The myriad of policies that may affect natural gas deployment across sectors provides impetus for assessing how different uses of natural gas may impact GHG emissions. Broad-scale energy systems modelling studies have projected that more abundant natural gas will only slightly impact aggregate GHG emissions, as the benefits of replacing dirtier fossil fuels are offset by the displacement of nuclear and renewable and the stimulation of energy consumption due to lower prices. While these economy-wide studies attempt to project the overall response of energy systems to growing availability of natural gas, direct substitution studies are needed to quantify the net emissions impacts of specific natural gas substitutions and thereby inform policies that could influence those substitutions. Three direct substitution studies all showed far more favorable net impacts for replacing coal-fired electricity than for replacing gasoline or diesel vehicles. However, residential uses and exports of natural gas were not considered in those comparisons.

Here, we conduct direct substitution analyses for a larger range of natural gas deployment options than has been considered previously, including home heating and exports along with vehicles and power plants that have been more widely studied. Substitutions are evaluated based on recent conditions in the USA, using the metric of grams CO₂ equivalent reduced per megajoule of natural gas consumed (gCO₂e/MJNG) to facilitate comparisons across sectors. We quantify how results vary with alternate assumptions for the ‘upstream’ emission rates from natural gas production and transport, given the substantial uncertainty in the rate of methane (CH4) leakage and other emissions from natural gas production and from distribution infrastructure. Thus, our results will show how the net GHG emissions impacts of natural gas depend on both uncertainty in upstream emissions and deployment choices, considering a wider range of substitutions than previous studies.

—Cohan and Sengupta (2016)

The researchers’ calculations considered emissions throughout the fuel cycle, from production and transport of each fuel through combustion and including leaks of methane. They made comparisons within the five sectors they studied—power plants, furnaces, exports for electricity generation overseas, buses and cars—and across sectors to see which use of natural gas pays the greatest dividend for reducing greenhouse gas emissions.

Though focused on end uses of natural gas, the paper also shows how results are affected by highly uncertain leak rates from natural gas production and delivery.

Comparisons for CNG- and gasoline-powered light-duty vehicles were conducted with the December 2012 release of Argonne National Lab’s GREET. The Rice researchers used the Honda Civic as the light-duty vehicle platform in the study, due to its availability in gasoline, gasoline hybrid, and CNG form. Fuel economy estimates for CNG and diesel powered 40-foot transit buses were taken from a National Renewable Energy Laboratory survey of US transit agencies.

GREET estimates that well-to-wheels emissions for a CNG Civic and a conventional gasoline Civic are each 220 gCO₂e/km; the lower tailpipe emissions from the CNG Civic are offset by its leaks of methane in-use and upstream. The hybrid gasoline Civic achieves 27% lower well-to-wheels emissions than the CNG or conventional gasoline options, due to its better fuel economy.

For buses, the analysis found that CNG buses emit 12% more well-to-wheels CO₂e than diesel, as the efficiency penalty of CNG more than offsets its smaller carbon footprint per joule.

Graphic by Tanyia Johnson/Rice University. Click to enlarge.

The “well-to-wire” research showed new natural-gas power plants are responsible for less than half as much greenhouse gas per kilowatt hour of electricity generated as existing coal power plants.

“Natural-gas vehicles yield the least savings, and require building out infrastructure that doesn’t transition into new renewable options,” Cohan said.

The researchers found replacing old oil-burning furnaces for residential heating with new natural-gas-burning models yielded emissions savings of up to 48 percent.

Finally, they estimated replacing coal-fired power plants in Japan with liquid-natural-gas plants that burn fuel imported from the United States would also be a net-plus for the environment, with a 15% emissions savings. Japan is the world’s leading importer of liquefied natural gas.

Resources

• Cohan, D.S. and Sengupta, S. (2016) “Net greenhouse gas emissions savings from natural gas substitutions in vehicles, furnaces, and power plants”, Int. J. Global Warming, Vol. 9, No. 2, pp.254–273 doi: 10.1504/IJGW.2016.074960