Steam cracking of plastic waste pyrolysis oil with focus on nitrogen compounds

In the context of plastic recycling, chemical recycling is an interesting opportunity, as it can regenerate high-quality polymers, instead of mechanical recycling, which often leads to downcycling. Among its options, plastic pyrolysis is very promising, as pyrolysis oils can be utilized in steam thermal cracking processes. However, pyrolysis oils differ significantly from fossil feedstocks, therefore their feasibility must be assessed. Understanding the heteroatom content and their behavior during steam cracking is critical for evaluating the applicability. This thesis addresses this challenge, investigating light plastic-waste pyrolysis oil fractions. First, a quantitative analysis of the main heteroatoms (N, S, O, Cl), and the compositional characterization of hydrocarbons and nitrogen species, was performed using Two-Dimensional Gas Chromatography (GC×GC) coupled to various detectors (FID, NCD, SCD, AED). These indicated the presence in traces of heteroatom-containing compounds, ranging from 100 to 3000 ppm. In particular, qualitative nitrogen analyses highlighted the presence of nitriles, which represent the 90 % of the total nitrogen content, half of them constituted by benzonitrile. The investigation of the hydrocarbon matrix determined consistent olefinic and naphthenic fractions, representing 42 and 24 %wt of the oil hydrocarbon composition respectively. Then, the analytical methodology was employed for evaluating pyrolysis oil suitability as alternative feedstocks for steam cracking and for understanding the behavior of nitrogen-containing compounds under high-temperature conditions. The industrial pyrolysis oil reached a severity of 0.44 (P/E ratio) only at 880°C, while a synthetic pyrolysis oil made of heptane and 2-ethylpyridine reached the same value at 820 °C. Nitrogen Chemiluminescence Detector analyses showed that nitriles and other pendant groups tend to abstract from the hydrocarbon matrix, leading mainly to ammonia and HCN, while aromatic nitrogen species tend to persist or condensate, due to resonance structure stabilization.