What Is A Blast Furnace Charging System?
The system of feeding materials into the blast furnace is one of the most important parts in the furnace operation, which has an impact on the functional efficiency, performance and reliability of the entire production cycle at a metallurgical enterprise.
The blast furnace charging system can be conventionally divided into two parts:
- Bottom loading
or charge-feeding system, which is meant for receiving and accumulating the required amount of raw materials in bunkers, as well as for preparing, dosing and forming of material batches for feeding directly into the charging hopper of the blast furnace charging device;
- Top loading,
which is designed to receive materials already prepared in the “bottom” charging system and feed them directly into the blast furnace according to predetermined technology guidelines. In other words, the “top” loading is responsible for correct distribution of charge materials throughout the blast furnace top.
Fig .1 Blast furnace bin trestle.
Why Is Blast Furnace Charging System Important?
A blast furnace is a continuous shaft furnace. For a modern furnace the period between overhauls is 15 ÷ 20 years, and during this time the furnace charging system should work without shutdowns as well.
The blast furnace performance depends on reliability of the charging system, and the furnace operation efficiency and environmental friendliness depends on the correct formation of charge material batches and proper distribution of the burden throughout the furnace top. An important indicator of the feeding system efficiency is reduction in coke consumption per ton of hot metal. This indicator directly affects the cost of the final product and the carbon footprint.
Therefore, very severe requirements are imposed on burden charge machines, since any interruption in feeding materials leads to switching the furnace to a slow-wind operation mode or to its complete shutdown. That is why the blast furnace charging equipment must have high efficiency, increased operational reliability and provide a possibility for full process automation. In the event of a significant decrease of the burden level in the furnace the machines should ensure a forced mode of feeding materials into the charger and their subsequent loading into the furnace.
How Does a Blast Furnace Charging System Work?
The burden material is first fed to the bin trestle, where separate bins for different types of material are located:
– agglomerate,
– pellets,
– coke,
– additives.
Then batches of charge materials are formed from the bins and fed by a conveyor system either into a skip or onto a conveyor, which in its turn feeds the material into the loading hopper of the charging device. Besides, agglomerate and coke fines are screened in the bunker trestle before being fed into the charging device.
After entering the receiving hopper of the charging device the charge material, passing through a system of shut-off and gas-sealing valves is distributed by means of a chute or cones over the top in accordance with a pre-set program.
Fig. Bin trestle and skip pit.
Types of Blast Furnace Charging Systems
There are two main methods of feeding charge materials into a blast furnace:
– by means of a skip
– by means of conveyors.
In the first case all materials are fed by skips from the skip pit of the stockhouse near the blast furnace, and in the second – by conveyors from the charge bins located at a distance from the blast furnace.
The main factors that determine the use of conveyors for loading large-volume blast furnaces are a significant increase in furnace productivity. Skip hoists cannot provide the required loading rate and the use of well-prepared burden. By conveyor supply of materials equipment maintenance and repair are easier, significant dynamic loads on load-bearing steel structures are eliminated, and a blast furnace and a charging system can be constructed separately.
Conveyor feeding of the burden to the furnace top is more suitable for complex automation of the blast furnace.
The production area around the blast furnace for removing liquid smelting products is bigger and the place for slag granulation plants inreases.
Blast Furnace Bottom-loading Technology
The bottom loading consists of a stockhouse and a skip hoist or conveyor for feeding the burden to the top.
The stockhouse is intended to create reserves of blast furnace charge components, their averaging and continuous supply to a skip or conveyor hoist to the furnace top. The location of the bin trestle and its design must ensure mechanization of material unloading from the bins and its subsequent intake in accordance with a pre-set program. Particular attention is paid to ensuring labor protection and safety standards.
The number of bins depends on the blast furnaces production rate, material consumption rates and material stocks.
The bunker trestle is constructed along or perpendicular (in the case of conveyor feeding of the charge to the top) to the front of blast furnaces with a double-row or a single-row arrangement of bins.
At modern blast furnaces conveyor systems are installed in stockhouses, which have replaced other options, such as charge feeding by means of scale-cars.
Two types of conveyors are used in the stockhouse:
– belt conveyors are the most common type. In this case the charge material is loaded onto the conveyor belt from the side bins of the bin trestle by feeders or screens and transported to the skip pit, where it comes to the skips or onto the main conveyor through weighing hoppers.
– slat-type conveyors are used for transportation of hot sinter, if it is impossible to install central bunkers for hot sinter.
Inclined conveyors are used instead of skip hoists at large-volume blast furnaces to feed charge materials into the receiving hopper of the charging device.
Fig. Charging burden materials into the blast furnace
Blast Furnace Top Charging Technology
The top charging system consists of a charging device with a receiving hopper or hoppers. Material is fed to the receiving hoppers by the bottom charging system with skips or conveyors.
There are 2 main types of charging devices:
- cone-type chargers,
- cone-less top chargers.
Currently chute-type cone-less charging devices are mainly used because bell-type chargers are technologically outdate and used less and less in blast furnace production.
Chute-type cone-less top chargers usually have the following structural elements, the presence of which is caused by technology requirements for material charging in normal BF operation conditions:
– a receiving hopper is used for receiving and direcingt the charge material flow from the skip into the bin of the charging device;
– an upper gas shut-off valve is meant for bunkers’ communicating with the atmosphere during their loading, and locking during the burden unloading;
– a charge hopper is for shutting off and on material charging into the blast furnace;
– weight measurement system is designed for weighing and controlling mass flow when loading the charge from a hopper into a blast furnace, as well as monitoring the presence of charging material in the hopper;
– block of bottom gas-sealing valves and charge gate valves. A charge gate valve is designed to hold the charge in the hopper and regulate its flow from the hopper. The bottom gas-sealing valve is designed to connect the hopper with the blast furnace during unloading and locking during its loading;
– a burden distributor. By rotation and changing the angle of inclination the burden distributor ensures rotation of the distribution chute about the the blast furnace axis and changing the angle of the chute inclination according to a pre-set program;
– a distribution chute is for loading burden materials along the circumference and radially along the blast furnace top.
The main advantage of a chute-type cone-less top charging device over a conical charger is the possibility of directed burden charging over the entire cross-section of the top of any diameter, to any point, including the center. Correct loading of a blast furnace, in its turn, makes it possible to avoid a number of problems, associated with the correct furnace operation, absence of peripheral gas channels and burden hanging, which in its turn affects the blast furnace performance and duration of its campaign, as well as its efficiency and environmental friendliness, due to a decrease in coke consumption per ton of hot metal produced.
Fig. Chute-type cone-less top charging device produced by DHM GROUP
Automated process control of the loading system is designed to effectively control the technological process of loading charge materials and coke into the blast furnace: uniform or specified distribution of materials around the circumference and radius of the furnace, ensuring its normal operation.
Burden distribution control is one of the most important ways to control the blast furnace smelting, as it provides for smooth operation of the furnace with the maximum possible use of chemical and thermal energy of the gas flow. Cone-less charging devices have broad capabilities for controlling charge material distribution on the top. Technological possibilities of the cone-less chargers in formation of a rational distribution of the burden on the top are largely determined by the functional features of the corresponding automated control system. The software makes it possible to expand functionality of the charging system, which ensures prompt and rational distribution of charge materials, aimed at achieving the greatest efficiency of the blast furnace smelting.
To provide reliable and actual information for the process control system a number of devices and systems for collecting process parameters can be installed on the blast furnace top, such as:
1. System of top gas sampling and analysis.
It is implemented through installation of gas sampling devices and an analytical system for blast furnace top gas analysis. Sampling devices are designed to take gas samples and perform temperature measuremente by introducing a gas probe into the working space of the furnace in the upper part of the shaft through a special embrasure, and the gas analysis system allows for prompt and accurate online analysis of the percentage composition of H2, H2O, N2, CO2, CO, O2, Ar, NOx, SO2, CH2, C2H6, C3H8 etc.
2. Gas flow distribution control.
It is implemented through instalation of four thermal probes and a system of peripheral thermocouples. This system is designed to display and record temperature fields above the stock line as well as temperature of the protective plates.
Fig. Installation of thermal probes for gas flow distribution control.
3. Installation of radar and mechanical level gauges (mechanical chain probes) on the blast furnace cone. A radar level gauge is a measuring device that is used to determine the actual burden surface profile on the cross-section of the blast furnace. Scanning the burden surface must be carried out continuously in order to determine the profile of the charged material and to use the capabilities of material distribution by means of the cone-less chargers.
4. Closed circuit television systems and burden surface control by means of an infrared video camera. This system gives the possibility of continuous visual monitoring the nature of gas distribution and technical condition of the chute-type distributor.
There is a number of similar monitoring systems which help manage blast furnace loading parameters to make its operation more efficient, reliable, economical and environment friendly.
Environmental Impacts of Blast Furnace Charging Systems
The main source of CO2 emissions in ferrous metallurgy is blast furnace production and especially the use of coke for reducing iron ore to iron. Coke has two main functions in blast furnace production: it is a reducing agent in the reduction reaction of iron oxides and it is a source of energy. Therefore, a very important issue is reducing consumption of coke per ton hot metal. Modern charging systems (conveyor feeding, instead of scale cars and cone-less chargers instead of cone-type top chargers) can reduce coke consumption to 20 kg/t of HM, which significantly decreases CO2 emissions.
Another important aspect is reduction of dust emissions during transportation, reloading and screening of materials. M HEAVY TECHNOLOGY has extensive experience in creating modern dedusting systems that can effectively capture dust emissions, make workplaces comfortable and safe for the personnel, minimize dust release at a vent stack and return collected dust into production as a raw material.
Fig. An example of a casthouse dedusting and charge mix feed systems with the use of bag filters.
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