| Description |
Oil shale is a complex material that is composed of organic matter, mineral matrix and trace amount of bound and/or unbound water. The endothermic decomposition of the organic matter generates liquid and gaseous products. The yield and the desired quality of the product (shale oil) are controlled by the operational conditions. Pyrolysis of a small batch of finely ground oil shale provides chemically controlled intrinsic kinetic rate of organic decomposition. Pyrolysis of large size block/core samples is governed by temperature distributions and the time required for product expulsion. Heat and mass transfer considerations influence the distribution of products and alter the yield and quality. The experimental studies on oil shale pyrolysis performed in this work were designed to understand the relevant coupled phenomena at multiple scales. Oil shale in the Mahogany zone of the Green River formation was used in all experiments. Experiments were conducted at four scales, powdered samples (100 mesh) and core samples of ¾", 1" and 2.5" diameters. Batch, semibatch and continuous flow pyrolysis experiments were designed to study the effect of temperature (300°C to 500°C), heating rate (1°C/min to 10°C/min), pressure (ambient and 500 psi) and size of the sample on product formation. Comprehensive analyses were performed on reactants and products - liquid, gas and spent shale. The activation energies of organic decomposition derived from advanced isoconversional method were in the range of 93 to 245 kJ/mol with an uncertainty of about 10%. Lighter hydrocarbons evolved slightly earlier and their amounts were higher in comparison to heavier hydrocarbons. Higher heating rates generated more alkenes compared to respective alkanes and as the carbon number increased, this ratio decreased. Oil yield decreased and the amount of coke formed increased as the sample size and/or pressure increased. Higher temperature, higher heating rate and low pressure favored more oil yield. The quality of oil improved with an increase in the temperature, pressure and size of the sample. A model in COMSOL multiphysics platform was developed. A general kinetic model was integrated with important physical and chemical phenomena that occur during pyrolysis. The secondary reactions of coking and cracking in the product phase were addressed. The multiscale experimental data generated and the models developed, provide an understanding of the simultaneous effects of chemical kinetics, heat and mass transfers on oil quality and yield. The comprehensive data collected in this study will help advance the move to large scale oil production from the pyrolysis of shale. |
