Still, a stationary power plant would have eminently doable in my not so humble opinion. Even stone for cylinder offers great compression strength. Make it big enough and the scale works out to your advantage. Bronze bearings could be made very durable.
The reason is simple - Romans tended to use wood as fuel source. Steam engines started way after we decided to use coal instead.
The wood heat is too low to get high pressure saturated steam, and lower pressure is not that useful and requires a huge boiler or specially prepared wood briquettes.
Metallurgically, Romans knew of brass and copper, which is enough for a decent steam boiler. It would be rather expensive to make.
