I would guess that to get O2 uptake, you'd have to use a large amount of char in order to be able to measure the resulting small weight percent increase. That could be inconvenient, or downright dangerous if spontaneous combustion is indeed a risk (large amount of char + atm of pure O2 ???).
Josh On Mon, Feb 25, 2013 at 1:36 PM, Frank Shields <[email protected]> wrote: > Stovers,**** > > ** ** > > I think wetting, keeping dry or covering new chars may just be putting of > the potential problem. **** > > ** ** > > A test might be:**** > > 1) Weigh a container**** > > 2) add biochar and weigh**** > > 3) purge with nitrogen (200c)**** > > 4) weigh tare and dry char**** > > 5) purge with air (oxygen) (25c)**** > > 6) weigh tare and biochar**** > > 7) purge with butane (25c)**** > > 8) weigh tare and biochar**** > > ** ** > > Step 4 you get the moisture**** > > Step 6 you get oxygen up-take and potential for spontaneous combustion**** > > Step 8 you get butane activity**** > > ** ** > > If this method looks good, when we get some time we will try it.**** > > We need to get relative O2 up-take values compared to potential for > Spontaneous Combustion to be able to report a SC potential Index. **** > > Butane activity value just easily added at this time. **** > > ** ** > > Regards**** > > ** ** > > Frank**** > > ** ** > > ** ** > > Frank Shields**** > > Control Laboratories, Inc.**** > > 42 Hangar Way**** > > Watsonville, CA 95076**** > > (831) 724-5422 tel**** > > (831) 724-3188 fax**** > > www.controllabs.com**** > > ** ** > > ** ** > > ** ** > > ** ** > > ** ** > > *From:* Stoves [mailto:[email protected]] *On > Behalf Of *Harold Annegarn > *Sent:* Saturday, February 23, 2013 12:50 AM > > *To:* Discussion of biomass cooking stoves > *Cc:* James Robinson; Glenn Stracher > *Subject:* Re: [Stoves] combustion of char**** > > ** ** > > Response from Harold Annegarn**** > > ** ** > > I make reference not to char, but to coal. Perhaps the same will apply to > freshly prepared char surfaces.**** > > ** ** > > The following information arises from the phenomenon of spontaneous > combustion in underground coal and in coal slag heaps. **** > > ** ** > > Exposure of freshly broken coal surfaces, or in situ coal that has been in > water filled voids that is subsequently exposed to air (oxygen) (as occurs > when previously mined areas by bord & pillar extraction is exposed by strip > mining) results in a chemical reaction known as oxysorption ==> oxygen > reacts with carbon/carbonaceous matter at the surface of the coal in an > exothermic reaction that results in the oxygen being absorbed into the > chemical structure of the coal without necessarily being emitted as CO.. > This oxysorption can occur at room temperatures and does not require > elevated temperatures or a spark to start. **** > > ** ** > > Thereafter, basic physics applies: if thermal energy is released from a > chemical reaction, that heat is either conducted, convected or radiated > away. If the rate of production is faster than the rate of dissipation, > then the temperature will rise in the zone where the oxysorption is taking > place. The balance between conduction, convection and radiation may change > as the local temperature increases; likewise the reaction rate will > increase. There must be some convection possible, otherwise the available > oxygen will be consumed and the process will stop, If the local convection > is large, then the surface remains cool and the reaction proceeds without > large temperature rise. If the convection is limited, local temperature > will increase, and eventually may reach the point where volatile matter > that is driven off reaches ignition point, i.e. spontaneous combustion has > commenced. **** > > ** ** > > It is not a matter of whether spon com occurs, it is just a matter of > when. For this reason, modern practise of stockpiling coal discards and > fines requires continual compaction of the surface to limit air ingress, > and final sealing of surfaces with a meter or more of soil.**** > > ** ** > > Although I I have not read anything about oxysorption on char, it is > logical that similar processes are occurring, The char is a freshly > prepared surface with high specific surface area due to the voids created > by driving off volatile material, and with many dangling bonds at these > surfaces. Not for nothing is this material also known as "activated > charcoal" with good properties for absorbing ions e.g. from water for water > purification.**** > > .**** > > To test this it would be possible to char wood in a dry nitrogen or helium > stream, although this would result in a different surface chemistry than if > the same material is passed through the same heating cycle in the presence > of oxygen. If the material is then cooled, and placed on a balance with > automatic recording, the the change of weight with time can be recorded. If > this is done in a closed system, with dry air passed over, then one could > isolate the effect of oxygen absorption from the absorption of H2O from the > atmosphere if this was done in open laboratory air with variable relative > humidity.**** > > ** ** > > In the case of the spontaneous combustion accident described by Frank > Shields, it appears that the raising of the temperature to just over 100 > degrees C assisted in creating a heat generation rate and oxygen supply to > the char samples that allowed the oxysorption to proceed to the point at > which full ignition commenced - yes, he is lucky to have a lab still!**** > > ** ** > > I have copied this message and thread to Glenn Stracher, an expert on > spontaneous combustion on coal, who has a much deeper understanding of > these matters than the qualitative handwaving that I have given above.**** > > ** ** > > Best regards**** > > Harold**** > > ** ** > > On 23 February 2013 03:43, Frank Shields <[email protected]> wrote:**** > > Stovers, > > FYI > After having about 30 compost samples in a drying oven go up in flame it > looked like it started with a single char sample placed in the corner. I > decided to do a little research by asking Hugh McLaughlin. Another question > I had for him was why does some biochar samples gain weight in a drying > oven > set at 105c. > > So it seems soon after a char is produced it wants to take up oxygen. If > bagged in a sealed container and sent to the lab soon after being made the > dry sample will take up oxygen in the oven therefore gaining weight. If a > sample is wetted soon after being made the water prevents oxygen from being > taken up. Then if it becomes dry it wants oxygen, increases temperature and > can catch fire under the right (wrong) conditions. Char is a great > insulator > so the heat can be trapped and build up. If there is organic materials > around it can flame. So before storage better give it time to take up > oxygen. Or if wetted do not let it dry out and keep bags of char in single > layers. > > The forced air drying oven was left on overnight and it was in the morning > we found all the burned up samples. It was placed against the wall so we > are > lucky we still have a lab! > > Regards > > Frank > > > > > _______________________________________________ > 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/**** > > > > **** > > ** ** > > -- > Harold Annegarn > Department of Geography Environmental Management and Energy Studies > University of Johannesburg > Mobile +27 (0)83 628 4210 > Fax +27 11 559 2430 > Office +27 11 559 3927**** > > _______________________________________________ > 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/ > > > -- Josh Kearns PhD Candidate, Environmental Engineering University of Colorado-Boulder Visiting Researcher, North Carolina State University Director of Science Aqueous Solutions www.aqsolutions.org Mobile: 720 989 3959 Skype: joshkearns
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