I guess what you are feeling is crystallised Adblue, which is a common problem in such systems, and presumably why you want to drain the tank.
In theory Adblue, which is principally urea, is very soluble in water so it should be possible to flush the crystals out with a stream of cold water. However I have never had anything to do with Adblue in my car so I do not know how practical the idea would be.