The chemistry of wood burning is a complicated subject in
itself, but in general the hydrocarbon molecules of the wood
fibers combine with oxygen from the available combustion air. The
hydrogen atoms combine with oxygen to produce water vapor. The
carbon atoms combine with the oxygen to form carbon monoxide
and/or eventually carbon dioxide. See other entries for description
of the carbon reactions.
In any case, all three of these chemical reactions are
exothermic and so give off more energy (heat and light) than
they use. This is why burning hydrocarbon fuels (wood, coal,
oil, gas, coke, peat, etc) gives off heat.
Wood contains about 6000 usable Btu/pound of latent heat.
(See "Amount of heat in wood" #311). This energy can only be
liberated at an interface between wood and air, i.e. at the
surface of the wood.
If ten pounds of wood is a single log (about 6" diameter
and 20" long), its surface area is about 3 square feet. If
the same wood is split in tiny pieces, it could have twenty times
the surface area. The second situation would burn (oxidize) at
potentially twenty times the rate, and therefore could produce
twenty times the heat output (for one twentieth as long). It would
not actually burn twenty times as fast because oxygen molecules
would not always be where they needed to be at the right time,
but fifteen times as fast is realistic.
There are some secondary considerations. Because fifteen
times the oxygen is necessary, then the fire-induced draft will
have fifteen times the velocity. This rapid draft shortens the
time that oxygen molecules are near the complex hydrocarbon molecules.
Some of the volatile gases released from the wood may not have
enough time to completely oxidize, so partially oxidized
hydrocarbon gases can escape the fire vicinity. In general, these
gases are called creosote.
The term creosote is somewhat generic. It refers to about 160
complex hydrocarbon chemicals which are gaseous at elevated
temperatures and solid at room temperature. If air supply is
restricted, as in an air-tight stove, the problem is exascerbated,
and huge amounts of creosote can be released which represents much
pollution and wasted fuel energy.
There is a simplification concept which says that three T's
are necessary for adequate combustion. Time, Temperature,
Assuming adequate air supply, if there is high enough
Temperature (over 1100°F) for all the necessary reactions
to occur; adequate Time to allow those reaction sequences to
occur; and enough Turbulence to ensure oxygen availability
when and where necessary; then proper and complete combustion
will occur. Virtually no wasted fuel and no pollution will be sent
out the chimney.
Air-tight stoves usually fulfill these requirement, but they do
not allow enough oxygen to burn completely. Therefore, they later
have to attempt secondary combustion with either a catalytic
combustor or by introducing secondary combustion air.
Both solutions can be very effective under some conditions, but
often fall far short of expectations under conditions found in
actual residential usage.
JUCA uses a design based on natural burning of large
diameter logs. By having relatively unlimited oxygen available,
JUCAs allow the possibility of complete combustion.
Large logs burn relatively leisurely, so therefore the amount of
oxygen used is minimal. Therefore, the flue gas velocity is slow,
allowing plenty of Time for the gases in the flame tips,
where the Temperature is around 2300°F, which is high
enough for even the complex hydrocarbons to break down into
Turbulence in natural burning fires is moderate, but
since so much oxygen is available, oxygen is nearly always
available for complete combustion.
Therefore, the JUCA accomplishes extremely high
The next consideration is the velocity of flue gases through
the wood burner. If a kindling (hot) fire is used, the rapid
burning creates much flue gas and high resultant velocity. This
causes the hot flue gas to whiz through the wood burner.
Therefore, heat transfer efficiency suffers.
If a JUCA uses full logs, it burns leisurely and therefore
flue gas velocity is low. The hot smoke spends more time in the
heat exchanger areas in the upper part of a JUCA, and
therefore gives up the highest possible amount of its heat. Therefore,
heat transfer efficiency is extremely high.
A final area to consider is the maintenance of adequate
temperature for proper combustion. Thin materials such as paper
have very low ignition temperatures and easily maintain
Wood pieces have a higher ignition temperature and so must be
continually heated by radiation from other burning. Two logs
burning next to each other radiate heat at each other and keep
each other hot enough for continued combustion.
This is why big logs nearly only burn between themselves.
Kindling fires have many radiation sources to keep each wood
piece heated from many sides. Most woodburners depend on
A complication arises if the stove has a small firebox. Then
there are "cold" steel walls right near the fire that effectively
steal heat from the fire. You probably know what it's like to stand in
front of a picture window in winter? You can feel your radiative
heat being "stolen" through the cold window. Since the 300°F
stove walls seem cold to a 2300°F flame, the same effect applies.
If the fire is therefore "chilled" then it burns more poorly.
JUCA uses large fireboxes in its products to make sure that
this effect cannot materially affect the fire's burning temperature
and the combustion efficiency.
The JUCA Home Page is at: