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This article presents an overview on carbon chemical structure transformation to understand kerogen thermal decomposition based on the chemical structure of kerogen. Formation of kerogen is highlighted to distinguish the typical types of kerogen containing in oil shale and coal. The oil production potential for oil shale and coal is found to little correlate with their organic amounts but to depend on the quality or chemical structure of organic matters. Aliphatic and aromatic carbons in kerogen are correlative with the yield of oil and carbon residue from Fischer Assay retorting, respectively. The aliphatic carbon moieties largely produce oil and gas, while aromatic carbon portion is apt to be converted directly to carbon residue during kerogen pyrolysis process. On this basis, an updated lumped mechanism model is proposed for viewing kerogen pyrolysis and provides a basis for understanding the transformation of carbon chemical structures. Further quantization and analysis conclude that: 1) 10–20% aliphatic carbon leaves in carbon residue as methyl groups and methylene bridges attached to aromatic rings, 2) 45–80% aliphatic carbon is directly distillated into oil, and 3) 15–40% aliphatic carbon is aromatized into aromatic carbon. The aromatization degree of aliphatic carbon varies with secondary reactions and its intrinsic chemical structure (alkyl chains, naphthenic and hydroaromatic hydrocarbons). Thus, the article justifies that primary pyrolysis determines the potentially maximal oil yield according to original carbon chemical structure, while the subsequent secondary reactions should be selective and minimized to determine the final oil yield and quality. [ABSTRACT FROM AUTHOR]

The chemical mechanisms and product distribution in the process of oil shale kerogen pyrolysis were investigated via reactive molecular dynamics (RMD) simulations employed a reactive force field (ReaxFF). A large-scale reactive system based on a Green River kerogen model was used to simulate the nonisothermal pyrolysis process. The results show that the weight loss curve shift to higher temperature with the increase of the heating rate. This tendency is corresponding to that of thermogravimetric experiments. The pyrolysis process was explored by dividing it into three stages according to the change of C 40+ compounds. The cleavages of weak bonds happened in stage I; long chain hydrocarbons were produced in abundance during stage II; the formations of small molecular hydrogen-containing gases were dominant in stage III. In addition, the bond breaking, typical products distribution and typical reaction pathways during the three stages were analyzed further to deeply understand the pyrolysis mechanisms of oil shale kerogen. [ABSTRACT FROM AUTHOR]

Highlights • The existing major heat conduction and radiation models are reviewed and evaluated. • For conduction, the Zehner-Bauer-Schlünder (ZBS) model is recommended in this work. • For radiation, Breitbach and Barthels (B-B) correlation can give acceptalbe results. • Temperature evolutions in four fixed-bed coal pyrolyzers are successfully predicted. Abstract The existing major heat conduction and radiation models for packed bed of particles are reviewed and evaluated by comparing the predicted results with experimental data. For low-temperature condition, it is found that the Zehner-Bauer-Schlünder (ZBS) model is less sensitive to the effect of contact area and is thus recommended for the calculation of effective thermal conductivity of packed bed. For high-temperature condition, although numbers of models can be used to calculate the radiative heat transfer behavior in packed bed, the Breitbach and Barthels (B-B) correlation is the optimal method applicable for different particle diameters, emissivities and voidages. The results of CFD simulations using the identified optimal heat transfer models agree well with the thermal measurements by thermocouple in four coal pyrolysis fixed-bed reactors mounted with or without particularly designed internals. [ABSTRACT FROM AUTHOR]

• Basic knowledges of kerogen and molecular simulation were introduced. • Six research areas of kerogen analyzed by molecular simulation were reviewed. • Larger scale model would be beneficial for molecular simulation studies on kerogen. • Future research directions of each research area were discussed. Kerogen is the most abundant source of natural organic matter in the world, and the research of kerogen to promote its utilization is of great significance to the development of unconventional fossil energy. Molecular simulation is a research method that is extremely suitable for investigating the detailed characteristics and physical/chemical processes of kerogen at atomic level. This work introduces some basic knowledge of molecular simulation that is useful in analyzing kerogen. We also review the molecular simulation studies of kerogen's structure, geological processes, interactions with minerals and shale gases, and pyrolysis. For each research application of molecular simulation on kerogen, directions for future research are proposed. [ABSTRACT FROM AUTHOR]

The product distribution and organic sulfur removal during direct pyrolysis and hydropyrolysis of oil shale kerogen were investigated via reactive molecular dynamics (RMD) simulations with reactive force field (ReaxFF). Two structural models for direct pyrolysis and hydropyrolysis of kerogen were constructed about kerogen extracted from Longkou oil shale to investigate the impact of H 2 at different temperatures on the product distribution and reaction processes of oil shale. The experimental results show that hydropyrolysis could increase light shale oil (the most important product in shale oil industry), and improve the removal rate of organic sulfur simultaneously. It was found that comparing to the direct pyrolysis, hydropyrolysis can provide more H free radicals to participate in the reaction and therefore promoting the pyrolytic reaction of kerogen. In addition, hydropyrolysis greatly promoted the desulfurization due to the contribution to the production of H 2 O molecules, and the transfer of sulfur to the gas products requires the participation of H 2 O molecules. This work is an intensive study on hydropyrolysis mechanism at different temperatures at the atomic level. These conclusions could be helpful for the clean utilization technology of oil shale industry. • Hydropyrolysis and direct pyrolysis of oil shale were studied using ReaxFF MD. • Hydropyrolysis is helpful for increasing light oil yield. • Hydropyrolysis has positive effects on the desulfurization. • Hydropyrolysis contributes to the production of H 2 O molecules. • The transfer of sulfur to the gas requires the participation of H 2 O molecules. [ABSTRACT FROM AUTHOR]

The secondary reactions occurring during the formation of shale oil are investigated with Huadian oil shale pyrolysis by solid heat carrier in a newly devised moving bed with internals (MBI). The effect of secondary reactions, which are mainly vapor-phase cracking, on shale oil property and yield at different temperatures is characterized on the basis of ultimate, 13 C NMR and GC–MS analyses of the produced oil. The increases in the production rate of hydrocarbon gas species and related molar ratios clarify that the oil vapor cracking, rather than coking, is the main secondary reactions at pyrolysis temperature over 495 °C. Ultimate analysis of shale oil shows that cracking increases the carbon and nitrogen contents of oil but decreases its hydrogen and oxygen contents. There are obvious decreases in the atomic H/C and O/C ratios of shale oil with increasing the pyrolysis temperature. NMR analysis shows that the aromaticity of shale oil increases considerably due to secondary reactions. Selective concentration of existing aromatics and aromatization of aliphatics are of roughly equal importance. The GC–MS results indicate that the aliphatic hydrocarbons (alkanes and alkenes) are the main detected components in shale oil and represent 75% of the detected peak areas. Via oil vapor cracking, a part of long-chain aliphatics is transformed into short-chain alkanes, alkenes and gas species to increase the low-carbon hydrocarbons. The increase in aromatic proportion with raising temperature is due to the selective concentration by cracking aliphatics and aromatization reactions such as dehydrogenation and their subsequent reactions. [ABSTRACT FROM AUTHOR]

Abstract: The reactions of atomic radical F and Cl with acetylene have been studied theoretically using ab initio quantum chemistry methods and transition state theory. The doublet potential energy surfaces were calculated at the CCSD(T)/aug-cc-pVDZ//CCSD/6-31G(d,p), CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) and compound method Gaussian-3 levels. Two reaction mechanisms including the addition–elimination and the hydrogen abstraction reaction mechanisms are considered. In the addition–elimination reactions, the halogen atoms approach C2H2, perpendicular to the Cbond, forming the pre-reactive complex C1 at the reaction entrance. C1 transforms to intermediate isomer I1 via transition state TSC1/1 with a negative/small barrier for C2H2F/C2H2Cl system, which can proceed by further eliminating H atom endothermally. While the hydrogen abstraction reactions also involve C1 for the fluorine atom abstraction of hydrogen, yet the hydrogen abstraction by chlorine atom first forms a collinear hydrogen-bonded complex C2. The other reaction pathways on the doublet PES are less competitive due to thermodynamical or kinetic factors. According to our results, the presence of pre-reactive complexes indicates that the simple hydrogen abstraction and addition in the halogen atoms reaction with unsaturated hydrocarbon should be more complex. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reactions proceed, we expect that the actual feasibility of the reaction channels may depend on the reaction conditions in the experiment. The present study may be helpful for probing the mechanisms of the title reactions and understanding the halogen chemistry. [Copyright &y& Elsevier]

• Oil shale pyrolysis in moving bed with internals (MBI) was enhanced by rapid preheating. • The quality of shale oil could be improved through the reaction regulation in stages. • Rapid preheating was beneficial to create the reasonable temperature filed in the MBI. • Temperature field in the MBI could match the flow and reaction of volatiles. The enhanced oil shale pyrolysis in a moving bed with internals (MBI) by rapid preheating in a drop tube was developed to improve the quality of shale oil through reaction regulation in stages. Initially rapid preheating of oil shale in the upper drop tube endowed the generation of massive free radical fragments and volatile precursors, which would simultaneously restrict the remarkable release of volatiles from particles to minimize the gas-phase secondary reactions. The preheated oil shale subsequently fell into the lower MBI to shape a granular bed with the cocurrent flow of heat and volatiles. In the MBI, the oil shale pyrolysis and volatiles reactions were directed and regulated by matching the flow and temperature fields to realize the selective cracking of heavy fraction in shale oil. The granular bed also served as a filter for the in-situ removal of dust entrained by the volatiles. The shale oil yield reached 83.0% of Fischer Assay oil yield, its light fraction was as high as 89.2 wt% and dust content was as low as 0.6 wt%. The advantages of pyrolysis in the MBI enhanced by rapid preheating were further justified by comparison with other pyrolysis processes. [ABSTRACT FROM AUTHOR]