Decarbonisation Technology - February 2022 Issue

Two major effects of using oxygen or oxygen- enriched air in place of air for combustion are higher temperatures and higher flame speeds. The degree of change depends on the degree of oxygen enrichment, but in the case of pure oxygen, temperatures may increase by 1,050°C and flame velocities by 10 times in round numbers. A combination of these two effects produces a hotter, shorter, more intense flame that is much better suited to the rapid destruction of combustible materials. The destruction of individual feed components in a Claus unit cannot be considered in isolation, since there is considerable molecular interaction. Both H 2 S and NH 3 dissociate quite readily, and the higher the temperature, the higher the level of dissociation. The result is that when oxygen is used, the hydrogen level in the reaction furnace increases greatly over that achieved in air-based systems. Most of this hydrogen will subsequently recombine with sulphur in the waste heat boiler (WHB), including hydrogen produced from ammonia dissociation. The ammonia must effectively be burned, even if the mechanism of destruction is initially dissociation, in order to preserve the Claus stoichiometry downstream of the WHB. It is possible to speculate that the hydrogen remaining in the gas after the WHB will be higher if the level in the reaction furnace before the boiler is higher. This must be true if the quench rate in the boiler remains constant. The effect may be small, however, and in the case of up-rating with oxygen, where the WHB sees a higher load, a fall in quench rate may reduce it still further. The AGRU is designed to separate H 2 S, where it is sent to the SRU to produce sulphur as the product and comprises three towers. The CO 2 is also separated and compressed to be used as product. The SWS and condensate tower also remove H 2 S and NH 3 , which is also sent to the SRU. The SRU receives two acid gas streams containing H 2 S and NH 3 . As we know, NH 3 can be destructed in the thermal section, where the reaction furnace and burner achieve the stable combustion temperature required for NH 3 burning. Unfortunately, H 2 S entering the SRU is very lean and cannot establish an adequate combustion temperature of at least 1050°C, which is needed to achieve the successful NH 3 burning with air operation. Therefore, this project

is designed using 100% oxygen enrichment in single combustion. Also, on account of the lean acid gases, even with Ti catalyst in the SRU converters, the significant COS and CS 2 produced in the reaction furnace would be incompletely converted to H 2 S (additional to any COS remaining in the feed compositions):

H 2 S + 3/2 O 2  SO 2 + H 2 O 2H 2 S + SO 2  3S + 2H 2 O Overall: 3H 2 S + 3/2 O 2  3S + 3H 2 O

(1) (2)

(3)

Ammonia, which can form undesirable sulphur compounds, must also be destroyed in the combustion process. This reaction is:

2NH 3 + 3/2 O 2  N 2 + 3H 2 O

(4)

The SRU design comprises the thermal section featuring 100% oxygen enrichment and two- stage catalytic reactors. The tail gas stream from the last condenser flows to the hydrogenation reactor in the TGU through an indirect steam reheater. The reactor is a low temperature hydrogenation unit with a cobalt/molybdenum or CoMo catalyst. H 2 and CO present in the tail gas from the Claus units react with the sulphur vapour and SO 2 in the tail gas over a catalyst bed to form H 2 S. The hydrogenation catalyst also promotes the hydrolysis, i.e., reaction of COS and CS 2 with water to form H 2 S. Hydrogenation and hydrolysis reactions for the four primary sulphur constituents are as follows:

Hydrogenation reactions: S + H 2  H 2 S SO 2 + 3H 2  H 2 S + 2H 2 O Hydrolysis reactions: CS 2 + 2H 2 O  2H 2 S + CO 2 COS + H 2 O  H 2 S + CO 2

(1) (2)

(3) (4)

CO does not react directly, but is converted to H 2 over the catalyst by the water shift reaction:

CO + H 2 O  H 2 + CO 2

(5)

These reactions are exothermic. The outlet of the hydrogenation reactor cools