[Stoves] oven-dried vs sun-dried biomass and TLUD stoves

[Frank Shields]

Dear AJH,

Back on the farm we would pile up very wet green wood and burn. A tractor
tire buried in the pile and a little gasoline would help get it started but
once going (and the tire gone) no amount of wet wood seemed to stop it. It
seems if calculating the energy needed to evaporate the water vs energy
produced from the biomass that it should not burn - or very well. So I
wonder if the process of water evaporating and then turning to H and CO
requiring a lot of energy then back to water at the outside releasing its
energy, all being energy neutral, make this all happen?

Thanks for the informative reply below.  


Frank


Frank Shields
42 Hangar Way
Watsonville,  CA  95076
(831) 724-5244 tel
(831) 724-3188 fax
frank at bioCharlab.com


 <snip>
I wouldn't make the distinction between sun dried and oven dried, it's
likely just to do with moisture content and how this modifies the rate of
release of offgas from the fire. 

Wood pellets made with no binders are around 10% moisture content and burn
with low particulates, probably because of their extra density and the fact
that there are few pellets in the fire basket at one time plus the secondary
air supply is adequate.

When you burn very dry wood the primary action is that of releasing a small
amount of heat from combustion of fresh char, this heat rapidly pyrolyses
the rest of the piece of wood, evolving lots of offgas in a short space of
time. You will note from the discussion on torrefied wood that pyrolysing
wood, a complex mixture of organic chemicals, has some fuzzy steps as the
chemicals break down at differing temperatures. IIRC ( and it will need
checking): up to 100C mostly water is evolved then from about 150 degrees
the wood gives off Volatile Organic Compounds as well as the small amount of
water weakly bound by hydrogen bonding to OH groups in the cellulose and
hemicellulose. Then starting around 230 degrees the chemicals start breaking
down, early products are things like water and acetic acid.
All these initial stages are endothermic, i.e. they need heat to be added to
the wood to keep them going, the gases given off will not support combustion
in this mixture. At about 330C further breakdown occurs but now the
reactions are exothermic, so they will continue without further input of
heat. This is where the chain reaction takes over, so as soon as the
adjacent unreacted bit of the wood particle reaches this temperature it
pyrolyses, releases hot offgas and causes an adjacent wood to follow suit.
The offgases at this stage are high oxygen content and hence do not release
lots of heat when oxidised but they will support a flame in the open all the
time the average calorific value is above about 2MJ/kg. As the temperature
reaches the top of the range ( around 440C) the reaction has moved into the
endothermic region again, loss of hot offgas is carrying away energy as
sensible heat and chemical energy. There will be secondary reactions taking
place inside the wood particles. The offgas now contains a high proportion
of vaporised light tars, carbon monoxide, methane and hydrogen and has a
high calorific value. At this stage the charcoal will still contain large
amounts of organic compounds  and be as much as 45% of the original dry mass
less the wood that has burned to initiate the reaction.

Further heating drives off the heavier tars and by about 900C the char
residue has dropped to around 15% of the original dry mass. The offgas at
this stage is largely CO and H2 and even a diffuse flame has a blue colour.

You will see from this that if a large amount of very dry wood is fired it
quickly releases a lot of offgas and delivering sufficient air to completely
burn this in a secondary flame can be a problem. If the flame can not
completely burn out because there is not enough chance of an oxygen molecule
reacting with all the offgas molecules then the easier high hydrogen
compounds get preferentially reacted leaving unreacted carbon and high
Products of Incomplete Combustion to clump together and leave the fire as
small sooty particle. Much the same happens if a hydrocarbon, like petrol,
is fired in the open. The major difference being it's easier to premix a
hydrocarbon liquid or gas in order to provide enough oxygen.

Consider how moisture content can affect this. Water has a high latent heat
of vaporisation, i.e. it needs a lot of energy to turn from a liquid in wood
to a gas compared with the amount of energy to raise its temperature. The
exothermy of pyrolysis in the 330-440C range is weak, there is not a lot of
energy given off. If the adjoining pieces of wood have some moisture this
first has to be evolved as vapour before the pyrolysis reaction can reach
330C and self sustain. So a small amount of water can modify the rate of
evolution of offgas such that the secondary combustion takes place in a
flame that is long enough for sufficient oxygen to diffuse into the flame
and react completely with fuel gases in the flame.

A good demonstration can be done by taking two freshly cut and similar
sticks, oven dry one and not the other, Place them in the middle of a
flaming fire and watch. The green stick is gradually consumed to ash from
the outside inward, shrinking to nothing. The oven dried stick rapidly
evolves a flame  and turns to char without changing shape much, then as the
flame subside the char gradually burns away.

AJH







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