Transit in China for a flight to Mongolia

I am a Belgian citizen and intend to travel to Mongolia via China.
My route is Paris-Beijing-Ulaan Batoor- Beijing-Paris.
I intend to get a 72h transit visa upon arrival at PEK airport, but I will leave the country from BJS airport within the allowed 72h. Similarly on my way back, I will arrive at BJS airport from Ulaan Batoor, but depart 1 day later for Paris from PEK airport. Can I apply for a 72h transit visa at BJS airport?
Looking forward to reading you.
Best regards