Dec. 8, 2014

Replacing old coal-fired power plants with new natural gas plants could cause climate damage to increase in coming decades, unless the new power plants are very efficient and have very low methane leakage rates, according to a new study published in the journal Environmental Research Letters.

In the study, authors Ken Caldeira and Xiaochun Zhang of the Carnegie Institution's Department of Global Ecology at Stanford University, and Nathan Myhrvold of Intellectual Ventures focused an ongoing debate: Does it makes sense to replace old, coal-fired power plants with new natural gas power plants today, as a bridge to a longer-term transition toward near zero-emission energy generation technologies, such as solar, wind or nuclear power?

A key issue in considering that question has been the potential climate effects of natural gas versus coal. 
 Natural gas power plants typically produce less carbon dioxide (CO2), a major contributor to global warming. than coal-fired plants. But uncombusted methane, the main ingredient in natural gas, is 34 times stronger a heat-trapping gas than CO2 over a 100-year time scale, according to the Intergovernmental Panel for Climate Change. 

Studies have yielded different results by focusing on power plants with different characteristics and using different definitions of what it means to be “better” for climate. Caldeira, Zhang and Myhrvold aimed to identify the key factors that are responsible for most of the difference in greenhouse gas emissions between individual gas and coal plants. The key factors, they found, are power plant efficiency and, in the case of natural gas plants, methane leakage during the supply process. The authors used these factors to derive a simple model for resulting temperature change caused by the CO2  and methane released by a particular plant.

The results showed that because natural gas plants are overall more efficient than coal plants, producing more energy per unit of carbon, they could cause less warming in the long term. However, it all depends on the amount of methane leakage that occurs, the authors said. Natural gas plants that leak a substantial amount of methane during their supply process can produce more warming than comparable coal plants.

“If there is substantial natural gas leakage, then building new natural gas plants would lead to more near- term climate damage than using the old, dirty coal plants,” explained Caldeira. “But natural gas plants would help reduce other types of air pollution that damage our health, and would be somewhat better for climate in the long term.”

The team found a short-term climate benefit to shutting down a typical coal plant versus a gas plant where methane leakage is typically below about 2 percent of total fuel. But if methane leakage is greater than 2 percent, there would be less warming in the near term if the natural gas plant were shut down instead of the coal plant.

The team emphasized that meeting global greenhouse gas emission targets will require deeper emissions cuts than just building natural gas plants with low methane leakage. If natural gas is to be a part of a future near-zero emission energy economy, methods for capturing and storing carbon from gas-fired power plants will likely be necessary, they said.  

This article is based on a news release from the Carnegie Institution for Science.