Evolution of sorptive and textural properties of CaO-based sorbents during repetitive sorption/regeneration cycles: Part II. Modeling of sorbent sintering during initial cycles

Highlights

• A new model for the evolution of CaO-based sorbent properties in sorption/regeneration cycles is proposed.

• The model takes into account the morphology of a sorbent and its surface sintering.

• The model predicts the recarbonation extent of sorbents during initial cycles.

• The contributions of volume and surface sintering mechanisms are estimated.

Abstract

A new model describing the evolution of sorptive and textural properties of a CaO-based sorbent during repetitive sorption/regeneration cycles has been developed. The proposed model considers the morphology of nascent monodisperse CaO and the sintering of sorbent grains upon the assumption of the surface mass-transfer mechanism. In addition, the obtained model allows predicting the change in the textural properties of the sorbent (e.g. the values of specific surface area and mean pore size) during the sorption/regeneration cycles. The obtained results show that the model of surface sintering is well applicable to the description of the decay of the sorbent sorption capacity in initial several cycles, while the previously developed model of the volume sintering provides good prediction of this decay only after several cycles. It can be assumed that during the cycling, the sintering of the sorbent switches from the surface to the volume regime.

Introduction

The investigation of calcium oxide sorption is of profound interest for many scientists. This material seems to be a promising solid sorbent for reversible CO₂ capture in many modern processes, such as biomass gasification or the reduction of greenhouse gas emissions from large stationary sources (Mohamed et al., 2016, Shi et al., 2017, Kierzkowska et al., 2013, Kenarsari and Zheng, 2015). 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 experiments have shown that the recarbonation is far from being reversible in practice and the main causes of the CaO capacity decay are sorbent sintering and pore blockage (Abanades, 2002, Abanades and Alvarez, 2003, Grasa and Abanades, 2006). The sintering is a complex process that includes such phenomena as the body shrinkage, the reduction of the surface area, and the redistribution of pore sizes (Geguzin et al., 1984, Kang, 2005). While CaO interacts with CO₂ on the solid/gas phase boundary, its surface area decreases because of sintering, which leads to an irreversible drop in the sorption capacity of CaO (Lysikov et al., 2007).

Initially, there appear point contacts between the grains of CaO, and then the mass transfer leads to the growth of “necks” between the grains. Finally, the mass transfer, which results from the surface energy gradient, leads to the coalescence of the grains and to the reduction in their total surface area. There are two opposite manifestations of sintering: the surface sintering and the volume sintering (Fig. 1). Both of them contribute to the neck growth, but only the volume transport leads to densification: it moves the mass from inside the solid to deposit it in the pores. In contrast, the surface transport mechanism does not lead to densification because the mass is repositioned on the pore surface. This leads to a lower surface area and removing curvature gradients. Often the mechanisms operate in collaboration (German, 2014).

The sintering of the CaO sorbent grains during the repetitive sorption/regeneration cycles was modeled in our previous work under the assumption of volume mass transfer (Bazaikin et al., 2016). The model predicted the sorbent sorption capacity and textural properties; however, its convergence with the experimental data was good only after first several sorption/regeneration cycles. At the beginning of the experiment (first 4–6 cycles), the model did not work. The current study is aimed at the development of a novel computer model that would describe the evolution of sorptive and textural properties of a CaO sorbent during initial sorption/regeneration cycles under the influence of the surface sintering mechanism. Another aim of this study is to compare the influence of the volume and surface sintering on the total textural and sorptive properties of the sorbent during the cycling.