Low-Temperature Oxidation Characteristics of Lignite Chars from Low-Temperature Pyrolysis

The oxidation characteristics of chars from low-temperature pyrolysis of two lignite coals have been systematically investigated using a dual fixed-bed quartz reactor and a thermogravimetric analyzer (TGA). During oxidation experiments, the temperature profiles of coal and char samples were recorded and CO2 and CO gases evolved were analyzed using gas chromatography. The physical and chemical structures of coals and chars were analyzed using Brunauer–Emmett–Teller (BET) adsorption isotherms and Fourier transform infrared spectroscopy (FTIR). During oxidation, the temperature of the coal sample may increase noticeably, exceeding the crossing point temperature (CPT). Chars prepared at 400 °C showed the lowest CPT, indicating the highest oxidation reactivity compared to that of parent coals and chars prepared at other temperatures. During oxidation, gaseous products are released, implying that oxygen-containing functional groups and solid oxygenated complexes decomposed and the yields increased with increasing the oxidation temperature. The amount of CO2 generation was proportional to that of CO with the molar ratio of CO2/CO at around 4.15 under the present experimental conditions. During non-isothermal oxidation, the concentration of OH and other oxygen-containing functional groups increased with increasing the oxidation temperature for char samples but decreased in the case of oxidized coal. During isothermal oxidation, the concentration of oxygen functional groups in coal decreased with increasing the oxidation temperature above 250 °C. Pre-oxidation of chars decreased their combustion reactivity measured on the TGA.