Quote20,000 Watts would take 6 seconds....annnd a 9V battery would take 10 days. XD9V at 150ma = 1.35W333Wh/1.35W = 246hours
20,000 Watts would take 6 seconds.
Also, if you go too slow, you will be losing heat faster than the battery can put it in. Theoretically a 1.35W heater should be able to boil water. Realistically it won't even come close for any substantial amount of water.
I think you will need a significantly larger battery, like a car battery. You can then buy a 200W car immersion heater to turn the charge in the battery into heat: http://www.langtoninfo.com/showitem.aspx?isbn=8000692729878&gclid=CI3EqfrQ1LMCFUOK4AodA0YARA
i'd go to a camping store and buy a small gas bottle
Perhaps an exothermic chemical reaction.