BOILER

Abstract: Boiler for extraction by combustion from random flammable material of thermal energy with a flow duct comprising an exhaust device connected to an outlet side of the duct, a first cylindrical combustion chamber (4) close to the intake side, a second cylindrical combustion chamber (5) connected to the first combustion chamber via a connecting duct portion whereby the connecting duct portion runs out tangentially into the second combustion chamber, and additional air supply means (11) for supplying additional combustion air into the second combustion chamber. The first and second combustion chambers are arranged coaxially and are separated from one another by a first dividing wall (6) comprising in the upper portion a tangential passage opening (7), a discharge chamber (10) with a discharge on the periphery connects onto the second combustion chamber via a second dividing wall (8) with a central passage opening (9), and the additional air supply means comprise an air supply pipe debouching into the discharge chamber at a distance opposite the central passage opening and having a smaller diameter than the passage opening.

Description: The invention relates to a boiler as specified in the heading to claim 1. Such a boiler for burning random material, in particular waste material, is suggested in EP-A-0 289 355 published after the filing date of this patent .The first and second combustion chambers are formed respectively in two cylinders adjacently positioned in a casing. The mixture of combustion gas with flammable products released in the first combustion chamber as a result of imperfect combustion is burned up completely in the second combustion chamber with the additional combustion air supplied by the additonal air supply means. Because the connecting ducts runs out tangentially into the second combustion chamber, an eddying of the combustion gases is caused in the second combustion chamber, which is favourable for a good combustion. As a result of the eddying movement any ash constituents which may be left behind are forced radially outward, as a result of which they remain behind in the second combustion chamber. The second combustion chamber thus acts in principle in the manner of a cyclone. The boiler is embodied such that cool air can flow along the cylinders. The air heats up as a result and the heated air can be used for any random heating purpose. A drawback of this boiler is that its manufacture is relatively complicated and its capacity is comparatively limited, since air is used as the heat transporting medium. The invention has for its object to improve a boiler of the above type such that it acquires a greater application potential as a result of a simpler construction. In a boiler according to the invention this is achieved according to claim 1. As a result of the coaxial arrangement of the first and second combustion chamber, separated by a dividing wall provided with a tangential passage opening, a very simple construction is obtained. A good operation, i.e. complete combustion, is ensured by the discharge chamber with the air feed pipe debouching opposite the central passage opening. It has been found that as a result of the tangential passage opening in the dividing wall a very good spiral flow is generated in the second combustion chamber. The central passage opening is likewise passed through in a spiral-like flow. The additional combustion air, which flows in via the air supply pipe running out at an interval before the central opening, flows, as a consequence of the underpressure generated by the exhaust device, unimpeded through the central "core" of the spiral-like flow in the passage opening into the second combustion chamber. Here this air mixes gradually with the combustion gases. At a comparatively short distance from the first dividing wall the unburnt products present in the combustion gases begin to burn. Volume increase occurs as a result, thus reinforcing the spiral flow. The supplied additional air is colder and therefore heavier than the combustion gases and is forced as a result of the generated centrifugal forces outward against the wall of the second combustion chamber. The thermal loading of the wall of the second combustion chamber consequently remains limited and the heat transfer can take place gradually over the whole surface area of the second combustion chamber. In the first combustion chamber the flammable material is in preference directly burnt on the bottom of the combustion chamber. Primary combustion air is supplied through an air supply pipe arranged just above the bottom and provided with small outflow openings. As a result of the supply of primary air the ash of the combustion material attains a fluidised state, which is a very suitable state for a good combustion and gasification. Flowing above the furnace hearth is extra air which has a cooling effect. As a result the temperature of the furnace hearth can if required be held at a low value. This occurs in particular when the material for combustion contains heavy metals and it is desirable that these remain behind in the ash. Because of the relatively low temperature these heavy metals will not vapourize. The temperature will however be such that hydrocarbons are broken down and it will therefore be possible for them to be completely burned in the second combustion chamber. Particles of flammable material carried along in the combustion gases burn up entirely in the second combustion chamber. It has been found that only minimal quantities of inflammable material reach the second combustion chamber. As a result of the centrifugal force field these solid particles are deposited on the wall of the second combustion chamber. The boiler according to the invention thus combines a simple construction with the possibility of a very complete combustion and minimal contamination of the exhaust gases. It has been found in the case of one embodiment that the content of carbon monoxide, nitrous oxides, unburnt hydrocarbons and dust in the combustion gases is minimal and lies well below the legally established requirements for these materials. Designated in claims 2-7 are steps which have been found to result in a very good operation of the boiler according to the invention. A favourable further development of the boiler according to the invention is characterized in claim 8. A maximum temperature indicates that the correct amount of additional air is being supplied. An excessive or deficient amount both result in a temperature lower than the maximum. The boiler embodied in this way thus achieves a maximum output. Using the step from claim 9 a good flow of the additional air through the central passage opening deep into the second combustion chamber is achieved. The boiler according to the invention is very suitable for embodiment as hot water or steam boiler. The coaxially arranged first and second combustion chambers thereby form in a favourable manner the fire tube of such a boiler. Because of the previously noted small heat loading of the walls of both the first and second combustion chamber, ordinary structural steel can be used. Achieved with the step from claim 11 is a favourable embodiment which can also be used for later conversion of already existing hot water or steam boilers. The invention will be further elucidated in the following description of a preferred embodiment with reference to the annexed figures. Fig. 1 is a partly broken away lengthwise section of a boiler according to the invention as a steam boiler. Fig. 2 is a partly broken away perspective view of the boiler from fig. 1. The steam boiler 1 according to the invention comprises in the usual manner a water jacket 2 and a fire tube 3. In accordance with the invention a first combustion chamber 4 and a second combustion chamber 5 are formed in coaxial connection in the fire tube 3. These two combustion chambers 4, 5 are separated from one another by a dividing wall 6 which comprises a tangential passage opening 7 in the upper portion. Connecting onto the second combustion chamber 5 via a second dividing wall 8 is a discharge chamber 10. The second dividing wall 8 is provided with a central passage opening 9 bounded by a cylindrical tube wall portion 19. Debouching into the discharge chamber 10 opposite the central passage opening 9 is an air supply pipe 11. This latter has a smaller diameter than the passage opening 9 and the diameter of the air supply pipe 11 preferably amounts to half that of the passage opening 9. The quantity of additional air supplied via the air supply pipe 11 is controlled with a control valve 28 arranged in this pipe 11. The discharge chamber 10 is embodied as a return flow chamber and leads to smoke tubes 12 debouching on the periphery thereof. These smoke tubes 12 guide hot combustion gases out of the return flow chamber 10 to the forward side of boiler 1 where these smoke tubes 12 run out into a discharge duct 13. This discharge duct 13 connects via an exhaust fan 14 driven by a motor 16 onto a flue tube 15. The boiler 1 is further provided in per se known manner with fittings for connection of mains water and steam lines. Fuel 21 can be introduced into the first combustion chamber 4 via a free intake opening 20 in the front wall of the boiler 1. A great variety of materials can serve as fuel. Waste substances in particular can be burned in suitable manner using the boiler according to the invention. As a result of the combustion an ash layer 22 is created at the bottom of the combustion chamber 4. Close to the bottom wall of the fire tube 3 an air pipe 23 provided with a large number of outflow openings protrudes into the first combustion chamber 4. A fan 24 connected to this pipe 23 supplies primary combustion air via the outflow openings in the pipe 23. The ash layer 22 is brought into fluidised state by the outflowing air, as a result of which all the flammable parts are provided with sufficient oxygen to be broken down completely. Completely burnt up ash 22 flows at the forward side of the fire tube 3 below a baffle wall 29 and over an overflow wall 30 onto an ash discharge belt 31. In this way a constant level of the ash layer is maintained in the first combustion chamber 4. The fuel 21 can be introduced into combustion chamber 4 either by means of a conveyor or manually. When a conveyor is used it can in a favourable manner also perform the function of keeping the layer of flammable material in loose state. Blades arranged for instance on the conveyor can "plough up" this layer at regular intervals. The disadvantageous effects of the tendency of some fuels to form a crust are eliminated as a result. The fans 24 and 14 are adjusted such that the primary combustion air supplied via the distributor pipe 23 and the additional air drawn in via the intake opening 20 are insufficient to bring about perfect combustion of the fuel. The temperature in the first combustion chamber remains relatively low as a result. Any heavy metals that may be present in the fuel 21 consequently do not vapourize but remain behind in the ash 22. The additional air flowing in via the intake opening 20 moreover has a cooling function, causing the temperature to remain relatively low. The combustion gases with partially gasified fuel flows via the tangential opening 7 into the second combustion chamber 5. As a result of this tangential supply a spiral-like flow is created in the second combustion chamber 5 along the cylindrical wall of this chamber 5. This flow is indicated with the arrow 25. Consequently a spiral-like flow into the central passage opening 9 in the dividing wall 8 likewise occurs, with the result that additional combustion air flowing in via the air supply pipe 11 can pass through. The inflow of additional combustion air through the air supply pipe 11 penetrates through the core of the spiral-like flow in the passage opening 9 deep into the second combustion chamber 5. This is indicated schematically in the figures with the lines 27. The spiral flow in the passage opening 9 is indicated with arrows 26. As a result of the tube wall 19 arranged at the location of the passage opening 9 a uniform flow is ensured. Shortly after the mixture of combustion gas and gasified fuel has found its way into the second combustion chamber 5, combustion of the gaseous fuel particles occurs through mixture with the additional air, which results in a great temperature increase and consequently volume increase. The spiral eddying is reinforced as a result. Because of the spiral eddying a centrifugal force field is moreover created, with the result that the supplied cool additional air, which is heavier than the hot combustion gases, displaces to the outside of the combustion chamber 5. Overheating of the wall of combustion chamber 5 is thus prevented and a uniform heat transfer ensured. In accordance with a preferred embodiment a temperature sensor 33 is arranged at the location where in practical tests the highest temperature has been found to occur. This sensor 33 is coupled to a control device 32 which actuates a control valve 28 in the additional air supply pipe 11. The control device actuates the control valve 28 such that a maximum temperature occurs at the location of sensor 33. Sensor 33 protrudes slightly from the wall of combustion chamber 5 so that the actual temperature in the combustion is determined and not that of the cooler air around it. As fig. 2 shows, the first dividing wall 6 can be formed in a favourable manner from a disc of sheet steel which is provided at the top with an incision extending from the edge to the central point. The portions of the disc on either side of the incision are deflected away from one another so that in between them the tangential opening 7 is formed and the adjoining portions define the walls of a partly helical channel. Good tangential admittance of the gas flow into the second combustion chamber 5 is achieved as a result. The two portions of the disc close to the incision are preferably deflected away from one another over a distance equal to a third of the diameter of the fire tube 3. The tangential opening 7 thus obtains a suitable section. It is further noted that the variation in air supply into the second combustion chamber 5 caused by the control valve 28 has hardly any or no effect on the air flow through the first combustion chamber 4. It has further been found that for good flow conditions a favourable ratio of the diameter of the central passage opening 9 to the diameter of the fire tube 3 can be selected of 1:2. The rear wall 34 of the discharge chamber 10 is formed in favourable manner as a removable cover in order to be able to clean discharge chamber 10 and the second combustion chamber 5 of ash remnants. The invention is not limited to the embodiment of a two-draught steam boiler shown in the figures. Single draught or for instance three-draught boilers can likewise be embodied in suitable manner as according to the invention. In addition, two fire tubes can be employed which are formed substantially identical to one another. Finally, it is noted that the stated dimensionings indicate preferences which have been found to result in a good operation of the boiler. The invention is not however limited to these dimensionings.

Claims: Boiler (1) for extraction by combustion from flammable material of thermal energy with an exhaust duct (13) comprising an exhaust device (14) connected to an outlet side of said duct (13), a first cylindrical combustion chamber (4) near the inlet side of said duct, a second cylindrical combustion chamber (5) connected to said first combustion chamber (4) via a connecting duct portion, whereby said connecting duct portion runs out tangentially into said second combustion chamber (5), and additional air supply means for supplying additional combustion air into said second combustion chamber (5), said first and second combustion chambers (4,5) are coaxially connected and are separated from one another by a first dividing wall (6) comprising in the upper portion a tangential passage opening (7), a discharge chamber (10) with a discharge on the periphery connects onto said second combustion chamber (5) via a second dividing wall (8) with a central passage opening (9), and said additional air supply means comprise an air supply pipe (11) opening out into said discharge chamber (10) at a distance from and opposite to said central passage opening (9) and having a smaller diameter than said passage opening (9). Boiler as claimed in claim 1, characterized in that the central passage opening (9) is bounded by a cylindrical wall (19). Boiler as claimed in claim 1 or 2, characterized in that the diameter of the central passage opening (9) amounts to substantially half the diameter of the second combustion chamber (5). Boiler as claimed in any of the foregoing claims, characterized in that the diameter of the air supply pipe (11) amounts to substantially half the diameter of the central passage opening (9). Boiler as claimed in any of the foregoing claims, characterized in that the air supply pipe (11) opens out at a distance in the order of magnitude of 20 cm from the central passage opening (9). Boiler as claimed in any of the foregoing claims, characterized in that the first dividing wall (6) is formed by a disc provided in its upper part with a radial incision, whereby the portions of said disc adjoining said incision are deflected on either side out of the plane thereof. Boiler as claimed in claim 6, characterized in that , the portions of said disc close to the outer diameter are deflected away from one another over a distance equal to substantially a third of the diameter of the first combustion chamber (4). Boiler as claimed in any of the foregoing claims, characterized in that a temperature sensor (33) is arranged in the second combustion chamber (5), that a control valve (28) is accomodated in the air supply pipe (11) and that a control device (32) is arranged connected at an intake to said temperature sensor (33) and at an outlet to said control valve (28) in order to maximise the temperature. Boiler as claimed in claim 8, characterized in that the control valve (28) is a diaphragm valve. Boiler as claimed in any of the foregoing claims, characterized in that the combustion chambers (4,5) are formed in the fire tube (3) of a hot water or steam boiler (1) and that the discharge chamber (10) is formed against the rear wall thereof. Boiler as claimed in claim 10, characterized in that said boiler (1) contains smoke tubes (12) and the discharge chamber (10) forms a return flow chamber connecting the fire tube (3) to the smoke tubes (12).

Inventors: VAN BREUKELEN, JOHANNES HUBERTUS
Applicants: MACHINEFABRIEK G. VAN DER PLOEG B.V.
Application year: 1989
Main classification: F23B5
IPC codes: F22B31, F23G7, F23G5