM> to a pressure sufficiently high to cause the liquefaction of the carbon dioxide.

The recovery of oxygen and any other inert, non-condensable gases (for example Argon or Nitrogen) along the recovery duct <NUM> allows the pressure in the liquefied carbon dioxide separator <NUM> to be reduced as the more oxygen and non-condensable gases are allowed to flow along the recovery duct <NUM>, due to the pressure difference between the carbon dioxide separator <NUM> and the mixing device <NUM>, the more the pressure in the separator <NUM> is reduced, tending the pressure in the separator <NUM> to approach the carbon dioxide saturation pressure. Accordingly, the opening of the regulating valve <NUM> (i.e., the flow of oxygen through the regulating valve <NUM>) is controlled by means of a pressure sensor <NUM> (i.e., a pressure controller) located in the liquefied carbon dioxide separator <NUM> such that the pressure in the separator <NUM> is always equal to a desired value within assigned tolerances from the reference value. In particular, the pressure sensor <NUM> is designed to measure a pressure in the first separator (<NUM>), the regulating valve <NUM> is located along the recovery duct <NUM>, and it is provided a control unit configured to drive the regulating valve <NUM> according to the pressure measured by the pressure sensor <NUM> and in such a way that the pressure measured by the pressure sensor <NUM> is always equal to a desired value. The regulating valve <NUM> may be continuously position-regulated (analogous to the regulating valve <NUM> and thus may be arranged in a series of intermediate positions between a fully open position and a fully closed position) or may be two-positioned (thus may assume only a fully open position or a fully closed position, i.e., "ON/OFF"), depending on the desired accuracy of pressure regulation in the separator <NUM>.

In accordance with <FIG>, the combined system <NUM> comprises a pump <NUM> which supplies pressurised water to the electrolysis plant <NUM> and is driven directly by the thermal engine <NUM>; alternatively, the pump <NUM> is driven by a dedicated electric motor.

The forms of implementation described here can be combined without going beyond the scope of protection of this innovation.

The combined system <NUM> described above has many advantages.

In the combined system <NUM> described above, the oxygen required for combustion is obtained from electrolysis as a high-value by-product of hydrogen production and is immediately used in the combined system <NUM>, which therefore has a maximised and integrated utilisation. Otherwise, the oxygen would have to be channelled for use either in cylinders or in pipelines (unless more complex integration of other processes requiring pure oxygen is possible).

Thus, the combined system <NUM> described above renders more flexible the use of renewable primary energies by producing hydrogen through an electrolysis process, known from the technique, effectively combined with a power generation system consisting of the thermal engine <NUM> which fully utilises the oxygen produced by the electrolysis and produces in an already sequestered form the carbon dioxide resulting from the combustion of the fuel <NUM> used.

Considering the stoichiometric ratios in the chemical reactions involved and the efficiencies of the internal combustion thermal engine <NUM>, the power generation, the electrolyser and the pumps and compressors forming part of the combined system <NUM> it can be stated that more than one fifth of the electrical energy required for the electrolytic process can be supplied by the thermal engine <NUM> using the oxygen produced, while all the carbon dioxide produced by combustion is condensed and made available in a pure state for stable and permanent sequestration or for industrial uses.

It is important to note that hydrogen can be produced by reforming with water vapour from methane or other hydrocarbons, producing carbon dioxide which, however, is diluted with other gases in the reaction products, both intermediate and final; this reforming process is still the most economical of all possible processes.

Claim 1:
Combined system (<NUM>) for the production of hydrogen, oxygen and segregated and sequestered carbon dioxide; the combined system (<NUM>) comprises:
an electrolysis plant (<NUM>) configured to receive water and electrical energy (E<NUM>, E<NUM>) as an input and supply hydrogen and oxygen in gaseous form as an output;
an electricity generation plant (<NUM>) which is equipped with an electrical generator (<NUM>) that generates at least part of the electrical energy (E<NUM>) used by the electrolysis plant (<NUM>) and a thermal engine (<NUM>) that drives the electrical generator (<NUM>) and comprises: an intake duct (<NUM>); an exhaust duct (<NUM>); and a recirculation duct (<NUM>) connecting the exhaust duct (<NUM>) to the intake duct (<NUM>) to recirculate the exhaust gases produced by the combustion;
a feed duct (<NUM>) that sends the oxygen generated by the electrolysis plant (<NUM>) to the intake duct (<NUM>) of the thermal engine (<NUM>); and
an extraction duct (<NUM>) configured to draw a portion of the exhaust gases from the exhaust duct (<NUM>) of the thermal engine (<NUM>) by withdrawing them from the recirculation duct (<NUM>);
the combined system (<NUM>) is characterised by the fact that it comprises:
a first heat exchanger (<NUM>) which is arranged along the extraction duct (<NUM>) and is configured to cool the exhaust gases to a temperature below a dew point of the gaseous mixture in order to condense the carbon dioxide;
a first separator (<NUM>) which is arranged along the extraction duct (<NUM>) downstream of the first heat exchanger (<NUM>) and is configured to separate liquefied carbon dioxide from the non-condensable gases in the exhaust gases; and
a recovery duct (<NUM>) connecting the first separator (<NUM>) to the feed duct (<NUM>) in order to introduce into the feed duct (<NUM>) the non-condensable gases present in the exhaust gas and extracted from the first separator (<NUM>).