On the use of industrial steel mill scale as a high-density energy carrier: Part II. Microstructural and chemical evolution over cycling

Abstract

Recently, iron powder has been considered as a high-density energy carrier, or sustainable metal fuel. The potential of low-cost steel processing by-product, mill scale powder, was investigated in the first part of this study through two cycles of hydrogen-based direct reduction and combustion in a metal cyclonic burner. This second part focuses on the microstructural and chemical evolution of the powder over its cycling, using a combination of state-of-the-art microscopy (scanning electron microscopy and scanning transmission electron microscopy, coupled with energy-dispersive x-ray spectroscopy), surface area BET measurements, x-ray diffraction and thermodynamic simulations. The distribution of the chemical elements is initially highly inhomogeneous and concentrated on the powder surface, but gets homogenised by the combustion process. New Ca and Si-rich oxide phase(s) form during the solidification of the combusted powders, while the remaining chemical elements (Mn, Cu, Ni, Ti, Al, Cr, Mg) remain in solid solution in the iron oxide phases. Enriched zones of Cu and Ni are observed in the Fe oxide nanoparticles, corresponding to their favoured evaporation, while Mn is homogeneously distributed in the Fe oxide nanoparticles. Si and Cr oxide nanoparticles are also detected, while Al hardly evaporates. A satisfactory heat release efficiency of 0.84–0.9 is measured for the reduced mill scale, even if slightly lower than pure Fe (0.88–0.91), confirming its potential for use as metal fuel