Abstract
Cycling high temperature CO2 capture using CaO–based solid sorbents, known as the calcium looping (CaL) process, is gaining considerable scientific and industrial interest due to the high theoretical sorbent capacity (0.78 gCO2/gCaO), the low specific cost, and the negligible environmental impact of the employed materials. In this work, we investigated the self–combustion synthesis of CaO–CaZrO3 sorbents with different CaO contents (40, 60, and 80 wt%) for use in the CaL process. CaZrO3 was used as a spacer to avoid CaO grains sintering at high temperature and to reduce the diffusional resistances of CO2 migrating towards the inner grains of the synthetic sorbent. Samples were characterized by X–ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and scanning electron microscopy (SEM) analyses. The reaction between CO2 and CaO (i.e., carbonation) was carried out in 20 vol% CO2 at 650 °C and calcination (i.e., decomposition of CaCO3 to CaO and CO2) at 900 °C in pure Ar or with 85 vol% CO2 using a thermogravimetric analyzer (thermogravimetric/differential thermal analysis (TG–DTA)). The most stable sorbent was with 40 wt% of CaO showing a CO2 uptake of up to 0.31 g CO2/gsorbent and 0.26 g CO2/gsorbent operating under mild and severe conditions, respectively. The experimental data corroborated the prediction of the shrinking core spherical model in the first phase of the carbonation. A maximum reaction rate of 0.12–0.13 min−1 was evaluated in the first cycle under mild and severe conditions of regeneration.
Highlights
Among different sorbents, metal oxides contained in naturally occurring minerals represent the cheapest option for the CO2 capture process; the calcined forms of limestone and dolomite [1,2,3] show a high reactivity within a temperature range from 550 to 750 ◦ C, which largely fits with reforming, gasification, and pyrolysis processes, and may be regenerated by thermal decomposition of carbonates at temperatures ranging from 850 to 900 ◦ C
After 12 cycles, the most stable sorbent was the one with 40 wt% of CaO (CCZ40), which maintained a CaO conversion to CaCO3 nearly 100% and a CO2 uptake of 0.31 g CO2 /gsorbent
The CCZ40 sample showed high performance even operating under severe conditions with a decrease of the CO2 uptake from
Summary
Metal oxides contained in naturally occurring minerals represent the cheapest option for the CO2 capture process; the calcined forms of limestone and dolomite [1,2,3] show a high reactivity within a temperature range from 550 to 750 ◦ C, which largely fits with reforming, gasification, and pyrolysis processes, and may be regenerated by thermal decomposition of carbonates at temperatures ranging from 850 to 900 ◦ C This process can be implemented for CO2 removal from product gases in fluidized bed systems for combustion gasification, or methane steam reforming with water–gas shift reaction [4,5,6]. Besides the cost for producing and separating the steam, steam itself was found to cause sintering of CaO, thereby reducing the CO2 uptake capacity of the regenerated sorbent; steam can lead to changes in the pore–size distribution (PSD) through the CaCO3 layer [11]
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