Study on the Volatiles and Kinetic of in-Situ Catalytic Pyrolysis of Swelling Low-Rank Coal

A new method combined solvent swelling with an in-situ catalytic effect of metal ions was developed and introduced in coal pyrolysis to increase the coal conversion and the tar yield as well as to improve the quality of the tar products. Low-rank coal of Shendong coal from China was used to investigate the effect of demineralization, swelling, and in-situ catalysis on pyrolysis reactivity and kinetic characteristics, yield distribution of products, and the tar composition. The experiments were performed using a thermogravimetric analyzer/Fourier transform infrared spectrometer (TG-FTIR), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), and a fixed-bed reactor to examine the pyrolysis behavior of raw coal, demineralized coal, methanol swelling demineralized coal, and methanol swelling with metal ions (Ca2+, Cu2+, and Co2+) in-situ impregnated coal, respectively. The results showed that coal conversion could be promoted by pretreatment of solvent swelling and in-situ-impregnated Cu2+ and Co2+ ions, respectively. The gas evolution results of FTIR indicated that the in-situ loading of Cu2+ and Co2+ ions had a catalytic effect on the evolution of CO2 CH4, and aromatics of the swollen coal. The tar yield of demineralized coal was improved by the methanol-swelling pretreatment. With the in-situ loading of Cu2+ and Co2+ ions, the tar yield of swelling coal further increased by 16.80% and 28.75%. The composition of tar analyzed by Py-GC/MS indicated that methanol swelling increased the relative content of the acidic compounds and also had a positive effect on the yields of PCX (phenol, cresol, xylenol). The in-situ loading of metal ions increased the relative content of the aromatic compounds but had a different effect on the formation of BTXN (benzene, toluene, xylene, and naphthalene). The Cu2+ and Co2+ ions had a catalytic effect on phenols decomposition during coal pyrolysis, resulting in a decrease of the relative content of the acidic compounds dramatically. The kinetic results showed that the in-situ impregnation of Ca2+, Cu2+, and Co2+ ions into the swollen coal could result in a decrease of the activation energy and pre-exponential factor at the corresponding temperature range of the first and the second prolysis stage. In addition, a possible mechanism on in-situ catalytic pyrolysis of swelling coal was discussed and proposed based on the evolution and composition of the evolved species investigated during pyrolysis. The impregnation of the metal ions may catalyze the primary reactions and secondary reactions during coal pyrolysis.