Which of the following is NOT typically used for burning coal?

The rotary cup stoker is typically not used for burning coal; instead, it is a technology more suited to burning liquids or finely atomized fuels. This choice involves a rotating cup that dispenses fuel into a combustion chamber, making it efficient for oil or other liquid fuels which require a specific delivery method that differs fundamentally from the bulk method needed for coal.

In contrast, the other types of stokers mentioned are designed for coal combustion. Under feed stokers, cross feed stokers, and chain grate stokers are all systems that facilitate the feeding of coal into a furnace efficiently. They allow for the necessary control over the fuel delivery and combustion process, which is essential for optimized burning of solid fuels like coal. Each of these systems utilizes various mechanisms to ensure a steady supply of coal to maintain combustion, ultimately enhancing efficiency and performance in coal-fired boilers.