Pollutants — NOₓ, soot, unburned hydrocarbons

5.3.9 · Chemistry › Combustion Chemistry (Propulsion Bridge)

1. NOₓ — nitrogen oxides

Routes KYA hain?

Teen mechanisms hain (80/20 ke liye teeno jaanani zaroori hain):

Route	USE trigger karta hai	KAHAN matter karta hai
Thermal (Zeldovich)	High temperature ( $> 1800$ K)	Hot lean flames, gas turbines
Prompt (Fenimore)	Fuel radicals (CH) jo N₂ par attack karte hain	Rich flame fronts
Fuel NOₓ	Fuel mein bound nitrogen	Coal, heavy oils

Thermal NOₓ KAISE banta hai — Zeldovich derivation

Extended Zeldovich chain: $O + N_{2} ⇌ NO + N (1)$ $N + O_{2} ⇌ NO + O (2)$ $N + OH ⇌ NO + H (3)$

NO production rate ka first principles se derivation.

(1) se kyun shuru karein? Reaction (1) rate-limiting step hai — iska activation energy bahut bada hai (N≡N todna padta hai). Reactions (2),(3) fast hain.

Step 1 — (1) ki rate likhein: $\frac{d [ NO ]}{d t} = k_{1} [O] [N_{2}]$ Yeh step kyun? Law of mass action: rate ∝ reactant concentrations ka product.

Step 2 — N atom ke liye quasi-steady-state invoke karein. N utni hi tezi se consume hota hai jitni tezi se banta hai: $\frac{d [ N ]}{d t} \approx 0$ Kyun? N atoms bahut zyada reactive hote hain — unki concentration tiny aur constant hoti hai. Isse hum unknown [N] ko eliminate kar sakte hain.

Step 3 — combine karein. (1) aur (2) dono ek-ek NO banate hain, toh steady N ko account karte hue: $\frac{d [ NO ]}{d t} = 2 k_{1} [O] [N_{2}]$ Factor 2 kyun? Ek NO (1) se, phir N atom turant doosra NO (2) ke through banata hai via N+O₂→NO+O.

Step 4 — temperature law. $k_{1}$ Arrhenius follow karta hai: $k_{1} = A exp (- \frac{E _{a}}{R T}), E_{a} \approx 319 kJ/mol$ Yeh kyun matter karta hai: woh bada $E_{a}$ hi wajah hai kyun NOₓ approximately ~ $70$ K rise ke saath double ho jaata hai $2000$ K ke paas. Temperature dominates, residence time nahi.

2. Soot — particulate carbon

Soot KAISE banta hai (woh pathway jo yaad rakhni hai)

Pyrolysis: fuel crack ho jaata hai (O₂ ke bina) chote radicals mein, khaas taur par C₂H₂ (acetylene).
Aromatic ring formation: pehli benzene ring banti hai (rate-controlling step).
PAH growth HACA mechanism se (H-Abstraction C₂H₂-Addition): ek ring ek H khoti hai, phir ek acetylene unit add karti hai, repeat karke grow karti hai.
Nucleation → tiny particles; surface growth + coagulation → visible soot.
Oxidation: agar soot phir O₂/OH se high T par milti hai, toh woh jal sakti hai. Net soot = formation − oxidation.

3. Unburned hydrocarbons (UHC) aur CO

Fuel unburned kyun bachta hai?

Wall quenching: thandi walls heat extract karti hain; reactions ek thin quench layer mein freeze ho jaate hain.
Crevices: fuel gaps mein chhupta hai (jaise piston ring crevice) jo flame ke liye bahut narrow hain.
Over-lean (flame-out) ya over-rich local mixtures: flammability limits se bahar.
Low temperature: $CO \to CO_{2}$ ko OH aur time chahiye; agar gases jaldi thandi ho jaayein, CO freeze ho jaata hai.

4. Master trade-off (THE 80/20 idea)

Figure — Pollutants — NOₓ, soot, unburned hydrocarbons

Common mistakes (Steel-man + fix)

Flashcards

NOₓ mein kaun se do species hote hain?

NO aur NO₂.

Thermal NOₓ mein nitrogen kahan se aata hai?

Air ke N₂ se, fuel se nahi.

NOₓ formation ke teen routes kaun se hain?

Thermal (Zeldovich), Prompt (Fenimore), Fuel NOₓ.

Thermal NOₓ ka rate-limiting step kya hai?

O + N₂ → NO + N (strong N≡N bond todna).

NOₓ temperature ke saath exponentially sensitive kyun hai?

Rate-limiting step ka activation energy bahut bada hai (~319 kJ/mol), toh k₁ ∝ exp(−Eₐ/RT).

2000 K ke paas thermal NOₓ double karne ke liye roughly kitna T rise chahiye?

Lagbhag 70 K rise (~319 kJ/mol activation energy ki wajah se).

d[NO]/dt mein factor 2 kahan se aata hai aur kyun?

Har O+N₂ event ultimately 2 NO molecules banata hai (ek directly, ek bache hue N se via N+O₂→NO+O).

Soot kis condition mein banta hai?

Hot AUR fuel-rich (oxygen-starved) zones mein.

Soot ka key precursor molecule kya hai?

Acetylene, C₂H₂.

HACA ka full form kya hai?

H-Abstraction, C₂H₂-Addition — PAH/soot growth mechanism.

Net soot = ?

Formation minus oxidation (soot jal sakta hai agar O₂/OH se high T par mile).

Unburned hydrocarbons ke do main causes kya hain?

Wall/quench-layer cooling aur crevices (plus over-lean/over-rich local mixtures).

CO burnout mein kaun si single reaction dominate karti hai?

CO + OH → CO₂ + H.

Peak temperature kam karne se CO aur UHC kyun badhte hain?

CO→CO₂ ka burnout aur fuel oxidation finish karna slow hai aur hot gas + radicals chahiye; cooling unhe freeze kar deti hai.

φ axis par NOₓ kahan peak karta hai?

Stoichiometric se thoda lean par (sabse high T aur O bhi kaafi).

φ axis par soot kahan dikhta hai?

Sirf rich side par (φ > 1), richness ke saath badhta hai.

Pollutant trade-off ek line mein?

Temperature kam karne se NOₓ ghatta hai lekin CO/UHC badhta hai; dono ko ek saath minimise nahi kar sakte.

Recall Feynman: 12-saal ke bachche ko samjhao

Ek campfire imagine karo. Agar aag super hot hai, toh air bhi (jo normally nahi jalti) "cook" ho jaati hai aur ek badbu-daari gas banati hai — yahi NOₓ hai. Agar tum bahut zyada lakdi daalo aur kaafi hawa nahi hoti, toh kaala dhuaan nikalta hai — yahi soot hai, carbon clump ho raha hai kyunki isse hawa nahi mil rahi. Agar aag ka koi hissa bahut thanda hai (jaise thande bartan ke paas), toh kuch lakdi ka dhuaan bina jale nikal jaata hai — yahi unburned fuel aur CO hai. Toh: bahut hot → NOₓ, bahut zyada fuel → soot, bahut thanda → dhuaan. Trick hai "bilkul sahi" wali middle ground dhundhna.

Connections

Adiabatic Flame Temperature — peak T set karta hai jo thermal NOₓ drive karta hai.
Equivalence Ratio and Flammability Limits — teeno pollutants ke peeche wala φ axis.
Arrhenius Equation and Activation Energy — kyun NOₓ T ke saath exp-sensitive hai.
Lean Premixed Combustion & Staging — NOₓ ke liye engineering fix.
Diffusion vs Premixed Flames — diffusion flames internally φ≈1 par baithe hain → zyada soot.
CO Oxidation and Chemical Kinetics — CO+OH burnout.
Quenching and Wall Heat Transfer — UHC ka origin.