I had a chance to visit some utilities "here", East of the Mississippi. Our company does a lot of replacement parts work, even offering to replace part or all of the section tube heat absorption surfaces, i.e. Economizer, Reheater, Primary and Secondary Superheat. We replaced entire furnace hoppers, many of these were on competitor-built units and they did the same to some of our fleet.
I got to see a few strategies, not as many as a service engineer since most of my career was back in the office behind a desk performing stress calculations.
At one site, the customer appeared to have (2) 100% capacity units and maybe a 50% capacity backup unit. It appeared they operated one of the 100% units while the other was in down time and perhaps out for an extended time undergoing repairs. They may have "harvested" some parts from the downed boiler to keep the active unit running and available, using the down time to order replacements for these parts for the downed boiler to ready the downed boiler when it was placed back in service. It seems logical that they used the 50% capacity boiler to allow the larger units to meet peak demands without running full tilt by filling in the peaks.
Another customer seemed very successful running, perhaps (2) larger, devoted boilers or maybe had one for backup like the first customer. They met peak demand by employing a combination of natural gas turbines and heat recovery steam boilers. The gas turbines I imagine were hooked directly to electric generators, supplying additional energy in that form. But the exhaust heat from these turbines started at over 1000 degrees F, plenty for attaching an HRSG (heat recovery steam generator) down stream in the flue work. Main steam lines from these small boilers fed back to the old steam turbine "house". I don't know if they introduced their steam at some intermediate pressure turbine or had a low pressure, devoted turbine. I believe the steam generators were plumbed to two gas turbines in dual flue feeds, two gas turbines feeding one HRSG. In total, they had (4) gas turbines and (2) steam generators (HRSG's).
We knew our large boilers and the competitors had a common weakness: The boilers, especially the pulverized coal burning type could not be run at loads less than, say, 30% capacity. This means the customer had to run the units at least at this load or higher and, I guess vent off or condense the excess steam at a significant loss of energy. The main cause of this limitation is the problem of cold end corrosion. If you run a large boiler at very low loads, the temperature in the convection pass drops low to prevent overabsorption in the convection pass tubes, you just don't need that much heat for a low steam demand. Down stream components of the exhaust flue suffer from the phenomenon of cold end corrosion. We saw entire scrubbers or precipitators reduced to "swiss cheese" in an alarmingly short time.
We and our competitors developed at least two solutions or methods to modify or retrofit the large boilers to keep their exhaust gases at a non-corrosive temperature, higher, of course during low load periods. Granted these "solutions" also waste energy by passing unabsorbed heat downstream of the convection pass tubes. In one method, we bypassed some of the tube sections or their portions by routing the flue gas so as to "miss" some of the heating surface. You can imagine this is not an easy retrofit, more of a rebuild of the unit's back end. The second solution is to modify, through valves, the circulation in some of the tube circuits so that not as much heat transfers to the water/steam even though the tube section remains fully wetted by the flue gas and the flue gas flow rate past the tubes is not reduced. In either solution, the flue gas leaving the boiler remains hot and non-threating to downstream components.
