The intensity of the heat output (calorific value) from burning different materials varies. For instance, different species of wood burn at different temperatures changing the calorific value or heat output.
Although both hardwoods, Ash wood burns at a much higher temperature than Alder wood.
Straw has a lesser calorific value than wood if compressed into a briquette. Coal on the other hand has an increased calorific value and in some cases can burn with a very intense heat. Over the years, a blacksmith chooses coal as a burning fuel because it burns very hot and will emit heat for much longer than wood.
The drier the fuel, the better the calorific value. Fuel should ideally have less than 20% humidity (water) content.