Earlier this year a video appeared online purportedly showing how you could turn a lemon into a simple battery and then use it to start a fire by igniting a piece of steel wool. Uh no, that's where the science falls apart.
Wilson and Adam, two amateur Mythbusters hoping to carry on Adam and Jamie's important work, decided to try and recreate the experiment as demonstrated in that video. They were met with zero success -- but not because they were doing it wrong. The problem is that a lemon stuffed with copper and zinc electrodes simply doesn't generate enough power to ignite steel wool.
The two even substituted a giant pineapple for the lemon and were still unable to generate enough power to start their fire. You need about two AA batteries worth of electricity to ignite steel wool. That would end up requiring an entire bag of lemons to be wired up in series.
So don't bother packing a lemon as part of your emergency supplies for the next time the power goes out. Just toss a box of matches in there and save yourself a lot of trouble.