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

[ajheggie at gmail.com]

On Sat, 25 Feb 2012 19:46:11 +0000 (UTC), rongretlarson at comcast.net
wrote:


>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. 

I've not meant to convey that, I have said it is difficult to get wood
dry enough for successful top lighting and having a char residue, and
I meant for wood collected outdoors. Inside my house wood seems to
settle to between 15% and  10% moisture content.

Freshly felled trees vary between 40% and up to 70% dependent on
species and whether foliage/needles are present, they will all self
sustain in a fire.
>
>Question for Andrew - are you arguing below in favor of torrefaction - and as low moisture content as possible? 

I wasn't considering torrefaction at all, just the query why oven dry
( close to nil% water) might produce a higher amount of PICs as
particulates compared with "sun Died" which I guess would be around
10% mc wwb.

>> 
>>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??] 

I'll butt in on your question to Rajan: Tom Reed has in the past
suggested that would with 12% mc (IIRC) burns "better" than oven dry
wood on this list.
>> 
>>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. 

Ron, I'm not doubting this statement, I think it may be so and am
trying to give a possible explanation; that all things in the stove
being equal oven dry wood will evolve offgas too fast for the
secondary air to cope with.

I have always advocated burning as dry a fuel as possible but
artificial drying has too high a cost (seldom bettering 4MJ input per
kg of water removed) compared with the small flue losses from burning
air dried biomass. Indeed burning 50% mc biomass in a large, non
condensing, furnace configured for it can be done cleanly and with a
total loss due to fuel moisture of ~15% which tends to be much smaller
compared with all the costs of rehandling and running a dryer.

There are of course many reasons why you might wish to pre dry things,
charcoal making and probably torrefaction are cases.

>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.] 

In the sense that these particulates are all formed in the secondary
flame then yes it applies to both complete combustion of pyrolysis,
conditions and air supply to the secondary flame determine
particulates. Char making  has a harder job to achieve these
conditions because the char retains a lot of the chemical energy (up
to half) that is available to keep temperature up in  full combustion.
Remember the three Ts for clean combustion. retention Time, Turbulence
and Temperature.


>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]. 

The flame from a fire with dry wood is not necessarily shorter, as
will be evidenced from a tlud open flaming with no secondary air
supply, nearly all the air necessary for the secondary combustion has
to diffuse through the flame: air interface and it tends to be a lazy
languid flame. Many things , mostly fuel gas related, determine
whether a diffuse open flame can burn out all the nascent carbon
particles within the flame length, if not it emits sooty particles.

Crispin has replied that reducing primary air would solve the problem,
in fact I would hold that reducing the amount of dry wood in the stove
as well as reducing primary air would be a solution. It's the runaway
evolution of offgas from the chain reaction of pyrolysis in the
exothermic stages that is heating the mass even in the absence of
primary air that it overwhelming the secondary combustion.


>
>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] 

Ronal I have nothing against torrefied wood, it costs less to
comminute, it doesn't re absorb water, it has a high bulk energy
density for transport, OTOH it's an industrial process to make it, I
suspect there are control issues, again Tom Reed would know because he
was involved in making it for its hydrophobic properties as an oil
absorbent, seasweep. there is an energy cost to making it and I can't
see the need in a rural setting.