When does the 3-hour period for startup officially begin?

The correct choice is when refuse is first placed on grates to burn. This marks the official beginning of the 3-hour startup period because it represents the moment when the combustion process actually starts. This is critical as it signifies that the system is actively engaged in incinerating waste, which is essential for the operational timeline of the facility.

The other options refer to actions or states that occur after the combustion process has commenced. Igniting the boilers and opening the main steam valve are procedures that follow the initial placement of refuse. Achieving operational pressure also occurs later in the startup sequence. Therefore, the critical startup time is defined by the actual initiation of the burning process with the refuse being placed on the grates.