If hydrogen production is the main goal, the carbon monoxide produced is
further subjected to a shift reaction (see Eq. 9.2), as described in the next
section, to produce additional hydrogen and carbon dioxide.
The ratio of hydrogen and carbon monoxide in the gasification product gas
is a critical parameter in the synthesis of the reactant gases into desired products
such as gasoline, methanol, and methane. The product desired determines that
ratio. For example, gasoline may need the H2/CO ratio to be 0.5 to 1.0, while
methanol may need it to be ~2.0 (Probstein and Hicks, 2006, p. 124). In a commercial
gasifier the H2/CO ratio of the product gas is typically less than 1.0, so
the shift reaction is necessary to increase this ratio by increasing the hydrogen
content at the expense of CO. The shift reaction often takes place in a separate
reactor, as the temperature and other conditions in the main gasifier may not
be conducive to it.