Been a long time since I worked those B707. I stand corrected then that no combuster on Boeings. The DC-8 when fitted with it had a combuster on Nbr 3 engine, also a 60-40 alcohol-water tank in Nbr 3 pylon that was injected into the hot gas to increase mass flow and probably cool the air a bit.
Usually though, in blowing down a 3000-psi air source, the gas gets extremely cold so loses energy. That I remember was why the combuster was fitted.
I remember the air flask on the B707 though and the noisy hydrauliclly driven air compressor.
In the old overhaul nose docks at EAL Miami base, the bays were fitted with a air connection that allowed plant air to be piped to the aircraft pneumatic system. There was an inline heater fitted using domestic gas to make up for the energy lost in dropping from 120 psi or so to 35 psi. Never saw it used though, in fact was long forgotten in fact. Only a nosey type like me asked those questions.