8.1 NOx, CO, and Particulate Matter Formation Mechanisms
3 min read•august 9, 2024
Pollutant formation in combustion is a big deal. , , and 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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Top images from around the web for Thermal NOx and Zeldovich Mechanism
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Thermal NOx forms at high temperatures through oxidation of atmospheric nitrogen
describes 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
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 of carbon-containing fuels
Incomplete combustion occurs when insufficient oxygen prevents full oxidation of carbon to CO2
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
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
affects CO formation
Poor fuel-air mixing leads to localized rich zones, increasing CO production
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
refers to particles with diameter ≤ 2.5 micrometers
Can penetrate deep into lungs and bloodstream
Primarily formed through combustion processes and atmospheric reactions
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
Sulfates and nitrates from SO2 and NOx emissions
Organic aerosols from
Non-combustion sources contribute to PM emissions
Mechanical wear (brake dust, tire wear)
Agricultural activities (dust from tilling, harvesting)