Temperature-Dependent Gas Evolution of Raw, Dried, and Moisture-Reabsorbed Coal: A Comparative Study of Three Lignite Samples

Moisture history plays an important role in the low-temperature oxidation of lignite, but its influence is not necessarily the same for every coal or temperature interval. This study compares the moisture uptake and gas-evolution behavior of BRXL, WJT, and XLT lignite in raw, vacuum-dried, and moisture-reabsorbed states. The dried samples were prepared at 40°C for 24 h, while the reabsorbed samples were conditioned for 48 h at 20°C and 90% relative humidity. Moisture contents at 48 h were 24.02% for BRXL, 23.52% for WJT, and 24.82% for XLT. For the oxidation tests, 15.00 g of 0.180–0.380 mm coal was heated from 30 to 180°C at 1°C/min under a dry-air flow of 100 mL/min, with O₂, CO, and CO₂ measured at 10°C intervals. CO and CO₂ increased nonlinearly with temperature, and the strongest acceleration generally occurred between 170 and 180°C. Dried BRXL gave the highest response at 180°C, reaching 21,700 ppm CO and 94,950 ppm CO₂. Moisture reabsorption reduced the 180°C CO concentration by 86.9%, 95.0%, and 71.1% for BRXL, WJT, and XLT, respectively; the corresponding CO₂ reductions were 79.5%, 79.9%, and 57.4%. Although the reabsorbed samples had similar moisture contents, their gas responses remained distinctly coal-dependent. Under the current experimental conditions, moisture reabsorption reduced the measured outlet CO and CO₂ concentrations, but the magnitude of the reduction remained coal-dependent. The findings highlight the need to consider both moisture-conditioning history and coal identity when interpreting oxidation indicator gases.