Effects of extractable compounds on the structure and pyrolysis behaviours of two Xinjiang coal

The extractable compounds are a very important part of coal and play an important role in the pyrolysis of coal. To further understand the effect of the extracts on the structure and pyrolysis behaviours of coal, Naomaohu (NMH) lignite and Hutubi (HTB) bituminous coal were extracted with pyridine and swollen with its solvent vapor. The pyrolysis process was performed in a fixed-bed reactor. Thermogravimetric analysis (TGA) of the coal samples was carried out on a thermogravimetric (TG) analyser. The pore structure characteristics of the coal samples and their chars obtained from pyrolysis at different temperatures were measured by the mercury intrusion porosimetry (MIP) method. The components of the pyrolysis gas were detected by gas chromatography (GC). It was found that the extraction and swelling processes enlarged the pore size and improved the porosity of the coals to some extent. Compared with the pyrolysis of the raw coal, that of the swollen coal resulted in a higher tar yield owing to the destruction of the cross-linked structure and the variations in the coal structure; the tar yield for the pyrolysis of all the residues decreased significantly, but the gas yield increased, where the volume of the CO gas mainly increased. The NMH residue char obtained from pyrolysis of the residue at different temperatures had a lower proportion of micropores than the HTB residue char. The extractable compounds will be converted to gas-phase (e.g. volatiles) and liquid-phase (e.g. colloids) products during the pyrolysis process, which could stabilise the free radical fragments, form coke in the pore structure of the char and increase the number of new micropores formed.