Effect of Swelling Treatment by Organic Solvent on the Structure and Pyrolysis Performance of the Direct Coal Liquefaction Residue

Five organic solvents, N-methyl-2-pyrrolidone (NMP), cyclohexanone (CYC), N,N-dimethylformamide (DMF), acetonitrile (ACN), and toluene (TOL) were used to treat Shenhua direct coal liquefaction residue (DCLR) by swelling processes. The treated residues were characterized by swelling degree (Q), SEM, FTIR, TG-DTG, and Py-GC/MS methods. The effect of swelling treatment on the swelling properties, surface morphology, functional group distribution, and pyrolysis performance of the residues was investigated. Swelling results showed that the swelling effect with different solvents on each residue was different and that the swelling degree of the residue treated by NMP was high, 2.4. Swelling treatment could significantly change the surface morphology and pore structure of DCLR, resulting in the increase of pores in the residue. FTIR spectra proposed that swelling treatment could effectively break the hydrogen bonds in residue and change the abundance of oxygen-containing functional groups. TG-DTG profiles demonstrated that the swelling procedure could increase the weight loss and pyrolysis rate of the residue, and the pyrolysis activation energy in high-temperature region (≥360 °C) was obviously reduced, while the energy in the low-temperature region (120–360 °C) was increased. Py-GC/MS proposed that the swelling pretreatment could change the pyrolysis product distribution of the residue, with the species of the products decreased, and that ketones and arenes compounds were the main products.