Hydrogen replaces coke in blast furnace: Challenges

Reduction with hydrogen, produced in the best-case scenario using renewable energies, is a forward-thinking alternative to commonly used fossil fuels. However, a blast furnace cannot operate without coke due to issues with hydrogen gas permeability in the reactor's cohesive zone, as well as gas, slag, and metal drainage in the metallurgical zone. It is not possible to replace all of the coke with hydrogen. A slew of studies [1-6] and tests have been carried out to investigate the effect of adding hydrogen to blast furnace gas; all of them yield similar results. Hydrogen is only useful to a limited extent due to its endothermic behavior. Higher hydrogen contents result in increased energy demand, necessitating the use of more coke to provide adequate energy. As a result, new hydrogen-only process concepts are required. Aside from that, the main text discusses the thermodynamic issues of iron oxide reduction with hydrogen.

The post is divided into two parts. [1] Existing steel production and [2] Challenges for replacing carbon with hydrogen.

A change in process concepts in the iron and steel industry is required within the next few years due to increasing environmental requirements. Currently, crude steel production is primarily based on fossil fuels, with significant emissions of the climate-relevant gas carbon dioxide. Substituting hydrogen for carbon as an energy source and reducing agent is one way to avoid or reduce greenhouse gas emissions. Hydrogen, which is produced using renewable energies, allows for carbon-free reduction while also avoiding the formation of harmful greenhouse gases during the reduction process.

The iron and steel industry accounts for 7% to 9% of direct carbon dioxide emissions from global fossil fuel use. This value demonstrates the significance of rethinking in the iron and steel industry. The only way to reduce emissions during iron ore reduction is to use more hydrogen as a reducing agent.

Existing iron-making by carbon

How are iron and steel manufactured?

Steel is made from iron ore, a naturally occurring compound of iron, oxygen, and other minerals. Steelmaking raw materials are mined and then transformed into steel via two different processes: the blast furnace/basic oxygen furnace route and the electric arc furnace route. Both processes are constantly being improved to meet the challenge of producing low-emission steel.

Iron making in blast furnace

Step 1: Pig iron making

The overall reaction for the production of iron in a blast furnace is as follows:

Fe2O3 [s] + 3C [c] = 2Fe [l] + 3CO [g]

The actual reductant is CO, which reduces Fe2O3 to produce Fe(l) and CO2(g). By reacting with excess carbon, the CO2 is reduced back to CO. As the ore, lime, and coke enter the furnace, any silicate minerals in the ore react with the lime to form slag, a low-melting mixture of calcium silicates that floats on top of the molten iron. The molten iron is then allowed to run out the furnace's bottom, leaving the slag behind. The iron was originally collected in pools called pigs, hence the name pig iron.

The first step in iron metallurgy is usually roasting the ore (heating it in air) to remove water, followed by decomposing carbonates into oxides and converting sulphides into oxides. The oxides are then reduced in a blast furnace 80-100 feet high and 25 feet in diameter, where roasted ore, coke and limestone (impure CaCO3) are continuously introduced into the top. At the bottom, molten iron and slag are withdrawn.

Step 2: Steelmaking

A large portion of the iron produced is refined and converted into steel. Steel is created by removing impurities and adding substances such as manganese, chromium, nickel, tungsten, molybdenum, and vanadium to iron to create alloys with properties that make the material suitable for specific uses. Most steels contain small but discernible amounts of carbon (0.04%-2.5%). However, a large portion of the carbon contained in iron must be removed during the steelmaking process; otherwise, the excess carbon would cause the iron to become brittle.

Sources of carbon dioxide emission: Three stages

The fundamental reaction of blast furnace

Fe2O3 [s] + 3C [c] = 2Fe [l] + 3CO [g]

The actual reductant is CO, which reduces Fe2O3 to produce Fe(l) and CO2(g). By reacting with excess carbon, the CO2 is reduced back to CO.

Decarburization

To achieve the desired carbon content of steel, the pig iron is re-melted and oxygen is blown through in a process known as basic oxygen steelmaking, which takes place in a ladle. A ladle is an open-topped cylindrical container made of heavy steel plates and lined with refractory.

 In this step, the oxygen binds with the unwanted carbon and transports it away in the form of carbon dioxide gas, which is another source of emissions.

Calcination:

 Further carbon dioxide emissions result from the use of limestone, which is melted at high temperatures in a reaction called calcination, which has the following chemical reaction:

CaCO3[s] ---- > CaO [s] + CO2 [g]

Carbon dioxide is an additional source of emissions in this reaction. However, modern industries use calcium oxide (CaO, quicklime) as a replacement for limestone to remove impurities (such as Sulfur or Phosphorus (e.g. apatite or fluorapatite) in the form of slag and keeps emissions of CO2 low. For example, calcium oxide can react to remove silicon oxide impurities:

SiO2 + CaO = CaSiO3.

Hot blast

Further carbon dioxide emissions result from the hot blast, which is used to increase the heat of the blast furnace. If the air in the hot blast is heated by burning fossil fuels, which often is the case, this is an additional source of carbon dioxide emissions.

Challenges for replacing carbon by hydrogen

The iron and steel industry accounts for 7% to 9% of global direct carbon dioxide emissions from the use of fossil fuels. Coke serves as both an energy source and a reducing agent in a blast furnace. As a result, large amounts of carbon monoxide and carbon dioxide escape from the furnace along with the top gas. Reduction with hydrogen, produced in the best-case scenario using renewable energies, is a forward-thinking alternative to commonly used fossil fuels.

There are two issues.

The first issue is the availability of hydrogen

The first is the availability of sufficient hydrogen. Due to a lack of renewable energy capacity, it will be impossible to produce all of the required hydrogen in the coming years. As a result, hybrid technologies will be used to produce hydrogen for the foreseeable future.

The second issue is a fundamental one.

The second issue is the permeability of H2 gas

As previously stated, the blast furnace is the primary process unit for producing pig iron. Due to issues with gas permeability in the reactor's cohesive zone, as well as gas, slag, and metal drainage in the metallurgical zone, a blast furnace cannot operate without coke. It is not possible to substitute hydrogen for all of the coke. A plethora of studies and tests have been conducted to investigate the effect of adding hydrogen to blast furnace gas; all of them produce similar results. Because of its endothermic behavior, hydrogen is only useful to a limited extent. Higher hydrogen contents result in higher energy demand, requiring more coke to provide adequate energy. As a result, new process concepts based solely on hydrogen are required.

Thermodynamics of Iron Oxide Reduction with Hydrogen

In general, the reduction of Fe2O3, known as hematite, does not directly result in metallic iron, Fe. If the reduction temperature is less than 570 °C, the reduction to Fe proceeds stepwise from Fe2O3 to Fe3O4, known as magnetite, and then to Fe. At temperatures below 570 °C, the intermediate oxide, wüstite Fe(1x) O, is not stable. Wüstite must be considered in the reduction process at temperatures higher than 570 °C. In this case, the reduction proceeds from Fe2O3 to Fe(1x) O via Fe3O4 and then to Fe.

The Baur-Glässner diagram, shown below, is useful for describing the thermodynamics of iron oxide reduction. The diagram depicts the stability areas of various iron oxide phases as a function of temperature and gas oxidation degree (GOD). The GOD is defined as the oxidized gas component ratio divided by the sum of oxidized and oxidizable gas components. A gas composition's GOD value is a good predictor of its reduction force; a lower GOD represents a higher reduction force for the gas mixture.

According to the diagram, reduction with hydrogen should always be done at the highest possible temperature because the stability area of iron expands with increasing temperature. This increases theoretical gas utilization as well as the thermodynamic driving force for reduction. The Fe-O-C system exhibits a variety of behaviors. At lower temperatures, iron's stability area expands. In this case, a low reduction temperature is preferable due to improved gas utilization.

Another distinction is that total hydrogen reduction is endothermic, whereas total carbon monoxide reduction is exothermic. This means that for hydrogen reduction, energy must be added to the system to ensure a constant reduction in temperature.

References

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