Abstract:
Sulfur vapor is formed by partial oxidation of hydrogen sulphide. A burner is operated so as to establish a flame in a furnace in or into which the burner fires. There is supplied to the flame from the first region of the mouth of the burner at least one flow of a first combustible gas comprising hydrogen sulfide. At least one second flow of a first oxidizing gas is caused to issue from the mouth of the burner and mix in the flame with the first combustible gas. There is supplied to the flame from a second region of the mouth of the burner surrounding and spaced from the said first region at least one third flow of a second combustible gas comprising hydrogen sulfide. At least one fourth flow of a second oxidizing gas is caused to issue from a region or regions of the mouth of the burner surrounded by said second region and mix in the flame with the second combustible gas. At least one fifth, outermost flow of a third oxidizing gas is caused to mix in the flame with the second combustible gas. A resultant gas mixture including sulfur vapor, water vapor, sulfur dioxide, hydrogen and residual hydrogen sulfide is withdrawn from the furnace.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the partial oxidation (partial combustion) of hydrogen sulphide and in particular to a method of and apparatus for forming sulphur vapour by partial oxidation of hydrogen sulphide. 
     Hydrogen sulphide containing gas streams (sometimes referred to as “acid gas streams”) are typically formed in oil refineries and natural gas processing units. Such streams cannot be vented directly to the atmosphere because hydrogen sulphide is poisonous. A conventional method of treating a hydrogen sulphide containing gas stream (which, if desired, has been pre-concentrated) is by the Claus process. In this process a part of the hydrogen sulphide content of the gas stream is subjected to combustion in a furnace so as to form sulphur dioxide. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide so as to form sulphur vapour. Thus, the hydrogen sulphide is effectively partially oxidised. The reaction between hydrogen sulphide and sulphur dioxide does not go to completion. The effluent gas stream from the furnace is cooled and sulphur is extracted, typically by condensation, from the cooled effluent gas stream. The resulting gas stream, still containing residual hydrogen sulphide and sulphur dioxide, passes through a train of stages in which catalysed reaction between the residual hydrogen sulphide and the sulphur dioxide takes place. Resulting sulphur vapor is extracted downstream of each stage. The effluent gas from the most downstream of the sulphur extractions may be incinerated or subjected to further treatment, e.g. by the SCOT or Beavon process, in order to form a gas stream which can be vented safely to the atmosphere. 
     Most Claus plants are equipped with right cylindrical furnaces having a length to internal diameter ratio in the range of from 2:1 to 4:1. The furnaces may be cross-fired or tangentially-fired by a burner or burners mounted at the side. Cross or tangentially fired burners achieve good mixing of the reacting chemical species. If desired, mixing can be enhanced by providing the furnace with baffles or checkerwork walls. 
     Air may be used to support the combustion of hydrogen sulphide in the initial part of the process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which is, of course, present in the air that supports the combustion) flow through the process and therefore place a ceiling on the rate at which the gas stream containing hydrogen sulphide can be treated in a furnace of given size. This ceiling can be raised by using commercially pure oxygen or oxygen-enriched air to support the combustion of the hydrogen sulphide. 
     If commercially pure oxygen or oxygen-enriched air having a mole fraction of oxygen above 0.65 is used to support the combustion of the hydrogen sulphide there is a relatively high risk of damage to the refractory lining of the furnace being created by the resulting increase in flame temperature depending on the composition of the Claus feed gas. There are a number of proposals in the art to solve this problem. Some proposals involve introduction of flame moderators such as water into the furnace; others involve recycling to the furnace gas from a downstream part of the plant so as to moderate the temperature in the furnace; and yet others employ a plurality of furnaces so as to limit the amount of combustion that is performed in each individual furnace, thereby obviating the need for an external flame moderator or to recycle gas from a downstream part of the plant. All these proposals, however, add to the complexity of the plant. 
     Axially or longitudinally fired burners mounted on the back wall may be used instead of cross or tangentially fired burners in Claus furnaces. Such axially or longitudinally fired burners can be designed to provide average residence times comparable with those of cross—or tangentially—fired burners at a specified throughput, and may be preferred at higher levels of oxygen-enrichment. 
     The use of such an axially or longitudinally fired burner is disclosed in European patent application 0 315 225 A, in which there is a central pipe for oxygen, at least one second pipe for hydrogen sulphide containing feed gas which coaxially surrounds the central pipe, and an external coaxial pipe for air. The burner is used when the hydrogen sulphide feed gas contains at least 5% by volume of carbon dioxide or hydrocarbons. The oxygen velocity at the outlet of the burner is in the range of from 50 to 250 metres per second (typically 150 metres per second) and the corresponding feed gas velocity is in the range of 10 to 30 metres per second. Temperatures in the range of 2000 to 3000° C. are generated in the core of the burner flame, and a gas mixture having a temperature in the range of 1350 to 1650° C. leaves the furnace. This gas mixture contains at least 2% by volume of carbon monoxide and at least 8% by volume of hydrogen. 
     During normal operation of, for example, an oil refinery the rate at which hydrogen sulphide containing gas streams are produced for treatment by the Claus process is not constant and can vary quite widely. It is therefore desirable that the furnace be capable of effective operation over a wide range of different rates of inflow of the hydrogen sulphide containing gas. 
     WO-A-96/26157 also discloses the use of an axially or longitudinally fired burner in the Claus process. Generally parallel flows of a first gas containing hydrogen sulphide and a second gas enriched in oxygen are supplied to the tip (i.e. mouth) of the burner. The ratio of the velocity of the first gas to the velocity of the second gas is selected so as to be in the range of 0.8:1 to 1.2:1. 
     Neither EP-A-0 315 225 A nor WO-A-96/26157 discusses the problem of how to handle a wide range of different rates of inflow of the hydrogen sulphide containing gas. In fact, neither discloses a method which is capable of effective operation if the rate of supply of the feed gas containing hydrogen sulphide varies considerably. 
     The method and apparatus according to the invention have it as aim to address this problem and to provide a solution superior to any possible when the disclosure of EP-A-0315 255 or WO-A-96/26157 is followed. 
     SUMMARY OF THE INVENTION 
     It is the primary object of the present invention to provide a method of forming sulphur by the partial oxidation of hydrogen sulphide. 
     According to the present invention there is provided a method of forming sulphur vapor by partial oxidation of hydrogen sulphide, comprising operating a burner so as to establish a flame having at least three stages in a furnace in or into which the burner fires, supplying to the flame from a first region of the mouth of the burner at least one flow of a first combustible gas comprising hydrogen sulphide, causing at least one second flow of a first oxidizing gas to issue from the mouth of the burner and mix in the flame with the first combustible gas, supplying to the flame from a second region of the mouth of the burner surrounding and spaced from said first region at least one third flow of a second combustible gas comprising hydrogen sulphide, causing at least one fourth flow of a second oxidizing gas to issue from a region or regions of the mouth of the burner surrounded by said second region and mix in the flame with the second combustible gas, causing at least one fifth, outermost, flow of a third oxidizing gas to mix in the flame with the second combustible gas, and withdrawing from the furnace a resultant gas mixture including sulphur vapor, water vapor, sulphur dioxide, hydrogen and residual hydrogen sulphide. 
     The invention also provides apparatus for forming sulphur vapor by partial oxidation of hydrogen sulphide, comprising a furnace, a port in the furnace, a burner having its mouth located in the port and operable to establish a flame having at least three stages in the furnace, and an outlet from the furnace for a resultant gas mixture including sulphur vapor, water vapor, sulphur dioxide, and residual hydrogen sulphide to exit the furnace; wherein the mouth of the burner has a first outlet or group of outlets for supplying to the flame at least one first flow of a first combustible gas comprising hydrogen sulphide, a second outlet or group of outlets for causing at least one second flow of a first oxidizing gas to issue from the burner and mix in the flame with the first combustible gas, a third outlet or group of outlets, surrounding and spaced apart from the first outlet or group of outlets, for supplying to the flame at least one third flow of a second combustible gas comprising hydrogen sulphide, and a fourth outlet or group of outlets surrounded by said third outlet or group of outlets for causing at least one fourth flow of a second oxidizing gas to issue from the burner and mix in the flame with the second combustible gas; wherein a passage or passages are defined between the burner and the port, or extend through or terminate in the mouth of the burner, and are able to cause an outermost fifth flow of a third oxidizing gas to mix in the flame with the second combustible gas. 
     Burning the hydrogen sulphide in three stages, namely an innermost stage, an outermost stage, and an intermediate stage, makes it possible to handle effectively a wider range of different rates of inflow of the hydrogen sulphide containing gas than if one or two such stages are employed. Other advantages accrue from such staging of the combustion. In particular, a relatively low temperature can be maintained in the outermost stage even though a temperature in excess of 2000° C. may be created in innermost stage, and therefore risk of damage to any refractory lining of the furnace can be kept to acceptable levels. A high temperature, that is a temperature well in excess of 2000° C., is particularly advantageous because it facilitates destruction of any ammonia in the first combustible gas and creation of conditions which increase the proportion of the resulting sulphur vapor that is formed directly by thermal cracking of hydrogen sulphide rather than by the indirect route involving oxidation of some hydrogen sulphide or sulphur to sulphur dioxide and then reaction of the thus formed sulphur dioxide with residual hydrogen sulphide. Destruction of ammonia is desirable because this gas tends to affect adversely downstream processing of the effluent from the furnace in catalytic reactors in which hydrogen sulphide and sulphur dioxide react together to form further sulphur vapor, the ammonia acting to block the catalyst by formation of ammonium salts. Moreover, the ammonia can be destroyed in the flame without resort to a combustion zone and a reaction zone separate from one another with some of the amine gas by-passed directly to the reaction zone. By avoiding the need to by-pass amine gas around the combustion zone even when using air for combustion, the ability of the burner to handle effectively a wide range of different flow rates of feed gases is enhanced due to a more effective use of its turndown range. In addition, forming sulphur vapor by thermal cracking reduces the rate at which oxygen-enriched air needs to be supplied to the burner to achieve a given recovery of sulphur vapor from the furnace, provided the furnace gases are cooled effectively downstream of the furnace. 
     Preferably the flame extends generally longitudinally within the furnace. The furnace is typically disposed with its longitudinal axis horizontal, and therefore the burner is typically also disposed with its longitudinal axis horizontal. Such arrangements can help to keep down the risk of damage to any refractory lining employed in the furnace. 
     The first and second oxidizing gases preferably have a mole fraction of at least 0.22 and may be oxygen-enriched air or pure oxygen. The third oxidizing gas is preferably atmospheric air neither enriched in nor depleted of oxygen, although enrichment up to 25% by volume of oxygen, or higher depending on the composition of the feed, is generally acceptable. 
     Preferably the mass flow rate of the first combustible gas and the mass flow rate of the second combustible gas to the burner are controlled independently of one another. Such an arrangement facilitates operation of the burner to handle variations in the total rate at which it is desired to feed combustible gas to the burner. The apparatus according to the invention therefore preferably additionally includes a first flow control valve in a first pipeline for supplying the first combustible gas to the burner, and a second flow control valve in a second pipeline for supplying the second combustible gas to the burner, the first and second control valves being operable independently of one another. 
     In a typical refinery there is more than one source of combustible gas comprising hydrogen sulphide. The sources typically have different compositions. Preferably the first combustible gas is of a different composition from the second combustible gas. By this means it is possible to optimise combustion of the combustible gas. Typically, both the first and second combustible gas streams contain at least 40% by volume of combustibles and at least 20% by volume of hydrogen sulphide. 
     If there are two separate sources of combustible gas comprising hydrogen sulphide, one containing ammonia, the other not, then all the ammonia containing gas is preferably employed in forming the first combustible gas. As a result it becomes possible to direct all the ammonia to a relatively inner region of the flame where a relatively high flame temperature can be maintained in order to destroy all the ammonia. For example, if one source of gas containing hydrogen sulphide is so-called “sour water stripper gas”, which typically contains about 20 to 35% by volume of hydrogen sulphide and 30 to 45% by volume of ammonia, and another source of gas containing hydrogen sulphide is so-called “amine gas” which typically contains over 80% by volume of hydrogen sulphide, the first combustible gas may comprise a mixture of some of the amine gas but all of the sour water stripper gas, and the second combustible gas may comprise the remaining amine gas. Preferably, the composition of the mixture is varied with the total rate of flow of combustible gas comprising hydrogen sulphide to the flame, with the proportion of amine gas in the first combustible gas being increased if the said total rate of flow is reduced below a chosen value. 
     Preferably the mass flow rate of the first oxidizing gas and the mass flow rate of the second oxidizing gas to the burner are controlled independently of one another. Such an arrangement facilitates operation of the burner to handle variations in the total rate at which it is desired to feed combustible gas to the burner and to cater for changes in the individual mass flow rates of the first and second combustible gas streams. The apparatus according to the invention preferably additionally includes a third flow control valve in a third pipeline for supplying the first oxidizing gas to the burner, and a fourth flow control valve in a fourth pipeline for supplying the second oxidizing gas to the burner. The third and fourth control valves are operable independently of one another. 
     The first and second oxidizing gases may be taken from the same or different sources of oxidizing gas. If different sources are employed, the first oxidizing gas may have a different composition from the second oxidizing gas. Employing first and second oxidizing gases of different composition adds to the flexibility of the method and apparatus according to the invention in effectively handling variable rates of supply of combustible gas. 
     The mole fraction of oxygen in both the first and second oxidizing gas is typically in the range of 0.3 to 1.0 depending on the proportion of combustibles in the first and second combustible gases. Care should be taken to avoid creating an excessive temperature at any location in any refractory employed to line the furnace. Modern commercially available refractories can typically withstand temperatures up to 1650° C. The oxygen-enriched air or pure oxygen of which either or both of the first and second oxidizing gases may be composed may be taken directly from an air separation plant. Depending on the purity of the oxygen product of the air separation plant, either or both of the first and second oxidizing gases may have a mole fraction of oxygen greater than 0.99. In general, however, particularly when handling sour water stripper gas, or amine gas, or mixtures of the two, it is preferred to form either or both of the first oxidizing gas and the second oxidizing gas by mixing an oxygen product of the air separation plant with atmospheric air, that is air which is neither enriched in nor depleted of oxygen. Forming either or both of the first and second oxidizing gases in this way makes it possible to vary the mole fraction of oxygen during operation of the method and apparatus according to the invention. Again, this ability to vary the mole fraction of oxygen adds to the flexibility of the method and apparatus according to the invention in effectively handling varying rates of supply of combustible gas. 
     Mixing of the first combustible gas with the first oxidizing gas is preferably facilitated by directing at least some of the first oxidizing gas along a path or paths which meet a path or paths followed by the first combustible gas. Accordingly, the second outlet or at least some of the second group of outlets each have an axis which extends at an angle to the axis of the first outlet or the axes of at least some of the second group of outlets. The angle is preferably in the range of 10 to 30°. Preferably, the flow of the first combustible gas is axial and the flow of the first oxidizing gas is at an angle to the axis of the burner. 
     Alternatively, mixing of the first combustible gas with the first oxidizing gas can be facilitated by directing at least some of the first oxidizing gas at a first linear velocity along a path or paths extending generally contiguous and generally parallel to a path or paths followed by the first combustible gas at a second linear velocity, and one of the first and second linear velocities is from 25 to 150% (and preferably from 25 to 100%) greater than the other thereof. Mixing is facilitated because the differential velocity between the first oxidizing gas and the first combustible gas creates forces of shear therebetween. Preferably, it is the first linear velocity which is selected to be the greater of the two said linear velocities. This arrangement facilitates design of the furnace to ensure that all the ammonia is destroyed in it. 
     A further alternative for facilitating mixing of the first combustible gas with the first oxidizing gas is to impart a swirling motion to one or both of the first oxidizing gas and the first combustible gas. Devices that are able to impart swirl to the gas are well known. 
     The natural curvature of the flame tends to facilitate mixing of the said fourth flow of second oxidizing gas flow with the said third flow of the second combustible gas. Nevertheless, it is preferred to arrange the supply of the said third and fourth flows so as to facilitate mixing. For example, at least some of the second oxidizing gas may flow at an angle to the second combustible gas such that the flow paths intersect, or at least bring the two gases closer together. Preferably, the second combustible gas leaves the mouth of the burner essentially axially, and at least some of the second oxidizing gas leaves the mouth of the burner at an angle to the axis. The angle is typically in the range of 10 to 30° to the axis of the burner. In another arrangement the third and fourth flows leave the burner alongside one another and at different velocities such that shear therebetween aids mixing. 
     If the total flow of the combustible gases becomes less than the maximum for which the burner is designed, then a reduction may be made in the flow of the first and second oxidizing gases and in the mole fraction of oxygen in both oxidizing gases. Further, in order to maintain a high temperature at the core of the flame when the burner is turned down, the proportional reduction in the rate at which the second combustible gas is supplied to the burner may be greater than that in the flow rate of the first combustible gas. In addition, if the first combustible gas is a mixture of sour water stripper gas and amine gas, the proportion of amine gas in the mixture may be changed. For this purpose, the first pipeline may communicate with a source of sour water stripper gas and the second pipeline with a source of amine gas, there being a conduit placing the second pipeline in communication with the first pipeline, and a further flow control valve in the further conduit for controlling flow of amine gas into the first pipeline. 
     In one preferred burner according to the invention there is a first tube defining a first passageway for the first combustible gas which terminates in the first outlet and within which extend a plurality of second tubes defining second passageways for the first oxidizing gas which each terminate in a respective second outlet, and a third tube surrounding and generally coaxial with the first tube, and defining therewith an annular third passageway for the second combustible gas which terminates in the third outlet and a plurality of fourth tubes for the second oxidizing gas which each extend within the third passageway and which define fourth passageways each terminating in a respective fourth outlet. Such an arrangement permits adjustment of the flow of the first oxidizing gas independently of the flow of the second oxidizing gas, and vice versa, and also adjustment of the flow of the first combustible gas independently of the flow of the second combustible gas, and vice versa. 
     Alternative preferred arrangements are possible. In one such arrangement, the first and second tubes are provided and disposed as described above. In addition, there is a third tube concentric with and surrounding the first tube to define therewith an annular third passageway for flow of the second oxidizing gas, the third passageway terminating in a nozzle in which the fourth outlets are formed, and a fourth tube concentric with and surrounding the third tube to define therewith an annular fourth passageway for the second combustible gas terminating in the said third outlet. Such an arrangement also permits adjustment of the flow of the first oxidizing gas independently of the flow of the second oxidizing gas, and vice versa, and also adjustment of the flow of the first combustible gas independently of the flow of the second combustible gas, and vice versa. 
     In yet another preferred arrangement, there are four concentric radially spaced apart tubes defining a central tubular passageway, and innermost intermediate, and outermost annular passageways. The central tubular passageway ends in the first outlet and the outermost annular passageway in the fourth outlet. The other two passageways both end in respective nozzles, the nozzles defining the second and third groups of outlets. This arrangement is another which permits adjustment of the flow of the first oxidizing gas independently of the second oxidizing gas, and vice versa, and also adjustment of the flow of the first combustible gas independently of the second combustible gas and vice versa. 
     In less preferred burners there may be three concentric tubes defining a central tubular passageway and inner and outer passageways. The central tubular passageway ends in the first outlet and the outer annular passageway in the third outlet. The inner annular passageway terminates in a nozzle in which both the second and fourth group of outlets are defined. Such an arrangement does not permit the flow rate and composition of the first oxidizing gas to be adjusted independently of the flow rate and composition of the second oxidizing gas. 
     The resultant gas mixture is preferably cooled in a waste heat boiler, and the cooled effluent gas stream is preferably passed to a condenser in which sulphur vapor is condensed therefrom. The resultant gas stream is preferably subjected downstream of the sulphur condenser to at least one stage of catalytic reaction between hydrogen sulphide and sulphur dioxide so as to enable further sulphur to be extracted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The method and apparatus according to the present invention will now be described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic sectional side elevation of a first burner for use in the method and apparatus according to the invention; 
     FIG. 2 is a schematic end view of the mouth of the burner shown in FIG. 1; 
     FIG. 3 is a schematic sectional side elevation of a second burner for use in the method and apparatus according to the invention; 
     FIG. 4 is a schematic end view of the burner shown in FIG. 3; 
     FIG. 5 is a schematic sectional side elevation of a third burner for use in the method and apparatus according to the invention; 
     FIG. 6 is a schematic end view of the mouth of the burner shown in FIG. 5; 
     FIG. 7 is a schematic sectional side elevation of a fourth burner for use in the method and apparatus according to the invention; 
     FIG. 8 is a schematic end view of the mouth of the burner shown in FIG. 7; 
     FIG. 9 is a schematic sectional side elevation of the fifth burner for use in the method and apparatus according to the invention; 
     FIG. 10 is a schematic end view of the mouth of the burner shown in FIG. 9; 
     FIG. 11 is a schematic flow diagram illustrating apparatus for supplying combustible gas and oxidizing gas to the burner in FIGS. 1 and 2; 
     FIG. 12 is a schematic flow diagram of a Claus plant for treating an acid gas including hydrogen sulphide which may use any of the burners shown in FIGS. 1 and 2, FIGS. 3 and 4, FIGS. 5 and 6, FIGS. 7 and 8, and FIGS.  9  and  10 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2 of the drawings, a burner  2  is of generally cylindrical shape and has a proximal end  4  and a distal end (or mouth  6 ). The burner  2  has a central passageway  8  defined by an innermost tube  10  for flow of a first combustible gas stream comprising hydrogen sulphide. The longitudinal axis of the burner  2  is coincident with the longitudinal axis of the tube  10 . The central passageway  8  has a first outlet  12  at its distal end. A second tube  14  is coaxial with the first tube  10 . The inner surface of the tube  14  makes a frictional engagement with the outer surface of the tube  10 . (Alternatively the tubes  10  and  14  may be joined to one another by means of an internal flange or welded connection.) A third tube  16  is spaced from and is coaxial with the tube  14 . Tubes  14  and  16  define a second, annular, passageway  18  for a second flow of combustible gas mixture comprising hydrogen sulphide terminating at its distal end in an outlet  20  (hereinbefore termed “the third outlet”) for the second combustible gas mixture. The tubes  14  and  16  terminate in the same plane as the tube  10 . 
     An array of tubes  22  extends from beyond the proximal end of the tube  10  therethrough and defines passages  24  for the flow of a first oxidizing gas mixture. Each passageway  24  has an outlet  26 . (These outlets are termed “the second group of outlets” hereinabove.) The tubes  22  terminate in the same plane as the tube  10 . The outlets  26  are typically disposed in a ring which is coaxial with the longitudinal axis of the burner  2 . 
     A second array of tubes  28  is disposed in the passageway  18  defined by the tubes  14  and  16 . Each tube  28  each defines a passageway  30  for a second flow of oxidizing gas terminating in respective outlet  32 . The tubes  28  each terminate in the same plane as the tube  10 . The outlets  30  of the tubes  28  are arranged in a ring which is coaxial with the longitudinal axis of the burner  2 . The respective tubes  22  and  28  may each be provided with a spider  34  to help support them when the burner is disposed with its longitudinal axis horizontal as shown in FIG.  1 . There is considerable flexibility in selecting the actual numbers of the tubes  22  and  28 . 
     The construction of the burner  2  so as to enable to the respective flows of combustible gas and oxidizing gas mixtures to be supplied to it is relatively simple. The outer tube  16  is provided with a first port  36  for the flow of the second combustible gas mixture comprising hydrogen sulphide. The proximal end of the outer tube  16  is formed with a flange  38  integral therewith or welded thereto. The flange  38  is bolted or otherwise secured to a similar flange  40  which is integral with or welded to the tube  14 . If desired, a gasket or other sealing member (not shown) can be engaged between the flanges  40  and  38  so as to ensure a fluid-tight seal therebetween. The flange  40  forms the distal end of a chamber  42  which receives the second oxidizing gas and which has a port  44  enabling it to be placed in communication with a source of such oxidizing gas. The proximal ends of the tubes  28  are all received fluid-tight in complementary apertures through the flange  40 . Thus, the tubes  26  communicate with the chamber  42 . The chamber  42  has an outer wall  46 , in which the port  44  is formed, which is provided at its distal end with a flange  48  which is fastened fluid-tight to the flange  40  and at its proximal end with a flange  50 . The flange  50  is bolted or otherwise secured fluid-tight to a complementary flange  52  which is integral with or is welded to the proximal end of the tube  10 . The flange  52  forms a proximal wall of the chamber  42 . It also forms a distal wall of a further chamber  54  having a side wall  56  with a port  58  formed therein which enables the chamber  54  to be placed in communication with a source of the first combustible gas. The wall  56  of the chamber  54  has a first flange  60  at its distal end which is bolted or otherwise secured fluid-tight to the flange  52  and a second flange  62  at its proximal end which is bolted or otherwise secured fluid-tight to an end plate  64  which forms a dividing wall between the chamber  54  and a yet further chamber  66  for the first oxidizing gas mixture and which receives fluid-tight in apertures formed therethrough the proximal ends of the tubes  22  so as to enable these tubes to receive a flow of the first oxidizing gas mixture. The chamber  66  is provided with a port  68  which is coaxial with the longitudinal axis of the burner  2  and is able to be placed in communication with the source of the first oxidizing gas mixture. 
     As shown in FIG. 1, the distal end  6  of the burner  2  extends into a port or quarl  70  of a furnace  72  for the partial combustion of hydrogen sulphide. An annular passage is defined between the distal end of the burner  2  and the port  70 . Air is supplied to this passage  74  as a third oxidizing gas. 
     If desired, the distal end of the outer tube  16  may be formed of a refractory metal. Other parts of the burner  2  may be formed of stainless steel. 
     In operation, the first combustible gas exiting the burner  2  from the outlet  12  becomes intimately mixed with the first oxidizing gas that leaves through the outlets  26  to form a first stage of a flame. Similarly, the flow of second combustible gas mixture leaving the burner  2  through the outlet  20  becomes intimately mixed with the flow of the second oxidizing gas which leaves the burner  2  through the outlets  32 , thus forming a second stage of the flame. A third stage of the flame is formed by intimate mixing of the air passing through the passage  74  with the second combustible gas mixture leaving the burner  2  through the outlet  20 . 
     Arrangement for supplying different gas flows to the burner  2  is shown in FIG.  11 . Referring to FIG. 11, a first pipeline  80  for sour water stripper gas (which includes both hydrogen sulphide and ammonia) terminates in the port  58  of the burner  2 . The first flow control valve  82  is disposed in the pipeline  80 . A second pipeline  84  for amine gas (which predominantly comprises hydrogen sulphide) terminates in the port  36  of the burner  2  and has a second flow control valve  86  disposed therein. A third pipeline  88  communicating with a source (not shown) of first oxidizing gas composed of air or oxygen-enriched air terminates in the port  68  of the burner  2 . A third flow control valve  90  is located the third pipeline  88 . A fourth pipeline  92  communicating with a source (not shown) of second oxidizing gas composed of air or oxygen-enriched air terminates in the port  44  of the burner  2 . A fourth flow control valve  94  is located in the fourth pipeline  92 . A fifth pipeline  96  communicating with a blower (not shown) or other source of compressed air (neither enriched in nor depleted of oxygen) terminates in an inlet  104  to a nozzle  106  which communicates with the annular passage  74  defined between the ports  70  and the burner  2 . The pipeline  96  has a fifth flow control valve  98  disposed therein. In addition, a pipeline  100  extends through a region of the second pipeline  84  upstream of the second flow control valve  86  to a region of the first pipeline  80  downstream of the first flow control valve  82 . A sixth flow control valve  102  is disposed in the pipe  100 . 
     In operation, the flow control valves described above may be set to determine the overall mole ratio of combustibles to oxygen supplied to the flame of the burner  2 , so as to enable different local ratios of the reacting species to be created in different regions of the flame, so as to enable a hot innermost region to be maintained in the flame at a temperature in excess of 1400° C., so as to enable a much lower temperature to be maintained at the periphery of the flame, to create within a localised region of the flame conditions which favour thermal dissociation of hydrogen sulphide, and to ensure that all ammonia is destroyed. Typically, the rates of supply of the reactants are controlled such that the mole ratio of hydrogen sulphide to sulphur dioxide and the gas mixture leaving the furnace is approximately 2:1. Within the respective regions of the flame, however, the mole ratio of hydrogen sulphide to sulphur dioxide can vary significantly. 
     The kind of flame that is formed in operation of the burner is shown schematically in FIG.  11  and is indicated therein by the reference numeral  110 . The flame has three stages  112 ,  114  and  116 . The innermost stage  112  is a high intensity zone into which the first oxidizing gas and the first combustible gas flow. In an example in which the first combustible gas is composed of a mixture of sour water stripper gas and amine gas, the first oxidizing gas is supplied at a rate that is sufficient to ensure the complete destruction of ammonia and any hydrocarbons in the first combustible gas and oxidation of more than one third of the hydrogen sulphide. A high temperature in the innermost stage  112  is thus ensured. The temperature can be controlled by the control valves  82 ,  90  and  102 . 
     The second intermediate stage  114  of the flame  110  receives the second oxidizing gas and part of the second combustible gas. This stage  114  is typically operated oxygen-poor that is to say that the relative rates of supply of hydrogen sulphide and oxygen molecules to this stage are such that less than one third of this hydrogen sulphide is oxidised to sulphur dioxide. The paucity of oxygen in this region together with the heat radiated from the inner stage  106  favour formation of sulphur vapor by thermal cracking of hydrogen sulphide. Since the thermal cracking of hydrogen sulphide proceeds endothermically, it provides a mechanism for moderating flame temperature and helps to prevent excessive temperatures being created in the outermost stage  116 . Further, it can reduce the demand for nitrogen molecules to moderate the flame temperature, and thereby enables the first and second oxidizing gases to have higher mole fractions of oxygen than would otherwise be possible. The temperature can be controlled in the stage  108  by the rate of flow of the first combustible gas and by its mole fraction of oxygen. The third outermost stage  116  of the flame receives the rest of the second combustible gas and the air which is supplied as the third oxidizing gas to the pipeline  96 . The rate of supply of air is controlled so as to ensure that an excessive flame temperature is not created in the stage on  116 . The total supply of oxidant is controlled such that the desired ratio of H 2 S to SO 2  is maintained after the waste heat revilers. 
     When the combined rates of supply of the first and second combustible gas are at a specified maximum, typically the rates of supply of the oxidizing gases are each at a maximum. If the total rate of supply of combustible gas falls, various control strategies are available to maintain suitable combustion conditions in the flame. Most simply, the control valves  94  and  98  may be reset to make a complementary reduction in the flow rate of the oxidizing gases. In addition, means (not shown) may be provided in the apparatus shown in FIG. 11 for adjusting the mole fraction of oxygen in the first and second oxidizing gases. For example, additional flow control valves (not shown) may be provided in pipes (not shown) which introduce commercially pure oxygen into the third and fourth pipelines  88  and  92 , respectively. Thus, the mole fraction of the oxygen in the first and second oxidizing gases may be controlled. 
     Care needs to be taken when operating the burner with a supply of combustible gas less than the specified maximum to ensure that all the ammonia is destroyed before the effluent gases leave the furnace  72 . Another control strategy which can be used when the rate of supply of the combustible gas is relatively low is to increase the proportion of the amine gas which is diverted through the sixth pipeline  100 . The sixth control valve  102  may be set accordingly. Supplying a greater proportion of the amine gas to the first combustible gas maintains the velocity of the first combustible gas and facilitates maintenance of a high ammonia and hydrocarbon destruction rate particularly when the first oxidizing gas is air. As a result, compared with operation at the maximum specified throughput of combustible gases, there is a disproportionate reduction in the rates at which the second combustible gas and the second oxidizing gas are supplied to the burner. 
     It is possible to increase the rate at which the burner shown in FIGS. 1 and 2 is able to handle the combustible gases by supplying amine gas rather than air to the passage  74 . In order to maintain the desired ratio of hydrogen sulphide to oxygen molecules entering the flame, the mole fraction of oxygen in the first and second oxidizing gases is correspondingly increased. Prior to employing the passage  74  to handle amine gas, this passage may be purged with nitrogen or other non-combustible gas so as to flush oxygen molecules therefrom. 
     Referring again to FIGS. 1 and 2, it will be appreciated that all the gas streams leave the burner  2  substantially axially. Mixing between the first oxidizing gas leaving the burner  2  through the outlets  26  and the first combustible gas leaving through the outlets  12  thereby takes place by virtue of shear between the respective gas streams. The degree of shear is enhanced by having a differential velocity between the first combustible gas and first oxidizing gas. In one illustrative example, the velocity of the first oxidizing gas leaving the burner  2  is 60 m/s and the velocity of the first combustible gas is 40 m/s. 
     Referring now to FIGS. 3 and 4 of the drawings, a burner  202  has a proximal end  204  and a distal end  206 . The burner  202  is formed with three coaxial tubes  208 ,  210  and  212  which are radially spaced from one another. The axis of the tube  208  is coincident with the longitudinal axis of the burner  202  itself. The tube  208  defines a passageway  214  for a first combustible gas mixture. The passageway  214  terminates at its distal end in an outlet  216  for the first combustible gas. 
     Extending within the tube  208  from beyond its proximal end is an array of tubes  218  each defining a respective passageway  216  for the flow of a first oxidizing gas. Each tube  218  terminates at its distal end in the same plane as the end of the tube  208 . Each tube  218  has an outlet  222  for the first oxidizing gas. The tube  210  defines with the tube  208  a passageway  224  for the flow of a second oxidizing gas. The passageway  224  terminates at its distal end in a nozzle  226  in which are formed at an angle to the axis of the burner outlets  228  for the second oxidizing gas. The outlets  228  terminate in the same plane as the distal end of the tube  208 . The nozzle  226  is engaged fluid-tight between the tubes  208  and  210 . If desired, the distal end of the nozzle  226  may be formed with an annular projection which is welded to the end of the tube  210 . The tubes  210  and  212  define an annular passageway  230  for the flow of a second combustible gas. The passageway  230  terminates at its distal end in an outlet  232  in the same plane as the outlet  216 . The outlets  228  formed in the nozzle  226  direct, in use, the second oxidizing gas into the second combustible gas comprising hydrogen sulphide issuing from the outlet  232 . 
     The burner shown in FIGS. 3 and 4 has relatively simple arrangements for the supply of the gas streams to it. Thus, the tube  212  has an inlet port  234  for the supply of second combustible gas mixture comprising hydrogen sulphide. The tube  212  has integral therewith or welded thereto at its distal end a flange  236  which is bolted or otherwise secured fluid-tight to a complementary flange  238  integral with or welded to the proximal end of the tube  210 . The flange  238  forms a distal wall of a chamber  240  for receipt of a second oxidizing gas. The chamber  240  has a cylindrical wall  242  having at its distal end a flange  244  which is bolted or otherwise secured fluid-tight to the flange  238  and at its proximal end another flange  244  which is bolted or otherwise secured fluid-tight to a complementary flange  246  integral with or welded to the proximal end of the tube  208 . The flange  246  forms the proximal wall of the chamber  240  and a distal wall of a further chamber  248  for the first combustible gas. The cylindrical wall  242  of the chamber  240  has an inlet port  250  enabling the chamber  240  to be placed in communication with the source of a second oxidizing gas. The chamber  248  has a cylindrical wall  252  which has at its distal end a first flange integral therewith or welded thereto which is bolted or otherwise secured fluid-tight to the flange  246  and at its proximal end a second flange  256  which is bolted or otherwise secured fluid-tight to an end plate  258  which forms the proximal wall of the chamber  248 . The cylindrical wall  252  has a port  260  for the admission of the first combustible gas comprising hydrogen sulphide. The end plate  258  also forms a wall of a yet further chamber  262  for the first oxidizing gas. The chamber  262  is defined between the end plate  258  and a generally conical member  264  which is bolted or otherwise secured fluid-tight thereto. The member  264  is formed with an axial port  266  which is coaxial with the burner  202 . 
     The distal end  206  of the burner  202  extends into a port  268  of a furnace  270  for the partial combustion of hydrogen sulphide. A passage  274  for air as a third oxidizing gas is defined between the outer tube  212  and the port  268 . 
     The tubes  214  and  222  may be supported by spiders or fins  276  when the burner is disposed with its longitudinal axis horizontal as shown in FIG.  3 . 
     Operation of the burner shown in FIGS. 3 and 4 is analogous to that shown in FIGS. 1 and 2. Mixing of the first oxidizing gas with the first combustible gas is facilitated by a differential velocity therebetween. Mixing of the second oxidizing gas with a second combustible gas is additionally facilitated by the angling of the outlets  228  towards the flow of gas issuing from the outlet  232 . 
     Referring now to FIGS. 5 and 6 of drawings, a burner  302  is of generally cylindrical shape and has a proximal end  304  and a distal end  306 . The burner comprises an array of four radially spaced apart, concentric, tubes  308 ,  310 ,  312  and  314  which are coaxial with the longitudinal axis of the burner  302 . The tube  308  is innermost and the tube  314  outermost. The tube  312  surrounds the tube  310 . The tubes  308  and  310  engage at their distal ends a first nozzle  316 . The tube  308  defines a first passageway  318  for a first combustible gas comprising hydrogen sulphide. The distal end of the tube  308  is set back relative to the distal end of the tube  310  and the nozzle  316  is provided with an inward annular projection which defines an outlet  320  from the burner  302  for the first combustible gas. The tubes  308  and  310  define an annular passageway  322  for a first oxidizing gas. The passage  322  terminates in the nozzle  316  which has formed therethrough, at an angle of 10 to 15 degrees to the horizontal, outlets  324  from the burner  302  for the first oxidizing gas. The outlets  324  direct the first oxidizing gas into the first combustible gas at a region downstream of the distal end  306  of the burner  302 . The tubes  310  and  312  define therebetween an annular passageway  326  for the flow of a second oxidizing gas. The passageway  326  terminates in a nozzle  328  which defines outlets  330  for the second oxidizing gas. The outlets are inclined at an angle of 10 to 15 degrees to the horizontal and extend radially outward in the direction of the flow of the gas therethrough. 
     The tube  312  and the outermost tube  314  define therebetween an annular passageway  332  for a second combustible gas comprising hydrogen sulphide. The passageway  322  has an outlet  334  at its distal end for the second combustible gas. In operation, downstream of the outlet  334  the second combustible gas flow becomes mixed with the flow of second oxidizing gas by virtue of the orientation of the outlets  330  relative to that of the outlet  334 . 
     The outermost tube  314  is provided with a port  336  for the flow of the second combustible gas mixture. The proximal end of the tube  314  is formed with a flange  338  integral therewith or welded thereto. The flange  338  is bolted or otherwise secured to a similar flange  340  which is integral with or welded to the tube  312 . If desired, a gasket or other sealing member (not shown) can be engaged between the flanges  338  and  340  so as to ensure a fluid-tight seal therebetween. The flange  340  bounds in part a chamber  342  which is contiguous to the passage  326 . The chamber  342  has a cylindrical wall  344  which is provided at its distal end with the flange  346  which is bolted or otherwise secured fluid-tight to the flange  340 . The wall  344  has a port  348  formed therein so as to enable the second oxidizing gas to be supplied to the chamber  342 . The proximal end of the wall  344  is provided with a flange  350  which is bolted or otherwise secured fluid-tight to a complementary flange  352  integral with or welded to the proximal end of the tube  310 . The flange  352  forms a common wall between the chamber  342  and a further chamber  354  for the first oxidizing gas. The chamber  354  is contiguous to the passageway  322 . The chamber  354  has a cylindrical wall  356  which has at its distal end a flange  358  which is bolted or otherwise secured fluid-tight to the flange  352 . The cylindrical wall  356  has a port  360  formed therethrough so as to enable the chamber  354  to receive first oxidizing gas. The proximal end of the wall  356  also carries a flange  362  which is bolted or otherwise secured to a flange  364  which is formed integral with or is welded to the proximal end of the innermost tube  308 . The flange  364  forms a common wall between the chamber  354  and a yet further chamber  366  which is contiguous to the passageway  318  and communicates therewith. The chamber  366  is provided with an axial port  368  to which the first combustible gas is able to be supplied. The port  368  is coaxial with the longitudinal axis of the burner  302 . As shown in FIG. 5, the distal end  306  of the burner  302  extends into a port or quarl  370  of a furnace  372  for the partial combustion of hydrogen sulphide. An annular space defined between the port  370  and the burner  306  provides a passage  374  for the flow of a third oxidizing gas, namely air. In operation, the air becomes mixed with the second combustible gas leaving the burner  302  through the outlet  334 . 
     Operation of the burner shown in FIGS. 5 and 6 is analogous to that shown in FIGS. 1 and 2 and that shown in FIGS. 3 and 4. 
     Referring now to FIGS. 7 and 8 of the drawings, a burner  402  has a proximal end  404  and a distal end  406 . The burner comprises an assembly of three coaxial radially spaced apart tubes  408 ,  410  and  412 . The tubes  408  defines a passageway  414  having at its distal end an outlet  416  for a first combustible gas comprising hydrogen sulphide. The tubes  408  and  410  define therebetween an annular passageway  418  for a flow of oxidizing gas, for example, oxygen enriched air. The passageway  418  terminates at its distal end terminates in an annular nozzle  420  which has a group of first outlets  422  formed therethrough and each inclined at the same angle in the range of 10 to 15 degrees to the longitudinal axis of the burner and which are inclined towards this axis in a direction of gas flow therethrough. The outlets  422  provide a flow of first oxidizing gas which mixes with the first combustible gas downstream of the distal end  406  of the burner  402 . 
     A second group of outlets  424  is also formed through the nozzle  420 . The outlets  424  are each inclined at the same angle in the range of 10 to 15 degrees to the longitudinal axis of the burner  402 , the gas diverging from the axis in its direction of flow. The group of outlets  424  therefore enable a second oxidizing gas to issue from the distal end  406  of the burner  402 . An annular passageway  426  is defined between the tubes  410  and  412 . The passageway  426  terminates at its distal end in an outlet  428 . In operation, the second combustible gas issues from the distal end  406  of the burner  402  through the outlet  428  and becomes mixed with the second oxidizing gas leaving the burner  402  through the outlets  424 . The flow ratio of the first oxidizing gas to the second oxidizing gas is determined by the relative cross-sectional areas of the outlets  422  and  424 . 
     The tube  412  has a port  430  formed therein to enable the passageway  426  to be placed in communication with a source of the second combustible gas comprising hydrogen sulphide. The proximal end of the tube  412  has a flange  432  formed integral therewith or welded thereto. The flange  432  is bolted or otherwise secured fluid-tight to a complementary flange which is welded to or formed integral with the tube  410 . The flange  434  is also bolted or otherwise secured fluid-tight to a complementary flange  436  extending from an end piece  438  which is provided with an inlet port  440  for the oxidizing gas. The tube  408  extends at its proximal end into the end piece  438  and carries a flange  442  to which the proximal end of the end piece  438  is welded or otherwise secured fluid-tight. Oxidizing gas flows, in operation of the burner  402 , into the port  440  and through the end piece  438  into the passageway  418  defined between the tubes  408  and  410 . The tube  408  is open at its proximal end and may be placed in communication with a source of the first combustible gas comprising hydrogen sulphide. 
     The distal end  406  of the burner  402  extends into a port  432  of a furnace  434  for the partial combustion of the hydrogen sulphide. An annular passage  438  is defined between the burner and the port  432  through which a third oxidizing gas, typically air, can be supplied to the burner flame in operation of the burner  402 . 
     The operation of the burner  402  is analogous to that of any of the burners shown in FIGS. 1 and 2, FIGS. 3 and 4 and FIGS. 5 and 6 of the accompanying drawings with the exception that there is no facility for varying the composition and the flow rate of the first oxidizing gas independently of the composition and flow rate of the second oxidizing gas, because both are taken from a common source supplied to the port  440  of the burner  402 . 
     Referring now to FIGS. 9 and 10 of the drawings, a burner  502  has a proximal end  504  and a distal end  506 . The burner  502  includes an assembly of two inner coaxial tubes  508  and  510 . The inner tube  508  defines a passageway for a first flow of gas mixture comprising hydrogen sulphide. The passageway  512  has at its distal end an outlet  514 . The tubes  508  and  510  define therebetween an annular passageway  516  for the flow of an oxidizing gas, typically oxygen-enriched air. The passageway  516  terminates at its distal end in an annular nozzle  518 . The nozzle  518  has a first group of outlets  520  formed therethrough. The outlets  520  are each inclined at the same angle towards the longitudinal axis of the passageway  512 , in the direction of flow of the gas. The nozzle  518  is also provided with a second group of inclined outlets  522 . The outlets  522  are each inclined at the same angle to the longitudinal axis of the tube  508 , the outlets  524  being arranged such that the gas is conducted away from the axis. The nozzle  518  is therefore able to divide the oxidizing gas into a first flow which is conducted into the gas leaving the outlet  514  and a second flow which is conducted into a second flow of combustible gas comprising hydrogen sulphide, as will be described below. 
     The assembly of the tubes  508  and  510  is located within an outermost tube  526 . The tube  526  is coaxial with the tubes  508  and  510 . It terminates at its distal end in the same plane as the distal end of the tube  512 . However, its proximal end is closer to the proximal end  504  of the burner  502  than the proximal ends of the tubes  508  and  510 . The tube  526  defines with the tube  510  an annular passage  528  for the flow of combustible gas mixture comprising hydrogen sulphide. The passageway  528  terminates at its distal end in an outlet  530 . In operation, the second flow of oxidizing gas from the group of outlets  522  becomes mixed with the flow of hydrogen sulphide containing gas leaving the burner  502  through the outlet  530 . 
     The tube  526  has formed integral therewith or welded thereto at its proximal end a flange  532 . The flange  532  is bolted or otherwise fastened fluid-tight to an end plate  534 . The tubes  508  and  510  engage fluid-tight an annular closure  536  at their proximal ends. The pipes  538  are engaged fluid-tight at the distal ends in the closure  536  and at their proximal ends by the end plate  534 , and communicate with a chamber  540  for oxidizing gas which is bounded at one end by the end plate  534  and which is secured fluid-tight thereto. The chamber  540  has an inlet  542  for oxidizing gas. Thus, in operation of the burner  502 , oxidizing gas is able to flow from the chamber  540  through the pipes  538  into the passageway  516 . It is then divided by the nozzle  518  into a first flow through the outlets  520  which mixes with the combustible gas issuing from the outlet  514  and a second flow through the outlets  522  which mixes with the combustible gas issuing from the outlet  530 . The combustible gas is itself supplied to the burner  502  through a port  550  in the tube  526 . The flow of combustible gas will naturally be distributed between the outlets  514  and  530 , relative proportions flowing to each outlet depending on the relative sizes of the outlets  514  and  530 . 
     The distal end  506  of the burner  502  is received in a port  560  of a furnace  562  for the partial combustion of the hydrogen sulphide. The burner  502  and the port  560  define therebetween a further annular passageway  564  for the flow of air to the burner flame, the air becoming mixed with the hydrogen sulphide, containing gas issuing from the outlet  530 . 
     If desired, the tube  510  may carry on its outer surface support fins  570  so as to enable the assembly of the tubes  508  and  510  to be centrally located within the outermost tube  526 . 
     The burner shown in FIGS. 9 and 10 may be operated similarly to the other burners described and illustrated therein. However, although a three stage flame is able to be formed, there is no facility for varying the composition and flow of the hydrogen sulphide-containing gas mixture exiting the burner  502  through the outlet  514  from that exiting through the outlet  530 . Similarly, there is no facility for varying the composition and flow rate of the oxidizing gas exiting through the outlets  520  independently of the flow rate and composition of the oxidizing gas exiting through the outlets  522 . 
     Any of the burners shown in FIGS. 1 and 2, FIGS. 3 and 4, FIGS. 5 and 6, FIGS. 7 and 8, and FIGS. 9 and 10 of the drawings may be employed as the burner  600  shown in FIG.  12 . With reference to FIG. 12, a combustible gas mixture which typically includes more than 40% by volume of hydrogen sulphide flows into the burner  600 . Partial combustion of the hydrogen sulphide is supported by the supply of oxygen enriched air and atmospheric air to the burner  600 . The burner  600  fires into a furnace  602 . A gas mixture comprising hydrogen sulphide, sulphur dioxide, sulphur vapor, water vapor, nitrogen, carbon dioxide and hydrogen leaves the furnace  602  typically in the range of 1100° C. to 1600° C. The effluent gas mixture passes through a waste heat boiler  604  in which its temperature is reduced to a little above the point at which sulphur vapor condenses. The mole ratio of hydrogen sulphide to sulphur dioxide in the effluent gas mixture is approximately 2 to 1 after some recombination of hydrogen and sulphur in the waste heat boiler  604 . 
     Downstream of the waste heat boiler  604  the effluent gas flows through a condenser  606  in which sulphur vapor is condensed. The resulting condensate is passed for storage. The residual gas mixture flows from the condenser  606  through successive catalytic Claus stages  608 ,  610  and  612 . Each of the stages  608 ,  610  and  612 , in accordance with the general practice in the art, comprises a train of units consisting, in sequence, of a reheater (not shown) to raise the temperature of the gas mixture to a temperature suitable for catalytic reaction between hydrogen sulphide and sulphur dioxide, a catalytic reactor (not shown) in which hydrogen sulphide reacts with sulphur dioxide to form sulphur vapor and water vapor, and a sulphur condenser (not shown). If desired, depending on the environmental standards which the plant shown in FIG. 12, one or more of the catalytic stages  608 ,  610  and  612  may be omitted. 
     The gas mixture leaving the downstream catalytic stage  612  may be subjected to any one of a number of known treatments for rendering Claus process effluent more suitable for discharge to the atmosphere. For example the gas may pass to a reactor  614  in which it is subjected to hydrolysis and hydrogenation. Any residual carbon oxysulphide and carbon disulphide are hydrolysed over a catalyst by water vapor to produce hydrogen sulphide. This catalyst may be, for example, alumina impregnated with cobalt and molybdenum. Such catalysts are well known in the art. At the same time, residual elemental sulphur and sulphur dioxide are hydrogenated to form hydrogen sulphide. The hydrolysis and hydrogenation take place on the impregnated alumina catalyst at a temperature typically in the range of 300 to 350° C. The resulting gas mixture consisting essentially of hydrogen sulphide, nitrogen, carbon dioxide, water vapor and hydrogen leaves the reactor  614  and flows first to a water condensation unit (not shown) and then to a separate unit (not shown) in which hydrogen sulphide is separated, for example, by chemical absorption/desorption. A suitable chemical absorbent is methyl diethylamine. If desired, the hydrogen sulphide thus recovered may be recycled to the burner  600 .