my husband walked out of our marriage 18 yrs ago, the children are not his can i still file for desertion and how quik is this kind of divorce.

Short answer: yes, although it is not necessary to plead or prove "desertion" in modern practice.  See the posted information at http://www.willicklawgroup.com/divorce/.  For more detailed guidance, consult with a family law specialist to go over all matters relating to your marriage -- and what an extended separation means to potential claims for property, debt, and support since you remain married, as well as the legal status and necessary support for the children.