I have a European piece of equipment that operates at 230 V, 50-60 Hz, 2900W. I have a 250 V NEMA L6-30 receptacle. Can I plug this equipment in the 250 V receptacle without worrying it would break it?

Based on the information you have provided, yes.
If you wish to provide more description of this European equipment,
i can be more confident of my answer.

The NEMA rating is max rating voltage not what is present. If you are on a regular US service, it should be producing 238-242 VAC or so at the outlet. Unless there is something different in the wiring, it should be OK.