Arch Coal: Triton Acquisition Completed  WASHINGTON (Reuters) - Arch Coal Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=ACI.N target=/stocks/quickinfo/fullquote"&gt;ACI.N&lt;/A&gt; said on  Friday it had completed the acquisition of rival Triton Coal  Co. after a U.S. appeals court denied the government's bid to  block the deal.