Even counting higher estimated emissions of methane (CH4) from shale gas production activities, natural gas-fired power generation still beats coal-fired power by a wide margin when it comes to overall greenhouse gas (GHG) emissions, according to a new study published by the gas-friendly American Clean Skies Foundation (ACSF).
The ACSF research joins the fray of the Environmental Protection Agency's (EPA) newly released 2011 U.S. GHG Inventory and a recent paper from a Cornell University researcher that claims CH4 emissions from gas are worse for the environment than those of carbon dioxide (CO2) from coal (see Shale Daily, April 13). ACSF refutes that claim and its 15-page paper pencils EPA's higher figures for CH4 emissions into full fuel chain GHG emissions estimates for gas and coal when used for power generation.
"On average, our paper shows that using the most current U.S. national inventory data, and standard international assumptions on the relevant time horizons for estimating the GWP [global warming potential] of methane and other GHGs, the large comparative GHG advantage of natural gas-fired power plants continues to outweigh the negative GHG impact from the estimated rates of methane leakage from natural gas production," wrote ACSF CEO Gregory Staple and Joel Swisher, director of technical services for carbon offset developer Camco International and a consultant to ACSF.
EPA recently increased its estimates of CH4 leaked during natural gas production activities, mainly due to emissions from unconventional plays where wells are stimulated using hydraulic fracturing. "Unlike completions and workovers without hydraulic fracturing (i.e., conventional workovers), the high-pressure venting of gas in order to expel the large volumes of liquid used to fracture the well formation results in a large emission of natural gas [CH4]," EPA said in its GHG Inventory.
However, ACSF noted that gas producers have criticized the new EPA estimates as being unrealistically high. "Even researchers who believe that the revised EPA estimates of fugitive emissions from unconventional production are too low acknowledge that the industry can reduce such leakages by up to 90% using available technologies," Staple and Swisher said.
Assuming the EPA's new emissions estimates are correct, gas-fired power production still produces 50% fewer GHG emissions than does coal-fired generation, ACSF said. The reduction in GHGs is even greater when compared to coal-fired plants built at least 30 years ago.
"The [Cornell research] compared the estimated GHG footprint of shale gas versus coal based solely on the theoretical amount of energy input for power generation," ACSF said. "This disregards the efficiency advantage of modern gas-fired generation in terms of the electric energy output: kilowatt hours."
The Staple-Swisher paper used the most recent EPA and nationwide Department of Energy data to calculate GHG emissions from both the production and combustion portion of the fuel chain for all natural gas-fired and coal-fired electric power. The comparison is based on GHG emissions per kilowatt hour generated and also uses conventional internationally accepted values for estimating the climate impact of various GHG emissions over 100 years, ACSF said.
"By contrast," ACSF said, "the Cornell team looked primarily at a 20-year time horizon and used novel values for weighting the comparative climate impact of methane and other greenhouse gases, rather than the 100-year period that climate scientists and researchers regard as most accurate and useful for estimating the impact of methane and other greenhouse gases."
The share of natural gas in the power generation stack has grown significantly since the late 1990s, according to the Energy Information Administration. Gas and coal account for the lion's share of generating capacity despite advances by renewables such as wind and solar. Numerous analysts have predicted that gas will pick up greater share as coal plants are retired. Gas-fired generators also are expected to be relied upon more heavily as a backup to intermittent renewables.