Is coal uneconomic? China still uses quite a lot of it, which makes me wonder why it would be economic there and not here.

The US has a lot of cheap natural gas and China does not.

Natural gas is hard to transport and often needs to be liquified to travel overseas, so global prices vary widely.

> wonder to what extent the economics of coal will render the relaxed rules for coal power plants moot

I'd guess nothing. This seems aimed at gas. Coal is sort of coming along for the ride.

(This is assuming they revoke the rules properly, which given the last 60 days and Trump 1.0 seems unlikely.)

The mercury and greenhouse gas rules seem more or less laser targeted at coal. The steam one also (where lots of gas plants are gas turbines rather than steam turbines).

Earlier this week there was talk of reopening coal units that already shut down:

https://www.reuters.com/business/energy/us-should-restart-sh...

The United States should restart shuttered coal-fired power plants under President Donald Trump’s national energy emergency declaration, Interior Secretary Doug Burgum said in a video interview with Bloomberg News on Monday.

...

"I think as part of the national energy emergency which President Trump has declared we’ve got to keep every plant open. And if there have been units at a coal plant that have been shut down, we need to bring those back on," he said.

I don't know who would want to operate the unprofitable units, or who would pay to make up the difference, or if permanently-closed units are even in a state such that they could be reactivated. Companies with recently-retired plants may want to expedite major equipment demolition to ensure that they can't be roped into a bad plan.