The process gas exiting the secondary reformer contains 12-15% (dry gas base) of CO. In shift conversion, most of the CO will be converted to CO₂. The performance of the shift conversion is very important for the overall efficiency of the ammonia plant, because unconverted CO will consume hydrogen and form CH₄ in the methanator, reducing the feedstock efficiency and increasing the inert content in the synthesis loop.

Conventionally, the conversion reaction is conducted in two steps with heat removal steps in between. Initially, the process gas passes through a bed of iron oxide/chromium oxide catalyst at around 350-380^oC (High Temperature Shift conversion) and then over a copper oxide/zinc oxide catalyst at around 200-220^oC (Low Temperature Shift conversion). The outcome is process gas with a residual CO content of 0.2-0.4% (dry gas base). New developments can employ an isothermal shift one-step conversion. The process gas exiting the low temperature shift converter is cooled and after most of the steam is condensed and removed it passes through the CO₂ removal section. Heat released during cooling and condensation can be used for other purposes such as the regeneration of the CO₂ scrubbing unit (IPTS/EC, 2007).

The steam consumption in the shift conversion and the CO₂ removal processes is about 0.8-1.2 GJ/tonne and the electricity consumption is about 0.2 GJ/tonne of ammonia (56 kWh/tonne) (Worrell et al., 2000 p.21).