Types of Energy used in a Steel Plants and Energy Conservation
Types of Energy used in a Steel Plants and Energy Conservation
Energy is needed to do any work. Energy can be in the form of potential energy or it can be in the form kinetic energy. Potential energy is stored energy and for doing the work it is to be released from the storage. Common example of potential energy are fuel energy, chemical energy and pressure energy etc. Kinetic energy is energy due to motion and example of kinetic energy is electric energy which is the movement of electrons. Energy conservation activities are the efforts made towards reduction in energy consumption for doing a work. These efforts can be in the form of reduction of energy wastages, recovery and recycling of waste energy, and/or by improving the energy efficiency of the processes.
Various forms of energy used in an integrated steel plant are described below.
Fuel energy is the potential energy which constitutes the major component of all the form of energies used in the steel plant. Fuel energy is used in the form of primary fuels and the byproduct fuels.
Primary fuels used in steel plant are (i) solid fuels, (ii) liquid fuels, and (iii) gaseous fuels. Byproduct fuels are mainly coke oven gas recovered during coking process of metallurgical coals, blast furnace gas recovered from blast furnace during the production of hot metal, converter gas recovered during the production of steel in the basic oxygen furnace process, and coal tar fuel produced during the cleaning of the raw coke oven gas produced while coking of the metallurgical coals.
Solid fuels used in the steel plant are coal and/or coke. Solid fuels constitute the highest percentage of energy consumed in steel plant. Both metallurgical (coking) coals and non coking coals are used in steel plants. Coking coals are used in coke ovens for the production of metallurgical coke to be used in blast furnace for producing hot metal. Different qualities of non coking coals are used in steel plant at various processes namely (i) pulverized coal injection in blast furnace, (ii) production of direct reduced iron (DRI) in rotary kilns, and (iii) production of steam and power in captive power plant. Non coking coal can also be used (i) in pelletizing process of iron ore, (ii) for calcining of lime stone, and (iii) for sintering of dolomite etc.
Liquid fuels used in the steel plant are mainly heavy fuel oil, light diesel oil, and low sulphur heavy stock (LSHS). Liquid fuels are mainly used as a source of heat in different heating furnaces. Besides diesel oil is used in locomotives and mobile equipments for their operation.
Gaseous fuels used in the steel plant are acetylene gas, liquefied petroleum gas (LPG), and natural gas. Acetylene gas and/or LPG is used for oxy-acetylene/oxy-LPG gas cutting and welding and also in flame cutting machines of continuous casting machines. They are sometimes used for carburization of steel, flame heating, flame gouging, flame hardening, flame cleaning, flame straightening, thermal spraying, spot-heating, brazing, texturing and profile-cutting, and carbon coating. LPG is also used as a source of heat in different furnaces. Natural gas is used for the production of gas based DRI and for auxiliary fuel injection in blast furnace. If available it can also be used as source of heat in various furnaces.
By product fuels in the steel plant are namely (i) coke oven (CO) gas, (ii) blast furnace (BF) gas, (iii) converter gas, (iv) coal tar fuel, and finer fractions of coke (nut coke and coke breeze). Coke oven gas is a rich gas and has a calorific value ranging between 4000 to 4600 kcal/N cum. It is used in the steel plant as a source of heat in various furnaces often mixed with other by product gases. It is also used in some steel plants for auxiliary fuel injection in blast furnace. It is also used in some plants in gas turbo generator in power plant as a combined cycle co- generation with heat recovery steam generation. BF gas is a lean fuel gas with a calorific value in the range of 650 to 900 kcal/N cum. Converter gas typically has a high carbon monoxide content and has a calorific value which varies in the range of 1600 to 2400 kcal/N cum. Converter gas is similar to BF gas except that its nitrogen percentage is lower. Hence both the gases are compatible to each other and usually they are mixed for use as a heat source in the steel plant. Coal tar fuel is usually recovered from the raw coke oven gas at the coke oven and by product plant and normally used for auxiliary fuel injection in the blast furnace. Finer fractions of coke are used in those places where size of the coke is not a specification requirement ( examples are use of coke breeze in sinter plant and use of nut coke in blast furnace along with ore burden to promote centre working of the blast furnace).
Types of fuel energy used in steel plant is shown in Fig 1.
Fig 1 Types of fuel energies used in steel plant
Other form of energy used in steel plants
After fuel energy, electricity constitute the next highest consumption of energy in the integrated steel plant. The electricity may be produced in captive power plant, or may be produced from waste energy or may be purchased from public utility companies. Electricity is used at various voltage levels in the entire plant for the running of the electric motors. The main consumption of electricity is in rolling mills where a large number of motors of different capacities are used for the rolling of steel.
Pressure energy is the other form of energy which is used in the steel plant. Blast furnace top gas pressure is used to generate electric power in top gas pressure recovery turbines (TRT). Pressure of compressed air as well as pressure of hydraulic oils is utilized to effect accurate movements up to 10 m of different objects.
Chemical energy is used during steel making process where the exothermic chemical reactions (oxidizing of impurities in hot metal) are utilized for supplying the necessary heat needed during the making of primary steel.
A large amount of waste energy is generated in the form of high temperature exhaust gases which are produced in various processes of iron and steel plant. This energy goes waste if not recovered. Part of this energy can be economically recovered for producing steam which in turn can generate electricity. This waste energy can also be used for preheating of combustion air as well as gaseous fuel used in the furnaces. Part of this energy is also used for preheating of raw materials and removal of moisture from the raw materials.
An integrated steel plant is a heavy consumer of energy. Hence any effort made in the direction of conservation of energy improves the economy of the plant. Important energy conservation measures are described below.
- Compact layout – In an integrated steel plant, huge amount of materials (raw materials, fuels, intermediate products and finished products) are required to be moved during the production processes. Every movement involve requirement of energy in the form of electricity and/or diesel oil. If the distances involved in the movement get reduced then the energy needed for the movement also gets reduced. Hence a compact lay out of the plant helps in the energy conservation.
- Raw material quality – The quality of the raw materials play a major and important role in the energy conservation efforts. Poor quality raw material increases the specific consumption of raw material per ton of the product. This means higher quantities of raw materials are to be moved/transported. This results into higher consumption of energy needed for their movement. Further with poor quality of raw materials, higher amounts of gangue material/ash are to be melted and converted into slag which then is to be cooled down for disposal. This process consumes a lot of heat energy which can be saved if higher quality of raw materials with lesser amount of gangue/ash are used.
- Recycling of solid wastes – Solid wastes like scrap, sludges, mill scale etc, are already in partially processed stage while getting generated during various production processes. Recycling of these solid waste materials saves energy since it reduces equivalent amount of raw materials usage and the energy needed for the processing of these raw materials.
- Improving process yields – Improvement in the process yields amounts to processing of lesser amount of raw materials for same quantity of the products. This in turn have positive effect on the consumption of energy in the production of the product.
- Line losses – Line losses take place due to the leakages as well as due to condensation. Line losses constitute a substantial amount of energy wastage. Stopping of leakages and keeping the line temperature above certain level to prevent condensation helps in the conservation of energy. In case of electric energy the line losses can be reduced substantially by transmitting the power at higher voltage and then reducing the voltage to the required level near the point of consumption.
- Cooling water – Many of the furnace elements need cooling to keep the equipment in the healthy condition. Most of this cooling is indirect cooling. Also in a number of processes the gases or materials comes in direct contact with water for the purpose of cooling or quenching and in such cases water gets contaminated. Many of such systems are once through systems while in others water is cleaned and re-circulated. Normally hot water is cooled down in cooling towers which utilize the heat of evaporation to cool the water. If chilled water is used for cooling in case of indirect cooling then the amount of water needed for cooling comes down drastically. The lower consumption of water means smaller sized pumps are needed for water pumping and hence the lower requirement of the energy for the pumping of water.
- Idle running of equipment – In many places in an integrated steel plant, the equipment are run idle to avoid production delays. In some cases, it is a safety requirement also. Idle running of equipment utilizes lot of energy which is unproductive. If suitable automation/ instrumentation are provided, this idle running can be prevented and can result into substantial saving in energy consumption. Further many a times equipment are run at a lower capacity levels which may be due to several reasons. Running of equipment below capacity means higher specific consumption of energy. From the view point of energy conservation, it is essential to run always the equipment to full capacity and to totally shut down the equipment if production is not needed.
- Energy conservation technologies – A large number of energy conservations technologies/energy efficient technologies have been developed for different production processes of the integrated steel plant. There is a long list of these technologies. Many of these technologies have a very low pay back periods. It is always advisable for the steel plants to find capital to invest in these technologies since the investments are paid back soon and makes the plant economically healthier for a very long time.
- Monitoring of energy consumption – Continuous monitoring of the energy at various processes has a positive effect on the energy conservation efforts. This monitoring indicates where the process is going off the normal and timely corrective action to bring the process back to normal has a very positive effect towards the energy conservation. These days supervisory control and data acquisition (SCADA) system is used for the energy monitoring. SCADA system gathers all plant site energy information and manages the load dispatch. Energy information to SCADA system is provided by remote PLCs and field instruments. Energy management optimization tools are used for high performance energy process data management and for the recording of time series – both historic and forecasts- of measured and calculated data. For this real time process data is collected from various data acquisition systems through interfaces and stored in the database as time histories.