Hydrogen concentration enhancement in co-pyrolysis of Chlorella vulgaris microalgae and empty fruit bunches: RSM optimisation and fixed-bed reactor comparison

Abstract

Hydrogen-rich syngas production from biomass remains constrained by limited hydrogen concentration and insufficient understanding of how reactor configuration influences hydrogen concentration under optimised conditions. This study systematically investigates hydrogen concentration (vol%) from Chlorella vulgaris microalgae and microalgae-empty fruit bunch (MA-EFB) co-pyrolysis using two fixed-bed reactor configurations: a custom-designed horizontal fixed-bed reactor (custom reactor) and a conventional tube furnace. Response surface methodology (RSM) was employed to optimise temperature (400–700 °C), residence time (20–60 min), particle size (150–300 µm), and MA blending ratio (0.25–0.75 g/g). Feedstock characterisation confirmed strong physicochemical complementarity, supporting favourable thermochemical interactions during co-pyrolysis. For microalgae pyrolysis, validated hydrogen concentrations of 48.28 vol% in the custom-designed horizontal fixed-bed reactor (custom reactor) and 44.81 vol% in the tube furnace were achieved under optimised conditions, with temperature identified as the dominant factor influencing hydrogen concentration. Co-pyrolysis further enhanced hydrogen concentration, achieving peak values of 58.94 vol% (custom reactor) and 56.48 vol% (tube furnace) at ∼666 °C, 60 min, MA ≈ 0.75, and 150 µm particle size. Statistical analysis produced highly significant quadratic models (R² > 0.996, CV < 5%), confirming the presence of an optimal operating window. Differences in hydrogen concentration between the two reactor configurations suggest that reactor configuration may influence hydrogen concentration, with the custom reactor consistently achieving approximately 2–3 vol% higher hydrogen concentration than the tube furnace under comparable conditions. These findings highlight the potential of MA-EFB co-pyrolysis under atmospheric, non-catalytic, fixed-bed conditions for enhancing hydrogen concentration (vol%) within the product gas, while further investigation incorporating absolute hydrogen productivity measurements and full gas composition analysis is required to evaluate overall process efficiency and scalability.