EP4367439A1

EP4367439A1 - Grate assembly

Info

Publication number: EP4367439A1
Application number: EP22735461.0A
Authority: EP
Inventors: Juha HILJANEN; Pekka Lehtonen; Juha OJANPERÄ; Tuomas KANNIAINEN
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2021-07-07
Filing date: 2022-06-29
Publication date: 2024-05-15
Anticipated expiration: 2042-06-29
Also published as: JP2024525089A; EP4367439B1; FI20215793A1; WO2023280646A1; FI130549B; PL4367439T3; CN117597546A; HRP20251007T1; US20240280258A1

Abstract

A grate assembly for use in a bottom section (28) of a combustion chamber(12) of a fluidized bed boiler (10), comprising: a grate bottom wall (32); a protective refractory material layer (36); a plurality of nozzle devices (38) for supplying fluidizing primary air; at least one grate module (40) formed on the grate bottom wall (32). Each grate module (40) comprises a solids removal opening (42); a plurality of concentric landings (46) formed in the refractory material layer (36) and separated by frontal surfaces (48) between the landings. Each frontal surface (48) surrounds one of the landings and follows the shape of the perimeter of a rectangle or a rectangle with at least one shaped corner. The landings (46) define a stepped structure. Each landing comprises a group of the nozzle devices embedded in the refractory material layer (36) and adapted to jet the air through one of the frontal surfaces (48)along one of the landings (46) that is adjacent to the frontal surface.

Description

GRATE ASSEMBLY

Technical field of the solution

The presented solution relates to a grate assembly for use in a bottom section of a combustion chamber of a fluidized bed boiler. The grate assembly includes a grate module or at least two adjacent grate modules constituting the bottom section. The presented solution relates to a fluidized bed boiler for use in steam production and having a combustion chamber.

Background of the solution

A bottom section of a combustion chamber of a fluidized bed boiler may include a sloping floor or sloping floor sections for facilitating removal of solids when transferring the solids into a solids removal opening by means of air jets and gravity, the sloping floor being formed by a protective refractory material layer. The air jets are brought about by nozzle devices providing air for combustion and fluidization. The nozzle devices extend from the protective refractory material layer to varying heights.

The nozzle devices may obstruct the removal of solids or the orientation of the air jets is not optimal. The nozzle devices may be abraded by the air jets carrying solids or fluidized bed material. The height differences in the sloping floor including the nozzle devices may vary strongly depending on the dimensions of the combustion chamber. Thus, the operation of the fluidized bed of the combustion chamber may be affected. The shape and structure of the protective refractory material layer may differ between boilers.

Brief summary of the solution

The grate assembly according to the solution is presented in claim 1 .

A fluidized bed boiler including the above-mentioned grate assembly is presented in claim 15.

Other claims present further details of the examples of the solution. The grate assembly according to the solution is for use in a bottom section of a combustion chamber of a fluidized bed boiler.

The grate assembly comprises a grate bottom wall having a plurality of cooling tubes that are attached to the grate bottom wall; a protective refractory material layer on the grate bottom wall and covering the plurality of cooling tubes; and a plurality of nozzle devices for supplying fluidizing primary air above the grate bottom wall and the protective refractory material layer into the combustion chamber for maintaining combustion of fuel and fluidization of bed material.

The grate assembly further comprises at least one grate module formed on the grate bottom wall, each grate module comprising a solids removal opening in the refractory material layer via which solids on the refractory material layer are guided downwards to a solids removal conduit adapted to guide the solids through the refractory material layer and the grate bottom wall.

Each grate module further comprises a plurality of concentric landings each formed in the refractory material layer, the landings being situated at intervals in a vertical direction and being separated by frontal surfaces between the landings. Each frontal surface surrounds one of the landings and follows the shape of the perimeter of a rectangle or a rectangle with at least one shaped corner. The landings define a stepped structure that descends towards the solids removal opening situated in the center of the landings. Each landing comprises a group of nozzle devices belonging to the plurality of nozzle devices and being embedded in the refractory material layer. The group of nozzle devices is directed to jet the air through one of the frontal surfaces along one of the landings that is adjacent to the frontal surface.

According to an example, the at least one shaped corner includes a chamfer, multiple chamfers, a step, multiple steps, a shape extending inwards the rectangle, and/or a shape extending outwards the rectangle.

According to an example, each landing comprises four rows of nozzle devices each with nozzle devices aligned in a row and belonging to the group of nozzle devices. According to an example, the frontal surface comprises four corners each having at least one nozzle device belonging to the group of nozzle devices and being between two of the rows of nozzle devices. The fluidized bed boiler for use in steam production comprises a combustion chamber with a bottom section including the above-mentioned grate assembly.

The presented solution is particularly advantageous and solves the above- mentioned problems.

In an example, the grate module of the presented solution provides a modular and expandable system for constructing the bottom section of the combustion chamber of the fluidized bed boiler.

In an example, the use of the grate modules of the presented solution provides a way of restricting the height differences between the nozzle devices. The height differences and the height of the stepped structure of the grate assembly and the grate module can be chosen in such a way that the motion of the fluidizing primary air above the grate assembly takes place in a desired or controlled manner.

In an example, rows of additional nozzle devices are easily integrated into or between the grate modules for facilitating the design of the layout of the grate assembly and the bottom section of the combustion chamber.

In an example, the central location of the solids removal opening in the grate module provides an efficient way of removing solids. The size and dimensions of the grate module can be chosen in such a way that solids are efficiently transferred to the solids removal opening.

In an example, the presented solution provides a simple structure in which the grate bottom wall and the plurality of parallel cooling tubes extend horizontally.

In an example, the presented solution provides the plurality of nozzle devices embedded in the refractory material layer constituting the stepped structure and thereby unobstructed removal of solids is facilitated by the surfaces of the stepped structure of the grate module.

In an example, the nozzle devices in corners of the grate module are oriented to remove solids efficiently from the surfaces of the stepped structure. These and other non-limiting features, characteristics and advantages of the presented solution are more particularly disclosed below.

Brief description of the drawings

The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

Fig. 1 shows in a side view an example of a steam generator or a fluidized bed boiler in which the present solution can be applied.

Fig. 2 shows in a cross-section, in a side view, an example of the grate assembly applied in the present solution and constituting one grate module.

Fig. 3 shows in a top view the example in Fig. 2 of the grate assembly applied in the present solution.

Fig. 4 shows in a top view an example of the grate assembly applied in the present solution and including two adjacent grate modules with additional nozzle devices.

Fig. 5 shows in a top view examples of shaped corners in the grate assembly of Fig. 3.

Detailed description of the solution

In the description, reference is made to the figures with the following reference numbers and denotations:

10 Fluidized bed boiler 12 Combustion chamber 14 Cyclone 16 Flue gas channel 18 Support frame 20 Flue gas passage 22 Column 24 Supporting beam 26 Hanger 28 Bottom section 30 Grate assembly 32 Grate bottom wall 34 Cooling tube 36 Refractory material layer 38 Nozzle device

381 First row of nozzle devices

382 Second row of nozzle devices

383 Third row of nozzle devices

384 Fourth row of nozzle devices 40 Grate module

42 Solids removal opening

44 Solids removal conduit

46 Landing

48 Frontal surface

50 Air plenum chamber

52 Additional landing

54 Additional frontal surface

56 Additional nozzle device

561 Additional row of nozzle devices

58 Corner

A more complete understanding of the features disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations and are not intended to indicate relative size and dimensions of the devices or components thereof or to define or limit the scope of the embodiments. The specific terms used in the following description are intended to refer only to the embodiments selected for the drawings and are not intended to define or limit the scope of the disclosure. In the drawings and the description below, like numeric designations refer to devices or components of like function.

In the following, the terms “horizontal” and “vertical” refer to the intended operating positions of the device or component in question when installed in place for implementing the functions of the described solution. The terms “horizontal” and “vertical” are used to indicate direction relative to an absolute reference, i.e. ground level. In the figures, the vertical direction is denoted by an arrow Z and the two orthogonal, horizontal directions are denoted by arrows X and Y. The horizonal directions are orthogonal in relation to the vertical direction.

Also, the terms “upper”, “lower”, “on top”, “below”, “upward”, and “downward” relate to the above-mentioned, intended operating positions. The terms “parallel” and “perpendicular” should not be construed to require structures to be absolutely parallel or absolutely perpendicular to each other. The term “opposite” should not be construed to require opposite directions to be absolutely parallel to each other.

Referring to Fig. 1 , one embodiment of a steam generator or a fluidized bed boiler 10 in which the present solution is applied is shown. The fluidized bed boiler 10 may be a part of a power plant, a steam boiler plant, or a hot water boiler plant, adapted for the production of electric energy, steam, and/or heating energy.

The boiler 10 includes a combustion chamber 12, i.e. a furnace, for the combustion of fuels and a flue gas channel 16 for conveying flue gases, i.e. combustion product gases, coming from the combustion chamber 12.

The boiler 10 may comprise further devices that are relevant for the design in question but are not necessarily shown in the figures. The boiler 10 may additionally comprise a cyclone separator 14 connected to the combustion chamber 12 for separating solid particles from the flue gases coming from the combustion chamber 12 and for guiding the flue gases to the flue gas channel 16. The boiler 10 may further comprise a support frame 18 for supporting the combustion chamber 12 and the flue gas channel 16 to the ground. The support frame 18 may, for example, include columns 22, supporting beams 24 and/or hangers 26 for supporting the combustion chamber 12 and/or the flue gas channel 16 to the support frame 18.

The boiler 10 may be a fluidized bed boiler of CFB design (circulating fluidized bed) or BFB design (bubbling fluidized bed). The fuel may be a gas, solid fuel or solid waste from various sources, e.g. municipal waste. Fluidizing air realizing a fluidized bed and to be used as primary air for combustion is fed into the combustion chamber 12 via a bottom section 28 constituting the lower part of the combustion chamber 12.

As shown in Figs. 2 and 3, the grate assembly for use in the bottom section 28 of the combustion chamber of the fluidized bed boiler 10 comprises a grate bottom wall 32, a protective refractory material layer 36, a plurality of nozzle devices 38, and at least one grate module 40.

The grate bottom wall 32 includes a plurality of cooling tubes 34 that are attached to the grate bottom wall 32.

According to an example in Fig. 2, the grate bottom wall 32 extends horizontally and the plurality of cooling tubes 34 are parallel, extend horizontally, and are attached to the grate bottom wall 32 at intervals along the grate bottom wall 32.

According to an example in Fig. 2, the grate bottom wall 32 is constituted by the plurality of cooling tubes 34 separated by fins attached between the cooling tubes 34.

The protective refractory material layer 36 is situated on the grate bottom wall 32 and covers the plurality of cooling tubes 34.

The plurality of nozzle devices 38 are for supplying fluidizing primary air for maintaining combustion of fuel and fluidization of bed material above the grate bottom wall 32 and the protective refractory material layer 36 into the combustion chamber 12. Each nozzle device 38 is adapted to guide primary air that arrives through the grate bottom wall 32 and the refractory material layer 36 and to jet the air to be used as the fluidizing primary air.

According to an example in Fig. 2, the nozzle device 38 includes a conduit section attached to and going through the grate bottom wall 32, and a mouth section for jetting out the air guided via the conduit section. The grate assembly 30 comprises, for example, one grate module 40 or 2 to 36 adjacent grate modules 40. According to an example, the grate modules 40 constitute a N x M grid, with N equaling 1 , 2, or 3 and M equaling 2, 12, or a whole number between 2 and 12.

According to an example in Fig. 4, all the grate modules 40 of the grate assembly 30 are similar. Alternatively, the grate assembly 30 may include similar and/or dissimilar grate modules 40.

Each grate module 40 comprises a solids removal opening 42, a solids removal conduit 44, and a plurality of concentric landings 46.

The solids removal opening 42 is formed in the refractory material layer 36 via which solids removal opening 42 solids on the refractory material layer 36 are guided, by means of air in motion and gravity, downwards through the refractory material layer 36 and the grate bottom wall 32.

According to an example in Fig. 2, the solids are guided downwards to a solids removal conduit 44 of the fluidized bed boiler 10 or the grate assembly.

Each landing 46 is formed in the refractory material layer 36. The landings 46 are situated at intervals in relation to a vertical direction and are separated from each other by frontal surfaces 48 that are situated between the landings 46.

The landings 46 constitute a stepped structure, e.g. a funnel, that descends towards the solids removal opening 42 that is situated in the centre of the landings 46.

According to an example in Fig. 2, the frontal surfaces 48 extend vertically.

Each frontal surface 48 surrounds one of the landings 46 and follows the shape of the perimeter of a rectangle, or the shape of the perimeter of a rectangle with at least one shaped corner. Alternatively, each frontal surface 48 surrounds one of the landings 46 and follows the shape of the perimeter of a square, or the shape of the perimeter of a square with at least one shaped corner. The square, representing a rectangle, is a special case of the rectangle with four sides of equal length.

According to examples in Fig. 5, the at least one shaped corner includes a chamfer (Fig. 5(b)), multiple chamfers (Fig. 5(c)), a step (Fig. 5(d)(g)), multiple steps (Fig. 5(e)(f)), a shape extending inwards the rectangle (Fig. 5(b)(c)(d)(e)), and/or a shape extending outwards the rectangle (Fig. 5(f)(g)). According to an example, all the four corners of the rectangle are similar. According to an example, the corner is formed of two perpendicular edges (Fig. 5(a)).

Each landing 46 comprises a group of nozzle devices 38 belonging to the plurality of nozzle devices 38. The group of nozzle devices 38 is embedded in the refractory material layer 36 and are configured, directed, or oriented to jet the air through one of the frontal surfaces 48 and further along one of the landings 46 that is adjacent to the one frontal surface 48. According to an example, the one landing 46 is between the one frontal surface 48 and another frontal surface 48 situated lower in relation to a vertical direction. According to an example, the one landing 46 is between the one frontal surface 48 and the solids removal opening 42 at the centre of the grate module 40.

According to an example in Fig. 3, the solids removal opening 42 is circular. Alternatively, the solids removal opening 42 may be, for example, of rectangular, e.g. a square, or polygon shape.

According to an example in Fig. 2, the nozzle devices 38 are adapted to jet the air to horizontal directions. According to an example in Fig. 3, each nozzle device 38 is adapted to jet the air to a predetermined horizontal direction specific to the nozzle device 38.

According to an example in Figs. 2 and 3, each frontal surface 48 is formed by surfaces of the group of nozzle devices 38 or by the refractory material layer 36, or by both the surfaces of the nozzle devices 38 and the refractory material layer 38. According to an example there are alternating nozzle devices 38 and sections of refractory material layer 36 in the frontal surface 48. According to an example, a front surface of the nozzle device 38 constitutes a part of the frontal surface 48. According to an example, the front surface is included in the mouth section of the nozzle device 38. According to an example, the air is jetted out via an opening in the front surface.

According to an example, the group of nozzle devices 38 is embedded in the refractory material layer 36 in such a way that the air is jetted out from the nozzle device 38 to a conduit and an opening formed in the refractory material layer 36 for jetting the air through the frontal surface 48.

According to an example in Fig. 2, the group of nozzle devices 38 is embedded in the refractory material layer 36 in such a way that a top surface of the nozzle device 38 constitutes a part of the landing 46. According to an example there are alternating nozzle devices 38 and sections of refractory material layer 36 on the landing 46. According to another example the nozzle device 38 is embedded below the surface of the landing 46.

According to an example in Fig. 3, each landing 46 comprises a first row 381 of nozzle devices 38, a second row 382 of nozzle devices 38, a third row 383 of nozzle devices 38, and a fourth row 384 of nozzle devices 38. The nozzle devices 38 in each row are aligned in a row and belong to the above-mentioned group of nozzle devices 38. The rows are arranged horizontally in such a way that the first and second rows 381 , 382 are parallel and are situated on opposite sides of the landing 46, and that the third and fourth rows 383, 384 are parallel, situated on opposite sides of the landing 46, and perpendicular to the first and second rows 381 , 382.

According to an example in Fig. 3, two or more nozzle devices 38 belonging to the plurality of nozzle devices 38 and being situated on different landings 46 are aligned in a row along a direction perpendicular to one of the rows 381 , 382, 383, 384.

According to an example in Fig. 3, each nozzle device 38 in each row 381 , 382, 383, 384 is configured, directed, or oriented to jet the air in a horizontal direction perpendicular to the above-mentioned opposite row towards the opposite row along the landing 46. According to an example, there are eight to eighty nozzle devices 38 in the above-mentioned group of nozzle devices 38.

According to an example, there are three to seven concentric landings 46 in the grate module 40. Alternatively, there are eight or more, for example at least ten, concentric landings 46. According to an example, the landings extend horizontally.

According to an example in Fig. 3, each frontal surface 48 comprises four corners 58 constituting the corners of the rectangle, each corner includes at least one nozzle device 38 belonging to the above-mentioned group of nozzle devices 38 and being between two of the rows 381 , 382, 383, 384 that are perpendicular to each other. The at least one nozzle device 38 is configured, directed, or oriented to jet the air in a horizontal direction towards the solids removal opening 42 or the one of the four corners 58 that is situated diagonally opposite. According to an example in Fig. 3, the at least one nozzle device 38 jets the air at an angle of 45 degrees or at an angle of 35 to 55 degrees, or at an angle of 20 to 70 degrees, in relation to the above-mentioned two rows that are perpendicular to each other.

According to an example, there are not more than one, two, or three nozzle devices 38 in the above-mentioned corner 58. Alternatively, there are more than three nozzle devices 38 in the above-mentioned corner 58.

According to an example in Fig. 4, the grate assembly 30 further comprises on at least one side of at least one of the grate modules 40 an additional landing 52. The additional landing 52 is formed in the refractory material layer 36. The additional landing 52 forms an extension to the stepped structure of the grate module 40. According to a first example in Fig. 4, the additional landing 52 is separated from the uppermost landing 46 of the grate module 40 by an additional frontal surface 54 between them. According to a second example, the additional landing 52 is separated from another additional landing 52 by an additional frontal surface 54 between them.

According to an example in Fig. 4, the additional landing 52 and/or the additional frontal surface 54 follows the shape of a line, i.e. the additional landing 52 and/or the additional frontal surface 54 extends rectilinearly. According to an example, the additional landings 52 extend horizontally.

According to an example, the additional landing 52 extends rectilinearly along one side of at least two of the grate modules 40 that are adjacent.

The additional landing 54 comprises a group of additional nozzle devices 56 embedded in the refractory material layer 36 and are configured, directed, or oriented to jet the air through one of the additional frontal surfaces 54 and further along the above-mentioned uppermost landing 46 or the above- mentioned other additional landing 52 that is adjacent to the one additional frontal surface 54. According to an example, the additional nozzle devices 56 are adapted to jet the air to horizontal directions.

According to an example in Fig. 4, the landings 46 of the grate module 40 and the additional landings 52 situated on one side, on two adjacent or opposite sides, or on three adjacent sides of the grate module 40 form a grate module 40 following a rectangular shape. According to another example, the grate module 40 follows a square shape which grate module 40 together with the additional landings 52 forms a non-square, rectangularly shaped structure. According to an example in Fig. 4, the additional landings 52 are to be situated between two adjacent grate modules 40.

According to an example in Fig. 4, the additional landing 52 comprises an additional row 561 of additional nozzle devices 56. The additional nozzle devices 56 in the additional row are aligned in a row and belong to the above- mentioned group of additional nozzle devices 56. The additional row is arranged horizontally in such a way that the additional row 561 is parallel to the first and second rows 381 , 382 or the third and fourth rows 383, 384.

According to an example in Fig. 4, each additional nozzle device 56 in the additional row 561 is configured, directed, or oriented to jet the air in a horizontal direction perpendicular to and towards the above-mentioned rows that are parallel to the additional row 561 . Alternatively, at least one additional nozzle device 56 at one or both ends of the additional row 561 is configured, directed, or oriented to jet the air in a horizontal direction towards the solids removal opening 42 or the one of the four corners 58 that is situated diagonally opposite, the at least one additional nozzle device 56 belonging to the above- mentioned group of additional nozzle devices 56. According to an example, the at least one additional nozzle device 56 jets the air at an angle of 45 degrees or at an angle of 35 to 55 degrees, or at an angle of 20 to 70 degrees, in relation to the other additional nozzle devices 56 of the additional row 561 .

According to some examples, the details of the structure, operation, and characteristics of the above-mentioned nozzle device 38 explained above, for example in relation to the refractory material layer 36, the frontal surface 48, and the landing 46, apply also to the additional nozzle device 56 in relation to the refractory material layer 36, the additional frontal surface 54, and the additional landing 52.

According to an example, the fluidized bed boiler 10 includes a solids collecting and handling system for receiving the solids coming via one or more of the solids removal opening 42 and/or the solids removal conduit 44.

According to an example, the fluidized bed boiler 10 or the grate assembly further comprises one or more air plenum chambers 50. The air plenum chamber 50 is adapted to receive the air to be supplied via the plurality of nozzle devices 38, 56 as the fluidizing primary air. The air plenum chamber 50 is situated below the grate bottom wall 32. According to an example, for conveying the air, the conduit section of the nozzle device 38, 56 is in communication with the air plenum chamber 50.

In this description, the singular form “a”, “an”, and “the” referring to a device or component does not exclude additional or a plurality of corresponding devices or components, unless where specifically specified.

In the description, various devices and components may be described as “comprising” other components. The terms “comprise(s)”, “comprising”, “include(s)”, “having”, “has”, and variants thereof, are intended to be open- ended phrases that do not exclude the possibility of additional components, unless where specifically specified.

The various aspects and embodiments of the present solution disclosed in this description are for the purposes of illustration and are not intended to be limiting. It is intended that the present solution be construed as including all such aspects and embodiments that are covered by the scope of the appended claims.

Claims

Claims:

1 . A grate assembly for use in a bottom section (28) of a combustion chamber (12) of a fluidized bed boiler (10), the grate assembly (30) comprising:

- a grate bottom wall (32);

- an arrangement for supplying fluidizing primary air above the grate bottom wall into the combustion chamber for maintaining combustion of fuel and fluidization of bed material;

- at least one grate module (40),

- wherein each grate module (40) comprises:

- a solids removal opening (42) via which solids are guided downwards through the grate bottom wall (32);

- a plurality of concentric landings (46) situated at intervals in a vertical direction and being separated by frontal surfaces (48) between the landings,

- wherein each frontal surface (48) surrounds one of the landings and follows the shape of the perimeter of a rectangle; and

- wherein the landings (46) define a stepped structure that descends towards the solids removal opening (42) situated in the center of the landings; and the grate assembly further comprising:

- a plurality of cooling tubes (34) of the grate bottom wall (32) and attached to the grate bottom wall; and

- a protective refractory material layer (36) on the grate bottom wall (32) and covering the plurality of cooling tubes (34),

- wherein the at least one grate module (40) is formed on the grate bottom wall (32) and each one of the plurality of concentric landings (46) is formed in the refractory material layer (36);

- wherein the solids removal opening is in the refractory material layer (36) to guide the solids on the refractory material layer downwards through the refractory material layer and the grate bottom;

- wherein the arrangement for supplying fluidizing primary air includes a plurality of nozzle devices (38) for supplying fluidizing primary air above the grate bottom wall and the protective refractory material layer into the combustion chamber; and - wherein each landing comprises a group of nozzle devices belonging to the plurality of nozzle devices (40) and being embedded in the refractory material layer (36), the group of nozzle devices being adapted to jet the air through one of the frontal surfaces (48) along one of the landings (46) that is adjacent to the frontal surface.

2. The grate assembly according to claim 1 , wherein the rectangle has at least one shaped corner, and wherein the at least one shaped corner includes a chamfer, multiple chamfers, a step, multiple steps, a shape extending inwards the rectangle, and/or a shape extending outwards the rectangle.

3. The grate assembly according to claim 1 or 2, wherein the landings (46) extend horizontally and the nozzle devices (38) are adapted to jet the air in horizontal directions.

4. The grate assembly according to any one of claims 1 to 3, wherein each frontal surface (48) is formed by surfaces of the group of nozzle devices (38), or by the refractory material layer (36), or by both the surfaces and the refractory material layer.

5. The grate assembly according to any one of claims 1 to 4, wherein the grate bottom wall (32) extends horizontally and the plurality of cooling tubes (34) are parallel, extend horizontally, and are attached to the grate bottom wall at intervals along the grate bottom wall.

6. The grate assembly according to any one of claims 1 to 5, wherein

- each landing (46) comprises a first (381 ), second (382), third (383) and fourth (384) row of nozzle devices aligned in a row and belonging to the group of nozzle devices and being arranged horizontally in such a way that the first and second rows (381 , 382) are parallel and are situated on opposite sides of the landing, and that the third and fourth rows (383, 384) are parallel, are situated on opposite sides of the landing, and are perpendicular to the first and second rows.

7. The grate assembly according to claim 6, wherein each nozzle device in each row is adapted to jet the air in a horizontal direction perpendicular to the opposite row towards the opposite row along the landing.

8. The grate assembly according to claim any one of claims 1 to 7, wherein

- each frontal surface (48) comprises four corners (58) constituting the corners of the rectangle, each corner having at least one nozzle device (38) belonging to the group of nozzle devices and being between two of the rows (381 , 382, 383, 384) of nozzle devices that are perpendicular to each other,

- wherein the at least one nozzle device is adapted to jet the air at an angle in a horizontal direction towards the solids removal opening (42) or the one of the four corners (58) that is situated diagonally opposite.

9. The grate assembly according to any one of claims 1 to 8, wherein the grate assembly further comprises at least one air plenum chamber (50) for receiving the air to be supplied via the plurality of nozzle devices (38) as the fluidizing primary air, wherein the at least one air plenum chamber is situated below the grate bottom wall (32).

10. The grate assembly according to any one of claims 1 to 9, wherein

- the grate assembly (30) further comprises on at least one side of at least one of the grate modules (40) an additional landing (52) formed in the refractory material layer (36), the additional landing forming an extension to the stepped structure of the grate module,

- the additional landing (52) is separated from the uppermost landing of the grate module or another additional landing by an additional frontal surface (54) between them;

- wherein the additional landing (54) comprises a group of additional nozzle devices (56) being embedded in the refractory material layer (36), the group of additional nozzle devices being adapted to jet the air through one of the additional frontal surfaces (54) along the uppermost landing or the other additional landing (52) that is adjacent to the additional frontal surface.

11. The grate assembly according to claim 10, wherein the additional landing (54) extends rectilinearly along one side of at least two of the grate modules (40) that are adjacent.

12. The grate assembly according to claim 10 or 11 , wherein

- the additional landing (54) comprises an additional row (561 ) of additional nozzle devices aligned in a row and belonging to the group of additional nozzle devices and being arranged horizontally in such a way that the additional row (561) is parallel to the first and second rows (381, 382) or the third and fourth rows (383, 384).

13. The grate assembly according to claim 12, wherein

- the additional landing (54) comprises at one or both ends of the additional row (561) at least one additional nozzle device (56) belonging to the group of additional nozzle devices and adapted to jet the air at an angle in a horizontal direction towards the solids removal opening (42).

14. The grate assembly according to claim 6, wherein two or more nozzle devices belonging to the plurality of nozzle devices (38) and being situated on different landings (46) are aligned in a row along a direction perpendicular to the first and second rows (381 , 382) or the third and fourth rows (383, 384).

15. A fluidized bed boiler (10) for use in steam production and having a combustion chamber (12) with a bottom section (28) comprising the grate assembly (30) according to any one of claims 1 to 14.