In a rapidly changing industrial landscape driven by electrification, carbon accountability, and shifting environmental priorities, the age-old cupola furnace remains a surprisingly modern solution. Its thermal efficiency for converting heat to molten iron is remarkable when applied efficiently. Often overlooked in the face of electric melting’s clean reputation, today’s high-efficiency cupola furnaces are not only highly effective recyclers of dirty, low-grade scrap—they’re also becoming serious contenders in the race to lower carbon emissions per ton of iron produced.

This article explores recent technical comparisons between cupola melting and electric melting, with emphasis on CO₂ emissions, energy efficiency benchmarks, and regional electric grid CO2 footprint variability. It also highlights how leading cupola producers are pushing innovation to make the process cleaner and more cost-effective, offering the foundry industry a stable and environmentally competitive path forward.

These authors believe and envision that induction melting and cupola melting are not in competition with each other––rather that they are alternative means of melting iron, and that both are essential to the future sustainability of the iron foundry industry in the U.S. and globally. It is further believed that cupola melting, with full optimization and new technical methods of operation can meet or exceed the future expectations of environmental sustainability, as compared to induction melting, while remaining economically competitive.

Still a Giant in U.S. Iron Production 

Despite the increase in electric melting, cupolas still account for approximately 32% of all foundry iron production in the U.S.—about 4.7 million tons per year.

The top 10 cupola producers alone melt about 3 million tons annually, showing this is not a fading legacy technology but rather a robust, large-scale method that continues to underpin much of the nation’s casting capacity. While the cast iron pipe shops and high tonnage foundries in particular rely on the low cost scrap advantage, low iron cost, low labor requirements, and steady output of a continuous stream of iron supply, there are a variety of smaller, relatively low melt rate cupolas, including some daily drop operations, that remain extremely competitive due to the benefits of cupola operation. It is worth noting that while cupolas are well suited especially for gray iron melting, current iron demand is shifting away from gray iron, toward ductile iron:

•    Gray iron production 
(~2.4 million tpy)
•    Ductile iron production 
(~2.1 million tpy)

Cupolas can easily process contaminated, dirty, oily, painted or mixed scrap efficiently and reliably. Electric furnaces on the other hand, generally require clean, dense, and homogenous charge materials. Electric furnaces cannot consume high amounts of loose, dirty, low-density, or zinc-coated scrap without suffering refractory lining failures or reduced melting efficiency.

Cupola TPK Efficiency: The Metric That Matters

At the heart of cupola performance lies the TPK efficiency metric—tons per hour of hot metal per 1,000 standard cubic feet of Equivalent Blast Rate (EBR). TPK reflects how well a cupola uses its fuel (primarily coke) by describing the amount of liquid iron produced relative to the amount of air and oxygen delivered through the tuyeres into the combustion zone. It takes into account furnace thermal efficiency, sensible heat losses, Boudouard reactions, slag generation rates, etc.

TPK ranges in U.S. cupola operations vary widely:

•    Low (2.3–2.8): Often unlined shell, or poorly operated cupolas using cold blast, short stack height, or poor charging practices.
•    Medium (2.8–3.3): Refractory-lined furnaces with oxygen injection/enrichment, relatively good hot blast temperature, good effective stack height, and good charging practices.
•    High (3.8 – 5.0+): Fully lined, hot blast/very hot blast, good stock height, advanced tuyere configurations, excellent control systems, and optimized charge preparation and control. TPKs of 5.0+ should be attainable with developing, new combustion technologies.

The best U.S. cupola installations have reached, or in some few cases, slightly exceeded 3.8 TPK, with a handful approaching 4.3 TPK, a level considered state-of-the-art, globally.

In general, the largest annual production cupolas rank the highest in TPK. Therefore, these authors estimate that the U.S. average TPK for all cupola iron produced, is in the range of 3.4–3.7 nationally.

Cupola Versus Electric Melting: A Carbon Footprint Comparison

To evaluate the environmental impact of melting iron, it’s essential to look at Scope 1 CO2 emissions (direct combustion), as well as Scope 2 CO2 emissions (indirect emissions from purchased electricity). For electric melting, Scope 2 CO2 emissions exclusively dominate. For cupola melting, CO2 emissions are predominantly Scope 1 (coke and afterburner gas combustion) and a small amount (perhaps 2%–4%) Scope 2 emissions, for motors, fans and other prime movers in the cupola plant.

Electric melting appears cleaner on paper, especially in areas with high renewable energy on the grid. But regional grid differences mean electric melting’s CO₂ footprint is not universally low. The U.S. EPA’s eGRID power profiler identifies the CO2 emission factor, per megawatt hour (MWhr) of electricity produced, for 27 sub-regions of electricity production in the 50 states and its territories. For the purpose of this study, only 20 of those sub-regions are considered with respect to supply of electricity, based on foundry industry locations. Significant disparities are apparent. For example, electric power in upstate New York emits only 241 lbs.

CO₂/MWhr, while the upper Midwest, including parts of Wisconsin and Michigan, can exceed 1,400 lbs. CO₂/MWhr.

When normalized to iron melted by induction melting (using 572 kWh/ston [short ton] as a representative electric melt nominal energy benchmark), using the national average of CO2 emission rate of 771.5 CO2 lbs./MWhr, this corresponds to the following emission factors for low, medium and high efficiency induction melting operations.

By comparison, cupola emissions—although variable—can rival or outperform electric melting, especially at high efficiency rates (TPK). With modern designs and best practice operation, cupolas fall into the following CO2 emission factor categories:

The CO2 emissions for the cupola above include Scope 1 primary coke and afterburner combustion and Scope 2 electrical motors usage. Scope 3 emissions for CO2 production of coke are not included. If they were, the approximately 500-600 lbs. CO2/ston of coke would only slightly increase total emission factors with no significant impact on the final conclusions of this analysis. Likewise, the Scope 1 CO2 emissions from the calcination of limestone, or metallurgical oxidation reactions within the cupola are not included, and would have an insignificant impact on CO2 emissions that would also have no impact on final conclusions.

Cupolas utilize coke, not just for combustion/melting heat, but also for metallurgical carbon pickup of carbon in the iron, where carbon (from coke) is dissolved and sequestered in the iron (castings). Typical cupola carbon pickup is in the range of 5%–18%, where only 82%–95% of the coke is used in combustion. In this analysis, we assumed a nominal carbon pickup of 12%, with 88% coke combustion (CO2 emissions). This analysis also assumes a 90% FC (fixed carbon) content in coke.

The CO2 emission factors above challenge the perception that electric melting is always cleaner than cupolas and illustrate how cupola optimization and biofuel substitution can significantly narrow or even reverse the carbon advantage typically attributed to electric furnaces.

Two Paths to Decarbonization for the Cupola

Cupola operations have two primary avenues to reduce CO₂ emissions:

1.    Fuel Reduction via TPK Improvement. Boosting TPK through hottest blast air possible, better oxygen use, best cupola thermal efficiency, improved thermal recovery, high-quality refractory linings, optimal tuyere design, longer campaigns, and optimal charge practices (best distribution and compensation weighing) can significantly lower coke (CO2) rates per ton.

2.    Carbon Offsetting via Bio-derived Fuels. Substituting fossil coke with biocoke––carbon-neutral biomass-based fuel––reduces net fossil-based CO2 emissions. Trials in a variety of European foundries show that substitution of fossil coke with up to 30%–50% replacement biocoke is easily feasible. One foundry in Japan recently ran an extended trial with proven success using a 100% biomass derived biocoke fuel. When paired with best hot blast heat recuperation, this can yield emission factors better than electric melting in many if not most subregions on the eGRID.

These strategies can be combined to a powerful effect. Recent case studies show that cupolas operating at medium-to-high TPK with 30% biocoke are capable of achieving CO2 emissions as low as 323 lbs./ston or less––greener than electric melting in most of the U.S.

The Electric Grid Storm: Why Electricity Isn’t Always Greener

While electrification is often promoted as a climate solution, the reality of electric grid emissions is complex. Of course, the electrification of transportation and industrial sectors will create significant future electric demand, but the exponential rise of data-driven power requirements appears to be an exponential boom. As AI data centers and cloud computing explode in demand, so too does the need for electricity. Planned data center growth in the U.S. is predicted to consume enough power by 2028 to melt an equivalent amount of iron on the order of 60–110 times more than the current total foundry iron annual production in the U.S. 
Much of this power will likely come from natural gas turbines, due to their speed and cost of deployment. In other cases, coal/fossil fuel-fired power plants are being ordered to stay open rather than being closed as planned. In addition, nuclear power plants (including 3 Mile Island!) are being made ready for re-opening after having been shuttered.

Renewable energy sources cannot be brought on line fast enough to meet the projected demand. With the exception of any nuclear plants that may be (re)opened, all the increase in power generation will cause CO2 emission factors per MWhr produced to be driven higher. 

When combined with aging power distribution infrastructure that will need to be upgraded to keep the grids reliable, this equates to higher power costs, electric market uncertainty, and reduced grid reliability, in addition to overall higher CO2 emission factors.
The net result? For the foreseeable future, electric melting’s advantage for reliability and reduced CO2 will likely shrink, especially in fossil-fuel-heavy regions. Meanwhile, high-efficiency cupolas with new technologies and biofuel supplements will likely continue to outperform in both emissions and cost stability.

It is also worth noting that the green incentives of past U.S. administrations have been stopped and/or rescinded to a great degree. Some iron foundries were formerly promised many millions of dollars in subsidies, which have been withdrawn.

Following is a graph that shows induction versus cupola melting CO2 emission factors, as a function of the variable CO2 emission factors in the 20 “significant” subregions of the eGRID. Commonly-accepted power levels for medium frequency furnaces (MFF) are used, showing low/medium/high for MFFs. Cupola efficiencies are shown, ranging from 2.8 to the highest level of 5.0 TPK with up to a modest 30% biocoke replacement. Note:  In both cases (MFF/Cupola) secondary electricity for motors, blowers, dust collection, charging systems, etc. are included. Since MFF electricity dominates CO2 emissions, the subregion essentially determines the combined Scope 1+2 emissions. For cupolas, Scope 1 emissions dominate, and the small percentage of Scope 2 emissions (approx. 2%–4%) result in almost “flat lines” for the cupola emissions. In other words, CO2 emissions for MFF are a function of eGRID factors and subregions, while cupola emissions are essentially only a function of cupola TPK/efficiency.

The Cost and Reliability Equation

Electric melting’s dependence on volatile electricity markets—and the regional eGRID’s unpredictable makeup—introduces significant cost and reliability risks:

•    Grid curtailments––particularly during peak loads, can disrupt operations.
•    Increasing electric prices––in some regions are rising faster than inflation.
•    Electric pricing volatility––near-future electricity demand will likely create unpredictable and highly variable pricing.
•    Production inflexibility––many induction shops already know very well that demand pricing fluctuates with the weather, supply, and demand and that electricity demand rates are highest during the day or during extreme weather. These things force foundries to alter schedules and/or limit production to times when the electricity is cheap.

Cupola furnaces, by contrast, offer predictable, continuous throughput. Their resilience to charge variability and independence from electricity price spikes make them an economic and operational anchor for many foundries, especially those targeting mid to high tonnage.

The Steel Scrap Conundrum

As steel mills have dramatically shifted to electrification in the last decade, nudging out blast furnace production, the demand for clean prompt scrap has risen as less “virgin iron ore units” are converted to steel. Clean steel prices have risen in stride, and some mini-mills have even purchased scrap suppliers for scrap security. This has put strain on scrap steel pricing, strain that will increase if electrification for iron melting continues. It is predicted that as pricing for clean steel rises, the pricing for “lower grade” cupola scrap will fall. This will likely benefit those remaining cupola shops with a great cost advantage, but, in the end, the “lower grade” scraps still need to be recycled. In all likelihood, this will promote scrap exporting to other countries, where cupolas still dominate, putting further pressure on global markets with “CO2 regulation-free” regions.

In Europe, these pressures are already being felt. There is great concern that the further export of low quality steel scraps will in fact increase global CO2 emissions by essentially exporting the high CO2 production of low efficiency plants to other countries.
Cupolas play an essential role in the recycling of many scraps that will continue to be generated by developed nations regardless of how much electrification takes place in the foundry industry.

Conclusions: The Cupola’s Role in a Sustainable Casting Future

Cupola melting is not a relic process. It is a high-throughput, flexible, and increasingly clean technology. While low-efficiency cupolas must be phased out or upgraded, the best-in-class cupola installations already rival or beat electric melting in environmental performance. 

More importantly, the cupola is a primary method to recycle dirty, mixed, or contaminated iron scrap at scale, a critical enabler for circular economy goals in casting and beyond.

As cupolas of the future are improved and optimized, the cupola will become not just competitive, but preferred in many foundry operations, especially in eGRID subregions where CO2 emission factors remain high.

Those cupola advancements include:

•    Biocoke/biofuel combustion
•    Increased hot blast temperatures.
•    Improved heat recovery. 
•    Advanced combustion and secondary combustion systems.
•    Improved charge system controls, compensation and automation.
•    Improved thermal efficiencies resulting from improved cupola and refractory design, combined with extended campaigns.

The foundry industry should embrace this future with clarity. Rather than viewing cupolas as obsolete, we should see them for what they are: essential, evolving, and increasingly aligned with sustainability and economic resilience.

In order to keep the U.S. iron foundry industry competitive and viable, we need to embrace both electric and cupola melting processes, where both are optimized to the fullest extent possible. Doing so will ensure the future sustainability of the industry while ensuring national security and maximizing global competitiveness.