The present invention relates to a stepped grate for a waste incineration plant of the kind set forth in the preamble of claim 1.

From GB-2120 764 it is known to provide a stepped grate of this kind, in which the grate bars and grate block stringers consist of cast iron. One of the advantages of this grate is simple maintenance as all grate girders and grate bars can be lifted vertically up into the furnace above the grate without anyone going under the grate and only simple hand tools are needed for cleaning and maintaining the grate. Furthermore, the grate is designed to maintain a low intrinsic pressure drop over the grate in the range 50-100 Pa. The typical maintenance and cleaning of this type of grate takes place 2-3 times a year, and the typical lifetime for a cast iron grate bar is approx. 3-4 years, both highly dependent on the calorific content of the waste to be incinerated. Traditionally, the grate bars having a width of 21 mm were filled into the grate block and possible tolerances were compensated by filling the open spaces with shims of a width of 6 and 9 mm. During the reciprocating movement of the movable grate beams these shims have shown a tendency of being pushed up and out of the grate block, whereby the grate bars can tilt and also be pushed out of the grate block by the waste during incineration, whereby a whole row of grate blocks may lose all its grate bars and thus develop unintended open holes eventually of a size corresponding to the size of the grate girders in the incineration grate.

It is the object of the invention to provide a stepped grate bar of the kind referred to above, with which it is possible to obtain a greater availability, i.e. longer lifetime and longer time between cleaning and maintenance, and which allows the burning of high calorific waste having a lower heating value in the range 10-15 MJ/kg. According to the present invention this is achieved with a stepped grate of said kind which comprises the features set forth in the characterizing part of claim 1. In this way, it is possible to avoid the provision of water cooling or the use of very costly high alloy steel materials for the grate bars, thereby maintaining the above mentioned advantages associated with the simple construction of the air-cooled grates. The grates, in accordance with the invention, has furthermore the advantage of remaining cleaner for significantly longer times than the traditional cast iron types. Advantageous embodiments of the invention, the nature of which will be evident from the following detailed description of the invention, are revealed in the subordinate claims.

In the following detailed part of the present description, the invention will be explained in more details with reference to the exemplary embodiment of a stepped grate according to the invention shown in the drawings, in which

• Fig. 1 shows a vertical section through three succeeding stepped grates of an incineration plant,
• Fig. 2 shows a grate block in perspective,
• Fig. 3 shows a grate bar seen from the side,
• Fig. 4 shows the grate bar of Fig. 3 seen from above, and
• Fig. 5 shows a grate bar for use at the lowermost grate block at the rear.

The stepped grate for a waste incineration plant shown in Fig. 1 consists of three sections, each section comprising stationary and reciprocating grate girders. Each girder consists of a grate beam 2, on which grate blocks 1 are fixed. Every second grate girder is adapted to make a reciprocating movement while the intermediate girders are stationary.

Each grate block, as shown in Fig. 2, consists of two cast stringers, 3, 4 mutually connected by two or four transverse steel rods 5. The grate block 1 is filled up with cast grate bars 6 which are resting on the steel rods 5. The grate bars are, as shown in Figs. 3-5, provided with suitable bearing elements for supporting the grate bars 6 on the transverse steel rods 5.

The grate bars 6, are preferably provided in two different widths in order to be able to fill out the grate blocks completely within a certain tolerance. In a preferred embodiment, the two different widths of the grate bars are 25 mm and 30 mm, respectively, whereby any dimension above 100 mm may be provided by combinations of those two different widths within a tolerance of 5 mm.

The primary air for the combustion, which enters into the furnace from below as seen in Fig. 1 and through the stepped grate, passes along the sides of the grate bars, these sides being provided with indentations as shown in Figs. 3-5. The indentations in adjacent grate bars are positioned to provide holes with an average distance of approx. 30 - 70 mm, and in the preferred embodiment the hole size in the tops of the grate bars has the dimensions 8 x 12 mm². As shown in Figs. 3-5 the holes in the grate bars have an opening slope down through the hole which in the preferred embodiment forms an angle (α) of approx. 14 °.

The form of the grate bar shown in Fig. 5 is suited for placement as the lowermost row of grate bars on a grate girder and comprises a hook-formed bearing at the front securing the grate bar against lifting up the front end of said grate bar when it is moving relative to the rear edge of the grate section. The hook shape is also adopted to follow closely the surface of the grate transition element thus providing an impediment to the ingress of undesired parasitic combustion air.

In accordance with the invention the grate bars 6 and the grate block stringers 3, 4 are cast from a steel alloy comprising 22-30% Cr, 2-10% Ni, 0.5-1.5% Mn, and 0.2-0.8% C. The specified ranges for these four ingredients allow for optimization of each of the above mentioned grate block stringers 3, 4 and grate bars 6 as concerns manufacturing costs, requirements for weldability (especially the grate block stringers, 3, 4), level of ductility, brittleness and durability inside the waste furnace.

These cast grate bars 6 and grate block stringers 3, 4 have better heat resisting properties than the traditional cast iron grate bars and stringers and furthermore provide a smoother surface, thereby also improving the self-cleaning features of the grate system compared to the cast iron types. Sticky ashes, molten metals and slag pieces have less tendency to clog the grate system in accordance with the invention.

Furthermore, the average durability of the grate bars 6 and the grate block stringers 3, 4 is also improved compared to the cast iron types to an average lifetime of 5-8 years, which together with the above mentioned selfcleaning properties keeps the grate system available for significantly longer times between cleaning and maintaining the grates.

With grate block stringers 3, 4 and grate bars 6, in accordance with the invention, it is possible to maintain a low intrinsic pressure drop over the grate in a range 50-100 Pa and at the same time allowing high calorific waste having a lower heating value in the range 10-15 MJ/kg to be burned on the grates which is cooled by the primary air only.

The grate in accordance with the invention is suited to be used in connection with larger furnaces and grates which are designed to be used for supplying heat to steam boilers with or without steam turbines for electric power generation, where long availability and low need for maintenance is requested, providing an availability greater than e.g. 8000 hours a year.