•Decarbonizing the industrial sector is a key part of economy-wide decarbonization•Decarbonizing process heat alone can mitigate about a fifth of global CO2 emissions•Cross-cutting R&D that can decarbonize industrial process heat is identified Industry is often termed “hard to decarbonize” because a vast, inhomogeneous array of processes comprise the sector. Nevertheless, decarbonization of the industrial sector is crucial to addressing economy-wide emissions. In 2010, a full 13.1 gigatons (Gt) of carbon dioxide (CO2) and 176 exajoules (EJ) of primary energy demand were attributed to the sector globally. Developing new, decarbonized process heating technologies represents a single, broadly applicable pathway to eliminating a large portion of sectoral emissions, and approximately one-fifth of global CO2 emissions, overall. In this perspective, we propose a cross-cutting research effort in (1) zero-carbon heat, (2) electrification of heat, (3) zero-carbon fuels, and (4) better heat management. If pursued, these distinct and cross-cutting areas for R&D will help drive technical advances that can help further reduce industrial heat emissions, such that neither economic nor climate progress are sacrificed. Industry is often termed “hard to decarbonize” because a vast, inhomogeneous array of processes comprise the sector. But developing new, decarbonized process heating technologies represents a single, broadly applicable pathway to eliminating a large portion of sectoral emissions—and approximately one-fifth of global CO2 emissions, overall. We begin this perspective with a brief review of the demand for and cost of industrial heat. Then, we highlight key challenges and R&D needs in developing zero-carbon industrial heating technologies. Technologies in four pathways are discussed: (1) zero-carbon fuels, (2) zero-carbon heat sources, (3) electrification of heat, and (4) better heat management. Finally, we identify cross-cutting challenges to the development and adoption of zero-carbon industrial heat technologies, the solution to any of which would constitute a significant breakthrough on the path to industrial decarbonization. Industry is often termed “hard to decarbonize” because a vast, inhomogeneous array of processes comprise the sector. But developing new, decarbonized process heating technologies represents a single, broadly applicable pathway to eliminating a large portion of sectoral emissions—and approximately one-fifth of global CO2 emissions, overall. We begin this perspective with a brief review of the demand for and cost of industrial heat. Then, we highlight key challenges and R&D needs in developing zero-carbon industrial heating technologies. Technologies in four pathways are discussed: (1) zero-carbon fuels, (2) zero-carbon heat sources, (3) electrification of heat, and (4) better heat management. Finally, we identify cross-cutting challenges to the development and adoption of zero-carbon industrial heat technologies, the solution to any of which would constitute a significant breakthrough on the path to industrial decarbonization. Decarbonizing, or the net elimination of carbon dioxide (CO2) emissions from, the industrial sector is widely acknowledged to be challenging,1Davis S.J. Lewis N.S. Shaner M. Aggarwal S. Arent D. Azevedo I.L. Benson S.M. Bradley T. Brouwer J. Chiang Y.-M. et al.Net-zero emissions energy systems.Science. 2018; 360: eaas9793Crossref PubMed Scopus (520) Google Scholar, 2Rissman J. Bataille C. Masanet E. Aden N. Morrow III, W.R. Zhou N. Elliott N. Dell R. Heeren N. Huckestein B. et al.Technologies and policies to decarbonize global industry: review and assessment of mitigation drivers through.Appl. Energy. 2020; 2070 (114848): 266Google Scholar, 3Bataille C. Åhman M. Neuhoff K. Nilsson L.J. Fischedick M. Lechtenböhmer S. Solano-Rodriquez B. Denis-Ryan A. Stiebert S. Waisman H. et al.A review of technology and policy deep decarbonization pathway options for making energy-intensive industry production consistent with the Paris agreement.J. Clean. Prod. 2018; 187: 960-973Crossref Scopus (143) Google Scholar but paramount to achieving IPCC climate goals. In 2010, worldwide, a full 13.1 gigatons (Gt) of CO24Fischedick M. Roy J. Abdel-Aziz A. Acquaye A. Allwood J.M. Ceron J.-P. Geng Y. Kheshgi H. Lanza A. Perczyk D. et al.Industry.in: Edenhofer O. Pichs-Madruga R. Sokona Y. Farahani E. Kadner S. Seyboth K. Adler A. Baum I. Brunner S. Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 2014: 739-810Google Scholar and 176 exajoules (EJ) of primary energy demand5IEAWorld Energy Outlook 2018. IEA, 2018Google Scholar were attributed to the sector—roughly a third or so of both global primary energy demand and emissions. From chemical to metallurgical, manufacturing to food processing, the dizzying array of processes that comprise the industrial sector make decarbonizing each one independently a dauntingly complex task.6Cunliff C. An innovation agenda for hard-to-decarbonize energy sectors.Issues Sci. Technol. 2019; 36: 74-79Google Scholar,7de Pee A. Pinner D. Roelofsen O. Somers K. Speelman E. Witteveen M. Decarbonization of Industrial Sectors: the Next Frontier. McKinsey & Company, 2018Google Scholar Since the number of processes comprising the sector is large, developing any single, decarbonized process will generally have limited impact on sectoral emissions, with no guarantee the new technology will be portable to the next process. Therefore, although it is important that researchers continue to work toward the development of new low-emission industrial processes, we choose to focus on a cross-cutting opportunity—the decarbonization of heat. A large portion of industrial sector emissions, estimated at about 7.5 Gt of CO2,8Savut I. Industrial Heat: Deep Decarbonization Opportunities. Bloomberg New Energy Finance, 2019Google Scholar or about 21% of global CO2 emissions in 2016,9Global Carbon Project Supplemental data of Global Carbon Budget 2019 (Version 1.0) [Data set]. Global Carbon Project.https://doi.org/10.18160/gcp-2019Date: 2019Google Scholar result from a single process: the generation of over 100 EJth of heat (we use the colloquial shorthand “heat” for the thermodynamic term “thermal energy” ).5IEAWorld Energy Outlook 2018. IEA, 2018Google Scholar As shown in Figure 1, the combustion of three fuels—coal, natural gas, and oil—generate the vast majority of this heat and its associated CO2 emissions. The heat is then used directly or indirectly, commonly via steam, to drive numerous processes like fluid heating, distillation, drying, and chemical reactions at temperatures ranging from only slightly above ambient to thousands of degrees Celsius (°C). In the US, just under 30% of heat is generated to make steam; the remainder is directly used in furnaces, ovens, kilns, and other unit operations.10U.S. Department of EnergyQuadrennial technology review.https://www.energy.gov/quadrennial-technology-review-2015Date: 2015Google Scholar The cross-cutting nature of heat generation and use in industry has led researchers to term industrial decarbonization a “grand challenge” in thermal science and engineering.11Henry A. Prasher R. Majumdar A. Five thermal energy grand challenges for decarbonization.Nat. Energy. 2020; 5: 635-637Crossref Scopus (38) Google Scholar A concerted research program to decarbonize the generation, transport, and use of heat in industry has the potential for great impact.12Friedmann S.J. Fan Z. Tang K. Low-carbon heat solutions for heavy industry: sources, options and costs today. Columbia/SIPA Center on Global Energy Policy, 2019https://www.energypolicy.columbia.edu/research/report/low-carbon-heat-solutions-heavy-industry-sources-options-and-costs-todayGoogle Scholar,13Haydock H. Napp T. Decarbonisation of heat in industry: a review of the research evidence.https://www.gov.uk/government/publications/decarbonisation-of-heat-in-industry-a-review-of-the-research-evidenceDate: 2013Google Scholar In this perspective, we first discuss contemporary technoeconomics of industrial heat, which provides a benchmark for nascent technologies and a goal for their development. We then discuss four R&D pathways by which industrial heat may be decarbonized: (1) zero-carbon fuels, (2) zero-carbon heat sources, (3) electrification of heat, and (4) better heat management. For each pathway, we discuss state of the art, review recent research efforts, and describe future needs to enable the pathway to compete with contemporary alternatives. Finally, we summarize key challenges and R&D needs (Table 3) in each pathway and identify opportunities that cut across pathways. Our view is that none of these pathways represent a silver bullet; rather, many technologies will find future use. For any zero-carbon heating technology to be viable, it must fulfill the end user’s heat demand at a competitive cost. Heat demand can be characterized by two variables—temperature and load. The temperature is set by the nature of the use and can exceed 1,400°C for processes like steelmaking and cement production. Heat transfer at higher temperature also has higher thermodynamic value, since it is able to deliver more exergy (the maximum amount of work that can be done by the heat source). Load is the rate of heat transfer required by the process, in kilowatts (kWth) or megawatts (MWth) thermal. In very large facilities, loads can be as high as thousands of MWth.14McMillan C.A. Ruth M. Using facility-level emissions data to estimate the technical potential of alternative thermal sources to meet industrial heat demand.Appl. Energy. 2019; 239: 1077-1090Crossref Scopus (11) Google Scholar For the purposes of cost comparisons, we use a simplified levelized cost of heat (LCOH), which is analogous to the levelized cost of electricity (LCOE) and is conceptually similar to other LCOH metrics in open literature15Louvet Y. Fischer S. Furbo S. Giovanetti F. Köhl M. Mauthner F. Mugnier D. Philippen D. Veynandt F. LCoH for solar thermal applications (IEA-SHC Task 54).https://task54.iea-shc.org/Data/Sites/1/publications/A01-Info-Sheet--LCOH-for-Solar-Thermal-Applications.pdfDate: 2019Google Scholar,16Kurup P. Turchi C. Initial investigation into the potential of CSP industrial process heat for the southwest United States. National Renewable Energy Laboratory, 2015https://www.nrel.gov/docs/fy16osti/64709.pdfCrossref Google Scholar:LCOH=AmortizedCapitalCostCapacityFactor+CostofEnergy×EnergyInHeatOut+OtherO&MHeatOut(Equation 1) where the “amortized capital cost” is normalized by the nameplate capacity of the heat generator. The “capacity factor,” or percentage uptime, accounts for utilization of incurred capital expenditures. The “energy in” can be in the form of a combustible fuel, electricity, and/or waste heat. The term “other O&M” captures all operation and maintenance costs other than energy input costs. The “heat out” is the actual heat output of the heat generator, which meets the thermal load of the process or facility. Like the LCOE, the LCOH has limitations. For example, intermittent resources like solar thermal without storage are not equivalent to an easily stored fuel. Distribution systems for steam or other heat transfer fluids add cost beyond what is considered here. In addition, the co-generation of heat and electricity, termed combined heat and power (CHP), uses one fuel source to generate both heat and electricity. Such methods do not as easily fit into the LCOH framework. Finally, any calculation of LCOH is only as good as its underlying data. Ultimately, though this framework and our analysis here has limitations, such technoeconomic analyses help technologists assess market constraints and derive performance metrics necessary for commercialization. Table 1 and Figure 2 illustrate the representative demand for and cost of industrial heat. Table 1, created from data by McMillan et al.17McMillan C. Ruth M. Industrial process heat demand characterization (NREL).https://data.nrel.gov/submissions/91Date: 2018Google Scholar shows the distribution of load sizes for facilities in the 14 top greenhouse gas emitting industries in the US. The top of Figure 2, created using the same dataset17McMillan C. Ruth M. Industrial process heat demand characterization (NREL).https://data.nrel.gov/submissions/91Date: 2018Google Scholar shows the amount of heat, in EJth/y, demanded and the associated CO2 emissions as a function of temperature for those same industries. Analogous data for the industrial sector in the EU, which show similar trends, is given by Naegler et al.18Naegler T. Simon S. Klein M. Gils H.C. Quantification of the European industrial heat demand by branch and temperature level.Int. J. Energy Res. 2015; 39: 2019-2030Crossref Scopus (54) Google Scholar The next plot shows ranges of approximate levelized capital costs as a function of temperature, the first term in Equation 1, of common equipment used to generate industrial heat, based on purchased equipment costs reported by Peters et al.19Peters M. Timmerhaus K. West R. Plant Design and Economics for Chemical Engineers. McGraw-Hill, 2002Google Scholar Capital costs presented here are in 2020 US dollars, inflated from reported values using the Chemical Manufacturing Producer Price Index,20U.S. Bureau of Labor StatisticsProducer Price Index by Industry: Chemical Manufacturing [PCU325325]. FRED, Federal Reserve Bank of St. Louis, 2020https://fred.stlouisfed.org/series/PCU325325Google Scholar are discounted at 5% over an assumed 30 year equipment lifespan and are based upon purchased equipment costs alone, before installation. The bottom plot shows approximate ranges for energy input costs as a function of temperature taken from a variety of global benchmarks.21BP BP Statistical Review of World Energy.https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.htmlDate: 2020Google Scholar, 22UK Department for Business, Energy & Industrial StrategyInternational industrial energy prices.https://www.gov.uk/government/statistical-data-sets/international-industrial-energy-pricesDate: 2020Google Scholar, 23UK Department for Business, Energy & Industrial StrategyPrices of fuels purchased by manufacturing industry.https://www.gov.uk/government/statistical-data-sets/prices-of-fuels-purchased-by-manufacturing-industryDate: 2020Google Scholar, 24Energy Information Administration, U.S. Department of EnergyHenry hub natural gas spot price.https://www.eia.gov/dnav/ng/hist/rngwhhdm.htmDate: 2020Google Scholar, 25Energy Information Administration, U.S. Department of EnergyAverage price of electricity to ultimate customers by end-use sector.https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_3Date: 2020Google Scholar, 26Brown T. The cost of hydrogen: Platts launches hydrogen price assessment.https://www.ammoniaenergy.org/articles/the-cost-of-hydrogen-platts-launches-hydrogen-price-assessment/Date: 2019Google Scholar For fuels, the temperature shown is the adiabatic flame temperature. Heat pumps are Carnot limited, with a source temperature of 30°C, that we have chosen as a compromise between air source heat pumps, which may have lower source temperatures, and heat pumps driven by waste heat, which may have higher source temperatures. Values for electrical resistive heating are computed using a 95% electrical-to-thermal efficiency, and values for combusted fuels are computed using a 95% chemical-to-thermal efficiency. The lower bound of the heat pump region is computed using a Carnot efficiency.Table 1Distribution of thermal loads as a fraction of facilities in 14 top greenhouse gas emitting industries in the USFacility annual average load (MWth)Fraction of facilities (%)0 to 139.6%1 to 1026.5%10 to 10018.9%100 to 1,0002.1%Unreported13.0%Data from McMillan and Ruth.17McMillan C. Ruth M. Industrial process heat demand characterization (NREL).https://data.nrel.gov/submissions/91Date: 2018Google Scholar Open table in a new tab Data from McMillan and Ruth.17McMillan C. Ruth M. Industrial process heat demand characterization (NREL).https://data.nrel.gov/submissions/91Date: 2018Google Scholar The benchmark costs for heat shown in Figure 2 may also subject to various sensitivities outside the ranges shown. The addition of a price on CO2 emissions, or an increase in that price in jurisdictions where such a price exists, would increase $/MWhth values for carbon-containing fuels, such as coal and natural gas. In such a scenario, raw material and construction costs could also increase, thereby increasing equipment capital costs. In addition, the efficiency of fuel or electricity use affects LCOH. Low-efficiency boilers or resistive heaters increase fuel costs on a $/MWhth basis in inverse proportion to efficiency (see “energy in/heat out” term in Equation 1). Finally, financial costs such as fuel hedges or costs of capital higher than those considered here would increase these LCOH benchmarks. Figure 2 displays three important takeaways. First, compared with familiar, residential retail electricity prices, industrial heat is a relatively cheap form of energy, particularly in locales with cheap coal and natural gas. Second, although renewable electricity prices are falling, most industrial electricity prices paid mean that in many cases the energy cost of electrified heating without using a heat pump (using, e.g., resistive heating) is still well over two to three times the cost of fossil-based heat generation. This means electrified heating technologies must gain proportionately in efficiency (as MWhth/MWhe), likely by using a heat pump where common coefficients of performance can yield MWhth/MWhe > 1, what they lose in operating cost—and do so at low capital cost—to be competitive. Finally, most fuels can generate flame temperatures far in excess of what is demanded. Ultimately, the low cost, and high performance of modern, carbon-based heat generation represents more than a century of science and engineering advancements—and a significant hurdle for developing economically competitive decarbonized alternatives. In this section, we discuss four pathways for decarbonizing heat production, as illustrated in Figure 3: (1) zero-carbon fuels, (2) zero-carbon heat sources, (3) electrification of heat, and (4) better heat management. In each subsection, we review state of the art and discuss R&D needs. We focus on technologies, both nascent and developed, that have the potential to impact the decarbonization of industrial heat but may not be focused on that application. Our aim is not to be comprehensive but to highlight R&D challenges which, if met, would have impact. A direct approach to decarbonize industrial heat is to substitute zero-carbon fuels directly for fossil fuels, and is a strategy that has been noted by other researchers.1Davis S.J. Lewis N.S. Shaner M. Aggarwal S. Arent D. Azevedo I.L. Benson S.M. Bradley T. Brouwer J. Chiang Y.-M. et al.Net-zero emissions energy systems.Science. 2018; 360: eaas9793Crossref PubMed Scopus (520) Google Scholar,2Rissman J. Bataille C. Masanet E. Aden N. Morrow III, W.R. Zhou N. Elliott N. Dell R. Heeren N. Huckestein B. et al.Technologies and policies to decarbonize global industry: review and assessment of mitigation drivers through.Appl. Energy. 2020; 2070 (114848): 266Google Scholar,12Friedmann S.J. Fan Z. Tang K. Low-carbon heat solutions for heavy industry: sources, options and costs today. Columbia/SIPA Center on Global Energy Policy, 2019https://www.energypolicy.columbia.edu/research/report/low-carbon-heat-solutions-heavy-industry-sources-options-and-costs-todayGoogle Scholar For applications where these fuels are combusted only for heat production—e.g., in boilers to generate steam—this is a relatively straightforward prospect. Zero-carbon substitutes such as hydrogen, ammonia, biofuels, and synthetic hydrocarbons can be employed, but R&D is needed to enable their production, handling, and utilization as a heat source in the industrial setting at a cost competitive with fossil fuels. However, in some cases direct fuel substitution is not straightforward. For many industrial processes, hydrocarbon fuels provide not only thermal energy but serve as a reactant as well, yielding CO2 emissions from their use. This is the case in steel production, where iron ore is reduced to atomic iron by reacting it with metallurgical coke. In a blast furnace the coke (C) is partially oxidized by oxygen to produce heat (Δ) and carbon monoxide (CO), which in turn functions as the main reducing agent converting iron oxide to iron:2C(s)+O2(g)→2CO(g)+Δ(Equation 2) Fe2O3(s)+3CO(g)→Δ2Fe(s)+3CO2(g).(Equation 3) For processes where a carbon-containing fuel is intrinsic to the process chemistry, either the effluent can be captured and geologically sequestered, or new process chemistries using carbon-free reactants must be developed. An example of the latter is the direct reduction of iron with hydrogen instead of coke,27Spreitzer D. Schenk J. Reduction of iron oxides with hydrogen—a review.Steel Research Int. 2019; 90: 1900108Crossref Scopus (86) Google Scholar yielding a global chemical reaction of:Fe2O3(s)+3H2(g)→2Fe(s)+3H2O(g).(Equation 4) where hydrogen provides both the heat to drive the reaction and serves as the reductive reactant. If the hydrogen is produced using a zero- or negative-emission process, then this approach would yield a decarbonized steel process. The development of new process chemistries that remove associated CO2 process emissions is critically important for decarbonizing industry since they account for a significant percentage of global emissions. For processes like cement production, such process-intrinsic emissions account the majority of associated emissions.2Rissman J. Bataille C. Masanet E. Aden N. Morrow III, W.R. Zhou N. Elliott N. Dell R. Heeren N. Huckestein B. et al.Technologies and policies to decarbonize global industry: review and assessment of mitigation drivers through.Appl. Energy. 2020; 2070 (114848): 266Google Scholar Other existing processes with associated process emissions include steam methane reforming to make hydrogen and the Hall electrolytic process for producing aluminum from alumina. However, in this perspective we focus on the use of fuels for heat production, and this is not directly addressed. In this section, two major approaches to decarbonizing industrial fuels are considered: (1) the production and use of carbon-free fuels such as hydrogen and ammonia and (2) zero-emission hydrocarbons derived from biofuels or synthesized from atmospheric CO2. Hydrogen (H2) and ammonia (NH3) are two combustible fuels that do not contain carbon, so when combusted yield primarily water, and in the case of ammonia, nitrogen (N2):2H2+O2→2H2O+Δ(Equation 5) 4NH3+3O2→6H2O+2N2+Δ.(Equation 6) Industry has extensive experience with hydrogen as a product and a process feedstock, but it has less experience burning hydrogen directly for heat. Of the more than 60 million tons of hydrogen produced annually around the world, most of it is used in industrial applications where it serves as a feedstock for the production of ammonia (53%), and in the oil and gas industry (40%) where hydrogen is used for refining via hydrocracking and hydrotreating and sweetening of natural gas.28Brandon N. Kurban Z. Clean energy and the hydrogen economy.Philos. Trans. A Math Phys. Eng. Sci. 2017; 375: 20160400Crossref PubMed Scopus (108) Google Scholar Ammonia is similarly a known quantity for industry as a major product, though it is not currently used in industry as a major fuel, recent work has identified its potential as an energy carrier and a path to being electrochemically synthesized cost-effectively.29MacFarlane D.R. Cherepanov P.V. Choi J. Suryanto B.H.R. Hodgetts R.Y. Bakker J.M. Ferrero Vallana F.M.F. Simonov A.N. A roadmap to the ammonia economy.Joule. 2020; 4: 1186-1205Abstract Full Text Full Text PDF Scopus (162) Google Scholar For hydrogen or ammonia to be adopted as industrial fuels they must first be produced cost-effectively at scale, using zero-emission technologies. An additional challenge is in their distribution and utilization since their physiochemical (density, mass diffusivity, etc.) and combustion properties (heat of combustion, flame speed, etc.) are different from natural gas, which they aim to replace. For example, due to its high flame speed, hydrogen combustors are hard to control relative to natural gas, leading to operability challenges.30Taamallah S. Vogiatzaki K. Alzahrani F.M. Mokheimer E.M.A. Habib M.A. Ghoniem A.F. Fuel flexibility, stability and emissions in premixed hydrogen-rich gas turbine combustion: technology, fundamentals, and numerical simulations.Appl. Energy. 2015; 154: 1020-1047Crossref Scopus (126) Google Scholar Ammonia has a flame speed much slower than methane, which also requires additional consideration in designing steady-state burners for its use.31Kobayashi H. Hayakawa A. Somarathne K.D. Okafor E.C. Science and technology of ammonia combustion.Proc. Combust. Inst. 2019; 37: 109-133Crossref Scopus (303) Google Scholar Research is needed to better characterize hydrogen and ammonia combustion properties in order to develop burners and combustion controls optimized that control their emissions of other pollutants like NOx and N2O. Hydrogen also has storage and distribution issues that must be addressed due to its ability to embrittle steel leading to leaks. Ammonia has fewer R&D needs on this front and has an existing distribution and storage supply chain owing to its use in agriculture. Finally, continued work is necessary to increase synthesis efficiency and decrease capital costs of the production of zero-carbon hydrogen and ammonia as a way to supply these clean fuels at competitive costs. Today industrial-scale hydrogen (which is also the precursor to ammonia synthesis via the Haber-Bosch process) is produced via steam methane reforming (SMR), which emits a sizable amount of CO2. Further work is required to drive down the cost of zero-emission hydrogen, whether through hydrogen synthesis from methane with carbon capture,32Parkinson B. Tabatabaei M. Upham D.C. Ballinger B. Greig C. Smart S. McFarland E. Hydrogen production using methane: techno-economics of decarbonizing fuels and chemicals.Int. J. Hydrogen Energy. 2018; 43: 2540-2555Crossref Scopus (82) Google Scholar thermally driven synthesis,33Randhir K. Rhodes N.R. Li L. AuYeung N. Hahn D.W. Mei R. Klausner J.F. Magnesioferrites for solar thermochemical fuel production.Sol. Energy. 2018; 163: 1-15Crossref Scopus (13) Google Scholar,34Randhir K. King K. Rhodes N. Li L. Hahn D. Mei R. AuYeung N. Klausner J. Magnesium-manganese oxides for high temperature thermochemical energy storage.J. Energy Storage. 2019; 21: 599-610Crossref Scopus (24) Google Scholar or electrolysis.35Miller H.A. Bouzek K. Hnat J. Loos S. Bernäcker C.I. Weißgärber T. Röntzsch L. Meier-Haack J. Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions.Sustain. Energy Fuels. 2020; 4: 2114-2133Crossref Google Scholar The development of direct electrosynthesis of ammonia offers a promising path for modular, distributed ammonia production,36Kyriakou V. Garagounis I. Vourros A. Vasileiou E. Stoukides M. An electrochemical Haber-Bosch process.Joule. 2020; 4: 142-158Abstract Full Text Full Text PDF Scopus (78) Google Scholar and recent work funded by ARPA-E’s REFUEL program has focused on technology to enable the cost-effective electrosynthesis of ammonia and synthetic hydrocarbons,37Soloveichik, G.L. (2018). ARPA-E REFUEL program: Electrochemical synthesis and utilization of sustainable fuels. In 233rd ECS Meeting (May 13-17, 2018). ECS.Google Scholar which are discussed in the next section. Biofuels and synthetic hydrocarbons derived from atmospheric CO2 can be drop-in ready, carbon-containing, net-zero-emission fuel substitutes. In contrast to hydrogen and ammonia, these fuels can be synthesized as one-to-one substitutes, fully fungible and integrable into today’s hydrocarbon infrastructure. Employing such a strategy could thereby avoid the massive capital investment necessary to build out a distribution infrastructure for a new fuel. However, using biofuels to decarbonize industry shifts the emissions reduction burden upstream and depends ultimately on the emissions intensity of the agricultural and forestry practices employed,38Robertson G.P. Hamilton S.K. Barham B.L. Dale B.E. Izaurralde R.C. Jackson R.D. Landis D.A. Swinton S.M. Thelen K.D. Tiedje J.M. Cellulosic biofuel contributions to a sustainable energy future: choices and outcomes.Science. 2017; 356Crossref Scopus (219) Google Scholar as well as land-use changes induced by increased demand for biofuels,39Sanchez S.T. Woods J. Akhurst M. Brander M. O’Hare M. Dawson T.P. Edwards R. Liska A.J. Malpas R. Accounting for indirect land-use change in the life cycle assessment of biofuel supply chains.J. R. Soc. Interface. 2012; 9: 1105-1119Crossref PubMed Scopus (73) Google Scholar which has been documented for charcoal use in Brazilian steel production.40Piketty M.-G. Wichert M. Fallot A. Aimola L. Assessing land availability to produce biomass for energy: the case of Brazilian charcoal for steel making.Biomass & Bioenergy. 2009; 33: 180-190Crossref Scopus (43) Google Scholar Currently, biofuel substitutes remain more expensive than their fossil-derived equivalents due to expensive processing and clean-up steps required. Biofuels can be broken into two categories: (1) the direct combustion of biomass to provide heat and (2) fuel