Seebeck generator to run a fan on a radiator
I have a feeling their data sheet is optimistic, but let's go with that.

I've got right here a computer fan, moving a stream of air which is miniscule compared to most home radiators I've ever seen. It is consuming 1.6W of power to accomplish this feat.

Let's say you're already actually doing a really good job of moving air away from this radiator, and so its immediate environment is 25C. You stick a giant whopping block of copper on the back side of this generator, and end up with just a 5C rise over ambient on the cool side of the generator. (After, of course, coming up with a very thermally conductive connection between your round pipe and this flat object.) The chart says you need your steam pipe to be about 150C to get 1.6W out.

As I understand it, steam in homes isn't that hot. So you're not going to get the power you want, even under the above generous assumptions. And I don't think the air circulation you'll get from 1.6W of fan would even be worth it.