Steam-Enhanced Calcium Looping Cycles with Calcium Aluminate Pellets Doped with Bromides

Abstract

This study explores the effect of calcium bromide (CaBr2) doping of lime-based sorbents in the presence of steam during calcination/carbonation cycles. Two sorbents were tested: natural limestone (Cadomin, Canada) and a synthetic sorbent (pellets) prepared from Cadomin limestone with addition of calcium aluminate cement. The mixture of calcined limestone and cement was pelletized in a mechanical granulator that uses spray water as the part of the pelletization process. Both the original limestone and the prepared pellets were impregnated with a dilute CaBr2 solution to achieve a Ca/Br mole ratio of 500:1. The CO2 carrying activities of the sorbents were tested during calcination/carbonation cycles in a thermogravimetric analysis (TGA) apparatus. Realistic calcination conditions during the reaction cycles were employed: 900 °C with a CO2 sweep gas. Multicycle tests were carried out with steam 15% and without steam present in the carbonation gas stream (20% CO2, 15% steam or 0% steam, N2 balance in both cases). The results showed that doping with CaBr2 has a beneficial effect on sorbent CO2 capture activity, and in particular, the conversion rate during the diffusion-controlled stage of carbonation was found to exhibit a strong synergic enhancement in the presence of steam. The effects of doping and steam were more pronounced in the case of synthetic pellets, resulting in an uptake of 23.8 g of CO2/100 g of sorbent after 31 cycles, which represents a conversion of 35.6%. This CO2 capture uptake is very high compared with that of pellets with no CaBr2 addition and no steam present during the reaction cycles, where only 15.0 g of CO2/100 g of sorbent (22.5% conversion) was seen after 10 cycles. These results suggest that the preparation of synthetic sorbents for calcium looping using solutions containing small amount of bromides would be beneficial in practical applications, and steam will either be produced by firing almost any fuel or be found in flue gas suitable for processing by calcium looping.

The Effects of Thermal Treatment and Steam Addition on Integrated CuO/CaO Chemical Looping Combustion for CO2 Capture

“…These results are consistent with those reported previously [26]. The higher proportion of cement, which contains high surface area alumina [18] in the CuO/CE pellets is consistent with their higher surface area. The recorded mass change during carbonation was attributed only to CaCO 3 formation as given in Equation (1) 00 (5) where m carb is the mass of carbonated pellets, m oxid is the mass of oxidized pellets, y CaO is the mass fraction of CaO in the pellets, MW CaCO 3 is the molecular weight of CaCO 3 and MW CaO is the molecular weight of CaO.…”

“…These results are consistent with those reported previously [26]. The higher proportion of cement, which contains high surface area alumina [18] in the CuO/CE pellets is consistent with their higher surface area.…”

“…In this paper, the deactivation behavior of CaO, CuO and CuO/CaO integrated pellets used for multiple (16)(17)(18)(19)(20) cycles in a thermogravimetric analyzer was studied. The impact of thermal treatment and the presence of steam on the deactivation were also investigated.…”

“…For example, Qin et al [17] studied various heat pre-treatment procedures with CaO/CuO pellet mixing to overcome the loss of performance and found that certain thermal treatments improved the multi-cycle CaL stability. Steam, which is present in post combustion flue gas, has also been demonstrated to have a positive effect on the stability of pure CaO pellets to capture CO 2 over multiple cycles [17][18][19][20][21][22][23][24]. Several theories have been proposed to explain how steam improves the CaL process.…”

CO2 capture by calcium aluminate pellets in a small fluidized bed

“…That said, pelletization would offer a method of re-using spent lime [46], if lime were not easily available in a given location, or the attrition resistance of a particular limestone was very poor. In addition, it offers the potential ability to use cheap additives which TGA tests have suggested are particularly effective in producing extremely reactive sorbents [47,48]. However, such tests must be done at the pilot plant level under realistic fluidizing velocities and temperatures if they are to provide the evidence that these approaches represent an advantage over using untreated lime-based sorbents and this represents the next goal of our research.…”