Two-stage hydrogasification of different rank coals with a focus on relationships between yields of products and coal properties or structures

The pressurized hydrogasification of different rank coals was carried out using a laboratory two-stage reactor to investigate the influences of volatile matter hydrocracking on the formations of gaseous products (CO2, CO, CH4 and C2–C3) and liquid products (water and light aromatics). Experiments were conducted by way of comparison under the N2 atmosphere. Fourier transform infrared spectroscopy (FTIR), solid state 13C nuclear magnetic resonance technique (13C NMR) and chemical titration analysis were used to semi-quantitatively or quantitatively determine the main functional groups in the coals. It was found that through the hydrocracking at 700°C, the summed yields of CH4 and C2H6 from two lignite coals reached as high as 21.2–22.9% (daf. coal), higher than those even from two subbituminous coals. This result was associated with the abundance of longer alkyl groups in two lignite coals. Two lignite coals were the most enriched with carboxyl groups, which was proved to be the principal precursor of CO2. The hydrocracking at 700°C allowed a distinct part of CO2 to convert into CO by the reversal gas water shift reaction, while the changes in the yields of H2O were indistinct. The hydrocracking at 700°C was also favorable for the production of BTX (benzene, toluene and xylene) and naphthalene. The simple linear regression was used to assess the correlations of the yields of main gaseous products (CH4, C2H6, CO2 and CO) and liquid products (H2O and BTX) obtained under varying conditions to some relevant coal properties such as volatile matter content and total oxygen content as well as to some structural parameters such as aliphatic ratio and carboxyl group content.