The Scripps team will be testing and developing crucial technology for open ocean farming of kelp. The PNNL team will be testing and developing hydrothermal liquefaction (HTL) and catalytic hydrothermal gasification (CHG) processes, followed by hydrotreating to cost-effectively convert kelp into biocrude and further to hydrocarbon liquids. The PNNL team will also generate a greenhouse gas life cycle analysis.

Earlier research published by Dr. Elliott and his PNNL colleagues evaluated HTL/CHG applied to Saccharina, a species of brown kelp similar to the giant kelp (Macrocystis pyrifera) considered for this project. The HTL was conducted in a continuous flow reactor at 350 ˚C and 20 MPa.

In contrast to fermentation—which often takes days—the average residence time in the reactor is less than an hour. The output from this process was a liquid that gravity-separated into a biocrude and an aqueous fraction. Just over half of the original carbon in the feedstock was found to be in the biocrude and just under half in the aqueous fraction.

The aqueous fraction was then catalytically processed by CHG at the same temperature and pressure as the HTL. The CHG caused the carbon in the aqueous fraction to go to methane and carbon dioxide in approximately the ratio of 2:1, with <1% remaining in the relatively clean waste water.

Some of this moderate heating-value gas will be used for process heat. Part of the gas can also be used to produce the hydrogen (by steam reforming and water-gas shift) needed to upgrade the biocrude via hydrotreating. This creates a high-value crude oil equivalent.

The biocrude coming out of the HTL process was found to have a C:H:O:S:N ratio of about 17.5:23:1:0.8:0.04. Hydrotreating of this biocrude will increase the H:C ratio closer to the approximate 1.9:1 value associated with desirable liquid fuels and will extract most of the remaining oxygen, sulfur, and nitrogen.

The surplus methane from CHG beyond that used for process heat and hydrogen synthesis can be sold, or in a future version of the concept, used to power the harvesters so that all processing can occur at sea and the harvesters do not need to return raw kelp to dock. Oil tankers can be filled with high-value biocrude at the harvesting sites in the open ocean, making the whole process carbon-neutral in the global environment.

Giant kelp is one of the fastest growing producers of biomass. The open ocean is an immense untapped region for collecting solar energy. Kelp does not grow naturally in the open ocean because it needs an attachment at ~ 5-25 meters depth and also needs key nutrients available in deep water or near shore but not necessarily available at the surface in the open ocean. The concept proposes an economical grid for attachment and access to nutrients, making it possible to farm kelp in the extensive regions of the open ocean.

This novel approach will grow kelp attached to large grids in the open ocean permanently towed by inexpensive robotic submarines. These submarines will keep the farms near the surface during the day to gather sunlight for photosynthesis. At night, the submarines will take the farms down to the deeper, cold water where the kelp can absorb nutrients that may be inadequate in the warmer surface waters. The farms will also be taken to deeper water during storms or to avoid passing ships.

Once every three months, the submarines will move the kelp farms to rendezvous with harvesters. The kelp will be processed into biocrude suitable for delivery to refineries. If this disruptive new technology works, kelp will become an abundant and affordable feedstock for biocrude, and will assist in meeting the ARPA-E goal of replacing imported energy.

Marine BioEnergy’s program was one of the 41 selected by ARPA-E for a total of $125 million in funding under the OPEN 2015 solicitation. The OPEN solicitations serve as an open call to scientists and engineers for transformational technologies outside the scope of ARPA-E’s existing focused programs.

Resources

• Douglas C. Elliott, Patrick Biller, Andrew B. Ross, Andrew J. Schmidt, Susanne B. Jones (2015) “Hydrothermal liquefaction of biomass: Developments from batch to continuous process,” Bioresource Technology, Volume 178, Pages 147-156 (open access) doi: 10.1016/j.biortech.2014.09.132

• Douglas C. Elliott, Todd R. Hart, Gary G. Neuenschwander, Leslie J. Rotness, Guri Roesijadi, Alan H. Zacher, and Jon K. Magnuson (2014) “Hydrothermal Processing of Macroalgal Feedstocks in Continuous-Flow Reactors” ACS Sustainable Chemistry & Engineering 2 (2), 207-215 doi: 10.1021/sc400251p