Fuel Composition Combustion Excess Air Balance Calculator

The Fuel Composition, Combustion and Excess-Air Balance Calculator balances ideal complete combustion for a fuel you describe yourself, and reports the air demand and the product gas that follow from that description.

You enter up to eight components in a grid. Each row carries a label, a composition percentage, a molecular weight, and the carbon, hydrogen, oxygen, nitrogen and sulfur atom counts for that component. The percentages are read as mole percent or as mass percent, and the active rows must total 100. From that grid the calculator sums the five element amounts, works out the stoichiometric oxygen requirement, and assigns every atom to a complete-combustion product: CO2, H2O, SO2, residual O2 and an N2-equivalent inert.

Excess air arrives one of two ways, chosen by a mode switch. Either you enter the excess air percentage directly, or you enter a measured dry product-gas oxygen reading and the model solves the same dry-product balance for the excess-air ratio lambda. Both paths feed the same twenty outputs, from stoichiometric and actual air through the five product amounts to the dry and wet totals, the dry O2 and dry CO2 percentages, and an element-balance closure residual. It works in kilograms and kilomoles, or in the matched pound and pound-mole display.

Combustion is assumed complete. There is no carbon monoxide, no unburned hydrocarbon, no NOx, no dissociation, no ash and no condensate anywhere in this model, so what it reports is the ideal product gas, not the one an analyser on a real stack will see.

Purpose and model boundary

This model converts an eight-row, user-authored fuel composition into an ideal complete-combustion atom balance. It reports stoichiometric and actual combustion air, ideal CO2, H2O, SO2, residual O2 and N2-equivalent products, dry-product concentrations, and an element-balance closure check. Excess air can be entered directly or inferred from measured dry O2.

It is a stoichiometric screening calculation. It does not calculate heating value, flame temperature, burner efficiency, equilibrium, dissociation, kinetics, CO, NOx, unburned fuel, ash, condensate, emission factors, permit limits, or compliance.

Inputs and units

The grid contains eight fixed component or pseudo-component rows. Each row supplies a label, composition percent, user molecular weight, and nonnegative C, H, O, N, and S atom coefficients. A zero-percent row is inactive; a positive-percent row must be complete. Composition basis is mole percent or mass percent, and all eight percentages must total 100.

Fuel mass is displayed in kg or lb. Combustion-air O2 and the specified or measured dry O2 are percentages. Air molecular weight is kg/kmol in SI or the numerically corresponding lb/lbmol convention in U.S. customary units. Amount outputs are kmol in SI and lbmol in U.S. customary display. The workbook uses 1 lb = 0.45359237 kg for physical parity.

Governing relationships

Let p_i be component percent, M_i its molecular weight, and c_i, h_i, o_i, n_i, and s_i its atom coefficients. For mole-percent input:

M_fuel = sum((p_i / 100) × M_i)

N_i = (m_fuel / M_fuel) × (p_i / 100)

For mass-percent input:

M_fuel = 1 / sum((p_i / 100) / M_i)

N_i = m_fuel × (p_i / 100) / M_i

The elemental atom amounts are C = sum(N_i c_i), H = sum(N_i h_i), O = sum(N_i o_i), N = sum(N_i n_i), and S = sum(N_i s_i). Stoichiometric oxygen and air are:

O2_st = C + H/4 + S - O/2

Air_st = O2_st / x_O2,air

where x_O2,air is entered air O2 divided by 100. In specified-excess-air mode, lambda = 1 + EA/100. In dry-O2 inference mode, with measured dry fraction y_O2:

lambda = [O2_st + y_O2 × (C + S + N/2 - O2_st)] / [O2_st × (1 - y_O2/x_O2,air)]

Actual air is lambda × Air_st; actual air mass is actual air amount times entered air molecular weight. Ideal products are:

CO2 = C

H2O = H/2

SO2 = S

Residual O2 = (lambda - 1) × O2_st

N2 equivalent = N/2 + lambda × O2_st × (1 - x_O2,air) / x_O2,air

Dry product is the sum of CO2, SO2, residual O2, and N2-equivalent. Wet product adds H2O. Dry O2 and CO2 percentages divide the corresponding product by dry-product total and multiply by 100.

Calculation sequence

1. Validate the selectors, visible scalar inputs, all eight grid rows, the 100-percent composition sum, and positive stoichiometric demand.
2. Convert the entered fuel mass to kilograms internally and convert each active row to component amount according to the selected composition basis.
3. Sum the five elemental atom amounts and calculate stoichiometric oxygen and air.
4. Determine lambda from specified excess air or the explicit dry-O2 relationship.
5. Calculate actual air, air mass, air-to-fuel ratio, ideal products, dry/wet totals, and dry concentrations.
6. Calculate the greatest absolute C/H/O/N/S closure residual and apply the derived numeric gate.
7. Convert public results to the selected display units and evaluate status in the order below.

Outputs and interpretation

Primary results report stoichiometric oxygen and air, actual air amount and mass, air-to-fuel mass ratio, excess-air ratio and percent, and dry O2. Supporting results show composition total, calculated fuel-mixture molecular weight, each ideal product, dry and wet totals, dry CO2, and the maximum element residual. The bar chart compares the workbook-calculated ideal product amounts; it is not a measured flue-gas analysis.

CB_Element_Balance_Max_Residual is a numerical closure diagnostic. A small residual confirms the implemented ideal atom assignment, not the validity of the user's chemical identities or operating assumptions.

Validation and status logic

The workbook evaluates status in this order:

The input gate requires listed modes; fuel mass greater than zero; air O2 greater than zero and below 100%; positive air molecular weight; nonnegative visible specified excess air; and visible measured dry O2 at least zero but below combustion-air O2. Exactly eight numeric percentage cells are required, at least one must be positive, and their sum must be within 0.0000001 of 100. Each positive row requires a trimmed label from 1 through 80 characters, positive molecular weight, and nonnegative atom coefficients. O2_st must be positive.

The derived gate requires lambda >= 1, positive air and dry/wet product totals, nonnegative products, dry O2 below entered air O2, dry CO2 from 0 through 100%, and element residual no greater than 1e-9 × max(1, wet product).

Assumptions and limitations

• Combustion is ideal and complete: carbon becomes CO2, hydrogen H2O, sulfur SO2, and fuel nitrogen N2-equivalent.
• The non-O2 balance of combustion air is represented as N2-equivalent inert. Air molecular weight and O2 content are user inputs.
• Labels, molecular weights, and atom coefficients are independent user data; the workbook does not identify or validate a chemical species.
• Dry-O2 inference assumes the same ideal product set, no air leakage, and a dry analyzer basis.
• All shipped values are synthetic examples. No fuel-property, atomic-weight, air-composition, emissions-factor, or regulatory table is embedded.
• Air leakage, wet measurements, analyzer bias, incomplete combustion, pollutant formation, equilibrium, phase change, and heat release require other models.

Restrictions and non-computing states

This calculator permits only the declared unit, composition, and excess-air modes and validates scalar/grid minima. The workbook supplies the stricter relational rules: a 100-percent composition, at least one active row, positive net stoichiometric oxygen, and measured dry O2 below entered air O2. Values hidden by the selected excess-air mode are inert.

Invalid inputs suppress public numeric results. There is no CHECK state: a valid finite ideal balance returns OK, while any input or derived-domain failure returns NOT VALID.

Errors and warnings

A rejected entry means the submitted values did not match the published input rules and no workbook conclusion was produced. Workbook NOT VALID identifies either an input-domain failure or a derived numeric/closure failure using the exact messages above. A connection or calculation-service failure is a service error, not a combustion result. OK does not certify safe combustion, equipment suitability, emissions, or regulatory compliance.

No fuel property table, ambient-air composition, emission factor or regulatory limit is embedded here. Every molecular weight, atom count and air property in the balance is one you enter, and the shipped defaults are synthetic values matching no real fuel or air. The workbook implements ordinary textbook stoichiometry of complete combustion. The sources below are cited for the relations and terminology; no prose, table, figure or dataset is reproduced.

• NIST, Users Guide to POST Plant Operations Simulation Template, NBSIR 88-3740, for the C/H/O/N/S atomic-ratio basis and the stoichiometric oxygen coefficient this workbook generalises as carbon plus hydrogen/4 plus sulfur minus oxygen/2. https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nbsir88-3740.pdf
• US Department of Energy, Operations and Maintenance Best Practices Guide, Release 3.0, for the general definition of excess air and the caution that a theoretical balance is not a real-combustion model. https://www.energy.gov/sites/default/files/2020/04/f74/omguide_complete_w-eo-disclaimer.pdf
• US EPA, Method 19, for dry-versus-wet product and dry-oxygen terminology only. None of that method's F factors, tables or criteria are implemented. https://www.epa.gov/sites/production/files/2017-08/documents/method_19.pdf
• NIST Handbook 133, Appendix E, for the exact identity 1 avoirdupois pound = 0.45359237 kilogram. https://www.nist.gov/document/14-app-e-14-hb133-finalpdf

Carbon monoxide and unburned species come from measurement; fuel analyses come from a laboratory. Where a result feeds a permit, an inventory or a compliance report, the air quality regulations and reference test methods that apply to your source and jurisdiction govern, and nothing on this page states what any of them require.