FR3030302A1 - THERMAL REDUCTION OF SULFUR - Google Patents

Abstract

Process and device for thermal reduction of sulfur dioxide present in a gas to be treated and in particular in industrial gaseous effluents, in which an oxygen-rich oxidant (6) and a gaseous fuel are injected into a reaction zone (201) of for generating an oxy-fuel flame (202) in the reaction zone (201) and in which the gas to be treated (5) is injected around the oxy-fuel flame (202), a reducing gas (7) being injected in the gas to be treated (5) in or upstream of the reaction zone (201).

Description

The present invention relates to the thermal reduction of sulfur dioxide and in particular the thermal reduction of sulfur dioxide present in industrial waste gases. Sulfur dioxide (SO2) is a pollutant that is very harmful to health and the environment. It is mainly generated by refineries during the desulphurisation of fuels and by the extraction mines of non-ferrous ores (nickel, copper, lead, etc.). For example, the extraction of one ton of Nickel emits 4 tons of SO2 and the extraction of copper produces 1 ton of 502. To treat these enormous volumes of S02, the thermal reduction is particularly interesting because it can be integrated in the SO2 emitter upstream process. In addition, at the end of the thermal reduction process, the sulfur is recovered mainly elemental form (Sx), and, in combination with a Claus-type process, almost all of the sulfur is recovered in elemental form. Thermal reduction is thus particularly interesting when SO2 sources do not have a sulfuric acid market nearby. The implementation of the thermal reduction requires the mixing of a reducing agent with the SO 2, and optionally with water vapor, at a high temperature.

The reactions which occur between 1000 ° C and 1500 ° C with, for example, methane as reducing agent, are: CH4 + 2SO2 = CO2 + 2H20 + 1 / x Sx (formation of elemental sulfur) 4CH4 + 4SO2 = 2CO + 4H2 + 2H25 + S2 + 2CO2 + 2H20 (formation of H25, elemental sulfur and syngas) The goal is to obtain a H25 / 502 flux in a 2: 1 ratio in order to recover a maximum of sulfur present in these two compound in solid form via a Claus type catalytic process.

Thermal reduction of SO2 is described in reference books such as Ullmann's Encylopedia of Industrial Chemistry. It is also present in the processes described in FR-A-2212290, US-A-4207304, WO-A-2012/177281 and WO-A-2013/190335.

The object of the present invention is to provide an improved method of thermal reduction of SO2 and means for its implementation. The present invention more particularly aims to allow a particularly effective thermal reduction of SO2 limiting the amount of soot and generated gaseous by-products, and this by means of a flexible process and easy to implement. To this end, the present invention provides a method of injecting reactants into a reaction zone in which the thermal reduction of SO2 occurs. Indeed, the realization of the mixture of reactants directly impacts the performance of the heat reduction process such as, in particular, the conversion rate of SO2 to H25 and the rate of production of soot (harmful pollutant for catalysts in the Claus downstream process) or finally the amount of by-products like COS, CO and H2 generated. According to the injection method according to the invention, the following reagents are injected into the reaction zone: a gaseous oxidant having an oxygen content of 50% vol to 100% vol; a gaseous fuel; a gas to be treated containing sulfur dioxide and a hydrogenated reducing gas. The oxidant and the fuel are injected into the reaction zone so as to generate, by combustion of the fuel with the oxidant, an oxy-fuel flame having a longitudinal axis. The gas to be treated is injected into the reaction zone around the oxy-fuel flame, and the reducing gas is injected into the gas to be treated inside or upstream of the reaction zone. In the first case, the hydrogenated reducing gas comes into contact with the gas to be treated in the reaction zone at or downstream of the injection point of the gas to be treated in the reaction zone.

In the latter case, the reducing gas comes into contact with the gas to be treated before the injection of the gas to be treated into the reaction zone. In the present context, the term "oxy-fuel flame" is understood to mean a flame generated by the combustion of a fuel with a gaseous (oxidizing) oxidant having an oxygen content of at least 50% vol. The reduction reaction between SO2 of the gas to be treated and the hydrogenated reducing gas is promoted by the heat generated by the oxy-fuel flame. The injection of the reducing gas into the gas to be treated ensures an intimate contact between the gas to be treated and the reducing gas, thus improving the efficiency of the thermal reduction of 502. According to one advantageous embodiment, at least one injection is injected. part or all of the oxidant around the fuel. Different injection orientations can be envisaged for the injection of the various reactants into the reaction zone: parallel to the longitudinal axis of the flame, convergent with respect to this axis or diverging with respect to this axis. It has been found, however, that the efficiency of the thermal reduction can be improved when at least one of the oxidant and the fuel and preferably both are injected into the reaction zone at a divergent angle to the reaction zone. longitudinal axis of the oxy-fuel flame, especially when fuel injection is surrounded by an oxidant injection. According to an alternative embodiment, the fuel is injected into the reaction zone around a first portion of the oxidant and a second portion of the oxidant is injected into the reaction zone around the fuel. In this case, the fuel and / or the second part of the oxidant and preferably both are advantageously injected into the reaction zone at a divergent angle with respect to the longitudinal axis of the flame. Other injection configurations for the fuel and the oxidant are, however, conceivable.

The injection speeds of the fuel gas and of oxygen are advantageously chosen between 30 and 60 m / s. In order to intensify the contact between the gaseous fuel and the oxidant, the fuel can be injected by means of a multitude of fuel injection openings. This multitude of fuel injection openings advantageously comprises a series of these injection openings positioned around the longitudinal axis of the oxy-fuel flame, in particular on a circular contour around the longitudinal axis and preferably concentric with the longitudinal axis. In this geometry and without central oxidant injection inside the fuel injections, it is preferable to add a central injection of combustible gas along the longitudinal axis to limit the soot deposition on the nose of the injection and thus to increase the service life. The efficiency of the thermal reduction can also be improved by the injection of the gas to be treated into the reaction zone with a rotational movement about the longitudinal axis of the oxy-fuel flame. Advantageously, the hydrogenated reducing gas is injected through a plurality of reducing gas injection openings which are disposed about the longitudinal axis of the oxy-fuel flame. Said reducing gas injection openings are preferably positioned axisymmetrically with respect to said longitudinal axis. In particular, the gas to be treated may be injected into the reaction zone through a passage, called the main passage, which ends in an annular injection opening concentric with the longitudinal axis of the oxy-fuel flame. In this case, said annular injection opening of the gas to be treated corresponds to the aforementioned point of injection of the gas to be treated. In this case, the reducing gas may more particularly be injected through a plurality of reducing gas injection lances, these lances being arranged in the above-mentioned main passage and ending in the main passageway so that the reducing gas comes into contact with the gas to be treated before its injection into the reaction zone through the annular injection opening of the main passage, or - In the annular injection opening of the main passage for the reducing gas to contact the gas to be treated during the injection of the gas to be treated into the reaction zone. The lances are advantageously arranged around the longitudinal axis of the oxy-fuel flame, preferably axisymmetrically. Advantageously, the injection speed of the gas to be treated will be chosen between 5 and 15 m / s, preferably 5 to 10 m / s while the injection speed of the reducing gas will be chosen between 10 and 50 m / s preferably between 20 and 40 m / s. The injection method according to the invention is generally part of a process for the recovery of sulfur from the sulfur dioxide-containing process gas. In this case, the gas to be treated is subjected to a partial reduction of SO 2 by the injection method according to any one of the embodiments described above. This gives a treated gas containing H25 and SO2. The treated gas is cooled downstream of the reaction zone, thereby recovering condensed sulfur on the one hand and a cooled gas containing H 2 O and SO 2 on the other hand. The treated gas can be cooled in a boiler, said recovery boiler, which captures and exploit a portion of the thermal energy of the treated gas from the reaction zone, for example by transforming it into the mechanical or electrical energy or to supply steam to other installations. It is also possible to exploit the captured thermal energy to heat one or more of the reagents before their injection into the reaction zone. The cooled gas is advantageously subjected to a desulfurization process in a Claus reactor. This makes it possible to recover more sulfur and a desulphurized and therefore less polluting gas. The injection method according to the invention has the important advantage that it makes it possible to regulate the H25 / 502 ratio of the treated gas simply by regulating the flow rate of the reducing gas injected into the reaction zone without impacting the fuel gas flow rate.

Thus, the release of heat from the flame to the gas to be treated is not affected by this regulation which allows greater flexibility in operation. In particular, when the treated gas is intended to be sent to a Claus reactor, the flow rate of the reducing gas is preferably adjusted so as to obtain in the treated gas a H2S / SO2 molar ratio between 1.9 and 2.1, a mol ratio of 2.0 being optimal for the reaction For a better regulation of said ratio, the H2S / SO2 ratio is advantageously determined in the treated gas (before cooling) or in the cooled gas. It is then possible to regulate the flow rate of the reducing gas injected into the reaction zone as a function of the ratio thus determined, preferably so that the molar ratio H2S / SO2 is between 1.9 and 2.1. The processes according to the invention make it possible to treat a large number of gases containing sulfur dioxide, in particular gaseous effluents resulting from industrial processes. The invention provides in particular a simple means for the partial thermal reduction of SO2 present in the gases containing SO2: - from refineries of petrochemical products, in particular during the desulfurization of petrochemical products such as fuels and other petrochemical products ; and - from melting furnaces (smelters) for non-ferrous ores, and in particular melting furnaces for nickel ores, copper, lead, etc. such as found near the mines, for example).

The oxygen content of the gaseous oxidant injected into the reaction zone is preferably greater than 80% vol, more preferably greater than 90% vol, or even greater than 95% vol. The gaseous fuel injected into the reaction zone is advantageously selected from methane, ethane, propane, hydrogen, a mixture of hydrogen and carbon monoxide and mixtures of at least two of these fuels. gaseous, such as natural gas. The hydrogen or the mixture of hydrogen and CO can in particular be derived from the gasification of biomass or waste.

The more hydrogen the fuel contains, the lower the soot formation. The presence of hydrogen in the gaseous fuel, in the form of H 2 or in chemically bonded form in the case of a hydrogenated gaseous fuel, is therefore generally desired.

Hydrogenated reducing gas comprises a gas capable of reacting with SO 2 at a temperature between 1000 ° C. and 1500 ° C. with the formation of elemental sulfur and H2S. The hydrogenated reducing gas is preferably selected from the gases listed above in connection with the gaseous fuel. The more the reducing gas contains hydrogen, the more effective the thermal reduction reaction will be. The H2 is therefore the most suitable reducing gas but generally also the most expensive. It is therefore wise and more environmentally friendly to use H2 as a reducing gas or a gas containing H2 from alternative sources such as gasification of waste or biomass. The reducing gas and the gaseous fuel may be identical and come from the same source. The reducing gas may also be different from the combustible gas. In this case, the reducing gas may comprise or not comprise a gas having the same composition as the fuel or vice versa: for example methane and natural gas. It may sometimes be advantageous to add water vapor (H2O) to the reaction zone to further limit soot formation if, for example, the type of fuel or reducing gas contains a large amount of carbon. This addition of water vapor can be done by mixing the steam with the reducing gas and / or by mixing it with the sulfur dioxide-containing gas to be treated. The invention has the advantage, however, that the formation of soot is particularly low, which is particularly important when the thermal reduction is followed by a Claus-type process, even without the addition of steam to the reducing gas and or the gas to be treated. The oxy-fuel flame advantageously has a richness (in English: "fuel-oxidant equivalence ratio") of from 0.5 to 2.0, preferably from 0.80 to 1.50 and even more preferably from 0.90 to 1, 10.

Wealth is defined as the ratio between, on the one hand, the actual ratio between the fuel flow and the oxidant flow, and on the other hand, the strictly stoichiometric ratio between the flow rate of this fuel and the flow rate of this oxidant. .

The adjustment of the richness is a setting parameter of the injection of the oxidant and the fuel on the release of heat by the oxy-fuel flame and thus on the performance of the thermal reduction reaction. According to a first embodiment, the oxy-fuel flame is stoichiometric (richness = 1). According to another embodiment, the oxy-fuel flame is low in oxygen (rich in fuel), that is to say with a richness greater than 1, preferably greater than 1 and less than or equal to 2. In this case, In this case, the combustion of the fuel in the oxy-fuel flame is a partial combustion and the unburnt thus formed, such as H2 present in the combustion gases generated by the oxy-fuel flame can act as an additional reducing gas for the thermal reduction of the fuel. S02. In yet another embodiment, the oxy-fuel flame is rich in oxygen (low in fuel), i.e., with a richness of less than 1, preferably greater than 0.5, and less than 1.0. In this case, the residual oxygen that has not taken part in the combustion due to the lack of fuel will burn the hydrogenated reducing gas to release more thermal energy in the heart of the reaction zone. The temperature of the gases in the reaction zone is typically from 1000 ° C to 1400 ° C, preferably from 1100 ° C to 1300 ° C. The injection method according to the invention fits perfectly in other steps to be part of a process for the recovery of elemental sulfur gas to be treated. The present invention therefore also covers a process for recovering sulfur from a sulfur dioxide containing process gas in which the gas to be treated is subjected to a partial thermal reduction of the SO 2 as described above. The gas resulting from the reaction zone thus obtained, having therefore undergone a partial thermal reduction of the SO 2, is hereinafter referred to as the "treated gas". The treated gas contains H25 and SO2. This treated gas is cooled to recover condensed sulfur and a cooled gas still containing H2S and SO2 of the uncooled treated gas. . The treated gas may more particularly be cooled in a heat recovery boiler located downstream of the reaction zone. This allows a thermal energy recovery of the treated gas in the form of steam, or in the form of mechanical or electrical energy. After separation of the elemental sulfur produced by the thermal reduction, the cooled gas is directed to a catalytic reactor of the Claus type, with recovery of sulfur and a desulphurized gas. The sulfur recovered in elemental form in the Claus process is derived from the residual sulfur present in the cooled gas. The gas recovered from the Claus process is essentially desulphurized and therefore significantly less polluting than the gas to be treated initially. The present invention more particularly allows the partial thermal reduction of the SO 2 in gases to be treated and in particular gaseous effluents: - from petrochemical refineries, in particular during the desulfurization of fuels or other petrochemical products, or - from melting furnaces for non-ferrous ores, and in particular melting furnaces for nickel, copper, lead ores, etc. such as found near mines, for example. Such a gaseous effluent may be subjected to a purification treatment, dedusting, etc. before being subjected to the process according to the invention for recovering elemental sulfur from said gases. The present invention also relates to an injection assembly adapted for carrying out the injection method according to the invention. The injection assembly includes a central burner, a peripheral injector and at least one additional injector. The central burner is adapted for the injection of an oxidant and a fuel into a reaction zone downstream of the injection assembly. It is capable of generating, by combustion of the fuel with the oxidant, a flame having a longitudinal axis. The central burner itself generally has a longitudinal axis called "burner" which coincides with the longitudinal axis of the flame. Unless otherwise indicated, reference to a "longitudinal axis" is a reference to the longitudinal axis of the flame. The peripheral injector of the injection assembly is suitable for injecting a gas to be treated containing sulfur dioxide into the reaction zone and this around the oxy-fuel flame generated by the central burner. The at least one additional injector of the injection assembly is adapted for injecting a hydrogenated reducing gas inside the peripheral injector, at an injection opening of the peripheral injector or still in the reaction zone. The at least one additional injector may open into the peripheral injector, at the or an injection opening of the peripheral injector and / or be positioned inside the peripheral injector so that the hydrogenated reducing gas mixes with the gas to be treated injected by the peripheral injector, in particular before or at the point of injection of the gas to be treated. The central burner advantageously comprises an oxidant injector and a fuel injector for injection into the reaction zone of the oxidant and the fuel, respectively.

In one embodiment, the oxidant injector surrounds the fuel injector. According to another embodiment, the central burner comprises a first oxidant injector surrounded by the fuel injector and a second oxidant injector surrounding the fuel injector.

Other configurations are however possible for the fuel injector (s) and the oxidizer nozzle (s) of the central burner. For the operation of the injection assembly, the oxidant injector or injectors are connected to an oxidant source and more particularly to a source of an oxidant having an oxygen content of 50% vol to 100% vol. the fuel injector (s) being connected to a source of a gaseous fuel. In the present context, two elements are "connected" when they are connected so as to allow the flow of a fluid from one of the two elements to the other of the two elements, for example by means of a pipe for the transporting said fluid. The oxidant and fuel injectors are preferably such that at least a portion of the oxidant and / or fuel is injected into the downstream reaction zone with a direction of injection diverging with respect to the longitudinal axis. Thus, when the central burner comprises a fuel injector that surrounds the oxidant injector, the fuel injector and / or the oxidant injector advantageously have an injection nozzle diverging with respect to the longitudinal axis in the directions of injection. When, on the other hand, the central burner comprises two oxidant injectors, one of which (the first) is surrounded by the fuel injector and the other (the second) surrounds the fuel injector, the injector fuel and the second oxidant injector are advantageously provided with such a diverging nozzle.

The central burner, and more particularly the fuel injector, preferably has a multitude of fuel injection openings. In this case, at least some of said fuel injection openings are advantageously positioned on a substantially circular contour around the longitudinal axis. In this geometry and without central injector of oxidant, it is preferable to add a central injector of combustible gas along the longitudinal axis to avoid the deposition of soot on the injector. The peripheral injector advantageously comprises a gyration device adapted for rotating around the longitudinal axis of the gas to be treated injected by the peripheral injector into the reaction zone. The turning device may in particular comprise rotating valves. The peripheral injector terminates in an injection opening, said injection opening being preferably an annular injection opening concentric with the longitudinal axis. In order to improve the efficiency of the thermal reduction of the effluent, the injection assembly advantageously comprises a multitude of additional injectors as described above.

The additional injectors of this mtiititucle thus open: inside the peripheral injector, and / or in the injection opening of the peripheral injector, that is to say in the same plane perpendicular to the longitudinal axis.

The additional injectors preferably open axisymmetrically about the longitudinal axis. For the use of the assembly according to the invention in a process for the thermal reduction of a gas to be treated containing sulfur dioxide: the central burner is connected to a source of an oxidant having an oxygen content of 50% vol at 100% vol and a source of a gaseous fuel; the peripheral injector is connected to a source of the gas to be treated containing sulfur dioxide; and the at least one additional injector is connected to a source of a hydrogenated reducing gas. As already indicated above with respect to the process according to the invention: the gas to be treated containing sulfur can be a gaseous effluent from a petrochemical refinery, a melting furnace for 20 non-ferrous ores; the gaseous fuel is advantageously chosen from methane, ethane, propane, hydrogen or mixtures of hydrogen and carbon monoxide, which may in particular be derived from biomass or waste, and the mixtures of at least two of these fuels such as natural gas; the oxidant, which has an oxygen content of at least 50% vol, preferably contains more than 80% vol of oxygen, more preferably more than 90% vol, or even more than 95% vol .; and the hydrogenated reducing gas is preferably selected from methane, ethane, propane, hydrogen or mixtures of hydrogen and carbon monoxide, and mixtures of at least two of these gases as per example natural gas.

The reducing gas and the gaseous fuel may be identical. The reducing gas may also be different from the combustible gas. In this case, the reducing gas may comprise or not comprise a gas of the same composition as the fuel or vice versa.

The injection assembly typically also includes controllers for regulating the flow rates of the different fluids to the injection assembly. When the reducing gas has the same composition as the gaseous fuel or comprises a gas having this composition (in combination with other components such as, for example, water vapor), the central burner (or even the fuel injector) this) and the at least one additional injector are advantageously connected to the same fuel source. The injection assembly is normally provided with a controller regulating the flow rates of fuel, oxidant, the gas to be treated and the hydrogenated reducing gas injected by the injection assembly. The controller therefore also regulates the richness of the flame (oxy-fuel flame) generated by the central burner. When the fuel and the reducing gas come from the same fuel source, the controller also regulates the ratio between: (i) the fuel flow to the central burner and (ii) the flow to the at least one additional injector fuel to be injected as the hydrogenated reducing gas, and (iii) the ratio of the two flows.

The present invention also relates to an installation for the recovery of sulfur from a gas called "gas to be treated" containing sulfur dioxide. The plant according to the invention comprises a sulfur dioxide thermal reduction chamber equipped with an assembly according to any one of the embodiments described above. Said thermal reduction chamber comprises an outlet for the evacuation of the treated gas, that is to say the gas which has been subjected to a thermal reduction of SO 2. The installation advantageously also comprises a cooling device for cooling the treated gas.

For this purpose the cooling device is connected to the treated gas outlet of the reduction chamber. The cooling device in turn comprises an outlet for evacuating the cooled gas from the cooling device and a sulfur outlet for discharging the condensed sulfur into the cooling system. The cooling device is advantageously a recovery boiler. The plant according to the invention advantageously also comprises an installation for further desulfurization of the cooled treated gas. Thus, the installation preferably comprises a Claus reactor 10 for the desulfurization of cooled gas from the cooling device and for the recovery of the sulfur thus obtained. For this purpose, the Claus reactor is connected to the cooled gas outlet of the cooling device. The plant according to any one of the embodiments described above conveniently comprises control equipment. Said regulating equipment includes a detection device for determining the ratio of H2S to SO2 in the treated gas at the treated gas outlet of the thermal reduction chamber or in the cooled gas at the cooled gas outlet of the device. cooling. The control equipment then advantageously also comprises a flow controller adapted to regulate the flow rates of the fluids supplied to the injection unit, the said flow rate controller preferably being able to regulate the flow of hydrogenated reducing gas supplied to the unit. injection assembly as a function of the ratio of H2S to SO2 determined by the above-mentioned detection device. The flow controller advantageously regulates the flow rates supplied to the injection assembly and in particular the flow rate of hydrogenated reducing gas supplied to the injection assembly so that the H2S / SO2 ratio detected is 1 , 9 to 2.1, preferably 2.0. The invention makes it possible to better control the reactions of the thermal reduction by uncoupling at least partially (a) the release of the heat necessary for the thermal reduction and (b) the mixing of the gas to be treated containing sulfur dioxide with the gas hydrogenated reductant. This also reduces the formation of soot. Indeed, the invention makes it possible to create a first heat-generating zone produced by an oxy-fuel flame (combustion zone), which can notably be close to stoichiometry (see above), and a second zone in which the gas to be treated containing SO2 is mixed with the products of combustion of the oxy-fuel flame and with the hydrogenated reducing gas and in which the SO 2 present in the gas to be treated reacts with the reducing gas with formation of elemental sulfur among others. It should be noted that the injection configuration of the method according to the invention and the structure of the injection assembly according to the invention have the advantage of allowing a particularly compact and easy to control system, for example with the together injections into the reaction zone on one side of a thermal reduction reactor. The present invention and its advantages will be better understood in the light of the example below, reference being made to FIGS. 1 and 2 in which: FIG. 1 is a diagrammatic representation in longitudinal section of a following injection assembly the invention. - Figure 2 is a schematic representation of the use of the injection assembly according to the invention in an installation for the recovery of sulfur from a gas to be treated.

The injection assembly 10 has a longitudinal axis X-X. It is composed in the center of an oxy-fuel central burner 20 for stoichiometric combustion (or non-stoichiometric, see above) of a gaseous fuel, such as natural gas with an oxygen-rich oxidant 50% vol) . The central burner comprises a first passage 21 located in the center of the burner and allowing the transport of a fuel and more particularly methane or natural gas to a first injection nozzle 22. This injection nozzle comprises a first opening of central injection 22a surrounded by a ring of injection openings 22b. The central burner also comprises a second passage 25 of annular section which surrounds the first passage 21 and which allows the transport of an oxidant rich in oxygen and more particularly oxygen of industrial purity (95% vol 02) to a second injection nozzle 26 which surrounds the first injection nozzle 22.

In the illustrated example, the injection direction of the first and second injection nozzles is parallel to the longitudinal axis X-X. It may, however, be preferable to use first and / or second nozzles 22, 26 such that the directions of injection of the fuel and oxidant jets injected by said nozzles 22, 26 into the reaction zone 1 are divergent relative to each other. to the axis XX (in the direction of injection of said jets). The gas to be treated containing SO2 is fed to the reaction zone 1 by a peripheral injector 30 terminating in a ring-shaped peripheral injection opening. The peripheral injector 30 comprises rotating fins 31 of the gas jet to be treated containing SO2 before it is injected into the reaction zone 1. Through this peripheral injector 30, additional injectors 40 of hydrogenated reducing gas are arranged axisymmetrically with respect to the longitudinal axis XX.

The whole is surrounded by a refractory block 50. Such a refractory block facilitates the integration of the injection assembly into one of the walls surrounding the reaction zone in a thermal reduction reactor. The swirling motion (often referred to as "swirl") given to the jet of the SO2-containing gas to be treated allows at least partly to dilute the combustion zone, i.e., the oxygen flame. fuel 2 with the gas to be treated containing SO2, thus limiting the formation of soot by lowering the temperature peaks. This desirable effect can be enhanced by injecting the gas to be treated with an injection speed whose component parallel to the longitudinal axis (called "longitudinal component") is less than or equal to the longitudinal component of the oxy-fuel flame speed. . In the illustrated case, an excess of the gaseous fuel injected by the central burner is injected as a hydrogenated reducing gas through the additional injectors 40. The injection direction of said reducing gas is parallel to the longitudinal axis XX and its speed of rotation. injection (and therefore the longitudinal component of this speed) is greater than the longitudinal component of the speed of the gas to be treated.

These two elements help to maximize the dilution of the reducing gas with the gas to be treated containing SO2 downstream and around the combustion zone, that is to say downstream and around the oxy-fuel flame 2 and thus limit the concentration of fuel in high temperature areas responsible for soot formation. As indicated above, the hydrogenated reducing gas corresponds to an excess of fuel. The mixture between the gas to be treated containing SO 2 and the hydrogenated reducing gas not being upstream, in or by means of the central burner, the present invention allows a great flexibility of operation on the ratio between, on the one hand, the S02 present in the gas to be treated and secondly, the reducing gas and in particular the hydrogen present in the reducing gas while ensuring the necessary heat input for SO2 thermal reduction reactions and this even for variable flow rates of gas to be treated and / or for variable SO2 contents of the gas to be treated. The S02 / hydrogen ratio drives the H2S / SO2 ratio at the outlet of the reaction zone. As illustrated in FIG. 2, an injection assembly 210 according to the invention can be installed in a thermal reduction reactor 200 having inside a reaction zone 201.

The injection assembly 210 and supplied with gas to be 5 containing SO 2, oxidant rich in oxygen 6 and natural gas 7. A portion of the natural gas supplied to the injection assembly 210 is the fuel supplied to the burner central to the injection assembly 210 to generate the oxy-fuel flame 202 and another portion of the natural gas is supplied to the injection assembly 210 as a hydrogenated reducing gas. As described above, it is also possible in some cases to supply the injection assembly with water vapor, particularly when the nature of the gas to be treated and / or the fuel is likely to promote the formation of water. soot.

The injection assembly 210, and more particularly its central burner, is operated so as to maintain the reaction zone 201 at a temperature between 1300 ° C and 1500 ° C suitable for the thermal reduction of the SO 2.

The treated gas 220 from the reactor 200, which contains SO2 and H2S, is directed to an energy recovery boiler 250 in which thermal energy is recovered from the treated gas 220, the gases are cooled and the elemental sulfur Sx obtained by the thermal reduction in the reactor 200 is recovered. Downstream of the energy recovery boiler 250, the cooled gas 230 is directed to a catalytic reactor of the Claus 260 type for the recovery of elemental sulfur Sx by reaction of the SO2 with H2S present in the cooled treated gas 230 in order to obtain a final gas 240, downstream of the Claus reactor 260 which is essentially free of sulfur. Thanks to the present invention, the production of soot in the thermal reduction reactor 200 is very limited, which greatly prolongs the longevity of the catalyst in the Claus 260 reactor and limits the consumption of natural gas and oxygen per unit of gas to be treated. (Better efficiency due to the absence of steam acting as a heat sink) The method and the injection assembly according to the invention were tested in an installation as illustrated in Figure 2 under different conditions. It has been found that the invention permits the thermal reduction of the SO 2 present in a gas to be treated with a good constant quality of the recovered sulfur and essentially without the production of soot, even without the addition of water vapor to the reducing gas. The sulfur recovery rate was high and the COS & CS2 content in the gas leaving the reaction zone was low (COS <4`) / 0 & 2 CS2 <2% by volume).

Claims (3)

REVENDICATIONS1. Injection method for thermal reduction of sulfur dioxide in a reaction zone (1, 201), wherein a gaseous oxidant (6) having an oxygen content of 50% vol to 100 vol% is injected, a gaseous fuel (7), a gas to be treated (5) containing sulfur dioxide and a hydrogenated reducing gas (7) and wherein: - the oxidant (6) and the fuel (7) are injected into the reaction zone (1) , 201) so as to generate an oxy-fuel flame (2, 202) having a longitudinal axis XX, - the gas to be treated (5) is injected into the reaction zone (1, 201) around the oxy-fuel flame (2, 202), and the reducing gas (7) is injected into the gas to be treated (5) in or upstream of the reaction zone (1, 201). 2. The method of claim 1, wherein at least a portion of the oxidant (6) is injected around the fuel (7). 20 A process as claimed in any one of the preceding claims, wherein the fuel (7) is injected by means of a plurality of fuel injection openings (22a, 22b), at least some of said fuel injection openings wherein the fuel (22b) is advantageously positioned on a circular contour about the longitudinal axis XX, preferably also having a fuel injection aperture (22a) in the center of the circular contour. 4. Process according to one of the preceding claims, in which the gas to be treated (5) is injected into the reaction zone (1, 201) with a rotational movement about the longitudinal axis XX of the oxy-fuel flame (2). , 202) .6. A process according to claim 5, wherein the reducing gas (7) is injected through a plurality of reducing gas injection lances (40) disposed in the passage of the gas to be treated (30), said lances (40) being terminating within the passage of the gas to be treated (30) and / or in the annular injection opening, said lances (40) preferably being axially symmetrical about the longitudinal axis XX of the oxy-fuel flame (2). 7. A process according to any one of the preceding claims wherein the gas to be treated (5) is a gas containing sulfur dioxide from a petrochemical refinery or a non-ferrous ore smelter. . 8. Injection assembly comprising: - a central burner (20) for the injection of an oxidant (6) and a fuel (7) into a reaction zone (1, 201), said burner (20) ) being capable of generating a flame having a longitudinal axis XX by the combustion of the fuel with the oxidant, - a peripheral injector (30) adapted for injecting a gas to be treated (5) containing sulfur dioxide into the reaction zone (1, 201) around the flame (2, 202), and - at least one additional injector (40) opening into or located in the peripheral injector (30) and adapted for the injection of a hydrogenated reducing gas (7) within the peripheral injector (30) or in the reaction zone (1, 201) downstream of the peripheral injector (30). 5. Method according to one of the preceding claims, wherein the gas to be treated (5) is injected into the reaction zone (1, 201) through a passage of the gas to be treated (30) ending in an opening of concentric annular injection with the longitudinal axis XX of the oxy-fuel flame (2) .9. An assembly according to claim 8, wherein the central burner (20) comprises an oxidant injector (25) and a fuel injector (21), the oxidant injector (25) preferably surrounding the fuel injector ( 21). 10. The assembly of claim 8, wherein the central burner (20) comprises a first oxidant injector, a fuel injector surrounding the first oxidant injector and a second oxidant injector surrounding the fuel injector. 11. An assembly according to one of claims 8 to 10, wherein the peripheral injector (30) comprises a gyration device (31) adapted for rotating about the longitudinal axis XX in the reaction zone (1). 201) of the gas to be treated (5) injected by said peripheral injector (30). 12. An assembly according to one of claims 8 to 11, comprising a plurality of additional injectors (40) opening (i) inside the peripheral injector (30) and / or (ii) with the peripheral injector. (30) in the same plane perpendicular to the longitudinal axis XX, said additional injectors (40) preferably axially symmetrical about the longitudinal axis XX. An assembly according to any one of claims 8 to 12 wherein the central burner (20) is connected to a source of an oxidant (6) having an oxygen content of 50% vol to 100% vol. and a source of a gaseous fuel; - the peripheral injector (30) is connected to a source of a gas A treat (5) containing sulfur dioxide; and the at least one additional injector (40) is connected to a source of a hydrogenated reducing gas (7). Apparatus for recovering sulfur from a sulfur dioxide containing process gas, comprising: - a sulfur dioxide thermal reduction chamber (200) having an assembly (210) according to any one of claims 8 at 13 and including a treated gas outlet (220), and - a cooling device (250) for cooling the treated gas (220) connected to the treated gas outlet (220) of the reduction chamber (200), said cooling device (250) having a cooled gas outlet (230) and a sulfur outlet for discharging condensed sulfur into the cooling plant (250). 15. Plant according to claim 14, comprising a Claus reactor (260) for the desulfurization of cooled gas (230) from the cooling device (250) and for the recovery of the sulfur thus obtained, said Claus reactor (260) being connected to the cooled gas outlet (230) of said cooling device (250).