Modeling the Life Cycle Environmental Impacts of Cellulosic Biofuel Production

Colin Murphy PhDResearch Affiliate – UC Davis Energy InstituteScience & Technology Policy Fellow – California Council on Science & Technology.

NEXTSTEPS Winter Symposium

12/10/2013

Outline1. Soil Organic Carbon Changes from Corn Stover

Harvest

2. Life Cycle Analysis of Biochemical Cellulosic Ethanol Production Systems

3. Implications for Biofuels

Soil Organic Carbon Changes from Corn Stover Harvest

Soil Organic Carbon

Soil Carbon Stock

Root Growt

h

Biomass

Incorp. Microbial Resp.

Erosion / Runoff

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

BF Results• Very robust and significant effect from residue removal.

• No other significant effects.

• No significant difference between tillage types.

BF Results• Very robust and significant effect from residue removal.

• No other significant effects.

• No significant difference between tillage types.

BF Results• Very robust and significant effect from residue removal.

• No other significant effects.

• No significant difference between tillage types.

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.

Life Cycle Analysis of Biochemical Cellulosic Ethanol Production Systems

Life-Cycle Model Flow Diagram

Results (Briefly)Scenario CO2e (g/MJ 100 year IPCC equivalents)

Corn Stover, Base Case 38.27Corn Stover, High SOC Change 157.29Corn Stover, Low SOC Change 91.05

Corn Stover, 5MW Electricity Surplus 28.28Corn Stover, High Conversion Process Efficiency 35.38

Corn Stover, with Dilute Acid Pretreatment 42.80Switchgrass, Base Case 41.32

Switchgrass, with SOC Change -6.81Switchgrass, 5MW Electricity Surplus 21.43

Switchgrass, High Conversion Process Efficiency38.41

Switchgrass, with Dilute Acid Pretreatment45.75

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%.

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

Questions?

Contact me:

colin.murphy@ccst.us

cwmurphy@ucdavis.edu

Corn Scenarios

SwitchgrassScenarios

WF Results• Very robust and significant effect from residue removal and initial SOC.

• Nitrogen rate gains significance.

• No significant difference between tillage types.

WF Results• Very robust and significant effect from residue removal and initial SOC.

• Nitrogen rate gains significance.

• No significant difference between tillage types.

WF Results• Very robust and significant effect from residue removal and initial SOC.

• Nitrogen rate gains significance.

• No significant difference between tillage types.

Key Issue: Power Generation

Energy From Process Byproducts