Most of the sulfur in the coal is converted to H2S during gasification. However, depending on the gasification temperature and moisture content, approximately 3 to 10% of the sulfur is converted to carbonyl sulfide (COS). In applications where very low sulfur (< 10 ppmv) syngas is required, converting COS to H2S before sulfur removal may be necessary, because most current commercial AGR processes do not efficiently remove sulfur in the form of COS. This is done by passing syngas from the water scrubber through a catalytic hydrolysis reactor where over 99% of the COS is converted to H2S according to the following reaction:
COS + H2O ↔ H2S + CO2
The scrubbed syngas feed is normally reheated to 30-50°F above saturation before entering the reactor to avoid catalyst damage by liquid water. COS hydrolysis uses an activated alumina-based catalyst, and is normally designed to operate at 350°F to 400°F. The reaction is largely independent of pressure. The equilibrium for COS conversion is favorable at low temperatures due to the exothermic nature of the reaction; however, the heat of reaction is normally dissipated throughout a large amount of non-reacting components, yielding nearly isothermal reactor conditions.
COS hydrolysis product gas is cooled in the LTGC system by generating low pressure steam, preheating boiler feed water, and heat exchanging against cooling water before going through the acid gas removal system for sulfur removal.