1. Definitions

• Higher heating value $Q_{HHV}$: water in combustion products is in liquid (condensed) form, including the latent heat of water vapor condensation.
• Lower heating value $Q_{LHV}$: water in combustion products stays as vapor, so condensation heat is not recovered (more common for industrial kilns).

2. Conversion core: latent heat of water vapor
   Hydrogen in the fuel forms water: each $1\ \text{kg H}$ produces $9\ \text{kg H}_2O$. If the fuel itself contains moisture $W$ (kg/kg fuel), total water vapor released is approximately $9H + W$.

Common engineering conversion (approx. on a $25^\circ C$ basis):

• $Q_{LHV} \approx Q_{HHV} - r\,(9H+W)$
  Where $r$ is the latent heat of vaporization of water at the reference temperature, commonly taken as:
• $r \approx 2.44\ \text{MJ/kg}$ (near $25^\circ C$)