Which statement correctly distinguishes fly ash and bottom ash?

• A. Bottom ash is captured in collectors; fly ash is collected in hoppers.
• B. Fly ash is captured in collectors; bottom ash is collected in hoppers.
• C. Fly ash is stored in the furnace; bottom ash is discharged with exhaust.
• D. Fly ash is captured in collectors and disposed or used; bottom ash is collected in hoppers at the furnace bottom and disposed or utilized.

Answer: (D)

Ash separation in a coal-fired boiler hinges on particle size and where the material ends up. Fly ash consists of very fine particles that are entrained in the flue gas and must be removed by pollution control devices—collectors such as electrostatic precipitators or baghouses. After capture, this fly ash is stored in hoppers or silos and then disposed of or used, often as a supplementary cementitious material.

Bottom ash is heavier and coarser, so it settles at the furnace bottom and is discharged into bottom ash hoppers located at the furnace, where it is then disposed of or utilized. This distinction—fly ash captured by collectors and stored/disposed or used, versus bottom ash collected in the furnace bottom hoppers and then disposed of or used—best reflects how these two types of ash are handled.