| CLAIMS 1. A boiler structure for burning solid fuel, wherein the boiler structure includes a combustion chamber (1) having a grate (2) at the bottom thereof; fuel supply means (3) ; combustion air supply means; and a discharge duct (4) for hot exhaust gases, characteri zed in that - the grate (2) includes a nozzle structure (5) provided with a primary combustion air feed- through in order to fluidize the fuel in the air flow above the grate, - the grate includes two hinged sections (6) which can be turned into the substantially vertical po- sition independently of each other in order to drop the ash from the top of the grate into a lower ash box, - the fuel supply means (3) include a substantially top part of the combustion chamber (1) to the space above the grate and surrounded by hot combustion gases during the combustion process, and that - the jacket of the tube (7) is provided with holes (8) disposed substantially along its entire height in order to lead the steam, released as the fuel flowing down the tube is drying, and the burning gases into the combustion chamber before the fuel falls into the layer of primary air on the grate. 2. The boiler structure according to claim 1, characteri zed in that the top end of the tube (7) includes a supply device (9) in order to supply fuel into the vertical tube in a controlled manner. 3. The boiler structure according to claim 1 or 2, characteri zed in that within the tube (7) there is a second supply device (10) in order to supply the fuel flowing onto the grate down the tube in a controlled manner. 4. The boiler structure according to any one of claims 1 to 3, characteri zed in that the combustion chamber includes at least two, preferably 4 to 8, vertical perforated tubes in order to drop the fuel onto the grate substantially evenly. 5. The boiler structure according to any one of claims 1 to 4, characterized in that the grate (2) includes flow openings for the primary combustion air disposed substantially evenly over its entire area in order to lead a strong primary combustion air flow under the fuel and to fluidize the fuel in the air flow during the combustion process. 6. The boiler structure according to any one of claims 1 to 5, characterized in that the hinged sections (6) of the grate (2) consist of a t o- layer structure constJjtnt-ina—a—sp -e—i-fi-fco—wki-eh—-h-e- primary combustion air is led via a pivoting shaft (11) of the hinged section. 7. The boiler structure according to any one of claims 1 to 6, characterized in that the combustion chamber (1) includes secondary combustion air nozzles (12) disposed substantially at the level of the bottom end of the tubes (7). 8. The boiler structure according to any one of claims 1 to 7, characterized in that the top end of the combustion chamber (1) includes ter- tiary combustion air nozzles (13) in order to supply tertiary combustion air to the hot combustion gases before they exit the combustion chamber. 9. The boiler structure according to any one of claims 1 to 8, characterized in that the vertical jacket of the combustion chamber is constituted by at least a two-layer steel sheet structure in order to cool the jacket and preheat the combustion air before supplying it to the combustion process. |
FIELD OF THE INVENTION
The invention relates to the boiler structure as defined in the preamble of claim 1 for burning solid fuel in a combustion chamber of a boiler provided with a grate.
BACKGROUND OF THE INVENTION
Currently, all available energy is to be recovered as efficiently as possible. For this reason, there have been attempts to develop various combustion processes where it would be possible to burn diverse masses in a versatile manner, such as sawdust and bark residue from the timber industry, straw and suchlike culms, brushwood from thinning, branches, tops and stumps fxom f nai logging, peat, pellets and briquettes made from different materials, municipal waste etc.
The boiler structures used in combustion processes are normally either boilers provided with different types of chain grates or fluidized-bed boilers. Both structures are well-functioning and efficient per se, but only with specific fuels. Clean and complete combustion would require high temperatures, which easily leads to melting of the ash. In addition, operation at high temperatures often approaches the thermal resistance limits of the structures.
Problems in the known boilers are caused particularly by the fuels having a high ash content and a relatively low melting temperature of the ash. This applies particularly to the peat-based fuels, but ash melting problems are present in more or less all bio-based solid fuels. Similarly, problems are caused by the moist fuels and particularly those exhibiting high moisture variation. Thereby, the combustion process is often unstable and difficult to control.
As the ash is melting and adhering to the grate structures, efficient combustion air flows change and fade away in such a way that the combustion becomes incomplete. Furthermore, cleaning the structures is slow and difficult, which leads to the molten ash causing long and expensive stoppages. In addition, in fluid- ized-bed boilers the sand forming the fluidized bed is consumed by tons per day. Being special sand, it is expensive. It requires a great deal of blowing energy to fluidize it. It requires a great deal of thermal energy to heat it. Furthermore, the sand in the combustion chamber and the smoke ducts wears heavily on the surfaces, thereby increasing the need for maintenance .
OBJECTIVE OF THE INVENTION
The objective of the invention is to eliminate the drawbacks of the prior art referred to above. In particular, it is an objective of the invention to disclose a new type of a combustion boiler for burning nearly any solid fuel effectively, completely, cleanly, economically and ecologically, without any special measures or adjustments.
SUMMARY OF THE INVENTION
The boiler structure according to the invention includes a combustion chamber having a grate on the bottom thereof. Furthermore, it includes fuel supply means and combustion air supply means as well as an outlet duct in order to lead the hot exhaust gases to the heat exchangers and other destinations according to the application and need. In addition, heat ex- changers, such as superheaters, may be disposed even at the top part of the combustion chamber.
According to the invention, the grate includes a nozzle structure provided with a primary combustion air feed-through, i.e. a set of holes directing a strong air flow upwards from the top of the planar grate. This way, merely by the primary air flow, the fuel is fluidized above the grate in an intensive combustion process. Furthermore, according to the invention, the grate includes two substantially horizontal hinged sections which may be turned to a substantially vertical position independently of each other and which provide for the dropping of the ash from the top of the grate into an ash box below the grate. As the grate consists of at least two separately turning sections, the combustion process can be made continuous, because the combustion may go on at one part of the grate while the other part_ of . the grate is being _t_urned and—cleaned-.—I-n—a-d-d-ifeion-—a-eee^-d-ift-—fe-o —the—i-n— vention, the fuel supply means include a substantially straight and vertical tube extending within the combustion chamber from the top part of the combustion chamber to the space above the grate, to a distance therefrom. This way, the tube is surrounded by hot combustion gases during the combustion process. As the tube jacket is additionally provided with holes substantially along its entire length, the fuel flowing down the tube is dried. In consequence, the steam released from the fuel at the top end of the tube is able to access the top end of the combustion chamber through the holes. In the same way, at the lower part of the tube, the burning gases gasifying from the drier fuel are discharged to the combustion chamber through the holes of the tube and are burnt therein. This way, the dry fuel which is already partially gasified or even charred falls from the bottom end of the tube onto the grate or, more specifically, onto a primary combustion air cushion on top of the grate, where it gasifies and burns cleanly and completely.
In a preferred embodiment of the invention the top end of the tube supplying fuel onto the grate includes a supply device, such as a screw conveyor, by which the fuel can be supplied in a controlled manner into the vertical tube with a desired mass flow. In the same way, a feed screw is preferably used in the vertical tube. This way, blockage of the tube is prevented and steady fuel supply is ensured with different fuels of varying properties and their mixtures.
In order to have the fuel as an even layer into the air cushion on top of the grate, it is advantageous to use at least two and possibly 4 to 8 vertical perfo ¬ rated tubes in the combustion chamber. When the number of tubes is selected—S-O—a.s t ma-tch—:to—th^g—s- r-f-a-e-e—a-r-- ea of the grate, and the bottom ends of the tubes are disposed evenly on different locations of the grate, quick and complete combustion is obtained.
In order to carry out fluidization of the fuel only by means of air, the grate has openings for the primary combustion air flow, distributed substantially evenly over its entire area. Preferably, the planar hinged sections of the grate consist of a two-layer structure, i.e. an upper jacket and a lower jacket. The primary combustion air is then led into the space formed therebetween, suitably for example via a pivoting shaft of the hinged section. The density of the holes and their size on the grate may vary in different applications. Their purpose is to provide a strong upwardly oriented primary combustion air flow forcing the fuel upwards and off from the surface of the grate. This way, the combustion takes place as fluid- ized combustion or at least as a swirling moving mass on top of the grate in the hot preheated primary corn- bustion air flow surrounded by the hot wall structures of the combustion chamber.
Complete combustion in the combustion chamber also requires additional combustion air in addition to the primary combustion air blown through the grate. Therefore, the combustion chamber also has secondary combustion air nozzles disposed above the grate and the fluidizing layer of fuel, preferably approximately at the level of the bottom end of the tubes supplying the fuel to the combustion chamber.
The final combustion of the compounds that have not yet burnt can be ensured by tertiary combustion air nozzles disposed at the top . end of the combustion -G -aicifeei— — h-i-e-h—tre tia-r-y—eembu-s-ti-e-— a-±£—i-s—dir-e-eted- to the hot combustion and exhaust gases before they exit the combustion chamber.
In the structure according to the invention the vertical tubes carrying out the fuel supply can be defined as having such a length that the fuel therein is always dried first and then gasified in such a way that the fuel discharging from the bottom end of the tube onto the air cushion is practically pure carbon. This way, a three-step combustion process is developed vertically in the combustion chamber, wherein drying of the fuel, gasifying and combustion of the carbon take place as substantially overlapping and sequential processes .
Preferably, the combustion chamber is a vertical and cylindrical structure, i.e. it has a circular horizon- tal cross-section. Preferably, the combustion chamber jacket has at least a two-layer steel sheet structure, wherein the primary, secondary or tertiary combustion air circulates between the jackets, cooling the jacket while the air is thus heating up before flowing into the combustion process. Similarly, the jacket may have other air circulations by which it is cooled down and the heated air is utilized in another way in the combustion process or externally.
This way, in the combustion chamber according to the invention the wall structures of the combustion chamber are intensively cooled by preheating the combustion air and drying the fuel, providing for high combustion temperatures and clean and complete combustion without the temperature of the combustion chamber structure being able to rise to a harmful level.
T & gxa£.e p.e —s-e—nd—the—&o^--e- on4 ftg—h-i-R-ging—mea-R-s- may constitute an assembly wherein the planar grate consisting of two sections and constituting the entire bottom of the combustion chamber can be turned in two separate sections independently of each other, the sections of the grate having for example the shape of a half of a circle. The essential feature is that the grate sections operate independently of each other in such a way that the combustion may go on at one half of the grate while the other half of the grate is turned in the vertical direction in order to clean it from the ash.
Thus, in the combustion chamber according to the invention the combustion takes place mainly by fluidiza- tion or at least as a strong swirling mass movement immediately above the grate, whereupon also the producing ash is mostly moving on top of the grate. Therefore, when the grate is turned down and the air flow carrying out the fluidization turns horizontal, the ash produced on top of the grate falls down into the ash box. The same also happens to ash possibly ad- hered to the grate, i.e. continuing the strong air flow through the grate also when it is turned into the substantially vertical position is sufficient to blow the grate clean. Of course, a vibrator can also be used in connection with the grate if necessary so as to be able to shake off any adhered matter from the grate. In the same way, scrapers scrubbing the surface of the grate or moving in other ways along its surface or other ar- rangements pushing or pulling the adhered impurities off from the grate surface can be used.
The boiler structure according to the invention provides considerable a Lantaxj S—a.s—compar-e-d-^ Q—t¾-e—pr-i-e-r- art. A single boiler structure can be used to burn any bio-based fuels and wastes. The combustion is efficient and complete, and the combustion process is controlled and stable, irrespective of the different moisture contents of different fuels, their ash con- tents and the melting temperatures of the ash. Thanks to the boiler structure, very high combustion temperatures can be used, ensuring clean combustion products. Furthermore, by the properly controlled heat flows in the structure, the structures are able to withstand even intensive heat.
LIST OF FIGURES
In the following section the invention will be described in detail with reference to the accompanying drawings, in which Fig. 1 is a schematic cross-sectional view from the side of one boiler structure according to the invention and
Fig. 2 is a schematic view of the grate of the structure illustrated in Fig. 1 as seen from the top .
DETAILED DESCRIPTION OF THE INVENTION
One boiler structure according to the invention pre- sented in Fig. 1 includes a combustion chamber 1 having the cross-section of a circular cylinder structure. The vertical walls of the cylinder are formed by a three-layer steel sheet structure forming air ducts. At the bottom of the combustion chamber 1 there is a grate 2, and the fuel is supplied to the combustion chamber and onto the grate from the top. More specifically, the structure is as follows.
The irsr 2 at the bottom of thie combustion chamber ϊ, as illustrated from the top in Fig. 2, is a circular structure consisting of two semi-circular hinged sections 6 having a bottom plate 14 and a top plate 15, whereby an air space is formed therebetween. Between the hinged sections there is a hollow pivoting shaft 11 by which the grate is supported to the sides of the combustion chamber. The pivoting shaft 11 may have for example two shafts within each other, one being coupled to the first hinged section 6 and the other one to the second hinged section 6 in such a way that the halves of the grate, i.e. the hinged sections, can be turned between a horizontal service position and a downward substantially vertically turned cleaning position independently of each other. In Fig. 2, between the hinged sections 6 there is the middle part 16 of the grate not turning with either of the hinged sections but instead remaining in place. However, it is possible to implement the grate also without the middle part, in which case the two hinged sections together constitute the entire top surface of the grate. The top plates 15 of the hinged sections 6 and the middle part are provided with a nozzle structure 5, i.e. they are provided with a large number of holes distributed substantially evenly over their entire surface area. This way, the primary combustion air blown from the hollow pivoting shaft 11 between the bottom plates 14 and the top plates 15 of the grate is discharged up into the combustion chamber 1 through the holes. The fuel supply means 3 of the boiler structure include fuel silos 17 from which the wood chips, peat, straw, pellets, sawdust or other solid fuel is supplied into the combustion chamber .1....Under, the fuel _s_i-Lo ]_7 thexe—is——fi-r-s-—suppl-y— ev-iee—9—p-ifov-drded—w-rth a rotatable coil by which the fuel is moved on top of the combustion chamber into the actual vertical fuel supply tube 7. The supply tube 7 is a long tube extending from the top of the combustion chamber to the interior thereof, to a distance from the grate above it. Inside the tube 7 there is also a second supply device 10 provided with a coil, the rotation of which may be controlled to control the amount of fuel falling onto the grate. The entire jacket of the tube 7 over the portion within the combustion chamber 1 is perforated, i.e. the tube jacket has a large number of holes 8 disposed along its entire length. This way, as the fuel flows down the tube at a speed determined by the second supply device 10, it simultaneously dries first, i.e. moisture evaporates from it. The moisture is able to discharge into the top part of the combustion chamber through the holes of the tube. After dry- ing, the fuel begins to gasify and, in the same way, the gases are able to access the combustion chamber through the holes 8 of the tube 7 where they burn. As seen in Fig. 2, there are preferably several tubes 7 supplying the fuel onto the grate; in Fig. 2 there are two tubes on top of each grate half. This way, the fuel can be distributed directly, as a sufficiently even layer into the air flow on the grate.
In the boiler, surrounding the combustion chamber 1 there is provided a multi-layer air circulation 20 in the following way. A primary air blower 18 blows pri ¬ mary combustion air around the ash box 19 and from there on around the bottom part of the combustion chamber 1, first upwards on the outside and then back down closer to the combustion process, after which the heated _combustion air is led into . the grate . via the fa arln-g-s of the—^i-v-oting—sii-aft—1-1—©ΐ—tfee—g-r-a4Ere 2—a-fi-d- there on out and upwards through the holes on the top plate 15 of the grate. The primary air flow heating up cools the bottom part of the combustion chamber as well as the grate structures, providing for a hot and efficient combustion process in the strong air flow carrying the fuel on top of the grate.
Above the primary air circulation 20, the combustion chamber is surrounded by a second two-jacket air circulation 21 where a secondary air blower 22 blows com- bustion air to the outer jacket 23 surrounding the combustion chamber, wherefrom the air is led to the inner air jacket 24. From the inner air jacket 24 the hot combustion air can be drawn in a controlled manner by a control device 25 to secondary combustion air nozzles 12 by which the secondary combustion air is led into the combustion chamber 1, approximately to the level of the bottom end of the tubes 7.
Furthermore, hot combustion air is drawn from the in- ner air jacket 24 by a second control device 26 into tertiary combustion air nozzles 13 disposed in the top part of the combustion chamber, i.e. approximately in the hottest part of the combustion chamber. Depending on the need of secondary and tertiary combustion air, the excess hot air flows are guided to another application through an outlet 27. Furthermore, in order to keep the maximum temperature at the top end of the combustion chamber at the allowed level of approximately 900°C in all conditions, water can be injected into the combustion chamber by nozzles 28 and a supply arrangement 29 connected thereto.
The structure according to Fig. 1 and 2 operates in the following way. Fuel_,_ siicli _as_ -W-O-od- -G-hi-p-s-r- i-s—ted- along the tubes 7 down to the grate 2 in a controlled manner. At the same time, the fuel dries and gasifies, and the steam and burning gases are able to discharge out into the combustion space through the holes 8 in the tubes 7. The partially or even completely charred fuel falls towards the grate 2 from the bottom end of the tubes. At the grate, the fuel burns as fluidized combustion on the intensive air cushion oriented upwards from the grate. The combustion is further intensified by the secondary combustion air to be led to the level of the bottom end of the tubes and assisted with the tertiary combustion air to be led to the top end of the combustion chamber. Depending on the humidity of the fuel, i.e. the amount of steam discharging therefrom to the combustion process through the holes 8 of the tube 7, additional water may also be led to the combustion process through the nozzles 28 in order to maintain the temperature of the combustion gases below the allowed maximum limits of the structures. An essential feature in the structure and operation according to the invention is also the removal of the ash carried out during the continuous combustion by tipping over the sections of the two-piece grate at different times according to need. The tipping over and the efficient cleaning of the grate is important for the reason alone that extremely hot and complete combustion bears a great risk that the ash would melt and block the grate. This way, by the strong air flow through the grate, also effected when the grate is disposed in the vertical position, the grate is kept clean and operational irrespective of the employed fuels and temperatures. Furthermore, during cleaning of one of the grate halves, the fuel supply and combustion goes on uninterrupted and at full power at the other grate section. This way, the new fuel to be sup- pii-e-d—onto—th-e—e-l-ea-ne- — ra-te—hai-f—ta-ke-s—f-i^-e—i-mrrted-i: ately and no substantial breaks in the operation of the combustion chamber occur.
The essential feature in the structure according to the invention is the extremely hot combustion as well as proper control of the heat-resistant materials having good heat-storing and conducting properties, and of the heat flows. The combustion may take place in extremely hot conditions because the wall structure limiting the combustion space is highly heat- resistant, thanks to the strong and efficiently cooling air flows. Furthermore, the combustion air to be led to the combustion space is very hot, ensuring quick and complete combustion. Due to the two-step operation of the grate, the process will not cool down when the grate is being turned, but instead the combustion is continuous and the burning hot combustion air flows into the combustion chamber steadily and continuously. This way, after turning the grate section back to the horizontal position, the steady and complete combustion is immediately continued.
The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims.