The park across the street doesn't have one. There are none in the town center (gotta buy Starbucks or similar).

And I don't recall seeing any public ones in downtown DC, other than the Mall and Smithsonian areas.

I already stated they do exist in quasi-public spaces (airports, offices, etc).

Like, if I'm running errands downtown, and I'm not in the tourist zone, there really isn't much without ducking into an office (that might be locked on the weekend).