Sebastiani, Alex; (2025) Modelling of fluidized bed gasification for hydrogen production from waste. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The need for sustainable waste management and the requirement for a reliable and constant source of renewable energy are the pillars of the current environmental agendas of many developing and developed countries. On a global level, bioenergy systems, and advanced thermochemical technologies, gasification in particular, are deemed to play a pivotal role in the quest for decarbonization of the energy sector, representing a strategic pathway towards reducing dependency on fossil fuel resources. This is especially true when waste feedstock is used, often favoured over other biomass-based fuels due to its low cost and abundance. Fluidized bed gasifiers offer the best opportunity for large-scale deployment of advanced thermochemical technologies due to the higher flexibility and application potential in comparison to other alternatives. However, despite gasification being a rather mature process, challenges still persist when dealing with unconventional solid feedstock such as waste-derived biomass and plastics. These heterogeneous materials, often containing high ash and moisture fractions, are highly volatile and can hinder the renowned features of fluidized bed reactors in terms of mixing and thermal stability. Additional challenges for the new-generation gasification systems arise from the choice of the most suited gasification agent for applications like hydrogen and low-carbon fuels production, where air is ruled out to avoid nitrogen dilution. Most of the industrial waste-fuelled gasifiers were developed as air-blown systems, and only a very limited experience exists in steam-oxygen operations, particularly on large (pilot and demonstrative scale) plants. For these reasons, the existence of a valid modelling tool is deemed essential for understanding the complexity of simultaneous phenomena occurring, as well as better predicting the system behaviour upon variation of process inputs and improving the design for future commercial plants. The current work aims at filling the gaps in existing kinetic models to accurately predict the performance of large-scale gasifiers, such as syngas composition and temperature profile, while maintaining low computational requirements and the inherent simplicity of 1D models. Particular focus is given to the main bottleneck of this technology represented by the prediction of light hydrocarbons, tars composition and gas energy throughput, investigating means of bolstering the performance within feasible operating conditions and technological constraints. A holistic approach is then adopted to investigate the potential of waste gasification in fluidized bed reactors as part of a waste-to-H2 plant, addressing technical challenges, environmental opportunities and the related strategies for large-scale deployment to support the transition to Net-zero.

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