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MIT study concludes lifecycle GHG emissions for biofuels should be presented to decision makers and the public as a range

Stratton1
LC-GHG emissions for the alternative diesel fuel pathways in the study. Uncertainty bars represent the variability captured by the low emissions, baseline, and high emissions scenarios. Note the different scales for the top and bottom portions of the figure. Credit: ACS, Stratton et al. Click to enlarge.

In a paper published in the ACS journal Environmental Science & Technology, researchers from MIT conclude that it is “paramount” that decision makers and the general public be given the range of LC-GHG emissions that could result from the production and use of a given bio or synthetic fuel, due to high variability within pathways. Among their findings in the study are that subjective choices such as coproduct usage and allocation methodology can be more important sources of variability in the life cycle greenhouse gas (LC-GHG) inventory of a fuel option than the process and energy use of fuel production.

Variability in lifecycle analysis (LCA), Stratton et al. say, must be distinguished from uncertainty. Variability—which is inherent due to both inexact LCA procedures and variation of numerical inputs—is a dispersion of discrete results, each of which has been measured or calculated with an inherent uncertainty in the result. The authors group the sources of variability into three categories: pathway-specific variability; coproduct usage and allocation; and land use change (LUC).

In their paper, they used specific examples from the production of diesel and jet fuels from 14 different feedstocks to demonstrate general trends in the types and magnitudes of variability present in life cycle greenhouse gas (LC-GHG) inventories of middle distillate fuels.

To understand how variability impacts LC-GHG inventories of transportation fuels, a new methodological approach was developed using screening level LCAs. Screening level analyses provide preliminary assessments of technology alternatives with the intent of informing research funding and decision makers.

A requirement of screening level LCAs is to identify the pivotal factors defining the LC-GHG emission profiles of fuel production for each LC step and each feedstock. Optimistic, nominal,and pessimistic sets of these key parameters were developed to formulate corresponding low LC-GHG emissions, baseline or nominal LC-GHG emissions, and high LC-GHG emissions scenarios for each feedstock-to-fuel pathway; hence, results for each feedstock-to-fuel pathway are a range of possible LC-GHG inventories intended to demonstrate variability in fuel production processes.

A requirement of screening level LCAs is to identify the pivotal factors defining the LC-GHG emission profiles of fuel production for each LC step and each feedstock. Optimistic, nominal, and pessimistic sets of these key parameters were developed to formulate corresponding low LC-GHG emissions, baseline or nominal LC-GHG emissions, and high LC-GHG emissions scenarios for each feedstock-to-fuel pathway; hence, results for each feedstock-to-fuel pathway are a range of possible LC-GHG inventories intended to demonstrate variability in fuel production processes.

—Stratton et al.

Transportation fuel pathways often result in coproducts; to allocate emissions among products, a usage must first be defined for the coproduct. In the study, the team examined four allocation methods to assign LC-GHG emissions between the primary fuel product and any coproducts: mass allocation; energy allocation; market-value allocation; and displacement (a.k.a., system expansion).

The choice of coproduct usage and allocation method may significantly affect the final results of the LCA. Several studies in the literature have acknowledged the variability introduced to LCA by different allocation methods. Three examples were chosen herein to demonstrate the variability introduced by coproduct treatment: (1) the oil and biomass coproduct system of soybeans where the biomass has an existing market as an animal feed; (2) the oil and biomass coproduct system of jatropha capsules where the biomass coproducts have a variety of potential uses; and (3) the liquid fuel product slate from a coal and biomass fed F-T facility. All three examples show a general shortcoming in the displacement approach. When the coproduct creation is large relative to the primary product, the LC-GHG inventory of the primary product depends more strongly on the LC-GHG inventory of the displaced product than the processes and energy flows of the product being examined.

—Stratton et al.

Stratton2
As an example, sensitivity of LC-GHG emissions from jatropha oil HRD to coproduct usage and allocation assumptions are shown. Baseline value indicates the chosen combination to represent HRD production from jatropha oil. Single usage and allocation entries indicate uniform application across all coproducts. Scenario 3 assumes meal is detoxified and used for animal feed with allocation by economic value, while all other coproducts are used for electricity with allocation by displacement of average grid electricity.
Credit: ACS, Stratton et al. Click to enlarge.

The team found that all of the biofuel options examined in their study could either potentially be produced with lower LC-GHG emissions than conventional diesel, or with LC-GHG emissions that exceed those of conventional diesel. The difference is due to the LC-GHG intensity of the processes and the emissions resulting from LUC.

For this reason, it is critical to emphasize that the use of renewable resources as feedstock does not guarantee an environmentally beneficial fuel. Knowledge of specific production details is required for any definitive conclusions to be drawn. This constitutes a strong argument for LC-GHG inventories of transportation fuels to be presented as a range.

...Three key conclusions can be drawn from the potentially dominating influence of variability due to coproduct usage and allocation and LUC assumptions: 1) minimizing variability across LCA results by maximizing methodological consistency is essential to making useful comparisons between fuel options; 2) the absolute result from attributional LCAs have a diluted physical meaning and are most effectively used as a comparative tool, given the condition from the first key conclusion; and 3) it is paramount that decision makers and the general public be given the range of LC-GHG emissions that could result from the production and use of these fuels.

Such an approach emphasizes the importance of understanding the key aspects that determine the LC-GHG emissions from fuel production and use. Furthermore, it can do so before any production actually occurs. Such knowledge would help to develop technologies and policies that diversify our energy supplies and stimulate economic development while mitigating the LC-GHG emissions from transportation.

—Stratton et al.

Resources

  • Russell W. Stratton, Hsin Min Wong, James I. Hileman (2011) Quantifying Variability in Life Cycle Greenhouse Gas Inventories of Alternative Middle Distillate Transportation Fuels. Environmental Science & Technology Article ASAP doi: 10.1021/es102597f

Comments

ai_vin

Sorry. Dido should read "Ditto."

Engineer-Poet

I don't like to deflate SJC too much, but the yields for switchgrass on the marginal lands which we can afford to devote to it are closer to 4 tons/ac (only Miscanthus gets to 10, and it takes good soil and lots of water). The energy cost of collecting it is considerable, too.

Word is that Chevy Volt owners are getting about 1000 miles between fill-ups. That's 100+ MPG of liquid fuel (plus electricity). The great part of the electric portion is that we can make it from a lot more things than we can make liquids from. If the US fleet jumped from ~22 MPG average to 100 MPG average and freight was electrified too (e.g. rail and battery-powered trucks for local deliveries), we'd just about wipe out US oil imports.

Reel$$

I think you are right EP. And even if the rail to regional transfers rely on trucks burning NG, alcohol and biodiesel - we are STILL far ahead of the massive imports today.

Now, to get the President on board with HSR for freight...

ai_vin

President Barack Obama is calling for a six-year, $53 billion spending plan for high-speed rail, as he seeks to use infrastructure spending to jump-start job creation and HSR for freight is a natural follow-on to this. The problem isn't the President, it's the dogmatists like Rick Scott who are getting in the way; http://www.tampabay.com/news/localgovernment/article1155233.ece

SJC

EP, deflate away, if you have real information that is more accurate, then present it. Those were the numbers I heard. One way or the other we can create fuels with biomass to a significant degree. Arguing never brought about the truth, just more arguing.

SJC

Switchgrass Yield Per Acre Growing Fast

"According to field trials conducted by energy crop company Ceres switchgrass can produce up to 10 tons of biomass per acre, as compared to the federal government’s projected yields of two to four tons."

http://www.matternetwork.com/2009/5/switchgrass-yield-per-acre-going.cfm

SJC

Switchgrass Benefits Are Greatly Underestimated

"highest yielding varieties across multiple trial locations, with average yields reaching nearly 10 tons."

"The highest yield was reported in California, where a Ceres experimental variety produced 19 tons per acre."

http://www.biofueldaily.com/reports/Switchgrass_Benefits_Are_Greatly_Underestimated_999.html

SJC

"Yields of switchgrass are high, averaging 7 tons per acre in unirrigated field trials with some lines yielding up to 10 tons per acre. Production costs are low because of the plant's low nutrient use, minimal pesticide requirements, propagation by seed, and perennial growth habit."

"Switchgrass can be harvested with conventional haying equipment, and its wide adaptability allows it to be grown productively across a large geographic area, including marginal regions that would otherwise be unproductive."

http://www.jgi.doe.gov/sequencing/why/50008.html

SJC

Cal researchers get 12-18 ton per acre switchgrass yields

"In California, researchers at the UC Desert Research and Extension Center in El Centro said that, following a three year trial, switchgrass test plot yields increased to an average of 12 tons per acre, with a high of 14 tons per acre in a Five Points trial in the third year trial."

http://biofuelsdigest.com/bdigest/2010/05/27/cal-researchers-get-12-18-ton-per-acre-switchgrass-yields/

SJC

"Test plots of Switchgrass at Auburn University have produced up to 15 tons of dry biomass per acre. Five-year yields average 11.5 tons, enough to make 1,150 gallons of ethanol per acre each year. Additional studies show that Switchgrass ethanol produces 94% less CO2 than oil."

http://www.buffalobrandseed.com/index.cfm?fuseaction=plants.main&alphaKey=P&whichName=genus&showIntro=0&typeID=19&alphaChange=1

Engineer-Poet

The question I have is "where is this unirrigated land where it shows such high productivity?" If it's in the truly marginal areas we've got to spare, GREAT! If it's in prime territory in Iowa, not so great.

SJC

I don't know, look it up. I am showing links that say today they get better than the 2-4 tons that were stated back in 1999.

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