FR3108709A1 - Ash grate system for the hearth of a combustion or gasification installation - Google Patents

Abstract

The invention relates to a ash removal grate system (1) for the hearth of a combustion or gasification installation supplied with solid fuel, said grate system comprising a lower circular plate (P1) of diameter D1 and of surface S1 rotatably mounted through 360° about a vertical axis of rotation (A), and characterized in that it further comprises at least one upper plate (P2, P3, P4 ), superimposed on said lower plate (P1), and rotatably mounted through 360° along the vertical axis of rotation (A), said at least one upper plate (P2, P3, P4) having a surface S2 lower than the surface S1 of the lower plate (P1) and a maximum dimension at most equal to the diameter D1 of this lower plate (P1). Abstract Figure: Fig. 1

Description

Ash grate system for the hearth of a combustion or gasification installation

FIELD OF THE INVENTION

The present invention relates to an ash removal grate system for the hearth of a combustion or gasification plant supplied with solid fuel, said grate system comprising a lower circular plate of diameter D1 and surface S1 mounted rotatable through 360° along an axis vertical rotation A, and an installation comprising such a grid system.

PRIOR ART

Combustion or gasification plants include a furnace inside which a fixed bed of a solid fuel, for example biomass, is burned to provide heat. They also generate residues, such as ashes, char or various unburnt matter, which must be continuously evacuated from the hearth, to avoid operational shutdowns. To this end, the combustion or gasification installations according to the prior art are equipped with ash removal grate systems.

Grate systems are advantageously designed to respond to various problems in combustion or gasification installations, including - accommodating the fuel injection system in the furnace; - setting the fuel in motion to move the fuel towards the ash removal system, the drive system having to be adapted to the heat and the presence of any dust, and to be protected from it; - control of the fuel bed advance speed to avoid the formation of unburnt matter (residence time too short) or bottom ash (residence time too long). This should also allow the fireplace to operate at different load levels; - the possibility of injecting primary air through the grille; - controlling the distribution of primary air over different portions of the grate to control combustion; - the evacuation of the ashes from the hearth as the combustion progresses, the largest pieces must not block this evacuation; - the grid must withstand high temperature levels without deformation; and - the control of the temperature and the speed of ash evacuation to avoid the formation of clinker.

Many ash grate systems have been developed to address some or all of the issues outlined above. Some systems include vibrating grates, some include rotating grates, some include tilting grates, and still others include step grates.

In view of the foregoing, a problem that the invention proposes to solve is to produce a ash removal grate system for the hearth of a combustion or gasification installation supplied with solid fuel, which overcomes the aforementioned problems of the state of the art.

The solution of the invention to this problem has as its first object an ash grate system for the hearth of a combustion or gasification installation supplied with solid fuel, said grate system comprising a lower circular plate of diameter D1 and surface S1 rotatably mounted through 360° along a vertical axis of rotation A, and characterized in that it further comprises at least one upper plate, superimposed on said lower plate, and rotatably mounted through 360° along the vertical axis of rotation A , said at least one upper plate having a surface S2 less than the surface S1 of the lower plate and a maximum dimension at most equal to the diameter D1 of this lower plate.

Advantageously, - the system comprises a plurality of upper plates superimposed on each other, all of the upper plates being superimposed on the lower plate, the surface S2 of the plate immediately above the lower plate fits into that S1 of said lower plate , and the surfaces S3 and S4 of the upper plates positioned above the plate immediately above the lower plate decrease according to their position in the superposition of the plates; - the upper plates have a shape different from the circular shape and a larger dimension substantially equal to the largest dimension of the plate on which they are superimposed; - the upper plates have an elliptical shape; - the center of gravity of the plates is on the axis of rotation; - the trays are perforated; - the trays have slots and/or openings; - a circular fuel inlet is arranged in the center of the stack of trays, the center of said circular inlet being positioned substantially on the axis A and in that a tube connected to this inlet allows the fuel to be admitted vertically , through superposed trays; - the system comprises two primary air inlets of two independent air circuits, a main air circuit and a cooling air circuit; - coaxial cylindrical tubes are fixed to the plates, a cylindrical tube being fixed to each plate, and in that the plates are driven in rotation by means of these tubes, which are themselves driven in rotation independently by means of 'coaching.

Its second object is a combustion or gasification installation supplied with solid fuel comprising a grid system as defined above.

BRIEF DESCRIPTION OF FIGURES

The invention will be better understood on reading the non-limiting description which follows, written with regard to the appended drawings, in which:

- Figure 1 shows, in perspective, an embodiment of the grid system according to the invention;

- Figure 2A is an illustrative diagram of a first embodiment of an upper plate of a grid system according to the invention;

- Figure 2B is an illustrative diagram of a second embodiment of an upper plate of a grid system according to the invention;

- Figure 2C is an illustrative diagram of a third embodiment of an upper plate of a grid system according to the invention;

- Figure 3A is a diagram which illustrates a superposition of two upper plates according to a first embodiment of a grid system according to the invention;

- Figure 3B is a diagram which illustrates a superposition of two upper plates according to a second embodiment of a grid system according to the invention;

- Figure 4 is a partial view, in perspective, of a set of stacked trays of a grid system according to the invention;

- Figure 5 is a top view of a plate of a grid system according to the invention, showing the slots formed inside this plate;

- Figure 6A shows, in perspective, the lower part of a grid system according to the invention, and the quick-opening hatches that this system includes, for cleaning the primary air injection circuits;

- Figure 6B shows, in section, a grid system according to the invention, as well as the main and cooling circuits of this circuit;

– is a diagram illustrating the positioning of the support tubes of the trays of the grid system according to the invention; And

- Figure 8 is a perspective and sectional view of a grid system according to the invention, which shows in particular the drive system of the plates of this system.

DETAILED DESCRIPTION OF THE INVENTION

In general, a gasification or combustion installation according to the invention is intended to burn or gasify a solid fuel, in particular from biomass, with a view to producing heat. Such an installation includes a hearth which is arranged around a vertical axis of symmetry. This hearth is delimited by a side wall, for example cylindrical. Fuel is admitted to the hearth through a fuel inlet in correspondence with a fuel supply channel. This supply channel is preferably formed by a vertical cylindrical tube centered on a set of superimposed turntables forming a grid. As the solid fuel is admitted into the hearth, vertically, from bottom to top, through the supply channel, to the fuel inlet, a mound of solid fuel forms on the grate. .

As shown in Figure 1, the grid system 1 comprises a lower plate P1 and one or more upper plates. In practice, the number of upper plates is advantageously two or three. In the example of figure 1, system 1 comprises three upper plates P2, P3 and P4.

The upper tray or trays P2 to P4 are positioned on the lower tray P1, stacked on this tray P1. All platters P1 to P4 are turntables, mounted to rotate around a vertical axis A at an angle of 360°. The lower plate P1 is mounted on a fixed circular base 2.

The lower plate P1 is circular with a diameter D. It is the largest of the plates. The surface of the upper plate P1 is equal to S1 (S=π(D/2) ² )). The surface of the upper plates S2, S3, S4 is less than S1. These surfaces S2, S3 and S4 are decreasing: S2 > S3 > S4.

Advantageously, the upper plates are not circular. Examples of shapes of these upper plates are shown in Figures 2A, 2B and 2C. In FIG. 2A, the plates P2 to P4 are in an elliptical shape. In Figure 2B they are triangular. In FIG. 2C, they are star shaped. Of course, other conformations of the upper plates are possible and in particular, an ellipsoidal conformation. Given that they are not circular, the upper plates P2 to P4 have at least one larger dimension and smaller dimensions. The largest dimension of the upper plate P2 positioned immediately above the lower plate P1 is equal or approximately equal to the diameter D of the lower plate P1. The surface S2 of the upper plate P2 is lower than that S1 of the lower plate. FIGS. 3A and 3B present a superposition of two upper plates, for example plates P2 and P3, plate P3 being superposed on plate P2. In FIG. 3A, the plates P2 and P3 are elliptical. They therefore each have a larger dimension, namely the major axis of the ellipse, and a smaller dimension: the minor axis of this ellipse. The major axis of the ellipse formed by plate P3 is approximately equal to or equal to the minor axis of the ellipse formed by plate P2. In FIG. 3B, the plates P2 and P3 are triangular and the triangles they define are equilateral. In this case also, the largest dimension of the triangle forming P3, namely the side of this triangle, is approximately equal to the smallest dimension defined by the triangle forming P2. Ultimately, it will be noted that the plate P3 fits into the plate P2 with a greater dimension, for P3 equal or approximately equal to the smallest dimension of P2. The aforementioned characteristics of the P2 and P3 trays apply in the same way to the P3 and P4 trays and so on.

It should be noted that the shape of the upper plate(s) can be different as long as it combines the following characteristics. First of all, the center of gravity of the platters is on the axis of rotation. This avoids having mechanical forces distributed asymmetrically on the supports of the plate. This improves the life of the system. In addition, this makes it possible to alternately cover and uncover the surface of the lower plate during rotation. This makes it possible to "scrape" the surface of a lower plate by an upper plate and thus to advance the fuel towards the perimeter of the reactor.

The thickness of the plates P1, P2, P3, P4 increases in the stacking of said plates in the grid system. The higher the tray is positioned in this stack, the greater its thickness. For example, the lower plate P1 has a thickness of 100 mm, that of the upper plate P2 which is immediately superior to it has a thickness of 80 mm, that of the plate P3 is 60 mm and that of the plate P4 is 60 mm. This makes it possible to train an equivalent quantity of fuel for each tray.

Trays P1 to P4 are made in such a way that they can be dismantled by portion to facilitate maintenance and installation operations. This also makes it possible to optimize the maintenance cost by replacing only the portion of a damaged tray and not the entire tray.

As shown in Figure 5, the plates P1 to P4 are perforated so that the primary air can pass through said plates. For this purpose, the plates have for example holes or slots. In the preferred configuration of Figure 5, they are provided with substantially radial slots, namely starting from the periphery of the plates and going towards the center thereof. Some F1 slots are not complete and do not extend to the perimeter of the trays. Other F2 slots are complete and extend to the periphery of the trays. They allow the trays to expand without creating distortion. The slots F1, F2 are advantageously extended, on the side of the center of the plates, by openings O wider than the slots F1, F2. These openings O allow air to pass through and prevent the formation of microcracks at the level of the slots F1, F2. The slots F1, F2 may not be strictly positioned radially on the plates. In other words, they may not pass through the center of these. They are then slightly inclined so that, for the same number of slots, the surface allowing the passage of air is greater. In addition, it improves the solidity of the trays. In addition, the slot surfaces are calculated in such a way as to optimize the air flow rates and thereby the combustion/gasification profile of the solid fuel, at the level of each tray. Finally, the section of the slots is calculated in such a way as to create a determined pressure drop for the air passing through them, ensuring a homogeneous flow per slot, independent of the local quantity of fuel located above, and in particular avoiding leaks. uncontrolled air in the event of a lack of fuel above the slots.

If one refers again to Figure 1, it appears that a circular fuel inlet 3 is arranged in the center of the trays P1 to P4 of the grid system 1. All the trays P1 to P4 can rotate around the axis of rotation A, identical to the axis of revolution of the fuel inlet 3. The rotational movement of a given plate creates a relative movement with respect to the plates located above and below and the elliptical shape, or other, upper plates P2 to P4, which makes it possible to sweep the surface of the plates on which they are superimposed and to advance the fuel towards the outside.

The platters P1 to P4 do not rotate at the same time and at the same speed so that there is relative movement between the platters.

A wide slot 4 is arranged around the perimeter of the grid to evacuate the ashes. The ashes are thus continuously evacuated from the hearth through this slot 4.

The grid system according to the invention is also designed so as not to create an accumulation point for unwanted ashes.

The trays have the function of spreading the fuel and bringing the ashes outwards, namely towards the wide slot 4, in order to evacuate them. The fact that all the trays are mobile has two advantages over systems with fixed trays. First, a single moving pan sweeps the fuel from the moving pan plus the fuel on the lower pan. There is therefore less need to run the engines for the same service. We save energy. In addition, the system is less sensitive to possible blockages. If a tray is blocked for any reason the trays above and below can be used to continue operating the grid and therefore the energy production unit while planning and carrying out a maintenance operation at the time. most conducive. Grid system availability and repair flexibility are higher.

The evacuation of the ashes present in the hearth is done by the wide slot or groove 4 positioned on the whole circumference of the grid system. The ashes fall through this slot onto a conveyor system not shown in the figures, which allows the ashes to be brought into a storage space. This wide slot 4 is made so that the largest pieces that can be injected into the hearth can be evacuated. The majority of the ashes are pushed by the rotary plates towards the outside of the grid and therefore towards this slot 4. However, small ashes can also be brought under the plates. Ultimately, these small ashes are likely to block or disrupt the flow of primary air. To avoid this phenomenon, scraping devices are fitted under the plates. These devices are not shown in the figures. They are oriented so that the ashes are pushed out of each tray, then collected on the lower tray, up to tray P1, where the ashes will then be pushed towards slot 4. One or more openings can additionally be arranged at a point around the tray(s) so that the ashes fall into the groove. The section of the opening is precisely calculated so as not to have a preferential air leak, which could affect the main air flow passing through the slots.

An injection system supplies the fireplace with primary air from two independent air inlets in the system. These inputs are referenced 5 and 6 in FIGS. 6A and 6B. These inlets 5 and 6 are primary air inlets for two independent air circuits: the cooling air circuit C1 and the main circuit C2. The cooling air circuit C1 allows atmospheric air to pass through the system for rotating the system and to cool it to protect it from damage due to a rise in temperature. The primary air admitted into inlet 5 thus cools the mechanical motion transmission system to preserve its integrity. This air also makes it possible to avoid any intrusion of ash dust into the mechanical part of the system according to the invention. Part of this primary air is injected through this inlet 5 and emerges at the level of each plate, via a set of dedicated lights, arranged in each coaxial tube. The injected air then joins the circuit under the grid system trays, before entering the solid fuel bed. The second main circuit C2 allows most of the primary air flow to be injected into the hearth through the grille. This circuit is independent of the cooling circuit and is not in contact with sensitive mechanical parts. This makes it possible to inject preheated air or directly recirculate fumes that can reach 200°C and be loaded with dust. The air admitted into inlet 7 therefore supplies the hearth through the grille. Its flow is controlled. The circuit corresponding to this input is never in contact with elements sensitive to temperature or dust. This makes it possible, if necessary, to inject other gases such as recirculated flue gases which still contain fly ash and can reach 200°C. Flue gas recirculation optimizes the combustion of very dry and volatile fuels and reduces the production of atmospheric pollutants such as NOx.

As shown in Figure 6, quick-opening hatches 7 are advantageously provided on the primary air circuit under the grid system. They allow the pipes of the air intake circuits to be quickly evacuated in the event of an accumulation of ashes.

As is more particularly shown in FIGS. 7 and 8, the rotational movements of the plates P1 to P4 are communicated to the plates by means of coaxial vertical tubes. The coaxial vertical tubes are integral with the plates. The axis of the tubes coincides with the axis A. In the case of a grid system comprising four plates P1 to P4, there are four coaxial tubes T1, T2, T3 and T4. A coaxial tube rotates a plate to which it is assembled. The tube T1 is assembled to the plate P1, the tube T2 is assembled to the plate P2, and so on until the tube T4 assembled to the plate P4. The coaxial tubes T1 to T4 are cylindrical of revolution and have different diameters as well as different heights. The diameter of the coaxial tubes increases from the smallest plate P4 to the largest P1. The coaxial tube T4, which is assembled in the lower part of the plate P4, has a smaller diameter than the diameter of the coaxial tube T3 and so on. The coaxial tubes T1 to T4 arranged around the central fuel injection tube 8. The smaller the diameter of a tube, the greater the height of the tube. Thus, the T4 tube is the tallest and the T1 tube is the shortest. As shown in Figure 8, the tubes T1 to T4 are equipped, at their base, with a drive system. This drive system is formed for example of toothed wheels 9 driven by a pinion 10, namely a toothed wheel of smaller diameter, fixed to a motor shaft 11 parallel to the coaxial tube, actuated by a motor-reduction unit 12. It However, it may be any other suitable drive system, for example, a similar system comprising an assembly provided with a toothed wheel, a chain, and a motor or piston.

Ultimately, the system according to the invention has many advantages.

The first advantages arise from the shape and design of the plates forming the grid of the grid system according to the invention. First of all, having trays with a center of gravity on the axis of rotation allows the use of larger and heavier grids. Indeed, this characteristic makes it possible to avoid wearing out the mechanical transmission system prematurely, while unbalanced chainrings create a damaging overhang for the entire mechanical strength over time. Then, the fact of having all the plates mobile makes it possible to operate the engines for a shorter time for the same effect of advancement of the fuel cell. This saves energy during operation and limits the wear of mechanical parts. Another advantage of this solution is that the grid is more resilient in the event of a blockage. It can continue to operate with a locked tray, the trays above and below the locked tray operating independently. The availability of the grid is thus improved and maintenance can be planned at the most favorable time. Then again, the trays are all removable in portions. This facilitates maintenance operations and can be carried out with a minimum of tools, even for a large-diameter grid. In addition, the complete slots in the trays and the use of refractory steel allow the trays to withstand the temperature well and to have thinner trays. This provides an economic gain in manufacturing but it also facilitates maintenance with trays that are lighter to handle. Finally, the variable height of the plates makes it possible to overcome the paradox of a circular grid fed from the center. Indeed, on this type of grate there is a lot of material in the center (unburned fuel) but little surface area, while on the outside of the grate there is much less material (ash) but a larger grate surface . The variable height of the trays makes it possible to compensate for this phenomenon and to better control the combustion.

Second advantages flow from the primary air injection system according to the system of the invention. First of all, the double primary air circuit makes it possible to inject recirculated flue gases or preheated air at a high temperature, for example of the order of 200°C, while ensuring that the mechanical system is cooled and not polluted by flying dust. The recirculation of fumes or the preheating of the air are an advantage for reducing the production of NOx and for fuels with a low calorific value. Then, the primary air injection system ensures the distribution of air under the grille and therefore better control combustion. It also allows the grid to be cooled with the primary air.

Third advantages arise from the management of the evacuation of the ashes according to the system of the invention. First of all, the ash evacuation groove all around the grate is wide and allows all the solid inputs or clinker produced to be evacuated. Then, the system of scrapers under the trays makes it possible to evacuate any small particles of ash that would slip under the trays continuously. This prevents mechanical blockages or air injection clogging.

Fourth advantages arise from the design of the drive system of the grid system according to the invention. The drive system based on concentric tubes allows great flexibility in the choice of the drive mechanism used. It can be adapted according to the characteristics of the grate and the fuel.

Claims (11)

System (1) of ash removal grate for the hearth of a combustion or gasification plant supplied with solid fuel, said grate system comprising a lower circular plate (P1) of diameter D1 and of surface S1 mounted rotatable through 360° according to a vertical axis of rotation (A), and characterized in that it further comprises at least one upper plate (P2, P3, P4), superimposed on said lower plate (P1), and rotatably mounted through 360° along the axis of vertical rotation (A), said at least one upper plate (P2, P3, P4) having a surface S2 less than the surface S1 of the lower plate (P1) and a maximum dimension at most equal to the diameter D1 of this lower plate (P1 ). System (1) according to claim 1, characterized in that it comprises a plurality of upper plates (P2, P3, P4) superimposed on each other, all of the upper plates (P2, P3, P4) being superimposed on the lower plate (P1), in that the surface S2 of the plate immediately above (P2) to the lower plate (P1) fits into that S1 of said lower plate, and in that the surfaces S3 and S4 of the upper plates (P3, P4) positioned above the plate (P2) immediately above the lower plate (P1) are decreasing according to their position in the superposition of the plates. System (1) according to one of the preceding claims, characterized in that the upper plates (P2, P3, P4) have a shape different from the circular shape and a largest dimension substantially equal to the largest dimension of the plate on which they are superimposed. System (1) according to one of the preceding claims, characterized in that the upper plates (P2, P3, P4) have an elliptical shape. System (1) according to one of the preceding claims, characterized in that the center of gravity of the plates (P1, P2, P3, P4) is on the axis of rotation (A). System (1) according to one of the preceding claims, characterized in that the plates (P1, P2, P3, P4) are perforated. System (1) according to one of the preceding claims, characterized in that the plates (P1, P2, P3, P4) have slots (F1, F2) and/or openings (O) System (1) according to one of the preceding claims, characterized in that a circular fuel inlet (3) is provided at the center of the stack of trays (P1, P2, P3, P4), the center of said inlet circular being positioned substantially on the axis (A) and in that a tube (8) connected to this inlet (3) allows the fuel to be admitted, vertically, through the plates (P1, P2, P3, P4) superimposed. System (1) according to one of the preceding claims, characterized in that it comprises two primary air inlets (5, 6) of two independent air circuits (C1, C2), a main air circuit ( C2) and a cooling air circuit (C1). System (1) according to one of the preceding claims, characterized in that coaxial cylindrical tubes (T1, T2, T3, T4) are fixed to the plates (P1, P2, P3, P4), a cylindrical tube being fixed to each plate, and in that the plates are driven in rotation by means of these tubes, which are themselves driven in rotation independently by drive means. Combustion or gasification plant fed with solid fuel comprising a grate system (1) according to one of the preceding claims.