The term hydrogasification is the reaction between carbonaceous mate­rial and hydrogen, strictly speaking. However, an augmented definition of "hydrogasification" involves the reaction of carbonaceous material in a hydrogen-rich environment to generate methane as a principal product. The gasification reaction in certain steam environments, such as in the copresence of steam and hydrogen, often qualifies for this hydrogen-rich environment for methane generation. The latter is called steam hydrogasifi­cation [38]. However, the steam gasification whose principal goal is to pro­duce syngas should still be referred to as "steam gasification," not simply as "hydrogasification." "Hydro-" as a prefix is used for "of water" or "of hydrogen," depending upon the situation. As far as the gasification of coal and biomass is concerned, the prefix "hydro-" means "of hydrogen," as in the case with "hydrocracking."

Carbonaceous materials undergo hydrocracking under high pressures of hydrogen at elevated temperatures. Hydrocracking generates lighter hydro­carbons as cleavage products from larger hydrocarbons. Although the
hydrocracking reaction is chemically distinct from hydrogasification of car­bon, the difference between the two becomes small when it is applied to coal or coal char whose molecular structure is deficient in hydrogen.

Unlike other gasification reactions involving steam and carbon dioxide, this gasification reaction is exothermic, that is, generating reaction heat, as

C(S) + 2 H2(g) = CH 4 (-ДН098) = 74.8 kj/mol

Coal char hydrogasification can be regarded as two simultaneous reac­tions differing considerably in their reaction rates [33], as also mentioned in the earlier section on steam gasification. This statement of apparent two — stage reactions of pyrolysis and gasification is valid for biomass gasification as well. Due to the high moisture content in raw untreated biomass, biomass hydrogasification always involves steam hydrogasification, where all three principal modes of gasification—including pyrolysis, steam gasification, and hydrogasification—take place simultaneously. Of these reactions, pyrolysis is by far the fastest chemical reaction at the operating conditions.

Hydrogasification of carbons and biomass can be catalyzed for faster and more efficient reactions [39]. Many metallic ingredients have been shown to have catalytic effects on hydrogasification of coal char and carbon and these catalysts include aluminum chloride [40], iron-based catalysts [41], nickel — based catalysts [42], and calcium salt-promoted iron group catalysts [43].

An interesting study was carried out by Porada [44], in which hydrogasifi­cation and pyrolysis of basket willow (Salix viminalis), bituminous coal, and a 1:1 mixture of the two were compared. Their study employed a nonisother­mal kinetics approach, in which the reaction temperature was increased at a constant rate of 3 K/min from ambient temperature to 1,200 K under the hydrogen pressure of 2.5 MPa. Of the test samples, the highest gas yields were obtained during hydrogasification of coal and the lowest yields were observed in the basket willow processing. It was also established that the conversion ratio to C1-C3 hydrocarbons from C under a relatively low H2 pressure was approximately five times higher than the pyrolysis conducted in an inert atmosphere. This clearly explains that the beneficial role of hydro­gen gas is very significant in gasification of biomass as well as coal.