>>628640
A flame isn't the same as an explosion. One is a prolonged burning process causing a slow release of energy in the form of heat to the surrounding area, the other is a quick expansion of hot gasses with the intent of creating a shock-wave.
>>628653
Lots. An average 737 has a fuel capacity of roughly 50 to 70 tonns.
Assuming that this is the exact amount of fuel required to weaken an average building to the point of collapse (which it is not), you would need 6.5 to 9,1 thousand tonns to destroy the 130 tallest buildings of New York City. However: there are more than 6000 registered "highrise" buildings and many many more just below that limit.
6.5 thousand tonns of Kerosene costs roughly 32.5 million US dollars, not counting for rebate on buying large quantities, or the cost of transport/storage.
However: this calculation is absolute dogshit.
How much fuel it takes to make a building collapse depends entirely on the building, and the way you start the fire. Using existing furniture (or cars in an underground garage) to keep the fire going will drastically increase the temperature of the fire. Smoke detectors and sprinklers can ruin your plans entirely. Closing all windows before starting the fire will mean that less oxygen enters the building, and thus it will not burn as hot.
Some older buildings still use wood, which will burn like hell, other modern ones have hallways designed in a way that heat is transferred away from structural elements. You would need to either douse the entire city in fuel, or autistically inspect each building, plan how you start the fire, and how you will ignite all of them at once, causing the fire services to be completely overwhelmed.