Study On The Pyrolysis Characteristics Of Oil Shale Based On Its Composition And Structure

In recent years,China's oil consumption increases in slow and medium speed,which makes foreign oil's dependence increases gradually.China's foreign oil dependence exceeded 60% in 2015 and reached 64.4% in 2016,it was expected that this value will exceed 65% in 2017.This situation seriously threats our sustainable safe energy supply.In order to alleviate the pressure of petroleum supplies,intensification of conventional oil's exploration and development is needed.Moreover,it is also essential to widely explore and utilize unconventional oil & gas resources,which involving oil shale,shale oil,shale gas,and coal bed gas,etc.China's oil shale reserves are about 720 billion tons,in situ converted to 47.6 billion tons of shale oil.Because of it large reserves and properties,oil shale has widely been considered as one of the most important potential substitute resources for petroleum.Oil shale is a sedimentary rock containing significant portions of kerogen and small amounts of bitumen.Pyrolysis is one of the most attractive conversion approaches,which involves the thermal degradation of organic matters resulting in the production of shale oil and fuel gas.Shale oil can be further hydrogenated and upgraded to produce gasoline and diesel or used to extract valuable chemicals.Many investigations have been done for the pyrolysis of oil shale in the past decades.Some researchers conducted pyrolysis experiments in a micro-reactor like thermogravimetric analyzer(TGA)to study the weight loss behavior of oil shale or the evolution feature of volatiles,while others investigate the effect of process conditions on the yield and properties of pyrolytic products with the application of large-scale reactors.However,few experimental studies have been conducted to explore the relationship between oil shale's composition and structure and its pyrolysis characteristics.Oil shale from different regions varies in type and content of organic matters and minerals,and thus holds different pyrolysis characteristics including decomposition behavior and product evolution.A fundamental study on oil shale's chemical structure and its pyrolysis behavior can help to design suitable pyrolysis reactors and processes.As pyrolysis of kerogen is a complex process involves a myriad of consecutive reactions and parallel reactions,it is necessary to conduct pyrolysis experiments on different oil shale samples in a reactor with appropriate scale to obtain enough pyrolytic products.Then characterize the pyrolytic products with the application of different analytical methods.Based on the transformation and variation of kerogen and thermal-induced products,the relationship between oil shale's composition and pyrolysis characteristics can be explored.In this study,Huadian,Taimu,and Beipiao oil shale samples were selected as raw materials.Firstly,several analytical methods including FTIR,XRD,13 C NMR,and XRF were applied to characterize and identify the chemical structure of kerogen,the mineral phase of oil shale and their occurrence features.Then from the point of view of kerogen,mineral matrix and their occurrence features,the relationship between oil shale's compostion and its pyrolysis characteristics were explored.Briefly,the dependence of pyrolysis behavior with kerogen was studied by probing the conversion of organic carbon,the transformation of kergen's chemical structure,and the variation of chemical compositions and properties of thermal bitumen and shale oil at different decomposition degree.The effect of inherent mineral matrix on kerogen pyrolysis was studied by comparing the kinetic parameters and pyrolytic products of raw oil shale samples and their demineralized products.Finally,the raw oil shale was separated into several fractions by sink-float processes and sieving tests.According to material compositions and pyrolysis characteristics of different fractions,the relationship between the occurrence of kerogen and minerals,and the pyrolysis behavior of oil shale was investigated.The main conclusions drawn from this research are as follows:I The composition and structure of oil shale samples(1)The content of kerogen present in Huadian(HD-R),Taimu(TM-R),and Beipiao(BP-R)oil shale are 19.1%,17.0%,and 9.4%,respectively.The kerogen of TM-R belongs to type II,while those contained in HD-R and BP-R belong to type I.For Huadian(HD-K),Taimu(TM-K),and Beipiao(BP-K)kerogen,the content of aliphatic carbon are 80.53%,52.83%,and 72.67%,while the content of aromatic carbon are 15.20%,44.05%,and 23.71%.With the metamorphic degree of kerogen deepens,the aliphatic carbon decreases and aliphatic chain shortens,while the aromatic carbon increases and aromatic rings become more condensed.(2)Silicates are the main component of mineral matrix of three oil shale,their contents are 60.5%,72.0%,and 68.1% respectively for HD-R,TM-R,and BP-R.In comparison,carbonates' content is lower or approximate to 20%,while the pyrite content is only ~2%.All oil shale samples contain quartz,calcite,pyrite,and mica,the main difference is the type and quantity of clay minerals.Montmorillonite is the dominant clay of HD-R,illite and illite/smectite are main clay of BP-R,and TM-R's clay is mainly composed of illite,kaolinite,and chlorite.(3)The amorphous kerogen will combine with clay minerals to form organic clay complexes,which constitute the matrix or continuous phase of oil shale.Quartz,calcite,pyrite and mica are embedded in the organic clay matrix as particles or laminates.Moreover,part of kerogen was present in oil shale as “pure kerogen” with extremely low mineral matter content.II The decomposition and conversion of kerogen during oil shale pyrolysis(1)With the increase of pyrolysis temperature at 360-530 °C,the semi-coke yield decreases while those of shale oil,water and gas increase,and the shale oil yield are 8.36%,4.48% and 3.95% at 530 °C for HD-C,TM-R and BP-R;the thermal bitumen yield firstly increases and then decreases,the maximum yield is 2.87% and 1.95% at 450 °C for HD-R and TM-R,and the corresponding value for BP-R is 0.75% at 435 °C.(2)On the basis of organic carbon,the average activation energy of kerogen decomposition at 360-530 °C for HD-K,TM-K and BP-K are 114.31,100.07 and 88.44 kJ·mol-1,which indicates that the reactivity of kerogen decreases as: BP-K>TM-K>HD-K.Accordingly,the conversion of aliphatic carbon for BP-K and TM-K was almost complete at 480 °C,while the decomposition of part aliphatic chain of HD-K needs high temperatures.(3)During the pyrolysis of HD-R,the accumulation of thermal bitumen and shale oil below 450 °C was mainly controlled by evaporation effect,which makes the boiling point increase with temperature increase.While,their accumulation above 450 °C are dominated by both evaporation and cracking,which makes the boiling point of thermal bitumen decrease and that of shale oil increase.According to the chemical composition of shale oil,kerogen pyrolysis at 435-450 °C incline to produce aliphatic compounds,while aromatic compounds prone to derive at 450-480 °C.(4)Compared with the thermal bitumen derived from HD-R,that produced from TM-R contains more asphaltene and is characterized by high nitrogen and low H/C.Saturates and resins are major constituent of Huadian and Taimu shale oil,respectively.The chemical composition and structure of thermal bitumen and shale oil reflects the chemical structure of kerogen.III The effect of inherent carbonates and silicates on kerogen pyrolysis(1)For three oil shale samples,removal of carbonates decreases shale oil yield while elimination of silicates increases oil yield,which suggest that carbonates and silicates could promote and suppress oil formation.The inherent carbonates and silicates could increase and decrease activation energy,respectively.As the minerals would decompose and cause weight loss during pyrolysis,the apparent activation energy for weight loss can not reflect the effect of minerals on kerogen pyrolysis.(2)The inherent carbonates will promote the formation of nitrogen-and oxygen-containing compounds,while silicates inhibit the formation of oxygenated compounds.In addition,both carbonates and silicates will decrease the H/C of shale oil,and thus lower the quality of shale oil.(3)The inherent silicates will catalyze the cracking of aliphatic hydrocarbons,which decrease the content of long-chain aliphatic hydrocarbons and increase that of short-chain aliphatic hydrocarbons,as well as increase the yield of hydrocarbon gases.In comparison,the inherent carbonates restrain aliphatic hydrocarbon cracking and exert an opposite role on the composition of shale oil and yields of gas components.(4)The inherent carbonates and silicates would respectively suppress and catalyze the combination of alkyl free radicals and hydrogen free radicals,thus increase and decrease the hydrogen yield,and make the shale oil derived from de-carbonate sample contains more alkanes than those from raw and de-silicate samples.In addition,the silicates of HD-R and BP-R would catalyze the aromatization of aliphatic hydrocarbons,which increase the content of aromatic hydrocarbons and H2 yield.The catalytic effect may be related to acidic sites of montmorillonite.IV The composition and pyrolysis characteristics of separated fractions(1)The kerogen and mineral matrix embedded with each other heterogeneously within oil shale.By using the sink-float method,HD-R can be separated into six fractions with different densities(i.e.,1.5-1.6,1.6-1.7,1.7-1.8,1.8-2.0,2.0-2.2,and 2.2-2.4 g/cm3),while TM-R and BP-R can be divided into four and two fractions,respectively.(2)With the increase of fraction density,both the kerogen content and the shale oil yield decrease,while the mineral matter content increases.For HD-R and TM-R,the 2.2-2.4 g/cm3 fraction takes up 8.82% and 12.49% of oil shale,respectively.However,their oil content is extremely low,which make that they only contain 0.99% and 2.53% of shale oil.The heavy fraction can be removed before pyrolysis to increase the process energy efficiency.(3)The chemical composition of oil shale,involving composition of mineral matrix and chemical structure of kerogen,varies with fraction density.For HD-R,with the increase of fraction density,the quartz,illite and kaolinite contents increase while those of montmorillonite and calcite decrease;meanwhile,the aliphatic carbon of kerogen firstly increases and then decreases and obtains the maximum value at 1.8-2.0 g/cm3.(4)During pulverization of oil shale,the organic matter and mineral matter distribute unevenly in different fractions.For HD-R and BP-R,the organic matter was concentrated in large particles,while the mineral matter selectively distribute in small particles,which cause the oil yield increase with the increase of particle size.For TM-R,organic matter preferentially distribute within moderate size fractions,thus their oil yields were greater than large and small size fractions.