MODELLO NON STAZIONARIO DI UN REATTORE DI CALCINAZIONE A LETTO FLUIDO CON RICIRCOLO DI CALCIO ALIMENTATO DA IMPIANTO SOLARE A CONCENTRAZIONE.

Increasing rates of electricity generation by variable renewable sources require the deployment of efficient technologies for energy storage. The Calcium-looping (CaL) process is an innovative and promising option for thermochemical energy storage (TCES) in concentrating solar power (CSP) plants. Energy from CSP can be stored by limestone or CaCO_3 calcination, which consists in an endothermic reaction at high temperatures that produces a pure stream of CaO and CO_2. This energy can be later released by means of carbonation reaction, an exothermic reaction where CaCO_3 is formed from CaO in presence of CO_2. The Calcium sorbent is very energy dense, carrying 1.8 MJ/kg of latent heat and 0.87 MJ/kg of sensible heat. In the case under analysis, the receiver of a CSP system will also act as a calcination reactor where the endothermic process occurs. There are several studies about the feasibility of the TCES process and possible receiver configurations. However, the simulation studies and the experimental studies about the calciners for this process are scarce and the technology needs to be further studied and improved in many aspects. The development of an efficient calcination receiver/reactor, adapted to the Calcium-looping process for TCES, is of crucial importance to demonstrate the potential of this technology. In this master thesis a new computational transient model is developed to describe the physical and chemical processes of a tubular fast fluidized bed calciner and to study the performance of the calciner under transient conditions. The transient effects due to changes in the concentrated solar irradiance, in the mass flow rates of the two phases and in the calciner inlet conditions are analyzed. For the heat transfer model in the lean zone of the reactor a core-annulus model is used, while for the hydrodynamic model a modified version of the Kunii-Levenspiel model is implemented. The model under analysis differs from previous works that are based on the Kunii-Levenspiel model because the variation of the mass fractions of the species and of the density of the gas species due to the reaction are taken into the account. The sensitivity study shows that the calcination efficiency strongly decreases if the concentrated solar irradiance decreases or if the solid mass flow rate increases, while it is not affected by the gas mass flow rate for the tested limits. The efficiency also increases with a larger mass fraction of the calcium oxide CaO, with a smaller mass fraction of CO_2 and with larger temperatures of the solids and gas phases. The results also show that complete calcination can be achieved with a 6 m high reactor, also when the boundary conditions are changed, for the tested range. This is true only if the solar irradiance is given to the system, otherwise, if the irradiance is removed, since the reaction occurs more slowly, only after 2 m the reaction rate increases due to the presence of the carbonate at the end of the reactor that still allows the reaction to happen. Finally, two effects with time separation and two different time constants of the process that occurs inside the system are found. The first effect is related to the solid species while the second one to the gas species; both depend on the velocity. Since the velocity of the gas is higher than the velocity of the solids, it was found that the time constant of the gas is lower than the time constant of the solids. In particular, for the inputs that were given to the system, time constants of τ_s<250 s and τ_g<20 s are found.