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JP-7 has a very high ignition temperature (it was actually used as a coolant/heat sink for the rest of the aircraft), so TEB combusting once it hit the air is used to light the engines.
SR-71s only had a limited amount of TEB to use each flight. I believe it was 13 shots per engine. TEB was used each time the engines were lit and also when afterburners were lit. While the SR-71's routes were built around refueling, the number of TEB shots remaining were the true limiting factor of the SR-71's flight time.