Article pubs.acs.org/EF
Characteristics of Estonian Oil Shale Kerogen and Its Pyrolysates with Thermal Bitumen as a Pyrolytic Intermediate Jian Shi,† Yue Ma,† Shuyuan Li,*,† Jianxun Wu,† Yukai Zhu,† and Jinsheng Teng‡ †
State Key Laboratory of Heavy Oil Processing, College of Science, China University of PetroleumBeijing, 18 Fuxue Road, Changping, Beijing 102249, People’s Republic of China ‡ Shandong Energy Longkou Mining Group Company, Limited, Longkou, Shandong 265700, People’s Republic of China ABSTRACT: Preparation and collection of thermal bitumen, a pyrolytic intermediate, are key factors in elucidating the mechanism of oil shale pyrolysis. Electron paramagnetic resonance (EPR), gas chromatography, Fourier transform infrared spectrophotometry, nuclear magnetic resonance (NMR) spectrometry, distortionless enhancement by polarization transfer (DEPT), and X-ray photoelectron spectroscopy were employed to investigate the thermochemical transformation in oil shale pyrolysis. Results showed that thermal bitumen was continuously generated and decomposed during the pyrolysis process. The maximum yield of thermal bitumen at 380 °C was 11.17%. EPR analysis showed that the g factor of kerogen and the pyrolysates was slightly higher than 2 and increased as pyrolysis progressed because of the aromatization of saturates and decarboxylation. CO2 and CO were mainly generated at temperatures lower than 340 °C, and less was obtained in the subsequent pyrolysis process. In contrast, C2−C5 organic gases were mainly generated at temperatures higher than 340 °C. NMR and DEPT analyses indicated that kerogen, thermal bitumen, and shale oil were mainly composed of aliphatic structures. During the pyrolysis process, aliphatic structures were constantly transformed into aromatic compounds, which were easily retained in shale oil and semi-coke. Pyrrolic, pyridinic, and quaternary compounds constituted 80% of the nitrogen compounds in kerogen and the pyrolysates. The sulfoxide content of thermal bitumen and semi-coke was considerably higher than that of kerogen, indicating that sulfoxide compounds present better thermostability during the pyrolysis process.
1. INTRODUCTION Oil shale is a highly important unconventional petroleum product that has attracted increasing attention in the past several decades because of its massive geological reserves.1,2 Complex macromolecular organic compounds in oil shale are collectively referred to as kerogen. These compounds can be pyrolyzed to shale oil and gas via a series of complex chemical reactions. In comparison to other fossil energy resources, such as coal tar and sand oil, shale oil possesses high levels of light components and can be used as marine kerosene. Generally, shale oil has a low viscosity and asphalt content. The properties of typical shale oil are similar to those of crude oil. Such advantages make oil shale stand out in the long term development of fossil resources. In contrast to crude oil, shale oil has higher olefin and non-hydrocarbon contents.3 In recent years, researchers have focused on the thermal transformation of kerogen to investigate the oil shale pyrolysis mechanism and obtain high-quality pyrolysates; however, research on the evolution of kerogen pyrolysis remains extremely limited, owing to the structural heterogeneity and different geographic origins of kerogen. Thermochemical conversion technology is the only recommended process to produce energy and chemical products from oil shales.4 Various studies have investigated the formation and molecular structure of oil shale kerogen,5 the pyrolysis mechanism and kinetic model of oil shale,4,6,7 and the heteroatomcontaining compounds in oil shale8 to investigate the pyrolysis mechanism of oil shale kerogen. The pyrolysis of oil shale is affected by numerous factors, such as shale origin and different pyrolysis modes. Pyrolysis processes have two steps. The first © XXXX American Chemical Society
step involves the depolymerization of macromolecular organic matter in kerogen into soluble extracts, including thermal bitumen and gas products. In this step, macromolecules and thermally unstable heterocyclic compounds disintegrate into small molecules.9 The second step involves the further decomposition of thermal bitumen into shale oil, gas, and semi-coke as the temperature increases. Prior studies have shown that kerogen has a more favorable atomic composition compared to coal. Kerogen, which contains twice the hydrogen content and less oxygen content compared to numerous coals,10 is mainly composed of long-chain aliphatic structures and a small amount of aromatic compounds.5,11 Therefore, the conversion of kerogen to oil, gas, and semi-coke via a low-temperature pyrolysis process is the most favorable oil shale thermochemical conversion process.12,13 The main pyrolytic products of kerogen are oil gas (CH4, H2, CO, CO2, H2S, and
