Usefulness of flame weapons post civilization
Fire is a powerful ally and a terrible enemy
If it can be bent to your will and harnessed as a weapon it can be unleashed against your enemy to his peril
Home made flame throwers are very effective weapons – especially against crowds of looters
and when combined with simple pumps can be a great deterrent to looters and raiders.
There range is limited and so is their fuel supply, but these can be over come when they are built in enough of a size.
in a nutshell these are base defensive weapons, and can be used in assault but are heavy and short ranged so best kept for teams with access to vehicles
and larger amounts of fuel.
The Fuel itself can be gas, liquid , and even solid, so long as it can be vaporised ( for instance solid wheat flour or coal dust works when combined with enough air)
Other uses of this would be in Fuel Air explosives
this in itself is a complex topic but can be achieved with little technology and is very effective