[Stoves] Saving the WBT
Ronal W. Larson
rongretlarson at comcast.net
Fri Aug 16 19:07:44 CDT 2013
Frank - I am worried you haven't maybe been playing with TLUDs. True?
See below.
On Aug 16, 2013, at 3:52 PM, Frank Shields <frank at compostlab.com> wrote:
> Ron,
>
> I look forward to hearing Jim’s, or anyone else’s approach to the difficult problem of accounting for the energy. Whoever comes out with a method there will be another right around the corner. This is non-ending so there is no need to wait. My suggested approach is not a comparison – just a different way of looking at it. Hopefully one that will work without all the errors regarding calculating the remaining chars.
[RWL: One measures, not calculates the "remaining chars". Can be pretty accurate - especially with TLUDs.
>
> I am thinking of a new approach where we do not need to handle char at all. I noticed when using the GEK and Tom Reeds TLUD that when fresh biomass ran out the secondary flame went out, or very poor flame. Just add more biomass and you are in business.
[RWL: This is not normally done at all with TLUDs. It is possible with BLDDs.
> Hot coals several inches below the pot did a poor job of heating the pot – so why even consider them?
[RWL: Right. One of the main purposes of TLUds is to stop the operation when the pyrolysis front hits the bottom.
> Its only the fresh tars that heat the pot and all that other energy just heats the stove body. Important to heat the stove body and aid in breaking the bonds to release lumps of tars and complex organics free to head to the secondary. But IF (Big IF) they do not significantly heat the pot we can rule them out it saves that problem of all the difficult calculating.
[RWL: If one purpose of the char was to make char, the measurement and calculating is relatively trivial.
> If you were to fill a rocket with char and blast air on the char would you get a secondary flame?
[RWL: Yes. This was demonstrated nicely at Stove camp by Kirk Harris, who had a special set of "intermediate" holes - so as to burn the chars nicely - from the top down.
> The stove body would get red hot but the pot only a few inches away would heat up slowly without the flames licking the bottom. Lots of useless heat.
[RWL: Nope - Kirk had a nice flame. His was a camping stove and not interested in producing char. Very clever mod.
>
> The question is can we take a block of wood and determine the weight fraction that will contribute to the secondary? And the fraction that sits and combusts in the stove body? I think the pipe will do that.
> [RWL: It might do it if you could reproduce all the stove operating temperature history. Running at high power will expose the biomass/char to higher temps (and less char) than if the run was all at low power.]
Ron
>
> Something different to talk about.
>
> Thanks Ron for the reply.
>
> Regards
>
> Frank
> Frank Shields
> Control Laboratories; Inc.
> 42 Hangar Way
> Watsonville, CA 95076
> (831) 724-5422 tel
> (831) 724-3188 fax
> frank at biocharlab.com
> www.controllabs.com
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> From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Ronal W. Larson
> Sent: Friday, August 16, 2013 1:27 PM
> To: Discussion of biomass cooking stoves
> Subject: Re: [Stoves] Saving the WBT
>
> Frank and list:
>
> Abut 1.25 hours after yours was a message fem Jim Jeffords on a webinar. He is obviously going to talk on Tuesday about how to handle remaining char when calculating efficiency. It was not obvious to me how you are handling char in your example. Could you give an example where the remaining char by weight was 25% of the input biomass weight. Maybe 30% into the cook pot. Or any numbers you want. Then we can hopefully compare your approach with Jim's
>
> Ron
>
>
> On Aug 16, 2013, at 12:12 PM, Frank Shields <frank at compostlab.com> wrote:
>
>
> Greetings Stovers,
>
> All this talk about the ocean water got me thinking about the Water Boiling Test.
> I would like to suggest a new way of testing and reporting results:
>
> 1) Procedure
> 2) Justification
> 3) Calculations
>
> Procedure: We take some oven dried wood and place in a pipe. Add both end caps, loosen one, weigh and place in an oven at ~450c. Then cool and weigh. The loss in weight is the volatile fraction of the fuel. This is the fraction that provides the energy to boil water. We determine the energy of this fraction and that is the energy of the fuel. Keeping track of the fuel weight we use we determine the total usable volatile-energy.
>
> We put the pot of water on the stove, measure the temperature of the water, start the fire and monitor the water temperature. We keep the fire going until the water is at ‘simmer’ then keep steady for 30 min. Adding no more fuel we then we manipulate the fire to keep the secondary burn going as long as possible. Soon as the secondary burn goes out we pull the pot off and measure the area under the temperature plot for energy that went into the pot. Energy in the pot / volatile-energy X 100 is the efficiency(?).
>
> Justification: When you are boiling water it is only good as long as the secondary burn is going. When that goes out, even with a glowing stove below, the water heating process slows way down because, as I learned in Stove Camp, we need the heat forced hitting the bottom of the pot to stick to it, go through the pot and heat the water.
>
> Biomass fuel has two types of energy; 1) the tars (C-H-O) that create the secondary burn and 2) the chars (C-C) that only heat the stove body. Important for the chars to heat the stove body but there is more than enough with a good insulated stove and all that extra heat is wasted – not used to heat the pot. When biomass is heated between 300c to 450c tars of massive C-H-O structures go to the secondary burn and ALL C > CO2, and all H > H2O releasing massive energy just at the pot bottom. The C-C bonds (chars) left need to go C (solid) – CO volatile) forms releasing energy only in the stove body. The CO (volatile) goes to the secondary burn (adding to the energy of the tars) to go CO > CO2 releasing a relatively small amount of energy. Under the best of conditions all the C goes to CO (not CO2) in the stove body but this is such a small amount of energy compared to the tars providing ALL their energy to heat the pot I suggest we can ignore (or estimate) the CO > CO2 added energy.
>
> This being the case if we use only the volatile fraction as the total energy then once the secondary burn stops all the rest of the material in the stove body can be ignored.
>
> Calculations: A block of 100 g dried wood contains 44g C, 50g O and 6g H. Let’s say 22g C goes to the secondary as tars to heat the pot and 22g C left behind to heat the stove. This can be determined (if needed) in the lab measuring C and H in the biomass and C and H in the char left. The weight loss in the pipe contains 28.2 % carbon and 7.7 % hydrogen for the starting energy value figuring all H and all O are included in the tar fraction.
>
> Now we need to use Bond Energy (I need help) to determine the energy value we give for all the tar carbon going all the way to CO2 and the hydrogen going all the way to H2O. We sum the Bond Energies in the tars as the Total Energy of the fuel. Add to it (ignore or estimate) the Bond Energy of the CO to CO2 in the chars.
>
> Bond Energies:
> C - - O = 360 kj/mol
> H - - O = 366 kj/mol
> What is C>CO2 and H>H2O?
>
> I realize if one has H2 and O2 that nothing happens until you provide enough energy (light a match) to break the H-H and O-O bonds to re-create H2O in an explosion. In this stove case there might be enough of the extra heat in the stove body to break apart the tars into C and H and O so we can just calculate them going completely to their end components.
>
> Regards
>
> Frank
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> Frank Shields
> Control Laboratories; Inc.
> 42 Hangar Way
> Watsonville, CA 95076
> (831) 724-5422 tel
> (831) 724-3188 fax
> frank at compostlab.com
> www.compostlab.com
>
>
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