8.1 NOx, CO, and Particulate Matter Formation Mechanisms

Pollutant formation in combustion is a big deal. NOx, CO, and particulate matter are major culprits. Understanding how they form is key to developing strategies to reduce emissions and meet air quality standards.

These pollutants form through different mechanisms. Thermal NOx needs high temps, while CO results from incomplete burning. Particulates can come from unburned fuel or form later in the atmosphere. Knowing these processes helps engineers design cleaner engines and power plants.

NOx Formation Mechanisms

Thermal NOx and Zeldovich Mechanism

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• Thermal NOx forms at high temperatures through oxidation of atmospheric nitrogen
• Zeldovich mechanism describes thermal NOx formation process
  • Consists of three main reactions involving nitrogen and oxygen atoms
  • Reaction rate increases exponentially with temperature
• Combustion temperature plays crucial role in thermal NOx formation
  • Temperatures above 1600°C significantly increase NOx production
  • Reducing peak flame temperature helps mitigate thermal NOx formation

Prompt NOx and Fuel NOx

• Prompt NOx forms rapidly in flame front through reaction of atmospheric nitrogen with hydrocarbon radicals
  • Occurs at lower temperatures compared to thermal NOx
  • Contributes smaller portion of total NOx emissions
• Fuel NOx results from oxidation of nitrogen-containing compounds in fuel
  • More prevalent in coal and heavy oil combustion
  • Can account for significant portion of total NOx emissions in some fuels

CO Formation

Incomplete Combustion and CO Production

• Carbon monoxide (CO) forms primarily through incomplete combustion of carbon-containing fuels
• Incomplete combustion occurs when insufficient oxygen prevents full oxidation of carbon to CO2
• Air-fuel ratio significantly impacts CO formation
  • Rich mixtures (excess fuel) lead to increased CO production
  • Lean mixtures (excess air) generally reduce CO formation but may affect combustion stability

Factors Affecting CO Formation

• Residence time influences CO formation and oxidation
  • Longer residence times in high-temperature zones promote CO oxidation to CO2
  • Short residence times may result in higher CO emissions
• Mixing quality affects CO formation
  • Poor fuel-air mixing leads to localized rich zones, increasing CO production
• Quenching of combustion reactions can freeze CO concentrations
  • Occurs when hot gases contact cooler surfaces or mix with cooler air

Particulate Matter

PM Characteristics and Formation

• Particulate matter (PM) consists of tiny solid or liquid particles suspended in air
• PM2.5 refers to particles with diameter ≤ 2.5 micrometers
  • Can penetrate deep into lungs and bloodstream
  • Primarily formed through combustion processes and atmospheric reactions
• PM10 includes particles with diameter ≤ 10 micrometers
  • Includes both fine and coarse particles
  • Sources include dust, pollen, and larger combustion particles

Sources and Formation Mechanisms

• Incomplete combustion contributes significantly to PM formation
  • Unburned fuel droplets and partially oxidized hydrocarbons form soot particles
  • Occurs in fuel-rich zones or areas with poor mixing
• Condensation of volatile compounds forms secondary PM
  • Sulfates and nitrates from SO2 and NOx emissions
  • Organic aerosols from volatile organic compounds (VOCs)
• Non-combustion sources contribute to PM emissions
  • Mechanical wear (brake dust, tire wear)
  • Agricultural activities (dust from tilling, harvesting)
  • Natural sources (sea spray, volcanic emissions)