9.4 Combustion

9.4 Combustion

1. Combustion Process and Phases

1. Combustion Definition:

   • Rapid chemical reaction between fuel and oxidizer (usually oxygen in air).

   • Releases energy as heat and light (exothermic reaction).

   • General equation: Fuel + Oxidizer → Products + Heat

2. Types of Combustion:

   • Complete Combustion: All carbon converts to CO₂, all hydrogen to H₂O.

     • Example: $C_xH_y + (x + \frac{y}{4})O_2 → xCO_2 + \frac{y}{2}H_2O$

   • Incomplete Combustion: Insufficient oxygen leads to CO, soot, unburned hydrocarbons.

   • Stoichiometric Combustion: Exact theoretical air required for complete combustion.

   • Rich Mixture: Air-fuel ratio less than stoichiometric (excess fuel).

   • Lean Mixture: Air-fuel ratio greater than stoichiometric (excess air).

3. Combustion Phases:

   • Ignition: Initial energy input to start combustion (spark, flame, compression).

     • Ignition temperature: Minimum temperature to initiate combustion.

   • Propagation: Self-sustaining reaction spreads through mixture.

   • Completion: Reaction continues until fuel or oxidizer is depleted.

4. Key Parameters:

   • Air-Fuel Ratio (AFR):

     • Stoichiometric: $AFR_{stoich} = \frac{m_{air}}{m_{fuel}}$

     • Rich: $AFR < AFR_{stoich}$

     • Lean: $AFR > AFR_{stoich}$

   • Equivalence Ratio ($\phi$):

     • $\phi = \frac{(F/A)_{actual}}{(F/A)_{stoich}}$

     • $\phi = 1$: Stoichiometric

     • $\phi > 1$: Rich

     • $\phi < 1$: Lean

5. Combustion Modes:

   • Premixed Combustion: Fuel and oxidizer mixed before ignition (gasoline engines).

   • Diffusion Combustion: Fuel and oxidizer mix during combustion (diesel engines).

   • Detonation: Supersonic combustion wave, extremely rapid pressure rise.

6. Heat of Combustion:

   • Higher Heating Value (HHV): Includes latent heat of water vapor condensation.

   • Lower Heating Value (LHV): Water remains as vapor.

   • Units: kJ/kg or MJ/kg of fuel.

2. Fire Control

1. Fire Tetrahedron:

   • Four elements required for fire:

     • Fuel (combustible material)

     • Oxidizer (usually oxygen)

     • Heat (ignition source)

     • Chemical chain reaction

   • Remove any one element to control/extinguish fire.

2. Fire Prevention Methods:

   • Fuel Control: Proper storage, leak prevention, substitution with less flammable materials.

   • Oxidizer Control: Inert gas purging, oxygen reduction systems.

   • Heat/Ignition Control: Temperature monitoring, spark/static control, proper electrical systems.

   • Chain Reaction Inhibition: Fire suppression agents.

3. Fire Suppression Techniques:

   • Cooling: Water application reduces temperature below ignition point.

   • Smothering: Removing oxygen (CO₂, foam, sand).

   • Starvation: Removing fuel source.

   • Chemical Inhibition: Interrupting chain reaction (dry chemicals, halons).

4. Fire Detection Systems:

   • Heat detectors: Fixed temperature or rate-of-rise.

   • Smoke detectors: Ionization or photoelectric.

   • Flame detectors: UV/IR sensors.

   • Gas detectors: For combustible gases.

5. Extinguishing Agents:

   • Water: Cooling effect, but conductive - not for electrical fires.

   • Foam: Forms blanket over liquid fuels.

   • CO₂: Displaces oxygen, non-conductive.

   • Dry Chemical: Interrupts chemical chain reaction.

   • Clean Agents: Halons (being phased out) and replacements (FM-200, Novec 1230).

6. Fire Classes:

   • Class A: Ordinary combustibles (wood, paper) → water, foam.

   • Class B: Flammable liquids (gasoline, oil) → foam, CO₂, dry chemical.

   • Class C: Electrical fires → CO₂, dry chemical (non-conductive).

   • Class D: Combustible metals → specialized dry powder.

   • Class K: Cooking oils/fats → wet chemical.

3. Engine Emissions

1. Major Pollutants from Engines:

   • Carbon Monoxide (CO): Product of incomplete combustion, odorless, colorless, toxic.

   • Hydrocarbons (HC): Unburned fuel, contribute to smog formation.

   • Nitrogen Oxides (NOₓ): NO, NO₂ formed at high temperatures (thermal NOₓ).

   • Particulate Matter (PM): Soot particles, especially from diesel engines.

   • Carbon Dioxide (CO₂): Product of complete combustion, greenhouse gas.

   • Sulfur Oxides (SOₓ): From sulfur in fuel (mostly in coal/HSD).

2. Emission Formation Mechanisms:

   • CO Formation: Insufficient oxygen or insufficient time for complete combustion.

     • $2C + O_2 → 2CO$ (incomplete)

     • $2CO + O_2 → 2CO₂$ (if conditions allow)

   • NOₓ Formation (Zeldovich mechanism):

     • $N_2 + O → NO + N$

     • $N + O_2 → NO + O$

     • $N + OH → NO + H$

   • Particulate Formation: Pyrolysis of fuel in oxygen-deficient zones.

3. Factors Affecting Emissions:

   • Air-fuel ratio (AFR)

   • Combustion temperature

   • Residence time at high temperature

   • Mixing quality

   • Engine load and speed

4. Emission Control Technologies:

   • Engine Design Improvements:

     • Electronic fuel injection (precise AFR control)

     • Exhaust gas recirculation (EGR) reduces NOₓ

     • Turbocharging and aftercooling

   • Aftertreatment Systems:

     • Catalytic Converters:

       • Three-way catalyst (gasoline): Reduces CO, HC, NOₓ simultaneously.

       • Oxidation catalyst (diesel): Converts CO and HC to CO₂ and H₂O.

     • Diesel Particulate Filter (DPF): Traps and burns soot particles.

     • Selective Catalytic Reduction (SCR): Uses urea solution to reduce NOₓ.

       • $4NO + 4NH_3 + O_2 → 4N_2 + 6H_2O$

     • Lean NOₓ Trap (LNT): Absorbs NOₓ during lean operation, releases during rich pulses.

5. Emission Standards:

   • Bharat Stages (India): BS-I to BS-VI (progressive tightening).

   • Euro Standards (Europe): Euro 1 to Euro 7.

   • EPA Standards (USA): Tier 1 to Tier 3.

   • Test Cycles: NEDC, WLTP, RDE (real driving emissions).

6. Emerging Technologies:

   • Homogeneous charge compression ignition (HCCI)

   • Variable compression ratio

   • Advanced aftertreatment systems

   • Alternative fuels (CNG, biofuels, hydrogen)