ENERGY_PERFORMANCE_ASSESSMENT FOR EQUIPMENT AND UTILITY SYSTEMS
(CHAPTER-12:ENERGY PERFORMANCE ASSESSMENT OF STEEL SECTOR)
Introduction
Iron and steel is the largest consumer of energy among all industrial sectors. The sector currently consumes about 10% of total electricity and 27% of coal used by Indian industry. The energy cost contributes to nearly 30 — 35% of this sector’s production cost. In a typical integrated steel plant the coking coal is the predominant source of primary energy. It has long been an established fuel economy policy for integrated steel mills to try their best in utilizing by-product gases derived from iron- and steelmaking, while limiting purchase of fuels and electric power from outside. Some 40% is accounted for by the byproduct gases.
Iron and Steel Making Routes
Currently there are two main routes for the production of steel: Production of primary steel using iron ores and scraps and production of secondary steel using sponge iron and scraps (Figure 12.1). A wide variety of steel products are produced by the industry, ranging from slabs and ingots to thin sheets, which are used in turn by a large number of other manufacturing industries.
Primary steel: Pig iron, the input for primary steel production is produced in a blast furnace, using coke in combination with injected coal, to reduce sintered or pelletized iron ore to pig iron. Limestone is added as a fluxing agent. Coke is produced in coke ovens. Reduction of the iron ore is the largest energy-consuming process in the production of primary steel. Besides iron, the blast furnace also produces blast furnace gas (used for heating purposes), electricity (if top gas pressure recovery turbines are installed) and slags (used as building materials). Further steel is produced by two processes: open hearth furnace (OHF) and basic oxygen furnace (BOF).
While OHF uses more energy, this process can also use more scrap than the BOF process. However, BOF process is rapidly replacing OHF worldwide, because of its greater productivity and lower capital mcosts. In addition, this process needs no net input of energy and can even be a net energy exporter in the form of BOF-gas and steam. The process operates through the injection of oxygen, oxidizing the carbon in the hot metal. Several configurations exist depending on the way the oxygen is injected. The steel quality can be improved further by ladle refining processes used in the steel mill.
Secondary steel: Sponge iron and Scrap are melted and refined, using a strong electric current, AC or DC, in an electric arc furnace (EAF) to produce steel. DRI is produced by reduction of the ores below the melting point and has different properties than pig iron.
Primary Steel Process
Steel production at an integrated steel plant using the primary process involves the following four basic steps i.e,
¢ Production of coke and sinter / pallets from iron fines - Material preparation
¢ Reduction of iron ore in blast furnace-Iron making Figure 12.2 represents the process flow chart
of an integrated steel plant.
¢ Processing of molten iron to produce steel (Steel making)
¢ Steel forming and finishing.
Coke ovens-sinter-BF-BOF route
The most common steel making technology is the BF-BOF route. Coke is used in Blast Furnace (BF) both as a reductant and as a source of thermal energy. It involves reduction of ore to liquid metal in the blast furnace and refining in convertor to form steel. The various stages of the steel plant is described below.
Coke making - coal carbonisation: In the coke-making process, bituminous coal 1s fed (usually after processing operations to control the size and quality of the feed) into a series of ovens, which are sealed and heated at high temperatures in the absence of oxygen. Volatile compounds that are driven off the coal are collected and processed to recover combustible gases and other by-products. The solid carbon remaining in the oven is coke. It is taken to the quench tower, where it is cooled with a water spray or by circulating an inert gas, a process known as dry quenching. The coke is screened and sent to a blast furnace or to storage. A schematic diagram of Coke Oven Battery is given in Figure 12.3.
The coal-to-coke transformation takes place as follows: The heat is transferred from the heated brick walls into the coal charge. From about 375 °C to 475 °C, the coal decomposes to form plastic layers near each wall. At about 475 °C to 600 °C, there is a marked evolution of tar, and aromatic hydrocarbon compounds, followed by resolidification of the plastic mass into semi-coke. At 600 °C to 1100 °C, the coke stabilization phase begins. This is characterized by contraction of coke mass, structural development of coke and final hydrogen evolution. During the plastic stage, the plastic layers move from each wall towards the center of the oven trapping the liberated gas and creating in gas pressure build up which is transferred to the heating wall. Once, the plastic layers have met at the center of the oven, the entire mass has been carbonized. The incandescent coke mass is pushed from the oven and is wet or dry quenched prior to its shipment to the blast furnace.
Sintering
Sintering is a technology for agglomeration of iron ore fines into useful Blast Furnace burden material. This technology was developed for the treatment of the waste fines in the early 20th century. Since then sinter has become the widely accepted and preferred Blast Furnace burden material.
The major advantages of using sinter in Blast Furnaces are :
¢ Use of iron ore fines, coke breeze, metallurgical wastes, lime, dolomite for hot metal production
¢ Better reducibility and other high temperature properties
¢ Increased BF productivity
¢ Improved quality of hot metal
¢ Reduction in coke rate in blast furnaces
A Sinter plant typically comprise the following sub-units as shown in Figure 12.4.
The raw materials used are as follows - Iron ore fines (-10 mm), coke breeze (-3 mm), Lime stone & dolomite fines (-3mm) and other metallurgical wastes. The proportioned raw materials are mixed and moistened in a mixing drum. The mix is loaded on sinter machine through a feeder onto a moving grate (pallet) and then the mix is rolled through segregation plate so that the coarse materials settle at the bottom and fines onto the top.
The top surface of the mix is ignited through stationary burners at 1200 °C. As the pallet moves forward, the air is sucked through wind box situated under the grate. A high temperature combustion zone is created in the charge -bed due to combustion of solid fuel of the mix and regeneration of heat of incandescent sinter and outgoing gases. Due to forward movement of pallet, the sintering process
travels vertically down.
Sinter is produced as a combined result of locally limited melting, grain boundary diffusion and recrystallisation of iron oxides.
On the completion of sintering process, finished sinter cake is crushed and cooled. The cooled sinter
is screened and + 6 mm fraction is despatched to blast furnace and -6 mm is recirculated as return
sinter.
Blast Furnace
The Blast furnace iron making process (Figure 12.5) basically consists of the conversion of iron oxide to iron in liquid form. This requires reductant for reduction of iron oxide and heat for the reduction reaction to take place and melting the products of smelting. The primary source to fulfill both these requirements is carbon (in the form of coke), which shares major portion of cost of hot metal production.
The raw materials passes through numerous chemical and physical reactions while descending to the bottom of the furnace. The iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a series of chemical reactions. These reactions occur as follows:
At the same time the iron oxides undergo these purifying reactions, they also begin to soften, melt and finally trickle as liquid iron through the coke to the bottom of the furnace.
The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows:
C+ O2 = CO2 + Heat
Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows:
CO2+ C=2CO
The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions.
The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows:
CaCO3= CaO + CO2
This reaction requires energy and starts at about 875°C. The CaO formed from this reaction is used to remove sulphur from the iron which is necessary before the hot metal becomes steel. This sulphur removing reaction is:
FeS + CaO + C = CaS + FeO + C
The CaS becomes part of the slag. The slag is also formed from any remaining Silica (SiO2), Alumina (AL2O3), Magnesia (MgO) or Calcia (CaO) that entered with the iron ore, pellets, sinter or coke. The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense.
Another product of the iron making process, in addition to molten iron and slag, is hot dirty gases. These gases exit the top of the blast furnace and proceed through gas cleaning equipment where particulate matter is removed from the gas and the gas is cooled. This gas has a considerable energy value so it is burned as a fuel in the “hot blast stoves” which are used to preheat the air entering the blast furnace to become “hot blast’. Any of the gas not burned in the stoves is sent to the boiler house and is used to generate steam which turns a turbo blower that generates the compressed air known as “cold blast” that comes to the stoves.
In summary, the blast furnace is a counter-current reaction where solids descend and gases ascend. In this reaction there are numerous chemical and physical reactions that produce the desired final product which is hot metal.
Hot metal produced in the blast furnace is sent to Basic Oxygen Furnace (BOF) for steel making or to Pig casting machines for pig iron casting in ladles.
Basic Oxygen Furnace
The basic oxygen process is the most common process for producing steel. The basic oxygen furnace is a pear shaped vessel lined inside with refractory bricks. The vessel lining consists of tar bonded dolomite /magnesia carbon bricks or other refractories. The vessel can be rotated 360° on its axis. The ‘heat’ begins with the addition of scrap into the slightly tilted convertor, hot metal is then added after straightening the convertor. Oxygen is blown into the bath through the water cooled lance. The necessary fluxes are added during blowing. Flux addition is done automatically and precisely through bunkers situated above the convertors. A sample is taken after blowing for 16-18 minutes and temperature is measured using a thermocouple. The steel is tapped by tilting the convertor to the tapping side and alloying elements are added via chutes while metal is being tapped. The convertor is tilted to the charging side in order to remove the floating slag.
During blowing operation, oxygen oxidises iron into iron oxide and carbon into carbon monoxide. The iron oxide immediately transfers the oxygen to the tramp elements. The center of the reaction has temperatures of around 2000-2500 °C. The development of carbon monoxide during refining process promotes agitation within the molten bath. The reaction of the tramp elements with the oxygen and the iron oxide developed in the center of reaction leads to formation of reactive slag. As blowing continues, there is a continuous decrease of carbon, phosphorous, manganese and silicon within the melt. Phosphorous is removed by inducing early slag formation by adding powder lime with oxygen. The refining process is completed when the desired carbon content is attained.
Continuous Casting (CC)
Continuous casting (Figure 12.6) technique accounts for more than 60% of total liquid steel in the world. The main advantages of steel processing through CC route are higher yield, lower energy consumption, elimination of primary mills. During continuous casting, the liquid steel passes from the pouring ladle, with the exclusion of air, via a tundish with an adjustable discharge device into the short, water-cooled copper mould. The shape of the mould defines the shape of the steel. Before casting, the bottom of the mould is sealed with a so-called dummy bar. As soon as the bath reaches its intended steel level, the mould starts to oscillate vertically in order to prevent the strand adhering to its walls. The red-hot strand, solidified at the surface zones, is drawn from the mould, first with the aid of a dummy bar, and later by driving rolls. Because of its liquid core, the strand must be carefully sprayed and cooled down with water. Rolls on all sides must also support it until it has completely solidified. This prevents the still thin rim zone from disintegrating.
Once it has completely solidified, the strand can be divided by mobile cutting torches or shears. Intensive cooling leads to a homogeneous solidification microstructure with favourable technological properties.
The cast material is sold as ingots or slabs to steel manufacturing industries. However, most of the steel is rolled by the steel industry to sheets, plates, tubes, profiles or wire.
Rolling and Finishing
Generally the steel is first treated in a hot rolling mill. The steel is heated and passed through heavy roller sections for reducing the thickness of the steel. Hot rolling produces profiles, sheets, or wire. After hot rolling, the sheets may be reduced in thickness by cold rolling. Finishing is the final production
step, and may include different processes such as annealing, pickling, and surface treatment. A more advanced technology, near-net-shape casting, reduces the need for hot rolling because products are cast closer to their final shape. (Figure 12.7).
Energy Consumption
Iron & Steel industry in India is highly energy intensive. Major energy inputs in the sector are coking coal, non-coking coal, coke & electricity. The specific energy consumption in Indian Steel plants is quite high. It ranges between 6.17 Gcal/ tcs to 8 Gcal/ tcs (tonne of crude steel). There is variation of specific energy consumption in different steel plants. This is mainly because of different processes, quality of coal, types of products produced & energy efficiency measures adopted by the plants. The details of specific energy consumption by the Indian steel plants (Gcal/ tcs) are given in Table 12.1. below:-The specific energy consumption for Indian plants are much above the World level of 4.5 Gcal/ tes.
The major energy consuming process in iron making are coking, sinter making & blast furnace. They consume about 61.3% of the total energy. The slabbing mill, hot strip mill and cold rolling mill together with others account for 36.5% energy consumption. The table 12.2 gives the major portion of energy consumption in iron making.
Secondary Steel Process
In the Electric Arc Furnace (EAF) (Figure — 12. 8) steelmaking process, the coke production, pig iron production, and steel “ production steps are omitted, resulting in much lower energy consumption. To produce EAF steel, scrap is melted and refined, using a strong electric current. Several process variations exist, using either AC or DC currents and fuels can be injected to reduce electricity use. The EAF is equipped with eccentric bottom tapping, ultra high power transformers, oxygen blowing, full foamy slag operation, oxy-fuel burners, and carbon injection.
In a typical heat cycle, commonly referred to as the “tap-to-tap cycle’, the cycle starts with the charging of the furnace with steel scrap. After the furnace is charged and the roof is in place, the operator lowers the electrodes, each of which has its own regulator and mechanical drive. Current is initiated and the electrodes bore through the scrap to form a pool of liquid metal. The scrap helps to protect the furnace lining from the high intensity arc during meltdown. Subsequently, the arc is lengthened by increasing the voltage to maximum power. Most modern furnaces are equipped with water-cooled panels in the upper half of the sidewall, rather than refractories, which allow for longer arcs and higher energy input to the furnace. In the final stage, when there is a nearly complete metal pool, the arc is shortened to reduce radiation heat losses and to avoid refractory damage and hot spots. After meltdown, oxygen is injected to oxidize the carbon in the steel or the charged carbon. The decarburization process is an important source of energy. In addition, the carbon monoxide that evolves helps to flush nitrogen and hydrogen out of the metal. It also foams the slag, which helps to minimize heat loss and shields the arc-thereby reducing damage to refractories.
Energy balance
A typical energy balance (Sankey diagram) for a modern EAF is shown in Figure 12.9 & 12.10. Depending upon the melt shop operation, about 60 to 65% of the total energy is electric, the remainder being chemical energy arising from the oxidation of elements such as carbon, iron, and silicon and the burning of natural gas with oxy-fuel burners. About 53% of the total energy leaves the furnace in the liquid steel, while the remainder is lost to the slag, waste gas, and cooling.
Sankey diagram for EAF
Case Example : Heat Balance of Rotary Hearth Furnace in a Secondary Steel Plant
In a rolling mill of a steel plant, slabs are reheated in a rotary hearth furnace (figure 12.11) for downstream rolling. The trial data is given below. Construct a heat balance.
Trial data
Solved Example:
In a steel plant, daily sponge iron production is 500 tons. The sponge iron is further processed in a steel melting shop for production of ingots. The yield from converting sponge iron into ingots is 88%. The plant has a coal fired captive power station to meet the entire power demand of the steel plant. The base year (2011) and current year (2012) energy consumption data are given below:
i) Calculate the specific energy consumption of the plant in Million kcals / Ton of finished product (Ingot) for the base year as well as for the current year
ii) Reduction in Coal consumption per day in current year compared to base year for the plant
Solution:
i) Specific energy consumption of the plant For Base Year
Specific energy consumption of the plant For Current Year
ii) Reduction in coal consumption
Energy saving in sponge iron plant = (6.42-5.82) x500 =300 million kcals/day
Energy saving in steel melting plant = (3.325-2.88) x 440 = 195.8 million kcal/day
Total energy saving = 300 + 195.8 = 495.8 million kcal/day
Equivalent coal reduction (saving) = 495.8 x 10°/5000 =99.16 Tons per day
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