Analysis of the products from mixed plastic waste pyrolysis.

Abstract

The utilization of plastic products has grown significantly in the last several decades and consequently expanded the production rate of plastic products, as well as increased pollution rates of waste plastic globally. Pyrolysis processes may be a feasible alternative for the management of non-recyclable plastic waste such as mixed plastic. During this process, plastics (individual or mixed types) are reacted at high temperatures (generally from 300°C to 900°C) to crack and breakdown. Products such as pyrolysis liquid, wax, gas, and char result from the process. South African legislation on pyrolysis emissions has caused several pyrolysis facilities in KwaZulu-Natal to halt operations due to non-compliance. Hence, pyrolysis technologies for the treatment of plastics needs to be analysed and redesigned/optimized to reduce emissions such that it complies with legislation. This work falls under a broader research project with the goal of developing an optimized pyrolysis process with novel aspects to manage mixed waste plastic through catalytic pyrolysis. This dissertation focuses specifically on the analysis of components generated from catalyzed lab-scale plastic pyrolysis. Plastic pyrolysis experiments were conducted using a feedstock of compounded low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) and mixtures of these plastics at 450°C and vacuum of approximately 42 kPa. Zinc oxide powder was used as the catalyst, and its effect was investigated in comparison to the thermal (uncatalysed) pyrolysis process. The selection of the conditions and catalyst used are described in detail within this dissertation. Product yields of the pyrolysis liquid, wax, char, and gas were determined through weighing and mass balance. Gas chromatography-mass spectrometry (GC-MS) was used to perform chemical analyses on the liquid and wax to determine the types of components constituting these products. Additionally, the emissions were analysed to identify the volatile organic compounds (VOCs) present. The results indicated that the zinc oxide catalyst effectively enhanced the pyrolysis experiments and increased the combined liquid and wax product yields, which are the desirable products. Pure LDPE and HDPE pyrolysis with catalyst to feed ratios of 5:100 produced 78.3 wt % and 78.2 wt % of combined liquid and wax yields respectively. This was relatively high compared to the other pure plastic experiments with and without catalyst. However, uncatalysed HDPE pyrolysis produced a combined liquid and wax yield of 85.4 % but required a longer reaction time. The use of zinc oxide catalyst (5:100 ratio) in LDPE pyrolysis experiments reduced reaction times by approximately 14 hours compared to the uncatalyzed experiment. Reaction times of catalysed (5:100) HDPE and PP pyrolysis experiments were reduced by 1 hour 30 minutes (HDPE) and 15 minutes (LDPE) compared to their respective uncatalyzed experiments. Pure PP pyrolysis produced the highest gas yields (ranging from 17.7 wt % to 19.2 wt %). Mixed plastic feed experiments with higher proportions of compounded LDPE produced the highest combined liquid and wax yields. The usage of catalyst caused a reduction of gas yields in many of the mixed plastic pyrolysis experiments. According to the GC-MS analyses, the use of 5:100 zinc oxide catalyst to feed ratio produced the lowest total VOC peak area percentages for many of the pure plastic feed types, however it seems the catalyst caused greater fractions of VOCs to form in the mixed plastic experiments. Many of the liquid and wax analysis results indicated a large proportion of components within the C12 to C27 range. This indicates that the liquid products potentially had similar properties to diesel fuel and correlates with the results of similar studies in the literature. Furthermore, the 5:100 zinc oxide catalyst to feed specification produced the highest combined liquid and wax yields in many of the experiments. A catalysed (5:100) experiment using a feedstock of 32.3 wt % LDPE, 34.52 wt % HDPE, and 33.18 wt % PP indicated a trade-off between high liquid + wax yields, reasonable VOC content in emissions, lower gas yields, and lower energy usage per gram of product formed relative to experiments with other feed compositions. A preliminary life cycle gate to gate analysis was conducted for the laboratory scale process, and it was determined that the major power consuming units, heating mantle, condensing circulator and vacuum pump contribute approximately equally to the indirect CO2 emissions for the process, assuming they are operating at capacity. Pure LDPE pyrolysis had the highest CO2 emissions per gram of desirable product obtained (3.83 𝑘𝑔 𝐶𝑂2𝑔 ) while PP pyrolysis with a catalyst ratio of 10:100 had the lowest CO2 emissions (0.31 𝑘𝑔 𝐶𝑂2𝑔). In the mixed plastic experiments, the CO2 emissions increased as the amount of LDPE in the feed was increased. The proportion of gas product streams that would be considered volatile organic compounds (VOCs) constituted between 32-83 % for each experiment. The total VOCs was lowest in mixed plastic pyrolysis experiments where approximately equal proportions of compounded LDPE, HDPE and PP was used as the feed, compared to the other sets. It was observed that the use of catalyst (5:100) generally increased the total VOCs and the carbon number of the most abundant VOCs in the gas stream in the mixed plastic experiments.