EXCEL and WORD are very and unmanaged objects, so GC is unable to collect them In EXCEL case first we need to we need to use 'Marshal.ReleaseComObject' method to release COM object or each sheet and then we need to dispose EXCEL Workbook first and then quit to EXCEL application object
see below cleanup group
GC.Collect();
GC.WaitForPendingFinalizers();
Marshal.FinalReleaseComObject(xlRng);
Marshal.FinalReleaseComObject(xlSheet);
xlBook.Close(Type.Missing, Type.Missing, Type.Missing);
Marshal.FinalReleaseComObject(xlBook);
xlApp.Quit();
Marshal.FinalReleaseComObject(xlApp);