Thermochemical and catalytic recycling of plastic waste from the rejected fraction of urban solid waste treatment plants to obtain commercial chemical products

Laburpena

In the last decades, energy recovery by incineration and deposits in landfills have been the main options for contaminated mixed plastic waste management unrecovered by mechanical means due to technical and economic reasons. Nevertheless, the legislation has changed, and a vast inversion is ongoing in plastic waste mechanical and chemical recycling new technologies. Thus, chemical recycling by pyrolysis appears as a featured technology, and although it is still little industrially implemented, many companies have announced pyrolysis plant constructions for different plastic waste types; it is expected an increase plats numbers along with the production capacity in the following years. Recycling plastic waste by pyrolysis allows processing for different waste flows. It is precisely for complex waste streams, which can be formed by a very heterogeneous mixture of polymers and present high contamination levels or waste with substances that need to be extracted from recycled plastics, where pyrolysis surge as an attractive alternative to mechanical recycling for valuable product generation. The investigation work, which results are presented in the present Doctoral Thesis, was developed in the “Waste Valorization Technologies and Catalytic Process” research group from the Department of Chemical Engineering of Granada University within the project “Converting the nonrecyclable mix plastic from municipal solid waste in chemical products and high-value carbonaceous materials” (reference PID2019-108826RB-I00) from Science and Innovation Ministry and “Valorization of the plastic waste coming from the rejected fraction of the urban solid waste treatment plants by pyrolysis” (reference B-RNM-78-UGR20) from University Counseling, Investigation and Innovation of Andalucía Board. In that context, this Doctoral Thesis pretends to develop a flex pyrolysis technology integrated to treat dirty plastic waste and mixed non-recyclable from urban solid litter, whose usual destination is the landfill or incineration, for fuel production through controlled pyrolysis conditions and the use of new catalysts. This Doctoral Thesis is presented for its evaluation as a paper grouping published organized in eight chapters. The first chapter is an introductory character, followed by a dedicated to the thesis objectives (chapter 2), later to continue with four additional chapters corresponding to published papers in impact journals indexed in the Journal Citation Reports. Appear, in addition, a conclusion chapter (chapter 7) and the last one with future works (chapter 8). A bit summary of the content of each one of the chapters corresponding to the published articles is shown as follows. Firstly, the results obtained in the study of the characterization of the liquid fraction obtained from the thermal pyrolysis of the mixture of non-recyclable post-consumer plastic waste from urban solid waste are presented. This study used a real mixture of plastic waste from the rejection fraction of an urban solid waste treatment plant belonging to the province of Granada (Spain). This waste mixture comprises polypropylene, expanded polystyrene, high-impact polystyrene, and film-type plastic (mainly low-density polyethylene). Different pyrolysis tests were carried out in a horizontal tubular reactor with a nitrogen flow of 100 L/h in the temperature range of 450 to 550 °C. The different products (solid, liquid, and gas) were collected, and the liquid fraction obtained was analyzed in detail (density, specific weight, refractive index, elemental analysis, simulated distillation curve, etc.). Subsequently, some mathematical correlations used in the refining industry were applied to estimate specific characteristic parameters of crude oil and analyze its suitability for pyrolysis oils. The results of this study are shown in Article I: “Characterization of liquid fraction obtained from pyrolysis of post-consumer mixed plastic waste: A comparing between measured and calculated parameters,” published in the journal Process Safety and Environmental Protection from the Elsevier Publishing House. with impact factor 7.8 (2022) and Q1 relative position in JCR. Secondly, the results of the study of the ex-situ thermal and catalytic pyrolysis of the mixture of non-recyclable post-consumer plastic waste from municipal solid waste on different catalysts (CaO, MgO, HY, HZSM-5) are presented. In this study, pyrolysis was carried out at 500 °C in a horizontal tubular reactor with the individual plastics and the mixture. The catalysts were characterized, and a detailed analysis of the composition of the liquid fraction with emphasis on the determination of the hydrocarbon groups present in the gasoline product. The results of this study are shown in Article II: “Thermal and catalytic pyrolysis of a real mixture of post-consumer plastic waste: An analysis of the gasoline-range product,” published in the journal Process Safety and Environmental Protection of the publisher Elsevier with impact factor 7.8 (2022) and Q1 relative position in JCR. Thirdly, as an alternative to the commercial catalysts analyzed in the previous study, the results of the study of the in-situ catalytic pyrolysis of the mixture of non-recyclable post-consumer plastic waste from urban solid waste on clays, which are especially abundant in Spain like sepiolite and montmorillonites K10 and K30, are shown. In this study, pyrolysis is repeated at 500 °C in the same horizontal tubular reactor with the mixture of plastic adding catalyst (in-situ); characterization of the catalysts used, and a detailed analysis of the composition of the gaseous and liquid fractions obtained was made. About the liquid fraction, the composition of the different products is determined in detail and compared by analogy with the fuels obtained from crude oil: gasoline and naphtha, kerosene, gas oils, and residual fuel oils. The results of this study are shown in Article III: “Towards fuels production by a catalytic pyrolysis of a real mixture of post-consumer plastic waste,” published in the magazine Fuel from Elsevier publishing house with impact factor 7.4 (2022) and Q1 relative position in JCR. In the fourth and final stage, the study of the ex-situ catalytic pyrolysis of the mixture of non-recyclable post-consumer plastic waste of urban solid waste on commercial zeolites impregnated with metals (nickel and cobalt) is performed. In this study, pyrolysis is carried out at 500 °C in a horizontal tubular reactor with a mixture of plastics, a characterization of the catalysts used, and a detailed analysis of the composition of the liquid fraction obtained is realized with emphasis on the determination of the groups of hydrocarbons present in the gasoline product. The metal-impregnated catalysts were prepared using the incipient humidity by precipitation on the zeolite (precursor) from a metal salt solution, evaporating the solvent to dryness. The results of this study are shown in Article IV: “Impact of metal impregnation of commercial zeolites on the catalytic pyrolysis of a real mixture of post-consumer plastic waste,” sent for publication to the journal Catalysts of the MDPI publishing house with impact factor 3.9 (2022) and relative position Q2 in JCR.