Glossary

12.3 Combustion kinetics and energy applications

3 min read•july 22, 2024

Combustion kinetics is all about understanding how fuels burn. It covers ignition, flame spread, and pollution formation. These processes are key to making engines, power plants, and other energy systems work efficiently and cleanly.

Factors like fuel type, air-fuel mix, and burning conditions affect how reactions happen. By tweaking these, we can create cleaner engines and use alternative fuels. This knowledge helps us design better energy tech for a greener future.

Combustion Kinetics Fundamentals

Principles of combustion kinetics

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  • Ignition
    • Initiates combustion reactions by providing sufficient energy to overcome the barrier
    • Influenced by fuel properties (octane number, cetane number), , and pressure
    • Examples: spark ignition in gasoline engines, compression ignition in diesel engines
  • Flame propagation
    • Spreads combustion reactions through the fuel-air mixture via heat and mass transfer processes
    • Affected by turbulence (enhances mixing), fuel composition (affects flame speed), and combustion chamber geometry (influences flow patterns)
    • Examples: laminar flame propagation in a Bunsen burner, turbulent flame propagation in a gas turbine combustor
  • Pollutant formation
    • Produces undesirable byproducts during combustion, such as (NOx), (CO), and particulate matter (PM)
    • Formation mechanisms depend on combustion conditions (temperature, pressure, air-fuel ratio) and fuel properties (sulfur content, aromatic content)
    • Examples: thermal NOx formation at high temperatures, soot formation in fuel-rich regions

Applications and Impact of Combustion Kinetics

Kinetics in energy systems

  • Internal combustion engines
    1. Spark-ignition (SI) engines: Ignition by spark plug, flame propagation through premixed fuel-air mixture
    2. Compression-ignition (CI) engines: Ignition by compression heating, flame propagation through diffusion-controlled combustion
    • Examples: gasoline engines in cars (SI), diesel engines in trucks (CI)
  • Gas turbines
    • Utilize continuous combustion process with high air-fuel ratios for power generation and propulsion
    • Kinetics influenced by turbulence (enhances mixing), fuel injection (affects atomization and evaporation), and combustion chamber design (influences flow patterns and residence time)
    • Examples: stationary gas turbines for electricity generation, aircraft jet engines
  • Power plants
    • Employ large-scale combustion systems for electricity generation using various fuels and technologies
    • Kinetics affected by fuel properties (coal rank, biomass composition), combustion technology (pulverized coal, fluidized bed), and emission control strategies (selective catalytic reduction, electrostatic precipitators)
    • Examples: coal-fired power plants, biomass-fired power plants

Factors affecting reaction kinetics

  • Fuel composition
    • Hydrocarbon structure and molecular weight influence ignition delay and flame propagation speed
      • Longer hydrocarbon chains and higher molecular weights tend to have longer ignition delays and slower flame speeds
    • Presence of impurities (sulfur, nitrogen) can affect pollutant formation
      • Sulfur can lead to SO2 emissions, while nitrogen can contribute to fuel NOx formation
  • Air-fuel ratio (ϕ\phi)
    • Stoichiometric ratio (ϕ=1\phi = 1): Theoretically with no or fuel
    • Lean combustion (ϕ<1\phi < 1): Excess air, lower flame temperatures, reduced NOx formation
    • Rich combustion (ϕ>1\phi > 1): Insufficient air, increased CO and PM formation due to
  • Combustion conditions
    • Temperature: Higher temperatures accelerate reaction rates but can increase thermal NOx formation
    • Pressure: Elevated pressures enhance reaction rates and flame propagation by increasing molecular collision frequency
    • Residence time: Longer residence times allow for more complete combustion but may increase pollutant formation if the mixture is not well-mixed

Kinetics for clean energy technologies

  • Low-emission engines
    1. Advanced combustion strategies (homogeneous charge compression ignition, lean burn) to reduce pollutant formation by controlling temperature and air-fuel mixing
    2. Improved fuel injection and air-fuel mixing (high-pressure injection, swirl) to optimize combustion kinetics and minimize local fuel-rich regions
    3. Exhaust gas recirculation (EGR) to lower flame temperatures and reduce thermal NOx formation by diluting the intake air with inert exhaust gas
    • Examples: HCCI engines, lean-burn gas engines
  • Alternative fuels
    • (ethanol, biodiesel), natural gas, and hydrogen offer cleaner alternatives to conventional fossil fuels
    • Each fuel type has unique combustion kinetics and challenges
      • Biofuels may have different ignition properties and flame speeds compared to conventional fuels
      • Natural gas has a higher octane number and can enable higher compression ratios for improved efficiency
      • Hydrogen has a wide flammability range and can enable ultra-lean combustion for reduced NOx emissions
    • Tailored combustion strategies can help reduce pollutant emissions and improve engine efficiency when using alternative fuels
    • Examples: flex-fuel vehicles running on gasoline-ethanol blends, natural gas buses, hydrogen fuel cell vehicles

Key Terms to Review (11)

Activation Energy: Activation energy is the minimum amount of energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome to be transformed into products, linking the concepts of kinetics and thermodynamics in the context of chemical reactions.
Biofuels: Biofuels are renewable energy sources derived from biological materials, such as plants and animal waste, that can be used to produce heat, electricity, or power vehicles. They serve as an alternative to fossil fuels, aiming to reduce greenhouse gas emissions and reliance on non-renewable energy sources. Their production and use involve various combustion kinetics principles that affect energy efficiency and emissions profiles.
Carbon monoxide: Carbon monoxide (CO) is a colorless, odorless gas that is produced during the incomplete combustion of carbon-containing fuels. This compound plays a significant role in combustion kinetics, as it can affect the efficiency and emissions of combustion processes, ultimately impacting energy applications and environmental considerations.
Chain Reaction: A chain reaction is a series of chemical reactions where the products of one reaction trigger subsequent reactions, often resulting in an exponential increase in reaction rate. This concept is particularly important in understanding combustion processes and the design of reactors, as it highlights how small changes in conditions can lead to significant effects on energy release and product formation.
Complete Combustion: Complete combustion is a chemical reaction in which a hydrocarbon fuel reacts with oxygen to produce carbon dioxide and water, releasing a maximum amount of energy. This process occurs when there is a sufficient supply of oxygen, ensuring that all carbon in the fuel is fully oxidized, leading to efficient energy conversion and minimal production of harmful byproducts.
Excess air: Excess air refers to the amount of air supplied to a combustion process that exceeds the theoretical air required for complete combustion of the fuel. This extra air is often used to ensure complete burning of the fuel, reduce emissions, and improve efficiency in energy applications. However, too much excess air can lead to heat loss and lower overall efficiency.
Incomplete combustion: Incomplete combustion occurs when a fuel burns in insufficient oxygen, leading to the formation of products other than carbon dioxide and water, such as carbon monoxide and soot. This phenomenon is significant as it directly relates to combustion kinetics and energy applications, affecting the efficiency and environmental impact of fuel usage.
Nitrogen Oxides: Nitrogen oxides (NOx) are a group of reactive gases that consist of nitrogen and oxygen, primarily including nitrogen dioxide (NO2) and nitric oxide (NO). These compounds are significant pollutants formed during combustion processes, such as those occurring in vehicles and industrial activities, and they play a crucial role in the formation of smog and acid rain.
Rate Constant: The rate constant is a proportionality factor in the rate law that quantifies the speed of a chemical reaction at a given temperature. It connects the concentration of reactants to the reaction rate, showing how quickly the reaction proceeds. The value of the rate constant is influenced by factors such as temperature, activation energy, and the presence of catalysts, making it a key element in understanding reaction kinetics and dynamics.
Stoichiometry: Stoichiometry is the branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It allows chemists to calculate how much of each substance is needed or produced based on balanced chemical equations, making it essential for understanding reaction yields and optimizing conditions in various processes, including combustion.
Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance, influencing how fast molecules move and collide. It plays a crucial role in determining reaction rates, as higher temperatures generally increase the frequency and energy of collisions between reactant molecules, thus affecting reaction kinetics across various chemical processes.
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