Fuel characterization and gasification of sunflower waste in sub- and supercritical water with influence of catalysts on optimum state

The amounts of H₂ production through subcritical water gasification (Sub-CWG) and supercritical water gasification (Super-CWG) have been revealed in the current study. Sunflower waste (SW) was selected as the biomass used in the gasification processes. To determine the fuel characteristics of SW, proximate, ultimate, and higher heating value (HHV) analyses were conducted. The response surface methodology (RSM) was applied to design the experimental runs, to determine and optimize the process parameters, and to explore the interactions between them, which included reaction temperature, feed concentration, and residence time. Furthermore, various catalyst additives (K₂CO₃, Na₂CO₃, and NaOH) were used at the optimum level of process parameters to investigate the effect of catalyst on H₂ production. According to the results of gasification processes, the amount of H₂ production in Sub-CWG was lower when compared to Super-CWG and the main gas yield was CO₂ and CH₄ due to low reaction temperature and residence time. On the other hand, it has been revealed that Super-CWG is much more efficient in H₂ production. Elevated temperatures and extended residence times enhance H₂ production by facilitating key reactions such as the water–gas shift and steam reforming. Higher feed concentrations were found to reduce H₂ production due to dilution effects and reduced water availability. The RSM explored that the created model matched the experimental data in H₂ production since the correlation coefficient values were high enough (R² = 99.83%, R²_Adj = 99.52%). In the second part of the study, catalytic Super-CWG experiments were conducted using the optimum process parameters determined by RSM. According to the results, NaOH significantly improves H₂ production by enhancing C–C bond cleavage and promoting the water–gas shift reaction, and the mechanistic basis for catalytic activity lies in the reduction of CO and CO₂ formation, thus maximizing H₂ production.