Carnegie Mellon Electricity Industry Center

2005-06 Seminars

Life Cycle Greenhouse Gas Emissions of Liquefied Natural Gas

Paulina Jaramillo, W. Michael Griffin, H. Scott Matthews, William R. Morrow, David Lewandowski

Abstract
Department of Energy (DOE) estimates suggest that in the coming decades U.S. demand for natural gas will increase. Estimates also suggest that supply of this natural gas will increasingly come in the form of liquefied natural gas (LNG) produced in countries like Russia and South Africa , and brought by oceanic tankers. During the same period, the awareness of global climate change will become more noticeable and the importance of reducing emissions of the greenhouse gases that contribute to this climate change will equally increase. Natural gas has been widely identified as a cleaner alternative to coal for electricity generation, as it has lower combustion emissions. Although emissions at the power plant make up most of the natural gas fuel cycle emissions, it is important to identify emissions from the entire fuel cycle.

Several studies have performed emission inventories for the natural gas lifecycle from production to distribution. Usually these analyses have been performed for domestic natural gas, so that emissions from the liquefaction to LNG, tanker transport, and re-gasification have not been considered. If, as the DOE estimates suggest, larger percentages of the supply of natural gas will come from these imports, emissions from these steps in the fuel cycle could influence the total fuel cycle emissions. Thus, comparisons between coal and natural gas that concentrate only on the emissions at the utility plant may not be adequate. The objective of this study is to perform an analysis of the natural gas fuel cycle greenhouse gas emissions taking the emissions from LNG into consideration. Different scenarios for the percentage of natural gas as LNG will be analyzed. Moreover, a comparison with the coal fuel cycle greenhouse gas emissions will be presented, in order to have a better understanding of the advantages and disadvantage of using coal versus natural gas for electricity generation. Our preliminary life cycle estimates suggest that advanced natural gas technologies produce lower greenhouse gas emissions compared to advanced coal technologies, even with high percentages of natural gas as LNG being used. However if the full fuel cycle is considered the difference in emissions decreases. While coal combustion at a power plant emits on average 54% more greenhouse gases (measured in pounds of CO₂ equivalents) than natural gas combustion, on a full life cycle basis coal generation emits on average 44% more than what is emitted with a natural gas mix that contain 20% LNG. In addition if 90% carbon capture and sequestration is achieved at the power plant, the range of emissions from natural gas is larger than the range of emissions from coal. These results should be considered when pursuing fuel and carbon policies at the national level when broader considerations such as energy dependence are relevant.