Gas Bubble 2020 notes
LNG's Greenhouse Gas Emissions
Global Energy Monitor calculated a range of life cycle emissions for liquefied natural gas (LNG) exported from the U.S. based on a study by the National Energy Technology Laboratory (NETL), ("Life Cycle Greenhouse Gas Perspective on Exporting Liquefied Natural Gas from the United States: 2019 Update") within the U.S. Department of Energy. GEM then adjusted the values in two ways:
- Modified the methane leakage rates to reflect a wide body of literature on methane leakage rates from gas production areas, and to show impacts of a range of possible values.
- Modified the power plant efficiencies to compare new power plants (whereas the NETL report compared existing coal-fired power plants to existing gas-fired power plants).
The NETL life cycle assessment included all steps in the process, including emissions due to:
- extracting gas
- processing and transporting gas to LNG export terminals
- building LNG export terminals
- liquefying the gas at the LNG export terminals
- shipping the LNG over long distances (e.g., from the U.S. to Europe or Asia)
- burning the gas in power plants
Methane leakage rates
NETL's emissions results are shown in their report, in Appendix A, Exhibits A-1 and A-2. The data is reported using both 100-year and 20-year global warming potentials (GWPs). Since any emissions other than CO2 and methane are negligible, and since the report states the GWPs used for methane, then it is possible to separately calculate the life cycle emissions from methane and from CO2.
The NETL report used an "upstream" methane leakage rate—for extraction, gathering, and processing—of 0.7%, and a total life cycle leakage rate of 1.1%, as as shown in Exhibit 6-8. (GEM's calculations based on the emissions data above matched these values, which helped validate the method.)
However, many measurements in U.S. gas production areas have found far higher rates of methane leakage. A 2018 study in Science ("Assessment of methane emissions from the U.S. oil and gas supply chain") used the data available up to that point to estimate that the methane leakage rate for US production areas—including extraction, gathering, and processing—was 1.9% of gas produced. That rate is about 2.7 times higher than the leakage rate NETL assumed.
Studies included in the 2018 Science paper, as well as others published later, show that different U.S. gas-producing areas have a wide range of methane leakage rates, from ~1% in the Marcellus basin in Pennsylvania and West Virginia, to 3.7% in the Permian basin of Texas and New Mexico, to 8.9% in the Uintah Basin in Utah. GEM's calculations of the U.S. average leakage rate incorporating these more recent measurements found a value that closely matched that of the 2018 Science paper, therefore we used that study's overall leakage rate of 2.2% (for all steps up to and including transmission) as our central estimate.
To show the effect of a range of values, GEM used leakage rates for the whole life cycle, including transmission of gas from producing areas to power plants, ranging from 1.2% to 3.2%, to reflect uncertainty in the estimates, as well as the differing leakage rates for gas originating from different basins.
The NETL study also included methane leakage from coal mines. Those emissions were a very small fraction of the total emissions from coal-fired power plants, so even if methane leakage from coal were substantially higher than assumed in the study, it would not have a large impact on the overall emissions from coal-fired power plants.
Power plant efficiencies
To compare emissions from coal-fired and gas-fired power plants, it's necessary to make assumptions about the efficiency of the power plants being compared. Less efficient plants use more fuel, which increases their direct emissions from burning fuel, as well as adding additional upstream emissions to extract the higher quantity of fuel and get it to the power plant.
The NETL report compared existing power plants, assuming that gas-fired power plants have an efficiency of 46.4% and coal-fired power plant an efficiency of 33%.
However, as the gas industry is pushing for a large expansion of the LNG system, including delivering LNG to countries that currently do not consume gas, GEM considered it more relevant to compare new coal-fired power plants against new gas-fired power plants. We found two studies that made this type of comparison. A 2014 study by Xiaochun Zhang and colleagues assumed new coal plants have 51% efficiency and new gas plants have 60% efficiency. A 2015 study by Zeke Hausfather assumed new coal plants have 43% efficiency and new gas plants have 50% efficiency. In both studies the gap in efficiencies between the efficiencies of coal and gas plants is much narrower for new plants than for existing plants, so gas has less of an advantage due to power plant efficiency in new plants as compared with older generations of plants.
To show the effect of a range of possibilities, GEM used either the efficiencies for gas and coal plants in Hausfather 2015 or in Zhang et al. 2014. Raising the efficiencies of power plants lowers the power plants' direct emissions from burning fuel, per unit of electricity generated, and also lowers other life cycle emissions such as from extracting, processing, and transporting the gas.
LNG is often transported long distances. The NETL report considered two scenarios for LNG from the U.S., with transport to either Europe (landing in Rotterdam, the Netherlands) or Asia (landing in Shanghai, China). The length of these trips adds significantly to the life cycle emissions for LNG.
To reflect a range of possibilities, GEM used the two scenarios from the NETL report for shipping emissions.
In the NETL report, LNG originating from the U.S. and used in power plants had emissions that were from 41% below that of coal (when transported to Europe) or 28% that of coal (when transported to Asia).
For gas-fired power plants, GEM adjusted the NETL life cycle emissions as described above, based on modified methane leakage rates and power plant efficiencies. For coal-fired power plants, GEM adjusted the total life cycle emissions based on power plant efficiencies.
GEM also considered two scenarios for transportation, as above, and calculated methane emissions as CO2 equivalents using global warming potentials (GWPs) over 20 years and over 100 years, since there is not a single accepted value to use for these.
GEM's scenario with the lowest emissions for gas-fired power plants found emissions would be 29% percent below that of coal-fired power plants (per unit of electricity generated), based on the following parameters:
- U.S. LNG transported to Europe
- 1.2% life cycle methane leakage rate
- coal plants of 51% efficiency compared with gas plants of 60% efficiency
- 100-year GWP
GEM's scenario with the highest emissions for gas-fired power plants found emissions would be 16% percent above that of coal-fired power plants (per unit of electricity generated), based on the following parameters:
- U.S. LNG transported to Asia
- 3.2% life cycle methane leakage rate
- coal plants of 43% efficiency compared with gas plants of 50% efficiency
- 20-year GWP
Given this range of possibilities for gas-fired electricity emissions—from 29% lower than coal to 16% higher than coal—we conclude that simply switching from coal to LNG cannot be used as a way to achieve deep emissions cuts consistent with the goals of the Paris Agreement to limit warming to 1.5 degrees C; in some cases, switching from coal to LNG could actually lead to an increase in greenhouse gas emissions.
- Selina Roman-White et al., "Life Cycle Greenhouse Gas Perspective on Exporting Liquefied Natural Gas from the United States: 2019 Update," National Energy Technology Laboratory, September 12, 2019
- Ramón A. Alvarez et al., "Assessment of methane emissions from the U.S. oil and gas supply chain," Science, 13 July 2018, Vol. 361, Issue 6398, pp. 186-188