Due to Neptune's high concentration of methane which is highly combustible, could it be ignited if you found a way to keep a continuous flame.This question is asked in the scope of figuring out if human settlement of Neptune would cause this. Would it explode if this flame was set for more than thirty min?
Short answer: there's no oxygen
Longer answer: as a rule, potential chemical reactions don't really hang around in the natural world unless living things are present. Most things that would react have reacted; Earth is of course the exception in this regard.
Fire requires oxygen - a highly-reactive substance, which only exists in its free form due to organisms which produce it as a waste product.