By Matt Jarzemsky and Joseph Checkler 

The ailing U.S. coal industry got another black mark Monday morning, when Alpha Natural Resources Inc. filed for chapter 11 bankruptcy protection.

The Bristol, Va., company filed for chapter 11 protection in U.S. Bankruptcy Court in Richmond, Va., brought down by plummeting coal prices and high debt from its 2011 acquisition of Massey Energy.

The Wall Street Journal reported Sunday that Alpha, with assets of $10.1 billion and liabilities of $7.1 billion, would be entering chapter 11 as part of a plan to cut its debt load, another victim of the severe slump in coal prices that continues to wreak havoc on the industry.

"While a difficult decision, this voluntary Chapter 11 filing is the right strategy at the right time for the future of our business," said Alpha Chairman and Chief Executive Kevin Crutchfield Monday.

Alpha, one of the largest U.S. coal producers, isn't entering chapter 11 with a restructuring plan in place, but did say in a filing accompanying its bankruptcy petition that it doesn't want to immediately sell itself "at a low point in the coal market." The Wall Street Journal reported Sunday that the company will likely sell some of its best mines or turn them over to creditors and close others during bankruptcy, citing people familiar with the matter.

The company is seeking approval of up to $692 million in bankruptcy financing from senior lenders and secured bondholders to fund its operations during its chapter 11 case, according to court documents. The company will ask for a judge's permission to keep operating normally at a first-day hearing yet to be scheduled, where it will seek approval of so-called first-day motions including ones that would allow it to keep using its bank accounts, paying its nearly 8,000 employees, and maintaining relationships with suppliers.

A steep drop in coal prices has Alpha and its rivals bleeding cash and choking on debt taken on to finance acquisitions from earlier this decade, when the industry's outlook was rosier. In 2011, Alpha paid $7.1 billion for rival mining company Massey Energy, a deal that extended Alpha's lead as the largest miner of the type of coal used in steelmaking.

But the price of metallurgical coal has hit an 11-year low amid an economic slowdown in China, the world's largest producer of steel. Thermal-coal prices have also plummeted as power plants switch to abundant and relatively clean-burning natural gas.

In court filings, the company said it has $1.96 billion in secured debt, including $714 million in second-lien bonds and a $611 million term loan. Alpha also reported $2.1 billion in outstanding unsecured bonds.

According to Fitch Ratings, bids on Alpha's bonds recently averaged 6%, "indicating the current market expectations of poor recovery rates on both unsecured and second lien debt claims."

The slump has a number of coal companies at risk. Walter Energy Inc. filed for bankruptcy protection last month with a plan to hand control of the company to senior creditors, after chapter 11 filings by Patriot Coal Corp. and Xinergy Ltd. earlier this year. Arch Coal Inc., meanwhile, is working with bankers and lawyers who specialize in helping struggling companies, The Wall Street Journal has reported, and is facing lender pushback on a proposed debt-for-debt exchange meant to reduce its borrowings and interest costs.

Alpha has posted four straight annual losses and disclosed in May that a Wyoming regulator had notified the company it no longer qualified for a "self-bonding" program that had freed the company from buying insurance to cover future mine cleanup costs. Given the financial burden, the company opted to file for bankruptcy rather than repaying a convertible bond due at the start of this month, despite having $476.3 million in cash as of March 31.

The Wall Street Journal reported in mid-July the company was in talks for bankruptcy financing with senior lenders such as Davidson Kempner Capital Management LLC and Citigroup Inc., represented by law firm Davis Polk & Wardwell LLP and financial adviser Ducera Partners LLC, and bondholders including Capital Research & Management Co. and Blue Mountain Capital Management LLC, working with law firm Kirkland & Ellis LLP. The bondholders have also since hired investment bank Houlihan Lokey, according to people familiar with the matter.

Peg Brickley contributed to this article.

Write to Matt Jarzemsky at matthew.jarzemsky@wsj.com and Joseph Checkler at joseph.checkler@wsj.com