Agglomeration mechanism of Tiebei lignite in supercritical water

Supercritical water gasification (SCWG) is a promising technology for clean, efficient conversion of lignite particles. Traditionally, reducing coal particle size has been thought to promote gasification by increasing specific surface area for reaction with supercritical water. However, our recent experiments revealed that overly fine particles can actually deteriorate SCWG performance under weak-shear conditions in a sealed batch reactor. In this work, the gasification performance and agglomeration behavior of Tiebei lignite (TBL) were experimentally investigated in a sealed batch supercritical water reactor across six initial particle-size ranges (0–50, 50–60, 60–75, 75–105, 105–150, and 150–355 μm) at a TBL concentration of 15 wt%. Stage-representative experiments were conducted from 25 to 650 °C and from 0.101 to 25 MPa, and final SCWG runs were performed at 650 °C and 25 MPa for 50 min. Carbon gasification efficiency (CE) showed a clear non-monotonic dependence on initial particle size, with the 60–75 μm fraction giving the highest performance. Quantitative image analysis showed that excessively fine particles underwent severe adhesive agglomeration, and the 0–50 μm fraction exhibited the largest floc growth with a 468% increase in mean floc size after reaction. Stage-representative experiments further showed that agglomeration intensified progressively along the pressurized heating path toward the final supercritical condition. XRD and SEM–EDS results suggest that, near the final supercritical condition, the exposure, redistribution, and transformation of Na-, Ca-, Si-, and Al-bearing mineral species contribute to ash-mediated inorganic bridging and promote the formation of dense agglomerates.