The Claus Process
The basic Claus process for substoichiometric combustion of H2S to elemental sulfur was developed as a single-stage process on the basis of reaction 8-5 at the end of the nineteenth century. During the 1930s it was modified into a two-stage process in which initially one third of the H2S was combusted to S02 and water, and in a second low-temperature catalytic stage, the S02 was reacted with the remaining H2S to sulfur. Operating the second stage at a comparatively low temperature (200-300°C) used the more favorable equilibrium to achieve much higher sulfur yields than had been possible with the original process.
H2S + Wi 02 < > S02 + H20 (8-3)
2 H2S + S02 <—> 2 H20 + 3/8 S8 (8-4)
3 H2S +IV2 02 <—> 3 H20 + 3/8 S8 (8-5)
Today there are innumerable Claus processes available, all of them ultimately variants of the modified Claus process. A typical standard Claus process is shown in Figure 8-13. In the first combustion stage all the H2S is combusted with an amount of air corresponding to the stoichiometry of reaction 8-5 at a temperature in the range 1000-1200°C. The thermodynamics of these three main reactions is such that about half the total sulfur is present in the outlet gas as elemental sulfur vapor, the rest as an equal mix of H2S and S02. The hot gas is cooled by raising steam and the sulfur already formed is condensed out. The removal of sulfur at this point assists in driving reaction 8-4 further to the right in the subsequent catalytic stage. The gas is reheated and passed over an alumina catalyst at a temperature of about 200-300°C, and cooled again to condense the sulfur formed. This may be performed a number of times to remove further amounts of sulfur. Typically, two (as shown in Figure 8-13) or three catalytic stages are used.
A standard air-blown Claus plant is limited in the dilution of H2S possible in the acid gas. At concentrations less than about 25-30 mol% H2S, the temperature in the
furnace is insufficient to maintain the reaction. Although this is only rarely a limitation for plants gasifying refinery residues, gasification of low-sulfur coal can easily produce an acid gas with such a low H2S concentration. Although it is possible to increase this concentration with some AGR systems, using oxygen instead of air as the oxidant in the Claus furnace often offers a more economic approach, especially since the oxygen demand for the Claus unit is only a fraction of that required for the gasifier itself. In addition to saving the cost of concentration within the AGR, the cost of the Claus unit itself is lower. The chief determinant for sizing the equipment in a Claus plant is the volume of gas throughput. Elimination of all or most of the nitrogen involved with air-blowing reduces the gas volume by between 30 and 60%, depending on the acid gas quality. The equipment is accordingly smaller.
There are a number of suppliers of oxygen-blown Claus technology, such as Lurgi, BOC, APCI/Goar. All use special burners to handle the oxygen. One example is the Lurgi OxyClaus burner, which is shown in Figure 8-14. The design of this multipurpose burner provides for operation on air, enriched air or pure oxygen. The burner has a series of acid gas burners arranged concentrically around a central burner muffle. Each acid gas burner consists of three coannular lances, with oxygen being injected through the inner lance, acid gas through the intermediate one, and air through the outer ring.
One aspect of processing acid gas in a Claus plant, which deserves special mention, is the problems that can occur, when excessive amounts of ammonia or HCN are present in the gas. These components must be oxidized fully to molecular
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Figure 8-14. Lurgi OxyClaus Burner (Source: Knab, Neumann, andNehb 1994) |
nitrogen as otherwise ammonium salts are formed downstream and can plug the liquid sulfur lines. Ensuring complete oxidation is essentially a matter of ensuring that these components are combusted stoichiometrically at a sufficiently high temperature, eventhough the H2S combustion is sub-stoichometric.
The main path of ammonia and HCN in a gasification unit is into the process condensate, from which they are removed in a sour water stripper (SWS). A feature of the Lurgi OxyClaus burner is the separate nozzle for the sour water stripper off-gas, which allows this gas to be combusted with a different stoichiometry and therefore a higher temperature than the bulk acid gas from the AGR unit.