Water-Gas-Shift / COS Hydrolysis
Depending on application, after particulate removal, raw syngas from gasification may need to be conditioned with a water-gas shift (WGS) reaction to adjust the hydrogen-to-carbon monoxide (H2/CO) ratio to meet downstream process requirements. In applications where very low sulfur (< 10 ppmv) synthesis gas (syngas) is required, converting carbonyl sulfide (COS) to hydrogen sulfide (H2S) before sulfur removal may also be necessary.
Water Gas Shift
In applications where a high H2/CO ratio is needed, syngas is passed through a fixed-bed reactor containing shift catalysts to convert carbon monoxide (CO) and water into additional H2 and CO2 according to the following reaction:
CO + H2O ↔ H2 + CO2
The shift reaction will operate with a variety of catalysts between 400°F and 900°F. The reaction is equal molar and therefore the effect of pressure on the reaction is minimal. The equilibrium for H2 production is favored by high moisture content and low temperature for the exothermic reaction. The WGS reactor can be located either before the sulfur removal step (sour shift) or after sulfur removal (sweet shift).
Sour shift uses a cobalt molybdenum catalyst and is normally located after the water scrubber, where syngas is saturated with water at about 450°F to 500°F, depending on the gasification conditions and the amount of high temperature heat recovery. The scrubber syngas feed is normally re-heated to 30°F to 50°F above saturation before entering the shift reactor to avoid catalyst damage by liquid water. An important benefit of sour shift is its ability to also convert COS and other organic sulfur compounds into H2S to make downstream sulfur removal easier. Therefore, syngas treated through WGS does not need separate COS hydrolysis conditioning.
A conventional high temperature (HT) sweet shifting operates between 550°F to 900°F and uses chromium or copper promoted iron-based catalysts. Because syngas from the sulfur removal process is saturated with water at either near or below ambient temperature, steam injection or other means to add moisture to the feed is normally needed for HT sweet shifting.
A conventional low temperature (LT) sweet shift, typically used to reduce residual CO content to below 1%, operates between 400°F to 500°F and uses a copper-zinc-aluminum catalyst. LT sweet shifting catalysts are extremely sensitive to sulfur and chloride poisoning and are normally not used in coal gasification plants.
Sweet shift is normally not used for coal gasification applications. This is due to the inefficiency of having to cool the syngas before sulfur removal, which condenses out all of the moisture gained in the water scrubber, and then reheating and re-injecting the steam into the treated gas after H2S removal to provide moisture for shift. Sour shift is normally preferred for coal gasification applications since the moisture gained in the water scrubber is used to drive the shift reaction to meet the required H2/CO ratio. For most slurry-fed gasifiers, a portion of the syngas feed may need to be bypassed around the sour shift reactor to avoid exceeding the required product H2/CO ratio. Depending on the gasification process and the required H2/CO ratio, additional steam injection before sour shift may be needed for dry-fed gasifiers.
Shifted syngas is cooled in the low temperature gas cooling (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.
Most of the sulfur in the coal is converted to H2S in the gasifier. Depending on the gasification temperature and moisture content, approximately 3 to 10% of the sulfur is converted to COS. To produce sweet syngas with less than 10 ppmv sulfur for downstream applications, the COS needs to be converted to H2S before sulfur removal in most current commercial acid gas removal (AGR) processes. 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 re-heated to 30°F to 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 equal molar and is therefore 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 among large amount of non-reacting components, yielding 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.
- Catalyst Handbook – Chapter 6: Water-gas-Shift Reaction (1996)
Edited by Martyn V. Twigg, Second Edition, Manson Publishing