Does a power plant burn more as the load is happening..or BEFORE the peak load?

Question

do they burn the most smoke a couple hours before peak time?

Answer 1

The Power Plant cannot exactly predict the load increase, as in the case of, say peak load hour:

As the load increases, the Generator tends to slow down, bringing the frequency and voltage down. If the load increase is gradual ( in relation to the capacity of the plant), as is usual, the controls in the plant take over, opens the steam inlet valve, letting in more steam (e.g. in a steam power plant) to compensate for the decrease in the speed, and pull up the generator speed. This in turn triggers the Boiler controls, which increases the fuel and air supply to the boiler to bring up the boiler pressure which would otherwise decrease.

Hence depending on the sensitivity of the controls and the magnitude of the change in load, there will be a minor, sometimes imperceptible, time delay as the rate of fuel burning is increased. In the worst case, when there is a sudden surge of Power demand, the controls may give up... shutting down the plant, (called tripping) leading to a power black out!