WO2020065496A1

WO2020065496A1 - High efficiency thermodynamic cycle in power applications

Info

Publication number: WO2020065496A1
Application number: PCT/IB2019/058045
Authority: WO
Inventors: Salvatore DE RINALDIS
Original assignee: Saipem S.P.A.
Priority date: 2018-09-24
Filing date: 2019-09-23
Publication date: 2020-04-02

Abstract

The present invention relates to the use of a carbon dioxide cycle in power applications.

Description

"High efficiency thermodynamic cycle in power

applications"

The present invention relates to the use of a high efficiency thermodynamic cycle using a working fluid, such as for example carbon dioxide, in power applications .

Technical field of the invention

A "basic" (supercritical) Brayton cycle comprises a compressor, a heater, an expander and a cooler and may be optimized by installing heat recovery units and other equipment (post-heater, inter-cooling, pre-compression, re-compression) , giving rise to various alternative layouts with increased thermodynamic efficiency and specific work as indicated in literature.

Among the various CO₂ Brayton cycles in literature, once the following are set:

the maximum cycle temperature, upstream of the turbine, and the minimum cycle temperature, in the cooling exchanger,

the compression ratio and the minimum cycle pressure, or start compression pressure, upstream of the compressor,

the best cycle from the thermodynamic efficiency and specific work/extractable power viewpoint is the (supercritical) CO2 Brayton cycle with mass split downstream of the low temperature recovery unit, re compression and two recovery units with two different heat levels, as depicted in figure 1.

Indeed, the specific heat of the cold current is about 2 or 3 times greater with respect to the hot current in the recovery units; therefore, the CO2 current is divided into two flows so as to compensate for this difference of specific heat in the low heat level recovery unit, avoiding pinch points, decreasing the work required for the compression and moreover recovering the heat generated.

In general, this decreases the irreversibility of the circuit with subsequent increase of the thermodynamic efficiency.

The main problem of the above-described cycle is represented by the fact that the fumes are released at a high temperature (about 250-300°C or also >350°C) after the exchange with the CO2.

Moreover, in calculating the thermodynamic performance, consideration should also be made of the energy consumption due to the fuel consumption for heating the air from ambient temperature to about 800-1.000°C (temperature of the combustible gases) before the heat exchange with the CO2.

The re-compressed cycle also needs a further compressor, complicating the system in terms of plant engineering; indeed as described above, since the specific heat of the cold current is about 2 to 3 times greater than the specific heat of the hot current, to have an efficient heat exchange in the recovery units at the low temperatures, there is a need to split the flow of mass and recirculate it in the re-compressor, heating it through a compression work .

Such a compression work is carried ou starting from a higher temperature with respect to the primary compression even if executed on a fraction of the flow .

The prior art document JP2011256818 (see figure 2) describes a cycle comprising a thermal system, a power cycle which utilizes the CO2 in supercritical conditions, and a cooling system.

A flow (5) exiting a first exchanger (]HX1) in the power cycle is sent to a turbine ( T1 ) for expansion, then it is sent to a first recovery unit (RHX1) and to a second recovery unit (RHX2); the flow exiting therefrom is sent to a cooler and then to a compressor (Cl ) . From the compressor, the flow is divided into two flows, one of which is sent to the recovery unit RHX2 and the other to exchanger ] HX3.

A flow exits from the recovery unit RHX2, which is divided into two flows, one of which goes to the recovery unit RHX1 and the other to the exchanger ]HX2, together with current 9 exiting ] HX3.

Once the portion sent to the exchanger (]HX2) has exited therefrom, it is sent to a second turbine (T2) for an expansion step, from which a flow (12) exits which is sent to the second recovery unit

(RHX2 ) .

The system is configured so as to adapt the cycle to the amount of thermal energy available, in particular managing, through mass splitting, the flow to be sent to the exchangers or to the recovery units .

Such a system penalizes the overall efficiency of the system because it causes a change in the thermodynamic states of the cycle, moving away from the condition of optimal design.

Summary of the invention

The inventors of the present patent application have surprisingly found that it is possible to utilize a high efficiency supercritical or transcritical carbon dioxide cycle to generate electricity .

Object of the invention

A first object of the invention is thus represented by a process for generating electricity by means of a power cycle utilizing a fluid which may be represented by supercritical or subcritical CO2.

A second object describes a system for implementing such a process.

Brief description of the drawings

Figure 1 shows the diagram of a cycle according to the prior art;

figure 2 shows the diagram of a cycle according to the prior art document JP2011256818 ;

figures 3A and 3B show the diagram of a cycle according to two alternative embodiments of the present invention;

figure 4 shows the diagram of a cycle according to an alternative embodiment of the present invention, applicable to several embodiments, in which there is a tank for regulating the CO2 flow; figure 5 shows the diagram of a cycle according to an embodiment of the present invention, in which there is a radiation heat exchange section;

figure 6 shows the diagram of a cycle according to an embodiment of the present invention, in which there is a gas turbine (GT) ;

figure 7 shows the diagram of an embodiment of the present invention in which the radiation heating section operates on fumes produced by a gas turbine (GT) ;

figure 8 shows the diagram of a cycle according to an embodiment of the present invention in which there are one or more heat recovery steps.

Detailed description of the invention

According to a first object, the present invention describes a process for generating electricity by means of a power cycle using a flow of a working fluid.

For the purposes of the present invention, such a working fluid is represented by a gas or a gas mixture selected from the group comprising: hydrocarbons, nitrogen, CO2, and refrigerant fluids.

In a particularly preferred embodiment of the present invention, the fluid is represented by CO2 in the supercritical or subcritical state.

"Fluid flow" refers to a flow of a working fluid in the power cycle circuit.

For the purposes of the present invention, reference is made to a "CO2 flow", meaning the CO2 as working fluid in the power cycle circuit.

For the purposes of the present invention, such a flow is >100 kg/s, preferably >120 kg/s and even more preferably >150 kg/s and also up to about 500 kg/s and higher.

"Supercritical" state means a CO2 flow in the supercritical temperature and pressure conditions.

In particular, such conditions correspond to higher temperature and pressure conditions than those of the critical state.

In more detail, the CO2 is in critical condition under the following conditions:

pressure temperature

73.83 bar a 31.06°C

Under the supercritical conditions, the CO2 has certain properties which are similar to liquids and other properties which are similar to gases.

For the purposes of the present invention, when reference is made to a transcritical CO2 cycle, reference is made to a supercritical CO2 cycle which takes place at a supercritical pressure and temperature, in which after an expanding step below the critical pressure (73.83 bar a), the CO2 itself is cooled to below the critical temperature (31.06°C)

(subcritical state) . Since the temperature decreases below the condensation point causing the condensation of the CO2, it is correct to indicate a transcritical CO2 cycle as a CO2 condensation cycle.

For the purposes of the present invention, the supercritical or transcritical CO2 power cycle is integrated with a "thermal system".

Thermal System

"Thermal system" means a system in which a gas burnt at a high temperature is produced from the combustion of a fuel.

For the purposes of the present invention, the fuel is represented by a gas or a gas mixture selected from the group comprising: methane, hydrogen, syngas, flare gas, biofuel, tail gas; or other liquid or solid fuels; or solid, liquid and gaseous waste.

In particular, with particular reference to the drawings, a system 100 is disclosed which comprises a heat source 101 (represented for example, by the fuel combustion product) , a heat exchanger 102 and a point in which the burnt gases are released into the environment 103.

In one embodiment of the invention, the heat source 101 originates from a furnace in which a combustion (or an oxy-combustion) occurs.

In a particular aspect, the combustion produces gases preferably having a temperature >800°C, and more preferably >850°C, and even more preferably >880°C and even up to 900°C and higher.

A heat exchange instead occurs between the high temperature gases in exchanger 102, which cool down, and a CO2 flow which is heated.

In particular, such a CO2 flow is represented by the CO2 circulating in a circuit (201) of the power cycle .

In one embodiment of the invention, the exchanger 102 comprises two sections:

- a high temperature exchange section or high temperature exchanger SAT 102', and

- a low temperature exchange section or low temperature exchanger SBT 102^,f.

In one embodiment of the invention, the exchanger 102 of the thermal system 100 may additionally comprise a radiant section SR (104) in which there occurs a heat exchange by radiation with the flame.

In a further embodiment of the invention, the thermal system 100 may comprise a gas turbine 105.

According to a further embodiment again, there may be provided both a gas turbine 105 and a radiation exchange section 104 for the further heating of the gases produced by a turbine.

The high temperature 102', low temperature 102^,f, and when present, by radiation 104, heat exchange sections are the points in which the thermal system 100 is integrated with the power cycle 200, i.e. they are common with the latter.

In a further embodiment (not represented in the drawings), the thermal system 100 may consist of the combustion gases from an existing furnace.

This condition utilizes the residual capacity thereof and may provide the addition of a further post-combustion and/or radiant section (not shown in the drawings) .

Such a configuration provides the advantage of increased plant engineering simplicity because it does not require installing a new furnace.

According to a still further embodiment, the thermal power may be provided by the products of a high temperature chemical reaction, such as for example, the steam methane reforming (SMR) reaction and gasification.

Alternatively, the thermal power may simultaneously be provided by the combustion gases and by the products of a chemical reaction.

Power cycle

For the purposes of the present invention, the supercritical or transcritical CO2 power cycle 200 (as defined above) comprises:

- a C0₂ circuit 201,

- a turbine 202,

- a heat recovery section 203,

- a cooling section 204,

- a compressor 205,

- a heat exchange section 206.

For the purposes of the present invention, the heat exchanger 206 is the part of power cycle which is integrated in the thermal system and is in common therewith .

In order to operate in a CO2 cycle in the supercritical or transcritical state, turbine 202 of the power cycle is designed and conceived - with particular reference to the profile of the blade and the number of stages - according to the construction methods and techniques of fluid dynamics applied to turbo machinery.

The material of which the turbine is made also is a material with mechanical features adapted to resist the fluid-dynamic, mechanical actions and the conditions of increased pressure, temperature at which the cycle operates.

With regard to the heat recovery section 203, in one aspect of the invention, this comprises:

- a high temperature recovery section 203' , and

- a low temperature recovery section 203 .

In one aspect of the present invention, the power cycle 200 further comprises a tank 211 (see figure 4 ) .

Such a tank 211 may be selected or sized according to needs; it is believed that a resistance up to about 70, 80 or also 200 bar a and higher is sufficient .

CO2 is stored in tank 211 at a higher pressure with respect to the minimum pressure of the CO2 in the power cycle circuit 201, in the sections upstream of compressor 205; or also at a higher pressure with respect to the pressure of the CO2 in the power cycle circuit 201, in the sections downstream of compressor 205.

In particular, a valve 212 may be provided, which regulates the introduction of CO2 into the circuit 201 and the increase of the flow thereof, and a valve 213 which allows the reduction of the flow; the technician may provide an additional compressor 214 for this purpose, if required.

In one embodiment of the present invention shown by way of example in figure 9, the power cycle 200 may further comprise one or more heat recovery sections 215, 216 (which work by utilizing fluids outside the power cycle) .

Such heat recovery sections 215, 216 preferably are downstream the compressor 205.

In a preferred embodiment of the present invention, each heat recovery section 215, 216 operates one portion alone of the CO2 flow exiting the compressor 205.

Such a configuration has the advantage of allowing a heat recovery by utilizing the relatively low temperature exiting compressor 205, especially when a condensation (or transcritical ) cycle is carried out due to the availability of a natural or artificial cold source (in the cooler) .

With regard to the cooler 204, it represents the point in which the power cycle 200 is integrated with the cooling circuit 301.

Cooling circuit

The cooling circuit 301 allows the temperature of the CO2 in the power cycle circuit 201 to be decreased. In a preferred embodiment of the invention, the cooling circuit 301 operates with water.

This may in turn be cooled by an air cooler, in a cooling tower; alternatively, it may be obtained from natural sources (sea, waterbed, river, etc.) .

In one embodiment of the present invention, a low temperature fluid may be used in place of the water, for example represented by LNG to be regasified, or by another gas to be regasified (e.g. hydrogen, nitrogen, liquid air, etc.) or again, by a gaseous, liquid or solid cryogenic storage.

In another embodiment of the present invention (not represented in the figures), the cold source may be obtained artificially by a refrigeration cycle.

If the circumstances are considered in which the ambient temperature is about 35°C, since the critical temperature of the CO2 is close to the ambient temperature, a refrigeration cycle would be carried out in order to liquefy it by cooling it, for example down to 25°C, the refrigeration cycle operating with a COP (coefficient of performance) value of about 10- 15, therefore very high; this would have the advantage of a reduced energy consumption required by the refrigeration cycle and pumping work of the CO2 (rather than increased compression work) , thus increasing the efficiency of the CO2 cycle.

The cooling circuit 301 operates on the power circuit 200, in particular downstream of the low temperature recovery unit 206' ' .

If the (natural or artificial) cold source is available at a sufficiently low temperature as described above, the cooling may lead to the liquefaction of the CO2.

For this purpose, the temperature of the cold source preferably is to be <25°C.

Therefore, a condensation cycle is achieved under such conditions.

As a consequence, compressor 205 may be replaced with a pump in the power cycle 200.

According to a first object of the invention, a process for generating electricity is described, comprising the steps of:

subjecting a first working fluid flow to an expansion and cooling step a) in a turbine 205 with production of electricity, obtaining a second flow of said fluid;

subjecting said second fluid flow obtained from step a) to a heat recovery step b) in which said flow cools down transferring heat, obtaining a fourth flow

(formed as it is better detailed in the continuation of the description) ;

subjecting said fourth fluid flow obtained from step b) to a further cooling step c) , obtaining a fifth fluid flow;

- subjecting said fifth fluid flow obtained from step c) to a compression step d) , obtaining a sixth flow;

carrying out a step e) comprising a step el), in which a first portion of said sixth fluid flow obtained from step d) is subjected to a heating step in a low temperature recovery unit 203' ' , obtaining a first portion of a seventh flow;

subjecting said seventh fluid flow obtained from step e) to a further heating step f) in a high temperature recovery unit 203' , obtaining an eighth flow;

subjecting said eighth fluid flow obtained from step f) to a heating step g) in a high temperature exchanger 102^,f, obtaining a ninth high temperature fluid flow.

In a particularly preferred aspect, the working fluid is represented by CO2.

For the purposes of the present invention and as described above, the CO2 flow is represented by a CO2 flow in the supercritical state or in the supercritical and then subcritical state (in the case of transcritical cycle) .

In particular, the CO2 flow subjected to step a) is a high temperature CO2 flow which has a temperature >500°C, preferably >550°C, and even more preferably >600°C and higher.

In one aspect of the invention, step a) may result in a decrease of the temperature of the CO2 flow from 660°C to about 480°C.

In the same step, the pressure may be decreased by about >75 bar a, preferably about >200 bar a, to about >10 bar a, preferably >70 bar a.

In one aspect of the invention, the compression ratio (initial pressure/ final pressure) is >1.1, preferably >2.5, and even more preferably >3.

Mechanical energy may be produced due to the expansion in turbine 205, which mechanical energy may be converted into electricity through an electric generation system (not shown in the figures); for example, the production is possible of a mechanical (or electric) power >10 MW, preferably >15 MW and again > 50 MW.

In a preferred aspect of the present invention, step b) comprises a heat recovery sub-step bl) in a high temperature recovery unit 203' . A third flow of the working fluid is obtained from such a sub-step bl) .

The temperature of the CO2 may be decreased from about 480°C to about 280°C in such a sub-step bl) .

In an even more preferred aspect, step b) further comprises a heat recovery sub-step b2), successive to bl), in a low temperature recovery unit 203' ' .

A fourth flow is obtained from such a sub-step b2), as described above.

The temperature of the CO2 may be decreased from about 280°C to about 80°C in such a sub-step b2) .

Instead in step c) , the temperature of the CO2 flow may be cooled from about 80°C to about 32°C.

In particular, the heat exchange may be obtained by transferring heat to the water.

The water preferably has a temperature of about 25°C, usually the cooling water has a temperature between 20°C and 40°C.

The compression step d) in the present invention may lead to an increase of the temperature from about 32°C to about 60°C.

In a particular aspect of the invention, the delivery pressure of compressor 205 in step d) may reach 150 bar a, or 180 bar a, or again 200 bar a, and preferably up to about 300 bar a and higher.

For the purposes of the present invention, the heat exchange in step el) occurs with the third flow from step b2 ) .

For the purposes of the present invention, the heat exchange step g) occurs in the high temperature exchanger 102' which is part of the thermal system 100.

According to a preferred embodiment of the invention, in addition to step el), the above- described step e) comprises a step e2), in which a second fraction of the sixth CO2 flow obtained from step d) is subjected to a heat exchange step in a low temperature exchanger 102''.

For the purposes of the present invention, the heat exchange step e2) occurs in the low temperature exchanger 102 which is part of the thermal system 100.

According to a preferred aspect of the invention, the second portion of the sixth CO2 flow in step e2) exiting compressor 205 represents about 13-50% of the total flow exiting the compressor.

According to another preferred aspect, the first portion of the sixth flow exiting the compressor in step el) represents about 87-50% of the total flow exiting compressor 205.

According to a further preferred embodiment, the second portion of the sixth flow exiting the compressor in step e2) represents about 0-13% of the total flow exiting compressor 205.

According to a further aspect of the invention, step el) and/or step e2) may represent from 0% to 100% of the total flow exiting compressor 205, independently of each other.

Thus, in step f) a flow comprising the CO2 exiting compressor 205 and subjected to a recovery step in a low temperature recovery unit 203' (step el)), and the CO2 exiting compressor 205 and subjected to a heat exchange step in a low temperature exchanger 102 (step e2)), is subjected to heating.

For the purposes of the present invention, said seventh CO2 flow is heated, acquiring heat from the third flow from step bl) .

In step f) the temperature of the CO2 preferably is increased from about 215°C to about 400°C.

With regard to step g) , this results in an increase of the temperature of the CO2 flow from about 400°C to about 600°C.

The ninth CO2 flow obtained from step g) corresponds to the CO2 flow to be subjected to the expansion step a) .

For the purposes of the present invention, the heat exchanges from steps g) and e2) are carried out with the product obtained from the combustion (or oxy-combustion) of a fuel and in particular, are carried out in the high temperature exchanger 102' and in the low temperature exchanger 102'', respectively, of the thermal system 100.

For the purposes of the present invention, the fuel is represented by a gas or a gas mixture selected from the group comprising: methane, hydrogen, syngas, flare gas, biofuel, tail gas (after separation of CO2 from biomethane or syngas); or other liquid fuels; or solid, liquid or gaseous waste .

After the combustion (or also oxy-combustion) of the fuel, the combustion gas involved in the heat exchange has a temperature of >800°C, preferably >850°C, and even more preferably >880°C and higher.

Even more specifically, at the end of the high and low temperature heat exchanges, the combustion gas has a temperature >100°C, and preferably 150°C, and even more preferably >150-200°C and higher.

Thus, the gases are released into the environment at such temperatures.

According to an embodiment of the invention, after the heat exchange step g) in a high temperature exchanger 102', a further radiation heat exchange step h) may be carried out.

According to the embodiment represented in figure 3B, the thermal system 100 indeed comprises an additional heat exchange section which is represented by a radiant exchange section 104.

In particular, such a heat exchange occurs due to the transfer of the heat by radiation between the flame generated by the combustion of the fuels feeding the radiant section.

In this case, the CO2 flow inlet in step a) corresponds to the CO2 flow exiting the radiant section 104.

According to a further embodiment reported in figure 6, the thermal power of the fumes produced by a turbine 105 is used as a heat source in the thermal system 100.

In a further alternative embodiment shown in figure 7, a radiation heat exchange step may instead be provided for the further heating of the gases produced by a turbine 105.

In a still further embodiment (not represented in the figures), the combustion gases of an existing furnace may be used, utilizing the residual capacity thereof and adding a further post-combustion, if required, with the advantage of an increased plant engineering simplicity as there is no need to install a new furnace .

According to a still further embodiment, the thermal power may be provided by the products of a high temperature chemical reaction, such as for example, the steam methane reforming (SMR) reaction and gasification such as for example a residual/waste of an industrial plant (for example, bottom of a refinery barrel), waste in general or biomass.

According to an even further embodiment, the thermal power may simultaneously be provided by the combustion gases and by the products of a chemical reaction .

As described above, according to a further embodiment reported for example in figure 9, the power cycle of the present invention may further comprise one or more heat recovery sections (215,

216) .

Such sections (215, 216) are downstream of the compressor and may operate on one or both the portions of the CO2 exiting the compressor (205) . In one embodiment of the invention, such a portion is the portion which is subjected to the heat exchange step in the low temperature exchanger (102'') (step e2); alternatively, it is the portion which is subjected to the heat recovery step in the low temperature recovery unit (203' ' ) (step b2); and more preferably, on both portions of CO2 (step e2) and step el ) .

According to one aspect of the invention which is applicable to all the above described embodiments, a step of varying the CO2 flow may be provided between the cooling step c) in the cooler (204) and the compression step d) .

In particular, such a variation may comprise an increase or a decrease of the CO2 flow.

This results in a pressurization or depressurization of the CO2 cycle by means of the use of the external tank (211) and the above-described valves (212, 213) .

More in detail, according to what is represented by example in figure 4, CO2 at a higher pressure is accumulated in the tank (211) with respect to the pressure of the CO2 in the section of the cycle upstream of the compressor (205) , allowing the introduction of CO2 into the cycle through the first pressure reducing valve (212); otherwise, the CO2 flow circulating in the cycle may be reduced (the technician may provide an additional compressor for this purpose, for said purpose) by acting on the second valve (213) .

The increase or decrease of the CO2 pressure allows the power of the cycle to be regulated.

It is worth noting that when present in an electric generation system which converts the mechanical energy into electric energy (P_ei) , the mechanical, electric power depends on the circulating mass (M) and the specific work (W) , which only depends on the physical states of the cycle:

P_el = M X W

In one embodiment of the invention, a condensation cycle may be operated, in particular when a natural and/or artificial cold source is available at a sufficiently low temperature in step c) , so as to allow the liquefaction of the CO2.

Such a condition occurs preferably for temperatures <25°C.

In a particularly preferred aspect of the invention, the flow exiting the low temperature exchanger 102 is completely sent to the high temperature recovery unit 203', joining the flow exiting the low temperature recovery unit 203' ' .

Moreover, according to another preferred embodiment of the invention, the flow exiting the low temperature exchanger 102' ’ is not directly sent to the high temperature exchanger 102', rather for example, is subjected to a heating step in a high temperature recovery unit 203' .

According to a further object, the present invention describes a system 400 for producing electricity by means of a thermal system 100 integrated with a power cycle 200 comprising a circuit 201 with a working fluid according to the above .

In a preferred aspect, said working fluid is represented by the supercritical or subcritical CO2.

In more detail, said system comprises a thermal system 100 as described above; this in particular comprises :

- a heat source represented by a furnace 101, in which the combustion or oxy-combustion of a fuel producing a high temperature gas occurs,

- a heat exchange section 102,

- an output section for said gases 103.

The system 400 further comprises a power cycle 200 which, as described above, comprises: - a circuit 201 of the fluid,

- a turbine 202 for expanding the fluid,

- a heat recovery section 203 for the fluid,

- a cooling section 204 for the fluid,

a compression section of the fluid comprising a compressor 205,

- and optionally, one or more of the following: a section 210 for varying the flow of the fluid

one or more heat recovery sections 215, 216 for the fluid.

The system 400 further comprises a circuit 300 for cooling the fluid, with a cooling section.

For the purposes of the present invention, as described above, the heat exchange section 102 of the thermal system and the cooling section of the cooling circuit 301 correspond to the heat exchange section 206 and the cooling section 204 of the power cycle, respectively .

The present invention was described with particular reference to the use of a CO2 flow, but other fluids or fluid mixtures may equally be used; for this purpose, those skilled in the art may adapt the system to the specific needs without however departing from the scope of the present invention. The invention is further described with reference to a specific application example.

EXAMPLE 1

A CO2 flow of about 150 kg/s at the pressure of about 200 bar a and temperature of about 600°C expands in turbine up to the pressure of about 78 bar a and temperature of about 480°C and carrying out a work of about 20 MW; the CO2 current in the recovery unit RAT cools to about 280°C and further cools to about 80°C in the recovery unit RBT and cools again in COOLER to about 32 °C; it is brought to about 200 bar a in the compressor, with a compression work of about 4 MW, and to the temperature of about 60 °C and is divided in SMI into a fraction of 20 kg/s and a fraction of 130 kg/s; the fraction of 20 kg/s is sent to exchanger SBT and heating with the exhaust fumes, is brought to 300°C; current 6a is heated in the recovery unit RBT up to 200°C and joins 7e, forming current 7 of 150 kg/s (complete flow) at 215°C; current 7 is heated in the recovery unit RAT up to 400°C and sent to the higher temperature exchanger SAT of the furnace, reaches about 600°C.

With regard to the fumes produced in the furnace at the temperature of about 900°C, they are cooled to about 530°C after the high temperature exchanger and to about 150-160°C after the low temperature exchanger .

The net electric power is about 16 MWe .

The electric efficiency is about 35-45%.

The cycle is executed by also providing the radiation heat exchange step in a radiant section of the furnace (SR) .

Such a step allows a flow of gas to be introduced into the high temperature exchange section (SAT) at a temperature >800°C, preferably >850°C, and even more preferably ³880°C.

The cycle is executed also with the insertion of a turbine GT .

Such a turbine is capable of heating ambient air up to a temperature >400°C, preferably >450°C.

In light of the above, the advantages of the present invention shall be apparent to those skilled in the art .

With regard to the advantages with respect to the cycle described by prior art document JP 2011256818, from a structural viewpoint, the cycle of the present invention comprises one turbine alone (rather than two) and two exchangers (rather than three) ; such a structure thus is structurally much more complex.

Moreover, such a document describes a cycle which always provides two mass splits; in particular:

- after compressor Cl, a portion of the flow is sent to exchanger RHX2 and a part to exchanger ]HX3;

- after exchanger RHX2, a portion of the flow is sent to exchanger RHX1 and a part to exchanger ] HX2.

Moreover, again with respect to JP 2011256818, the cycle of the present invention allows the net power of the cycle to be regulated as the thermal power of the fumes varies, without modifying the thermodynamic states of the cycle, remaining within an optimal design condition, i.e. the maximum efficiency, allowing the mass circulating by pressurization or depressurization and therefore, the power, to be varied.

In the configuration of the invention providing the realization of a condensation cycle, the temperature of the exhaust gases may be decreased to 150-200°C, further increasing the efficiency of the cycle, also due to the lesser pumping work with respect to the compression work.

The cycle described by the present invention can be adapted to different sources; in the case of gases, it allows the most to be made of flare gas or tail gas, which would otherwise be discarded.

If hydrogen is used instead, electricity may be produced without toxic emissions for the environment and additionally, since the combustion gases from hydrogen do not result in acid dew problems, the temperature of the combustion gases themselves may be further reduced, also recovering the condensation heat of the water therein contained.

The cycle of the present invention may also be adapted to thermal sources consisting of the combustion (or also oxy-combustion) of gaseous, liquid or solid fuels or solid, liquid and gaseous waste .

A further advantage is the possibility of regulating the net power of the cycle as the thermal power of the fumes varies while remaining in optimal design condition (i.e. of maximum efficiency) by varying the circulating mass: for example, by pressurizing or depressurizing the cycle, the density increases/decreases and therefore, the circulating mass increases/decreases and again the power increases/decreases .

Therefore, it pressurizes if the thermal power of the fumes increases, while it depressurizes if it decreases, without changing the physical states of the cycle.

Claims

CLAIMS :

1. A process for generating electricity, comprising the steps of:

subjecting a first working fluid flow to an expansion and cooling step a) in a turbine (202) with production of electricity, obtaining a second working fluid flow;

subjecting said second flow of said fluid obtained from step a) to a cooling step b) , comprising a cooling step bl) in a high temperature recovery unit (203' ) , obtaining a third flow of said fluid and a cooling step b2) in a low temperature recovery unit (203' ' ) , obtaining a fourth flow;

subjecting said fourth flow of said fluid obtained from step b) to a further cooling step c) , obtaining a fifth flow;

subjecting said fifth flow of said fluid obtained from step c) to a compression step d) , obtaining a sixth flow;

carrying out a step e) comprising a step el) , wherein a first portion of said sixth flow of said fluid obtained from step d) is subjected to a heating step in the low temperature recovery unit (203' ) , obtaining a first portion of a seventh flow;

subjecting said seventh flow of said fluid obtained from step e) to a further heating step f) in the high temperature recovery unit (203' ) , obtaining an eighth flow;

subjecting said eighth flow of said fluid obtained from step f) to a heating step g) in a high temperature exchanger (206' ' ) , obtaining a ninth high temperature fluid flow, wherein

said step e) further comprises a step e2), wherein a second portion of said sixth flow of said fluid obtained from step d) is subjected to a heating step in a low temperature exchanger (206^,f), obtaining a second portion of said seventh flow, and wherein

said step e2) in the low temperature exchanger (206' ) and said step g) in the high temperature exchanger (206' ) are carried out by acquiring heat from a heat source, which can be represented by the fuel combustion product.

2. A process according to the preceding claim, wherein the flow of said working fluid is a CO2 flow at the supercritical or subcritical state.

3. A process according to the preceding claim, wherein said fuel is preferably selected from the group comprising: methane, hydrogen, syngas, flare gas, biofuel, tail gas, and solid, liquid or gaseous waste .

4. A process according to any one of the preceding claims from 1 to 3, wherein, in step c) the heat exchange is obtained by transferring heat to a low temperature fluid selected from the group comprising water, LNG, hydrogen, nitrogen, liquid air .

5. A process according to any one of the preceding claims from 1 to 3, wherein in step c) the heat exchange is obtained by transferring heat to a gaseous, liquid or solid cryogenic storage.

6. A process according to any one of the preceding claims from 2 to 5, wherein said step c) is a step of liquefying said working fluid represented by C0₂.

7. A process according to any one of the preceding claims, wherein in step el) the heat exchange occurs with said third flow from step b2) .

8. A process according to any one of the preceding claims from 2 to 7, wherein the first portion of said sixth flow of CO2 in step el) represents about 50-85% of the total flow exiting the compressor .

9. A process according to any one of the preceding claims from 2 to 7, wherein in step f) said seventh flow of CO2 flow acquire heat from the flow from step bl ) .

10. A process according to any one of the preceding claims, wherein after the heating step g) in a high temperature exchanger (206' ) , a further radiation heat exchange step h) is carried out.

11. A process according to the preceding claim, wherein in said radiation heat exchange step h) , there are used the gases produced by the combustion of a fuel.

12. A process according to any one of the preceding claims, wherein said heat source is replaced by a heated gas produced by a turbine (202) .

13. A process according to the preceding claim, wherein said heated gas produced by a turbine (202) is further heated by radiation.

14. A process according to any one of the preceding claims, wherein a step of varying the amount of said fifth CO2 flow is carried out after step c) and before step d) .

15. A system (400) for producing electricity by means of a thermal system (100) integrated with a power cycle (200) comprising a circuit (201) with a supercritical or subcritical working fluid represented by CO2, said system comprising:

I) a thermal system (100) comprising: - a heat source represented by a furnace (101), wherein the combustion or oxy-combustion of a fuel producing a high temperature gas occurs,

- a heat exchange section (102, 206) comprising one or more exchangers (102', 102", 206', 206''),

- an output section for said gases (103);

II) a power cycle (200) comprising:

a fluid circuit (201),

a turbine (202) for expanding said fluid, a section (203) for the heat recovery comprising one or more recovery units (203' , 203' ' ) of said fluid,

a section (204, 301) for cooling said fluid,

a section (205) for compressing said fluid, and optionally, one or more of the following :

a section (210) for varying the amount of the flow of said fluid in the circuit,

one or more sections (206, 102) for the heat exchange of said fluid, comprising one or more exchangers (102', 102", 206', 206"),

III) a circuit (300) for cooling said fluid, comprising a cooling section (204, 301),

wherein said heat exchange section (102) of the thermal system (100) and said cooling section (301) of the cooling circuit (300) correspond to the heat exchange section (206) and the cooling section (204) of the power cycle, respectively.

16 . A system for producing electricity according to the preceding claim, which implements the process according to any one of the claims from 1 to 14.