Dear Paul Thanks very much for your detailed explanation.
Fundamentally, a "Perfect" TLUD will work perfectly if it has perfect fuel, perfect, required air volume, and perfect fan pressure (or stack vacuum) to deliver the perfect air flow. Given that you have "off-standard" fuel, with greater average void space diameter, the pressure loss across the fuel bed will be lower. The fan or stack will thus deliver more air through the bed. Thus, you will get excess primary air flow, and if your secondary air porting was designed for a bed with a greater pressure drop, you will get less secondary air delivery; this will mess up your intended Secondary/Primary air ratio. You can burn virtually any fuel in a TLUd, as long as it is uniform, and the stove was designed to handle it. Change the fuel significantly, and you need to change the design, to maintain the desired SA/PA ratio. If you "change the fuel", but don't change the design, then the only way to restore the system to "good operation" is to "modify the fuel" to one having similar a pressure drop across the bed, similar to one for which the system was designed. This is what you are effectively trying to do. Assume, for example, that you have 4" long pieces of straw as fuel. This will likely give you all the problems you note. Adding char can, in theory, help increase the bed pressure drop. However, I am guessing that it will be a real stinker to get the char particles distributed through the bed. There will likely be "too much" in one area, and "too little" in another. Channelling is thus very likely. What you will probably end up with is a fuel bed having non-uniform flow properties, because of the large differences between straw properties and char properties. Try mixing various percentages of char with the straw, in a mixing bucket, then try to take "mixed fuel" from the "mixing bucket" and place it in the stove. I am guessing that you should see the non-uniformity of the fuel bed even before you ignite it. My guess is that your best bet would be to chop the straw, so that it is free-flowing. This alone will make life very much easier for the operator, and will enable a greater weight of fuel to be added to the stove. Longer burn times between re-fuelling. Play with the air flow, simply by obstructing the fan intake with a piece of paper or cardboard. Then see if you can get your usual good combustion. If not, then consider re-drilling or partially plugging, the SA air holes to get back to the correct PA/SA ratio for good combustion. Is chopping the straw (or whatever the fuel is) an option you can consider? Best wishes, Kevin ----- Original Message ----- From: Paul Anderson To: [email protected] Cc: Kevin ; Discussion of biomass cooking stoves ; James S. Schoner ; Hugh McLaughlin Sent: Monday, May 06, 2013 11:23 AM Subject: Re: [Stoves] [biochar] Charcoal as space filler in TLUD reactors Kevin, The objective of the filler is two-fold: One reason is precisely to reduce the maximum flow of primary air. Without resistance, too much primary air can race through the loose pile of biomass, reach too much of the biomass at the same time, and have an excessive fire without much control. Control simply by a "gate" at the entrance of the primary air is usually insufficient. Second reason is that burning embers at the top of loosely packed fuel can sometimes fall to lower areas of the fuel bed and ignite the raw fuel there. This defeats the process of the pyrolysis front that starts at the top and should progress slowly and uniformly downward through the bed of fuel. That migrating pyrolytic front is THE most important and distinguishing feature of the TLUD stoves. Ignition at the top and having updraft are not the single-most defining characteristics of TLUD stoves (even though that is what the name says). Maybe I should have called it Migrating Pyrolytic Front Gasification (or MPFG), but TLUD is the accepted name now. [And Tom Reed always thanks me for getting away from the Inverted DownDraft (IDD) name that was not well understood.] Important note: When the pyrolytic front correctly reaches the bottom of the batch of fuel, the combustion style changes to be Bottom-Burning UpDraft (call it BBUD if you must have an acronym, but note that at the start it was NOT IGNITED or lit at the bottom). And there is no more migration/movement of a "gas-making" zone. Also note: When the batch has been pyrolyzed, the burning at the bottom is "char-gasification" and can be at forge temperatures that can damage the metal pieces. There is still restricted flow of primary air. The hot gases go upward. IF additional raw biomass fuel is placed onto the top of that charcoal, it will be heated, dried, torrified, and eventually pyrolyzed, giving additional pyrolytic gases that can be combusted where the incoming secondary air enters. But this is NOT operating as a TLUD stove (with MPFG). This type of bottom-burning gasifier is well illustrated by the Oorja stove (former BP, now First Energy) in India. It has a cast-iron cup in the bottom to protect the other metal parts, and that cup glows red-hot after continual use. [Technical note: Stove testing should measure separately the emissions during each of the different combustion modes instead of just reporting averages that include emissions from two or more combustion modes. I think we can do some of that at this summer's Stove Camps at CREEC - Uganda and at Aprovecho - Oregon-USA where emissions equipment is available.] About terminology: A bucket stove or mud stove or Rocket stove and many others can be ignited at the bottom of a container and they do have updraft, BUT they are NOT GASIFIER devices. So the designation BLUD is not relevant. UD and DD and TLUD are designations historically for gasifiers, which means that the gases are created in one place that is NOT the same place as the combustion or other use of the gases. Paul (James, please get this onto the drtlud.com website in edited format.) Paul S. Anderson, PhD aka "Dr TLUD" Email: [email protected] Skype: paultlud Phone: +1-309-452-7072 Website: www.drtlud.comOn 5/6/2013 1:17 AM, Kevin wrote: Dear Paul ----- Original Message ----- From: Paul Anderson To: Discussion of biomass cooking stoves Cc: Kevin ; [email protected] ; James S. Schoner ; Hugh McLaughlin Sent: Sunday, May 05, 2013 6:33 PM Subject: Re: [Stoves] [biochar] Charcoal as space filler in TLUD reactors Kevin, The "charcoal as filler" is not about consuming the charcoal. # Sorry, I missed that. The charcoal is "almost" non-active in the pyrloysis of the new biomass. This is a discussion about limiting air flows with a filler that mostly is inactive in environments that are at 650 C without oxygen. # Why do you feel it would be advantageous to limit air flow with an inert filler? If the char was significantly larger or smaller than the biomass fuel, it could significantly increase pressure drop through the bed, and would likely reduce maximum flow. # Thanks. [ Note that I avoid using the word "inert" in this discussion.] Best wishes, Kevin Paul Paul S. Anderson, PhD aka "Dr TLUD" Email: [email protected] Skype: paultlud Phone: +1-309-452-7072 Website: www.drtlud.com
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