Fuel & Air Inputs
Examples
What the Calculator Does
- Accepts a fuel in empirical form (e.g., CxHyOz) or as a named common fuel.
- Computes stoichiometric oxygen demand, required dry air, and the stoichiometric air–fuel ratio (AFR) by mass and by moles.
- Allows specification of excess air (λ) or equivalence ratio (ϕ) and returns the fully balanced products including any residual O2.
- Estimates dry and wet flue‑gas composition (volume %) and basic performance checks such as carbon and oxygen balances.
Core Chemistry and Balancing
For a generic hydrocarbon with oxygen, written as CxHyOz, the complete, ideal combustion with dry air is:
CxHyOz + a (O2 + 3.761 N2) → x CO2 + (y/2) H2O + a·3.761 N2
The required moles of O2 for complete combustion are:
Oxygen demand: astoich = x + y/4 − z/2
Assuming air contains 21% O2 (molar) and 79% N2, the accompanying nitrogen is 3.761 moles N2 per mole O2. When the process uses excess air, λ = actual O2 supplied / astoich, the balanced reaction becomes:
CxHyOz + λ·a_st (O2 + 3.761 N2) → x CO2 + (y/2) H2O + (λ·a_st − a_st) O2 + λ·a_st·3.761 N2
By mass, the stoichiometric air–fuel ratio is AFRst = mair,st / mfuel. For gases and vapors at standard conditions, calculators typically report both mass‑ and mole‑based AFRs.
Key Inputs and Outputs
| Input | Description | Typical Range |
|---|---|---|
| Fuel formula or name | Empirical composition (x, y, z) or a preset (e.g., methane, propane, gasoline surrogate). | Hydrocarbons and oxygenates |
| Excess air (λ) or ϕ | λ > 1 means extra oxygen is provided; ϕ = 1/λ. | 0.8–2.0 |
| Moisture basis | Dry or wet flue‑gas reporting; water may be included or excluded from percentages. | Dry or Wet |
| Air composition | Default 21% O2, 79% N2; advanced tools allow Ar/CO2 traces. | Fixed or user‑defined |
| Output metrics | AFR, O2 demand, flue‑gas composition, elemental balances, and sometimes adiabatic flame temperature. | Computed |
Reference Values for Common Fuels
These representative values help you sanity‑check results. Use them as indicative references; exact values depend on composition.
| Fuel | Empirical Formula | Stoich AFR (mass) | Higher Heating Value (MJ/kg) |
|---|---|---|---|
| Methane | CH4 | ~17.2 | ~55.5 |
| Propane | C3H8 | ~15.7 | ~50.4 |
| Gasoline (typ.) | CH1.87 | ~14.7 | ~46.4 |
| Diesel (typ.) | C12H23 | ~14.5 | ~45.5 |
| Ethanol | C2H6O | ~9.0 | ~29.7 |
| Hydrogen | H2 | ~34.3 | ~141.9 |
Worked Example: Propane with 10% Excess Air
Given fuel C3H8 and λ = 1.10.
- Oxygen demand: ast = x + y/4 − z/2 = 3 + 8/4 − 0 = 5 mol O2 per mol fuel.
- Actual O2 supplied: λ·ast = 1.1 × 5 = 5.5 mol.
- N2 accompanying air: 5.5 × 3.761 = 20.686 mol.
- Products: 3 CO2 + 4 H2O + 0.5 O2 + 20.686 N2 (ideal, complete burn).
| Quantity | Value | Notes |
|---|---|---|
| Stoich O2 (mol/mol fuel) | 5.000 | From ast formula |
| Air moles (stoich) | 5 × (1 + 3.761) = 23.805 | O2 + N2 components |
| AFRst (mass) | ≈ 15.6 | Using MO2=32, MN2=28.013, Mfuel=44.10 |
| AFR at 10% excess | ≈ 17.2 | AFR = λ × AFRst |
| Flue‑gas (dry, vol%) | CO2 ≈ 12.7%, O2 ≈ 1.8%, N2 ≈ 85.5% | Excluding water vapor |
How to Use the Calculator Effectively
- Define the fuel: Enter a measured or assumed empirical formula. For blends, the calculator may accept a molar mix and compute an equivalent formula.
- Choose λ or ϕ: For burners and engines operated lean of stoichiometric, specify λ > 1; for rich mixtures, use ϕ > 1 (i.e., λ < 1).
- Select dry or wet basis: Decide whether you want flue‑gas fractions reported including water.
- Set air composition: Use defaults unless high‑altitude or oxygen‑enriched conditions apply.
- Review balances: Confirm that carbon, hydrogen, and oxygen balances close within numerical tolerance.
Energy, Temperature, and Efficiency (Overview)
While a stoichiometric calculator balances atoms, practical design also considers heat release. Using a higher heating value (HHV) or lower heating value (LHV) paired with inlet and exhaust temperatures, the tool can estimate an adiabatic flame temperature by equating reaction enthalpy to sensible heats of products. Real devices exhibit losses and dissociation at high temperatures, so adiabatic estimates tend to be upper bounds. If the calculator provides both HHV and LHV, choose the basis that matches your test data and reporting standard.
Validation and Quality Checks
- Carbon balance: Total carbon in fuel must equal carbon in CO2 (and CO if modeled).
- Oxygen residual: At λ > 1, the product stream must include free O2; at λ = 1, it should be zero.
- Sensitivity: Small errors in hydrogen count (y) materially change water generation and HHV/LHV comparisons.
- Units: Confirm whether AFR is mass‑ or mole‑based and whether gas fractions are on a dry or wet basis.
Assumptions and Limitations
| Area | Assumption | Implication |
|---|---|---|
| Chemistry | Complete combustion to CO2 and H2O only | Real systems may form CO, NOx, unburned HC; advanced models add these species. |
| Air Model | 21% O2, 79% N2, no water | Humidity slightly reduces available O2 and affects wet‑basis reporting. |
| Thermal | Ideal gas with fixed specific heats | At high T, variable Cp and dissociation reduce adiabatic temperatures. |
| Measurement | Accurate fuel formula or certified composition | Surrogates (e.g., “gasoline”) introduce uncertainty; validate with flue‑gas analyzers. |
Frequently Asked Questions
How does equivalence ratio relate to excess air?
Equivalence ratio ϕ is the ratio of stoichiometric to actual air (or the inverse of λ). Thus, ϕ = 1/λ. A lean burn has λ > 1 and ϕ < 1.
Why do my calculated flue‑gas percentages differ from analyzer readings?
Instruments report either dry or wet basis and include measurement uncertainty. Real combustion can produce CO and NOx, and ambient humidity dilutes the stream. Align the calculator’s basis and species list with the analyzer.
Can the calculator handle oxygenated fuels?
Yes. The oxygen term (z) reduces the external O2 demand via ast = x + y/4 − z/2, which is why alcohols have lower AFRs.
What AFR should I expect for gaseous natural gas?
For methane-dominant gas, AFRst ≈ 17.2 by mass. Mixture variations (ethane/propane content) shift this slightly.
Does altitude matter?
Yes. Lower ambient pressure reduces O2 density, changing volumetric air requirements. The chemical balance is unchanged, but blower sizing may differ.