Shifts in Product Distribution in Microwave Plasma Methane Pyrolysis Due to Hydrogen and Nitrogen Addition

Methane pyrolysis can produce many valuable products besides hydrogen, e.g., C2 compounds or carbon black. In the conditions provided by microwave plasma, the distribution of these products might be shifted by the addition of hydrogen and nitrogen. In this work, different ratios of H2:CH4, ranging from 0:1 to 4:1, were tested. The most unambiguous and promising result was obtained for the highest H2:CH4 ratio. For this ratio, a significant improvement in methane conversion rate was observed (from 72% to 95%) along with the increase in C2H2 and C2H4 yield and selectivity. The results support the hypothesis that the H radicals present in the plasma are responsible for improving methane conversion, while the presence of molecular hydrogen shifts the product distribution towards C2 compounds. Based on the carbon balance, the increase in the output of C2 compounds was obtained at the cost of solid carbon. At the same time, the addition of hydrogen resulted in the formation of bigger carbon particles. Finally, with the addition of both nitrogen and hydrogen, the formation of carbon was completely inhibited. Hydrogen cyanide was the main product formed instead of soot and some of the acetylene.