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Introduction

Waste streams containing carbon dioxide, methane, and biogas¹ represent large flows of emissions into the atmosphere. These emissions, which have risen sharply over the past several centuries due to anthropogenic activities, are a hazard primarily due to their greenhouse gas effect. Carbon capture, utilization, and sequestration technologies may play a critical role in reducing global greenhouse gas emissions. Carbon capture paired with sequestration for reducing CO₂ emissions from large point sources has received considerable attention as one approach to greenhouse gas mitigation. More recently, strategies that pair carbon capture with utilization have been receiving increased attention from researchers and policy makers as an alternative path for captured carbon dioxide or methane.

Carbon utilization is based on the concept that gaseous carbon waste streams can have value as feedstocks for producing fuels, construction materials, plastics, or other useful materials. For the purposes of this report, carbon utilization is defined as the manufacture of valuable products from a gaseous carbon waste feedstock (carbon dioxide and methane) that results in a net reduction of greenhouse gases emitted to the atmosphere.² This value may be captured by different actors and may represent economic value stemming from production and use of the product made from the carbon waste stream, or it may represent the value of reducing emissions that results from carbon utilization. The ultimate value and extent of carbon utilization will depend on future technology, economics, and policy drivers. This report identifies key scientific and technological barriers to carbon utilization and the factors that will influence the extent of carbon utilization and suggests a research agenda that would further enable carbon utilization.

The sources and types of gaseous carbon waste are illustrated in Figure 1-1 and discussed in more detail in Chapter 2. In the United States, carbon dioxide emissions are primarily due to fossil fuel combustion while methane emissions stem primarily from oil and gas production, landfills, livestock, and manure (NASEM, 2018). A third source of gaseous carbon waste,

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¹ Biogas is a mixture of methane, carbon dioxide, and other trace species. It is generated by anaerobic biological processes, often in landfills or sewage treatment facilities.

² The calculation of net greenhouse gas emissions is described in Chapter 8.

biogas, contains both carbon dioxide and methane and is generated by the degradation of biomass in the absence of oxygen.

Mitigating greenhouse gas (GHG) emissions is and will be an important goal for many who are exploring carbon utilization technology development and implementation. There are many scenarios for mitigating GHG emissions to the atmosphere, ranging from decarbonization of energy and other economic sectors to carbon capture with sequestration and/or utilization. The long-term mitigation impact of carbon utilization technologies will depend on the life-cycle emissions from production through use of the product, through the fate of the carbon at end of product use. Key aspects determining the mitigation potential of carbon uti-

lization technologies and resulting products include the greenhouse gas emissions associated with product production, the volume of carbon embedded in the product (depending on the size of the product market and amount of carbon incorporated), the product lifetime during use, and the ultimate fate of the carbon in the product at end of use. Different products and processes will have different resulting mitigation impacts, and carbon waste stream utilization may be pursued for reasons other than long-term carbon mitigation.

Currently, capture of carbon waste gases is limited, with 45 large-scale carbon dioxide capture projects operating with a total capture capacity of 80 million tons per annum, globally (Global CCS Institute, 2015). The majority of the captured gases are sequestered, and the utilization of gaseous carbon waste is limited to niche applications. There are many scenarios for how carbon flows will operate in the future, including scenarios with varying mixes of deep decarbonization of energy and other sectors, as well as scenarios with carbon capture and/or utilization. Some envision a much larger role for carbon utilization in the future management of carbon emissions. For example, Shell’s latest energy-system scenario, Sky, depicts the potential achievement of net-negative emissions by 2100 through a combination of large-scale (11 Gt/yr) capture and geologic storage of carbon dioxide and even larger scale removal of carbon dioxide with photosynthesis (26 Gt/yr). The Sky scenario relies on a complex combination of reinforcing drivers, accelerated by society, markets, and governments, and follows three distinct routes (see Figure 1-2). In such a scenario, carbon wastes could become inputs to a cyclic industrial system that transforms the wastes into useful products through biological and chemical processes.

STUDY CHARGE

At the request of the U.S. Department of Energy (DOE) and Shell, the National Academies of Sciences, Engineering, and Medicine (the Academies) assembled an ad hoc committee to assess the economics, benefits, risks, scale, energy, and life-cycle requirements for carbon utilization technologies and develop a detailed research agenda to increase the technical and commercial viability of carbon utilization (Box 1-1).

STUDY APPROACH

In addressing its charge, the committee limited the scope of the study by carbon waste stream, type of carbon use, and geography. Only carbon waste streams that contain carbon dioxide, methane, or biogas were considered, and the committee focused on the utilization of only these components of gaseous waste streams. While gaseous waste streams commonly contain other gases and particles, these other constituents were only considered insofar as they impact the utilization of carbon dioxide, methane, or biogas. The committee did not focus extensively on the capture of carbon dioxide and methane, as that is being examined by a

parallel study under way at the National Academies (Box 1-2). Carbon utilization technologies that are already mature or do not chemically transform carbon dioxide or methane into products or materials were excluded from the study.

The study focuses on advancing carbon utilization within the United States but recognizes that U.S. carbon utilization takes place within a larger global context. For example, it is assumed that waste gas feedstocks and other inputs may come from other countries, and that international market demand plays a role in the commercial viability of products produced

in the United States. It is also assumed that domestic utilization technologies would take advantage of state-of-the-art research and technologies developed anywhere; as a result, the committee’s information-gathering process included input from speakers involved in carbon utilization research and development outside the United States.

Although the environmental and economic evaluation of carbon utilization technologies is discussed in Chapters 8 and 9, the committee did not attempt to conduct detailed environmental and economic evaluations of any particular technology. The committee’s focus was on identifying scientific and technological barriers to carbon utilization (Chapters 3-6 and 11) and on identifying factors and criteria that should be used to evaluate the commercialization potential of various technologies (Chapter 10). The prioritized research needs for waste carbon utilization range from fundamental research to research needed for commercialization. The committee attempted to identify broad progressions of research, starting with fundamental research, proceeding to proof-of-concept activities at the bench scale, pilot scale, and demonstration scale, and concluding with research needed at near-commercial stages. The boundaries between the stages are not distinct.

To support its deliberations, the committee gathered information from experts and stakeholders from government agencies, industry (including trade associations), and academia. A list of those experts and stakeholders may be found in the Acknowledgments. Information about current utilization technologies at all levels of technology readiness was provided to the committee during several data-gathering meetings and follow-up communications. The committee also gathered information from public documents, including the scientific literature and government reports.

The committee structured its activities by examining carbon dioxide utilization, methane utilization (including biogas), and enabling technologies, resources, and analyses. Each of these segments of the carbon utilization system is described below.

CARBON DIOXIDE UTILIZATION

A wide variety of potential uses have been identified for carbon dioxide waste, as illustrated in Figure 1-3. Carbon dioxide has been used for decades in enhanced oil recovery, as a refrigerant, as an extractive solvent, and as an additive in food and beverage products; as technologically mature processes that do not involve chemical transformations, these uses were outside the study scope.³ Instead, the committee focused on emerging carbon dioxide utilization waste stream technologies that offer the promise of a net reduction in greenhouse gas emissions, including the mineral carbonation to produce construction materials and the chemical or biological conversion of carbon dioxide to fuels and chemicals.

A number of studies have attempted to estimate the market for carbon utilization products and one published study has reported the future market could be as high as $800 billion by 2030, utilizing 7 billion metric tons of carbon dioxide per year (CO₂ Sciences, Inc., 2016). Three broad categories of carbon utilization were considered by the committee:

1. Conversion to inorganic products (mineral carbonation). This process converts stable CO₂ into an even more stable form of carbon, typically a carbonate, which can be used to produce construction materials such as concrete. Mineralization involves reaction of minerals (mostly calcium or magnesium silicates) with CO₂ to give inert carbonates. The reaction to form carbonates itself requires no energy inputs and actually releases heat, although significant energy is typically required to generate the requisite feed minerals. The current bottleneck, however, for viable mineral carbonation processes on an industrial scale is the reaction rate of carbonation. Addition-

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³ CO₂ is already used in commercial processes, both in its pure form and as a feedstock in the synthesis of bulk chemicals such as urea. In the pure form, CO₂ is presently used in the food industry with applications that vary from carbonation of drinks to accelerated production of greenhouse tomatoes. Likewise, bulk CO₂ is also used as solvents in processes such as dry cleaning of fabric and decaffeination. CO₂ has also been used in enhanced oil and gas recovery by pumping it under near-critical or supercritical conditions into oil fields where conventional recovery has become uneconomical or impractical.

1. new formulations of materials such as concrete will require testing and property validation before being accepted by users and regulators for the market.
2. Chemical utilization. It is possible to use CO₂ for the production of fuels and chemicals by reacting it with other molecules and/or providing electrochemical, photochemical, or thermal energy. These conversions require catalysts to overcome kinetic barriers. Because carbon in CO₂ is in its most highly oxidized form, many of the resulting reactions are reductions, either through the addition of hydrogen or electrons. Catalysts are critical not only for making the transformation possible, but also for reducing the energy inputs to (ideally) the minimum amount dictated by the thermodynamics of the transformation, and discovery of appropriate catalysts and development of energy-efficient processes are current bottlenecks.
3. Biological utilization. Biological conversion involves using photosynthetic and other metabolic processes inherent to plants, algae, bacteria, and fungi to produce higher-value chemicals. Several factors have expanded the repertoire of biobased products that can be synthesized directly from CO₂, including the large number of CO₂utlizing microorganisms, genetic modification of microorganisms, and tailoring enzymatic/protein properties through protein engineering. Biogical utilization has a large range of potential uses in the development of commercial products, including various biofuels, chemicals, and fertilizers. However, biogical utilization rates and scalability remain challenges.

These carbon utilization approaches are discussed in detail in Chapters 3-5.

METHANE UTILIZATION

Although carbon dioxide waste gas utilization is a main focus of this report, methane also serves as a source of carbon for making products, as described in Figure 1-4. The two gases differ in physical and chemical properties and in the locations, magnitudes, and properties of the waste gas streams (see Chapter 2). Carbon dioxide is fundamentally a low-value, low-energy waste gas, which is often available in large quantities in single locations. Methane, in contrast, is a high-value, high-energy molecule. Because of its high-value and high-energy of combustion, methane as a waste gas is only available in low quantities, or with intermittent flows. If methane is available as a waste gas, chemical uses of the waste gas must compete against the value of methane as a fuel, limiting possible nonfuel utilization pathways.

Methane from waste gases, including biogas, can be collected and used directly as a fuel. Direct use of methane from waste gases, without chemical modification, is a relatively mature technology and was considered by the committee only in the context of improved gas cleaning operations that would make these energy recovery processes more economical. In addition to direct use as fuel, methane in waste gases can be used as a feedstock in chemical

and fuel manufacturing technologies. Some chemical pathways that use gaseous methane to make liquid fuels (e.g., Fischer-Tropsch synthesis) are relatively mature technologies and were outside the study scope. Instead, the committee focused on pathways for using methane to produce fuels and chemicals that are at an earlier stage of research and development.

Like carbon dioxide, methane and biogas can also be converted to products such as fuels, fine chemicals, polymers, and other materials through chemical or biological means. However, because methane is so chemically different from carbon dioxide, different utilization pathways and technologies are generally needed for the utilization of methane or biogas versus carbon

dioxide. Technologies and research needs for the utilization of methane waste and biogas are discussed in Chapter 6.

ENABLING RESOURCES, TECHNOLOGIES, AND ANALYSES

Enabling resources, technologies, and analyses are required to accomplish carbon utilization at scale with a net reduction in greenhouse gas emissions. For example, carbon dioxide utilization typically requires hydrogen, heat, or electricity; these resources will need to come from renewable or other low-carbon sources to achieve a net reduction in carbon emissions. Enabling technologies such as separation and purification, processing, and transportation are also vital for developing efficient and low-emissions utilization systems. Spatial disconnects between the sources of the waste streams, the sources of the enabling resources, and the facilities where valuable products are being produced can necessitate improved infrastructure and process design solutions. These and other enabling resources and technologies are discussed in Chapter 7.

Life-cycle assessment (LCA), discussed in Chapter 8, is a critical tool to evaluate the total energy requirements and environmental impacts involved in producing chemicals, fuels, construction materials, and polymers from waste carbon feedstocks. LCA is particularly relevant for evaluating carbon utilization technologies when a goal of the technology is to achieve a net reduction in greenhouse gas emissions.

Determining the technical and economic viability of a new technology is initiated through a technoeconomic analysis (TEA). Components considered in a TEA, discussed in Chapter 9, include defining a conceptual process design, performing a feasibility assessment through material and energy balances in combination with economic projections, examining the thermodynamic and kinetic models governing the technology process, estimating capital and operating costs, and classifying the technology according to its technical readiness. Key considerations for carbon waste gas stream utilization include cost, quantity, and purity of the waste stream; costs and carbon footprints of other reactants and inputs; product quality and marketability; and availability of capital with an appropriate risk tolerance. A TEA may also consider how the evaluation will change over time and across geographies as conditions vary. However, determining the commercialization potential of a new product or process requires consideration of factors beyond the technology and economic viability such as market-focused components and legal components. Market-focused components to be considered include an evaluation of the end market size and competitiveness and the perception of the technology offering, while legal components to be considered include consideration of regulatory issues, intellectual property, and standards and norms in the market of interest.

Chapter 10 describes factors and criteria that can be used to evaluate the commercialization potential of carbon dioxide and methane utilization technologies and presents illustrative cases demonstrating the use of those factors and criteria.

Chapter 11 provides a comprehensive research agenda for removing technical barriers and developing viable pathways for carbon dioxide and methane utilization.

REFERENCES