Is it possible to decompose all steam to hydrogen and C to CO?

Typically, gas producer, operates in a counter-current mode i.e. coal is charged from top and a mixture of air and steam is blown through the coal bed simultaneously and continuously from the bottom. The flow rates of air and steam are adjusted so that the heat evolution in the reaction of with C of coke/coal balances the heat absorption due to endothermic reaction of decomposition of steam. Thus

• Not all steam decomposes to H₂;i.e. some amount of steam remains undecomposed and
• Not all C is converted to CO.

The extent of decomposition of steam to hydrogen and conversion of C to CO depends on temperature, residence time of the reactants in the reactor, reactivity of carbon of coal, reaction surface area etc. The gaseous fuel produced after gasification with a moisture of air +Steam and carbon will always contain undecomposed steam and carbon dioxide besides other components.