Rapid co-pyrolysis of lignite and biomass blends: Analysis of synergy and gasification reactivity of residue char

Abstract

This work investigated the structural evolution of rapid co-pyrolysis char and its influence on co-gasification characteristics. The chars were prepared in a high frequency furnace at 900 °C with the heating rate of 300 °C/min, using Xinjiang lignite and cotton straw as raw materials. Blends of coal and biomass were prepared in proportions of 25:75, 50:50 and 75:25 wt.%. Compared to the individual pyrolysis, co-pyrolysis enhanced the transformation of ion-exchanged alkali and alkaline-earth metals (AAEMs). Moreover, the surface area of co-pyrolysis chars were enhanced, and the carbon structure of biomass char in mixtures became more ordered. The gasification reactivity of co-pyrolysis char was evaluated by thermogravimetric analyzer (TGA), including isothermal (800, 850 and 900 °C) and non-isothermal (5 °C/min) gasification using CO2 as agent. For isothermal gasification, the synergistic effect of reactivity changed from inhibition to promotion with the increasing conversion. The synergy index changed obviously at 800 °C. For non-isothermal gasification, the maximum reaction rate was increased. The blends with 75% biomass had the minimum activation energy. The co-gasification reactivity varied with the temperatures, biomass blending ratios and conversions, which were well related to the char structural evolution caused by rapid co-pyrolysis.