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Charcoal for terra preta
Michael J. Antal, Jr, Goro Uehara, Jonathan Deenik,and Tai McClellan
Hawaii Natural Energy Institute andThe College of Tropical Agriculture & Human Resources
University of Hawaii at Manoa
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Modern Biomass Refineries• Ethanol from corn grain and biocarbons
from corn stover (USA)• Biodiesel from sunflower oil and biocar-
bons from sunflower shells and stalks (EU)• Biodiesel from coconut oil and biocarbons
from coconut shells, fronds, etc. (Malaysia)• Biodiesel from marine algae and biocarbons
from residual (dry) algal material (Hawaii)
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Fuel Costs
$18-24/GJHydrogen$6-17/GJGas$14/GJEthanol$15/GJOil$8/GJCharcoalCoal
RENEWABLEFOSSIL
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How can we use charcoal?• Potting soil (orchids and ornamentals)• Cooking (barbeque) fuel• Ultra clean coal (power production)• Activated carbon (water treatment)• Metal reductant• Terra preta (carbon sequestration!)• Biocarbon fuel cell
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Some questions concerning the pro-duction of biocarbons:1. In theory, what limits the yield of bioC
(charcoal) from biomass?2. In theory, what is the energy conversion
efficiency of biomass into bioC? 3. In practice, what yield and energy
conversion efficiency can be achieved?4. In practice, how quickly can we convert
biomass to bioC?
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Useful definitions:
1. ychar = mchar / mbio
2. 100 = % VM + % fC + % ash; where
VM = volatile matter; fC = fixed carbon
3. yfC = ychar × {% fC / (100 - % feed ash)}
4. ηchar = ychar × (HHVchar / HHVbio)
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Thermochemical equilibrium predictions for the products of cellulose pyrolysis at 400 C (Ind. Eng. Chem. Res. 2003, 42, 3690-3699).
• C, H2O, CO2, and CH4 are the only significant products.
• The theoretical charcoal (i.e. C) yield is 28 wt%.
• The gas contains significant energy (i.e. CH4).
Pressure (MPa)
0.001 0.01 0.1 1 10
Mas
s fr
actio
n (%
)
0
10
20
30
40
50
C(s)
CO2
H2O(g)
CO
CH4
(a)
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Reaction stoichiometry for the products of cellulose pyrolysis at 400 C & 1 MPa (Ind. Eng. Chem. Res. 2003, 42, 3690-3699)
C6H10O5 → 3.74 C +
2.65 H2O + 1.17 CO2 + 1.08 CH4
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Energy balance for cellulose pyrolysisfollowing thermochemical equilibrium (Ind. Eng. Chem. Res. 2003, 42, 3690-3699)
0 5000 10000 15000 20000
input
output
Energy [kJ/kg-cellulose]
cellulosespecific heat
carbongas
worksensible heat
exotherm
0 5000 10000 15000 20000
input
output
Energy [kJ/kg-cellulose]
cellulosespecific heat
carbongas
worksensible heat
exotherm
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Plot of charcoal yield from cellulose pyrolysis vs. pressure (Thermochim. Acta, 1983, 68, 165-186).
• Pressure strongly favors formation of charcoal.
• Low gas flow rates also favor the formation of charcoal.
• Elevated pressure and low flow rates together double the yield of charcoal.
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Flash CarbonizationTM reactor schematic (U.S. patent # 6,790,317; September 14, 2004).
PG
DDV
SRD
PRV
flare
IV UDV
MMV
R
H
ATW
C
A
HIC
DS
TCTC
TCTC
TCTC
TCTC
IV
TC
PT
PT
GSP
WT
WT
H
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Parity plot of Flash CarbonizationTM fixed-carbon yields from various biomass feedstocks(Ind. Eng. Chem. Res. 2003, 42, 3690-3699)
• Fixed-carbon yields from corn cob, oak, and macshellapproach the theoretical limit.
• Leucaena offers almost 90% of the theoretical limit.
yfC - experimental (%)
20 25 30 35 40 45
y fC
- th
eore
tical
lim
it (%
)
20
25
30
35
40
45
90%
80%
100%
MSLW-A1
LW-OOW-2
OW-1CC
LW-A2
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Flash CarbonizationTM demo reactor on the UH campus
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Picture source: http://www.gerhardbechtold.com/TP/gbtp.php
Terra Preta (Amazonian Dark Earths): Highly Fertile Anthropogenic Soils
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Terra Preta SoilTypical Upland Amazonian Soil
Photo source: University of Bayreuth Photo source: University of Bayreuth
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16Terra Preta Unamended Soil
Effect of Terra Preta on Plant Growth
Photo source: http://tinselwing.wordpress.com/tag/terra-preta/
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Volcanic ash soil treated with flash carbonizedmacadamia nut shell charcoal
0% (w/w) 5% (w/w) 10% (w/w) 20% (w/w)
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Control 20% (w/w) charcoal
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Lettuce Shoot Biomass
0% 5% 10% 20%
Plan
t wei
ght (
gram
s/po
t)
0
20
40
60
80
100
120
140
160
a a
b
c
Charcoal Rate (w/w)
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Charcoal Effect in an Acid, Infertile Soil
0% 5% NPK + Lime
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NPK + Lime 5% + NPK + Lime
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Preliminary Conclusion:
Charcoal used in the experiment caused a negative effect on plant growth
But why?• Crop?• Soil??• Charcoal???
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• Volatile Matter (VM) content: a measure of the susceptibility of charcoal to further decompose and form carbon when heated
Hydrophobic Hydrophilic
22.5% VM Content 6.3% VM Content
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Effect of High Volatile Matter (22.5%) Charcoal on Plant Growth
0% 10% High VM
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Effect of Low Volatile Matter (6.3%) Charcoal on Plant Growth
0% 10% Low VM
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Low Volatile Matter Charcoal (6.3%) versus High Volatile Matter Charcoal (22.5%)
Low Volatile Matter
High Volatile Matter
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Combined Effect of Low Volatile Matter Charcoal Plus Fertilizer
NPK + Lime
NPK + Lime + 10% Low VM
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Volatile Matter or Feedstock?• VM content affected plant growth in macnut
shell charcoal• Does feedstock make a difference?• Repeat trial with corn cob charcoal
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Control High VM
Low VM
Effect of corn cob charcoal on soybean
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Waller-Duncan K Ratio t-test
Control HVM LVM
Soy
bean
Fre
sh W
eigh
t (g/
pot)
0
2
4
6
8
10
12
14
a
bb
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Control ControlLimeNPK
High VM
NPK
Low VM
NPK
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Waller-Duncan K Ratio t-test
Control Lime+NPK HVM+NPK LVM+NPK
Soy
bean
Fre
sh W
eigh
t (g/
pot)
0
10
20
30
40
abb
c
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Conclusions
• Volatile matter content influences a charcoal’s effectiveness as a soil amendment
• Low volatile matter charcoals are more effective soil amendments than high volatile matter charcoals
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Future Studies
• Will the positive effects observed in greenhouse tests carry over into field trials?
• Will the positive effects persist or diminish with time?
• Will the negative effects of high volatile matter charcoal persist or diminish with time?