Alpha Natural Resources, once a powerhouse of the American coal industry, filed for Chapter 11 bankruptcy protection on Monday so it may emerge from the grip of a $3 billion debt at a time when utilities are switching to natural gas and coal prices are plummeting.

Alpha Natural Resources borrowed heavily in 2011 to buy Massey Energy for $7.1 billion, seeking, like many coal companies at the time, to acquire an empire of Appalachian coal mines in the hope that China would be a growth market for exports.

That deal turned out to be a disaster for the company when demand for both steelmaking and power-generating coal declined domestically and in several important international markets in recent years.

“The change and challenges the U.S. coal industry has experienced over the last several years are greater than any in the past three decades,” Kevin S. Crutchfield, Alpha’s chairman and chief executive, said in a statement released by the company.