Boiler Fuel Use — LHV Basis

Estimate fuel mass from useful boiler duty, operating time and an explicitly LHV-based efficiency.

How this calculation works

This fuel estimate keeps efficiency and heating value on the same lower-heating-value basis. Enter the useful thermal duty delivered by the boiler, its LHV-based efficiency, the fuel's lower heating value in MJ/kg and the operating duration. Dividing useful output by fractional efficiency gives the fuel energy-input rate. A conversion of 3.6 MJ per kWh then turns that power into an hourly mass rate, which is multiplied by operating hours. An HHV or AFUE rating should not be inserted into the LHV efficiency field. Efficiencies above 100% require a matching LHV convention with recovered condensation heat. Constant duty and efficiency are assumed; separate electrical auxiliaries, startup, cycling and standing losses not represented in the chosen efficiency are excluded.

Inputs and units

Useful thermal output (kW)
Thermal efficiency on LHV basis (%): Use efficiency and fuel heating value on the same LHV basis; condensing values may exceed 100%
Fuel lower heating value (MJ/kg)
Operating duration (h)

Method and formula

Fuel-input power = useful duty/(ηLHV/100); fuel mass rate = 3.6 × fuel-input kW/LHV in MJ/kg; fuel mass = rate × hours.

Worked example

Example inputs

Useful thermal output: 100 kW
Thermal efficiency on LHV basis: 90 %
Fuel lower heating value: 42.5 MJ/kg
Operating duration: 8 h

Calculation steps

Convert LHV efficiency to a fraction: 90/100 = 0.90.
Calculate fuel-input power: 100/0.90 = 111.111111 kW on an LHV basis.
Convert to fuel mass rate: 111.111111 × 3.6/42.5 = 9.411765 kg/h, using the unrounded input power.
For 8 h, fuel mass = 9.411764706 × 8 = 75.294118 kg.

Example results

Fuel energy-input rate, LHV: 111.1111111 kW
Fuel mass rate: 9.411764706 kg/h
Fuel mass for period: 75.29411765 kg

Assumptions

User supplies the same lower-heating-value basis for efficiency and fuel energy
Useful duty and efficiency remain constant over the period

Limitations

Do not enter an HHV/AFUE rating into the LHV efficiency field
Excludes separate electrical auxiliaries, startup, cycling and unrepresented standing losses

Sources

U.S. DOE: Steam Generation Efficiency Definition: Slide 11, direct boiler efficiency and fuel flow times heating value
U.S. DOE: Fuel Glossary: Heating value definitions distinguish LHV and HHV