[nffc]

Firstly it'd probably mean compounds.

Sulfur is a common impurity in organic assays - such as extracts from fossil fuels which would then oxidise or add into combustion products- they may use sulfur-containing reagents as part of the synthesis...

Nitrogen is easy, that will come from the air, or again from a reagent or by-product in the synthesis.

Just because the end product doesn't contain N or S then that doesn't mean that either is not used at the plant - and of course when an incident like this occurs, it's the feedstock and everything else that explodes.

And incidentally, S and N oxides are produced in burning of fossil fuels and it's not likely that alkanes - hydrocarbons - used in petrols etc will contain that unless it's an impurity.