Free enthalpy for pyrolytic reactions

The course of a pyrolytic reaction may be determined kinetically or thermodynamically. When the reaction is able to reach equilibrium, thermodynamic factors control the reaction outcome. The calculation of a reaction free enthalpy using rel. (2.2.25a) allows the prediction of the reaction course. Since reactions are displaced toward the formation of products when AG° 0, the calculation of the temperature for which AG° = 

0 allows the determination of which reactions take place and which do not occur at the specific temperature. For pyrolytic reactions, the temperature T satisfying relation AG° = 0 is defined as the ceiling temperature Tci 

Many reactions occurring during pyroiysis invoive the breaking of covalent bonds. These reactions are frequently endothermic. For example, pyrolysis of aliphatic hydrocarbons (also pertinent for the pyrolysis of polyethylene) may take place as a dehydrogenation or as a fragmentation. These reactions are shown below  

The reaction with the formation of an alkene is endothermic. In the case of hydrogen elimination, the C-H bond requires considerable energy, and also the breaking of a C-C single bond is not fully compensated by the formation of a new C=C double bond [23]. For a small molecule such as butane, the reaction  

The expression for the free enthalpy for the dehydrogenation reaction can be written as follows  

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