Influence of ball milling on CaO crystal growth during limestone and dolomite calcination: Effect on CO2 capture at Calcium Looping conditions

Abstract

The multicycle CO2 capture performance of CaO derived from the calcination of ball-milled limestone and dolomite have been tested under high temperature and high CO2 concentration environment for the first time. Here it is shown that the CO2 capture capacity of CaO is inversely related to the milling power applied to the starting mineral and the size of nascent CaO nanocrystals. In situ X-ray diffraction analysis used to follow the average crystallite size of CaCO3 and CaO during the calcination process as a function of temperature demonstrates that crystal growth is notably enhanced in a CO2-rich atmosphere for milled sorbents. Contrary to early reports suggesting improved reactivity towards carbonation of CaO from milled sorbents, promoted agglomeration, and crystal growth under these more “realistic” conditions lead to a severe deterioration of both capture capacity and recyclability, as observed from the multicyclic carbonation/calcination experiments. Yet the negative effect of milling is less pronounced in dolomite due to the constrained sintering effect of the inert MgO grains that results in smaller CaO crystallite sizes, reduced crystal growth rate, and improved performance. These results provide insight on the role of CaO crystallinity on the carbonation reaction, useful for devising strategies to improve sorbents performance.