|Monthly operating margins for the average coal plant. (Bloomberg graph)|
Plants that didn't make enough money were able to stay open because most were located in areas where rates were determined by regulators rather than market forces. Regulators and utilities often keep struggling plants open to ensure stability of the electricity grid, but grids may face upheaval as more unprofitable plants close and natural gas continues to have a price advantage over coal.
The report's authors, William Nelson and Sophia Lium, wrote in the study that they were "awestruck by the resilience of U.S. coal . . . Plants persist even when they cost more to run than replace."