The influence of CO on the carbon isotopic composition of CH4 in closed pyrolysis experiment with coal

Abstract

A low-mature coal (R(subscript o)=0.4%, from the Manjia'er depression, Tarim Basin, China) was subjected to closed system pyrolysis, in sealed gold tubes, under isothermal temperature conditions. The carbon isotopic compositions of the pyrolyst fractions (hydrocarbon, CO2, CO, etc.) at two temperature points (350℃ and 550℃) were measured. The results showed that δ 13C(subscript CH4) value is generally heavier at 350℃ than that at 550℃, because the high abundance of CO generated at low temperature would greatly influence δ 13C(subscript CH4) value, and the retention time of CO in gas chromatograph is close to that of CH4. But CO is formed through chemical reaction of the oxygen-containing functional group -C=0, e. g. lactones, ketones, ether, etc. at low temperature, while CO2 comes mainly from decarboxylization. The carbon isotopic composition of coal gas from Lanzhou Coal Gas Works was definitely different from that of thermally pyrolysed products from coal. The 6 1311 value of coal gas was abnormally heavier than δ 13(subscript CO) At the same time, the reversed sequence (δ 13C1＞δ 13C2) of δ 13C1 and δ 13C2 happened. The bond energy of free ions generally decides the sequence of generation of hydrocarbon fractions according to the chemical structure, whereas the stability of pyrolysate fractions and their carbon isotope fractionation are affected by the C-C bond energy.