Dr. Derevshchikov is a researcher at Boreskov Institute of Catalysis, Novosibirsk. Field of interests: regularities in the formation of the texture of catalysts and sorbents.
4. Carbon dioxide separation from gas mixtures is critical process in several emerging energy-related industries, including in hydrogen production by means of biomass utilization and exhaust gas clean up. Some studies on this topic suggest using calcium-based sorbents for effective CO2 capture from gases at high temperatures because of potential advantages including: a wide range of operating temperatures; reduced energy penalties; avoidance of liquid wastes; the relatively inert nature of solid wastes.
However, the use of CaO as a regenerable CO2 sorbent is limited by the rapid decay of the carbonation conversion with the number of carbonation/calcination cycles. The main reason for CaO capacity decay is sorbent sintering during sorption/retgeneration cycles.
The evolution of sorptive and textural properties of CaO-based sorbents during repetitive sorption/regeneration cycles has been mathematically simulated in this study. The proposed model takes into account the morphology of nascent CaO, sorbent sintering physics and CO2 sorption kinetics. The results show that the model is in good agreement with the experimental data for real sorbents and predicts the dependence of the recarbonation extent on the number and duration of the sorption/regeneration cycles well. The model allows predicting the change of the textural properties of the sorbent (e.g. the values of specific surface area and mean pore size) during the sorption/regeneration cycles. The structure of the sorbent was modeled with the dense random packing of spheres using the Lubachevsky–Stillinger compression algorithm. The sintering was simulated under the following assumptions: the sintering proceeds via the lattice diffusion mechanism, and the sintering rate of CaCO3 is higher than that of CaO. Note that, depending on the carbonation duration, our model predicts that the maximal thickness δ of the CaCO3 layer constitutes 7% or 11% of the mean diameter of grains. Thus, taking into account that the radius of original CaO was near 300 nm, we can obtain that the effective thickness of the CaCO3 layer is near 20–30 nm that is close to the experimental estimations. It should be noted that the calculated value 2.8 m2 /g of the specific surface area of the parent sorbent is close to the experimental value of 4.2 m2 /g for the sorbent produced by the CaCO3 decomposition (Lu et al., 2006). To estimate the change in the sorbent pore size distribution during the cycling, we used the chord-length probability distribution (CLPD) method (Lu and Torquatoa, 1993). In this method, the mean chord length lC estimates a characteristic length of the porous space and can be interpreted as a first approximation of the mean pore half-size. Diminishing of the pore size from initial to final cycle also means the reducing of pore volume in the set of spheres i.e. its shrinkage. In this way sintering in the system drives to decrease the size and the volume of small pores inside the sets, but to increase of the pore volume which size comparable to the size of the sets in a multisets system. Similar pore size redistribution during cycling confirmed experimentally (Alvarez and Abanades, 2005). Indeed, some more advanced simulations approaches are needed to extract the exact morphological characteristics of porous systems (pore volume, geometry and size distribution) and it will be a task for our subsequent work.
The work was performed under support of RFBR (Research project No. 16-33-00436 mol_a)
Audience Take away:
• A method will be presented for modeling the change in the texture of porous bodies during the sintering process. The approach allows to monitor changes in the parameters of the porous medium during the sintering process: porosity, pore size distribution and sorption capacity.
• This approach can be used to describe the deactivation of catalysts and sorbents