Ca(OH)₂ Superheating as a Low-Attrition Steam Reactivation Method for CaO in Calcium Looping Applications

Abstract

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.

CO2 capture by calcium aluminate pellets in a small fluidized bed

“…These were particles to subsequent carbonation [39,40]. A similar process will have this occurred here upon aging; though the carbonation mechanism would be different, likely going through a Ca(OH)2 intermediate, which has also been shown to increase sorbent reactivity [33,34]. However, once steam is introduced a marked improvement in reactivity was observed for aged pellets over the original pellets.…”

“…In addition, some carbonation occurred as a result of reaction with CO2 in the atmosphere during the pelletization process. Further hydration and carbonation can be expected from further exposure to the air, and these are in fact a potential reactivation method for spent natural sorbent [2,33,34] , although it is much less clear what they might do to pellets produced by a partial hydration method.…”

In situ X-ray diffraction of CaO based CO2 sorbents

“…Bench scale reactors can be sized to allow for larger sample mass so that reactor outlet gas concentrations and steam partial pressures can be accurately quantified. 19,24,31,33 With this technique, outlet gas concentrations and steam partial pressure measurements are related to known inlet concentrations and partial pressures, allowing for rates of production or consumption to be calculated. To obtain extents of CO 2 /H 2 O intakes or releases, the rates require integrating over the reaction time, a calculation prone to error propagation and over-reliant on accurate knowledge of the duration of the reactions.…”

“…26,29,30 However, by hydrating in a CO 2 atmosphere, the reduction in mechanical strength can be retarded. 31 Currently, thermogravimetric analysis (TGA) and bench scale reactors are the most common techniques for studying CO 2 capture by CaO based materials. TGA relies on monitoring the sample mass over time which can be used to measure formation of CaCO 3 and Ca(OH) 2 from CaO and vice versa.…”

Effect of CaO hydration and carbonation on the hydrogen production from sorption enhanced water gas shift reaction

“…Here, the [CO 2 ] ad is formed directly on the CaO surface, and this is really in situ sorption. CO%CO ad (10) OH ad þ CO ad %COOH ad (11) COOH ad %½CO 2 ad þ H ad (12) 2H ad %H 2…”