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Direct conversion
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As of today, it seems that no 'direct conversion' method is available for DeCO2 application, but only for selective removal of H2S as sulfur from gases such as COG, coal gas and Claus tail gas. No significant amount of CO2 is removed practically.
Direct conversion processes is divided into 'wet (liquid phase catalytic) oxidation' and 'scavenging' methods. In the wet oxidation process, H2S removal is accomplished by so-called 'redox (reduction and oxidation)' reactions based on the proprietary liquid redox catalyt for individual process to convert H2S to solid sulfur.
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Stretford process used to employ (seems not to be licensed any more) a redox liquod catalyst of an alkaline solvent containing sodium carbonate, ADA and, as activator, sodium metavanadate. Both LO-CAT process, originally developped by ARI Technologies and once aquired by US, having been licensed by Merchem group since 2001, and SulFerox process (also seems not to be licensed any more) use iron chlete as the liquid redox catalyst. Ionic iron used in solution is used as the electron receptor for H2S.
What is the difference between SulFerox and LO-CAT ? First difference is catalyst concentration. SulFerox iron concentration is usually 2%; LO-CAT is usually about 0.1% to 0.05%. The more dilute catalyst system of the LO-CAT has benefits in system stability, ease of operation and catalyst consumption. LO-CAT also uses a prprietary chelate system that is more resistant to oxidation and further enhancing system stability. However, the LO-CAT unit will have higher circulation rates and slightly higher power costs.
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As scavenging of H2S applying irreversible reaction, zinc oxide (ZnO) is used for desulfurization of NG, containing very low sulfur, prior to steam reforming. ZnO may become more important for final removal of all sulfur components in coal gasification plant. Byproduct ZnS shall be disposed. One of Johnson Matthey's Puraspec series catalyst using mixture (Cu/Zn) oxides reacts with COS asfollows and can be used for much deeper desulfurization of NG as low as the order of ppb.