CAREER: Manipulating Polarity to Enhance Hydrothermal Liquefaction of Biomass for Biofuels

Cornell University, Ithaca NY

Investigators

Abstract

Cost-effective waste-to-energy technologies are critical components of a future green economy powered by renewable fuels. Hydrothermal liquefaction (HTL) transforms organic wastes into liquid biofuels by processing the waste in water at high temperature and pressure. One advantage of HTL over other biomass conversion techniques is that it directly treats wet wastes without an energy-intensive pre-drying step. To date, the majority of HTL research has explored the impact of process conditions on products generated from a range of biomass feedstocks. However, this research approach cannot overcome the primary challenges to widespread application of HTL for biofuels, namely that (1) product distributions cannot be accurately controlled or predicted, resulting in the need for significant downstream upgrading of the biocrude, and (2) the byproduct process water requires considerable treatment. These two challenges combine to make current large-scale HTL economically infeasible. The new approach to process design proposed in this project, however, could enable widespread implementation of HTL for wet waste to biofuel conversion. The production of such renewable fuels could help transform the U.S. into a green energy exporter and job opportunity creator. To fill such high-tech positions, a diverse workforce is needed. The Principal Investigator (PI) of this proposal has a strong track record training students from under-represented groups; at least one graduate student, one post-doctoral associate, and two undergraduate students will be trained as part of this work. The PI will develop a new course on Sustainable Engineering Design specifically aimed at encouraging and retaining women engineers. Garnering widespread public acceptance and support for policy initiatives promoting biofuels is critical to widespread deployment. To do this, the PI will conduct original survey research to benchmark public knowledge and opinion on biomass-based renewable fuels. Then, by incorporating results from the research into a first-of-its kind survey, the PI will uncover how scientists can best frame messages to increase public support for green energy technologies. Hydrothermal liquefaction (HTL) is a reactive and multiphase biomass conversion process producing supersaturated solutions of aqueous-organic mixtures and a separate organic biocrude phase. HTL process water (PW) contains organic molecules that are either insoluble or beyond their solubility limits, suggesting that as the dielectric constant of the HTL aqueous reaction media changes, it can create a supersaturated solution where sparingly soluble organics remain even after the PW is cooled. It is hypothesized that the selectivity and yields of HTL reactions are a function of this supersaturation degree, which is controlled by the dielectric constant and reaction enthalpies. To confirm and develop this concept, a new apparatus will be constructed to measure the dielectric constant of model molecules and mixtures at HTL-relevant conditions and solubilities of HTL intermediates and products. Using a Design of Experiments approach, the statistically significant relationships between dielectric constant and solubility parameter, and the selectivity and yields of HTL reactions will be explored. The accuracy of thermodynamic models used to predict the solubility of the organics in the aqueous phase and partitioning between the biocrude and organics phase will be improved by incorporating the dielectric constant and solubility measurements. Finally, the research team will investigate how organic compounds in the PW produced in the HTL process can be recovered through a combination of kosmotropic salting out and liquid-liquid extraction using a variety of salts with varying kosmotropic strength. Model HTL products will be examined individually at their solubility limits to estimate the maximum recoveries of each product as a first step towards estimating the separation selectivity in mixtures of these products. This work will lead naturally to the investigation of mixtures of model compounds to determine the degree to which these organics can form their own phase separate from the PW, and the abilities of kosmotropes to salt-out the organic products in a selective manner. This fundamental thermodynamic approach to HTL and product recovery could accelerate the transition to a renewable energy future by facilitating the design of more efficient and selective HTL processes. In addition to the advances made in terms of HTL, the Educational Component of this CAREER proposal will advance the way Introductory Environmental Engineering courses are taught to retain women and minorities in STEM. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.