(31a) Advances in Kinetic Modeling of Hydrothermal Liquefaction of Microalgae

A growing demand for sustainable energy has emerged in the 21st century in an effort to both diversify our nation’s energy portfolio and reduce net carbon dioxide emissions. Algal biofuels are an attractive option, as algae cultivation does not require arable land and algal biomass is more energy-dense than terrestrial plants [1]. One biomass conversion technology, called hydrothermal liquefaction (HTL), uses the properties of high-temperature water (HTW), including an increased ion product, lowered density, and lowered dielectric constant, to directly convert wet microalgae to an energy-dense "biocrude" oil which resembles petroleum crude oil [2]. HTW facilitates acid-catalyzed hydrolysis of biomacromolecules [3] and solvation of compounds with significantly less polarity than otherwise possible at ambient temperature [4], facilitating the production of biocrude. This process is advantageous from an energy return standpoint because it only requires wet algae to be concentrated (5 - 35 wt % algae solids) rather than completely dried. Furthermore, HTL has the potential to be carbon neutral when implemented in a well-integrated biorefinery [5].

Recent advances by other researchers in the field have demonstrated that the kinetics of HTL occur predominantly at short reaction times (< 10 min) [6]. Moreover, a subset of HTL called Fast Hydrothermal Liquefaction (FHTL) that features high heating rates (75 - 275 ^oC min^-1) and short reaction times (< 3 min) has been shown to produce equivalent or greater biocrude yields; this would allow for substantial reductions in reaction energy requirements compared to more conventional isothermal HTL, which typically operates at 300 - 350 ^oC for 30 - 60 min [7].

In this presentation, we present results that expand previous work [6] on the FHTL of Nannochloropsis sp. to cover additional reaction times and water loadings. These data represent the most comprehensive study of HTL reaction conditions to date in the literature. We calculated Arrhenius parameters for each reaction pathway from these data with an adapted form of the kinetic model developed by Valdez and Savage [8] that uses temperature measurements. We demonstrate that our new model predicts data from our experiments and the literature with more accuracy than any existing model from the literature. This model advances the understanding of the underlying kinetics occurring during HTL and brings algal biofuels one step closer to commercial viability.

References

1. Patil, V.; Tran, K. Q.; Giselrod, H. R. International Journal of Molecular Sciences 2008, 9, 1188–1195.

2. Brown, T. M.; Duan, P.; Savage, P. E. Energy & Fuels 2010, 24, 3639–3646.

3. Toor, S. S.; Rosendahl, L.; Rudolf, A. Energy 2011, 36, 2328–2342.

4. Akiya, N.; Savage, P. E. Chemical reviews 2002, 102, 2725–2750.

5. Williams, P. J. L. B.; Laurens, L. M. L. Energy & Environmental Science 2010, 3, 554.

6. Faeth, J. L.; Valdez, P. J.; Savage, P. E. Energy & Fuels 2013, 27, 1391–1398.

7. Jena, U.; Das, K. C.; Kastner, J. R. Bioresource Technology 2011, 102, 6221–6229.

8. Valdez, P. J.; Savage, P. E. Algal Research 2013, 2, 416–425.