THE 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 hotplate 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.
Please see the section on page 10 on seasoning and storing wood, which can help to minimise these
problems.
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.
The resulting residue that sticks to the inner walls of the chimney is called creosote. Creosote is formed
by unburned, flammable particulates present in the smoke. It is black or brown in appearance. It can be
crusty and flaky, tar-like, drippy and sticky or shiny and hardened. Quite often, all forms will occur in one
chimney system (see an extreme example of this on page 3).
If the wood you are using is water logged, or green, the fire will tend to smoulder and not warm the chimney
sufficiently. Wet wood causes the whole system to be cool, and inefficient. In contrast: dry wood means a
hot fire, which results in a hot flue, and a hot flue means much less creosote clogging up your chimney.
The Woodfired cooker’s firebox is designed to operate at very high temperatures to burn the gases and
particles released from the wood, which means less air-pollution.
The air supply to the afterburner in your cooker is always open but works most effectively in conjunction
with a hot fire. We strongly recommend running the cooker up to a high temperature at least once per day.
Charcoal
- When most of the tar and gasses have burned the remaining substance is charcoal (ash in it’s
finer form). A hot bed of charcoals and ash can enhance the combustion process when burning larger
pieces of wood. Start with a small fire to develop a bed of glowing embers. As the charcoal bed develops
and the cooker heats up, slowly add larger and larger pieces of wood. It takes time to build a good
charcoal bed, but it is well worth the effort. only empty excess ash periodically and always leave a bed of
ash on which to light the next fire.
WHAT TYPE OF WOOD IS BEST
The difference between 'hard' and 'soft' woods is the density of their cells or fibres. The harder a wood, the
greater the density and quantity of fibres in any square inch of that wood.
As a general rule, the deciduous trees (those that lose their leaves in the autumn) are usually thought of
as hardwoods and the evergreen trees (such as pines, firs and larches) as the softwoods. But
generalisations are of course always subject to many exceptions. Some evergreens may well be harder
than some deciduous trees. Birch, for example, is not very hard at all. So we should understand that there
is a whole range of densities amongst our tree species, including medium dense woods, which cannot be
satisfactorily classed as hard or soft.
For construction purposes and different wood working projects, the shape and arrangement of the
individual cells and its growth patterns are also important considerations, but when judging the value of a
bit of wood for its use as a firewood, we look especially at the density of fibres.
One of the reason for this is that firewood tends to be sold by volume rather than weight. Assuming that
the wood is reasonably dry, the weight of a square foot of good hardwood may be double of that of a
square foot of softwood. This means that the same volume of hardwood will provide you with more fuel to
burn than an equal amount of softwood, simply because it contains more substance.
Yet the price of that hardwood will normally not be double that of softwood, because it took the same
amount of labour to prepare. So, if a trailer full of softwood costs £100 and the same size trailer full of
hardwood costs £120, the more expensive option may well be the most economical.
The other advantage of good hard firewoods are that the cooker does not need to be fed as often and the
charcoal-beds made by the glowing wood may burn more easily overnight.
However, the ideal situation would be to have a store of both hard and soft woods, because the softer
woods also have distinct advantages. They light more easily than the slower burning hardwoods and if the
softwoods are dry, they create a hotter, more intense fire. The draught created by the hotter fire moves the
air up the chimney faster.
After reading the notes opposite about the burning process, you will understand that means less pollution
in the form of smoke and less creosote condensation in your chimney.
The denser hardwoods tend to smoulder more easily when the fire is lit first, so their flue gas temperature
will be much cooler.
Because softwoods like pine and larch contain a lot of resins and pitch, a popular misconception is that
CHOOSING THE RIGHT WOOD & WOOD BURNING GUIDELINES
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