[Terrapreta] Economics of biochar

[Sean K. Barry]

Hi Andrew,

This is a very interesting post.  I like both the application and the process descriptions.  I have some (lots of) questions.

Is the total energy, 15.4 MJ/kg, estimated from the BTU of the dry arisings?

2.97 MJ is the estimated energy that is left remaining in the ~11% charcoal yield (dw% = 100% x (0.323 - 0.210)/1.22)) (with >80% fixed Carbon (fC) in the charcoal byproduct)?

I see, (15.4 MJ/kg) * (1.22 kg) = ~18.8 MJ total energy content of the sample and ~18.8 MJ - 2.97 MJ = ~15.8 MJ released ?

How do you arrive at the amount of energy released?  You state some dry matter was lost in the carbonization because of combustion.
Was that, 0.22 kg, so that ...

(15.4 MJ/kg) * (1.22 kg - 0.22 kg)  - 2.97 MJ = ~12.5 MJ ?  

You also stated that the other released heat energy of the process was enough to dry 3-5 times the biomass, plus bring that to the exothermic temperature.  What is the energy conversion efficiency of the pyrolysis process then?

    processing efficiency = 1 - (MJ needed to pyrolyze the biomass / MJ in biomass)?

    = MJ needed to pyrolyze the biomass = ~12.5/3 to ~12.5/5 = ~4.2 to ~2.5 MJ?
       (the amount of energy required to dry and bring a kg of biomass to exothermic,
        not including loading/unloading, transportation, or other handling)

    = 1 - (MJ needed to pyrolyze the biomass / MJ in biomass)?

    = 1 - (~4.2/15.4) to 1 - (~2.5/15.4)?

    = ~73% to ~84% energy conversion process efficiency?

With only 11% of the energy left in the charcoal yield, then what % of the energy did this pyrolysis reaction release as heat and gases?

~73-11% = 62 to 73% = 84-11% ?

81% = ~12.5 MJ / 15.4 MJ ?

84% = ~15.8 MJ / ~18.8 MJ ?

Anything from 62 to 84% !?

What portion of that released energy (12.5 to 15.8 MJ) is in the product gas BTUs as combustible fuel gases and what portion is heat?

Would it be possible to tune down this reaction by moving the biomass feedstock through the reaction faster, producing less gases, less heat loss, and more charcoal?  Assuming that we wish to pyrolyze completely (react until the exothermic stops), then how long does the biomass need to stay above the exothermic temperature?  Once it reaches that temperature, then how long before we move it out of the reaction zone, where it is cooler, or has less oxygen?

I guess what I am grappling with trying to ask here, is if there is a way to throttle this reaction, to control the relative amounts, composition, and qualities of the byproduct outputs; charcoal, gases, and heat, by adjusting feedstock, gas, and/or charcoal flow controls, insulating the reaction, and while using temperature as the feedback?

It has been discussed here, as well as in some referenced papers, that control over maximum internal particle temperature can best help manage VM% and porosity of the resulting charcoal byproduct.  Is seems that control over these properties is desirable for potential users of agrichar.  Shouldn't we then maintain the quality of the charcoal product, primarily by controlling the process temperature?

The 62 to 84% energy "released" as heat and gases is another concern.  Should more of that energy go into the charcoal or should some of it be harvested and converted to usable work, heating, or electrical power?

It seems to me, that the ratio of the two byproducts; charcoal and/or usable process energy could depend on the economic values?  But I think the value of the energy harvested should only cover the cost of producing the charcoal yield.  By this, I mean minimize the harvested energy to only that required to process the biomass into charcoal.  This will maximize the charcoal yield.  This goes counter to the economic argument, though, because the energy may perhaps be worth more than the charcoal (in the short term).

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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