Acid Gas Removal

Raw synthesis gas (syngas) produced from coal gasification needs to be cleaned of sulfur-bearing acid gases (primarily hydrogen sulfide (H₂S) and carbonyl sulfide (COS)) to meet either environmental emissions regulations, or to protect downstream catalysts for chemical processing applications. For integrated gasification combined cycle (IGCC) applications, environmental regulations require that the sulfur content of the product syngas be reduced to less than 30 parts per million by volume (ppmv) in order to meet the stack gas emission target of less than 4 ppmv sulfur dioxide (SO₂)^[1]. In IGCC applications, where selective catalytic reduction (SCR) is required to lower NO_X emissions to less than 10 ppmv, syngas sulfur content may have to be lowered to 10 to 20 ppmv in order to prevent ammonium bisulfate fouling of the heat recovery steam generator's (HRSG) cold end tubes. For chemical production, the downstream synthesis catalyst sulfur tolerance dictates the sulfur removal level, which can be less than 0.1 ppmv.

Acid gases in a gasification process typically consist of H₂S, COS, and carbon dioxide (CO₂). Current processes of removing these gases from the raw syngas typically involve countercurrent absorption with a regenerative solvent, in an absorber column. Acid-gas-rich solvent from the absorber bottom is then stripped of its acid gas in the regenerator by applying heat through reboiling. Lean solvent from the regenerator bottom is cooled and recycled to the top of the absorber and the cycle is repeated. Depending on the solvent used, COS may first need to be converted to H₂S via a COS hydrolysis unit. H₂S and CO₂ can be removed either simultaneously or selectively, depending on the raw syngas composition and conditions, and the end syngas specifications.

H₂S removed by the regenerator is sent to a sulfur recovery unit, such as a Claus Plant, to recover the sulfur as a salable byproduct. Regenerative solvent based acid gas removal (AGR) processes are commonly used in refining, chemical, and natural gas industries. These processes can be grouped into three general types: chemical solvents, physical solvents, and hybrid mixtures of chemical and physical solvents.

Chemical Solvents
Chemical solvents include primary, secondary and tertiary amines, and potassium carbonate as listed in Table 1, below. Through acid-base reactions, aqueous solutions of these basic alkanolamines, or alkaline salts, capture and remove acid gases by forming weak chemical bonds with dissolved acid gases in the absorber. The bonds are broken by heat in the regenerator to release the acid gases and regenerate the solvent for reuse. Chemical solvent absorption processes normally operate at slightly above ambient temperature. Chemical solvents are more effective for low acid gas partial pressure applications than physical solvents.

Physical Solvents
Physical solvents are organic solvents that have a high affinity for acid gases. Some of the solvents that are commercially available are listed in Table 2. Acid gases are removed from sour syngas (syngas which contains significant amounts of H₂S and COS) by dissolving the acid gases into the solvent under high partial pressure and low temperature in the absorber. Solvent, rich with acid gases from the absorber, is then subjected to multistage controlled pressure decreases, followed with hot stripping in the regenerator, to release the acid gases and regenerate the solvent for reuse. Physical solvent absorption processes normally operate at cryogenic temperatures (below –150 °C, –238°F or 123 K). In general, physical solvents are more effective for high acid gas partial pressure applications.

Hybrid Solvent Systems
Hybrid solvents are mixtures of amine chemical solvents and physical solvents to take advantage of the high treated-gas purity performance of chemical solvents, and the low energy requirement associated with flash regeneration of physical solvents. Hybrid solvents commercially available include Sulfinol-D (aqueous DIPA plus sulfolane), Sulfinol-M (aqueous MDEA plus sulfolane), FLEXSORB SE (aqueous hindered amines plus unspecified physical solvents), and FLEXSORB PS (aqueous MDEA plus unspecified physical solvents), Amisol (methanol with MDEA or diethylamine), and Selefining (methanol and toluene). Hybrid solvents allow for better acid gas absorption at high partial pressures, and feature higher solubility of COS and organic sulfur compounds than aqueous amines. The amine portion allows AGR under very low partial pressures. In general, hybrid solvents are effective over a wide range of acid gas partial pressures at near room temperatures.

Applications
There are over 30 AGR processes available commercially, but only four of these have been demonstrated or implemented in the 18 commercial-size coke or coal-based gasification plants worldwide, as reported by SFA Pacific in 2002:^[2]  Rectisol, Selexol, Sulfinol, and MDEA. Half of these18 plants are for chemical production while the other half are IGCC applications.

Eight of the nine chemical production plants, in operation as of 2002, use Rectisol (typically operates at -40°F to -80 °F), and one uses Selexol (typically operates at 20°F to 40 °F). This is consistent with the general perception that physical solvent-based AGR is normally selected to protect synthesis catalysts against poisoning from sulfur and other trace contaminants in chemical production from coal applications. While Rectisol is more costly, it is preferred for treating coal-based syngas because it allows for very deep sulfur removal (<0.1 ppmv H₂S plus COS), and also because it can remove HCN, NH₃, and many metallic trace contaminants (including iron- and nickel-carbonyls, and mercury) to provide additional catalyst protection.

Six of nine IGCC plants, in operation as of 2002, use MDEA with the remaining three using Rectisol (Sokolovska Uhelna, Lurgi gasifier), Selexol (Cool Water, GE gasifier), and Sulfinol (Nuon Buggenum, Shell gasifier). Figure 1 shows a simplified process flow diagram (PFD) of a typical MDEA-based AGR process. Figure 2 shows a simplified PFD for the Selexol process. Due to the need for refrigeration, as well as a more complex flashing arrangement, a typical physical solvent process can be two to three times more costly than an amine based chemical solvent system.

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Figure 1: Simplified MDEA Flow Diagram

Figure 2: Simplified Selexol Flow Diagram

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