How upstream methane leakage further weakens the argument for natural gas trucks

Authored by Gonca ?eber , Researcher, ICCT

A recent report by the North American Council for Freight Efficiency (NACFE) highlighted the role of natural gas as a transport fuel and estimated that the greenhouse gas (GHG) emission savings from a natural gas engine are in the range of 13%–18% compared with diesel fuel. However, NACFE “focused most [its] discussion on the tank-to-wheels effects of the alternate fuels” in comparing a natural gas-powered truck with a diesel truck doing the same route. An analysis of the complete fuel-cycle GHG emissions (i.e., well-to-wheel) would cover emissions associated with all the steps of producing, transporting, and consuming the natural gas and diesel used for those trucks. As I’ll show here, the emission impacts of the upstream natural gas supply chain complicate the climate benefits of using natural gas for trucks.

The primary issue is methane leakage. Natural gas is mostly methane (85%–90% by volume) and its production involves multiple steps during which methane could be released into the atmosphere through leaks and venting. This happens all along the supply chain and these upstream emissions are noteworthy because methane is a potent GHG.

Upstream methane emissions can be substantial and they’re not easy to estimate. For example, using ground-based measurements validated by aircraft observations, researchers have estimated that methane emissions from the oil and natural gas (O&NG) industry are much higher than previously estimated by the U.S. Environmental Protection Agency (EPA). Methane emission estimates reported in EPA’s national GHG inventory are based on adding up the emissions from individual components of natural gas production equipment. Although this kind of bottom-up methodology provides detailed data from routine equipment behavior, it does not detect super-emitters, which can be unpredictable and can emit unusually large amounts of methane (one example is malfunctioning equipment). Alternate measurement approaches such as remote sensing of methane emissions via satellites or aerial surveys can help cover vast areas and detect these super-emitters, but such top-down emission estimates can also overestimate emissions. For instance, this technique might not be able to differentiate between O&NG sites and other sources of methane, such as landfills or dairy farms.

It’s also important to differentiate between emissions from combined O&NG production and emissions from producing just natural gas. For sites that produce both fuels, part of the methane emissions should be attributed to the oil produced alongside natural gas on an energy-weighted basis. The left column in Figure 1 illustrates the range of methane losses from O&NG production normalized by natural gas production using data from recent literature. These losses are calculated by dividing methane emissions by the amount of methane produced. The data from both bottom-up (e.g., EPA) and hybrid methodologies (i.e., a mix of bottom-up data and satellite or aerial surveys) were used for these estimates. The methane loss estimates in the right column in Figure 1 illustrate the emissions allocated solely to natural gas production, so they are allocation-adjusted loss rates. When the O&NG sector is considered, the methane loss rate ranges between 0.4% and 9.6%, with a mean of 3.4%. When losses are allocation adjusted, it ranges between 0.4% and 4.8%, with 1.8% as the mean.

Figure 1. Methane emissions from oil and natural gas (O&NG) production and emissions allocated to natural gas (NG) production from recent literature

To understand the climate impacts of upstream methane losses, let’s explore the fuel cycle GHG emissions of natural gas-powered heavy-duty trucks. Figure 2 illustrates the differences in well-to-wheel GHG emissions for 40-tonne trucks that run on compressed natural gas (CNG), normalized per mile, for each fuel option analyzed. We used the mean methane loss rate for natural gas production (1.8%) as well as the minimum (0.4%) and maximum (4.8%) loss rates from Figure 1 to provide the range of emissions estimates indicated by the error bar. The fuel economy of a heavy truck running on natural gas of 6.5 miles per diesel gallon equivalent was taken from the NACFE report. To compare our analysis with diesel-powered trucks, we used the U.S. national average for the carbon intensity of diesel fuel from the U.S. Renewable Fuel Standard, 91.9 g CO2e/MJ. Non-CO2 tailpipe emissions (methane and nitrous oxide) from GREET 2023 were included as equivalent amounts of CO2 in the combustion emissions for diesel and natural gas-powered trucks. The system boundary for natural gas includes extraction, processing, transport, fuel refining and distribution, and methane leakage for all steps. As illustrated in Figure 2, with the mean methane emissions rate of 1.8%, our estimates are a 6% GHG emission savings from CNG trucks compared with diesel ones. However, the same estimate shows that if there is a methane leakage rate greater than 2.5%, that would make CNG trucks worse than diesel ones from a climate perspective.

Figure 2. Fuel-cycle greenhouse gas emissions from a 40-tonne tractor-trailer for diesel and compressed natural gas (CNG)

Thus, even with optimistic assumptions for upstream methane leakage, we estimate that CNG trucks only offer mild GHG reductions, if any, compared with petroleum diesel. This means that the estimated GHG savings for switching to natural gas trucks are marginal at best. However, there is also a long-term problem: Purchasing natural gas trucks may create technology lock-in. The CNG trucks purchased today and in the next several years could be on the road well into the 2030s, when zero-emission vehicles that provide much larger emission benefits could be more widely available. Battery electric trucks using grid-average electricity already generate deeper GHG savings than CNG trucks in many regions, and these GHG savings will grow over time as the grid decarbonizes. Adopting CNG could mean foregoing substantial GHG savings in the future from zero-emission vehicles.