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

Sat Feb 25 13:46:11 CST 2012

Andrew, Rajan, stoveslist 

This is not clearly in the current thread related to Torrefaction - but it bears directly on it. Also, when I started the topic, I was especially thinking of Andrew, as he has often reported difficulty in the UK getting wood dry enough to even combust - much less get useful heat out. 

Question for Andrew - are you arguing below in favor of torrefaction - and as low moisture content as possible? 

See few inserts below. 

----- Original Message -----
From: ajheggie at gmail.com 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Sent: Saturday, February 25, 2012 3:38:40 AM 
Subject: Re: [Stoves] oven-dried vs sun-dried biomass and TLUD stoves 

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. 

[RWL1: I wonder if you can back up your "feeling" with any data? I have not seen this statement in any peer-reviewed literature. Anyone able to comment on whether low moisture gives this undesired effect??] 
> 
>So, probably a moisture content of around 10 to 15 % ( not more ) in the fuel has a positive role to play. 

[RWL2: I also haven't seen this anywhere. I interpret Andrew below to be doubting this - so hope everyone will look closely at this topic. In a different note I just sent out, the citation by Kleinschmidt gives moisture content for torrefied pellets and charcoal in the range of 1-5 % . These are desired fuels. I think Paul Anderson has also recently reported preferring as dry as he can get. 
> 
>The best sun-drying may be giving us this moisture content. 

[RWL3: I am not arguing against sun-drying. But I see no reason to think it is an optimum - and optimum seems to be zero moisture. Especially if you are paying by the kilo. 
> 

Rajan 

[RWL4 The rest from Andrew - which I am interpreting to be in the spirit also of talking about torrefaction (mentionned in next to last paragraph.] 

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. 
[RWL4: Andrew uses the term "burn", but I think this applies equally or more so to pyrolysis.] 

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 off gas 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. 
[RWL: And this seems to be the rationale for minimum moisture - a shorter, less-wispy flame, with oxygen better able to reach the pyrolysis gases. . Trouble might occur with primary and secondary air tuned for a moist fuel, I suppose - but well designed TLUDs will have controllable primary air]. 

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. 
[RWL: And this is what we desire in a char-making stove. We can replace the words" oven-dried" by "torrefied". Much less chance of undesired particulates when you have char left. 
Andrew - this last is new to me - I will try to get the needed two pieces and test this. Thanks. Ron] 

AJH 

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