In recent months, several sources have called natural gas’s greenhouse gas (GHG) emissions reductions potential over coal into question. Because the GHG benefit of a coal-to-natural gas shift in the power sector is a critical assumption in our common conclusion that natural gas can facilitate a reduction in power sector GHG emissions, the Worldwatch Institute and Deutsche Bank Climate Change Advisors are embarking on a joint study to assess the currently available emissions data, develop a rigorous and transparent life cycle assessment of electricity generated from gas and coal, and identify data gaps that must be prioritized in further research.
Why is it commonly held that electricity generated from natural gas is 50-60 percent cleaner than coal, and why is this belief being challenged?
Traditionally, comparisons of emissions from different types of power plants have focused on emissions from the combustion of fuels at power plants themselves. Combustion emissions are a product of two main factors – the fuel’s carbon content and the efficiency with which power plants convert that fuel into electricity. In 2009, U.S. coal steam turbines (the type of plant in which almost all coal is burned in the U.S.) had an average efficiency of 34 percent, while natural gas combined cycle units had an average efficiency of 45%. Combined cycle plants, the technology most likely to displace coal for baseload power generation, are more efficient and can be throttled up and down more quickly and cleanly. Nationally, the EPA reports that burning natural gas in a power plant releases 53 kilograms of carbon dioxide per million Btu (kg CO2 per MMBtu) of fuel, while burning coal releases 96 kg CO2 per MMBtu. When combined with their greater conversion efficiency, this means that natural gas combined cycle power plants emit about 60 percent less CO2 than coal steam turbines.
We are not aware of any controversy surrounding the comparisons of point-of-use GHG emissions between electricity generated by single cycle coal power plants and natural gas combined cycle plants. Rather, attention has focused on the greenhouse gases—especially methane (CH4), the main component of natural gas—emitted during the processing, transport, and especially the production of coal and natural gas.
The need to consider full life-cycle greenhouse gas emissions when comparing different power sources has emerged in recent years. Several life-cycle assessments available by the end of 2010 estimated the greenhouse gas emissions per unit of electricity generated from coal and gas plants. (See Figure.) A 2007 life-cycle assessment by Dr. Paulina Jaramillo of Carnegie-Mellon University concludes that electricity generated by natural gas–fired power plants emitted about 361 to 774 kg CO2e/MWh on a life-cycle basis, compared to coal, which the study concludes emitted about 892 to 1142 kg CO2e/MWh.
In October 2010, the National Energy Technology Laboratory (NETL) released a series of life-cycle assessments placing life-cycle GHG emissions from combined cycle power plants using natural gas from a mixture of domestic sources, including shale gas and coalbed methane, at 467 kg CO2-equivalent per Megawatt-hour—51 percent less than a modern supercritical pulverized coal plant and 58 percent less than a representative existing coal power plant in the U.S.
|Jaramillo 2007||NETL 2010|
|North American NG
|361 to 774||467|
|892 to 1142||1109|
|Implied Natural Gas Emissions Savings Versus Coal||-44%||-58%|
|Notes||Jaramillo 2007 uses a top-down life-cycle analysis approach to compare emissions from natural gas and coal plants. Their range of natural gas plant efficiencies may capture a range of generating technologies, including but not limited to natural gas combined cycle units. Jaramillo 2007’s upstream emissions calculations do not account for unconventional natural gas.||NETL 2010 uses a bottom-up life-cycle analysis approach to compare emissions from natural gas and coal plants. They use representative natural gas combined-cycle and existing pulverized coal steam turbine plants rather than looking at the entire U.S. fleet. NETL 2010’s upstream emissions calculations include estimates for conventional onshore, associated onshore, offshore, coalbed methane, and Barnett Shale natural gas.|
Many other life cycle assessments used emissions factors from the Environmental Protection Agency, which were originally developed in 1996 with the Gas Research Institute, and have been revised slightly over the years. In November 2010, however, the EPA released a Technical Support Document for Subpart W of its Greenhouse Gas Reporting Rule, the portion pertaining to reporting from the oil and gas industry. In it, the EPA attempts to account for previously understated sources of emissions from certain processes during natural gas production, resulting in significantly higher overall methane emissions from natural gas systems.
Plugging these new numbers into a life-cycle analysis methodology developed by Carnegie Mellon professor Paulina Jaramillo in 2007, online investigative journalism outlet Propublica, calculated in January that “the median gas-powered plant in the United States is just 40 percent cleaner than coal.” Another 2-page document posted to the web in January by Dr. Robert Howarth of Cornell University uses the same EPA source to state, “Compared to coal, the footprint of shale gas is 1.2-to 2.1-fold greater on [a] 20-year time frame and is comparable when compared over 100-years.”
Meanwhile, the United Kingdom-based Tyndall Centre for Climate Change Research released a report (which did not rely on the numbers in the EPA’s 2010 TSD) estimating the amount of additional GHG emissions that might be associated in the production of unconventional gas versus conventional gas. Using what they acknowledge to be “non-peer reviewed data from a limited number of site measurements,” the authors conclude that “CO2 emissions from shale gas are likely to be only marginally higher than those from conventional gas sources.” The Tyndall Centre’s analysis does not tackle fugitive or vented GHG emissions (non-combustion emissions that are unintentionally or intentionally released during production, processing, and transport of natural gas).
For its part, the EPA has so far not endorsed any of these life cycle analyses, releasing the following response to the Propublica article on 31 January 2011:
EPA has not conducted an analysis of coal versus natural gas, and there is no new report. The information referred to in the article was developed based on information from a Technical Support Document, however, which was developed as support for the Greenhouse Gas Reporting Program. The reporter used that data and did his own calculations to arrive at the figures used in the article. EPA has not reviewed the analysis described in the article in detail, but we have not seen any indication that the benefits of natural gas have been called into question. Available data demonstrate that switching from another fossil fuel to natural gas reduces emissions of carbon pollution and other harmful pollutants that threaten Americans’ health.
In February, the EPA released a review draft of its 2011 Greenhouse Gas Inventory, which confirms that the agency has significantly upwardly revised its estimates of methane emitted during natural gas production.
With all of these new numbers floating around, including multiple life cycle assessments that come to very different conclusions based on at least somewhat overlapping sources of data, it’s clear that the emissions factors that have been used for life cycle analyses involve greater uncertainty than previously appreciated. Both the natural gas industry and its critics agree that original data are urgently needed to provide an accurate picture of how much greenhouse gas is currently being emitted. In the meantime, a transparent, comprehensive life cycle analysis using the best available data for both gas and coal is critical to understanding the implications of new emissions numbers. We will be conducting such an analysis over the coming months, and will make it publically available when it is complete. Until then, we must reserve judgment on the life-cycle comparison of natural gas and coal and urge our readers to do the same. Stay tuned for updates.
 Jaramillo et al. 2007 does not give specific numerical ranges, but represents them graphically. These ranges were calculated following the methodology described in Jaramillo et al. 2007 and its supporting material, and appear to match the graphical representation in Jaramillo 2007. The midpoints of these ranges also corresponded closely with midpoints named in Jaramillo 2007. Paulina Jaramillo, et al. “Comparative Life-Cycle Air Emissions of Coal, Domestic Natural Gas, LNG, and SNG for Electricity Generation,” Environmental Science and Technology 41, 6290-6296 (2007).
 This figure is based on the differential between the midpoints of the ranges Jaramillo 2007gives for life-cycle emissions from natural gas and coal.