Hot Air

The big problem with vehicles that run on natural gas: methane.

A Metro bus in DC operating on "clean natural gas."
Over the past decade, there has been an increase in the adoption of natural gas vehicles in bus and truck fleets.

Courtesy of Jason Lawrence/Flickr

On a sweltering Tuesday afternoon in June in Washington, D.C., President Obama unveiled “a plan to lead the world in a coordinated assault on a changing climate.” His speech was complemented by a plethora of visual exclamation points. Acknowledging the heat, Obama took off his jacket and paused several times to wipe the sweat off his brow. After a first term in which many environmental supporters felt neglected, the speech suggested that the Obama administration is finally ready to spend some political capital on climate change.

A large tenet of Obama’s energy plan is to promote the use of shale natural gas that has recently become accessible due to advances in drilling technology (that is, fracking). This appears to be working. A recent report by the U.S. Energy Information Administration shows that carbon dioxide emissions in the United States are steadily decreasing in large part to the substitution of coal with natural gas. More recently, the Economist has also enthusiastically embraced the potential of natural gas to replace demand for oil.

However, shale exploitation is not without critics, and rightfully so. In terms of climate change, there are two main objections to shale gas. One is in the process of digging up the gas and moving it to fuel stations through pipelines, and the second is in the use of the gas itself. The former gets most of the attention, but the latter could also have significant implications, especially if predictions of increased natural gas vehicle adoption come to fruition.

The first argument against shale gas is that the extraction and pipeline transport process of shale results in leaked methane, a greenhouse gas that is stronger than carbon dioxide. In April 2011, researchers at Cornell published a study that concluded that if methane leaks are considered, natural gas contributes more to global warming than oil or coal.

The exact quantity of so-called “fugitive methane” continues to be hotly debated, but the good news is that there seems to be an emerging consensus that containing the leaks is neither difficult nor expensive. In April 2013, the EPA lowered its estimates of methane leakage from natural gas production in part due to voluntary industry-imposed methane control measures, such as better-sealed replacement gaskets.

But there’s a second problem here, one that hasn’t received nearly as much attention as fugitive pipeline methane leaks: methane emissions from in-use natural gas.

Over the past decade, there has been an increase in the adoption of natural gas vehicles in bus and truck fleets. Most recently in December 2012, the San Diego Metropolitan Transit System Board of Directors announced a planned $343 million and 500-bus transformation of its transit bus fleet to compressed natural gas. The New York Times recently reported that there is momentum in the trucking industry to adopt natural gas vehicles, as the increase in shale gas supply has made the fuel cheap and economically attractive. As an added marketing incentive, most transit agencies trumpet their natural gas vehicle investments to burnish their green credentials.

What those bragging campaigns fail to mention, however, are the methane emissions that have been reported by several studies. The emissions occur when the natural gas is actually combusted to create energy (such as in the engine of a natural gas vehicle). Just as hydrocarbons, for example, are belched out by gasoline and diesel vehicles today, methane is a pollutant that is emitted by natural gas vehicles. Vehicle emissions are highest when operating conditions are unsteady or dynamic, such as when a cool engine is first turned on or when an engine has to speed up. That’s particularly worrisome if natural gas vehicles like transit buses or freight trucks are operated in urban environments and are subject to a lot of stops and starts that could result in methane emissions. One study of compressed natural gas buses even suggests that when methane is considered, the total global warming impact for natural gas vehicles could be greater than current diesel buses. The methane impact from smaller vehicles (such as passenger cars) is unclear, as most studies have been conducted on heavy-duty trucks and buses. However, since the current interest in natural gas vehicles is predominantly coming from freight and bus companies, tailpipe methane emissions from these studies are still relevant. Yet, amid the optimism for natural gas, it seems that these results have been outshined by the bright promise of a fuel that can ease the tension between economic cost and environmental protection.

The threat of methane emissions should not necessarily spell the end of natural gas vehicles. One clear solution is to develop a catalyst that is able to reduce methane. Catalysts are devices that cause chemical reactions in a vehicle’s exhaust to eliminate many of the toxic air pollutants that result from cars and trucks. Much in the same way that catalysts are used in diesel or gasoline engines to chemically convert exhaust gases to meet federal emissions limits, a catalyst could be designed to break down methane to solve this suffocating issue.

But this is easier said than done because existing catalysts are not effective at reducing methane, a very stable compound that is resistant to chemical reaction. Researchers at the University of Trieste, the University of Pennsylvania, and the University of Cadiz recently published promising findings of a new catalyst using palladium and ceria that can reduce methane. The solution is not yet commercially feasible, but the results are promising. Methane emissions limits have also been developed by the U.S. EPA and will be implemented starting with model year 2014 vehicles. These regulatory actions will spur on new research programs to comprehensively measure methane emissions from natural gas fleets and ensure that natural gas is a truly climate-friendly alternative fuel.

To be clear, there are political, economic, and environmental advantages to natural gas use, which make it a potentially important tool in the fight against climate change. But in the context of urban transportation fleets, we need to be mindful that natural gas could be a methane-filled Trojan horse, a deceptively veiled prop that ultimately proves destructive.