Studys On Structure Of Oil Shale Organic Matter And Bond Cleavage During Pyrolysis

With increasing consumption and shortage of petroleum resources,the exploitation and utilization of petroleum alternative energy has been paid more and more attention.Oil shale is considered as an important oil alternative energy due to its advantages such as splendid reserve and strong oil-generating capacity.Its efficient exploitation and utilization can substantially supplement conventional oil resources and has important significance for alleviating the shortage of oil resources in China.Although oil shale refining has one hundred years of history in China,the technology progress is still small and there are still some problems remained,such as low refining efficiency and oil yield,poor oil quality and severe pollution.The key to solve these industrial problems is to gain an in-depth understanding of the scientific basis among them.Oil shale is an organic rock composed of organic matter and inorganic minerals.The process of oil shale refining is the conversion of organic matter by pyrolysis into shale oil.In terms of chemical reaction,organic matter is a kind of macromolecular polymer constituted by covalent bonds.Pyrolysis of organic matter is the process of covalent bonds cleavage to produce free radical fragments and free radical fragments reaction to form shale oil,gas and semicoke.The producing areas of oil shale in China are widely distributed and the structure of organic matters is complex and different.Therefore,understanding the change rule of organic matter structure and the relationship between the bond cleavage-radical reaction and structure is the basis of optimizing the reactor and pyrolysis process and achieving efficient refining of oil shale.According to the above analysis,eight representative oil shale organic matters were selected as the research objects in this paper.The bonding structure models of organic matters were constructed based on the ultimate analysis and 13C NMR results,and the characteristic and change rule of structure of organic matters were analyzed.The influence of pyrite removal by nitric acid and chromous chloride on the structure of Huadian organic matter was investigated in order to facilitate the study of the organic matter structure.The behaviors of bond cleavage and radical reaction during organic matter pyrolysis and their relationships with organic matter structure were explored based on the method of determining the quantity of active radicals generated and bonds cleaved proposed by our research group.The main conclusions are the followings(1)The metamorphic grade of oil shale organic matter is quite low.The aliphaticity and aromaticity of organic matters are mainly around 45%-77%and 18%-49%,respectively.The H/C molar ratio is in direct proportion to aliphaticity of the organic matters while in inverse proportion to the aromaticity.The aliphatic carbons mainly exist in the form of methylene bridge chain in the three-dimensional macromolecular structure of organic matter.The average length of methylene chain ranges from 2.3 to 21.2,the average aromatic cluster is 2-4 rings and the substitutive degree of aromatic ring is 0.4-0.6.The organic matter itself contains some stable radicals and the concentration of stable radicals,in a range of 4×10-4 mmol/g to 8×10-3mol/g,is exponentially proportional to the aromaticity,which can be used to estimate the concentration of average aromatic cluster in the organic matters.(2)The concentrations of different covalent bonds in organic matters are distributed between mmol/g and tens of mmol/g.Among them,the Cal-Cal concentration and Cal-H concentration,31-57 mmol/g and 29-74 mmol/g respectively,are higher comparatively.The H/C molar ratio is in direct proportion to the concentration of Cal-Cal and Cal-H of the organic matters while in inverse proportion to that of Cal-Car,Car-Car and Car-H.The weak bonds in organic matter are mainly Cal-Cal in ? site,O-Cal/ar in(3 site and Cal-O-Cal,and the concentration of weak bonds(5-10 mmol/g)is negatively linear correlated with H/C.The main distinction of organic matter structure is the difference of relative concentration of Cal-Cal and Car-Car.Compared with the existing chemical structure model,the bonding structure model can provide a basis for the quantitative study of kinetics and thermodynamics of organic matter of organic matters during pyrolysis.(3)The removal effect of chromous chloride on pyrite in oil shale is better than that of nitric acid with removal efficiency of 96%and 66%,respectively.However,the organic matter loss of the chromous chloride treatment is significantly higher than that of nitric acid treatment(13%and 7%respectively).The lost organic matters are all mainly O-and H-containing substances,resulting in a slight decrease of H/C but a marked decrease of O%and O/C in the remainder of organic matter.The relative content of aliphatic,aromatic,and carboxyl/carbonyl carbons of organic matter remain about the same after pyrite removal by chromous chloride or nitric acid,so does the pyrolysis characteristic temperature,indicating the carbon skeleton of organic matter is less affected by chromous chloride or nitric acid.However,the chromous chloride treatment can break the C-O bond in organic matter and reduce the content of C-O/C-OH and O=C-O.In general,except for the introduction of a small amount of nitrogen,the influence of nitric acid removal of pyrite on organic matter is relatively small compared with that of chromous chloride.(4)The quantity of bond cleavage increases with the increase of temperature and time during organic matter pyrolysis,and the rate of bond cleavage decreases exponentially with the increase of pyrolysis time.The bond cleavage mainly ccurres within the first 5 minutes during pyrolysis.The bond cleavage can be described by the 1 st-order reaction kinetic.The amount of cleavable covalent bonds per unit mass of organic increases exponentially with the increase of temperature,but is negatively correlated with H/C.The covalent bonds cleaved during organic matter pyrolysis before 420? are basically weak bonds and the primary radical fragments generated are difficult to volatilize.The volatiles are mainly generated from Cal-Cal cleavage,and the cleavable Cal-Cal at 420? only accounts for the 2%-10%of the total Cal-Cal in organic matters.The activation energy of weak bond cleavage is 78-104 kJ/mol,which decreases linearly with the increase of weak bond amount and is independent of H/C of organic matters.(5)The number of active radicals is 3 orders of magnitude higher than that of stable radicals under the same pyrolysis conditions,indicating more than 99.9%of the active radicals generated diminish by coupling.The quantity of stable radicals survived in the pyrolysis product correlates linearly with that of active radicals generated during organic matters pyrolysis and one stable radical is formed among around 1700-5500 active radicals.The primary radical fragments produced during weak bond cleavage can be solved in tetrahydrofuran.Compared with aliphatic radical fragments,aromatic radical fragments are more prone to condensation to produce large size products,and thereby lower the volatile content.The formation probability of stable radicals,macrocosmically,is positively correlated with the number and size of aromatic radical fragments.