Formulating and optimizing a combustion pathways for oil shale and its semi-coke

Abstract

Enhanced technologies from oil recovery to unconventional fuels - oil shale, oil sands and extra-heavy oil – have in common complex chemical reactions processes. This paper is about the formulation and optimization of the chemical mechanism especially in oil shale and semi-coke combustion. The Levenberg–Marquardt algorithm was used to minimize the error between estimated values and the thermogravimetric data for combustion mechanisms of 4-steps and 3-steps proposed for the oil shale and its semi-coke respectively. The kinetic parameters such as reaction order, pre-exponential factor, activation energy and stoichiometric coefficients that affect drying, pyrolysis, oxidation and decarbonation reactions were estimated with success. The values of activation energies were 54–67kJmol−1 for oil shale drying, 62–65kJmol−1 for pyrolysis reaction, up to 100kJmol−1 for Fixed Carbon (FC) oxidation reaction, and 162–418kJmol−1 for decarbonation reaction. Regarding to the semi-coke combustion, the activation energies were 33kJmol−1 for drying reaction, 211kJmol−1 for oxidation reaction and 291kJmol−1 for decarbonation reaction. The chemical reactions suggest reaction order superior to one, except to the decarbonation reaction at 3Kmin−1. Considering the estimated parameters, as well as a heating rate at 3Kmin−1, an oil shale containing about 20wt.% of organic matter and 34.6wt.% of CaCO3, the species mass fractions formed during combustion process were 3.4wt.% of FC, 10.6wt.% of Oil, 3.3wt.% of HC and 1.8wt.% of CO. The fraction of CO2 formed accounts a total of 21.6wt.%. For a semi-coke containing 3.4wt.% of FC and 40.6wt.% of CaCO3, its combustion formed 2.1wt.% of CO. The CO2 fraction from oxidation and decarbonation reactions accounts 10.2wt.%, considering that the stoichiometric mass coefficient γ=0.75 in decarbonation reaction.