Cold Wet Gas Cleaning or Conventional Gas Cleaning
Cold gas cleaning (CGC), or conventional gas cleaning, is another common technique used in the purification of biomass-derived syngas. In this method gas is washed by exposure to a solvent or a liquid adsorbent. The operating temperature in cold gas cleaning may be as high as the condensation point of the water used for tar and particulate scrubbing, or as low as -62°C for chilled methanol used in removing acid gases. Particulate matter is typically removed at ambient temperatures using water as a "wet scrubbing" agent. Wet scrubbing is widely deployed in industry given its relative simplicity and effectiveness. Cold gas scrubbing can be characterized according to operating principles: spray scrubbers, wet dynamic scrubbers, cyclonic spray scrubbers, impactor scrubbers, venturi scrubbers, and electrostatic scrubbers [1].
Wet gas cleaning can be used for removal of particulate matter, tars, acid gases like HCl and other trace organic and inorganic components. Conventional acid gas removal processes can be carried out with techniques such as the Rectisol process. In the Rectisol process (licensed by both Linde AG and Lurgi AG), cold methanol at -40°C is used to absorb the acid gases from the feed gas at relatively high pressure, usually 2.7 to 7.0 MPa. Another widely used scrubber is the solvent identified in the trade name Selexol, and the use of this solvent is known as the Selexol process. In the Selexol process (now licensed by UOP LLC), the Selexol solvent absorbs the acidic gases such as carbon dioxide and hydrogen sulfide from the feed gas at relatively high pressure, usually 2 to 14 MPa. The pressure in the rich solvent containing the acid gases is then reduced or steam stripped to release the acid gases from the solvent. This process can operate selectively to recover H2S and CO2 as separate streams so that the H2S can be sent to separate reactors for conversion to elemental sulfur or sulfuric acid [1].
Scrubbers that undergo chemical transformations with impurities in biomass-derived syngas are known as chemical scrubbers. Alkanol amine has been used to remove acidic gases such as HCl, H2S, and CO2. One such solvent is aliphatic amine methyldiethanol — amine (MDEA). For example, HCl in the gas stream can be removed by reacting with MDEA as shown in the equation in Figure 11.6, which separates as the ammonium salt. Chemical scrubber solvents
СНз Hx zCH3
HO^^N^^OH + HCl ————- ► HO + ^"OH
cT
Figure 11.6 Methyldiethanolamine (MDEA) as a chemical scrubber for removal of HCl in biomass-derived syngas.
are favorable at low acid-gas partial pressures, whereas physical solvents are typically used at high acid-gas partial pressures.