[Terrapreta] Strong warning against "simple" charcoal kilns

[Sean K. Barry]

Hi Richard,

"Burning" 500 pounds of biomass down to nothing but ash produces complete combustion byproducts, with very nearly an entire consistency of only CO2 and H2O.  This plus lots of heat.  Pyrolizing 500 pounds of biomass to produce 150 pounds of charcoal will (because of the reduced amount of oxygen available [reduced air] during pyrolysis) generate "producer gas", as I describe below.  The Methane-CH4 content in "producer gas" is the most important difference between these resultant emissions.

Biomass basically consists of molecules with a chemical composition of N x (CH2O).  For the nit pickers, it's not exactly that, but its close!

"Burning" generates "complete combustion byproducts" only

    n(CH2O) + n(O2) => n(CO2) + n(H2O)

Pyrolysis generates "producer gas"

    n(CH2O) + m(O2) => ~2n * [0.20(CO) +  ~0.20(H2) +  ~0.13(CO2) + ~0.05(H2O) + ~0.03(CH4)], where m < ~0.25n

The GHG Methane-CH4 makes up ~2-3% of the "producer gas" content.  Methane-CH4 is non-existent in "complete combustion byproducts".
150 pounds of charcoal @ ~90% carbon is 135 lbs of carbon.

So, bury the 150 lbs of charcoal and you offset the emissions of ~495 lbs of CO2 [0.90 * 150 lbs * (12 + 16 + 16) / 12, (12 + 16 + 16) is the atomic weight of CO2].  Since the feedstock has molecules with an atomic weight of ~(12 + 1 + 1 + 16 = 30) and the carbon in the charcoal has an atomic weight of 12, then ~12/30 (~0.40) of the total weight of the biomass is carbon.  So, the weight of the original carbon in the 500 pounds of biomass is then about ~200 lbs (= 0.40 * 500 lbs).  The weight of the carbon retained in the charcoal is 135 lbs (0.90 * 150 lbs).  With a "fixed Carbon" yield of 135/500 = 0.27, the yield of carbon from that pyrolysis would be quite high (0.27/0.40) = ~68%!.  This would be outstanding for "open air" pyrolysis, but let's go with it.

"Producer gas" released during pyrolysis has in it (~20% CO  + ~13% CO2 + ~3% CH4).  The ratio then of carbon in Methane-CH4 molecules in the "producer gas" to carbon NOT in Methane-CH4 molecules (that is CO or CO2) in the gas is then, conservatively, about 9% (= 0.03/(0.20 + 0.13) = 0.03/0.33 = ~0.09).  This results, then, in 65 lbs (= 200 - 135 lbs) of carbon being released in the "producer gas" with ~6 lbs (0.09 * 65 lbs) of that as carbon in Methane-CH4 molecules.

On the weight vs weight balance then, it looks like 135 lbs of carbon sequestered into the ground and only ~65 lbs of carbon released into the air, with ONLY ~6 lbs of that carbon being in Methane-CH4 molecules.  Cool, huh?

NOT QUITE!  On a molecule vs molecule basis Methane-CH4 is a far more potent contributor to the green house warming effect than CO2 is.
A molecule of Methane-CH4 is 62 times more potent than a molecule of CO2 in the first 10 years of life in the atmosphere.  Methane-CH4 is 23 times more potent than CO2 over a 100 year life span in the atmosphere.  Methane-CH4 (contrary to what some believe) is a long lived gas in the atmosphere.  It has a half-life of ~12 years.  This means that after ~12 years only half will be gone.  It can linger in the atmosphere for many decades, even upwards of 100 years for some of it.  It DOES NOT just all disappear in a few years!

Now, use the "greenhouse warming potential" scale and balance these emissions versus the buried carbon.  135 lbs of carbon into the ground.
59 lbs (= 65 lbs  - 6 lbs) of carbon in CO or CO2 molecules and 6 lbs of carbon in CH4 molecules.  Let's say all the CO burns (or decays) to CO2, then the 59 lbs of carbon will look like only ~216 lbs of CO2 (= 59lbs * 44/12) released to the atmosphere.  Carbon monoxide-CO burns more readily than CH4 and is decays (intoCO2) much more readily in the atmosphere than does CH4 (into CO2 and H2O).

Still we buried 135 pounds of carbon in charcoal (sequestered it) into the ground.

The ~6 lbs of carbon in the ~8 lbs of Methane-CH4 (= 6 lbs * (12 + 1 + 1 +1 +1 )/12) is also potentially ALL released in "open air" pyrolysis.
That 8 lbs of Methane-CH4 will effect the atmosphere like adding ~1364 lbs of CO2 into the air (= 62 * ~6 lbs (44/12)) for ten years.
It will be like adding ~506 lbs oc CO2 (= 23 * ~6 lbs (44/12)) for one hundred years.

The GHG balance is then 65 lbs of carbon emitted out into the atmosphere, which acts like 1580 lbs of CO2 (= 216 + 1364 lbs) for the next 10 years, and like 722 lbs of CO2 (216 + 506 lbs) for the next 100 years.  In neither case (1580 * 12/44 = 430 lbs), nor (722 * 12/44 = 196 lbs) will the negative effect of carbon containign GHG molecule introductions (as CO2 and CH4) to the the atmosphere be LESS than benefit to be had for burying the 135 lbs of carbon.

ANYONE who would like to challenge me on this analysis, feel free to do so.  I think it is sound and it does point out very emphatically, that we cannot pyrolyze biomass in "open air", semi-open-air", or  "charcoal-in-a-barrel" kilns and EVER hope to manage GHG concentrations in the atmosphere and bring them down.  If we hurry up and do this anyway, ignoring what I suggest, then the immediate future will get far worse much faster than it is even now.


Regards,

SKB



  ----- Original Message ----- 
  From: Richard Haard<mailto:richrd at nas.com> 
  To: Sean K. Barry<mailto:sean.barry at juno.com> 
  Sent: Sunday, April 27, 2008 8:11 PM
  Subject: Re: [Terrapreta] Strong warning against "simple" charcoal kilns




  On Apr 27, 2008, at 5:23 PM, Sean K. Barry wrote:


    Now, the "charcoal-in-a-barrel" kiln?  Well, when biomass if pyrolyzed in open air (or limited amounts of air).  This pyrolysis reaction generates "producer gas" (H2:~20~, CO:~20%, CO2:~10-15%, H2O:~5%, Methane-CH4:~2-3%, and Nitrogen gas-N2:~40-45%)





  I'm still puzzled by this. Today we burned tree debris that came down in winter storms. little or no charcoal. Lets say 500 lbs of debris. Now  it' s all ash'


  If we would have pyrolysed this with 30% yield then we would have  about 150 lbs of charcoal and  the fire would have consumed 350 lbs of wood. Seems like no matter how you slice this one any kind of pyrolysis is better than burning debris ??


  Rich
-------------- next part --------------
An HTML attachment was scrubbed...
URL: /attachments/20080427/0364bc17/attachment.html