Pyrolysis characteristics and evolution of the organics in Nong’an oil shale lump during sub-critical water extraction

Sub-critical water extraction is being considered as a potential in-situ conversion process for deep-lying oil shale. In this study, low-grade oil shale cores from the Nong’an oil shale in-situ conversion pilot were extracted by sub-critical water at 350 °C for 5–100 h. The results show that the maximum oil yield obtained at 50 h was 3.68 %, higher than the 3.45 % yield from Fischer assay analysis. It shows that sub-critical water extraction can obtain more organic matter than retorting. Additionally, a large amount of bitumen remained in the shale matrix after sub-critical water extraction. Its content reached maximum value when the kerogen was completely cracked at about 70 h. The gas chromatography-mass spectrometry (GC–MS) was conducted to analyze the components of the shale oil and residual bitumen. The results showed that n-alkanes, iso-hydrocarbons, aromatics, and n-alkenes were the major components of the kerogen cracking products. More aliphatic hydrocarbons could be extracted by sub-critical water, but more heteroatomic compounds would remain in the shale residue. This illustrates the selective extraction of more aliphatic hydrocarbons by sub-critical water during hydrolysis. The solid-state 13C NMR results of kerogen indicated that, under the action of sub-critical water, the long aliphatic chain in kerogen could be easily pyrolysis and released. The condensation of aromatic carbons was inhibited during the cracking of kerogen. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) analysis revealed that, some sulfur-containing functional groups would be oxidized or cyclized to sulfone and thiophene, and some oxygen-containing components might form hydroxyl and oxy heterocycle after cracking in sub-critical water. Anyway, sub-critical water extraction can more effectively present the in-situ conversion process.