Payoff from Idling Coal Plants Overestimated, Researchers Say

Four researchers from Carnegie Mellon University’s Green Design Institute discuss their more conservative estimates of greenhouse gas emission reductions in two papers this month.

A quartet of researchers at Carnegie Mellon University's Green Design Institute conclude in two new papers that ignoring uncertainty in any coal-to-natural-gas transition for generating electricity can make a substantial difference in estimating the net environmental effect of the change. Researchers Aranya Venkatesh, W. Michael Griffin, H. Scott Matthews, and Paulina Jaramillo concluded life cycle assessment (LCA, the study of impacts that occur from cradle to grave) can be useful in these analyses.

Their papers appear in the October issue of Environmental Research Letters and in Environmental Science and Technology, according to a news release posted by the university.

While many studies simply examine different emissions from coal and natural gas plants, suggesting roughly a 50 percent reduction in greenhouse gas emissions, these researchers conclude the reduction is likely to be 7-15 percent instead because of changes in grid operation in response to price changes in natural gas.

"As natural gas prices go down, it becomes cheaper to operate natural gas plants, and some of these plants start being operated more often. This results in some coal plants being operated less often. However, given certain technical constraints related to the operation of existing power plants, the displacement of coal-based generation is limited," said Jaramillo, an assistant research professor in CMU's Department of Engineering and Public Policy. To cut emissions by 50 percent using natural gas would require a significant retirement of coal plants and building new natural gas plants.

The second paper examined the uncertainty in emissions that could be expected from retiring coal-fired power plants. While it suggests reductions in greenhouse gas emissions from limited retirement of coal plants will be minimal, emissions of sulfur and nitrogen oxides would be substantial, up to 25 percent, in some areas. (The paper focuses on up to 7 gigawatts of coal capacity being retired without building new power plants to replace them.) "We found that if expected coal plants retire, that alone will not bring us dramatic reductions in climate change inducing greenhouse gas emissions," said Matthews, a professor in CMU's Civil and Environmental Engineering and EPP departments.

"In addition, the benefits achieved from reducing emissions of sulfur and nitrogen oxides, while substantial in aggregate measures, will not be evenly distributed; and while some counties will see reductions in the emissions of these criteria air pollutants, some counties will see increases," Jaramillo said.