Investigation of coal-biomass interaction during co-pyrolysis by char separation and its effect on coal char structure and gasification reactivity with CO₂

The interaction between coal and biomass has been widely investigated. However, the mechanism is always proposed based on physicochemical structure and reactivity of char mixture. In this work, char mixture after co-pyrolysis of anthracite and biomass was separated based on different shape and size, and then structure and reactivity of the coal char were analyzed to reveal mechanism of coal-biomass interaction. Anthracite char samples with different corn straw (CS) blending ratios were prepared by pyrolysis in a fixed bed reactor at 600 and 900℃. The AAEM concentration and microcrystalline structures of coal char were examined by inductively coupled plasma-optical emission spectrometry (ICP-OES) system and X-ray diffraction (XRD). The gasification reactivity of char sample after separation was analyzed by TGA under CO₂. The results show that concentration of active K and Mg in coal char samples gradually increased and more disordered carbon structure formed as the CS proportion in the blending increased from 0 to 80%. The coal char in the blending captured more AAEM species by volatile-char interactions instead of escaping with volatile from biomass during co-pyrolysis process. Meanwhile, higher pyrolysis temperature led to volatilization and inactivation of K and Na, and also decrease in graphitization degree. Moreover, both addition of CS and low pyrolysis temperature could promote gasification reactivity of coal char sample. Furthermore, a satisfactory linear correlation (R²=0.9009) between alkali index AI and R_0.5 of the char samples was established. This indicated that AAEMs performed the dominate effect to enhance gasification reactivity of coal char during co-gasification of coal and biomass.