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 CO 2 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 CO 2 during the dehydration step reduces attrition during subsequent cycles. Experiments performed in a small fluid bed reactor show that the presence of 40−100% CO 2 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 CO 2 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% CO 2 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 CO 2 capture and in thermal energy storage applications.