Thermochemical co-liquefaction of waste plastics and biomass for the production of fuels and further chemicals

Abstract

There is increasing concern over the levels of plastic waste that are entering the environment or being collected with no appreciable route to disposal. While recycling, as an alternative to disposal, offers one route, the need for uncontaminated sources and the lower quality of recycled plastics presents huge challenges. Alternatively, an increasing body of research has aimed to use plastic waste as a feedstock in a circular economy methodology to produce more valuable products such as fuels. Plastic waste could potentially be co-processed with biomass to create biofuels and chemicals, decreasing dependence on fossil fuels and remediating the plastic problem. The aim of this thesis is to explore the valorisation of plastic waste, through co-processing with biomass. To this end thermochemical co-liquefaction was explored.
Initially a novel type of liquid based pyrolysis was assessed on the lab and pilot scale. The technique is known as the pressure-less catalyst depolymerisation (KDV, in the original German), and has been claimed to depolymerise organic feedstocks to produce a hydrocarbon biofuel, in one step, without the need for hydrogen or chemical upgrading. However, despite a number of pilot plants in operation, no systematic mechanistic studies have been reported and it is unclear how this pyrolysis can be achieved. To determine these outstanding questions, pistachio hulls were liquefied using a KDV process in the lab and through collaboration with the Wonderful Company to assess the pilot scale mass balance and determine the suitability of the KDV approach. The process was carried out using an aluminosilicate 4Å zeolite catalyst at atmospheric pressure and temperatures at 300 °C. In this process the biomass and catalyst are suspended in a petroleum carrier oil and the fuel recovered though distillation from the reaction vessel. The process has a stated productivity of 32.8 L distillate oil/1,000 kg pistachio hull on pilot plant scale. However, the 14C analysis demonstrated that most of the product came from the fossil carrier oil. The process was then mimicked on the lab scale, on both 1L and 5L scales and the optimal catalyst type was tested using aluminosilicate 4A zeolite, zeolite, aluminosilicate catalyst, and a calcium hydroxide neutraliser at atmospheric pressure and temperature at 300 °C. Despite the lab scale tests, the yield was not improved through using the more stable zeolite catalyst. The maximum distillate obtained when using a heavier carrier oil was relatively low (approximately 6.5v/w%) with less than half of the pistachio hull used. The liquid product contained not only pyrolysis oil but also 30-50 wt.% water. The bio-content as determined through 14C analysis was remarkably low for all reaction conditions, demonstrating that the majority of the product came from the carrier oil. The concerted efforts on lab and pilot plant scale demonstrated the unsuitability of this one step process for fuel production, and therefore was not used to co-process plastics.
Hydrothermal liquefaction is another promising, low-energy route for the bio-crude conversion of biomass which can be upgraded to advanced biofuels. The co-processing of common plastic waste (including; polyethylene, polypropylene, PET and nylon-6,) with pistachio hulls was therefore assessed to investigate the suitability of the HTL approach at 350 °C. High yields of up to 35% bio-crude were achieved. Synergistic effects between plastics and pistachio hulls conversion were stronger in the presence of nylon-6 and PET. Nylon-6 almost completely depolymerised under the optimal HTL conditions and generated the caprolactam monomer. PE and PP were less reactive; a limited degree of decomposition formed oxidised products, which distributed into the bio-crude phase. The HHV of the bio-crudes increased substantially in the presence of plastic blends.
The recalcitrant polyolefins also need to be converted before plastic waste can become an integral part of a biorefinery. In order to enhance the conversion of these plastics, a possible solution through the catalytic co-liquefaction of a model waste (pistachio hulls) and polypropylene (PP) was assessed. Pure PP did not break down under HTL conditions, and only small synergistic effects occurred when placed with biomass. In the presence of typical HTL catalysts including Fe, FeSO4, MgSO4, ZnSO4, ZSM-5, aluminosilicate, Y-zeolite, and Na2CO3, the PP almost exclusively broke down into a solid phase product with no enhancement of the bio-crude fraction. However, the plastic conversion was enhanced up to 50% through the addition of the hydrogen donor formic acid. This reduced the amount of carbon going to the solid phase and the volatile organic produced was increased in the gas phase. The gaseous products were an array of short chain hydrocarbons, which could be repolymerised as a polyolefin or combined with the bio-crude for further processing.

Several broad classes of plastics have a high possibility of ending up in the ocean environment. Extensive fishing, recreational and maritime uses of the ocean increase the influx of plastic waste into the oceans. The concept of implementing HTL as a route to processing marine plastic and macroalgae was therefore investigated. Co-processing of marine macroalgae was undertaken with a range of different nylons (nylon-6, nylon6/6, nylon6/12, and nylon12) including an actual sample of marine macroalgae collected at sea, entangled with nylon fishing line. Due to the variation in macroalgae composition, synergetic effects between macroalgae and nylon conversion were observed, producing bio-crudes in higher yields and with better fuel properties. Co-processing of marine macroalgae and contaminant marine plastic therefore is a potentially useful method to solve the ocean environmental problem and create value for fuel production.
Based on the investigations, HTL has been demonstrated to be a highly promising route to convert the energy from biomass and plastic to valuable bio-crude, liquid products, gaseous and bio-char. The HTL could be achieved in both thermal and catalytic processes. However, catalytic processes provide higher plastic conversion with greater yield of gaseous products. With the potential HTL method, waste management can become more efficient, with reduced need of waste disposal, less pollution, and is seemingly cost effective.

Date of Award	24 Mar 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Chris Chuck (Supervisor) & Hannah Leese (Supervisor)