Comparative pyrolysis and combustion kinetics of oil shales

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Abstract

In this research, thermal characteristics and kinetic parameters of eight Turkish oil shale samples were determined by thermogravimetry (TG/DTG) at non-isothermal heating conditions both for pyrolysis and combustion processes. A general computer program was developed and the methods are compared with regard to their accuracy and the ease of interpretation of the kinetics of thermal decomposition. Activation energies of the samples were determined by five different methods and the results are discussed.

Introduction

Thermogravimetric analysis (TG/DTG) of oil shale samples has been extensively used as a means of determining the characteristics of devolatilization and kinetic parameters. Thermal methods providing information about net results of mass loss and calculation of kinetic parameters are based on simplifying assumptions, which do not correspond to the complex chemical reactions in the thermal degradation of the oil shales. Thakur and Nuttall [1] studied the pyrolysis kinetics of the thermal decomposition of oil shale by isothermal and non-isothermal thermogravimetry. Their results showed that the thermal decomposition of oil shale involves two consecutive reactions with bitumen as an intermediate. Both reactions follow first-order kinetics. Three models using the Antony–Howard model yield lower deviation and thus provide a better fit of the data. Skala and Sokic [2] developed a kinetic expression commonly used in the thermal analysis of oil shale pyrolysis that was derived on the basis of a simple first-order kinetic equation of kerogen decomposition. There was an increase in the activation energy with an increase in content of paraffinic structures in the oil shale. Rajeshwar [3] studied the pyrolysis kinetics of thermal decomposition of Green River oil shale kerogene by non-isothermal thermogravimetry. He critically reviewed the factors influencing kinetic data such as sample order geometry, heating rate and atmosphere. He analysed the weight loss data by the Coats and Redfern and the Freeman and Carroll techniques. Shih and Sohn [4] used non-isothermal thermogravimetry with a variety of heating rates to the determination of kinetic parameters for Green River oil shale pyrolysis. Four different methods were employed for kinetic analysis and the results appear to be in fair agreement. Lee et al. [5] studied the thermal behaviour of oil shale as a function of grade, gas composition and particle size. Two exothermic peaks are generated by oil shale heated in air. The first peak can be assigned to the combustion of light hydrocarbon fractions from the shale organic matter whereas the second peak arises from the oxidation of char. Skala et al. [6] investigated the pyrolysis kinetics of oil shales under non-isothermal conditions using thermal methods. The results obtained were incorporated into the multi-step kinetic model which was adjusted according to the specific purposes of particular oil shale samples and tested by comparison of the experimental and simulated thermogravimetry (TG/DTG) and differential scanning calorimetry (DSC) curves. Levy and Stuart [7] obtained thermogravimetry (TG/DTG) curves of Australian oil shales and kerogen concentrates heated in a dynamic air atmosphere. It was observed that the combustion of kerogen occurs in two stages, indicated by two sharply defined peaks.

The objective of this research was to investigate the pyrolysis and combustion kinetics of oil shale samples. For this reason five different kinetic models were used and the results discussed and compared.

Section snippets

Experimental

In this research, thermogravimetry (TG/DTG) experiments were performed with a DuPont 9900 thermal analysis system. A sample size of 10 mg, heating rate of 5°C min−1, temperature range of 20–600°C and 50 ml min−1 gas flow rate (air and nitrogen) were used. The oil shale samples used in all experiments had a particle size <60 mesh and prepared according to ASTM Standards (ASTM D 2013-72). It is believed that for such a small particle size the effect of temperature distribution within the sample

Results and discussion

Non-isothermal kinetic study of weight loss under pyrolysis and combustion processes is extremely complex for oil shales because of the presence of the numerous complex components and their parallel and consecutive reactions. In the course of this research, five different models (Arrhenius, Coats and Redfern, Maximum Point, Horowitz and Metzger and Ingraham and Marrier) all based on Arrhenius kinetic theory were used for kinetic analysis of the data generated by the TG/DTG experiments.

The

Conclusions

In the course of this research, kinetic parameters (both pyrolysis and combustion) of oil shale samples were determined using five different methods. Among the kinetic models studied, the Coats and Redfern method involved a trial and error procedure, where the reaction order is estimated until the best straight line is obtained. The correct order presumed to lead the best linear plot, from which the activation energy is determined. Differences between the activation energy values of the samples

Acknowledgements

The authors would like to express heir appreciation for the financial support of The Scientific and Technical research Council of Turkey, TUBITAK (YDABCAG-141).

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