The present invention relates to a grate, which comprises essentially concentrically arranged stationary and rotating grate rings, which alternate from the centre outwards to the peripheral ring and in which each grate ring is lower than the next one inwards and has a greater diameter than it, so that fuel fed from underneath in the centre flows while it burns towards the peripheral ring, where the ash drops through openings arranged in it into an ashbox.

A grate of the type referred to above is described in Finnish patent No. 95964. The rotating grate rings are set slightly eccentrically, when they also move the downwards flowing mass radially. In addition, the grate rings rotate in opposite directions, which improves the transport of the ash over the grate rings. An opening has been left between the peripheral ring of a grate of this kind and the wall, from which the ash drops into an ashbox beneath it. The opening must be quite wide, because the ash contains some large clinkers. As the fuel feed is increased, the proportion of unburnt material in the ash increases. This limits the available output of the grate, though the grate could otherwise burn a much larger amount of fuel.

The purpose of the invention is to create an improvement over the case referred to above. The characteristic features of the invention are described in the accompanying Claims. With the aid of units according to the invention, the intermediate space that was previously a free opening is covered with a grid construction, in which openings can be formed if necessary. The grate can then be loaded much more than previously, particu- larly for short periods, with no fear of considerable quanti- ties of unburned material flowing with the ash. Even large lumps of clinker can be removed through the openings formed at regular intervals between the units, though otherwise the

elements form a grid structure, on top of which combustion continues, with the fine ash nevertheless dropping through.

Other advantageous embodiments of the invention and advantages are described later concerning the example of application.

In the following, the invention is described with the aid of the accompanying drawings, which show one grate according to the invention.

Figure 1 shows a vertical cross-section of a combustion cham- ber, to which a grate according to the invention has been adapted.

Figure 2 shows a partial enlargement of the cross-section in Figure 1 in the area of the peripheral ring, in which there is the new arrangement for removing ash.

Figure 3 shows an adaption arranged on the peripheral ring, comprising stationary and moving units with the ash openings closed; the Figure includes a partial en- largement.

Figure 4 shows the arrangement in Figure 3 with the units overlapped and forming ash openings; the Figure includes a partial enlargement.

Figure 5 shows the vertical bracket in figure 2, seen from in front.

In Figure 1, a grate 1 according to the invention is adapted to a combustion chamber with a circular horizontal cross-section, which in this adaptation acts as the front chamber of a boiler.

Grate 1 conforms to the patent publication referred to above, except for the arrangement of the periphery. The lower part of combustion chamber 2 is cylindrical, while the upper part is conical. The conical upper part is connected to flue 3, which is connected to the after combustion chamber and to a boiler (not shown) for recovering heat from the flue gases. The jacket 4 of the combustion chamber has thermal insulation. Grate 1 is

arranged in the lower part of combustion chamber 2. An annular ashbox surrounds grate 1. Grate 1 according to the invention includes stationary and rotating grate sections. The stationary grate componens are grate rings 6a, 6b, 6c, and 6d, while the rotating grate sections are correspondingly grate rings 7a, 7b, and 7c. These are mutually arranged so that every other grate ring is stationary, while the others rotate. The inner and outermost grate rings 6a and 6d are stationary grate rings.

The fuel feed channel 8 is arranged to run horizontally beneath grate 1 from the outside of the combustion chamber 2 to its centre. Fuel, e. g. woodchips, conveyed by feed screw 9 travels along the feed channel 8 to the centre of the grate and rises through the feed cone to the top of grate 1. There the fuel ignites and while burning flows over the grate rings 6a-d, 7a- c. Ash and possible burning residue move to the peripheral ring, in which there is an arrangement according to this invention.

The enlargement in Figure 2 shows the new arrangement according to the invention in detail. In this arrangement too, the ash openings 30 are formed between stationary grate ring 6d and the wall of the combustion chamber, but the openings can be closed by stationary and moving units 15 and 16. The moving units are installed on arm 19, which is in turn attached to support ring 14. This is supported vertically on five bearings 13 from the support structure of the grate. Figure 2 shows one bearing 13 in cross-section, while Figure 5 shows it seen from the front.

Units 15 and 16 comprise circumferentially oriented bars 15.1 and 16.1. According to Figure 2, bars 15.1 and 16.1 interlap, so that the bars 16.1 of moving unit 16 push between the stationary bars 15.1.

Figure 3 shows the general arrangement of the units 15 and 16 that cover the entire ring, with the ash openings closed.

However, fine ash can flow down, due to the grid construction

of units 15 and 16. The partial enlargement shows the construc- tion of the units in greater detail. They each comprise three bars 15.1 and correspondingly 16.1 set at a distance to one another. Bars 15.1 are attached to arm 18 and correspondingly bars 16.1 are attached to arm 19. In general, each unit 15, 16 comprises 2-4 bars 15.1,16.1, at a radial distance of 2-4 cm.

Support ring 14 is not only supported vertically, but also horizontally on five rollers 20, which roll along the cylindri- cal support structure. The support ring is driven by operating device 21, for example, a pneumatic cylinder, which is in- stalled in air intake duct 22. In this way the operating device remains cool.

In this example 16 pairs of units are used, but advantageously these are 10-20.

The ash openings 30 are formed according to Figure 4 by using operating device 21, by means of which support ring 14 is turned slightly, so that the attached units 16 interlap with the stationary units. Even large lumps of clinker will fit through the ash openings 30 that are formed. According to Figure 5, the rolling surface of the support ring incorporates a step 17, which raises the units 16 when they are moved. This improves the cleaning of the units and reduces the risk of them jamming.