Engineering biomass combustion fly-ash derived zeolites for post-combustion CO₂ capture

Abstract

Mitigation of CO₂ emissions through the use of carbon removal technologies is widely recognised as a pivotal tool on the path to net-zero GHG emissions. Beyond these targets, net-negative emissions will be essential to stabilise global temperatures. Technologies such as bioenergy with carbon capture and storage, considered a net-negative emission technology is therefore poised for a significant share in the power generation mix. However, this is associated with the production of a significant quantity of waste ash residues such as fly ash. Valorisation of this waste stream provides an opportunity to simultaneously mitigate the requirement for waste disposal (i.e. landfilling) and provide a pathway to value-added products, zeolites. In this thesis, industrially produced biomass combustion fly ashes have been comprehensively characterised and subsequently investigated for their potential as zeolite precursors. Suitable design of experiment techniques have been employed to systematically assess the influence of various factors on the alkaline fusion assisted hydrothermal synthesis to maximise the CO₂ equilibrium adsorption capacity. The bulk biomass combustion fly ash has been shown to present a CO₂ adsorption capacity of over 1.8 mmol·g⁻¹ at 50 °C and 1 bar, with a stable capacity of 87% of that after 40 cycles. This adsorbent was then produced at a larger scale to facilitate breakthrough performance assessments in a fixed-bed temperature swing adsorption system designed and built during this research. The process was optimised via Taguchi design of experiment to reveal the influential factors on the bed utilisation efficiency. The results indicate a usable bed capacity of approximately 0.6 mmol·g⁻¹ corresponding to a bed utilisation efficiency of 62% under the optimal factor and level configuration. These findings underscore the feasibility of industrial biomass combustion fly ashes as feedstocks in the preparation of zeolitic adsorbents/catalyst for post-combustion CO₂ capture.