Correlation between the structure of low-rank coal and its yield and characteristics of thermal dissolution extracts

Abstract

To investigate the effect of structure difference of coals on its yield and characteristics of thermal dissolution extracts (TDEs), four Xinjiang low-rank coals were chosen for thermal dissolution extraction using 1-methylnaphthalene (1-MN) as the solvent at 380 °C. The low-rank coals were divided into TDEs consisting of deposit (DP) precipitated at room temperature in 1-MN and soluble (SL) dissolved in 1-MN, along with a minor proportion of liquid and gas products, as well as residue. The results showed that TDEs yields of Qidaowan (QDW), Heishan (HSC) and Runbei (RBC) coal samples increased with the increase of volatiles. However, almost the same TDEs yield was obtained for Hefeng coal (HFC) as RBC despite of higher volatiles content of the former. Further analysis suggested that both higher volatiles content and aromatic carbon ratio contributed to the formation of TDEs when 1-MN was used as the solvent. Besides, significant differences of carbon types for the above coal samples were observed, wherein, HFC possessed the lowest aromatic carbon of 45.59%, yet the value for all the DPs and SLs were above 60%, falling in the range of 60%-68% and 66%-71%, respectively. Furthermore, the aromatic ring cluster of DPs was composed of 2–3 rings, while that for SLs was 1–2 rings on average. Moreover, the TDEs obtained from coals with higher oxygen content were also characterized by a relatively higher oxygen content, and a higher content occurred on DPs. The molecular weight, polydispersity index and methylene chain length of DPs and SLs located in the range of 1000–1800 amu, 2.0–4.4, and 0.8–2.3, respectively. And it was found that good caking properties similar to coking coals occurred on the DPs and SLs, ranging from 88 to 97. Results also exhibited that there was a positive synergistic effect for caking index of DP & SL mixture. This work provides a theoretical basis for low-rank coal as an alternate to coke coal by thermal dissolution extraction.