Biomass Chemical Looping Gasification: Preliminary Assessment of Fluidization Issues

This work is part of the Horizon2020 research project CLARA (Chemical looping gasification for the sustainable production of biofuels, G.A. 817841). CLARA has investigated the usage of biogenic residues (e.g., wheat straw) as fuel in Chemical Looping Gasification (CLG), producing N₂-free syngas and high-purity CO₂ flue-gas. CLARA’s CLG is based on a dual interconnected fluidized bed configuration, with an oxygen carrier (OC, i.e., a solid with redox properties) circulating between the gasification and combustion reactors. An adequate fluidization quality is critical to the correct CLG operation. Ashes of biogenic residues can jeopardize the desired fluidization regimes as they can agglomerate, therefore increasing the average diameter of OC bed particles. CLARA has investigated pretreatments of biogenic residues to prevent this issue. On the other hand, OC particles can fragment because of continuous attrition by circulation down to cohesive powders.

Measurement of pressure fluctuations signals in fluidized beds generally allows assessment of fluidization quality. As to bubbling beds:

• the signal’s standard deviation is related to the size of erupting bubbles;
• the dominant frequency of the signal’s power spectral density function (PSDF) is related to the number of bubbles.

Analyzing those parameters in laboratory-scale bubbling beds allows for detecting defluidization tendencies of mixed beds made of OCs and ashes from pretreated and untreated biomasses.

This research group used this experimental methodology to study beds made of OC/ash couples: three OCs (i.e., ilmenite, calcined sibelco, and LD-slag) and several ash types were investigated at relevant CLG temperatures (700-1000°C). The methodology has proved to be a suitable tool for detecting incipient changes in bubbling fluidization, as it quickly caught the fading of bubbles, then related to the modifications of particles size, confirmed by the cross-checking with characterizations of after-test samples (scanning electron microscopy SEM, particle size distributions).