I'd like to prefix this all by pointing out that coal is absolutely terrible to use in several ways.

However: most thermal plants get about 45% efficiency, based on using very high steam temperatures. We all know that the theoretical max efficiency for a thermal process is limited by the Carnot cycle, which explicitly depends on the difference in temperature between the working fluid and the surroundings.

I'd also like to point out an important point: carbon plants are not constricted by the need to keep the engine lightweight, we can capture most fly ash and other process exhaust.

I again, do not care to bring such an arcane tech back online, it's terrible to mine, process and use. Just remember there's a bit more to all of this that engineers have indeed thought of.

E: https://www.sciencedirect.com/science/article/abs/pii/S0196890415007657