PREVENTING CONGLOMERATION OF REDUCED FINE POWDER IN SOLAR THERMOCHEMICAL REDOX CYCLES BASED ON METALS WITH LOW MELTING AND HIGH BOILING POINTS

This paper describes a new method that enables avoiding conglomeration of small metal drops after reduction from their oxides. These solidified metal micro drops can be hydrolyzed with steam to generate hydrogen and close a solar redox cycle. Metals with low melting points and low vapor pressure at the reduction temperature do not evaporate and their drops tend to agglomerate. This phenomenon severely reduces the yield of the hydrogen production in the hydrolysis reaction. To avoid this agglomeration the mixture of metal oxide and carbon is blended with an additional small amount of inert ceramic powder that does not take a part in either reaction. This inert powder minimizes the amount of the reduction agent necessary for high conversion of the oxide and allows producing micron and submicron metal particles which are suitable for the hydrolysis step when performed in a fixed bed flow reactor. Experimental results with tin dioxide powder as a representative material are presented. Tests were done with different proportions of tin dioxide powder, charcoal and alumina powder as agglomeration retardant. Product morphology as a function of the components content was analyzed by TEM and SEM. Conversion was controlled by measuring of weight losses, amount of oxygen in output gases and X-ray Diffraction Quantitative Analysis.