The invention relates to a grate structure for a furnace, which comprises a grate, which has holes for the passing through of gaseous materials and ash and a dome-shaped lid part on the first side of the grate, which lid part is dimensioned to cover the grate at least partly.

Solid fuels, such as wood chips, can be made into burning product gas with a gasifier, which has a fuel silo and beneath it a furnace. On the bottom of the furnace there is a grate. The fuel is fed from above into the upper part of the fuel silo, from where it flows due to gravity downwards into the furnace, where combustion of the fuel takes place. The combustion air needed in the gasification is fed to the middle part of the gasifier and the generated product gas is suctioned out from underneath the grate of the furnace. Inflow of combustion gas is achieved with the aid of suction generated remove the product gas. One gasifier functioning according to this principle is presented in reference publication WO 2008/145814 A1.

There are also problems related to the operation of the gasifier described in reference publication WO 2008/145814 A1. The grate in the bottom of the furnace has gaps or holes, through which the ash generated in the combustion flows. Caked ash and incompletely burned fuel brands among the ash may obstruct the gaps of the grate, whereby the flow of product gas through the grate is hindered. There may sometimes also be non-flammable metal pieces, such as nails, among the fuel, which may obstruct the gaps of the grate. A disturbance in the exit of the product gas affects the inflow of combustion air, which may lead to an incomplete combustion reaction, a decreased gas production and an increase in contaminants in the product gas. In order to keep the gaps of the grate open, an ash scraper rotated by an actuator may be arranged in the furnace, with which ash scraper the ash and brands are directed through the gaps of the grate. Constructing an ash scraper however raises the manufacturing costs of the grate. Furthermore, possible non-flammable metal parts among the ash may disturb the operation of the ash scraper.

It is an object of the invention to provide a grate structure for a furnace, by which the drawbacks and disadvantages relating to the prior art can be removed.

The objects of the invention are obtained with a grate structure for a furnace, which is characterised by what is presented in the independent claim. Some advantageous embodiments of the invention are presented in the dependent claims.

A grate structure for a furnace according to the invention comprises a grate, which has holes for the passing through of gaseous substances and ash. On the first side of the grate there is a dome-shaped, advantageously substantially hemispherical lid part, which is dimensioned to cover the grate at least partly. The first side in this context means the upper side of the grate. The lid part prevents the holes of the grate from clogging, whereby the gaseous substances can flow continuously through the grate. In one advantageous embodiment of the grate structure the lid part is dimensioned to cover the grate substantially completely. The grate structure further comprises a flow channel leading from the second side of the grate through the grate to the first side of the grate, to the space between the grate and the lid part, and a trough-like bottom part surrounding the flow channel, on the second side of the grate. The flow channel forms a flow route for the gaseous substances through the grate.

An advantageous embodiment of the grate structure according to the invention further comprises a support axis advantageously fitted into the flow channel for supporting the lid part. The grate structure advantageously further comprises an actuator for rotating the support axis and/or for moving the support axis in the longitudinal direction.

In a second advantageous embodiment of the grate structure according to the invention, there is a first ring-shaped gap between the edge of said bottom part and the edge of the lid part and a second ring-shaped gap between the edge of the lid part and the wall of the flow channel, and the grate is fitted in said second ring-shaped gap. The gaseous substances can flow via the first gap underneath the grate and via the second gap through the grate above the grate to the space between the lid part and the grate. From this space the gaseous substances exit along the flow channel. The grate structure is advantageously the grate structure of the furnace of a co-current gasifier. It is an advantage of the invention that it does not require an ash scraper, wherefore the grate with regards to its manufacturing costs is affordable and functionally reliable.

It is further an advantage of the invention that its gaps remain open, even if there were non-flammable metal parts, such as nails, among the ash. In the following, the invention will be described in detail. In the description, reference is made to the appended drawings, in which

Figure 1 a shows as an example a furnace, which has a grate structure according to the invention, and Figure 1 b shows as an example a grate belonging to the grate structure according to the invention seen diagonally from above.

Figure 1 a shows as an example a furnace, which has a grate structure according to the invention, as a vertical cross-sectional view. The furnace shown in Figure 1a is the furnace of a co-current gasifier, above which there is a fuel silo (the fuel silo is not shown in the figure). The fuel, such as the wood chips, flows from the fuel silo into the furnace, where gasification of the fuel takes place. The furnace has a triple wall structure comprising an internal jacket 10, an intermediate jacket 12 and an external jacket 14. The internal jacket forming the innermost wall of the furnace is a cylindrical part, in the downward pointing first edge of which there is a support collar 16 directed toward the inner part of the furnace, on which collar the grate structure 20 of the furnace is supported. The radius of the hole outlined by the support collar is Ri. Around the internal jacket there is an intermediate jacket 12, which forms a closed wall surface outside the cylindrical sidewall of the internal jacket and below the grate. The intermediate jacket turns inwards by its upper edge and is joined by its other edge to the internal jacket. The intermediate jacket is advantageously made of steel and attached at its upper edge to the internal jacket by welding. Around the intermediate jacket there is an external jacket 14, which forms the outermost casing of the furnace. A space surrounding the furnace is formed between the external jacket and the intermediate jacket, which space functions as a preheating space for the gasification air.

The grate structure 20 of the furnace comprises a bottom part 22 and a lid part 30. The bottom part is a trough-like part, which has a solid wall. In the edge of the bottom part there is a support flange 24 settling against the upper surface of the support collar. The radius of the outer edge of the support flange is larger than R-t , but smaller than the radius of the internal jacket of the furnace. In the middle of the bottom part there is a through-hole, as a continuation of which there is a cylindrical flow channel 26, the radius of which is R ₂ . The flow channel is attached at its first end to the edges of the hole penetrating the wall of the bottom part and its second end is directed from the bottom part toward the lid part 30. The second end of the flow channel settles above the height level of the support flange, when seen in the height direction. The wall of the bottom part thus forms a ring-like trough surrounding the flow channel, the lowest part of the wall of which trough extends clearly beneath the level of the support collar, when seen in the direction of an imaginary vertical central axis K. The distance h between the level of the support collar and the lowest level of the bottom part in the direction of the central axis is advantageously in the range of 50-100 mm.

In the lid part 30 above the bottom part there is a substantially hemispherical wall. The lid part is supported in place so that it opens downwards toward the bottom part, i.e. the free edges of the wall of the lid part settle on a lower height level than the middle part of the wall of the lid part, when seen in the height direction. The lid part thus forms a dome, protecting the bottom part, on the bottom part. In the middle of the flow channel there is a support axis 40 in the direction of the longitudinal axis of the flow channel, which support axis is supported by its one end to the wall of the lid part. The other end of the support axis extends through the holes in the intermediate and external jacket of the furnace to outside the furnace. On the outer surface of the external jacket there is an actuator 50 in connection with the support axis, by means of which actuator the support axis and the lid part in its other end may when necessary be rotated around the central axis K and/or moved upwards or downwards in the direction of the central axis. The free edge of the lid part forms a ring, the radius R ₃ of which is smaller than the radius Ri of the support collar but larger than the radius R ₂ of the outer surface of the wall of the flow channel. A first ring-shaped gap 32 thus remains between the edge of the bottom part and the edge of the lid part, through which gap the gaseous substances, ash and non-flammable metal parts, such as nails, among the ash can pass to the trough-like part of the bottom part. Between the wall of the flow channel 26 and the edge of the lid part there is a second ring-shaped gap 34. Around the flow channel there is a ring-shaped grate 38, the radius of the outer edge of which is slightly smaller than the radius of the edge of the lid part and the radius of the inner hole of which is slightly larger than the radius of the flow channel (Figure 1 b). The grate thus covers the ring-shaped gap between the edge of the lid part and the outer surface of the flow channel essentially completely. The grate has gaps going through it, through which the gaseous substances and the fine-grained substances passing with the gas, such as ash, can pass underneath the lid part and onward along the flow channel to the space between the internal jacket and the intermediate jacket of the furnace. The ash exits from the space between the internal jacket and the intermediate jacket along a product gas suction pipe 36 to the outside of the furnace. The width of the gaps in the grate is dimensioned so that larger non-flammable pieces, such as nails, possibly passing with the product gas, cannot fit to pass through the gaps. Thus they cannot exit from the furnace along with the product gas. Figure 1 b shows as an example the grate 38 of the grate structure of Figure 1a shown diagonally from above. The grate is a ring-shaped part, the radius of the inner hole of which is slightly larger than the radius F½ of the outer surface of the wall of the flow channel 26. The grate can thus be fitted around the flow channel. The radius of the outer periphery of the grate is slightly smaller than the radius R3 of the inner surface of the wall of the lid part 30. The lid part can thus be fitted on top of the grate, so that the grate remains essentially completely inside the perimeter outlined by the free edge of the lid part. A group of elongated, curved holes 42 lead through the grate. The holes are arranged in two circular lines of holes, so that a neck 44 is left between adjacent holes. The holes are formed so that the necks of the inner and outer lines of holes are not aligned. The grate is manufactured from some material which can withstand high temperatures, such as cast iron or steel. In Figure 1 b the holes leading through the grate have an elongated shape. It is clear to someone skilled in the art that the holes can also have another shape, such as round or oval. Some advantageous embodiments of the grate structure of a furnace according to the invention have been described above. The invention is thus not limited to the solutions described above, but the inventive idea can be applied in numerous ways within the scope of the claims.