Greening the Steel: Energy Efficiency Strategies for Sustainable Sponge Iron Manufacturing
- November 29, 2023
- Posted by: BiconAdmin
- Categories: Energy Efficiency, Iron and Steel, Steel Making Technology
Steel is considered as the major tool to showcase the development of the modern economy and it acts as the backbone of any civilization. The contribution of steel sector to the country’s GDP growth is substantial. The overall study of Indian market indicates that a large growth in steel consumption is required to raise the Indian GDP per capita and improve the welfare of its citizens.
The demand rise in the use of steel in the upcoming years requires a modern solution. One of the biggest challenges is to cut down the emission of CO2 in the long run basis. Implementing energy efficient technologies in the existing steel production may lead to a reduction in CO2 emission. When it comes to coal based iron manufacturing plants (Sponge Iron plants) has a significant potential to implement it also these plants are smaller in capacities when compared to larger integrated plants.
Implementing energy efficient technologies has a vital importance in cost reduction as well as reducing the environmental impact of the iron & steel sector. Like solid coal in the rotary kilns are the dominant technology today can be replaced with gas-based technologies, which can substantially reduce the environmental impacts.
Sponge iron, also called as direct reduced iron (DRI), direct reduction of iron can be defined as a process in which metallic iron is directly produced by reduction of iron ore in solid state at temperature below the melting point of iron. The reduction of iron ore can be achieved using either carbon-bearing material, such as non-coking coal, or a suitable reducing gas in the form of reformed natural gas. The reducing gas in DRI process mainly consists of hydrogen and carbon monoxide. The iron thus produced in a solid state is known as direct reduced iron. The DRI resembles a honeycomb structure, which looks spongy in texture when viewed in the microscope and hence is also called ‘sponge iron’.
Iron ore, coal and dolomite of required proportions are fed into kiln continuously from feed end using weight feeders. The row material are moving along with the kiln on its rotation basis. Secondary air is blown into the kiln through air pipes located along the kiln length. Initial heating of the kiln refractory is carried out above the ignition temperature of coal using oil-fired system at the discharge end of the kiln. The temperatures of different heating zones are measured and controlled using thermocouples mounted on the kiln. Fine coal is injected at the discharge end of the kiln to meet additional carbon requirements for the reactions. The preheating zone accounts for about 30% of the kiln length, wherein both moisture and volatile matter present in feed mixture are removed. The heat required in preheating zone is provided by combustion of part of coal.
The section of rotary kiln after preheating zone is called ‘reduction zone’. Here, the oxygen present in the iron ore dissociates and oxidizes, reducing carbon element in non-coking coal to form carbon monoxide, leaving the metallic iron. The residence time for iron ore inside the kiln is about 8–10 hours to form metallic iron. The quality of sponge iron is measured in terms of metallization, which is the ratio of metallic iron to total iron present in sponge iron.
The sponge iron and solid waste discharge are transferred to water-cooled rotary cooler. Water is sprayed on outer shell of rotary cooler to indirectly reduce the temperature of kiln discharge to about 100–120°C. This helps in avoiding re-oxidation of sponge iron on exposure to atmosphere as it is quite unstable at high temperatures.
The discharge material from the rotary cooler is transferred through conveyors for screening of fines and coarse materials. The discharge material of grain size less than 3 mm is separated out and passed through an electro-magnetic separator, wherein sponge iron is separated from char and other impurities. The sponge iron is screened in size fraction to separate lumps and fines.
TYPES OF PLANTS
- Sponge Iron production solid coal based
- Sponge Iron production gas based
KEY PERFORMANCE INDICATORS
Key performance indicators in a plant can notify the effectiveness for performance improvements with respect to the design or best performance values.
The KPIs of a direct reduction of iron (DRI) process include:
- Capacity utilization- Performance of the sponge iron plant largely relies on the overall utilization of the installed capacity.
- Yield- Yield of sponge iron is dependent on factors such as tumbler index, abrasion index, and thermal degradation.
- Specific energy consumption (SEC) – Is defined as the ratio of total energy consumption to the corresponding total production.
- Material balance- The input materials to rotary kiln include iron ore, dolomite, and coal. Air is supplied for combustion of coal and to maintain the set temperature along the entire length of the rotary kiln for sustaining reduction reactions towards formation of iron.
- Energy balance- The energy balance considers total energy as input into the process and reveals useful quantity heat and energy losses as output for a specific time period under steady state conditions.
- Energy performance Assessment of key equipment like:
- Rotary Kiln
- Waste Heat Recovery Boiler (WHRB)
- Cooling Tower
- Air Compressors
ENERGY EFFICIENT TECHNOLOGY
Various energy losses occurs in sponge iron plant during the production process. Waste heat in off-gases formed in rotary kiln forms the major share of heat loss. Waste heat recovery (WHR) has been identified as one of the viable options towards maximizing the utilization of heat energy in a direct reduction of iron (DRI) plant.
1) Waste Heat Recovery in power generation
The off-gases generated during the process leave the kiln at very high temperatures of about 950–1025°C, carrying away a significant quantity of sensible heat. The off-gases need to be cooled down to about 180°C before being transferred to electrostatic precipitator (ESP). The dust-free off-gases are let out from the chimney top at about 120°C. In place of cooling, the high sensible heat in off-gases can be recovered using a WHR boiler to generate high-pressure steam for power generation.
2) Iron ore preheating using Waste Heat Recovery
The plants having capacity less than 200 tpd (Tonnes Per Day) capacity generally let out off-gases without any heat recovery. In such cases, the sensible heat of off-gases can be recovered for pre-heating of iron ore, resulting in lower coal consumption.
3) Coal gasification for partial substitution
The rotary kiln uses solid coal for both thermal energy requirements and reduction reactions for transformation of iron ore into sponge iron. In a rotary kiln, 40%–50% coal of size 8–20 mm is fed along with raw materials at the feed end. About 50%–60% of coal of size 0–8 mm is injected along with low pressure air at the discharge end.
The coal injected at the discharge end can be converted to producer gas in a gasifier which can be supplied to the rotary kiln from discharge end to improve the overall efficiency of the rotary kiln.
4) Waste Heat Recovery based absorption chiller
Direct reduction of iron plants require comfort cooling in control rooms and administrative buildings. The cooling load is met primarily through stand-alone air-conditioning system. One of the potential options is to use a fraction of the sensible heat available in off-gases in a WHR-based system, that is vapour absorption machine (VAM) to replace existing cooling arrangements. The WHR-based VAM system can be installed in all DRI plants irrespective of existing WHR systems such as WHR-based power generation or WHR-based iron ore preheater.
5) Decentralized control for shell air fans
The typical dimensions of a rotary kiln of 100 tpd capacity are 42 metre length and 3 metre diameter. The raw materials comprising iron ore, non-coking coal, and dolomite are mixed in required ratio before feeding into the kiln. All shell air fans in the kiln are controlled centrally by a single VFD. However, finer and regular adjustment of air flow across the kiln is ensured through manual control of mechanical dampers in each fan on daily/shift basis. The use of damper at delivery side increases air flow and affects the power consumption. Precise control of temperatures can be attained with dedicated VFD for each fan, which will ensure air flow with least resistance.
6) Mullite-based kiln lining
The radiation heat loss of the rotary kiln typically accounts for about 5% of the total heat input, which can be reduced with application of low-thermal conductivity material such as mullite-based kiln lining.
7) Switch over to iron ore pellets
The mined iron ore available to DRI plants has generally low-iron content. Further, the processing of lumps results in formation of fines and requires agglomeration to maintain the yield. Iron ore pellets can be used in place of lumps to increase the yield.
8) Moisture Reduction from coal
The coal-based DRI plants in India use either domestic coal or imported coal, depending on landed costs and quality. Generally, the moisture content in coal is in the range of 8%–12%, depending on ambient conditions. The moisture content is reduced to 4%–5% before feeding into the kiln. Open sun drying is practiced to reduce surface moisture. Reduction or minimization of surface moisture present in coal would help in bringing down heat losses due to moisture content in coal and effectively reduces the overall coal consumption.
9) VFDs for air compressors
The VFD is used to minimize electricity consumption during unload in rotary screw compressor. VFD enabled air compressors can deliver variable air flow to maintain set pressure based on end-use points. The energy consumption and air flow of a VFD-based air compressor is directly proportional to motor speed and, hence, results in higher energy savings as compared to fixed-speed compressors with partial loading.
10) Fibre- reinforced plastic blades for cooling tower fans
The metal blades in cooling tower fan can be replaced with lighter fibre-reinforced
Plastic (FRP) blades, which would reduce the power consumption of the cooling tower system. Further, it increases the possibility of de-rating or resizing the motor capacity of cooling tower fan to allow the use of lower-sized motor.
11) Thermostatic controller for cooling tower
The cooling tower is used to reduce the temperature of incoming water based on wet bulb temperature and relative humidity of ambient conditions. Most of the cooling towers are not equipped with automatic control system to regulate the fan operation. Some of the plants use manual controls based on outlet temperature of cooling water. The seasonal variations in ambient temperatures and relative humidity clearly indicate that continuous monitoring of temperatures is required in cooling tower for effective operation.