Re: 2nd batch flame cap trench method #technology #flamecap

Paul S Anderson



Some quick notes.     50% of carbon atoms of biomass remain in the biochar, but by weight that is only about 20%.   Retorts and TLUDs do not produce char that is, in general, different from flame-cap / trench / cavity char.


I do not trust an Infrared thermometer to be relevant for the temperatures of charcoal making.   If other readers disagree, I am interested in their responses.


You could get variation in your trench-made chars if you do not have some  mixing to be sure that no biomass has been protected from pyrolysis heat because of other biomass on top of it.  


Any type of blower on a long metal pipe can give you some thermal boost.  


Best wishes,




Doc / Dr TLUD / Paul S. Anderson, PhD --- Website:

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From: <> On Behalf Of Eli Fishpaw via
Sent: Monday, October 12, 2020 9:28 AM
Subject: [Biochar] 2nd batch flame cap trench method


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I am preparing for my second flame cap trough batch for creating charcoal to process into biochar.  This 6’ long 20” wide, 20” deep at bottom trench, reduces the processing needed of the feedstock as compared to retort chambers such as 55 gallon TLUD.  I have heard the figure of only 18% or carbon contained in wood becomes recalcitrant (not biologically available).  I have heard 50% for retorts and TLUDs.  There is something very primal about this low-tech method (earth, wood, fire, water).  As a novice, this method delivers significant quantities of char (120 gallons in first batch). 


I want to review lessons learned from first batch as well as integrating info learned from this list, my only interactive source of information about biochar.  According to Nando Brieter on , temperature of processing should be between 300 C – 900 C (572 F-1652 F) to achieve maximum Cation Exchange Capacity.  According to Paul Andersen’s paper, max surface area is achieved between 500 C -700 C (932 F-1292 F).  I am going to try to somewhat control the temp.  My infrared thermometer only goes up to 1015F.  On first batch there were a few times when temp was above that.  However, I doubt it ever got into activated charcoal territory. 


I have gathered wood and brush from my surroundings, sorting by size and dryness.  I want to start with small and dry material and incrementally work up to 8-12” diameter x 16” logs that are well cured.  When the fire is really hot, I noticed that even the somewhat green or even partially rotted material that is added, quickly vaporized and became a source of heat.  I want to monitor the temperature with my infrared thermometer.  When the fire is not as hot as I want it, I will add smaller or dryer material to bring it back.  One discovery from first run was dry grass clippings would quickly raise temp of fire.  I will note Frank Strike’s suggestion of using a leaf blower to spike the fire when it is cooler than desired.  To avoid the need to purchase that equipment, I want to use a shop vac by taping a galvanized down spout to the end of plastic tube to give some non-flammable reach for air injection into the fire. 


I have larger material that is well cured. Once that is burning, those pieces should add volatile gases for a while.   After that is added, if temp allows, I will add less cured logs.  Then, I will incrementally add smaller material that will achieve the flame cap when I see ash forming (white color).  Incrementally smaller feedstock will require incrementally less time to achieve pyrolysis.  Adding increasingly smaller material until eventually, I am adding kindling.  The smaller material requires less time for pyrolysis.  I am hoping to reduce the amount of partial pyrolysis pieces. 


I will dowse with garden hose to end the process.  One thing I noticed when I used a 30 gallon metal tub for flame cap method, when I doused for cooling off the charcoal to avoid turning into ash, it was hard to determine how much water was enough.  When I came back the next day, the total volume had shrunk because some of the deep feedstock continued to burn with access to air without the flame cap going and turned to ash.  On the other hand, I do not want to over water it.  I splurged for a grain grinder to crush the charcoal.  The dowsing benefits that process by reducing the dust that would occur with fully dry charcoal.  However, I am hesitant to flood trench with standing water visible, because I think grinder would have trouble with that also.  I am considering after initial dowsing, loading into metal trash cans that I can put a tight lid on to prevent the free flow of air to prevent conversion to ash. 

Your thoughts will be appreciated. Thanks for reading my long message.  

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