Electric arc furnaces are increasingly used for melting and refining iron or steel, and especially scrap metal. However, these furnaces are very energy intensive and typically require at least one high energy electrode and a high pressure oxygen lance to obtain and maintain a molten metal bath. Because of the large amounts of energy used in an electric arc furnace, steel mill operators are increasingly looking for ways to use all potential energy during the steel making process.
Recently, electric arc furnace operators have used post combustion technology that was originally developed for basic oxygen furnaces many years ago. During the steel making process, oxygen reacts with the carbon in the scrap and/or molten steel and forms carbon monoxide, which bubbles out of the molten metal and then into the exhaust gas. In a post combustion process, secondary oxygen is injected into the melting vessel and reacts with the generated carbon monoxide and forms carbon dioxide. Also, any hydrogen and hydrocarbons could also react with secondary oxygen in a post combustion reaction and form water and carbon dioxide. All these reactions are an exothermic reaction and release energy to be used in the steel making process.
Various types of post combustion systems have been designed. Some post combustion systems inject oxygen into the upper shell of the melting vessel, and other post combustion systems inject oxygen into the foamy slag. Still others inject the post combustion oxygen just above the foamy slag. In order to ensure adequate control over oxygen flow, some of the post combustion systems measure the amount of carbon monoxide present in the melting vessel, such as the generated gas, and depending on the carbon monoxide content, adjust any oxygen injectors to vary the amount of oxygen injected into the melting vessel.
Various systems attempt to improve upon post combustion systems used in steel making, especially with electric arc furnaces. One post combustion system is disclosed in U.S. Pat. No. 5,572,544 to Mathur, et al., where a gas stream is formed within the electric arc furnace, which concentrates the carbon monoxide. Post combustion oxygen is injected into the gas stream where the carbon monoxide is concentrated. However, the sampling of any exhaust gas to determine what post combustion oxygen is required is time consuming and does not operate in real time.
Additionally, many post combustion systems use standard oxygen injectors that can be subject to overheating during this exothermic post combustion processes. Some of the post combustion processes also are not as accurate because it is difficult to constantly measure the gas concentration of carbon monoxide and other gases within a melting vessel. It is also sometimes difficult to adjust oxygen burners in amounts sufficient for adequate post combustion when the gas concentration of various constituents, such as oxygen, carbon dioxide and carbon monoxide change so often during the steel making process.