Comprehensive study of structure model, pyrolysis and liquefaction behaviour of Heidaigou lignite and its liquefied oil

Abstract

An integrated study involving structure model, pyrolysis and liquefaction behaviour of Heidaigou lignite (HL) and its liquefied oil was carried out to have an in-depth and comprehensive understanding of lignite and its direct liquefaction behaviour. Detailed information on HL was obtained using a molecular structural model constructed on the basis of ultimate analysis combined with solid-state 13C nuclear magnetic resonance (NMR), X-ray photoelectron spectrometer (XPS) and X-ray diffraction (XRD) studies. The HL model has few aromatic ring structures, which are composed of naphthalene, benzene, pyridine and pyrrole. The aromatic structures are linked by the aliphatic chains, and the carbon content in each chain is not less than three. Pyrolysis is the basis of liquefaction; as such, the pyrolysis behaviour of HL was investigated by TG/DTG. Results show that the maximum weight loss rate is about 425 °C. At below 500 °C, some Cal–Cal and Cal–Car broke, but Car–Car did not. The effects of time and temperature on the liquefaction behaviour of HL were investigated in a PARR-stirred high-pressure reactor. The liquefaction took a certain amount of time. With increasing temperature, the coal conversion rate and oil yield remained stable or increased slowly. The optimal time and temperature for liquefaction of HL are 40 min and 440 °C, and the maximum oil yield and HL conversion rate reach 56.68% and 89.79%, respectively. In addition, the optimal temperature of liquefaction (440 °C) is slightly higher than that of the maximum weight loss rate (425 °C). The composition of aromatics in the liquefied oil was determined using column chromatography and GC–MS. Most of the aromatics are tetraline, naphthalene and its derivatives and monocyclic benzenes. Agreements and consistency exist between the HL model, pyrolysis and aromatics in the liquefied oil.