BURNER FOR BURNING SOLID FUEL, PARTICULARLY PELLETS

Abstract: The invention relates to a burner intended for burning solid fuel, particularly pellets, which is intended to be installed in an opening in the side of the firebox. The burner includes hollow grate tubes (9) extending into the firebox, arranged parallel to and at a distance to each other. The fuel is burned on top of the grate tubes (9) and secondary air (SEC) is led through them. The burner also includes a pulsed and devices for feeding primary air (PRI) under the grate tubes (9) and for feeding secondary air into the grate tubes. The grate tubes (9) are arranged to be moved to and fro over at least two areas of movement and the burner includes an operating device (26, 30, 31) to operate them.

Description: BURNER FOR BURNING SOLID FUEL, PARTICULARLY PELLETS The present invention relates to a burner intended for burning solid fuel, particularly pellets, which is intended to be installed in an opening in the side of a firebox and which includes - hollow grate tubes extending into the firebox, arranged parallel to and at a distance from each other, for burning the fuel in a controlled manner on top of them, and for leading secondary air through them into the firebox, and - an operating device and slide for moving the grate tubes to and fro, and - a pulsed feeder for the fuel, comprising a fuel doser and a piston ram that moves to and fro, to push a dose of fuel from the opening in front of the burner into the firebox and on top of the grate tubes , and - devices for feeding primary air under the grate tubes and secondary air into the grate tubes . So-called stoker-burners have been on the market for a long time already. In these, fuel, especially wood-chips, is fed directly to the firebox by means of a screw feeder, their operation at full load being fair. Problems appear at part-load and during various breaks. Among other things, back-fires take place, i.e. the fire spreads along the feeder device towards the fuel silo. Finnish patents 65852 and 70316 disclose various burner devices, in which ram feeders are used to push the fuel into the firebox. Such a ram feeder pushes a does of a certain size into the firebox, the fuel feeder channel remaining otherwise empty, preventing possible back-fires. Norwegian patent 874876 (= WO 89/04939) discloses a burner equipped with grate tubes, in which the grate tubes are arranged to move backwards and forwards. Recently, it has been observed to be advantageous to form the grate on top of hollow tubes, with the primary air led beneath them and the secondary air, which also cools the grate tubes, led through them. This technique can be applied in burners in the size-class 50 kW - 1 MW. Burners of this type, equipped with hollow grate tubes, are already on the market and have proved to function well at full power. A scraper, which moves to and fro, may be used on top of the tubes and cleans combustion waste that has adhered to the tubes. Operation is not, however, entirely satisfactory. At part-load in particular, combustion does not take optimally and fuel does not move over the grate in a controlled manner. The present invention is intended to eliminate the drawbacks referred to above and create a burner according to the preface, in which the movement of the fuel over the grate is controlled and which operates at a good efficiency, even at part-load. The characteristic features of a burner according to the invention are stated in the accompanying Claims. In small burners of a size class of 100 kW, it is possible to use a combined operating device for both the piston ram feeding the fuel and for the grate tubes. Usually, however, it is preferable to use a separate operating device for the piston ram, so that the area of movement of the grate tubes can be divided according to the power used. Thus, at full power the largest grate area is used and is reduced according to the power. Secondary air is then always led to the correct place in relation to combustion. Other benefits and embodiments of the invention will appear in connection with the later examples of applications. In the following, the invention is described with reference to the accompanying figures, which show one burner according to the invention. Figure 1 shows the burner in partial cross-section. Figure 2 shows a cross-section of the burner at the point II - II. Figure 3 shows a top view of the burner and a cross-sectional top view of the screw drive device. Figure 4 shows a top view of the burner, with the radiation guard, operating device, and connections removed. In Figures 1 - 3, the burner is generally marked with the reference number 10. Burner 10 is attached by means of an attachment plate 11 to the side of the burner, covering the opening left for a burner. The fuel doser (not shown) comprises, for example, a screw conveyor and its operating device, by means of which doses 14 can be fed in front of the piston ram 13. The piston ram pushes the dose through an opening, which is normally covered by a cover 15. Burner 10 has a casing, into which a fan 19 blows air, so that the interior of the burner has a continual excess pressure. The combustion the fuel takes place mainly on top of the grate tubes 9, which are hollow, so that secondary air (SEC) can be fed through them into the firebox. Grate tubes 9 are secured together with the aid of a beam 29, to the ends of which arms 28 are attached, their other ends being attached to operating plate 21. These form a slide permitting the grate tubes to move. In Figure 2, operating plate 21 and nut component 27 are shown in their entirety. Nut component 27 of drive screw 26 is attached to this operating plate 21, so that operating plate 21 can be moved by rotating the drive screw. At the end of the drive screw, there is a shouldered shaft 32, which is driven through a gearbox 30 by the power of an electric motor 31. Electrical motor 31 is connected to a motor control, by means of which the operating device can be controlled as desired. The apparatus includes an operating screw revolution counter, with the aid of which the motor control can control the distance moved. In this case, operating plate 21 operates both piston ram 13 and support beam 29 of the grate tubes 9. It is not then possible to vary the stroke for different loading ranges. If, on the other hand, piston ram 13 is equipped with its own operating device, for example, a hydraulically, pneumatically, or electrically-operated cylinder, which moves to and fro according to its own control, the grate tubes can be controlled independently and be set to move backwards and forwards in, for example, three different areas of movement. In Figure 4, the single area of movement is marked with the reference number S and the areas of movement of the improved model with the reference number S'. As stated above, the casing of the burner is pressurized, so that secondary air can be fed to the grate tubes through the holes 25 in the bottom 23. This also cools the grate tubes. Primary air (PRI) is led to the upper space of the cover 24 through side holes 33, among other routes. Tube channels can be installed between covers 23 and 24, if the amount of air of the side holes 33 is insufficient. From this space, the air is guided under the grate tubes 9 by a fixed scraper plate 34 and from there with the aid of bottom support 35 to the fuel burning on top of the grate tubes. The grate tubes 9 are preferably covered by a radiation guard 36, which covers most of them, Figure 1. During the fuel-feeding phase, the upper corner of piston ram 13 opens cover 15, which closes by gravity when the piston returns to its space. There is no possibility of a back-fire. In the screw-feeder device according to the invention, the direction of rotation of the motor is converted to achieve a to and fro movement. In addition, the distance of travel is controlled by means of the aforesaid revolution counter. The burner according to the invention is equipped with an igniter that is, as such, known, comprising, for example, a resistor installed in the air pipe. During the ignition stage, air is led only through this pipe to a small amount of fuel. After this, the air-intake openings 33 and 25, shown in the figures, are opened to the required extent, for example, by means of a motor-operated damper (not shown) . Usually, this has three positions: ignition (openings 25 and 33 closed), openings 25 and 33 half open, and full power. In Figure 4, the grate area is at its smallest, being at its greatest at the other far end of the area of movement S, at which the end of one grate tube is shown by a broken line. The figure also shows clearly that primary air is led to most of the surface area of the grate, i.e. to the area of bottom support 35. The fan duct is connected to opening 1 ' . The fuel feed channel, however, is led through opening 12' to the top of the stroke space of piston ram 13.

Claims: Claims 1. A burner intended for burning solid fuel, particularly pellets, which is intended to be installed in an opening in the side of a firebox and which includes - hollow grate tubes (9) extending into the firebox, arranged parallel to and at a distance to each other, for burning fuel on top of them in a controlled manner and for leading secondary air (SEC) through them to the firebox, and - an operating device (26, 30, 31) and a slide (21, 28, 29) for moving the grate tubes (9) to and fro, and - a fuel feeder, to feed the fuel on top of the grate tubes, and - devices for feeding primary air (PRI) under the grate tubes (9) and secondary air into the grate tubes, characterized in that the arrangement includes devices for moving the grate tubes (9) over at least two areas of movement, to adjust the grate surface area according to the load. 2. A burner according to Claim 1, characterized in that the said operating device for operating the grate tubes is an drive screw (26) with an electric motor (30, 31) driving it. 3. A burner according to Claim 1 or 2 , characterized in that the said operating device (26, 30, 31) operates both the grate tubes (9) and the piston ram (13) of the feed device. 4. A burner according to Claim 1 or 2 , characterized in that the burner includes an auxiliary operating device for operating the piston ram independently of the grate tubes. 5. A burner according to Claim 2 , characterized in that the operating device includes a drive-screw (26) revolution counter, which is arranged to control the operating device of the drive screw.

Inventors: VALKONEN PERTTI, VALKONEN, PERTTI
Applicants: HT ENGINEERING LTD OY, VALKONEN PERTTI, OY HT ENGINEERING LTD, VALKONEN, PERTTI
Application year: 2000
Main classification: F23B1
IPC codes: F23H1, F23H3, F23H17, F23L9