A study on the effects of particle size and crack formation on the hydration kinetics of K2CO3
Thermochemical materials (TCMs) have the potential to store large quantities of heat for a long time and in a nearly loss-free way. The principle of storage involves a reversible hydration/dehydration reaction with water vapor. Adding water vapor discharges the TCM, supplying low-temperature thermal energy. Adding heat (re)charges the TCM, removing absorbed water in the process. Exploiting this principle, TCMs such as potassium carbonate (K2CO3) can be stored in granular form in a steel vessel, creating a heat battery.
Repeated charging and discharging K2CO3 results in material degradation. Internal stresses caused by temperature and pressure cause the material grains to crack and break open, increasing in size. This is partly desired as the water vapor can now enter and exit the material structure easier, resulting in faster kinetics and therefore, higher thermal power output. On the other hand, since the particles increase in size the effective energy density decreases. It is unclear if the reaction kinetics keep increasing or if the kinetics become independent on (de)hydration conditions after dis-/recharging cycles.
This work focuses on characterizing K2CO3 grains by means of TGA/DSC and microscopy experiments. K2CO3 particles are alternated between hydration and dehydration conditions and the kinetics and morphology are evaluated. Additionally, a hydration model based on nucleation and growth phenomena is established to investigate mechanisms limiting the reaction rate of hydration.
Repeated cycling of K2CO3 particles (0.7-1.0 mm in diameter) by means of TGA/DSC resulted in faster kinetics after each cycle. Typical conversion rates were increased by a factor 15 during the first 12 TGA cycles. No ceiling to reaction speed was observed after 43 cycles. This may be caused by size effects and crack formation. To exclude these effects, smaller particles (25-38 µm in diameter) were also cycled. It is assumed that these particles are single-crystalline without initial cracks. Also cycling these particles did not result in stable kinetics even after 25 cycles. A numerical model was employed to investigate the mechanism limiting the reaction rate. The conversion of K2CO3 particles measured with TGA experiments were well modelled using semi-instantaneous nucleation, limited by diffusion through the solid TCM.