Low-temperature pyrolysis upgrading of lignite: Three-phase product distribution and characterization

Lignite is one of the most abundant fossil energy resources worldwide. However, its high moisture and oxygen contents and low calorific value lead to low efficiency and severe pollutant emissions during direct combustion. Here, Indonesian lignite was upgraded via low-temperature pyrolysis under nitrogen in the range of 200-600°C, with systematic quantification and characterization of the gas, liquid and solid products. The results show that as the pyrolysis temperature increased, the solid yield gradually decreased, while the yields of liquid and gas products increased. In the gas products, the CO2 content decreased noticeably, while the content of combustible components, including CO, CH4, and H2, increased. Specifically, CH4 increased markedly beyond 400°C, and H2 rose significantly above 500°C. The liquid products were dominated by phenolic compounds, with low-molecular-weight phenols (such as phenol) as major constituents. A temperature of 400°C represents a key node where phenols become predominant and polycyclic aromatic hydrocarbons (PAHs) begin to emerge. Proximate/ultimate analyses and lower heating value (LHV) measurements of the chars demonstrated systematic fuel-property upgrading, with decreased volatile matter, increased fixed carbon, and an LHV increase from 18,723.4 to 30,714.74 kJ•kg-1. Thermogravimetric analysis (TGA) in air showed a single dominant derivative thermogravimetric (DTG) peak, and Coats-Redfern fitting over the main DTG peak interval indicated increasing apparent kinetic parameters with increasing pyrolysis severity. Overall, this study elucidates the temperature-dependent evolution of three-phase products during lignite low-temperature pyrolysis and provides data support for clean upgrading and graded utilization of lignite.