Correspondence/Rebuttal pubs.acs.org/est
Comment on “Methane Emissions from Process Equipment at Natural Gas Production Sites in the United States: Pneumatic Controllers”
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llen et al. (2014) recently published measurements of natural gas (NG) emissions from pneumatic controllers (PCs) at NG production sites in the United States in ES&T.1 Limitations to this work include (1) faulty flow meters; (2) sensor failure in their Hi-Flow instrument; and (3) potential reset of malfunctioning PCs when installing flow meters. These problems could cause the authors’ emission estimates to be underestimated by a factor of 3. Most measurements were made by installing flow meters in the supply gas lines of the devices, which required temporarily turning off the supply gas. On March 18, 2014, in collaboration with the authors, I tested these meters (designated Fox A and Fox C) while the project was underway. The Fox A meter read an average of 65% lower than the Fox C meter (which agreed with a rotameter to within 10%). The University of Texas (UT) team replaced the meter cable, but problems persisted and the Fox A meter was not tested further that day. The authors continued using the Fox A meter during their program but without incorporating the March 18 test results; the authors only noted that Fox A read 37% lower than Fox C during post project calibration due to “condensation of an oily substance on the sensor”. However, even with this correction, measurements made with Fox A were a factor of 2 lower than Fox C on March 18. Volatiles likely accumulated and dissipated on both meters throughout the project; with calibrations only before and after the project, and the large difference in Fox A’s measurements on March 18 compared to these corrections, the accuracy of these meters is highly questionable. Although the authors cite the agreement of five Hi-Flow sampler and meter measurements for emission rates >6 scfh, at least four of those comparisons were for Fox A. Additionally, four of these comparisons were made at either the same or nearby sites. Consequently, these comparisons primarily show meter performance for a short time frame and for one meter. In contrast, all available comparisons of Hi-Flow and meter measurements show on average the meters read only 71% of the Hi-Flow measurements; removing one Hi-Flow measurement that the authors thought low due to incomplete capture indicates on average the meters read only 58% of the Hi-Flow measurements. Also during the March 18 tests, the UT Bacharach Hi-Flow sampler failed to transition between low and high range sensors, underestimating emission rates by up to 2 orders of magnitude.2 The instrument was recalibrated after this occurred, eliminating this failure for the rest of that day, but the performance of the instrument before and after our testing is unknown. This could cause underestimates in the Hi-Flow measurements made during this project1 and previously.3 Finally, in my experience, manual actuation of malfunctioning PCs with high bleed rates may reset them to lower bleed rates; turning the supply gas off and on to install meters might have similarly reset devices. Figure 1 compares intermittent PC © XXXX American Chemical Society
Figure 1. Intermittent pneumatic device emission rates measured by meters and Hi-Flow Sampler. The lower peak emitters observed in the studies made by installing meters is likely due to resetting malfunctioning controllers by turning the supply gas to devices off and back on during meter installation.
bleed rates from three studies: Allen et al. (2013);3 Prasino (2013),4 which also installed meters, and the study under discussion; and Allen et al. (2014).1 The two studies which installed meters have significantly lower peak emitters than the previous study done by Hi-Flow,3 which did not require stopping the supply gas. The current study1 noted that intermittent PCs with bleed rates larger than 40.2 scfh were malfunctioning; only 1.3% of the intermittent PCs in (1) were above this bleed rate compared to 11% in the previous study.3 Consequently, the scarcity of high emitters in the current study1 is likely due to reset devices, because other factors, that is, greater liquid production rates, would not be sufficient to cause the greater occurrence of high emitters observed in (3). The effect of these missing malfunctioning controllers can be estimated by adding the emission rates for intermittent PCs observed by (3) that are greater than the largest emission rate for intermittent PCs observed in (1). This would increase the average emission rate from intermittent PCs in the current study by 81%. If the number of missing high emitters is adjusted for the increased sample size of the current study compared to (3), then the average emission rate would increase by 298%.
Touché Howard*
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Indaco Air Quality Services, Inc., Durham, North Carolina 27713, United States
AUTHOR INFORMATION
Corresponding Author
*Phone: 919-943-9406; e-mail:
[email protected].
A
DOI: 10.1021/acs.est.5b00507 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Notes
The author declares no competing financial interest
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REFERENCES
(1) Allen, D.; Pacsi, A.; Sullivan, D.; Zavala-Araiza, D.; Harrison, M.; Keen, K.; Fraser, M.; Hill, D.; Sawyer, R.; Seinfeld, J. Methane emissions from process equipment at natural gas production sites in the United States: Pneumatic controllers. Environ. Sci. Technol. 2014, 49, 633−640. (2) Howard, T., Ferrara, T. W., Townsend-Small, A. Sensor transition failure in the high flow sampler: Implications for methane emissions estimates from natural gas infrastructure. J. Air Waste Manage. Assoc., in press. (3) Allen, D. T.; Torres, V. M.; Thomas, J.; Sullivan, D.; Harrison, M.; Hendler, A.; Herndon, S. C.; Kolb, C. E.; Fraser, M. P.; Hill, A. D.; Lamb, B. K.; Miskimins, J.; Sawyer, R. F.; Seinfeld, J. H. Measurements of methane emissions at natural gas production sites in the United States. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 17768−17773. (4) Prasino Group. Final Report for determining bleed rates for pneumatic devices in British Columbia. Report to British Columbia Ministry of Environment, December 2013. http://www2.gov.bc.ca/ gov/DownloadAsset?assetId= 1F074ABD990D4EFB8AE555AEB3B8D771&filename=prasino_ pneumatic_ghg_ef_final_report.pdf.
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DOI: 10.1021/acs.est.5b00507 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
