1. Fuel - wood and its calorific value.

The heat locked up in a fuel is expressed as its 'calorific value.' To measure the calorific value of wood in a laboratory, you take a small sample and oven dry it until all moisture has been driven off. Then you weigh it carefully and place the sample in a device called a 'bomb calorimeter' which enables you to burn the sample completely while measuring the amount of heat produced. With wood, the number arrived at is likely to be around 8,600 BThUs.
XXThis is all very academic and bears little relation to what goes on when you throw a log on the fire! But the results are of interest nevertheless in that they show show us that there is close correspondence between the calorific values of all types of wood fibre. In other words you can take sap wood or heart wood from oak, spruce, beech, rhododendron, balsa, teak, pear tree – pretty well any wood you can lay your hands on – and they all turn out to match each other quite closely in terms of calorific value.
XXNOTE - wood and peat are both famous for giving you "two heats" – the first when you harvest and render them ready for use and the second when you burn them! Although different species of tree may deliver equal calorific values, some will always be much more cost-effective to process than others because they are much easier to saw and split.