Iron and Steel plants have traditionally had a low level of automation. However, this is no longer true today. Major Iron and Steel owners are making large investments in process computers and modern generation distributed control systems to boost product quality and plant productivity. This modernization of steel plants is occurring in many countries in the world today. This technology urbanization has made Gas Phase Filtration a necessity in Iron and Steel Plants.
Process of Air Contamination in a Iron and Steel mill
Iron and Steel mills can basically be broken down into two process areas :
- The metallurgical area
- The mills section
Corrosive and Toxic gas filtration necessity in Coking plants
In the metallurgical area, coal, coke, iron ore, manganese and other basic core materials from which steel is made, are brought into the plant. If the mill is integrated, these components are then pre-processed in various plant sections (sintering plant, briquetting plant, coking plant). Main source of corrosive and toxic gases is coking plant. In the coking plant, coal is converted to coking coal.
During the coking process, the coal is subjected to high temperatures in an oxygen depleted atmosphere, and many volatiles are driven off the coal. As coal also contains a great deal of nitrogen-based compounds, which are released to the atmosphere during the coking process, high concentrations of oxides of nitrogen (NOx) and ammonia are found. NOx level inside coking plant can peak at around 5 PPM and ammonia levels are also very high, with levels between 20 to 50 PPM at peaks, which makes air and gas purification a necessary criteria.
Usually the first stage in an iron and steel plant is the blast furnaces. Due to high temperatures at furnaces sulphur dioxide is produced in high quantities from hydrogen sulphide , as well as carbon monoxide and ammonia.
Acidic Gases control in Annealing Furnace and Galvanizing Lines
In mills section, the steel is converted from continuous cast slabs or rolled slabs to a variety of plates, rods or sheets in various dedicated mills. Within the mills section, the galvanizing/tinning mill is often associated with high concentrations of sulphuric acid and hydrochloric acid fumes, which is a major source of air contamination. If the plant has galvanizing/tinning lines, then there may also be a hydrochloric acid re-generation plant. These acid re-generation plants tend to have extremely high concentrations of chlorine gas, chlorine and hydrochloric acid have been measured. Also, in the mills section, use is often made of annealing furnaces. These annealing furnaces tend to have higher sulphur levels around them.
Electronic corrosion at Control / Switchgear rooms and Motor Control Center
Electronic process controls are found throughout iron and steel plants, controlling most of the various processes in the conversion of raw materials into finished iron and steel products. Control rooms, switchgear rooms and motor control center are mostly present in metallurgical plant area which makes them exposed to electronic corrosion due to corrosive and toxic gases emitted in metallurgical area. This makes corrosion control a necessary measure especially in metallurgical area.
Corrosion Control in the Control/MCC Rooms
In case of rooms with very low movement of personnel coming in and out, only pressurization with chemically cleaned air is sufficient. Provide from 3 to 6 air changes per hour, to attain approximately 2.5 to 5 mm WC positive pressure inside the room.
Rooms with high pedestrian traffic, such as, operator control rooms would require air purification by re-circulation mode in addition to air pressurization. This is due to the absorption of contaminants on clothes and body surface of plant personnel while attending to duties at the different process locations, which are generally highly contaminated. Such absorbed gases would immediately de-sorb upon entry to a relatively cleaner area.
