Carbon dioxide has been identified as the major greenhouse gas contributing to global climate change. As a result, various processes are being explored that would allow CO2 to be captured and ultimately sequestered from processes, such as combustion, gasification, and steam methane reforming, where carbon dioxide is a major by-product. Use of amine solvents for CO2 capture, while common in industry, is expensive and unlikely to be feasible on a large scale. Looping cycles involving calcium sorbents, where CO2 is released at high temperature by calcinations and captured at lower temperature by carbonation, are of particular interest. However, evidence has shown that the ability of the sorbents to maintain their utilization over repeated cycles is quite limited.
One of the characteristics that influences the capture capability of limestones is the particle size distribution. Most previous studies report that particle size slightly affects the cyclic process and the performance of limestones (Sun et al., 2005; Li et al., 2005). Grasa and Abanades (2006) who focused on small particle fractions, of the order of 100-1000 pm, concluded that sorbent particle size did not influence the sorption capacity of the sorbents tested, instead only depending on the number of cycles performed. On the other hand, Ye et al. (1995) suggested that particles smaller than 5 pm be considered optional due to the increased cost of the milling.
Another experimental parameter that affects the sorption ability and the decay of the lime-based sorbents is the temperature of the reaction between CaO and CO2. Typical temperatures in the carbonator exceed 600°C, while calcination requires higher temperatures. Grasa and Abanades (2006) proposed that the temperature of 950°C during the calcination stage could be beneficial for the sorbent cyclic performance. Sun et al. (2005) investigated sorbent performance at 850°C
I. Dincer et al. (eds.), Global Warming, Green Energy and Technology,
DOI 10.1007/978-1-4419-1017-2_20, © Springer Science+Business Media, LLC 2010
and reported that lower temperatures (750°C) enhance the carbonation of the CaO during the first two cycles. The reaction time constitutes another crucial parameter affecting the sorption capability during continuous cycling of the sorbents. Previous research studies indicate that relatively brief sorption times result in decreased calcium utilization efficiencies, but in slightly increased reactivity, mostly for the few first cycles of the process.
In fluidized bed combustion (FBC) plants the presence of SO2 is unavoidable. Therefore, the sulfation of the sorbents is of major importance as also shown from the numerous works performed on the behavior of sorbents under SO2 (Adanez and Garcia-Labiano, 1993; Badin and Frazier, 1985; Cheng et al., 2004; Ghosh-Dastidar et al., 1996; Laursen et al., 2001, 2002; Stanmore and Gilot, 2005; Sun et al., 2007; Ye et al., 1995). Most of these studies claimed that sulfation mainly occurs on the surface of the particles forming a CaSO4 layer, which impedes further sulfation and/or carbonation of the particle. Stanmore and Gilot (2004) reported that sulfite is also involved in the reaction scheme and concluded that the sulfation ability is reduced by the presence of the high proportions of O2 and SO2 in the gas stream. The sulfation extent is based on the Ca/S ratio. According to the chemical reaction the absorption of 1 mol of SO2 requires 1 mol of Ca. Nevertheless, the utilization of Ca is most of the times higher. It is common notion that the pore structure strongly affects the sulfation process. Ghosh-Dastidar et al. (1996) underlined the existence of an optimum pore structure of the parent limestone, which after calcination will produce the optimum pore size for sulfation.
The aim of this study is to elucidate the factors leading to the deactivation of two individual limestone types. For this reason, two limestones originating from two different Greece locations are subjected to cyclic sorption/calcination investigations. The sorbents are characterized for their CO2 caption ability under a specific number of carbonation/calcination cycles. The effect of particle size, reaction time, and reaction temperature on the CO2 capture ability and the decay of the limestones is also investigated. Another purpose of the study reported here is to explore possible ways to diminish the sorbents deactivation. In practice, carbona-tion always occurs in the presence of SO2, and therefore the comprehension of the interaction phenomena is of great importance. The durability of the limestones under simultaneous carbonation and sulfation is determined. Among the priorities of this work is to figure out the effect of SO2 on the carbonation and simultaneously the effect of CO2 on the sulfation process.
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