Effects of Microwave Irradiation on Combustion and Sodium Release Characteristics of Zhundong Lignite

Lignite can be upgraded by using microwave irradiation (MI), which may change physical and chemical properties of the coal, affecting its combustion characteristics. In this work, MI was used to upgrade coal samples, and the changes in combustion characteristics of the upgraded coal samples were analyzed by thermogravimetric analysis (TGA) and surface temperature measurement. The method of sequential extraction was employed to investigate the changes of various Na classes in different coal samples, and laser-induced breakdown spectroscopy was then used to measure the temporal sodium release during combustion of the different coal samples. Results show that the MI process significantly reduces the moisture concentration, thereby increasing the carbon content and calorific value of the coal. The decrease of the oxygen carbon atomic ratio indicates an improvement of the coal rank. The TGA shows that the upgrading process will delay the combustion process toward higher temperatures. The comprehensive combustion parameter calculated from the TGA shows the combustion performance of upgraded coal samples became worse, which is also seconded by the measured surface temperatures. MI produces interconversion among different sodium classes, particularly between NH4Ac-soluble sodium and water-soluble sodium. The sodium release characteristics are similar to the surface temperature profiles, indicating that, despite the MI process, sodium release is predominantly controlled by the combustion process. However, the MI process is found to increase both the mass fraction of sodium in coal samples and the amount of sodium release during the combustion process.