Which statement is true regarding the operation of an overfeed stoker?

The operation of an overfeed stoker involves feeding coal from above the grates. This design is specifically intended to allow a continuous and controlled flow of coal onto the combustion surface. By feeding the coal from above, it helps maintain a sufficient bed of fuel on the grates, allowing for efficient combustion and better utilization of the heat generated.

In an overfeed stoker system, the coal descends onto the grates where air is introduced from below, promoting efficient combustion. This method is advantageous because it facilitates the burning of the coal while minimizing unburned carbon losses. Additionally, this approach allows for easier ash removal since the ash produced during combustion can fall through the grates.

The other options refer to different methods of feeding coal or processes that are not characteristic of an overfeed stoker. For example, feeding coal from below the grates is typical of underfeed stokers, while high-pressure injection and preheating of coal are not standard practices associated with overfeed stokers. This understanding is crucial for those operating and maintaining boiler systems, as it highlights key operational principles and design features that optimize combustion efficiency.