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Primary gasification involves thermal decomposition of the raw feedstock via various chemical processes (Table 2.6) and many schemes involve pressures ranging from atmospheric to 1000 psi. Air or oxygen may be admitted to support combustion to provide the necessary heat. The product is usually a low heat content (low-Btu) gas ranging from a carbon monoxide/hydrogen mixture to mixtures containing varying amounts of carbon monoxide, carbon dioxide, hydrogen, water, methane, hydrogen sulfide, nitrogen, and typical tar-like products of thermal decomposition of carbonaceous feedstocks are complex mixtures and include hydrocarbon oils and phenolic products (Dutcher et al., 1983; Speight, 2011a, 2013, 2014b).

Table 2.6 Types of reactions that occur in a gasifier.

2 C + O2 → 2 CO

C + O2 → CO2

C + CO2 → 2 CO

CO + H2O → CO2 + H2 (shift reaction)

C + H2O → CO + H2 (water gas reaction)

C + 2 H2 → CH4

2 H2 + O2 → 2 H2O

CO + 2 H2 → CH3OH

CO + 3 H2 → CH4 + H2O (methanation reaction)

CO2 + 4 H2 → CH4 + 2 H2O

C + 2 H2O → 2 H2 + CO2

2 C + H2 → C2H2

CH4 + 2 H2O → CO2 + 4 H2

Devolatilization of the feedstock occurs rapidly as the temperature rises above 300°C (570^oF). During this period, the chemical structure is altered, producing solid char, tar products, condensable liquids, and low molecular weight gases. Furthermore, the products of the devolatilization stage in an inert gas atmosphere are very different from those in an atmosphere containing hydrogen at elevated pressure. In an atmosphere of hydrogen at elevated pressure, additional yields of methane or other low molecular weight gaseous hydrocarbon derivatives can result during the initial gasification stage from reactions such as: (i) direct hydrogenation of feedstock or semi-char because of any reactive intermediates formed and (ii) the hydrogenation of other gaseous hydrocarbon derivatives, oils, tars, and carbon oxides. Again, the kinetic picture for such reactions is complex due to the varying composition of the volatile products which, in turn, are related to the chemical character of the feedstock and the process parameters, including the reactor type.

A solid char product may also be produced, and may represent the bulk of the weight of the original feedstock, which determines (to a large extent) the yield of char and the composition of the gaseous product.