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: [email protected] To: "Discussion of biomass cooking stoves" <[email protected]> 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, [email protected] 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 _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://www.bioenergylists.org/
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