(24a) Nitrogen Recovery and Biocrude Formation from Hydrothermal Liquefaction of Protein

James D. Sheehan and Phillip E. Savage

Hydrothermal liquefaction is a thermochemical process that uses water at elevated temperatures (200-370°C) and pressures (10-22 MPa) to transform wet biomass into an energy dense “biocrude” oil and aqueous-phase nutrient coproducts.^1–3 The biocrude can be upgraded and co-refined with petroleum crudes into liquid fuels while the N- and P-containing nutrient coproducts can be recovered and recycled to cultivate additional biomass (e.g., microalgae). N is an essential element for cultivating microalgae and its demand can be met in part by recycling recovered N-containing nutrients from HTL, which are mostly derived from microalgal proteins. Proteins are major biochemical constituents of microalgae and contain approximately 90% of microalgal N.⁴The N atoms in the biomass proteins are ideally recovered as nutrients, however N-containing compounds can also partition into biocrude, which lowers the fuel quality and reduces the heating value of biocrude. Thus, the sustainability of HTL can be highly dependent on proteins and their transformation into valuable nutrient coproducts or detrimental N-containing compounds that partition into biocrude.

We performed HTL of bovine serum albumin (BSA), a well-defined model protein, and observed its transformation into valuable nutrient coproducts (e.g. amino acids, ammonia) and biocrude. We used temperatures ranging from 200 °C to 400 °C and holding times between 30 s to 1 hour. BSA readily decomposed during HTL into smaller peptides. The presence of peptides with molecular weights exceeding 5 kDa was no longer observable at temperatures at 250°C or higher. At 200°C, BSA aggregated into aqueous- and organic solvent-insoluble solids, in yields up to 80 wt%, that eventually decomposed into peptides which persevered in the hydrothermal media. The yields of amino acids were less than 6 wt% under all conditions, however, at temperatures of 350°C or higher the yields of primary and secondary amines in the aqueous-soluble products were upwards of 50 wt% and over 40% of N was recovered as ammonia, a preferred N-source for cultivating microalgae. The highest yields of biocrude were between 17 and 24 wt% and were achieved at temperatures ranging 300 to 400°C and holding times of 1 to 10 min. Mild (200°C) and severe (400°C, >10 min) conditions resulted with biocrude yields less than 5 wt%. This presentation will also include a discussion of the reaction pathways, kinetics, and mechanisms and thereby breaks new ground by connecting operationally defined lumped products (e.g., biocrude) with molecular-level chemistry for HTL of proteins.

References

(1) Peterson, A. A.; Vogel, F.; Lachance, R. P.; Fröling, M.; Antal, Jr., M. J.; Tester, J. W. Thermochemical Biofuel Production in Hydrothermal Media: A Review of Sub- and Supercritical Water Technologies. Energy Environ. Sci. 2008, 1, 32–65.

(2) Toor, S. S.; Rosendahl, L.; Rudolf, A. Hydrothermal Liquefaction of Biomass: A Review of Subcritical Water Technologies. Energy 2011, 36, 2328–2342.

(3) López Barreiro, D.; Prins, W.; Ronsse, F.; Brilman, W. Hydrothermal Liquefaction (HTL) of Microalgae for Biofuel Production: State of the Art Review and Future Prospects. Biomass and Bioenergy 2013, 53, 113–127.

(4) Becker, E. Microalgae Biotechnology and Microbiology; Cambridge University Press: New York, 1994.