Raw Gas Shift
For applications where it is desired to perform CO shift on raw syngas, a cobalt — molybdenum catalyst, variously described as a “sour shift” or “dirty shift” catalyst, can be used. In some parts of the literature this catalyst is described as sulfur tolerant. This is actually a misnomer, since the catalyst requires sulfur in the feed gas to maintain it in the active sulfided state. It is generally applied after a water quench of the raw syngas, which typically will provide a gas at about 250°C saturated with sufficient water to conduct the shift reaction without any further steam addition. For an ammonia application the raw gas shift is typically configured as two or three adiabatic beds with intermediate cooling resulting in a residual CO of about 1.6 or 0.8 mol%, respectively.
An important side-effect of the raw gas shift catalyst is its ability to handle a number of other impurities characteristic of gasification. COS and other organic sulfur compounds are largely converted to H2S, which eases the task of the downstream AGR. HCN and any unsaturated hydrocarbons are hydrogenated.
Carbonyls are decomposed and deposited as sulfides, which increases the pressure drop over the bed. Selective removal of arsenic in the feed is also claimed (BASF undated).