What type of coal feeder requires the least excess air?

The type of coal feeder that requires the least excess air is indeed pulverizers. Pulverizers are designed to grind coal into a fine powder that can be easily burnt in a furnace. This fineness allows for better combustion efficiency as the larger surface area of the powdered coal facilitates faster and more complete combustion.

When using a pulverizer, the coal is mixed with air in such a way that the fuel-air mixture can ignite and burn more effectively. Since the coal is already in a finely divided state, it needs less excess air to achieve optimal combustion compared to coarser fuels. This reduces the amount of unreacted oxygen in the flue gases, leading to lower emissions and higher efficiency.

In contrast, other feeder types like volumetric and gravimetric feeders tend to provide coal in larger chunks and often require more excess air to ensure proper combustion, as they are not as efficient in promoting thorough mixing between fuel and air. Chain feeders also dispense coal but do not achieve the same particle fineness and mix as effectively as pulverizers. Thus, pulverizers are superior in this aspect, requiring less excess air for efficient combustion.