All types of wood can be used as firewood but some are better than others. Besides investigating how much heat is produced you also need to consider the aroma of the wood. Most wood produces lovely aromas, with fruity, nutty or piney fragrances. But several woods produce aromas that smell bad and should be avoided when looking for firewood.
Fresh Red Oak
Red oak is normally a very good firewood, but fresh red oak smells like vinegar when it burns. This is because fresh red oak contains a lot of tannic acid.
Elm is typically stinky when it burns because of disease and a beetle that lives in elm bark. If you can find elm without disease, then it can be used as firewood. However elm is very difficult to split and takes a very long time to season. Elm also can absorb a lot of water which results in a lot of smoke when burning.
Eucalyptus wood produces a medicinal like smell that most individuals find unpleasant. This aroma is produced by the gums that are found in the wood. But some individuals enjoy the smell of burning eucalyptus and it is a favorite for smoking meat and cooking.
The smoke of walnut is very bitter and walnut wood should not be burnt on its own. To counteract this bitterness you should mix the walnut with other more pleasant smelling wood.