Protection of Environment in an Integrated Steel Plant
Protection of Environment in an Integrated Steel Plant
The processes of an integrated steel plant are highly resource intensive, consequently emitting and discharging pollutants and therefore, the cause of environmental concerns. They have a variety of impacts on the environment. The main impacts come from the use of energy and raw materials, which result in emissions such as carbon dioxide (CO2), sulphur oxides (SOx), nitrogen oxides (NOx), dust emissions to air. Water is used throughout the plant for cooling or heat transfer of heat processing equipment. Water is also required for descaling, dust scrubbing, quenching and other processes. Direct contact water gets contaminated during use and when this contaminated water is discharged from the plant, it impacts the environment. Also solid wastes generated during the plant operations, need dumping in the land filling area. These wastes also have its effects on the environment. One more factor which affects the environment is the noise which is produced during the plant operations. In short, thera are four types of pollutants which are affecting the environments (Fig 1). These are (i) air emissions, (ii) liquid effluent discharges, (iii) solid waste dumping, and (iv) generation of noise.
Fig 1 Types of pollutants affecting environment
The steel plant is required to be in compliance with regulations for meeting both generic standards for air, water, noise, waste management as well as specific standards for steel industry. The plant is required to meet the environmental norms for coke ovens, sinter plant, blast furnace, steel melting shop, rolling mills etc. for stack emissions, fugitive emissions and effluent discharges. It is also to comply with the ambient air quality standards. The steel plant is to take measures to improve its environmental performance not only for complying with the statutory environmental norms but also for further improvement on these norms to satisfy the social objectives towards clean and sustainable environment.
The steel plant is to manage the environmental impacts of the plant operation. For example, the plant is to try to reduce the amount of water it uses and make sure that the water that goes back into the environment from the steel plant is clean. The plant is also to ensure that biodiversity and air quality is not threatened by the activities of the plants. Further the plant is to control the noise levels within the limits. Management of the steel plant is to take all the measures to minimize and reduce steel plant’s environmental footprint. The protection of the environment is essential for the long term survival of the plant.
Protection of environment in an integrated steel plant means taking responsible decisions and finding innovative ways that help to mitigate negative impacts and enhance positive impacts on the environment. The following are the factors/techniques which are important for the protection of the environment.
- Resource planning – The major resources used in steel plant are raw materials, energies, water, consumables and utilities, equipment, and personnel. Resources if not planned properly can have adverse impact on the environment. For example if the raw materials quality is not up to the mark, then the process will generate more fines and solid wastes besides emitting higher amount of the pollutants in the air. Similarly if the quality of energy deteriorates then it will have adverse effect on specific energy consumption which in turn means emitting of higher amount of pollutants in the air. Also if the equipments of steel plant are not properly maintained then the processes in the plant may run in unstable mode which will have its effect on the raw material and energy consumption and consequential adverse impact on the environment.
- Resource efficiency – It means using of natural resources (raw materials, energy, water and land) responsibly and efficiently, so that more value is created with less input. Lower resource efficiency results into higher load of pollutants on the environment.
- Energy efficiency – Efficient use of energy has its impact on the environment. Plant processes are to be energy efficient to reduce the specific energy consumption. Higher energy consumption means generation of higher amount of pollutants which are emitted to the air.
- Material efficiency – Material efficiency is another factor which impacts the environment. It makes management of the processes and the recycling of the waste more efficient. Higher material efficiency means that lower amount of input materials are to be used for the production with lower amount of material losses in the yield. Lower use of materials results into lower generation of wastes and lower emissions of the pollutants. Material efficiency has three major components namely (i) the reduction of material inputs and waste, (ii) the efficient use of by-products, and (iii) recycling of waste.
- Process integrated measures – There are several measures which can be incorporated in the steel plant processes for the reduction of pollution from the process. Many of these process integrated measures needs capital investments. Examples are process automation and instrumentation for close monitoring of the processes, dry fog system for dust suppression, use of consumable materials with lower emissions, dry quenching of coke, waste gas recirculation in sinter plant, and top pressure recovery turbine in blast furnace etc. In fact there is a big list of such measures which can be incorporated in the steel plant.
- End of the pipe solutions – There are several end of the pipe techniques which are used in the steel plant for treating pollutants before they are emitted/discharged in the environment. The purpose of these techniques is to reduce their harmful effect of the pollutants on the environment. End of the pipe solutions also need capital investments and usually they also have got substantial cost of operation.
- Environmental management system – Environmental management system or EMS is a set of processes and practices which enables the steel plant to manage the impacts of its organizational activities on the environment and also to increase its operating efficiency. It is a framework which helps the steel plant to achieve its environmental goals through consistent control of its operations. The framework includes environmental programs of the plant in a comprehensive, systematic, planned and documented manner and includes the organizational structure, planning and resources for developing, implementing and maintaining organizational policy for the protection of the environment. It provides a structured approach to planning and implementation of the environment protection measures.
The integrated steel plant is highly intensive in both materials and energy. Important subjects for action in response to environmental concerns are generally related to controlling air emissions and liquid effluent discharge as well as managing of the solid wastes. Air pollution remains an important issue. In the integrated steel plant, sinter plants dominate the overall emissions for most atmospheric pollutants, followed by coke-oven plants. Blast furnaces, steelmaking furnaces, and coke ovens also have considerable relative percentages of dust emissions.
The first step towards air pollution control is dust collection and removal. In case of effective dust removal (especially secondary dedusting), there is reduction of the directly related heavy metal emissions except in the case of those with high vapour pressure such as mercury. Efforts to minimize SOx and NOx emissions have also been made. In addition the emission of organo-halogen compounds such as polychlorinated dibenzo-p-dioxins and -furans (PCDD/F), hexachlorobenzene (HCB) and polychlorinated biphenyls (PCB) together with polycyclic aromatic hydrocarbons (PAH) and monocyclic aromatic hydrocarbons, especially benzene, are of increasingly importance. The so-called diffuse emission from plants and emission from open yard storage is also subject to control.
The greenhouse gas which has become very relevant to the integrated steel plant because of global warming is carbon dioxide (CO2), as it makes up of more than 90 % of all steel plant greenhouse gas emissions. CO2 emissions vary by production route. On an average, around 1.8 tons of carbon dioxide is emitted for every ton of steel produced. CO2 generated by the steel industry results mostly from the chemical interaction of coal and coke (carbon) with iron ore in a blast furnace. This process is called ore reduction and produces hot metal which is then converted to steel. There is no large-scale commercially available substitute for carbon in iron and steelmaking. Technological advancements over the past thirty years have enabled substantial reductions in CO2 emissions from steel plant. These advancements include (i) energy efficiency improvements in the steel plant processes, (ii) improved recycling rates of iron and steel, (iii) increased recycling and utilization of byproducts, and (iv) extensive process automation for precise control. A modern integrated steel plant has production processes which are now very close to their theoretical minimum CO2 intensity per ton of steel output.
End of the pipe equipment and processes for environment protection
The following is a partial list of the end of the pipe pollution control equipment and processes normally used in a steel plant.
- Cyclones or centrifugal collectors – Centrifugal collectors operate by separating dust particles from the gas stream. In a typical cyclone, a centrifugal force is created when the gas stream is spun rapidly. The centrifugal force throws the dust particles toward the wall of the cyclone, which then fall into a hopper located underneath.
- Wet scrubbers – In wet scrubbers a scrubbing liquid which is usually water, comes into contact with a gas stream containing dust particles. Dust removal efficiency increases with greater contact between the gas and liquid streams. In general, all wet scrubbers have three basic operations consisting of (i) the gas-humidification process, which increases the size of fine particles so that they can be collected more easily, (ii) the gas-liquid contact process, which provides for contact between the particle and water droplets, and (iii) the gas-liquid separation process, which enables dust particulates and water droplets to combine into larger particles and settle into a collector.
- Electrostatic precipitators – Electrostatic precipitator is popularly known by its abbreviation ESP. In an ESP, electrostatic forces separate dust particles from exhaust gases using a number of high-voltage, direct-current discharge electrodes, which are placed between grounded electrodes. The contaminated gases flow through a passage formed by these two sets of electrodes, which produces negatively charged particles as they pass through an ionized field created by the electrodes. The charged particles are then collected using a grounded or positively charged electrode, and disposed of by rapping or vibrating the collecting electrodes either continuously or at a predetermined interval.
- Bag houses – Fabric collectors, which are commonly known as bag houses, separate dust particulates from dusty gases through filtration. To capture the particulates, dust-laden gases pass through fabric bags made of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape which act as filters. With a collection rate of more than 99 % for very fine particulates, they are one of the most efficient and cost effective types of dust collectors available. The high efficiency of these collectors is due to the dust cake formed on the surfaces of the bags.
- Solids separation from waste water – This process removes most solids using simple technique of sedimentation. This technique recovers solids as slurry or sludge. However, very fine solids and solids with densities close to the density of water are more difficult to separate from waste water. In these instances additional filtration may be required. For example, certain salts or poly-electrodes may be used to agglomerate and then filter out the particles.
- Oils and grease removal – Frequently, oils can be recovered from water surfaces simply by using skimming devices. However, additional treatment is often required for hydraulic oils and the majority of oils that have degraded to any extent, which have a soluble or emulsified component that is more difficult to remove.
- Treatment of acids and alkalis – These wastes are often eliminated through neutralization under controlled conditions. However, this process frequently produces a precipitate that needs to be treated as a solid residue since it may be toxic. In other instances, gases may be generated which require separate treatment as well.
- BOD units – Biochemical oxygen demand (BOD) is a measure of the amount of oxygen that bacteria will consume while decomposing organic matter under aerobic conditions. Biochemical oxygen demand is determined by incubating a sealed sample of water for five days and measuring the loss of oxygen from the beginning to the end of the test. Samples often must be diluted prior to incubation or the bacteria will deplete all of the oxygen in the bottle before the test is complete. The main focus of wastewater treatment plants is to reduce the BOD in the effluent discharged to natural waters. Wastewater treatment plants are designed to function as bacteria farms, where bacteria are fed with oxygen and organic waste. The excess bacteria grown in the system are removed as sludge, and this ‘solid’ waste is then disposed of on land.
- Treatment of metals or other toxic materials – Many materials such as zinc, silver, cadmium, thallium acids, alkalis, arsenic and selenium cannot be eliminated using biological processes unless the materials are much diluted. Instead, they are usually removed by altering the pH or treated with additional chemicals. In cases where the materials are resistant to these processes, they may require disposal through landfilling or recycling.
- Magnetic separation – Magnetic separation technique is employed for the separation of ferrous scrap from the solid waste.
- Crushing, screening and sorting – This technique is employed for processing of air cooled blast furnace and steel melting slags. By this process air cooled slag is sorted in different size fractions.
- Solid waste processing – Processing of solid waste is normally carried out for removal of refractories and other useful materials from the blast furnace cast house and steel making muck.
A key aspect of steel plant environmental protection is to minimize emissions to the air and the discharges of effluents to the environment. Emission/discharge sources are to be mapped and monitored. Process improvements can then be identified and implemented with the goal of reducing emissions/discharges.
The management of the steel plant is to demonstrate its commitment towards environmental protection by (i) integrating sound environmental management practices in all the plant activities, (ii) conducting of the plant operations in an environmentally responsible manner to comply with applicable statutory and other requirements related to its environmental aspects and strive to go beyond, (iii) adopting of cleaner and energy efficient technologies, (iv) minimizing of waste generation and promoting recovery, recycle and reuse, (v) increasing greenery in and around the plant, (vi) striving for continual improvement of the environmental performance by setting challenging targets, measuring progress, taking corrective action and communicating the environmental information, and (vii) enhancing the environmental awareness amongst the plant employees.