Steam reacts with carbonaceous materials including hydrocarbons, carbohy­drates, oxygenates, natural gas, and even graphite at elevated temperatures and generates carbon monoxide and hydrogen. The stoichiometric chemical reactions in this class of reaction include:

C(s) + H2O(g) = CO(g) + H2 (g)

Coal + Steam = CO + H2

CH4 + H2O = CO + 3 ■ H2

V

The first reaction represents steam gasification of carbon, whereas the second reaction is steam gasification of coal. The third reaction is steam ref­ormation of methane (or, methane steam reformation, MSR), whereas the fourth reaction is known as reformation of hydrocarbon fuels. The chemi­cal equilibrium favors the forward reaction of steam gasification of carbon, if the temperature of reaction exceeds 674°C, as explained in Section 5.3.1 and Table 5.9. This threshold temperature (and its vicinity) for forward reac­tion progress is nearly universally applicable to all hydrocarbon species including coal. As clearly shown, product hydrogen in these reactions at least partially originates from water (steam) molecules. Without separately going through a water-splitting reaction, this reaction efficiently extracts hydrogen out of water molecules and carbon atoms in the hydrocarbon molecules react with oxygen atoms from water molecules. As expected, the forward reactions, that is, steam gasification reactions as written, are highly endothermic at practical operating temperatures, requiring high energy input.

As mentioned earlier in the pyrolysis section, even when hydrocarbons are reformed or gasified by steam at elevated temperatures, thermochemical conversion due to pyrolysis is also taking place as a competing and parallel reaction to the steam gasification reaction. If a hydrocarbon feedstock, such as coal and biomass, is introduced into a reactor where steam gasification is desired at an elevated temperature, the resultant reaction usually proceeds as an apparent two-stage reaction, viz., appearing to be the pyrolysis reac­tion followed by steam gasification. Mathematically, this apparent two-stage reaction process can be explained by the result of superposition of two par­allel reactions between one very fast reaction (pyrolysis) and one slow reac­tion (gasification). In this explanation, easily pyrolyzable components are rapidly broken down in the early period (e. g., a matter of a few seconds), whereas much slower gasification takes place more steadily over a much lon­ger period of time (e. g., a matter of 0.5-3 hours). In coal gasification studies, some researchers interpreted this early stage pyrolysis result as an initial conversion [33].

Because biomass generally contains a high level of moisture, steam gas­ification reaction is nearly always present with or without a separate feed of steam into the reactor, except in the case of fast pyrolysis. In the fast pyrolysis of biomass, the typical temperature of operation is around 500°C, which is substantially lower than the steam gasification temperature, and therefore the biomass moisture is not involved in the steam gasification reaction.