Using model compound studies to investigate the hydrothermal carbonisation behaviour of components in municipal waste

Municipal solid waste (MSW) is a complex and heterogeneous mixture that poses significant challenges to conventional recycling and waste management strategies. Among its numerous components, waste textiles, predominantly composed of cotton and polyester fibres, represent a growing fraction due to the global increase in textile consumption. Current disposal methods for waste textiles, such as landfilling and incineration, are environmentally unsustainable, contributing to pollution, greenhouse gas emissions, and resource depletion. Hydrothermal carbonisation (HTC) offers a promising alternative by converting waste into value-added products such as hydrochar, which can be used for soil amendments, as precursors for advanced materials or as fuel. However, the HTC process is highly influenced by feedstock composition and process conditions. MSW are particularly challenging to process due to their diverse constituents and interactions with reaction pathways during HTC. This study uses model compounds to investigate the HTC of components in MSW. Attention was given to waste textile (WT) to understand how it directs the reaction pathways. The role of inorganic metal salts in directing the HTC process was also investigated. Inorganic compounds are considered to negatively affect the combustion of the solid product (hydrochar). However, there are tendencies that these compounds have the potential to direct the reaction pathway. Metal salts generally act as catalysts, influencing reaction pathways and the physicochemical properties of hydrochar. A proper understanding of the reaction pathway is crucial to improving the efficiency of textile waste conversion and to the production of bespoke materials tailored for specific applications. The linear additive method was employed to predict the behaviour of three theoretical MSW models: A (typical UK MSW composition), B (MSW without textile) and C (MSW rich in textile). A realistic MSW obtained from Fiberight Limited was used to validate the model. At mild temperatures (200°C), the model accurately predicts hydrochar yields for cellulose and protein-rich wastes within a 10% error margin, though accuracy decreases at higher temperatures (250°C) for complex materials like polymers and ash-heavy components. The discrepancies observed in complex waste streams suggest that the model requires specific calibration factors and corrections for inert fractions and other organic extractables not accounted for to improve the model. Samples of cotton, polyester and their blend were carbonised at different HTC severities factor (SF) for temperatures of 200, 230 and 250 oC at different residence times of 1 hr and 2 hr. The result showed that HTC induces divergent degradation pathways for cotton and polyester. As a result, a potentially novel density‑based aqueous separation method was investigated. The study demonstrated that HTC could function as a thermochemical "sieve", transforming mixed textile waste into distinct, density-stratified layers. The method was promising for effectively partitioning the resulting hydrochars but requires further development to actualise. Further investigations revealed that introducing metal salts in the HTC of cellulose feedstock influenced reaction pathways in the order NaCl < KCl < CaCl₂ < FeCl₃, but temperature ultimately showed more impact on degradation. The study further examined the extraction of humic-like acids (HLA) from model compounds of biomass (cellulose, xylan and lignin) and cotton hydrochar produced under different HTC conditions and in the presence of metal salts. Results demonstrated a successful humification process. HLA yield was influenced by feedstock composition and process severity, with lignin-rich materials yielding the highest HLA levels. However, HLA was also obtained from cellulose and cotton textiles, which, usually, are overlooked as precursors for recovering humic materials from biomass. Metal salts in the precursor hydrochar promoted aromatic compound formation, such as furans and aldehydes. This was evident in the higher HLA yield compared to hydrochar, which contains more carbohydrates and esters. This research provides a deeper understanding of the HTC reaction pathways for waste textiles and highlights the potential for producing tailored hydrochars and humic substances for sustainable applications.