Evolution Of Heavy Components In Oily Sludge During Pyrolysis

At present,the annual output of oily sludge in China has exceeded 9 million tons,and the treatment and disposal of oily sludge is imminent.The treatment of oily sludge by pyrolysis technology can realize the resource utilization of oil and gas.It is a clean,efficient and environmentally friendly treatment technology,so it has been widely concerned by scholars at home and abroad.However,the reaction mechanism involved in the pyrolysis process of oily sludge and the migration and transformation of C,H,S,N and other elements in the pyrolysis process are not clear,which has become one of the bottlenecks restricting the development of oily sludge pyrolysis technology.Therefore,in this paper,aiming at the problems of unclear pyrolysis mechanism and element migration and transformation of oily sludge,simulated sludge was selected as the research object.The pyrolysis kinetics,thermodynamics,solid-phase reaction mechanism of simulated sludge and different pyrolysis conditions（pyrolysis temperature,pyrolysis time,catalyst addition）were studied.The formation characteristics of three-state products such as pyrolysis gas,oil and residue and the migration and transformation of C,H,S and N elements in the pyrolysis process reveal the mechanism of Ca O catalyst in the pyrolysis process of simulated sludge.The main research contents are as follows:The pyrolysis process of simulated sludge was studied by TG and TG-FTIR-MS.The kinetics（activation energy,pre-exponential factor）and thermodynamic parameters of simulated sludge at different heating rates were explored by combining the isoconversional model method and the principal function diagram method.The reaction mechanism was proposed.The main pyrolysis mechanism of simulated sludge conforms to the surface limited reaction R2 model between the two phases in the low conversion range and the three-dimensional diffusion model（D3）model in the high conversion range.Based on the qualitative and semi-quantitative analysis of pyrolysis products by TG-FTIR-MS,the formation process of main products was speculated.The pyrolysis products were relatively complex.The gas products were mainly CO₂,CH₄,H₂O,etc.The liquid products were mainly aromatic hydrocarbons and alkanes such as phenols,alcohols,esters,acids,aldehydes,ketones,etc.The content of oxygen-containing and sulfur-containing compounds was relatively high.Based on four isoconversional methods（FWO,FAS,Friedman,Starink method）,the average activation energy of simulated sludge pyrolysis was 210.71,212.02,209.88 and 218.67 k J/mol,respectively.The effects of pyrolysis temperature and time on the distribution of pyrolysis products and the evolution of C,H,S and N elements were studied by tube furnace pyrolysis experiments.The gas phase,liquid phase and residue were characterized by GC,GC-MS,XPS and elemental analyzer,respectively.The distribution characteristics of three-phase products and the evolution of the content and morphology of C,H,S and N elements with pyrolysis conditions were revealed.In the pyrolysis temperature range of 400～700℃,the maximum yield of pyrolysis residue,pyrolysis oil and pyrolysis gas was 83.00%,18.60%and 28.98%,respectively.In addition,with the increase of pyrolysis time,the contents of H₂and CH₄in gas yield were the highest at 120 min,which were 17.83 vol.%and 26.00 vol.%,respectively.With the increase of pyrolysis temperature and the extension of pyrolysis time,the yield of residue showed a downward trend.It can not only decompose and transform pyrrole,quaternary nitrogen and oxidized nitrogen,but also make S element migrate to pyrolysis oil and pyrolysis gas.The content of S in the three-phase product decreased from 70.26%to49.81%.CaO was selected as the pyrolysis catalyst of simulated oil sludge.The effect of Ca O addition on the pyrolysis process of simulated oil sludge was studied,and the mechanism of Ca O in the catalytic pyrolysis process was preliminarily analyzed.With the increase of Ca O addition,the yield of pyrolysis residue increased gradually,while the yield of pyrolysis gas and pyrolysis oil decreased relatively.The addition of Ca O is beneficial to the formation of high-quality pyrolysis gas such as H₂,CO and CH₄.When the amount of Ca O is 8%,the maximum contents of CO,H₂and CH₄in the gas phase products are 1.68 vol.%,18.60 vol.%and 26.87 vol.%,respectively.In addition,the presence of Ca O can not only indirectly fix CO₂by reacting with acid substances,catalyze the decomposition of macromolecular oxygen-containing substances in pyrolysis oil by dehydration,decarbonylation and cracking of hydrocarbon side chains,but also promote the conversion of aliphatic sulfur,thiophene sulfur,sulfone and other organic sulfur into volatiles during pyrolysis,effectively absorb H₂S and other sulfur-containing gases produced during pyrolysis,and generate inorganic calcium sulfide,sulfate,etc.The fixation rate of sulfur in pyrolysis residue increased by 6～16%.When the mass fraction of Ca O exceeds 4%,the fixation rate of sulfur in the pyrolysis residue is stable at more than 60%.