A novel reactor concept for supercritical water gasification was developed where cold biomass is mixed with supercritical water for heating. This approach enhanced the gasification efficiency and reduced solid deposition. Following proof of concept over 100 h, a parametric study was conducted, varying biomass type, dry matter content (1.3-8.2 wt %), temperature (550-700 °C), pressure (240-300 bar), potassium addition (0-3750 ppm), and residence time (0-42 s). Results showed that biomass type, pressure, and potassium addition had a minimal impact on the process. Higher biomass concentration reduced carbon efficiency (CE) but maintained levels around 80%. Increasing the residence time at the reaction temperature significantly improved CE up to 8 s at 650 °C (τ = 0 s, CE = 27%
τ = 8 s, CE = 72%), beyond which gains plateaued. Temperature strongly influenced CE (