Steam hydration is reported to be an effective method for reactivating spent sorbents in calcium looping applications; however, uncertainties remain regarding the optimal method of returning the hydrated sorbent to the CO 2 capture loop. Carbonation conversions were found to be higher when Ca(OH) 2 was directly carbonated at high temperatures compared to conversions reached when Ca(OH) 2 was dehydrated prior to carbonation. This observation can lead to improved hydration based reactivation techniques for calcium looping applications. Upon heating in CO 2 , calcium hydroxide remained stable at temperatures >450 °C and the extent of carbonation was controlled by temperature only. The carbonation mechanism of Ca(OH) 2 at high temperatures appears to be more complex than the expected simple mechanism comprising the dehydration reaction of Ca(OH) 2 and the subsequent carbonation of the resulting CaO. An alternate mechanism was proposed, involving the formation of liquid like layers of water on the surface of Ca(OH) 2 .

Steam hydration of lime is an effective method for restoring CO2 capture activity but gives rise to high particle attrition rates in a fluid bed reactor. This paper describes the phenomenon of Ca(OH)2 superheating, also referred to as superheated dehydration (SD). The potential of an attrition-free lime reactivation process using this phenomenon is also investigated. Attrition rates of the sorbent are measured when a reactivation step using steam hydration is implemented every three carbonation/calcination cycles. It has been shown that the presence of CO2 during the dehydration step reduces attrition during subsequent cycles. Experiments performed in a small fluid bed reactor show that the presence of 40−100% CO2 during the dehydration step increases the initiation temperature of the decomposition of Ca(OH)2 from 445 to 618 °C. The thermodynamic equilibrium water vapor pressure for the dehydration reaction at 618 °C is 516 kPa, whereas no water vapor was detected in the reactor during the dehydration step before the temperature reached 618 °C. Under these circumstances it is proposed that the Ca(OH)2 is in a nonequilibrium “superheated state”. A CO2 capture cycling experiment, with a reactivation step every three carbonation/calcination cycles, maintained an average activity of 60%, creating only 3.25% of fines < 150 μm after 28 carbonations. The reactivation step consisted of hydrating the sorbent at a temperature of 270 °C and dehydrating it in 100% CO2 with a 23 min hold at 520 °C. It is proposed that the SD phenomenon may be a key step in the development of an industrially feasible method of lime reactivation for use in CO2 capture and in thermal energy storage applications.

An electrochemical impedance analysis of pure aluminum in 4M KOH at 25~ is reported. Impedance spectra have been obtained at 30-80 mV intervals extending from the hydrogen evolution region at -1.96V (vs. Hg/HgO, 4M KOH) to the transpassive dissolution region at -1.35V. The impedance spectra are found to consist of two intersecting capacitive semicircles with a loop at intermediate frequencies. The low-frequency capacitive arc and the loop become increasingly dominant with respect to the high-frequency relaxation as the potential is shifted in the positive direction. The impedance spectra and the steady-state current/voltage characteristics (including the partial anodic and cathodic curves) are accounted for by a model involving the stepwise addition of hydroxyl groups to surface aluminum atoms, culminating in the chemical dissolution of AI(OH)~ to form AI(OH)4-. This anodic process is coupled to hydrogen evolution via competition for bare surface sites. Comparison of the experimental and predicted impedance spectra indicate that the total concentration of reactive sites at the surface varies with potential in a manner that parallels the anodic partial current. This variation is attributed to the existence of a porous corrosion product film on the surface. The impedance analysis also indicates small values (<0.1) for the transfer coefficients for elementary charge transfer reactions; these are attributed to the highly asymmetric nature of the reaction coordinate for reactions involving reactive species (A1 § OH ) or to strong repulsive interaction between adsorbed species, as embodied in the Temkin adsorption isotherm.

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