string.replace() does not work for # (pound) sign?

I have a string that I'm pulling from an Excel spreadsheet that is x.firstName
When I pull it in it gets converted to x#FirstName
 Fine. So I tried string.replace as shown below and it change my string, any ideas?
 
here's my code:
string

stFormattedColumnName = objDataset1.Tables[0].Columns[i].ToString().ToLower();

//value is x#firstName

stFormattedColumnName.Replace(Go to the complete details ...