Patent Publication Number: US-2010122643-A1

Title: Modular grate block for a refuse incinerator

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to grate blocks, which are a key feature of modern waste to energy plants that incinerate refuse and capture the energy released as steam for generating electricity. As a result of changes in the composition of refuse or garbage, and particularly due to the increase in caloric value of such material, the combustion grate, which is made up of a plurality of individual grate blocks, is exposed to high thermal stresses, particularly certain individual portions thereof such as the front face of the individual grates. Furthermore, the operator of municipal waste mass burning applications typically has no control over the composition of the trash being fed into the system. At any given moment, one section of the grate can have a pile of wet yard waste while another section can have bags of high caloric or energy content plastic containers. 
     Due to the dual function of the combustion grate as a combustion support with ventilating means and also as a transfer or conveyance means for the material to be burned, the grate structure often includes such features as alternating fixed and movable grate sections and is a relatively complex multi-part structure. By having a uniform distribution of air beneath the grate, the basic design and operation ensures adequate oxygen for good combustion and cooling. The grate area and length is selected for sufficient residence time to allow for complete burnout, generally less than 2 percent unburned carbon content remains in the ash residue. 
     There are numerous factors in the combustion process that are monitored and/or attempted to be controlled. One such factor or boundary condition that is attempted to be controlled is the grate temperature. The specific control intervention involves establishing combustion temperature controls such that the average temperature of the grate layer does not exceed 300° C. with a combustion temperature of, for example, 1000° C. Local overheating of the grate layer due to heat accumulation leads to increased corrosion and an increased scale formation rate. This results in excess wear of parts of the grate within a relatively short time and extensive annual maintenance. In these annual maintenance periods, large segments of grate parts are replaced. 
     The prior art has recognized one preventative measure for preventing high corrosion or scaling rates and the resulting increased mechanical wear which leads to the premature destruction of larger segments of grate block is provided by cooling off the grate blocks. There are several techniques for cooling including passing a coolant such as water through a chamber in the grate blocks and forcing air through the grate blocks. Generally, when cooling air is used, the cooling air is additionally used as the primary combustion air. Thus, the control of the primary combustion air is also a temperature control measure. For forced cooling purposes, the under grate blast generally flows against the grate layer first and air passage openings in the layer, which allows the cooling medium to pass into the refuse bed to be burned where it then participates in the combustion process as the primary combustion air. Clogging of the air openings, however, leads to reduced flow and increased back pressure in the cooling air path and, consequently, to accumulation of heat at the particular point of the grate layer. This leads to thermal overstressing of the grate part, increased wear, higher scaling rates and, within a short time, the failure of portions of the grate. 
     Our invention solves the above-stated problems by providing an improved modular grate block that has at least one wear plate attached to the front face of the grate block. This wear plate is designed to be removed and replaced with a new wear plate and thus avoiding the cost and waste associated with replacing the entire grate block. 
     SUMMARY OF THE INVENTION 
     Our invention eliminates the wasteful and expensive practice of discarding individual grate blocks that are worn from the high temperatures and corrosive environments found in refuse incinerators. More specifically, our invention is directed to providing individual modular grate blocks that have at least one wear plate, which is preferably attached to the front face of the grate block. The grate system of our invention preferably has a plurality of rows of fixed grate blocks and a plurality of rows of movable grate blocks alternating back and forth with each individual block having a removable wear plate as described in more detail below. A reciprocal mechanism is connected to each of the rows of moveable grate blocks for moving the rows relative to the rows of the fixed grate blocks. Each of the modular grate blocks has a top section, a front face, and a pair of side walls. Each side wall extends from the top section and the front wall. Each of the side walls of the grate blocks engage the side wall of the adjacent grate block. Although it is preferred that the at least one wear plate is attached to the front face, it is within the scope of our invention to have a wear plate attached to the top section. Each grate block has a paw portion located at the lower surface of the side section wall and front face. In a preferred embodiment, the wear block extends below the paw and engages a top section of a grate block directly in front and underneath. 
     The wear plate our invention is preferably made of a material that is different than the material used to fabricate the grate block body. In particular, it is preferred that the wear plate comprise a harder material and more corrosion resistant than the block body. Although harder or hardened materials are typically heavier and have higher costs associated therewith, these negatives are minimized because only the wear plate is made of such hardened materials. Indeed, it would be cost prohibiting to fabricate the unitary prior art blocks from hardened materials. With regard to the wear plates of our invention the preferred materials of construction that resist wear and corrosion include chrome-nickel alloys, stainless steels, ceramics, titanium and like materials. 
     Another feature of the modular grate blocks of our invention is the removability of the wear plates, especially when worn wear plates must be replaced with new wear plates. Although this removability feature can be accomplished by any known connection method, it is preferred to use a press fit connection between the backside of the wear plate and the front surface of the face wall of the grate block. One type of press fit connection that is particularly preferred is where a male protrusion or nub on the wear plate engages a corresponding slot in the front wall of the block body. Preferably, the dimensions of the nub and slot are chosen such that nub is held in the slot by friction, thus preventing the wear plate from moving in either a vertical or horizontal manner. A most preferred configuration is where the nub slides into a cup shaped slot that is tapered to provide the friction press fit. Alternatively, the nub and slot could form a “dovetail” type joint or connection. 
     The wear plate can be fabricated to match exactly the dimensions of the front wall of the grate block or it can be smaller or larger than the front surface of the front wall. Preferably, the wear plate should match the side walls and the top wall, but extend beyond the paw or lower edge of the front face. In this manner the wear plate becomes the bearing surface for contacting the top surface of the grate block positioned in front and underneath. This will prevent the paw of the grate block from wearing because the bottom of the wear plate makes the contact with the top wall of the grate block disposed beneath. 
     Our invention also encompasses methods for retrofitting an existing incinerator grate system where prior art grate blocks, which do not have wear plates, are inspected, identified as being worn, and then removed and replaced with the grate blocks of our invention that have at least one wear plate. Alternatively, the entire grate system can be changed out with a system having the modular grate blocks of our invention. In addition, our invention is directed to a method where an incinerator grate block system is inspected to identify blocks having worn wear plates and then replacing those worn wear plates with new wear plates without having to remove the individual grate blocks. Basically, the method involves locating worn wear plates, popping off the worn wear plate and snapping on a new wear plate, while leaving the grate block body attached to the system. This method of repair is cost effective in that labor is greatly reduced as well as the cost of the wear plate compared to the cost of replacing the entire grate block. Moreover, the incinerator downtime is greatly reduced as is the frequency of the planned maintenance because the wear plate can be fabricated with a longer lasting material of construction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a schematic of a combustion furnace; 
         FIG. 2  is a perspective view of a portion of the grate blocks with a portion of the grate blocks removed; 
         FIG. 3  is a partial side elevation, in partial section, illustrating grate blocks in accordance with the invention assembled in a grate layer; 
         FIG. 4  is a partial perspective side and top view of the front portion of a grate block of our invention having attached a wear plate; 
         FIG. 5  a partial perspective side and bottom view of the front portion of a grate block of our invention having attached a wear plate; 
         FIG. 6  is a partial perspective side and bottom view of the front portion of a grate block of our invention showing the wear plate removed from the front wall and showing the nub and slot connection; 
         FIG. 7  is a partial perspective side and top view of the front portion of an alternative grate block of our invention having attached a wear plate; 
         FIG. 8  a partial perspective side and bottom view of the front portion of an alternative grate block of our invention having attached a wear plate; 
         FIG. 9  is a partial perspective side and bottom view of the front portion of an alternative grate block of our invention showing the wear plate removed from the front wall and showing an alternative nub and slot connection; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings in detail, there is illustrated a a grate block in accordance with the present invention designated generally as  40 . In a preferred embodiment, the grate block according to the invention is a modular block with at least one removable wear plate. The overall design of the complete grate block is to direct air flow to allow for generally uniform burning of trash or refuse without thermal stress caused by intense combustion and cooling. 
     Referring to  FIG. 1 , one possible design of a combustion furnace  20  has trash, also referred to as refuse or fuel, fed via a refuse feed chute  22 . The trash is typically not homogeneous and can include wet yard waste, non-combustible material, and high energy content or caloric material. The trash drops upon a feed table  24 , on which a pusher ram  26  is moved back and forth by a drive  28 . The feed table  24  is adjoined at the same height by the start of a grate  32  having a plurality of grate blocks  40  which consists of fixed rows  44  arranged stepwise and movable rows  46  arranged in-between the fixed rows  44 . The movable rows  46  are shown in  FIG. 1  in a center position, in which the movable rows  46  are positioned over the fixed rows  44  arranged below them in between a retracted position and an extended position. 
     Underlying the grates  32  are a plurality of hoppers  34 . Each of the hoppers  34  is capable of gathering any trash or ash that falls through the grate  32 . It is not typical for large amounts of trash or ash to fall through the grate  32  unless one of the grate blocks  40  fails. In addition, each of the hoppers  34  is connected to an air source, such as a primary air fan  36  as seen in  FIG. 1 . The air from the air source passes through openings in the grate block  40 , as described below, to a combustion chamber  38 . FIG.  1  shows two hoppers  34 , but the combustion furnace  20  typically has as many as four hoppers  34  in a trash conveying direction. Depending on the width of the combustion furnace, the furnace can typically have  1  to  6  hoppers in the direction perpendicular to the conveying direction. By means of a back and forth movement of the movable rows  46 , the trash, i.e., the fuel, is moved slopingly downwards on the grate  42  until it drops, completely burned, into an ash receiver  52 , from which the ash is transported away, for example, by means of a conveyor  54 . 
     The movement of the movable rows  46  is accomplished by hydraulics or a motor driven actuator. The movable rows  46  over each hopper  34  are controlled as a unit and the units can each be controlled individually. The combustion furnace  20  can have the rate of movement of each section or unit of movable rows  46  be at a different rate. The combustion furnace  20  has the combustion chamber  38  arranged above the grate  42 . The combustion chamber  38 , on the left side of  FIG. 1 , towards the tray  24  and the pusher tray ram  26  is defined by a wall  58  which starts slightly above the start of the grate. The combustion gases reach an exit  60  of the combustion furnace  20  through a passage  62 . Heat exchangers, such as the boiler tubes  64  as shown in  FIG. 1 , filters, and the like can adjoin the exit  18  of the boiler. The grate according to the present invention is designed such that the combustion takes place with primary air passing through the grate blocks  40  from the hoppers  34 .. Secondary air is admitted to the combustion chamber  38  above the grate  32  and the trash through the upper portion of the chamber such as represented by an arrow  66 . 
     The combustion furnace  20  with the grate block  40  arrangement as described above operates with combustion air which passes through openings in the grate blocks  40 . The combustion chamber  38  is under reduced pressure which causes combustion air from the hopper, which is under positive pressure by the primary air fan  36 , to be forced through the openings  120  in the grate blocks  40  as seen in  FIGS. 4 &amp; 7 . Sharply defined combustion conditions can be set by means of proper air distribution. For example, the combustion chamber  38  can be operated at −0.1 inches of pressure, which maintains a negative pressure that prevents smoke and exhaust from entering the building through penetration and openings in the combustion furnace and the hopper  22 . The combustion furnace  20 , can preferably be designed with an after-burning chamber in which very high temperatures decompose any unburned pollutants thermally to produce harmless gases and are generated as a result of radiant heat and good insulation. The combustion furnace  20  can also operate without an additional flame, due to the controlled trash feed and transport on the grate; the trash rate can be reliably controlled at any time, so that defined temperatures and combustion conditions can be achieved even with trash having widely varying properties. However, it is typical to have starter burners in order to have the combustion chamber  38  reach sufficient temperature prior to the introduction of trash for environmental reasons. 
     The basic structure of the trash combustion grate  32  of this invention with its essential elements is shown most clearly in  FIG. 2 .  FIG. 2  shows a portion of the grate  32  in a perspective view, with some of the grate blocks  40  removed. The grate  32  is sloped downwards in the direction of the conveyance, as represented by an arrow  68 . The grate  32  can be formed of several modules  80  in the direction perpendicular to the conveying direction, wherein each module overlies a hopper. Each module  80  has a pair of side wall blocks  70  and  72  that are stably connected to each other by a plurality of tensioning rods  74 . These tensioning rods  74  extend perpendicular and extend across the inside width between the pair of side wall blocks  70  and  72 . The tensioning rods  74  are threaded at each end and extend through openings in the pair of side wall blocks  70  and  72 . The tensioning rods  74  are secured to the pair of side wall blocks  70  and  72  by a plurality of nuts on the threaded ends. The tensioning rods  74  also serve as supporting rods for the group of stationary grate blocks  40  that receive the rod  74  through a support rib. A shorter tensioning rod extends through the grate blocks  40  of the movable row  46 . A movable row  46  of grate blocks  40 , moving in the direction opposite the conveyance, is located on the first fixed row  44 . The front under edge of grate blocks  40  of the movable row  46  rests on the grate blocks  40  of the first fixed row  44  below. The front under edge of the next highest fixed row  44  rests in turn on the movable grate blocks  40  and so on. While the grate  32  is shown having a slope, such that there is a change in vertical height from one end to the other of the grate, it is recognized that the slope can be horizontal (i.e., having no slope). 
     The grate blocks  40  for both the moveable rows  46  and the fixed rows  44  have a hook portion at the rear of the block that are each received by a respective block holding tube  92 . The block holding tube  92  for the fixed rows  44  are each supported by at least a pair of support ribs  93 . Each support rib  93  is carried by a support rail  94  as seen in  FIG. 2  that extends parallel with the conveyance direction. Likewise the block holding tube  92  for the movable rows  46  are each supported by support ribs  95  and a carriage rail  96 . The block holding tube  92 , the support ribs  93  and  95  and the rails  94  and  96  are shown in further detail in  FIG. 3 . 
     As indicated above with respect to  FIG. 1 , the area underneath the grate  32  has a plurality of hoppers  34 . These hoppers define several distinct zones as represented by the grate modules  80 . In addition to being able to vary the stroke rate of the movable rows  46 , the hoppers are distinct in that the air flow underneath the grate can be adjusted to each region defined by the hoppers  34 . Primary air is blown into the individual zones by means of the primary air fan  36  with adjustable dampers, and this air then reaches the combustion chamber through the openings in the grate block  40 . As further illustrated in  FIG. 3 , the combustion furnace  20  has the plurality of block holding tubes  92 . The block holding tubes  92  for the fixed rows  44  are each supported by the support ribs  93  carried by the support rail  94 . The block holding tubes  92  for the movable rows  46  are each supported by the support ribs  95  carried by the carriage rail  96 . The grate blocks  40  are mounted on bearing means  92  which are supported on supports  94  and  96 , and the blocks  40  being rotatable relative to the block holding tube  92 . 
     As indicated with respect to  FIG. 2 , the movable rows  46  can be adjusted in stroke rate by the movement of the carriage rail  96  by the actuator  92 . The tensioning rods  74  are provided to support the blocks  40  and are coupled together so that the blocks are movable in groups and are combined together perpendicular to the longitudinal direction or the direction of conveyance of the grate assembly  32 . 
     Referring to  FIG. 3 , the grate block  40  has an top wall  100 , a front wall  102 , and can have an angle corner wall  104 , which is interposed between the top wall  100  and the front wall  102 . In addition, the grate block has a projecting arm  106  that extends under the overlying grate block  40 . The arm has a hook  108  that receives the support rod  92 . The top wall  100  has a thickened portion  110  on which a paw  112  of the front wall  102  of the block above moves relative to the lower block. The grate block  40  also has a pair of side walls  114 . The projecting arm  106  has the hook  108  for receiving the support rod  92 . The top wall  100  has a thickened portion  110  upon which the paw  112  of the overlaying grate block  40  rests. The side wall  114  has an alignment pin hole  130  for accepting an alignment pin for securing adjacent grate blocks together. Front wall  102  has attached wear plate  200  each with a bottom edge  203 . 
       FIGS. 4-6  and  FIGS. 7-9  show two of the many possible configurations of wear plates  200  removably attached to front wall  102  of grate block body  40 . As mentioned, it is also within the scope of our invention to have wear plates attached to the top wall of the grate block. In both embodiments shown in the figures the wear plate extends down below the bottom edge of paw  112  to act as a bearing surface for contact on the top wall of another grate block in the system as shown in  FIGS. 2 &amp; 3 . Because bottom edge  203  of wear plate  200  is the only portion of the grate block in contact with the top wall of the other grate block, this prevents wear to paw  112 . This is clearly shown in  FIGS. 5 &amp; 8  where when viewed from the underneath side of grate block  40 , lower edge  203  of wear plate  200  extends beyond the bottom edge of paw  112 . 
     Top edge  201  of wear plate  200  is shown matching the angle of inclination defined by corner wall  104  of top wall  100 , however, other designs where the angle is not matched are possible. Wear plate  200  also has holes  120  to allow combustion air to flow from underneath grate block  40 . This flow of combustion air not only supplies the oxygen necessary for combustion, but also acts a heat transfer medium to cool the grate block and attached wear plate.  FIGS. 6 &amp; 9  illustrate two possible connection designs to removably secure backside  207  of wear plate  200  to front surface  206  of grate block  40 . As mentioned, any connection design can be used to secure the wear plate to the front wall of the block body, provided that it is not a permanent connection. It is important that the connection between the wear plate and the grate block is releasable so that during a shutdown of the incinerator a maintenance worker can replace a worn wear plate with a new wear plate without removing individual grate blocks from the system. 
     The means, materials, and steps for carrying out various disclosed functions may take a variety of alternative forms without departing from the invention. Thus, the expressions “means to . . . ” and “means for . . . ”, or any method step language as may be found in the specification above or the claims below, followed by a functional statement, are intended to define and cover whatever structural, physical, chemical or electrical element or structure, or whatever method step, which may now or in the future exist which carries out the recited function, whether or not precisely equivalent to the embodiment or embodiments disclosed in the specification above, i.e., other means or steps for carrying out the same function can be used; and it is intended that such expressions be given their broadest interpretation within the terms of the following claims. Likewise, the claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.