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

[ajheggie at gmail.com]

On Sat, 25 Feb 2012 07:44:11 +0530, rajan_jiby at dataone.in wrote:

>Dear All,
>
>I have a feeling that oven-dried biomass if used in a TLUD stove can lead to higher "particulate matter" emissions from the stove - which can be a health hazard.
>
>So, probably a moisture content of around 10 to 15 % ( not more ) in the fuel has a positive role to play.
>
>The best sun-drying may be giving us this moisture content.
>

Rajan

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