Syngas Contaminant Removal and Conditioning

Water Gas Shift & Hydrogen Production

Water Gas Shift
In applications where scrubbed syngas hydrogen/carbon monoxide (H2/CO) ratio must be increased/adjusted to meet downstream process requirements, the syngas is passed through a multi-stage, fixed-bed reactor containing shift catalysts to convert CO and water into additional H2 and carbon dioxide (CO2) according to the following reaction known as the water-gas shift (WGS) reaction:

CO  +  H2O   ↔  H2  +  CO2

The shift reaction will operate with a variety of catalysts between 400°F and 900°F. The reaction does not change molar totals and therefore the effect of pressure on the reaction is minimal. However, the equilibrium for H2 production is favored by high moisture content and low temperature for the exothermic reaction. Normally, excess moisture is present in the scrubber syngas from slurry-fed gasifiers sufficient to drive the shift reaction to achieve the required H2-to-CO ratio. Indeed, for some slurry-fed gasification systems, a portion of the syngas feed may need to be bypassed around the sour shift reactor to avoid exceeding the required product H2-to-CO ratio. On the other hand, additional steam injection before the shift may be needed for syngas output by dry-fed gasifiers.

In any case, the scrubber syngas feed is normally reheated to 30 to 50°F above saturation temperature to avoid catalyst damage by condensation of liquid water in the shift reactor. 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.

There is some flexibility for locating the WGS reactor: it 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. An important benefit of sour shift is its ability to also convert carbonyl sulfide (COS) and other organic sulfur compounds into hydrogen sulfide (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, given the problems of sulfur and chloride poisoning as mentioned above, in addition 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.

Hydrogen Production
As explained above, water gas shift is commonly used to adjust H2 to CO ratios in syngas for many end products or purposes of coal gasification. However, in the production of hydrogen it is an essential post-gasification operation and used to convert all CO present in the syngas to CO2, yielding the maximum possible amount of hydrogen. The principles of shift reactions, catalysts used, and reactor setups are the same as discussed above but with an emphasis of process configurations to take the shift reaction to a complete extent. Discussion on hydrogen production from coal contains descriptions of process arrangements to generate hydrogen as a primary product or coproduct from coal.

References/Further Reading
  • Catalyst Handbook – Chapter 6: Water-gas-Shift Reaction (1996)
    Edited by Martyn V. Twigg, Second Edition, Manson Publishing

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