FR2555710A1 - Method for burning or gasifying solid fuels in a fluidised bed, and device for the implementation of this method - Google Patents

Abstract

Method for burning or gasifying solid fuels in a fluidised bed, consisting in making, on a rectangular grid 6, a fluidised bed L having, seen from the top, an elongate shape, and in transverse cross-section a trapezoidal shape widened towards the top, and in providing in the bed L a longitudinal systematic circulation of the fuel and of the solid residues of the combustion or of the gasification, from its front end where the bed L is supplied with fuel, to its rear end where the said residues are discharged. In order to achieve as economic an operation as possible, the depth of the bed L increases from front to rear or is essentially constant, so that the product h.t is kept essentially constant or decreases slightly, h representing the depth of the bed L and t its carbon content at the point in question.

Description

 The present invention relates to the combustion or gasification in a fluidized bed of solid fuels.

 In the field of combustion, the use of a fluidized bed hearth with ascending mechanical grate, of the type operating with agglomeration of ash, known under the name of IGNIFLUID, has so far given satisfaction.

In reality the geometry of the bed, with decreasing depth, has some drawbacks. For a given bed height, therefore a given pressure drop, and a given surface grid, the overall useful volume of the bed is smaller than if the grid had a lower slope. Furthermore, the variable depth requires the use of multiple blowing boxes provided with registers generating sterile pressure drops and a complication of regulation. In addition, it is in the zone where the depth is the smallest and the lowest carbon concentration1 that the air flow tends to be greatest, while the opposite would be preferable. Finally, the rear zone shallow depth leads to a high local temperature and sometimes annoying agglomerates
We thought of replacing the movable grid by a fixed grid similarly inclined and possibly provided with tilting bars. Such embodiments have been described in French patent applications NO 83/16290 and 83/18009 filed respectively on October 13 and November 14, 1983 in the name of the applicant company.

 The above drawbacks have not all been eliminated, and additional drawbacks have appeared.

 In the case of a grid with tilting bars, the latter have the difficult task of bringing the slope back up to the agglomerates of clinkers formed.

 In gasification, the rear zone produces an oxidizing gas which burns part of the gas produced in front. The gas obtained is thereby significantly degraded.

 The main object of the present invention is to eliminate the above-mentioned drawbacks.

 More specifically, it relates to a process for the combustion or gasification in a fluidized bed of solid fuels, consisting in creating on a rectangular grid a fluidized bed having, seen in plan, an elongated shape, and in cross section, a trapezoidal shape flared upwards, and to ensure in the bed a methodical longitudinal circulation of the fuel and solid residues from combustion or gasification, from its front end where the bed is supplied with fuel, to its rear end where said residues are discharged, characterized in that the depth of the bed increases from front to rear or is substantially constant, so that the product ht is kept substantially constant or decreases slightly, h denoting the depth of the bed and t its content in carbon at the point considered.

 It also relates to a device for implementing the method, comprising at least one blowing box covered by a fixed rectangular grid forming a distributor of the blowing gas, characterized in that said grid is inclined downward from its front end where the bed is supplied with fuel, up to its rear end - where the residues from combustion or gasification are discharged.

 According to a variant, the grid can be substantially horizontal.

 The grid is bordered by a fuel slope on all four sides.

 The grid consists of fixed bars or tilting bars capable of moving the solid residues from combustion or gasification longitudinally.

 According to a first case, where there is no agglomeration of the ashes, there is provided a tube for evacuating the latter, which receives them by overflowing above the rear slope.

 According to a second case, where there is agglomeration of the ashes, there is provision, under the rear slope, for a periodic evacuation hatch of agglomerated clinkers, whether this agglomeration is accidental or systematic.

 The evacuation of the agglomerates can be ensured continuously by a selective valve having a venturi neck, the divergent of which opens into the fluidized bed in the vicinity of the rear slope, a continuous injection of pressurized gas upstream of the convergent, making it possible to discharge the fine particles and allowing the fall of agglomerates.

 When there is no agglomeration of the ashes, a blowing nozzle is provided to prevent direct and untimely projections of unburnt fuel in the ash discharge tube.

The invention will be better understood by referring to the description which follows, given with reference to the appended drawings, concerning several embodiments given by way of nonlimiting examples
Figure 1 is a longitudinal sectional view along line BB of Figure 2, the device according to a first form of the invention.

 Figure 2 is a cross-sectional view along line A-A of Figure 1.

 Figure 3 is a longitudinal sectional view of the device according to a second form of the invention.

 Figure 4 is a longitudinal sectional view, along line D-D of Figure 5, of the device according to a third form of the invention.

 Figure 5 is a cross-sectional view along line C-C of Figure 4.

 Figure 6 is a longitudinal sectional view of the device according to a fourth form of the invention.

 FIG. 7 is a view in cross section along the line E-E of FIG. 6.

 Figures 1 and 2Jon shows the device provided for cases where there is no agglomeration and where the temperature is such, 8000C for example, that we can avoid the formation, even accidental, of everything agglomerate.

 In these figures, the reference 1 designates the front wall of the fireplace, and the references 2 and 3 its side walls. The wall 1 is provided with one or more fuel supply chutes 4. The reference 5 designates the rear wall.

 The bottom of the chamber C delimited by said walls is constituted by a fixed rectangular grid 6 inclined downwards from the front towards the rear. Said grid is bordered on its four sides with flat surfaces 7, 8, 9 and 10 on which fuel slopes rest. The reference 7 'designates the slope formed against the front wall 1, the references 8' and 9 'respectively designate the slopes formed against the side walls, while the reference lOt designates the rear slope formed at a certain distance from the wall 5.

 Below the grid 6 are arranged two blowing boxes 11 and 12, each of which can be divided into two transversely twinned compartments, such as 12 'and 12 ", each having their supply pipe 13' and 13" in blowing gas.

 The reference L designates the fluidized bed from which the ashes are evacuated by overflowing above the slope 10 'in a tube 14 of rectangular section arranged between the rear wall 5 and the slope 10' to which it serves as a support. The bed L has, seen in plan, an elongated shape, and in cross section, a trapezoidal shape flared upwards as clearly appears in FIG. 2. A hatch 15 is provided at the base of the slope 10 'in order to be able to drain the bed L. if necessary. Below each box, such as 12, there is a hopper 16 provided with an airlock 17 for the evacuation of particles (arches) which could have passed through the grid 6.

 A U-shaped cooling tube 26 runs along the three slopes 7t, 8 'and 9' at their base. At least one nozzle 27, arranged through the rear wall 5, is provided to prevent untimely direct projections of unburnt fuel, into the tube 14. This nozzle is supplied with a gas which may be air in the case of combustion, or recycled gas in the case of gaZification.

 In Figure 3 there is shown the device provided for the case where there is not normally agglomeration of ash, but where agglomerates can form accidentally.

 In this figure, as in the following, the same references have been adopted as in Figures 1 and 2 to designate identical or equivalent elements. The description will be limited to essentially different elements.

 The grid 6 here consists of tilting bars, such as 18, arranged parallel to each other with a small interval. These bars are advantageously in even number for each box II and 12, and the bars of the same parity are coupled together, so as to lift alternately all of the bars 18 of the same parity by a conventional control means, such as that described in the aforementioned application 83/18009.

 The ashes are removed in the same way as in the previous case. On the other hand, the hatch 15 is capable of periodically removing the agglomerates liable to form accidentally, said agglomerates being transported by the grid 6 which moves them longitudinally.

 Figures 4 and 5 show the device provided for the case where there is permanent agglomeration.

 The grid 6 transports, thanks to these tilting bars 18, from the front to the rear of the grid, the solid residues from combustion or gasification, essentially agglomerates which are periodically removed by the hatch 15. The tube 14 here has no reason to be, so that the slope lO 'bears directly against the rear wall 5.

 In Figures 6 and 7, there is shown the device provided for a use case similar to that of Figures 4 and 5, but with continuous evacuation of agglomerates.

 For this purpose) there is provided a selective valve, known per se, referenced 19 as a whole. This valve comprises a venturi the neck of which is marked 20. The diverging portion 21 of said venturi opens into the fluidized bed L in the vicinity of the rear slope 10 'between said slope and the grid 6. The marking 22 designates the convergent of the venturi preceded by a tube cylindrical 23. On the latter is connected a tube 24 supplied with pressurized gas. Under the effect of the injection of gas under pressure through the venturi 20, 21, 22 the fine particles are forced back into the fluidized bed L, while the agglomerates fall through the tube 23 into an ashtray 25.

 Whatever the scenario envisaged, circulation in bed L is always methodical. This methodicity is ensured by giving the fluidized bed L a significant minimum length relative to the aq> litude of the longitudinal turbulence, and by providing for the supply of fuel at only one end of the bed, and the extraction of the residues at l 'other end. Because of this method, the residues discharged at the end of the grid 6 with tilting bars 18 contain only a very low carbon content.

 The grid 6 is inclined so that the product h x t of the height of the bed at any point by the carbon content t is kept substantially constant.

 In the case where only one blowing box is provided, if the product h.t is rigorously constant, the highest hydrostatic resistance at the rear leads to a lower natural gas flow. This has two drawbacks; First the gas flow would be variable from one, vertical to the other with a constant product h.t, therefore a gas flow not proportional to h.t. Then the fluidization speed would be heterogeneous between the different verticals. This can be offset, at least for a given load, by the choice of a height h such that h.t is not substantially constant, but slightly decreasing. It may thus be necessary to provide a substantially horizontal grid.

 Although the invention has been described with reference to several embodiments, it goes without saying that it is in no way limited to them, and that modifications may be made thereto without departing from its field.

 The blowing gas can be air, enriched air, or oxygen with water vapor.

 We can, of course, replace any of the means described by a technically equivalent means.

 The invention therefore covers, in addition to the examples described, their different possible variants, within the limits defined by the claims.

Claims (10)

REVENDICA ~ IONS 1. Method for the combustion or gasification in a fluidized bed of solid fuels, consisting in creating on a rectangular grid (6), a fluidized bed (L) having, seen in plan, an elongated shape, and in cross section, a shape trapezoidal flared upwards, and to ensure in the bed (L) a methodical longitudinal circulation of the fuel and solid residues of combustion or gasification, from its front end where the bed (L) is supplied with fuel, up to at its rear end where said residues are discharged, characterized in that the depth of the bed (L) increases from the front towards the rear or is substantially constant, so that the product ht is kept substantially constant or decreases slightly, h denoting the depth of the bed (L) and t its carbon content at the point considered. 2. Device for implementing the method according to claim 1, comprising at least one blowing box (11, 12) covered by a fixed rectangular grid (6) forming a distributor of the blowing ga, characterized in that said grid is inclined downwards, from its front end where the bed (L) is supplied with fuel, to its rear end where the residues of combustion or gasification are evacuated. 3. Device for implementing the method according to claim 1, comprising a blowing box (11 or 12) covered by a fixed rectangular grid (6) forming a blowing gas distributor, characterized in that said grid is substantially horizontal , from its axis end where the bed (L) is supplied with fuel, to its rear end where the residues of combustion or gasification are discharged. 4. Device according to one of claims 2 or 3, charac terized in that the grid (6) is bordered by a slope of combus tible (7 ', 8', 9 ', 10') on its four sides. 5. Device according to one of claims 2 to 4, characterized in that the grid (6) consists of fixed bars. 6. Device according to one of claims 2 to 4, characterized in that the grid (6) consists of tilting bars (18) capable of longitudinally moving the solid residues from combustion or gasification. 7. Device according to one of claims 5 or 6, characterized in that there is provided a powder ash evacuation tube (14) which receives the latter by overflowing above the rear slope (10 '). 8. Device according to claim 6, characterized in that it is provided under the rear slope (10 '), a hatch (15) for periodic evacuation of clinkers which form accidentally or systematically. 9. Device according to claim 6, characterized in that the evacuation of the agglomerates is ensured continuously by a selective valve (19) having a venturi neck (20) of which the diverging portion (21) opens into the fluidized bed (L) in the vicinity of the rear slope (-10 '), a continuous injection of pressurized gas upstream of the convergent (22-) allowing the pulverulent particles to be forced back into the bed (L) and allowing the agglomerates to fall. 10. Device according to claim 7, characterized in that there is provided a blowing nozzle (27) to avoid direct and untimely projections of unburnt fuel into the tube (14) for discharging the ash.