Valorisation of oily sludge, municipal solid waste incineration fly ash, and secondary aluminium dross for fuel recovery and calcium sulfoaluminate cement production through thermochemical co-processing

Abstract

The co-processing of hazardous solid wastes offers an innovative solution to global challenges in waste management, resource recovery, and environmental sustainability. Integrating these wastes into thermochemical processes mitigates ecological risks and transforms solid wastes into valuable products, such as alternative fuels and construction materials. However, the inherent complexity of hazardous waste makes it difficult to elucidate the synergistic interactions during co-processing, which imposes challenges on obtaining universally applicable scientific insights. This thesis explores the potential of co-processing oily sludge (OS), municipal solid waste incineration fly ash (IFA), secondary aluminium dross (SAD), and other solid wastes, focusing on process optimisation and synergistic interactions, demonstrating the feasibility of converting hazardous wastes into energy-rich products and eco-friendly cement.
This study first investigates the co-pyrolysis of OS and IFA to achieve fuel recovery, CO2 mitigation, and heavy metal (HM) immobilisation. The optimal condition with 20 wt% IFA addition at 600°C enhances light oil formation, and increases aromatic hydrocarbons selectivity to 30.72%, and reduces coke yield to 106.13 mg/g OS. Additionally, 50 wt% IFA addition increases H2 yield from 21.02 to 60.95 L/kg OS and facilitates CO2 sequestration via Ca-bearing minerals. HMs in co-pyrolysis char can be well immobilised into stable fractions with reduced environmental risks.
Further study systematically investigates the catalytic roles of individual IFA components in co-pyrolysis, revealing their distinct contributions to product distribution and reaction mechanisms. Notably, CaCl2 and KCl significantly enhance pyrolysis oil upcycling compared to IFA, while IFA components collectively exhibit a positive catalytic effect on pyrolysis gas and coke production. Ca(OH)2 greatly boosts H2 yield by 137.16%. Alkali chlorides facilitate gaseous hydrocarbon formation and convert oxygen-containing compounds to CO and CO2 which are subsequently consumed and absorbed by CaO and Ca(OH)2. However, CaSO4 and CaCO3 hinder catalytic reactions. Importantly, IFA compounds aid the dispersion of inherent Fe-based species from OS on char surface, enhancing in-situ catalytic pyrolysis.
The water-washed co-pyrolysis char (WCPC) is utilised as a CaSO4 substitute in calcium sulfoaluminate (CSA) cement production through calcination at 1300°C, along with water-washed IFA (WIFA), and SAD. Results show that the low availability of calcium phases in WIFA limits the formation of hydraulically active phases, requiring a higher alkalinity modulus (Cm) of 1.10 to maximise the 28-day compressive strength (e.g., 76.39 MPa). However, excess WCPC weakens long-term performance due to inert phase formation, such as C2AS and Fe2SiO4. HMs are fully immobilised in the clinkers and hydrated pastes due to enhanced volatilisation and solidification in aluminosilicate. Moreover, Fe2O3 in feedstock contributes to the formation of C4A3-xFxS¯ . In addition, increasing Cm reduces the Fe/(Al+Fe) ratio in ye’elimite, particularly for low-aluminium clinkers. This contributes to the transformation of C4A3S¯ -o to C4A3S¯ -c and slowed hydration. This provides valuable insights into regulating Fe2O3 incorporation in ye’elimite during waste-based CSA cement clinker production.
This study offers an effective strategy for converting hazardous wastes into valuable products while stabilising HMs and mitigating CO2 emissions. The findings enhance the understanding of synergistic treatment mechanisms for hazardous wastes and expand waste utilisation possibilities for achieving sustainability goals.

Date of Award	13 Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Jun He (Supervisor), Bo Li (Supervisor) & Yin Wang (Supervisor)