New LCA of Four Different Soybean-Based Fuels Finds Potential for Significant Reduction in Fossil Energy Use and GHG Relative to Petroleum Fuels

26 December 2008

Well-to-wheels GHG emissions and emission changes relative to conventional fuels, using five different approaches for co-product allocation: 1, displacement; 2, energy-value-based allocation; 3, market-value-based allocation; 4, hybrid I; 5, hybrid II. Click to enlarge. Credit: ACS

A new energy and greenhouse gas (GHG) emission life-cycle analysis (LCA) by Michael Wang and colleagues at Argonne National Laboratory assesses the impacts of four soybean-derived fuels: biodiesel fuel produced via transesterification; two renewable diesel fuels produced from different hydrogenation processes; and renewable gasoline produced from catalytic cracking. A paper on their work was published online 23 December in the ACS journal Environmental Science & Technology.

The researchers found that, although the production and combustion of soybean-based fuels might increase total energy use, they could have significant benefits in reducing fossil energy use (>52%), petroleum use (>88%), and GHG emissions (>57%) relative to petroleum fuels.

Wang and his colleagues used an expanded and updated version of Argonne’s GREET model (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation), and employed five approaches to allocate the co-products: a displacement approach; two allocation approaches, one based on the energy value and the other based on the market value; and two hybrid approaches that integrated the displacement and allocation methods.

The team evaluated six fuel pathways: conventional gasoline and low-sulfur diesel as the baseline fuels, along with the four renewable soybean based fuels:

• Soybean biodiesel.

• Soybean-based renewable diesel produced by the SuperCetane process developed by the CANMET Energy Technology Centre of Natural Resources Canada. SuperCetane is a high-cetane, low-sulfur, diesel fuel blending stock. (Earlier post.)

• Soybean-based renewable diesel produced by the UOP green-diesel process. Green diesel is an isoparaffin-rich diesel substitute. (Earlier post.)

• Soybean-based renewable gasoline produced by a UOP fluidized catalytic cracking process.

The results in the study were based on 1 million Btu of fuel produced and used.

Given the fact that fuel consumption by diesel engines could be 15-20% less than consumption by gasoline engines per distance traveled, to compare WTW results on a per-mile basis, researchers could reduce energy use and GHG emissions for the four diesel fuel options.

—Huo at al. (2008)

The LCA found that all soybean-derived fuels offer significant reductions (52-107%) in fossil energy use. The largest reduction (107%) was delivered by the green gasoline when evaluated with the displacement approach; large quantities of co-products produced were assumed to displace fossil energy, resulting in a large credit in fossil energy. Biodiesel, SuperCetane and Green Diesel can achieve WTW fossil energy reductions of 84%, 90%, and 55%, respectively under the displacement approach. With the allocation and hybrid approaches, the reduction ratios are around 63-71% and 52-61%, respectively.

When the displacement approach was used, all four soybean-based fuels achieved a modest to significant reduction in WTW GHG emissions (64-174%) versus petroleum- based fuels. SuperCetane and Green Gasoline achieved a much larger reduction in GHG emissions (-130% and -174%) because they have a significant amount of co-products and because the production and combustion of the replaced fuels (natural gas, diesel fuel, and residual oil) could release large amounts of GHGs.

With an allocation approach, soybean-based fuels achieve a more modest reduction in GHG emissions (57-74%). The results from using the hybrid approaches are similar to the results obtained from using the allocation approach.

The new Argonne results highlight the difference the choice of allocations approach can make. Earlier studies reported a 72-80% reduction in WTW GHG emissions for biodiesel and an 84-85% reduction for green diesel when a mass allocation approach was used.

These reductions were higher than those in our study, 66-68% for BD and 74% for RD-II, when the two allocation approaches were used. The major reason is that using energy-value-based and market-value-based approaches can allocate more of the energy and emission burden to the primary product than does the mass-based allocation approach, because soy oil and biodiesel have a higher energy value and market value per pound than soy meal and glycerin, respectively.

...When the choice is between the displacement method and the allocation method, the displacement method tends to be chosen if the uncertainties and difficulties associated with it are solved, because it can reflect the energy use and emissions actually saved as a result of the co-products replacing other equivalent products. Nevertheless, the allocation approaches have been more widely used, because they are less data-intensive and less challenging than the displacement approach.

The energy-value-based allocation method is a favorable choice for a system in which the value of all the primary product and co-products can be determined on the basis of their energy content, such as the production processes of renewable fuels. If a “nonenergy” coproduct is involved and there are difficulties associated with using the displacement approach, the market-value-based allocation method could be an acceptable choice, although the fluctuation of prices could affect the results.

In addition, the allocation method is a better choice than the displacement method if the amount of the co-products is relatively large in comparison to the amount of the primary product, because the displacement method could generate distorted results for the primary product, as can be seen in the case of RG, the WTW fossil and petroleum use of which are negative.

...Hybrid approaches are the most preferable for a complicated comparison system like our study.

—Huo at al. (2008)

Resources

• Hong Huo, Michael Wang, Cary Bloyd, and Vicky Putsche (2008) Life-Cycle Assessment of Energy Use and Greenhouse Gas Emissions of Soybean-Derived Biodiesel and Renewable Fuels. Environ. Sci. Technol., Article ASAP doi: 10.1021/es8011436