The Refining of Algal Oils into Fungible Transportation Fuels
As part of the National Alliance for Advanced Biofuels and Bioproducts consortia, Honeywell’s UOP has been tasked with converting algal oils produced by consortia partners into fungible transportation fuels such as trademarked Green Jet Fuel using the UOP Renewable Jet Fuel process and Green Diesel using the trademarked UOP/ENI Ecofining process. These processes are based upon UOP’s more than 90 years of experience and expertise in the petrochemical refining industry and hydrotreating technology. Hydrotreatment utilizes hydrogen at elevated temperatures and pressures, in the presence of a suitable catalyst, to remove hetero-atoms such as oxygen, nitrogen, sulfur, and halides such as chlorine, from organic molecules to produce a purely hydrocarbon fuel.
UOP received a number of algal-derived oils from NAABB partners for evaluation as feedstocks for both the Ecofining and Renewable Jet Fuel processes to produce drop-in hydrocarbon fuels. The analysis of these oils indicated that the level of contaminants such as metals, phosphorous, nitrogen, chlorine and sulfur varied widely depending upon both methods of cultivation and oil extraction. Some oils were obvious candidates for conversion to hydrocarbon fuels whereas other algal oils presented challenges for processing without additional pretreatment. Pretreatment of the algal oils may include processes such as degumming as is commonly used in the pretreatment of vegetable oils. Pretreatment of algal oils not only serves to remove catalyst poisons, it also provides a mechanism by which valuable nutrients such as phosphorous, nitrogen, and divalent cations such as calcium and magnesium can be recycled back to the cultivation ponds from the extracted oils, especially if pretreatment is done near the source of cultivation.
UOP successfully converted three algal oils supplied by NAABB members to hydrocarbon fuels. One of the oils was a fatty acid methyl ester (FAME) oil produced by an NAABB member using a proprietary process to convert whole algal biomass to FAME oil and byproducts. The other two oils were crude triacylglyceride (TAG) oils extracted from algae grown in outdoor ponds. All the algal oils required pretreatment to remove metals and phosphorous before conversion to fuels. A two-step process was used for conversion to produce jet and diesel fuels with the required freeze and cold flow properties. The first step involved removal of oxygen, along with nitrogen and sulfur. This step produced straight-chain normal (n-) alkanes from the fatty acid component of the algal oil. The second step involved the cracking and isomerization of these straight-chain alkanes into a mixture of highly branched (iso-) alkanes. Some of the larger molecular weight paraffins were also cracked into smaller molecular weight paraffins, which increased the amount of material that was in the jet fuel boiling range. This flexibility allows the yield of jet and diesel product to be tuned towards either fuel, depending upon the current market value of jet fuel versus diesel fuel. The treated product was then fractionated by distillation into naphtha, synthetic paraffinic kerosene (SPK) jet fuel and diesel fractions. Along with naphtha, some light fuel gas is also produced. Both of these products have value as fuels, and the naphtha fraction can be a feedstock for a reforming unit for gasoline or polymer-grade olefins.
The SPK fraction from all three algal oils met the recently published ASTM D7566 specifications for bio-SPK jet fuel component for density, freeze point, flash point and distillation profile.
The diesel product made from these algal oils also met the specifications for both the U.S. Navy F-76 Marine Distillate fuel and the ASTM D 975 No. 2-Ultra Low Sulfur Diesel.
Outside of the NAABB project, UOP has converted considerable quantities of heterotrophically produced algal oils to jet and naval distillate fuels. More than 100,000 gallons of finished hydrocarbon fuels have been produced from oils extracted from heterotrophically grown algae. These fuels are currently being evaluated by the U.S. Navy in a number of test demonstrations by naval aircraft and surface ships. UOP is committed to working closely with NAABB partners, as well as other algal oil feedstock producers, to hasten the commercialization of renewable algal jet and renewable algal diesel fuels for both civilian and military use.
Author: F. Stephen Lupton
Senior Research Associate,
Renewable Energy & Chemicals
UOP LLC, A Honeywell Company