A new report from the US Department of Energy (DOE) Co-Optima initiative identifies the top 13 diesel fuel blendstocks from biomass resources that could reduce harmful emissions from medium- to heavy-duty diesel vehicles. The top 13 blendstocks include hydrocarbons, esters, and ethers that have the potential to reduce GHG emissions by at least 60% and be produced at a competitive cost.

The blendstocks could also make the emissions control system simpler and less expensive to operate. Together, the improvements could translate to lower costs for industry and consumers.

The 13 top-performing blendstocks with the potential to reduce criteria and GHG emissions comprise 6 hydrocarbons, 3 esters, and 4 ethers. (Figure by Dan Gaspar | Pacific Northwest National Laboratory)

Lower engine-out emissions can be cheaper to operate and potentially a little more efficient, and using blendstocks made from biomass and waste means lower well-to-wheels GHG emissions.

—Dan Gaspar, a bioenergy researcher at Pacific Northwest National Laboratory (PNNL) and lead author of the report

Co-Optima (Co-Optimization of Fuels & Engines) is a collaboration among multiple DOE national laboratories, universities, and industry to accelerate the development of affordable, scalable, and sustainable biofuels along with high-efficiency, low-emission vehicle engines. The work is sponsored by DOE’s Vehicle Technologies Office and Bioenergy Technologies Office.

Gaspar, a member of the Co-Optima project Leadership Team, coordinated development of the report, merging contributions from more than three dozen researchers from nine national laboratories, including PNNL. The team focused on blendstocks to improve cold-weather operability, ensure good combustion performance, reduce GHG emissions, and reduce soot.

Along with minor engine operation adjustments, reducing soot produced in the engine can also decrease production of NO_x.

To determine their top 13 blendstocks, the researchers first conducted modeling, small-scale experiments, and single-cylinder testing to establish target values of key fuel properties for reducing soot production and improving engine operability. These properties include the cetane number; the propensity to generate soot related to criteria emissions; cold-weather operations—a key limitation of current biodiesel mixtures; and energy density.

Researchers then used computational and experimental methods to screen thousands of blendstocks against the target values to determine which of them had the greatest potential to improve performance and reduce emissions. To evaluate the potential of different classes of molecules to reduce soot and NO_x, the team conducted tests in single-cylinder engines, including conventional and a new type of engine called ducted fuel injection. Next, the team conducted technoeconomic and life cycle analyses on the most promising candidates to evaluate their market potential, scalability, and sustainability.

The most promising blendstocks fell into three categories:

1. Market or near-market biofuels with minimal barriers to market introduction;

2. Candidates offering improved properties but with at least one significant barrier to market introduction; and

3. Biofuels that initially looked promising but were determined to be unsuited for commercial adoption for fuel property or operability reasons.

The eight blendstocks that met the fuel property requirements and have minimal barriers to adoption include six hydrocarbons and two esters:

• Hydrocarbons:

  1. Farnesane;

  2. A diesel-boiling-range hydrocarbon blendstock produced via the Fischer-Tropsch process;

  3. Hydrothermal liquefaction bio-oils derived from sewage sludge, algae, and algae/wood blends;

  4. An isoalkane mixture catalytically derived from ethanol;

  5. An isoalkane mixture produced by catalytic conversion of volatile fatty acids (VFA) produced from food waste; and

  6. Hydroprocessed esters and fatty acids (also called renewable diesel).

• Esters:

  1. Fatty acid methyl esters/biodiesel; and

  2. Fatty acid fusel esters.

One additional ester and five ethers were identified with significant potential to reduce emissions but with at least one significant barrier to adoption and use identified. These blendstocks include short-carbon-chain esters produced from oilseed crops, 4-butoxyheptane, polyoxymethylene ethers and end-exchanged derivatives, a family of alkoxyalkanoates derived from fusel alcohols and lactate esters, and fatty alkyl ethers.

The Co-Optima team determined that the identified blendstocks have the potential, consistent with industry targets, to increase indicated brake thermal efficiency by up to 1%, reduce particulate matter emissions by 50% to 99% and NO_x emissions by 50% to 99%, and are likely compatible with the fuel infrastructure and legacy fleet, while reducing GHG emissions by 62 to 89%.

Co-Optima previously identified the top 10 bio-blendstocks for turbocharged engines using a similar tiered screening approach. (Earlier post.)

Resources

• Gaspar, D. J. 2021. Top 13 Blendstocks Derived from Biomass for Mixing-Controlled Compression-Ignition (Diesel) Engines: Bioblendstocks with Potential for Lower Emissions and Increased Operability. PNNL-31421. Pacific Northwest National Laboratory, Richland, WA.