Parametric evaluation of carbon dioxide and steam co-gasification of sewage sludge and palm kernel shell in a downdraft fixed bed reactor: Computational Fluid Dynamics (CFD) approach

Thermally converting sewage sludge (SS) to harness its energy potential brings challenges like high ash content, which can cause system instability. However, co-gasifying SS with carbon-rich materials has shown to be more advantageous. This study analyses a two-dimensional Eulerian CFD numerical model for a CO₂ and steam cogasification of SS and palm kernel shell in a downdraft gasifier by solving the governing equations of mass phases, turbulence, energy, and momentum on a high-resolution mesh model. The devolatilization phase is defined by comprehensive solid carbon pyrolysis kinetic mechanisms and secondary gas reaction mechanisms, whilst the gasification and combustion processes are governed by applying detailed heterogeneous and homogeneous rate-controlled reactions. An experimental assessment with temperature of 1000 °C; ER of 0.28; fuel feed rate of 0.00061 kgs⁻¹; at atmospheric pressure was conducted to validate the current model considering the temperature distribution and gas composition. The validated model is further used to evaluate the effect of mixing ratios, steam-to-fuel ratio (S/F), carbon dioxide-fuel ratio (CO₂/F), and gasification temperature on syngas composition, lower heating value, hot gas efficiency, gas yield, H₂/CO and synergistic coefficient. The predicted results agree well with the experimental data with the maximum deviational error of 9.52 %. The highest H₂ production was recorded during steam gasification, whilst CO was favored by CO₂ gasification. Synergistic analysis presented the highest synergy coefficient for the SS mixing ratio of 25 % at 1.91 and 50 % at 1.93 for steam and CO₂ gasification respectively.