Uncertainty in Life Cycle Greenhouse Gas Emissions from United States Natural Gas End-Uses and its Effects on Policy

Abstract

Increasing concerns about greenhouse gas (GHG) emissions in the United States have spurred interest in alternate low carbon fuel sources, such as natural gas. Life cycle assessment (LCA) methods can be used to estimate potential emissions reductions through the use of such fuels. Some recent policies have used the results of LCAs to encourage the use of low carbon fuels to meet future energy demands in the U.S., without, however, acknowledging and addressing the uncertainty and variability prevalent in LCA. Natural gas is a particularly interesting fuel since it can be used to meet various energy demands, for example, as a transportation fuel or in power generation. Estimating the magnitudes and likelihoods of achieving emissions reductions from competing end-uses of natural gas using LCA offers one way to examine optimal strategies of natural gas resource allocation, given that its availability is likely to be limited in the future. In this study, the uncertainty in life cycle GHG emissions of natural gas (domestic and imported) consumed in the U.S. was estimated using probabilistic modeling methods. Monte Carlo simulations are performed to obtain sample distributions representing life cycle GHG emissions from the use of 1 MJ of domestic natural gas and imported LNG. Life cycle GHG emissions per energy unit of average natural gas consumed in the U.S were found to range between -8 and 9% of the mean value of 66 g CO(2)e/MJ. The probabilities of achieving emissions reductions by using natural gas for transportation and power generation, as a substitute for incumbent fuels such as gasoline, diesel, and coal were estimated. The use of natural gas for power generation instead of coal was found to have the highest and most likely emissions reductions (almost a 100% probability of achieving reductions of 60 g CO(2)e/MJ of natural gas used), while there is a 10-35% probability of the emissions from natural gas being higher than the incumbent if it were used as a transportation fuel. This likelihood of an increase in GHG emissions is indicative of the potential failure of a climate policy targeting reductions in GHG emissions.

Liquefied natural gas for the UK: a life cycle assessment

“…For all scenarios, almost 18% of the total GWP is due to the distribution of the evaporated gas. The total GWP calculated in this study for the upstream processing (including extraction and drying, liquefaction, transport and evaporation but not distribution) equals 0.0174 kg of CO 2 eq., and this value is consistent with values already reported in literature~0.016-0.018 kg of CO 2 eq by (Tamura et al 2001;Edwards et al 2006;Venkatesh et al 2011;NETL 2012;Safaei et al 2015).…”

Limited impact on decadal-scale climate change from increased use of natural gas

“…The rates used in the five models all fall within the range 0.3-0.6 kg of CH 4 per GJ (Extended Data Table 3). These values are similar to the values reported in conventional literature 23,24,33 . However, some recent literature suggests that the fugitive methane rate may be substantially higher by up to a factor of four 7,27,28 .…”

“…On the one hand, conventional estimates for natural gas methane leakage rates have been less than 2% of production 23,24 , and studies have shown that the leakage rate is not considerably different between conventional and unconventional sources 25,26 . On the other hand, other studies have reported substantially higher leakage rates 7,27,28 .…”

Greater focus needed on methane leakage from natural gas infrastructure

“…Recent reports in the scientific literature and popular press have produced confusion about the climate implications of natural gas (1)(2)(3)(4)(5). On the one hand, a shift to natural gas is promoted as climate mitigation because it has lower carbon per unit energy than coal or oil (6).…”