Pulverised coal firing practice: Why does the ratio of primary air to secondary air generally increase with an increase in the rank of coal?

Introduction / Context:
In pulverised-coal combustion, primary air transports and begins drying/ignition of coal particles, while secondary air provides additional oxygen for complete burnout. Coal rank affects volatiles, ignition characteristics, and air staging strategy.

Given Data / Assumptions:

• Comparison among low-rank (high-volatile) and high-rank (low-volatile, high fixed-carbon) coals.
• Conventional burner with distinct primary and secondary air streams.
• Goal is stable ignition and complete burnout.

Concept / Approach:
As coal rank increases, volatile matter decreases and fixed-carbon proportion increases. More primary air (relative to secondary) helps transport, dry, and ensure ignition/burnout of the higher fixed-carbon fraction within the burner zone, hence the primary/secondary air ratio tends to rise with rank.

Step-by-Step Solution:
High rank → higher fixed carbon / volatile ratio.Lower volatiles reduce early flame stability; more primary air promotes ignition and transport.Therefore, the primary-to-secondary air ratio increases.

Verification / Alternative check:
Combustion handbooks show burner air-staging tuned to fuel volatiles; low-volatile coals need stronger primary stream and longer residence for char burnout.

Why Other Options Are Wrong:
(a) Reverses the trend; higher rank generally has lower volatiles. (c) While oxygen content may vary, it is not the primary control for the air split. (d) Calorific value trends are not the direct reason for air staging changes.

Common Pitfalls:

• Assuming one air split suits all coals; burner tuning must match fuel rank.
• Confusing transport (primary) air function with overall excess air requirements.

Final Answer:
Because the ratio of fixed carbon to volatile matter increases.