sources and carbon sequestration and utilization at an unprecedented rate and scale, as well as consideration of trade-offs in such areas as water consumption, cost, and environmental justice.

This chapter describes the impetus for prioritizing decarbonization of energy systems and the important role chemical engineers will have in advancing technologies that minimize the climate impact of the energy sector. The chapter is organized from sources to end uses. Opportunities for chemical engineers are explored in energy sources; energy carrier production; energy storage; energy conversion and efficiency; and carbon capture, use, and storage.

THE NEED FOR DECARBONIZATION

The international climate science community has established a link between global greenhouse gas (GHG) emissions and human actions and energy usage. The concentration of CO₂ in the atmosphere has tracked closely its rate of anthropogenic production since the start of the Industrial Revolution in 1750 (Figure 3-1). Emissions rose to about 5 billion metric tons per year in the mid-20th century before reaching more than 35 billion metric tons per year by 2000 (NOAA, 2020). In parallel, global surface air temperatures have increased by 1 °C. Oceans absorb a large amount of CO₂ released into the atmosphere. This absorbed CO₂ reacts with seawater to form carbonic acid. Thus, increased CO₂ levels in the atmosphere increase the acidity of the ocean, harming shellfish and other marine life.

International efforts to mitigate climate change began in 1992, culminating in 2015 with the Paris Agreement (UNFCC, 2015), whose primary goal is to keep the global

temperature rise during this century well below 2 °C above preindustrial levels. This is a monumental challenge that will require decarbonization of the energy sector, net-zero emissions, and fast-paced removal of GHG from the atmosphere. Transitioning to net-zero emissions in the energy sector is likely to cost trillions of dollars and will require efforts at all levels of government and across all sectors (e.g., the coordinated, systems-level approach proposed as part of the Sustainable Energy Corps; Alger et al., 2021). It will take decades, and may never be complete. The time required to decarbonize energy systems will depend on technological advances, government policies, changing economics of energy carrier options, and essential modifications in consumer behavior. Given the magnitude and complexity of the global energy system, no single energy carrier will be able to satisfy requirements across all sectors in the foreseeable future. Thus, achieving the goals of the Paris Agreement will require a wide range of energy sources and carriers.

The question of viable energy mixes was addressed in a comprehensive multimodel study coordinated by the Energy Modeling Forum 27 (EMF27), which examined 13 scenarios for keeping the global temperature increase below 2 °C in this century (Kriegler et al., 2014). Because of the significant number of uncertainties and assumptions involved in these 13 scenarios, it is best to use a notional average of the scenario outcomes to approximate the various energy trends. A notional average view suggests that in 2040,

• petroleum and natural gas will continue to play a significant role in the energy mix;
• coal usage will decrease significantly;
• energy carriers from nonbiogenic renewables (e.g., solar, wind, hydroelectric power) and biogenic sources (bioenergy) will grow significantly; and
• carbon capture, use, and storage (CCUS) will become a key technology for decreasing CO₂ emissions.

The generation, distribution, and use of electrons from renewable energy sources represent some of the most robust enablers of decarbonization. In its Sustainable Development Scenario for 2019–2070, the International Energy Agency (IEA, 2020d) concluded that the share of electricity in end-use energy demand will grow from about 20 percent to more than 50 percent. One-third of that electricity demand is expected to be met by solar power in the form of photovoltaic devices deployed at scale in decentralized form (Figure 3-2), with another 20 percent met by modular wind resources.

Opportunities exist worldwide across all sectors (electricity generation, industrial, transportation, and residential/commercial) to decrease energy-related emissions. Meeting the challenge of keeping the global temperature increase below 2 °C will require advances in four key areas:

• energy efficiency;
• increased use of lower-carbon energy sources;
• development and deployment of novel energy and energy storage technologies; and
• government policies to promote cost-effective solutions.

stored and transported to consumers and markets are inseparable. The path to clean energy from photons, mediated by electrons and molecules that can be stored and transported, will require decentralized capture within “photon conversion factories,” akin in concept to the integrated refineries used to transform fossil resources into fuels and chemicals today, but in much smaller and modular forms and deployed at diverse points of photon capture. These factories will produce heat, electrons, energy carriers, and chemicals as part of modular integrated systems designed to capture the largest possible fraction of the solar flux with high quantum efficiencies and convert it into useful forms of chemical energy.

Direct photon capture and conversion to electrons as energy carriers.

There are several paths to photon capture and utilization. Direct solar conversion to electrons is carried out using semiconductors that first capture photons through electronic excitations across their band gap and then collect the excited electrons in the form of photovoltaic (PV) solar panels. Together, the decrease in the cost of PV panels and the expected increase in the electrification of energy systems are driving the deployment of global PV capacity at a rate that could not have been envisioned just a few years ago. This global capacity, only about 500 GW in 2018, will double by 2022 and is predicted to exceed 10 TW by 2030 and 30–70 TW by 2050 (the current global demand is 18 TW; Haegel et al., 2019).

Silicon (Si)-based PV cells represent about 80 percent of the currently installed solar capture capacity in the United States, the rest consisting of cadmium-telluride (CdTe) semiconducting thin-film PV cells (DOE, 2021a). High-purity amorphous and polycrystalline Si PV cells are manufactured using energy-intensive purification processes first developed for the processing of Si wafers for electronic devices. State-of-the-art Si PV cells operate at near-theoretical capture efficiencies (~30 percent), a limit set by the solar spectrum and the balance between the band gaps accessible by doping and the attainable current densities. The low-absorption cross-section of Si requires thick wafers, precluding the use of tandem devices designed to collect different components of the solar spectrum through systematic doping. Si PV cells will continue to evolve through incremental improvements in device architecture and design options at the cell/module scale, as well as through lower manufacturing costs; greater reliability/durability; and the development of infrastructure for their installation, maintenance, and seamless insertion into advanced electrical grids. Chemical engineers will continue to enable the evolution of Si PV cells, their deployment at a global scale, and the optimization and control strategies required to integrate them into the grid. Si PV cells represent the medium-term choice for deployment at scale in direct photon-to-electron conversion.

CdTe thin-film PV cells absorb photons at energies near the maximum flux in the solar spectrum. Recent improvements in efficiency and manufacturing costs have made them competitive with Si PV cells. These modules consist of micron-thick films of CdTe held within layers of conducting transparent oxides. Environmental concerns about the toxicity of the components in CdTe PV cells will need to be addressed through improvements in reliability and durability, thinner films, and higher efficiencies. These advances will enable greater market penetration as PV-based solar capture devices become more

prevalent in practice. Such advances in manufacturing, cell/module architecture, and systems integration will be driven by chemical engineering as an enabling discipline, as illustrated by recent efforts to coordinate research and manufacturing capabilities through a National Renewable Energy Laboratory–led consortium.¹

The parallel developments in dye-sensitized PV cells have recently been punctuated by the emergence of mesoscopic architectures, in which coatings of n-type semiconductor nanoparticles, such as titania, act as mesoporous anodes that provide 1,000-fold increases in dye-anode connectivity (Hardin et al., 2012; O’Regan and Grätzel, 1991). These molecular photovoltaics have emerged in parallel with perovskite solar cells (PSCs; Grätzel and Milić, 2019), leading to a significant disruption in the nature of research on PV cells and to very rapid advances in capture efficiencies. PSC devices also provide the benefits of roll-to-roll solution-based manufacturing processes, a tolerance for reagents lower in purity, and much smaller amounts of active materials relative to Si PV cells. PSC devices have evolved rapidly in photon capture efficiency, from 4 percent in 2010 to >25 percent in 2019 (Grätzel and Milić, 2019; Figure 3-3). These PSC devices are now approaching photocurrents near their theoretical maximum, but improvements in efficiency, open-circuit voltages, and long-term durability, as well as replacement of the toxic water-soluble components ubiquitous in the best-performing perovskite materials, need to be addressed before significant commercial deployment at scale can occur (Correa-Baena et al., 2017). These cells suffer from operational instabilities, short useful lives, and significant environmental and health concerns related to the long-term containment and ultimate disposal/recycling of their toxic constituents. These challenges are being addressed through significant funding from federal programs² and an influx of entrepreneurial capital.

Concerns about toxicity and long-term durability continue to prevent PSC devices from displacing Si PV cells in the marketplace. Recent developments have led to more stable perovskite compositions, to the identification and mitigation of extrinsic degradation mechanisms, and to device configurations that ensure more reliable containment to prevent the release of toxic components in the most efficient perovskites (e.g., methylammonium lead trihalides and formamidinium analogs). Durability and containment, however, remain formidable challenges (Correa-Baena et al., 2017; Rong et al., 2018). Significant ongoing research focuses on perovskite compositions that minimize intrinsic degradation processes and on modular device architectures that ensure reliable long-term operations. As in the case of Si PV cells, chemical engineering is well positioned to address and resolve these challenges, and to deploy the improvements in practice at scale. The development of advanced solution-based processes for perovskite cell manufacture, integration of PSC systems into existing grids, and life-cycle assessment (LCA) of the full environmental impact of these devices are also encompassed by fundamentals and practice of chemical engineering.

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¹ See https://www.energy.gov/eere/solar/cadmium-telluride-research-and-development-consortium-coordination.

² See https://www.energy.gov/eere/solar/solar-energy-technologies-office-fiscal-year-2020-perovskite-funding-program.

of these challenges are being addressed as part of the work of large multidisciplinary centers, such as the Joint Center for Artificial Photosynthesis³ and its recently announced successors, the Liquid Sunlight Alliance and the Center for Hybrid Approaches in Solar Energy to Liquid Fuels.⁴ These centers aim to design in concert the different components required and develop hybrid photoelectrodes that can combine photon capture and molecular catalysis to generate carriers from a broad range of wavelengths in the solar spectrum. These advances require a bridge between length and time scales inherent in photon-driven excitation and molecular transformations induced by emitted photoelectrons at a catalytic function. Systems-based integration, control, and design; reaction-transport formalisms; and knowledge of the catalytic properties of active surfaces and centers will play an enabling role in the design and selection of cost-effective devices for the direct generation of energy carriers from photons in a manner that avoids toxic and scarce elements, as well as containment and sustainability concerns (Montoya et al., 2017). Bringing such considerations into chemical, biochemical, and electron-driven processes is a domain of chemical engineers.

Formidable challenges remain for the development of electrochemical systems for direct or sequential conversion of photon energies into chemical energy in the form of H₂ (from H₂O), CO (from CO₂), small alcohols and hydrocarbons or H₂–CO mixtures for subsequent thermochemical conversion to such molecular carriers (from CO₂–H₂O), and ammonia (from N₂–H₂O). The most enduring and significant of these challenges are

• the modular nature and complex interconnections among functions and the integration of electrical and chemical processes at scale;
• the ubiquitous requirement for scarce precious metals as electrodes, and toxic or rare elements as semiconductors, dye sensitizers, dopants, and connectors;
• the need for process intensification and high photon capture efficiencies limited by transport processes within electrolytes, electrodes, or semiconductors;
• the durability of modules during extended field use and their recyclability after their useful life;
• the costly extraction of dissolved product molecules from dilute aqueous media and the separators required to avoid the recombination of photocatalytic or electrocatalytic products; and
• the energy requirements in fabrication and recovery of the component elements after use.

These matters involve catalysis and kinetics in complex and nonideal liquid systems (thermodynamic and hydrodynamic); transport of molecules, ions, and electrons in fluids and solids; materials assembly with precise nanoscale and mesoscale architectures; process integration, control, and optimization; and LCA. The challenges, fundamentals, and coping/solution strategies lie firmly within the domain of chemical engineering,

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³ See https://solarfuelshub.org/.

⁴ See https://www.energy.gov/articles/department-energy-announces-100-million-artificial-photosynthesis-research.

which has in the past adeptly tackled challenges of similar character for complex chemical, biochemical, and electrochemical conversion processes mediated by heterogeneous, molecular, or biological catalysts.

Electrolysis remains the proven technology for electrochemical generation of H₂ via modular systems based on acid polymer, liquid alkaline, or ionic-transport solid electrolytes (Ardo et al., 2018; Moss et al., 2021). Progress has recently been made in the scaling up of electrolysis, and chemical engineers have an opportunity to contribute to the development of applications at the scale required to disrupt the energy landscape. Electrolysis systems can be operated in acidic or alkaline regimes, although the rate-limiting nature of the O₂ evolution half-reaction (H₂O oxidation) has led to a preference for alkaline electrolyzers, which also avoid the platinum-based materials required to prevent electrode dissolution in acidic media, thus allowing the use of nonprecious metals (e.g., Ni, Fe, Cu) as electrodes. Acidic electrolyzers use polymer electrolyte membranes that minimize contact between H₂ and O₂ through fast proton transport and short anode–cathode distances. Alkaline electrolyzers have relied on microporous physical barriers that impose larger anode–cathode distances and ohmic losses. Recent developments in selective anion transport membranes prevent contact between H₂ and O₂ and have led to more compact membrane–electrode modules.

The challenges of deploying electrolyzers at scale include their efficient integration and control as multimodule stacks, the development of earth-abundant electrode materials, and thinner and more efficient separator membranes. The challenges are similar but even more formidable for electrochemical reduction of CO₂ via concurrent electrolysis with H₂O to form mixtures of H₂, CO, alcohols, carboxylic acids, and hydrocarbons (Hori, 2008; Nitopi et al., 2019). These products can be used directly as energy carriers or as precursors to such carriers on heterogeneous catalysts (e.g., H₂–CO conversion to liquid transportation fuels via Fischer-Tropsch or methanol synthesis). Electrochemical systems face similar challenges in meeting the scale required for impact:

• more efficient and robust electrodes based on earth-abundant elements;
• thinner and more selective membrane separators;
• higher-temperature electrolyzers;
• integration of electrocatalytic and thermocatalytic systems in sequence or within a single device to form more suitable energy carriers; and
• the development of supply chains to lower the costs of manufacturing and integrating the modular devices into molecular weight (MW)–scale distributed deployments for the synthesis of H₂ and other energy carriers.

Conversion of photons to heat as an energy carrier.

The modular nature of PV and electrochemical cells, whether in separate or combined forms, poses significant challenges, including process integration and intensification and deployment at scale. These challenges can be addressed by using solar thermal strategies that capture the energy of photons as heat, which can be delivered to users as thermal energy, or converted to chemical energy for transport or for storage during diurnal or intermittent fluctuations in solar flux. The end use of the captured thermal energy depends on the temperatures accessible

through solar collectors and heat transfer media and on the location of markets relative to the point of capture. The specific end-use option of generating H₂ at scale has been highlighted in recent reports because of its relevance to a hydrogen economy and its inherent advantages over electrolyzers in large-scale deployment (Gonzalez-Portillo et al., 2021; Moss et al., 2021; NREL, 2017).

This capture and storage of photon energies as heat can be used directly in ambient temperature control in commercial or residential spaces; as process heat in existing chemical processing plants or manufacturing operations; or for conversion into electrons or hydrogen, typically through the generation of high-pressure steam or through conversion cycles commonly termed “chemical looping.” The latter approach involves the design of reactors based on the chemical engineering principles of kinetics, transport, chemical absorbents, and construction materials that can withstand the extreme temperatures required for thermal and chemical efficiencies.

The achievement of very high temperatures (~1400 °C) has been enabled by advances in parabolic solar thermal concentrators (Sargent & Lundy LLC Consulting Group, 2003); these systems have in turn allowed the generation of very high–pressure steam for power generation in high-temperature turbines. At such high temperatures, chemical looping using redox-active oxides enables the cycling of such solids between a reduced state that reacts with water to form H₂ and an oxidized state that evolves O₂ in a spatially separate stage. Such processes use oxides of earth-abundant elements and form separate streams of H₂ and O₂ from H₂O, thus avoiding the need for gas separators and the use of costly metals as electrodes, as well as the transport limitations inherent in the use of liquids as electrolytes.

These cyclic processes can be deployed via large-scale devices that allow process integration and intensification strategies unavailable for modular systems but ubiquitous in conventional refining, chemical manufacturing, and power generation processes, albeit at somewhat lower temperatures. The high temperatures required for efficient solar thermal capture pose significant challenges with respect to the design, use, and handling of the heat transfer media and the durability of the required redox-active oxides. Molten salts are typically used as heat transfer fluids, but solid particles in fluidized systems have recently emerged as attractive alternatives (Gonzalez-Portillo et al., 2021).

Fossil Fuels

Fossil fuels (coal, petroleum, and natural gas) have been essential for society’s development and progress, having powered the Industrial Revolution and shaped the modern world. Technological advances based largely on the ingenuity of chemical engineers have enabled the efficient extraction, processing, and conversion of fossil fuel raw materials into useful products. The expertise of chemical engineers remains essential in enabling a transition from the current energy landscape to one based on renewable and sustainable energy sources. However, most studies have concluded that fossil fuels will continue to play a key role in the energy mix at least until 2050, and in the meantime, the imperative is to reduce the carbon footprint of fossil fuels—a key opportunity for chemical engineers.

Coal.

Coal is the most abundant and least expensive of all fossil fuel resources. It can be converted into gas or liquids to produce chemicals and fuels, but is used primarily for direct combustion. As a solid fuel, coal generates more CO₂ per unit energy than other fossil fuels—from approximately 30 percent more than diesel to nearly twice as much as natural gas (EIA, 2021i).⁵ Coal resources are used mainly to generate electricity, and while reliance on coal has increased in low- and middle-income countries (e.g., China and India), the opposite has been true in higher-income countries (e.g., United States, European Union, Japan). In the United States, electricity generation from coal has been declining since 2008, with the biggest drop (~16 percent) taking place in 2019; in contrast, the use of natural gas has increased dramatically since 1995, and the use of renewables has increased since 2005, albeit at a slower pace (Figure 3-5).⁶

Most investments in coal-fired plants in 2019 (almost 90 percent) were for higher-efficiency (supercritical and ultrasupercritical) plants; the remaining small portion of investments were in inefficient subcritical plants, mainly in Indonesia (IEA, 2020c). High-efficiency coal-fired power plants use water at high, above-critical temperatures and pressures (373 °C and 220 bar, respectively). The efficiency gains thus achieved reduce by about 20 percent both the amount of coal needed and CO₂ emissions. These plants also emit substantially lower amounts of nitrogen oxides and sulfur oxides (IEA, 2012).

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⁵ Pounds of CO₂ emitted per million Btu (gJ): coal, 215 (227); diesel, 161 (170); gasoline, 157 (166); propane, 139 (147); natural gas, 117 (123) (EIA, 2021).

⁶ Preliminary IEA analysis indicates a sharp drop in power-sector demand in 2020 as a result of the COVID-19 pandemic, with demand for coal having the greatest uncertainty of all fuels used for power.

economic, environmental, and technical analyses—hallmarks of the chemical engineering profession—to bear on the selection of viable options for biofuel production.

Lignocellulosic feedstocks.

An estimated 1 billion tons of lignocellulose could be sustainably harvested for biofuel production annually in North America (DOE, 2016). The conversion of lignocellulosic biomass into biofuels and other organic coproducts—which represents the foundation of the biorefinery concept—poses considerable challenges for chemical engineers. Lignocellulose is a complex and heterogeneous composite material whose carbon content is predominantly in the form of two polysaccharides—cellulose and hemicellulose—as well as the aromatic polymer lignin. Numerous processing options have been considered over many decades; they entail either fractionating biomass into its constituents (thus enabling selective processing options, akin conceptually to the methods used in petroleum processing) or processing biomass directly into liquid or gaseous intermediates for subsequent conversion to transportation fuels and chemicals. Unlike petroleum, biomass is a solid, polymeric material that can be substantially heterogeneous, a feature that presents feedstock-associated challenges beyond the processing of liquids and gases that form the bedrock of the chemical engineering discipline.

The combined challenges of converting lignocellulosic biomass and offsetting fossil resources as a feedstock for transportation fuels create considerable opportunities for chemical engineering to continue making enabling contributions to the at-scale conversion of lignocellulose into biofuels and other products (flexible feedstocks are discussed in Chapter 6). These opportunities begin with the crops themselves. Now that biology is a core component of the chemical engineering discipline, modern molecular biology techniques are now ubiquitous in the toolkit of many chemical engineers. These techniques can be used to modify plants so they can be more efficient photon collectors (Kromdijk et al., 2016), to produce target chemicals in planta (Yang et al., 2020), and to

reduce the recalcitrance of plant components in chemical conversion in a biorefinery (Chen and Dixon, 2007).

Once plants have been harvested and transported to a biorefinery or centralized depot, there are many opportunities for overcoming recalcitrance to enable conversion of plant-based biomass to biofuels. The diversity of conversion pathways precludes a comprehensive review; therefore, this section focuses on the challenges and opportunities for chemical engineers to achieve cost-effective and sustainable biofuel production.

In the conventional biochemical conversion pathways, thermochemical treatment processes increase the reactivity of biomass, mainly by improving physical access to polysaccharides. These processes apply acid, base, steam, organic solvents, ionic liquids, or deep eutectic solvents, usually at temperatures in excess of 100 °C. Pretreatment approaches can also take the form of fractionation methods that separate polysaccharides from lignin for more direct and selective processing in parallel process trains. Polysaccharides are subsequently converted into monomeric sugars via carbohydrate-active enzymes or sugars and dehydration products, such as furanics or levulinic acid, through further use of acid catalysts. Soluble carbohydrates and derivatives are then converted into fuel molecules or precursors through biological and/or chemical catalysis. In the pioneer cellulosic ethanol plants built in the 2000s, lignin is commonly used to provide heat and power via on-site combustion.

Many attempts have been made to bring biochemical conversion–focused biorefineries to scale (1,000 to 2,000 metric tons/day), especially with the aim of converting nonfood crops or agricultural residues (e.g., wheat straw, corn stover), supported by substantial government investments. Yet the formidable challenges of economical feedstock collection, feedstock handling, biomass pretreatment, aseptic solid–liquid separation, and sterile bioconversion continue to prevent these facilities from achieving the required capacity factors for economic viability at scale of impact.

The lessons learned from these early facilities inform the many opportunities for chemical engineers to advance this field in moving from the process paradigm described above. For example, because on-site lignin combustion is estimated to be the most expensive single unit operation in a biorefinery, as well as a major source of non-GHG emissions, the development of alternative uses for lignin, 40 percent of which can be made of biomass carbon, is a major opportunity for research (Davis et al., 2013; Eberle et al., 2017). Successful conversion of lignin into value-added biofuels or biorefinery coproducts represents a major frontier for the chemical engineering community. Process intensification (PI) through the consolidation of biomass deconstruction into fewer unit operations and a focus on the elimination of costly steps is critical. An important component of PI involves separations, which are often key cost drivers in biorefineries. The challenges in lignocellulosic separations differ from those in petroleum processes because they involve the handling of solids and because biomass-derived compounds consist of high–boiling point oxygenates. Thus, separation technologies that operate wholly in the condensed phase will likely be necessary for the biorefinery.

Beyond biochemical conversion strategies, other lignocellulosic conversion routes employ fast, thermal deconstruction of the whole biomass or fractions thereof. Hydrothermal liquefaction uses liquid water at temperatures above 250 °C to produce a liquid

biocrude stream that can be catalytically converted into biofuel molecules. Alternative biomass pyrolysis routes use oxygen-free environments at or above 500 °C to produce bio-oil, light gases, and char from lignocellulosic biomass; some of these products are deoxygenated catalytically, either in the pyrolysis reactor or in subsequent process steps. At an even higher temperature (>700 °C), synthesis gas (CO and H₂) can be produced from biomass via gasification in mildly oxidizing hydrothermal environments. Research opportunities common to these high-temperature biomass conversion processes include the need to understand the complex reaction networks, the design of catalysts and catalytic processes for substantial deoxygenation and operation in the presence of common catalyst poisons entrained in and originating from biomass, and the challenges of operating continuous high-pressure processes.

In most biofuel production processes, chemical coproducts are often invoked as a requirement for economic viability, with the associated challenges of the very large–scale disparity between these two value streams. Even ethylene, the chemical produced in largest amounts from fossil sources, is produced in amounts approximately an order of magnitude smaller than diesel and gasoline, while other commodity-scale chemicals are dwarfed by the scale of ethylene. Ultimately, expensive, small-volume coproducts cannot serve as adequate justification for expensive biofuel production processes, and chemical engineers will play an important role in finding realistic, scalable solutions. It is important to note the annual scale of global petroleum production: 5.0–5.5 billion metric tons (100.69 million bbl per day) of crude oil and 4.1 trillion cubic meters (3.6 billion tons of oil equivalent) of natural gas (IEA, 2021c,d; EIA, 2021d). Petroleum refineries have scales ranging from 10 million to 130 million metric tons per year globally. For biofuels to compete economically with fossil fuels, they need to be produced at similar scales, and even that may not be sufficient because of their unfavorable (oxygen) stoichiometry. Nonetheless, government regulations and/or the implementation of a carbon tax may bring the production of biofuels to a scale of impact, as in the recent case of ethanol in the United States and Brazil.

Feedstocks beyond lignocellulose.

Waste plant biomass is not the only source of renewable or waste carbon for producing biofuels, as is evident from global efforts to use algae, which can grow on marginal lands and in ocean or brackish water. The economical conversion of algae to lipids and other biofuel precursors has not been achieved, however, despite extensive research over decades. Many engineering challenges remain, including cost-effective cultivation in open ponds or controlled photobioreactors, greater CO₂ and photon capture efficiency, separation of target products from cells, and catalysts and processes for the downstream conversion of algae-based intermediates (e.g., fatty acids and carbohydrates) to biofuels.

Some organic waste feedstocks are also of potential use in biofuel production (see the discussion of feedstock flexibility in Chapter 6). Given that municipal solid waste (MSW) contains substantial organic matter (e.g., food waste, paper, and cardboard), chemical engineering has many opportunities to increase the use of such feedstocks through research in fractionation and separations, as well as combined conversion approaches. Similarly, industrial and consumer-based food production yields substantial, often highly reduced (deoxygenated) waste feedstocks, such as oils and fats, that can be

catalytically converted to biofuels, although that process poses substantial challenges. In moving toward a zero-waste society, these organic waste feedstocks, among others, offer substantial opportunities for chemical process development.

Lastly, it is noteworthy that the conversion of CO₂ or gaseous mixtures, such as flue gas from coal-fired power plants or gases from steel processing, is of interest to the chemical engineering community. Considerable effort is currently devoted to realizing the potential of CO₂ and other gas conversions via biological, electro-, and thermal catalysis routes (e.g., Ye et al., 2019). It will be critical to consider substrate concentrations (e.g., direct CO₂ air capture and conversion is a major challenge), the source of reducing equivalents, and the cost and sustainability of any type of process in this vein, as discussed earlier in this chapter in the context of photon capture.

Overall, biofuels will play a role in reducing GHG emissions associated with transportation fuels. However, judicious analyses of process feasibility, economics, and environmental impact will be critical for deciding among the many options; LCA will provide the rigor needed for such analyses. The challenge for chemical engineers is to identify those options that will ultimately be economically successful and sustainable at the scale required to meet society’s fuel needs.

Intermittent Energy Sources

Wind.

Wind has provided a source of power for centuries. Three main types of wind turbines are used today:

• distributed or “small” wind turbines (<100 kW), which are used to power a home, farm, or small business directly and are not integrated into the electrical grid;
• utility-scale wind turbines (100 kW to several MW), which deliver electricity to the grid for distribution to end users; and
• larger offshore wind turbines (up to 15 MW).

Wind energy, a niche option a few decades ago, is now the largest source of renewable electricity in the United States. In 2019, wind energy output represented about 7 percent of the U.S. electricity supply, with 100 GW of capacity—equivalent to powering about 32 million homes (AWEA, 2020). On a global scale, wind energy accounts for about 5 percent of electricity demand (IEA, 2020d).

Chemical engineers are involved in several areas of wind energy production (Veers et al., 2019). Specific challenges in materials research, development, and implementation include

• carbon composites and/or recycled materials for turbine blades;
• cement and steel manufacturing with lower CO₂ emissions for wind turbine structural units, such as motors and gear boxes; and
• metallurgy and lubricants for state-of-the-art wind turbines.

2021, the United States had 94 operable reactors (96,550 MW); 39 inactive reactors (18,140 MW); and two new reactors under construction in Georgia, with a planned electricity generation capacity of about 1,100 MW each (WNA, 2021).

Advanced nuclear industrial cogeneration offers a potential pathway with sufficient heat and energy intensity to address the problems of industrial emissions at scale. Traditional nuclear reactors rely on large light water reactors operating at maximum temperatures below 300 °C—a temperature high enough to make steam for power generation but too low to drive industrial processes. Consequently, the nuclear power industry is currently focused solely on power generation. Advanced reactors have higher output temperatures relative to light water reactors—up to 600 °C for molten salt reactors and 900 °C for high-temperature gas reactors. These higher temperatures are sufficient to drive most petrochemical processes. During the past decade, DOE has explored using this heat for industrial processes with the Next Generation Nuclear Plant.

Advanced nuclear reactors have the potential to provide the heat required by various industrial processes. Significant cost reduction is required for this advanced technology to be affordable for industrial heat generation, but with some new concepts based on low-cost natural gas, competitive, cost-effective solutions are not out of reach. Efforts to drive down cost are focused in three areas:

• New qualified fuels—TRISO (TRi-structural ISOtropic) particle fuel and molten salts—offer fundamentally better process safety profiles. Each TRISO particle is made up of a uranium, carbon, and oxygen fuel kernel, which is encapsulated by three layers of carbon- and ceramic-based materials that prevent the release of radioactive fission products. These fuels are designed so that processing shuts down automatically if they overheat, thus allowing for inherently safer reactors that are much simpler to operate relative to traditional reactors.
• Safer fuel allows for extensive or complete automation, which significantly lowers operating costs.
• Well-supervised factory production of standard reactors attempts to drive down unit costs by applying the fixed manufacturing facility costs over many units and driving annual improvements in efficiency. This factory-built approach has been used to achieve dramatic cost improvements in the wind and solar industries.

Increased demand for nuclear reactors and efficiency gains associated with the corresponding manufacturing learning curve could lower the cost of nuclear reactors. The petrochemical industry sector is capable of driving demand for these units for decades. High-temperature reactors provide high-quality heat directly to industrial facilities, and the integration of this heat will require chemical engineers working within an interdisciplinary team that understands process safety, integration, and intensification.

While existing as a separate discipline, nuclear engineering borrows heavily from physics, as well as from mechanical and chemical engineering. Setting aside the particle physics associated with fission and fusion reactions, a nuclear reactor is effectively a

chemical reactor that takes in fuel as a feed, produces fission or fusion products as waste, and produces heat as a product. Process integration and process design are necessary to extract energy most efficiently from the steam that is generated by a nuclear reactor. Viewed through the lens of the fundamental pillars of chemical engineering (transport phenomena, reaction engineering, thermodynamics, and applied mathematics), the design and safe operation of nuclear power plants are a good match for the skillset of a well-trained chemical engineer. Optimal thermodynamics and heat transfer are key to an efficient process design, as is process control to operate a power plant effectively and safely. Further development of advanced reactor designs, as well as storage solutions for nuclear waste, will also benefit from the same chemical engineering fundamentals. Advances in nuclear energy present a clear opportunity for chemical engineers to collaborate with nuclear and other engineering disciplines.

ENERGY CARRIER PRODUCTION

Energy carriers are intermediates in the energy-supply chain, located between primary and/or secondary sources (Thollander et al., 2020) and end-use applications. For convenience and economy, energy carriers have shifted continually from solids to liquids, recently from liquids to gases, and more recently to electricity, a trend that is expected to continue and even accelerate to address climate change concerns. Currently, about one-third of final energy carriers reach consumers in solid form (as coal and biomass), one-third in liquid form (consisting primarily of oil products used in transportation), and one-third through distribution grids in the form of electricity and gas. It is projected that the share of all grid-oriented energy carriers could increase to about 50 percent of all consumer energy by 2100 (Sims et al., 2007).

The following sections describe the opportunities for chemical engineers to contribute to electricity generation and the production of low-carbon fossil fuels, hydrogen, and synthetic fuels; production of advanced liquid biofuels was discussed previously in this chapter.

Electricity

Reduction of GHG emissions during electricity generation, as well as electrification of light- and medium-duty vehicles and residential/commercial heating, is crucial for decarbonization of the energy sector in the near and medium terms.

Electricity generation from coal-fired plants, the largest CO₂ emitters, has decreased in the United States since 2009, while electricity generation from both natural gas and renewable energy sources has increased. In 2020, about 4,000 TWh of electricity was generated at utility-scale electricity generation facilities in the United States, with about 60 percent of that total being generated from fossil fuels—coal, natural gas, petroleum, and other gases; about 20 percent from nuclear energy; and about 20 percent from renewable energy sources (EIA, 2021b).

The shift from coal to natural gas and renewables was made possible by a steep decline in the cost of key technologies associated with shale gas, wind power, solar power,

and grid-connected electricity storage (DOE, 2015). New wind and solar technologies offer the lowest levelized cost⁷ of electricity over most of the Earth’s surface (IRENA, 2020). Since 2009, the levelized cost of wind has declined by 70 percent and that of solar photovoltaics by almost 90 percent, providing an important means of supplying electricity with no direct CO₂ emissions (Lazard, 2019).

Recent decarbonization studies (e.g., IEA, 2021b; NASEM, 2021a) indicate that deep decarbonization of electricity generation can be accelerated, but further innovation is required. Chemical engineers are contributing to the scale-up, cost reduction, and reliability of improved and novel technologies, particularly in the solar and wind energy sectors. (Specific opportunities in these sectors were discussed earlier in this chapter.)

Low-Carbon Liquid Fossil Fuels

Liquid hydrocarbons from crude oil have been the preferred energy carrier for the transportation sector because of their high energy density, easy distribution and storage, low cost, and well-established and extensive infrastructure along the value chain. If they are to play a role in the low-carbon energy mix of the future, however their carbon footprint will need to be significantly reduced (Figure 3-11).

In 2019, the global demand for liquid hydrocarbons was about 100 million barrels per day, approximately 58 percent of which was for the transportation sector, 14 percent for feedstock for chemicals, 12 percent for power generation/residential/buildings, and 16 percent for other industrial use (ExxonMobil, 2019). Demand is expected to increase until at least 2040, although not uniformly across all sectors. Demand for hydrocarbon liquid fuels for industrial use and for power generation, residential uses, and buildings is projected to decrease, being replaced by energy carriers from renewable sources. In the chemical sector, demand for liquid hydrocarbons used as feedstock to manufacture consumer products is expected to increase. In the transportation sector, overall demand is projected to grow, but not uniformly across transportation types. Gasoline demand for light-duty vehicles is projected to decrease as a result of greater market penetration of EVs, while demand for liquid fuels in commercial transportation (heavy-duty, aviation, and marine) is expected to increase, particularly in the heavy-duty long-haul sector. Changes in global demand for oil, based on new policy scenarios, are projected to follow similar trends (IEA, 2021a).

Petroleum Refining

Most of the CO₂ emissions associated with liquid fuels come from the use/combustion of the fuel itself, but there are also opportunities to reduce emissions during oil production, extraction, and refining. The majority of current refineries were designed and optimized to manufacture primarily gasoline; diesel, aviation, and other heavy fuels and

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⁷ Levelized cost is the sum of total lifetime costs divided by the amount of energy produced, and thus represents the present value of the total cost of building and operating a power plant over an assumed lifetime.

chemicals are normally secondary products. To maintain a low cost of production, the amount of lower-cost heavy (high C/H ratio) and “dirty” crude oils in the feedstock mix is maximized. Refineries will require significant reconfiguration not only to satisfy product demand and meet challenges associated with GHG emissions, but also to maintain low production costs for liquid fuels. Chemical engineering is already playing a central role in addressing these challenges.

Opportunities to reduce CO₂ emissions during refinery operations include the following:

• Increase energy efficiency through further improvement of energy management systems to ensure that refineries are run according to the most energy-efficient standards.
• Replace combustion of liquid fuel for heat generation with renewable sources (e.g., green electricity).
• Produce renewable (green) hydrogen with electrolyzers using imported or self-generated renewable electricity.
• Use low-grade heat generated during operations to produce electricity for internal and external use.
• Eliminate flaring in refineries.
• Replace steam-driven rotating machines and fired heaters with electric counterparts.
• Deploy CCUS from refinery flue gases.

that cannot be met by electrification, including fuel for heavy-duty long-haul transportation; synthetic fuels for aviation and shipping, for which available low-carbon fuel options are limited; ammonia as fuel for shipping; and a source for heat generation in the industrial and buildings sector. Hydrogen is a promising option for storing renewable energy.

Today, hydrogen is produced primarily by steam reforming of natural gas; partial oxidation (catalytic and not) and autothermal reforming of natural gas technologies are also used to a lesser extent. In some countries, particularly in Asia, gasification is used commercially for hydrogen production from coal. Production by water electrolysis is also a commercial technology, but at a much smaller scale and higher cost. Methane pyrolysis, or methane splitting of natural gas using renewable electricity to produce hydrogen and black carbon, is currently in the commercial demonstration scale.⁸ Many other routes to low-carbon hydrogen, such as thermochemical water splitting, direct photocatalysis, and biological production from microorganisms, are in various stages of R&D.

It is generally accepted that hydrogen from fossil fuels without CCUS will remain the main source of hydrogen production. After 2030, it is estimated that almost all of the growth in hydrogen production will come from low-carbon hydrogen (IEA, 2020d) made from renewables-based electricity or from fossil fuels, particularly natural gas, in combination with CCUS. By 2070, electrolytic hydrogen is projected to account for nearly 60 percent of global hydrogen production (IEA, 2020d).

Hydrogen produced from different feedstocks is identified by colors. “Black,” “gray,” and “brown” refer to hydrogen produced from coal, natural gas, and biomass, respectively. “Blue” is commonly used for hydrogen produced from fossil fuels, with CO₂ emissions reduced by the use of CCUS. “Green” hydrogen is produced from water electrolysis using renewable electricity.

Blue and green hydrogen have a path to competitiveness with gray hydrogen (Hydrogen Council, 2019). The competitiveness of blue hydrogen depends primarily on scale-up of CCUS facilities and the value attributed to sequestered CO₂. Carbon prices or taxes of USD 50/ton—a figure consistent with near-term milestones of major economies with net-zero commitments (e.g., in the European Union by 2030; Argus, 2020)—would make blue hydrogen competitive. The competitiveness of green hydrogen will require steep cost reductions for electrolyzers, as well as reductions in renewable energy costs. The production of green hydrogen is projected to break even with that of gray hydrogen before 2030 in regions with low renewable-energy costs, and before 2035 in regions with average renewable-energy costs (Hydrogen Council, 2019). A combination of green and blue hydrogen production pathways will be required to satisfy the potential demand for low-carbon hydrogen. Their supply mix will depend on a range of technical and societal factors, production costs, existing infrastructure (such as power supply and transmission networks), and emerging hydrogen trade routes.

Many challenges and opportunities related to the production of low-carbon hydrogen can be addressed by chemical engineers. For blue hydrogen, the challenges and opportunities center on improved CCUS (i.e., at lower cost) and assessment of geological

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⁸ See www.monolithmaterials.com.

sites for long-term CO₂ storage; for green hydrogen, they center on the need for a source of sustainable, low-cost, renewable electricity and access to freshwater.

Larger-scale electrolysis plants, whose development will depend on achieving lower costs and improved electrical efficiency for the electrolyzers, are also needed. Three main electrolyzer technologies exist today: alkaline electrolysis, proton exchange membrane (PEM) electrolysis, and solid oxide electrolysis cells (SOECs). Alkaline electrolysis is a mature, commercial-scale technology with relatively low capital costs. PEM electrolyzers use pure water as an electrolyte solution and avoid the recovery and recycling of electrolyte solution necessary with alkaline electrolysis; however, they use expensive electrode catalysts and membrane materials. Lifetimes of PEM electrolyzers are currently shorter than those of alkaline electrolyzers, and overall costs are higher. SOECs are the least-developed electrolysis technology and not yet commercialized. Ceramics serve as the electrolyte, resulting in lower material costs. Because steam is used for electrolysis, SOECs require a heat source. If the hydrogen produced were to be used for the production of synthetic hydrocarbons (in power-to-liquid or power-to-gas schemes), the waste heat from these synthesis processes (e.g., Fischer-Tropsch synthesis and methanation) could be recovered to produce steam for further SOEC electrolysis (IEA, 2019).

Synthetic Fuels

Synthetic fuels are produced by converting hydrogen and a carbon source into compounds that can be used as energy carriers, such as methane; methanol; ethanol; and such higher-carbon-number products as gasoline, diesel, and aviation fuel. Ammonia is also increasingly seen as a non–CO₂-generating synthetic fuel, although ammonia production itself is currently a major source of CO₂ emissions. Electrification and advances in electrochemical routes to ammonia synthesis may increase the potential for ammonia as a promising fuel. Low-carbon or carbon-neutral synthetic fuels require green hydrogen and carbon from a bioenergy source or from CO₂ captured from flue gases or the atmosphere using direct air capture (DAC) technologies. In the case of DAC, however, significant energy and technology advances will be necessary to make this route viable. The low-carbon or carbon-neutral synthetic fuels are referred as power-to-fuels (PtF) or electrofuels (e-fuels).

The production of e-fuels requires significant amounts of electricity, and from a thermodynamic point of view, the electricity should be used directly. For example, 25 kWh of energy is required to produce 1 L of synthetic kerosene from electrolytic hydrogen together with CO₂ captured through DAC. More than 80 percent of the energy is used to produce hydrogen; around 15 percent is used for the capture of CO₂ through DAC; and the rest is used in the Fischer-Tropsch synthesis. Currently, only about 40 percent of the energy input is stored in the final liquid product, although with process optimization, the overall conversion efficiency could potentially increase beyond 45 percent (IEA, 2020d). However, e-fuels have the potential to help some sectors—particularly the aviation transportation sector, which is difficult to decarbonize and for which electrification is not an

option because of high energy-density requirements for the energy carrier. These considerations would need to be important enough to justify the thermodynamic inefficiencies, however.

A few aviation e-fuel demonstration plants have been announced. For example, the Norsk e-fuel project is planning a first plant in Herøya, Norway. That plant is expected to become operational in 2023, with a production capacity of 10 million L annually, scaling up to 100 million L annually by 2026.⁹ Cost reduction for e-fuel production is expected to benefit from the economy of scale and experienced manufacturing learning curve. It should be emphasized that DAC technology is in its infancy, with substantial room for technology and process improvement and cost reduction. For e-fuels to be competitive with conventional fossil fuels and even bio–jet fuel, a combination of low electricity costs and a high CO₂ tax or cost will be needed.

ENERGY STORAGE

Energy storage occurs when an energy carrier is held in a fixed location until it can be deployed. When the energy carrier uses chemical bonds or potential energy, as is the case for liquid fuels and pumped hydropower, respectively, energy storage is conceptually simple. When the energy carrier is electrons, storage requires batteries or capacitors or conversion of the electrical energy into another energy carrier. Because many of these are mature technologies, chemical engineers have limited potential to impact some modes of energy storage, especially those involving mechanical energy, such as pumped hydropower and compressed gas storage. For energy carriers that involve electrons or chemical bonds, however, chemical engineers can play a critical role in the development and deployment of scalable and economical energy storage.

The changing nature of the world’s energy system has continued to create significant demand for energy storage, and this trend is likely to accelerate in the coming years. One obvious example involves intermittent renewable electricity sources such as solar and wind, whose full contribution to decarbonization of electricity generation can be realized only if they are coupled with grid-scale storage. A second example is the trend toward electrification of light-duty vehicles, which will require massive deployment of on-board batteries.

The properties of an energy storage system can vary with both scale and application. In vehicle applications, for example, the weight and size of batteries are critical; in grid-scale storage, by contrast, the battery weight is unimportant. The typical storage time and desired charging and discharging rates are also very different for these two applications. This example illustrates why future energy storage needs will be met by a wide array of technologies—there is no one-size-fits-all approach to energy storage. The rapid deployment of electrical storage systems is underpinned by the commercial sector’s large investments in technology development. If academic research is to have an impact in this environment, researchers will have to seek solutions in frontier areas in which well-

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⁹ See https://www.norsk-e-fuel.com/en.

funded development efforts in industry are less likely to overwhelm the scale of any academic effort.

An important distinction between energy storage in electrons and chemical bonds lies in the number of times a particular piece of storage medium can be cycled. For fuels based on chemical bonds, storage simply involves a tank, and little to no change in the storage medium is expected even after thousands of cycles. Here, chemical engineers have a role to play in understanding evaporation and erosion, as well as improving leak detection methods. Batteries, however, tend to degrade during cycling. Many of these degradation processes stem from chemical reactions or nucleation and growth of phases that are driven by fundamental chemical engineering principles. This observation indicates that chemical engineers can play a key role in developing concepts that increase battery lifetimes. At the same time, for batteries to be a core part of a truly sustainable energy system, their end-of-life disposal or recycling needs to be considered. Similarly, the global availability of battery components (e.g., lithium) is an important consideration in LCA comparisons of competing technologies. The intentional design of batteries for end-of-life disposal and the use of earth-abundant elements are areas in which chemical engineering researchers are poised to play an important role.

As discussed in the previous section, hydrogen is a versatile energy carrier with significant potential to contribute to a clean, low-carbon energy system. Given its low energy density, however, its long-distance distribution and storage poses challenges, particularly if it needs to be shipped overseas. It has been estimated (IEA, 2019) that for distances above ~1,000 miles, shipping hydrogen as ammonia or liquid organic hydrogen carriers (LOHCs) is likely to be more cost-effective than shipping liquified hydrogen. Chemical engineers have opportunities to significantly lower the cost of conversion before export and reconversion back to hydrogen in the case of LOHCs, and before consumption or the direct use of ammonia as fuel.

Vast sectors of the global energy system rely on liquid or solid energy carriers, such as gasoline and coal. The energy density and ease of transporting these carriers relative to gases or electrons give them strong intrinsic advantages. Chemical engineering played a central role in what might be termed the hydrocarbon economy of the 20th century, and the discipline will also be central to the deployment of more sustainable and environmentally benign liquid energy carriers in the 21st century. Unlike high-value-added products such as pharmaceuticals, new or improved liquid energy carriers need to be competitive with low-cost alternatives and deployable at massive scale in order to be viable. In many cases, achieving these goals will require integrated process development rather than a singular focus on improved catalysts or similar chemical steps. The ability of chemical engineers to use LCAs and technoeconomic assessments (TEAs) to focus research on approaches with a plausible path to economic viability ensures that their contributions will have impact in the domain of energy storage.

ducing CO₂ emissions. Advances in ICE technology have traditionally been led by mechanical engineers, making this a key opportunity for chemical engineers to collaborate with another engineering discipline. Areas for such collaboration include

• advanced combustion schemes (e.g., low-temperature combustion);
• electrified accessories and waste-heat recovery;
• hybridization;
• lightweighting, with new, aerodynamic cabs and trailer components;
• advanced communication and logistics;
• energy-efficient tires and monitoring systems; and
• advanced GPS-based predictive control technology.

GHG emissions could be reduced in ICE-powered heavy-duty vehicles by the use of low-carbon liquid fuels. Codevelopment of more efficient fuels and vehicle engines is a major collaborative opportunity for chemical and mechanical engineers, with chemical engineers bringing expertise in reaction mechanisms and transport and mechanical engineers bringing expertise in transport, computational fluid dynamics, and engine design. Opportunities in this area include

• blends of high-quality, low-carbon fossil-diesel fuel with advanced biofuels and/or synthetic fuels (e.g., e-fuels) to achieve specific tailpipe GHG emission levels; and
• increased control of the variability of diesel-fuel properties.

Industrial Sector

The industrial sector produces the goods and raw materials people use every day. Industry is energy intensive—nearly half of the world’s energy is dedicated to industrial activity; accordingly, it is also responsible for a large portion of global CO₂ emissions. Production of cement, steel, and chemicals accounts for the largest portion of industrial CO₂ emissions—about 70 percent (IEA, 2020d). Reducing manufacturing-related emissions in the United States and China will be critical to reducing CO₂ global emissions from manufacturing (Figure 3-13).

The lack of commercially available and scalable low-carbon alternatives to fossil fuels makes deep reductions in CO₂ emissions from industry highly challenging in the short and medium terms. This fact is reflected in the Sustainable Development Scenario projections (IEA, 2020d), in which the industrial sector emerges as the second-largest CO₂ emitter in 2070, after the transportation sector, accounting for around 40 percent of residual emissions, even though its emissions are projected to be 90 percent lower overall than in 2019 (Figure 3-13). Currently, energy inputs to the industrial sector are approximately 70 percent from fossil fuels (IEA, 2020d). In the Sustainable Development Scenario projections, the use of fossil fuels in industry would be reduced by more than 60 percent by

2070, being replaced primarily by electricity and bioenergy, while more than 75 percent of the remaining CO₂ emissions would be captured and stored permanently. The following sections focus on opportunities for chemical engineers in the cement, steel, and chemical production subsectors, as well as cross-cutting approaches for decarbonization of the industrial sector.

Cement Production

More than 70 percent of the energy used in the U.S. cement industry comes from coal and petroleum coke. More than 50 percent of the total CO₂ emissions attributable to cement production are process related (from calcination of limestone in the kiln), not energy related. To advance decarbonization of the cement industry, in addition to energy-efficiency improvements, these process-related CO₂ emissions will need to be reduced. Key strategies for deep decarbonization of the cement industry are clinker substitution (supplementary cementitious materials [SCMs]), a switch to lower-carbon fuels, CCUS, low-carbon cement and concrete chemistries, and kiln electrification. While several of these strategies can be combined to approach net-zero emissions, decarbonization efforts related to demand reduction, use of SCMs, and waste-carbon utilization will also be needed. For example, Hasanbeigi and Springer (2019) recently showed that, compared with 2015, total CO₂ emissions from California’s cement industry could decrease by 68 percent by 2040 even though the state’s cement production is projected to increase by 42 percent (Figure 3-14).

ethanol or chemicals); and green hydrogen plasma smelting reduction. Although some of these decarbonization technologies have been commercialized, some require further RD&D, such as

• industrial-scale plant design for producing iron by electrolysis, and development of detailed cost models for assessing the commercial viability of the process; and
• development of methods to analyze and evaluate the BF operation continuously when extending the use of hydrogen to all tuyeres (injection nozzles for air/H₂) in BFs.

Chemical Manufacturing

The chemical industry is highly diverse, producing more than 70,000 products globally. Yet 18 large-volume chemicals—including light olefins, ammonia, BTX (benzene, toluene, xylene) aromatics, and methanol—account for 80 percent of the energy demand and 75 percent of the total GHG emissions attributable to global chemical manufacturing (IEA, ICCA, and DECHEMA, 2013). For perspective, the global production volumes in 2012 were 220 million metric tons for ethylene and propylene, 198 million metric tons for ammonia, 58 million metric tons for methanol, and 43 million metric tons for benzene. Together, production of these four products used about 7.1 EJ of energy per year (or a specific energy consumption of about 13.7 MJ/kg of product), and the top 18 large-volume products used a total of about 9.4 EJ of energy per year (Figure 3-15).

Because about 90 percent of chemical manufacturing processes use catalysts, catalyst and catalyst-related process improvements could reduce the energy intensity of these 18 products by 20–40 percent by 2050, amounting to energy savings of about 13 EJ and a CO₂ emissions reduction of 1 metric gigaton (IEA, ICCA, and DECHEMA, 2013). Incremental improvements will suffice in the short to medium term; in the longer term, however, the deployment of new technologies, such as biomass feedstocks and green hydrogen, will be necessary. The challenges for chemical engineers are clear: to identify top catalyst and catalyst-related process opportunities for these 18 and other chemicals.

Key technologies for decarbonization of the chemical industry include energy systems engineering; fuel switching; novel catalytic processes (e.g., olefin production via catalytic cracking of naphtha or renewable methanol); advanced separation processes; electrification; transitions to low-carbon feedstocks and processes (e.g., biomass, hydrogen-based production of ammonia and methanol, artificial photosynthesis, renewable-energy electrochemistry, biobased plastics production, gas-to-liquids gas); and CCUS. Chemical engineers can contribute to the RD&D necessary to achieve these technological advances, including the following examples:

• Achieve viability for natural gas crackers and improve catalyst production; improve the efficiency of olefin production via catalytic cracking of naphtha or via methanol; and address environmental issues for used catalysts, including regeneration and catalyst selectivity and lifetime.

• Reduce technical and economic hurdles for such low-carbon processes as anaerobic digestion and gasification, biobased production of plastics, and hydrogen-based production of ammonia and methanol.
• Advance processes related to water electrolysis, such as optimized processes for variable operation, improved stability for operations under pressure (30–40 bar), electrodes with low-content noble metals and other rare elements, and photocatalytic water splitting (e.g., non–noble metal electrodes, corrosion-resistant photoelectrode materials improved over potential).
• Reduce technical hurdles for fuel switching to hydrogen and other lower-carbon fuels, electrification, and CCUS.

Petroleum Refining

The United States is one of the largest producers of liquid transportation fuels and refined petroleum products in the world, and energy engineering could reduce the fuel used in producing these products by 50 percent (Morrow et al., 2015). Key technologies for decarbonization of petroleum refining include fuel switching, electrification, biomass hydrothermal liquefaction and biocrude oil, synthetic fuel synthesis, reduction or elimination of flaring, selective membranes, advanced control and improved monitoring, CCUS, catalytic cracking, progressive distillation, self-heat recuperation, and biodesulfurization. Further RD&D is critical for these technologies, and chemical engineers can make contributions to

• advance synthetic fuel synthesis;
• integrate biofeedstocks;
• scale up technology for conversion of CO₂ and H₂ to hydrocarbons with lower electricity consumption;
• improve chemical separations with lower energy demand for selective membranes;
• apply advanced modular nuclear reactors for low-temperature steam generation; and
• improve electric heating to achieve high temperatures and large scales efficiently and economically.

Food and Beverage Industry

The food and beverage industry is the third-largest consumer of energy in the United States; its energy consumption is dominated by mechanical systems, compressed air, refrigeration, and process heat in the moderate-to-low temperature range. Key technologies for decarbonization of the food and beverage industry include fuel switching to lower-GHG sources, such as electricity and renewables, and plant efficiency measures, particularly because many of these plants tend to be operated by small- and medium-sized manufacturers with limited energy-management capacity. Electrification of dewatering,

drying, and process-heating applications using heat pumps, hybrid boilers, induction heating, dielectric heating, and advanced cooling/refrigeration represents an important opportunity for this subsector. In addition, advanced processing and preservation to reduce degradation in processing, along with improvements at the supply chain and consumer levels, are important because on a global scale, about a quarter of the food supply is wasted (see Chapter 4; Buzby et al., 2014; D’Odorico et al., 2018; Finley and Seiber, 2014). A key challenge for this subsector is that food and beverage processors must comply with multiple regulations that complicate the implementation and slow the adoption of new technologies. Although some decarbonization technologies are commercially available, further RD&D is needed in such areas as

• shifting from steam and fossil fuels to electric and solar heating technologies;
• demonstrating and certifying alternative processing technologies to reduce GHG emissions while maintaining product safety and quality, and reducing degradation in the supply chain;
• using waste to produce bioenergy; and
• adopting fuel switching and expanded implementation of energy efficiency.

Cross-Cutting Approaches for Decarbonization of the Industrial Sector

To achieve the net-zero emissions goal for industry, five broadly applicable decarbonization pillars require vigorous pursuit in parallel over the next decades: demand reduction, energy efficiency, fuel switching and electrification, transformative technologies in sectors, and abatement. The applicability and selection of these pillars will vary across sectors, with weighing of trade-offs in costs and accessible resources (de Pee et al., 2018). Except for sector-specific transformative technologies, these approaches can be considered cross-cutting and can facilitate reduction of GHG emissions across multiple sectors.

Barriers abound across the landscape of deployment, development, scale-up, and whole-system integration of current, emerging, and transformative technologies that can advance these cross-cutting approaches. Nonetheless, chemical engineers have opportunities to help overcome these barriers (e.g., in the areas of competitiveness, carbon capture and use, and advanced materials). Many low-carbon technologies are in the early stages of development and will require extensive RD&D for effective deployment. RD&D needs range from advancing cross-cutting technologies (e.g., improved electrolysis of water to lower-cost H₂) to making radical changes (e.g., applying high-temperature heat for ethane crackers). Figure 3-16 summarizes a more comprehensive set of recommendations from a recent study by the National Academies of Sciences, Engineering, and Medicine on low-carbon technologies, approaches, and infrastructure needing RD&D investment in the 2020–2050 period (NASEM, 2021a). A portfolio of collaborative RD&D initiatives, multigeneration plans, agile management, and durable support will be needed to face these challenges successfully and drive progress going forward (NASEM, 2019f, 2021b).

development of technologies that can drive progress. As is the case with other topics discussed in this chapter, putative technologies will need to be deployable at low cost with high reliability to have any chance of success.

Although systems for refrigeration and air conditioning are ubiquitous, improvements in their efficiency and sustainability will require overcoming key technological challenges. Chemical engineers can play a pivotal role in the development of new heat-transfer fluids that are nontoxic and nonflammable and lack the very high GHG intensity of many chlorofluorocarbons (CFCs) and other halocarbons. Nontraditional heat-transfer cycles, such as adsorption cooling, and materials with caloric properties (e.g., Moya and Mathur, 2020) also have considerable potential that intersects strongly with chemical engineering applications in other domains.

Improving the properties of building materials is a key path toward improving commercial and residential energy efficiency. Opportunities include the development of passive materials, such as paints for roofs that reflect the solar spectrum more effectively and, as a result, reduce the cooling load required for buildings (Li et al., 2021a); and active materials, such as “smart glass,” which adapts to external sunlight to reduce the net energy demands in buildings (Alias et al., 2019). Chemical engineers have many opportunities to contribute creatively in these areas, provided that researchers remain focused on meeting the cultural and economic needs of end users rather than on fashioning “pure technology” solutions.

CARBON CAPTURE, USE, AND STORAGE

CCUS will be centrally important to controlling the concentration of carbon in the atmosphere. Achieving net-zero emissions to halt growth in the concentration of atmospheric CO₂ will not require achieving zero anthropogenic CO₂ emissions, but rather balancing anthropogenic CO₂ emissions with natural and anthropogenic CO₂ sinks. An extensive portfolio of mitigation strategies for GHG emissions could contribute to the achievement of net-zero emissions (Figure 3-17). Six negative emissions technologies (NETs) remove carbon from the atmosphere and sequester it (Fuss et al., 2018; NASEM, 2019b): coastal blue carbon, terrestrial carbon removal and sequestration, bioenergy with carbon capture and sequestration, DAC, carbon mineralization, and geological sequestration. Other approaches have been proposed, as well, such as cloud alkalinity, biomass burial, enhanced ocean upwelling and downwelling, DAC by freezing, marine bioenergy with carbon capture and sequestration, and electrochemical lining (The Royal Society, 2018b). The removal of other gases, such as methane, N₂O, and CFCs with significant global warming potential will also be important in the overall effort; however, the concentration of these gases in the atmosphere is much smaller than that of CO₂, making them very difficult to remove once they have been released.

The broad area of CCUS is a very rich one for chemical engineering; it presents numerous opportunities for major contributions, as well as challenges to address over the next years and decades, in the technical, economic, LCA, and integrated assessment management (IAM) areas. IAM is a quantitative tool for combining information from diverse

fields (i.e., science, economics, and policy) to assess the impact of emissions or their reduction. To address these challenges effectively, however, chemical engineers will need to partner with engineers from other disciplines, such as environmental engineering (e.g., NASEM, 2019c).

DOE has identified priority research directions for CCUS (DOE, 2018c). Of these priorities, chemical engineers could contribute in the following areas:

• Capture—designing high-performance solvents and developing environmentally friendly solvent processes, designing sorbent materials and integrated processes, developing membranes and related processes, and producing hydrogen from fossil fuels with CO₂ capture.
• Utilization—designing interfaces for enhanced hydrocarbon recovery with carbon storage; valorizing CO₂ from catalytic, electrochemical, and photochemical transformations to fuels, chemicals, and new materials; and tailoring microbial and bioinspired approaches to CO₂ conversion.
• Storage—advancing multiphysics and multiscale fluid flow to achieve Gt-per-year capacity; locating, evaluating, and remediating existing and abandoned wells; and optimizing injection of CO₂.
• Cross-cutting—integrating experiments, simulations, and machine learning across multiple length scales to guide the development of materials and processes; intensifying CCUS processes; incorporating social aspects into decision making; and integrating LCA and technoeconomic assessment (TEA), along with environmental and social considerations, to guide technology portfolio optimization.

Near-zero- and positive-emissions technologies emit almost zero GHGs or emit GHGs to a lesser extent compared with alternative technologies, respectively. They include enhanced energy efficiency, clean or renewable electrification, bioenergy, hydrogen and hydrogen-based fuels, and CCUS technologies. LCA and TEA (including scalability of the specific technology) are two primary tools used to evaluate and rank these technology options. Chemical engineers can play a role in the development of many, technologies that have been reviewed extensively elsewhere (e.g., Bui et al., 2018; Fuss et al., 2018; Hepburn et al., 2019; IEA, 2020d; NASEM, 2019b). Opportunities for chemical engineers in the areas of DAC and CO₂ and CH₄ utilization are briefly summarized in this section.

Direct Air Capture

DAC appears to be a relatively easy fix for climate change and has the additional advantage that it can be located close to the sequestration reservoir, thus avoiding the need to use a pipeline for CO₂ transportation. However, dilute systems, such as those with CO₂ in the air at a concentration of about 400 ppm, require more energy than concentrated systems, such as those with CO₂ in flue gas from ammonia manufacture at a concentration exceeding 98 percent; from coal-fired power plants at a concentration of 12–15 percent; from cement, iron/steel, and glass production at a concentration of 20–35 percent; and

from natural gas–fired power plants at a concentration of 3–4 percent on a volume basis (NASEM, 2019a). Thus, the concentration of CO₂ in flue gases is between almost 100 and 300 times that of CO₂ in the air, and as a result, the energy required to capture CO₂ from the air is 2 to 3 times greater than that required to capture CO₂ from the flue gases (Bui et al., 2018).

Areas on which chemical engineers will focus in the future include the development of low-cost solid sorbents, highly CO₂-selective materials that require reduced regeneration energy, materials that are highly active in ambient conditions, and processes with increased mass-transfer coefficient and high throughput and low pressure drop (NASEM, 2019b). Other areas of focus will include packing designs, process intensification, catalytic additives, and long-term stability of sorbent matrices.

plants is primarily methane, whereas that from waste management operations, called biogas, is a mixture of CO₂ and methane. In contrast with CO₂, methane is a high-value and high-energy chemical and has no equivalent pathways to mineral carbonation. Because of its high energy, it is used primarily as fuel, and thus any conversion to chemicals needs to compete with the fuel value of methane. The cost trade-offs are likely to change with increased use of CCUS technologies. Challenges and opportunities for chemical engineers in this area include the development of catalysts, integration of catalyst and reactor technology, and identification of new chemical targets. Tools such as LCA and TEA will also be critical in identifying situations in which methane conversion is cost-competitive with the use of methane as fuel.

Biological Conversion of CO₂

Biological conversion of CO₂ holds great promise because some microorganisms have a natural ability to capture and covert CO₂. Photosynthetic pathways to CO₂ utilization include approaches using algae (products include biofuels, dietary protein and food additives, and commodity and specialized chemicals), green algae (products include biodiesel, dietary protein, polyunsaturated fatty acids, pigments, lipids, and terpenoids), and cyanobacteria (products include ethanol, butanol, fatty acids, heptadecane, limonene, bisabolene, ethylene, isoprene, squalene, and farnesene). By using CO₂ as feedstock, these photosynthetic pathways mitigate the problem of the high-cost sugar feedstocks needed for microbial pathways; however, the slow growth rates of algae and cyanobacteria prevent them from achieving industrially relevant productivity and scale-up. Challenges and opportunities for chemical engineers in this area include bioreactor and cultivation optimization, analytical and monitoring tools, genome-scale modeling and improvement of metabolic efficiency, bioprospecting, valorization of coproducts, genetic tools, and pathways to new products.

Biological Conversion of CH₄

Methanotrophs can use methane as their carbon and energy source. However, significant challenges arise, such as the risk of contamination during fermentation, buildup of toxic intermediates, and the high cost of additives. Some commercial activity has taken place in this space. Calysta™ has commercialized FeedKind^® protein as an alternative feed for fish, livestock, and pets, using no agricultural land and less water than is required for similar agricultural products. Intrexon™ has used methanotrophs to produce high-value chemicals, such as isobutanol and farnesene. And Mango Materials™ plans to convert biogas into polyhydroxyalkanoate (PHA), which is a biodegradable plastic. Challenges and opportunities for chemical engineers in this area are the same as those for biological conversion of CO₂.

increased recovery to extend well life, data-driven approaches to reservoir management, and improved methane management. Decreasing the GHG emissions associated with all types of fossil fuels will require the demonstration of cost-effective and secure carbon capture and storage methods, a critical opportunity for chemical engineers.

Because most biogenic feedstocks have lower energy content than their fossil-based counterparts, the greatest challenge for increased use of biofuels is the production of high-density fuels at a reasonable cost that is competitive with that of existing, fossil-based fuels. This challenge, combined with the need to account for the environmental consequences of harvesting crops for energy use, creates opportunities for chemical engineers to use systems-level economic, environmental, and technical analyses to select the most viable biofuel options. Furthermore, increasing the market penetration of EVs for personal transportation will require reimagining petroleum refineries that were designed to produce gasoline or diesel fuel as their main products. Refineries will require reconfiguration to shift their product slate toward petrochemicals and low-carbon liquid fuels needed for the difficult-to-decarbonize commercial transportation sector (e.g., heavy-duty long-haul ground, aviation, and marine transportation). Full integration of existing petroleum refinery assets with biorefineries and the greater use of renewable energy will enable significant reductions in carbon footprints and lower-cost low-carbon liquid fuels, another area that presents considerable opportunities for chemical engineers.

For intermittent energy sources, chemical engineers can contribute to the development of advanced materials that can increase the viability of wind and marine energy. Chemical engineering research will also be critical to advancing low-carbon fuels; improving petroleum refining; advancing clean hydrogen production; and developing improved synthetic fuels for sectors, such as aviation, that are difficult to decarbonize. A successful transition to low-carbon energy systems presents the challenge of energy storage. Chemical engineers can enable the development of new battery materials, as well as contribute to LCAs of competing battery technologies and the design of batteries for safe end-of-life disposal.

For end uses, the production of cement, steel, and chemicals presents the clearest opportunities for chemical engineers to contribute to decarbonization of the industrial sector. To achieve the net-zero emissions goal for industry, five broadly applicable decarbonization pillars require vigorous pursuit in parallel over the next decades: demand reduction, energy efficiency, fuel switching and electrification, transformative technologies in sectors, and abatement. All will benefit from the contributions of chemical engineers.

Finally, CCUS will be centrally important to controlling the concentration of carbon in the atmosphere. This broad area is rich with opportunities for chemical engineers, including LCA, integrated assessment management, and the science and technology advances necessary to advance direct air capture and carbon utilization.

Recommendation 3-1: Across the energy value chain, federal research funding should be directed to advancing technologies that shift the energy mix to lower-carbon-intensity sources; developing novel low- or zero-carbon energy technologies; advancing the field of photochemistry; minimizing water use associated with energy

systems; and developing cost-effective and secure carbon capture, use, and storage methods.

Recommendation 3-2: Researchers in academic and government laboratories and industry practitioners should form interdisciplinary, cross-sector collaborations focused on pilot- and demonstration-scale projects and modeling and analysis for low-carbon energy technologies.