The synthesis gas produced by the BCL and IGT gasifiers has a low H2:CO ratio. The water gas shift (WGS) reaction (Equation 2.4) is a common process operation to shift the energy value of the carbon monoxide to the hydrogen, which can then be separated using pressure swing adsorption. If the stoichiometric ratio of H2, CO, and CO2 is unfavorable for methanol production, the water gas shift can be used in combination with a CO2 removal step. The equilibrium constant for the WGS increases as temperature decreases. Hence, to increase the production to H2 from CO, it is desirable to conduct the reaction at lower temperatures, which is also preferred in view of steam economy. However, to achieve the necessary reaction kinetics, higher temperatures are required (Armor 1998; Maiya et al. 2000).

CO + H2O о CO2 + H2 (2.4)

The water gas shift reaction is exothermic and proceeds nearly to completion at low temperatures. Modern catalysts are active at temperatures as low as 200°C (Katofsky 1993) or 400°C (Maiya et al. 2000). Due to high-catalyst selectivity, all gases except those involved in the water-gas shift reaction are inert. The reaction is independent of pressure.

Conventionally, the shift is realized in a successive high temperature (360°C) and low temperature (190°C) reactor. Nowadays, the shift section is often sim­plified by installing only one CO-shift converter operating at medium temperature (210°C) (Haldor Topsoe 1991). For methanol synthesis, the gas can be shifted partially to a suitable H2:CO ratio; therefore, “less than one” reactor is applied. The temperature may be higher because the reaction needs not to be complete and this way less process heat is lost.

Theoretically the steam:carbon monoxide ratio could be 2:1. On a lab scale good results are achieved with this ratio (Maiya et al. 2000). In practice extra steam is added to prevent coking (Tijmensen 2000).