Optimization and modeling of process parameters during hydrothermal gasification of biomass model compounds to generate hydrogen-rich gas products

Abstract

Hydrothermal gasification in subcritical and supercritical water is gaining attention as an attractive option to produce hydrogen from lignocellulosic biomass. However, for process optimization, it is important to understand the fundamental phenomenon involved in hydrothermal gasification of synthetic biomass or biomass model compounds, namely cellulose, hemicellulose and lignin. In this study, the response surface methodology using the Box-Behnken design was applied for the first time to optimize the process parameters during hydrothermal (subcritical and supercritical water) gasification of cellulose. The process parameters investigated include temperature (300–500 °C), reaction time (30–60 min) and feedstock concentration (10–30 wt%). Temperature was found to be the most significant factor that influenced the yields of hydrogen and total gases. Furthermore, negligible interaction was found between lower temperatures and reaction time while the interaction became dominant at higher temperatures. Hydrogen yield remained at about 0.8 mmol/g with an increase in the reaction time from 30 min to 60 min at the temperature range of 300–400 °C. When the temperature was raised to 500 °C, hydrogen yield started to elevate at longer reaction time. Maximum hydrogen yield of 1.95 mmol/g was obtained from supercritical water gasification of cellulose alone at 500 °C with 12.5 wt% feedstock concentration in 60 min. Using these optimal reaction conditions, a comparative evaluation of the gas yields and product distribution of cellulose, hemicellulose (xylose) and lignin was performed. Among the three model compounds, hydrogen yields increased in the order of lignin (0.73 mmol/g) < cellulose (1.95 mmol/g) < xylose (2.26 mmol/g). Based on the gas yields from these model compounds, a possible reaction pathway of model lignocellulosic biomass decomposition in supercritical water was proposed.