Mechanism of microwave-enhanced quality improvement in the production of coal-based specialty fuels from low-rank coal

Coal plays an irreplaceable role in optimizing the energy structure and ensuring national energy security. Upgrading abundant low-rank coal reserves is a vital pathway to reduce dependence on petroleum. However, the inherent high moisture, high porosity, and low calorific value of low-rank coal necessitate rigorous to meet the specifications for direct hydrogenation liquefaction in coal-based specialty fuel production, i.e., moisture <5%, particle size <74 μm, and calorific value > 20 MJ/kg The mechanism of microwave-enhanced quality improvement for fine-particle low-rank coal is investigated systematically. Through the system optimization condition parameters, the moisture content was reduced to 1.05%. Notably, the C:H ratio increased by 20.59%, indicating that microwaves have an upgrading effect on low-rank coal. The calorific value of the coal increased remarkably by 44.49%, significantly enhancing fuel energy density. Drying kinetic analysis revealed a two-stage falling rate process. Kinetic analysis reveals that the activation energy stands at 17.43 W/g for the first deceleration stage and 28.61 W/g for the second one. The effective diffusion coefficient increased by approximately one order of magnitude. After microwave drying, the BET specific surface area increased by 10.75%, and the micropore volume decreased by 42.1%, indicating that microwave treatment promoted the optimization of pore structure and helped to improve the suitability and yield of direct hydrogenation liquefaction. Furthermore, the C:O ratio increased, indicating the reduction of reactive oxygen sites, which is conducive to cracking. These upgrades provide favorable conditions for improving the conversion efficiency and economic viability of the direct coal hydrogenation liquefaction process.