im not saying use fire as a primary weapon in itself, more of a distance trap to alert survivors, and possibly confuse zombies. I would only use them at night, and at a good distance, so i'd be able to see them coming. here's my design:
have a primary explosive (either IED or military grade) large enough to create a fairly big explosions. Now surround that explosive with flammable liquids/gels such as germ-x in bottles around .5-1 liters. So THEORETICALLY when the explosive goes off it sends flaming goo in a large radius, creating an explosion and flame weapon. just an idea i came up with in like 2 minutes today, tell me what you think and/or include an idea of your own.