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Fuels Combustion Efficiency

Reference data and engineering information about fuels combustion efficiency for combustion applications.

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Overview

Engineering reference data for Fuels Combustion Efficiency in combustion engineering.

Key Formulas

Heat Release

Q=m˙⋅HVQ = \dot{m} \cdot HV

Fuel energy release rate.

Air-Fuel Ratio

AF=mairmfuelAF = \frac{m_{air}}{m_{fuel}}

Mass of air per mass of fuel.

Excess Air

EA=O221−O2×100%EA = \frac{O_2}{21 - O_2} \times 100\%

From flue gas oxygen measurement.

Variables

Symbol Description Unit
QQ Heat release rate W
m˙\dot{m} Mass flow rate kg/s
HVHV Heating value J/kg
AFAF Air-fuel ratio —

Excess Air Reference Data

Combustion Theory & Excess Air

Stoichiometric Combustion

Stoichiometric combustion (also called theoretical or ideal combustion) is the chemically correct mixing proportion between air and fuel where no fuel or oxygen remains unreacted after combustion. In practice, process heating equipment almost never operates at stoichiometric conditions.

Excess Air

Excess air is the amount of air supplied beyond the theoretical minimum required for complete combustion. It is expressed as a percentage of the stoichiometric air requirement:

Excess Air (%)=mair, actual−mair, stoichmair, stoich×100\text{Excess Air (\%)} = \frac{m_{\text{air, actual}} - m_{\text{air, stoich}}}{m_{\text{air, stoich}}} \times 100

where mair, actualm_{\text{air, actual}} is the actual mass of air supplied and mair, stoichm_{\text{air, stoich}} is the stoichiometric (theoretical minimum) mass of air required per unit mass of fuel.

Even so-called "on-ratio" combustion used in boilers and high-temperature furnaces incorporates 10 to 20% excess air to ensure complete fuel burnout.

Effects of Insufficient Air

When insufficient air is supplied to the burner, incomplete combustion produces:

  • Unburned fuel and soot
  • Smoke and carbon monoxide (CO)
  • Heat transfer surface fouling
  • Reduced combustion efficiency
  • Flame instability and explosion risk

Practical Operating Guidelines

Application Typical Excess Air
Power plant boilers 10 – 20%
Natural gas-fired boilers ~5%
Pulverized coal-fired boilers ~20%
Gas turbines Up to 300%

Operating with adequate excess air provides a safety margin against variations in fuel composition and control system tolerances.

References