Evolution of pore structure in bituminous coal during in-situ supercritical water gasification: Experimental study and mechanistic understanding

Coal's pore structure governs both mass-transfer behavior and chemical reaction rate during in-situ supercritical water gasification (ISCWG). Understanding pore structure evolution is critical to comprehending gasification processes. This study examines changes in pore structure and the controlling factors during gasification in bituminous coal. Results indicate that the effect of gasification on pore structure exhibited systematic variations with increasing coal rank. The pore volume (PV) of high-, medium-, and low-volatile bituminous coal increases by 7.2 x 10-2, 1.4 x 10-2, and 1.2 x 10-2 cm3/g, respectively. The average pore diameter (APD) increases by 1973 %, 33.75 %, and-35.8 %, while the fractal dimensions decrease by 17.77 %, 6.36 %, and-1.83 %. Pore structure evolution is primarily driven by heterogeneous gasification reactions, controlled by coal reactivity (RA) and specific surface area (SSA). Increases in RA and SSA enhance both PV and APD, with RA being the dominant factor, exerting an influence 6 to 7 times greater than SSA. The micro-mechanism behind the pore structure evolution in coals of varying ranks was further revealed. With increasing coal rank, coal reactivity decreases, weakening both pyrolysis and steam reforming, gradually shifting the dominant reaction from steam reforming to pyrolysis. This results in smaller PV increments and a shift in APD from growth to reduction.