Many automotive OEMS and others have committed to dramatically reducing greenhouse gas emissions throughout the supply chain. As part of the supply chain, some metalcasters have differentiated themselves, increased market share, and improved their social and environmental profile by transitioning to “green metals.” Momentum for this is growing globally. But what is green metal, and how is it determined?
“Green metal” refers to a metal with lowest total life cycle carbon (Clc) emissions over its useful period. 

The measure of “green” (Clc) includes carbon (Cm) related to the mining of raw materials, carbon (Ct) from transporting the metals, carbon (Cp) from refining and purifying the metals for use, carbon (Cf) from melting and forming the metal, carbon (Cr) from the recovery and recycling of the metal so it can be used again, and last, the carbon (Cd) associated with disposal of the metal when it is no longer usable. The Life Cycle Carbon equation of a metal could be written in simple terms as:

Clc = Cm + Ct + Cp + Cf + Cr +Cd 

The metals industry accounts for about 8% of the global carbon dioxide equivalent (CO2e) emission annually with almost all coming from the iron and steel industry due to coke and limestone used in melting. Electric arc furnaces and oxygen blast furnaces can greatly reduce this impact. Aluminum is the next largest source of metals CO2e, at less than 1% of the industry contribution.
The green metals CO2e intensity could then be written as: 
    
Clc Tons / Metal Produced Tons

Globally, some areas are defining green metals by the CO2e intensity. For example, green aluminum intensity is being projected as less than 2 metric tons of CO2e per ton of metal, which is about half of current CO2e emission estimates. Similar reductions are being looked at for most metals, as well. The carbon emission content is complex to determine, depending on various factors involved to produce the metal; such as (Cm) mining methods and the distance between the mine or source of the metal (Ct) and the foundry, the process to purify the ore (Cp), the fuels/energy used to melt or form the metal (Cf), the ratio of raw metal to recycled content (CR) and the distance the finished metal needs to be shipped to the consumer (Ct). 

The more carbon-based fuels used in the melting process, the higher (Cf) the Scope 1 onsite emissions. The more melting done with electricity, the higher (Cf) the offsite Scope 2 CO2e content. The higher the raw metal content, the higher (Cm, CP, CT) the Scope 3 supplier-based CO2e emissions, and so on. To add more complexity to this determination, the source of electric power has different CO2e impacts depending on the green/renewable content of the power generation. As foundries electrify operations (Cf) CO2e onsite emissions can be eliminated and moved offsite to the power producer, but the total (Cf) CO2e Scope 2 emissions may not go down with traditional coal-based electric generation in the Midwest.  

If a foundry chooses to use low carbon fuels for melting, like hydrogen, (Cf) Scope 1 emissions will be drastically reduced. A caution though—not all hydrogen produced is green; hydrogen produced via hydrolysis can be green if the power source is green. Currently this is the most expensive method to produce large volumes of hydrogen and it is not readily available in most areas. The typical source of hydrogen production is from the steam, methane, reformation process, which is the least expensive and yet can only be considered gray in terms of emissions from the steam production and the breaking of methane into carbon and hydrogen. 

Efforts to reduce CO2e should include eliminating energy that is not needed or is wasted in the process, as this yields the quickest and most cost-effective means to reduce the CO2e content in metals. Increasing the recycled content of the metal feed stock helps tremendously to reduce the (Clc) of the metal being produced. Typical recycling rates for metals are about 50/50 and this varies by metal. The energy saved (Cr) (CO2e saved) by using recycled material ranges from 50% for steel and iron to over 90% for aluminum. The next step in the journey to reduce CO2e  content in the metal comes from efficiency opportunities that include improved lighting, improved motors with variable frequency drives, right-sizing operations to best match production requirements, improved shutdowns when production is down or limited, utilization of waste heat, etc.

The use of burner improvements like pulse burner technologies, oxygenated fuels and recuperative burners can go a long way in initial emission reductions at a reasonable cost with current technology.  
The final big hurdle will come with conversions from fossil fuels to electrification and green hydrogen, as well as new undetermined sources of heat. Electric melting is available today and it is anticipated to be as efficient as fuel-based melting. Converting to electric means a change in a plant’s infrastructure that is not cheap or easy to do. Metal quality improvements and melt loss reductions are expected with a move from burners to heating elements. The heating element energy flux (watts/sq.in.), the element life and the cost of fuel is a limiting factor with this technology today, but it is improving rapidly. The electrification (Cf) Scope 2 CO2e improvements assumes the grid can provide green sources that support the increases in load by installing more solar, wind, hydro and nuclear sources. Renewable fuel sources like landfill gas and biofuels only provide a small improvement over conventional fossil fuels, so this option will not reduce the (Cf) CO2e content very much.

The CO2e content of metals is also impacted by how the metal is used. If the molten metal can be made into the final shape or configuration without additional steps and with minimal waste in the forming process, the (Cf) CO2e will be reduced. Reduced gating and sprues, improving product quality to eliminate rejects, continuous casting runs, and avoidance of repeated heating and cooling of the metal are all additional methods to improve the (Cf) CO2e content of the metal. How high-value, high-heat-content metal can be utilized before it becomes scrap is a question each of us needs to ask. Other smaller sources of (Cf) CO2e in a foundry include fluxing agents, dirty scrap, lost foam, sand binders, releasing agents and material handling devices that can be minimized in some cases.

Decarbonization

The need for green metals is only going to grow, and this will put more strain on reducing CO2e emissions from existing operations while meeting increased product demands. Other methods to improve the total carbon emission intensity include decarbonization by producing and using renewable energy sources, utilization of waste heat from other industries, solar melting, carbon capture that puts carbon back into the ground instead of the air, or the purchase of carbon offsets from a source that can reduce CO2e more economically than a foundry may be able to do. Additional work is needed before these ideas will become mainstream in the foundry industry. As new technologies emerge that are lower emitting than what what foundries use today, these advancements will need to be reviewed, validated, and made practical on an industrial level, which takes time and vision.  

It is hard to determine what is a “green metal” because standards are being developed that are not final and the definition is not the same in many places around the world. Clearly China and other geographies consuming coal and fossil fuels will be able to produce low cost “brown metals,” so whatever is done for green metals will need to be competitive with external sources of metals. Low CO2e or CO2e neutral supply chains are a tremendous opportunity for North American metalcasters in the future.      CS

Click here to view the article in the November/December 2021 Casting Source digital edition.