Fuel Flow Calculator — Gas Flow from Output & Heat Rate

Net output (MW)

Heat rate (kJ/kWh, LHV basis)

Fuel LHV (MJ/kg)

Gas specific gravity (air = 1)

Quantity	Value
Fuel thermal power input	–
Implied efficiency	–
Fuel mass flow	–
Fuel mass flow	–
Fuel volume flow (normal m3, 0 °C)	–
Fuel volume flow	–
Fuel energy flow	–

The Calculation, Explained

The derivation

Everything follows from one balance: fuel energy in = electrical energy out / efficiency, and heat rate is efficiency in disguise.

Fuel thermal power (MW) = P_net × HR / 3,600

Mass flow (kg/s) = Fuel thermal power (kJ/s) / LHV (kJ/kg)

Volume flow needs the gas density at the reference state. This calculator uses normal conditions (0 °C, 1.01325 bar), where air density is 1.293 kg/m³, so ρ_gas = SG × 1.293:

Volume flow (Nm³/h) = Mass flow (kg/h) / (SG × 1.293)

MMSCFD converts the daily volume to cubic feet (1 ft³ = 0.0283168 m³); MMBTU/h converts thermal power at 1 MW = 3.41214 MMBTU/h.

Assumptions and applicability

Normal vs standard — read this before comparing with a bill. Gas is also commonly metered at 15 °C ("standard" m³, Sm³) or 60 °F. The same gas occupies 5.4% more volume at 15 °C than at 0 °C. A flow that "disagrees" with the fiscal meter by ~5% is often just two different reference temperatures arguing with each other.
Ideal-gas behaviour is assumed in the density conversion — adequate at near-atmospheric reference conditions, which is where reference volumes are defined.
The heat rate and the LHV must be on the same basis (both LHV here). An HHV heat rate with an LHV heating value silently invents or destroys ~10% of the fuel.
Steady operation at the stated output; start-up, shutdown and ramping fuel are outside this balance.

Worked example (the defaults above)

An 850 MW plant at 9,850 kJ/kWh: thermal input = 850 × 9,850 / 3,600 = 2,325.7 MW thermal. At LHV 49.0 MJ/kg, mass flow = 2,325,700 / 49,000 = 47.5 kg/s (170.9 t/h). At SG 0.60 (ρ = 0.776 kg/Nm³): ≈ 220,000 Nm³/h, i.e. ≈ 187 MMSCFD, or 7,936 MMBTU/h. If the station meter reads far from these numbers at this load, either the meter, the heat rate, or the LHV is wrong — and finding out which is exactly what test day is for.

Practitioner's note. Run this calculation before the test, at the expected test load, and write the expected fuel flow on the test log sheet. A meter that disagrees is discovered in the first hour, while it can still be fixed — not in the report, three weeks later.

References

ASME PTC 46 (test boundary and fuel measurement); ISO 6976 (heating values from composition — see the companion Gas Heating Value & Wobbe Index calculator for the LHV and SG of your actual gas).

Related calculators: Gas Heating Value & Wobbe · Heat Rate & Efficiency

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