Coupling and reforming of methane by means of low pressure radio-frequency plasmas

Non-oxidative coupling of CH4/H2 mixtures was carried out by means of radio frequency (rf) glow discharges for the first time. A central composite design was employed to determine the best experimental conditions for methane transformation into higher hydrocarbons and to fit the experimental data. rf power was the factor showing the highest effect on the results while CH4/H2 mole ratio showed the lowest. Conversion was 46.4% at 100 W, 0.07 mbar and CH4/H2 mole ratio of 1/2. Selectivity was 56.9% for C2, 6.9% for C3, and 36.2% for C4 hydrocarbons. Least squares fits of quadratic equations yielded approximating functions permitting to predict results of random experiments with errors of about 5%. The same rf system was used for the reforming of methane with CO2, O2, and steam plasmas, respectively. The highest oxidation was observed with oxygen whilst steam plasma produced the best results. H2/CO mole ratio was adjusted by setting specific experimental parameters of the latter. CO2 free synthesis gas was produced at higher H2O and CH4 flow rates, i.e. 0.8 mmol/h and higher power, i.e. 100 W. CO2 and CO free H2 was produced at 0.3 and 0.6 mmol/h flow rates of H2O and CH4, respectively, and 50 W.