Experimental investigation, non-isothermal kinetic study and optimization of oil shale pyrolysis using two-step reaction network: Maximization of shale oil and shale gas production

Production of petroleum from pyrolysis of oil shale could offer a solution to meet the current high energy demands. In this respect, optimizing reaction-dependent parameters during pyrolysis kinetics is crucial for achieving this goal commercially. Thus, in this study, experimental pyrolysis of oil shale samples was conducted in the temperature range of 270–495 °C, with the reaction time of 10–235 min, and a heating rate of 2 °C/min in a fixed bed reactor. Composition and weight of different hydrocarbon fractions (bitumen, shale oil, shale gas, water, and char) based on the reaction pathways were measured. Desired reaction conditions to achieve the maximum petroleum production from the pyrolysis reactions were specified based on the design of the experiment (DOE) following response surface methodology (RSM). Empirical correlations were developed for the prediction of the amount of shale oil and gas, by considering the reaction time, reaction temperature and the char amount as the main governing factors. For the first time, multi-objective optimization was applied to determine the optimal operational parameters, with the aim of maximizing petroleum production. Finally, mass balance equations coupled with second order reactions based on two-step pyrolysis reaction pathway (intermediate bitumen as the transition fraction) were implemented for kinetic modeling. Statistical and graphical evaluations of the kinetic model as well as the proposed correlation, verified an excellent agreement between the models and the experimental composition of pyrolysis products. Moreover, multi-objective optimization revealed that a concurrent maximum shale oil (1.92 gr) and gas (5.72 gr) production from the oil shale (20 gr), reaction temperature, reaction time, and char amount of 457.34, 198.356, and 16.385, would be the optimized reaction-dependent factors. Interactive plots indicated that interactions between time and temperature are not significant for pyrolysis reaction; however, there is a single intersection point between the reaction temperature-char weight and the reaction time-char weight. This, delineated that reaction time (t > 190 min), reaction temperature (T > 440 °C) and higher values of char amount (24.4 gr) would be more favorable to attain higher oil production. Optimal reaction temperature, time, char amount for the maximum oil (5.72 %) and gas (1.92 %) production would be 457.34 °C, 198.35 min, 16.38 %, respectively, while conversion of kerogen to intermediate bitumen (preliminary step) followed by the oil production from the intermediate bitumen (secondary step) were found to be the most significant reactions taking place during pyrolysis. Finally, reaction temperature and time would have a positive relationship with the production of desired yield, while char amount has an inverse relationship with these outputs. Overall, this study should provide specific guidelines for implementing in-situ pyrolysis operations underground in immature organic-rich shale and coal beds.