Parametric studies on hydrothermal gasification of biomass pellets using Box-Behnken experimental design to produce fuel gas and hydrochar

Abstract

This study involves optimization of H2-rich fuel gas yield from canola hull pellet via supercritical water gasification at variable temperatures (375–575 °C), biomass-to-water ratio (1:5–1:20) and residence time (10–60 min). The optimization study was accomplished using the Box-Behnken Design approach based on response surface methodology. The temperature was the most important process parameter for H2 yield followed by biomass-to-water ratio and residence time. The highest H2 (3.43 mol/kg) was obtained at 575 °C in 45 min with a biomass-to-water ratio of 1:20. A comparative evaluation of gas yield was performed for oat hull pellets and torrefied pellets at optimum gasification conditions. Torrefaction pretreatment of pellets significantly enhanced the fuel gas yield and gas composition. H2 yield decreased in the following order: torrefied canola hull + mustard meal pellets (4.6 mol/kg) > canola hull + mustard meal pellets (3.3 mol/kg) > torrefied oat hull + mustard meal pellets (3.3 mol/kg) > oat hull + mustard meal pellets (3.1 mol/kg). Hydrochar produced at higher gasification temperatures had greater levels of carbon, thermal stability, alkalinity and structural deformity.