An improved method for feeding ash model compounds to a bubbling fluidized bed – CLC experiments with ilmenite, methane, and K2CO3

Abstract

Biomass conversion with carbon capture and storage (Bio-Energy CCS; BECCS) is one of the options considered for mitigating climate change. In this paper, the carbon capture technology chemical-looping combustion (CLC) is examined in which the CO₂ is produced in a stream separate from the combustion air. A central research topic for CLC is oxygen carriers; solid metal oxides that provide oxygen for the conversion process. Biomass and waste-derived fuels contain reactive ash compounds, such as potassium, and interactions between the oxygen carrier and the ash species are critical for the lifetime and performance of the oxygen carrier. This work develops and demonstrates an improved method for studying the interactions between ash species and oxygen carriers. The method uses a lab-scale reactor operating under fluidized conditions, simulating CLC batch-wise by switching between feed gas. The novelty of the setup is the integrated system for feeding solid particles of ash model compounds, enabling the simulation of ash species accumulating in the bed. Ilmenite is a benchmark oxygen carrier for solid fuel conversion and was used in this study to evaluate the method using K₂CO₃ as a model ash compound. Experiments were done at 850 and 950°C. Methane conversion in CLC cycles and fluidization was evaluated with gas analysis and pressure drop measurements. Scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD) analysis of bed particles were done after the experiments to establish changes in the morphology and composition of the ilmenite. The method for feeding the ash model compound was concluded to be satisfactory. At 950°C, K accumulated in the particles forming K-titanates and agglomeration was enhanced with K₂CO₃ addition. The agglomeration mechanism was solid-state sintering between the Fe-oxides forming on the particle surfaces. The bed defluidized at 950°C, but no such effect was seen at 850°C. The method is suitable for studying the Fe-Ti-K system with ilmenite and potassium without the influence of other ash species. © 2023 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.