Study on the physicochemical structure and gasification reactivity of chars from pyrolysis of biomass pellets under different heating rates

The physicochemical properties of chars produced from biomass pellet pyrolysis under different heating rates were characterized including basic fuel properties, physical properties, chemical structure, and gasification reactivity. Results showed that after pyrolysis, the volume density and compressive strength of pellet were significantly decreased, especially for the isothermal pyrolysis. The char yield was decreased from 26.09 to 23.54 wt% with the heating rate raising from 2 to 25 K/min, and it would further slightly decrease by about 3 wt% after holding for 1 h. The chemical structure analysis of chars explained that with the increase of heating rate, the crystal stacking height and surface area were decreased, probably due to the limited volatile-char interaction within pellets and the destroyed pore structure by violently released volatiles at higher heating rate. On the other hand, the increased heating rate led to the drastic cracking and diffusion of volatiles which resulted in the increase of surface area to 327.17 m2/g at 25 K/min. The inverse factors explained that the effects of heating rate on crystal interlayer spacing, microcrystalline size, aromatic structure, pore volume and gasification reactivity were not regular. After holding process, the chars were further carbonized and condensed, which resulted in the increased fused ring structure and the reduction of about 0.004–0.010 min−1 for gasification reactivity index. This study provided the basic knowledge on the physicochemical structure of char produced from biomass pellet and further understanding of its gasification reactivity.