Effect of the product layer on the kinetics of the CO₂‐lime reaction

Abstract

The kinetics of reaction between COZ and lime is investigated in the range of 673 to 998 K with a view to examining the effects of product layer deposition and variations in the limestone calcination atmosphere. The reaction is initially rapid and chemically controlled and goes through a sudden transition to a much slower regime controlled by diffusion in the product CaCOs layer. The magnitude of the estimated product layer diffusivity is in the range of m2/s, the corresponding activation energy is 88.9 f 3.7 kJ/mol below 688 K and 179.2 f 7.0 kJ/mol above that temperature, suggestive of solid state diffusion. Plausible mechanisms are discussed. to SCOPEThe effect of pore structure and the influence of product layer diffusion on the CO&me reaction has been examined in the temperature range of 673 to 998 K under COz levels varying from 10 to 42% in nitrogen. The data is analyzed by means of the random pore model (Bhatia and Perlmutter, 1980, 1981a), and intrinsic rate constants as well as product layer diifusion coefficients are determined. Product layer diffusion coefficients for calcines prepared under varying levels of COZ in nitrogen are compared, and the role of solid state processes in the product layer diffusion process is examined.

Modelling of the kinetics of sulphation of CaO particles under CaL reactor conditions

“…For the diffusion through the product layer regime, we applied the Arrhenius equation above 850 ºC but below 850 ºC separately because a slight variation in the E aD fitted under these conditions was detected. This effect has been reported elsewhere in relation with the carbonation reaction of lime [57]. The explanation given in that case was that above the Tammann temperature (861 ºC [63]), there is a change in the properties of the solid CaSO 4 formed, which affects the diffusivity of SO 2 through the product layer, and therefore the E aD .…”

“…One of the most widely used pore models is the model developed by Bhatia and Perlmutter [52,53]: the random pore model (RPM), which assumes that the particle is traversed by random size cylindrical pores with intersecting and overlapping surfaces as reaction proceeds. This model was applied successfully to gas solid reactions [54] including the carbonation and sulphation of CaO [33,[54][55][56][57].…”

Additive effects of steam addition and HBr doping for CaO-based sorbents for CO 2 capture

“…This contradicts somewhat some previous work (Arias et al, 2012) where only a very limited extension of the kinetically-limited regime was observed when steam was present. The explanation is most likely that diffusion is a more highly activated process than surface reaction, as has been found previously by Grasa et al(Grasa et al, 2009), who adapted the work of Bhatia and Perlmutter (Bhatia and Perlmutter, 1983) and found an initial first-order surface reaction, with an activation energy of 21.3 kJ/mol for Imeco limestone and 19 kJ/mol for Katowice limestone, but an activation energy for the subsequent diffusion reaction of 163 kJ/mol (i.e. around an order of magnitude higher).…”

In situstudies of materials for high-temperature CO2capture and storage

“…This lling of smaller pores before larger pores are completely lled supports observations from many earlier studies that measured the changes in the pore size distribution upon carbonation. 5,32 Their conclusions were that the carbonation reaction can be divided into two segments: the rst, rapid segment consisting of the lling of smaller pores <100 nm, with the slower, diffusion limited segment beginning once these smaller pores were lled and layers of CaCO 3 begin to be deposited on the surfaces of the larger pores. It appears that our results support these earlier measurements with direct evidence of more complete carbonation seen for smaller pores in CaO, with the smaller pores lling completely during the initial rapid segment of carbonation, before the onset of the slower diffusion controlled rate.…”