Characteristics And Mechanism Of Oil Shale Pyrolysis

In this paper,effects of carrier gas flow rate,heating rate,particle size and pyrolysis temperature on yields and characteristics of pyrolysates from Longkou oil shale were investigated using a fixed bed.Temperature remarkably affected the oil yield and composition of retorting gas.The oil yield increased and H/C ratio decreased with the growth of temperature.Increasing carrier gas flow rate accelerated fluxion of pyrolysates,but shale oil was descended if it was larger than 0.6 L·min^-1.Higher heating rate aggravated the rate of heat transfer and improved the volatilization of pyrolysates.Larger particle size prolonged the escaping path of pyrolysates and aggravated secondary cracking reaction,descending the oil yield.So the optimum operating condition in laboratory have been found,including particle size of 2-3mm,heating rate of 15℃·min^-1,temperature of 550℃ and carrier gas flow rate of 0.6 L·min^-1.In optimized condition,the oil yield was 17.39wt%,including 50.03wt% saturated fraction,25.63wt% aromatic fraction and total content of C8-C31 n-Alkanes and C8-C23 n-Alkenes with 582 mg·goil-1.Electron paramagnetic resonance spectroscopy（EPR）was used to characterize the properties of free radical in oil shale pyrolysates.EPR properties of shale oil and semi-coke obtained at different temperatures ranging from 370 oC to 550 oC were also investigated to understand the behavior of free radicals at different temperatures.The concentrations of free radical（Ng）of shale oil and semi-coke became higher with increasing temperatures.The Ng of shale oil was lower than that of semi-coke due to the formation of new molecules before being condensed.Furthermore,the g-values and linewidths of shale oil and semi-coke were also influenced by temperature,ascribing to the spin-spin and spin-lattice interactions of free radicals and surrounding compounds.Secondary pyrolysis processing of Longkou oil shale was studied in a two-stage fixed bed.It demonstrated that temperature and residence time of two-stage fixed bed were the mainly influencing factors on secondary cracking reaction.The oil yield decreased from 12.66% at the pyrolysis temperature of 300°C to 8.47% at 500 °C.Besides,increasing cracking temperature and prolonging residence time promoted secondary cracking reaction,increasing retorting gas yield.Due to secondary reaction,the shale oil had lower IBP point,higher light fraction and H/C ratio,tending to simpler carbon number distribution.Besides,the coking reaction was also taken place and occupied the main position at higher temperature.Mechanism study of oil shale fast pyrolysis was carried out on a Pyrolysis-Gas Chromatography and Mass Spectrometer（Py-GC/MS）.The detected pyrolysates included five components: n-Alkanes and n-Alkenes;Aromatics compounds;Phenol series;Fatty acid series;oxygen compounds and isoprene alkanes.Kinetic parameters（apparent activation energy E and frequency factor A）of each n-alkane,n-alkene and alkyl-benzenes were determined by differential method.The values of apparent activation energy E for n-Alkanes were found to range from 16 kJ·mol-1 to 86 kJ·mol-1,n-alkenes with 50-98 kJ·mol-1 and alkyl-benzenes with 88-99 kJ·mol-1.E of n-alkanes and n-alkenes decreased with the growth of carbon number.The variation of reaction rate constant was influenced by apparent activation energy.Besides,effect of heating rate from 10°C·min^-1 to 10000°C·min^-1on the content of n-alkanes,n-alkenes and alkyl-benzenes were also studied.It was found that decreasing heating rate extended heating time and aggravated the secondary cracking reaction,leading to increasing content for C8-C20 compounds.Total content of n-alkanes,n-alkenes and alkyl-benzenes was increased by 40% with decreasing heating rate.Terahertz（THz）technique was used to realize the identification and determination of gas components from oil shale,including methane（CH4）,ethane（C2H6）,carbon monoxide（CO）and carbon dioxide（CO2）.An obvious variation can be observed among the components in THz spectra.In addition,back propagation artificial neural network was employed to quantitatively characterize the components’ content.The correlation coefficient of the prediction set in the quantitative model equaled 0.9950,showing THz technique combined with mathematical method was an effective tool for gas detection.What’s more,THz technique was also used to the identification of semi-coke obtained from different pyrolysis conditions.The results displayed that terahertz response for each semi-coke was different,due to the changed structure of semi-coke during the pyrolysis process.The relationship of peak amplitude vs pyrolysis condition was in accordance to that of oil yield vs pyrolysis condition,providing theoretic and experimental basis for research on component and structure of semi-coke information using Terahertz（THz）technique.