Natural gas used for power generation emits about half as many greenhouse gases as coal during its production, distribution and use, environmental research organization Worldwatch Institute said.

The group’s report is the latest contribution to a burgeoning debate on the environmental advantages of natural gas.

The Institute said it applied revised methodology from the US Environmental Protection Agency to investigate whether emissions of methane – the principal component of natural gas and a more potent GHG source than carbon dioxide – would diminish the perceived advantage of natural gas over coal.

It found that over a life-cycle analysis, natural gas emits less than half the GHGs of coal.

“Despite a substantial increase in the methane assumed to be emitted during natural gas production, we found that U.S. natural-gas fired electricity generation still released 47% fewer greenhouse gases than coal from source to use,” said Saya Kitasei, one of the authors of the Worldwatch report.

Critics Target Methodology

Critics of the current boom in natural gas production from shale beds say that although natural gas emits GHGs at about half the rate of coal when burned, additional emissions during production and distribution make it at least as damaging as coal when considered on a life-cycle basis.

Among them is Cornell University professor Robert Howarth, who said the Worldwatch report underestimates methane emissions by relying on incomplete revisions to the EPA methodology, and by focusing only on the 30% of US natural gas that’s used to generate electricity.

“Electric power plants that burn gas are more efficient than those that burn coal, but for the other 70 percent use of gas (heating and industrial use), there is no greater efficiency for gas over other fossil fuels,” Howarth wrote in an email.

“Therefore, focusing just on electricity is misleading, and tends to make natural gas look more favorable than it truly is overall,” he said.

The EPA’s analysis is based on inventory data from 2008 when only 15% of US natural gas came from shale, and therefore ignores the growing share of shale gas, which is expected to account for 45% of the total by 2035, Howarth said.

The Potent Pollutant

He also argued that Worldwatch has adopted a low-end estimate for the GHG potency of methane compared with carbon dioxide.

The report assumes methane is 25 times more powerful as a GHG than carbon dioxide over 100 years, an estimate used by the Intergovernmental Panel on Climate Change in 2007 but which is exceeded by a more recent estimate of 33 times, published in the journal Science in 2009, Howarth said.

But in another study, IHS Cambridge Energy Research said the EPA “grossly overestimates” GHG emissions from hydraulically fractured shale-gas extraction. The research company said the agency was incorrect to base its findings on four “green” well completions, in which the amount of captured methane was assumed to be equivalent to that which escapes into the atmosphere.

“The assumption that all methane recovered from these sample wells would otherwise have been flared or vented is questionable at best, given that common industry practice is to capture gas for sale as soon as it is technically feasible,” said Surya Rajan, co-author of the IHS report, issued on Aug. 24.

The EPA did not immediately respond to a request for comment on the IHS report.

Although natural gas emits fewer GHGs than coal, steps should still be taken to cut methane emissions, Worldwatch said in the report issued last week. It argued that technologies to reduce smog-forming emissions can also be used to prevent methane from entering the atmosphere.

The role of coal in the next generation of the electricity system is controversial, but the fossil fuel remains a vital component in the contemporary energy market, even as natural gas takes a larger share of new generation. Read more of Breaking Energy’s coal coverage here, and more of our natural gas coverage here.