An experimental study of the influence of temperature on char separation in a moving granular bed

Abstract

Char removal at high temperature in order to improve the quality of bio-oil plays an important role in biomass fast pyrolysis. The goal of this study was to evaluate the performance of a moving granular-bed filter designed for biomass fast pyrolysis. The study deals with the influence of temperature on the separation efficiency of such a filter for particles that undergo thermal decomposition and particles that do not. Char of different particle sizes and Al2O3 powder were employed to investigate the separation mechanisms acting in a moving granular bed at high temperature. The results indicate that particle removal efficiencies of up to 98% can be achieved at high temperature. The removal efficiency for char was more sensitive to temperature than that for Al2O3 particles. The operating temperature enhanced the diffusion separation effect, but decreased the inertial-collision separation effect. The regime of combined diffusion and inertial-collision effects dominated the separation efficiency for char, to a greater extent than for Al2O3; in this regime, the separation efficiency decreases with increasing temperature, because the char particles decompose at mid to high temperatures (500–600K) so as to release volatile substances. This reduces the weight and size of the particles, which changes the separation mechanism so as to reduce the separation efficiency. The moving granular bed used in this study can operate with a high collection efficiency and low pressure drop during biomass fast pyrolysis, and we propose that it can be used for the design of a moving granular-bed filter and for the optimization of the operating variables.