But it is a great idea to burn any fuel whose oxidized product is not a gas. The only reason why burning hydrocarbons is problematic is because carbon dioxide is a gas, otherwise it could be captured and recycled like this.
If you wanted to convert that heat back to electricity, there would be losses, but nowhere near as much as some people are saying. 1800 degrees minus ambient is a very big delta T. You are not bound to a closed cycle, so Carnot does not apply. So it is likely 80+% would be achievable, maybe over 65% round trip.
Burning hydrocarbons is problematic for a host of reasons besides CO2 release: fracking, oil spills, release of other pollutants, political issues with the supplier countries, pipelines, ...
Emphasis mine. Don't be a pedant.
70% vs 40% is a big difference, especially when you consider it from the losses column instead of the wins column. If I gave you a system that was losing 30% of a resource to replace a system that lost 60%, you've cut your losses in half.
If I told you a mechanical system could get that to 15%, you'd replace that again. Anyone who tells you they can get a heat engine down to 15% losses is a charlatan and should be reported for fraud.