EP2665971A2

EP2665971A2 - System and method for cooling and extraction of heavy ashes with increase in total boiler efficiency

Info

Publication number: EP2665971A2
Application number: EP12703161.5A
Authority: EP
Inventors: Mario Magaldi; Alberto Carrea; Rocco Sorrenti
Original assignee: Magaldi Power SpA
Current assignee: Magaldi Power SpA
Priority date: 2011-01-21
Filing date: 2012-01-18
Publication date: 2013-11-27
Also published as: EA201300851A1; BR112013018427A2; WO2012098504A3; IT1405071B1; MX2013008443A; WO2012098504A2; CN103477152A; US20130284079A1; AR085084A1; JP5539598B2; KR20140008347A; JP2014509378A; TW201237332A; ITRM20110023A1

Abstract

A cooling system (1 ) for heavy ashes of the type apt to be used in association with a combustion chamber (2), in particular for large flows rates of ashes deriving for example from solid fossil fuel in an energy-production unit, which system (1 ) comprises: - a transport belt (31 ) for transporting the heavy ashes, apt to be arranged below the combustion chamber (2) and having a containment casing (3) and a transport surface (311) equipped with openings (9) for the transit of cooling air, which transport surface (311) is apt to receive the ashes produced in the combustion chamber (2) substantially in the form of continuous bed; and - cooling means for cooling the heavy ashes received on said transport surface (311), which cooling means comprises at least one partitioned region (4) arranged below said transport surface (311) and forced feeding means (11) for a forced feeding of cooling air at said partitioned region (4), wherein said partitioned region (4) is configured so as to limit outflows of air fed therein, and wherein the overall arrangement is such that, in use, the cooling air fed into said partitioned region (4) crosses said openings (9) in said transport surface (311) and the bed of ashes received on the latter (Fig.1).

Description

SYSTEM AND METHOD FOR COOLING AND EXTRACTION OF HEAVY ASHES WITH INCREASE IN TOTAL BOILER EFFICIENCY

DESCRIPTION

Field of the invention

The present invention relates to a cooling system for heavy ashes of the type apt to be used in association with a combustion chamber, in particular for large flow rates of ashes deriving for example from solid fossil fuel in an energy-production unit.

Background of the invention

Well-known dry cooling and extraction systems for heavy ashes produced in combustion chambers (or boilers) from solid fuel are based upon air cooling of an ash bed . To this end, the latter is transported on a belt resistant to high temperatures placed just below the throat of the boiler. Cooling air is drawn into the extraction system by the negative pressure present within the boiler, passing through proper openings of a containment casing of the belt. The air then runs the system and the ash bed in countercurrent to the sense of direction, thus operating the cooling of ashes and equipments.

A system for extraction and cooling of that type just described is disclosed in EP 0 252 967.

The above-mentioned mechanism of heat exchange between ashes and air in countercurrent affects the size of the extraction system, in terms of:

air flow rate used, where the speed of air itself within the system has to be maximized to increase, as a result, the heat exchange coefficient with the ash; and

wheelbase and speed of transport belt, parameters that are to be optimized to increase the residence time of the ash in contact with the air and to limit the height of ash layer.

In fact, the efficiency of ash cooling is limited to the exposed and available surface for the heat exchange with the air. In particular, given the isolating nature of ash, the first layers licked by the air get cool whereas the inner layers of the ash remain at temperature. Therefore, a perfectible first aspect of the known system is related to the heat exchange mode between ash and cooling air.

The relationship between the amount of cooling air and ash is typically 3: 1 , i.e. 3 tons of air are needed to cool 1 ton of heavy ashes. However, since, downstream of the ash cooling, all the air introduced into the boiler is drawn from the bottom of this, the amount of cooling air must not exceed 0.5 to 2, 0% of the total air of combustion. In fact, if it is alter over such limit, the stechiometric ratio between fuel and air results in a reduction of combustion efficiency and in an increase of losses to the fireplace.

In particular, in the known systems mentioned above, factors that contributes to the increase in combustion efficiency (positive terms) are:

- the chemical energy of the, recovered by sensitive heat of cooling air thanks to the post combustion of unburned on the extracting belt favoured by the air;

- the sensitive ash heat, recovered through the sensitive heat of the cooling air reintroduced into the boiler; and

- the recovery of radiant floe at the throat of the boiler, absorbed by irradiated components of the system and transferred to the cooling air and extracted ashes.

The factor that instead determines a decrease of the efficiency of the boiler (negative term) is the loss of efficiency at the air / smoke pre-heater. The latter involves the use of ambient air which precisely cools the combustion smokes, by pre-heating them. Such preheated air is sent into the combustion chamber. However, this specific amount of air has to be reduced to take account of the air introduced from the bottom of the boiler, and thus the lower intake of cooling air in the pre-heater determines higher temperatures of output fumes from the latter.

Therefore, from the point of view of the overall combustion efficiency the system of heat exchange between ash and air cooling is perfectible.

It should also be noted that the continued growth in demand for solid fossil fuels for the production of electric energy makes it even more frequent also the combustion of coals or lignites with high ash grade. The combustion of these latter in high power boilers leads to a significant production of heavy ash, even up to 100 tons / hour, often containing high percentages of unburned. The dry cooling or mostly dry of these quantities requires considerable flow of cooling air, even two or three times greater than fossil fuels with high calorific value. This leads to the important drawback that, in the prior art systems considered above, the amount of air required for cooling of ashes is much higher than the maximum percentage re-introduced into the combustion chamber through the throat of the boiler.

Summary of the invention

According to the explanations in the previous section, the technical problem posed and solved by the present invention is to provide a system and a method that are optimized in terms of heat exchange between extracted ashes and air cooling, and that allow to overcome the drawbacks mentioned with reference to the prior art.

This problem is solved by a system according to claim 1 and by a method according to claim 17.

Preferred features of the present invention are present in the dependent claims thereof.

As explained in more detail below, the invention provides that an insufflating system - i.e. forced feeding system - is associated to the main extractor belt connected to the throat of the boiler - that takes the ambient air and pushes it inside the containing casing of extractor, in correspondence of one or more partitioned regions below the transport surface of the belt. The latter has dedicated openings - typically holes or cracks in the form of millings - which allows the passage of cooling air through it and then through the ash layer.

In this configuration, the partitioned that allows directing the cooling air through the holes-cracks, minimizing air outflows of the regions of interest. Therefore, when the cooling air crosses the bed of ash realizes with it a heat exchange of called "cross flow" type, characterized by a heat exchange efficiency much superior to that of the dry extraction systems already known, and that due to the increased area surface of ash affected to the heat transfer. The cooling air is then introduced into the boiler from the bottom of this.

In a preferred configuration, any outflows of cooling air, coming out from lights present between the transport surface and said partitioned region below without crossing the bed of ash, are recycled to the partitioned region of the main extractor, preferably by means of the same insufflating system. Thus, the invention allows to maximize the efficiency of heat exchange between the ash on the extracting belt and air cooling and so the cooling efficiency of the ashes.

This implies a strong reduction on the ratio between the quantity of cooling air and flow of extracted ash and therefore the possibility to minimize the amount of cooling air reintroduced into the combustion chamber from the bottom, with a consequent strong reduction in losses associated with the air / smoke exchanger and an increase in net overall efficiency of combustion.

I n add ition , the configuration of the i nvention , particu larly in the preferred embodiments illustrated below, allows making the extraction and cooling system extremely compact and simple, even for the possibility of eliminating a auxiliary wet cooling system present in some known systems.

In a particularly preferred configuration, there is also a further stage of cooling, preferably located downstream of a stage of crushing and based on an auxiliary heat exchanger of tube bundle type crossed by a working fluid cooling.

In said further stage of cooling is expected a fluidization of the ash contained in a cooling volume, preferably performed by the same ambient air recovered for cooling on the main extraction belt. This fluidization allows an improved heat exchange with the surfaces of the tube sheaf. The head ash to be fluidized can be obtained by making a transport belt placed downstream the cooler and similar to the main extracting belt working in an extraction manner.

According to another preferred feature of the invention, the main extracting belt connected to the throat of the boiler by means of a hopper selectively closable to block the ash flow. During the accumulation stage of the ash on the closed bottom of the hopper, there is a cooling of the ash by the same forced feeding system of cooling air, preferably obtained by the same main forced feeding system associated with the cooling on the belt.

It will be appreciated that said cooling system allows avoiding an important drawback of the known systems, namely the fact that at the opening of the hopper, large amounts of ash at high temperature have to be cooled on the belt, potentially out of the nominal parameters of the system.

Brief description of the drawings

Other advantages, features and conditions of employment of the present invention wi ll be apparent from the followi ng detailed description of certain preferred embodiments presented for illustrative purpose without limiting the scope of the invention. Reference will be made to the figures of the annexed drawings, wherein:

-Fig 1 shows a schematic representation in side view of an extraction, cooling and transporting system of ashes according to a preferred embodiment of the invention;

-Fig 2 shows a schematic cross-sectional view of the system of Figu re 1 , performed along the line AA of the latter and suitable to highlight a first preferred embodiment of a partitioned region of said system;

-Fig 2A shows a schematic perspective view of part of the region partitioned by lateral baffles of Figure 2;

- Fig 3 shows a schematic cross-sectional view of the system of Figure 1 , performed along the line AA of the latter and suitable to highlight second preferred embodiment of a partitioned region of the system;

- Fig 3A shows a magnified view of a detail of the partitioned region of Fig. 3;

- Fig 4 shows a schematic magnified view of a detail of the system of Figure 1 , showing the presence of additional cooling stages of the ashes;

- Figure 5 shows a cross section view of the system of Figure 1 performed at the throat of the boiler, showing a cooling circuit of the ash in the hopper;

- Fig 6 shows a plan view of a detail of the transport belt, showing the passage openings for a cooling air flow, and

- Fig 6A shows a cross section view of the belt of Fig. 6, performed along the line AA of the latter figure.

Detailed description of preferred embodiments

With reference to Figure 1 , a preferred embodiment of extraction and cooling system of heavy ashes of the invention is globally indicated with 1 . The system 1 is of the type apt to be used in association with a combustion chamber or boiler 2, in particular for large flow rates of ashes deriving for example from solid fossil fuel in an energy-production unit.

The boiler 2 may be an integral part of the system 1 or provided separately from it and is equipped with an extraction hopper 21 , the latter typically lined inside with refractory material. The hopper 21 is associated with a system that allows the closing of its bottom and then of the boiler 2 throat, which will be described in greater detail later.

The initial part of a continuous transport belt 31 , moving along a closed path, is placed in correspondence of the bottom of the boiler 2. During the use, the belt 31 receives from the hopper 21 the ashes produced by the boiler 2 and substantially carries them in the form of continuous bed. In particular, the ashes are received on a upper transport surface 31 1 of the belt 31 during a return run of it. On this transport surface 31 1 , during the movement away of the ashes from the boiler bottom 2, takes place the dry cooling of the ashes themselves, by means of a flow ambient air which is sent into a containment casing system 3 of the belt 31 according to ways that will be shortly described.

The transport belt 31 and its casing 3 may have a globally construction of the kind described in EP 0 252 967 or EP 0 931 981.

Moreover, as shown in Figure 6 and 6A, passing openings for cooling air 9 are made on the continuous belt 31 for example in the form of holes or, as represented, cracks obtained by milling.

Always with reference to Figure 1 , the system 1 is equipped with means of cooling of the ashes received on the transport belt 31 , apt to determine a feeding of cooling air in correspondence of those ashes.

Such cooling means include means of forced feeding of air, for example based on a blower or compressor 1 1 and on an associated intake pipe of ambient air 1 1 1 , the latter preferably equipped with appropriate control means selectively operable, in particular a valve 1 12. Such aspirated ambient air is sent to a feeding pipe, globally denoted by 13, that conduct it to a partitioned region 4 associated with the belt 31 . Even the feeding to the partitioned region 4 is preferably controlled by appropriate control means selectively operable, in this case in particular a valve 134.

In Figure 1 , a single partitioned region 4 was represented for simplicity, arranged downstream of the bottom boiler with respect to the direction of advancement of the transport surface 31 1 and below to the latter, interposed between the there and back tract of the belt 31 . However, the partitioning preferably extends longitudinally to cover the whole bottom of the transport surface 31 1 (as shown in Figures 2, 2A and 3, 3A). Moreover, in one embodiment, different partitioned regions may be made discretely distributed along said transport surface 31 1 , below it.

The partitioned region 4 is apt to minimize outflows of alleged cooling air in it, so that such air passes almost entirely through the openings 9 of the transport belt 31 , thereby effectively cooling the bed of ashes received on the transport surface 31 1 .

The blower or compressor 1 1 then generates a suitable pressure gradient to overcome the losses d istributed and concentrated along the circuit 1 3 and associated with the transport belt 31 and the overlooking layer of ash.

In a first embodiment shown in Figures 2 and 2A, the partitioned region 4 is affected by transverse baffles 6, arranged transversely to the transport surface 31 1 with respect to the d irection of th is advancement, and bounded laterally by two longitudinal baffles 7, spanning according said progress direction.

The lateral baffles 7 are arranged near the transport surface 31 1 and its roller support 14, so as not to interfere with the movement of each body but at the same time minimizing light outflows through the air cooling alleged in the partitioned region 4.

In addition, the arrangement of the transverse baffles 6 of the partitioning of the region below the transport surface 31 1 ensures a labyrinth seal to the cooling air, assisting the sealing action to the lateral air outflows by means of baffles 7.

Always in this example, the partitioned region 4 is bounded below by a tilted-surface plate 5 for recovering of any lost fines during the transport on the surface 31 1 . The longitudinal baffles 7 have each a corresponding lower end door 72, selectively openable to the outside through a mechanism 71 , preferably hinged , for the downflow of fines toward the bottom of the containment casing 3 (where they can be recovered by a cleaning system not shown). Preferably, the downflow system based on door items 72 - mechanisms 71 is timed.

In a second embodiment of the partitioned region 4 shown in Figures 3, 3A, transverse baffles 6 are still provided, in this case associated with bulkheads or lateral baffles 51 which extend longitudinally along the belt 31 , substantially parallel to it, upper to the transport surface 31 1 and each along a respective side of this, where the contact or proximity of said baffles 51 with the containment ends 81 of the transport belt 31 , allows limiting the passage of air that does not pass through the hole belt 31 . ln this second embodiment, each of the tilted-surface plate 5 presents a lower end door 725 selectively openable to the outside through a mechanism 715, preferably hinged, for the purposes of downflow of fines toward the bottom of said containment casing 3. Thus, when - during normal operation - the door 725 is closed, it keeps a tight air, being in direct contact with the side wall of the casing 3.

A further embodiment may provide the combined presence of these side walls arranged at the upper transport surface, of the lateral baffles placed lower to the latter and with selectively openable doors and of a tile, the latter typically without doors as in the first embodiment described above.

The globally configuration of the cooling means is such that the ash layer transported on the surface 31 1 is cooled by a flow of ambient air passing through it transversely from the bottom to the top along the entirely length of the cooling forced region consists of the partitioned region 4 and comprises between the first and the last transverse baffle 6. The cooling air which has passed through the bed of ash, is attracted in the boiler 2 from the bottom of it being this, as well known for a skilled person in the technical filed, at pressure values lower to the environment of the casing 3.

As already mentioned, the mechanism of heat exchange between air and ash thus obtained is characterized by high thermal efficiency, thanks to the large ash surface available for the contact with the ambient air.

In Figures 2, 2A and 3, 3A typical containing lateral edges 8 flanking longitudinally the entire transport surface 31 1 are also represented.

Always with reference to Figure 1 , to prevent uncontrolled entry into the boiler 2 of cooli ng air that escapes from the lights of the partitioned region 4, in this embodiment are provided means of air re-circulation in that region, preferably openable by the same forced feeding means 1 1 . In the present example, these means provide a pipe for extracting air 131 from the casing 3 in communication with the feeding pipe 13.

In particular - and also with reference to Figures 2, 2A / 3, 3A - the circuit 13-131 can take air from an region 15 between the casing 3 and the lateral baffles 7 (Figure 2) or between the casing 3 and bottom faces of the tile 5 (Fig. 3) and send it back to the partitioned region 4 below the transport surface 31 1 . The recirculated air, not having passed the bed of ash, will have a temperature close to the environment one. Also in this preferred embodiment example, said air outflows can be intercepted by pressure control 16 means, apt, in use, to detect by sensors a pressure difference between a first area 161 in the casing 3 arranged above the transport surface 31 1 and a second area 162 arranged in said casing 3 lower the transport belt 31.

In Figure 1 , these areas 161 and 162 have been depicted as arranged, for example, at a portion of the casing 3 immediately below the combustion chamber 2. The area 1 62 may also coincide with the above-mentioned area 1 5, being the pressures substantially equal in the two areas.

The pressure control means 1 6 are in communication with the air recirculating means and then with the feeding forced means 1 1 by a control valve 132 selectively openable. Preferably, there is an automatic control mean associated with the system 1 and with the means 16 that, if a overpressure in the second zone 162 is detected, operated the valve 132 so to determine an extraction of air through the pipe 131 and its return within the partitioned region 4, bringing the pressure difference between the two areas 161 and 162 essentially at zero. In this way, the transfer of air outflows from the area 162/15 to the area 161 is prevented.

Always with reference to Figure 1 and now also to Figure 5, preferably the system 1 also provides feeding means of cooling air to the extraction hopper 21 of the boiler 2, apt to allow a cooling of the ash held on that said hopper when it is closed, for example, during short periods of maintenance of the belt 31 or any other operational need or discontinuous management arrangements of the system 1 . Preferably such means are operated by the same forced feeding means 1 1 and are based on feeding means 100 even in this case with selectively adjustable air flow rate, for example by one or more valves 101.

As mentioned above, the hopper 21 provides a locking system that allows accumulation of the heavy ash on it. This system is formed preferably by one or more refractory valve 212 preferably servo-controlled and operated according to a rotating closing movement.

Such feeding means of cooling air to the hopper 21 allow the cooling of the ash during said accumulation phase in the hopper and are preferably operated automatically by closing the bottom valves 212. The pipe circuit 100 feeds one or more air inlets 213 made on the bottom valves 212, resulting in an homogenous distribution of the air from the bottom of the hopper 21 . The entering air to the hopper 21 is of course sent at a pressure such to overcome the loss of load generated by the layer of ash accumulated, thus procuring a suitable cooling of the bed of ash present on the valves.

Always with reference to Figure 1 and now also to Figure 4, in this configuration the system 1 also includes a second assembly / casing transport belt, globally denoted by 30 and similar to the first, arranged downstream of the main belt 31 by means of interposition of an ash crusher 1 7 and of an auxiliary cooling device 18 of tube bundle type 183.

The presence of the second transport belt 30 may be advisable depending on the amount and size of the ash. It may be associated with it forced feeding air means of one or more partitioned regions and eventually recirculation air means similar to those already described with respect to the first transport belt 31 and preferably integrated with these. In such a configuration, the cooling air introduced into the area below the belt 30 is then drawn into the boiler 2 by the pressure running existing therein.

The crushing device 17, which may also include multiple stages of fragmentation in sequence, allows increasing the ash surface available for the cooling, thus increasing the overall efficiency of the latter.

The auxiliary cooling device 18 provides that the ash is accumulated within a volume 181 defined by walls 182 preferably metal and associated with these tube bundles 183, also preferably metal and constantly traversed by a fluid at low temperature, preferably water. Still in a preferred configuration, these bundles 183 are arranged horizontally or however that develop in the direction substantially orthogonal to that of a fluidizing gas flow that will be introduced shortly.

The second transport belt 30 is controlled by fed speed and transport width such to realize an ash head within the cooling device18 associated with it, working as ash puller from the latter. At the basis of the cooling volume 181 , there is a feeding circuit of a fluidizing gas 133, preferably also with a selectively adjustable flow rate through appropriate means such as a valve 135.

In the present example, the fluidization gas is air, and in particular the same cooling air fed by force by the means 1 1 and through the valve 134 and the pipe circuit 13.

The feeding of fluidizing air affects preferably the entire outer perimeter of the walls 1 82. The air sent in this way within the volume 181 fluidizes the ash present, promoting a high number of collisions of ash particles with the surfaces of the tubes 183 cooled by the water. In this way an effective additional cooling of the ash is obtained all the more appreciated as much as the smaller size of the particles of the fluidised ash.

Another object of the invention is a method of extraction, cooling and recovery of heavy energy ash as described so far in relation with the system 1 .

The ash cooling means into the hopper and its method as described above and as an object of the following dependent claims could also be protected independently from the invention as defined in claims 1 and 17, and in particular independently form the expectation of air cooling means based on a partitioned region.

Similarly, the fluidization system of the ash in a tube bundle cooler and its method as described above and as an object of the following dependent claims could be protected independently from the invention as defined in claims 1 and 17, and in particular independently form the expectation of air cooling means based on a partitioned region.

The present invention has been described so far with reference to preferred embodiments. It is intended that there may be other embodiments which refer to the same inventive nucleus, all falling within the protection of the claims set out below.

Claims

1. A cooling system (1 ) for heavy ashes of the type apt to be used in association with a combustion chamber (2), in particular for large flows rates of ashes deriving for example from solid fossil fuel in an energy-production unit, which system (1 ) comprises:

- a transport belt (31 ) for transporting the heavy ashes, apt to be arranged below the combustion chamber (2) and having a containment casing (3) and a transport surface (31 1 ) equipped with openings (9) for the transit of cooling air, which transport surface (31 1 ) is apt to receive the ashes produced in the combustion chamber (2) substantially in the form of continuous bed; and

- cooling means for cooling the heavy ashes received on said transport surface (31 1 ), which cooling means comprises at least one partitioned region (4) arranged below said transport surface (31 1 ) and forced feeding means (1 1 ) for a forced feeding of cooling air at said partitioned region (4),

wherein said partitioned region (4) is configured so as to limit outflows of air fed therein,

and wherein the overall arrangement is such that, in use, the cooling air fed into said partitioned region (4) crosses said openings (9) in said transport surface (31 1 ) and the bed of ashes received on the latter.

2. The system (1 ) according to claim 1 , wherein said partitioned region (4) develops longitudinally along said transport surface (31 1 ) substantially for the entire extension of the latter.

3. The system (1 ) according to claim 1 or 2, wherein said partitioned region (4) is laterally delimited by one or more pairs of longitudinal baffles (7) extending along the direction of advancement of said transport belt (31 ).

4. The system (1 ) according to the preceding claim, wherein one or both of said longitudinal baffles (7) have a door (72) selectively openable for the downflow of fines toward the bottom of said containment casing (3).

5. The system (1 ) according to any one of the preceding claims, wherein said partitioned region (4) is delimited below by a tilted-surface plate (5) for fines recovery.

6. The system (1 ) according to the preceding claim, wherein said plate (5) comprises one side door (725) or a pair of side doors (725), selectively openable for the downflow of fines toward the bottom of said containment casing (3).

7. The system (1 ) according to any one of the preceding claims, comprising one or more pairs of side bulkheads (51 ) longitudinally extending along the flanks of said transport surface (31 1 ), above thereto, and at said partitioned region (4) so as to limit air leaks.

8. The system (1 ) according to any one of the preceding claims, wherein said partitioned region (4) comprises a plurality of transverse baffles (6) arranged transversally to said transport surface (31 1 ) with respect to the direction of advancement of said belt (31 ) and apt to define a substantially labyrinth-like seal for air fed in said partitioned region (4).

9. The system (1 ) according to any one of the preceding claims, comprising air recirculation means (131 ) for air recirculation in said partitioned region (4), apt to extract air from said containment casing (3) and preferably operable by said same forced feeding means (1 1 ).

10. The system (1 ) according to the preceding claim, comprising pressure control means (16) in communication with said air recirculation means (131 ), which control means is apt, in use, to detect a pressure difference between a first area (161 ) in said casing (3) arranged above said transport surface (31 1 ) and a second area (162) in said casing (3) external to said partitioned region (4) and arranged below said transport surface (31 1 ).

11. The system (1 ) according to any one of the preceding claims, comprising feeding means (100) for feeding cooling air into an extraction hopper (21 ) of the combustion chamber (2), apt to allow a cooling of the ashes retained on said hopper (21 ) when the hopper is closed, wherein preferably said air feeding means (100) is operable by said same forced feeding means (1 1 ).

12. The system (1 ) according to any one of the preceding claims, comprising fluidization air feeding means (133) for feeding fluidization air into an ancillary cooling device (18) arranged downstream of said transport belt (31 ), apt to determine a fluidized moving of the ashes received therein, wherein preferably said fluidization air feeding means (133) is operable by said same forced feeding means (1 1 ).

13. The system (1 ) according to the preceding claim, wherein said ancillary cooling device (18) is of tube bundle (183) type.

14. The system (1 ) according to claim 12 or 13, comprising a crusher (17) arranged upstream of said ancillary cooling device (18).

15. The system (1 ) according to any one of the claims 12 to 14, comprising a second transport belt (30) arranged downstream of said ancillary cooling device (18).

16. The system (1 ) according to claims 9, 1 1 and 12 or according to any one claim dependent therefrom, wherein said forced feeding means (1 1 ), said air recirculation means (131 ), said air feeding means (100) for feeding air into a hopper and said fluidization air feeding means (133) are connected to form a single circuit equipped with selectively operable flow adjustment valves.

17. A method for cooling heavy ashes of the type apt to be used in a system comprising a combustion chamber (2), in particular for large flows rates of ashes deriving for example from solid fossil fuel in an energy-production unit, which method comprises the steps of:

- extracting and transporting the ashes produced in the combustion chamber (2) on a transport surface (31 1 ) substantially in the form of a continuous bed, which transport surface (31 1 ) is equipped with openings (9) for the transit of cooling air; and

- forcedly feeding cooling air into a partitioned region (4) arranged below said transport surface (31 1 ) and configured so as to limit outflows of air fed therein, so that the cooling air fed into said partitioned region (4) crosses said openings (9) in said transport surface (31 1 ) and the bed of ashes received on the latter and be then returned in the combustion chamber (2) from the bottom thereof.

18. The method according to claim 17, providing a recirculation of air in said partitioned region (4) by means of an extraction of air that has not crossed the ash bed by a containment casing (3) of said transport surface (31 1 ).

19. The method according to the preceding claim, wherein said recirculation of air is selectively activated depending on a value of a pressure difference between a first area (161 ) in said casing (3) arranged above said transport surface (31 1 ) and a second area (162) in said casing (3) external to said partitioned region (4) arranged below said transport surface (31 1 ).

20. The method according to any one of the claims 17 to 19, providing a feeding of cooling air into an extraction hopper (21 ) of the combustion chamber (2), so as to allow a cooling of the ashes retained on said hopper (21 ) when the hopper is closed.

21. The method according to any one of the claims 17 to 20, providing a feeding of fluidization air into an ancillary cooling device (18) arranged downstream of said transport surface (31 1 ), which air feeding determines a fluidized moving of the ashes received in said cooling device.

22. The method according to the preceding claim, comprising a step of crushing upstream of said ancillary cooling device (18).