Climate change impacts are lessened when power plants burn natural gas instead of coal, but the same is not necessarily true when vehicles burn natural gas instead of traditional fuels because methane leakage more than offsets the clean-burning benefits of gas, according to a methane leakage model developed by the Environmental Defense Fund (EDF) and university researchers. However, the gas industry countered that the model relies on "outdated and incomplete" data.
"Natural gas burns cleaner than other fossil fuels when combusted, but methane leakage from production and transportation of natural gas has the potential to remove some or all of those benefits, depending on the leakage rate," EDF said.
Assuming the Environmental Protection Agency's (EPA) 2009 leakage rate of 2.4% (from well to city), new gas combined-cycle power plants reduce climate impacts compared to new coal plants. This is true as long as leakage remains under 3.2%, the group said. However, compressed natural gas vehicles are not a viable mitigation strategy for climate change because of methane leakage from natural gas production, delivery infrastructure and from the vehicles themselves, EDF said.
The EDF findings are in a paper published in the Proceedings of the National Academy of Sciences (PNAS). The paper "illustrates the importance of accounting for methane leakage across the value chain of natural gas (i.e., production, processing and delivery) when considering fuel-switching scenarios from gasoline, diesel fuel and coal to natural gas," EDF said.
"The model allows users to plug in different variables and observe the outcome," EDF said. "Thus the paper does not draw hard and fast conclusions about the future implications of any kind of fuel-shifting, nor does it answer the question of whether natural gas generation or natural gas-powered vehicles will be better or worse for the climate. What it does do is provide those answers in terms of the leak rates at which fuel-switching produces climate benefits at all points in time."
The industry-funded America's Natural Gas Alliance (ANGA) disputed the validity of the EPA data used in the model.
"The Environmental Defense Fund study is based on EPA data that the EPA itself has publicly said is outdated and incomplete," ANGA's Dan Whitten said. "In fact, EDF knows new and additional data is being provided by natural gas producers. We are surprised EDF would release such a report with this knowledge in that they are aware it is premature to draw any conclusions.
"It is in everyone's best interest that this clean-burning domestic energy is developed safely and responsibly, and producers are taking extraordinary measures to do so every day."
For light-duty CNG vehicles to become a viable short-term climate strategy, methane leakage would need to be kept below 1.6% of total natural gas produced (approximately half the current amount for well to wheels), EDF said. Further, methane emissions would need to be cut by more than two-thirds to immediately produce climate benefits in heavy-duty natural gas-powered trucks.
"At current leakage rate estimates, converting a fleet of heavy duty diesel vehicles to natural gas would result in nearly 300 years of climate damage before any benefits were achieved," EDF said.
American Gas Association spokesman Jake Rubin said the industry trade association agrees that methane emissions reductions should continue. "America's natural gas utilities are committed to enhancing safety and lowering emissions through upgrading our natural gas infrastructure," he said. "More robust data is needed to address confusion regarding the benefits of increasing the use of natural gas, and we believe that the data that EPA receives from its greenhouse gas reporting requirement is likely to demonstrate that methane emissions are vastly lower than those assumed in this paper.
"As technology continues to improve the way we produce natural gas, the pipelines and wellheads of tomorrow will have even lower methane emissions than we have today. It would be premature to draw any policy conclusions about natural gas vehicles based solely on this report."
There have been several papers published over the last year or so that reach different conclusions about the amount and effects of methane emissions (see related story; NGI, Jan. 23). "The PNAS paper tries to clear up some of the confusion by addressing the analytical challenge of comparing the time-dependent effects on climate of methane by using the 'technology warming potential' approach," EDF said.
"Failing to reduce methane leaks has the potential to eliminate much, if not all, of the greenhouse gas advantage of natural gas over coal," said EDF chief scientist Steven Hamburg, coauthor of the paper. "If we want natural gas to be an accepted part of a strategy for achieving energy independence and moving to a clean energy future, it's critical that industry, regulators and other stakeholders work together to quantify the existing methane leakage rate and commit to reducing it to 1% or below if, as expected, the leakage is currently higher than that."
EDF is collaborating with partners on a study designed to quantify the methane leakage rate across the natural gas value chain in five discrete modules, the first of which -- emissions from the production sector -- has already been launched. EDF said it aims to complete the entire study by December 2013.
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