Effect of hydrothermal upgrading on the pyrolysis and gasification characteristics of baiyinhua lignite and a mechanistic analysis

Abstract Although poly-generation technology based on lignite pyrolysis has become an important research direction, how to increase the yield of pyrolysis tar and decrease the yield of pyrolysis water is still a challenge, and the pretreatment of lignite is essential. Here, we proposed to improve the quality of lignite though hydrothermal treatment dewatering (HTD) and then change the pyrolysis products distribution. A series of HTD and pyrolysis experiments as well as measurements of lignite and HTD samples were conducted in this paper, and the results indicated that HTD is an effective means to improve the quality of lignite. The moisture (air dried basis) decreased from 26.55% to 5.27% and O/C atomic ratio reduced from 0.26 to 0.17 as the temperature increased to 310 °C. The atomic ratio of H/C increase from 0.85 to 0.90 of HTD-250 and decrease subsequently. HTD caused great changes in its physical and chemical structure, promoting the conversion of aliphatic structural unit to the aromatic structure followed by aromatization of this organic structure, and thus increased coal rank for HTD samples. The pyrolysis tar yield increased from 7.27% of raw coal to 8.49% of HTD-250 and subsequently decreased. The yield of CO2 components and pyrolysis water continued to decrease as the temperature increased, and the amount of decrease reached 24.9% and 39.5%, respectively. The degree of cross-linking reaction of upgraded coal is lower that of raw coal, leading to a reduction in the yield of pyrolysis water and an increase in the yield of tar.