Pulverized coal transport safety: Preheated air for conveying pulverized coal to boiler burners is typically limited to about 300°C mainly to avoid the __________.

Introduction:
Pulverized coal is conveyed pneumatically with hot air from mills to burners. While preheating improves drying and ignition, excessive air temperature can create a fire/explosion hazard by igniting coal dust within the transport lines.

Given Data / Assumptions:

• Coal dust–air mixtures can be explosible within specific concentration and temperature ranges.
• Typical plant practice limits primary air to around 300°C.
• Lines include bends, valves, and potential ignition sources (hot spots).

Concept / Approach:
Coal dust has a minimum ignition temperature; raising the carrier air too high increases the chance of autoignition or ignition from smoldering deposits. The catastrophic consequence is a deflagration in ducts or mills. Therefore, a conservative temperature limit mitigates explosion risk.

Step-by-Step Solution:
Identify hazard: dust–air mixture with ignition source + high T.Note industry limit ~300°C as a safe upper bound.Conclude primary reason is explosion risk minimization.

Verification / Alternative check:
NFPA guidance and utility boiler practices emphasize temperature control of primary air and mill outlet to prevent fires/explosions.

Why Other Options Are Wrong:

• Chamber size, clinker, incomplete combustion, erosion: Influenced by many factors, but not the main reason for the strict 300°C cap.

Common Pitfalls:
Overlooking smoldering coal in mills as an ignition source when air is too hot.

Final Answer:
risk of explosion