FR2546272A1 - PROCESS FOR DESULFURIZING FUEL GASES FROM THE COMBUSTION OF A SOLID FUEL AND IMPROVING THEIR COMBUSTION, AND FIXED BED BOILER FIREPLACES INCLUDING APPLICATION - Google Patents

Abstract

Method wherein a fixed combustion bed is used to desulphurate and improve combustion gas resulting from the combustion of solid fuel containing sulphur and/or with a high content of volatile materials or water. The invention relates more particularly to a primary hearth for a staged combustion boiler comprising a pre-hearth (5) with reversed combustion arranged between the solid fuel supply system (4) and the primary hearth (1) and supplied with a mixture of air and burnt gas provided from the secondary hearth (8), means for circulating the gas provided from the pre-hearth through the talus of the primary hearth so as to avoid a direct evacuation of volatile materials and means for adjusting the recycling ratios of burnt gas from the secondary hearth towards the pre-hearth and the primary hearth so as to obtain a liberation of volatile materials in the pre-hearth (5) and a fixation of sulphur to solid ashy materials present in the primary hearth (1) and/or an improvement of the quality of fuel gas following reactions of dissociation of volatile materials an/or water vapour.

Description

 The present invention relates to a process for desulfurizing gases from the combustion in a fixed bed of a sulfur-containing solid fuel or improving their combustion and to boiler stoves comprising application of this process.

 Desulphurization of combustion gases has become a major problem. Various methods have been proposed for this purpose. Some processes use the reactions of alkaline products and milk of lime, which leads to relatively large and expensive installations. To reduce operating costs, thought has been given to using the alkaline elements of fly ash from spray or fluidized bed fireplaces.

 The present invention aims to provide a particularly economical solution to the problem of desulfurization of combustion gases, in particular in the context of the use of stoves of staged combustion boilers, such as those described in patents FR-A-895,629 and US-A-2,554,092, FR-A-1,020,525 and French patent application 82/08 603, where it not only avoids the rejection of sulfur fumes, but also improves secondary combustion.

 The subject of the present invention is a process for desulfurizing combustible gases originating from fixed bed combustion of a solid fuel and / or improving their combustion, characterized in that reverse combustion of the solid fuel is carried out in a precombustion zone. and the gases obtained in this zone are passed through a combustion zone proper of solid fuel, adjacent to the pre-combustion zone, the temperatures in the pre-combustion zone and in the combustion zone itself being adjusted so as to obtain a release of volatile matter in the pre-combustion zone and a fixation of the sulfur on the ashy solid matter in the combustion zone proper and / or an improvement in the quality of the combustible gases as a result of dissociation reactions of the volatile matter and / or water vapor.

 The temperatures in the precombustion zone and in the combustion zone proper can be easily adjusted by the controlled mixture of burnt gas rich in CO2 and at suitable temperature, respectively with the air sent to the precombustion zone and with l air sent to the combustion zone proper.

 The temperature of the recycled burnt gases can be regulated by sampling these gases at different points in their circuit before their evacuation to the atmosphere.

 The present invention also relates to a primary hearth for sulfur-containing solid fuel, and / or with a high volatile matter or water content, in a boiler of the type comprising a solid fuel supply system, a primary hearth with grate on which the solid fuel forms a thick slope of slope, this hearth functioning in gasogenic or semigazogenic, and a secondary hearth of gas combustion coming from the primary hearth, whose burnt gases (smoke for example) are partially recycled in the combustion air of the primary hearth, characterized in that it comprises a reverse combustion pre-hearth disposed between the solid fuel supply system and the primary hearth and supplied with a mixture of air and burnt gases coming from the secondary hearth, means for passing gases from the pre-hearth through the landslide and means for separately adjusting the recycling rates of the burnt gases from the secondary hearth to the pre hearth and the primary hearth so as to obtain a release of the volatile matters in the hearth and a fixing of the sulfur on the ashy solid matlers present in the primary hearth and / or an improvement of the quality of the combustible gases as a result of reactions of dissociation of the volatile matter and / or water vapor.

 In the present invention, by "reverse" precombustion or "reverse" combustion, is meant a preliminary combustion in which the combustion air progresses in the same direction as the solid fuel (whereas, generally, the combustion takes place against current or cross).

 In practice, this preliminary combustion can generally be carried out at a temperature below 7000C. At this temperature, the evolution of volatile matter occurs. These volatiles contain a large part of the sulfur present in solid fuels.

 In conventional fireplaces, these volatile materials mix with the fumes, burn (moreover very badly) if an excess of air is present and leave the fireplace, thus polluting the environment.

 In the present invention, on the contrary, all the gases which come from the preliminary combustion are forced through a more coughed combustion zone of the solid materials, thus avoiding any direct and irregular evacuation of the volatile materials and of the water vapor from the fuel. . The sulfur contained in volatile matter binds to solid matter during combustion to combine with elements such as calcium, maynesium and iron contained in these ashy solid matter.

 The controlled additions of a burnt gas (rich in CC2> at a suitable temperature sent separately to the two combustion zones in admixture with the air make it possible to easily regulate the optimum reaction temperatures of the two zones thanks to the endothermic CO2 dissociation reactions which avoid the fusion of ashes.

 Thus, in the present invention, the desulfurization is carried out on the gases before their actual combustion and therefore avoids an expensive treatment of the fumes while improving the quality of this combustion.

 The invention is set out below in more detail with the aid of drawings representing only embodiments.

On these drawings
Fig. 1 is an arc in vertical section of a primary hearth according to the present invention;
Figs. 2 and 3 are vertical section views showing variants of the present invention.

 The primary focus 1 shown in FIG. 1 comprises an enclosure 2 provided with a refractory lining. Inside this enclosure is arranged a simple stepped grid 3. This grid 3 is supplied with granular solid fuel by gravity from a hopper 4. At the entrance of the hearth 1, that is to say between the hopper 4 and the hearth 1, the fuel passes into a hearth 5 separated from the hearth 1 by a wall 6 having an L-shaped section dipping into the hearth 1. The exit of the hearth 5 is relatively wide so as to allow the formation of a large slope of landslide on the stepped grid 3.

 A descender-7 ensures the evacuation of the ash accumulating at the bottom of the hearth 1.

 The primary hearth 1 supplies combustible gas to a secondary hearth burner 8 via a duct 9. The secondary hearth is supplied with air by a duct 10.

 The gases used for the combustion of solid fuel are brought under the grate 3 by a duct 11, while the gases used for the preliminary combustion of the reverse type are brought to the upper part of the pre-hearth 5 by a duct 12.

 These two types of gas are obtained by a controlled mixture of air taken off through a pipe 13 and burnt gases (smoke) from the secondary hearth brought in through a pipe 14.

 The air entry into the duct 12 is controlled by a valve 15.

 The burnt gases are brought to the conduits 11 and 12 respectively by means of conduits 16 and 17, the inlet of which is controlled by valves 18 and 19.

The operation of hearth 1 is as follows:
The solid fuel undergoes a reverse type preliminary combustion in the prefoyer 5.

The temperature in the pre-hearth 5 is adjusted using the valves 15 and 19 by adjusting the relative proportions of air and burnt gases of the secondary hearth containing CO2. This adjustment is advantageously carried out so as to obtain only a release of volatile matter in the preheater, that is to say that the temperature is maintained below 7000C to 8000C taking into account the possible presence of water vapor.
The temperature in the primary hearth 1 is regulated by the valve 18 by the rate of recycling of the fumes in the primary hearth 1. This temperature is advantageously maintained between 700 and 9500C, which avoids the melting of the ashes.

 In addition, the overall flow rate of gas sent through line 12 is adjusted so as to have a sufficiently strong current of gas in the preheater so that it has a sufficient path inside the layer of solids during combustion. forming a landslide slope in the primary focus 1.

 During this path, the sulfur reacts with the solid materials in the course of combustion and binds to these materials containing powdery ash.

In addition, during this path, and in the presence of the water vapor of the fuel, there occurs a cracking of the volatile materials in the pre-hearth 5 and a certain reforming of the recycled CO 2, which improves the subsequent combustion thanks to the presence of CO and
H2 in relatively homogeneous mixture.

 In addition, this cracking improves the regularity of combustion.

 To cover the landslide slope, the pre-hearth 5 could be separated from the primary hearth 1 and fed independently with a different fuel, in particular in order to allow easier combustion in the pre-hearth.

 The foci shown in Figs. 2 and 3 are variants.

 The focus 21 shown in FIG. 2 is a circular hearth comprising a rotating grate 22, supplied with gas for combustion by a conduit 23 opening out under the rotating grate 22.

 At the upper part of the hearth 21 is arranged a preheater 24. This preheater is separated from the hearth 21 by a narrowing 25. it is supplied with solid fuel by a hopper 26 disposed at the upper part and with gas used for reverse combustion by a conduit 27 opening out in the vicinity of its upper part.

 The combustible gases resulting from the combustion in the hearth 21 are evacuated through a lateral duct 28.

 The focal point 31 shown in FIG. 3 is a hearth comprising a double tiered grate 32, supplied with gas for combustion by a conduit 33 opening under the grate 32. At the upper part of the hearth 31 is arranged a preheater 34. This preheater is separated from the hearth 32 by a narrowing 35.

 The prefoyer comprises, at its upper part, a stepped grid 36 under which is injected by a conduit 37 the gas used for reverse combustion.

The grid 36 rests on a base 38 integral with the narrowing 35.

 The prefoyer 34 is supplied with solid fuel by a hopper 39 disposed at its upper part.

 The combustible gases resulting from combustion in the hearth 31 are evacuated through a conduit 40 placed just under the base 38.

 In the context of the present invention, it is also possible to promote the sulfur fixation reactions and the cracking, reforming or other reactions by the addition of desulphurization products or volatile matter conversion catalysts and / or water vapor such as cracking or reforming catalysts. These additives can be added separately to the reverse combustion pre-hearth and to the primary hearth proper. they can be added in particular to the different fuel that can be used to pre-fuel it.

 The present invention finds a particularly advantageous application in the combustion of lignite which generally contains high sulfur contents, as well as with fuels with a high content of volatile matter and water.

Claims (7)

 1. A method for desulfurizing combustible gases from fixed-bed combustion of a solid fuel and / or improving their combustion, characterized in that reverse combustion of the solid fuel is carried out in a pre-combustion zone and the gases obtained in this zone through an actual combustion zone of solid fuel, adjacent to the precombustion zone, the temperatures in the precombustion zone and in the combustion zone itself being adjusted so as to obtain a release of volatile matter in the precombustion zone and a fixation of the sulfur on the ashy solids in the combustion zone proper and / or an improvement in the quality of the combustible gases as a result of reactions of dissociation of the volatile materials and / or of the vapor of water.  2. Method according to claim 1, characterized in that the optimum temperatures in the precombustion zone and in the combustion zone proper are regulated by the controlled mixture of burnt gas rich in CO2 at suitable temperature respectively with the air sent into the pre-combustion zone and with the air sent to the combustion zone proper.  3. Method according to claim 2 t characterized in that to regulate the temperatures, the recycled burnt gases are sampled at different points in their circuit before their evacuation to the atmosphere.  4. Method according to any one of the preceding claims, characterized in that additives chosen from desulphurization products and catalysts for the conversion of volatile materials and / or water vapor are added to the solid fuel.  5. Method according to any of claims 1 to 4, characterized in that the precombustion zone is fed separately with a different fuel.  6. Method according to claims 4 and 5, characterized in that the additives are added to the fuel of the pre-combustion zone.  7. Primary fireplace with fixed bed for solid sulfur fuel and / or with a high volatile matter or water content in a boiler of the type comprising a system (4,26,39) for supplying solid fuel, a primary fireplace (1 , 21,31) grate (3,22,32) on which the solid fuel forms a thick slope of slope, this hearth working in gasogenic or semi-gasogenic, and a secondary hearth (8) of combustion of gases coming from the primary hearth (1,21,31), the burnt gases of which are partially recycled into the combustion air of the primary hearth, characterized in that it comprises a reverse combustion preheater (5,24,34) disposed between the system supply (4,26,39) with solid fuel and the primary hearth (1,21,31) and supplied with a mixture of air and burnt gases coming from the secondary hearth (8), means for passing the gas from the prefoyer through the landslide so as to avoid direct evacuation of volatile matter and means for separately regulate the recycling rates of the burnt gases from the secondary hearth to the pre-hearth and the primary hearth so as to obtain a release of volatile matter in the pre-hearth (5,24,34) and a fixation of the sulfur on the ashy solid matter present in the primary hearth (1,21,31) and / or an improvement in the quality of combustible gases as a result of reactions of dissociation of volatile matter and / or water vapor.