Effects of Acetylene and Hydrogen Cofeeding in Microwave Plasma Methane Pyrolysis

Methane pyrolysis offers low-carbon hydrogen production by decomposing CH4 into H2 and solid carbon. However, the formation of stable hydrocarbon intermediates, particularly acetylene (C2H2), can limit the overall conversion efficiency and associated carbon-emission benefits. In this work, C2H2 cofeeding and combined C2H2 and H2 cofeeding were investigated in a microwave plasma reactor to emulate product recirculation and evaluate their impact on process performance and carbon formation. Experiments were conducted over microwave powers of 1.1–2.5 kW with C2H2 feed fractions ranging from 0 to 100%. Three distinct operational regimes were identified based on normalized energy input (NEI). At low NEI (<1), H2 selectivity remained constant up to 50% C2H2 before decreasing sharply due to preferential CH4 decomposition. At high NEI (>2), efficient conversion of both species was measured, resulting in a linear decrease in H2 selectivity with increasing C2H2 fraction. Intermediate NEI values (1–1.8) exhibited transitional behavior, characterized by a gradual selectivity decay beyond 20% C2H2. Combined C2H2 and H2 cofeeding reduced total hydrocarbon conversion by up to 20 percentage points and decreased H2 selectivity while increasing C2H4 selectivity to as high as 23%, consistent with equilibrium shifts favoring reverse reactions. Carbon characteristics were strongly influenced by cofeeding. C2H2 addition increased primary particle sizes from 50–70 nm to 100–200 nm, while combined C2H2 and H2 cofeeding yielded particles as large as 0.5–1 μm. Surface areas increased from ∼100 m2/g to 400–550 m2/g with C2H2, accompanied by an increase in microporosity. While cofeeding does not improve the overall reactivity, C2H2 incorporation enables tunable carbon morphology and porosity, offering pathways to produce amorphous carbon with varying properties within a single reactor system.