Experimental and mechanistic study on the impact of methane addition on the explosion limits of H 2 /CO syngas

Syngas, a mixture primarily composed of hydrogen and carbon monoxide, is widely used in energy and chemical industries. In practical scenarios such as catalytic reforming, methane is often blended into syngas streams, significantly altering its combustion and explosion characteristics and posing new safety challenges. In this study, the effects of methane addition on the explosion limits, critical oxygen concentration (COC), and explosion hazard of syngas were investigated through closed-vessel explosion experiments and chemical kinetic simulations. The results show that as the methane volume fraction increases from 0% to 50%, the lower explosion limit (LEL) drops from 8.75% to 5.15%, while the upper explosion limit (UEL) decreases sharply from 59.15% to 17.05%, leading to a substantial narrowing of the flammability range. The COC also increases from 6.85% to 11.55%, indicating a reduced explosion risk and enhanced inerting resistance. An explosion hazard index was further established based on pressure rise data, confirming that methane blending significantly lowers both the maximum pressure and pressure rise rate. Reaction kinetic analysis using the NUIG-Mech 1.1 and the Aramco-Mech 3.0 mechanisms reveals that OH radicals play a dominant role in sustaining chain reactions and promoting radical propagation, whereas CH3 radicals, generated from methane, act as chain inhibitors by scavenging active species such as ∙OH and ∙H. These findings demonstrate that methane admixture weakens the explosivity of syngas by suppressing radical chain branching, narrowing the explosion envelope, and improving the intrinsic safety of syngas-based processes.