Excel instances: release and kill
Since Excel runs via COM, it won't be released from memory until you remove all references to it.  Your example (above) does a pretty good job, but after you say wBook.Close(), you should say wBook = null.  Likewise Excel won't gracefully close-down while your xCel object refers to an instance of Excel.
This article on CodeProject shows the recommended/industry-standard way of closing-down excel. ffwd down to Sections 13 and 14.
http://www.codeproject.com/Articles/404688/Word-Excel-ActiveX-Controls-in-ASP-NET