combustion

name	combustion
description	Combustion — stoichiometry, air-fuel ratio, adiabatic flame temperature, products of combustion, incomplete combustion, flame speed, emissions (NOx, CO), fuel properties.
metadata	{"priority":6,"promptSignals":{"phrases":["combustion","stoichiometry","air fuel ratio","adiabatic flame temperature","AFR","equivalence ratio","flue gas","excess air"],"minScore":4}}

Combustion — Complete Skill

Stoichiometry

General Hydrocarbon (CₓHᵧ)

Complete combustion reaction: CₓHᵧ + (x + y/4)(O₂ + 3.76N₂) → x CO₂ + (y/2) H₂O + 3.76(x + y/4)N₂

Oxygen required: n_O₂ = x + y/4 [mol O₂ per mol fuel] Air required: n_air = n_O₂ × (1/0.21) = n_O₂ / 0.21 [mol air, dry basis]

Stoichiometric Air-Fuel Ratio (AFR_s)

AFR_s = n_air × M_air / (n_fuel × M_fuel) [by mass] M_air = 28.97 g/mol

Common fuels:

Fuel	Formula	AFR_s (mass)	LHV [MJ/kg]	HHV [MJ/kg]
Methane (CH₄)	CH₄	17.2	50.0	55.5
Propane (C₃H₈)	C₃H₈	15.7	46.3	50.3
Octane (gasoline)	C₈H₁₈	15.1	44.4	47.9
Diesel (C₁₂H₂₆)	C₁₂H₂₆	14.5	43.4	45.8
Hydrogen (H₂)	H₂	34.3	119.7	141.8
Ethanol (C₂H₅OH)	C₂H₆O	9.0	26.8	29.7
Coal (bituminous)	CHₓ (approx)	11-13	29-34	30-35

HHV = Higher Heating Value (water condenses); LHV = Lower Heating Value (water stays vapor) LHV ≈ HHV - m_water × h_fg [h_fg = 2.44 MJ/kg at 25°C]

Equivalence Ratio φ

φ = (AFR_s) / (AFR_actual) = (m_air/m_fuel)_stoich / (m_air/m_fuel)_actual

φ = 1: stoichiometric (λ = 1 in European notation) φ < 1 (λ > 1): lean (excess air) φ > 1 (λ < 1): rich (fuel-rich, incomplete combustion)

Excess air percentage: EA% = (1/φ - 1) × 100% = (λ - 1) × 100%

Adiabatic Flame Temperature (AFT)

For adiabatic, complete combustion at constant pressure: Q_release = ΔH_combustion = Σ[ṅ_products × (h̄°_f + (h̄_T - h̄_298)) _products] - Σ[ṅ_reactants × h̄°_f]

Iterative solution: assume T_ad → compute enthalpy of products → check energy balance

Approximate formulas: T_ad ≈ T_inlet + (LHV × η_c) / (ṁ_products × c_p,products) c_p,products ≈ 1.15-1.25 kJ/(kg·K) for lean combustion

Typical values at stoichiometric, 25°C reactants: CH₄/air: T_ad ≈ 2230 K (1957°C) H₂/air: T_ad ≈ 2480 K (2207°C) C₃H₈/air: T_ad ≈ 2270 K (1997°C) Gasoline/air: T_ad ≈ 2280 K

Effect of excess air: reduces T_ad (more N₂ absorbs heat) At 20% excess air: T_ad drops ~150-200°C

Products of Combustion

Complete combustion, dry products (Orsat analysis): CO₂%: x/(x + y/4 + 3.76(x + y/4) + excess air terms) (vol%) O₂%: excess O₂ only for lean combustion

Flue gas composition (methane, stoichiometric, vol% dry): CO₂: 9.5%, H₂O: 19% (wet), N₂: 71.5%, O₂: 0%

CO₂ (max, dry) for various fuels: CH₄: CO₂,max = 11.7% C₃H₈: CO₂,max = 13.7% H₂: CO₂,max = 0% (no carbon) Coal: CO₂,max = 18-20%

O₂ measured in flue gas → excess air calculation: EA% = O₂% / (0.21 - O₂%) × 100% [for hydrocarbon fuels, approximate]

Incomplete Combustion

CO formation (rich mixture or quenching): CO + ½O₂ → CO₂ (fast at high T, quenched at low T) At T < 1000 K: CO freezes (slow kinetics → CO emission)

Rich mixture combustion products (equilibrium): CO₂, H₂O, CO, H₂, OH, O, H, N₂, NO at equilibrium Minimize Gibbs energy to find equilibrium composition

Equivalence ratio corrections: φ > 1: CO increases sharply (rich cutoff at φ ≈ 1.15 for stability) φ < 0.7: combustion quenches (lean blow-off) Stability range: φ = 0.7-1.4 for most flames (wider for stirred reactors)

Flame Speed

Laminar flame speed S_L: CH₄/air: S_L,max ≈ 0.40 m/s (at φ ≈ 1.05) H₂/air: S_L,max ≈ 3.0 m/s C₃H₈/air: S_L,max ≈ 0.45 m/s

Turbulent flame speed: S_T/S_L ≈ 1 + (u'/S_L)^0.7 (Peters correlation) S_T increases with turbulence intensity u'

Autoignition temperature (AIT): CH₄: 580°C, H₂: 500-571°C, Gasoline: 257-280°C Flash point (liquid fuels): min T for ignitable vapor mix (gasoline: -43°C)

Emissions

NOx Formation (Zeldovich mechanism): Thermal NOx: N₂ + O ↔ NO + N; N + O₂ ↔ NO + O Exponential in temperature: T_flame > 1800 K → rapid thermal NOx Control: reduce T (lean combustion, EGR, water injection), staged combustion

NOx reduction: Lean premix combustion (GTs): T_flame < 1700 K → <25 ppm NOx SCR (Selective Catalytic Reduction): NH₃ + NOx → N₂ + H₂O (95% reduction) EGR (Exhaust Gas Recirculation): dilutes charge, reduces O₂ concentration

Particulate/Soot: Rich combustion + high C/H fuel → soot formation Soot oxidizes at T > 1600 K (if O₂ present) DPF (Diesel Particulate Filter) + DOC (Diesel Oxidation Catalyst)

Output

Provide: stoichiometric AFR, equivalence ratio φ, T_adiabatic [K], product composition (CO₂%, O₂%, N₂%), fuel mass flow ṁ [kg/s], heat release Q [MW].