<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: Arial; font-size: 12pt; color: #000000'>Andrew, Rajan, stoveslist<br><br> 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.<br><br>Question for Andrew - are you arguing below in favor of torrefaction - and as low moisture content as possible?<br><br>See few inserts below.<br><br><br><hr id="zwchr"><b>From: </b>ajheggie@gmail.com<br><b>To: </b>"Discussion of biomass cooking stoves" <stoves@lists.bioenergylists.org><br><b>Sent: </b>Saturday, February 25, 2012 3:38:40 AM<br><b>Subject: </b>Re: [Stoves] oven-dried vs sun-dried biomass and TLUD stoves<br><br>On Sat, 25 Feb 2012 07:44:11 +0530, rajan_jiby@dataone.in wrote:<br><br>>Dear All,<br>><br>>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.<br><br> <strong>[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??]</strong><br>><br>>So, probably a moisture content of around 10 to 15 % ( not more ) in the fuel has a positive role to play.<br><br> <span style="font-weight: bold;">[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.</span><br>><br>>The best sun-drying may be giving us this moisture content.<br><br style="font-weight: bold;"><span style="font-weight: bold;"> [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.</span><br>><br><br>Rajan<br><br> <span style="font-weight: bold;">[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.]</span><br><br>I wouldn't make the distinction between sun dried and oven dried, it's<br>likely just to do with moisture content and how this modifies the rate<br>of release of offgas from the fire. <br><br>Wood pellets made with no binders are around 10% moisture content and<br>burn with low particulates, probably because of their extra density<br>and the fact that there are few pellets in the fire basket at one time<br>plus the secondary air supply is adequate.<br> <span style="font-weight: bold;">[RWL4: Andrew uses the term "burn", but I think this applies equally or more so to pyrolysis.]</span><br><br>When you burn very dry wood the primary action is that of releasing a<br>small amount of heat from combustion of fresh char, this heat rapidly<br>pyrolyses the rest of the piece of wood, evolving lots of off gas in a<br>short space of time. You will note from the discussion on torrefied<br>wood that pyrolysing wood, a complex mixture of organic chemicals, has<br>some fuzzy steps as the chemicals break down at differing<br>temperatures. IIRC ( and it will need checking): up to 100C mostly<br>water is evolved then from about 150 degrees the wood gives off<br>Volatile Organic Compounds as well as the small amount of water weakly<br>bound by hydrogen bonding to OH groups in the cellulose and<br>hemicellulose. Then starting around 230 degrees the chemicals start<br>breaking down, early products are things like water and acetic acid.<br>All these initial stages are endothermic, i.e. they need heat to be<br>added to the wood to keep them going, the gases given off will not<br>support combustion in this mixture. At about 330C further breakdown<br>occurs but now the reactions are exothermic, so they will continue<br>without further input of heat. This is where the chain reaction takes<br>over, so as soon as the adjacent unreacted bit of the wood particle<br>reaches this temperature it pyrolyses, releases hot offgas and causes<br>an adjacent wood to follow suit. The offgases at this stage are high<br>oxygen content and hence do not release lots of heat when oxidised but<br>they will support a flame in the open all the time the average<br>calorific value is above about 2MJ/kg. As the temperature reaches the<br>top of the range ( around 440C) the reaction has moved into the<br>endothermic region again, loss of hot offgas is carrying away energy<br>as sensible heat and chemical energy. There will be secondary<br>reactions taking place inside the wood particles. The offgas now<br>contains a high proportion of vaporised light tars, carbon monoxide,<br>methane and hydrogen and has a high calorific value. At this stage the<br>charcoal will still contain large amounts of organic compounds and be<br>as much as 45% of the original dry mass less the wood that has burned<br>to initiate the reaction.<br><br>Further heating drives off the heavier tars and by about 900C the char<br>residue has dropped to around 15% of the original dry mass. The offgas<br>at this stage is largely CO and H2 and even a diffuse flame has a blue<br>colour.<br><br>You will see from this that if a large amount of very dry wood is<br>fired it quickly releases a lot of offgas and delivering sufficient<br>air to completely burn this in a secondary flame can be a problem. If<br>the flame can not completely burn out because there is not enough<br>chance of an oxygen molecule reacting with all the offgas molecules<br>then the easier high hydrogen compounds get preferentially reacted<br>leaving unreacted carbon and high Products of Incomplete Combustion to<br>clump together and leave the fire as small sooty particle. Much the<br>same happens if a hydrocarbon, like petrol, is fired in the open. The<br>major difference being it's easier to premix a hydrocarbon liquid or<br>gas in order to provide enough oxygen.<br><br>Consider how moisture content can affect this. Water has a high latent<br>heat of vaporisation, i.e. it needs a lot of energy to turn from a<br>liquid in wood to a gas compared with the amount of energy to raise<br>its temperature. The exothermy of pyrolysis in the 330-440C range is<br>weak, there is not a lot of energy given off. If the adjoining pieces<br>of wood have some moisture this first has to be evolved as vapour<br>before the pyrolysis reaction can reach 330C and self sustain. So a<br>small amount of water can modify the rate of evolution of offgas such<br>that the secondary combustion takes place in a flame that is long<br>enough for sufficient oxygen to diffuse into the flame and react<br>completely with fuel gases in the flame.<br> <span style="font-weight: bold;">[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].</span><br><br>A good demonstration can be done by taking two freshly cut and similar<br>sticks, oven dry one and not the other, Place them in the middle of a<br>flaming fire and watch. The green stick is gradually consumed to ash<br>from the outside inward, shrinking to nothing. The oven dried stick<br>rapidly evolves a flame and turns to char without changing shape<br>much, then as the flame subside the char gradually burns away.<br> <span style="font-weight: bold;">[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.<br> Andrew - this last is new to me - I will try to get the needed two pieces and test this. Thanks. Ron]</span><br><div style="text-align: right;"> <span style="font-weight: bold;"></span><br></div>AJH<br><br><br><br><br><br><br><br>_______________________________________________<br>Stoves mailing list<br><br>to Send a Message to the list, use the email address<br>stoves@lists.bioenergylists.org<br><br>to UNSUBSCRIBE or Change your List Settings use the web page<br>http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org<br><br>for more Biomass Cooking Stoves, News and Information see our web site:<br>http://www.bioenergylists.org/<br><br></div></body></html>