Calcium-looping for post-combustion CO2 capture. On the adverse effect of sorbent regeneration under CO2

Abstract

The multicyclic carbonation/calcination (c/c) of CaO solid particles at high temperature is at the basis of the recently emerged Calcium-looping (CaL) technology, which has been shown to be potentially suitable for achieving high and sustainable post-combustion CO 2 capture efficiency. Despite the success of pilot plant projects at the MW th scale, a matter of concern for scaling-up the CaL technology to a commercial level (to the GW th scale) is that the CaO carbonation reactivity can be recovered only partially when the sorbent is regenerated by calcination at high temperatures (around 950 • C) as required by the CO 2 high concentration in the calciner. In order to reactivate the sorbent, a novel CaL concept has been proposed wherein a recarbonator reactor operated at high temperature/high CO 2 concentration leads to further carbonation of the solids before entering into the calciner for regeneration. Multicyclic thermogravimetric analysis (TGA) tests demonstrate the feasibility of recarbonation to reactivate the sorbent regenerated at high calcination temperatures yet at unrealistically low CO 2 partial pressure mainly because of technical limitations concerning low heating/cooling rates. We report results from multicyclic c/c and carbonation/recarbonation/calcination (c/r/c) TGA tests at high heating/coling rates and in which the sorbent is regenerated in a dry atmosphere at high CO 2 partial pressure. It is shown that that at these conditions there is a drastic drop of CaO conversion to a very small residual value in just a few cycles. Moreover, the introduction of a recarbonation stage has actually an adverse effect. Arguably, CaCO 3 decomposition in a CO 2 rich atmosphere is ruled by CO 2 dynamic adsorption/desorption in reactive CaO (111) surfaces as suggested by theoretical studies, which would preclude the growth of the regenerated CaO crystal structure along these reactive surfaces and would be intensified by recarbonation. Nevertheless, the presence of H 2 O in the calciner, which is also adsorbed/desorbed 2 dynamically in CaO reactive planes, would shield CO 2 adsorption/desorption thus mitigating the deeply detrimental effect of CO 2 on the carbonation reactivity of the regenerated CaO structure. Oxy-combustion, which produces a significant amount of H 2 O, is currently used in pilot-scale plants to raise the temperature in the calciner although alternative techniques are being explored since it represents an important penalty to the CaL technology. Our study suggests that steam injection would be necessary in a dry calciner environment to avoid a sharp loss of CaO conversion if the sorbent is regenerated at high CO 2 partial pressure.

Economic evaluations of coal-based combustion and gasification power plants with post-combustion CO 2 capture using calcium looping cycle

“…For instance, sorbent make-up rate is a technical parameter having major influence on plant economics. The sorbent used in CaL unit has to be continuously renew due to two important factors: solid attrition caused by high operating temperature which produce the degradation of sorbent capacity to capture CO 2 and sorbent contamination with ash from additional coal used in the calciner [32,33]. As an illustrative example, Fig.…”

On the negative activation energy for limestone calcination at high temperatures nearby equilibrium

“…On the other hand, the calciner reactor in pilotscale tests is operated in the fast fluidization regime, which provides a high efficiency of heat and mass transfer in the typically short residence times (of just a few minutes). Calcination of the partially carbonated solids (with a typical molar carbonate content of 15% after many cycles) is conceivably attained at about T C 900 1C in relatively short times as inferred from thermogravimetric analysis (TGA) tests (Martinez et al, 2012a;Valverde et al, 2014a). Yet, the calciner temperature in practice has to be increased up to T C 930 1C (or, equivalently, the ratio P=P eq decreased to P=P eq C 0:6) in order to achieve a high calcination efficiency of the make-up flow of natural limestone (Charitos et al, 2011;Arias et al, 2013;Ströhle et al, 2014), which imposes an important energy penalty to the technology (Martinez et al, 2012b.…”

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

“…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.…”