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.
CREATION OF CONTAMINANT GASES IN A STEEL MILL
Iron and Steel mills can basically be broken down into two process areas ie. the metallurgical plant and the mills.
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).
In the coking plant, coal is converted to coking coal. In the coking process, the coal is subjected to high temperatures in an oxygen depleted atmosphere, and many volatiles are driven off the coal. The pollution in this area is enormous and typical total sulphur concentrations can be defined as average 2 PPM and peaks of 300 PPM. Because 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. Although not definitely known, it is thought that NOx levels can peak at around 5 PPM. Ammonia levels are very high, with levels between 20 to 50 PPM at peaks.
Usually the first stage in an iron and steel plant is the blast furnaces. Coke and iron, in conjunction with a blend of other minerals, are fed into the blast furnace and reduced to molten iron.
In the above iron and steel making areas, high temperatures are used (up to 2000°C in the blast furnace). High concentrations of particulate and gaseous pollution, especially sulphur dioxide are produced from hydrogen sulphide in high temperature, as well as carbon monoxide and ammonia. (Ammonia is normally at the coal-coking area).
Around the furnaces, the most dominant gas found is sulphur dioxide. Hydrogen sulphide generated in the furnaces, is converted into sulphur dioxide because of the high temperatures.
Within this metallurgical section, very high levels of sulphur dioxide are found with average concentrations around 0.2-0.3 PPM range. Peak concentrations go up to 5 PPM.
Sulphur dioxide is universally high throughout the metallurgical plant, and will also be found in areas such as the sinter plant and briquetting plant.
Crude steel is produced via the casting of ingots or continuous casting (concast) in a dedicated plant. The above individual plant sections are typically grouped together inside the main plant. These are all areas of very high fume and gas generation.
The next major plant area on a steelworks is the mills section. Here, the steel is converted from continuous cast slabs or rolled slabs to a variety of plates, rods or sheets in various dedicated mills.
This plant area is not dealing with a lot of molten steel, and the steel will have already been cleaned and purified in the metallurgical section. As a result, sulphur generation in these areas tend to be very low. In the mill section, very little hydrogen sulphide is around 0.04 PPM with peaks going up to around 2 PPM. Virtually no oxides of nitrogen or ammonia are found in these areas, and hydrogen sulphide concentrations tend to be in the region of 0.015 PPM, with peaks of around 0.3 PPM.
Within the mills section, the galvanizing/tinning mill is often associated with high concentrations of sulphuric acid and hydrochloric acid fumes. It is thought that the concentrations of sulphide and chlorine gases will be in the region of 3-10 PPM, depending on how far the electrical/electronics room is from the process.
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, and peak levels of 120 PPM of chlorine and hydrochloric acid have been measured. However, the average level is thought to be in the 5 PPM area.
In the mills section, use is often made of annealing furnaces. Although the general atmosphere in the mills area has peaks of 2 PPM of sulphur dioxide, the annealing furnaces tend to have higher sulphur levels around them.
LOCATION OF ELECTRONIC PROCESS CONTROLS
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.
In the metallurgical plant area, control rooms, distributed process control systems, electrical switchgear rooms and motor control centers (containing Programmable Logic Controllers) are found in the following areas:
Coke ovens; Control and gas analyzer laboratories; Central metering or energy control centers; Briquetting plant; Sinter plant; Arc furnace area; Basic oxygen furnace area; Direct reduction works; Continuous casting plant; Soaking pits; Overhead crane operator’s pulpits; Central laboratories.
In the mills section of the plant, all the various mills will have distributed control drives, which require some form of environmental protection. Additionally, the Hot Strip mill will have process controls, which must be protected.
AIR PURIFICATION METHODS FOR IRON & STEEL PLANTS
For controlling gaseous contaminants infiltrating in the Control/MCC rooms from outside air
Two types of solutions are available for control and elimination of contaminants, depending on the size and frequency of pedestrian traffic of the room. They are as follows:
- 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. By this method, there will be a net outflow of clean air from inside the room to outside atmosphere only, thereby eliminating the leakage of outside contaminated air into the room.
Mostly, all centralized Computer/Control rooms, Rack rooms and MCC locations would require this type of air purification.
In locations having high suspended coal dust levels, use 10 or more air changes per hour to maintain a higher 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.
For such areas, it is advisable to provide re-circulation type air purifiers working independently, in addition to the pressurization equipment. The equipment should be designed to handle 6 to 10 air treatment cycles or air changes per hour, wherein the inside room air is continuously cleaned of contaminants that are being carried in due to movement of personnel. The pressurization equipment here, should provide 3 to 6 air changes per hour, and 10 or more air changes per hour for dusty areas, as already explained above.
The types and configuration of media to be used here is similar to that of refinery and petrochemical industry. However, the media quantities required may vary due to the different concentrations of the contaminant gases.
Other important design requirements
- Less than 50% Relative Humidity with less than 6% change per hour.
- 21-23°C, with less than 1°C change.
Room Construction & Integrity
- Well Sealed Room (not leak-proof – small amount of leakage is desired and is a requirement).
- Provision of airlock entryways, with door/ windows provided with weather-stripping.