UBC Theses and Dissertations
Sorbent attrition in fluidized carbon dioxide capture systems Knight, Andrew
As part of a collaborative Carbon Management Canada project, the extent and causes of attrition for three proposed carbon dioxide capture sorbents for fluidized power generation systems was investigated. The sorbents are crushed limestone, lime-based pellets containing calcium aluminate cement binder, and the same pellets with a mesoporous silica coating replacing the binder. The tested sorbents were provided by the University of British Columbia, the University of Ottawa/CANMET, and Laval University, respectively. The attrition testing equipment was an air-jet apparatus based on the ASTM D5757 standard, with several modifications to improve testing capabilities. The experiments were conducted under varying attrition test periods, gas velocities, temperatures, humidities, and initial particle sizes. Following each test cycle, particle size distributions of the produced fines and remaining bed material were analysed, and scanning electron microscopy was utilized to provide additional insight on attrition mechanism. The strengths and weaknesses of the ASTM standard were evaluated, and a new attrition testing standard is proposed to improve operability and reduce the equipment size and material requirements. The experimental results indicate that the cement-bound pellets attrit to the same extent or worse than crushed limestone, and are highly sensitive to humidity because of the hygroscopic cement and formation of calcium hydroxide. Calcination substantially increases the friability of the cement-bound pellets and limestone. Both materials approached complete degradation after 24 h during the 500°C tests. However, the silica coated pellets were found to have high attrition resistance in dry air at 500°C. After 24 hours of operation, compared to the coated sorbents, the lime mean diameter reduction and Air Jet Index were greater by factors of 2.4 and 3.4, respectively. The 1 μm thick and 5 μm thick-coating pellets experienced nearly the same mean reduction in diameter and production of fines, a key finding for future studies of the economic viability of the coating process. The tested materials experienced significant fragmentation, resulting in deviations from fines production models from the literature. The experimental results also displayed large variance in fines production rates during replicate trials. A novel model for mean diameter reduction rate is proposed, based on particle momentum.
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