Response to Comment on “Environmental Implications of United

Correspondence/Rebuttal pubs.acs.org/est

Response to Comment on “Environmental Implications of United States Coal Exports: A Comparative Life Cycle Assessment of Future Power System Scenarios”

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Mr. Gilbert states that our comparative metric of emissions per megawatt hour (MWh) is not adequate because “potential importers of U.S. coal, including South Korea, use significant levels of coal for industrial and other uses.” We disagree. For a comparative LCA to be meaningful, the estimates of impacts air emissions in this caseneed to be normalized by the f unctional unit, that is, by the units of the function or service the resource accomplishes.2 Since we explicitly defined the export scenario as one in which PRB coal is exported to South Korea for electricity generation, the functional unit we use is MWh, as common in other LCA studies.3,4 The assumption that PRB coal would be used for electricity generation in South Korea is plausible (the majority of coal mined globally is used for power generation5) and necessary in light of the many uncertainties we identified in the paper: “Comparing the life cycle emissions of burning U.S. coal domestically with those of burning it abroad is naturally complicated by uncertainty with regard to a number of factors, including: destination of exported coal, amounts of exported coal, uses for exported coal (steam versus metallurgical), and foreign power plant technology and emissions factors.” Mr. Gilbert says that “normalizing to tons instead of MWh allows cross-sectoral emissions quantification and is more comprehensive.” Although it may be possible to compare environmental impacts using a functional unit of tons, this would complicate the analysis and interpretation of results given that cross-sectoral uses involve different functions/ services and a unit of tons would obscure important factors such as different efficiencies in converting coal into different products (e.g., electricity vs steel). Having said this, we actually do in fact present emissions per ton of exported PRB coal. However, we include the caveat that comparing the emissions per ton of coal burned in the U.S. vs South Korea stops short of the goals of our analysis and addresses a different question. As stated, we sought to investigate the impacts of scenarios where PRB coal was or was not exported. To make a fair comparison we held constant the amount of power that would be delivered in each scenario (in both jurisdictions). Therefore, in our case, an average of emissions per MWh for each scenario is a better way to summarize the combined impacts. Finally, we indeed left out of the study the emissions associated with market dynamics which is common in attributional LCAs.6,7 A consequential LCA (cLCA) would likely be more capable of addressing these issues and we agree that this would be an interesting analysis to conduct. However, it does not negate or replace the value we argue we have added by taking a scenario based approach. Finally, until this cLCA is

e thank Mr. Gilbert for this opportunity to clarify three important points about our study. Mr. Gilbert’s first comment is that because both Centralia and Boardman are scheduled to close, a scenario that considers retrofitting both plants with emissions controls and continued operation is not appropriate. Mr. Gilbert states that “The authors do not explicitly acknowledge that these plants are already scheduled to be shut down and have an extremely low likelihood of being retrof itted. Rather, they identif y their baseline scenario as “hypothetical,” which is misleading as it implies a business as usual case to readers.” We respectfully reject the first sentence of this statement as false. Our paper is explicit and clear about this: “The MPP takes advantage of existing rail routes historically used to transport PRB coal to the 585 MW Boardman coal plant in Boardman, Oregon and the 1340 MW Centralia Coal Plant in Centralia, Washington. The two plants are the only existing coal-f ired power plants in Oregon and Washington, and both are being closed by 2025 to avoid expensive upgrades required as part of stricter emissions regulations.” Because we are explicit about the fate of Boardman and Centralia, we disagree with the claim that identifying the baseline as hypothetical is misleading. Also, although Boardman and Centralia are being closed, it is worth considering that stricter emissions regulations are prompting other coal plants around the U.S. to decide between retrofitting their environmental controls or shutting down.1 While coal plants retrofits might not be the most likely scenario, it is certainly a possibility with impacts worth investigating. Mr. Gilbert says that we omitted “a third scenario where coal is not exported or used domestically.” We indeed decided not to explicitly consider this leave-the-coal-in-the-ground scenario for several reasons including the fact that it was not a scenario that would serve our purpose of comparing air emissions between domestic burning and exporting PRB coal. However, our study actually provides the LCA estimates to answer the question of whether the leave-the-coal-in-the-ground scenario is better than the mine-the-coal scenario under certain boundary choices. For example, if we drew a boundary to include only the PRB mine, it would be clear that leaving the coal in the ground would result in zero LCA emissions, while mining the coal would not. If we expanded the boundary of the system considered to include the Boardman and Centralia power plants, then the leave-the-coal-in-the-ground scenario would need to include the emissions of building the replacement plants and extracting and transporting the replacement fuel. All these emissions estimates can be obtained from our paper, and as our results indicate, the leave-the-coal-in-the-ground scenario would be more favorable than the mine-the-coal scenario assuming current combustion in Boardman and Centralia and replacement of these plants with NGCC. © 2014 American Chemical Society

Published: November 18, 2014 14055

dx.doi.org/10.1021/es5049682 | Environ. Sci. Technol. 2014, 48, 14055−14056

Environmental Science & Technology

Correspondence/Rebuttal

conducted, it is impossible to conclude whether the economywide emissions would go up or down.

Barrett Bohnengel† Dalia Patiño-Echeverri*,† Joule Bergerson‡ †

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Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, United States ‡ Department of Chemical and Petroleum Engineering, Schulich School of Engineering, Institute for Sustainable Energy, Environment and Economy, Centre for Environmental Engineering Research and Education, University of Calgary, Calgary, Alberta T2N 1N4, Canada

AUTHOR INFORMATION

Corresponding Author

*Phone: 919 358-0858; fax: 919.684.8741; e-mail: dalia. [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Planned coal-fired power plant retirements continue to increase; Today in Energy −03-20-2014; U.S. Energy Information Administration, 2014; http://www.eia.gov/todayinenergy/detail.cfm?id= 15491. (2) ISO. 14044 International Standard: Environmental Management Life Cycle AssessmentRequirements and Guidelines; International Organization for Standardization: Geneva, Switzerland, 2006. (3) Jaramillo, P.; Griffin, W. M.; Matthews, H. S. Comparative lifecycle air emissions of coal, domestic natural gas, LNG, and SNG for electricity generation. Environ. Sci. Technol. 2007, 41 (17), 6290−6296 DOI: 10.1021/es063031o. (4) Davidsson, S.; Hook, M.; Wall, G. A review of life cycle assessments of wind energy systems. Int. J. Life Cycle Assess. 2012, 17, 729−742. (5) International Energy Agency. 2011. Key World Energy Statistics. http://www.iea.org/publications/freepublications/publication/key_ world_energy_stats-1.pdf. (6) Plevin, R. J.; Delucchi, M. A.; Creutzig, F. Using attributional life cycle assessment to estimate climate-change mitigation benefits misleads policy makers. J. Ind. Ecol. 2014, 18 (1), 73−83. (7) Anex, R.; Lifset, R. Life cycle assessment: Different models for different purposes. . J. Ind. Ecol.. 18 (3): 321−323.

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dx.doi.org/10.1021/es5049682 | Environ. Sci. Technol. 2014, 48, 14055−14056