TYPES OF FUEL
SEASONING AND STORING FIREWOOD
Wood, which has recently been cut and is still full of sap and water is known as "green" wood.
Greenwood will generally burn poorly and inefficiently, because it can have over 50% water in its cells.
It may be hard to light, smoulder, not put out any heat and cause more than the usual amount of
creosote to build up in your chimney.
So your aim should be to dry the wood out to below 25% moisture content, this process is called
seasoning. As the name im-plies, you should store your wood for a season or so, while it dries, but there
are things you can do to speed up seasoning by cutting the wood now rather than just before you use it.
Wood is composed of bundles of microscopic tubes that were used to transport water from the roots of
the tree to the leaves. These tubes will stay full of water for years even after a tree is dead. This is why it
is so important to have your firewood cut to length for 6 months or more before you burn it, it gives this
water a chance to evaporate since the tube ends are finally open and the water only has to migrate a
foot or two to escape. Splitting the wood helps too by exposing more surface area to the sun and wind,
but cutting the wood to shorter lengths is of primary importance.
Here’s how you can tell whether your wood is ready or not: Well seasoned firewood generally has
darkened ends with cracks or splits visible, it is relatively lightweight, and makes a clear "clunk" when
two pieces are beat together. Green wood on the other hand is very heavy, the ends look fresher, and it
tends to make a dull "thud" when struck.
Another thing you can do to help is store your wood properly. Store if off the ground by building the pile on
some longer logs (or whatever method you can devise). A shed or shelter with an open side makes an ideal
storage place, as the air can circulate around the logs and help to dry them out. Unventilated spaces or
plastic tarps, which never get taken off will prevent the drying and evaporation process and cause moulds
and rot. So, if a tarp is your only option, take it off frequently to air the wood on fine days. And remember to
put it back on again. Seasoned firewood will reabsorb large amounts of water if exposed to rain, snow and
excessive dew, which is liable to make it rot and be unfit for making a good fire.
When you build up a store of firewood, remember that the wood may start to deteriorate after 4 to 5
years, although this is of course variable and depending on storage conditions and species involved.
6
THE WOOD BURNING PROCESS
Understanding what happens when wood is burnt will enable you to burn wood in a more environmentally
friendly way, reduce the maintenance required for your chimney and get more out of your wood. There are
3 stages in the wood-burning process:
Evaporation
When you light the cooker a lot of energy will be needed at first to boil away any moisture, which is left
in the wood. Using energy to drive off excess water in firewood robs the cooker of energy needed for an
efficient and clean burn. Also, much of the energy wasted in evaporating water is energy that could have
heated the hot-plate and ovens. This is a waste of wood, money and effort. The presence of all that
moisture tends to keep "putting out" the fire, and therefore making it burn very poorly, which tends to
produce a lot of creosote and pollution.
Emissions
As the heat of the fire intensifies, waste-gases (smoke) are released from the wood.
Unburned smoke is emitted into the air either as pollution, or condensed in the chimney causing
creosote build-up. It takes time for the air in your chimney to heat up. When it is still cold you get an
effect similar to the condensation of hot breath on a colder window or mirror. So when the by-products
of combustion (smoke in the form of gases) exit the cooker, and flow up into the relatively cooler
chimney, condensation occurs.
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