The Wikipedia article on heat of combustion says:
The calorific value [...] may be expressed with the quantities:
- energy/mole of fuel
- energy/mass of fuel
- energy/volume of the fuel
Is the energy obtained by the combustion mainly proportional to the mass of the gas, being then values in $\pu{J/mol}$ and $\pu{J/kg}$ a characteristic of the gas composition and basically independent of the pressure and temperature?
That is, for a specific gas, values in $\pu{J/mol}$ and $\pu{J/kg}$ will be mainly independent of the pressure/temperature of the gas in combustion, while value in $\pu{J/m^3}$ will be strongly dependent due to the relation between volume, pressure and mass.
In the latter case, given the heating value, $f_0$ (in $\pu{J/m^3}$) for a gas at $\pu{°C}$ and pressure $p_0 = \pu{101 kPa} = \pu{1 atm}$, we could (?) find new heating value $f$ (in $\pu{J/m^3}$) at some other temperature $t$ (in $\pu{°C}$) and pressure $p$ (in $\pu{Pa}$) applying the expression: $$f = f' \frac{p}{p_0} \frac{273}{273+t}$$