Study on mechanism of C–H radicals’ recombination into acetylene in the process of coal pyrolysis in hydrogen plasma

According to computation results of C–H equilibrium systems, C₂H₂ and C₂H are the main hydrocarbon in the C–H equilibrium system at the temperature of approximately 3500 K. Because hydrogen plasma has the advantage of high temperature (over 3500 K), acetylene can be directly produced by coal pyrolysis in hydrogen plasma. In order to obtain high yields of acetylene, a quenching process is needed to fix the acetylene produced at high temperature. It is proved that an adequate quenching rate (0.775.8×10⁸ K/s) can avoid the decomposition of acetylene, but will not prevent C₂H radicals recombining into acetylene Chem. Eng. Sci. 54 (1999) 957]. A dynamic chemical method is employed in this paper to study the mechanism of C₂H radicals’ recombination into acetylene in the quenching process. Primary experiments have also been carried out to study the process of coal pyrolysis in hydrogen plasma. It is shown by the calculation results that: (1) the reaction that really has an effect on acetylene yield in the quenching process is the recombination of C₂H and H₂, and not that of C₂H and H in traditional opinions; (2) if the recombination of C₂H and H₂ is taken into account, the total mass content of acetylene in the quenched gas may increase from 58% to 78% at the quenching rate which can prevent acetylene from decomposing. The experimental results prove that C₂H radicals really recombine into acetylene in the quenching process.