Take a taxi to Beijing South Railway Station to catch the first inter-city bullet train departing at 6:35 am and get to Tianjin Railway Station at 7:05 am. Also take a taxi to Binhai Airport，about 40 minutes, then you can reach the airport at 8am, 45 minutes left for check-in. The counter may stop checking 30 minutes before departure. Or you should take earlier trains which may cost 1.5 hours on the way. Take a taxi from Beijing to Tianjin is the simpleast but expensive. Hope it helps.