[Terrapreta] Some clarifying answers

[Ron Larson]

Hi to T-P list members

1. Erich Knight ("Shengar") wrote to me off line about the ASES report (http://www.ases.org/climatechange/toc/07_biomass.pdf ) (that I sent information about on Feb. 28) that mentioned Terra Preta. Erich said: "This statement [below as #2, last sentence of p117] is contrary to the potential that char has as a soil carbon sink." RWL response: I can see why Erich thinks the sentence to be a negative one, but I believe it is really positive - as discussed next.

2. The sentence (emphasis added) in question reads "Because char residue from pyrolysis processes would contain about 10% of the original carbon, the carbon storage component that would arise from the incorporation of the char into soils would likely be about 1 tC/ha/yr, compared with offset rates from Table 1 for power systems on the order of 4 to 6 tC/ha/yr."

3. First, we must understand that the top entries of Table 1 (p 116) gives growth rates in terms of CO2 potential avoidance (14 to 23 tCO2/ha/yr) that are higher than the 4-6 tC/ha/yr (stated above) by the CO2/C ratio of 44/12= 3.67. 

4. The charcoal production value of 1 tC/ha/yr is not explained anywhere, but seems to be consistent with central - high estimates (9 to 20+ tC/ha/yr) in Table 2 of annual biomass production in the US and the 10% charcoal estimate in the first part of the item #2 sentence. The 10% estimate is consistent with present-day gasification conversion processes, but I think we should place more emphasis on the unexplained 1 tC/ha/yr –which can be increased considerably.

5. One can change sentence #2, without changing the meaning, to read: "Because char residue from pyrolysis processes would contain about 10% of the original carbon, the carbon storage component that would arise from the incorporation of the char into soils would likely be about 1 tC/ha/yr, which can be added to offset rates from Table 1 for power systems on the order of 4 to 6 tC/ha/yr, giving a first year total offset of 5 to 6 tC/ha/yr." 

6. But we can also think of maximizing, rather than minimizing, charcoal production – to say 50% - or 5 tC/ha/yr. then the same sentence could read: "Because char residue from pyrolysis processes can contain as much as 50% of the original carbon, the carbon storage component that would arise from the incorporation of the char into soils would likely be about 5 tC/ha/yr, which can be added to half the offset rates from Table 1 for power systems on the order of 2 to 3 tC/ha/yr, giving a first-year total offset of 7 to 8 tC/ha/yr". 

7. Caution – the above large offset improvement is probably not real. However, I believe that there is no change in meaning from the original #2 – and shows that the authors of this chapter were not downplaying the TP approach. Note my inclusion of "first-year".

8. Thus, I hope Erich (and all) will agree that the above sentence #2 is in fact NOT a contrary statement. 

9. In a second follow-up off-line message, Erich also asked three more questions: 

  1.. "? how does any combustion or IGCC system store carbon in soil?" RWL answer: I agree that they can’t –but gasification/pyrolysis schemes of course can. 
b."? Are they just talking here about "Offsets"?" RWL answer: Yes and no. I think I have rewritten their sentence appropriately to show that they are talking of FIRST-YEAR offset reductions. The important point is that, unlike the direct offset, carbon in the soil has a MULTI-YEAR impact. If the overall impact is a C-doubling for 10 years, the soil carbon (charcoal) should be interpreted as having the huge offset value of 2*10 = 20 times the C-amount placed in the ground. This is the area where we need lots of help from soil science economists. I will try to explain this more ASAP in a separate message. Hopefully, we will see lots of discussion of this at the Terrigal conference.

c. "?The range of Tons of Charcoal per hectare I have read has been from 1 to 16 ton/ha?" RWL answer: I have seen similar - with your "ton" (in English units: 2000 lb = short ton) generally being interpreted to mean in metric units: "tonne" = 1000 kg = 2200 lb = long ton. Note that 10 t/ha = 10^4 kg/10^4 sqm = 1kg/sqm. If we assume the soil has a density of 1 (meaning = 1g/cm^3 = 1 kg in 1 liter (10cm on a side) = 1000 kg (1 tonne) /cubic meter), then 1 kg/sqm is equivalent to a 1 % loading over the top 10 cm or a 10% loading over the top 1 cm (i.e. - 1 kg uniformly distributed over 0.1 or 0.01 cubic meter). It appears that the ancient Amazonians added perhaps 100 times more than 10 tonnes/ha (but took many centuries or a few millenia to accomplish this). Anyone have alternative numerical analyses?

d. (in a third message) "I still don't understand why any offsetting technology, which does not place carbon in the soil, would be characterized by a tons per hectare per year designation, I find this confusing. Wouldn't it be clearer if offsets were described as just tons per year? 

RWL answer: I agree with Erich that these units can confusing. We in this Terra Preta discussion group need to and can learn much from this Biomass chapter about present and future biomass growth rates - the likelihood of increasing biomass metrics in units such as tC/ha/yr. [And we on the "terrapreta" list especially should note that there will always be confusion with these units between growth and sequestration rates!!] The authors of this ASES report chapter (who have an interest in both topics) report that we have in the US a present national growth capability of one gigaton per year of biomass. This large number apparently is roughly the same as 10 tC/ha/yr (on average, with a wide range). Can the terra preta sequestration rate concept double that growth rate number? 

My friend Ron West, ChE Professor-Emeritus, has noted in helping me answer Erich that carbon is only about 40% of bone dry biomass – so we need also to be clear in whether tonnes are of carbon or total biomass.

Main RWL conclusion: It is time to add a major terra preta research category to our world (and especially US) energy RD&D efforts to answer these coupled sequestration and growth rate questions – much as we have been supporting some limited research on the growth rate question. Note the frustration of several other recent messages on the falure to get US Federal funding in these areas.

     Thanks to Erich for asking for these clarifications.

Ron


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