Download PDF Summer Bridge on Engineering the Energy Transition June 26, 2023 Volume 53 Issue 2 This issue explores the energy transition needed to address the mounting threats of climate change. The articles are an excellent resource to help inform meaningful decisions and steps for energy-related contributions to reduce carbon emissions. Producing Transportation Fuels, Electrical Power, and Chemicals in a Circular Bioeconomy Wednesday, June 7, 2023 Author: Timothy J. Donohue Renewable raw materials can be exploited as alternatives to fossil fuel–based liquid transportation fuels, electrical power, and chemicals. Among today’s greatest challenges is the development of sustainable and cost-effective ways to produce sufficient transportation fuels, electrical power, and chemicals while reducing greenhouse gas (GHG) emissions. In 2019 fossil fuels (petroleum, natural gas, and coal) supplied roughly 80 percent of the energy used in the United States, with the remainder derived from a combination of renewable resources (nuclear, wind, hydroelectricity, and biofuels; Kretchmer 2020). Adding to the challenge is the ever-growing demand for numerous products derived from fossil fuels. Tens of billions of gallons of fossil fuel–derived hydrocarbons are used every year to generate liquid transportation fuels, electrical power, and petrochemicals. In the United States in 2021, liquid transportation fuels accounted for about 30 percent of fossil fuel use—-combining the commercial and military needs of the aviation, marine, shipping, automotive, and industrial sectors—and the electrical power sector accounted for about 10 percent. Using raw materials as a source of fuels, electrical power, and chemicals could move society to a circular bioeconomy that minimizes waste while generating products, services, and processes from this and other renewable resources (Gallo 2022). Plugging abundant renewable raw materials into the economy will require significant technical advances and changes in existing agricultural and industrial practices, but the environmental, health, social, and economic benefits are enormous (Northrup et al. 2021; Robertson et al. 2022). Renewable Biological Sources There is a growing call to develop methods to produce significant quantities of liquid transportation fuels, electrical power, and chemicals from abundant renewable resources. Successful efforts could reduce net GHG emissions, lead to a decarbonized industrial base, and provide other environmental, economic, and societal benefits. To achieve such goals, advances are needed to enable cost-competitive and low net GHG production of products to replace those derived from fossil fuels as well as the generation of new products from renewable raw materials. Studies indicate that sufficient renewable raw materials are available to produce sizable amounts of liquid transportation fuels, electrical power, and other products currently derived from fossil fuels (Burger and van Nimwegen 2008; Lizundia et al. 2022). These renewable raw materials include billions of tons of organic residues in nonfood animal and plant material, purpose-grown crops (e.g., switchgrass, poplar) used for conversion into these products, manure, microbes, and residues derived from other agricultural, municipal, and industrial activity. There is also an opportunity to tap abundant gaseous carbon sources (carbon dioxide)—produced by the biological process of respiration, sequestered from the atmosphere, or released by fuel combustion—to help reduce net GHG emissions (Elhacham et al. 2020). Approaches to a Circular Bioeconomy The potential role of a circular bioeconomy (figure 1) in the 21st century industrial evolution is large. For example, it has been estimated that up to 60 percent of the inputs to the global economy could, in principle, be produced biologically (Chui et al. 2020). Nonfood plant and animal residues, when combined with inedible materials generated by agriculture, food, biotechnology, or other industries, can provide a significant portion of the raw materials needed to feed a circular bioeconomy (Lizundia et al. 2022; Zhao et al. 2021). For instance, industries could convert plant and animal residues that are not suitable for or needed as food into numerous products (e.g., feed, fiber, chemicals, materials, pharmaceuticals, and food replacements or additives). Such practices can increase the future profitability of agriculture and reduce net GHG emissions from industrial synthesis of these products when compared to existing practices. In addition, advances in breeding of so-called dedicated energy crops can increase biomass productivity and enhance above- and underground carbon sequestration from the atmosphere (Northrup et al. 2021). The ability to increase the yield and quality of biomass per acre from purpose-grown nonfood cropping systems on fallow, unreclaimed pastureland and acreage that is not suitable for food production is crucial to realizing the vision of a circular bioeconomy. There are also calls for the adoption of zero-carbon farming practices that can lower the net GHG footprint of crop production by reducing, or even avoiding, the use of fertilizer and pesticides since these agrochemicals have a high energy production cost, contribute directly or -indirectly to GHG emissions, and can have other detrimental impacts on air, soil, or water quality (Northrup et al. 2021; Robertson et al. 2022). Catalytic, biological, and hybrid technologies can be used to convert raw materials derived from existing industries (food, chemicals, pharmaceutical, biotechnology, and other sectors) into liquid transportation fuels, electrical power chemicals, and materials. This conversion can replace or supplement fossil fuel use as sources of these products while lowering the net GHG footprint of generating them (Liu et al. 2021; Nielsen and Keasling 2016; Schwartz et al. 2016). Challenges of Renewables for Liquid Transportation Fuels Given the growing global need to move people and products and current estimates of raw materials supply, liquid transportation fuels and electrical power generated by a circular bioeconomy cannot totally replace hydrocarbons derived from fossil fuels in the near future (Liu et al. 2021). It is therefore necessary to develop liquid fuels that fit the needs of different parts of the transportation sector. For example, electrification can be a viable replacement for fossil-derived liquid fuels for most light vehicles, medium- and possibly heavy-duty trucks, and a significant fraction of rail transport (Tamor and Stechel 2022). However, for commercial and military air transport and ocean-going ships, there is a growing consensus that liquid fuels will be needed in the near to long term, because of the added weight of airline batteries and the long distances traveled by most marine transports. Development and acceptance of drop-in fuels will minimize or prevent the need to design and deploy entirely new engine systems. For air and marine transport, “drop-in” liquid fuels—derived from renewable resources (e.g., sustainable aviation fuel, renewable diesel, and renewable gasoline) that can be mixed with fossil-derived fuels—are needed until other non-GHG-intensive petroleum replacements can be developed. Of course, combustion of drop-in liquid fuels should release minimal particulates or pollutants to prevent unwanted environmental impacts of their use in different engines. And the renewable hydrocarbons in drop-in fuels should be cost-competitive and compatible with existing pipeline, shipping, and engine systems. Development and acceptance of drop-in fuels will also minimize or prevent the need to design and deploy entirely new engine systems. Renewable natural gas (RNG) can be a major source of fuel for some vehicles and for electrical power. RNG is typically obtained by isolating and purifying the natural gas (methane) generated by microbial activity in anaerobic digestors, although landfill sites have recently become a significant source (Burger and van Nimwegen 2008; Keogh et al. 2022). Many additional sources of RNG could be developed if anaerobic digestion of the raw materials in agricultural, wastewater, and industrial residue streams could be made cost effective (Krohn et al. 2022). Renewable Chemicals and Materials in the Circular Bioeconomy Technoeconomic analyses predict the benefits of generating liquid transportation fuels, electrical power, and chemicals from renewable raw materials as much as possible (Perez et al. 2022; Scarborough et al. 2018). In this vision, a circular bioeconomy can support the sustainable and renewable production of high-demand chemicals and materials (figure 1) (Liu et al. 2021). Existing carbon capture and storage (CCS) technologies can sequester CO2 underground either in plant roots (biological CCS; Northrup et al. 2021) or in geological formations (Raza et al. 2019). Emerging technologies can capture CO2 and store it in insoluble material (the type used to reinforce concrete and other materials; Ragipani et al. 2022). Syngas, a mixture of carbon monoxide and hydrogen generated by industrial activity, could also be converted into useful chemicals and other materials (Sun et al. 2019). Deployment of these approaches at industrial scale would allow abundant gaseous carbon sources to be used as renewable raw materials in a variety of applications. Public and private investments are essential to generate the game-changing advances in biology, chemistry, computation, and engineering needed for success. The potential to convert organic matter into chemicals and materials is seemingly endless if one considers the combined use of existing or improved enzymes, genome-enabled synthetic biology to build new bio-synthetic pathways, and new chemical catalysts. The suite of products that might be derived from these renewable raw materials include building blocks for synthesis of biodegradable plastics, lubricants, polyesters, adhesives, and new microbial foods. They may also be used to develop additives to stimulate growth or productivity of crops and animals; compounds with pharma-ceutical, antimicrobial, or health-beneficial effects; and myriad other specialty (small-scale) or commodity (large-scale) chemicals (Donohue 2022; Jahn et al. 2023; Schwartz et al. 2016). Industries in the circular bioeconomy would operate like petrochemical refineries where chemicals and materials can be lower-volume and higher-profit per unit products, generating revenue to lower the cost of liquid transportation fuels and electrical power (Huang et al. 2020; Wu and Maravelias 2019). In addition, recent advances in computational, catalytic, and genomic techniques can facilitate the development of renewable chemicals and materials that cannot yet be generated from fossil fuels in a cost-effective manner (Liu et al. 2021; Nielsen and Keasling 2016; Schwartz et al. 2016). Technology, Investment, Modeling, and Communication Needs There are differences in the readiness of individual technologies given the range of advances needed to satisfy the world’s ever-growing need for transportation fuels, electrical power, chemicals, and materials. -Anaerobic digestion, for example, is a fairly long-standing and well-developed technology. In contrast, the production of drop-in biofuels and chemicals from lignocellulosic biomass, other agricultural residues, or municipal waste is at a lower technology readiness level. While some technologies have been commercialized and deployed at industrial scale, improvements could remove existing technical bottlenecks and make these alternative approaches even more cost effective at industrial scale. Given these differences in technology readiness, it could be helpful to set priorities for scientific development and the transition to industrial deployment that would stimulate government and industrial investment in individual approaches. To address knowledge and technology gaps, public and private investments are essential to generate the likely game-changing advances in biology, chemistry, computation, and engineering needed for success. Investments could include single investigator awards and center-scale initiatives that assemble teams to make breakthroughs that occur when researchers work across disciplines. Programs are needed to support high-throughput approaches, mining and modeling of large datasets by machine learning and other computational techniques, and the promotion of transitional research advances from field and laboratory studies to pilot and industrial scales. Biological, physical, computational, and engineering professional societies as well as practitioners, researchers, policymakers, and educators can both contribute to and reap the benefits of a circular bioeconomy. Lifecycle analysis predicts that maximizing the economic and evironmental benefits of converting raw materials into products will depend on the strategic placement of refineries close to their sources and the infrastructure needed to move materials from producers to end users (Gelfand et al. 2013). This in turn will require integrating remote, satellite, and land-based tracking systems with other datasets. The data will inform models that can accurately predict the supply of raw materials, model the costs of purchase and transport to refineries, and forecast expenses to produce, purify, and distribute products at scale (Gelfand et al. 2013; Robertson et al. 2017). Modeling of the economic, GHG, and other environmental benefits associated with the circular bioeconomy is needed to inform industrial, community, and consumer dialogue and acceptance of this new industrial ecosystem. The cost of building and operating a new generation of refineries means that public and private sector investors will need to be convinced about cost-effective access to raw materials and the technology to generate useful products. Looking to the Future A significant fraction of the raw resources needed to provide cost-effective renewable liquid transportation fuels, electrical power, chemicals, and materials can be derived from existing nonfood products of agricultural, industrial, or other societal activities. The siting of next-generation refineries close to the supply of raw mate-rials can create economic opportunities for industries and communities. By producing liquid transportation fuels, power, and chemicals from local raw materials, rural communities that have traditionally not been part of the fuel and chemical industries can participate in a multitrillion-dollar-per-year economy. In this model, the refineries that power a circular bioeconomy can become a cornerstone of a new industrial ecosystem for the country and the world. 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Baldwin and UW Foundation Fetzer-Bascom Professor of Bacteriology at the University of Wisconsin–Madison, and director, Wisconsin Energy Institute and Great Lakes Bioenergy Research Center.