Formation behaviors of gas and liquid products during the two-stage hydrogasification of low-rank coal

• The study is focused on the effects of post hydrocracking on the gas and liquid formations.
• The intermingled effects of temperature, pressure and vapor residence time are separated.
• The yields of water by hydrogasification of various coal and biomass samples are determined.
• CH4 and benzene substantially increase in yield with aggravating hydrocracking.
• CO2, toluene, xylene, phenol decrease in yield with aggravating hydrocracking.

The hydrogasification of a low-rank coal was carried out in a two-stage fixed bed reactor with a focus on the separate effects of hydrocracking temperature, hydrogen pressure and vapor residence time on the formations of gaseous and liquid products, particularly the oxygen-containing products. While the yields of CH4 increased with the increase of hydrocracking temperature (500–750 °C) and hydrogen pressure (0.1–5.0 MPa), the yields of CO2 showed a downtrend. Both the yield of CH4 and that of CO2 changed insignificantly with the vapor residence time (20–100 s). The yield of CO varied somewhat complicatedly with the hydrocracking conditions. The gas generation was influenced not only by the hydrocracking of volatile matter but also by the gas-phase reactions. The increased severity of hydrocracking substantially enhanced the yield of BTX (benzene, toluene and xylene), even with the strengthened decomposition of toluene and xylene. Phenol was hydrocracked to an increased extent, with increasing hydrocracking temperature and hydrogen pressure, whereas naphthalene was barely hydrocracked till the hydrogen pressure was elevated to 5.0 MPa. The pressurized hydrogen intensified the hydrogenation of oxygen atoms in coal forming more water. The yields of water arising from the hydrogenation reactions were found to have a good correlation with the oxygen contents in various coal and biomass samples.