WO2024008930A1

WO2024008930A1 - Plate-formed grate element for a movable grate of a furnace

Info

Publication number: WO2024008930A1
Application number: PCT/EP2023/068868
Authority: WO
Inventors: Hans BØGH ANDERSEN; Morten Ryge BØGILD; Thomas Schaldemose NORMAN
Original assignee: Babcock & Wilcox Vølund A/S
Priority date: 2022-07-07
Filing date: 2023-07-07
Publication date: 2024-01-11
Also published as: EP4303492A1

Abstract

The plate-formed grate element (1) has a top wall (12), a front end (14) and a back end (15). The front end has a lower inwardly curved wall portion (16) adapted to maintain a predetermined clearance with a back tip edge of a corresponding grate element. The grate element has an outwardly curved front wall (22) having a nominal wall thickness varying by less than ±35 per cent and extending from the top wall of the grate element to the lower inwardly curved wall portion of the front end, and a front tip edge (23) of the front end is formed by the outwardly curved front wall at its connection with the lower inwardly curved wall portion. At least one downwardly projecting cooling rib (19) extends into an area (13) formed between the outwardly curved front wall and the lower inwardly curved wall portion and is connected to these.

Description

PLATE-FORMED GRATE ELEMENT FOR A MOVABLE GRATE OF A FURNACE

The present invention relates to a plate-formed grate element for a movable grate of a furnace, the movable grate including a number of pivotal grate shafts carrying plate- formed grate elements and thereby defining an inclined grate surface, the movable grate including a drive mechanism being arranged for pivoting back and forth neighbouring grate shafts in opposite rotational directions so as to impart a wave-like movement to material on the grate surface in order to transport such material downwards, and the movable grate including a synchronising mechanism being arranged to maintain a predetermined clearance between edge portions of plate-formed grate elements of neighbouring grate shafts during the pivoting movement of the grate shafts, the plate-formed grate element having a top wall, a front end and a back end, a longitudinal direction of the plate-formed grate element extending between the front end and the back end, the front end of the plate-formed grate element having a lower inwardly curved wall portion being adapted to maintain said predetermined clearance with a back tip edge of the back end of a corresponding plate-formed grate element during part of said pivoting movement of the grate shafts when said plate-formed grate elements are arranged on neighbouring grate shafts, and the plate-formed grate element being adapted to be cooled by means of combustion air coming from a lower chamber formed below the movable grate in the furnace in that the top wall of the plate-formed grate element is provided with at least one cooling rib projecting downwardly from the top wall and extending in the longitudinal direction of the plate-formed grate element and being freely exposed to combustion air coming from below.

WO 2018/007854 Al discloses movable grates for a combustion furnace. The movable grate includes a number of pivotal grate shafts carrying plate-formed grate elements, neighbouring grate shafts being arranged for pivoting back and forth in opposite rotational directions so as to maintain a predetermined clearance between edge portions of the plate-formed grate elements of the neighbouring grate shafts. The plate-formed grate elements have a front end with a relatively pointed front tip edge and a back end with a relatively pointed back tip edge. Each plate-formed grate element has a top wall and a straight front wall forming an oblique angle with the top wall and extending from the top wall to a position of the pointed front tip edge which is below or at level with a general lower surface of the plate-formed grate element. At the pointed front tip edge, the straight front wall is connected with the general lower surface of the plate-formed grate element by means of a lower inwardly curved wall portion adapted to form said predetermined clearance with a back end of another plate-formed grate element. During operation, the front end of a plurality of plate-formed grate elements overlaps a corresponding back end of a plate-formed grate element of a neighbouring grate shaft. The predetermined clearance between the individual plate-formed grate elements, on which material intended for combustion is placed, provides for supplying primary air for the combustion. To make the supply of primary air as uniform as possible, it is important that the size of said predetermined clearance does not change when the plate-formed grate elements pivot in relation to each other or due to wear. Wear is caused by abrasive wear by the material which is burnt, this wear being further increased if the surface temperature of a plate-formed grate element is approaching the point of softening of the grate material because of the combustion heat. Therefore, at least some of the plate-formed grate elements include an internal cooling fluid chamber adapted for water cooling in order to reduce wear. On the other hand, some of the plate-formed grate elements being less subjected to wear are adapted to be cooled by means of combustion air rising freely from a lower chamber formed below the movable grate.

However, when burning some kinds of particularly aggressive fuel and/or high heat value fuel, such as fuel including predominantly shredded waste wood, the prior art plate-formed grate elements may suffer from excessive wear of the pointed front tip edge of the front end of the grate elements. In some cases, significant compressive stress may cause plastic deformation of the pointed front tip edge during operation. Subsequently, during cool down, the pointed front tip edge may experience high tensile stress due to the plastic deformation which may result in micro cracks in the front tip edge. Corrosion caused by high concentrations of heavy metals in the fuel may further aggravate the wear of the front tip edge.

In a combustion furnace of for instance a large waste incineration plant, the service life of the components of the movable grate is of utmost importance. Regular maintenance intervals of a combustion furnace may for instance be a year or so, and unexpected breakdown in between scheduled maintenance operations may seriously influence the economy of the plant.

US 4,275,706 relates to air-cooled grate bars, in particular for mechanically conveying mechanical grates such as pivot step grates. A cap of U-shape which is provided over the respective grate bar forms an air channel on top of the grate bar. Cooling air is injected into the channel through an inlet air tube extending downwards from a lower open side of the grate bar. The air exits from the channel at either end of the grate bar, whereby the air is guided through holes to the lower side of the grate bar. From there, the air flows up through gaps between neighbouring grate bars to the combustion chamber. As it is understood, the air channel on top of the grate bar therefore forms part of an open air cooling path and is not adapted for or suitable for cooling by means of combustion air rising freely from a lower chamber formed below the grate bars. At a back end of the grate bar, the grate bar is articulately mounted, and at this end, considered in side view, it has a lower curved section forming a bearing. At a front end of the grate bar, the grate bar is adapted to overlap a corresponding front end of another grate bar. DE 3343024 Al relates to similar air-cooled grate bars.

US 5,033,396 discloses a grill arrangement, particularly for stepped pivoting grills, comprising feeding means, possibly drying and firing grills and at least one vertical grill with associated horizontal grill as well as following burnout grill. The air supply of the pivoting fuel elements is provided in particular for cooling the grill bars. To each grill bar in the centre cooling air is supplied from below, for example from an air distributing box which can be provided within or outside an air funnel. Generally, the air is guided via a cap welded for example onto the grill bar upper side. The centrally supplied air leaves at the end face on the one hand in the region of the pivot mounting for the grill bars and on the other hand in the region of the opposite grill bar end. As it is understood, because the air is guided via a cap welded onto the grill bar, an air channel of the grate bar therefore forms part of an open air cooling path and is not adapted for or suitable for cooling by means of combustion air rising freely from a lower chamber formed below the grate bars. The object of the present invention is to provide a plate-formed grate element being less prone to wear.

In view of this object, the plate-formed grate element has an outwardly curved front wall extending from the top wall of the plate-formed grate element to the lower inwardly curved wall portion of the front end, a front tip edge of the front end is formed by the outwardly curved front wall at its connection with the lower inwardly curved wall portion, the outwardly curved front wall has a nominal wall thickness varying by less than ±35 per cent, and the at least one cooling rib extends further into an area formed between the outwardly curved front wall and the lower inwardly curved wall portion of the front end and is connected to both the outwardly curved front wall and the lower inwardly curved wall portion.

In this way, it may be achieved that combustion air flows into a cooling area or chamber formed in the front end of the plate-formed grate element whereby the cooling combustion air may contact the inside of the outwardly curved front wall and the inside of the lower inwardly curved wall portion of the front end and thereby efficiently cool the front end and in particular the area at the front tip edge. Furthermore, combustion air contacting the at least one cooling rib extending into said cooling chamber and being connected to the outwardly curved front wall and the lower inwardly curved wall portion of the front end may further cool the area of the front tip edge. In particular, the cooling effect of the combustion air is evened out over the outwardly curved front wall, as compared to the prior art grate elements, thereby cooling the front tip edge better and more efficiently. A better cooling of the front tip edge may result in less wear of the front tip and therefore a longer service life of the plate-formed grate elements. Furthermore, a smooth curvature of the entire outwardly curved front wall may result in a stronger front wall without weak areas in which tension may build up.

In an embodiment, the nominal wall thickness of the outwardly curved front wall varies by less than ±30 per cent, preferably less than ±25 per cent, and most preferred less than ±20 per cent. By reducing the variation of the nominal wall thickness of the outwardly curved front wall even further, it may be possible to further even out the cooling effect of the combustion air over the outwardly curved front wall and thereby to a higher degree obtain even cooling of the front wall. In particular, it may be possible to avoid insufficient cooling of the front tip edge.

Preferably, the outwardly curved front wall has an at least substantially constant wall thickness.

In a structurally particularly advantageous embodiment, the part of the outwardly curved front wall extending from the top wall of the plate-formed grate element to the front tip edge has an outer contour with a first nominal radius of curvature varying by less than ±40 per cent, and preferably less than ±20 per cent, the front tip edge has an outer contour with a second nominal radius of curvature varying by less than ±20 per cent, and the first nominal radius of curvature is more than 2 times larger, preferably more than 3 times larger, more preferred more than 4 times larger and most preferred more than 5 times larger than the second nominal radius of curvature. Thereby, it may in particular be possible to concentrate the cooling effect of the combustion air to the area at the front tip edge, because the combustion air may be directed much closer to the front tip edge of the plate-formed grate element than according to the prior art. An even better cooling of the front tip edge may result in less wear of the front tip and therefore a longer service life of the plate-formed grate elements.

In a structurally particularly advantageous embodiment, the at least one cooling rib includes a first cooling rib forming a first side wall of the plate-formed grate element and a second cooling rib forming a second side wall of the plate-formed grate element, the second side wall being opposed to the first side wall. Thereby, in particular, a cooling chamber may formed in the front end of the plate-formed grate element whereby the cooling combustion air may contact the inside of the outwardly curved front wall and the inside of the lower inwardly curved wall portion of the front end and thereby efficiently cool the front end and in particular the area at the front tip edge, thereby resulting in less wear at the front tip edge. In an embodiment, the at least one cooling rib further includes a number of intermediate cooling ribs arranged between the first cooling rib and the second cooling rib. Thereby, an even better cooling effect and therefore less wear may be experienced at the area of the front tip edge.

In an embodiment, the cooling ribs are sealingly connected to both the outwardly curved front wall and the lower inwardly curved wall portion so that separate cooling chambers are formed between neighbouring cooling ribs at the front end of the plate- formed grate element, and each one of said separate cooling chambers is accessible for combustion air through an opening formed between respective neighbouring cooling ribs and behind the front end of the plate-formed grate element. Thereby, an even better cooling effect and therefore less wear may be experienced at the area of the front tip edge.

In an embodiment, the top wall of the plate-formed grate element is at least substantially flat when seen in longitudinal cross-section, and a part of the top wall extends over at least a part of the lower inwardly curved wall portion of the plate-formed grate element. Thereby, a volume of said cooling area or chamber formed in the front end of the plate-formed grate element may be maximised, thereby facilitating access of combustion air and thereby maximising the possible cooling effect of the combustion air in the front end and in particular in the area at the front tip edge. Therefore, less wear may be experienced at the area of the front tip edge.

In an embodiment, the top wall extends over at least 30 per cent, preferably at least 40 per cent, and most preferred at least 50 per cent, of the extent of the lower inwardly curved wall portion in the longitudinal direction of the plate-formed grate element. Thereby, access of combustion air may be even better facilitated, and the possible cooling effect of the combustion air in the front end and in particular in the area at the front tip edge may be even further increased. In an embodiment, seen in a longitudinal cross-section of the plate-formed grate element, a connection point between the top wall and the outwardly curved front wall is located in front of an apex of the lower inwardly curved wall portion by a distance in the longitudinal direction of the plate-formed grate element. Thereby, access of combustion air may be even better facilitated.

In a structurally particularly advantageous embodiment, seen in a longitudinal cross-section of the plate-formed grate element, the outwardly curved front wall is at least substantially symmetric about a symmetry line forming an angle of at least substantially 45 degrees with the longitudinal direction of the plate-formed grate element. Thereby, a smooth at least substantially symmetric curvature of the entire outwardly curved front wall may result in a stronger front wall without weak areas in which tension may build up.

The present invention further relates to a furnace with a movable grate including a number of plate-formed grate elements as described above.

The invention will now be explained in more detail below by means of examples of embodiments with reference to the very schematic drawing, in which

Fig. 1 is a longitudinal cross-section through a prior art air-cooled plate-formed grate element for a movable grate of a furnace;

Fig. 2 is a bottom view of the prior art air-cooled plate-formed grate element of Fig. 1;

Fig. 3 is a bottom view of an air-cooled plate-formed grate element according to the present invention, for a movable grate of a furnace;

Fig. 4 is a side view of the plate-formed grate element of Fig. 3;

Fig. 5 is a cross-section taken along the line V - V of the plate-formed grate element as illustrated in Fig. 3; Fig. 6 is a cross-section taken along the line VI - VI of the plate-formed grate element as illustrated in Fig. 4;

Fig. 7 is a cross-section taken along the line VII - VII of the plate-formed grate element as illustrated in Fig. 4;

Fig. 8 is a cross-section taken along the line VIII - VIII of the plate-formed grate element as illustrated in Fig. 4;

Fig. 9 is a cross-section taken along the line IX - IX of the plate-formed grate element as illustrated in Fig. 4;

Fig. 10 is a cross-section taken along the line X - X of the plate-formed grate element as illustrated in Fig. 4;

Fig. 11 is a perspective view seen obliquely from below of the plate-formed grate element according to the present invention as illustrated in Figs. 3 to 5;

Fig. 12 is a perspective view seen obliquely from above of the plate-formed grate element according to the present invention as illustrated in Figs. 3 to 5;

Fig. 13 is a bottom view of a so-called first half plate-formed grate element according to the present invention, for a movable grate of a furnace;

Fig. 14 is a longitudinal cross-section taken along the line XIV - XIV of the first half plate- formed grate element as illustrated in Fig. 13;

Fig. 15 is a perspective view seen obliquely from below of the first half plate-formed grate element according to the present invention as illustrated in Figs. 13 and 14; Figs. 16A-C illustrate cross-sectional views of a section of a movable grate including a number of plate-formed grate elements according to the present invention, in different stages of operation;

Fig. 17 illustrates a longitudinal section through a movable grate including a number of plate-formed grate elements according to the present invention;

Fig. 18 illustrates a perspective view seen obliquely from above of the movable grate as illustrated in Fig. 17;

Fig. 19 illustrates a transverse section through part of the movable grate illustrated in Fig. 17;

Fig. 20 is a cross-section taken along the line XX - XX of the movable grate as illustrated in Fig. 19;

Fig. 21 is a cross-sectional view corresponding to that of Fig. 20, but illustrating a so- called half plate-formed grate element according to the present invention; and

Fig. 22 illustrates a drive and synchronising mechanism being arranged for pivoting back and forth grate shafts of a section of the movable grate illustrated in Fig. 17.

In the following, generally, similar elements of different embodiments have been designated by the same reference numerals.

Figs. 3 to 12 illustrate a full-sized air-cooled plate-formed grate element 1, according to the present invention, for use in a movable grate 5 of a furnace of the type illustrated in Figs. 17 and 18. By air-cooled is understood cooled by means of combustion gas or air. As seen, the movable grate 5 includes a number of pivotal grate shafts 6 carrying plate- formed grate elements 1, 2, 3 and thereby defining an inclined grate surface 7. The pivotal grate shafts 6 are illustrated in further detail in Figs. 16 and 19 to 21. Referring to Fig. 22, the movable grate 5 further includes a drive mechanism 8 being arranged for pivoting back and forth neighbouring grate shafts 6 in opposite rotational directions so as to impart a wave-like movement to material on the grate surface 7 in order to transport such material downwards. The drive mechanism 8 is arranged so that each grate shafts 6 is provided with a crank arm 63, the crank arms of every other grate shafts 6 are connected by means of a first linking rod 61 and the crank arms 63 of the remaining grate shafts 6 are connected by means of a second linking rod 62, the actuator of said drive mechanism is a linear actuator 60, such as a hydraulic piston actuator, and the first linking rod 61 and the second linking rod 62 are interconnected by means of the linear actuator 60. Instead of being provided on the grate shafts 6, the crank arms 63 may be mounted on separate shafts connected to the respective grate shafts 6 via separate crank systems or via any other suitable mechanical drive connection.

Referring still to Fig. 22, the movable grate 5 further includes a synchronising mechanism 9 being arranged to maintain a predetermined clearance 10 (so small that it is not distinguishable in the figures) between edge portions 11 of plate-formed grate elements 1, 2, 3 of neighbouring grate shafts 6 during the pivoting movement of the grate shafts 6. The synchronising mechanism 9 includes a first synchronising lever arm 58 having a first end fixedly connected to one of the grate shafts 6 connected to the first linking rod 61 and a second synchronising lever arm 59 having a first end fixedly connected to one of the grate shafts 6 connected to the second linking rod 62. The second ends of the respective first and second synchronising lever arms 58, 59 are pivotally connected to respective ends of a synchronising rod 57. Thereby, the synchronising mechanism 9 may maintain said predetermined clearance between edge portions of plate-formed grate elements 1, 2, 3 of neighbouring grate shafts 6.

By means of the drive mechanism 8 and the synchronising mechanism 9, the mutual relative pivotal positions of the respective grate shafts 6 of the movable grate 5 may be individually elastically biased towards respective predetermined relative pivotal positions by means of respective biasing mechanisms in the form of disc springs 64 arranged in respective mounting brackets of the crank arm 63 on the grate shafts 6. Thereby, if the movement of a grate shaft 6 is prevented, the movement may wholly or partly be taken up by the biasing mechanisms. The plate-formed grate elements 1, 2, 3 on each grate shaft 6 coincide with the plate- formed grate elements 1, 2, 3 on the neighbouring shaft 6 without touching these, thereby forming the practically cohesive inclined grate surface 7. The gap between two coinciding plate-formed grate elements 1, 2, 3 in the form of the predetermined clearance 10 mentioned just above may for instance be approximately 1 to 3 millimetres. The grate function is such that the grate shafts 6 alternately turn to their respective outer positions, as illustrated in Figs. 16A and 16 C, respectively, thereby passing their intermediate position, as illustrated in Fig. 16B, and the inclined grate surface 7 thus forms a stair-shaped surface where the steps change direction. This produces a rolling movement to material present on the movable grate 5, which may have the effect of breaking it up and agitating it, while at the same time moving it forward in downward direction, thus achieving good exposure to radiant heat from the combustion chamber above the movable grate 5 and good exposure to combustion air. In particular, the access of primary combustion air through the gaps formed between edge portions 11 of plate- formed grate elements 1, 2, 3 of neighbouring grate shafts 6, from below the movable grate 5 to the combustion chamber above the movable grate 5, is controlled by the predetermined clearance 10 between neighbouring plate-formed grate elements 1, 2, 3.

Fig. 18 illustrates a complete movable grate 5 for a not shown furnace. The movable grate 5 has a left grate lane 41 and a right grate lane 42. However, the illustrated type of movable grate 5 may have any suitable number of grate lanes, such as one, two, three, four or even more grate lanes. Fig. 17 illustrates a longitudinal section through the right grate lane 42 of the movable grate 5 of Fig. 18. Each grate lane 41, 42 has a first section 43, on which the fuel enters, a second section 44, a third section 45, and a fourth section 46, from which the fuel finally exits. More sections may be provided. Naturally, temperatures may get higher in the first and second sections 43, 44 than in the third and fourth sections 45, 46 due to the fact that fresh fuel enters the first section 43. The first and second sections 43, 44 may therefore typically include mainly plate-formed grate elements provided with internal cooling fluid chambers through which a cooling fluid, typically a liquid, such as water, is circulated. Such plate-formed grate elements are de- scribed in detail in applicant's pending application PCT/EP2021/086204. However, preferably the plate-formed grate elements arranged along the sides of the grate lanes 41, 42 of the first and second sections 43, 44 may be air-cooled and of the type according to the present invention. This may be due to the fact that less fuel reaches the side areas of the grate lanes 41, 42 and therefore cooling requirements may be reduced.

The third and fourth sections 45, 46, on the other hand, may typically be cooled entirely by means of primary combustion air and all full-sized and so-called first half plate- formed grate elements 1, 2 of these sections may therefore be of the type according to the present invention.

Figs. 16A, 16B and 16C illustrate different stages of operation of the third section 45 of the right grate lane 42 of the movable grate 5 illustrated in Fig. 18. It is noted that the third section 45 of the right grate lane 42 includes from left to right, a so-called first half plate-formed grate element 2, four full-sized plate-formed grate elements 1 arranged in succession and a so-called last half plate-formed grate element 3. In this connection, the designation "half" simply refers to a reduced length of the first and last plate-formed grate elements 2, 3, as compared to the full-sized plate-formed grate elements 1. In addition, it is seen that the first half plate-formed grate element 2 has a specific design of its back end 15 and the last half plate-formed grate element 3 has a specific design of its front end 14, as it will be explained in further detail in the following. Comparing with Fig. 17, it is noted that a back end 15 of the first half plate-formed grate element 2 of the fourth section 46 cooperates with a fixed plate-formed grate element 4. Likewise, a back end 15 of the first half plate-formed grate element 2 of the third section 45 cooperates with a fixed plate-formed grate element 4, however, in Fig. 17, the first half plate- formed grate element 2 of the third section 45 and the corresponding fixed plate- formed grate element 4 are not visible, because the movable grate 5 has been illustrated broken up in parts as illustrated by broken lines. In order for the back end 15 of the first half plate-formed grate element 2 of the third section 45 to cooperate with the corresponding fixed plate-formed grate element 4, the back end 15 of the first half plate-formed grate element 2 is shorter and has a rounded contour as compared to the back end 15 of the full-sized plate-formed grate elements 1. The first half plate-formed grate element 2 according to the present invention is illustrated in Figs. 13 to 15. Referring again to Fig. 16, it is noted that the front end 14 of the first half plate-formed grate element 2 cooperates with the back end 15 of the first one of the four full-sized plate- formed grate elements 1 in the same way as the front end 14 of each of the first, second and third full-sized plate-formed grate element 1 cooperates with the back end 15 of a neighbouring full-sized plate-formed grate element 1. Furthermore, it is noted that the front end 14 of the last (fourth) full-sized plate-formed grate element 1 cooperates with a back end 15 of the last half plate-formed grate element 3 in the same way as the front end 14 of a full-sized plate-formed grate element 1 cooperates with the back end 15 of a neighbouring full-sized plate-formed grate element 1. However, referring to Fig. 17, it is noted that a front end 14 of the last half plate-formed grate element 3 of the third section 45 cooperates with a fixed plate-formed grate element 4 arranged between the third section 45 of the grate lane 42 and the fourth section 46 of the grate lane 42. In order to do this, the front end 14 of the last half plate-formed grate element 3 is shorter and has a different contour as compared to the front end 14 of the full-sized plate- formed grate elements 1.

Because the front end 14 of the last half plate-formed grate element 3 during operation is located below the fixed plate-formed grate element 4, the front end 14 of the last half plate-formed grate element 3 is subjected to lower temperatures than the front end 14 of the first half plate-formed grate element 2 and the front end 14 of each of the four full-sized plate-formed grate elements 1. Therefore, the requirement for cooling of the front end 14 of the last half plate-formed grate element 3 is relatively low and is not designed according to the present invention.

However, the front end 14 of the first half plate-formed grate element 2 is during operation located above the back end 15 of the first one of the four full-sized plate-formed grate elements 1 in the same way as the front end 14 of each full-sized plate-formed grate element 1 is during operation located above the back end 15 of a neighbouring full-sized plate-formed grate element 1 or above the back end 15 of the last half plate- formed grate element 3. Therefore, the front end 14 of the first half plate-formed grate element 2 and the front end 14 of each full-sized plate-formed grate element 1 are subjected to extremely high temperatures caused by the combustion of fuel on the movable grate 5 during operation. Therefore, the requirement for cooling of the front end 14 of the first half plate-formed grate element 2 and the front end 14 of each full-sized plate- formed grate element 1 is very high in order to avoid excessive wear. An embodiment of the full-sized plate-formed grate element 1 according to the present invention is illustrated in Figs. 3 to 12 and 20, and an embodiment of the first half plate-formed grate element 2 according to the present invention is illustrated in Figs. 13 to 15 and 21. The plate-formed grate elements 1, 2 according to the present invention are less prone to wear of in particular the front tip edge 23, as it will be explained in further detail below.

Referring to Figs. 4 and 5, the plate-formed grate element 1 according to the present invention has a top wall 12, a front end 14 and a back end 15. As indicated in Fig. 3, a longitudinal direction L of the plate-formed grate element 1, 2 extends between the front end 14 and the back end 15. The front end 14 of the plate-formed grate element 1 has a lower inwardly curved wall portion 16 being adapted to maintain said predetermined clearance 10 with a back tip edge 17 of the back end 15 of a corresponding plate-formed grate element 1 during part of said pivoting movement of the grate shafts 6 when said plate-formed grate elements 1 are arranged on neighbouring grate shafts 6. The pivoting movement of the grate shafts 6 is illustrated in Figs. 16A-C.

The plate-formed grate element 1, 2 according to the present invention is adapted to be cooled by means of combustion air coming from a lower chamber 51 formed below the movable grate 5 in the furnace as indicated in Figs. 17 and 18. As illustrated in Figs. 3 to 12, this is achieved in that the top wall 12 of the plate-formed grate element 1 is provided with at least one cooling rib 18, 19, 20, 21 projecting downwardly from a lower side of the top wall 12 and extending in the longitudinal direction L of the plate-formed grate element 1 and being freely exposed to combustion air coming from below. As seen, the at least one cooling rib 18, 19, 20, 21 is freely exposed to combustion air coming from below, because the plate-formed grate element 1 is not provided with any bottom wall or similar element below the at least one cooling rib 18, 19, 20, 21 which could hinder the airflow to or from said at least one cooling rib. As seen, in the illustrated embodiment, the plate-formed grate element 1 is provided with four cooling ribs 18, 19, 20, 21, however, any suitable number of cooling ribs may be employed.

The furnace as illustrated in Figs. 17 and 18 comprises a not shown air supply configured for supplying primary air for the combustion from beneath and through the movable grate 5 and through the layer of fuel situated on the grate during operation. This combustion air coming from the lower chamber 51 formed below the movable grate 5 also serves as cooling air for the plate-formed grate elements of the grate as mentioned above. Said air supply is often referred to as underfire air. Furthermore, likewise not indicated, overfire air may be supplied to the furnace above the grate 5.

Figs. 1 and 2 illustrate a known air-cooled plate-formed grate element 52. This prior art plate-formed grate element 52 also has a top wall 12 being generally flat, a front end 14 and a back end 15. The front end 14 of the plate-formed grate element 1 has a lower inwardly curved wall portion 16 being adapted to maintain said predetermined clearance 10 with a back tip edge 17 of the back end 15 of a corresponding plate-formed grate element 1 during part of said pivoting movement of the grate shafts 6 when said plate- formed grate elements 1 are arranged on neighbouring grate shafts 6. The top wall 12 of the prior art plate-formed grate element 52 is provided with four cooling ribs 18, 19, 20, 21 projecting downwardly from the top wall 12 and extending in a longitudinal direction of the plate-formed grate element 52.

The prior art plate-formed grate element 52 of Figs. 1 and 2 has a straight front wall 53 extending from the top wall 12 of the prior art plate-formed grate element 52 to the lower inwardly curved wall portion 16 of the front end 14. As seen, the straight front wall 53 forms an oblique angle with the top wall 12 and forms a pointed front tip edge 54 at its connection with the lower inwardly curved wall portion 16. As seen in Figs. 1 and 2, the entire front end 14 of the prior art plate-formed grate element 52, i.e. an area 24 formed between the straight front wall 53 and the lower inwardly curved wall portion 16 of the front end 14, is formed by solid material. The solid material extends along the front end 14 from a left side 65 of the plate-formed grate element 52 to a right side 66 of the plate-formed grate element 52. As seen in the longitudinal cross-section of Fig. 1, the thickness of this solid material varies greatly along the straight front wall 53 from a connection 25 between the top wall 12 and the straight front wall 53 to the pointed front tip edge 54. At the connection 25 between the top wall 12 and the straight front wall 53, said thickness is relatively small, at the middle of the straight front wall 53, said thickness is relatively large, and at the pointed front tip edge 54, said thickness is relatively small, although thicker than the thickness at the connection 25 between the top wall 12 and the straight front wall 53.

The pointed front tip edge 54 of the prior art plate-formed grate element 52 may during operation be subject to a significant temperature gradient due to a substantial mass concentration at the front end 14 and at the pointed front tip edge 54 in the form of the solid material in the area 24 formed between the straight front wall 53 and the lower inwardly curved wall portion 16 of the front end 14. Furthermore, as seen, the four cooling ribs 18, 19, 20, 21 of the prior art plate-formed grate element 52 end before reaching the front end 14 which is relatively distant from the pointed front tip edge 54 at which the temperature may be elevated. The temperature of the pointed front tip edge 54 may during operation reach up to about 900 degrees Celsius.

As illustrated in Figs. 4 and 5, according to the present invention, on the contrary, the plate-formed grate element 1 has an outwardly curved front wall 22 extending all the way from the top wall 12 of the plate-formed grate element 1 to the lower inwardly curved wall portion 16 of the front end 14. A front tip edge 23 of the front end 14 is formed by the outwardly curved front wall 22 at its connection with the lower inwardly curved wall portion 16, and the outwardly curved front wall 22 has a nominal wall thickness varying by less than ±35 per cent. The at least one cooling rib 18, 19, 20, 21 extends further into an area 13 formed between the outwardly curved front wall 22 and the lower inwardly curved wall portion 16 of the front end 14 and is connected to both the outwardly curved front wall 22 and the lower inwardly curved wall portion 16. Combustion air may therefore flow into one or more cooling areas or cooling chambers 32 formed in the front end 14 of the plate-formed grate element 1 whereby the cooling combustion air may contact the inside of the outwardly curved front wall 22 and the inside of the lower inwardly curved wall portion 16 of the front end 14 and thereby efficiently cool the front end 14 and in particular the area at the front tip edge 23. Furthermore, combustion air contacting the at least one cooling rib 18, 19, 20, 21 extending into said cooling chamber or chambers and being connected to the outwardly curved front wall 22 and the lower inwardly curved wall portion 16 of the front end 14 may further cool the area of the front tip edge 23. In particular, due to the fact that the outwardly curved front wall 22 has a nominal wall thickness varying by less than ±35 per cent, the cooling effect of the combustion air is evened out over the outwardly curved front wall 22, as compared to the prior art grate elements, thereby cooling the front tip edge 23 better and more efficiently. A better cooling of the front tip edge 23 may result in less wear of the front tip edge and therefore a longer service life of the plate-formed grate elements 1. Furthermore, a smooth curvature of the entire outwardly curved front wall 22 may result in a stronger front wall without weak areas in which tension may build up.

As an example, the temperature of the front tip edge 23 of the plate-formed grate element 1 according to the present invention may during operation reach no more than 300 degrees Celsius in a furnace setup in which the pointed front tip edge 54 of the prior art plate-formed grate element 52 of Figs. 1 and 2 would reach almost 900 degrees Celsius. This means that a temperature reduction of up to about 600 degrees Celsius may be obtained by means of the plate-formed grate element 1 according to the invention.

According to the present invention, preferably, the top wall 12 of the plate-formed grate element 1 is generally at least substantially flat. Preferably, the top wall 12 has an at least substantially constant wall thickness. A nominal wall thickness of the top wall 12 of the plate-formed grate element 1 may advantageously vary by less than ±35 per cent, preferably less than ±30 per cent, more preferred less than ±25 per cent, and most preferred less than ±20 per cent.

According to the present invention, preferably, the outwardly curved front wall 22 is continuously rounded from the top wall 12 of the plate-formed grate element 1 to the lower inwardly curved wall portion 16 of the front end 14 so that the outwardly curved front wall 22 forms a convex part of the front end 14 and the lower inwardly curved wall portion 16 forms a concave part of the front end 14.

As seen in particular in the cross-sections of Figs. 6, 7 and 10, an upper surface of the top wall 12 of the plate-formed grate element 1 slopes from both sides towards a central area of the upper surface when seen in a transverse cross-section. A central line 50 illustrated in Fig. 12 forms the lowest point of said upper surface as seen in the crosssections. Said slope of the top surface forms a kind of trough leading possible fluid further down the plate-formed grate element 1 and away therefrom.

As further illustrated in Figs. 13 to 15, according to the present invention, the first half plate-formed grate element 2 also has an outwardly curved front wall 22 extending from the top wall 12 of the plate-formed grate element 2 to the lower inwardly curved wall portion 16 of the front end 14. A front tip edge 23 of the front end 14 is formed by the outwardly curved front wall 22 at its connection with the lower inwardly curved wall portion 16, and the outwardly curved front wall 22 has a nominal wall thickness varying by less than ±35 per cent. The at least one cooling rib 18, 19, 20, 21 extends further into an area 13 formed between the outwardly curved front wall 22 and the lower inwardly curved wall portion 16 of the front end 14 and is connected to both the outwardly curved front wall 22 and the lower inwardly curved wall portion 16. Combustion air may therefore flow into a cooling area or chamber formed in the front end 14 of the plate- formed grate element 2 whereby the cooling combustion air may contact the inside of the outwardly curved front wall 22 and the inside of the lower inwardly curved wall portion 16 of the front end 14 and thereby efficiently cool the front end 14 and in particular the area at the front tip edge 23. Furthermore, combustion air contacting the at least one cooling rib 18, 19, 20, 21 extending into said cooling chamber and being connected to the outwardly curved front wall 22 and the lower inwardly curved wall portion 16 of the front end 14 may further cool the area of the front tip edge 23. As it will be understood, the design of the front end 14 of the first half plate-formed grate element 2 as illustrated in Figs. 13 to 15 corresponds to the design of the front end 14 of the full-sized plate-formed grate element 1 as illustrated in Figs. 3 to 12. Therefore, the same advantages as explained above in relation to the full-sized plate-formed grate element 1 may also be achieved by means of the first half plate-formed grate element 2.

On the other hand, as mentioned above, the design of the back end 15 of the first half plate-formed grate element 2 differs from the design of the back end 15 of the full-sized plate-formed grate element 1. Comparing Figs. 5 and 14, it is seen that the back end 15 of the first half plate-formed grate element 2 is shorter than the back end 15 of the full- sized plate-formed grate element 1. Furthermore, the back end 15 of the first half plate- formed grate element 2 is rounded with a relatively large radius of curvature in order to cooperate with a fixed plate-formed grate element 4 as discussed above and as illustrated in Figs. 16A to 16C.

The plate-formed grate element 1, 2 according to the present invention may preferably be produced in one single piece of metal in a sand casting process. Subsequently, the casting may be machined to accurate measurements. The sand casting process may for instance be of the lost foam type or any other suitable sand casting process. However, of course, the plate-formed grate element 1, 2 according to the present invention may be produced in any suitable way, such as by any suitable casting process or machining process or even in a 3D printing process. The plate-formed grate element 1, 2 may also be assembled from any suitable number of elements.

The nominal wall thickness of the outwardly curved front wall 22 of the plate-formed grate element 1, 2 according to the present invention may advantageously vary by less than ±30 per cent, preferably less than ±25 per cent, and most preferred less than ±20 per cent. By reducing the variation of the nominal wall thickness of the outwardly curved front wall 22 even further, it may be possible to further even out the effect of the cooling fluid over the outwardly curved front wall 22 and thereby to a higher degree obtain even cooling of the front wall. In particular, it may be possible to avoid insufficient cooling of the front tip edge 23. Preferably, the outwardly curved front 22 wall has an at least substantially constant wall thickness.

Referring to Fig. 5, the part of the outwardly curved front wall 22 extending from the top wall 12 of the plate-formed grate element 1, 2 to the front tip edge 23 may advantageously have an outer contour with a first nominal radius of curvature R varying by less than ±40 per cent, and preferably less than ±20 per cent. The front tip edge 23 may advantageously have an outer contour with a second nominal radius of curvature r varying by less than ±20 per cent. Advantageously, the first nominal radius of curvature R is more than 2 times larger, preferably more than 3 times larger, more preferred more than 4 times larger and most preferred more than 5 times larger than the second nominal radius of curvature r. Thereby, it may in particular be possible to concentrate the cooling effect of the combustion air to the area at the front tip edge 23, because the combustion air may be directed much closer to the front tip edge 23 of the plate-formed grate element 1, 2 than according to the prior art. An even better cooling of the front tip edge 23 may result in less wear of the front tip edge and therefore a longer service life of the plate-formed grate elements 1, 2.

According to the invention, the outwardly curved front wall 22 of the plate-formed grate element 1, 2 may advantageously have an outer contour with a first nominal radius of curvature R, wherein the first nominal radius of curvature R is constant, constantly increases or constantly decreases, from the top wall 12 of the plate-formed grate element 1, 2 to the front tip edge 23.

Referring to Figs. 3 to 12, it is seen that in the illustrated embodiment, the at least one cooling rib 18, 19, 20, 21 includes a first cooling rib 18 forming a first side wall of the plate-formed grate element 1, 2 and a second cooling rib 21 forming a second side wall of the plate-formed grate element, the second side wall being opposed to the first side wall. Thereby, in particular, one or more cooling chambers 32 may be formed in the front end 14 of the plate-formed grate element 1 whereby the cooling combustion air may contact the inside of the outwardly curved front wall 22 and the inside of the lower inwardly curved wall portion 16 of the front end 14 and thereby efficiently cool the front end 14 and in particular the area at the front tip edge 23, thereby resulting in less wear at the front tip edge. It is furthermore seen that in the illustrated embodiment, the at least one cooling rib 18, 19, 20, 21 further includes two intermediate cooling ribs 19, 20 arranged between the first cooling rib 18 and the second cooling rib 21. Thereby, an even better cooling effect and therefore less wear may be experienced at the area of the front tip edge 23. According to the invention, more or less than two intermediate cooling ribs may be provided.

Preferably, the cooling ribs 18, 19, 20, 21 are sealingly connected to both the outwardly curved front wall 22 and the lower inwardly curved wall portion 16 so that separate cooling chambers 67 are formed between neighbouring cooling ribs 18, 19, 20, 21 at the front end 14 of the plate-formed grate element 1, 2, and wherein each one of said separate cooling chambers 67 is accessible for combustion air through an opening 68 formed between respective neighbouring cooling ribs 18, 19, 20, 21 and behind the front end 14 of the plate-formed grate element 1, 2. Thereby, an even better cooling effect and therefore less wear may be experienced at the area of the front tip edge 23.

As seen in Figs. 4 and 5, preferably, the top wall 12 of the plate-formed grate element 1 is at least substantially flat when seen in longitudinal cross-section, and a part 69 of the top wall 12 extends over at least a part of the lower inwardly curved wall portion 16 of the plate-formed grate element. The part 69 of the top wall 12 is connected to the outwardly curved front wall 22 at a connection point 33 illustrated in Figs. 4 and 5. Thereby, a volume of said cooling area or chamber 32 formed in the front end 14 of the plate- formed grate element 1 may be maximised, thereby facilitating access of combustion air and thereby maximising the possible cooling effect of the combustion air in the front end 14 and in particular in the area at the front tip edge 23. Therefore, less wear may be experienced at the area of the front tip edge.

Preferably, the top wall 12 extends over at least 30 per cent, preferably at least 40 per cent, and most preferred at least 50 per cent, of the extent of the lower inwardly curved wall portion 16 in the longitudinal direction L of the plate-formed grate element 1, 2. Thereby, access of combustion air may be even better facilitated, and the possible cooling effect of the combustion air in the front end 14 and in particular in the area at the front tip edge 23 may be even further increased.

As illustrated in Fig. 5, in an embodiment, a connection point 33 (as seen in the illustrated cross-section) between the top wall 12 and the outwardly curved front wall 22 is located in front of an apex 36 or upper point (as seen in the illustrated cross-section) of the lower inwardly curved wall portion 16 by a distance d in the longitudinal direction L of the plate-formed grate element 1. Thereby, access of combustion air may be even better facilitated. It is noted that although said connection point 33 and said apex 36 are seen as points in the illustrated cross-section, theses points are in reality lines on the plate-formed grate element 1, 2.

As illustrated in Fig. 4, in an embodiment, seen in a longitudinal cross-section of the plate-formed grate element 1, 2, the outwardly curved front wall 22 is at least substantially symmetric about a symmetry line S forming an angle of at least substantially 45 degrees with the longitudinal direction L of the plate-formed grate element 1, 2.

As seen in particular in Figs. 3 and 11, the at least one downwardly projecting cooling rib 18, 19, 20, 21 may be interconnected to a neighbouring downwardly projecting cooling rib 18, 19, 20, 21 by means of a transverse rib 26, 27 or the like element. In the illustrated embodiment, a transverse rib 26 connects all four downwardly projecting cooling ribs 18, 19, 20, 21 at a back part of the plate-formed grate element 1, and three transverse ribs 27 connect respective pairs of neighbouring downwardly projecting cooling rib 18, 19, 20, 21 at a front part of the plate-formed grate element 1. It is noted that the transverse rib 26, in the illustrated embodiment, so to say runs through all four downwardly projecting cooling ribs 18, 19, 20, 21. As seen, on the other hand, in the illustrated embodiment, the three transverse ribs 27 are arranged between the respective pairs of neighbouring downwardly projecting cooling ribs 18, 19, 20, 21. In other not shown embodiments, one or more of the downwardly projecting cooling ribs 18, 19, 20, 21 may be broken, have cut-outs or the like, at one or more positions along the longitudinal direction of the plate-formed grate element 1. Furthermore, three transverse ribs 35 connect respective pairs of neighbouring downwardly projecting cooling rib 18, 19, 20, 21 at a central part of the plate-formed grate element 1, of which three transverse ribs 35 the two outermost ones are provided with respective threaded mounting holes 39 for mounting the plate-formed grate element 1 on girders 48 as illustrated in Figs. 20 and 21.

As seen in Fig. 11, the first cooling rib 18 forming the first side wall of the plate-formed grate element 1 and the second cooling rib 21 forming the second side wall of the plate- formed grate element are provided with a relatively short front extension 29 of the respective cooling rib. Likewise, the third cooling rib 19 and the fourth cooling rib 20 are provided with respective relatively long front extensions 28 of the cooling ribs. The downwardly extending front extensions 28, 29 of the cooling ribs help to guide cooling air into the separate cooling chambers 67 formed between respective neighbouring cooling ribs in the front end 14 and should preferably have a certain height. However, the relatively short front extension 29 of the first and second cooling ribs 18, 21 provide room for elements such as bolts arranged at side walls 34 of the grate lanes when the plate-formed grate element 1 is arranged next to such side wall 34.

As illustrated in Figs. 19, 20 and 21, the plate-formed grate elements 1, 2 are mounted on the girders 48 by means of not shown bolts screwed into threaded mounting holes 39 of the plate-formed grate elements.

List of reference numbers d longitudinal distance from connection point between top wall and outwardly curved front wall to apex of lower inwardly curved wall portion

R first nominal radius of curvature r second nominal radius of curvature

S symmetry line of outwardly curved front wall

1 full-sized plate-formed grate element

2 first half plate-formed grate element

3 last half plate-formed grate element

4 fixed plate-formed grate element

5 movable grate of furnace

6 pivotal grate shaft

7 inclined grate surface

8 drive mechanism

9 synchronising mechanism

10 predetermined clearance between plate-formed grate elements

11 edge portion of plate-formed grate element

12 top wall of plate-formed grate element

13 area formed between outwardly curved front wall and lower inwardly curved wall portion

14 front end of plate-formed grate element

15 back end of plate-formed grate element

16 lower inwardly curved wall portion of front end

17 back tip edge of back end

18 first cooling rib

19 third cooling rib

20 fourth cooling rib

21 second cooling rib

22 outwardly curved front wall of plate-formed grate element

23 rounded front tip edge of front end 24 area formed between straight front wall and lower inwardly curved wall portion of prior art plate-formed grate element

25 connection between top wall and straight front wall of prior art plate- formed grate element

26 transverse connecting rib 1 transverse connecting rib

28 relatively long front extension of cooling rib

29 relatively short front extension of cooling rib

30 first side of front end of plate-formed grate element

31 second side of front end of plate-formed grate element

32 cooling chamber in front end of plate-formed grate element

33 connection point (seen in cross-section) between the top wall and outwardly curved front wall (in reality a connection line)

34 side wall of he grate lane

35 transverse rib

36 apex of lower inwardly curved wall portion (in reality an apex line)

39 threaded mounting hole of plate-formed grate element

41 left grate lane

42 right grate lane

43 first section of grate lane

44 second section of grate lane

45 third section of grate lane

46 fourth section of grate lane

47 stationary inlet connection plate

48 girder carrying plate-formed grate elements

49 cooling air inlet

50 central line of upper surface of top wall

51 lower chamber

52 prior art full-sized plate-formed grate element

53 straight front wall of plate-formed grate element

54 pointed front tip edge of prior art plate-formed grate element

56 frame of movable grate 57 synchronising rod

58 first synchronising lever arm

59 second synchronising lever arm

60 linear actuator of drive mechanism 61 first linking rod

62 second linking rod

63 crank arm

64 disc springs of biasing mechanism

65 left side of plate-formed grate element 66 right side of plate-formed grate element

67 separate cooling chambers formed between respective neighbouring cooling ribs

68 opening formed between respective neighbouring cooling ribs

69 part of top wall extending over at least a part of the lower inwardly curved wall portion

Claims

1. A plate-formed grate element (1, 2) for a movable grate (5) of a furnace, the movable grate (5) including a number of pivotal grate shafts (6) carrying plate-formed grate elements (1, 2, 3) and thereby defining an inclined grate surface (7), the movable grate (5) including a drive mechanism (8) being arranged for pivoting back and forth neighbouring grate shafts (6) in opposite rotational directions so as to impart a wave-like movement to material on the grate surface (7) in order to transport such material downwards, and the movable grate (5) including a synchronising mechanism (9) being arranged to maintain a predetermined clearance (10) between edge portions (11) of plate-formed grate elements (1, 2, 3) of neighbouring grate shafts (6) during the pivoting movement of the grate shafts (6), the plate-formed grate element (1, 2) having a top wall (12), a front end (14) and a back end (15), a longitudinal direction (L) of the plate-formed grate element (1, 2) extending between the front end (14) and the back end (15), the front end (14) of the plate-formed grate element (1, 2) having a lower inwardly curved wall portion (16) being adapted to maintain said predetermined clearance (10) with a back tip edge (17) of the back end (15) of a corresponding plate-formed grate element (1) during part of said pivoting movement of the grate shafts (6) when said plate-formed grate elements (1, 2) are arranged on neighbouring grate shafts (6), and the plate-formed grate element (1, 2) being adapted to be cooled by means of combustion air coming from a lower chamber (51) formed below the movable grate (5) in the furnace in that the top wall (12) of the plate-formed grate element (1, 2) is provided with at least one cooling rib (18, 19, 20, 21) projecting downwardly from the top wall (12) and extending in the longitudinal direction (L) of the plate-formed grate element (1, 2) and being freely exposed to combustion air coming from below, characterised in that the plate-formed grate element (1, 2) has an outwardly curved front wall (22) extending from the top wall (12) of the plate-formed grate element (1, 2) to the lower inwardly curved wall portion (16) of the front end (14), in that a front tip edge (23) of the front end (14) is formed by the outwardly curved front wall (22) at its connection with the lower inwardly curved wall portion (16), in that the outwardly curved front wall (22) has a nominal wall thickness varying by less than ±35 per cent, and in that the at least one cooling rib (18, 19, 20, 21) extends further into an area (13) formed between the outwardly curved front wall (22) and the lower inwardly curved wall portion (16) of the front end (14) and is connected to both the outwardly curved front wall (22) and the lower inwardly curved wall portion (16).

2. A plate-formed grate element according to claim 1, wherein the nominal wall thickness of the outwardly curved front wall (22) varies by less than ±30 per cent, preferably less than ±25 per cent, and most preferred less than ±20 per cent.

3. A plate-formed grate element according to claim 1 or 2, wherein the part of the outwardly curved front wall (22) extending from the top wall (12) of the plate-formed grate element (1, 2) to the front tip edge (23) has an outer contour with a first nominal radius of curvature (R) varying by less than ±40 per cent, and preferably less than ±20 per cent, wherein the front tip edge (23) has an outer contour with a second nominal radius of curvature (r) varying by less than ±20 per cent, and wherein the first nominal radius of curvature (R) is more than 2 times larger, preferably more than 3 times larger, more preferred more than 4 times larger and most preferred more than 5 times larger than the second nominal radius of curvature (r).

4. A plate-formed grate element according to any one of the preceding claims, wherein the at least one cooling rib (18, 19, 20, 21) includes a first cooling rib (18) forming a first side wall of the plate-formed grate element (1, 2) and a second cooling rib (21) forming a second side wall of the plate-formed grate element, the second side wall being opposed to the first side wall.

5. A plate-formed grate element according to claim 4, wherein the at least one cooling rib (18, 19, 20, 21) further includes a number of intermediate cooling ribs (19, 20) arranged between the first cooling rib (18) and the second cooling rib (21).

6. A plate-formed grate element according to claim 4 or 5, wherein the cooling ribs (18, 19, 20, 21) are sealingly connected to both the outwardly curved front wall (22) and the lower inwardly curved wall portion (16) so that separate cooling chambers (67) are formed between neighbouring cooling ribs (18, 19, 20, 21) at the front end (14) of the plate-formed grate element (1, 2), and wherein each one of said separate cooling chambers (67) is accessible for combustion air through an opening (68) formed between respective neighbouring cooling ribs (18, 19, 20, 21) and behind the front end (14) of the plate-formed grate element (1, 2).

7. A plate-formed grate element according to any one of the preceding claims, wherein the top wall (12) of the plate-formed grate element (1, 2) is at least substantially flat when seen in longitudinal cross-section, and wherein a part (69) of the top wall extends over at least a part of the lower inwardly curved wall portion (16) of the plate-formed grate element.

8. A plate-formed grate element according to claim 7, wherein the top wall (12) extends over at least 30 per cent, preferably at least 40 per cent, and most preferred at least 50 per cent, of the extent of the lower inwardly curved wall portion (16) in the longitudinal direction (L) of the plate-formed grate element (1, 2).

9. A plate-formed grate element according to any one of the preceding claims, wherein, seen in a longitudinal cross-section of the plate-formed grate element (1, 2), a connection point (33) between the top wall (12) and the outwardly curved front wall (22) is located in front of an apex (36) of the lower inwardly curved wall portion (16) by a distance (d) in the longitudinal direction (L) of the plate-formed grate element (1, 2).

10. A plate-formed grate element according to any one of the preceding claims, wherein, seen in a longitudinal cross-section of the plate-formed grate element (1, 2), the outwardly curved front wall (22) is at least substantially symmetric about a symmetry line (S) forming an angle of at least substantially 45 degrees with the longitudinal direction (L) of the plate-formed grate element (1, 2).

11. A furnace with a movable grate (5) including a number of plate-formed grate elements (1, 2) according to any one of the preceding claims.