How far is it from dusseldorf airport to cologne city center?
And how much by taxi?
try google maps for question 1.
By regional train 50 minutes and EUR 11,50.
From: Flughafen Terminal A/B/C, Düsseldorf
To: Köln Hbf
This ticket is good to anywhere in Cologne (by train, tram or bus).
Taxi will be approx. EUR 110. Under ideal circumstances as fast as by train. During rush hours surely not.
Use local transportation/train. Much faster and convenient.