How to select free wheeling ultra fast recovery diode for 30VDC coil with 10 amperes of current
Selecting flywheel diode: 

Voltage rating should be at least two times the supply voltage.  For your case , use diode rating of least 60V. 

Current rating should be at least 1.5 times load current. So if use 10A load then use 15A diode.

Also here is an example device MBR1660