Continuous Solar Carbothermal Reduction of Aerosolized ZnO Particles Under Vacuum in a Directly Irradiated Vertical-Tube Reactor

Abstract

A solar-driven aerosolized particle reactor under vacuum was tested for carbothermal reduction of zinc oxide using concentrated solar power. The reactor concept is based on the downward flow of zinc oxide and carbon particles, which are indirectly heated by an opaque intermediate solar absorption tube. The particles are rapidly heated to reaction temperature and reduced within residence times of less than 1 s. In the continuous feeding experiments, maximum sustained temperatures close to 2000 K and heating rates as fast as 1400 K min−1 could be achieved for pressures between 1 and 1000 mbar. Reactant conversions of up to 44% were obtained at 1000 mbar. It was found that a reduction in system pressure leads to a decreased particle residence time (as low as 0.09 s), and therefore low conversion (as low as 1%), thus partially diminishing the positive thermodynamic effects of vacuum operation. Experimental results validate the robust and versatile reactor concept, and simultaneously highlight the necessity of balancing the system design in order to optimize the conflicting influence of vacuum operation and reacting particle residence time.