Question bank — Pollutants — NOₓ, soot, unburned hydrocarbons

False ::: Kerosene is a clean fuel with negligible bound nitrogen; its NOₓ comes from the N₂ in the air cracked by the thermal (Zeldovich) route. Fuel-bound N only dominates for coal and heavy oils.

False ::: Heat does finish CO/UHC burnout, but the thermal-NOₓ rate carries an $e^{-E_a/RT}$ factor with a huge $E_a$, so extra heat explodes NOₓ. There is an optimum temperature, not a "hotter is cleaner" rule.

False ::: Backwards. Soot forms in oxygen-starved, fuel-rich pockets where carbon clumps before it finds O; oxygen actually destroys soot via the oxidation step.

False ::: NOₓ scales linearly with time but exponentially with temperature through $e^{-E_a/RT}$. Temperature dominates by orders of magnitude; residence time is a secondary knob.

False ::: CO is fuel oxidised only partway ($\text{C}\to \text{CO}$, not to ${\text{CO}}_2$). It signals incomplete combustion and is a toxic pollutant, not a finished product.

False ::: N atoms are so reactive they are consumed as fast as made — we invoke the quasi-steady-state $d[\text{N}]/dt\approx 0$, so [N] stays tiny and roughly constant.

False ::: The adiabatic peak temperature is at φ ≈ 1, but NOₓ also needs plentiful O atoms, which favours the lean side. The combined effect puts the NOₓ peak slightly lean of stoichiometric.

False ::: Soot needs both richness and heat. Too cold and pyrolysis never cracks the fuel; the "soot window" is a hot, rich pocket — ideally one quenched before oxidation cleans it up.

$\text{CO}+\text{OH}\to {\text{CO}}_2+\text{H}$ is fast, so CO always burns out. False ::: It is comparatively slow and needs hot gas plus OH radicals. If the gas cools quickly (quench), CO is frozen in and escapes — hence high CO from quenched or cold zones.

$d[\text{NO}]/dt=2k_1[\text{O}][{\text{N}}_2]$ is because there are two nitrogen atoms in N₂." Error ::: The 2 counts product NO molecules, not atoms in the reactant. Reaction (1) makes one NO and one leftover N; that N immediately makes a second NO via $\text{N}+{\text{O}}_2\to \text{NO}+\text{O}$.

$\text{N}+{\text{O}}_2\to \text{NO}+\text{O}$, is the rate-limiting step because it consumes oxygen." Error ::: Reactions (2) and (3) are fast. The rate-limiter is (1), $\text{O}+{\text{N}}_2\to \text{NO}+\text{N}$, because it must break the very strong N≡N triple bond ($945$ kJ/mol) — hence the enormous $E_a$.

Error ::: They are distinct. Thermal (Zeldovich) is O atoms cracking N₂ at high T; prompt (Fenimore) is CH fuel radicals attacking N₂ in the flame front, and it happens even in rich, cooler zones where thermal is weak.

Error ::: HACA is H-Abstraction, C₂H₂-Addition: a ring first loses an H (abstraction), then adds an acetylene (C₂H₂) unit, repeating to grow the PAH sheet.

Error ::: The opposite — the wall is cold and pulls heat out, freezing the reactions in a thin layer near it, so fuel there escapes unburned.

Error ::: Net soot = formation − oxidation. Soot that later meets O₂ or OH at high T can burn off; the worst emissions come when a rich, sooty pocket is quenched before that oxidation finishes.

Why ::: The Zeldovich rate-limiter has a giant activation energy ($\approx 319$ kJ/mol) sitting inside $e^{-E_a/RT}$, making rate roughly double per ~70 K near 2000 K. Soot depends more on mixture richness and available carbon than on a single high-$E_a$ step.

Why ::: The same coolness that starves the high-$E_a$ NOₓ step also starves the slow CO+OH burnout and can drop mixtures below the flammability limit, leaving fuel and CO unfinished — the central engineering trade-off.

peak temperature rather than average? Why ::: Because NOₓ is set by the hottest local pockets (exponential in T), flattening the temperature profile — removing hot spikes — cuts NOₓ far more than lowering the mean would suggest.

Why ::: In a diffusion flame fuel and air meet only at a thin interface, so the fuel side is locally very rich and hot — an ideal soot-forming pocket — whereas premixing spreads oxygen throughout, avoiding those extreme-rich zones (see Diffusion vs Premixed Flames).

Why ::: Pyrolysis of rich fuel cracks it into small radicals dominated by C₂H₂, which is the building block that both forms the first aromatic ring and then adds on repeatedly (the "A" in HACA) to grow PAHs.

Why ::: At low load the gases are cooler and residence times shorter, so the slow, radical-hungry $\text{CO}+\text{OH}\to {\text{CO}}_2$ step freezes out before completing — cold, quenched conditions trap CO.

Edge ::: NOₓ falls (low temperature starves the high-$E_a$ step), but CO and UHC rise sharply because the flame is on the edge of extinction and cannot complete oxidation — flame-out territory.

Edge ::: No. Even uniform-T stoichiometric combustion is hot enough to make thermal NOₓ and leaves some equilibrium CO; the ideal only removes soot (needs richness) and the extra NOₓ from hot spots.

Edge ::: Thermal NOₓ is suppressed by the low T, yet fuel-NOₓ can still form because its nitrogen is already unlocked in the fuel and does not need the N≡N bond broken — a route that survives cool flames.

Edge ::: Bad — the cooling quenches oxidation before the freshly-formed soot can burn off, so net soot rises. This is the classic "hot-rich-then-quenched" worst case.

$T\to \infty$ in the Arrhenius factor $e^{-E_a/RT}$, what does the NOₓ rate constant approach, and is that physical? Edge ::: The exponential tends to 1, so $k_1\to A$, its pre-exponential ceiling. Physically real flames never reach that limit — dissociation and heat loss cap the temperature well before, but it shows why NOₓ saturates rather than growing without bound (see Arrhenius Equation and Activation Energy).

Edge ::: NOₓ cannot (no O atoms, and no N₂ if it's pure fuel), and CO burnout is moot, but soot and UHC thrive — pyrolysis cracks fuel into C₂H₂ and PAHs with nothing to oxidise them.