Download PDF Summer Bridge on Engineering the Energy Transition June 26, 2023 Volume 53 Issue 2 This issue explores the energy transition needed to address the mounting threats of climate change. The articles are an excellent resource to help inform meaningful decisions and steps for energy-related contributions to reduce carbon emissions. Decarbonization of Chemical Process Industries via Electrification Wednesday, June 7, 2023 Author: Rakesh Agrawal and Jeffrey J. Siirola Numerous existing and emerging technologies can help chemical process and petroleum refining industries decarbonize their operations. In 2018[1] the US manufacturing sector used 19.4 percent of the country’s primary energy and emitted 17.5 percent of its total greenhouse gases (GHGs). The top two US energy-use processing industries—chemicals and petroleum refining—use nearly half of the manufacturing sector’s primary energy and emit half of its GHGs. The majority of the process energy use in these sectors is for process heating (chemicals ~60 percent, refining ~90 percent). Electrical energy constitutes only 22 percent of total US chemical process energy use, and nearly three-fourths of that percentage is for machine drives, process cooling, and refrigeration. Historically, most electricity has been generated from the combustion of fossil fuels, and for a unit quantity of energy its cost as electricity is nearly three times that of heat from combustion. As a result, almost all process heating needs are met either by the direct combustion of fossil fuels or through the use of steam raised directly or indirectly through the combustion of such fuels. With emerging solar and wind, energy is directly harvested as electricity, making it important to examine the impact of replacing combustion heat with electrical energy. We discuss challenges and opportunities associated with the potential use of electricity in the US chemical manufacturing and petroleum refining industries.[2] Electrification of the US Chemical Manufacturing and Petroleum Refining Sectors According to the US Energy Information Administration (EIA 2022a), in 2022 total US electricity genera-tion was 4,243 terawatt-hours (TWh), of which 580 TWh were from wind and solar using photo-voltaics (PV). EIA estimates that by 2050 total US electricity generation from wind and solar will exceed 1,900 TWh—more than the total anticipated increase in electricity generation of 1,157 TWh. The generation of zero carbon electricity from sources such as hydroelectric and nuclear is expected to remain virtually unchanged. Examining the electrification of the US chemical manufacturing sector against this evolving electricity generation landscape, we see two macrolevel barriers. The total process energy used by the two sectors in 2018 was 1,784 TWh. If we assume the process efficiencies to be similar if heat were provided by electricity, then the electricity demand in 2018 for these two sectors would have been 42 percent of the total US electricity generated that year and close to the anticipated solar and wind generation in 2050. The first challenge is the strain this demand would place on electric power infrastructure (i.e., production, transmission, and distribution). The second stems from the fact that most large-scale chemical manufacturing and petroleum refining plants operate around the clock, whereas wind and solar electricity are variable in nature and their average available time ranges from 20 percent to 40 percent of a 24-hour period. The variable renewable electricity (VRE) of wind and solar presents another challenge: the need for either massive energy storage or process plant redesign to enable load following. The latter would mean increasing plant size nearly threefold to meet the average production rate and add greatly to product cost. There would also be nontrivial design and operational challenges for such load-following processes. The variable renewable electricity of wind and solar requires either massive energy storage or process plant redesign to enable load following. But the changing energy landscape provides many opportunities for innovation. For example, units such as reactors and separators need to be redesigned with increased process intensification for improved productivity and energy-efficient operation. Chemical synthesis needs to be reinvented using electrochemical reactions under much milder processing conditions of temperature and pressure. And the potential of large-scale hydrogen generation through the electrolysis of water using VRE (so-called “green hydrogen”) could act as an energy carrier, a reductant in various chemical reactions and an enabler for the use of byproduct CO2. Process Heating Through Electricity The process industries evolved with the exploitation of fossil resources to meet their energy needs (Agrawal 2019). Given economic reasons and the ease of using a central boiler to generate steam to supply heat at multiple locations in a plant, fuel combustion became the norm. Current Status If all the steam and combustion heat used by the chemical manufacturing and petroleum refining sectors in 2018 were provided with resistive electrical heating, then, assuming similar efficiencies, this would translate to 1,512 TWh of electricity—about one-third of total US electricity generation in that year. If, on the other hand, all this additional electricity were produced by natural gas power plants, then, given their efficiency at 50–55 percent, the net amount of fossil combustion would nearly double, leading to an equivalent increase in GHG emissions. It is therefore essential to investigate process synergies based on electricity use, and to envision new unit operation designs to increase process efficiencies and greatly reduce electric power demand for heating. One method to reduce the environmental impact of electricity use for heat is VRE. But although it would reduce GHG emissions, the intermittent availability of VRE presents a huge challenge because of the requirement for thousands of TWh of electricity storage. Heat and mass flow between various units in a chemical plant are interconnected, so the entire plant must be considered rather than an isolated process unit for electrification. For example, if VRE from solar or wind is on average available for 30 percent of a day, then at least 70 percent of daily energy (1,058 TWh of electricity) needs to be stored for around-the-clock operation of chemical plants. Battery storage, based on 100 kWh of the battery pack in a Tesla Model S electric car, would require the battery capacity of 10.6 billion Tesla Model S cars! (For reference, the total number of Tesla cars sold in 2022 was 1.3 million; Goldman 2023.) The actual amount that would need to be stored is likely one to two orders of magnitude greater because of daily and seasonal weather variations. Clearly, the use of VRE for process heating purposes would require prodigious innovations in electricity storage technologies. Our Envisioned Options Our vision for using electricity to provide process heat for the chemical manufacturing and petroleum refining industries is shown in figure 1. Because of the diversity of applications (far right), the temperature levels at which heat is needed span a wide range, from low (below 100°C) to medium (100–400°C) and high (above 400°C) (Lechtenböhmer et al. 2016). Most applications (e.g., distillation, melting, drying) involve low to medium temperatures, while certain endothermic reactors (e.g., ethane crackers and steam methane reformers) require heat at temperatures greater than 800°C. As shown in figure 1, electricity offers multiple options for providing heat. For example, conventional resistive heating can be used to generate steam or warm hot oil or gas streams as heating media. These -methods mostly use existing plant technology and should be relatively easy to implement, although, as noted above, electricity generated using natural gas combustion would nearly double GHG emissions. On the other hand, innovative methods and equipment design may be pursued to use secondary energy forms such as induction, dielectric, plasma, infrared, arc, and laser. But conversion of electricity to these secondary forms may entail efficiency loss, so process synergies and efficiency gains must accompany their use for heating applications to make them attractive. Process Intensification Using Electricity For certain heating applications, electricity may improve process efficiency and intensification. For example, the use of microwaves or other electromagnetic waves to directly heat a catalyst for reaction could eliminate unnecessary heating of the rest of the equipment and yield energy savings (Mallapragada et al. 2023). Steam methane reforming (SMR) has been demonstrated with resistive heating of a Fe-Cr-Al alloy reactor tube at near ambient operating pressure and 800°C (Wismann et al. 2019). Such a reactor eliminates both combustion volume and the equipment associated with heat recovery in a conventionally fired SMR furnace flue. The study authors projected that the use of the electrically heated tube reactors would reduce the SMR volume by a factor greater than 200. Conventional SMR and ethane cracking furnaces are more than 90 percent energy efficient as they recover heat from the flue gas by raising steam. This steam stream is absent from the electrically heated reactor, so if generated steam is needed for heating by other operations in the process, then energy savings may be minimal. In other words, in a chemical plant, heat and mass flow between various units are interconnected, and care must be taken to evaluate overall impact by considering the entire plant rather than an isolated process unit for electrification (Chavez -Velasco et al. 2021). Electricity-Driven Separation Processes Separation processes account for about 40 percent of the energy consumption in chemical manufacturing, and petroleum refining and distillation constitute most separation applications (NASEM 2019). It is estimated that 2.5 percent of US energy consumption is for distillation (Chapas and Colwell 2007). Almost all of the above ambient-temperature distillations are driven by heat, and electricity can efficiently pump heat from the condenser to the reboiler. For most distillations, heat pumping may reduce energy consumption by a factor of 3 to 10, making it quite attractive in reducing energy consumption, fossil fuel–related GHG emissions, and VRE-related energy storage (Chavez Velasco et al. 2022). Research and development are needed for the selection of energy-efficient distillation configurations and for the implementation and operation of heat-pumped distillation columns (Mathew et al. 2022). Additionally, where energy efficient and cost effective, pressure-driven separation processes such as membranes and pressure swing adsorption need to be pursued. It is also worthwhile to consider heat pumps to upgrade heat from one unit operation for use at a higher temperature in another. Traditionally, heat exchanger network analysis is performed, but going forward it will be important to perform heat and power network analysis for the entire plant for optimal results (figure 1). Possibilities for VRE Storage One very efficient method of VRE storage is batteries with an overall efficiency of about 80 percent from storage to delivery. But with a low energy density of storage and self-discharge over time, batteries are more suitable for short-term storage. An alternative way to store VRE is with a water electrolyzer and stored compressed hydrogen (H2). The use of pressurized H2 provides higher energy density and allows much longer storage, but it is less efficient than batteries because of losses first in the electrolyzer and then in the fuel cell. To produce 1 kg of H2, 9 liters of fresh water and about 51 kWh of electricity are needed (Rissman et al. 2020). Demand for large quantities of electrolytic H2 for process electrification could tax the clean water supply. Compression or liquefaction of electrolytic H2 provides higher storage density. However, with current technology, compressed stored electrolytic H2, upon combustion, provides only about 63 percent of the energy used in its generation and storage, and when used with a fuel cell to supply electricity this number drops to about 45–50 percent. Deficiencies in VRE availability could be offset by using natural gas turbines. Thus, the preferred order of VRE use efficiency for continuous heating would consider batteries first, then H2 combustion, and finally fuel cells. Conversion of electrolytic H2 to other chemicals such as liquid ammonia and methanol has also been suggested to increase storage density and transportability but, compared to compressed H2, such storage systems add equipment and energy inefficiencies. A hybrid system of VRE and fossil fuels may reduce GHG emissions while decreasing the amount of energy to be stored. For example, VRE could be used when available with no energy storage. A hybrid steam boiler would be heated with renewable electricity when available and otherwise use efficient natural gas combustion, decreasing GHG emissions proportionally to the period of VRE availability. Alternatively, supplementary VRE storage may be used. Battery storage may be sized based on average VRE availability during a 24-hour period. Any deficiency in electricity availability beyond that (e.g., due to extended weather patterns such as cloudy days for PV farms or low-wind days for wind turbines) could be offset by using natural gas turbines. Depending on the storage capacity of the batteries, an electrically heated SMR or ethane cracker could operate with a sub-stantial reduction in GHG emissions and still enjoy process intensification with electricity. The use of VRE in conjunction with optimally sized storage is an interesting process system engineering problem. Electrochemical Synthesis of Chemicals and Hydrogen Chlorine and some small-scale chemicals (e.g., adiponitrile, ozone, and perchlorates) are produced via electrochemical route, but the use of electricity to produce most large-scale chemicals is rare. Electrochemical processes for the production of ammonia and methanol and for CO2 conversion are being developed. They operate at lower temperatures and pressures and are more amenable to VRE load following. However, energy inefficiency, long-term stability, durability under dynamic operating conditions, and the absence of electrocatalysts for high selectivity and yield are challenges that need to be addressed. One exception is electrolytic H2 generation, which is quite advanced—several dozen projects with a power rating between 100 MW and 1 GW are under development around the world (Mallapragada et al. 2023). Electrolytic hydrogen enables VRE storage for eventual supply as electricity or fuel for combustion, and it can be used as an energy carrier for fuel cell vehicles, transported long distances via pipelines, and used as a reductant in various reactions and processes that generate CO2. Electrification to Reduce or Eliminate CO2 Release While Using Fossil Feedstock As noted, primary energy use for process heat, cooling, refrigeration, and plant machinery operation releases most of the CO2 from chemical and petroleum refining plants. Endothermic chemistries such as SMR and ethane cracking release CO2 because of combustion in their furnaces; only a few chemical processes, such as lime and hydrogen via reforming, directly generate CO2 as a coproduct. Of the 332 million metric tons (MT) CO2eq of GHG released from chemical plants in 2018, only 71 MT were coproducts of chemical synthesis; the remaining MT were due to combustion. For the petroleum refining sector, the entire 244 MT CO2eq of GHG emissions were due to fuel use. Clearly, the coproduct GHG release of both sectors combined is a tiny fraction of the total 2018 US GHG emissions of 6,677 MT CO2-eq. There are two ways to mitigate coproduct CO2. In one method, CO2 is recovered from the product stream and either directly sequestered or upgraded to a usable chemical using zero carbon electricity. The second method is to change or modify the chemical synthesis to eliminate CO2 formation. When feasible, this method is likely to be more energy efficient than the first. A classic example of a chemical process that co-produces CO2 is the production of ammonia. With an annual production of 16.41 MT in 2019, ammonia constitutes the second-largest chemical production in the United States (Statista 2023). On average, the emission intensity of an ammonia plant is 2.4 T of CO2 per T of ammonia. Hydrogen for ammonia synthesis is generally produced from SMR. In addition to the CO2 release in the SMR furnace flue gas, CO2 is a coproduct of the water gas shift reaction to convert SMR carbon monoxide to CO2 and H2. The use of green H2 in conjunction with nitrogen from a zero carbon electricity-driven air separation plant would avoid the release of CO2, resulting in “green ammonia.” However, the use of electrolytic hydrogen in conjunction with VRE for CO2-free chemical production comes with steep challenges in terms of the cost and volume associated with the electrolyzer and batteries, both of which require innovations for large-scale deployment. For example, a 1,000 T/day green ammonia plant would require 12.5 MWh of electricity per ton of ammonia, of which 10 MWh would be needed by the electrolyzer (DECHEMA 2017). Based on the DECHEMA data, the footprint of the electrolyzer to produce the needed amount of H2 would be about 470 × 295 m2, with an anticipated electrolyzer cost of about $500 million. To meet annual US ammonia demand, 48 such green ammonia plants would be needed. Challenges associated with the number of H2 electrolyzer units and energy storage become apparent. Electrification to Replace Fossil Feedstock The two primary constituent elements of organic chemicals are carbon and hydrogen. If fossil resources are not to be used, then alternative sources will be needed for these elements. Green H2 is available, and carbon could be sourced from lignocellulosic biomass or CO2 emissions from processes. Each presents different opportunities, challenges, and outcomes. Use of Biomass as a Feedstock A benefit of using biomass as a feedstock is that the process from conversion to the end use of the chemical products enables no net release of CO2 to the atmosphere. Generally, though, the supply of sustainable residual/waste lignocellulosic biomass that does not compete with food is limited. Furthermore, the collection efficiency of solar energy as biomass is quite low (less than 2 percent). So it is best to view biomass as a source of carbon and not as a source of energy or hydrogen (Agrawal et al. 2007; Agrawal and Mallapragada 2010). This also implies that during the conversion of biomass to chemicals and fuels, biomass carbon should be preserved and energy needed for the conversion supplied by zero carbon electricity. Gasification and fast hydropyrolysis of biomass using green H2 and zero carbon electricity may be used in creating a number of major organic chemicals. Since a biomass molecule contains oxygen, its conversion to non-oxygen-containing organic molecules would need green H2 to avoid the release of coproduct CO2 and maximize the yield of the desired products by preserving carbon. Accordingly, gasification and fast hydropyrolysis of biomass using green H2 and zero carbon electricity have been proposed as potential routes to creating a number of major organic chemicals (Agrawal 2019). But the following challenges are associated with the use of lignocellulosic biomass: (1) seasonal availability and its low volumetric energy density limit long--distance transportation because of the associated carbon footprint, cost, and logistics; (2) biomass can contain up to 70 percent water, and drying before use is energy intensive; (3) the presence of char, tar, and ash during conversion often presents processing challenges; and (4) capital and production costs, even from standalone plants that do not use VRE or green H2, are generally substantially higher than those of natural gas–based plants. Nevertheless, compared to fossil resource–based chemicals, the use of renewable carbon in chemicals will reduce net CO2 release through the lifecycle use and disposal of chemicals and could be quite attractive from an environmental perspective. Green Hydrogen and VRE The second option, converting CO2 to chemicals in conjunction with green hydrogen and VRE, raises questions about cost and energy efficiency. In this process, H2 is needed to remove oxygen (contained in the CO2 as water) as well as H2 atoms for incorporation in the chemical molecules. This leads to high demand for VRE and associated electrolyzers and energy storage. It is imperative that processes be highly efficient to minimize the use of electricity. Consider the major building block chemicals: methanol, ethylene, propylene, benzene, toluene, and xylenes. Based on the DECHEMA (2017) data, 243 MT of CO2 and 2014 TWh of VRE would have been needed for 2019 US production of these chemicals (Statista 2023). This electricity demand is nearly half the entire US electricity generation of 2019 (EIA 2022b). This route would put a huge stress on power generation, distribution, and storage, not to mention the volume of electrolyzers needed to produce green H2. Moreover, collecting CO2 via a process that uses fossil fuel as an energy source (or as a reducing agent) and then converting the collected carbon to a chemical or fuel using zero carbon electricity is generally less energy efficient than directly using zero carbon elec-tricity for the process, avoiding the release of CO2 in the first place. When the reducing function is needed, H2 generated from zero carbon electricity should be used. If there is still a need for a specific chemical, then a fossil resource should be considered for the conversion to the desired chemical. The energy needed to convert collected CO2 to most chemicals that do not contain oxygen is greater than the energy supplied through the combustion of fossil fuel, and extra energy is required to separate and collect the CO2 in the first place. A Systems View According to the EIA (2022a), by 2050 renewable electricity’s share will grow from the current 21 percent to 44 percent of total US electricity generation. Almost all of this increase will come from wind and solar; zero carbon electricity from nuclear and hydroelectric are expected to remain flat. This is encouraging, as it implies that zero carbon electricity will become more cost effective and readily available for the large-scale electrification of the chemical manufacturing and petroleum refining sectors. The Imperative of Efficiency Because all the increase in zero carbon electricity will be due to wind and solar, it will be variable. It is therefore imperative that processes be highly efficient to minimize electricity use. This reasoning is based on several factors. The cost associated with batteries and electrolyzers scales directly with the amount of electricity used. Any process energy inefficiencies will further increase the cost associated with these units. The estimated amount of electricity needed by the chemical manufacturing and petroleum refining sectors will be about half of current US electricity production, and this large amount of VRE is very likely to strain VRE harvesting and distribution. -Considered in conjunction with the electrification of other sectors such as transportation, land for solar energy collection may compete with agricultural use (Miskin et al. 2019). Finally, availability of VRE may not be plentiful in all regions, and its inefficient use will have to be minimized. Nearly 80 percent of the chemical industry’s CO2 emissions are associated with the supply of heat and power using fossil fuels. Replacing these energy needs with zero carbon electricity may dramatically reduce GHG emissions from chemical manufacturing—while maintaining current use of fossil resources as feedstock. A Circular Economy To further reduce GHG emissions over the production and use stages of chemicals, consider the life of chemicals after they leave the plant (figure 2). In a circular economy, a portion of chemicals (e.g., polymers) will be recycled to reduce the use of fresh feedstock. Some chemicals and their derivatives at the end of their use cycle could be safely buried in a landfill; for these products, the use of fossil feedstock in conjunction with low-carbon electricity will not contribute to GHG emissions. But a third portion will eventually be released into the atmosphere as a greenhouse gas. This portion includes the combustion of chemicals such as methanol, vaporization of some volatile chemicals during use, and release into the environment during their application and use (e.g., urea). To avoid net GHG emissions associated with this third category of chemicals, an equivalent amount of renewable carbon (e.g., residual/waste lignocellulosic biomass) would be needed as a chemical feedstock. For certain large-volume chemicals, such as ammonia, that do not contain carbon atoms but are responsible for a large fraction of process CO2 release, the use of zero carbon resources such as green H2 must be pursued. Basically, for net zero GHG emissions during chemical production and use, thanks to zero carbon VRE and prudent recycling and disposal of chemicals at the end of their lifecycle, it may not be necessary to replace the entire fossil feedstock for chemicals production. This will likely result in a much lower requirement for renewable carbon. This system needs to be analyzed and evaluated. Challenges and Opportunities New equipment design may help achieve process intensification and reduced cost even when a unit quantity of fossil heat is replaced with a unit quantity of electricity. Heat pumping of above-ambient temperature distillations and use of electromagnetic waves to supply energy where it is needed (rather than heating the entire equipment) should be explored to use electricity advantageously and greatly reduce the total energy requirement. The use of heat and power networks to optimize energy flow throughout the plant may enhance process synthesis. Integration of VRE to accommodate around-the-clock operation of chemical plants presents unique challenges and opportunities in innovation and analysis. Chemical plants that just load follow VRE and remain idle most of the 24-hour day not only will be costly per unit quantity of product but also could be challenging to operate and maintain. These concerns could be addressed with VRE use, storage, and periodic, limited use of energy/power from natural gas. Concluding Observations The lifetime of typical chemical plants is long—30 to 50 years or more—and it will take a great deal of innovation to implement new electrification ideas at existing plants. The direct use of electricity and secondary forms (e.g., electromagnetic waves of various frequencies, plasma) will introduce new physics in the design of chemical reactors and separation processes and spur new and exciting development in the analysis of such equipment. Electrification will result in new dynamic behaviors of unit operations and affect associated sensors and control strategies. For example, an inductively heated steam boiler or a reactor could be heated much faster, or its output or temperature rapidly adjusted. The availability of both electrolytic green H2 and byproduct process H2 will enhance flexibility in the use of various types of carbon resources (Chen et al. 2022). In summary, given the changing energy landscape and environmental concerns, the electrification of chemical manufacturing and petroleum refining sectors presents a generational opportunity for engineers. Successful implementation will depend on the 24-hour availability of hundreds to thousands of TWh of zero carbon electricity at low cost. Otherwise sustainable electrification will remain limited. Acknowledgments This work was supported by the National Science Foundation under Cooperative Agreement No. EEC-1647722, an Engineering Research Center for Innovative and Strategic Transformation of Alkane Resources (CISTAR). The authors would like to acknowledge Edwin Andres Rodriguez Gil for help with the drawing of figures. References Agrawal R. 2019. Chemical engineering for a solar economy (2017 P. V. Danckwerts Lecture). 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Wismann ST, Engbaek JS, Vendelbo SB, Bendixen FB, Eriksen WL, Petersen KA, Frandsen C, Chorkendorff I, Mortensen PM. 2019. Electrified methane reforming: A compact approach to greener industrial hydrogen production. Science 364(6442):756–59. [1] This is the latest year for which data are available from the Advanced Manufacturing Office of the US Department of Energy (https://www.energy.gov/eere/amo/-manufacturing-energy- and-carbon-footprints-2018-mecs). In this article, all the US manufacturing data are for the year 2018 unless specified otherwise. [2] In terms of global sales dollars associated with chemicals in 2021, the US share was only 11 percent, thus the global scope of the challenges discussed in this article is likely to be an order of magnitude higher (Statista, US chemical industry revenue 2005–21, Mar 10, 2023, update). About the Author:Rakesh Agrawal (NAE) is Winthrop E. Stone Distinguished Professor and Jeff Siirola (NAE) is professor of engineering practice, both in the Davidson School of Chemical Engineering, Purdue University.