A METHOD FOR MOVING A STEPPED GRATE IN A FURNACE FOR SOLID FUEL

Abstract: The invention relates to a method for feeding solid fuel on a grate forward in a furnace (10), whose grate (4) comprises two or more movable grate elements (4a, 4b, 4c) which are arranged stepwise after and underneath each other. In the invention, the grate elements (4a, 4b, 4c) are first moved together forward, after which the grate elements (4a, 4b, 4c) are returned backwards, starting from the lowermost one (4c). With the invention, more even combustion is achieved than in prior art, and the effective forward movement of the fuel is secured.

Description: A method for moving a stepped grate in a furnace for solid fuel The present invention relates to a method for feeding solid fuel on a grate forward in a furnace, as set forth in the preamble of the appended claim 1. The invention also relates to a furnace grate according to the preamble of the appended claim 5. The invention further relates to a furnace for solid fuel, as set forth in the preamble of the appended claim 8. Various arrangements for moving stepped grates are presented, for example, in US patent publication 4,598,651 , in which the grate is formed of transverse grate elements arranged in a step-like manner. So that the solid fuel supplied into the furnace would move forward and downwards during the combustion, the successive grate elements are simultaneously moved in opposite directions by means of a mechanical transmission. Combustion air is supplied to the fuel through and between the grate elements. Another solution is presented in US patent publication 4,563,959, in which the grate consists of elongated grate elements, each of which is arranged to be step-like. For moving the fuel forward, every other of the adjacent grate elements is arranged to move in a reciprocating manner, and every other grate element remains in a fixed position. The combustion air is supplied to the fuel through the grate elements. Another solution is known from Finnish published application 94284, in which transverse fixed grate elements are arranged one after another, and each grate element comprises several longitudinal grate bars. The adjacent grate bars are arranged to move in opposite directions to move the fuel. A problem with prior art grates is that the combustion is uneven, particularly that the combustion is intensified at the stage when the longitudinal or transverse grate elements are moving. The unevenness causes changes in the output of the furnace, wherein it is also difficult to control the combustion and, for example, a more precise control of the combustion air supply will be required. Also, the composition of the flue gases will be harmfully changed, when the straight flow of the combustion air through the fuel is intensified. With the intensification, also the quantity of suspended ash and other particles will increase and cause problems. It is an aim of the present invention to present a solution to the above- presented problems by means of a novel movement solution. In par- ticular, the aim is to keep the combustion process even but simultaneously to secure the effective progressive movement of the fuel on the grate of the furnace. The method according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 1. The furnace grate according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 5. The furnace according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 8. The significant advantage of the invention is the evenness of the combustion in spite of the fact that all of the fuel is first fed forward. Because of shifts, unburnt material is only exposed on a small area at a time, wherein the combustion is also intensified only slightly at a time. Because of a shift, for example, a thinner layer or an area totally free from fuel can be formed at the initial end, from where the combustion air escapes, causing the above-mentioned problems., The effect on the whole combustion process is considerably small or may be nonexistent, because there is more fuel at the initial end in any case. Fuel is also exposed when the grate elements move in relation to each other, but the area is small in size and has an insignificant effect on the output. The changes can also be easily influenced by the length of the movement of the grate elements and by the evenness of the cycle of the shifting movement. Moreover, thanks to the movement, the fuel is efficiently spread on the grate, and the gaps between the grate elements remain free from ash. In the following, the invention will be described in more detail with reference to the appended drawings, in which Fig. 1 shows a furnace and a grate to be applied in the invention, seen from the side, and Fig. 2 shows the movement of grate elements in a grate to be applied in the invention in more detail, seen from the side. With reference to Fig. 1 , fuel is supplied into a furnace 1 by feeding means, for example along a feeding pipe 2 by means of a feed screw 3. The fuel is made of combustible solid fuels, such as wood, straw and peat, for example by cutting, compressing or crushing. The fuel is in the form of wood chips, briquets or pellets. The grate 4 is preferably formed of three uniform cast iron elements 4a, 4b, 4c with an identical shape, which are interlocked, for example horizontally, to form a stepped grate 4. Each grate element forms a step of its own. In their width direction, which is the direction perpendicular to the plane of the drawing, the grate elements extend from one side of the furnace 1 to the other. Figure 1 also shows an example of, for example, actuator means which are intended to move the grate element back and forth. The means comprise a movable rod 7 supported by guides 5 and 6. The rod 7 is moved by a cylinder 8 operated by a pressurized medium, for example a hydraulic cylinder. The grate element 4c is coupled to the rod 7 by means of a pin, and the grate element 4c can also be supported to other structures in a movable manner. The other grate elements 4a and 4b are provided with their respective actuator means. The grate 4 is illustrated in a situation according to the invention, in which all the grate elements 4a, 4b and 4c are shifted backwards. In a feeding movement, all the grate elements are first moved simultaneously to the right, in the fuel feeding direction, and after that, they are returned to the left preferably one at a time, starting with the outermost and lowermost movable grate element 4c, then the element 4b and finally the element 4a, after which the cycle is repeated. Two or more steps consisting of one grate element will be required. The control system controlling the movement can simultaneously control other functions, such as blower means 11 and the motorized feed screw 3. The cylinder 8 coupled to the pressurized medium system is controlled by means of a control system, such as controllable logics 9 and controllable valve means 10. The valve means 10 comprise, for example, electrically controlled directional valves for controlling the hydraulic fluid and its pressure. The control system 9 is used for storing, or timing in another way, the desired movement cycle and its repetition frequency, which may vary according to the fuel used. The grate elements 4a, 4b and 4c are moved forward at intervals of, for example, about 5 minutes. Combustion air is supplied by the blower means 11 into the space underneath the grate 4, from which it flows past the grate elements into the fuel. The grate elements are preferably also provided with holes for the combustion air. When the grate elements are moving, the gaps between them remain open, wherein the combustion air can pass through the gaps into the fuel. The fuel coming onto the grate 4 is fed by the movement of the grate elements forward and stepwise downwards towards an ash space. When combusted, the fuel turns into ash which is then removed from the furnace. Figure 2 also illustrates the movement of the grate elements 4a, 4b and 4c of Fig. 1 , which takes place in the following way. In the first step A, all the grate elements 4a, 4b and 4c move simultaneously forward to feed fuel forward to the right. After this, in step B, the returning of all the grate elements to the opposite direction is started from the outermost single movable grate element 4c, wherein fuel, in this case primarily ash, is dropped off. In the next step C, the grate element 4b moves in the opposite direction, to the left, wherein fuel is dropped to the support of the grate element 4c. After this, in step C, for example after a desired delay, the grate element 4a moves backwards, after which the elements are in their initial position. The movement in the presented order, in a wave-like manner, towards the fuel feeding point, has the advantage that for example the rear part of the grate element 4b does not need to extend far underneath the grate element 4a. The rear part would be exposed, if the grate element 4a moved first. In this way, it becomes easier to arrange the flow of the combustion air and to support the grate elements. In the pre- sented order of shifting, the fuel also moves more evenly and efficiently. The fuel transferred to the support of the next element is carried by the fuel on the support of the preceding element. The grate 4 can be provided with several parallel sequences of grate elements, wherein the elements 4a, 4b and 4c of Fig. 1 constitute one sequence, and wherein all the sequences are moved forward simultaneously. To keep the changes smaller and more even, the adjacent grate elements are returned backwards one sequence at a time. The sequences are preferably separate, but the grate elements of the sequence can be placed at least partly underneath the adjacent sequence. The grate elements can also be arranged in a tilted position. The grate element may also consist of several parts, for example grate bars or the like. In case the grate comprises a large number of grate elements, the above-described system can also be applied in such a way that the returning takes place in sets of two or more successive grate elements, starting from below. The present invention is not limited solely to the advantageous embodiments presented and used as examples above, but it can be modified within the scope of the appended claims.

Claims: Claims: 1. A method for feeding solid fuel on a grate (4) forward in a furnace (1), the grate (4) comprising two or more movable grate elements (4a, 4b, 4c) which are arranged in a step-like manner after and underneath each other, characterized in that the grate elements (4a, 4b, 4c) are first moved together forward, after which the grate elements (4a, 4b, 4c) are returned backwards, starting from below. 2. The method according to claim 1 , characterized in that the grate (4) comprises two or more adjacent sequences of grate elements, which are moved together forward. 3. The method according to claim 2, characterized in that the adjacent sequences of grate elements are returned backwards simultaneously or one sequence at a time. 4. The method according to any of the claims 1 to 3, characterized in that the grate elements (4a, 4b, 4c) are returned backwards one by one, starting from below. 5. A furnace grate (4) comprising two or more movable grate elements (4a, 4b, 4c) which are arranged stepwise after and underneath each other, actuator means (5, 6, 7, 8) for moving the grate elements (4a, 4b, 4c), control means (9, 10) for controlling the actuator means and the grate means (4a, 4b, 4c) in a desired manner, characterized in that to feed solid fuel on the grate (4) forward, the actuator means (5, 6, 7, 8) and the control means (9, 10) are arranged first to move the grate means (4a, 4b, 4c) together forward and then to return them back- wards, starting from the lowermost one. 6. The furnace grate (4) according to claim 5, characterized in that the actuator means (5, 6, 7, 8) and the control means (9, 10) are arranged to move forward two or more adjacent sequences of grate elements together. 7. The furnace grate (4) according to claim 6, characterized in that the actuator means (5, 6, 7, 8) and the control means (9, 10) are arranged to return the adjacent sequences of grate elements backwards simultaneously or one sequence at a time. 8. A furnace for solid fuel, the furnace (1) comprising a grate (4) which is, for feeding the fuel forward, provided with two or more movable grate elements (4a, 4b, 4c) which are arranged stepwise after and underneath each other, actuator means (5, 6, 7, 8) for moving the grate elements (4a, 4b, 4c), control means (9, 10) for controlling the actuator means and the grate elements (4a, 4b, 4c) in a desired manner, feeding means (2, 3) for feeding fuel to the initial end of the grate (4), and blower means (11) for feeding combustion air through the grate (4) to the combustible fuel, characterized in that for feeding the solid fuel on the grate (4) forward, the actuator means (5, 6, 7, 8) and the control means (9, 10) are arranged first to move the grate elements (4a, 4b, 4c) together forward and then to return them backwards, starting from the lowermost one. 9. The furnace (1) according to claim 8, characterized in that the actuator means (5, 6, 7, 8) and the control means (9, 10) are arranged to move forward two or more adjacent sequences of grate elements together. 10. The furnace (1) according to claim 9, characterized in that the actuator means (5, 6, 7, 8) and the control means (9, 10) are arranged to return the adjacent sequences of grate elements backwards simulta- neously or one sequence at a time.

Inventors: PAEMPPI TOPI, PAEMPPI, TOPI
Applicants: PAEMPPI TOPI, PAEMPPI, TOPI
Application year: 2001
Main classification: F23H7
IPC codes: F23H7