Conversion of HFO Firing Steam/Power Plant to Gas-Efficiency Impact

There won't be much difference overall between these two fuels.

It'll be hard to calculate the efficiency difference between fuels for most industrial boilers, as few plants will spend the money necessary for any kind of accurate steam metering. Many plants have worn-out old orifice plates for metering steam that were likely never properly sized in the first place. A good number of those orifice plates are installed backwards anyway. Even with properly sized vortex shedding meters, proper pressure compensation is very important for accurate readings. Without accurate steam production readings, any efficiency calculations are pretty much based on somebody's "best guess".