Effect Of Supercritical Carbon Dioxide On Gasification Of 3D Printed Polylactic Acid Waste

As an important type of the information in modern life and production,the use of plastics is closely related to energy,economy,and environment.In response to national policies and to ease environmental pressures,researchers have turned their attention to biodegradable plastics with green,extensive,and less polluting source of raw materials.Among them,poly-lactic acid（PLA）has a promising prospect as its main type.At the same time,3D printed as an advanced manufacturing technology has attracted great attention in the world.In the“14^thFive-Year Plan”,China proposed that 3D printed technology should be an important position in the field of future development planning.PLA is one of the ideal materials in 3D printed technology.However,PLA can maintain a good state without external interference at room temperature and pressure,and does not degrade for a long time,resulting in environmental impact and waste of resources.Therefore,this work compared and analyzed the physical and chemical properties of PLA before and after 3D printed.Thermogravimetric-Foutiertransforminfraredspectroscopy/gas chromatography-mass spectrometry（TG-FTIR/GC-MS）technology was used to analyze the basic characteristics of 3D printed poly-lactic acid waste（3D-PLAW）such as thermal weight loss and pyrolysis mechanism,so as to fill the research gap in this field.3D-PLAW gasification was designed by response surface method to clarify the interaction between gasification products and reaction conditions.On the basis,the effect of supercritical carbon dioxide（SCCO₂）pretreatment technology on 3D-PLAW gasification was studied.The results show that the content of C,O elements and volatiles in 3D-PLAW was higher,and the micro-structure connection was closer.It is speculated that some long carbon chains were broken during 3D printed.The TG/DTG curves were different from PLA,and there are two pyrolysis regions.It is speculated that the second pyrolysis region is caused by the slow heat conduction inside the feedstock.The pyrolysis kinetics of3D-PLAW was calculated by DAEM,FWO,Starink,and Friedman methods.The analysis results showed that the activation energy was low(the average activation energy was 206k J·mol^-1),which was conducive to heat recovery research.Based on the previous studies on PLA pyrolysis,the pyrolysis mechanism and products of 3D-PLAW were analyzed.The free radical reaction,transesterification reaction,and homeopathic elimination reaction produced CO,CO₂,CH₄,acetaldehyde,esters（medium lactide and D,L-lactide）,and carbon-based compounds,which may contain ethylene,other esters,alcohols,phenols,and organic acid compounds.According to the calorific value of gasification gas,peak area of hydrocarbon,and gasification gas products characterization results,the atmosphere,final temperature of tube furnace,and flow rate of gasification agent in 3D-PLAW gasification were compared.The atmosphere has the most significant effect on the calorific value and hydrocarbon peak area of gasification gas.The optimum conditions for the tubular furnace to reach 700°C in the CO₂atmosphere and the gasification agent flow rate of 0.6 L·min^-1were obtained by fitting.Gas chromatography-mass spectrometry（GC-MS）analysis of gasification gas components mainly includes CO,CH₄,CO₂,C₂H₄,C₂H₆,C₃H₆,C₄H₆and C₂H₄O,among which combustible gas occupies the main part.A small amount of liquid products were produced for 3D-PLAW gasification,mainly including lactide and carbon chain scission.In this paper,based on the dissolution and swelling effect of SCCO₂on 3D-PLAW,the calorific value of gasification gas,the yield of hydrocarbons,and the composition of gasification gas were discussed by setting different pretreatment experimental pressure,temperature,and time.The optimal experimental conditions were selected to occur at 30MPa,30°C pretreatment for 4 h,and the gasification gas calorific value and hydrocarbons obtained by gasification with 3D-PLAW were optimal.Compared with 3D-PLAW gasification,the proportion of combustible gas components produced by SCCO₂-3D-PLAW gasification increases,and the calorific value is higher.In addition,SCCO₂-3D-PLAW was gasified to obtain a small amount of liquid phase products,which were mainly lactide,accompanied by cyclic compounds with stable structure after full reaction.The promotion effect of SCCO₂pretreatment on 3D-PLAW gasification was verified.In this study,the physical and chemical properties of the feedstocks under the optimal gasification conditions after pretreatment were analyzed.According to scanning electron microscope（SEM）and brunauer emmett telller（BET）analysis,cracks and mesoporous pores appeared on the surface and inside,indicating that 3D-PLAW had structural and chemical changes during pretreatment.