Presentation given at Fall 2013 NEXTSTEPS research symposium

1. Modeling the Life Cycle Environmental
Impacts of Cellulosic Biofuel
Production
Colin Murphy PhD
Research Affiliate – UC Davis Energy Institute
Science & Technology Policy Fellow – California Council on Science & Technology.
NEXTSTEPS Winter Symposium
12/10/2013

2. Outline
1.
Soil Organic Carbon Changes from Corn Stover Harvest
2.
Life Cycle Analysis of Biochemical Cellulosic Ethanol
Production Systems
3.
Implications for Biofuels

3. Soil Organic Carbon
Changes from Corn
Stover Harvest

4. Soil Organic Carbon
Root
Growth
Biomass
Incorp.
Soil Carbon Stock
Microbial
Resp.
Erosion / Runoff

5. Research
•
Literature review: 21 studies which measure SOC changes with at least
two levels of residue removal.
•
Used linear and logistic regression to identify effect of removing stover on
SOC.
•
Collaborators: Gabriel Lade, Lindsay Price, Boon-Ling Yeo, Alissa Kendall
•
Within-Field (WF) SOC Change:
 Final SOC – Initial SOC
•
Between-Field (BF) SOC Change:
 Final SOC(stover removed) – Final SOC (stover retained)
•
Residue removal rate: Likely rates 20-50%, but limited data exists for this
range.
•
Parameters of interest: Tillage, Fertilization, Soil Texture

6. BF Results
•
Very robust and
significant effect
from residue
removal.
•
No other significant
effects.
•
No significant
difference between
tillage types.

7. BF Results
•
Very robust and
significant effect
from residue
removal.
•
No other significant
effects.
•
No significant
difference between
tillage types.

8. BF Results
•
Very robust and
significant effect
from residue
removal.
•
No other significant
effects.
•
No significant
difference between
tillage types.

9. Implications
•
For this set of studies, the average effect of increasing residue removal
from zero to 30% (all other things equal) is to remove ~200-750 kg SOC
per hectare per year, from the top 30cm of soil.
 This yields about 30 g CO2e per MJ of delivered fuel @ 70 gal/Mg.
•
SOC eventually equilibrates, so this loss would be transient. But it would
be rapid and immediate.
•
Still some effects to be added to this model – temperature and rainfall.
•
Major multi-center study underway – Sungrant Partnership – to do
experimental work on this subject.

10. Life Cycle Analysis of
Biochemical Cellulosic
Ethanol Production
Systems

11. Life-Cycle Model Flow Diagram

13. Results (Briefly)
Scenario
Corn Stover, Base Case
CO2e (g/MJ 100 year IPCC equivalents)
38.27
Corn Stover, High SOC Change
157.29
Corn Stover, Low SOC Change
91.05
Corn Stover, 5MW Electricity Surplus
28.28
Corn Stover, High Conversion Process Efficiency
35.38
Corn Stover, with Dilute Acid Pretreatment
Switchgrass, Base Case
42.80
41.32
Switchgrass, with SOC Change
-6.81
Switchgrass, 5MW Electricity Surplus
Switchgrass, High Conversion Process Efficiency
Switchgrass, with Dilute Acid Pretreatment
21.43
38.41
45.75

14. Conclusions
•
Both corn and switchgrass based ethanol have the potential to achieve
RFS GHG targets, but it is by no means a sure thing.
•
Assuming energy neutral production facilities, feedstock production and
processing dominate the GHG footprint.
•
Energy neutrality appears to be a reasonable assumption and some
electricity surplus may be expected
 @ 70 gallons/ton, each MW of surplus generation reduces GHG emission by
~2 g CO2e / MJ for a 40 Million Gallon per year facility.
•
Enzyme production is energy intensive and poorly understood, but has
the potential to be a major source of emissions.
•
If SOC loss occurs, it likely dominates the system.
•
Building emissions are insignificant. Transportation is 10-20%.

15. Making Biofuels and Bioenergy Work
•
Residues are not necessarily waste.
 Must use consequential LCA principles. “What would have happened to
them, otherwise?”
 Be VERY careful about SOC loss. If you screw up SOC, it doesn’t matter
how good your technology is.
•
Don’t overlook the value of putting carbon back in the ground.
 Landfilling organic material may be a good option, especially if LFG
capture and combustion is present.
•
If a biofuel facility has to import energy, the carbon balance is going
to be ugly.
•
Not all “efficiency enhancing” additions to the process necessarily
pay off, e.g. pelletization

16. Questions?
Contact me:
colin.murphy@ccst.us
cwmurphy@ucdavis.edu

17. Corn Scenarios

18. Switchgrass
Scenarios

19. WF Results
•
Very robust and
significant effect
from residue
removal and initial
SOC.
•
Nitrogen rate gains
significance.
•
No significant
difference between
tillage types.

20. WF Results
•
Very robust and
significant effect
from residue
removal and initial
SOC.
•
Nitrogen rate gains
significance.
•
No significant
difference between
tillage types.

21. WF Results
•
Very robust and
significant effect
from residue
removal and initial
SOC.
•
Nitrogen rate gains
significance.
•
No significant
difference between
tillage types.

22. Key Issue: Power Generation

23. Energy From Process Byproducts