Study on combustion performance of hydrothermally dewatered lignite by thermal analysis technique

Abstract

The high water content of lignite has raised serious environmental and efficient issues, which are obstacles to its large scale utilization. In this study, hydrothermal dewatering (HTD) method was used to upgrade a lignite from Inner Mongolia in China at 230–330 °C. The combustion behaviors of raw lignite and HTD samples were investigated by thermogravimetry with differential scanning calorimetry (TG-DSC). By changes of mass and heat flow, the lignite combustion process was described from three stages: water evaporation, coal devolatilization and char combustion. Compared with raw lignite, the combustion reactivity of HTD samples upgraded at 230–300 °C was improved, while that of sample upgraded at 330 °C decreased. The temperatures at maximum combustion rate of 230–300 °C samples (below 397.6 °C) were lower and that of 330 °C samples (448.9 °C) was higher than that of raw lignite (399.8 °C). The released heat at char combustion stage increased for HTD samples. Then the physico-chemical structures of lignite before and after HTD process were analyzed to investigate the effect of HTD on coal combustion characteristics. Results showed that HTD process could affect the heat at water evaporation and initial devolatilization stages during coal combustion through removing oxygen functional groups in lignite. The enhancement in aromaticity of lignite after HTD increased activation energy of intrinsic reaction, whereas the large pore volume and high surface area of 230–300 °C samples decreased diffusion activation energy at char combustion stage. The effect of pore structure predominated on combustion reactivity compared with aromaticity for HTD samples.