The water gas shift reaction plays an important role in manufacturing hydro­gen, ammonia, methanol, and other chemicals. Nearly all synthesis gas reac­tion involves a water gas shift reaction in some manner. The WGS reaction is of a reversible kind, whose proceeding direction can be relatively easily reversed by changing the gaseous composition as well as varying the tem­perature of the reaction. As shown in Table 5.9, the temperature dependence of chemical reaction equilibrium constant (Kp) is the mildest of the important syngas reactions considered in the table. Furthermore, the temperature when Kp becomes unity is around 814°C, where most carbon gasification reactions begin to be kinetically active.

The water gas shift reaction has dual significance. The forward water gas shift reaction converts carbon monoxide and water into additional hydrogen and carbon dioxide. This reaction is utilized to enhance the hydrogen yield from raw or intermediate syngas, such as raw product of steam reformation of hydrocarbons. If the water gas shift reaction is exploited in its reverse direction, that is, as a reverse water gas shift reaction, carbon dioxide can be reduced to carbon monoxide that is far more reactive than carbon dioxide. This enables further chemical conversion of carbon monoxide into other use­ful petrochemicals, instead of direct conversion of carbon dioxide, which is much more difficult as a task. The catalytic reverse water gas shift reaction could be very useful as a reduction method of carbon dioxide.

In many industrial reactions, the water gas shift reaction is a companion reaction to the principally desired reaction in the main stage, as evidenced in methanol synthesis and in steam reformation of methane. Whenever deemed appropriate, WGS is also carried out as a secondary stage reaction to result in additional conversion of water gas into hydrogen, as desired by a fuel reformer to generate hydrogen for proton exchange membrane (PEM) fuel cell application. Because the forward water gas shift is an exothermic reaction, low temperature thermodynamically favors higher CO conversion, and its intrinsic kinetic rate without an aid of an effective catalyst is inevi­tably low at low temperatures. Therefore, most water gas shift reaction is carried out catalytically at a low temperature such as 180-240°C. This type of catalyst is called a low-temperature shift (LTS) catalyst, which has long been used industrially. One of the LTS catalyst formulations is coprecipitated Cu/ ZnO/Al2O3 catalyst, whose formulation is better known for the low-pressure methanol synthesis catalyst [24].

In biomass gasification for generation of biosyngas, the water gas shift reaction plays an important role, inasmuch as it can produce additional hydrogen and it can also be used to control the ratio of H2:CO in the syn­thesis gas composition. The WGS reaction not only enhances the targeted gas composition with a higher selectivity, but also prepares a syngas more suitable for the next-stage conversion by adjusting it for an optimal syngas composition.