Study On Structure Composition And Thermal Fracture Behavior Of Oil Shale Organic Matter

With the rapidly growing of global energy consumption and petroleum and natural gas increasingly depleted, oil shale, as an important part of unconventional oil and gas resources, are getting more and more world attentions. Oil shale is a kind of important source rock, which consists of lots of inorganic and organic matter; in the process of temperature programmed, kerogen molecular structure will crack and release shale oil which similar to crude oil. The core technology of oil shale refining is pyrolysis reaction and the pyrolysis volatile is the source of shale oil and gas; pyrolysis factors, such as residence time, particle size, heating rate, mineral matter and others, all will affect the devolatilization process; but volatile maximum limit depends upon organic matter structure composition. However, among numerous oil shale pyrolysis literatures, rarely have investigated the relationship of oil shale with its pyrolysis products from the perspective of bonding structure, and more failed to speculate the bond structure in the organic matter.This work combines TGA-MS, FTIR and 13C-NMR techniques to investigate the evolution features of pyrolysis gaseous products and the effects of heating rate, particle size and mineral matrix on organic matter decomposition. It is found that there are two types of organic matter in oil shale, the structure composition and decomposition behavior of two kinds organic matters in Yilan, Huadian and Longkou oil shales are investigated, and the interaction effects for mixed-pyrolysis are exploratory studied. Pyrolysis volatile secondary reactions to generate solid matter whether could take place in thermogravimetric crucibles is also studied. Moreover, oil shale organic bond structure characteristics and its fracture behavior in pyrolysis process are preliminarily explored. The main conclusions of this paper can be drawn as follows:(1) Particle size affects mineral content and decomposition of organic matter in oil shale. Heating rate when less than 30℃·min-1 almost does not affect total TG loss. Alkali, alkaline earth metals and pyrite have a promoting effect on TG loss, while silicate clays play an inhibitory effect; the overall mineral matrix in Yilan oil shale does not substantially affect the breakage of organic matter carbon skeleton structure.(2) There exists two forms of organic matter in Yilan, Huadian and Longkou oil shales; one exists in the form of shale combined with minerals (gray blocks) and another in the form of enriched organic matter (black block). Grey block contains more aliphatic carbon chain and ether oxygen bridge bond structure, black block contains more aromatic structure and short alkyl chain; grey blocks organic matter TG loss is greater than black blocks and generate more C2/C3 hydrocarbons and CO2, while black block produce more CH4.(3) There is a synergetic effect during grey block and black block mixed-pyrolysis and gaseous products increased significantly compared to pyrolysis individually. For Yilan oil shale, when grey block and black block mixed-pyrolysis at the mass ratio of 5:1, gas yield increased by 5%(dry ash-free basis) and oil (containing water) yield decreased by about 9%(dry ash-free basis) compared to gray block alone pyrolysis.(4) Total pyrolysis weight loss decreased gradually along with the increase of crucible height, and the extent of volatile polycondensation reaction to generate solid enhanced. TG loss reduced mainly occurred in high temperature stage and between 300 and 600℃ changes little; indicating that volatile polycondensation reaction has little effect on oil shale pyrolysis weight loss in this temperature zone, reflecting the bond cleavage behavior in organic matter.(5) The fraction of aliphatic carbon structure in Yilan oil shale (grey block) kerogen accounts for 53% and the aromaticity is about 42%. Aliphatic structures are dominated by methylene, methine chain and multi-branched carbon chain, about accounting for 79% of total aliphatic carbon; followed by methyl carbon on aromatic ring and short chain alkyl. The organic oxygen in kerogen mainly exists in Car-O-, Cal-O- and carboxyl or carbonyl groups. The breakage of aliphatic chain starts about 350℃ and concentrated at 400-500℃, about 93% of the methylene and methine bridges breaked at 500℃, when temperature rise to 600℃, almost all of aromatic ring methyl cleaved.(6) The first breaking reaction is ascribed to the breakage of weak bonds (Cal-Cal, Cal-O/S, Cal-H) with aliphatic carbon as the center, the second to the cleavage of strong bonds (Car-Cal, Car-O/S) with aromatic carbon as the center. Oil shale pyrolysis process was dominated by the first bonds breaking reaction, especially the breakage of Cal-Cal bond. The contribution of first rupture reaction to the pyrolysis volatile for Yilan oil shale is 68.6% and for Longkou oil shale is about 72%.