Effect of deep eutectic solvent on combustion characteristics of lignite under thermal treatment conditions

Abstract The substantial moisture content of lignite imposes considerable constraints on its deep processing and subsequent utilization. A deep eutectic solvent (DES), synthesized from choline chloride (ChCl) and zinc chloride (ZnCl₂), was used to upgrade lignite under thermal treatment conditions. The effects of the DES molar ratio and its addition on the physicochemical and combustion characteristics of lignite were systematically analyzed at 280°C. The results demonstrated that the moisture content of lignite decreased from 22.63% to 5.18%, and then to 6.05%, as the molar ratio of DES was adjusted from 1:1 to 1:3, with the addition of 3 g of DES. This suggests that a DES molar ratio of 1:1 is more effective for lignite upgrading. As the DES addition increased from 1 g to 3 g, the moisture content of lignite decreased from 9.19% to 5.18%. As the molar ratio of ZnCl₂ in DES increased, the maximum combustion rate (V max ) and ignition index (C) of the upgraded coal sample gradually decreased, indicating that lignite upgraded with DES at a 1:1 molar ratio exhibited good combustion performance. The sample treated with 3 g of DES demonstrated the best overall combustion characteristic index ( S = 1.8), optimal ignition index (C), and combustion index ( R j ), when the DES addition increased from 1 g to 3 g. Compared to raw coal, the activation energy of the upgraded lignite was lower ( E ₐ = 63.99 kJ/mol), indicating enhanced combustion reactivity.