I know this answer is a bit late but your question is a really good one. the reason is efficiency and it’s cousin, Heat Rate. The best the coal units could do is 30 to 32% efficiency while the new combined cycle plant is probably in the high 50’s to low 60’s. This means that the new plant puts out nearly twice as much electrical energy for the same amount of energy burned. The new plant also requires a fraction of the staff to maintain and operate so it’s cheaper overall to operate. The new plant can also go from a cold state to full load and back in a much quicker time so it can operated when needed and not burning fuel just to be available. Even though a conversion is possible of the coal plant, the amount of mechanical, electrical, and control system changes required to implement would pay for half the new gas plant and you would still have a 60 year old, not nearly as efficient plant that mechanically, probably only has another 10 years in her at best. It would be nice to be able to reuse the old iron since it’s there. It’s just not very cost effective to do so. Sometimes it’s possible to re-power the old steam turbines of an older fossil plant by running them with steam produced by a gas turbine combined cycle plant but it’s a tricky proposition and is very site and even unit specific. TVA probably looked at this option and realized that the steam turbines they had at the coal plant just didn’t make sense to repower given their age and efficiency. Since the limiting factor in how quickly a thermal plant can get up to full load is usually the steam turbine, that was probably the reason they ruled out this option. Newer GTCC steam turbines can get to full load in little over an hour while the coal units probably took close to 20.
