Fuel Lower Heating Value Adjustment¶
When a user changes the lower heating value (LHV) of a gas fuel in the UI, OST does not simply store the entered value. Instead, it recalculates the gas composition so that the resulting mixture produces the desired LHV. This ensures that all other combustion properties derived from the composition — such as density, higher heating value (HHV), flue gas volumes, and NOx calculations — remain physically consistent.
How the recalculation works¶
The recalculation is performed by adjusting the volumetric fractions (vol-%) of the gas components in the fuel mixture. The algorithm works differently depending on the fuel sub-type.
Natural gas¶
Natural gas consists mainly of methane (CH₄) with smaller amounts of nitrogen (N₂) and heavier hydrocarbons (ethane C₂H₆, propane C₃H₈, n-butane C₄H₁₀).
To increase the LHV: The methane fraction is raised and the nitrogen fraction is lowered. If the target LHV cannot be reached even when methane is at its maximum (i.e. all the nitrogen has been displaced), the algorithm begins substituting methane with heavier hydrocarbons (ethane, propane, n-butane), which have higher heating values.
To decrease the LHV: The methane fraction is lowered and the nitrogen fraction is raised.
Biogas¶
Biogas consists mainly of methane (CH₄) and carbon dioxide (CO₂).
To increase the LHV: The methane fraction is raised and the carbon dioxide fraction is lowered. If the target LHV cannot be reached at maximum methane, heavier hydrocarbons are introduced in the same manner as for natural gas.
To decrease the LHV: The methane fraction is lowered and the carbon dioxide fraction is raised.
LPG¶
LPG consists mainly of n-butane (C₄H₁₀) and propane (C₃H₈).
To increase the LHV: The n-butane fraction is raised and the propane fraction is lowered.
To decrease the LHV: The n-butane fraction is lowered and the propane fraction is raised.
Numerical solving¶
The composition adjustment is not a simple linear calculation, because the relationship between composition and heating value is non-linear when multiple components are involved. OST uses a root-finding algorithm (Brent’s method) to find the primary component fraction at which the computed LHV of the mixture equals the user’s target value.
The algorithm iterates by:
Proposing a new fraction for the primary component (e.g. CH₄ for natural gas).
Setting the secondary component fraction to fill the remaining share (e.g. N₂ = original combined share − new CH₄ share).
Recalculating the LHV of the resulting mixture as the weighted sum of each component’s individual heating value.
Comparing the result to the target and adjusting the primary component fraction until the difference is within tolerance.
Note
The solver tolerance is 0.1 MJ/nm³ (absolute) and 0.1 % (relative). If the target LHV is outside the physically achievable range for the selected fuel sub-type, the composition is clamped to the nearest feasible boundary.
After convergence, the full fuel state is recalculated: LHV, HHV, and density are all recomputed from the new composition so that every downstream calculation uses a consistent set of fuel properties.
Higher heating value adjustment¶
The same mechanism is used when the user changes the higher heating value (HHV) instead of the LHV. The only difference is that the solver targets the HHV of the resulting mixture rather than the LHV. All other steps are identical.
Heating value per mass¶
The user can also enter the LHV or HHV on a mass basis (MJ/kg). In that case, the entered value is first converted to a volumetric basis (MJ/nm³) using the current normal density of the fuel before the composition recalculation is performed. The normal density itself is also recalculated after convergence, so the per-mass values displayed in the UI remain consistent with the updated composition.