Life cycle assessment of biochar production from corn stover, yard waste, and switchgrass

1
Life cycle assessment of biochar production from
corn stover, yard waste, and switchgrass

• Kelli G. Roberts,1 Brent A. Gloy,2 Stephen
  Joseph,3
• Norman R. Scott,4 Johannes Lehmann1
• North American Biochar Conference
• Boulder, Colorado
• August 11, 2009

1 Department of Crop and Soil Sciences, Cornell
University 2 Department of Applied Economics and
Management, Cornell University 3 School of
Materials Science and Engineering, University of
New South Wales, Australia 4 Department of
Biological and Environmental Engineering, Cornell
University
2
What is Life Cycle Assessment (LCA)?

• Methodology to evaluate the environmental burdens
  associated with a product, process or activity
  throughout its full life by quantifying energy,
  resources, and emissions and assessing their
  impact on the global environment.
• LCA has been standardized by the ISO
  (International Organization for Standardization).

• Life cycle of a product

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Goals of the LCA

• To conduct a cradle-to-grave analysis of the
  energy, greenhouse gas, and economic inputs and
  outputs of biochar production at a large-scale
  facility in the US.
• To compare feedstocks (corn stover, yard waste,
  switchgrass).
• To understand the role of transportation in the
  life cycle.

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Scope the functional unit

• The functional unit
• A measure of the performance or requirement for a
  product system.
• Provides a reference so that alternatives can be
  compared.
• Our functional unit
• The management of one tonne of dry biomass.

5
Scope system boundaries
Dashed arrows with (-) indicate avoided
processes. The T represents transportation.
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LCA of biochar industrial scale

• The slow pyrolysis process has four co-products
• Biomass waste management
• Biochar soil amendment
• Bioenergy production
• Carbon sequestration

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Energy flows feedstock to products
Sankey diagram, per dry tonne stover
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Feedstocks

• Corn stover
• Late and early harvest (15 and 30 mcwb).
• Second pass collection, harvest 50 above ground
  biomass.
• Yard waste
• No environmental burden for production.
• 45 mcwb
• Assumed to be diverted from large-scale
  composting facility.
• Switchgrass
• Scenarios A and B
• 12 mcwb

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Feedstocks (cont.)

• Switchgrass A
• Lifecycle emissions model (Deluchi), informally
  models land-use change.
• Assumes land conversion predominantly temperate
  grasses and existing croplands, rather than
  temperate, tropical or boreal forests.
• Net GHG of 406.8 kg CO2e t-1 dry switchgrass
  harvested.
• Switchgrass B
• Searchinger et al (2008) global agricultural
  model.
• Assumes land conversion in other countries from
  forest and pasture to cropland to replace the
  crops lost to bioenergy crops in the U.S.
• Net GHG of 886.0 kg CO2e t-1 dry switchgrass
  harvested.

Deluchi, M. A lifecycle emissions model (LEM)
UCD-ITS-RR-03-17 UC Davis, CA, 2003.
Searchinger, T. et al. Science 2008, 319
(5867), 1238-1240.
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Energy balance

• All feedstocks are net energy positive.
• Yard waste has the highest net energy.
• Agrochemical production and drying consume
  largest proportion of energy.
• Biomass and biochar transport (15 km) consume lt
  3.
• Other category includes biochar transport,
  plant dismantling, avoided fertilizer production,
  farm equipment, and biochar application.

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GHG emissions balance

• Stover and yard waste have net (-) emissions
  (greater than -800 kg CO2e).
• However, switchgrass A has -353 kg CO2e of
  emissions reductions, while B actually has net
  emissions of 127 kg CO2e.
• Other category includes biomass transport,
  biochar transport, chipping, plant construction
  and dismantling, farm equipment, biochar
  application and avoided fertilizer production.

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GHG emissions (cont.)

• Biomass and biochar transport (15 km) each
  contribute lt 3.
• The stable C sequestered in the biochar
  contributes the largest percentage ( 60) of
  emission reductions.
• Avoided natural gas also accounts for a
  significant portion of reductions (30).
• Reduced soil N2O emissions upon biochar
  application to the soil contributes only 2-4 of
  the total emission reductions.

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Economic analysis

• High revenue scenario
• 80 t-1 CO2e
• Syngas product
• Low revenue scenario
• 20 t-1 CO2e
• Electricity product

• The high revenue of late stover (35 t-1
  stover).
• Late stover breakeven price is 38 t-1 CO2e.
• Neither switchgrass A nor B is profitable.
• Yard waste biochar is economically profitable
  even with no C price.
• Highest revenues for waste stream feedstocks with
  a cost associated with current management.

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Transportation sensitivity analysis

• The net revenue is most sensitive to the
  transport distance, where costs increase by 0.80
  t-1 for every 10 km.
• The net energy is more sensitive than GHG
  emissions to distance.
• Transporting the feedstock and biochar each 200
  km, the net CO2 emission reductions decrease by
  only 5 of the baseline (15 km).
• Biochar systems are most economically viable as
  distributed systems with low transportation
  requirements.

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Conclusions

• Careful feedstock selection is required to avoid
  unintended consequences such as net GHG emissions
  or consuming more energy than is generated.
• If energy crops are grown on land diverted from
  annual crops, indirect land-use change impacts
  could mean that more GHG are actually emitted
  than sequestered.
• Waste biomass streams have the most potential to
  be economically viable while still being net
  energy positive and reducing GHG emissions (over
  800 kg CO2e per tonne feedstock).
• Valuing greenhouse gas offsets at a minimum of
  38 t-1 CO2e and further development of
  pyrolysis-biochar systems will encourage
  sustainable strategies for renewable energy
  generation and climate change mitigation.

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Acknowledgements

• Cornell Center for a Sustainable Future (CCSF)
• John Gaunt (Carbon Consulting) Jim Fournier
  (Biochar Engineering)
• Lehmann Biochar Research Group, especially Kelly
  Hanley, Thea Whitman, Dorisel Torres, David
  Guerena, Akio Enders

Thank you!
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Scaling up first approximations
Calculated based on late stover model
Pacala Socolow 2004, Science, 305, 968
Currently unused crop and forestry residues
(Perlack 2005, US DOE and USDA The technical
feasibility of a billion-ton annual supply)
Currently unused crop residues (Krausmann
2008, Ecol. Econ. 65, 471)
IPCC recommendation to stabilize warming at
2.0-2.4C is to decrease fossil fuel GHG
emissions by 50 in 2050
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Inventory results
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Alternative biomass uses

• biochar-to-soil (-838 kg CO2e t-1 dry stover) vs.
  biochar-as-fuel (-593 kg CO2e t-1 dry stover)
• 41 more reduction for biochar-to-soil
• biomass direct combustion (not including avoided
  fossil fuels) the resulting net GHG are 76 kg
  CO2e t-1 stover vs. -539 kg CO2e for
  biochar-to-soil.
• This indicates that emission reductions are
  greater for a biochar system than for direct
  combustion.
• If natural gas is used as the avoided fossil fuel
  in both scenarios, the net GHG are -985 and -838
  kg CO2e t-1 dry stover for the biomass combustion
  and biochar-to-soil, respectively.
• Net GHG look comparable.
• However, in the biochar-to-soil scenario, 589 kg
  of CO2 are actually sequestered in soil vs.
  avoided emissions.
• Transparent system boundaries!

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Process data baseline assumptions

• Transport feedstock 15 km farm to plant.
• Pyrolysis facility, 10 dry tonnes/hr.
• Emissions from pyrolysis and syngas combustion
  are biogenic CO2 and H2O only.
• Excess syngas replaces natural gas production and
  combustion.
• Process heat included only heat used in drying,
  excess heat does not contribute to net energy.
• Transport biochar 15 km plant to farm.
• Apply biochar at 5 Mg C per ha.

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Switchgrass A versus B

• The differences between the models arise from a
  number of factors
• LEM does not utilize a formal partial-equilibrium
  model for agricultural change as does Searchinger
  et al
• LEM assumes that crop yields abroad will equal
  those in the US
• LEM does not account for the loss of annual
  sequestration on converted forest
• and LEM uses a discount rate for emissions
  accounting of 30 years, while Searchinger et al
  calculate the net impact over a 30 year period

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Biogenic CO2 emissions
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Biochar application

• Assumed fuel use is similar to fertilizer
  application
• 506 MJ/ha diesel
• 60 MJ/ha embodied energy in manufacturing,
  transport and repair of machinery

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Sensitivity analysis summary

• GHG emissions are relatively insensitive to
  rather large changes in biochar properties such
  as between 80 and 90 of stable C (9 change) and
  between 80 and 0 decrease in N2O emissions from
  soil.
• The GHG balance is more sensitive to feedstock
  collection (change from -13 to 3 depending on
  assumptions), in contrast to the energy balance
  (lt3 change).
• The net energy is very sensitive to the syngas
  energy yield however, even a conservative
  estimate of 50 of the baseline results in a net
  positive energy balance, even though it is 66
  less than the baseline.
• The net energy is also sensitive to the avoided
  fossil fuel process (9 and 23 increased for
  diesel and coal, respectively), while the GHG
  balance changes only 7.

(b)
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Composting avoided process
Net CO2e 19.96 kg CO2e per tonne yard waste
(45 mcwb)