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

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.

1 . A combustion plant adapted to be used in a thermoelectric solid fuel power plant in association with a combustion chamber for said fuel, comprising:
 a dry extractor of heavy ashes and unburnts from the combustion chamber, adapted to be arranged downstream of the combustion chamber and comprising a post-combustion portion for the unburnts; and   control means for controlling unburnts post-combustion occurring on said post-combustion portion, the control means comprising means for feeding hot air and means for feeding combustion fumes, adapted to provide a flow of heated air and of combustion fumes, respectively, in correspondence of said post-combustion portion in order to respectively foster and inhibit post-combustion, said control means being adapted to selectively adjust the flow rate of hot air and/or fumes fed into said post-combustion portion.   
   
   
       2 . The plant according to  claim 1 , wherein said post-combustion portion is adapted to be arranged at least partially facing the bottom of the combustion chamber so that the post-combustion exploits irradiation heat coming from the combustion chamber itself. 
   
   
       3 . The plant according to  claim 1 , wherein said means for feeding hot air and/or means for feeding combustion fumes are configured to supply, in correspondence of said post-combustion portion, an air flow countercurrent with respect to a moving direction of said extractor. 
   
   
       4 . The plant according to  claim 1 , wherein said control means comprises automatic means for adjusting said flow rates of air and/or fumes depending on temperature detected in correspondence of said post-combustion portion. 
   
   
       5 . The plant according to  claim 1 , wherein said means for feeding hot air and/or means for feeding combustion fumes are adapted to exploit, in order to foster air and/or fumes circulation, negative pressure present in the combustion chamber. 
   
   
       6 . The plant according to  claim 1 , wherein said control means comprises means for controlled inletting of outside air to be mixed to the hot air of said feeding means. 
   
   
       7 . The plant according to  claim 1 , wherein said means for feeding hotair collects or is adapted to collect heated air from an air chamber or from a pre-heater. 
   
   
       8 . The plant according to  claim 1 , wherein said means for feeding combustion fumes collects or is adapted to collect said fumes downstream of an electrostatic precipitator. 
   
   
       9 . The plant according to  claim 1 , wherein the temperature of the combustion fumes fed by said means for feeding combustion fumes is equal to or lower than about 150° C. 
   
   
       10 . The plant according to  claim 1 , wherein said means for feeding combustion fumes comprises a fan for increasing head of the flow. 
   
   
       11 . The plant according to  claim 1 , wherein said dry extractor has side inlets for controlled inletting of outside air. 
   
   
       12 . The plant according to  claim 1 , wherein said dry extractor comprises an extractor belt comprising, at least in correspondence of said post-combustion portion, a plurality of perforations adapted to foster passage of air through the conveyed material. 
   
   
       13 . The plant according to  claim 1 , comprising means for feeding cooling water in said extractor, arranged at least in correspondence of an end portion thereof. 
   
   
       14 . The plant according to  claim 1 , comprising means for adjusting residence time of unburnts in said post-combustion portion. 
   
   
       15 . The plant according to  claim 14 , wherein said means for adjusting is based on control of speed of said extractor. 
   
   
       16 . The plant according to  claim 1 , comprising a cooling conveyor arranged downstream of said dry extractor. 
   
   
       17 . The plant according to  claim 16 , comprising pressure insulation means adapted to create a pressure separation between environments of said extractor and said cooling conveyor. 
   
   
       18 . The plant according to  claim 17 , wherein said pressure insulation means comprises means for accumulating the conveyed material, adapted to allow forming of a head of material between said environments creating said pressure separation. 
   
   
       19 . The plant according to  claim 1 , comprising means for re-circulating, light ashes in the combustion chamber. 
   
   
       20 . The plant according to  claim 19 , wherein said means for re-circulating comprises means for collecting light ashes from or immediately downstream of an electrostatic precipitator. 
   
   
       21 . The plant according to  claim 1 , comprising means for re-circulating, in the combustion chamber, unburnts contained in heavy ash. 
   
   
       22 . The plant according to  claim 1 , comprising means for feeding said fumes into an ash cooling zone arranged downstream of said post-combustion portion of said extractor. 
   
   
       23 . The plant according to  claim 1 , comprising an unburnt biomass feeder, arranged upstream or in correspondence of said post-combustion portion of said extractor and independent from the combustion chamber, said feeder being adapted to lay unburnt biomass directly on said extractor. 
   
   
       24 . The plant according to  claim 23 , comprising means for feeding hot air to said feeder, adapted to perform a first drying of the unburnt biomass. 
   
   
       25 . The plant according to  claim 24 , wherein said means for feeding hot air to the biomass is associated with an air/combustion fumes exchanger. 
   
   
       26 . The plant according to  claim 1 , comprising dedicated crushing means, arranged or adapted to be arranged upstream of the combustion chamber and suitable for crushing biomass according to a preset maximum outlet grain size. 
   
   
       27 . The plant according to  claim 26 , wherein said dedicated crushing means comprises one or more hammer mills. 
   
   
       28 . The plant according to  claim 26 , comprising bypass means adapted to supply the biomass from storage bunkers to said dedicated crushing means. 
   
   
       29 . The plant according to  claim 26 , comprising screening means arranged downstream of said dedicated crushing means and adapted to intercept biomass particles of a grain size greater than a predetermined threshold to resend said biomass particles to said crushing means. 
   
   
       30 . A combustion method adapted to be used in a thermoelectric solid fuel power plant, comprising a combustion chamber for said fuel, said method comprising:
 providing dry extraction of heavy ashes and unburnts from the combustion chamber by an extractor arranged downstream of the combustion chamber, and   performing post-combustion of the unburnts occurring on a post-combustion portion of said extractor,   said performing post-combustion being controlled by 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, wherein flow rate of hot air and/or of fumes fed into said post-combustion portion is adjusted.   
   
   
       31 . The method according to  claim 30 , wherein said post-combustion portion is arranged at least partially facing the bottom of the combustion chamber, so that the post-combustion exploits irradiation heat coming from the combustion chamber itself. 
   
   
       32 . The method according to  claim 30 , wherein said feeding of hot air and/or of fumes supplies, in correspondence of said post-combustion portion, a flow countercurrent with respect to direction of advancement of combustion residues. 
   
   
       33 . The method according to  claim 30 , wherein said flow rates of air and/or fumes is automatically adjusted depending on temperature detected in correspondence of said post-combustion portion. 
   
   
       34 . The method according to  claim 30 , wherein said feeding of hot air and/or of fumes exploits, in order to foster air and/or fumes circulation, negative pressure present in the combustion chamber. 
   
   
       35 . The method according to  claim 30 , wherein a controlled feeding of outside air is provided into said post-combustion portion. 
   
   
       36 . The method according to  claim 30 , wherein said feeding of hot air comprises collecting heated air from a pre-heater or from an air chamber. 
   
   
       37 . The method according to  claim 30 , wherein said feeding of combustion fumes provides collecting of said fumes downstream of an electrostatic precipitator. 
   
   
       38 . The method according to  claim 30 , wherein temperature of the combustion fumes fed into said post-combustion portion is equal to or lower than about 150° C. 
   
   
       39 . The method according to  claim 30 , wherein said feeding of combustion fumes comprises using ventilation means for increasing head of the flow. 
   
   
       40 . The method according to  claim 30 , further comprising providing controlled feeding of outside air into the dry extractor. 
   
   
       41 . The method according to  claim 30 , further comprising feeding cooling water into said extractor, at least in correspondence of an end portion thereof. 
   
   
       42 . The method according to  claim 30 , further comprising adjusting residence time of unburnts in said post-combustion zone. 
   
   
       43 . The method according to  claim 42 , wherein said adjusting is based on speed control of said extractor. 
   
   
       44 . The method according to  claim 30 , comprising a step of cooling downstream of the extracting carried out by said dry extractor. 
   
   
       45 . The method according to  claim 44 , further comprising providing an option of activating a pressure insulation between environments of said extractor and said cooling. 
   
   
       46 . The method according to  claim 45 , wherein said pressure insulation is activated by means for accumulating the conveyed material, adapted to allow forming of a head of material between said environments creating said pressure separation. 
   
   
       47 . The method according to  claim 30 , further comprising re-circulating light ashes in the combustion chamber. 
   
   
       48 . The method according to  claim 47 , wherein the light ashes to be re-circulated are collected from or immediately downstream of an electrostatic precipitator. 
   
   
       49 . The method according to  claim 30 , further comprising re-circulating, in the combustion chamber, unburnts contained in the heavy ash. 
   
   
       50 . The method according to  claim 30 , further comprising cooling the ashes carried out by said combustion fumes. 
   
   
       51 . The method according to  claim 50 , wherein said cooling is carried out on said extractor. 
   
   
       52 . The method according to  claim 30 , further comprising directly feeding unburnt biomass on said extractor, upstream or in correspondence of said post-combustion portion. 
   
   
       53 . The method according to  claim 52 , further comprising feeding hot air to an unburnt biomass feeder, to perform a first drying of the unburnt biomass feeder. 
   
   
       54 . The method according to  claim 53 , wherein said hot air is obtained by an air/combustion fumes heat exchange. 
   
   
       55 . The method according to  claim 30 , further comprising providing dedicated crushing means, suitable for crushing biomass according to a preset maximum end grain size. 
   
   
       56 . The method according to  claim 55 , further comprising
 storing the biomass into a same type of bunkers used for solid fuel and   supplying the biomass from said bunkers to said dedicated crushing means.   
   
   
       57 . The method according to  claim 56 , wherein said bunkers are associated to burners of the combustion chamber arranged at topmost levels. 
   
   
       58 . The method according to  claim 55 , further comprising providing, downstream of said dedicated crushing means, a screen for intercepting particles of a grain size greater than a predetermined threshold, and resending said particles to said dedicated crushing means.