Analysis of the behaviour of limestone sorbents for sorption-enhanced gasification in dual interconnected fluidised bed reactor

Sorption-Enhanced Gasification (SEG) is a promising technology based on the use of Ca-based sorbents (like limestone) to selectively remove CO2 from the gasification environment, for production of hydrogen rich syngas. This process benefits from the extensive understanding of “calcium looping”, a post-combustion technique aimed at removing CO2 from flue gas. Calcium looping is most typically carried out in Dual Interconnected Fluidised Bed (DIFB) reactors. The correct design of sorbent looping processes in DIFB reactors must consider: 1) sorbent deactivation (i.e., decay of CO2 capture performance) over repeated cycling; 2) the loss of sorbent material due to elutriation, that may be enhanced by attrition and fragmentation. The aim of this study is to investigate six different commercial limestones in terms of sorbent performance and attrition/fragmentation tendency under simulated SEG conditions. The experimental campaign was carried out in a lab-scale DIFB reactor, electrically heated. The six sorbents were limestones coming from different parts of Europe. A synthetic gas including air, CO2 and N2 was used to simulate SEG conditions. A “test” consisted of ten complete cycles of calcination/carbonation. Calcination was performed at 850 °C, fluidising the bed with a stream of 10 % CO2 (balance air) to simulate oxidising conditions typical of the combustor-calciner. In the carbonation stage, the temperature was kept at 650 °C and the CO2 concentration was set at 10 % (balance nitrogen) to account for reducing conditions typical of the gasifier-carbonator. During each carbonation stage, the CO2 concentration at the exhaust was continuously monitored to calculate the CO2 specific capture performance. The sorbent attrition rate was determined by working out the mass of fines elutriated at the exhaust and collected in filters, for each calcination and carbonation stage. After a test, each exhaust sorbent sample was sieve-analysed to obtain the particle size distribution and the fraction of generated fragments. Moreover, the characterisation was extended by carrying out, in an ex situ apparatus, impact fragmentation tests on samples preprocessed in DIFB. Results were critically analysed in the light of the adopted operating conditions, by also including: (a) a fitting equation of conversion data, able to give indications on the sorbent resistance to sintering; (b) a carbonation reaction model, that allowed the estimation of the decrease in the sorbent specific surface area as long as the number of cycles increases.