Multicyclic conversion of limestone at Ca-looping conditions: The role of solid-sate diffusion controlled carbonation

Abstract

Limestone derived CaO conversion when subjected to multiple carbonation/calcination cycles is a subject of interest currently fueled by several industrial applications of the so-called Ca-looping (CaL) technology. The multicyclic CaO conversion at Ca-looping conditions exhibits two main features as demonstrated by thermogravimetric analysis (TGA). On one hand, carbonation occurs by two well differentiated phases: a first kinetically-driven fast phase and a subsequent much slower solid-state diffusion controlled phase. On the other, carbonation in the fast phase usually exhibits a drastic decay with the cycle number along the first carbonation/calcination cycles. This trend can be reversed by means of heat pretreatment, which induces a marked loss of fast conversion in the first carbonation but enhances diffusion of CO 2 in the solid. Upon decarbonation the regenerated CaO skeleton shows an increased conversion in the fast carbonation phase of the next cycle, a phenomenon which has been referred to as reactivation. Nonetheless, sorbent reactivation is hampered by looping carbonation/calcination conditions as those to be likely found in practice such as carbonation stages characterized by low CO 2 concentrations and short duration and calcination stages at high temperatures in a CO 2 enriched atmosphere, which causes a sintering and loss of activity of the regenerated CaO skeleton. We analyze in this work sorbent reactivation as affected by heat pretreatment and carbonation/calcination conditions. Aimed at shedding light on the role played by these conditions on reactivation we look separately at the multicyclic evolution of conversion in the kinetic and diffusive phases. Generally, the evolution of multicyclic conversion after the first cycle can be described by a balance between the surface area gain due to diffusive carbonation and the surface area loss as caused by sintering in the calcination stage. A significant gain of relative surface area after the first cycle, which is favored by harshening the heat pretreatment conditions, leads however to a marked decay of it during subsequent cycles, which precludes reactivation for an extended interval of cycles. On the other hand, sorbent grinding, if performed before heat pretreatment, leads to a less marked but more sustainable reactivation along the cycles. A novel observation reported in our work is that pretreatment of limestone in a CO 2 atmosphere leads upon a subsequent quick decarbonation to a CaO skeleton with extraordinarily enhanced reactivity in the kinetically-driven carbonation phase and with a high resistance to solid-state diffusion, which can be attributed to annealing of the crystal structure as reported by independent studies. 2

A new model of the carbonator reactor in the calcium looping technology for post-combustion CO2 capture

“…As seen in Figure 4 (a) the time for the kinetic stage (t k ) decreases with the number of cycles as conversion in the diffusive phase is enhanced, which is however dismissed in the theoretical model (solid lines). Figure 4(b) illustrates the kinetic curves proposed in our work as representative of the experimental curves reported in [42] in which calcinations were carried out under high CO 2 partial pressure. It is assumed that the time evolution of CaO conversion can be approximated by two 20 lines of constant slope, one corresponding to the kinetically controlled carbonation stage and another for the diffusive carbonation stage.…”

“…TGA results show that the solid-state diffusion controlled carbonation is greatly enhanced when CaO is regenerated under high CO 2 partial pressure as compared to calcination under low CO 2 concentration [26,42,43]. Figure 1 shows 15 the time evolution of sorbent weight during the first calcination/carbonation cycles of two TGA tests in which calcination was carried out either under high CO 2 partial pressure or air.…”

“…However, as pointed out above, the diffusive carbonation phase plays a relevant role for calcination under high CO 2 partial pressure whereas the kinetically controlled carbonation stage is severely hampered. Using the multicyclic TGA results reported in [34,35,42], we develop below a modified carbonator model built upon the model proposed by Alonso et al [18]. 10 The main novelty of our model is that carbonation in the diffusion controlled phase will be considered to predict the carbonation efficiency.…”

“…The inset shows the time evolution of sorbent wt% in the first 3 cycles. Obtained from data originally reported in [42].…”

“…Moreover, an increase of the carbonation phase above 5 min would lead to higher conversion in the diffusive phase since carbonation in this stage grows roughly linearly with time until carbonation is completed [42].…”