The Climate Impacts of Bioenergy Systems Depend on Market and Regulatory Policy ContextsClick to copy article linkArticle link copied!
- Derek M. Lemoine
- Richard J. Plevin
- Avery S. Cohn
- Andrew D. Jones
- Adam R. Brandt
- Sintana E. Vergara
- Daniel M. Kammen
Abstract
Biomass can help reduce greenhouse gas (GHG) emissions by displacing petroleum in the transportation sector, by displacing fossil-based electricity, and by sequestering atmospheric carbon. Which use mitigates the most emissions depends on market and regulatory contexts outside the scope of attributional life cycle assessments. We show that bioelectricity’s advantage over liquid biofuels depends on the GHG intensity of the electricity displaced. Bioelectricity that displaces coal-fired electricity could reduce GHG emissions, but bioelectricity that displaces wind electricity could increase GHG emissions. The electricity displaced depends upon existing infrastructure and policies affecting the electric grid. These findings demonstrate how model assumptions about whether the vehicle fleet and bioenergy use are fixed or free parameters constrain the policy questions an analysis can inform. Our bioenergy life cycle assessment can inform questions about a bioenergy mandate’s optimal allocation between liquid fuels and electricity generation, but questions about the optimal level of bioenergy use require analyses with different assumptions about fixed and free parameters.
This publication is licensed for personal use by The American Chemical Society.
Synopsis
Whether bioelectricity generation or biofuels production avoids more greenhouse gas emissions depends on the electricity source that bioelectricity generation displaces.
Introduction
Bioelectricity versus Biofuels: The Importance of Assumptions about Displacement
Figure 1
Figure 1. Net GHG savings per area of cropland are sensitive to displacement assumptions. The X marker shows ethanol displacing gasoline. The red circle follows ref 10 in assuming that bioelectricity is used to power electrified vehicles and displaces gasoline. The diamond, square, and triangle (coal, natural gas combined cycle, and wind electricity) show the GHG benefit (or cost) when bioelectricity displaces each of these types of power. See Supporting Information for details.
Figure 2
Figure 2. Net GHG savings per area of cropland under varying assumptions about emissions from indirect land use change due to cultivation of corn for biofuels or bioelectricity. The X marker shows ethanol displacing gasoline. The red circle follows ref 10 in assuming that bioelectricity is used to power electrified vehicles and displaces gasoline. The diamond, square, and triangle (coal, natural gas combined cycle, and wind electricity) show the GHG benefit (or cost) when bioelectricity displaces each of these types of power. See Supporting Information for details.
Policy Questions Should Guide Assumptions about Decision-Making Contexts
Figure 3
Figure 3. Assumptions in a bioenergy- and transportation-focused analysis about whether bioenergy use and the vehicle fleet are fixed (taken as given) or free (allowed to vary) strongly influence the questions modelers can address and the results they obtain. For each pair of assumptions, the cells contain examples of the types of questions that analyses can answer and examples of the types of analyses that can contribute to answering the questions.
Supporting Information
Complete description of the Energy Displacement Model. This material is available free of charge via the Internet at http://pubs.acs.org.
Terms & Conditions
Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.
Acknowledgment
We acknowledge the support of the Robert and Patricia Switzer Foundation Environmental Fellowship Program (to D.M.L.), the Energy Biosciences Institute (to A.S.C.), a U.S. Environmental Protection Agency Science to Achieve Results (STAR) fellowship (to A.D.J.), and a National Science Foundation Graduate Research Fellowship (to S.E.V.). We thank the Energy Foundation and the Karsten Family Foundation Endowment of the Renewable and Appropriate Energy Laboratory (to D.M.K.).
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Figure 1
Figure 1. Net GHG savings per area of cropland are sensitive to displacement assumptions. The X marker shows ethanol displacing gasoline. The red circle follows ref 10 in assuming that bioelectricity is used to power electrified vehicles and displaces gasoline. The diamond, square, and triangle (coal, natural gas combined cycle, and wind electricity) show the GHG benefit (or cost) when bioelectricity displaces each of these types of power. See Supporting Information for details.
Figure 2
Figure 2. Net GHG savings per area of cropland under varying assumptions about emissions from indirect land use change due to cultivation of corn for biofuels or bioelectricity. The X marker shows ethanol displacing gasoline. The red circle follows ref 10 in assuming that bioelectricity is used to power electrified vehicles and displaces gasoline. The diamond, square, and triangle (coal, natural gas combined cycle, and wind electricity) show the GHG benefit (or cost) when bioelectricity displaces each of these types of power. See Supporting Information for details.
Figure 3
Figure 3. Assumptions in a bioenergy- and transportation-focused analysis about whether bioenergy use and the vehicle fleet are fixed (taken as given) or free (allowed to vary) strongly influence the questions modelers can address and the results they obtain. For each pair of assumptions, the cells contain examples of the types of questions that analyses can answer and examples of the types of analyses that can contribute to answering the questions.
References
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- 15Searchinger, T.; Heimlich, R.; Houghton, R. A.; Dong, F.; Elobeid, A.; Fabiosa, J.; Tokgoz, S.; Hayes, D.; Yu, T. Use of U.S. croplands for biofuels increases greenhouse gases through emissions from land-use change Science 2008, 319, 1238– 124015https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXisVSksLg%253D&md5=2296fcdc224e70c0e7da4d9af5115d41Use of U.S. croplands for biofuels increases greenhouse gases through emissions from land-use changeSearchinger, Timothy; Heimlich, Ralph; Houghton, R. A.; Dong, Fengxia; Elobeid, Amani; Fabiosa, Jacinto; Tokgoz, Simla; Hayes, Dermot; Yu, Tun-HsiangScience (Washington, DC, United States) (2008), 319 (5867), 1238-1240CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Most prior studies obsd. that substituting biofuels for gasoline reduces greenhouse gases since biofuels sequester C through the growth of the feedstock. These analyses failed to count C emissions which occur as farmers respond to higher prices and convert forest and grassland to new cropland to replace grain (or cropland) diverted to biofuels. Using a worldwide agricultural model to est. emissions from land-use change, the authors detd. corn-based ethanol, instead of producing a 20% savings, nearly doubles greenhouse emissions over 30 years and increases greenhouse gases for 167 years. Biofuels from switchgrass, if grown on US corn lands, increase emissions 50%. This result raises concerns about large biofuel mandates and highlights the value of using waste products.
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- 19Foley, J. A.; et al. Global consequences of land use Science 2005, 309, 570– 57419https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXmsFChtrs%253D&md5=6c5699de2c42011adcebf8b4c9589b2cGlobal Consequences of Land UseFoley, Jonathan A.; DeFries, Ruth; Asner, Gregory P.; Barford, Carol; Bonan, Gordon; Carpenter, Stephen R.; Chapin, F. Stuart; Coe, Michael T.; Daily, Gretchen C.; Gibbs, Holly K.; Helkowski, Joseph H.; Holloway, Tracey; Howard, Erica A.; Kucharik, Christopher J.; Monfreda, Chad; Patz, Jonathan A.; Prentice, I. Colin; Ramankutty, Navin; Snyder, Peter K.Science (Washington, DC, United States) (2005), 309 (5734), 570-574CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Land use has generally been considered a local environmental issue, but it is becoming a force of global importance. Worldwide changes to forests, farmlands, waterways, and air are being driven by the need to provide food, fiber, water, and shelter to more than six billion people. Global croplands, pastures, plantations, and urban areas have expanded in recent decades, accompanied by large increases in energy, water, and fertilizer consumption, along with considerable losses of biodiversity. Such changes in land use have enabled humans to appropriate an increasing share of the planet's resources, but they also potentially undermine the capacity of ecosystems to sustain food prodn., maintain freshwater and forest resources, regulate climate and air quality, and ameliorate infectious diseases. We face the challenge of managing trade-offs between immediate human needs and maintaining the capacity of the biosphere to provide goods and services in the long term.
- 20Fargione, J.; Hill, J.; Tilman, D.; Polasky, S.; Hawthorne, P. Land clearing and the biofuel carbon debt Science 2008, 319, 1235– 123820https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXisVSksLs%253D&md5=236c65084d106d27e3624b2374f2511eLand Clearing and the Biofuel Carbon DebtFargione, Joseph; Hill, Jason; Tilman, David; Polasky, Stephen; Hawthorne, PeterScience (Washington, DC, United States) (2008), 319 (5867), 1235-1238CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Increasing energy use, climate change, and CO2 emissions from fossil fuels make switching to low-C fuels a high priority. Biofuels are a potential low-C energy source, but whether biofuels offer C savings depends on how they are produced. Converting rain forest, peatland, savanna, or grassland to produce food crop-based biofuels in Brazil, southeast Asia, and the US creates a biofuel C debt by releasing 17-420 times more CO2 than the annual greenhouse gas (GHG) redns. these biofuels would provide by displacing fossil fuels. In contrast, biofuels made from waste biomass or from biomass grown on degraded and abandoned agricultural lands planted with perennials incur little or no C debt and can offer immediate, sustained GHG advantages.
- 21Gerbens-Leenes, W.; Hoekstra, A. Y.; van der Meer, T. H. The water footprint of bioenergy Proc. Natl. Acad. Sci. 2009, 106, 10219– 1022321https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXot1Gkur8%253D&md5=c373dc11bcf85b36345fceac0cdd58f0The water footprint of bioenergyGerbens-Leenes, Winnie; Hoekstra, Arjen Y.; van der Meer, Theo H.Proceedings of the National Academy of Sciences of the United States of America (2009), 106 (25), 10219-10223, S10219/1-S10219/10CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)All energy scenarios show a shift toward an increased percentage of renewable energy sources, including biomass. This study gives an overview of water footprints (WFs) of bioenergy from 12 crops that currently contribute the most to global agricultural prodn.: barley, cassava, maize, potato, rapeseed, rice, rye, sorghum, soybean, sugar beet, sugar cane, and wheat. In addn., this study includes jatropha, a suitable energy crop. Since climate and prodn. circumstances differ among regions, calcns. have been performed by country. The WF of bioelectricity is smaller than that of biofuels because it is more efficient to use total biomass (e.g., for electricity or heat) than a fraction of the crop (its sugar, starch, or oil content) for biofuel. The WF of bioethanol appears to be smaller than that of biodiesel. For electricity, sugar beet, maize, and sugar cane are the most favorable crops [50 m3/GJ (GJ)]. Rapeseed and jatropha, typical energy crops, are disadvantageous (400 m3/GJ). For ethanol, sugar beet, and potato (60 and 100 m3/GJ) are the most advantageous, followed by sugar cane (110 m3/GJ); sorghum (400 m3/GJ) is the most unfavorable. For biodiesel, soybean and rapeseed show to be the most favorable WF (400 m3/GJ); jatropha has an adverse WF (600 m3/GJ). When expressed per L, the WF ranges from 1,400 to 20,000 L of water per L of biofuel. If a shift toward a greater contribution of bioenergy to energy supply takes place, the results of this study can be used to select the crops and countries that produce bioenergy in the most water-efficient way.
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Supporting Information
Supporting Information
Complete description of the Energy Displacement Model. This material is available free of charge via the Internet at http://pubs.acs.org.
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