Combustion air requirements:\nWhich type of fuel typically requires the greatest percentage of excess air to achieve complete combustion in practice?

Introduction / Context:
“Excess air” is the percentage of air supplied above the stoichiometric requirement. Real burners and furnaces never mix fuel and oxidizer perfectly; some extra air ensures complete oxidation of combustibles and low CO/soot emissions. Understanding which fuels need more excess air guides burner design, stack O2 targets, and efficiency estimates.

Given Data / Assumptions:

• Comparison of typical industrial fuels operated in air.
• Focus on practical plant conditions, not ideal laboratory mixing.
• Complete combustion target with reasonable safety margin.

Concept / Approach:
Mixing quality and reaction homogeneity largely dictate excess air. Gases mix at the molecular level, so they require the least excess air (often 0–10%). Liquids atomize into droplets; mixing is good but less perfect than gases (10–20% typical). Solid fuels (e.g., coal, biomass) introduce heterogeneity: variable particle size, ash, moisture, and diffusion limits around particles. As a result, solids need substantially higher excess air (20–50% or more) to ensure burnout and avoid CO generation.

Step-by-Step Solution:

Verification / Alternative check:
Combustion handbooks and boiler guidelines target higher stack O2 for coal stokers/grates than for natural gas burners, confirming higher excess-air needs for solids.

Why Other Options Are Wrong:

• Liquids and gases achieve better mixing and atomization/dispersion, requiring less excess air.
• “Nuclear” does not involve oxidation of chemical fuel with air.
• Biomass slurries behave as solids in terms of heterogeneity but are a subset; “solid fuels” is the broad, correct category.

Common Pitfalls:
Assuming stoichiometric air is sufficient in real furnaces; ignoring ash, moisture, and particle burnout times for solids.

Final Answer:
Solid fuels.