Patent Application
20110297061
The present invention relates to a plant and to a method for extracting, cooling and recovering thermal energy for large flows of heavy ashes produced by solid fuel boilers.
The continuous increasing demand for solid fossil fuels for the production of electric energy makes more and more frequent the combustion of coals or lignites with high ash content, too. The combustion of the latter in high power boilers involves a huge production of heavy ashes, even up to 100 tons/hour, often containing high percentages of unburnt materials. The dry or mainly dry cooling of such quantities requests huge cooling air flows, even two or three times higher than the fossil fuels with high heat-producing power.
As illustrated in EP 0 471 055 B1, in some known extraction and dry cooling of ashes the cooling air, once heated under the effect of the thermal exchange with the latter, is introduced into the boiler from the bottom thereof. Therefore, in principle the greater is the produced ash quantity, the greater is the recovery of the heat which is supplied to the boiler by the cooling air in the above-mentioned way, both for the thermal exchange with the air and for the combustion of the unburnt materials.
However, in order to avoid the combustion efficiency both negatively influenced by the air introduced into the combustion chamber from the bottom rather than by the burners or other specific air entrances and/or to avoid analogous unwished effects on the generation of nitrogen oxides (NOx), the boiler designers prefer limiting this quantity to a maximum value of 1.5% of the total combustion air.
As to what just illustrated, the known cooling systems do not succeed in implementing in an effective and efficient way the dry or mainly dry cooling of the heavy ashes and the disposal of the related cooling air, above all if said ashes are in large flows, with high content of unburnt materials and therefore at high temperature. In particular, even if one succeeds in obtaining said cooling, thermal energy recovery and disposal, they are reached with huge plant complications and consequent very high implementation and handling costs. WO2008/023393—the content thereof is incorporated herein by means of this reference—provides that, when the air flow needed for cooling the extracted heavy ash exceeds the maximum flow admissible in the combustion chamber, the exceeding air can be sent to the fume duct and this thanks to a pressure separation of the cooling environments made by the ash itself. Still in WO2008/023393, a possible site for introducing said exceeding air is placed on the mentioned duct in a position upstream or downstream of air/fume exchanger.
A potential system limit of WO2008/023393 consists in the loss of the thermal content associated to the cooling air used downstream of the pressure separation system and in the increase in the whole flow of the fumes which has to be processed by the apparatuses downstream of the air introduction site.
In fact, in case of inletting hot air downstream of the air/fume exchanger the thermal content of the cooling air is wholly lost and it translates in an overall temperature increase in the flue fumes, apart from increasing the power absorbed by the apparatuses necessary for the fume process.
In case of inletting the hot air into the fume duct upstream the exchanger, as the cooling air is at a lower temperature than the combustion fume flow, a whole increase in the flow of hot air flow+fumes inletting the exchangeresults, which worsens the efficiency of environment air/fume thermal exchange of the device.
In particular, the above-mentioned cooling air can reach a temperature of about 200° C., and therefore, as just said, the inletting of the same in the fumes the temperature thereof is about 400° C. can result to be antieconomic on balance on the air/fume exchanger, in fact based upon the operating principle of the air/fume exchanger, even if the thermal content of the flow entering on the fumes' side increases, this does not succeed in being transferred to the air unless for a negligible portion.
Furthermore, the hot air inletting into the fume duct (made upstream or downstream of the exchanger) makes that the electrostatic precipitators arranged downstream of the air/fume exchange receive a higher whole flow than the design data, apart from an overall increase in the temperature of the fume flow to be processed. Such circumstance determines a worsening in the efficiency of the electrostatic separators caused by an increase in the inletting velocity and above all by an increase in the ash resistivity.
Based upon what illustrated in the previous section, the technical problem placed and solved by the present invention is to provide an apparatus and a method allowing to obviate the drawbacks mentioned above with reference to the known art.
Such problem is solved by a plant according to claim 1 and by a method according to claim 22.
Preferred features of the present invention are present in the claims depending upon the same.
The present invention provides important advantages which will be wholly appreciated in the light of the detailed description shown hereinafter.
The main advantage lies in the fact that the invention allows to maximize the recovery of the sensible heat contained in the exceeding cooling air used in the second plant portion downstream of the pressure insulation.
The configuration proposed in the present invention, in fact, provides the inletting of the cooling air into the environment air flow sent to the air/fume exchanger before inletting the combustion chamber. The environment air mixed with the hot cooling air before entering the exchanger undergoes an increase in temperature and it implements an efficient pre-heating of the same. This configuration leaves almost unchanged the efficiency of the air/fume heater and it allows the recovery of the sensible heat of the cooling air thereof. The invention, in fact, allows the total recovery of the sensible heat acquired by the cooling air in the plant portion downstream of the pressure separation and at the same time it leaves unchanged the fume flow and the temperature thereof crossed by the electrostatic precipitators, so as not to lower the separation efficiency of the same.
The invention also allows to keep the advantages already present in the system of WO2008/023393, that is to obtain an efficient dry or mainly dry cooling of the ashes without exceeding the above-mentioned limit of 1.5% for the cooling air introduced into the combustion chamber from the bottom.
The invention practically allows an optimization of the system described in EP 0 471 055 B1 and in WO2008/023393, by widening the potentiality of the thermal recovery thereof in case of application to large quantities of heavy ashes coming from coals or lignites with high ash content.
By synthesizing the detailed description of embodiments shown hereinafter, the present invention relates to an air or twofold, air/water, extracting and cooling system for large flows of heavy ashes produced by solid fuel boilers, able to decrease the final temperature of the extracted ash without increasing the air flow inletting the boiler flue, usually fixed by the boiler designers at a value around 1.5% of the total combustion air. When the air flow needed to the cooling exceeds the maximum quantity allowable in the boiler, the system allows the exceeding air to be sent to the combustion air sucking duct and preferably to the secondary air duct, thanks to a separation of the cooling environments made preferably by the ash itself.
According to the plant configurations and therefore to the concentrated and distributed load losses of the connecting line of the cooling air, it can be useful providing a fan pushing in line, so as to confer the right push to the intubated fluid.
The whole increase in temperature of the environment air inletting the exchanger translates into a negligible decrease in temperature delta between fumes and environment air, affecting in a negligible way onto the overall performance of the air/fume heater.
The separation of the environments of the cooling system is handled automatically based upon a signal of temperature and/or ash flow at the discharge from the system.
If the cooling air is not sufficient to cool the ash, the cooling efficiency can be increased by the addition of nebulised water. The usually added water quantity is dosed based upon the flow and the ash temperature so as to guarantee the complete evaporation of the water injected to obtain if necessary dry ash at the discharge, suitable to be milled and transported pneumatically.
Based upon a preferred configuration, the proposed, used system is mainly constituted by:
Other advantages, features and application modes of the present invention will be evident from the following detailed description of some preferred embodiments, shown by way of example and not for limitative purposes. The figures of the enclosed drawings will be referred to, wherein:
By referring to the above-mentioned figures, a plant for extracting and cooling the combustion residues, of the type used for example in solid fossil fuel thermo-electric plants and according to a preferred embodiment of the invention, is designated as a whole with 1. As it will be better appreciated in the following description, the plant 1 is particularly suitable to handle large flows of heavy ashes, produced for example by the combustion of coals or lignites with high ash content.
For a better explanation clarity, the different components of the plant 1 will be described as follows by referring to the path followed by the combustion residues as from the extraction thereof from the bottom of the combustion chamber (or boiler), designated with 100, until the disposal thereof.
Immediately downstream of the combustion chamber 100, or better of a transition hopper 105 of the latter, the plant 1 provides a first extraction and transport unit, in particular a dry extractor 9 mainly implemented in steel with high thermal resistance. Said extractor 9 is of the type known on itself and described for example in EP 0 252 967, incorporated herein by means of this reference. The extractor 9 gathers the heavy ashes which precipitate downwards in the combustion chamber 100 through the above-mentioned transition hopper 105.
The extractor 9, at the side walls of its own casing, has a plurality of holes for the outer cooling air entrance, distributed in a substantially uniform way along the development of the extractor 9 itself and each one designated with 10. Said entrances 10 may be equipped with means for regulating the flow or may be made active or de-activated. The extractor 9 can further have an additional outer cooling air entrance 19, preferably regulated, too, by an automatic valve or by equivalent flow regulating means and arranged substantially at an end portion of the extractor 9 itself.
The cooling air is attracted through the entrances 10 and 19 within the extractor 9 and in countercurrent with respect thereto under the effect of the depression existing in the combustion chamber 100. More in detail, the air entrance takes place thanks to the depression existing in the transition hopper 105, on the bottom thereof there is a depression regulated by the control system of the combustion chamber 100 (generally around 300-500 Pa under the atmospheric pressure).
Downstream of the extractor 9 the ashes are fed to a mill 3, which crushes the most coarse fractions thereof so as to increase the thermal exchange surface and thus improving the efficiency of such exchange and therefore the cooling.
Downstream of the mill 3 an additional outer cooling air entrance is provided, designated with 17 and in case equipped, too, with flow regulating means as those already described. Also in this case, the air coming from the entrance 17 is fed in countercurrent through the mill 3 itself and along the first extractor 9 under the effect of the depression existing in the combustion chamber 100. Said cooling air results useful not only for cooling the ash but also for cooling the machines.
As illustrated in greater detail in
Minimum and maximum level sensors, designated with 7, and a layer leveller 18, the latter arranged at an initial portion inletting the conveyor 6, are also associated to the hopper 8.
The position indication of the layer regulator 18 connected to the velocity indication of the belt of the conveyor cooler 6 provides information about the ash volumetric flow, useful together with the temperature indication for regulating the cooling fluids.
On the conveyor 6 the ash continues to be cooled both by means of the air attracted from outside through additional entrances 11 arranged on the side walls of the extractor 6 itself in a way analogous to what already illustrated for the first extractor 9, and analogously it can have an additional outer cooling air entrance, equivalent to 19, preferably regulated, too, by an automatic valve or by equivalent flow regulating means and arranged substantially at an initial portion of the conveyor 6 itself.
When needed, the cooling on the conveyor 6 can take place by means of water finely dosed by means of delivery nozzles 12 positioned inside the cover of the conveyor 6.
At this point it will be then understood that the plant 1 can be equipped with a twofold, air-water cooling system, among other things implemented by the air entrances 10, 11, 17 and 19 and by the water delivery nozzles 12.
The plant 1 further provides means for feeding part of the cooling air, heated after the heat exchange with the combustion residues, into an environment air discharge duct 50 associated to the air/fume exchanger 102. In the present embodiment said feeding means comprises a duct 51, properly insulated and thermally traced to avoid condensate, apt to be selectively regulated and however interdicted/enabled by means of an automatic valve 150 (or equivalent means) arranged along the development thereof.
More in detail, the duct 51 connects, or better is apt to connect, the discharge area of the conveyor 6 (
The exchanger 102 can be of the type commonly called Ljungstrom.
A cyclone separator 55 or an equivalent apparatus, apt to gather the fine ash in case existing in the cooling air flow outletting the conveyor 6 and/or the mixer 2 and suitable regulating valves 150, 59 can be associated to the line of the duct 51.
Still in line with the duct 51 there may be a fan 56 in case the concentrated and distributed load losses of the cooling air are higher than the depression provided in the inletting site on the duct upstream of the fan 54 or the head available on the same.
In general terms, the optimum inletting site of the cooling air is represented by the sucking duct of the combustion air fan which picks up the air from the environment and it sends it to the air/fume exchanger. If—as in FIG. 1—the exchanger is of the trisector type that is it has two entrances, respectively 61 and 62, dedicated to the combustion air (divided into primary and secondary) and an entrance dedicated to the fumes, the preferential site for inletting the cooling air is detected, as said, by the sucking duct of the secondary air. Said site, in fact, is preferred with respect to the sucking line of the fan of the primary air 58, as the pressure level of the primary fan 58 is considerable higher than the secondary one and therefor the energy lost in the pumping is higher.
In case the plant configuration does not allow the connection to the sucking line of the secondary air one could choose the primary air line which however offers advantages in terms of thermal recovery, even if to a smaller extent.
The advantage of inletting the cooling air upstream of the (primary 58 or secondary 54) fan results also in making that the same always processes the same amount of inletting air and therefore that it is not subjected to operating variations.
In an alternative embodiment, the cooling air can be sent directly entering the air/fume exchanger 102 on the air side.
The ash cooling on the conveyor 6 can be made more effective thanks to the presence of specific re-mixing means, in particular substantially cuneiform members 14 fixed with respect to the conveyor belt 6 itself and which in the present example are shaped like a share. Said share-like members 14 are distributed in a substantially uniform way along the development of the conveyor 6 and they are arranged at the transport section of the ashes. As mentioned above, the share-like members 14 plough the ashes by making a continuous re-mixing during the transport on the belt, by exposing in such way the maximum surface thereof available for the thermal exchange with the air and/or the cooling water.
Downstream of the conveyor 6 an automatic diverting valve 16 (or equivalent means for deviating selectively the ash flow) is provided, which allows selectively the cooled ash feeding to a discharge means 13 directed outwards or to a continuous mixer 2, in turn in the present example in communication outwards and shown in greater detail in
The discharge conveyor 13 is equipped with a device for controlling the entering air, not illustrated, to eliminate the uncontrolled entering of air from outside (or, in embodiment variants, to connect the system to other transport or storage closed environments).
The mixer with water 2 allows completing the ash cooling if necessary to reach temperature values compatible with the downstream processes or to humidify the ash to decrease the powder emissions under certain transport and disposal conditions. The mixer 2 is equipped with a discharge casing 21, equipped with means able to allow the ash discharge from the system by preventing at the same time an uncontrolled outer air return. Such device can be constituted for example by a double clapet valve or rubber boards which, upon deforming under the ash weight, allow the discharge thereof into the needed minimum passage section.
Based upon a preferred embodiment variant, a pipe 66 connecting the mixer 2 to the duct 51 is provided, for the air and vapour vent into the latter with the valve 59 or equivalent in line means.
The plant 1 then comprises sensors of temperature and/or volumetric and/or ponderal flow sensors of the ashes which in the present example are arranged at the end or discharge portion of the conveyor 6 and/or on the main extractor 9 or more preferably at the ash discharge at the conveyor 13. Advantageously, sensors of the above-mentioned type are provided also at the hopper/reservoir 8.
Still at said hopper 8, load cells or equivalent means can be provided to control the ash level in the hopper/reservoir.
Furthermore, temperature sensor means can be provided, arranged at the duct 51. The plant 1 comprises a control system, in communication with said sensor means, apt to control the operation modes of the plant 1 related to the quantity and temperature of the ashes.
The operation modes of the plant 1 and in particular those of the cooling system thereof controlled by the above-mentioned control means, will be now illustrated in greater detail.
First of all, the ash temperature and/or flow values provided by the sensor means are compared to values pre-fixed and stored by the control system and based upon the result of such comparison, the operating mode most suitable to the operation of the plant 1 is determined. Regarding the need of performing temperature and/or flow measurements, it is to be noted that the increase in the ash temperature usually is linked to the increase in the flow thereof in the plant 1 considered herein. The plant in the starting phase is configured in the mode illustrated in
Such operating mode is performed until the ash temperature at the exhaust of the conveyor 6 reaches the predetermined value Tminimum,
In such operating mode the control means acts on the related velocity of the belt of the extractor 9 and of the belt of the conveyor 6, substantially by making so that the conveyor 6 has a greater potential ash flow than the extractor 9 so as to avoid the formation of a material head within the hopper 8.
When the value Tminimum is exceeded, the system acts on the velocity of the conveyor 6, in particular by decreasing it and regulating it so as to determine an ash accumulation in the hopper 8 and therefore the creation of a continuous ash plug and furthermore it opens the valve 150 of the duct 51 so as to create two different atmospheres, respectively in the extractor 9 and in the conveyor 6, the first one linked to the pressure existing in the boiler and the second one connected to the pressure existing in the environment air feeding duct 51.
In said operating mode the air entrance valves 10, 19 and 17 of the extractor 9 and of the hopper 8 are regulated automatically so as to concentrate in the extractor only the whole 1.5% of cooling air which can be inlet into the boiler and the valves 11 and in case subsequently the nozzles 12 of the conveyor 6 by adding at first air until a percentage calculated so as not to influence the operation of the downstream air/fume exchanger and subsequently water if needed to reach the wished cooling. On this regard, it is to be noted that for choosing the site of inletting the cooling air, the fan 54 processes always the same air quantity, upon increasing the cooling air through the duct 51 the air attracted from the environment will decrease.
In said environment separation configuration, the cooling air acting on the main extractor 9 introduced by means of the entrances 10, 17 and 19 crosses such countercurrent extractor and it enters the combustion chamber 100 in the limit of 1.5%. The cooling air exceeding 1.5% is picked up from outside through the entrances 11 and equivalent to 19 if present of the conveyor 6, and it crosses the latter in equicurrent and it is sucked through the duct 51, together with the vapour produced by the possible water local cooling, by the depression generated by the air fan 54 and in case by the supporting fan 56 positioned in line to the duct 51.
In this way the maximum possible combustion of the possible unburnt materials on the belt of the extractor 9 is obtained, by bringing the related energy back to the boiler and the maximum cooling on the belt of the conveyor 6, by reducing to the minimum the cooling water intervention.
Such operating modes are exemplified in
In presence of the above-mentioned ash head, the emptying of the load hopper 8 is avoided by controlling the velocity of the conveyor 6 depending upon the detections of the maximum and minimum level sensors 7. In particular, if the level reaches the minimum one, the slowing down is provided until stopping the conveyor 6, whereas when the minimum level is exceeded, the re-start of the conveyor 6 is provided and upon reaching the maximum level the increase in velocity and therefore in the flow of the belt of the conveyor 6.
In the configuration considered herein the control means can have available additional information detected by specific sensor means, in particular related to the ash temperature in the hopper 8 and to the forwarding velocity of the conveyor 6. The latter, together with the (fixed) value of the extraction section defines exactly the ash volumetric flow. It has to be specified that the extraction level, in order to avoid possible obstructions in the extraction section itself, will have to be higher than a suitable margin in the size of the ash pieces outletting the mill 3.
Furthermore, in an additional operating mode exemplified in
In the mixer 2 an additional water quantity could be introduced so as to bring the ash at the provided end temperature (typically approximately 80° C.) with a suitable humidity content (preferably around 10) to guarantee the absence of powders in the following motion operations.
In order to avoid that the vapour generated by such cooling in the mixer 2 goes up again towards the conveyor 6 (with the risk of generating condensate), an upside-down “Y” connection can be provided directly between the conveyor 6, the mixer 2 and the duct 51. Thanks to this so-made configuration, the air and in case the vapour arriving from the conveyor-cooler 6 goes towards the duct connecting the fume line by joining to the vapour which has generated into the mixer 2. This connection duct (between the mixer 2 and the main duct 51), which could be suitably heated if the design conditions should perceive the risk of the condensate formation and related ash incrustations, remains with the risk of condensing.
It will be understood that said prefixed values of temperature and/or flow or quantity of predetermined combustion air can be set selectively by an operator handling the plant 1.
It will be further understood that the previously described operating modes constitute only one of the possibilities of handling the plant 1. A simpler operating mode can provide, for example, that the ash head is created upon reaching a prefixed temperature value and that the handling takes place for the remaining part by properly modulating the cooling air and water flow, the latter if needed.
A series of operating modes like those considered so far can be set manually or automatically by means of a handling and controlling system which, based upon the ash temperature/flow value, determines the cooling mode of the ash itself by acting onto the formation of the separation area, on the air flows inletting the extractor 9 and the conveyor 6, on the possible dosing of nebulised water and on the activation of the diverting valve.
Generally, it will be appreciated at this point that the plant 1 has a total operating versatility and therefore the capability of practically handling any ash flow, and this without the problems associated to the introduction of an exceeding quantity of cooling air from the bottom of the boiler 100. As mentioned above, such versatility is obtained by introducing even very high cooling air flows and feeding the additional air flow which is not suitable to introduce from the boiler bottom into the environment air inletting duct to the air/fume exchanger and by means of the possibility of adding even cooling water, if needed.
As far as this last aspect is concerned, preferably the plant 1, through the control means thereof, can dose adequately the used water quantity so that it wholly vaporizes during the cooling process and the at the outlet of the conveyor 6 substantially dry ashes are then obtained, suitable to be milled and transported automatically. This can be obtained by making that the ash end temperature keeps above 100° C. The water flow to be nebulised and injected will be controlled by means of a thermal balance leading to equal on one side the heat to be removed from the ash (produced of the flow for the specific enthalpy variation requested between the temperature in the hopper 8 and the discharge end temperature) and on the other side the sum of the water evaporation heat and of the enthalpy variation experienced by the cooling air.
It will be also appreciated that sending part of the cooling air in the environment air inletting duct to the air/fume exchanger allows the maximum heat recovery associated to the cooling air, allowing to emphasize the performance advantages already associated to the dry extractors used herein and described in the already mentioned patent EP 0 471 055 B1.
It will be also appreciated that the presence of the share like members or means equivalent thereto, also together with the possibility of activating selectively the water cooling by means of the nozzles 12, allows uniforming the ash temperature.
It will be further appreciated that the temperature sensors arranged at the duct 51, apart from allowing a more complete control of the plant parameters, further allow to verify the formation of possible condensate sites at the whole duct 51 due to the vapour deriving from the cooling water. In fact, knowing both the temperature of the air itself and of the nebulised water quantity allows easily to calculate the related humidity of the cooling air and to verify that:
In order to avoid any risk of forming condensate in the system, an additional connection duct (or equivalent means) can be provided between the transition hopper 105 and the conveyor 6 near the hopper 8, by moving selectively the outer air entrance on such duct and providing valves for regulating the flow both of the hot air arriving from the transition hopper 105 and of the cold environment air. This allows raising the air temperature in the system up to levels so as to eliminate the condensate formation risk. The above-mentioned regulation of inletting hot and cold air flows could then take place based upon the detections of the above-mentioned temperature sensor positioned onto the duct 51.
At last, it will be then comprised that the above-mentioned separation into two environments can be also obtained by means of devices different from those described above. For example between the extractor 9 and the conveyor 6 additional devices can be provided such as clapet valves or equivalent devices, moreover the separation of the two environments can be obtained by applying under the hopper/reservoir 8 a second crushing stage with variable flow with respect to the mill 3, so as to produce in the hopper the necessary ash head apt to separate the environments.
It will be appreciated that the invention allows an efficient energy recovery deriving from having sent the maximum outer air flow on the extractor 9 and having decreased drastically the air quantity on the second extractor 6 (for the water addition) and therefore the energy necessary to the fume treatment.
The invention object is also a method for extracting, cooling and recovering energy of heavy ashes as described so far with reference to the plant 1.
The present invention has been so far described by referring to preferred embodiments. It is to be meant that other embodiments may exist belonging to the same inventive core, all within the protection scope of the claims illustrated hereinafter.
| Number | Date | Country | Kind |
|---|---|---|---|
| RM2008A000662 | Dec 2008 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB09/55604 | 12/9/2009 | WO | 00 | 8/25/2011 |
