Comparative study on the one‐step and two‐step pyrolysis of lignocellulosic biomass: Effects of two‐step pyrolysis on improving product quality

Abstract

Two-step pyrolysis (TSP) and one-step pyrolysis (OSP) of rice husk was conducted using a fixed-bed reactor, and the product distribution and physicochemical property were analyzed. TSP had two fast pyrolysis processes, which were, respectively, set at lower temperature (400°C) in the first step and high temperature (550°C) in the second step, while OSP was only set at high temperature (550°C). Compared with OSP, TSP increased the total yields of char and gas but decreased the total yield of bio-oil. The pyrolysis process of the first step facilitated the cross-linking and polycondensation reactions in the char of the first step, thus inhibiting the decomposition of the char from the first step and decreasing the yield of bio-oil in the second step. Due to the high pyrolysis temperature, there was no significant difference between the char from the second step and the char from OSP. The first step improved the selectivity of acids, furans and phenols with complex side chains in bio-oil. The bio-oil from the second step had lower acidity, and higher selectivity of phenols and hydrocarbons. Compared with the gas product of OSP, the H₂ and CH₄ contents of the gas product from the second step were improved by more than 70% and 35%. TSP could not only improve the selectivity of value-added chemicals but also achieve the higher quality of gas and bio-oil as fuel.