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Stepwise pyrolysis of mixed plastics and paper for separation of oxygenated and hydrocarbon condensates
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-4047-5444
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-1837-5439
2018 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 229, p. 314-325Article in journal (Refereed) Published
Abstract [en]

Mixed plastics and papers are two of the main fractions in municipal solid waste which is a critical environmental issue today. Recovering energy and chemicals from this waste stream by pyrolysis is one of the favorable options to achieve a circular economy. While pyrolysis products from plastics are mainly hydrocarbons, pyrolysis products from paper/biomass are highly oxygenated. The different nature of the two pyrolysis products results in different treatments and applications as well as economic values. Therefore, separation of these two products by multi-step pyrolysis based on their different decomposition temperatures could be beneficial for downstream processes to recover materials, chemicals and/or energy. In this work, stepwise pyrolysis of mixed plastics and paper waste was performed in a batch type fixed bed reactor using two different pyrolysis temperatures. Neat plastic materials (polystyrene, polyethylene) and cellulose mixtures were used as starting materials. Then, the same conditions were applied to a mixed plastics and paper residue stream derived from paper recycling process. The condensable products were analyzed by GC/MS. It was found that pyrolysis temperatures during the first and second step of 350 and 500 °C resulted in a better separation of the oxygenated and hydrocarbon condensates than when a lower pyrolysis temperature (300 °C) was used in the first step. The products from the first step were derived from cellulose with some heavy fraction of styrene oligomers, while the products from the second step were mainly hydrocarbons derived from polystyrene and polyethylene. This thus shows that stepwise pyrolysis can separate the products from these materials, although with some degree of overlapping products. Indications of interaction between PS and cellulose during stepwise pyrolysis were observed including an increase in char yield, a decrease in liquid yield from the first temperature step and changes in liquid composition, compared to stepwise pyrolysis of the two materials separately. A longer vapor residence time in the second step was found to help reducing the amount of wax derived from polyethylene. Results from stepwise pyrolysis of a real waste showed that oxygenated and acidic products were concentrated in the liquid from the first step, while the product from the second step contained a high portion of hydrocarbons and had a low acid number. 

Place, publisher, year, edition, pages
Elsevier Ltd , 2018. Vol. 229, p. 314-325
Keywords [en]
Cellulose, Hydrocarbons, Oxygenated products, Paper reject, Plastics, Stepwise pyrolysis, Chemical reactors, Economics, Elastomers, Liquids, Municipal solid waste, Paper, Plastic products, Polyethylenes, Polystyrenes, Separation, Styrene, Decomposition temperature, Different treatments, Environmental issues, Liquid compositions, Pyrolysis temperature, Vapor residence time, Pyrolysis, biomass, condensation, decomposition, environmental issue, hydrocarbon, oxygenation, plastic, recycling
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-236622DOI: 10.1016/j.apenergy.2018.08.006ISI: 000449891500026Scopus ID: 2-s2.0-85051140419OAI: oai:DiVA.org:kth-236622DiVA, id: diva2:1263978
Funder
Swedish Energy Agency
Note

QC 20181119

Available from: 2018-11-19 Created: 2018-11-19 Last updated: 2019-04-08Bibliographically approved
In thesis
1. Pyrolysis of mixed plastics and paper to produce fuels and other chemicals
Open this publication in new window or tab >>Pyrolysis of mixed plastics and paper to produce fuels and other chemicals
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

As the world population and economy grow, higher consumption results in higher waste packaging, plastics and paper residues. Pyrolysis offers a way to recover fuels and other chemicals from this waste fraction. By applying heat to these materials in the absence of oxygen, pyrolysis process can convert these feedstocks into more valuable products in the forms of gas, liquid and char.

One important issue in the pyrolysis process which requires an investigation is the interactions between the feedstocks which consist of cellulose as the main component of paper and different types of plastics. Regarding this topic, 3 subtopics were investigated which are: the effect of mixing methods on the co-pyrolysis products, the interactions between the plastics and cellulose, and the formation of H, OH and water during cellulose pyrolysis. All these experimental investigations were based on microscale pyrolysis experiments using Py-GC/MS technique.

In the first work, polyethylene and cellulose were mixed by melting and by putting side-by-side. It was found that some interactions occurred during co-pyrolysis of these materials which slightly altered the yields of some anhydrosugars, aldehydes and ketones when the two feedstocks were mixed together by melting. Nevertheless, the main pyrolysis products from each feedstock were not affected. 

In the second study, the investigation continues on the interactions between different types of plastics (PE, PP, PS, PET) and cellulose. By using Py-GC×GC/MS, a good separation of the mixed pyrolysis products could be achieved, thus assisting the analysis. It was found that although the main pyrolysis products from each feedstock were not affected by the co-pyrolysis, small interactions occurred such that the interactions between different plastics were more pronounced than the interactions between plastics and cellulose. Nevertheless, some hydrogen transfer reactions occurred when PS was co-pyrolyzed with cellulose. However, the source of hydrogen was not clear.

Therefore, the investigation on the formation of H and OH radicals during cellulose pyrolysis was performed. This work combined first-principle calculations with experimental investigations. The author of the thesis was responsible for the experimental part. It was found from the first-principle calculations that it is energetically more favorable for the generation of a pair of H and OH radicals with subsequence formation of water than to generate a single radical because the formation of a double bond on the resulting cellulose helps stabilize the structure. With Gibbs free energy calculations, it was predicted that the water would be released at 280 °C. This agree well with the experimental findings from multistep pyrolysis of cellulose in Py-GC/MS which showed that water was generated at two different temperature ranges with the first peak around 280 °C.

As the interactions between the feedstocks during co-pyrolysis do not much improve the liquid products’ properties, and the nature of the products produced from plastics and paper pyrolysis are significantly different; it might be more beneficial to separate the pyrolysis products from the two feedstocks. Moreover, the hydrocarbons produced from plastics pyrolysis and the oxygenated products from paper pyrolysis require different upgradation methods. Stepwise pyrolysis was then proposed to produce and collect these two products separately. With simulated feedstock mixtures (PE, PS, cellulose) and real waste fractions which are paper rejects, it was successfully demonstrated that the stepwise pyrolysis with a temperature of the first step of 300-350 °C and a temperature of the second step of 500 °C could be used to produce two products streams as previously described. However, an optimization of the process and further investigations on product properties and upgradation are still required.

As a continuation on the investigation of the stepwise pyrolysis, an upgradation of the products from the first pyrolysis step was studied. When PVC plastic is present in the feedstock, dehydrochlorination of PVC occurs in the temperature range of the first pyrolysis step together with the pyrolysis of cellulose. Calcium oxide (CaO) was then tested for the simultaneous adsorption of HCl and reforming of cellulose pyrolysis products. The experiments were performed in a two-stage reactor system which was a pyrolysis reactor connected in series to a catalytic reactor containing CaO. It was found that the catalytic temperature should be between 300-350 °C because the desorption of HCl occurred when the temperature was higher than 400 °C. This was partly due to a reaction between water and CaCl2 which caused the desorption of HCl.

From all the studies, stepwise pyrolysis has a great potential to produce fuels and other chemicals from mixed plastics and paper. Further investigations are needed to develop, evaluate and realize this promising process.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. p. 65
Series
TRITA-ITM-AVL ; 2019:10
Keywords
Pyrolysis; Mixed plastics and biomass; Cellulose; Refuse derived fuels (RDF); Paper rejects; Interactions; Stepwise pyrolysis; Calcium oxide (CaO)
National Category
Engineering and Technology Chemical Process Engineering Energy Engineering
Research subject
Chemical Engineering; Energy Technology
Identifiers
urn:nbn:se:kth:diva-248391 (URN)978-91-7873-148-0 (ISBN)
Public defence
2019-05-09, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2019-04-15 Created: 2019-04-07 Last updated: 2019-04-15Bibliographically approved

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