This is the exact control of a typical waste fuel boiler, nothing other than conveyor control is necessary. Steam pressure is controlled by the conveyor speed. Unless someone is standing on the conveyor with a fire hose, the moisture content should vary day to day, but not minute-to-minute.
What you may find is that is is difficult to control the flame at high moisture content and require an additional burner of fossil fuel to maintain a good flame condition. My experience with just such a boiler was the flame because unstable at very high moisture; the boiler at my plant had an oil burner for such instances.