Photo: Coyote Springs Generating Station by Portland General Electric
On August 25, my colleagues at the Deutsche Bank Climate Change Advisors and I released a new greenhouse gas (GHG) life-cycle analysis of U.S. coal and natural gas-fired electricity. If you have been following my posts on ReVolt over the last year, you’ll know we began studying this issue after the Environmental Protection Agency (EPA) announced revisions to its methodology for estimating emissions from natural gas systems (basically from the production, processing, transmission, and distribution of natural gas) that resulted in a more than doubling of its estimate for methane emissions from those sources. Methane, in addition to being the primary component of natural gas, is a GHG some 25 times more potent than carbon dioxide over a hundred-year period. Consequently, some analysts have raised concerns that when the actual amount of methane emitted during the entire life cycle of natural gas (an amount which the EPA’s previous methodology apparently underestimated) is taken into account, natural gas might lose its GHG advantage over coal.
Over the past year, a number of new life-cycle analyses have come out that all ask different versions of the question, “How clean is natural gas really, on a life-cycle basis?” Some focus on GHG emissions from shale versus conventional natural gas, while others focus on all natural gas produced in the United States. The life-cycle analyses use different underlying assumptions, methodologies, and sources of data, and nearly all comment on the implications of their findings for the GHG comparison between coal and gas. After all, if the Obama administration is (or at least was) considering a clean energy standard that gave natural gas-fired electricity a half-credit on the basis of its GHG savings over coal, this should be reflected by actual GHG savings.
The studies have offered a bewildering array of answers to this question. A March 2011 study by researchers from Cornell concludes, “Compared to coal, the footprint of shale gas is at least 20 percent greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years.” On the other hand, a May 2011 presentation from the National Energy Technology Laboratory (NETL) concludes, “Average natural gas baseload power generation has a life-cycle [GHG footprint] 54 percent lower than average coal baseload power generation on a 100-year time horizon.” And an August 2011 study by researchers from Carnegie Mellon University finds, “Natural gas from the Marcellus shale has generally lower life-cycle GHG emissions than coal for the production of electricity in the absence of any effective carbon capture and storage processes, by 20-50 percent depending upon plant efficiencies and natural gas emissions variability.”
The results of our life-cycle analysis are generally consistent with the findings of NETL and Carnegie Mellon. Since the most important question for us was how new information about methane emissions from natural gas systems changed the GHG comparison between natural gas- and coal-fired electricity, we analyzed the impact of EPA revisions on the average GHG footprint of a unit of electricity produced from natural gas versus coal. We found that, on average, U.S. natural gas-fired electricity emits 47 percent less GHGs than coal over a hundred-year timeframe.
It’s important to note that looking at the emissions associated with average U.S. natural gas is different from looking at those associated with shale gas only, which currently accounts for 15 percent of U.S. natural gas production but whose share is projected to rise to more than 45 percent by 2035. Although all three life-cycle studies mentioned above attempt to estimate shale gas emissions alone, all acknowledge significant uncertainty around certain segments of the life-cycle stemming from inadequate data. This lack of data is due in part to the relatively recent expansion of shale gas development, but it is compounded by the rapid evolution of technologies, practices, and regulations. For example, one of the main drivers of the increase in EPA’s methane emissions estimates was the amount of emissions during the flowback period of wells that receive hydraulic fracturing treatment—emissions that new EPA regulations will require to be captured.
A major push to collect better data on the methane emissions from shale gas production will be necessary before we can develop a truly accurate picture of the life-cycle GHG footprints of shale versus conventional natural gas—beginning but not ending with the EPA’s Greenhouse Gas Reporting Program. More robust data will make it possible not only to more clearly assess the GHG footprint of natural gas production, but also to identify where in the life cycle of natural gas control technologies and practices can most cost-effectively reduce methane venting and leaking. Many such control technologies and practices have been documented by the EPA’s Natural Gas STAR program and are already being employed in the industry today.
Methane emissions during the entire life cycle of natural gas may not be enough to negate the GHG savings that natural gas has over coal at the point of combustion, but they still pose a serious risk to the climate. Rather than allowing significant quantities of methane to escape during production, companies should be capturing it for sale—and indeed, those who have done so have often reported payback times of less than three years through Natural Gas STAR. Moreover, because some of the same technologies that prevent methane from entering the atmosphere also reduce emissions of smog-forming compounds, tackling methane emissions is a win-win-win proposition for the natural gas industry, local air quality, and the climate.