WO2008102387A1

WO2008102387A1 - Plant and method for dry extracting / cooling heavy ashes and for controlling the combustion of high unburnt content residues

Info

Publication number: WO2008102387A1
Application number: PCT/IT2007/000118
Authority: WO
Inventors: Mario Magaldi; Rocco Sorrenti
Original assignee: Magaldi Ricerche E Brevetti S.R.L.
Priority date: 2007-02-20
Filing date: 2007-02-20
Publication date: 2008-08-28
Also published as: KR20100014491A; JP2010519493A; CN101641552A; JP5216783B2; EP2126469A1; EA200901137A1; BRPI0721307A2; KR101428831B1; EA016107B1; MX2009008840A; CA2678727A1; AU2007347640A1; US20100180807A1

Abstract

The present invention relates to a system for dry extracting / cooling heavy ashes and for controlling the combustion of high unburnt content residues, allowing to: extract heavy ash from the boiler bottom (12), foster and adjust post-combustion on the extractor belt (14) by combined use of comburent hot air and inert combustion fumes, already available in boiler, cool the ashes present on the belt and optionally re- circulating them - all or in part - in boiler, along with the fraction of light ashes of higher unburnt content.

Description

PLANT AND METHOD FOR DRY EXTRACTING / COOLING HEAVY ASHES AND FOR CONTROLLING THE COMBUSTION OF HIGH UNBURNT CONTENT

RESIDUES

Description

Field of the invention

The present invention refers to a plant and a method for dry, or mainly dry extracting / cooling and reducing unburnts in heavy ashes produced by boilers of the type used in solid fuel thermoelectric power plants. Such plant and method are specifically suitable in the case of boilers burning, under co-combustion, conventional solid fuel (typically coal dust) and non-conventional fuel, especially biomasses and/or fuel derived from municipal solid waste (RDF).

Background of the Invention The need to reduce CO₂ emissions urged to use alternative fuels in lieu of coal, such as biomasses and the so-called "RDF" (fuel derived from municipal solid waste).

If on the one hand the use of biomasses, and of non-conventional fuels in general under co-combustion with coal dust, allows a reduction in total CO₂ emissions in the atmosphere, on the other it entails a series of problems related to the combustion system, among which that of fuel pulverizing. In fact, though being very reactive with respect to coal, such alternative fuels, and particularly biomasses, require remarkable energy amounts in order to be crushed to an appropriate degree/level, thereby ensuring high combustion efficiency. Moreover, an excessive level of crushing causes a greater wear of the grinding members.

Therefore, in common practice, both in order to limit energy consumption and increase the life of said wearing parts of the grinding members, it is preferred to crush biomasses to a coarser grade, thereby reducing, however, the combustion efficiency. Accordingly, a merely partial combustion of the coarser biomass particles causes an increase in the amount of unburnts in heavy and light ashes.

Said heavy ashes are removed from the bottom of the combustion chamber by a dry extracting / cooling system typically made as illustrated in European Patent EP 0 471 055 B1 , and may undergo an at least partial post-combustion on such an extracting system, so as to reduce unburnt content in end ashes. However, in known-art plants the management modes of such a post-combustion, above all in the most critical applications associated to the use of the above- illustrated biomasses and RDF, are less than optimal and allow a merely limited reduction of total unbumts. In particular, in known plants there is anyhow the risk of uncontrolled post-combustion phenomena on the extractor; said risk limits, also implicitly, the (full) usability of measures for triggering or fostering such a post- combustion.

Summary of the Invention Hence, on the basis of what has been disclosed in the preceding section, the technical problem set and solved by the present invention is to provide a plant and a method for the post-combustion of unbumts in a system for dry - or mainly dry - extracting heavy ashes, overcoming the drawbacks mentioned above with reference to the known art. Such a problem is solved by a plant according to claim 1 and a method according to claim 30.

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

The present invention provides several relevant advantages. The main advantage lies in that it allows, above all in the mentioned case of biomass or RDF co- combustion, an effective and efficient post-combustion of unbumts on the primary extractor, thereby reducing the total content thereof, concomitantly allowing to avert the risk of an uncontrolled post-combustion.

In particular, to foster unburnt reduction the invention preferably acts both on the temperature of the heavy ashes extracted and on their residence time in an environment with a suitable temperature, typically the belt-equipped extractor portion arranged immediately downstream of the combustion chamber and facing thereon. With the increase of the temperature in the post-combustion zone and of the related residence time of the fuel therein, the combustibility rate of the latter increases proportionally. To increase the temperature, the invention provides the inletting of hot air into the extractor and, in a preferred embodiment thereof, fuel residence time is adjusted by acting on the speed of the conveyor belt. According to the invention, to control the combustion process and prevent an excessive amount of biomass or RDF from leading to uncontrolled development of heat, combustion exhaust gases (fumes) - preferably collected downstream of the electrofilter (or electrostatic precipitator) typically provided in plants to which the invention applies - are used to partially or completely replace comburent air. Hence, the invention allows to increase the post-combustion capacity of dry or mainly dry extractors, making the extracting environment where ashes are moved more favorable to the reduction of unburnts present in the ashes themselves..

The combined use of hot air and combustion fumes according to the invention allows to have total control on unburnt combustion on the extractor-afterburner belt.

As mentioned above, the post-combustion process preferably occurs and is completed in the extractor zone corresponding to the throat at the boiler bottom and optionally, if necessary and advisable for plant requirements, also in the subsequent section thereof. Moreover, in a preferred embodiment it is provided, downstream of a suitable cooling of the extracted heavy ashes, the in-boiler re-circulating thereof, along with the fraction of light ashes of higher unburnt content.

Always on the basis of a preferred embodiment, downstream of the post- combustion process there starts the cooling of the ash by air, which, in a controlled and adjusted amount, is let into the extractor, in the end portion of conveying, and/or in a secondary conveyor/cooler (post-cooler) arranged downstream of the primary extractor. Preferably, at the outlet of the conveyor-cooler an ash crushing stage (step) is provided, and, downstream thereof, a screening station allowing to eliminate any plastics or metallic residue present in the ashes. Thus, it is possible to obtain ash suitable to be stored or optionally re-circulated in a boiler.

Preferably, the permanence time of light ashes in said electrofilter (or electrostatic precipitator) is reduced by continuously re-circulating in combustion chamber the unbumt-richer fraction of light ashes. Thus, the invention allows to reduce the risks of fires in the electrofilters when as fuels there are utilized biomasses or RDF that, by accumulating in the hoppers of the electrofilters themselves can cause fires by self-combustion, biomass or RDF unburnts being very reactive with respect to those from coal.

Total unburnt reduction, attained both through post-combustion on the conveyor belt and re-circulating in combustion chamber, allows to save on consumption of ammonia utilized for NO_x reduction in catalyzers. In fact, to attain the same unburnt content in the total ashes, in the absence of re-circulation, a higher excess of combustion air would be required, with the entailed increase of NO_x rate in exhaust gases and of the amount of ammonia required for their reduction. Summing up the detailed description of preferred embodiments reported hereinafter, the present invention relates to a system allowing to: extract heavy ash from the boiler bottom, foster and adjust post-combustion on the extractor belt by - A - combined use of comburent hot air and inert combustion fumes, already available in boiler, cool the ashes present on the belt and optionally re-circulate them - all or in part - in boiler, along with the fraction of light ashes of higher unburnt content.

Brief description of the figures

Other advantages, features and the operation modes of the present invention will be made evident from the following detailed description of some preferred embodiments thereof, given by way of example and without limitative purposes. Reference will be made to the figures of the annexed drawings, wherein: - Figure 1 shows a general exemplary diagram of a preferred embodiment of the plant of the invention;

Figure 2 shows a cross section of an extractor - afterburner of the plant of Figure 1 , taken along line A-A of the latter figure; and

Figure 3 depicts a top plan view of a detail of a drilled extractor belt of the plant of Figure 1.

Detailed description

Referring to said figures, a combustion plant made according to a preferred embodiment of the invention is generally denoted by 1. The plant 1 is of the type used in thermoelectric solid fuel power plants firing solid fuel, in particular coal dust, and it is suitable for the (co-)combustion of biomasses and/or fuel derived from municipal solid waste (RDF).

In the present example, the plant 1 will be described with reference to the co- combustion of biomasses. For clarity's sake, the various components of the plant 1 will hereinafter be described mainly with reference to the path followed by biomasses, starting from their collection from storing means down to their combustion, and with reference to the path followed by combustion residues (heavy and light ashes) starting from their collection from the bottom of a combustion chamber (or boiler) 12 of the plant 1 down to their post-combustion, optional re-circulating in the combustion chamber itself and discharge.

First of all, the plant 1 provides as biomass storing means the same bunkers, per se known, already used for coal. In the present embodiment, the biomass-dedicated bunkers are the two depicted on the left in Figure 1 and denoted by 21 and 22, respectively. The other bunkers and the associated additional components depicted in Figure 1 are understood as used for coal, and therefore will not be further considered hereinafter, their structure and use being of a per se known type. Preferably, the bunkers 21 and 22 are those feeding the burners of the combustion chamber 12 at topmost levels, so as to have for the heavier particles a longer residence time in the combustion chamber itself during their falling to the bottom.

From the dedicated bunkers 21 and 22, the biomass is extracted by one or more conveyors 3 analogous to those already used for coal, or by augers. Thus, biomass is supplied, by an intercepting valve 4, to a dedicated meter 5, in this case with three outlets, respectively denoted by 51 , 52 and 53, and therefrom to one or more dedicated crushers, in this case three, respectively denoted by 61 , 62 and 63. In the present embodiment said crushers 61-63 are implemented by per se known hammer mills. Hence, said conveyors 3 constitute bypass means of known coal crushers, here denoted by F, associated to the bunkers 21 and 22. Such a bypass therefore allows the plant 1 not to overly modify its own standard configuration with respect to that related to the combustion of coal alone, allowing to leave installed also said known crushers F. The crushers 61-63 are apt to reduce the biomass to a desired maximum end (outlet) grain size. As it will be better appreciated at the end of the description, it is not required that said final grain size be particularly fine, since the overall structure of the integrated plant 1 allows anyhow a complete combustion of the biomass even at such "coarse" grain sizes. Downstream of the mills 61-63 there are respective screening means 71 , 72 and 73 apt to intercept biomass particles of a grain size greater than a predetermined threshold, in order to resend them, through dedicated mechanical or pneumatic conveyors 8, to the meter 5 and then into the same hammer mills 61-63 for a new crushing thereof. Finer biomass particles crossing the screening means 71-73 are carried by a common (shared) conveyor 9 to a single meter 10 and then, by means of a two-way valve 91 , fed to known-type pneumatic conveyors 93, the latter already present in existing solid fuel plants in association with the crushers F. I.e., the pneumatic conveyors 93 are those developing from the coal crushing mills F (not used) associated to the bunkers 21 and 22. Then, crushed biomasses are introduced in the feed pipes feeding coal dust, and therefrom fed to the burners of the boiler 12, it also of a type already present in existing solid-fuel plants.

Once the fuel comprised of said coal and biomasses is fed to the combustion chamber 12, the plant 1 and the extraction and post-combustion process carried out therefrom develop as described hereinafter.

Fly ashes leave the combustion chamber 12 through traditional ducts (flues) for expelling combustion fumes, generally denoted by 13 in Figure 1. Instead, the heavy portion of ashes and of any unburnt precipitates bottomwise and is collected on a conveyor-type dry extractor 14, of the kind subject-matter of EP 0 471 055 and not further described herein.

According to the invention, on said extractor 14, and in particular on a post- combustion or afterburning portion 141 thereof facing onto the combustion chamber 12, unburnt post-combustion goes on. To this end, the top surface of the extractor belt, and in particular of the portion 141 , receives heat by irradiation from the burners of the combustion chamber 12.

According to a variant embodiment, the plant of the invention may also comprise a biomass meter 18 independent of the combustion chamber 12 and a feeder 19, depicted also in Figure 1 , arranged upstream of the extractor 14 (or at least upstream or in correspondence of the portion 141 thereof), to supply unburnt biomass directly to the extractor 14 itself, for a first combustion of the latter biomass in the post-combustion zone 141. In this case, a first drying of said biomasses may be carried out by a feeding of hot air on the feeder 19; advantageously, said hot air may come from an air/fumes exchanger 29, already present in existing thermoelectric plants and that will be introduced hereinafter.

According to the invention, to allow a higher post-combustion efficiency and concomitantly avoid the development of uncontrolled phenomena, it is provided control means 100 for controlling unburnt post-combustion occurring on said post- combustion portion 141.

Said means 100 in turn comprises means 15 for feeding hot air and means 150 for feeding combustion exhaust gases (fumes), apt to provide a flow of heated air and of combustion fumes, respectively, in correspondence of said post-combustion portion 141 in order to respectively foster and inhibit post-combustion.

In the present embodiment, the feeding means 15 and 150 comprises respective ducts for the collecting respectively of heated air from an air chamber 151 associated to the boiler 12 and of exhaust fumes downstream of an electrostatic precipitator (electrofilter) 28 of the plant 1.

Typically, hot air in the air chamber 151 comes from the abovementioned exchanger 29, which in the present embodiment is a fumes/air exchanger arranged downstream of the combustion chamber 12 and exploiting just the residual heat of the combustion fumes to heat outside air. According to a variant embodiment, hot air fed by the means 15 may also be bled directly from the latter exchanger 29.

Said air chamber 151, air pre-heater 29 and electrostatic precipitator 28 are well- known to a person skilled in the art and already present in known plants; therefore, a further description thereof will be omitted. The means 15 for feeding hot air comprises means 143 for the controlled (adjusted) inletting of outside air, e.g. inlets made on the casing of the extractor 14 and associated to one or more valves preferably controlled by the control means 100, allowing to attain a desired oxygen content and an appropriate temperature of the air inlet into the extractor 14. Hence, heated air fed by the means 15 and suitably proportioned with atmospheric air allows to attain on the extractor/afterburner belt 14 a temperature optimal for post-combustion.

Said means 143 can also exploit the negative pressure present in the combustion chamber 12 for the feeding of air from the outside. Preferably, the overall arrangement is such that the hot-air feeding means 15 and the fume feeding means 150 is apt to supply a flow countercurrent with respect to the direction of advance of the belt of the extractor 14 itself, at least in the above- introduced post-combustion portion 141.

Both the air feeding means 15 and the fume feeding means 150 may be equipped with respective means for automatically adjusting the flow rate, respectively denoted by 102 and 103 in Figure 1 , controlled by the control means 100.

The control means 100 is apt to adjust the flow rate of hot air and/or of fumes fed into the post-combustion portion 141, and for this purpose it comprises automatic means for adjusting said flow rates of air and fumes depending on the temperature detected by suitable sensors, preferably arranged in correspondence of said post- combustion portion 141. When the temperature value exceeds a preset threshold, fume flow rate is increased and accordingly hot air flow rate is reduced: Thus, oxygen concentration is reduced, reducing the combustion rate. In particular, the fumes produced by the combustion process of the boiler may integrate or replace comburent air to adjust or stop on-belt combustion thanks to the low (< 6%) O₂ concentration in the fumes, enabling its use as inert gas. Vice versa, when the temperature is lower than a preset limit, fume inletting is inhibited and hot-air flow rate is increased, optionally reducing also the flow rate of room-temperature air introduced by the means 143. For moving the fumes an additional fan may be installed, when required, to provide them with the head required for the inletting into the extractor/afterburner 14.

In order to avoid acid condensation problems, the piping for the fumes of the feeding means 150 should be insulated, just to hold a temperature higher than the condensation one. The fumes utilized for controlling the combustion, in addition to the extinguishing power exhibit also an appreciable cooling capacity, since their temperature is no higher than 150°C. In fact, in the present embodiment said fumes come from the zone downstream of the electrofilter, i.e. are collected when they have already lost their thermal content. Hence, according to a variant embodiment, the post-combustion portion 141 is concentrated exclusively or nearly exclusively in the irradiated zone below the throat of the boiler 12, and controlled by feeding hot air and/or fumes in the manner disclosed above. The extractor belt portion not facing onto the boiler bottom is instead dedicated to the cooling, which may be carried out by the feeding of combustion fumes (with dedicated means) and cold air (with the above-introduced means 143), taking care to inlet the fluid in the conveying zone by exploiting the negative pressure in the boiler, and so as to have the fluid lick the cover of the extractor 14, cooling it.

Lastly, as a further option for controlling the combustion there may be provided the use of water finely metered by means of delivery nozzles104 preferably provided in plural zones of the extractor/afterburner 14, and in particular (at least) in the primary crushing zone (i.e. in the end portion of the extractor 14 with respect to the direction of advance of the ashes).

According to a preferred variant embodiment, hot air fed by the means 15 is supplied below the belt of the extractor 14, and in particular below the portion 141 thereof, as mentioned above in countercurrent to the flow of heavy ashes and unburnts. In this case, in order to make heat exchange and post-combustion more effective and efficient, the belt of the extractor 14 may be provided with perforations (holes) or slots 142, shown in Figure 3. Thus, hot air, in addition to heating the bottom of the extractor 14, crosses the bed of ash and unbumts and partially returns into the combustion chamber 12, thanks to the negative pressure present in the latter, re-feeding heat therein. Such a passage of air is fostered by the pressure difference existing between the bottom portion of the belt conveyor and the bottom of the boiler. Hot-air transit through the holes of the belt conveyor of the extractor 14 allows a greater and more effective contact of air itself with the ash present on the belt, with the result of enhancing the combustion efficiency of the unburnt material.

As already mentioned above, in the present embodiment onto the lateral walls of the extractor 14 - and in particular in correspondence of an end portion of the belt typically uninvolved in the post-combustion - it is provided further air inletting means 143 for the controlled inletting of outside cooling air. From the extractor 14, heavy ashes and unbumts are supplied to a secondary belt conveyor 16 serving as post-cooler, and this through a primary crusher 20, preferably water-cooled to resist high temperatures, downstream of which it is positioned a transition hopper 201 , merely sketched in Figure 1.

On the cooling conveyor 16 an air-assisted cooling of the ashes is carried out by a system in countercurrent, exploiting the negative pressure present in the combustion chamber to feed outside air via controlled inlets 160 present on the lateral walls of the casing of the conveyor 16 itself, as already described also in EP 0 471 055.

According to a preferred variant embodiment of the invention, said hopper 201 forms part of a pressure insulation system apt to create just a pressure separation between the environments of the extractor 14 and of said cooling conveyor 16. For this purpose the hopper 201 forms means for accumulating the conveyed material, allowing the forming of a head of material between said environments, creating said pressure separation. Said pressure insulation system allows a more effective management of the air- assisted cooling of the ashes, as it allows - when required and typically on the basis of ash temperature and flow rate detections performed, e.g., at their discharge onto the extractor 14 - to avoid the introduction of an excessive amount of cooling air into the combustion chamber 12, just by selectively activating such a pressure separation, as described also in PCT/IT2006/000625.

The head of material forming at the level of the hopper 201 may be adjusted by acting on the relative and absolute advance speeds of the conveyors 14 and 16.

When the pressure insulation is activated, the heated air outlet from the conveyor 16 is fed into a section 26 of the fume ducts 13 of the plant 1 by a further suitable duct 25 originating from the end portion of the conveyor 16 itself and provided with means for automatically adjusting the flow rate. Such a connection between the post-cooler 16 and the boiler side may occur upstream or downstream of the abovementioned air pre-heater 29 (fumes side) arranged along the fume ducts 13. Through said connection, cooling air inlet to the post-cooler 16 is recalled by the negative pressure value existing in said boiler-side zone. Preferably, the plant 1 provides also a bypass piping or duct, (for simplicity's sake omitted from the figures) connecting the extractor/afterburner 14 with the conveyor- cooler 16 and provided with automatic opening/closing valve.

Downstream of the cooling conveyor 16 it is provided a secondary crusher 202 of opposing roll or equivalent type, suitable for crushing ash only, and in particular capable of reducing the grain size of the latter without altering the grain size of any plastics material mixed to the ash and deriving from RDF combustion. In this typology of crusher, already known to a person skilled in the art, plastics particles transit through the rolls deforming, yet without breaking.

Hence, it is provided a mechanical or pneumatic screening system 203 located downstream of the second crushing stage (step) associated to the device 202, for separating ash from plastics particulate and any metallic parts present in the RDF that are stored or forwarded to disposal by dedicated means.

Crushed ash is forwarded to coal pulverizing mills via a mechanical or pneumatic conveyor 204, so as to be re-circulated in combustion chamber through the coal pulverizing mills and the boiler burners as described in the International Application PCT/EP2005/007536.

Concerning the path of the abovementioned combustion fumes generated in the combustion chamber 12 and of the light ashes carried thereby, said fumes cross the zone of the economizers 27 and are then inlet through said fume ducts 13 in the hereto-mentioned light-ash electrostatic precipitator 28 or equivalent means, optionally by crossing first the hereto-mentioned air/fumes exchanger 29.

The light ashes precipitated into said electrostatic precipitator 28 are collected by suitable means 30 for their re-circulating in combustion chamber.

In short, therefore, after the post-combustion on the belt of the extractor 14 any unburnt still present in the heavy ash, after a suitable cooling by the conveyor 16, are re-circulated in combustion chamber along with the light ash having a higher unburnt content, to attain a full unburnt conversion.

Moreover, the plant 1 incorporates a central adjustment and control system, capable of ensuring the automated carrying out of the steps described hereto.

Therefore, by now it will be appreciated that the described plant allows to attain a reduction in unburnts present in heavy ashes, by a controlled post-combustion process on the extractor belt and by re-circulating in boiler the heavy ashes thus produced and the light ashes having higher unburnt content, the latter ashes coming from the electrofilters.

Moreover, it will be appreciated that the above-described integrated system for the dry extraction, crushing, biomass post-combustion and heavy ashes and unburnt re- circulating allows to enhance the combustion capacity of biomasses (and generally of non-conventional fuel) and also to increase the combustion efficiency thereof. The described system concomitantly allows the use of the apparatuses already present in the existing thermoelectric power plants, reducing plant installation costs and requiring minimum adjustment measures of the existing ones. In particular, also biomasses (and, in general, non-conventional fuel masses) continue to be moved with the means used for moving coal and heavy ashes. Moreover, boiler bunkers currently representing the means for accumulating traditional solid fuel material can be utilized for biomasses as well.

By now, it will also be appreciated that, unlike in the state of the art, in which it is provided the re-circulation in combustion chamber of heavy ashes added to light ashes of higher unburnt content, using coal mills for their pulverizing, the present invention provides biomass combustion directly on the extractor belt, utilizing part of the pre-heated air, and it optimizes mixed combustion of biomasses in coal dust boilers, by heavy ash re-circulating.

It will be appreciated that the invention also refers to a method for the (co- )combustion of biomasses in a solid-fuel thermoelectric power plant of the above- described type, said method providing the dry extraction of heavy ashes and unburnts from the combustion chamber 12 by an extractor 14 of the above- mentioned type, and wherein in correspondence of the portion 141 of the latter there occurs a post-combustion of the unburnts controlled by the selective feeding of a flow of hot air and of a flow of combustion fumes in order to respectively foster and inhibit the post-combustion, the flow rate of hot air and/or of fumes fed into said post-combustion portion being adjusted.

Preferred features of said method have already been described with reference to plant 1.

Lastly, it will be understood that, even though the integrated system described hereto optimizes the combustion efficiency of biomasses (and of non-conventional fuel in general) and plant management, separate protection might be required for the different aspects of the invention, and in particular for the system for controlling the combustion by air and fumes, for the dedicated crushing, for the biomass post- combustion system and for the means for the direct combustion of biomasses on the extractor by the dedicated meter, each of said aspects allowing however a substantial improvement of said efficiency.

The present invention has been hereto described with reference to preferred embodiments thereof. It is understood that other embodiments might exist, all comprised within the protective scope of the claims hereinafter.

Claims

CLAlMS

1. A combustion plant (1) apt to be used in a thermoelectric solid fuel power plant in association with a combustion chamber (12) for said fuel, in particular of the type suitable for the (co-)combustion of biomasses and/or fuel derived from urban solid waste (RDF), comprising:

- a dry extractor (14) of heavy ashes and unbumts from the combustion chamber (12), apt to be arranged downstream of the latter and having a post-combustion portion (141) for the unburnts; and

- control means (100) for controlling unburnts post-combustion occurring on said post-combustion portion (141), in turn comprising means (15) for feeding hot air and means (150) for feeding combustion fumes, apt to provide a flow of heated air and of combustion fumes, respectively, in correspondence of said post- combustion portion (141) in order to respectively foster and inhibit post- combustion, said control means (100) being apt to selectively adjust the flow rate of hot air and/or fumes fed into said post-combustion portion (141).

2. The plant (1) according to claim 1 , wherein the overall arrangement is such that said post-combustion portion (141) is apt to be arranged at least partially facing the bottom of the combustion chamber (12) so that the post-combustion exploits irradiation heat coming from the combustion chamber (12) itself.

3. The plant (1) according to claim 1 or 2, wherein said hot air (15) and/or fumes (150) feeding means is configured so as to supply, in correspondence of said post- combustion portion (141), an air flow countercurrent with respect to the moving direction of said extractor (14).

4. The plant (1) according to any one of the preceding claims, wherein said control means (100) comprises automatic means for adjusting said flow rates of air and/or fumes depending on the temperature detected in correspondence of said post- combustion portion (141).

5. The plant (1) according to any one of the preceding claims, wherein the overall arrangement is such that said hot air (15) and/or fumes (150) feeding means is apt to exploit, in order to foster air and/or fumes circulation, the negative pressure present in the combustion chamber (12).

6. The plant (1) according to any one of the preceding claims, wherein said control means (100) comprises means (143) for the controlled inletting of outside air to be mixed to the hot air of said feeding means (15).

7. The plant (1) according to any one of the preceding claims, wherein said hot-air feeding means (15) collects or is apt to collect heated air from an air chamber (151) or from a pre-heater (29) apt to supply heated air to the combustion chamber (12).

8. The plant (1) according to any one of the preceding claims, wherein said means (150) for feeding combustion fumes collects or is apt to collect said fumes downstream of an electrostatic precipitator (28).

9. The plant (1) according to any one of the preceding claims, wherein the overall arrangement is such that the temperature of the combustion fumes fed by said means (150) is equal to or lower than about 150 ⁰C.

10. The plant (1) according to any one of the preceding claims, wherein said means (150) for feeding combustion fumes comprises a fan for increasing the head of the flow.

11. The plant (1) according to any one of the preceding claims, wherein said dry extractor (14) has side inlets (143) for the controlled inletting of outside air.

12. The plant (1) according to any one of the preceding claims, wherein said dry extractor (14) comprises an extractor belt having, at least in correspondence of said post-combustion portion (141), a plurality of perforations (142) apt to foster the passage of air through the conveyed material.

13. The plant (1) according to any one of the preceding claims, comprising means (104) for feeding cooling water in said extractor (14), arranged at least in correspondence of an end portion thereof.

14. The plant (1) according to any one of the preceding claims, comprising means for adjusting the residence time of unbumts in said post-combustion portion (141).

15. The plant (1) according to the preceding claim, wherein said adjusting means is based on the control of the speed of said extractor (14).

16. The plant (1) according to any one of the preceding claims, comprising a cooling conveyor (16) arranged downstream of said dry extractor (14).

17. The plant (1) according to the preceding claim, comprising pressure insulation means (201) apt to create a pressure separation between the environments of said extractor (14) and of said cooling conveyor (16).

18. The plant (1) according to the preceding claim, wherein said pressure insulation means comprises means (201) for accumulating the conveyed material, apt to allow the forming of a head of material between said environments creating said pressure separation.

19. The plant (1) according to any one of the preceding claims, comprising means (30) for re-circulating, in the combustion chamber (12), light ashes.

20. The plant (1) according to the preceding claim, wherein said re-circulating means (30) comprises means for collecting light ashes from or immediately downstream of an electrostatic precipitator (28).

21. The plant (1) according to any one of the preceding claims, comprising means (204) for re-circulating, in the combustion chamber (12), unburnts contained in heavy ash.

22. The plant (1) according to any one of the preceding claims, comprising means for feeding said fumes into an ash cooling zone arranged downstream of said post-combustion portion (141) of said extractor (14).

23. The plant (1) according to any one of the preceding claims, comprising an unburnt biomass feeder (18, 19), arranged upstream or in correspondence of said post-combustion portion (141) of said extractor (14) and independent from the combustion chamber (12), said feeder (18, 19) being apt to lay unburnt biomass directly on said extractor (14).

24. The plant (1) according to the preceding claim, comprising means for feeding hot air to said feeder (19), apt to perform a first drying of the unburnt biomass.

25. The plant (1) according to the preceding claim, wherein said means for feeding hot air to the biomass is associated with an air / combustion fumes exchanger (29).

26. The plant (1) according to any one of the preceding claims, comprising dedicated crushing means (61-63), arranged or apt to be arranged upstream of the combustion chamber (12) and suitable for crushing the biomasses according to a preset maximum outlet grain size.

27. The plant (1) according to the preceding claim, wherein said dedicated crushing means (61-63) comprises one or more hammer mills.

28. The plant (1) according to claim 26 or 27, comprising bypass means (3) apt to supply the biomass from storage bunkers (21, 22) to said dedicated crushing means (61-63).

29. The plant (1) according to any one of the claims 26 to 28, comprising screening means (71-73) arranged downstream of said dedicated crushing means (61-63) and apt to intercept biomass particles of a grain size greater than a predetermined threshold to resend them to said crushing means (61-63).

30. A combustion method apt to be used in a thermoelectric solid fuel power plant, comprising a combustion chamber (12) for said fuel, in particular of the type suitable for the (co-)combustion of biomasses and/or fuel derived from urban solid waste (RDF), said method providing the dry extraction of heavy ashes and unburnts from the combustion chamber (12) by an extractor (14) arranged downstream of the latter, said method further comprising a step of post-combustion of the unburnts occurring on a post-combustion portion (141) of said extractor (14), said step of post-combustion being controlled by the selective feeding of a flow of hot air and of a flow of combustion fumes in order to respectively foster and inhibit the post- combustion, the flow rate of hot air and/or of fumes fed into said post-combustion portion (141) being adjusted.

31. The method according to claim 30, wherein said post-combustion portion (141) is arranged at least partially facing the bottom of the combustion chamber (12), so that the post-combustion exploits irradiation heat coming from the combustion chamber (12) itself.

32. The method according to claim 30 or 31 , wherein said feeding of hot air and/or of fumes supplies, in correspondence of said post-combustion portion (141), a flow countercurrent with respect to the direction of advance of the combustion residues.

33. The method according to any one of the claims 30 to 32, wherein said flow rates of air and/or fumes are automatically adjusted depending on the temperature detected in correspondence of said post-combustion portion (141).

34. The method according to any one of the claims 30 to 33, wherein said feeding of hot air and/or of fumes exploits, in order to foster air and/or fumes circulation, the negative pressure present in the combustion chamber (12).

35. The method according to any one of the claims 30 to 34, wherein it is provided a controlled feeding of outside air into said post-combustion portion (141).

36. The method according to any one of the claims 30 to 35, wherein said feeding of hot air provides the collecting of heated air from a pre-heater (29) or from an air chamber (151) both apt to supply heated air to the combustion chamber (12).

37. The method according to any one of the claims 30 to 36, wherein said feeding of combustion fumes provides the collecting of said fumes downstream of an electrostatic precipitator (28).

38. The method according to any one of the claims 30 to 37, wherein the temperature of the combustion fumes fed into said post-combustion portion (141) is equal to or lower than about 150 ⁰C.

39. The method according to any one of the claims 30 to 38, wherein said feeding of combustion fumes provides the use of ventilation means for increasing the head of the flow.

40. The method according to any one of the claims 30 to 39, wherein it is provided the controlled feeding of outside air into the dry extractor (14).

41. The method according to any one of the claims 30 to 40, providing also a feeding of cooling water into said extractor (14), at least in correspondence of an end portion thereof.

42. The method according to any one of the claims 30 to 41 , providing an adjusting of the residence time of unbumts in said post-combustion zone (141).

43. The method according to the preceding claim, wherein said adjusting is based on the control of the speed of said extractor (14).

44. The method according to any one of the claims 30 to 43, comprising a step of cooling downstream of the extracting carried out by said dry extractor (14).

45. The method according to the preceding claim, providing the option of activating a pressure insulation between the environments of said extractor (14) and of said step of cooling carried out downstream thereof.

46. The method according to the preceding claim, wherein said pressure insulation is activated by means for accumulating the conveyed material (201), apt to allow the forming of a head of material between said environments creating said pressure separation.

47. The method according to any one of the claims 30 to 46, providing a recirculation, in the combustion chamber (12), of light ashes.

48. The method according to the preceding claim, wherein the light ashes to be re-circulated are collected from or immediately downstream of an electrostatic precipitator (28).

49. The method according to any one of the claims 30 to 48, providing a re- circulating, in the combustion chamber (12), of unburnts contained in the heavy ash.

50. The method according to any one of the claims 30 to 49, providing a step of cooling the ashes carried out by said combustion fumes.

51. The method according to the preceding claim, wherein said step of cooling is carried out on said extractor (14).

52. The method according to any one of the claims 30 to 51 , wherein it is provided the direct feeding of unburnt biomass on said extractor (14), upstream or in correspondence of said post-combustion portion (141).

53. The method according to the preceding claim, wherein it is provided a feeding of hot air to the unburnt biomass feeder (18, 19), so as to perform a first drying of the latter.

54. The method according to the preceding claim, wherein said hot air fed to the biomass is obtained by an air /combustion fumes heat exchange.

55. The method according to any one of the claims 30 to 54, providing the use of dedicated crushing means (61-63), suitable for crushing the biomasses according to a preset maximum end (outlet) grain size.

56. The method according to the preceding claim, providing the storing of the biomass into the same type of bunkers (21 , 22) used for solid fuel and the supplying of biomasses from said bunkers (21, 22) to said dedicated crushing means (61-63).

57. The method according to the preceding claim, wherein said biomass storage bunkers (21 , 22) are associated to burners of the combustion chamber (12) arranged at topmost levels.

58. The method according to any one of the claims 55 to 57, providing, downstream of said dedicated crushing means (61-63), a screen for intercepting particles of a grain size greater than a predetermined threshold, and the resending of the latter to said dedicated crushing means (61-63).