The steel manufacturing industry is one of the highest carbon emission sources globally, leading to the highest CO2 emissions into the atmosphere. The process from converting iron ore to graded steel includes a blast furnace, followed by a basic oxygen furnace and an electric arc furnace. The highest emissions are generated during coke production, blast furnace, i.e., Energy demand and GHG emissions in the Iron and Steel sector principally result from the large consumption of coal/coke used in conjunction with the blast furnace.
What is Green Steel
Green steel refers to the process of steel manufacturing with reduced GHG emissions into the atmosphere as well as potentially reducing cost and improving steel quality, as compared to conventional steel production. A study indicates that steel demand will keep on rising until the end of the 21st century, so there is a huge motivation to look for an alternative method of steel production that emits low greenhouse gas (GHG) emissions into the atmosphere.
Scrap steel recycling is a positive step toward alleviating emissions. However, based on the available scrap, this route can contribute 44% of the total steel production by the end of 2050, which is not sufficient to meet the growing demands.
Also, the issue with recycled steel is that they are contaminated with copper and tin, which causes surface cracking during the hot rolling process. An integrated steel recycling process with innovative routes can bring down the global warming to a manageable threat.
Blast furnace (BF) and basic oxygen furnace (BOF) contribute to 70% of total GHG emissions into the environment. The process reduces iron into ores, sinter and pellets using carbon-based lowering agents. Fluxes (or steel scrap) are added to the blast furnace to maintain the slag temperature and separate the impurities. The hot metal produced contains sulphur, phosphorous, manganese and silicon. The impurities are heated/reduced in BOF to produce high-quality steel with carbon below 2%.
According to research, hydrogen-based and electricity-based steel production have minimal emissions into the atmosphere. However, this technology is still under investigation, some small-scale development has been done in the past, but large scale development is still under development phase.
Pathways for Green Steel Production – Opportunities and Challenges
Various alternative ways exist to produce low-grade carbon products such as carbon capture and storage (CCS), renewable hydrogen and high utilisation of biomass resources. The use of artificial iron units (AIUs) in iron steel production can reduce significant carbon emissions and high-grade steel production.
To minimize emissions, scrap use must be incorporated into the manufacturing process. The use of bioenergy resources in steel production can be a good option, but that goes through a long list of concerns, such as biomass availability, the capital cost of replacement of existing technology.
An Integrated Iron and Steel Mill (ISM) consists of many complex series of interconnected plants, where emissions come out from many sources (10 or more). Huge amount of CO2 is produced by the reduction reaction reactions occurring in the blast furnace and the combustion reaction in sintering, blast furnace and basic oxygen furnace.
Biomass can be used for steel production in place of coal, but this is discouraged by most industries, mainly because of huge biomass requirement, transportation, and storage requirement. Another alternative is the use of natural gas, which at present accounts for 20% of overall steel production in the world. Natural gas produces GHG emissions, which is feasible for small scale goals. If the end target is to achieve significant scale goals, then natural gas use integrated with carbon capture technology is beneficial.
The absorption process is another method used to separate CO2 from gas streams using chemical solvents. However, this process is very expensive because of the high thermal energy required to break the strong bond between solvents and CO2.
Adsorption is also a process to reduce CO2 where a gas stream is passed through the solid adsorbent (such as zeolites, activated carbon). The bed loaded with reduced pressure, increased temperature, and low voltage electric current is challenging to maintain to also expensive.
Gas separation is also a method to reduce GHG emissions, which works on the development of gas separation membranes (polymers, ceramics, zeolites and metals), depending on the difference in physical and chemical interactions. The reducing efficiency reaches up to 80% CO2 separation. In 2007, a simulation study revealed 97% of CO2 recovery from blast furnace gas. Ongoing research in Australia where researchers are developing new technology for gas separation membrane. The research aims to test a number of separation strategies, investigate the influence of syngas and minor gas components.
Hydrogen-based steel making route is another positive step toward green steel. Two different routes exist, direct hydrogen reduction and hydrogen plasma reduction. Small scale utilisation of hydrogen with up to 70% volume reduction was achieved, but the large-scale application is still under development.
The challenge lies mostly with the hydrogen-based DRI process, it produces 0% carbon which does not fulfil the carbon demand of the downstream process. The second issue is the supply of sufficient hydrogen. According to the study, the electricity cost for hydrogen production, considering the electrolysis to produce the hydrogen, should be less than 0.02 USD/kWh to make the process economically feasible. However, hydrogen storage supply and transportation costs are other scopes that still need to be explored.
As on closing comments, steel production is one of the highest GHG emitting sources globally. If not controlled, the commitment at Paris Climate Summit 2015 to hold global temperature below 2℃ seems lost way before the set target date of 2050.
Promoting green steel production can be majorly significant with the targets. Technologies exist that can reduce GHG emissions, and some of them are under commission at a small scale; however, large scale implementation is yet to get approval from research integrity.
Existing technologies are very expensive, or they do have technical challenges which are economically costly to manage. Hydrogen-based steel production is a technology that looks very promising. Researchers are working on the project to analyse the economic and technical feasibility at a large scale.