High-temperature CO2 capture by fly ash derived sorbents: Effect of scale-up on sorbents performance

Abstract

Dry sorbent high temperature CO2 capture process is a promising technology to remove CO2 efficiently from flue gas. Here we report the performance of fly ash derived potassium-aluminium silicates sorbents (K-FAs) for high-temperature CO2 capture at different scale setups, from milligrams to hundred kilograms. Li2CO3 and Na2CO3 were evaluated as promoters by their incorporation at 10 – 20 wt% on the sorbents for pure (100 vol%) or diluted (14 vol%) CO2 capture. The properties of the K-FAs sorbents were characterised by a number of techniques and linked to their performance and cyclability. The maximum CO2 uptake of 2.82 mmol CO2/gsorbent was achieved with K-FAs_20%Li with pure CO2 using a thermogravimetric set-up, while K-FAs_20%Na exhibited the faster CO2 adsorption and no CO2 capture decay in 10 consecutive cycles when tested in a fixed bed reactor, which was linked to enhanced diffusion of CO2 and Na due to the smaller crystals (33 nm), smaller particles (56.3 µm) and larger pores (16.9 nm) compared to the Li and Li-Na doped K-FAs sorbents. SEM-EXD analyses indicate formation of K/Na eutectic melt on surface, which also facilitates CO2 and K diffusion and protect from strong sintering. The capability of deploying the K-FAs_20%Na sorbent at 100 kg scale was also demonstrated with the separation of 6 kg CO2/h (from a 50 kg/h biomass pyrolysis unit) at 700 °C in 30 min cycles. The gas recovered in the desorption stage had a purity of 55 mol% in wet basis and 80 mol% in dry basis, suggesting improvements are required to achieve ≥ 90% efficiency. Detailed kinetics analysis concluded that the adsorption of CO2 on K-FAs sorbents could be described by double exponential kinetics and the diffusion was found to be governed by a mixture of different mechanisms with intramolecular particle being the prominent. The accessibility and cost-effectiveness of K-FAs_20%Na and its performances render it as interesting candidate sorbents for high temperature CO2 capture only at small scale.