Energy Conservation In Coal Based Dri Plant

Energy is lost in terms of waste heat , discharge of char material (waste) etc in the process. By integration of heat utilization and using radiator, the loss of energy may be reduced... - Robbin Pramanick

The process of Coal based DRI plant is thoroughly studied and a few initiatives were taken to reduce energy consumption of the plant. Innovative development and drives have been taken in this area – and that have enriched the desired results. By adopting the ways and means – energy conservation and carbon emission, both were improved substantially. Further scope of development has also been identified in this article.

Opportunities for energy conservation in coal based DRI plant

  • Provision of covered coal storage to minimize moisture content and improve Injectability.
  • Procurement of reliable, good quality coal.
  • Modification of the coal injection system.
  • Improvement of the draft for transmission efficiency.
  • Provision of variable speed drives for the kiln discharge fan.
  • Modification of blowers for kiln off gas to optimize efficiency.
  • Reduction of radiation loss from KILN shell.
  • Preheating of feed by extracting heat from waste flue – emission through chimney.
  • Different improvement in electrical installations.
  • Modifying coal handling, procurement, and injection has the potential of reducing the thermal load From 25 GJ/metric ton of DRI to between 15 and 19 GJ/metric ton of DRI.

The coal consumption ranges from 950 to 1,000 kg/metric ton DRI as charged. This is equivalent to an energy consumption of 21 to 23 GJ/metric ton DRI, using 23 GJ/metric ton of Indian sub-bituminous coal.

Coal consumption at ISCOR is reported to be 800 to 900 kg/metric ton of DRI using 28 GJ/metric ton for South African sub-bituminous coal, equivalent to an energy consumption of 22 to 25 GJ/metric ton DRI.

The installation of a coal washing system, which will reduce the ash content of the coal feed and reduce coal consumption as well. Electrical consumption has been minimized in the plant process through the use of high-voltage motors, proper sizing of motors and compressors, and close monitoring of consumption to ensure identification of efficiency problems.

Energy conservation measures for sponge iron plants using process integration principles

During the operation a tremendous amount of heat is generated in coal based sponge iron plant and significant part of this heat associated with the waste gas, remains unutilized. While utilizing this heat in the process the energy demand of the process may be reduced, which decrease the coal consumption as coal is the only source of energy in this plant. Further, it is seen that existing plants consume 5.57 times more energy than theoretical value. The best design includes preheating of air as well as feed materials to rotary kiln and cooling of kiln outlet using waste gas. It consumes 12.5% less coal in comparison to existing system. This design also satisfies the practical condition of the process.

Cost details for one tone production of sponge iron

The cost of sponge iron production varies with different inputs such as iron ore., coal, dolomite, fuel, maintenance, depreciation, power etc., the cost of these input, taken directly from the plant, for one tone production of sponge iron are drawn in the fig. below. It is clear from this fig. that 31.8% cost is due to the coal, which is a considerable amount.

In fact , the energy demand of the process can be reduced by proper integration of heat within the process, which results less energy to be supplied by coal. Consequently, coal consumption is decreased.

In sponge iron making process, coal is the prime source of energy utilised. About 98% of energy input in the process is coal and 2% electrical energy is consumed in the process.

SPONGE Iron Making Process…

Sponge iron is produced by direct reduction of iron ore carried out in rotary kiln, which involves following chemical reactions:

Equation ( IV ) indicates the heat required for desired rate of reduction of iron oxide to iron. Thus, generated by equation IV is utilized for further reduction for iron ore.

So, energy conservation in the process primarily depends on coal. Energy is lost in terms of waste heat , discharge of char material (waste) etc in the process. By integration of heat utilization and using radiator, the loss of energy may be reduced, as suggested by Mr. Prasad [Refer: A. K. Prasad, R. K. Prasad, S. Khanna, Design modifications for energy conservation of Sponge Iron Plants, J. Thermal Sci. Eng. Appl. 3 (2011)].

Waste heat is utilised for generation of electricity through waste heat recovery boiler. In this process, chimney discharge of waste heat is about 170 degree centigrade. This heat may be further extracted up to 140 degree centrigrade, and can be used for preheating of coal feed to kiln. This process can reduce moisture content in coal, hence reduce coal consumption. Additional benefit in the process is smooth operation of rotary kiln, without jamming. As a result trouble free operation of sponge iron and power plant operation can be achieved.

Coal catalyst / additives has also been tried in some plant to reduce coal consumption in sponge iron making. And about 2.5 % reduction in coal consumption is expected / estimated for this initiative.

Reduction of radiation loss

Reduction of radiation loss from the shell of kiln is investigated and analysed by this author for a period of three years. Heat shield paint (coating) was developed in collaboration with M/S Novata Industries & applied on kiln shell, suitable for application on the shell body. By the application of the coating, the radiation loss was reduced and 2.7% coal saving was observed.

The surface temperature of the kiln shell varies from 300 degree to 400 degree centigrade. It is understood that about 15 to 20% of heat is lost through the shell body by radiation and convection process. For reduction of losses as well as reduction of coal consumption – an attempt was taken and accordingly a special coating was developed (Temshield) in collaboration with M/S Novata Industries, Mumbai and applied a thin coating (300 micron) on 500 TPD and 375 TPD kiln shell. Different trials, modification and studies were done and finally a savings of 2.7% of coal consumption was achieved.

Small white patch: 1st trial of the coating…

Energy conservation initiative was taken in electrical installation as well.

Energy Efficient Technologies in Electrical Systems has a major role in the industry. Various methods and instrumentations have been developed for the purpose. They are:

  1. Maximum demand controllers.
  2. Automatic power factor controllers.
  3. Energy efficient motors
  4. Soft starters with energy saver.
  5. Variable speed drives.
  6. Energy efficient transformers.
  7. Electronic ballast.
  8. Energy efficient lighting controls.
  9. Auto Star delta starter.

Induction motor is used most widely in the world. The reasons are:

  1. Low cost
  2. Simple and rugged construction
  3. Absence of commutator
  4. Good operating characteristic
  5. Good power factor
  6. High efficiency
  7. Good speed regulation
The dotted section: The kiln shell temperature recorded before application of the paint in the highlighted zone…
Coated Kiln Shell
The dotted section:On application of the paint, the surface temperature was observed to have reduced by 40°C as seen on the online thermography screen…

It is a great challenge now-a-days to operate the induction motor efficiently and economically. The following points are considered for these.

There are several methods of starting of squirrel cage induction motor which depends upon the following factors:

  1. The size and design of motor
  2. The kind of application
  3. The location of the motor in the distribution system
  4. The capacity of the power system

In the industry 90% load is consumed by motor. Out of which 95% are induction motor.

Energy efficient motors

Energy-efficient motors are now available in India, which operate with efficiencies that are typically 3 to 4 percentage points higher than standard motors. Energy-efficient motors are designed to operate without loss in efficiency, at loads between 75 to 100 % of rated capacity. This may result in major benefits in varying load applications. The power factor is about the same as that of standard motors or may be higher than that. Furthermore, energy efficient motors have lower operating temperatures and noise levels, greater ability to accelerate at higher-inertia loads, and are less affected by supply voltage fluctuations.

Soft starter

Soft starter provides a reliable and economical solution of motor operation by delivering a controlled release of power to the motor, thereby providing smooth, steeples acceleration and deceleration. Motor life is extended because damage to windings and bearings is reduced.

Advantages of soft start

  1. Less mechanical stress
  2. Improved power factor
  3. Lower maximum demand
  4. Less mechanical maintenance

Variable speed drives

Variable Frequency Drive (VFD) is a power electronics device, which generates variable voltage and frequency output from a fixed (constant) input of voltage and frequency. While the maximum variable output voltage is equal to the input voltage in magnitude, maximum output frequency can be as high as 8 to 10 times of input frequency. Thus, if a VFD is fed from a supply voltage of 415V, 50Hz, then maximum output voltage from the VFD shall be 415V, but maximum output frequency can be 400Hz. VFD is one of the options when we look for the operation of process equipment with variable speed. This can be achieved by varying the speed of the connected motor.

There are numerous applications, which involve blower, fan extruder, lift, spinning machine and paper machine where variable speed is needed and scope of energy consumption is available (throttled operation).

Induction motor…

VFD is used for smooth starting and stopping for energy conservation of the system by controlling speed. It can be represented as follows:
Flow α speed
Head α (speed)²
Power α (speed)³

Motor running with under load condition and scope of energy reduction.

The induction motor with a percentage loading below 50% would operate at lower efficiency in delta mode. This efficiency at low loading can be improved by converting delta connection into star connection. The reported savings due to this conversion varies from around 3% to 10% because the rated output of motor drops to 1/2 of delta configuration without affecting performance and the percent loading increases as compared to delta mode. This option does not require any capital investment and is one of the least cost options available for the energy conservation in induction motors.

Loading of motors have been studied and observed under-loading of motors in some equipment

The observed features are:

  •  Less efficiency
  •  Low power factor
  •  High voltage related iron losses
  •  Capacity optimization by voltage reduction.

Under-loaded motors

  1. Converted Delta connection to star (method of voltage reduction)
  2. Implemented Auto star-delta-star converters

Benefits obtained are:

  •  Reduction in voltage related Iron losses
  •  Reduction in copper losses which means less energy conversion
  •  Operates with improved Power Factor
  •  Improved motor efficiency

Calculation of motor loading

Motor loading = input power drawn by the motor (kW) at existing load
=√3 x KV X I COSф
Where KV=Rated Kilo Voltage
I =Rated Current
COSф= Power Factor

Under-load results in lower efficiency and power factor, and requires higher initial cost of the motor and related control equipment. Under loading is common for several reasons. Original equipment manufacturers tend to use a large safety factor in motors they select. Under loading of motor may also occur from under utilization of equipment.

For motors, which consistently operate at a load below that of 50% of rated capacity, an inexpensive and effective measure might be to operate in star mode. A change from the standard delta operation to star operation involves re-configuring the wiring of the three phases of power input at the terminal box.

Operating in star mode leads to a voltage reduction by a factor of √3. motor is electrically downsized by star mode operation, but performance characteristics as a function of load remain unchanged. Thus, full load operation in star mode gives higher efficiency and power factor than partial load operation in the delta mode. However, motor operation in the star mode is possible only for applications where the torque to speed requirement is lower at reduced load.

As speed of the motor reduces in star mode this option may be avoided in case the motor is connected to a production facility whose output is related to the motor speed. (By Bureau of Energy Efficiency)

A study of squirrel cage induction motor’s efficiency and energy saving

Among the several methods available for reduction of energy consumption of induction motor, we have chosen the improvement of motor efficiency and loading factor by star-delta-star connection as the method doesn’t require any investment. It require minimum down time for implementation.

  star- dealta – star starter..

Besides the technical information collected from different sources, we have investigated and tested an 18.5 kW secondary blower at different loads, both with delta and star connection at our test bench of W/S. We started the blower from a star/delta starter and made it star again, after the blower reached a stable condition. Thus, we improved the system and eliminated the problem of low torque at star connection during starting.

In our investigation, we found the best result at about 45% of loading. RPM in star mode is reduced by (30 rpm ) 1% and energy output is reduced by 1%. But total energy saving is around 14%. Net saving is around 10%.

The graphical representations are shown above for convenience.

We are operating a number of motors in star connection and getting the benefit of energy conservation.

By adopting the above initiative energy consumption of per ton sponge iron making was reduced remarkably. Electrical energy consumption was reduced from 100 kWh/ MT to about 94.4 kWh / MT.

Energy conservation and efficient operation of plant and machinery are the essential requirement in our daily life. It reduces our energy cost and carbon-di-oxide emission as well.

Innovation in this field adds value and improves our life cycle parameters. 

Further R&D in this field is required, particularly use of CBM (Coal Bed Methane) for sponge iron making process with a mix with coal etc. to reduce carbon-di-oxide emission.

The author of this article has taken initiative and practised the above processes in sponge iron industry to reduce energy consumption to a great extent.

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