Do natural gas-fired power plants contribute less greenhouse warming than coal ?

Emissions Associated with Electric Power Production

Methane represents 9.9 % of U.S. greenhouse gas emissions and is at least 20 more times more potent in warming the atmosphere than carbon dioxide.

It has been asserted that natural gas power plants, which are responsible for 21% of power generation in the US, produce 50% less emissions than coal-fired plants. However, in the past the amount of methane gas that is lost (vented or leaked) during production, distribution and in plants has not been included in the calculation. Now a new study suggests that methane emissions from power plants and refineries may be much larger than current estimates.

A recent EPA analysis doubled its previous estimates for the amount of methane gas that leaks from pipes and is vented from gas wells, which significantly changes the emissions picture.  Methane (CH₄) levels from hydraulic fracturing of shale gas were found to be 9,000 times higher than previously reported.  Based on the new numbers, the median gas-powered plant in the United States is estimated to be 40 % cleaner than coal-fired plants, according to calculations ProPublica has made. In addition about half of the 1,600 gas-fired power plants in the US operate relatively inefficiently. In the past these plants were estimated to be 32 % cleaner than coal, but with the revised EPA estimates, these ~800 inefficient plants are estimated to produce 25 percent less emissions than coal.

But there is another issue.  Methane is one of the more potent greenhouse gases for global warming, but it is not clear just how much more potent methane is than CO₂.  The EPA has estimated a factor of  21 times compared to carbon dioxide. But Robert Howarth, an environmental biology professor at Cornell University, has suggested that it is actually 72 times as powerful as carbon dioxide in terms of its warming potential.  This is critical, because if the climate effect of methane from natural gas is 72 rather than 21 times that of carbon dioxide from burning coal, natural gas may even turn out to be worse than coal in terms of global warming.  Howarth has suggested that the type of shale gas drilling taking place in Texas, New York and Pennsylvania generates particularly high emissions of methane and could be as dirty as coal.

New estimates of methane emissions from power plants

Now a new study reports on overflights of power plants and refineries and finds that methane emissions are much larger than current estimates.  Power plants and oil refineries are large consumers of natural gas. The EPA has collected data contributed by operators and estimated the methane (CH4) emissions from these plants, but there is high uncertainty in these estimated. In this study an airborne chemistry lab was used to estimate the methane emissions from three gas-fired power plants and three oil refineries. The average methane emission rates were larger than than the operator-reported estimates by 21 to 120 times for the power plants and by 11 to 90 times for the refineries. By looking at the pattern of methane emissions compared to carbon dioxide (CO2) and water vapour, the researchers were able to determine that the methane emissions were primarily from non-combustion processes suggesting leaks and venting as the sources. Scaling these result to the national level suggests that methane emissions from these types of facilities are 4.4 to 42 times larger than current estimates. The results indicate that gas-fired power plants and oil refineries could contribute significantly to U.S. methane emissions. The estimated contribution of 0.61 teragrams of methane annually (Tg CH4/yr) is significant, representing about 2% of total U.S. annual emissions of methane of about 30 Tg CH4/yr.

A recently published study has assessed the spatial distribution of anthropogenic methane sources in the United States by combining comprehensive atmospheric methane observations, extensive spatial datasets, and a high-resolution atmospheric transport model. Based on the results of this analysis the authors conclude that the EPA underestimates methane emissions nationally by a factor of about 1.5.  Generally the study found that methane emissions due to the animal husbandry and fossil fuel industries have larger greenhouse gas impacts than indicated by existing inventories.

One of the motivations from switching from coal to natural gas for these types of facilities is that natural gas delivers the same amount of energy but with significantly reduced emissions. But that does not take into account leaks and other processes releasing methane during production, distribution and within plants.  These latest results further reduce the advantage of gas-fired over coal-fired power production.

A study by Robert W. Howarth, Renee Santoro, and Anthony Ingraffea has concluded that the emissions of methane from shale gas wells are between 30% and 100% more than methane emissions from conventional natural gas wells.  The study estimates that between 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well.

As a result the study found that the greenhouse gas (GHG) footprint for shale gas is greater than that for conventional gas or oil. When compared to coal, which is responsible for nearly 50% of electric power generation in the US,  the GHG footprint of shale gas is estimated to be 20% to 100% greater than coal over a 20 year period.  Over a 100 years, the study concludes that the GHG impact of shale gas is comparable to coal.