Study On Pyrolysis Characteristics And Reaction Kinetics Of Soybean Soapstock

In 2021,China’s soybean oil production reached 16,666 thousand tonnes,accounting for 28.12%of the world’s total production,ranking first in the world.Soybean soapstock is waste oil biomass produced in the refining process of soybean oil and accounts for～6 vol%of the crude soybean oil.Hence the annual output of soybean soapstock is large.Soybean soapstock contains more than 30%fatty acids,which has high utilization value.Up to now,the resource treatment of soybean soapstock has not attracted widespread attention.In this paper,a relatively new pyrolysis-catalytic pyrolysis-co-pyrolysis technology was adopted to study the resource recovery and harmless utilization of soybean soapstock.The influences of two key pyrolysis parameters,pyrolysis temperature and heating rate,on the pyrolysis process were investigated.On this basis,the influences of two different environmentally friendly catalysts,iron slag and aluminum scrap,on the pyrolysis process of soybean soapstock and the optimization and modification of the products were explored.The co-pyrolysis of soybean soapstock and anaerobic fermentation biogas residue were studied to improve the utilization potential of products.The main research results are as follows:（1）The effects of temperature（500℃～800℃）and heating rate（10℃/min,20℃/min,40℃/min,60℃/min）on the characteristics of pyrolysis products in N₂atmosphere were studied,and the mechanism of pyrolysis reaction was studied.With the increase of pyrolysis temperature,the yield of pyrolytic carbon decreased 11.79%,the yield of pyrolysis gas increased,and the yield of pyrolysis liquid reached the maximum at 700℃.After pyrolysis,the contents of C,H and N in soybean soapstock decreased,while the contents of S remained stable.Pyrolysis temperature can promote the conversion of oxygen-containing compounds to alkanes,alkenes and aromatic hydrocarbons.At800℃,the content of aromatic hydrocarbons in pyrolysis oil was more than 6times that of soybean soapstock.The heating rate had little effect on the yield of pyrolytic carbon.With the increase of heating rate,the yield of pyrolysis oil increased and the yield of pyrolysis gas decreased.Higher heating rate can improve the aromatization level of pyrolytic carbon.The content of aromatic hydrocarbons in pyrolysis oil increased with the increase of heating rate.Lower heating rate was more favorable for alkenes generation in pyrolysis oil.The pyrolysis of soybean soapstock can be divided into pyrolysis of saturated fatty acid salts and unsaturated fatty acid salts.Ketones,alkanes and alkenes were generated from saturated fatty acid salts through condensation reaction,rearrangement reaction and terminal alkene reaction.Butadiene and other alkenes were formed by decarboxylation andβ-breaking reaction of unsaturated fatty acid salts,and cycloalkanes and aromatic hydrocarbons were formed by Diels-Alder reaction and radical intramolecular cyclization.（2）Pyrolysis characteristics and kinetics were studied in combination with thermogravimetric data.The pyrolysis of soybean soapstock can be divided into four stages,and the weight loss ratio was the highest in the range of 380℃～565℃,which can reach more than 27.9%.With the increase of heating rate,ignition index,devolatilization index,burnout index,flammability index,comprehensive combustion index and pre-index factor increased continuously.The apparent activation energies calculated by Flynm-Wall-Ozawa method and Kissinger-Akahira-Sunose method were similar,ranging from 55 k J·mol^-1to 227 k J·mol^-1,the maximum activation energy was obtained when the conversion was 0.5.For Coats-Redfern and Malek method,the three dimensional Jander 3D（n=1/2）equation was used to determine the pyrolysis mechanism of soybean soapstock.（3）The catalytic pyrolysis of soybean soapstock by iron slag and aluminum scrap was studied.Iron slag played a significant catalytic role in gas production.When the mass ratio of soybean soapstock to iron slag was 4:1,gas production was twice that of the pyrolysis of soybean soapstock.Aluminum scrap catalyst could reduce the content of oxygen compounds in soybean soapstock by 54.19%.The content of aromatic hydrocarbons in the pyrolysis oil catalyzed by aluminum scrap was 3.39 times that of the pyrolysis oil,and the content of polycyclic aromatic hydrocarbons reached 21.81%.The contents of alkanes,cycloalkanes and oxygen-containing compounds in catalytic pyrolysis oil decreased,while the contents of alkenes,cycloalkenes and aromatic hydrocarbons increased.After 600℃,the production of pyrolysis gas increased gradually.In the early stage,pyrolysis reaction and water gas reaction were dominant,while in the later stage,Boudouad and water gas reaction became important.N₂adsorption-desorption isotherms of catalytic pyrolysis carbons were type IV,showing priority to mesoporous structure.BET specific surface area was the largest for catalytic pyrolysis carbon in the presence of iron slag.（4）The co-pyrolysis of biogas residue and soybean soapstock was studied,and the effects of co-pyrolysis carbon on anaerobic fermentation gas production were explored.According to thermogravimetric data,with the increase of biogas residue addition,the maximum weight loss rate moved to lower temperature zone.When the mass ratio of soybean soapstock and biogas residue was 4:1,the synergistic effects reached the maximum.When the addition amount of biogas residue was small,it can promote the generation of co-pyrolysis oil,but inhibit the generation of co-pyrolysis gas.The specific surface area of biogas slag pyrolytic carbon was more than 4 times that of soybean soapstock,the specific surface area and average pore size of co-pyrolytic carbon were in between.In terms of the effect of co-pyrolytic carbon on anaerobic fermentation,the co-pyrolytic carbon with a mixture ratio of soybean soapstock and biogas residue of 4:3 had a short reaction start-up time in the early stage of anaerobic digestion reaction.The modified Gompertz model was used to conduct kinetic fitting analysis on the cumulative methane gas production,and the maximum methane production rate of the co-pyrolytic carbon was 6.39%higher than that of the blank group.The lag time was9.59%earlier than that of the blank group,and this co-pyrolytic carbon had the best anaerobic digestion performance.