This came up a while back here; the idea is that you would burn the iron powder, then re-generate the iron oxide waste with electrolysis preferably using renewable energy. So iron becomes an energy transport and storage mechanism instead of a single-use fuel.
Seems like it would be more efficient to use the renewable energy directly or failing that, use the electrolysis to create hydrogen and burn that, but I guess that comes down to how efficient the regeneration process is and cost of transport.
https://gizmodo.com/a-dutch-brewery-is-burning-iron-powder-a...