Experiment evaluation and thermodynamic analysis of pulping modification on ordered conversion of bark in supercritical water

Major obstacles of supercritical water gasification (SCWG) of lignocellulosic biomass, including ineffective pulping for energy self-sufficiency and a high conversion barrier significantly inhibited its large-scale development. In this study, the pulping effect of alkaline and salt pulping for high concentration bark slurry were compared systematically. The SCWG performances of bark (non-catalytic/catalytic) and bark slurry were evaluated. Based on energy self-sufficiency, a novel SCWG process of 50 wt% bark slurry was designated with a multistage reactor configuration and pulping process. The findings suggest that alkaline pulping had a wider applicability (reaching 50 wt%) and better rheological properties than salt pulping. Only 30 wt% bark slurry was capable for applying salt pulping because of solid residue with the excessive salt addition. Alkaline pulping resulted in an ordered conversion of 5 wt% and 10 wt% bark slurries during SCWG, with complete gasification of 5 wt% bark slurries prepared by KOH and NaOH at 650 and 680 °C, respectively. Besides, alkaline and salt pulping reduced the 26 % and 33 % carbon emissions, respectively. In the novel process, the optimal ratio of preheated water to bark slurry was 4 (water diversion to the primary and secondary gasification reactors of 1:3), which exhibits in energy and exergy efficiencies of 47.47 % and 48.63 %, respectively. This configuration reduces exergy losses in the gasification reactor and heat exchanger, aiding in the large-scale industrial output of SCWG.