Board Thread:Questions and Answers/@comment-2600:387:5:803:0:0:0:90-20190713200400/@comment-38876330-20190723105523

Bit late but...

Pyrokinetic decompose air molecules into the chemical reactants required to synthesize them into fuel bonded with oxygen molecules. They then increase the temperature of the fuel, which is a measure of thermal energy (vibrational kinetic energy), until ignition is achieved. This would be the scientific rational behind their skill, but energy is being violated in doing so because espers are supernatural, as implied by the previous respondant.