Patent Publication Number: US-8522697-B2

Title: Waste disposal plant with modular frame and guide assembly

Description:
This application is a National Stage Application of PCT/EP2010/007026, filed 19 Nov. 2010, which claims benefit of Serial No. TO2009A000919, filed 26 Nov. 2009 in Italy and which application(s) are incorporated herein by reference. To the extent appropriate, a claim of priority is made to each of the above disclosed applications. 
     BACKGROUND 
     The present invention relates to a combustion grate, having a modular frame, and being installed in the furnace of a waste disposal plant, in which the aforesaid waste is burnt and so disposed of as ashes. 
     Such disposal generally comprises also an energy recovery system through the production of overheated steam and the exploitation of steam in a turbine, which in turn is coupled to an electric generator. 
     Such plants generally comprise a combustion chamber inside which the waste laid on a combustion grate is burnt, through which an adequate quantity of air is inputted. The combustion grate is adapted to sustain and advance the waste during the combustion permitting at the same time the insufflation of combustion air under the waste bed. The grate forms the lower portion of the combustion chamber. The combustion chamber begins physically immediately above the grate. In some cases, the walls of the combustion chamber are completely or partially cooled, by means of evaporation tube bundles protected by the refractory material itself. The region at the interface between grate and combustion chamber is made by the refractory-carrying beam. Inside the combustion chamber, the flame produced by the combustion of waste is spread, reaching temperatures over 1400° C. The surface of the grate is hit only occasionally by the radiation of the flame, as it is normally protected by the waste bed in transit. 
     The surface of the grate is made by plates (typically called “fire bars”) which are normally made of molten steel having a high chromium content, in order to show high wear characteristics when hot. The advancement of waste is obtained through the relative movement of the fire bars which can have several characteristics. The actuation system is normally made of hydraulic pistons. The fire bars are provided with apertures or holes to allow the combustion air to flow from underneath the plane of the grate, through the waste. The combustion air has in fact the double function of providing the oxygen for the oxidation of the waste and of cooling the fire bar by maintaining it at an acceptable temperature in order to maintain the mechanical characteristics. The cooling is necessary, as the grates normally work covered by the forwarded fuel, but they can also be directly exposed to the combustion flames. 
     The steps making the grate can also be provided with an additional cooling system with water, particularly when they are used for the combustion of fuels with high calorific power. Such cooling is obtained by a liquid circulation which is forced to lap the surfaces which do not contact the fuel of each fire bar, through a liner or an equivalent apparatus for the accumulation of liquid. 
     The fire bars at the initial and terminal ends of each step are separated from the carrying structure of the grate by means of plates, generally of the same material of the fire bar, which are approached with pressure against the side of the side bar itself. Such plates have the function of laterally containing the fuel forwarded in the region immediately above the fire bars, and of separating it from the lateral portions of the grate, which do not tolerate the direct exposition to the burning material. The lateral plates eventually join the plane made by the fire bars with the vertical surfaces of the combustion chamber, which are disposed immediately above the grate. 
     The grate further comprises a plurality of handling groups, each formed by the cited fire bars organized in bundles, which relatively move one with respect to the other advancing the waste on the grate. In particular, the fire bars are divided into fixed and movable fire bars, which through slides cause a back-and-forth movement, by sliding one on another and determining the advancement of the waste in each handling group, and so in general on the grate. 
     The disposal of the solid urban waste, even if it is not characterized by a highest technological content, is an activity particularly sensible from a point of view of the reliability and the guarantees of the function. The complex integrated system of the waste disposal (the accumulation, the collection, the transport, the stockpiling and the disposal) requires that the technological components employed in the last stage of the supply chain permit a continuous operation over 24 hours and minimize the risks of stopping due to damages (minor or catastrophic damages as they can be). The operator of the disposal plant requires reliability, robustness and simplicity of the component “grate”, both in the operation and in the maintenance stage. 
     Patent application MI2004A001746 describes a plant of this kind having a movable combustion grate, in which the handling group is formed by a plurality of fire bars, which alternatively move one with respect to the other by advancing the waste on said grate, which realizes a substantially horizontal and at least partially continuous combustion plane. 
     The movable fire bars are bound to a movable frame, which is pushed by two pistons, one on each side. The fixed fire bars are in turn connected with a fixed frame, common to all fixed fire bars. 
     The relative movement between the fixed frame and the movable frame is of a simple alternate translation. 
     The grate is further provided with sliding elements adapted to determine the sliding between the two frames and elements limiting the movement between them which determine its stroke. 
     In the forward stroke the movable fire bar pushes the waste on the back of the fixed fire bar until causing its fall onto the subsequent movable fire bar, and at the same time it drags the waste on its own back. In the return stroke, the waste on the back of the movable fire bar finds an obstacle on the front of the fixed fire bar and, instead of moving back, it is pushed onto the back of the fixed fire bars downwards and then it is pushed forwards in the subsequent displacement, so determining the advancement of the waste in each handling group on the grate in general. 
     The sliding elements are substantially made by bearings or rolls upon which a pad slides substantially integrally with the moveable frame with an inclination dependent on the direction of movement which the moveable frame must communicate to the fire bars. 
     The movement limiting elements comprise a track integral with the carrying or fixed frame, whereas to the movable frame of the grate two wheels are in turn connected, bound to a fixed axis. The wheels are mounted with a transversal clearance with respect to the track. When the movable frame moves in different directions with respect to that required for the correct feeding of the fire bar, the clearance between the wheels and the frame is reduced until becoming zero by stopping the movement of the movable frame. 
     Such typical sliding elements are unavoidably subjected to dust accumulations between the surfaces of the rolls, which can impair the correct lubrication of the roll bearings and cause a rapid wear of the bearing themselves or finally their seizure. Furthermore, such movement limiting elements are bulky and have a remarkable constructive complexity. Their encumbrance requires that they be positioned proximal to the symmetry axis of the module of the grate, so rendering them accessible with difficulty for the lubrication and the maintenance. 
     SUMMARY 
     The Applicant has realized a waste disposal plant in which in each handling group the sliding elements and the movement limiting elements are mutually integrated in a single ergonomic assembly. 
     Consequently the present invention solves the above mentioned drawbacks by realizing a waste disposal plant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics and advantages of the plant according to the present invention will be better clarified and evident from the following, by way of example and non limitative, description of an embodiment with reference to the annexed figures, in which: 
         FIG. 1  is a schematic presentation of a waste disposal plant according to the known art, which provides for a three-level combustion grate; 
         FIG. 2  is a schematic presentation of a handling group of the grate of the plant of  FIG. 1 ; 
         FIG. 3  is a perspective view of the upper portion of the grate according to the present invention, usable in the plant of  FIG. 1 ; 
         FIG. 4  is a perspective view of the lower portion of the grate according to the present invention usable in the plant of  FIG. 1 ; 
         FIG. 5  shows a handling assembly comprising sliding elements and movement limiting elements. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to cited figures a typical waste disposal plant comprises a combustion chamber  2  inside which the waste disposed on a combustion grate  3  are burnt, through which an adequate quantity of combustion air is inputted. 
     The grate forms the lower region of the combustion chamber, above which a beam  4  is present, which has also the function of supporting lateral refractory walls  5 . Preferably, the walls of the combustion chamber are completely or partially cooled, through evaporating tube bundles  6  which are protected by the refractory material itself. 
     The combustion grate comprises at least a handling group made by a plurality of fire bars  7 , which move alternately one with respect to the other by advancing the waste on the grate. 
     In the example of embodiment shown in  FIG. 1  there are three handling groups disposed offset one with respect to the other, in order to obtain a horizontal discontinuous placement (stepped or with jumps). Alternatively, still within the present invention, it is possible to obtain a slightly inclined placement (up to 15°), without substantial modifications. 
     Each handling group is actuated by handling means  8  made for example by at least a hydraulic piston. Such handling means permit the alternate movement of fire bars  7  which are divided in movable fire bars  7   a  and fixed fire bars  7   b , alternately disposed one with respect to the other, on transversal rows resting one on the other according to a longitudinal disposition with alternate steps, respectively with one fixed and one movable. 
     The movable fire bars are connected to movable beams  9   a  and are bound to a movable frame  10   a , which is pushed by two pistons, one on each side. The fixed fire bars are in turn connected through fixed beams  9   b  with a fixed frame  10   b  common to all fixed fire bars. 
     The relative movement between the fixed frame and the movable frame is of a simple alternate translation. The relative movement of the fire bars is of an alternate translation preferably according to a direction inclined of 20° on the horizontal. 
     The grate is also provided with sliding elements  11  adapted to determine the sliding between the two frames and elements  12  limiting the movement between them, which determine their movement according to predefined trends. 
     In the forward stroke the movable fire bar pushes the waste on the back of fixed fire bar  7   b  until causing its fall from the subsequent movable fire bar, and at the same time it drags the waste on its own back. In the return stroke the waste on the back of the movable fire bar finds an obstacle in the front of the fixed fire bar, and instead of going back, it is pushed downwards onto the back of the fixed fire bar and therefore it is pushed forward in the subsequent movement, by determining the advancement of the waste in each handling group and then on the grate in general. 
     In  FIGS. 3 and 4  a handling group of the grate is illustrated according to the present invention, comprising a carrying frame including two lateral beams  36  and two transversal cross-beams  37 . Lateral beams  36  are preferably realized with a closed square or rectangular cross section and are connected to cross-beams  37  by means of a knot  38  structurally equivalent to a joint, positioned on the upper portion of lateral beams  36  themselves. Knot  38  also comprises a reference (plug or equivalent device—non represented) which permits to precisely position cross-beams  37  with respect to lateral beams  36  during the assembly of the module of the grate. Inside lateral beams  36  axes of wheel assemblies  39  are mounted, which permit the relative movement between the carrying frame and movable frame  40 . To the lower portion of beams  36  the supports of bearings  41  of actuating shafts  42  are on the contrary fixed. 
     To the upper portion of beams  36  an upper sheet  43  is fixed. Such sheet separates the lateral plates from the outside and forms the connecting element between the carrying frame and the upper portion of the furnace (non represented), for example comprising thermal insulating panels and respective fixing sheeting. On upper sheet  43  apertures  44  are formed which permit the access to the sealing elements of the lateral plates. Apertures  44  must be air-tightly closed through flanges  45 , doors or other closures of equivalent function. To the lower portion of beams  36  a lower non-structural sheet  47  is instead fixed. Sheet  47  together with beams  36  delimits from the two sides of the module of the grate the pressurized volume which provides supply air to fire bars  7 . To the front and back ends of beams  36  two flanges  48  and  49  are fixed, whose outer surfaces are made to match in order to couple two successive modules, for example through bolts with gasket or other equivalent air-tight connection. Flanges  48  and  49  are provided with upper extensions  50  and lower extensions  51 , which permit to directly couple two successive modules also at upper sheets  43  and of lower sheets  47 . Front flanges  48  are finally provided with a lateral extension  52  to which a connection element  53  is fixed, for the hydraulic cylinder moving actuating shaft  42 . The handling of the actuating shaft takes place through a lever  54  fitted flush on shaft  42  itself. 
     Beams  36  as well as having a structural function incorporate the seats of all the inner and outer mechanical parts of the module of the grate, lateral sheets  43  and  47 , which make the sides of the module itself and connecting flanges between successive modules. 
     Due to the fact that the support cross-beams of the fire bars are structural elements in the present invention, the assembly of the carrying frame of each module requires exclusively the coupling of beams  36  specular for the two sides of the module, to cross-beams  37  by means of knot  38 , provided with reference elements (non represented) for the correct coupling. On beams  36  nearly all the working operations on the machine tools are further concentrated, which are necessary in order to provide for the assembly of the module of the grate, as beams  36  are integral with the seats of all the inner and outer mechanical parts, flanges  48  and  49  coupling the successive modules and the coupling and reference surfaces to cross-beams  37  (non represented). As beams  36  have a reduced length in order to permit the transport of the modules without resorting to exceptional transport means, they can be worked with reduced times and costs with respect to the completely assembled modules of the today produced grates. The only working operations at the machine tools which are not positioned on beams  36  are those of the coupling surfaces of knot  38  which are integral with cross-beams  37  and which must be referred to the corresponding surfaces on beams  36 . 
     In  FIG. 5  a guide assembly of the movement of the movable frame is shown, comprising a sliding element and a movement limiting element integrally mutually coupled. Such assembly  39  comprises essentially an axis  55 , which is associated to the fixed or carrying frame, formed by an outer shaft  56 , which engages in a seat  57  realized inside lateral beams  36  of the carrying frame, and an inner shaft  58  instead cantilevered under the plane of the grate. On such inner shaft one or more wheels  71  with a substantially horizontal axis are fitted flush, eventually separated by a spacer  95 , on which the movable frame  40  slides (non represented in this figure). On the outer shaft a bushing  72  is instead fitted flush with a sliding coupling along axis  55 , to which one or more wheels  73  with a substantially vertical axis are fixed, which encounter a movable edge of the frame, in order it to be forced to move on a seat inferiorly delimited by wheels  71  and laterally by wheels  73 . In a transversal direction, bushing  72  is separated by the side of seat  57  by an elastic member  97 , for example realized by a cup spring or an equivalent device. The slipping of the axis from seat  57  is prevented by two ring nuts  74  which clamp between them sequentially wheels  71 , spacer  95 , bushing  72 , elastic element  97  and seat  57 . 
     The deformation of the elastic element permits to regulate the transversal position of the axis in order to bring wheel  73  in contact with movable frame  40 . Wheels  73  are aligned with the direction of movement of the movable frame by utilizing a reference mark (non represented) integral with the bushing to which the wheels themselves are anchored. The reference mark is blocked in a seat made in the carrying frame (non represented). When movable frame  40  moves in different directions with respect to that provided for the correct advancement of fire bars  7 , wheels  73  come in contact with movable frame  40  preventing its further deviation from the predetermined trajectory. Elastic element  97  permits to bushing  72  a limited sliding on outer shaft  56  following the contact with movable frame  40 , and with its deformation, continuously increases starting from zero the force which maintains the movable frame  40  within the correct trajectory. At the inside of axis  55  lubrication ducts  98  are advantageously provided, for sending greases or oils to sliding wheels  71  and to movement limiting wheels  73 . 
     The guide assembly according to the present invention has a remarkably simpler construction with respect to the known ones. First, it integrates in a single assembly made from few pieces both sliding elements  11  and movement limiting elements  12  of the movable frame. The entire assembly is further bound to the carrying frame through a single coupling shaft-hole between axis  55  and seat  57 . 
     Assembly  39  is installed at the lateral beams  36 , and so it is more protected from the dust present in the pressurized volume underneath the plane of fire bars  7 , and it is immediately accessible for inspection. Wheels  71  are further cantilevered inside the pressurized volume underneath the plane of fire bars  7  and do not have any support structure, in which dust can accumulate. The result is a greater predictable feasibility and duration of wheels  71  themselves. The assembly in the indicated position allows the positioning of the inlet hole of the lubricant on the surface of the axis facing the outside of the module. In this way it is not necessary to install any lubrication tubing inside the machine. 
     Finally, if necessary the substitution of an entire guide assembly can be done with extreme rapidity. It is in fact sufficient to remove ring nut  74  mounted at the outside of the machine and slip the entire group from seat  57 . The operation frees automatically the remaining parts, i.e. bushing  72  and the elastic member  97 . With the same simplicity it is possible, by repeating in reverse the above indicated operations, to install a new assembly  39 . In this way the machine shutdown times for the ordinary and extraordinary maintenance of the wheels assemblies are minimized.