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Title:
ORGANIC RANKINE CYCLE POWER PLANT
Document Type and Number:
WIPO Patent Application WO/2013/050803
Kind Code:
A1
Abstract:
A plant for co - generation of electricity and heating of water. Said plant comprises a combustion chamber (10) for combustion of biomass or of some other fuel. This plant comprises a circulating line (11) for the flue gases produced in said combustion chamber. The plant also comprises an operational circuit in which an organic fluid circulates, operating according to a Rankine cycle. The plant according to the invention further comprises a heat exchange group comprising at least one heat exchanger unit which comprises in its turn at least one first casing (31 ) that can be traversed by said combustion flue gases and at least one second casing (41), physically separate from said first casing (31 ), that can be traversed by said organic fluid. Said unit comprises a plurality of heat pipes (151, 15", 15'"), separated from one another, arranged in such a way that for each of said heat pipes (15 ', 15", 15"') a first portion (16 ') and a second portion (16") are housed respectively in said first casing (31) and in said second casing (31).

Inventors:
PALENA, Matteo (Viale Buffoli 4, Cusano Milanino, I-20095, IT)
TANZI-MIRA, Gabriele (Via L. Frapolli 21, Milano, I-20133, IT)
FIORI, Andrea (Via Durazzo 5, Milano, I-20134, IT)
Application Number:
IB2011/054388
Publication Date:
April 11, 2013
Filing Date:
October 05, 2011
Export Citation:
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Assignee:
SPIRAX-SARCO S.R.L. (Via per Cinisello 18, Nova Milanese, I-20834, IT)
PALENA, Matteo (Viale Buffoli 4, Cusano Milanino, I-20095, IT)
TANZI-MIRA, Gabriele (Via L. Frapolli 21, Milano, I-20133, IT)
FIORI, Andrea (Via Durazzo 5, Milano, I-20134, IT)
International Classes:
F01K25/08; F22B1/16; F22B1/18
Attorney, Agent or Firm:
GERVASI, Gemma et al. (Corso di Porta Vittoria 9, Milan, I-20122, IT)
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Claims:
CLAIMS

Plant (1) for co-generation of electricity and heating of water, said plant (1 ) comprising:

a combustion chamber (10) for combustion of biomass or of some other fuel;

a circulating line (11 ) of the combustion flue gases produced by said combustion of biomass in said combustion chamber (10);

an operational circuit (2) in which an organic fluid circulates, operating according to a Rankine cycle, a generating unit (2') of electricity and a condensing unit (2") of said organic fluid downstream of said generating unit (2') of electricity,

in which said plant (1 ) comprises a heat exchange group (5) comprising at least one heat exchange unit (5', 5", 5'") comprising:

a first casing (31 ) that can be traversed by said combustion flue gases in a direction of crossing (101 ) defined by an inlet section (21 ') of said combustion flue gases and by an outlet section (21") of said combustion flue gases;

at least one second casing (41 ) physically separate from said first casing (31 ), and said second casing (41 ) can be traversed by said organic fluid, said second casing (41 ) comprising an inlet section (22') of said organic fluid and an outlet section (22") of said organic fluid;

a plurality of heat pipes (15', 15", 15"') separated from one another, said heat pipes (15', 15", 15"') being arranged in such a way that for each of said heat pipes (15', 15", 15"') a first portion (16') and a second portion (16") are housed respectively in said first casing (31 ) and in said second casing (31 ).

Plant (1 ) according to claim 1 , in which said at least one heat exchanger unit (5', 5", 5"') comprises a third insulating casing (23) physically interposed between said first casing (31 ) and said at least one second casing (41 ), said heat pipes (15', 15", 15"') being arranged in such a way that, for each of them, a third portion (16"') between said first portion (16') and said second portion (16") traverses said third casing (71 ). Plant (1 ) according to claim 2, in which said at least one heat exchanger unit (5', 5", 5'") comprises aspirating means able to keep the pressure inside said third casing (23) below a predetermined threshold.

Plant (1 ) according to any one of claims 1 to 3, in which said heat pipes

(15', 15", 15'") are arranged in such a way that, for each of them, said first portion (16') is substantially perpendicular to said direction of crossing

(101 ).

Plant (1) according to any one of claims 1 to 4, in which said first casing (31 ) has a prismatic configuration comprising two substantially vertical parallel walls (49), a bottom (48) which supports said parallel walls (49) and a plate (51 ) above and opposite to said bottom (48), said plate (51 ) supporting said pipes (15', 15", 15'") vertically in such a way that they are suspended inside said first casing (31 ).

Plant (1 ) according to claim (49) in which said parallel walls (49) are removable.

Plant (1) according to any one of claims 1 to 6, in which said second casing (41 ) of said at least one heat exchanger unit (5', 5", 5"') comprises a plurality of containments (41', 41") and a second plate (52) which supports each of said containments (41', 41"), each of said containments (41', 41") being closed at the top by a corresponding closing flange (53).

Plant (1 ) according to claim 7, in which said containments (41', 41") of said second casing (41 ) are separated from one another and interconnected by at least one connecting line (44) to allow passage of said organic fluid from a first containment (41') to a second containment (41").

Plant (1 ) according to claim 7, in which each of said containments (41 ', 41") comprises an inlet section (22') and an outlet section (22") for said organic fluid.

Plant (1 ) according to any one of claims 1 to 9, in which said heat exchange group (5) comprises a first heat exchanger unit (5') for evaporating said organic fluid.

Plant (1) according to claim 10, in which said heat exchange group (5) comprises a second unit (5") of heat exchange (5) arranged upstream of said first heat exchange unit (5'), said second unit (5") being able to preheat said organic fluid before evaporation thereof.

12. Plant (1 ) according to claim 10, in which said heat exchange group (5) comprises a third heat exchange unit (5"') arranged upstream of said electricity generating unit (2') and downstream of said first heat exchange unit (5'), said third heat exchange unit (5'") being able to superheat said organic fluid in the vapour state.

13. Plant (1) according to any one of claims 10 to 12, in which said plant (1 ) comprises a further heat exchanger unit (5"") for performing a further heat exchange between said combustion flue gases and the combustion air intended for said combustion chamber (10), said further heat exchange being subsequent to heat exchange between said combustion flue gases and said organic fluid.

14. Plant (1 ) according to any one of claims 1 to 13, in which said plant (1 ) comprises:

a mixing chamber (18) of the combustion flue gases arranged along said circulating line (11 ) of said combustion flue gases, said mixing chamber (18) being downstream of said combustion chamber (10) and being upstream of said heat exchange group (5);

- a flue gas recycling line (11 ') in which a fraction of the flow of flue gases circulating in said flue gas circulating line (11 ) circulates, said recycling line (11') conveying said recycled flow to said mixing chamber (18).

15. Plant (1 ) according to claim 14, in which said recycling line (11') comprises regulating means (71 ) for regulating the flow of flue gases conveyed to said mixing chamber (18), said plant (1 ) comprising first temperature detecting means (91 ) and a control unit for controlling said regulating means (71 ) as a function of a temperature signal sent from said detecting means (91) to said control unit.

16. Plant (1 ) according to claim 15, in which said regulating means (71 ) increase the flow of flue gases conveyed to said mixing chamber (18) when said first temperature detecting means (91 ) detect a temperature above a first predetermined threshold.

Plant (1 ) according to claim 16, in which said plant (11) comprises an emergency line (11") for circulation of cooling air, said emergency line (11") conveying said cooling air in said circulating line (11 ) of said flue gases in order to keep their temperature below a second predetermined threshold above said first predetermined threshold.

Description:
ORGANIC RANKINE CYCLE POWER PLANT

FIELD OF THE INVENTION

The present invention relates to the field of the construction of plant for co- generation of electricity and hot water for example for industrial use or for use in district heating. In particular the invention relates to electricity generating plant of the ORC (Organic Rankine Cycle) type, i.e. comprising a circuit for generating electricity operating according to a Rankine cycle with organic fluid. In particular the ORC plant according to the invention comprises at least one heat exchanger unit which performs heat exchange between high-temperature flue gases produced by combustion of biomass or of some other fuel and the organic fluid operating according to the Rankine cycle.

THE PRIOR ART

As is known, plants of the ORC type, i.e. based on the use of a first circuit in which an organic fluid operates according to a classical Rankine cycle, are used, among others, for co-generation of electricity and hot water, for industrial use or for district heating.

In general these plants comprise a line for circulation of flue gas received from a combustion chamber normally fed with biomass or other fuel. In particular the flue gas is conveyed to a heat exchanger so that it gives up some of its heat energy to a heat-transfer fluid, normally thermal oil, circulating in a second circuit. By means of a second heat exchanger, this heat-transfer fluid (thermal oil) gives up its heat energy to the organic fluid circulating in the operating circuit of the plant. In particular, the heat-transfer fluid (thermal oil) preheats and vaporizes the organic fluid which then, in the vapour state, enters a turbine connected to an electricity generator. The organic fluid leaving the turbine is then condensed so as to transfer its heat content to water, which can be used, for example, for district heating.

Fig. 1 is a schematic view relating to a conventional configuration of an ORC plant as mentioned above. As shown, the plant comprises a first circuit 100 in which an organic fluid operates according to a Rankine cycle. The plant also comprises a second circuit 200 in which a transfer fluid, consisting of thermal oil, circulates. The latter is heated in an exchanger-heater 110 by the flue gases produced by combustion of biomass or of some other fuel. The thermal oil gives up its heat to the organic fluid circulating in the first circuit 100 by means of an exchanger group 120, on leaving which the organic fluid is evaporated. The first circuit 100 comprises at least one turbogenerator (or turbine) 150 for generating electricity, downstream of which an exchanger-regenerator 151 and an exchanger-condenser 152 are arranged in series. The superheated organic fluid leaving the turbogenerator 150 gives up some of its heat energy to the organic fluid in the liquid state leaving the exchanger-condenser so as to increase its heat content before entering a preheater 130 upstream of the exchanger-evaporator 120. By means of the exchanger-condenser 152, the organic fluid circulating in the first circuit gives up its heat energy to the water circulating in a further circuit 300 for industrial use or for district heating.

The plant just described, like others of similar design, has a number of drawbacks due principally to the use of thermal oil as heat-transfer fluid. The use of thermal oil necessarily requires the provision of an expansion tank to compensate on the one hand the variations in volume caused by thermal expansion of the circuit and on the other hand to separate the liquid phase of the oil from the vapour phase. This requirement has a considerable effect on plant construction costs and on plant operating and maintenance costs.

In order to guarantee circulation of the thermal oil in the circuit, it is moreover essential to have two or more pumps (indicated in Fig. 1 with reference 140) for circulation in normal operating conditions and at least one motor-driven pump, typically diesel or driven by an electric motor, to guarantee circulation in emergency conditions. Obviously the necessary presence of these devices also constitutes a critical factor in terms of costs of construction and maintenance. In this sense it is further observed that filling of the circuit with thermal oil gives rise to high initial costs and further costs of renewal when said oil needs to be regenerated/replaced .

Further negative aspects of the use of thermal oil are also seen in the need to provide sealing means that are reliable, and therefore of high cost, for preventing leakage of diathermic fluid. In this sense, moreover, comprehensive inspection devices are also required, on the one hand for detecting possible leaks and for monitoring the conditions of the thermal oil to prevent flammability thereof. It is observed, moreover, that the heat exchangers between the combustion flue gases and the thermal oil are characterized by appreciable dimensions, which on the one hand complicates operations for installing them and on the other hand the operations of maintenance and/or operation. The thermal oil is moreover particularly flammable per se and so is relatively hazardous. Moreover, it requires the use of processes for storage and disposal that are particularly expensive and complex.

Based on these considerations, it is therefore clear that the use of thermal oil as heat-transfer fluid, even if effective from the functional standpoint, represents a critical factor accounting for high capital costs and high operating costs. It should be emphasized that the problem of costs also arises in plants of the ORC type that do not envisage the use of thermal oil as transfer fluid. In this sense, patent application EP 1555396 describes a solution in which a gaseous fluid, such as for example inert gas or gas with low oxygen content, is used as heat-transfer fluid. The use of a gaseous heat carrier in fact makes it possible to overcome some of the problems noted above relating to thermal fluid. In particular those relating to circulation of the fluid appear to be more or less overcome.

However, it has been seen that this further solution is still not entirely satisfactory because although, on the one hand, circulation of a gas is simpler than a diathermic fluid, on the other hand it still requires the provision of devices for controlling and monitoring the state of the fluid, safety devices and more generally a circulating circuit that guarantees the required safety conditions. Essentially, this solution is also associated with high costs of construction, operation and maintenance and at the same time is also disadvantageous in terms of design and thus of arrangement. Generally, in fact, the circuit for circulating the heat-transfer fluid (whether liquid or gaseous) also represents a critical factor in terms of costs of design and obviously consequently in costs of installation.

Based on these consideration, there is therefore a clear need for novel technical solutions for overcoming the drawbacks described above connected with the use of a heat-transfer fluid and the provision of a suitable circuit for circulation thereof. Therefore the main problem to be solved by the present invention is to provide a novel technical solution that allows the drawbacks presented above to be overcome. In the context of this problem, a first aim of the present invention is to eliminate, in electricity generating plants of the ORC type, the problems relating to the use of thermal oil as heat transfer fluid and more generally connected with the use of a heat-transfer fluid for transferring heat energy from the flue gas to the organic fluid. In this sense, another aim of the present invention is to eliminate, in electricity generating plants of the ORC type, the problems relating to the presence and to the operation of a circuit for circulating a heat-transfer fluid. A further aim of the present invention is to reduce the operating and maintenance costs of a plant of the ORC type relative to conventional plants. Last but not least, another aim of the present invention is to make a plant of the ORC type reliable and easy to construct at competitive costs. SUMMARY OF THE INVENTION

The present invention also relates to an electricity generating plant of the ORC type comprising a combustion chamber for combustion of biomass or of some other fuel, a line for circulation of the combustion flue gases produced in said combustion chamber and an operational circuit in which an organic fluid circulates, operating according to a Rankine cycle. In particular the operational circuit comprises at least one electricity generating unit and at least one unit for condensing said organic fluid downstream of said electricity generating unit. The plant according to the invention comprises a heat exchange group for effecting heat exchange between said combustion flue gases and said organic fluid circulating in said operational circuit. The heat exchange group comprises at least one heat exchanger unit (hereinafter simply "unit") in its turn comprising a first casing that can be traversed by the combustion flue gases in a direction of crossing defined by an inlet section of said combustion flue gases and by an outlet section of said combustion flue gases.

The heat exchanger unit further comprises at least one second casing that can be traversed by said organic fluid. More precisely this second casing comprises an inlet section of said organic fluid and an outlet section of said organic fluid. The heat exchanger unit also comprises a plurality of heat pipes arranged in such a way that for each of said heat pipes a first portion of the pipe and a second portion of the pipe are housed respectively in the first casing and in the second casing of the heat exchanger unit.

The use of one or more heat exchanger units, as stated above, in a plant of the ORC type, makes it possible for the aforementioned drawbacks of the prior art to be remedied completely. In essence, heat exchange between the combustion flue gases and the organic fluid is advantageously effected by means of the heat exchanger unit, according to the invention, without the need to provide complicated circuits for circulation of heat-transfer fluids. In particular the criticality due to thermal oil as heat-transfer fluid, as well as all the other problems connected with its use, is advantageously overcome by the use of heat pipes that provide highly efficient heat exchange, with very low operating and maintenance costs.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will become clear from the description of particular embodiments of plant according to the present invention illustrated as non-limiting examples in the appended drawings in which: - Fig. 1 is a schematic view of an electricity generating plant of the ORC type known from the prior art;

- Fig. 2 is a view of a plant of the ORC type according to the present invention;

- Fig. 3 is a first side view of a first embodiment of the heat exchanger unit of a heat exchange group of a plant according to the present invention;

- Fig. 4 is a second side view of the heat exchanger unit of Fig. 3;

- Fig. 5 is a sectional view of the heat exchanger unit of Fig. 3;

- Figs. 6 and 7 are respectively a front view and a plan view relating to a second possible embodiment of a heat exchanger unit of a heat exchange group of a plant according to the present invention;

- Figs. 8 and 9 are respectively a front view and a plan view relating to a third possible embodiment of a heat exchanger unit of a heat exchange group of a plant according to the present invention; - Figs. 10 and 11 are respectively a front view and a plan view relating to a further possible embodiment of a heat exchanger unit of a heat exchange group of a plant according to the present invention;

- Fig. 12 is a front view of a heat exchange group of a plant according to the present invention.

The same reference numbers in the figures identify the same elements or components.

DETAILED DESCRIPTION

Fig. 2 is a schematic view of a plant 1 according to the present invention advantageously arranged for co-generation of electricity and of water for industrial use or for district heating. The plant 1 according to the invention comprises a combustion chamber 10 for combustion of biomass or of some other fuel. This combustion chamber 0 is supplied with combustion air through a feed line 9 for combustion air preferably delivered by means of a fan 19. The flue gases leaving the combustion chamber are conveyed in a flue gas circulating line 11 which preferably envisages a mixing chamber 18. Inside the latter, flue gases produced in the combustion chamber 10 are mixed with recycle flue gases circulating in a flue gas recycling line 11 '. More precisely, a fraction of the flow of flue gases circulating in the flue gas circulating line 11 circulates in this flue gas recycling line 11 '. The function of the mixing chamber 18 and of the flue gas recycling line 11 ' will be described in more detail later.

The plant 1 comprises an operational circuit 2 in which an organic fluid, known per se, circulates, operating according to a Rankine cycle. Thus, the operational circuit 2 comprises an electricity generating unit 2", comprising one or more turbines, and a condensing unit 2" downstream of said generating unit 2'. According to a preferred embodiment the operational circuit 2 also comprises a regenerative exchanger 3 interposed between the generating unit 2' and the condensing unit 2". This regenerative exchanger 3 acts essentially as desuperheater of the organic fluid leaving the generating unit 2 * and transfers the heat energy of the superheated fluid to the organic fluid in the liquid state leaving the condensing unit 2". Downstream of the condensing unit 2", pumping means 2"' are arranged, by means of which the organic fluid is made to circulate in the operational circuit 2 overcoming the pressure losses of said circuit.

Plant 1 according to the invention comprises a heat exchange group 5 (designated hereinafter simply as "group 5") for effecting heat exchange between the combustion flue gases, produced in combustion chamber 10, circulating in circulating line 1 1 , and the organic fluid circulating in the operational circuit 2. Group 5 comprises at least one heat exchanger unit 5', 5", 5'" comprising a first casing 31 comprising an inlet section 21 ' of said flue gases and an outlet section 21 " of said flue gases. The first casing 31 can thus be traversed by the combustion flue gases in a direction of crossing defined by the mutual positioning of the inlet section 21 ' and of the outlet section 21 " of the flue gases.

Said at least one heat exchanger unit 5', 5", 5'" further comprises a second casing 41 that can be traversed by said organic fluid circulating in the operational circuit 2 of plant 1 and physically separated from the first casing 31. The expression "physically separated" means a condition according to which the organic fluid cannot come into contact with the combustion flue gases, or with the walls that define the first portion 21 of passage. The second casing 41 also comprises an inlet section 22' for the organic fluid and an outlet section 22" for the same fluid. Said at least one heat exchanger unit 5', 5", 5" of said group 5 further comprises a plurality of heat pipes 15', 15", 15'" (designated hereinafter simply as pipes 15', 15", 15'") separated from one another. As stated later, these pipes 15', 15", 15'" can be collected together in groups or batteries. For the purposes of the present invention the expression "heat pipe" means a pipe, hollow internally, in which an operational fluid is arranged, which is able to evaporate and condense in said pipe.

According to the invention, said pipes 15', 15", 15"' are arranged in such a way that, for each of them, a first portion 16' and a second portion 16" are housed, respectively, in the first casing 31 and in the second casing 41 of the same heat exchanger unit 5', 5", 5"'. In this way the first portion 16' of pipes 15', 15", 15"' is intended to be licked externally by the combustion flue gases, while the second portion 16" is intended to be licked externally by the organic fluid. Through this arrangement of the pipes 15', 15", 15'", part of the heat energy contained in the combustion flue gases is transferred to the organic fluid. More precisely the combustion flue gases externally licking the first portion 16' of pipes 15', 15", 15"' inside the first casing 31 cause evaporation of the operational fluid contained inside each pipe 15', 15", 15"'. This evaporation generates in its turn convective motion inside said pipe so that the evaporated part of the fluid moves towards the respective second portion 16". Because of the lower temperature of said second portion 16", the operational fluid condenses, releasing heat energy that is transferred firstly by conduction, through the metal wall of the second portion 16", and then by convection to the organic fluid that licks externally the second portion 16" of pipes 15', 15", 15'" arranged in the second casing 41.

It has been found that the use of a heat exchanger unit 5', 5", 5"' based on the principle described above in fact makes it possible to remedy all of the drawbacks stated above connected with the use of a circuit for circulation of a heat-transfer fluid and in particular of a diathermic liquid. The heat exchanger unit 5', 5", 5"' in fact allows heat exchange between the combustion flue gases and the organic fluid that is more efficient and decisively more favourable in terms of operation. Moreover, the unit 5', 5", 5"' operates in conditions of complete safety and the operating and maintenance costs are much lower compared with the conventional solutions that use circuits for circulating heat-transfer fluids.

Still referring to Fig. 2, group 5 comprises a first exchange unit (indicated with reference 5') by means of which evaporation of the organic fluid is effected. In other words, by means of the first heat exchanger unit 5' the fraction of heat energy transferred from the combustion flue gases to the organic fluid, via the heat pipes 15', 15", 15"', allows said organic fluid to pass from the liquid state to the vapour state. Group 5 preferably also comprises a second heat exchanger unit 5" (designated hereinafter simply as "second unit 5") arranged upstream of the first heat exchanger unit 5' for preheating the organic fluid leaving the exchanger- regenerator 3 and before it enters the first unit 5'. In particular this second unit 5" has an operating principle similar to that of the first unit 5" and also uses a plurality of heat pipes arranged on the one hand in a first casing traversed by the combustion flue gases and on the other hand in a second casing traversed by the organic fluid. Consequently the first unit 5' and the second unit 5' are substantially equivalent, providing technical solutions and operation corresponding to that described above.

According to the present invention, group 5 preferably also comprises a third heat exchanger unit 5'" (designated hereinafter simply as third unit 5"') that is arranged operationally in the operational circuit 2 downstream of the first heat exchanger unit 5' and upstream of the generating unit 2'. The third unit 5"' has the function of superheating the organic fluid in the vapour state leaving the first unit 5' and before it reaches the generating unit 2'. The same considerations made in relation to the first unit also apply to the third unit 5"'. Thus, the third unit 5"' performs heat exchange based on the same principle implemented by the first unit 5' and by the second unit 5".

Based on the foregoing, group 5 advantageously integrates three heat exchanger units 5', 5", 5'", one of which (the first unit 5') is intended to produce a change of state in the organic fluid (from liquid to vapour), while the others are intended to raise the temperature of the organic fluid in the liquid state (second unit 5") or in the vapour state (third unit 5"').

Still referring to the plant in Fig. 2, it can be seen that first temperature detecting means 91 of the combustion flue gases are provided downstream of mixing chamber 18. These means are connected operationally to regulating means 71 of the flow of the recycle flue gases. In particular said regulating means 71 , comprising for example a fan, are arranged operationally along the flue gas recycling line 1 V for sending the recycle flue gases to the mixing chamber 10. The first detecting means 91 send a signal to a control unit (not shown) which in its turn controls the regulating means 71 for modulating the flow of the recycle flue gases. In the case of a fan, for example, this modulation is obtained by adjusting the fan speed. More precisely, when the first detecting means 91 detects a temperature above a first predetermined threshold, then the control unit controls the regulating means so that the latter increase the flow of the recycle flue gases sent to the mixing chamber (increase in fan speed). Vice versa, when the temperature drops below this first predetermined threshold the flow of the recycle flue gases is reduced (decrease in fan speed). Thus, essentially, the flow of the recycle flue gases is modulated as a function of the temperature of the combustion flue gases.

This solution makes it possible advantageously to control the temperature of the combustion flue gases upstream of the heat exchanger group 5. In particular it has been found that by using organic fluids belonging to the siloxanes family, the ideal flue gas inlet temperature in group 5 is between 450 and 550°C and preferably about 500°C. Obviously, by varying the typology of organic fluid the value of the flue gas inlet temperature in the exchanger group 5 can be established appropriately.

It can be seen that the plant in Fig. 2 advantageously also comprises a fourth heat exchange unit 5"" which could also be integrated in group 5. This fourth unit 5"" has the function of preheating the combustion air intended for the combustion chamber 10 and circulating in feed line 9. In particular it can be seen that the combustion air is preheated by the combustion flue gases leaving group 5. It can be seen that downstream of the fourth heat exchange unit 5"", the flue gas recycling line 11' mentioned above departs from the flue gas circulating line 11. The fraction of the flow of combustion flue gases not circulating in the recycling line 11' (and therefore not intended for the mixing chamber 18) is sent to a flue gas treatment device 97 before being discharged to atmosphere employing aspirating means 94.

Still as shown in Fig. 2, the plant 1 according to the invention advantageously also comprises an emergency line 11 " for admitting cooling air in emergency. In particular this emergency line 11" envisages a fan 95 (or functionally equivalent means) for admitting cooling air. Second temperature detecting means 92 are arranged, downstream of the first detecting means 91 , for sending a signal, characteristic of the temperature level reached by the combustion flue gases upstream of heat exchange group 5, to a control unit (not shown). Based on this characteristic signal, the control unit operates fan 95 to allow rapid admission of cooling air into the flue gas circulating line 11 for instantaneously stopping the process of preheating, evaporation and superheating of the organic fluid operated by the respective units 5', 5" and 5"' of group 5. The presence of the emergency line 11" makes the plant according to the invention particularly safe and able to W

react to any emergencies. Essentially, this emergency line 11 " further improves the conditions of safety in which plant 1 operates.

Figs. 3 to 5 relate to a possible embodiment of a heat exchanger unit 5', 5", 5'" of group 5 of plant 1 according to the present invention. In particular, the unit shown in these figures can be used for preheating, evaporation or superheating of the organic fluid circulating in the operational circuit 2 of an ORC plant and preferably in plant 1 according to the invention. Purely for clarity, the heat exchanger unit illustrated in Figs. 3 to 5 will be indicated with the reference 5' already used for indicating the first heat exchanger unit of group 5 of plant 1.

In addition to the first casing 31 , traversed by the combustion flue gases, and the second casing 41 traversed by the organic fluid, the heat exchanger unit 5' shown in Figs. 3 to 5 also comprises a third insulating casing 23. This third casing 23 is physically arranged between said first casing 31 and said second casing 41 in such a way as to be traversed by a third portion 16'" of the pipes 15', 15", 15'" between the first portion 16' and the second portion 16" indicated above. In other words the third casing 23 keeps the second casing 41 physically separate and apart from the first casing 31 defining a volume of separation between them (first casing 31 and second casing 41 ).

The exchange unit 5' preferably also comprises aspirating means for keeping the pressure inside the third casing 23 below a predetermined minimum pressure. In this way any losses of organic liquid from the second casing 41 are "aspirated" by the volume of separation actually defined by the third casing. This arrangement advantageously prevents the organic fluid (relative to said losses) coming into contact with the high-temperature surfaces that define the first casing 31. Essentially this solution allows the heat exchanger unit 5' to operate in conditions of maximum safety, since the risks of ignition of the organic fluid are eliminated. Figs. 3 and 4 are respectively a front view and a side view of a heat exchanger unit 5' according to the present invention. The first casing 31 , hollow internally, has a substantially prismatic configuration defined by two substantially vertical parallel walls 49 (indicated in Fig. 3). At the bottom, the first casing 31 is closed by a bottom 48 which supports the vertical walls 49 and at the top by a first plate 51 opposite and above said bottom 48. The latter essentially defines a supporting plane 9 for the exchange unit 5. The heat exchanger unit 5', via the bottom 48, preferably rests on a frame (not shown), under which a belt conveyor can be arranged for removing dust. Moreover, the bottom 48 can be equipped with a device for extraction and/or separation of said dust.

The first plate 51 supports pipes 15', 15", 15'" vertically so that they are suspended inside the first casing 31 or so that the pipes are not supported on the bottom 48. This solution allows the pipes 15', 15", 5"' to expand freely in the vertical direction as a result of the heating to which they are subjected. With regard to this expansion, it can be seen that the vertical walls 49 of the first casing 31 are connected to the first plate 51 in such a way that the latter is free to expand in directions substantially parallel to the supporting plane 9 defined by bottom 48. The inlet section 21 ' and the outlet section 21 " of the combustion flue gases of the first casing 31 have a substantially rectangular configuration and extend on planes parallel to one another and orthogonal to the planes along which the vertical walls 49 extend. It can thus be seen that said sections 21 ' and 21 " define a direction of crossing 101 (indicated in Fig. 4) of the combustion flue gases which is also substantially parallel to the planes along which the vertical walls 49 extend.

Referring to Fig. 3, it can be seen that the heat pipes 15', 15", 15"' are arranged so that their longitudinal axis is orthogonal to the direction of crossing 101 noted above. It has been found that this arrangement makes it possible to optimize the heating of the first portion 16' of the heat pipes 15', 15", 15"' and thus obtain more efficient heat exchange.

According to a preferred embodiment of the invention, the vertical walls 49 of the first casing 31 are advantageously removable to allow inspection and/or maintenance of the first portion 16' of the heat pipes 5', 15", 15'". In this sense it can be seen that the operations of inspection and/or maintenance are extremely quick and easy, in contrast to said operations in the heat exchangers used in the known ORC plants.

Still referring to Figs. 3 to 5, the second casing 41 is preferably defined by a plurality of containments 41 ', 41" separated from one another and connected by one or more connecting lines 44 that allow the organic fluid to pass from one containment to another. The second casing 41 further comprises a second plate 52 that supports said containments 41 ', 41 ". This second plate 52 is traversed by the heat pipes 15', 15", 15'" and fixes the position of said pipes and in particular the position of the second portion 16" of the heat pipes inside the containments 41 ', 41 ".

More precisely, in the solution illustrated in Figs. 3 to 5, the second casing 41 comprises a first containment 41 ' and a second containment 41" of substantially cylindrical shape delimited at the bottom by the second flange 52 and at the top by a corresponding closing flange 53. The latter is preferably removable to allow inspection and/or maintenance of the second portion 16" of the heat pipes 15', 15", 15'" contained in each containment 41 ', 41 ".

Referring in particular to Fig. 5, the plurality of heat pipes 15', 15", 15'" comprises a first group 15' of heat pipes, a second group 15" of heat pipes arranged in such a way that a first portion 16' is housed in the first casing 31 , a second portion 16" is housed in the first containment 41 ' of the second casing 41 and a third portion 16'" traverses the third, insulating containment 23. Unit 5' further comprises a third group 15"' and a fourth group of pipes (not visible) arranged in such a way that, for each pipe, a first portion 16' is housed in the first casing 31 , a second portion 16" is housed in the second containment 41 " of the second casing 41 and a third portion 16'" traverses the third, insulating containment 23.

It has been found that this particular arrangement in groups makes it possible to optimize heat exchange between pipes 15', 15", 15"' and the organic fluid, at the same time allowing easy access to the second portion of heat exchange. In particular the containments 41 ', 41 " of the second casing 41 make it possible to optimize the transfer of heat from the second portion 16" of pipes 15', 15", 15"' to the organic fluid itself as they "concentrate" the organic fluid actually inside this second portion 16".

Figs. 6 and 7 relate to a second possible embodiment of a heat exchanger unit (called "second unit 5" hereinafter) particularly suitable for preheating the organic fluid (in the liquid state) circulating in the operational circuit 2 of plant 1 before the next evaporation phase. In particular this second heat exchanger unit 5" comprises a first portion 91 ', a second portion 91 " connected to said first portion 91 ' and a third portion 91 "' connected to said second portion 91 ". Each of the three portions 91 ', 91 ", 91 '" has a configuration substantially equivalent to that of the heat exchanger unit 5' shown in Figs. 3 to 5. In particular it has a first casing 31 , a second casing 41 and a plurality of heat pipes contained in the two casings 31 and 41 as described above. It can be seen that the second heat exchanger unit 5" shown in Figs. 6 and 7 has a substantially "modular" structure in which each module is equivalent, essentially, to the heat exchanger unit (indicated with 5') shown in Figs. 3 to 5 and described above.

Referring in particular to Fig. 6, it can be seen that for each of the three portions 91 ', 91 ", 91 "' the first casing 31 is connected to the corresponding first casing of an adjacent portion in order to allow a continuous flow for the combustion flue gases that develops between an inlet section 21 ', defined by the first casing 31 of the first portion 91', to an outlet section 21 " defined instead by the first casing 31 of the third portion 91 '".

Referring to Fig. 7, it can be seen that for each of the three portions 91 ', 91 ", 91 "' at least one of the two containments 41 ', 41 " of the second casing 41 is connected to one of the containments of the second casing of an adjacent portion. This connection is made by a corresponding connecting line 44'. This solution allows a continuous flow for the organic fluid from an inlet section 22' (defined by one of the containments of the second casing 41 of the third portion 91 "') to an outlet section (defined by one of the containments of the second casing 41 of the first portion 91 ').

Figs. 8 and 9 relate to a further possible embodiment of a heat exchanger unit (still indicated with the reference 5') of the group 5 of plant 1 according to the present invention. In particular this heat exchanger unit is particularly suitable for evaporation of the organic fluid and will therefore be indicated hereunder as the first unit 5'. It can be seen from Figs. 8 and 9 that this first unit 5' also has a substantially "modular" configuration (similar to the second unit 5" in Figs. 6 and 7) in which each module is defined essentially as a heat exchanger unit substantially similar to that shown and described in Figs. 3 to 5.

More precisely the first unit 5' has a first portion 91 ' and a second portion 91 " connected to said first portion 91 '. Each of these portions 91 ', 91 " comprises a first casing 31 and a second casing 41 inside which a plurality of heat pipes is arranged (not shown in the drawings). The first casing 31 of the first portion 9 ' is connected to the first casing 31 of the second portion 91" in order to allow a continuous flow for the combustion flue gases from an inlet section 21', defined by the first casing 31 of the first portion 91 ' to an outlet section 21 " defined by the first casing 31 of the third portion 91"'.

Referring to Fig. 9, for each of the portions 91', 91" of the first unit 5', the corresponding second casing 41 comprises a plurality of containments 41', 41" each in its turn comprising an inlet section 22' for the organic fluid in the liquid state and an outlet section 22" of the organic fluid in the vapour state.

Figs. 10 and 11 relate to a further possible embodiment of a heat exchanger unit (indicated hereinafter as third unit 5"') of a group 5 of a plant 1 according to the present invention that is particularly suitable for superheating the organic fluid in the vapour state leaving the first heat exchanger unit 5'. This third unit 5"' corresponds substantially to that shown in Figs. 3 to 5, differing from it by a different configuration of the second casing 41. More precisely the latter comprises a first containment 41 ' that defines an inlet section 22' of the organic fluid in the vapour state. This first containment 41' communicates with a second containment 41" in its turn communicating with a third containment 41'". The latter finally also communicates with a fourth containment 41 "" that defines an outlet section 22" of the organic fluid in the vapour state.

Fig. 12 shows a group 5 of a plant 1 according to the present invention that comprises a first heat exchanger unit 5' corresponding to that shown in Figs. 6 and 7, a second heat exchanger unit 5" corresponding to that shown in Figs. 8 and 9 and a third heat exchanger unit 5'" corresponding to that shown in Figs. 10 and 11. The particular structure of the heat exchanger units 5', 5", 5"' described above allows easy assembly thereof and hence construction of a particularly compact group 5.

Still referring to Fig. 12, it can be seen that group 5 comprises a central portion 81 , a first lateral portion 81' and a second lateral portion 81 ". The first central portion 81 consists of the three heat exchanger units 5', 5" and 5"' indicated above, while the first lateral portion 81 ' has a different configuration, extending from a first section 85' to a second section 85" of substantially rectangular shape and having an extension, reckoned on a plane orthogonal to the direction 101 , markedly greater than that of the first section. The first section 85' instead has a shape geometrically conjugated to that of the section of the flue gas circulating line 11 of plant 1 upstream of group 5 relative to the direction of crossing of the flue gases. In this sense the rectangular shape represents, for example, an advantageous design possibility for the first section 85' and for the relevant section of the flue gas circulating line 11.

The first lateral portion 81' is connected to the central portion 81 near said second section 85" of rectangular shape. In essence, the first lateral portion 81 ' has a divergent configuration, relative to the direction of crossing 101 , in order to assist the expansion of the combustion flue gases or in order to assist their slowing down in the central portion 81.

The second lateral portion 81" has a configuration similar to that of the first lateral portion 81 ', but a substantially opposite orientation. In fact it extends between a first section 86', of substantially rectangular section and a second section 86" of shape geometrically conjugated with the section of the flue gas circulating line downstream of group 5. In this case the first section 86' of the second lateral portion 81" is markedly greater than the second section thereof according to a convergent configuration towards the direction of crossing 101.

The structure of group 5 proves to be particularly advantageous in terms of overall dimensions as it concentrates, in a relatively restricted space, all the heat exchanger units 5', 5", 5"' provided for transferring heat energy from the combustion flue gases to the organic fluid. In particular, group 5 also appears to be more advantageous in terms of operation and maintenance compared with the conventional circuits with thermal oil. In this sense it can again be seen that each heat exchanger unit 5', 5", 5'" permits maintenance interventions that are easy and rapid, and targeted owing to the possibility of removing some structural components (side walls 49, second plate 52) of the two casings 31 , 41 of which it is composed. It can also be seen, moreover, that the operation of the heat pipes 15, 15', 15" of each of the units 5', 5", 5"' can be constantly checked by the use of thermocouples connected to said pipes. In particular, by means of said thermocouples it is possible to check the external temperature level of the heat pipes to verify that they are operating correctly.

The technical solutions adopted for the plant mean that the problems and aims established can be achieved completely. In particular the use of heat exchanger units, like those described above, in an ORC plant provides efficient heat exchange between the combustion flue gases, which is reflected in high efficiency of said plant. The use of a heat exchanger unit based on the use of heat pipes ensures, moreover, far lower costs of repairs and maintenance compared with the conventional solutions. In this connection, it is noted that any failure or malfunction of one or more heat pipes, advantageously, does not lead to shutdown of the ORC plant, which can instead remain in operation. The heat exchange group of the plant thus proves extremely reliable from the functional standpoint.

The heat exchanger unit, the heat exchange group and the ORC plant thus conceived are amenable to numerous modifications and variants, all falling within the scope of the inventive concept; moreover, all the details can be replaced with others that are technically equivalent. In practice, the materials used as well as the dimensions and contingent shapes can be varied at will according to the requirements and the state of the art.