[Terrapreta] Economics of biochar

[Sean K. Barry]

Hi Greg,

I get a lot of my information from the Biomass Energy Foundation library, at www.woodgas.com<http://www.woodgas.com/>

Regards,

SKB
  ----- Original Message ----- 
  From: Greg and April<mailto:gregandapril at earthlink.net> 
  To: terrapreta at bioenergylists.org<mailto:terrapreta at bioenergylists.org> 
  Sent: Tuesday, January 08, 2008 6:05 PM
  Subject: Re: [Terrapreta] Economics of biochar


  Ok I admit it, I'm in over my head.

  Does anyone else want to fess up too, and tell me where I can find a copy of " Terra Preta for Dummies ",  " The Idiots Guide to Char from Gasifiers " or perhaps even " Char in 3 easy steps " ?

  LOL

  Greg H.

    ----- Original Message ----- 
    From: Sean K. Barry<mailto:sean.barry at juno.com> 
    To: terrapreta at bioenergylists.org<mailto:terrapreta at bioenergylists.org> ; andrew<mailto:list at sylva.icuklive.co.uk> 
    Sent: Tuesday, January 08, 2008 15:14
    Subject: Re: [Terrapreta] Economics of biochar


    Hi Andrew,

    What happens if the number for the yield of charcoal to dry weight of the feedstock is raised to ~30-40%, approaching stoichiometric?  I've read that the energy content of the charcoal goes to maybe ~60%?

    In your pyrolysis reaction (carbonization) 2.97 MJ / 18.8 MJ = ~16% of the total energy is in the 11% yield of charcoal ?

    Or, is it 2.97 MJ / 15.4 MJ = ~19% of the total energy is in the 11% yield of charcoal ?

    The speed of the pyrolysis reaction is driven by the particle size and the ratio of feedstock to oxidant (lambda).  The reactor can be insulated, to maintain a more uniform "core" temperature (in the "exothermic" temperature zone).  Then the diameter & speed (kg/h) of the biomass flow through the reaction zone and same diameter of oxidant flow can be used to measure the reactor's operating lambda.

    Most carbohydrates in biomass have a C:H:O ratio of 1:2:1, like sugar C6H12O6.  With the temperature maintained at or below the stoichiometric for complete combustion of carbohydrates, picking the lambda to run at is basically dictated by the general reaction sequence, ...

        nCH2O + mO2            <=>     nH2 + nCO + mO2     <=>      nH2O + nCO2,
                                    pyrolysis                          combustion

        feedstock + oxidant    <=>  "synthesis gas"            <=>      complete combustion exhaust gases

    As m approaches n, the lambda (n/m) approaches 1.0 (m:n -> 1:1).  Because air is only 19% oxygen, the stoichiometric lambda value is ~0.25 when air used as the oxidant.

    When the oxidant flow is held below the stoichiometric value (operating lambda held above ~0.25), then the reaction is maintained at just entering "exothermic" temperature zone.  The reaction is more in a pyrolysis phase; producing more BTU containing gases, less complete combustion gases, less heat loss, and more charcoal.  If more oxidant is allowed (lower lambda), then combustion ensues and more heat is liberated (a more "exothermic" phase).

    I am looking at a design for a down-draft gasifier.  The feedstock and the air-oxidant are both entered through the top of the reactor.  The pyrolysis reaction proceeds below inside a charcoal bed.  It lies in a reaction zone near the top, but below fresh feedstock and above a cooler, moving (downward) charcoal bed.  The charcoal and "producer" gas both exit out the bottom of the reactor.

    "Holding" the pyrolysis reaction temperature at (or below) the "exothermic" reaction temperature is accomplished by moving biomass out of the reaction zone, as it begins to get hotter.
    This is the same as removing some charcoal at the bottom and adding some new feedstock at the top (get this, in the desired yield ratio?!).  The biomass in the bed above the reaction is un-pyrolyzed because it is still below the reaction temperature.  The charcoal bed below will stop pyrolyzing when it drops below the "exothermic" reaction temperature.

    With this down-draft reactor, the issue of leaving energy in the charcoal versus harvesting available process energy for use has become a matter of the range adjustment on lambda.  Setting the desired charcoal yield of the reactor, by unloading charcoal and loading fresh biomass feedstock (at the yield ratio), will dictate when the temperature rises into the "exothermic" temperature zone, when to move the bed, and the overall biomass processing speed of the reactor.  The biomass processing speed and lambda dictate the oxidant flow rate.  With lambda held close and below 0.25, or ~0.24, then the reaction will release the least amount of excess energy.

    The oxidant flow rate is then,  (moles/h air / mole density of oxidant in air) = (grams(biomass)/h / mole density of biomass reactant) * lambda.

    Because air is only 19% O2, then the mole density of the oxidant in air is 19% of the mole density of pure O2 (0.19 * 32).

    The mole density of the biomass reactant is 30, because CH2O has C:H:O at 1:2:1, with mole density of 12 + 2 + 16 = 30.

    So, (moles/h air / (0.19 * 32)) = (grams(biomass)/h / 30) * 0.24

    Air flow, then, in moles/h @ lambda = 0.24 would be ...

    kg(biomass)/h * (0.24 * 0.19 * 32) / (30E-3 kg/mole (biomass)) = ~49 * kg(biomass)/h

    There is another value, called the "superficial velocity" of the reaction.  It is the cross-sectional flow of biomass through the reactor in, measured MJ/m^2*h.  Some how this relates
    to what I am trying to discuss here.  Controlling this "superficial velocity" changes how the reactor is operated to produce the products.  Changing this number changes the relative amounts, composition, and speed of production of all the byproducts.

    Regards,

    SKB

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