On Sat, 25 Feb 2012 19:46:11 +0000 (UTC), [email protected] 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. _______________________________________________ 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/
