Estimates of emissions from natural gas-fueled plants much too low, study finds

Purdue researchers flew an airborne chemistry laboratory over natural gas-fueled power plants and refineries to measure greenhouse gases. They found that although these facilities are much better for the environment than coal or oil-fueled plants, the actual amount of pollutants produced is as much as 42 times higher than estimates by the Environmental Protection Agency. (Purdue University photo) Download image

Overall, natural gas plants are still better than coal for the environment

WEST LAFAYETTE, Ind. — Power plants that burn natural gas produce significantly less pollutants and greenhouse gases than coal-burning plants, according to current estimates of how much methane escapes from such power plants, as well as from oil refineries, and estimates could be off by a wide margin, a new Purdue University study finds.

For the past decade, natural gas has been replacing coal as a fuel for electric power plants. It's become relatively inexpensive, and it's much less damaging to the environment if – and it's an important "if" – it doesn't leak out of the system before it is burned to make power.

That's because although burning natural gas is much cleaner than coal or oil, methane (which is mostly what natural gas consists of) has the potential to be even more damaging over the short term than coal or oil if it isn't handled properly, says Paul Shepson, Purdue's Jonathan Amy Distinguished Professor of Analytical and Atmospheric Chemistry.

"Methane is a 34 times more potent greenhouse gas than is carbon dioxide," he says. "It's a better fuel all around as long as you don't spill it. But it doesn't take much methane leakage to ruin your whole day if you care about climate change."

The breaking point for natural gas leakage is about 3 percent. If more than that leaks, the fuel has a bigger climate effect than burning coal.

"The good news from our study is that while emissions are greater than anticipated, natural gas-burning power plants are still cleaner, relative to burning coal" Shepson says. Shepson said this pilot study found that the amount of methane escaping from the plants was only 0.3 percent on average.

Even taking into account previous estimates of methane leakage in the supply chain of 1.7 percent, the total methane emissions are still below the 3 percent threshold, the study found.

The study also found that methane emission rates were significantly higher than two sets of estimates reported by the Environmental Protection Agency; the EPA’s Greenhouse Gas Inventory of Emissions and Sinks estimated that total methane emissions from all U.S. refineries and natural gas power plants was negligible in 2014.

However, this study estimated that annual methane emissions may actually be 11-90 times higher for refineries and 2 -120 times higher for natural gas power plants than those calculated from data provided by facility operators and reported to the EPA’s Greenhouse Gas Reporting Program, and used in the Greenhouse Gas Inventory of Emissions and Sinks. 

"There is much more methane being released into the atmosphere by leaky compressors, valves, and industrial hardware," Shepson says. "But the good news here is that you can take a specialized infrared camera around the plant to find the leaks and then patch the them with a wad of bubblegum. I'm joking about that, of course, but the point is that it's a relatively easy thing to fix."

The study's paper was released today by the journal Environmental Science & Technology, which is produced by the American Chemical Society.

The study conducted in collaboration with the New York-based Environmental Defense Fund, with funding provided by the Alfred P. Sloan Foundation.

Joseph Rudek, a lead senior scientist at the Environmental Defense Fund and a co-author on the paper, says that natural gas power plants and refineries could be a significantly unaccounted-for source of methane emissions. "More measurements are needed to better understand the methane emissions from these sectors."

Steve Hamburg, chief scientist at the Environmental Defense Fund, says that the leaking methane will especially diminish the environmental effects of using natural gas over the first few critical decades.

"There is the capacity to cost-effectively reduce methane emissions associated with use and production of natural gas, so there’s no excuse for the waste and serious long-term impacts” he says.

The study was conducted using Purdue's flying atmospheric chemistry laboratory, the Airborne Laboratory for Atmospheric Research, or ALAR. The ALAR is a modified Beechcraft 76 Duchess that flies at a height of 6,000 to 12,000 feet (2 to 4 kilometers) collecting air samples and conducting sophisticated measurements.

"ALAR is a unique machine, and it was created by combining three of Purdue's major strengths: atmospheric sciences, analytical chemistry, and aviation technology," Shepson says.

Shepson says the benefit of this research is that everyone involved will be able to improve the emission factor formulas used in calculating the amount of methane entering the atmosphere based on the total emissions of the plants, not just the amount going up the smokestacks.

"But the important overall message of the study is to say while natural gas power plants appear to provide a climate benefit, it can still be easily improved'" he says. 

This research study and materials are based, in part, upon work supported by the National Science Foundation.

Writer: Steve Tally, 765.494.9809, steve@purdue.edu, @sciencewriter 

Sources: Paul Shepson, 765-490-9352, pshepson@purdue.edu

EDF media contact Stacy MacDiarmid, 512-691-3439, smacdiarmid@edf.org

ABSTRACT

Presently, there is high uncertainty in estimates of methane (CH₄) emissions from natural gas-fired power plants (NGPP) and oil refineries, two major end users of natural gas. Therefore, we measured CH₄ and CO₂ emissions at three NGPPs and three refineries using an aircraft-based mass balance technique. Average CH₄ emission rates (NGPPs: 140 ± 70 kg/h; refineries: 580 ± 220 kg/h, 95% CL) were larger than facility-reported estimates by factors of 21–120 (NGPPs) and 11–90 (refineries). At NGPPs, the percentage of unburned CH₄ emitted from stacks (0.01–0.14%) was much lower than respective facility-scale losses (0.10–0.42%), and CH₄ emissions from both NGPPs and refineries were more strongly correlated with enhanced H₂O concentrations (R²_avg = 0.65) than with CO₂ (R²_avg = 0.21), suggesting noncombustion-related equipment as potential CH₄ sources. Additionally, calculated throughput-based emission factors (EF) derived from the NGPP measurements made in this study were, on average, a factor of 4.4 (stacks) and 42 (facility-scale) larger than industry-used EFs. Subsequently, throughput-based EFs for both the NGPPs and refineries were used to estimate total U.S. emissions from these facility-types. Results indicate that NGPPs and oil refineries may be large sources of CH₄emissions and could contribute significantly (0.61 ± 0.18 Tg CH₄/yr, 95% CL) to U.S. emissions.