Every fuel carries a fixed amount of chemical energy per unit mass, released when it burns completely. This heat of combustion, written \( H \), is measured in megajoules per kilogram: oven-dry wood, grass and forest litter sit near \( 18 \) to \( 20 \ \mathrm{MJ/kg} \), while polyurethane foam, polystyrene and other petroleum-derived plastics reach \( 25 \) to \( 40 \ \mathrm{MJ/kg} \). Real fires never achieve the laboratory value, so fire scientists use an effective heat of combustion, lower than the ideal because some fuel leaves as soot and unburned gas rather than as carbon dioxide and water. The rule that makes it usable is that energy released equals mass burned times \( H \): the same hectare releases far more energy carrying ten tonnes of fuel than carrying two. The confusion to clear is that \( H \) is a property of the fuel alone and says nothing about speed; a slow smoulder and a fast flame in the same wood release the same energy per kilogram, just at different rates. After this Concept you can convert a mass of fuel into the energy a fire can release from it.
Heating value is defined as the energy content of a fuel minus the energy retained in the products of its combustion, representing the net heat released when a hydrocarbon fully oxidizes into carbon …