Iannello, Stefano; (2024) A fundamental investigation on thermochemical conversion of highly volatile solid feedstocks in fluidized bed reactors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The global escalation of population and demand for resources has provoked a critical revaluation of conventional technologies for energy production and manufacturing, and their associated environmental impacts. Simultaneously, the rampant surge in waste generation and lack of adequate disposal practises have posed a substantial threat to ecosystems and human health in the last two decades. Fluidized bed reactors (FBR) offer a promising technological opportunity and potential solution for conversion of non-recyclable waste to clean energy and chemical feedstock for the manufacturing industry. The utilization of FBRs in new advanced thermochemical processes, such as gasification and pyrolysis, enable efficient conversion of a wide range of feedstocks into chemicals and alternative fuels with reduced emissions and improved efficiency. The ability of FBRs to accommodate diverse waste streams, including biomass, plastics, and municipal solid waste underscores their versatility and effectiveness in waste valorisation. Nonetheless, some challenges need to be addressed to encourage the scale up and wider deployment of FBRs applied to advanced thermochemical conversions of waste feedstocks. These materials are highly heterogeneous and the prediction of both their conversion and mixing behaviours in FBRs is not trivial. During thermochemical operations the establishment of flaming combustion, feedstock stratification, agglomeration, and poor mixing between solid and gas phases pose major challenges for the design and operation of FBRs. The overarching objective of this PhD Thesis is to gain fundamental understanding of how different types of waste feedstocks behave in industrial gasification or pyrolysis fluidized bed reactors and how the key operating variables may impact the thermochemical conversion and mixing of the materials studied. The experimental investigations were conducted in a large laboratory scale (146 mm inner diameter) bubbling fluidized bed reactor (BFBR), operating at temperatures in the range of 500 – 730 °C, and fluidization conditions ranging from the minimum fluidization to the bubbling regime. The kinetics of devolatilization of ligno-cellulosic and plastic particles have been investigated, with particular attention on how the nature of the fluidizing medium (inert or oxidizing) may affect the devolatilization rate. Most of the experiments were carried out using the unique high – powered X-ray imaging facility available at UCL. This allowed direct visualization of both gases and solid patterns within the fluidized bed during operation. Specifically, the evolution and properties of endogenous bubbles released by a single devolatilizing particle were assessed for the first time to estimate the lift effect that they induce on the feedstock itself, enhancing its segregation and stratification at the surface of the bed. Experiments focused on the incipient agglomeration between bed material and a single polypropylene particle were also conducted, aimed at understanding how this phenomenon may affect the mixing/segregation behaviour of waste plastic feedstock, in particular. Two models were proposed in this research study. The first one is a one-dimensional semi-empirical model linking the thermochemical conversion kinetics of the feedstock to its axial mixing/segregation pattern within the BFBR. Overall, the model showed accurate predictive capabilities for the biomass feedstock, whilst it fails in predicting the behaviour of the plastic material. A second model based on a physics-assisted Monte Carlo approach was developed to predict the evolution of molten plastic properties during the incipient agglomeration with the particles of the bed inventory, with the aim of understanding the dynamic behaviour of plastics in FBR conversion. This model appears to reasonably predict the formation of a single sand-plastic agglomerate. However, its validation is challenging due to the difficulties in producing agglomerates during real thermochemical process conditions. The outcomes obtained in this Thesis highlight the importance of linking the thermal decomposition to the mixing/segregation behaviour of highly volatile feedstocks in FBRs. In fact, without the first information, the second aspect cannot be fully characterised and understood. Furthermore, this work shows that a fundamental investigation at the single particle level provides important information on the behaviour of different types of feedstocks, including biomass and plastic, which would be nearly impossible to assess on a different level of investigation, such as during a continuous operation. The findings reported in this work could be used to inform reactor design and operating procedure depending on the nature of the feedstock processed. Furthermore, they may serve as a basis to develop alternative methodologies to better understand the behaviour of more complex and heterogeneous feedstocks, including municipal solid waste and refuse derived fuel.

Type:	Thesis (Doctoral)
Qualification:	Ph.D
Title:	A fundamental investigation on thermochemical conversion of highly volatile solid feedstocks in fluidized bed reactors
Language:	English
UCL classification:	UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Chemical Engineering
URI:	https://discovery.ucl.ac.uk/id/eprint/10191933

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