Impact of initial pressure and heat flux on hydrogen and carbon monoxide production in supercritical water gasification of biomass: A molecular dynamics study

Abstract

This study investigates the effects of initial pressure and external heat flux on hydrogen and carbon monoxide production during the supercritical water gasification of biomass. According to the results, after one nanosecond of equilibration, the system reached thermal equilibrium and structural stability, with potential and total energies stabilizing at –83.84 and –83.77 kcal/mol, respectively. The results show that increasing the initial pressure from 0 to 2.5 bar caused a decrease in the number of CO molecules from 86 to 71 and H₂ molecules from 574 to 543, indicating that higher pressure suppressed gas formation. Combustion efficiency also declined from 32 % to 25 % with increasing pressure, suggesting more complete reactions under elevated pressure conditions. Conversely, heat flux slightly increases from 3.92 to 4.06 W/m², likely due to enhanced gas production, while thermal conductivity rose from 0.30 to 0.37 W/m·K, reflecting improved heat transfer resulting from denser atomic packing. Furthermore, increasing the external heat flux from 0.001 to 0.005 W/m² intensified molecular dissociation, raising CO and H₂ counts from 93 to 112 and 605 to 692, respectively, which corresponded with an improvement in combustion efficiency from 49 % to 69 %. However, the heat flux decreases from 3.89 to 3.76 W/m², and thermal conductivity dropped from 0.28 to 0.19 W/m·K with higher heat flux, attributed to structural degradation and disrupted conductive pathways. Overall, these findings demonstrate the complex interplay between pressure and heat flux on gasification efficiency, molecular product distribution, and thermal properties, providing valuable insights for optimizing hydrogen production through supercritical water gasification of biomass.