Thermo-chemical coupling and transport phenomena in single biomass particle gasification within supercritical water crossflow: A comparative kinetic study

The thermochemical conversion of single cylindrical biomass particles in supercritical water crossflow is numerically investigated using a multi-region CFD model that couples heat transfer, mass transfer, and chemical reactions. This study conducts a comparative analysis between a global single-step mechanism (Lumped biomass model) and a multi-step component-based mechanism (Three components biomass model) to quantify their impact on conversion characteristics under crossflow conditions. The results reveal that while both models predict gasification as the rate-limiting step, the Three components biomass model predicts significantly longer conversion times and a higher gasification time proportion (∼ 80%) compared to the Lumped biomass model (> 60%), attributed to the broad decomposition temperature range and slow kinetics of lignin. Particle size is identified as the dominant factor controlling conversion, with the Three components biomass model exhibiting a stronger sensitivity to diameter (t_c ∝ d_p1.6971) than the Lumped biomass model (t_c ∝ d_p1.38). This magnified sensitivity stems from a “production delay” in the Three components biomass model, where thermal lag in larger particles retards the activation of refractory lignin, thereby delaying char formation and coupling the entire reaction sequence to internal heat transfer. Conversely, convection intensity has a limited impact, with the Three components biomass model showing reduced sensitivity (t_c ∝ U_in-0.086) compared to the Lumped biomass model (t_c ∝ U_in-0.118). These findings underscore that accurate modeling of supercritical water gasification requires explicit consideration of component-specific kinetics and their interplay with intra-particle transport phenomena.