Research published by a team at Pacific Northwest National Laboratory has evaluated HydroThermal Liquefaction and Catalytic Hydrothermal Gasification (HTL/CHG) applied to Saccharina,1 a species of brown kelp similar to the Macrocystis considered for this project. The HTL was conducted in a continuous flow reactor at 350oC and 20 MPa. In contrast to fermentation, which often takes many 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. About half of the original carbon in the feedstock was converted into 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 (respectively), with <1% remaining in the relatively clean waste water.
Some of this moderate heating-value gas will be used for the needed 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.