Investigation of electrostatic precipitation to avoid particles deposition on a solar reactor window

Kurzfassung

Solar production of Calcium Oxide (CaO) is a promising way to reduce the CO2 emissions from the cement industry. To carry out this thermochemical process, closed solar reactors with continuous processing are needed. Indirect heated solar reactors to carry out this process have been designed and tested at lab-scale. However, the main drawback of these designs is the low reactor efficiency, because of the formation of a solid material layer in the inner reactor wall that works as a thermal resistance of contact. Therefore a higher energy input and higher temperatures at absorbing surfaces are needed to reach the reaction temperature inside the reactor. On the other hand, in designs where the material is directly heated, a window is needed to allow the access of solar radiation into the reactor. After some time of operation, this window tends to be damaged due to the overheating of solid material deposited on it [1]. For this reason, one of the major challenges here is to prevent the migration of particles to the reactor window. The most well-known method to protect the window of solar reactors is implementing a gas curtain or a gas vortex flow, as shown in the solar reactor of Koepf et al. [2]. Nevertheless, results of a 100 % protected window have not been reported yet, and the low reactor efficiency of these approaches due to the high amount of inert gas used is still a topic of discussion. The focus of the present work is the investigation of using electrostatic precipitation, to solve the task of keeping the window free of particles and fines in a solar rotary kiln for the solar production of CaO. An Electrostatic Precipitator (ESP) works using the corona discharge effect. Here, an electrical field between two electrodes ionizes the gas molecules which charge the solid particles in the gas. The charged particles can be collected on the surface of the collecting electrode. ESPs have been operated in high concentrated CO2 atmospheres, as well as in other gas media at temperatures over 900 °C, showing separation efficiencies higher than 90 %. The electrical resistivity of CaO at temperature values of 950 °C is in the range where an ESP can be operated (104-1011 Ohm-cm) [3]. However, the construction of an ESP to separate CaO from a 100 % CO2 atmosphere at temperature values over 900 °C and the possibility of its later implementation in a solar reactor has to be demonstrated. [1] T. Litterst, Investigation of window damage by hot particles in solar heated circulating fluidized beds, Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR), 1993 [2] E. Koepf, W. Villasmil, A. Meier, Demonstration of a 100-kWth High-Temperature Solar Thermochemical Reactor Pilot Plant for ZnO Dissociation, AIP Conference Proceedings 1734, 2016. [3] N. A. Surplice, The electrical conductivity of calcium and strontium oxides, Brit. J. Appl. Phys, 1966.