Gas generation and its isotope composition during coal pyrolysis: The catalytic effect of nickel and magnetite

Abstract

Pyrolysis experiments were conducted at 370–550 °C for 72 h in closed gold capsules to investigate the effect of nickel and magnetite on gas generation from coal in the absence and presence of water.Nickel had a great catalytic effect on the yield, molecular and carbon and hydrogen isotope composition of generated gases. The methane yields increased significantly while heavy hydrocarbon gas yields decreased substantially due to the catalysis of nickel both in the absence and presence of water. The nickel catalysis caused an increase of δ13CCH4 value in the absence of water and a decrease of δ13CCH4 value in the presence of water. Results of molecular and carbon isotopic composition of hydrocarbons indicate that nickel could catalyze the decomposition of high-molecular-weight organic matter and Fischer-Tropsch-type (FTT) reactions both of which were responsible for the increased methane yield. FTT reactions catalyzed by transition metals are an important pathway for methane generation, especially in the presence of water. The catalytic decomposition of organic matter and FTT reactions also caused an increase of δ2HCH4 value both in the presence and absence of water. CO2 yield and 13CCO2 value increased obviously in the presence of nickel due to the catalytic decomposition of organic matter.In experiments without added water, addition of magnetite enhanced CO2 yield at all temperatures but only enhanced methane yield slightly at high temperature (430–550 °C). The increased CO2 and methane yields can be attributed to the oxidative decomposition of long-chain hydrocarbons via redox reactions between magnetite and hydrocarbons. In experiments with added water, addition of magnetite only caused a slight increase of methane and CO2 yields at 500 °C and 550 °C but did not influence their yields at 370–450 °C, indicating that water inhibited the effect of magnetite on methane and CO2 generation at low temperature. The addition of magnetite did not have systematic influence on carbon and hydrogen isotopes of hydrocarbon gases and carbon isotopes of CO2. Generally, the influence of magnetite on gas generation was very small.