Study on the pyrolysis characteristic of mild liquefaction solid product of Hami coal and CO2 gasification of its char

Abstract

Efficient utilization of mild liquefaction solid product (MLS) of Hami coal is critical to improve energy efficiency of the whole process, which accounts for >30 wt% of raw coal. In this work, the structure features of MLS were characterized by the solid-stage 13C NMR (SS-NMR) and its pyrolysis behavior was measured by a thermogravimetric analyzer (TGA) coupled with a mass spectrometer (MS). Additionally, the chars of MLS were prepared in a quartz tube fixed-bed reactor at different temperatures. Then the physico-chemical structure and CO2 gasification reactivity of the MLS chars were measured by multiple methods, including Fourier Transform Infrared Spectrometer (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD) and TGA. According to the SS-NMR analysis, the aromaticity of MLS is relatively high even though it contains ca. 47.90% aliphatic carbon. The evaporation of hexane soluble fraction (HS) and cleavage of CalCal and CarCal bonds are dominant in the process of MLS pyrolysis according to the TGA-MS analysis. With the increase of final pyrolysis temperature, the yield of char and H/C atomic ratio gradually decrease from 69.4% to 54.9% and from 0.664 to 0.110, respectively. In addition, the side chains and functional groups of char gradually reduce, the char becomes dense and dim, and the degree of ordering of char increases. Well quantitative linear relationships (R2 > 0.95) between gasification reactivity, carbon crystalline stacking layer number (n) and H/C atomic ratio are established, which could be used to finely evaluate the CO2 gasification reactivity of MLS and pyrolysis chars.