Furnace combustion diagnostics: black smoke from a stack indicates what primary combustion condition in the furnace operation?

Introduction / Context:
Stack plume appearance is a quick diagnostic for combustion quality. Black smoke typically signals incomplete combustion with soot formation, indicating a deficiency of oxygen relative to fuel feed or poor mixing/atomization.

Given Data / Assumptions:

• Observation: black smoke from furnace chimney.
• Fuel may be oil, gas, or pulverized coal.
• Focus: primary combustion condition implied by plume color.

Concept / Approach:
When excess air is too low, carbonaceous particles and unburnt hydrocarbons form and exit as visible black smoke. Adequate excess air and proper burner setup (atomization, turbulence, residence time) minimize soot and CO formation.

Step-by-Step Solution:

Verification / Alternative check:
Combustion tuning uses oxygen/CO analyzers; low O2 with elevated CO/opacity correlates with black smoke, confirming air deficiency or atomization issues.

Why Other Options Are Wrong:

• Large excess air: typically yields pale plume and higher stack losses, not black smoke.
• Use of only hydrocarbon gas fuel: clean gas with proper air rarely produces black smoke.
• Pulverised coal as fuel always: coal can smoke if air is low, but the color specifically points to air deficiency, not the mere use of coal.
• Perfect stoichiometric firing: would not create a black plume if mixing is proper.

Common Pitfalls:
Assuming fuel type alone dictates smoke color; ignoring burner maintenance and atomization quality; confusing white steam plume with black carbonaceous smoke.

Final Answer:
Low amount of excess combustion air