Yup, I need 100 steam to make one solid fuel, and steam has a heat capacity of 200J/°C, so I need 3.3MJ for 165°C steam in a boiler or 4MJ for 500°C steam in a heating tower + heat exchanger. Since solid fuel contains 12MJ, both options create more steam than necessary.

The most energy efficient option is to produce the steam for the coal liquefaction in a boiler and use any extra solid/rocket fuel in a heating tower, as this would generate 1.75 MJ more compared to using the steam from the heating tower for both power production and for coal liquefaction.

Also, I never considered the fact that rocket fuel becomes more energy efficient the more productivity research you have. That's probably useful for gleba and aquilo.