Understanding Ash Sintering Variation Behaviors of Low-Rank Coals with Municipal Sludge Addition Based on Mineral Interactions

Abstract

Co-gasification with coal provides an economically viable way to use sludge. To investigate the effect of municipal sludge (MS) on the sintering behaviors of low-rank coals (LRCs) and their modification mechanisms, the initial sintering temperature (Ts) of three LRCs and their mixtures with MS addition were tested by a Ts analyzer, an X-ray diffractometer, and FactSage calculation. The results show that the Ts values of Xiaolongtan coal (XLT), Xiangyuan coal (XY), and Daliuta coal (DLT) all increase with MS addition. The 9–12% MS mass ratio is suitable during LRC fluidized-bed gasification to mitigate ash-related issues. The Ts is closely related to the liquid-phase content or the transmissions of microparticles (e.g., atoms and ions) or blank spots during heating, while the ash fusion temperatures (AFTs) are mainly determined by acid/base ratios. The Ts values of high-Fe XLT and XY mixed ashes increased gradually with increasing MS proportion because the sintering mechanisms transferred from liquid phase to solid phase, while for relatively high-Mg DLT ashes, the Ts values increased with increasing MS proportions, which might result from the formations of high-melting-point minerals (e.g., Ca3(PO4)2 and Mg2SiO4). The results deepen the understanding of ash sintering behaviors and provide references to alleviate ash-related issues during gasification.