WO1997045675A1

WO1997045675A1 - Process for recirculating ashes generated by steam producing boilers

Info

Publication number: WO1997045675A1
Application number: PCT/EP1997/002954
Authority: WO
Inventors: Alberto Carrea
Original assignee: Magaldi Ricerche e Brevetti SRL
Current assignee: Magaldi Ricerche e Brevetti SRL
Priority date: 1996-05-31
Filing date: 1997-05-29
Publication date: 1997-12-04
Anticipated expiration: 1998-11-30
Also published as: ITMI961114A0; ITMI961114A1; AU3032897A; IT1282773B1

Abstract

A process for treating ashes generated by steam producing boilers. The process according to the present invention fundamentally provides for the reintroduction of ashes into the combustion chamber, preferably using a conveyor belt type dry ash extractor system. Through the use of the process according to the invention the unburnt matter percentage in ashes is highly reduced thus increasing the boiler efficiency and having as a final result ashes which are more easily treatable, usable and disposable and having a very small size.

Description

"PROCESS FOR RECIRCULATING ASHES GENERATED BY STEAM PRODUCING BOILERS"

Description The present invention relates to a process for treating ashes which are generated by steam producing boilers. More particularly the present invention relates to a process readmitting the ashes into the combustion chamber, preferably by using an ash dry extraction apparatus.

It is known that in steam producing plants using solid fuel such as for example coal, xylite, petroleum coke and anthracite, ashes as products of combustion are generated.

Paniculariy by using the above solid fuels which, as it is known, are generally completely burned with difficulty, ashes containing a high percentage of unburnt matter and which can have a quite large size are generated.

The process according to the invention provides a considerable improvement of the efficiency of a coal dust or other solid fuel boiler when the bottom ashes which are there generated have a high unburnt percentage or contents.

The process includes a set of steps, which will be better disclosed in detail in the following, such steps allowing the bottom ashes which have been generated during the combustion step, to be recirculated to the fuel silos for a reintroduction thereof into the boiler.

By using such a process the following main advantages are obtained: - the pulverized bottom ashes which are rich of unburnt matter are reintroduced into the combustion chamber, with a consequent use of a relevant part of the combustible matter contained therein; and - the bottom heavy ashes are eliminated due to the fact that all the ashes leave the combustion chamber as light ashes, thus their treatment will be easier and a consequent save of money will be obtained.

The above second advantage makes the use of the present process convenient also when the unburnt matter contents of the bottom ashes is not high, thus providing a broader possibility of using such a process.

When the bottom ash percentage is negligible with respect to the light ashes (indeed the general trend in this field is just towards a higher pulverization of the fuel in order to balance the effects due to the so called "NOx Ports"), the ashes treatment could be simplified by introducing into the boiler also the bottom ashes, in view of their negligible effect on the operating conditions of both the combustion chamber and the downstream apparatus.

Another convenient situation could be found when there is a difference between the commercial values of the light and bottom ashes: if such a difference is for the light ashes, it could be effective to change the bottom ashes into light ashes and then let them recirculate into the combustion chamber.

The above and further advantages are brilliantly obtained by a process having the features and comprising the steps of the characterizing part of independent claim 1. Further advantageous features of such a process are set forth in the dependent claims.

Claims 13-16 set forth particular embodiments of plants for carrying out the process according to the present invention.

The characteristics, objects and advantages of the present invention will become promptly clear from the detailed following description and the annexed drawings relating to some embodiments, given by way of non limiting example, of the plants which could be used in order to implement the process according to the invention.

In the various figures of the drawings:

- Figure 1 diagrammatically shows one plant embodiment which allows to carry out the process according to the invention; - Figure 2 diagrammatically shows a second plant embodiment which allows to carry out the process according to the invention;

- Figure 3 diagramatically shows a third plant embodiment which allows to carry out the process according to the invention; and

- Figure 4 diagrammatically shows a fourth plant embodiment which allows to carry out the process according to the invention.

The process according to the present invention, as set forth in the above introduction part of the present description, is for use in steam producing boilers.

In the following description, all the apparatus and machinery which are considered known for a skilled man in the art will not be described in detail. Further, for a better understanding of the invention reference is to be made to the European patents EP

0,252,967 and EP 0,471,055 both to Mario Magaldi, the Inventor of the instant application.

The process according to the present invention includes the following operations or main steps: a) dry extracting the ashes from the combustion chamber bottom and possibly cooling them to such temperatures which are compatible with the characteristics of the machinery which are in turn required and used for the further treating operations; b) crushing the ashes to a size which is compatible with the characteristics of the conveying system used for the ash recirculation, or with the fuel milling system, according to which of the above situations is more restrictive; and c) conveying the ashes from the discharge port of the crushing system to the fuel feeding silos for their reintroduction into the boiler. For each of the above operations the following apparatus and systems, according to the plant characteristics, could be used.

A system using a metal conveyor belt according to EP 0,252,967 (and commonly known as MAC system) could be conveniently used in order to carry out the dry ash extraction, or also any other dry extraction systems in use could be used. As far as the system configuration is concerned, in addition to common evaluations

(such as the room availability, the system costs, the system reliability etc.) the ash temperature at the extractor discharge port with respect to the temperature allowed by the downstream apparatus (crushing and conveying systems) should be considered.

The dry extraction of the present process has the great advantage in respect of the wet systems that it allows the immediate reintroduction of the extracted bottom ashes into the fuel silos without having to remove water from the extracted material or to properly locate the ashes, for its drying in the open air. In addition, the water contents of the wet ashes is in any case an energy loss because of the heat which will be taken away by the water evaporation. Thus, while the inherent drawbacks of the wet systems did not allow the reintroduction of the unburnt matter rich ashes into the combustion chamber, the now available dry systems allow the implementation of such a process.

As far as the bottom ash crushing is concerned, such a step is considered necessary mainly in order to reduce the ash size to such dimensions which are compatible with the requirements of the conveying system which will be used for the ash recycling and, in any way, with the requirements of the solid fuel crushing system.

A single crushing step (and for this purpose a single or double roller crusher could be used) or a double crushing step could be used in order to further reduce the ash size, if required by their output size (this situation could be when the ash conveying system to the silos is pneumatic-type).

As far as the step of conveying ashes from the crushing system discharge port to the fuel feeding silos is concerned, it is recommended that the reintroduction of ashes into the combustion chamber be carried out according to the following criteria, i.e. that said reintroduction is carried out by solid fuel pulverization mills. One must pulverize the recirculated ashes in order to avoid the progressive increase of the recycling matter amount: the pulverization will provide the proper separation between light ashes and bottom ashes. Because of the need of passing the recycling matter through the fuel mills a homogenous blend of the recycling matter into the fresh fuel is also requested order to avoid fuel pockets having different characteristics.

Under the above criteria, the proper time for reintroducing ashes into the fuel cycle could be chosen in the most suitable way according to the circumstances, complying with the configuration of the fresh fuel feeding system.

A possible way complying with such criteria is to mix the recycling ashes with the fresh fiiel on the belts feeding fuel to the fuel silos. This system, on the one hand provides a homogenous blend of the ashes and fuel, on the other hand needs some attention in order to avoid dust production when the dry and powdery ashes are discharged on the fresh fuel and further it requires that the system of feeding fresh fuel to the silos must be always operating. In order to avoid the first problem (dust production), it will be convenient to use an ash wetting apparatus placed upstream from the discharge port, such an apparatus will prevent the dust production by adding just a small amount of water. Instead, the second problem will be solved by inserting, at the most convenient position, a storage tank which is able to contain the whole matter which is produced during the maximum stop time of the fresh fuel feeding apparatus.

When the best position for reintroducing the ashes into the fresh fuel feeding apparatus has been fixed, according to the above criteria, the matter conveyance from the discharge port of the crushing system to said position could be carried out either by a mechanic apparatus or a conveying pneumatic one.

A possible change within the object of the present invention could be a process in which a very high crushing (pulverization) is carried out thus avoiding the use of fresh fuel mills in order to obtain such a pulverization (such an embodiment could be advantageous when the mills are considered a bottle-neck in the energy production).

In such a case the produced pulverized ashes could be introduced directly, by a pneumatic conveying system, into the feeding tubes of the mixture (primary air and fuel dust) upstream from the boilers. As a man skilled in the art could realize, designing of the present process is extremely important whenever a temporary storing tank is interposed between two process apparatus (as it could be convenient in order to limit any charge peaks or discontinuously use the downstream conveying system as to limit its wear), particularly as far as the proper tank is concerned. This is because of the inherent danger (due to the possible content of unbumt matter in the ashes) of causing a self combustion phenomenon which while it will not be hard because of the lack of flying matter into the ashes, is always a possible danger to be avoided. Thus such temporary storing tanks will be missing or, if the plant design makes them preferable, they should be provided with an inerting system or should be so designed as to avoid the. above phenomena unless the matter dwell time being so short that the self combustion start will not be possible.

A further possible feature of the present invention which is of main importance, is the possibility of providing the system with an alternative path, possibly also to be used as a redundance conveying system, for discharging the matter from the recirculating circuit. While the experience teaches that the recirculate inert matter is like the inert fresh fuel (from the point of view of the percentage between bottom and light ashes) it is always advisable to provide a matter purge system whenever there is a progressive tendency of the recirculating matter to increase.

Such an alternative path could also be a simple matter discharge port and in such a case it should be placed, while in accordance with other requirements, in a position as upstream as possible in the system component sequence in order to use it also as a discharge port in emergency situations.

The process of readmitting ashes into the combustion chamber thus is comprised of the above disclosed steps, which are chosen according to the above mentioned requirements and features, in order to physically and functionally interface each other. As an exemplification and not limiting example some of the possible embodiments of plants which could be used for carrying our the process according to the invention will be given in the accompanying drawings.

Each plant will be generally given the reference number 100, 200, 300 and 400 according to the particular embodiment, respectively; the same or equivalent parts will be given the same reference number.

In Fig. 1 a plant 100 is shown using a belt extractor 18, a crusher 20 able to reduce the ash size to a size which is compatible with both the next conveying system and the pulverization solid fiiel system requirements and further a mechanical conveying system comprised of a cup elevator 24 and an ash wetting apparatus 28 which is used only in order to avoid dust at the extractor itself discharge port 26. The wetting apparatus discharges the ashes directly on fresh fuel feeding belts 30, 32 in order to obtain a good mixing of fresh fuel and recirculated ashes. Starting from such a point the fresh fuel and ashes mix will travel along the usual feeding path toward the combustion chamber, firstly being discharged into the various silos 34.

For completeness of disclosure, 12 is a boiler provided with a proper (mechanic or hydraulic) seal, 14 a transition hopper, 16 a set of bottom valves, 22 a separator valve placed between the crusher and the cup elevator and 22A an emergency discharge port for valve 22.

In Figure 2 a plant 200 similar to the above plant 100 is shown but further having a storage tank 36 for temporary storing ashes because the fresh fuel feeding apparatus is not continuously operating; such a storage tank 36 is provided with a proper inerting system 38 in order to eliminate the possible danger of ashes, burning or self burning. In such an embodiment the emergency discharge port is indicated by 36A because it is provided in the storage tank. Between the storage tank 36 and the cup elevator, in such a case a storage tank extractor 22 is provided.

In Figure 3 a plant 300 using a pneumatic conveyor system 44 to reintroduce the ashes to silos 34 is shown; the system also uses a belt extractor 18 placed under the combustion chamber 12 and further a crusher 20 in order to reduce the ash size; a storage tank 36 of a proper volume provides a temporarv storing means for complying with the possible not continuous operation of the pneum: cic conveying apparatus, which indeed will operate discontinuously. A further crusher 20A operates as a pneumatic transport feeder/batcher, such a further crusher is advisable when a further reduction of the ash size is required by the pneumatic conveyor apparatus (which is able to convey only not too large grains) requirements or whenever ash reaggregation into the storage tank 36 is possible, such a reaggregation will cause clogging of the pneumatic conveying system.

The pneumatic conveyance (which in such a case is a vacuum type one) provides the ash transport to a separator provided with a filter 42 for the de- pulverization of the transport air; the ashes will fall on the separator bottom from which they will be extracted (at the end of the pneumatic conveying step) by the wetting apparatus itself which has in such a case a dual function. The wetting apparatus 28 will discharge the wet ashes on the belt feeding coal to silos.

Finally in Figure 4 a recirculating plant 400 is shown, such a plant being used whenever the dry ashes are collected in a hopper 14 placed under the combustion chamber and discontinuously evacuated therefrom and using a vacuum pneumatic conveying apparatus 44. The hopper is evacuated according to a preset cycle, by using the crushing system (generally comprised of a number of parallel placed crushers 20) and the exhaustor 40 of the pneumatic conveying apparatus, from such a point the system is like the embodiment of the previous figure, where a set of ash admitting valves has been assigned the reference number 22. It will be now clear that the process according to the present invention brilliantly attains the above objects and further remarkably increases the solid fuel boiler efficiency by causing the generated ashes to be recirculated.

Further burnt matter which can be treated and used in a easier way is obtained.

It is clear that several modifications, amendments, additions and changes of parts with functionally equivalent parts could be carried out in the disclosed embodiments, without anyway departing from the scope of the invention as defined in the appended claims.

Claims

1. Process of treating ashes which are generated in steam producing boilers, such a process comprising at least the following steps: a) extracting said ashes from the bottom of a chamber of combustion; b) conveying said extracted ashes to a fresh fuel mixing station; c) storing said ashes and fresh fuel mixture in one or more silos; and d) readmitting such a mixture of ashes and fresh fuel into the combustion chamber.

2. Process according to claim 1, characterized in that it further comprises one or more steps of crushing of the ashes having a too large size.

3. Process according to claim 1 or 2, characterized in that it further comprises one or more steps of cooling ashes to such a temperature which is compatible with the characteristics of the apparatus to be used for the further steps.

4. Process according to any of the preceding claims, characterized in that the ash extracting step is carried out in a substantially dry condition by a metal conveyor belt system, the ashes being possibly cooled during their conveyance.

5. Process according to any of the preceding claims, characterized in that the step of reintroduction into the boiler is carried out by solid fuel mills so as to carry out the usual separation between light and bottom ashes.

6. Process according to any of the preceding claims, characterized in that said ashes are mixed with the fresh fuel on belts feeding fuel to the fuel silos.

7. Process according to claim 6, characterized in that ash wetting means are provided upstream from the discharge port of the ashes on said feeding belts so as to avoid dust formation.

8. Process according to any of the preceding claims, characterized in that at least a storage tank for temporary storing ashes is provided along the ash conveying path.

9. Process according to claim 8, characterized in that said at least one storage tank is provided with ash inerting means in order to prevent the ash self combustion and possible reaggregation thereof.

10. Process according to claims 8 or 9, characterized in that said at least one storage tank is provided with emergency ash discharge means.

11. Process according to any of the preceding claims, characterized in that at least a portion of said ashes is pneumatically vacuum conveyed.

12. Process according to any of the preceding claims, characterized in that at least a portion of said ashes is mechanically conveyed, by a cup elevator.

13. Plant for treating ashes generated in a solid fuel energy producing boiler, characterized in that it comprises: first conveyor belt means, crushing means, valve means, mechanical lifting means, wetting means, means for mixing ashes with fresh fuel, second conveyor belt means and means for storing said fresh fuel and ashes mixture before the reintroduction thereof into the boiler.

14. Plant for treating ashes generated in a solid fiiel energy producing boiler, characterized in that it comprises: first conveyor belt means, crushing means, storing tank means provided with an inerting apparatus and an apparatus for discharging dust in emergency situations, mechanic elevator means, wetting means, means for mixing ashes with fresh fuel, second conveyor belt means and means for storing said fresh fuel and ashes mixture before reintroducing it into the boiler.

15. Plant for treating ashes generated in a solid fuel energy generating boiler, characterized in that it comprises: first conveyor belt means, first crushing means, storage tank means, second crushing means, valve means, means for pneumatically transporting the ashes, exhaustor means, mixing means, wetting means, second conveyor belt means and means for storing said mixture of fresh fuel and ashes before reintroducing it into the boiler.

16. Plant for treating ashes generated in a solid fiiel energy generating boiler, characterized in that it comprises: valve means, ash conveyor pneumatic means, exhaustor means, mixing means, wetting means, conveyor belt means and means for storing said fresh fuel and ashes mixture before reintroduction thereof into the boiler.