Evolution of char structural features during fast pyrolysis of corn straw with solid heat carriers in a novel V-shaped down tube reactor

Investigating structural properties of the chars derived from biomass pyrolysis and their evolution mechanisms is of potential interest because they play crucial roles not only in biomass thermochemical conversion but also for bio-char processing and high value-added utilization. In this study, the evolution of char structural features during fast pyrolysis of corn straw with solid heat carriers in a novel V-shaped down tube reactor was studied within a wide temperature range of 200-900 °C. The char fractal properties were explored to evaluate the complexities of pore development. In addition, the volatilization behaviors of the alkali and alkaline earth metallic species (AAEMs) during rapid pyrolysis were analyzed in detailed. The H/C and O/C ratios were reduced by 85.5% and 89.8% respectively in response to increasing temperature until 900 °C. Great loss of O-containing structures, particularly ether and CO groups, resulted in more aromatic and ordered structure of the chars at high temperature. The evolution of pore structure exhibited a three-stage development. Increasing temperature gave rise to progressive increase in porosity development, whereas the maximum BET surface area and pore volume appeared at 500 °C. The pore fractal dimension (Ds) and BET surface area (SBET) demonstrated an excellent positive linear correlation. The volatilization extent of AAEMs significantly depended on temperature in the order of Na > K > Mg > Ca. The K and Na concentration in chars reached maxima of 1.123 wt% and 0.049 wt% at 350 °C, respectively.