Gujarat is the largest sponge iron plain in India. You never knew the possibility and potential that this city has. The production of sponge iron in Gujarat has now surpassed that of Chhattisgarh, making it the most productive state in the country. During the period of four months (April 6 through July 6), Gujarat produced 1.1 million tons, while Chhattisgarh produced 1.084 million tons, placing it marginally behind Gujarat.
According to data provided by the Center for Monitoring Indian Economy, the combined output of sponge iron in these two states accounts for roughly 62 percent of India’s overall production (CMIE).
Essar Steel plans to boost its sponge iron capacity at Hazira from 3.5 t/y to 5 t/y in the next three to four months, which will have a positive impact on Gujarat’s production.
According to SS Bhatnagar, managing director of the Sponge Iron Manufacturers Association (SIMA), the factory at Essar is the largest of its kind. This information was shared with ET. In addition to Essar, there are two additional, much smaller makers of sponge iron in the state.
Coal-based DRI plants are mainly located in mineral-rich states such as Chhattisgarh, Odisha, West Bengal, Jharkhand and Karnataka.
Significant changes in SEC levels can be observed in coal-based DRI production. The SEC for coal-based DRI production using rotary kilns was 4.10 to 5.26 Gcal/t-DRI (average: 4.51 Gcal/t-DRI). SEC levels are plant specific and their variation can be attributed to factors such as iron content of iron ore, solid carbon and volatiles in coal, furnace temperature distribution and operating practices. On-line measurement and control of key operating parameters is essential to optimize the SEC level of a DRI plant.
Plant-level SEC can be reduced in two cases: (i) if the sensible heat in the flue gas is recovered, reused or recycled to meet the plant’s energy needs, and (ii) if dolochar (from a rotary by-product kiln ) ) is recycled at the plant level. In modern coal-based DRI plants, the flue gas is used to generate electricity and the dolochar is used to generate additional steam, thereby increasing the SEC level.
In a 400 t/d DRI plant, heat recovery and coal recovery from off-gas will reduce SEC levels from 5.20 Gcal/t-DRI production to 3.19 Gcal/t-DRI. It can be noted that a more realistic SEC assessment method would be based on the iron content of the mushroom iron. For example, given that the iron content of sponge iron is 89%, the calorific value of coal is 5200 kcal/kg, and the ratio of coal to iron is 0.95, SEC calculates a value of 5.54 Gcal/t iron.
material balance
Input materials for the rotary kiln include iron ore, dolomite and coal. Air is supplied to the combustion of the coal and maintains the set temperature along the entire length of the rotary kiln to sustain the reduction reaction to form iron. The output of the rotary kiln includes sponge iron, solid waste emissions and flue gas.
The resulting exhaust gas is passed through the afterburner and transferred to a waste heat recovery (WHR) system or extracted through a stack where appropriate. Typical mass balances are based on 64% yield.
Production process
Direct reduction of iron can be defined as the process of direct production of metallic iron by reducing solid iron ore at temperatures below the melting point of iron. The reduction of iron ore can be achieved by using carbonaceous materials (eg non-coking coal) or a suitable reducing gas in the form of reformed natural gas. The reducing gas in the DRI process consists mainly of hydrogen and carbon monoxide.
The solid iron produced in this way is called directly reduced iron. DRI looks like a honeycomb structure and looks like a mushroom-like structure under the microscope, so it is also called “sponge iron”. DRI plants in India follow one of these approaches and obtain the reducing agent in the form of hydrogen or carbon monoxide during the reduction of solid coal, coal gasification or natural gas.
DRI production using solid coal Most coal-based DRI production in India has tailored plant specifications using one of the standard DRI processes such as SL/RN, KRUPP-REIN, KRUPP-CODIR, DRC (DAVY Reduction Corporation), ACCAR, DRC or customized such as TDR (TISCO Direct Reduction), SIIL (Sponge Iron India Limited), JINDAL, OSIL (Orissa Sponge Iron Limited) and Popuri Engineering Limited. Solid coal-based DRI production uses horizontal rotary kilns in the production process.
The furnace is equipped with a 150-200 mm refractory lining to protect the furnace body, which slopes 2.5%-3.0% towards the outlet end. Combustion air requirements for the feed are provided by fans along the length of the heating zone. The main raw materials used in the production of sponge iron include iron ore (hematite or Fe2O3), non-coking coal and limestone/dolomite. Iron-rich hematite of 65% or more is the preferred choice for mushroom iron plants. Iron ore is available in lumps and pellets. Most DRI plants in India use smaller capacity rotary kilns (100 t/d or less).
largest sponge iron plant in India
Gujarat has the largest sponge iron plant in India and the production of this product is very energy consuming. Iron ore is reduced to sponge iron
Iron ore and non-coking coal are crushed to the desired size in the crusher. The required proportions of iron ore, coal and dolomite are continuously fed into the furnace from the feed end through a gravimetric feeder. The raw material moves along the length of the oven in a preset rotation. Secondary air is blown into the furnace through an air duct located along the length of the furnace. The initial heating of the furnace refractory is carried out at the outlet end of the furnace using a fuel oil system above the ignition temperature of the coal. The temperature in the different heating zones is measured and controlled using thermocouples mounted on the oven.
Fine coal is injected at the outlet end of the furnace to meet the additional carbon demand of the reaction. As the charge moves along the length of the furnace, it gradually absorbs heat from the hot gases flowing in the opposite direction of the charge. The preheating zone, which is approximately 30% of the oven length, is where moisture and volatiles are removed from the feed mixture. The necessary heat in the preheating zone comes from part of the coal combustion.
The rotary kiln section after the preheating zone is called the “reduction zone”. Here, the oxygen present in the iron ore dissociates and oxidizes, reducing the elemental carbon in the non-coking coal to carbon monoxide, leaving behind metallic iron. The rotation of the furnace and its tilt ensure better mixing and movement of the charge to the outlet end of the furnace at the desired speed.
The temperature in the reduction zone is kept at around 900-1050°C. The higher the temperature, the faster the oxygen is removed from the hematite. reduction of iron ore
Occurring in the solid state, the key factor is the “controlled combustion of coal” to form carbon monoxide (endothermic Boudoir reaction). The residence time of iron ore in the furnace is about 8-10 hours to form metallic iron. The quality of sponge iron is measured by metallization, which is the ratio of metallic iron to total iron in the sponge iron.
Cooling of sponge iron
Discharges of sponge iron and solid waste (including charcoal, spent limestone/dolomite) are diverted to water-cooled rotary coolers. The slope of the rotary cooler is approx. 2.5%–3%. Spray water on the rotary cooler shell to indirectly reduce the oven temperature to around 100-120°C. This helps avoid reoxidation of sponge iron when exposed to the atmosphere, as it is very unstable at high temperatures.
Electromagnetic separation and shielding
The material discharged from the rotary cooler is passed through a conveyor for screening fine and coarse materials. The discharge material with a particle size of less than 3 mm is separated, and the iron sponge and carbon, and other impurities are separated through an electromagnetic separator.
Sponge iron is sieved by size fraction to separate lumps and fine particles.
The waste gas produced by the rotary kiln flows into the feed in a countercurrent direction, exits from the feed end of the rotary kiln, and passes through the gravity settling chamber. It then passes through a post-combustion chamber (PCC) or post-combustion chamber (ABC), where the remaining carbon monoxide in the exhaust is converted to carbon dioxide. The flue gas is passed through a wet scrubber or a combined gas conditioning tower to reduce the flue gas temperature below 150°C. The cooled exhaust gas passes through an electrostatic precipitator (ESP) before being discharged through the stack.
largest sponge iron plant
in the largest sponge iron plant or smallest one, the procedure for iron production through DRI methods are not very different. Gas-based DRI manufacturing processes include:
(i) using gasification or reforming pathways, as appropriate, to generate and purify reducing gases such as hydrogen and carbon monoxide; and
(ii) reduction of iron ore in a vertical shaft furnace.
The vertical shaft furnace is the heart of the gas-based DRI process and consists of a cylindrical, refractory-lined vessel. It uses reducing gas from natural gas, syngas from coal, coke oven gas, or waste gas from the COREX process.
The shaft furnace works on the principle of a counter-current system, where iron ore (charge) is moved downwards by gravity and reduced by the upward-flowing reducing gas.
Most DRI plants in India follow the fixed coal-based route. In light of this, this technical compendium focuses primarily on energy-saving technologies and practices related to the rotary kiln DRI process using solid coal.
A key performance indicator (KPI) is a measurable parameter that indicates the effectiveness or efficiency of a system. KPIs indicate the potential for performance improvement relative to a design or optimal performance value. KPIs for the direct reduced iron (DRI) process include capacity utilization, yield, specific energy consumption (SEC), material balance, and energy balance. This section provides KPIs for the production of direct reduced iron or sponge iron using solid coal in the process.
Utilization of available capacity
The degree to which an industry makes full use of the capacity it has established is a significant factor in determining its overall performance. It is the ratio of total DRI production to the entire capacity that has been installed. The average capacity utilization rate for DRI plants was 62% over the years 2014-2015 and 2019-2020 (ranging from 52.0% to 74.5%).
It is abundantly obvious from the many different types of energy losses that occur in coal-based rotary kilns that there is a significant amount of potential to cut SEC levels and increase energy efficiency. The majority of the heat that is lost occurs as waste heat, which is contained inside the flue gas that is produced by the rotary kiln. Waste heat recovery, often known as WHR, is one of the possible approaches that has been identified to maximize the utilization of thermal energy in direct reduced iron plants (DRI plants). In addition to water recirculation, there are other energy efficiency methods that may be used to rotary kilns and the associated ancillary equipment in order to improve the overall performance of the DRI manufacturing process.
The generation of energy background on the recovery of waste heat
The flue gas that is created during the production of coal-based DRI utilizing rotary kilns exits the kiln at a very high temperature, roughly 950-1025 degrees Celsius, carrying away a substantial quantity of sensible heat in the process. Before the exhaust gas can be delivered to an electrostatic precipitator, it needs to be cooled to around 180 degrees Celsius (ESP).
Exhaust air that is around 120 degrees Fahrenheit and free of dust is released from the top of the chimney. A WHR boiler can be used to recover the high sensible heat from the flue gas, which can then be used to generate high pressure steam for use in power generation rather than chilling the flue gas.
Introduction to the technical aspects
Flue gas volume produced by a rotary kiln with a capacity of 100 tons per day
often falling anywhere between 24,000 and 1,500 Nm3 per hour. The amount of volatile chemicals that make up 25%–28% of the coal and the amount of solid carbon that makes up 48%–50% of the coal determine the volume of the off-gas.
About forty percent of the total heat input comes from the form of heat that can be felt in the exhaust gas. The pulverized coal that is injected and fed into the flue gas is completely combusted thanks to ABC, which also contributes to an increase in the sensible heat contained in the flue gas. In power generation systems that use WHR as the primary fuel source, economic viability can be achieved for cumulative installed capacities of more than 200 tons per day.
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