Gasifipedia
Supporting Technologies
Sulfur Recovery and Tail Gas Treating
The sulfur compounds from the coal feed of a gasification process are generally removed from the synthesis gas (syngas) via an acid gas removal (AGR) process as a concentrated hydrogen sulfide (H2S) stream. Sulfur is then recovered as either liquid or solid elemental sulfur, or as sulfuric acid, depending on market demands. For an elemental sulfur product, a Claus sulfur recovery unit produces elemental sulfur from H2S in a series of catalytic stages, achieving about 98% recovery of the sulfur in the syngas. Part of the H2S is oxidized to produce sulfur dioxide (SO2), which is then reacted with the remaining H2S to give elemental sulfur and water. Tail gas from the Claus process is sent for further treatment to an amine-based Shell Claus Offgas Treatment (SCOT) unit to achieve an overall sulfur recovery of 99.8%.
For recovery as a sulfuric acid, H2S is first oxidized to SO2, then to sulfur trioxide (SO3), which is then scrubbed with water or a recycled weak sulfuric acid stream to yield saleable 98% sulfuric acid. Typically, 99.8% of the H2S can be recovered in the sulfuric acid plant.
The Claus Process
The basic Claus process for sub-stoichiometric combustion of H2S to elemental sulfur follows the following reactions:
H2S + 1 ½ O2 → SO2 + H2O |
2 H2S + SO2 → 2 H2O + 3 S |
3 H2S + 1 ½ O2 → 3 H2O + 3 S |
Figure 1 shows a typical block flow scheme of a 3-stage split-flow Claus sulfur recovery unit (SRU). Acid gas from the AGR process, along with the recycle gas stream from the tail gas treating unit and from the sour water stripping plant, is burned with sufficient air to produce an overall SRU feed with the desired 2 to 1 stoichiometric ratio of H2S to SO2 for conversion to sulfur and water. The hot burner exhaust is cooled in the waste heat boiler (WHB) before being mixed with the remaining AGR acid gas prior to entering the first stage catalytic converter. Approximately 75% of the sulfur conversion occurs in the 1st stage catalytic converter. The remaining sulfur species in the 1st stage catalytic converter exhaust are converted in subsequent catalytic converters. Reaction heat produced in the burner is recovered in the integrated WHB by generating 650 psig steam.
Sulfur products are cooled and condensed by generating low pressure steam. Condensed sulfur product is stored in an underground molten sulfur pit, where it is later pumped to truck loading for shipment. Claus tail gas from the last stage sulfur condenser is sent to the SCOT tail gas treatment unit to remove unconverted H2S, SO2, and carbonyl sulfide (COS) before disposal.
Figure 1: A Typical Claus Process Block Flow Diagram
SCOT Tail Gas Treating
Figure 2 shows a simplified SCOT tail gas treating unit (TGTU). Tail gas from the Claus SRU is heated in an in-line burner before entering the hydrogenation reactor, where all sulfur species are converted to H2S. Hydrogenation reactor effluent is then cooled by generating low pressure (LP) steam, followed by cooling with heat exchange between cooling water. Residual H2S in the cooled tail gas is removed with amine in a counter-current packed absorber. The treated tail gas from the absorber top is incinerated before being vented to the atmosphere.
The rich solvent from the amine absorber is pumped to the regenerator after heat exchange against the hot lean solvent from the regenerator. Acid gases are stripped from the solvent in the trayed regenerator via a steam reboiler. The hot lean solvent from the regenerator bottom is pumped back to the absorber after being heat exchanged with rich solvent and cooling water to lower its temperature. Acid gas from TGTU amine regenerator overhead is recycled back to the Claus plant for sulfur recovery.
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 Figure 2: A Simplified SCOT Tail Gas Treating Scheme |
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Sulfuric Acid
The option to recover sulfur in the form of sulfuric acid is practiced at Tampa Electric’s IGCC demonstration plant. Figure 3 shows a simplified flow of the Tampa Electric IGCC sulfuric acid plant. The sulfuric acid plant receives the H2S from the AGR unit and H2S and ammonia from the water stripper. The gas streams are then burned in a decomposition furnace, where the H2S produces primarily SO2 with trace amounts of SO3, sulfuric acid and elemental sulfur and the ammonia is converted to N2 and water. The decomposition furnace exit gas is cooled from about 1,950°F to 650°F in a waste heat boiler to produce medium pressure steam for in plant use. The gas is then further cooled and dried. This step produces a 'weak acid' waste stream which needs to be neutralized before discharging into the cooling pond. The SO2 and oxygen (from either air or an air separation plant) then react over a vanadium based catalyst bed in a converter according to the reaction;
SO2 + ½ O2 → SO3 |
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| The produced SO3 is then reacted with water as follows: |
SO3 + H2O → H2SO4 |
The catalytic oxidation of SO2 to SO3 is highly exothermic, and the equilibrium becomes increasingly unfavorable for SO3 formation as temperature increases to about 800 °F. For this reason, special catalytic converters (reactors) are designed as multistage reactor bed units with air cooling between each bed for temperature control.
Gas from the final reactor beds enters the absorbing towers, where the produced SO3 reacts with the excess water in a circulating, strong (98%) sulfuric acid stream, creating additional H2SO4. This incrementally raises the concentration of the sulfuric acid so that water is introduced as needed to maintain the H2SO4 at 98.5% as the final product.
The Tampa Electric sulfuric acid plant is very efficient, converting over 99.5% of the incoming H2S to H2SO4.
Figure 3: Tampa Electric IGCC Sulfuric Acid Plan Flow Diagram
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