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Power Eng 4th Class 4A study materials
Power Eng 4th Class 4A study resources

Power Eng 4th Class 4A: Fuels, Combustion, and Environmental Controls

Power Eng 4th Class 4A · Work through your exam, one topic at a time.

All guide sections

Fuels, Combustion, and Environmental Controls

What this domain tests

This domain covers fuel properties, combustion chemistry, and the emission controls that keep a plant within environmental limits. Questions test stoichiometry, excess air, heating values, and the operation of NOx and SOx controls.

Core rules to have at your fingertips

  • Fuel properties: calorific value (HHV and LHV), density, viscosity, sulphur content, ash, moisture, and volatility. LHV = HHV − latent heat of water vapour formed.
  • Combustion stoichiometry: carbon burns to CO₂ (C + O₂ → CO₂), hydrogen to H₂O, sulphur to SO₂. Theoretical air is the minimum for complete combustion; excess air is the percentage above theoretical.
  • Three T's: temperature, turbulence, and time - all needed for complete combustion.
  • NOx control: low-NOx burners, staged combustion (air or fuel staging), flue gas recirculation (FGR), selective catalytic reduction (SCR), and selective non-catalytic reduction (SNCR).
  • SOx control: low-sulphur fuel, flue gas desulphurisation (FGD - wet limestone, spray dry), and sorbent injection.
  • Particulates: baghouse, electrostatic precipitator (ESP), and cyclone.

Worked example

A coal has 70 % carbon by mass. Theoretical oxygen for carbon = 32/12 = 2.667 kg O₂ per kg C, so O₂ required = 0.70 × 2.667 = 1.867 kg O₂/kg coal.

Air is 23.2 % O₂ by mass, so theoretical air = 1.867 / 0.232 = 8.05 kg air/kg coal.

At 15 % excess air, actual air = 8.05 × 1.15 = 9.26 kg air/kg coal. If the boiler burns 5 kg/s coal, air flow = 5 × 9.26 = 46.3 kg/s.

For NOx, if the uncontrolled emission is 400 mg/Nm³ and SCR achieves 80 % reduction, the outlet is 80 mg/Nm³ - check against the permit limit.

Traps that cost marks

  • Using HHV when the question specifies LHV.
  • Forgetting the hydrogen and sulphur contributions to theoretical air.
  • Confusing excess air with air/fuel ratio.
  • Assuming FGD removes NOx - it removes SO₂.
  • Ignoring the effect of FGR on flame stability at low load.

Exam strategy

For stoichiometry, work in mass per kg fuel and show each step. For emissions, state the pollutant, the control technology, and the expected reduction.