An approach for upgrading lignite to improve slurryability: Blending with direct coal liquefaction residue under microwave-assisted pyrolysis

Highlights: • Direct coal liquefaction residue was used as consumable microwave absorbent. • The absorbent was eventually converted into gases with highly added value. • Infrared and Roman parameters were combined to reflect the char structure. • The absorbent promoted aromatization of organic structure while reduced porosity. • The maximum solid concentration of lignite water slurry reached 65.42 wt%. As the composition of direct coal liquefaction residue (DCLR) is complex and difficult to handle, more importantly its dielectric properties are excellent, it is used as consumable wave-absorbents to enhance the microwave pyrolysis of lignite for slurryability improvement. The effects of the different additions of DCLR on the evolution of pyrolysis products were studied by using gas chromatography, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and low-temperature N₂ adsorption analysis. The results indicated that microwave-assisted pyrolysis with DCLR was a promising method for improving slurryability of lignite and an effective method for utilizing DCLR. The introduced DCLR accelerated the heating rate of the process of microwave upgrading lignite, facilitating the decomposition of active oxygen-containing functional groups and aliphatic hydrocarbons and then their transformation into stable ether groups and aromatic structures. In the meantime, it was converted into gaseous products, mainly composed of H₂, CO, and CH₄, and solid products with high quality. Additionally, the cyclization and aromatization of organic structures were improved, as well as the order degree of aromatic systems, especially with 12 wt% of DCLR added. Furthermore, the existing and newly formed structures of micropores and mesopores in the upgraded lignite (UL) were remarkably reduced with the increase of DCLR contents. The re-absorption capacity was dramatically reduced and the slurryability of UL was improved as a result of the changes in chemical properties and pore structures. The maximum solid concentration of lignite water slurry (LWS) increased from 41.73 wt% to 65.42 wt% with lower pseudo-plasticity and static stability.