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Reducing the industrial carbon footprint has been at the forefront of much discussion, heat treat industry-specific or otherwise. How can heat treaters dealing with vacuum operations consider sustainability in a carbon-conscious market? 

This Technical Tuesday Sustainability Insight article was written by Bryan Stern, the product development manager at Gasbarre Thermal Processing Systems, for Heat Treat Today's November 2023 Vacuum Heat Treating print edition.

There is a growing understanding that changes in environmental policy and corporate initiatives will have an increasing impact on the landscape of domestic processing and manufacturing industries in the near future. This is of particular interest to the heat treating industry as thermal processing intrinsically consumes large amounts of energy.

Energy has always been a financial reality for heat treaters, but the impact of transitioning environmental reform will reach beyond monthly utility bills. This is because large players in primary heat treat markets will seek to integrate low-carbon service and equipment suppliers into their direct and indirect supply chains to meet decarbonization objectives.

As a result of this impending trajectory, there has been more attention on furnace design and energy sources within the thermal processing industry. One topic that has received a great deal of focus is the potential benefit of vacuum furnaces as a less emissions-intensive approach to heat treating. Although fundamentally based on electrification, it would be difficult to argue that at least some of the interest in vacuum does not stem from a reactionary desire to distance thermal processing from the image of fire-breathing fossil fuel furnaces given the current political environment.

But beyond the undeniably more marketable aesthetic, the legitimate question remains: Does vacuum heat treating provide tangible environmental advantages over combustion-fired atmosphere alternatives?

The soundness of the argument for electrification and vacuum is not as obvious as it might first appear. To start, eliminating on-site combustion does not eliminate CO₂ emissions. Electrical utilities still have emissions factors (reported in CO₂ equivalent emissions per kWh) that must be accounted for as part of Scope 2 supply emissions. Counterintuitively, the national average emissions factor for electric power is 2.2 times that of natural gas to produce an equivalent amount of thermal energy.^1,2 This is primarily due to the inefficiencies associated with generating and transporting electricity versus converting fossil fuels directly to thermal energy on site.

In addition to having higher emissions, electricity is 3.6 times the cost of natural gas for an equivalent amount of energy based on national averages for 2022.^3,4

The cost effectiveness of gas fired atmosphere furnaces historically has been the motivator behind their use, unless the process benefitted in some other way from vacuum processing.

If electricity has a greater carbon footprint and is more expensive per unit of energy than fossil fuels, why is the industry transitioning to electrification and increasingly favoring vacuum processing? The answer lies with several factors both internal and external to the equipment itself.

Within the scope of the equipment, gas fired furnaces are intrinsically inefficient. Burners exhaust hot gas which continuously siphons energy away from the process. Although less significant for direct fired burners, this effect is amplified for indirect burners, which are commonly used. Recuperators and regenerators can dramatically improve efficiencies by recycling exhaust to pre-heat combustion air, but additional energy is always required for burner systems beyond what is needed to heat the work and overcome losses through insulation. Electric furnaces, on the other hand, have no such additional demand, and the energy they consume is more directly applied to the process. Although the type of energy used is more financially and environmentally costly per unit, electric vacuum equipment uses that energy more efficiently.

In addition to the demands from the burner exhaust, gas fired furnaces usually depend on a blanketing atmosphere to protect the work from oxidation. Endothermic gas is commonly used for this purpose, and in addition to the heat input required for endothermic gas generation, CO and CO₂ are products of the reaction. Although it is an objective of endothermic gas generation to minimize the amount of CO₂ present in the furnace, the CO exhausted to the atmosphere eventually reacts to form CO₂, leading to a higher effective emissions rate. The use of a vacuum as a protective atmosphere is less carbon-intensive as it relies primarily on the power required to operate the vacuum pumps. This leads to much lower emissions to create the processing atmosphere.

Looking outside of the equipment at the overall manufacturing process, heat treating in vacuum can often eliminate post processing steps required when using other types of equipment. This may come in the form of less oxidation or scale, meaning less part cleanup, or low distortion gas quenching, allowing final machining to be moved forward in the manufacturing process or removed altogether. These potential production cost savings are not new, but the value of eliminating the emissions associated with additional manufacturing steps will only serve to further incentivize vacuum equipment moving forward.

There is one final dynamic outside the scope of the equipment that contributes to the explanation of the industry’s push toward vacuum. The emissions factors associated with electric power generation are decreasing, a trend which is expected to continue. The contribution of renewable energy to the domestic power grid is projected to more than double in the next seven years.⁵

Although the contribution from renewable sources is still significantly less than fossil fuels, changes in generation are not the only factors at play. Significant efforts are being made to develop grid-scale energy storage solutions. Although most often associated as a prerequisite for intermittent production from renewables, these storage solutions serve an important function for the existing infrastructure. By storing excess power during low demand and releasing it during peak hours, grid scale energy storage would allow fossil fuel power plants to run at more optimized efficiencies without having to ramp up and down to match demand.

Beyond the process efficiencies of vacuum discussed above, investing in electric fired equipment is the only way to capture the benefits of ongoing improvements to electric supply and generation infrastructure. While the benefits of electrification may currently depend on contextual variables such as geographic location and equipment design, natural gas fired processing has a relatively fixed ceiling for future improvement. As an added advantage of electrification, the carbon accounting reductions from the improvement in emissions factors can be captured passively after the initial investment.

While the above advantages of electrification and vacuum do help explain the industry’s push in that direction, it is worth considering how vacuum equipment will continue to evolve to maximize energy efficiency and reduce emissions. Historically, the majority of vacuum furnaces have been single chamber batch style pieces of equipment. This configuration usually requires that loading and unloading occur at, or near, room temperature to avoid oxidation of sensitive materials. In addition to longer floor-to-floor times, this means that the energy required to heat the furnace is thrown away at the end of each cycle.

The competitive demand for low-carbon solutions will drive the use of multi-chamber batch and continuous style furnaces that allow stored energy to be conserved between cycles. This will be especially true as we see more high-volume manufacturing shift away from traditional continuous atmosphere heat treating. In the past, batch vacuum processing has been too restrictive to both part cost and throughput to be competitive. As emissions concerns gain prominence, vacuum furnace configurations that offer higher energy efficiencies and throughput will begin to close that gap.

The processing and energy advantages of electric vacuum furnaces have positioned them well to meet the low-carbon demands of an increasingly emissions-conscious market. It will be exciting to see how the equipment continues to develop to meet those needs in the future.

References
[1] “Data Explorer: CO₂ total output emission rate (lb/MWh),” United States Environmental Protection Agency, last modified September 26, 2023, https://www.epa.gov/egrid/data-explorer.
[2] “Carbon Dioxide Emissions Coefficients,” U.S. Energy Information Association, released September 7, 2023, https:// www.eia.gov/environment/emissions/co2_vol_mass.php.
[3] “Natural Gas Summary,” U.S. Energy Information Association, released September 29, 2023, https://www.eia.gov/ dnav/ng/ng_sum_lsum_a_EPG0_PCS_DMcf_a.htm.
[4] “Electricity Data Browser,” U.S. Energy Information Association, accessed October 3, 2023, https:// www.eia.gov/electricity/data/browser/#/topic/7?agg=0,1&- geo=g0fvvvvvvvvvo&endsec=6&freq=A&start=2001&end=2022&ctype=linechart&ltype=pin&rtype=s&pin=&rse=0&maptype=0.
[5] “Renewables,” International Energy Agency, last modified July 11, 2023, https://www.iea.org/energy-system/renewables.

About the author:

Bryan Stern is the product development manager at Gasbarre Thermal Processing Systems. He has been involved in the development of vacuum furnace systems for the past 7 years and is passionate about technical education and bringing value to the end-user. Bryan holds a B.S. in Mechanical Engineering from Georgia Institute of Technology and a B.A. in Natural Science from Covenant College. In addition to being a member of ASM, ASME, and a former committee member for NFPA, Bryan is a graduate of the MTI YES program and is proud to have been included in Heat Treat Today's 40 Under 40 Class of 2020.

For more information:
Contact Bryan at bstern@gasbarre.com or IHEA.org.

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