[Terrapreta] Economics of biochar

[Sean K. Barry]

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


  ----- Original Message ----- 
  From: andrew<mailto:list at sylva.icuklive.co.uk> 
  To: terrapreta at bioenergylists.org<mailto:terrapreta at bioenergylists.org> 
  Sent: Tuesday, January 08, 2008 6:16 AM
  Subject: Re: [Terrapreta] Economics of biochar


  On Monday 31 December 2007 13:08, andrew wrote:
  > I have collected a sample by
  > scraping and 3kg wet has reduced to kg air dry, I'll carbonise
  > this and then ash it just to see what the energy balances look
  > like with inevitable soil contamination.

  Just in case anybody was following this, it's a bit UK centric but 
  here goes:

  I was looking at "other" woody residues that would normally be left 
  to rot on the ground to consider their possible worth as a carbon 
  sink.

  As I had been working shredding standing invasive species 
  (rhododendron) within a 60 year old pine plantation I collected a 
  small 3kg sample of the arisings. I estimated that this represented 
  30Tonnes/ha of material.

  I then dried, charred and ashed the sample to see what the yields 
  were, bear in mind I expected fairly heavy soil/stone contamination. 
  In fact I only found 2 small stones in the ash.

  The figures:
  Wet sample    2.971kg
  air dry       1.556kg
  oven dry      1.220kg
  char [1]      0.323kg
  ash           0.210kg

  [1] there was some loss of dm in the charring as the tlud burn went 
  into a combustion phase and the carbonising was completed in a 
  biscuit tin retort in the wood stove, I would estimate the max 
  temperature to be 600C and the fixed carbon >80%

  This shows that the mid winter harvested shredded material and forest 
  litter had a wet basis water content of ~60% and the ash free yield 
  of char was 11% of total dry matter.

  Given the high ash/soil contamination and reduced to a per kg basis 
  it looks like there was a total energy of 15.4MJ/kg oven dried matter 
  of which 2.97MJ was left in the char. The process gave off 12.5MJ of 
  energy which would have been adequate to supply the drying and 
  heating to pyrolysis temperature of between 3 and 5 times this 
  amount of wet material depending on process efficiency and process 
  temperature. Lower temperature would retain more energy in the char, 
  increase the gross amount of lower fixed carbon char but reduce 
  process energy available.

  Now whether it would be economic to do this in order to sequester 
  just over 1 tonne of 80% fixed carbon char/ha whilst handling 30 
  tonnes of material is another matter.

  AJH

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