Formation Behaviors Of Tar And Gas In Oil Sand Pyrolysis

Oil sand,as one kind of unconventional petroleum resources,has abundant reserves and extensive distributions around the world.During oil sand retorting,in-situ thermal mining,bitumen refinements,and other recoverings and processings,pyrolysis is an important part of reactions.The fundamental issues during oil sand pyrolysis are on the formation of tar and gas,which are also the key problems for related technical researches and developments.In order to study the generation behavior of tar and gas in oil sand pyrolysis,an Indonesian(IO),a Philippine(PO),and a Chinese(XO)oil sand were studied.Related research works are as followings:(1)By Soxhlet extraction,XRD,1H/13C NMR,XPS,and other methods,the composition and structure of the three oil sand samples were characterized.The contents of bitumen in the oil sands are all more than 8%,belonging to the ones with a medium or higher grade.IO’s bitumen contains higher proportion of asphaltene,and the sulfur content is more than 6%,of which>65%is as aliphatic forms.The mineral composition of IO is relatively simple,mainly composed by calcium carbonate.The asphaltene and sulfur content in PO’s and XO’s are relatively low,but the mineral composition is more complicated,and the extremely fine mineral particles with a particle size of less than 44 μm occupy a higher proportion.Through the calculations of the average structure of bitumen,it is found that the degrees of condensation in PO’s and XO’s bitumen aromatic cores are mild,but their aliphatic substituents contain more carbon number.IO’s bitumen aromatic cores are mainly as forced condensation,and the cores contain more but less carbon number of substituents.(2)TGA-MS and Py-EGA-MS were used to study the volatile release during pyrolysis of the oil sands and their bitumen sub-fractions.From the weight loss process and the release of volatiles during pyrolysis,it was observed that the producings of tar and gas included an desorption stage(<350 ℃)and an thermal-cracking stage(350～550 ℃).The main reaction modes and volatile compositions of the two stages were different.Each sub-fraction of bitumen had specific characteristics in volatile release during pyrolysis,and the yields at the two stage were mainly determined by the composition of organics in oil sand.The tar and gas produced in the desorption stage mainly originated from the saturate fractions in maltene,and macromolecular compounds such as polycyclic biomarkers evolved at 200～350 ℃.The tar and gas in the thermal-cracking stage mainly originated from the asphaltenes and resins,during which gases and light hydrocarbons(olefins and alkanes in the majority)were produced in large quantities.Aromatic fraction in oil sands was found volatile releasing in both stages.During oil and gas producing process,minerals had few effects on volatile release,but interactions between the subfractions were suggested during the formation of the final products.(3)Through the pyrolysis experiments of 10 and PO in fixed-bed reactor,the effects of temperature on the compositions and structures of char containing bitumen,and on the oil and gas composition were studied and to revealed the evolution of bitumen during pyrolysis and the formations of pyrolytic products.In the desorption stage,the thermal effects removed some of the light components of bitumen,but did not severely damage the main structures.Although heavy components such as asphaltenes would not obviously produce volatiles in the low temperature stage,they were converted into maltene fractions by dissociations of weak bonds,and became precursors for formation of tar and gas.As temperature increased into thermal-cracking stage,the crackings of alkyl side chains became the main source of oil and gas producings,leading to the rapid increase of their yields.Meanwhile,some parts of bitumen turned to be heavier components by dehydrogenation and condensation under the effect of heat,and gradually till to be coke left in the chars.When the temperature is higher than 450 ℃,the tar did not change much in the yield and distillation range.At 500 ℃,about 80%bitumen of the two oil sands converted into tars.During pyrolysis of IO,although a little sulfur containing compounds evolved below 300 ℃,the aliphatic sulfur in oil sand gradually converted into aromatic sulfur under the effect of heat.The sulfur containing compounds mainly escaped at 300～400 ℃.At 500 ℃,over 90%of the sulfur escaped from the oil sand particles in the form of volatiles,and mainly presented as aromatic sulfur compounds in the tar.(4)By the means of GC/MS and Py-GC/MS,the effects of temperature and heating rate on yield and product distribution of oil sand pyrolysis were studied.The results showed that the tar derived from oil sand pyrolysis was a complex mixture mainly in aliphatic hydrocarbons,and its composition was related to the structure of oil sand bitumens.During pyrolysis,temperature determines the decomposing degree of bitumen.At the same final temperature,the increasing heating rate would shorten the time required for the volatile release,but the volatiles would evolved into higher temperature environment.Due to the difference in thermal stability of the tar and gas,the degree of secondary reaction is different,which affects the carbon number distribution of the chain hydrocarbons,the olefin/alkane ratio,the length of the side chain of the aromatic hydrocarbons,and the form of the hetero atoms.Among them,increasing heating rate or temperature would strengthen the secondary cracking and dehydrogenation of volatiles,contributing the increase of unsaturates hydrocarbons.Moreover,with the increasing heating rate,the desorption stage and thermal-cracking stage would gradually coincide with each other,which made the desorbed macromolecules evolved into the high temperature zone and had more severe cracking reactions to generate smaller molecule compounds.(5)By TGA and fixed bed pyrolysis apparatus,the influence of the high proportion addition of calcium oxide on the oil sand pyrolysis was studied.When 1:1(mass ratio)of calcium oxide was added,the heating and pyrolysis volatile evolving of the oil sands was hindered,and the pyrolysis activation energy during oil and gas producing stage increased to 168～270 kJ/mol from 66～173 kJ/mol.However,as the pyrolysis temperature increased,this inhibition was gradually weakened.Pyrolyzed with high proportion addition of calcium oxide at 480 ℃ in fixed bed reactor,calcium oxide showed obvious effects of dehydration,decarburization,and sulfur fixation.Tar yield had small difference compared to pyrolysis alone,but the proportion of saturate and resin increased,while the aromatic fraction decreased by about 10%.Through GC/MS analysis,the proportion of olefins in saturate decreased,while the content of alkanes,especially long-chain alkanes,increased significantly.The proportion of PAHs in aromatics increased,and acidic compounds,such as phenols and carboxylic acids,tended to dehydroxylate with calcium oxide,forming more ketones and esters.