An integrated hot air-microwave drying strategy for lignite: Achieving enhanced efficiency and CO2 reduction potential

Lignite drying continues to be majorly hindered by high energy consumption and low efficiency. A laboratory-scale combined hot air–microwave drying system integrating convective and dielectric heating was developed to address these limitations and thus enhance performance. Drying characteristics and energy efficiency were systematically evaluated. Zhaotong lignite contained abundant oxygen-containing functional groups and a well-developed mesoporous structure, which facilitated significant moisture retention, with a maximum bound water content of 0.600 g/g. Its initial activation energy (40.30 kJ/mol) resulted in slow drying kinetics during the initial drying stage. Experimental results demonstrated that the proposed system significantly outperformed conventional hot-air drying, achieving a 16.1-fold increase (Drying rate , DR ) under optimized conditions (microwave power: 500 W; microwave intervention: 25 min). The maximum exergy evaporation efficiency ( η ex,evp ) reached 27.02%, while specific energy consumption ( SEC ) decreased from 26.60 to 6.45 kJ/g (a four-fold reduction). Four key indicators: average drying rate ( ‾DR ), average exergy evaporation efficiency (‾ η ex,evp ), SEC , irreversible exergy loss ( η ex,loss ) were integrated into an energy efficiency factor ( EEF ), and process conditions were optimized using response surface methodology (RSM). Experimental validation confirmed the optimal EEF and demonstrated a 75% reduction in CO 2 emissions, achieving a balance between drying kinetics and energy utilization. This study provides a novel energy-efficient drying strategy for lignite, leveraging the synergistic effects of hot-air and microwave heating. The findings have practical implications for optimizing industrial drying processes, reducing carbon emissions, and enhancing the sustainability of coal-based energy systems.