Process Description :
Process Description:- Blast furnaces are usually tall shaft-type steel vessels, up to ten stories high, internally lined with refractory brick, and superimposed over a crucible-like hearth. The necessary charge to produce molten pig iron consists of iron-bearing materials, coke, and flux. The charge is introduced into the furnace at the top. Blasts of heated air from large blast stoves, and in most cases gaseous, liquid, or powdered fuel, are injected into the furnace through openings (tuyeres) at the bottom of the shaft just above the hearth crucible. As the hot air encounters the coke, the coke is burned along with the injected fuels, producing the necessary heat and reducing gas to remove oxygen from the ore in the reduction process. As the iron melts, it descends and accumulates in the crucible. The molten pig iron and slag are drained from the crucible through different tapping holes. The gas that exits from the top of the furnace goes through a cleaning process. The cleaned hot gas is then used in other operations of the plant, e.g. to pre-heat the blast air, while the collected dust is sent to the sintering plant for recycling back into the blast furnace. Once fired-up, a blast furnace burns continuously until the lining needs replacement (approximately 5-6 years).




Glossary :
Iron-bearing materials :- The iron-bearing materials are usually iron ore, sinter, pellets, mill scale, steelmaking slag, scrap, and other materials.
Coke:- The coke is added to provide the main chemical reagents (carbon and carbon monoxide) for the iron ore reduction.
Flux:- The flux, limestone and/or dolomite, is added to combine with ash in the coke and gangue in the ores, to produce a slag that rises to the top of the pool of molten pig iron that collects in the crucible.


Equipment :
Blast furnaces are large intricate systems that are constructed from a combination of off-the-shelf equipment and custom constructed components. The largest components of the blast furnace proper include the furnace shell, the furnace internal refractory lining, and the crucible-like hearth. Most of the stand-alone equipment such as motors, burners, etc., are associated with the following:

Blast furnace

  • Raw material assembly and transport
  • Hot metal and slag transfer
  • Off-gas cleaning
  • Hot-blast stoves

While all of the equipment used in the above operations may be of interest, the focus here is placed on the equipment and components specific to the blast furnace.
Skip Car (conveyors):- The skip cars, and in some installations conveyors, are used to deliver the blast furnace charge to the top of the furnace.

Bosh:- The bosh is an inverted conical section in which the melting starts.

Receiving Hopper:- The blast furnace charge is loaded into the receiving hopper, which in turn delivers the charge to the rotating distributor. The rotating distributor helps assure a uniform distribution of the charge in the furnace stack.

Hearth:- The hearth is an intricately constructed crucible-like vessel upon which the vertical shaft portion of the furnace sits. All the molten metal and slag collect in the hearth before being drained.

Bells (large and small):- the large and small bells are conically shaped devices that form a gas-tight lock hopper. The hopper prevents gas from escaping from the furnace while it is being charged.

Bustle Pipe:- The bustle encircles the blast furnace and delivers the hot blast air from the hot-blast line to the furnace.

Stack:- The stack is the upper portion of the furnace where the burden is pre-heated.

Injection Lance:- The injection lance is inserted into the blowpipe that leads up to the tuyeres. The supplemental fuel is delivered to the furnace through the injection lance.

Iron and Slag Notches:- The molten metal is removed from the hearth through the iron notches. The metal is placed into transfer ladles, while the slag may be transferred to slag pots, drawn off into dry pits for solidification, or granulated with a stream of water and flushed into a well pit.

Tuyeres:- The hot blast air is delivered to the furnace through water-cooled openings called tuyeres. The tuyeres are located at the top of the hearth.


Combustion Technology:
The pressures on blast furnace operations to improve efficiencies while simultaneously reducing emissions are tremendous, and they continue to mount. The focus of the efforts to achieve these ends has been the combustion processes within the hot-blast stoves and the blast furnace proper.
Combustion of carbon monoxide-rich blast furnace off-gases within the hot-blast stoves is used to produce the hot-blast air. In older blast-stoves, metallic burners are typically used. These burners are located externally to the combustion chamber, and are placed at the bottom of the stoves. In newer blast-stoves, ceramic burners are employed. These burners include a mixing chamber and are installed directly within the combustion chamber.
Combustion within the blast furnace proper is being significantly affected by increased adoption of supplemental fuel injection. Supplemental fuels such as natural gas, pulverized coal, and oil, are typically injected through a lance inserted into the blowpipe leading up to the tuyere. The use of supplemental fuel injection, particularly natural gas, can introduce significant amounts of hydrogen into the furnace, which in turn affects the thermal balance. Due to high velocities, combustion of the hot-blast air and injected fuels only occurs once they are well into the furnace.


Hot blast stoves for blast furnace


Energy Consumption :

By far the largest source of energy for the blast furnace is coal. Total coal consumption in the iron and steel industry in 1994 was 694.41 trillion Btu, of which roughly 96% was used for coke production, while the remainder was used in such operations as electricity generation. For each year from 1990 through 1994, total coke consumption in the industry exceeded production, with the balance assumed purchased from other sources. Of the total coke consumption for each of these years, greater than 99% was by blast furnaces. Four other sources of energy play significant role in blast furnace operation, namely natural gas, blast furnace gas, fuel oil, and coke oven gas. Through the next decade at least, the energy consumption profile will continue to change significantly.



Blast Furnace Operation:

By far the largest source of energy for the blast furnace is coal. Total coal consumption in the iron and steel industry in 1994 was 694.41 trillion Btu, of which roughly 96% was used for coke production, while the remainder was used in such operations as electricity generation. For each year from 1990 through 1994, total coke consumption in the industry exceeded production, with the balance assumed purchased from other sources. Of the total coke consumption for each of these years, greater than 99% was by blast furnaces. Four other sources of energy play significant role in blast furnace operation, namely natural gas, blast furnace gas, fuel oil, and coke oven gas. Through the next decade at least, the energy consumption profile will continue to change significantly.





Glossary:

Electricity Generation - Most of the generated electricity is used to power air compressors and other electrical systems.
Energy Consumption Profile - From 1990 through 1994, the blast furnace energy consumption profile changed as follows:

  • Coke consumption down 12%
  • Blast furnace gas consumption relatively flat
  • Natural Gas consumption is up 52%
  • Fuel oil consumption down 62%
  • Coke oven gas consumption down 56%