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Carbon control for DRI products

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The level of carbon content in DRI is a key factor in the value it provides in use. DRI plant builder and solutions provider Midrex has developed adjustable carbon technology for use with its plants, which will soon see its first commercial application

When Cleveland-Cliffs starts up its new $830 million, 1.6 million tonnes per year MIDREX Plant® with high-carbon HBI in Toledo, Ohio, USA later this year, they will also start operating the first commercial plant equipped with the adjustable carbon technology developed by Midrex (MIDREX ACT™).

The carbon content needed in DRI products depends on the types of steel that it will be melted to produce, usually in an EAF, and what other steelmaking raw materials (scrap hot metal, pig iron) it is mixed with in the melt. The exact percentage of carbon needed varies with melt-shop, so the ability to tailor the carbon content to the exact needs of each DRI product consumer is valuable.

For example, a steelmaker looking to replace pig iron with HBI in a mixture of steelmaking raw materials also containing a high proportion of scrap will typically need to use a high-carbon DRI product. Steelmakers using less scrap usually need less carbon in complementary DRI products melted into the scrap, and they are very unlikely to need more than 2% carbon if they are using 100% DRI as their feedstock. Too much carbon in the melt can actually waste energy.

MIDREX ACT, which can be incorporated as part of new plants or retrofitted to existing Midrex shaft furnaces, enables production of DRI products with a wider range of carbon content (0.5-4.5%) than previously available.

Midrex notes that a crucial advantage of the technology is its ability to fine-tune carbon content, and particularly higher carbon content, in the DRI without sacrificing product discharge temperature.

How to add carbon

In the shaft furnace, iron oxide pellets and/or lump are fed to the top of the furnace and flow downward. The iron oxide is heated and converted to DRI by a high-temperature reducing gas. The products can be discharged hot or cold in combinations that include cold DRI (CDRI), hot briquetted iron (HBI) or hot DRI (HDRI).

To control carbon content in the products, carbon is added to DRI in three places: the reduction, the transition and the cooling zones.

In the reduction zone, iron is metallized by using carbon monoxide and hydrogen produced in a gas reformer. Some carbon is added in this zone from methane (CH4) and carbon monoxide (CO).

In the transition zone, a controlled flow of natural gas is added, which is the main method of adding and controlling the amount of carbon in Midrex DRI products. The natural gas feedstock contains a variety of hydrocarbons, but mostly methane.

Midrex plants that have a cold discharge furnace use cooling gas, which also contains hydrocarbons that add carbon in a similar manner to those in the transition zone.

While all these steps provide an effective means of adding carbon to the DRI produced, their drawback for plants producing HDRI or HBI is that many of the carbon-adding chemical reactions that they use – notably methane’s reaction with iron to create iron carbide in the transition zone – are endothermic, and therefore absorb heat.

To counteract the consequential undesirable loss of product temperature, ACT boosts it by adding a CO-rich gas stream, made in the gas reformer, to the transition zone. The carbon monoxide makes contact with the DRI and a set of exothermic chemical reactions that ensue provide extra energy to maintain temperature.

By adjusting the amount of carbon monoxide in the transition zone, the plant operator can adjust the amount of energy added to the HDRI. Adjusting the addition of natural gas controls the carbon content of the DRI. Midrex explains that use of these principles by ACT allows independent control of temperature and the amount of carbon added.

How much is needed?

The amount of carbon needed in a given DRI product is determined by the requirements of the downstream steelmaking operations in which it is used on site or, for merchant DRI producers, the different markets that they serve.

ACT enables Midrex plants to produce CDRI, HBI or HDRI with higher carbon content while maintaining a consistent discharge temperature. About 85-90% of the carbon created by the technology is in the form of iron carbide (Fe3C). Adding essential carbon in steelmaking in that form, with the appropriate recycling of off-gases, has the potential to contribute to carbon-neutral steelmaking.

The benefits of the application of the technology at a new or existing Midrex plant vary according to each plant’s purpose. For example, CDRI can be produced with a higher carbon content.

HDRI can be produced with either higher carbon, higher temperature or both. Since HDRI is usually charged straight to an EAF for steelmaking on site, it is an advantage to avoid charging cooler material.

HBI can be produced with higher carbon content without a detrimental loss of temperature at the briquetting machine or with a higher briquetting temperature at a given carbon content. The quality and strength of briquettes produced improves with higher temperatures. HBI products typically contain up to about 1.5% carbon, but the higher temperature DRI production provided by ACT is designed to produce strong briquettes with 3% carbon content.

Merchant producers of CDRI and HBI can tailor their product chemistry and produce a value-added product made to the specifications of their customers. While ACT can work with any type of pellet feed, its nature and quality does influence the amount of carbon that can be added to a given DRI product.

Some Midrex plant operators may choose to run them with ACT when higher carbon levels are needed, or without the new technology when lower carbon levels are needed. Midrex has already received enquiries about retrofitting ACT from existing Midrex plant operators and expects further interest in the technology after it has proved its value in the first commercial application for Cliffs.

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