[Stoves] TLUD theory -- Fuel Bed Temperatures

[Josh Kearns]

Using a 1-gal TLUD "Toucan" model in natural and forced air modes, we have
seen, pretty consistently, ND peak temps. 600-650 C, and FD peak temps
900-950 C. This has been across feedstocks - wood pellets, sugarcane
bagasse pellets, bamboo chips, jatropha presscake pellets, pecan shells,
cherry pits.

Temperature is controlled by draft, and we were lucky to achieve high draft
conditions in our 55-gal drum TLUD without applying a fan. Feedstock
granularity, density, and bed density had to be within an optimal range
however. High draft 55-gal TLUD chars from dense, uniform feedstocks (e.g.
eucalyptus branches 1-2-" dia cut into 4-6" segments) seem to offer
equivalent sorption performance as 1-gal FD chars made from wood pellets.

Interestingly, temperatures have been more or less independent of
feedstock, but burn duration is longer with denser feedstocks. If
feedstocks are insufficiently dense, e.g. the bagasse pellets I made with
our crappy benchtop pelletizer, then they tend to burn up and the char
yield is very low if draft is high.

Also, interestingly, feedstock did not seem to matter much for sorption of
our probe compounds (e.g herbicide), where peak temperatures were
consistent. This agrees with other work that finds peak temperature, and to
a lesser extent heating duration, to be controlling factors in product char
properties, with feedstock identity exerting less of an influence (except
insofar as feedstock identity exerts intrinsic effects on the pyrolysis
regime).

JK



On Mon, Jun 17, 2013 at 8:56 AM, Alex English <english at kingston.net> wrote:

>
>
> Julien,
> Josh Kearns has posted graphs of bed temperatures.
>
> I data logged a T-LUD on wood pellets
> http://www.bioenergylists.org/stovesdoc/English/bigtop2.htm
>
> Switching units I have choked primary air as low as possible while
> maintaining a flame. That corresponded to a peak fuel bed temperature of
> 480 C.
> My impression, though unmeasured, that the gas composition was different
> than at high temperatures. I hav also pushed the TLUD with a high pressure
> blower and at no time did it fail to produce combustible gasses.
>
> Alex
>
>
>
> On 16/06/2013 10:57 PM, Julien Winter wrote:
>
> Dear stovers;
>
> What do we know about the temperature in the fuel bed of top-lit
> updraft gasifier stoves (TLUDs)?
>
> Almost all features of TLUD function are affected by the temperature
> of the fuel bed.  This includes the combustion quality of the effluent
> gases, the stability and momentum of the reaction, water boiling rate,
> the quality of biochar, the yield of biochar, the nature of effluent
> particles, production of crystobalites, production of polynuclear
> aromatic hydrocarbons, etc.
>
> In the last 30 years, temperature data has been published for
> laboratory bench-top reactors that were, ostensibly, forced-air TLUDs.
>  Most of this work was done to understand the function of moving grate
> gasifiers for urban waste and biomass fuels (and even earlier research
> on coal).  Along with other measurements, temperature observations
> were used to calibrate computer simulations.
>
> An example of bench-top TLUD data is attached as a pdf.
>
> These bench-top TLUDs do not cover the range of conditions experienced
> in the real world of feral TLUD cookstoves.
>
> What observations are there on fuel bed temperatures in TLUD cookstoves?
>
> What range of temperatures are possible for a functional stove?  If
> the reaction is too cool, we just get incombustible shouldering smoke.
>  Will the stove function poorly if the temperature is too high and
> most of the gases oxidized to CO2 within the bed?  Is there a sweet
> spot temperature range, and does it vary between fuels (e.g., grass,
> chips, pellets, briquettes)?
>
>
> Just as an aside, someone should make virtual TLUD stove for testing
> theories and teaching.  That shouldn't be too hard for pellets.
>
> Cheers,
> Julien-the-eyebrowless
>
>
>
>
>
>
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-- 
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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