PROCESS FOR COMBUSTING HYDROGEN SULPHIDE-CONTAINING GASES AND BURNER FOR USE IN SUCH A PROCESS

Abstract: Process for partially combusting H₂S-containing gases in a Claus unit by means of a burner comprising a burner gun sur­ rounded by an air register provided with openings for the sup­ ply of an oxygen-containing gas. in which process the pres­ sure drop of the oxygen-containing gas over the air register is maintained at or above a predetermined minimum value by par­ tially blanking off the openings in the air register, e.g. by a mov­ able sleeve. A burner for use in this process is also claimed.

Description: PROCESS FOR COMBUSTING HYDROGEN SULPHIDE CONTAINING GASES AND BURNER FOR USE IN SUCH A PROCESS The invention relates to a process for combusting hydrogen sulphide (H2S)-containing gases in the thermal zone of a sulphur recovery unit, and a burner for use in such a process. The recovery of elemental sulphur from H2S-containing gases by means of the Claus reaction is well known and different procedures for carrying out this reaction are being applied in practice. In one of these procedures the Claus reaction is carried out by partially combusting the H2S-containing gases with air or another free oxygen-containing gas in a thermal zone to form a gas mixture containing sulphur dioxide (SO2) and H2 S in a preferred molar ratio of 1:2, whereupon the gases are cooled after leaving the thermal zone in order to condense the formed sulphur vapour and recover the sulphur as liquid sulphur. Subsequently, the gases are conducted to one or more catalytic reaction zones where an extra quantity of sulphur is formed. Before introduction into a catalytic reaction zone the gases are first brought to the desired reaction temperature and after leaving this zone they are re-cooled for recovery of the formed sulphur. The H2S-containing gases mostly originate from desulphurization processes in a refinery or from natural gas from which the acid gas components have been removed in gas purification units by means of absorption by certain absorbents. After regeneration of the absorbent used, an H2S-enriched gas is obtained which possibly contains minor amounts of hydrocarbons. Since it is generally not acceptable that the throughput of the main processing units where the H2 S is produced is restricted by the available Claus reaction capacity, the design capacity of the Claus reaction apparatus is normally determined by the maximum amount of H2 S which might be produced in the main processing units. Actual practice is that Claus reaction units operate most of the time below the design capacity. A yearly average utilization of only 35% of the installed Claus capacity is to be considered as normal. Furthermore, the increasing legislation on environmental pollution abatement urges more and more to the installation of spare Claus reaction capacity, which reduces the utilization of the Claus reaction units even further. To prevent corrosion problems which may easily occur in an idle Claus unit it is necessary, even in case of a low H2 S production, that all availabe Claus units remain in operation. It is evident that with smaller amounts of H S produced a actual turndown load 2 higher turndown ratio (design load) of the Claus units, sometimes even about 0.10, is required to avoid the necessity of flaring the H 2S. The ability of a Claus unit to operate at a reduced actual load is in first instance limited by the performance in the thermal zone of the Claus installation, where the H2S-containing gases are partially combusted with air to form a mixture ofSO2 and H2S in substantially stoichiometric proportions. In order to achieve the attainable conversion/sulphur recovery in the thermal zone in combination with the production of bright yellow sulphur without soot formation, it is essential for the H2S-containing gases to be intensively mixed with the combustion air. To this end it is known to bring the combustion air into a strong rotary motion, and supplying the whirling combustion air to the H2S-containing gases. The required mixing action may suitably be obtained in a burner comprising a burner gun provided with a supply tube for the supply of H2S-containing gases, an air register surrounding the burner gun for the supply of free oxygen-containing gases into an air chamber, wherein the air chamber debouches into a combustion chamber via an air nose having an inner converging wall viewed in the direction of the combustion chamber. The air register normally consisting of a plurality of substantially tangentially arranged blade-shaped elements gives a gas upon passing therethrough a rotary motion. This motion, accelerated as the gas flows to the air nose, has its maximum velocity when the gas leaves the air nose. During operation of the above-mentioned burner an H2S-containing gas is supplied via the supply tube in the burner gun to the air nose, whereas free oxygen-containing gas or gas mixture, normally air, is supplied to the air nose via the air register and the air chamber where the gas is brought in a rotary motion. The highly turbulent free oxygen-containing gas and the H2S-containing gas are intensively mixed in the air nose prior to entering into the combustion chamber. When the amount of H2S-containing gases to be combusted for sulphur recovery decreases, the amount of free oxygen-containing gas should be reduced proportionally to maintain the for the process preferred molar ratio of 1:2 for the gas mixture of S02 and H2 S which is formed in the combustion chamber. However, reduction of the applied amount of free oxygen-containing gas implies a decrease of the rotary motion imparted to the gas upon passing through the air register. From the practice, it is known that the reduction of the applied amount of free oxygen-containing gas cannot be freely chosen but is restricted to a particular minimum below which the rotary motion of the gas by passing the air register is too small to obtain an intensive mixing of free oxygen and H2 S in the air chamber, necessary for the process of the sulphur recovery. According to the practice, the process for combusting H2S-containing gases can be satisfactorily applied up to throughputs of H2 S-containing gases of about 10% of the maximum throughput for which the applied burner in the process has been designed. Below the minimum throughput the pressure drop over the air register becomes too low to bring the free oxygen-containing gas in a sufficiently rotary motion. To guarantee a sufficient rotary motion of the free oxygencontaining gas upon contact with the acid gas the pressure drop over the air register and the air nose should have a certain minimum value. It will be clear that when the amount of applied free oxygen-containing gas is reduced the pressure drop over the air register and the air nose is also reduced. Since the applied amount of free oxygen-containing gas is substantially proportional with the amount of supplied acid gas, the amount of acid gas is bound to a minimum which can be treated in the known process for combusting acid gas in the thermal zone of a sulphur recovery unit. This limitation in operating range has generally been recognized, and a well-known method to overcome this disadvantage is to supply a support fuel gas in the thermal zone of a sulphur recovery unit. As a result thereof the total consumption of free oxygen-containing gas is no longer proportional with the amount of supplied acid gas. A number of disadvantages are however encountered with the supply of support fuel gas in addition to acid gas, such as for example an impaired controllability of the required amount of air increasing the risk of soot formation, and an impaired sulphur recovery efficiency. It is an object of the present invention to provide a process and a burner for use in this process enabling a satisfactory treatment of even very small amounts of H2S-containing gas, up to and below 10% of the design throughput, without the application of support fuel gas in addition to the acid gas and without substantially reducing the efficiency of the treatment. The invention relates to a process for combusting hydrogen sulphide-containing gases in the thermal zone of a sulphur recovery unit, in which hydrogen sulphide-containing gases are partially combusted to a mixture of sulphur dioxide and hydrogen sulphide in a molar ratio of substantially 1:2 by supplying said hydrogen sulphide-containing gases to a burner comprising a burner gun, surrounded by an air register provided with openings for the supply of a free oxygen-containing gas or gas mixture to an air chamber, which burner gun debouches into a mixing zone of the air chamber, said air chamber being in fluid communication with a combustion chamber via an air nose, in which process a hydrogen sulphide-containing gas is mixed in the mixing zone with a free oxygen-containing gas or gas mixture supplied to the mixing zone via the air register, wherein the pressure drop of the free oxygen-containing gas or gas mixture over the air register and the air nose is maintained at or above a predetermined minimum value by partially blanking off the openings in the air register. The burner for use in the process according to the invention comprises a burner gun, surrounded by an air register provided with openings for the supply of a free oxygencontaining gas or gas mixture into an air chamber, which burner gun debouches into a mixing zone of the air chamber, said air chamber being provided with an air nose for forming a fluid communication with a combustion chamber, the burner further comprising means for partially blanking off the openings in the air register. Since in the process for combusting H2S-containing gases according to the invention the pressure drop over the air register and air nose can be artificially maintained at or above a predetermined minimum value, even very small amounts of H2S-containing gases below 10% of the design throughput can be satisfactorily combusted. According to a suitable embodiment of the invention, the openings in the air register for the supply of gas into the air chamber are blanked off by displacing a sleeve along the air register, said sleeve being substantially co-axially arranged with respect to the air register. The displaceably arranged sleeve is pre-eminently suitable for suppressing resonation/vibration effects which may occur. Such effects loosen the fire-resistant ceramic lining and subsequently cause collapsing of the loosened lining. According to a further suitable embodiment of the invention, the openings in the air register for the supply of gas into the air chamber are partially blanked off by turning rotatably mounted blade-shaped elements forming the air register. The invention will now be described by way of example in more detail with reference to the accompanying drawing, wherein: Figure 1 shows a longitudinal cross section of the thermal zone of a sulphur recovery unit with a burner according to the invention; Figure 2 shows a longitudinal cross section of an alternative for the burner shown in Figure 1; and Figure 3 shows cross-section III-III of the burner shown in Figure 2. It is to be understood that identical elements shown in the Figures have been indicated with the same reference numeral. In Figure 1, reference numeral 1 indicates the thermal zone of a sulphur recovery unit. This thermal zone 1 is formed by a burner 2, a combustion chamber 3 and a reaction chamber 4. The burner 2 comprises a burner gun 5, having as main components a supply tube 6 for the supply of H2S-containing gases and a supply tube 7 for the supply of support fuel gas. The supply tube 6 is substantially concentrically arranged around the supply tube 7. The burner 2 further comprises an air register 8 surrounding the burner gun 5 and being provided with openings through which air or another free oxygencontaining gas may be blown into an air chamber 9. The air register 8 may consist of a plurality of not separately shown blades substantially tangentially arranged with respect to the circumference of the air chamber 9, and spaced apart from each other to form openings for the passage of gas. An inlet 10 is provided for the supply of air into the burner 2. The air chamber 9 is provided with an air nose 11, forming a fluid communication between the air chamber 9 and the combustion chamber 3. The flame-faced end of the burner gun 5 is arranged in the air chamber 9 at some distance from the air nose 11 to provide a mixing zone 12. For mixing purposes the said flame-faced end of the burner gun 5 is provided with a conically shaped diffuser 13, and the air nose 11 converges in the direction of the combustion chamber 3. Both the combustion chamber 3 and the reaction chamber 4 are provided with a ceramic lining 14 which is applied on a heat-insulating layer 15. For controlling the pressure drop over the air register 8 and the air nose 11, the burner 2 is provided with a sleeve 16, arranged in the air chamber 9 at the inner side of the air register 8 and positioned co-axially therewith. The length of the sleeve 16 is chosen substantially equal to the length of the air register 8. The end of the sleeve 16 away from the air register 8 is connected to a disc-shaped wall 17, via a plurality of rods 18. The disc-shaped wall 17 is slidably arranged over the burner gun 5. One or more pneumatic or hydraulic actuators 19 are arranged at the front side of the burner 2 for displacing the wall 17. The dimensions of the openings in the air register 8 are determined by the maximum amount and quality of H2S-containing gas which has to be treated in the thermal zone 1. At the maximum load the pressure drop over the air register 8 and the air nose 11 should for practical reasons not exceed a predetermined maximum value. Reference is now made to Figures 2 and 3 showing an alternative of the burner 2 shown in Figure 1. In this embodiment of the invention the air register 8 consists of a plurality of rotatable blade-shaped elements 20 spaced apart from each other. Rods 21 and 22 are mounted on the outer ends of each of the elements 20. The rods 21 are rotatably mounted in the vertical part of the wall of air nose 11. The rods 22 fit in grooves of an annular wall 23 rotatably mounted on the burner gun 5. The blade-shaped elements 20 can be rotated by turning the annular wall 23 by means of a pneumatic or hydraulic actuator 24. Rotation of the elements 20 results in a variation of the width of the opening between each pair of adjacent elements 20. The process for combusting H2S-containing gas by means of the apparatus shown in the Figures is as follows. H2 S-containing gas is supplied via the supply tube 6 to the mixing zone 12 in the air chamber 9. A proportional amount of air or another free oxygen-containing gas is blown by a fan via the inlet 10 and the openings in air register 8 into said mixing zone 12. H2S-containing gas and air brought in a rotary motion by passing the air register 8 are intensively mixed prior to entering into the combustion chamber 3, and subsequently the reaction chamber 4, which provides a certain residence time in order to ensure that the various reactants will react with each other. The amount of air supplied is adjusted to the amount and quality of H2S-containing gas to be treated, in such a manner that the gas mixture leaving the reaction chamber 4 contains SO2 and H2 S in a preferred molar ratio of 1:2. The amount of supplied air may be automatically readjusted when a variation in the supply of acid gas occurs. To this end a flow meter, installed in the supply tube 6, can be coupled to a flow control in the means for supplying the air to the inlet 10. When the amount of acid gas is about the maximum for which the thermal zone has been designed, the air register 8 is in a fully opened position. In the embodiment of Figure 1, this means that the sleeve 16 is arranged in a fully retracted position. In the embodiment of Figure 2, the fully opened position of the air register 8 is obtained by rotating the annular wall 23, so that the blades are substantially tangential to the air chamber 9. To ensure a sufficient mixing of air and acid gas in the mixing zone of the air chamber 9, the pressure drop of the air stream over the air register 8 and the air nose 11 should preferably have a predetermined minimum value. When the amount of supplied air adjusted to the amount of supplied acid gas is reduced below a certain extent the pressure drop over the fully opened air register and air nose will become insufficient for ensuring the for the process necessary intensive mixing of acid gas and air. In this case the air register 9 is partially blanked off by displacement of the sleeve 16 or turning the blade-shaped elements 20, so that the pressure drop of the air over the air register is increased. The blanking off of part of the openings in the air register may be carried out automatically. To this end a pressure drop meter for measuring the pressure drop over the air register and the air nose is coupled to the actuators for partially blanking off the openings in the air register. In this manner it is possible to maintain the pressure drop over the air register and the air nose at a constant value above the predetermined minimum value. When the velocity of the acid gas is again increased, the sleeve 16 or the blade-shaped elements may be moved for increasing the free passage in the air register 8. The invention is not restricted to the means for blanking off the air register as shown in the drawings. Instead of a displaceable sleeve or rotatable bladeshaped elements, for blanking off the openings in the air register, the air register may be formed by fixed blades provided with elongated elements hingeably connected to said blades, wherein compression spring means are arranged between said blades and elongated elements. The spring tension and the position of the elongated elements are chosen so that the openings for the passage of air are reduced when the air velocity is diminished. The invention is not restricted to the application of pressure actuators for moving the sleeve 16 or the bladeshaped elements 20. Instead of such pressure actuators, manually operated handles may for example be applied. The invention is not restricted to a process and apparatus wherein an air nose is applied having a conical shape. The shown conical-shaped air nose 11, forming a constriction between the mixing zone of the air chamber and the combustion chamber, however, ensures a better mixing of air and acid gas than, for example, a cylindrically-shaped air nose. Although in the above-described process, the quantity of supplied air is controlled outside the burner, it is also possible, without departing from the present invention, to control the air supply with the aid of the above-described means for partially blanking off the openings in the air register. In this procedure it is possible to maintain a constant pressure drop over the air register and the air nose independent of the amount of supplied air.

Claims: CLAIMS 1. A process for combusting hydrogen sulphide-containing gases in the thermal zone of a sulphur recovery unit, in which hydrogen sulphide-containing gases are partially combusted to a mixture of sulphur dioxide and hydrogen sulphide in a molar ratio of substantially 1:2 by supplying said hydrogen sulphide-containing gases to a burner comprising a burner gun, surrounded by an air register provided with openings for the supply of a free oxygen-containing gas or gas mixture to an air chamber, which burner gun debouches into a mixing zone of the air chamber, said air chamber being in fluid communication with a combustion chamber via an air nose, in which process a hydrogen sulphide-containing gas is mixed in the mixing zone with a free oxygen-containing gas or gas mixture supplied to the mixing zone via the air register, wherein the pressure drop of the free oxygen-containing gas or gas mixture over the air register and the air nose is maintained at or above a predetermined minimum value by partially blanking off the openings in the air register. 2. A process as claimed in claim 1, wherein the openings in the air register are partially blanked off in the longitudinal direction of the air register. 3. A process as claimed in claim 1 or 2, wherein the openings in the air register are partially blanked off by displacing a sleeve, along the air register, said sleeve being substantially co-axially arranged with respect to the air register. 4. A process as claimed in claim 1 or 2, wherein the openings in the air register are partially blanked off by turning rotatable blade-shaped elements forming the air register. 5. A process as claimed in any one of the preceding claims, wherein the pressure drop of the free oxygen-containing gas or gas mixture is maintained at a s-tbstantially constant value above the predetermined minimum value. 6. A process for combusting hydrogen sulphide-containing gases in the thermal zone of a sulphur recovery unit, substantially as described with particular reference to the accompanying drawings. 7. A burner for use in the process as claimed in claim 1, comprising a burner gun, surrounded by an air register provided with openings for the supply of a free oxygencontaining gas or gas mixture into an air chamber, which burner gun debouches into a mixing zone of the air chamber, said air chamber being provided with an air nose for forming a fluid communication with a combustion chamber, the burner further comprising means for partially blanking off the openings in the air register. 8. A burner as claimed in claim 7, wherein the means for partially blanking off the openings in the air register comprises a sleeve substantially co-axially arranged with the air register, said sleeve being displaceably arranged in longitudinal direction of the burner. 9. A burner as claimed in claim 8, wherein the sleeve is arranged at the inner side of the air register. 10. A burner as claimed in claim 7, wherein the air register is formed by rotatable blade-shaped elements, and wherein means are provided for turning the rotatable blade-shaped elements for partially blanking off the openings in the air register. 11. A burner for use in a process for combusting hydrogen sulphide-containing gases in the thermal zone of a sulphur recovery unit, substantially as described with particular reference to the accompanying drawings.

Inventors: LOOF, PHILIPPUS, VAN DEN BOSCH, ALBERTUS VINCENTIUS PETRUS
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1982
Main classification: C01B17
IPC codes: F23N3, F23D14, C01B17