I have to spend 21 hours(11 am to 8 and next morning) in Tianjin.

Oh, you can make full use of time to tour around this city. Ancient Cultural Street and Tianjin Eye are favorable choices. In addition, you can also go to Nanshi Food Street to taste local foods, such as Shiba Jie Fried Dough Twist, Goubuli Steamed Stuffed Bun and Ear-hole Fried Cake.

Thanks for prompt reply! Is it possible to go Huangyaguan Great Wall and how can I get there?
My plane arriving Tianjin International airport 10:20 am and immigration process will take 1 hour then around 11:30 am... Can you please advise me the best way to get the Great Wall? I think I may visit some city sightseeing places which you recommend in the evening or on the way back to NZ.

You are advised to take a taxi or hire a car to get there and back to the airport because there is no direct bus and the bus ride takes too much time. The car ride may need about 2.5 hours. You may get to the great wall at about 14:00. The opening time of the great wall is from 08:30 to 17:00. So you only have about 1 hour to visit the great wall.