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Is natural gas cleaner than coal?

By Stephen Lacey on May 25, 2011 at 12:07 pm

"Is natural gas cleaner than coal?"

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The debate about the cleanliness of natural gas continues, as the National Energy Technology Laboratory weighs in with its own analysis comparing coal and gas.

Last month Climate Progress wrote about a new study from Cornell University ecology professor Robert Howarth, which found that shale gas was potentially as big a contributor to climate change than coal. By examining the impact of “fugitive emissions” of methane – a greenhouse gas that traps heat far better than CO2 – Howarth came to the conclusion that shale gas “may aggravate rather than mitigate global warming”:

Shale gas is extracted by high-volume hydraulic fracturing. Large volumes of water are forced under pressure into the shale to fracture and re-fracture the rock to boost gas flow. A significant amount of this water returns to the surface as flowback within the first few days to weeks after injection and is accompanied by large quantities of methane.

He goes on to conclude:

The GHG footprint of shale gas is significantly larger than that from conventional gas, due to methane emissions with flow-back fluids and from drill out of wells during well completion. Routine production and downstream methane emissions are also large, but are the same for conventional and shale gas. Our estimates for these routine and downstream methane emission sources are within the range of those reported by most other peer-reviewed publications inventories (Hayhoe et al. 2002; Lelieveld et al. 2005). Despite this broad agreement, the uncertainty in the magnitude of fugitive emissions is large. Given the importance of methane in global warming, these emissions deserve far greater study than has occurred in the past. We urge both more direct measurements and refined accounting to better quantify lost and unaccounted for gas.

The study was reported in the New York Times shortly after it was released, stirring the debate further and raising the questions about the strength of Howarth’s data.

Now the National Energy Technology Laboratory has recently released its own study showing a different result. NETL looked at both a 100-year timeframe and a 20-year time frame, and in both cases found that life-cycle greenhouse gas emissions from natural gas were substantially lower than coal. According to the findings, natural gas emitted 50% fewer emissions than coal over a 20-year GWP and emitted 55% fewer emissions over a 100-year GWP.

NETL admits that there’s uncertainty in its data collection of production rates, flaring rates and factors related to pipeline transport. But the study does show that natural gas has considerably lower GHG emissions than coal over both a short and long time horizon — when you compare coal burned in a typical (inefficient) coal plant with gas burned in a typical (efficient) baseload plant.  So-called simple (or single) cycle gas plants running on unconventional gas may have only marginally lower GHG emissions that coal plants.

Also, NETL uses the IPCC’s 2007 GWP numbers.  While that is not unreasonable, Howarth uses the higher GWP of gas from more recent research, which is also reasonable and probably more accurate.  NETL also blurs the numbers a little by its use of calculations based on “average generation” and a “domestic mix” of natural gas (conventional and unconventional).

It looks like Howarth’s claim that natural gas vehicles may not have lower lifecycle greenhouse gas emissions than petroleum-fueled vehicles may hold up.  It would be good to see industry provide more data and a credible independent group do a more comprehensive analysis.  As Climate Progress noted last month, this would be a very ripe topic for the National Academy of Sciences to review.

These studies do not, however, factor in the concerns about methane release into water or whether fracturing fluids will cause environmental and health problems. That is another separate issue which has caused concern among environmental groups and communities close to drilling sites. As NETL concludes in the study:

“All opportunities need to be evaluated on a sustainable energy basis: Environmental performance, economic performance and social performance.”

So while greenhouse gas emissions are an extremely important part of this debate as it relates to climate change, we need to keep our attention on the other impacts of drilling as well.

Finally, few activities on the face of the planet are more harmful to humans than the use of coal from extraction through combustion.  For now, it seems safe to say that natural gas is indeed ‘cleaner’ than coal.  Of course, that isn’t saying much (see Life-cycle study: Accounting for total harm from coal would add “close to 17.8¢/kWh of electricity generated”).  We need to get off of fossil fuels of all kinds ASAP.

– Stephen Lacey and Joe Romm

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