Methane–hydrogen–air mixtures present a viable alternative to conventional fuels, reducing CO2 emissions while maintaining high energy density. This study numerically investigates their combustion characteristics in millimeter-scale reactors, focusing on flame stabilisation and combustion dynamics in confined spaces. A species transport model with volumetric reactions incorporated a detailed kinetic mechanism with 16 species and 41 reactions. The simulations employed a laminar flow model, second-order upwind discretisation, and SIMPLE algorithm for pressure–velocity coupling. The key parameters analysed include equivalence ratio, hydrogen volume fraction, inlet velocity, and gas pressure and their impact on fuel conversion efficiency and heat release was evaluated. The results indicate that hydrogen enrichment enhances flame stability and combustion efficiency, with optimal performance over 40% hydrogen content. Additionally, increased outlet pressure raises flame temperature by 15%, while larger reactor diameters reduce heat losses, improving combustion efficiency by 20%. Emissions of CO decrease significantly at higher hydrogen fractions, demonstrating the potential for cleaner combustion. These findings support the integration of methane–hydrogen mixtures into sustainable energy systems, providing insights for designing efficient, low-emission micro-combustors. © 2025 by the authors.