Process Description :
In a direct reduction process, lump iron oxide pellets and/or lump iron ore, are reduced (oxygen removed) by a reducing gas, producing direct reduced iron (DRI). If the cooling stage is omitted, the DRI can be immediately briquetted into hot briquetted iron (HBI). The reducing gas can be generated externally to the reduction furnace, or can be generated from hydrocarbons introduced into the reduction zone of the furnace. In the former case, the reducing gas is produced from a mixture of natural gas (usually methane) and recycled gas from the reducing furnace. The mixture is passed through catalyst tubes where it is chemically converted to a gas that is rich in hydrogen and carbon monoxide. Examples of processes that use variations of this general procedure include Midrex and HYL. When the reducing gas is generated from hydrocarbons in the reduction zone of the furnace, it is typically a rotary kiln furnace that uses hydrocarbon fuels (primarily coal, but sometimes oil and natural gas) without prior gasification in the reduction chamber. Examples include the ACCAR and SL/RN processes.


Glossary :
DRI - also known as direct reduced iron, is a virgin iron source that is relatively uniform in composition, and virtually free from tramp elements. It is used increasingly in electric furnace steelmaking to dilute the contaminants present in the scrap used in these processes. It has an associated energy value in the form of combined carbon, which has a tendency to increase furnace efficiency. For captive DRI production facilities, there is the added advantage that the delivery of hot DRI to the furnace can reduce energy consumption 16 to 20%.




Processes

Midrex - The charge is fed in continuously from the top of the furnace, passing uniformly through the preheat, reduction, and cooling zones of the furnace. The reducing gas consists of about 95% combined hydrogen plus carbon monoxide. It is heated to a temperature range of 1400° to 1700°F and is fed in from the bottom of the furnace, below the reducing section. The gas flows countercurrent to the descending solids. At the top of the furnace, the partially spent reducing gas (approximately 70% hydrogen plus carbon monoxide) exists and is recompressed, enriched with natural gas, preheated to 750°F, and transported to the gas reformer. The reformer reforms the mixture back to 95% hydrogen plus carbon monoxide, which is then ready for re-use by the direct reduction furnace. In the cooling zone, the cooling gases flow countercurrent to the DRI. At the top of the cooling zone, the cooling gases exit, are sent to recycling, then return to the bottom of the cooling zone. The cooled direct reduced iron (DRI) is discharged through the bottom of the furnace, after which it is screened for removal of fines, and treated to minimize the danger of spontaneous ignition during extended storage. The reduced fines are briquetted to produce a usable DRI product.

HYL - The HYL process uses reformed natural gas to reduce lump ore and fixed pellets in fixed-bus retorts. Prior to the gas reformer, natural gas is mixed with excess steam (over and above stoichiometric requirements) and is then passed over nickel-based catalysts. The excess steam server to prevent carbon formation and promote catalyst life. After the reformer, the water vapor in the reformed gas is removed by quenching, to achieve a hydrogen-rich reducing gas. The HYL process utilized four reactors in the reducing section. The reduction of the charge occurs in an initial and main reduction stage, while the third stage is used for cooling, carburization, and the final adjustment of metallization. The charge reduction takes place at temperatures above 1800°F, the advantages of this being that the reduction efficiency is raised, and the result is a more stable product with a reduced pyrophoric tendency. The HYL process uses cold reducing natural gas for product cooling as well as carburization. Product cooling occurs at temperatures of around 1020°F, during which time carbon is deposited to form a cementite (Fe2C) shell that retards re-oxidation.

ACCAR - The Allis-Chalmers Controlled Atmosphere Reactor (ACCAR) was designed to produce highly metallized DRI in an intricately ported rotary kiln. The liquid and/or gaseous fuels are injected under the bed, and air above it. The charge (e.g. coal, lump ore, and/or iron oxide pellets) is heated to reduction temperature by counter-flowing hot gas. The carbon and carbon monoxide reducing gases are formed from the hydrocarbons present in the reduction zone, and initiate the reduction. Additional liquid and/or gaseous fuel introduced through the kiln shell ports near the product end of the kiln brings about the final degree of reduction. The product is then discharged into a rotary cooler which is externally spray-cooled. Magnetic separation is used to separate the DRI and coal ash, followed by screening to separate the coarse and fine product.

SL/RN - The charge, preheated to 1800°F by counter-flowing freeboard gases, usually consists of lump ore (or pellets), coal, recycled char, and flux if sulphur need to be removed from the coal. Reduction is brought about by reducing gases generated from hydrocarbons present in the reduction section. In order to raise kiln efficiency, the preheat zone is usually limited to 40 to 50% of total kiln length. To ensure a uniform temperature throughout the reduction zone, and to prevent accredation formation due to overheating in hot zones, all the coal is introduced at the feed end of the kiln. As the charge moves into the reduction zone, reduction begins when it has reached roughly 1650°F. After reduction, the solids are discharged into a sealed rotary cooler, where water is sprayed on the cooler shell to reduce the temperature of the solids to about 200°F in a non-oxidizing atmosphere. The cooled material is then separate into DRI, DRI fines, and non-magnetic by a series of screens and magnetic separators. The fines are briquette to be used later with the DRI.

Equipment:- For the Midrex and HYL processes, the vertical reduction furnaces represent the largest piece of "equipment" utilized in these two processes. For the ACCAR and SL/RN processes, it is the rotary kiln furnaces that are inclined at roughly 3 to 4% of the horizontal.


Midrex
In addition to its main vertical shaft furnace, the Midrex process utilizes the following major pieces of equipment in its production process.
Charge Feed System - the charge feed system introduces the charge into the top of the furnace.

Heat Exchanger - the heat exchanger preheats the gases prior to reforming.

Reformer - the reformer converts the natural and recycled gases into the reducing gas (hydrogen and carbon monoxide)

Cooling Gas Scrubber - the cooling gas scrubber recycles the cooling gases that exit from the DRI cooling zone of the furnace.

Top Gas Scrubber - the top gas scrubber recycles the furnace exhaust gases prior to combustion in the gas reformer.

Ejector Stack - the ejector stack rejects the scrubbed flue gases to the atmosphere.
HYL
The equipment most directly related to the DRI production in the HYL process is contained in the reduction and cooling circuits. Following is a list of major pieces of equipment:
Reduction Circuit:
Reduction Zone - the reduction zone of the vertical reactor primarily removes oxygen from the iron ore with the reducing gases.

Quench Tower - the quench tower removes water from the reactor (top) exhaust gases.

CO2 Scrubber - the CO2 scrubber recycles reducing gas by removing CO2, before mixing with the reducing gas make-up.

Gas Heater - the gas heater heats the reducing gases to between 900-960°C before being fed into the reduction reactor.

Cooling Circuit:
Cooling Zone - the cooling zone of the vertical reactor follows the reduction zone. This zone is used to cool and carburize the metallized DRI. The flow of DRI is controlled by a rotary valve at the exit of the cooling zone.

Quench Tower - the quench tower cools and removes water from the exhaust gas exiting from the cooling zone. The gas exits the cooling zone at 490-540°C and is cooled to 40-45°C before entering the bottom of the cooling zone.

ACCAR
In addition to its main controlled atmosphere horizontal rotary kiln furnace, the ACCAR process utilizes the following major pieces of equipment in its production process:
Solid and Liquid Fuel Feed System - introduces the fuels into the rotary kiln.

Rotary Cooler - the rotary cooler, normally externally spray cooled with water, to cool the DRI before discharging.

Magnetic Separator - separate the DRI from the coal ash.

Exhaust Gas Scrubbers - the exhaust gas scrubbers clean the exhaust gases before rejecting them to the atmosphere.

Ejector Stack - the ejector stack rejects the scrubbed furnace exhaust gases.
SL/RN
In addition to its main rotary horizontal kiln furnace, the SL/RN process utilizes the following major pieces of equipment in its production process:

Solids Feed System - introduces the solid fuels and charge into the feed end of the furnace.

Underbed Air Injection System - allows introduction of process air into the preheating zone of the kiln.

Shell-Mounted Fans - the shell-mounted fans introduce air into the freeboard to aid in maintaining a uniform reduction zone temperature.

Rotary Cooler - the rotary cooler, normally externally spray-cooled with water, cool the DRI before discharge.

Magnetic Separator - separate the final product into DRI, DRI fines, and non-magnetics; the char is separated using gravity separation.

Combustion Technology :

The DRI processes outlined in the Process Equipment section were either natural gas or coal-based. A combination of fuel feed at the charge end of the reducing furnace, and fuel injection at various stages of the process were employed. The combustion processes in these furnaces are highly dependent upon the manner in which the fuels are introduced into the furnaces.

Additional information:
Midrex - the Midrex gas-based process utilizes a vertical shaft furnace that has preheat, reduction, and cooling zones. The reducing gas is ported into the reduction zone through a bustle pipe that is located at the bottom of the reduction zone, while the charge is introduced at the top of the furnace. Combustion occurs as the reducing gases flow countercurrent to the charge. The excess top gases are combusted as fuel for the reformer burners. The hot flue gases from the reformer are passed through heat recuperators to preheat combustion gases for the reformer burners, and also to preheat the process gases before reforming. Usage of the heat recuperators have significantly raised the efficiency of the process.

HYL - The HYL gas-based process uses four vertical reactors. The main combustion processes occur in the initial and main reduction reactors, as well as in the gas reformer. The fresh reducing gas that originates in the reformer is gradually heated by passing it through the cooling reactor, the main reduction reactor, and then finally the initial reduction reactor. Before entering the reduction stages, the reduction gases are indirectly heated by a gas-fired furnace. To further heat up the reducing gases, air is injected at the entrance to the reactor and is used during the combustion of unreformed hydrocarbons. The process tail gases are used as fuel for the reformer and for the gas-heating furnaces.

ACCAR - The ACCAR coal-based process uses a rotary kiln furnace that is inclined at 3-4% of the horizontal. The charge (coal, lump ore, and/or iron oxide pellets) is fed in at the upper end of the kiln, while liquid and/or gaseous fuels are injected into the kiln from beneath the bed. Air is injected into the kiln from above the bed, and is used in the combustion of the coal and liquid and/or gaseous fuels during the reduction process. The coal feed is carefully controlled to ensure total coal combustion as the burden enters the final reduction stages. The additional fuel injected through the ports is also closely monitored in order to maintain an optimal temperature profile and gas composition along the length of the kiln. Combustibles released from the bed are combusted in the kiln freeboard. The final reduction occurs with liquid and/or gaseous fuel introduced through the kiln shell ports near the product end of the kiln as they pass under the solids bed.

SL/RN - The SL/RN coal-based process also uses a rotary kiln inclined at 3-4% of horizontal. All the coal feed is introduced at the upper end of the kiln to ensure a uniform temperature distribution along the length of the reduction zone, and to ensure that combustion takes place at the bed surface. In addition, a uniform reduction zone temperature is maintained by introducing air into the kiln for the combustion of combustibles released from the bed. Auxiliary fuels such as natural gas and oil are burnt at the discharge end for plant start-up.
Energy Consumption Statistics:- Energy usage for DRI production is dominated by natural gas and coal-based processes. Natural gas based processes accounted for 91.8% of total DRI production in 1992.