Abstract:
The present invention relates to an additional cooling system ( 1 ) for the dry extraction of large flow of heavy ashes produced by boilers ( 100 ) with solid fuel apt to decrease the temperature of the ashes. The system comprises an extractor with metallic belt ( 2 ) gathering the ash which deposits onto the bottom of the boiler ( 100 ), a crushing system ( 3 ), having the purpose of increasing the thermal exchange surface of the material, one or more metallic conveyors ( 4, 6 ) having the cooling function by introducing countercurrent air-flow running through transported ashes, an in-line cooling device ( 5 ) having the function of putting into contact the ash several times with additional countercurrent air in order to increase the possible exchange without necessarily increasing the air-flow entering the combustion chamber. Such additional air can be sent preferably upstream of the air heater or in atmosphere upon fines&#39; captation.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a plant and to a method for the dry extraction and cooling of combustion residues coming from a combustion chamber, in particular for large quantities of heavy ashes originating, for example, from fossil fuel used in thermo-electric energy-producing plants. 
       BACKGROUND OF THE INVENTION 
       [0002]    The constant growth in the request for solid fossil fuels for producing electric energy makes more frequent the combustion also of coals and lignites with high ash content. The combustion of the latter in high-power boilers involves a considerable production of heavy ashes gathered at the bottom of the boiler itself, production which can reach quantities even near to 100 tons/hour. The dry cooling of such quantities requires large quantities of cooling air, even twice or three times greater than the conventional fossil fuels. 
         [0003]    As illustrated in EP 0 471 055 B1, in some known ash extraction and dry cooling systems, the cooling air, once heated due to the thermal exchange with the latter, is introduced into the boiler from the bottom thereof. Therefore, at first the greater is the quantity of the produced ash, the greater is the potential heat recovery which is provided in the boiler by the cooling air in the above-mentioned way. 
         [0004]    However, in order to avoid that the combustion efficiency and/or the boiler efficiency be influenced negatively by the air introduced into the combustion chamber from the bottom rather than from the burners or from other specific air entrances and/or in order to avoid similar unwished effects on the production of nitrogen oxides (NO x ), some boiler designers prefer limiting this quantity to a maximum value of 1.0-1.5% of the total air introduced in the combustion chamber. 
         [0005]    For what just illustrated, the known cooling systems do not succeed in implementing, in an effective and efficient way, the dry cooling or mainly dry, cooling of the heavy ashes and the disposal of the latter and of the related cooling air, above all if such ashes are in large quantities and at high temperature. In particular, even when such cooling and disposal are succeeded to be obtained, they are achieved with considerable plant complications and with consequent very high implementation and handling costs. 
         [0006]    Therefore, the technical problem underlying and solved by the present invention is to provide a system and method for the dry extraction and cooling of combustion residues coming from a solid fuel combustion chamber which allow obviating to the drawbacks just mentioned with to reference to the known art. 
       SUMMARY OF THE INVENTION 
       [0007]    The above-mentioned problem is solved by a system according to claim  1  and by a method according to claim  28 . 
         [0008]    Preferred features of the present invention are present in the claims depending from the same. 
         [0009]    The present invention provides some important advantages which will be appreciated in full in the light of the detailed description reported hereinafter. The main advantage consists in that the invention allows, in case of coals with high ash content, to carry out an adequate and effective dry cooling of the ash itself without exceeding the above-mentioned limit of 1.0-1.5% of cooling air introduced in the combustion chamber from the bottom. Such advantage is particularly important in the above-mentioned case of coals with high content of heavy ashes. This is obtained mainly by providing an air/ash dry exchanger of gravitational type and allowing only to a controlled quantity of cooling air to be introduced into the combustion chamber from the bottom; whereas the air excess coming from such gravitational exchanger can be discharged into the atmosphere upon dedicated filtration, carried to the system for filtering the boiler fumes or—preferably—sent upstream of the combustion air heater, on the fume side, thus recovering great part of the energy transferred by the ash to the air. 
         [0010]    In order to guarantee the effectiveness in the heat recovery under all conditions of quantity and temperature of the ashes, the cooling air quantity introduced into the system can be adjusted based upon a combination of measures of ash quantity and/or temperature. 
         [0011]    Upon summarizing the detailed description of preferred embodiments reported hereinafter, the present invention mainly relates to an additional cooling system for the dry extraction of heavy ashes produced by solid fuel boilers, apt to reduce the ash temperature. The system mainly comprises, arranged subsequentially:
       a metallic belt extractor gathering the ash depositing onto the boiler bottom; of the type subject of already mentioned patent EP 0 471 055 B1 and known with the tradename “MAC”;   a crushing system, having the purpose of increasing the ashes&#39; thermal exchange surface;   one or more metallic conveyors in line with said extractor, having the transport and cooling function by introducing air in countercurrent; and   an in-line cooling device, having the function of placing in contact several times the ash with additional air in countercurrent in order to increase the possible exchange without necessarily increasing the air quantity coming back into the combustion chamber—as mentioned above, such additional air is then preferably sent upstream of the air heater or in atmosphere upon captation of fines.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Other advantages, features and application modes of the present invention will be evident from the following detailed description in some preferred embodiments, shown by way of example and not for limitative purposes. The figures of the enclosed drawings will be referred to, wherein: 
           [0017]      FIG. 1  shows a general layout exemplifying a first embodiment or preferred operation mode of the invention system, providing to send the cooling air coming from a gravitational air/ash exchanger into the fume duct associated to the combustion chamber, upstream of the air heater; 
           [0018]      FIG. 2  shows a general layout exemplifying a second embodiment or preferred operation mode of the invention system, providing that the air coming from a gravity air/ash exchanger is moved by an auxiliary fan and discharged into the atmosphere; 
           [0019]      FIG. 3  shows a general layout exemplifying a third embodiment or preferred operation mode of the invention system providing to send the cooling air coming from a gravity air/ashes exchanger into the fume duct associated to the combustion chamber, upstream of the dust removal system; and 
           [0020]      FIGS. 4   a  and  4   b  refer to the gravity air/ash exchanger of the previous figures equipped with an air-dosing system, showing a side view and a front view thereof, respectively. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    By firstly referring to  FIG. 1 , a system for extracting and cooling combustion residues, of the kind used for example in solid fossil fuel thermo-electric plants and according to a first preferred embodiment of the invention, is designated as a whole with  1 . As it will be better appreciated hereinafter in the description, the system  1  is particularly suitable to handle large flow of heavy ashes, produced, for example by the combustion of coals or lignites with high content of ashes. 
         [0022]    For greater illustration clarity, the different component of the system  1  will be described hereinafter by referring to the path followed by the combustion residues from the extraction thereof from the bottom of the combustion chamber (or boiler), designated with  100 , to the disposal thereof. 
         [0023]    Immediately downstream of the combustion chamber  100 , or better of a transition hopper  105  thereof, the system  1  provides a first extraction and/or transport unit, in particular a dry extractor  2  mainly made of steel with high thermal resistance. Such extractor  2  is of an already known type and described for example in EP 0 252 967, herein incorporated by means of this reference. The extractor  2  gathers the heavy ashes which precipitate downwards into the combustion chamber  100  through the transition hopper  105  mentioned above. 
         [0024]    Extractor  2 , at the side walls of its own casing, has a plurality of entrance holes for the outer cooling air, distributed in a substantially regular way along the development of the extractor  2  itself and each one designated with  13 . Such entrances  13  preferably are equipped with means for regulating cooling air-flow, for example one or more gate valves, apt also to wholly interdict one or more selected entrances. 
         [0025]    Cooling air is sucked through the entrances  13  inside the extractor  2  and in countercurrent with respect to the ashes&#39; transportation because of the depression existing in the combustion chamber  100 . More in detail, the air enters thanks to the depression existing in the transition hopper  105 , on the bottom thereof there is a depression adjusted by the control system of the combustion chamber  100  (generally around 300-500 Pa under the atmospheric pressure). Such cooling air involving the extractor  2  enters then the boiler  100  from the bottom thereof. 
         [0026]    Downstream of the extractor  2  the ashes are fed to a breaker or crusher  3 , which triturates the coarsest fractions thereof so as to increase the thermal exchange surface and thus to improve the potentials of such exchange and therefore the cooling process. 
         [0027]    Downstream of the crusher  3 , the ashes are conveyed to a second extraction and/or transport unit, in particular a steel-belt conveyor-cooler  4 . 
         [0028]    Onto the conveyor  4  the ash continues to be cooled by means of air in countercurrent resucked from the outside through additional entrances  13  arranged onto the side walls of the conveyor  4  itself in a way analogous to what already illustrated for the first extractor  2 . In particular, also at such entrances the air is resucked from the mentioned-above depression existing in the combustion chamber  100  and also such entrances can be equipped with means for flow—regulation of the already described kind. 
         [0029]    Also the cooling air involving such second conveyor  4  enters the boiler from the bottom thereof. 
         [0030]    A duct for the passage for the cooling air, designated with  42 , can be provided between the conveyor  4  and the extractor  2  for the bypass of the crusher  3 . 
         [0031]    At this point it will be understood that the system  1  is equipped with a dry cooling system, implemented among other things by the air entrances  13 . 
         [0032]    Downstream of the second conveyor  4  such cooling system comprises an air/ash dry gravitational thermal exchanger, preferably of the plate-like type  19 , designated as a whole with  5  and shown in greater detail in the  FIGS. 4   a - 4   b . The plates  19  preferably are made of wear-resistant metal. 
         [0033]    Immediately upstream of the exchanger  5  the system  1  can provide an additional crusher  10 , which can be selectively driven in case of need. 
         [0034]    Immediately downstream of the gravitational exchanger  5  an additional cooling air entrance  17  is then provided, equipped with means for cooling air-flow regulation as well, of the already described kind. 
         [0035]    Downstream of the exchanger  5  the system  1  further provides a third extraction and/or transport unit, in particular a third conveyor  6  ending into a silo  11  for discharging the ashes for their disposal and/or possible reuse thereof. 
         [0036]    At an entrance portion of the silo  11  an additional air entrance  14  is provided, equipped with means for cooling air-flow regulation as well, of the already described type. 
         [0037]    The additional air of the entrances  17  and  14  crosses in countercurrent the gravitational exchanger  5  and, considering the air introduced through the entrance  14 , also the third conveyor  6 . 
         [0038]    In such gravitational exchanger  5 , the ash crushed by the crusher  3  and in case if necessary by the crusher  10  mixes intimately with the air introduced in countercurrent by the entrances  14  and  17  during the falls from plate to plate, by increasing the thermal exchange and thus increasing the heat quantity transmitted by the ash to the air. The more the number of the falls and the air/ashes ponderal ratio are and the less the ash granulometry is, the better the thermal exchange, and consequently the obtainable cooling degree, will result. 
         [0039]    The system  1  then comprises means for sensing the temperature and/or volumetric and/or ponderal flow of the ashes which in the present example are arranged at the ending portion or the exhaust of the conveyor  4  and/or on the main extractor  2  or more preferably at the ashes&#39; discharge at the conveyor  6 . 
         [0040]    The system  1  further comprises control means, in communication with said sensor means and apt to control the cooling system mentioned above as well as the extraction and/or transport units  2 ,  4  and B. 
         [0041]    The system  1  then comprises feeding means apt to send part of the cooling air—and in particular the additional air introduced through the entrances  17  and  14  and which crosses the exchanger  5 —downstream of the ashes&#39; cooling process, into the atmosphere or in a fume duct  101  associated to the combustion chamber  100 . 
         [0042]    In particular, said additional air necessary to cool the ashes in the gravity air/ash exchanger  5  and introduced through the additional entrances  14  and  17  can follow three different paths depending upon the specific embodiment or considered construction configuration. 
         [0043]    In the here considered case by referring to  FIG. 1 , the cooling air introduced through the additional entrances  14  and  17  is resucked, downstream of the countercurrent crossing of the exchanger  5 , by the depression of the fume duct  101  upstream of an exchanger  102  associated to the boiler  100 . Such exchanger  102 , usually existing in the known systems, is used to pre-heat the combustion air. Said cooling air, heated by the ashes, is then sent in such exchanger  102  (fume side) and used for pre-heating the boiler combustion air. 
         [0044]    In the present example, said feeding means comprises then a duct  20  to connecting the entrance of the exchanger  5  with the fume duct  101 . Such duct  20  must be selectively adjusted and however interdicted/enabled by means of an automatic valve  15  (or equivalent means) arranged along its development thereof. 
         [0045]    The duct  20  then connects, or better is apt to connect, the exchanger  5  with the economizers&#39; area of the combustion system, under negative pressure too with respect to the one of the exchanger  5  itself. 
         [0046]    Preferably, in order to avoid transporting excessive quantities of fines, immediately after the gravity air/ash exchanger  5  the air crosses a cyclone dust collector  7  arranged in line onto the duct  20  and apt to discharge said exceeding fine dusts onto the third conveyor  6 . 
         [0047]    This configuration allows then an effective recovery of the heat ceased by the ash to the air during the contact time in the gravity air/ash exchanger  5 . 
         [0048]    In order to guarantee that the ash cooling process be not influenced on the extractor  2  and on the conveyors  4  and  6  and in order to avoid uncontrolled air entrance from the boiler bottom, before the entrance of the gravity air/ash exchanger  5  (that is upstream of the latter with respect to the ash flow) a valve with double clapet (not illustrated) or an equivalent pressure control means can be installed, for example a differential pressure transmitter measured upstream of and downstream of the entrance to the gravity air/ash exchanger  5  which, upon acting on the valve  15  of the duct  20 , brings the pressure difference back to zero. 
         [0049]    The air flow entering the gravitational air/ash exchanger  5 , that is in the present example the one fed into the system through the entrances  14  and  17 , can be adjusted by the above-mentioned control means based upon the ash temperature and/or quantity detected by the sensors mentioned above, also based upon thresholds which can be set selectively by an operator managing the system  1 . 
         [0050]    In the second construction configuration shown in  FIG. 2 , the additional cooling air entering the gravitational exchanger  5 , downstream of thereof and of the possible dust remover  7 , follows a path different from the one described by referring to the first embodiment. In such case, in fact, in the ending tract the air instead of being sucked by the depression existing in the fume line  101  of the thermo-electric plant, is sucked by a dedicated fan  16  or by equivalent means along a duct  200  equipped with adjusting means  150  to analogous to those already described and then discharged into the atmosphere after having passed through a dedicated filter  9  arranged upstream of the fan  16 . In this case a fan work is necessary helping the air to cross at first the third conveyor belt  6  and then the gravity air/ash exchanger  5 . 
         [0051]    In the third construction configuration shown in  FIG. 3 , the additional cooling air entering the exchanger  5 , upon possible passage into the cyclone  7  mentioned above for the fines separation, is brought to a system for treating the combustion fumes  104  associated to the boiler  100 , by entering the fume duct  101  downstream of the air exchanger (heater)  102  mentioned above. In such case, the duct of the above-mentioned feeding means has been designated with  201  and the related adjusting means with  151 . 
         [0052]    It will be understood that even if the configurations of the  FIGS. 1 ,  2  and  3  have been described separately, they can exist simultaneously in a same system as different operation modes, which can be activated depending upon the specific needs. 
         [0053]    At this point it will be appreciated that the system  1  has a great operating versatility and therefore the capability of handling even very large flow of ashes and this without the problems associated to the introduction of an excessive cooling air-flow from the bottom of the boiler  100 . As mentioned above, such versatility is obtained by allowing the controlled introduction of even very large quantities of cooling air and feeding the additional cooling air-flow (in particular the ratio exceeding 1.0-1.5% of the total combustion air) in the fume duct or outwards since it is not appropriate to introduce said air-flow in the boiler from its bottom. 
         [0054]    The invention has also as object a method for extracting and dry cooling combustion residues as described so far with reference to the system  1 . 
         [0055]    The present invention has been so far described by referring to preferred embodiments. It is to be meant that other embodiments belonging to the same inventive core may exist, all comprised within the protective scope of to the herebelow reported claims.