Evaluation of Moisture Readsorption and Combustion Characteristics of a Lignite Thermally Upgraded with the Addition of Asphalt

Thermal upgrading is a promising way to use lignite efficiently and safely. However, moisture readsorption properties of the upgraded lignite would partially offset the upgrading effect. Asphalt is so repellent that it could be used as an additive in lignite thermal upgrading to prevent moisture readsorption of upgraded lignite. In this study, a Chinese lignite upgraded at various temperatures (200–500 °C) with various concentrations of asphalt addition (0–10 wt %) was thoroughly investigated. The changes in chemical structures of the upgraded lignites were investigated using Fourier transform infrared spectroscopy (FTIR). The changes in physical structures were analyzed by N₂ adsorption isotherm and scanning electron microscopy (SEM). The moisture readsorption and combustion characteristics of the upgraded lignites were studied using a constant temperature/humidity chamber and a thermogravimetric analyer (TGA), respectively. The results indicate that the abundance of oxygen-containing groups (i.e., hydroxyl and carboxyl) decreased with the increasing upgrading temperature, while it was not significantly influenced by the addition of asphalt. The pore volume and surface area of the upgraded lignites increased with the increasing temperature, while the pore diameter decreased. SEM images revealed that some pores in upgraded lignites were covered by asphalt and the surface became much smoother with the increasing asphalt concentration, which resulted in the decrease of the pore volume and surface area. The moisture readsorption ratio and spontaneous combustion tendency to the upgraded lignites decreased with the increasing temperature, while the influence of asphalt on the moisture readsorption and combustion characteristics was temperature-dependent.