Application of concentrated solar radiation to chemical looping combustion

Abstract

The novel concept of the application of the Oxygen Carrier (OC) particles in a Chemical Looping Combustion (CLC) system for diurnal storage of the concentrated solar thermal energy is presented here. Two innovative configurations of Hybrid Solar CLC (Hy-Sol-CLC) systems for continuous base-load power generation are proposed and assessed. The systems seek to take the key features of a CLC system that are desirable for a hybrid solar GTCCs; notably the production of an industrially pure stream of CO₂, inherent chemical and sensible heat storage, relatively low temperature of the fuel reactor relative to that of the air reactor and the potential to operate the fuel and air reactors at different pressures. In the first proposed configuration, three reservoirs have been added to a conventional CLC system to allow storage of the OC particles, while a cavity solar receiver has been chosen for the fuel reactor. In this Hy-Sol-CLC system the flow rates of the fuel and OC particles were considered to be constant. The calculations demonstrated that the solar thermal energy can be stored using CLC components. However, this configuration is limited to a low solar share of about 6.5% when averaged over the whole day. Besides, the variations in fuel reactor temperature, owing to the diurnal variations of the input concentrated solar thermal energy, might result in damage to the OC particles. The second Hy-Sol-CLC system addresses the limitations associated with the first configuration. In this system, as for the first one, a cavity solar reactor has been chosen for the fuel reactor while two reservoirs have been added to a conventional CLC for the storage of OC particles. A direct air-particle heat exchanger has been also proposed to provide independent control of the temperature of the OC particles in the air reactor from those stored in the storage reservoirs. In this process the operating temperature of the solar fuel reactor is controlled through varying the flow rates of fuel and OC particles proportional to the variations in the input concentrated solar thermal energy. This hybrid cycle is estimated to achieve a solar share of up to 60% when averaged over the whole day. The performance of this hybrid system in a GTCC cycle was evaluated with and without the application of an after-burner. The after-burner was added to further increase the gas turbine inlet temperature. The calculations predict a first law efficiency of 50.0% for the cycle employing the after-burner, compared with 44.0% for that without the after-burner. However, this higher thermal efficiency is achieved at the cost of decreasing the solar share from 60.0%, without the after-burner, to 41.4% with it. Applicability of the combinations of natural gas, CO and H₂, as fuel with the oxides of Co, Cu, Fe, Mn and Ni, as oxygen carriers in the proposed Hy-Sol-CLC system was also evaluated. The calculations demonstrated that, from all of the assessed metal-based oxygen carriers, only the pairs of CoO/Co, NiO/Ni and Fe₂O₃/Fe₃O₄ are potentially suitable for use in a Hy-Sol-CLC system working with natural gas. However, none of these materials allow any significant chemical storage of solar energy for the oxidation of CO and H₂. An unsteady-state model of an OC particle exposed to high intensity solar heat flux was also developed to provide the fundamental knowledge required for the selection of an efficient solar cavity reactor for the Hy-Sol-CLC systems. The model was validated against the available data in the literature. The numerical analysis demonstrated that the application of direct heat transfer is desirable. However, it must be combined with a high convective cooling to avoid excessive heating rates, which would result in overheating of the particles.