WO1984004577A1

WO1984004577A1 - Method and boiler hearth for the desulphuration of fuel gas

Info

Publication number: WO1984004577A1
Application number: PCT/FR1984/000134
Authority: WO
Inventors: Poray Marcel Cyprien De; Roland Kissel
Original assignee: Elf France
Priority date: 1983-05-18
Filing date: 1984-05-17
Publication date: 1984-11-22
Also published as: FR2546272B1; EP0145735A1; FR2546272A1

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

Process and boiler furnace for desulfurizing combustible gases.

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 method 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 precombustion zone, the temperatures in the precombustion zone and in the combustion zone proper said being regulated so as to obtain a release of the volatile materials in the pre-combustion zone and a fixation of the sulfur on the ashy solid materials in the combustion zone proper and / or an improvement in the quality of the combustible gases as a result of reactions of dissociation of 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 CO ₂ and at suitable temperature, respectively with the air sent to the precombustion zone and with the 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 working in gasόgene or semigazogen, 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 prefoyer 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 pass the gases from the prefoyer through the bank slope and means for separately regulating the recycling rates of the burnt gases from the secondary hearth to the prefoyer and the primary hearth so as to obtain a release of volatile matter in the pre-concentrate and fix sulfur on the ashy solids present in the primary hearth and / or improve the quality of combustible gases as a result of dissociation reactions of volatile materials 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 700 ° C. 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 firebox 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 intense combustion zone of the solid materials, thus avoiding any direct and irregular evacuation of the volatile materials and of the water vapor. combustible. The sulfur contained in volatile matter binds to solid matter during combustion to combine with elements such as calcium, magnesium and iron contained in these ashy solid matter.

Controlled additions of a burnt gas (rich in CO ₂ ) at a suitable temperature sent separately to the two combustion zones mixed with air allow to easily regulate the optimum reaction temperatures of the two zones thanks to the endothermic reactions of dissociation of CO ₂ which avoid the fusion of the ashes. Thus, in the present invention, the desulphurization 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 explained below in more detail using drawings representing only embodiments.

In these drawings: FIG. 1 is a vertical section view of a primary hearth according to the present invention; Figs. 2 and 3 are views in vertical section 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 to the hearth 1, that is to say between the hopper 4 and hearth 1, the fuel passes into a hearth 5 separated from hearth 1 by a wall 6 having an L-shaped section plunging into hearth 1. The exit of hearth 5 is relatively wide so as to allow the formation of a large slope of landslide on the stepped grid 3. A de-ashtray 7 ensures the evacuation of the ash accumulating at the lower part 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 grid 3 by a conduit 11, while the gases used for the preliminary combustion of the reverse type are brought to the upper part of the prefoyer 5 by a conduit 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 inlet in the duct 12 is controlled by a valve 15, the burnt gases are brought to the ducts 11 and

12 respectively via conduits 16 and 17, the inlet of which is controlled by valves 18 and 19. The operation of the hearth 1 is as follows: The solid fuel undergoes a preliminary combustion of the reverse type in the preheater 5.

The temperature in the preheater 5 is adjusted using the valves 15 and 19 by adjusting the relative proportions of air and burnt gases from the secondary hearth containing CO ₂ . 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 700 ° C. to 800 ° C. taking into account the possible presence of steam. of water. The temperature in the primary hearth 1 is adjusted by means of the valve 18 by the rate of recycling of the fumes in the primary hearth 1. This temperature is advantageously maintained between 700 and 950 ° C., 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 solid matter during combustion. forming a landslide embankment in the primary hearth 1. During this path, the sulfur reacts with the solid materials during combustion and settles on 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 preheater 5 and a certain reforming of the recycled CO ₂ , which improves the subsequent combustion thanks to the presence of CO and H ₂ in a relatively homogeneous mixture.

In addition, this cracking improves the regularity of combustion.

To cover the landslide slope, the preheater 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 preheater.

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 shrinking 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 by a lateral duct 28. The hearth 31 shown in FIG. 3 is a hearth comprising a double tiered grate 32, supplied with gas for combustion by a duct 33 opening out 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 shrinking 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 disposed just under the base 38.

In the context of the present invention, it is also possible to promote the sulfur fixing reactions and the cracking, reforming or other reactions by the addition of desulphurisation 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 prefoyer and to the primary fireplace 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

1. A method for desulfurizing combustible gases from combustion in a fixed bed of solid fuel and / or improving their combustion, characterized in that reverse combustion of 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 proper 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 pre-combustion zone and in the combustion zone proper are regulated by the controlled mixture of burnt gas rich in CO ₂ at suitable temperature respectively with the air supplied in the pre-combustion zone and with the air sent to the combustion zone proper.

3. Method according to claim 2, characterized in that to regulate the temperatures, the recycled burnt gases are sampled at different points of 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 one of claims 1 to 4, characterized in that the precombustion zone is supplied 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) with a grate (3, 22, 32) on which the solid fuel forms a thick slope of slope, this hearth operating as gasifier or semi-gasifier, and a secondary hearth (8) for combustion of the 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) of solid fuel and the primary hearth (1, 21, 31) and supplied with a mixture of air and burnt gases from the secondary hearth (8), means for passing the gas from the prefoyer through the landslide so as to avoid direct evacuation volatile matter 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 pre-hearth (5, 24, 34) and a fixation of the sulfur on the ashy solids present in the primary hearth (1, 21, 31) 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 water vapor.