ABSTRACT

The aim of this work is to determine the molecular and stable carbon isotope compositions of pyrolysates from normal octadecane and, then, to study methane generation kinetic and carbon isotope fractionation of gaseous hydrocarbons generated from n-octadecane cracking. Pyrolyses were carried out in an anhydrous closed system (gold tubes) under a constant pressure of 50 MPa at heating rates of 20°C/h and 2°C/h. Results show that secondary cracking of pyrolysates from n-octadecane largely contributes to the amount of methane generation, much more than primary cracking of n-octadecane. Cracking and polymerization at relatively low temperatures and disproportionation reactions leading to light hydrocarbons and polyaromatic hydrocarbons at high temperatures are probable causes for the carbon isotope reversal of gaseous hydrocarbons that is commonly observed in pyrolysis experiments. This study of methane generation kinetics suggests that n-alkane hydrocarbon cracking begins to generate methane at 170°C but a great deal would be generated at 200°C in sedimentary basins.