Abstract

•The biomethane and biogas supply chain may emit up to 18.5 Tg CH4 per year•Biomethane and biogas emit much less CH4 than oil and natural gas•CH4 loss rates in biomethane and biogas supply chain exceed those in oil and natural gas•The top 5% of emitters account for 62% of CH4 emissions An immediate shift away from coal and oil for energy is necessary to limit rising temperatures but is challenging due to energy needs, particularly in areas like heating and cooling that require substantial energy supply all year round. Natural gas is presently being used as a bridging fuel. It delivers the same performance as coal and oil but has lower CO2 emissions. However, natural gas releases methane (CH4), which is a more powerful warming agent than CO2. Biomethane and biogas have emerged as strong candidates to replace gas and lower CO2 and CH4 emissions. However, these replacement fuels are not CH4 emission free. Indeed, CH4 is released at various points during production and distribution, but a thorough understanding of where, when, and how much CH4 is released remains absent. A synthesis and analysis of existing biomethane and biogas CH4 emission data reveal that CH4 emissions throughout the supply chains have been underestimated. The majority of CH4 comes from just a few super-emitters and mainly at the digestate stage. Mitigating CH4 throughout biomethane and biogas supply chains is urgently needed if we are to limit global warming to 1.5°C. Although natural gas generates lower CO2 emissions, gas extraction, processing, and distribution all release methane, which has a greater global warming potential than CO2. Biomethane and biogas that use organic wastes as a feedstock have emerged as alternatives to natural gas, with lower carbon and methane emissions. However, the extent to which methane is still emitted at various stages along biogas and biomethane supply chains remains unclear. Here, we adopt a Monte Carlo approach to systematically synthesize the distribution of methane emissions at each key biomethane and biogas supply chain stage using data collected from the existing literature. We show that the top 5% of emitters are responsible for 62% of emissions. Methane emissions could be more than two times of greater than previously estimated, with the digestate handling stage responsible for the majority of methane released. To ensure the climate benefits of biomethane and biogas production, effective methane-mitigation strategies must be designed and deployed at each supply chain stage. Although natural gas generates lower CO2 emissions, gas extraction, processing, and distribution all release methane, which has a greater global warming potential than CO2. Biomethane and biogas that use organic wastes as a feedstock have emerged as alternatives to natural gas, with lower carbon and methane emissions. However, the extent to which methane is still emitted at various stages along biogas and biomethane supply chains remains unclear. Here, we adopt a Monte Carlo approach to systematically synthesize the distribution of methane emissions at each key biomethane and biogas supply chain stage using data collected from the existing literature. We show that the top 5% of emitters are responsible for 62% of emissions. Methane emissions could be more than two times of greater than previously estimated, with the digestate handling stage responsible for the majority of methane released. To ensure the climate benefits of biomethane and biogas production, effective methane-mitigation strategies must be designed and deployed at each supply chain stage. IntroductionAs we move further into the 21st century, energy systems must move away from fossil fuels and grow in renewable energy capacity if Paris Agreement temperature targets are to be met. However, due to challenges in adopting low-carbon technologies, certain areas of global energy systems are difficult to decarbonize. These include heavy industry, transport, and heating and cooling systems, which together account for a significant portion of carbon dioxide (CO2) emissions.1The Royal SocietyLow-carbon Heating and Cooling: Overcoming One of World’s Most Important Net Zero Challenges.https://royalsociety.org/-/media/policy/projects/climate-change-science-solutions/climate-science-solutions-heating-cooling.pdfDate: 2021Google Scholar Natural gas has therefore been used as an important alternative fuel, which can offer large-scale energy supply, especially for domestic space heating and hot water needs, electricity generation, and industrial applications, with much lower CO2 emissions compared with oil and coal. Although replacing oil and coal with natural gas reduces CO2 emissions, fugitive emissions from the supply chain of natural gas—gas extraction, processing, and distribution—can all release CH4. Around 39.6 million tonnes of CH4 were emitted in 2021,2IEAGlobal Methane Tracker 2022. IEA, Paris2022https://www.iea.org/reports/global-methane-tracker-2022Google Scholar representing 61% of oil and gas emissions and 30% of total-energy-sector CH4 emissions. Since CH4 has a much stronger global warming potential than CO2 and is currently responsible for at least one-quarter of global warming, there are strong calls for natural gas use to be reduced by at least 35% by 2050 and 70% by 2100 relative to 2019;3Speirs J. Dubey L. Balcombe P. Tariq N. Brandon N. Hawkes A. The Best Uses of Natural Gas within Paris Climate Targets. Sustainable Gas Institute, Imperial College London, 2021Google Scholar therefore, alternative clean-energy methods are vital to replace natural gas to limit global warming to 1.5°C.An alternative method of decarbonizing natural gas is via replacing it with biomethane or biogas, which is a mixture of gases (mostly CH4 and CO2) produced from biodegradable materials. Biomethane and biogas production and use have been put forward as part of mitigation efforts,4Nisbet E.G. Fisher R.E. Lowry D. France J.L. Allen G. Bakkaloglu S. Broderick T.J. Cain M. Coleman M. Fernandez J. et al.Methane mitigation: methods to reduce emissions, on the path to the Paris Agreement.Rev. Geophys. 2020; 58 (e2019RG000675)https://doi.org/10.1029/2019rg000675Crossref Google Scholar with up to 37 exajoule (EJ)/year of biomass-based gases in Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5°C (IPCC SR1.5C) scenarios,5Huppmann D. IAMC 1.5° C Scenario Explorer and Data Hosted by IIASA. Integrated Assessment Modeling Consortium & International Institute for Applied Systems Analysis, 2018https://doi.org/10.22022/SR15/08-2018.15429Crossref Google Scholar which limits temperature rises to below 2°C. The International Energy Agency (IEA)6IEAOutlook for Biogas and Biomethane: Prospects for Organic Growth. IEA, Paris2020Google Scholar reported that global biomethane and biogas production could satisfy nearly 20% of global gas demand if its sustainable potential was fully utilized.6IEAOutlook for Biogas and Biomethane: Prospects for Organic Growth. IEA, Paris2020Google Scholar Because biomethane is similar to natural gas, it can be easily stored and injected into the existing natural gas infrastructure, potentially providing reliable and affordable energy.7Marc-Antoine E.M. Mathieu C. Biogas and Biomethane in Europe: Lessons from Denmark, Germany and Italy. Études de l’Ifri, 2019https://www.ifri.org/en/publications/etudes-de-lifri/biogas-and-biomethane-europe-lessons-denmark-germany-and-italyGoogle Scholar At the time of writing, Europe is the world leader in biomethane production by upgrading biogas, followed by the United States, China, and Canada.8The World Biogas Association Global Potential of Biogas. World Biogas Association, 2019https://www.worldbiogasassociation.org/wp-content/uploads/2019/07/WBA-globalreport-56ppa4_digital.pdfGoogle Scholar According to the World Biogas Association (2019), 700 biogas-upgrading plants are operating worldwide, with 195 in Germany (the largest producer), with biogas currently dominating biomethane production. Biomethane and biogas production are expected to grow further, with demand predicted to grow 9-fold by 2040 compared with 2018 levels,6IEAOutlook for Biogas and Biomethane: Prospects for Organic Growth. IEA, Paris2020Google Scholar,9ADBAAnaerobic Digestion Policy Report. Anaerobic Digestion & Bioresources Association, 2019Google Scholar driven by increases in the volume of organic waste generated by modern societies, changes in waste practices, and the phasing out of fossil fuels aimed at reducing greenhouse gas (GHG) emissions and meeting government targets. Given this host of commitments, investments, and developments, biomethane and biogas could be crucial in helping to establish a clean, reliable, and affordable global energy system.However, large quantities of CH4 can still be emitted from the biomethane and biogas supply chains, including digestate handling, anaerobic digesters, upgrading units, feedstock storages and transmission, and storage and distribution stages.4Nisbet E.G. Fisher R.E. Lowry D. France J.L. Allen G. Bakkaloglu S. Broderick T.J. Cain M. Coleman M. Fernandez J. et al.Methane mitigation: methods to reduce emissions, on the path to the Paris Agreement.Rev. Geophys. 2020; 58 (e2019RG000675)https://doi.org/10.1029/2019rg000675Crossref Google Scholar CH4 is a relatively short-lived GHG but has a global warming potential (GWP) 27.2 ± 11 times larger than CO2 over a 100-year horizon and 80.8 ± 25.8 times larger over a 20-year time horizon for biogenic sources.10IPCCAR6 Climate Change 2021: The Physical Science Basis. Cambridge University Press, 2021Google Scholar The importance of reducing CH4 emissions to meet Paris Agreement11Paris AgreementUnited Nations Framework Convention on Climate Change. Paris Agreement, 2015Google Scholar targets has been demonstrated by Rogelj et al.,12Rogelj J. Meinshausen M. Schaeffer M. Knutti R. Riahi K. Impact of short-lived non-CO2 mitigation on carbon budgets for stabilizing global warming.Environ. Res. Lett. 2015; 10: 075001https://doi.org/10.1088/1748-9326/10/7/075001Crossref Scopus (55) Google Scholar as it is an important GHG in terms of potential overshooting of Paris Agreement targets, where warming exceeds “well below 2°C” and then returns to the target level by 2100,10IPCCAR6 Climate Change 2021: The Physical Science Basis. Cambridge University Press, 2021Google Scholar leading to potential tipping points in physical and socio-economic systems. The IPCC (Intergovernmental Panel on Climate Change) Sixth Assessment Report (AR6) (Working Group III)13IPCCClimate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 2022https://report.ipcc.ch/ar6wg3/pdf/IPCC_AR6_WGIII_FinalDraft_FullReport.pdfGoogle Scholar highlighted CH4 as playing a significant role in determining whether or when 1.5°C is achieved, as reducing CH4 emissions will offset global temperature increase much more quickly than CO2, due to its relatively short lifetime and higher GHG potency. The AR6 report also noted that reductions to CH4 emissions will need to occur more rapidly than CO2 and that reducing CH4 (and other non-CO2 GHG) emissions is essential for lowering warming.13IPCCClimate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 2022https://report.ipcc.ch/ar6wg3/pdf/IPCC_AR6_WGIII_FinalDraft_FullReport.pdfGoogle Scholar As the AR6 scenarios predict biomethane capacity to increase by up to 200-fold between 2020 and 2050,14Edward B. Volker K. Elmar K. Keywan R. Roberto S. Jarmo K. Robin L. Zebedee N. Marit S. Chris S. et al.AR6 Scenarios Database Hosted by IIASA. International Institute for Applied Systems Analysis, 2022Google Scholar understanding where CH4 emissions occur and how much is emitted is crucial.There are some emissions-measurement studies to date focusing on specific biomethane facilities,4Nisbet E.G. Fisher R.E. Lowry D. France J.L. Allen G. Bakkaloglu S. Broderick T.J. Cain M. Coleman M. Fernandez J. et al.Methane mitigation: methods to reduce emissions, on the path to the Paris Agreement.Rev. Geophys. 2020; 58 (e2019RG000675)https://doi.org/10.1029/2019rg000675Crossref Google Scholar,15Reinelt T. McCabe B.K. Hill A. Harris P. Baillie C. Liebetrau J. Field measurements of fugitive methane emissions from three Australian waste management and biogas facilities.Waste Manag. 2022; 137: 294-303https://doi.org/10.1016/j.wasman.2021.11.012Crossref PubMed Scopus (2) Google Scholar, 16Reinelt T. Liebetrau J. Monitoring and mitigation of methane emissions from pressure relief valves of a biogas plant.Chem. Eng. Technol. 2020; 43: 7-18https://doi.org/10.1002/ceat.201900180Crossref Scopus (9) Google Scholar, 17Reinelt T. Delre A. Westerkamp T. Holmgren M.A. Liebetrau J. Scheutz C. Comparative use of different emission measurement approaches to determine methane emissions from a biogas plant.Waste management. 2017; 68: 173-185https://doi.org/10.1016/j.wasman.2017.05.053Crossref PubMed Scopus (28) Google Scholar, 18Liebetrau J. Reinelt T. Agostini A. Linke B. Methane Emissions from Biogas Plants. IEA bioenergy, 2017Google Scholar, 19Liebetrau J. Reinelt T. Clemens J. Hafermann C. Friehe J. Weiland P. Analysis of greenhouse gas emissions from 10 biogas plants within the agricultural sector.Water Sci. Technol. 2013; 67: 1370-1379https://doi.org/10.2166/wst.2013.005Crossref PubMed Scopus (72) Google Scholar, 20Bakkaloglu S. Lowry D. Fisher R.E. France J.L. Brunner D. Chen H. Nisbet E.G. Quantification of methane emissions from UK biogas plants.Waste Manag. 2021; 124: 82-93https://doi.org/10.1016/j.wasman.2021.01.011Crossref PubMed Scopus (25) Google Scholar, 21Scheutz C. Fredenslund A.M. Total methane emission rates and losses from 23 biogas plants.Waste Manag. 2019; 97: 38-46https://doi.org/10.1016/j.wasman.2019.07.029Crossref PubMed Scopus (41) Google Scholar, 22Flesch T.K. Desjardins R.L. Worth D. Fugitive methane emissions from an agricultural biodigester.Biomass Bioenergy. 2011; 35: 3927-3935https://doi.org/10.1016/j.biombioe.2011.06.009Crossref Scopus (101) Google Scholar which have measured on site (measurement of emissions at each individual point source) and off site (measurement of emissions based on observations made away from the site). These can also be referred to as bottom-up (on-site) and top-down (off-site) studies. These have found that emissions from biomethane facilities can be up to 97 kg h−1 CH4.4Nisbet E.G. Fisher R.E. Lowry D. France J.L. Allen G. Bakkaloglu S. Broderick T.J. Cain M. Coleman M. Fernandez J. et al.Methane mitigation: methods to reduce emissions, on the path to the Paris Agreement.Rev. Geophys. 2020; 58 (e2019RG000675)https://doi.org/10.1029/2019rg000675Crossref Google Scholar,16Reinelt T. Liebetrau J. Monitoring and mitigation of methane emissions from pressure relief valves of a biogas plant.Chem. Eng. Technol. 2020; 43: 7-18https://doi.org/10.1002/ceat.201900180Crossref Scopus (9) Google Scholar, 17Reinelt T. Delre A. Westerkamp T. Holmgren M.A. Liebetrau J. Scheutz C. Comparative use of different emission measurement approaches to determine methane emissions from a biogas plant.Waste management. 2017; 68: 173-185https://doi.org/10.1016/j.wasman.2017.05.053Crossref PubMed Scopus (28) Google Scholar, 18Liebetrau J. Reinelt T. Agostini A. Linke B. Methane Emissions from Biogas Plants. IEA bioenergy, 2017Google Scholar, 19Liebetrau J. Reinelt T. Clemens J. Hafermann C. Friehe J. Weiland P. Analysis of greenhouse gas emissions from 10 biogas plants within the agricultural sector.Water Sci. Technol. 2013; 67: 1370-1379https://doi.org/10.2166/wst.2013.005Crossref PubMed Scopus (72) Google Scholar, 20Bakkaloglu S. Lowry D. Fisher R.E. France J.L. Brunner D. Chen H. Nisbet E.G. Quantification of methane emissions from UK biogas plants.Waste Manag. 2021; 124: 82-93https://doi.org/10.1016/j.wasman.2021.01.011Crossref PubMed Scopus (25) Google Scholar, 21Scheutz C. Fredenslund A.M. Total methane emission rates and losses from 23 biogas plants.Waste Manag. 2019; 97: 38-46https://doi.org/10.1016/j.wasman.2019.07.029Crossref PubMed Scopus (41) Google Scholar, 22Flesch T.K. Desjardins R.L. Worth D. Fugitive methane emissions from an agricultural biodigester.Biomass Bioenergy. 2011; 35: 3927-3935https://doi.org/10.1016/j.biombioe.2011.06.009Crossref Scopus (101) Google Scholar, 23Daniel-Gromke J. Liebetrau J. Denysenko V. W S K. J H. The humelock hemiarthoplasty device for both primary and failed management of proximal humerus fractures: a case series.Open Orthop. J. 2015; 9: 1-6https://doi.org/10.2174/1874325001509010001Crossref PubMed Google Scholar, 24Balde H. VanderZaag A.C. Burtt S.D. Wagner-Riddle C. Crolla A. Desjardins R.L. MacDonald D.J. Methane emissions from digestate at an agricultural biogas plant.Bioresour. Technol. 2016; 216: 914-922https://doi.org/10.1016/j.biortech.2016.06.031Crossref PubMed Scopus (47) Google Scholar However, a comprehensive evaluation by characterizing the distribution of CH4 emissions at each biomethane and biogas supply chain stage remains unclear.Here, we bring together the published emissions data from CH4-measurement studies to assess and synthesize the distribution of emissions from each supply chain stage in order to characterize the emissions profile of the biomethane and biogas supply chain (see experimental procedures and Figure S1 for the selected supply chain route). A Monte Carlo aggregation examines the distribution of supply chain emissions. This allows for the emission profile of biomethane and biogas supply chains to be characterized. We find that, while the biomethane and biogas supply chain emits less CH4 than the oil and natural gas supply chain, the emission rate is higher. Furthermore, we find that 62% of cumulative emissions are released by just the top 5% of emitters. We also find that methane emissions could be more than two times higher than previously estimated, and the digestate-handling stage contributed to the largest CH4 emissions along the supply chain. Our results will allow for a greater understanding of how to improve the sustainability of biomethane and biogas production by providing plant operators, investors in the supply chain, and policymakers with information on where improvements can be made in biomethane and biogas supply chains to reduce CH4 emissions, as well as whether existing or proposed CH4 regulations are sufficient or need to be revised.ResultsMethod summaryTo assess overall supply chain emissions, the biomethane supply chain is divided into five major stages: (1) feedstock; (2) biogas production; (3) biogas upgrading; (4) transmission, distribution, and gas storage; and (5) digestate storage. This study was compiled from several published studies and the data from on-site (taken at each individual emission source) and off-site measurements (reported for the entire site). The kernel density estimation (KDE) function was used to assess the characteristics of the data distribution gathered from individual sources for each stage of the supply chain. Following that, a Monte Carlo simulation was performed to estimate total supply chain emissions, which were then compared with the off-site emissions reported from whole-site measurements in previously published studies (see the experimental procedures for further details).Total supply chain emissionsThe cumulative distribution of the supply chain CH4 emissions is shown in Figure 1A. Median and mean emissions are 40.0–42.3 g CO2-eq./MJHHV (41.1–41.3 at the 95% confidence interval [CI]) and 51.4–52.7 g CO2-eq./MJHHV (52.2–52.4 at the 95% CI), respectively, with a 5th percentile of 11.0–16.3 g CO2-eq./MJHHV (15.6–15.7 at the 95% CI) and a 95th percentile between 118.2 and 144.0 g CO2-eq./MJHHV (131–133 at the 95% CI) using GWP100 values. Each curve defines the cumulative distribution for a single Monte Carlo simulation and shows that total supply chain emissions range from 2.5 to 343 g CO2-eq./MJHHV. The emissions distribution is highly upward skewed (Figure 1A), which is indicative of disproportionately high emitting sites referred to as “super-emitters” (see the identification of super-emitters section for details). Our findings are consistent with those observed for oil and natural-gas supply chains.25Balcombe P. Brandon N. Hawkes A. Characterising the distribution of methane and carbon dioxide emissions from the natural gas supply chain.J. Clean. Prod. 2018; 172: 2019-2032https://doi.org/10.1016/j.jclepro.2017.11.223Crossref Scopus (53) Google Scholar, 26Balcombe P. Anderson K. Speirs J. Brandon N. Hawkes A. The natural gas supply chain: the importance of methane and carbon dioxide emissions.ACS Sustain. Chem. Eng. 2017; 5: 3-20https://doi.org/10.1021/acssuschemeng.6b00144Crossref Scopus (70) Google Scholar, 27Balcombe P. Anderson K. Speirs J. Brandon N. Hawkes A. Methane and CO 2 Emissions from the Natural Gas Supply Chain. Sustainable Gas Institute, 2015Google Scholar, 28Brandt A.R. Heath G.A. Cooley D. Methane leaks from natural gas systems follow extreme distributions.Environmental science & technology. 2016; 50: 12512-12520https://doi.org/10.1021/acs.est.6b04303Crossref PubMed Scopus (127) Google Scholar, 29Omara M. Zimmerman N. Sullivan M.R. Li X. Ellis A. Cesa R. Subramanian R. Presto A.A. Robinson A.L. Methane emissions from natural gas production sites in the United States: data synthesis and national estimate.Environmental science & technology. 2018; 52: 12915-12925https://doi.org/10.1021/acs.est.8b03535Crossref PubMed Scopus (48) Google Scholar, 30Zavala-Araiza D. Alvarez R.A. Lyon D.R. Allen D.T. Marchese A.J. Zimmerle D.J. Hamburg S.P. Super-emitters in natural gas infrastructure are caused by abnormal process conditions.Nat. Commun. 2017; 8: 14012https://doi.org/10.1038/ncomms14012Crossref PubMed Scopus (85) Google Scholar Using global biogas and biomethane production of 35 megatonnes of oil equivalent (Mtoe) (1.47 × 1012 MJ) in 2018,6IEAOutlook for Biogas and Biomethane: Prospects for Organic Growth. IEA, Paris2020Google Scholar our model-based estimate of 2018 biomethane supply chain emissions may account for up to 18.5 teragram (Tg) CH4 per year (6.4–7.8 Tg CH4 year−1 at the 95th percentile and an average of 2.8–2.9 Tg CH4 year−1), which is more than two times greater than the International Energy Agency’s (IEA’s) estimate of CH4 emissions from bioenergy (9.1 Tg in 2021).2IEAGlobal Methane Tracker 2022. IEA, Paris2022https://www.iea.org/reports/global-methane-tracker-2022Google Scholar Our estimate of global biogas and biomethane CH4 emissions is significantly lower than in the global oil and natural-gas supply chain (82.5 Tg in 2021);2IEAGlobal Methane Tracker 2022. IEA, Paris2022https://www.iea.org/reports/global-methane-tracker-2022Google Scholar on the other hand, it is comparable to the production segment of the US oil and natural-gas supply chain (6.1–7.1 Tg year−1)31Rutherford J.S. Sherwin E.D. Ravikumar A.P. Heath G.A. Englander J. Cooley D. Lyon D. Omara M. Langfitt Q. Brandt A.R. Closing the methane gap in US oil and natural gas production emissions inventories.Nat. Commun. 2021; 12: 4715https://doi.org/10.1038/s41467-021-25017-4Crossref PubMed Scopus (22) Google Scholar based on site measurements.The cumulative distribution of emissions as a percentage of total CH4 produced is shown in Figure 1B. The 5th percentile is 1.7%–2.0% (1.94%–2.0% at the 95% CI) of CH4 production, and the 95th percentile is 12.3%–13.4% (12.6%–12.8% at the 95% CI) of total gas production. The ranges in minimum, median, mean, and maximum values were fairly consistent across all estimates (Figure 1). While the low and median estimates are nearly identical, the disparity between biomethane and natural gas varies widely in the highest estimates. The median ranged from 5.1% to 5.3% (5.1%–5.2% at the 95% CI), with mean emission rates of 5.90%–6.04% (5.9%–6.0% at the 95% CI) of total CH4 production, which is higher than natural gas (0.8%–2.2% of CH4 production).25Balcombe P. Brandon N. Hawkes A. Characterising the distribution of methane and carbon dioxide emissions from the natural gas supply chain.J. Clean. Prod. 2018; 172: 2019-2032https://doi.org/10.1016/j.jclepro.2017.11.223Crossref Scopus (53) Google Scholar,26Balcombe P. Anderson K. Speirs J. Brandon N. Hawkes A. The natural gas supply chain: the importance of methane and carbon dioxide emissions.ACS Sustain. Chem. Eng. 2017; 5: 3-20https://doi.org/10.1021/acssuschemeng.6b00144Crossref Scopus (70) Google Scholar Rutherford et al.31Rutherford J.S. Sherwin E.D. Ravikumar A.P. Heath G.A. Englander J. Cooley D. Lyon D. Omara M. Langfitt Q. Brandt A.R. Closing the methane gap in US oil and natural gas production emissions inventories.Nat. Commun. 2021; 12: 4715https://doi.org/10.1038/s41467-021-25017-4Crossref PubMed Scopus (22) Google Scholar found CH4 emissions in the oil and natural-gas-production segment to be 1.3% (1.2%–1.4% at the 95% CI), which is significantly lower than our findings. On the other hand, despite declining gas production, one of the highest reported CH4 emissions from oil and gas production (Uinta Basin from a multi-year record of in-site observations) reveals a higher emission rate than our results (6%–8%).32Lin J.C. Bares R. Fasoli B. Garcia M. Crosman E. Lyman S. Declining methane emissions and steady, high leakage rates observed over multiple years in a western US oil/gas production basin.Sci. Rep. 2021; 11: 22291https://doi.org/10.1038/s41598-021-01721-5Crossref PubMed Scopus (5) Google Scholar Although emissions from the biomethane supply chain are comparable to oil and natural-gas production in terms of Tg CH4 year−1, the production-normalized emission rate is considerably higher. This could be due to a variety of factors, including poorly managed production facilities; a lack of attention to the biomethane industry resulting in lower investments for modernization, operation, and monitoring; and employment of highly skilled plant operators16Reinelt T. Liebetrau J. Monitoring and mitigation of methane emissions from pressure relief valves of a biogas plant.Chem. Eng. Technol. 2020; 43: 7-18https://doi.org/10.1002/ceat.201900180Crossref Scopus (9) Google Scholar,21Scheutz C. Fredenslund A.M. Total methane emission rates and losses from 23 biogas plants.Waste Manag. 2019; 97: 38-46https://doi.org/10.1016/j.wasman.2019.07.029Crossref PubMed Scopus (41) Google Scholar when compared with oil and natural gas. In addition, poor design and management of feedstock and digestate storage units33Paolini V. Petracchini F. Segreto M. Tomassetti L. Naja N. Cecinato A. Environmental impact of biogas: a short review of current knowledge.J. Environ. Sci. Health A Tox Hazard Subst. Environ. Eng. 2018; 53: 899-906https://doi.org/10.1080/10934529.2018.1459076Crossref PubMed Scopus (169) Google Scholar as well as a limited interest in infrastructure emissions may result in higher emission rates compared with the amount of gas produced. Because oil and natural-gas supply chains have been primarily operated by large companies for decades, they have invested more in leak detection and repair.34EDFMethane Mitigation in the Oil & Gas Industry.https://business.edf.org/insights/methane-mitigation-in-the-oil-gas-industry/Date: 2019Google Scholar,35IEADriving Down Methane Leaks from the Oil and Gas Industry. IEA, Paris2021Google Scholar On the other hand, given the growth in biomethane generation due to national decarbonization strategies, more urgent efforts are also needed for the biomethane supply chain to address not only CH4 emissions but also the sustainability of biomethane.Identification of super-emittersA small proportion of facilities or equipment with disproportionately large emission rates are labeled super-emitters,36Zavala-Araiza D. Lyon D. Alvarez R.A. Palacios V. Harriss R. Lan X. Talbot R. Hamburg S.P. Toward a functional definition of methane super-emitters: application to natural gas production sites.Environ. Sci. Technol. 2015; 49: 8167-8174https://doi.org/10.1021/acs.est.5b00133Crossref PubMed Scopus (89) Google Scholar,37Brandt A.R. Heath G.A. Kort E.A. O'Sullivan F. Pétron G. Jordaan S.M. Tans P. Wilcox J. Gopstein A.M. Arent D. et al.Methane leaks from North American natural gas systems.Science. 2014; 343: 733-735https://doi.org/10.1126/science.1247045Crossref PubMed Scopus (597) Google Scholar causing the heavy-tailed distribution (see Figure S4). A small number of high emitters may cause under- or overestimations of emissions rates38Duren R.M. Thorpe A.K. Foster K.T. Rafiq T. Hopkins F.M. Yadav V. Bue B.D. Thompson D.R. Conley S. Colombi N.K. et al.California’s methane super-emitters.Nature. 2019; 575: 180-184https://doi.org/10.1038/s41586-019-1720-3Crossref PubMed Scopus (110) Google Scholar if they have intermittent emissions patterns, insufficient process equipment usage, or inadequate operations and maintenance strategies. In this study, super-emitters have been investigated at various stages across the supply chain, including feeding systems; substrate storage; runoff ponds; pressure relief valves on the anaerobic digesters and gas holders; exhausts and aeration lines of upgrading units; ventilation of units, such as compressors or closed digestate tanks; open digestate storage; and flaring. Within the heavy-tail distribution (Figures 1A and 1B) and the boxplot of each stage’s emissions (Figure 2), the mean emission rate is higher than the median because of super-emitters (see Table S1 for details). Since we lack informati

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