Combustion Kinetics

FUEL & ENERGY TECHNOLOGY

• Course Code: CHE 557

Pollution Engineering and Technology

• Topic focus: Combustion and Incineration Processes

• Applications in Environmental Engineering by Walter R. Niessen

Combustion Kinetics

• Combustion occurs at a finite rate, influenced by:

  • Temperature

  • Concentrations of reacting species

  • Static pressure

• Combustion kinetics studies the relationship between reaction rate and various parameters.

• In practical combustors, reaction rates often do not control burning rates due to:

  • High maintained temperatures allowing rapid reactions compared to mean residence time.

  • Rate-limiting factors typically include mixing rates of fuel and oxidant.

• Important species for understanding kinetics:

  • Carbon monoxide (CO)

  • Soot (carbon)

Introduction to Kinetics

Overall Kinetics

• General gas-phase reaction example:

  • Forward reaction: bB + CC → dD + eE

  • Reaction rate:

    • r = k[B]^b[C]^c

• Backward rate expression for equilibrium conditions:

  • r' = k'[D]^d[E]^e

• At equilibrium:

  • r = r' and k[B]^b[C]^c = k'[D]^d[E]^e

Equilibrium Constant

• Equilibrium constant (K): K = k/k’

Van't Hoff's Analysis

• Variation of the equilibrium constant Kp with temperature T given by:

  • dlnKp/dT = ΔΗ/RT²

• Interpretation of energy changes in reactions:

  • E = Average energy reactants must possess for reactions to occur.

  • A is the pre-exponential factor accounting for molecular collision frequency and steric factors.

Energy Changes in Reactions

• Illustrated through energy diagrams for:

  • Exothermic reactions: average energy of products is lower than reactants.

  • Endothermic reactions: average energy of products is higher than reactants.

Temperature and Reaction Rate

• Reaction rate increases with rising temperature.

• Ignition temperature is a significant concept in combustion reactions.

Mechanism of Reactions

• Reaction rate expressions may involve complex mechanisms rather than simple stoichiometric coefficients.

• Free radicals play a vital role in initiating and propagating combustion reactions.

Example Reaction: Hydrogen and Bromine

• Stoichiometric reaction: H₂ + Br₂ → 2HBr

• Reaction steps include formation of bromine radicals leading to HBr formation.

Importance of Free Radicals in Combustion

• Heptane (C7H16) serves as a standard test fuel with high reactivity leading to knock in engines.

• Octane is branched, stabilizing free radicals and thus reducing combustion speed and knock potential.

Tetraethyl Lead

• Acts as a reaction inhibitor reducing radical concentration but increases lead pollution and negatively affects catalytic converters.

Mechanism Problem Example

• Nitryl chloride (NO₂Cl) decomposes into nitrogen dioxide (NO2) and chlorine gas (Cl2).

Kinetics of Carbon Monoxide Oxidation

• CO: important air pollutant, can store combustion energy.

• Kinetic expressions mirror those of methane oxidation with dependencies on mole fractions.

Kinetics of Soot Oxidation

• High temps, low oxygen can lead to soot formation in burning carbon.

• Soot impacts emission limits due to poor combustion conditions.

Control of Soot Burnout

• Examined through kinetics of combustion of carbonaceous particles.

• Soot burnout is influenced by both kinetic and differential resistances to reaction.

Kinetics of Nitrogen Oxides (NOx) Formation

• NO produced at high temperatures in combustion systems.

• NOx also form from oxidizing nitrogen from combustion air and fuel bonds.

• Influenced by both equilibrium and kinetic processes.

Strategies to Reduce NOx Emissions

• Techniques include:

  • H2O injection

  • Low excess air operation

  • Flue gas recirculation

  • Staged combustion

Fuel Nitrogen Conversion Details

• High conversion efficiency for fuel nitrogen is observed under certain conditions.

Emission Estimation

• NOx emissions correlated with flame temperature and fuel heating value.