Hydrogen plasma smelting reduction process monitoring with optical emission spectroscopy – Establishing the basis for the method

In the world of ever-increasing demand for carbon-free steel, hydrogen and recycling have an undeniable role in achieving net-zero carbon dioxide emissions for the steel industry. However, even though steel is one of the most recycled materials globally, the quantity of steel that can be made from recycled steel will probably not match the demand in the future. This in turn means that steel must be also produced from the conventional resource, the iron ore. Hydrogen has been proposed as an environmentally friendly alternative to carbon as a reducing agent. To tackle the problems related to the usage of hydrogen for this purpose, hydrogen plasma smelting reduction has been studied extensively in the last few years. This article aims to provide means for process control of the hydrogen plasma, which may show erratic and chaotic behavior during the smelting process. The method used is optical emission spectroscopy, with which the plasma can be characterized, its composition can be evaluated, and its temporal evolution can be assessed. This study sheds light on how the plasma behaves with different electrode gaps and flow gas compositions together with how the position of the arc with respect to the center of the crucible can be assessed. In Ar/H2 plasma, the plasma temperatures derived with OES varied between 4000 and 10000 K, and up to a 26% decrease in electron density was observed when increasing the electrode gap in 1 cm increments.