Experimental Investigation of Different Limestones As Sorbents for Sorption Enhanced Gasification in a Lab-Scale Dual Interconnected Fluidized Bed System

Sorption Enhanced Gasification (SEG) represents a promising technology enabling to couple a CO₂ sequestration technique with the production of hydrogen-rich syngas by removing CO₂ from the reaction environment. Ca-based sorbents allow the capture of CO₂, being applied in the well-known “calcium looping” post-combustion technique, typically carried out in Dual Interconnected Fluidized Beds (DIFB).

Six commercial limestones were investigated in terms of CO₂ capture capacity and attrition/fragmentation tendency under simulated SEG conditions and over repeated cycling typical of a looping process. A series of tests were carried out in a lab-scale DIFB system, each of which involving ten complete cycles of calcination/carbonation. Calcination was performed at 850 °C with a stream of 10% CO₂ in air (simulated oxidizing conditions of a combustor-calciner). As for the carbonation, a range of temperatures from 600 °C to 700 °C was investigated, which is the possible operation window of SEG found in the literature considering suitable gasification temperatures. Carbonation was carried out fluidizing the reactor with 10% CO₂ (balance nitrogen, to reproduce reducing conditions of a gasifier-carbonator). CO₂ capture performance during each carbonation stage was calculated by monitoring the CO₂ concentration at the exhaust. The sorbent attrition rate was evaluated based on the fraction of fines elutriated and collected in filters during each stage of a test. Exhaust samples, sieve-analyzed, were characterized obtaining the particle size distribution to evaluate the in-bed sorbents fragmentation extent. Moreover, the impact fragmentation extent was additionally evaluated by means of an ex-situ apparatus.

The results showed that, despite the similar chemical composition, the six sorbents behave differently. Overall, the attrition/fragmentation tendency is predicted to be relatively limited under simulated DIFB-SEG conditions, leading to conclude that the sorbent make up rate would mostly depend on the decay of CO₂ capture capacity due to sintering.