| Oily sludge is a viscous solid waste generated during the production,transportation,storage,cleaning and refining of crude oil.It is a complex mixture with highly emulsified water,oil and solid particles.Inappropriate disposal of oily sludge will not only occupy a lot of land resources,but also cause severe soil,air and water pollution,and even threaten human health.It has already been classified as a hazardous waste in many countries.On the other hand,oily sludge is also a potential energy resource due to its high oil content?3080%?.Compared with other methods and technologies,pyrolysis has a more efficient energy recovery rate and a lower pollutant emission level,and catalytic pyrolysis can upgrade the products using catalysts.Therefore,catalytic pyrolysis of oily sludge has become a research hotspot in recent years.However,at present,the mechanism of catalytic reaction of oily sludge is still unclear,and there is insufficient research on the regulation of pyrolysis products,utilization of pyrolysis residues,and migration and transformation characteristics of sulfur pollutions.The purpose of this paper is to achieve the target-oriented catalytic pyrolysis of oily sludge.A comprehensive and in-depth study was conducted on the basic pyrolysis characteristic of oily sludge,directional regulation of oil and gas products,catalytic mechanism of common catalysts and char catalyst,pyrolysis behavior of sulfur compounds.Besides,the co-pyrolysis of oily sludge and rice husk was proposed,and the influence of synergetic effect on the pyrolysis products was studied.The results of this paper could provide a theoretical and technical support for the resource utilization of oily sludge and other heavy oil wastes in the petrochemical industry.First,the pyrolysis characteristics of oily sludge was investigated.The pyrolysis of organic components in oily sludge can be divided into three stages,the corresponding temperature ranges were 100180°C,180350°C and 350500°C.The pyrolysis of heavy components happened at higher temperature ranges.The kinetic analysis of the three volatilization stages showed that the pyrolysis reaction can be simplified as first order reaction.The activation energy was the highest during the decomposition of heavy components,which reached 67.78 kJ/mol.The oil yield was related to the oil content of the raw sludge,but the oil recovery rate of different sludge was similar,about 5960%.The oil product from direct pyrolysis still had low quality with 15.6%asphaltenes content,large average molecular weight and high viscosity,but the calorific value was higher than that of raw sludge.With the addition of KOH,the activation energy during the decomposition of heavy fractions was significantly lowered.Meanwhile,the asphaltenes content in the oil product was reduced to 6.9%,the average molecular weight was reduced by 50%,the viscosity was significantly reduced and the calorific value was increased to 41.1 MJ/kg,the content of short-chain saturates also increased.In all,the quality of the oil product was improved.The catalytic pyrolysis for producing saturates-enriched light oil was studied.Results showed that higher pyrolysis temperature led to higher gas yield and lower oil yield,meanwhile,the polymerization was promoted that the polyamromatics was increased in the oil product.Dolomite was applied as catalyst for catalytic pyrolysis of oily sludge under 500°C.When the residence time was 4.4 s,a light oil with 56.1%saturates and 2.0%asphaltenes was obtained.Compared with the oil phase in the raw sludge,the content of saturates was increased by 43%and that of asphaltenes was reduced by 85%.It was supposed that the inherent moisture in the oily sludge could participate in the steaming reforming reaction,promoting the decomposition of aromatics and leading to an increase of saturates as well as H2.Dolomite also inhibited the release of H2S and reduced the sulfur content of oil product.The deactivation of dolomite was caused by the deposition of polyaromatic coke and carbonation of CaO but can be regenerated by calcination.However,the catalytic performance was slightly weakened after regeneration.The catalytic pyrolysis for producing aromatics-enriched oil was studied.HZSM-5 was applied as catalyst during two-stage pyrolysis of oily sludge.Results showed that HZSM-5 has a strong aromatization effect on the volatiles under 500°C.Longer residence time led to a higher aromatics production,however,aromatics were more concentrated on polyaromatics with three or more benzene rings under 7.6 s.By contrast,3.8 s was regarded as the optimum residence time for a high aromatics production?84.4%?and the highest naphthalenes yield?57.2%?.The catalytic effects of Zn modified HZSM-5 were investigated under 1.9 s.The incorporation of 3%Zn enhanced the aromatics yield from 58.7%to 81.0%,as well as the naphthalenes yield?from 31.5%to 67.5%?.This was because a new Lewis acid site was generated after loading Zn,which promoted the dehydrogenation and deoxygenation of hydrocarbons.The deposited coke was determined as polyaromatic coke due to the polycondensation reaction over HZSM-5.Moreover,longer residence time promoted the formation of coke while loading of Zn slightly reduced it.The catalytic pyrolysis for producing hydrogen-enriched gas was studied.Fe-char catalyst was prepared from oily sludge and was further applied to the catalytic cracking of heavy oil in oily sludge.The char exhibited a porous structure and the inherent iron oxides were directly reduced to Fe2+and metallic iron?Fe0?after carbonizaiton.The catalytic effects of the char prepared at different temperatures were compared at 600°C in terms of oil conversion efficiency and gas yield.Results showed char catalyst with Fe0 showed better catalytic activity than that with Fe2+.The oil conversion efficiency reached 90.2%and more polyaromatics in the oil were cracked into monocyclic aromatics,meanwhile,the H2 yield increased to 0.29 m3/kg sludge.The effects of temperature on the performance of Fe-char catalyst were also investigated.At 800°C,the oil conversion efficiency reached the highest?95.8%?,the CO yield was 9.5 times higher than that under uncatalyzed conditions,and the H2 yield increased to 0.48 m3/kg sludge.Fe-char catalyst not only provided a porous surface for the adsorption of hydrocarbons,but also played as a catalytic medium for the hydrocarbons reforming and water-gas-shift reactions,enhancing the production of syngas.Moreover,the metallic iron will not be oxidized during the reaction,therefore,the catalyst still showed good activity after several uses.The transformation behavior of sulfur during pyrolysis of oily sludge was investigated.Organic sulfur of oily sludge was mainly existed in the oil phase as aliphatic sulfur and heterocyclic sulfur,and inorganic sufur was mianly presented in the form of sulfate in the solid particles.During pyrolysis,H2S was the main sulfur-containing gas in the gas product,which was from the decomposition of mercaptan and thioether.The sulfur remaining in the residues was mainly in the form of thiophene and sulfate.The increase of pyrolysis temperature promoted the release of H2S,also increased the sulfur content of the oil product.On the contrary,the addition of 10%KOH inhibited the migration of sulfur into the liquid and gas phases.The sulfur content in the oil was decreased and the release of H2S was reduced by 75%.Moreover,KOH promoted the decomposition of thiophene and its content in the residues was reduced by 85%,but a large amount of metal sulfide was generated and the total sulfur remaining in the residues was increased by 39.5%.Overall,more organic sulfur was converted into inorganic sulfur and was fixed in the pyrolysis residues after addition of KOH.At last,the co-pyrolysis of oily sludge and rice husk was conducted in a fixed-bed reactor to investigate the effects of interactions on the products and improve the quality of the pyrolysis oil.Results showed that the physical state of oily sludge and rice husk mixture was fluffy and solid-like,which was easy to feed continuously.The synergy can be attributed to the catalytic effects of ash and alkali metals derived from biomass,which made the compositions of oil product ligher.Compared to the theoretical value,the contents of saturates and aromatics in oil product were increased by 1555%and 5586%respectively,while the contents of resins and asphaltenes were decreased by 2531%and 1168%respectively.Moreover,the content of oxygenates in oil product was reduced by 4693%than expected.The gas yield was improved due to the promotion of secondary reactions and more H2,CO and C1-C2 hydrocarbons were generated.Additionally,though the emission of H2S was restrained,the distribution of sulfur in oil and gas phases was promoted.Through theoretical calculation,the energy recovery efficiency could reach 52.560.9%during co-pyrolysis.Therefore,the results could provide a theoretical basis and technical support for the industrialization of oily sludge pyrolysis. |
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