Crispin and stoves list
See below. For background, here are the two responses and my comments from which Crispin has picked one sentence . >From Dean (with my responses, with two emphases added.) Hi Ron, 1.) For the EPA we tried to burn all the fuel about 2/3 as wood and 1/3 as charcoal to get the best fuel economy. [RWL: Most char-making estimates are that (if char had been retained), the char weight would have been about 1/5 to 1/3 (average of 1/4??) of the initial weight - leaving (maybe) 1/2 of the initial carbon in the biomass. But because pyrolysis removes most of the hydrogen, more than half of the initial energy content has appeared as gases/liquids when pyrolysis stops and gasification begins. Just saying that the 2/3 and 1/3 values could be right on, but the analysis is pretty complicated. ] 2.) As far as I can tell, no primary air can make it up through the pellets until 2/3 fuel is gone and the bottom air gets up through the remaining charcoal. [RWL: It sounds like there was no "door" placed over the usual Stove-Tec opening, so perhaps other owners of Stove-Tecs can also test this in some way. It sounds like it would have been difficult to capture much char - but you were seeing several nice advantages from top lighting. For those not having experience with TLUDs, Dean's reference to "no primary air can make it up", means that the oxygen is "entirely" used to produce carbon monoxide. I am presuming that for Dean's specific geometry, there is some side air entry when the pyrolysis front has moved down by 2/3 (but I may not yet have the geometry right).] [RWL1. I should have stated more - my reason for wanting to respond on this topic was that the number 2/3 was so foreign to me. I know of no other TLUD with this characteristic - and so my question was trying to understand Dean's twice talking about 2/3. I still can only guess at what is going on because I have no figure/photo from which to deduce air flow patterns. More below. From: "Crispin Pemberton-Pigott" <[email protected]> To: "Discussion of biomass cooking stoves" <[email protected]> Sent: Monday, October 24, 2011 7:03:38 AM Subject: Re: [Stoves] Hi TLUDers -- and EPA testing questions Dear Ron Addressing only one sentence of your very interesting list: “For those not having experience with TLUDs, Dean's reference to "no primary air can make it up", means that the oxygen is "entirely" used to produce carbon monoxide.” The Hydrogen in biomass (about 5.6%) requires almost exactly the amount of Oxygen available in it (46%) to create water. While CO can form at a low temperature, the tendency of Oxygen to react with Hydrogen is so strong that given a chance, biomass heated in a TLUD environment creates H2O. Lots of it. The thick fog of ‘smoke’ coming of a totally choked TLUD does have lots of CO in it but it has a heck of a lot more water (which we usually don’t measure). It is really rare to find a normalised CO emissions factor (not concentration in the emerging gases) above 100,000 ppm. I have only see it once and I work with some of the wildest devices the imagination has produced. [RWL: This part I don't understand. Neither Dean or I were talking about anything other than primary air. For TLUDs, the most important defining characteristic is separation of the air stream into primary and secondary components. The Rocket, before this newest modification, didn't have that separation. I presume (could be wrong, but think Dean said so) that the "StoveTec TLUD" tested by Jim Jetter had this separation. So I repeat - I don't know what Crispin is talking about here or below. Or more accurately - why. Obviously photosynthesis requires energy, water and CO2 - and "reversing" that in a fire we expect all three back. So we need to talk about the produced numbers: If you multiply the measured CO number (the concentration) by the Excess Air (EA) present at the time adding 100% to the EA figure you get the CO emission factor CO(ppm) * (EA+100%) = CO(EF) at O2=0% (the O2 is factored out). [RWL3: The subject of excess air for testing the completeness of combustion (after adding secondary air and releasing the majority of the energy) is extraneous to the sentence under discussion.] An emission factor calculated in this manner makes it possible to compare any stove emission to any other without worrying that ‘the conditions’ were different. This normalises the conditions. So, back to the CO. If the O2 available in the biomass (about 46% by mass) were turned into CO, it would create far more than 10% of the total non-O2 component of the total emissions. That 5.6% H2 mass is a heck of a lot of H atoms. Normally it is unusual to see a CO(EF) above 50,000. In a single case I have seen +130,000 briefly during the test of an ‘improved stove’ which put it unfortunately into the category where many ‘improved stoves’ belong. That high a value does not seem to be able to be created without first heating the fuel quite a bit so I am expressing doubts that level could be created in a TLUD that was not first run as a regular fire . I mention this to support my conclusion that the O2 tends to create ‘fuel moisture’ very easily. [RWL4: I have no issue with the last sentence - but not sure why it is raised. But I am not sure where these CO numbers are coming from - or why. I have no problem with them, but they are not the issue. The question is what are the various pyrolysis gas (especially CO) levels before the pyrolysis gases see the secondary air. I have personally measured the gases coming up through the fuel bed in a TLUD (a borrowed high quality tool) and the dominant gas was CO (many millions of ppm). The initial 21 % oxygen is down to zero - as is the CO2 level. My equipment didn't measure water content and I have forgotten most as this was more than 15 years ago. But it would be easy to make such a CO test - maybe also for H20. But the equipment for testing the hot pyrolysis gases before their full combustion is not cheap.] RWL4: My main concern is with Crispin's above next-to-last sentence: " I am expressing doubts that level could be created in a TLUD that was not first run as a regular fire. The word "TLUD" should say to all that the test operation was NOT run as a regular fire. They are as near to polar opposites as the stove world can get. So this is to ask Crispin what he is saying here and what part of my response he is objecting to ? Biomass needs just a little more air (Oxygen) to completely use up the H2 and then breathe in whatever additional air would burn all the Carbon. In any real file, some of the C becomes CO and CO2 (surface reactions mentioned by Dr Tom Reed in a previous discussion). [RWL: I hope we can get a specific citation for/from Tom here. At the hot surface from which (very complicated and numerous [1000's of species??]) pyrolysis gases are emerging, my understanding of the pyrolysis surface effect literature is that "all" (given control of the incoming oxygen flow) are turned into CO and water . The relatively small amount of CO2 that is produced near the surface (not ON) is converted back to CO as it interacts with the hot char above it, while rising to meet the secondary air (which through combustion is providing the needed natural draft). The theoretical literature is not one I claim to understand, but the more thorough talk about several transformation zones. The important point is that the oxygen molecules are converted and therefore cannot reach the hot blackening surface. This is why we make char - rather than consume it in TLUDs. In the Rocket, the 3-stone and other fires, there is so much air that pyrolysis and gasification can both occur. For most fires (and all TLUDS worth than name), char is consumed while nearby fuel particles are still pyrolyzing (but will themselves soon also be consumed because the air supply is too large). So I repeat - I am mystified by this message and about what is at dispute. Ron Regards Crispin _______________________________________________ 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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