by Catherine Ortega Klett, NM WRRI Senior Program Manager
Zheng Cui is a PhD student in the NMSU Department of Chemical and Materials Engineering, expecting to graduate in Spring 2020. Last year she received an NM WRRI Student Water Research Grant entitled: Energy and Nutrient Recovery from Co-Solvent Hydrothermal Liquefaction of Wastewater-Grown Algae.
This research addresses aspects of finding economically viable ways to convert organic waste material into biofuels, in this case, wet algal biomass obtained through the treatment of wastewater into biofuels. The thermochemical conversion process Zheng is studying for this purpose is hydrothermal liquefaction (HTL), whereby biomass is “cooked” in a reactor at elevated temperature and pressure. The efficiency of the process depends on several factors, such as the composition of the biomass, the moisture water content, the rate and duration of heating, and the presence of other substances that can catalyze or participate in the chemical conversion processes. The focus of Zheng’s research has been to investigate the effect that alcohols have on the yield and quality of the fuel intermediate: bio-crude oil. She is working under the guidance of two faculty advisors, both from Chemical and Materials Engineering: Dr. Catherine Brewer and Dr. Umakanta Jena.
In HTL, long carbon chain molecules in biomass are thermally broken into simpler molecules, and oxygen is removed in the form of water and carbon dioxide. These reactions result in the production of high hydrogen-to-carbon ratio bio-crude oil, which has higher energy density than unprocessed algae. To increase bio-crude oil yields and thus enhance the energy recovery from the wastewater treatment process using Galdieria sulphuraria algae, Zheng has experimented with the addition of three alcohols (ethanol, isopropanol, and glycerol) as HTL co-solvents. Among these, glycerol is favored from the standpoint of being a low-cost residual material produced from the biodiesel industry.
Alcohols react with acidic components in the bio-crude oils to produce compounds that inhibit the formation of carbon-based residues (char) and the re-formation of large molecules. For this reason, the yield of bio-crude oil from co-solvent HTL is significantly higher than that from traditional water-only HTL. The influence of these alcohols on the optimized HTL conditions have been studied with respect to the resulting product chemistry, using an array of instrumental techniques: mass spectrometry, IR spectrophotometry, and gas chromatography. The main objectives were to evaluate: (1) the yield and quality of bio-crude oil from G. sulphuraria when alcohols are added; (2) the mechanisms of different alcohols promoting bio-crude oil production; and (3) the feasibility of wastewater treatment algal HTL with crude glycerol so as to integrate the wastewater treatment process with the production of biodiesel.
The type of co-solvent did influence the chemical composition of the produced bio-crude oils and co-solvent addition was effective at improving bio-crude oil production. At low HTL temperatures (310 °C), isopropanol and glycerol achieved the highest yields (about 25% by weight). The decrease of nitrogen recovery in the aqueous phase suggests that co-solvents promote the transfer of organic compounds into the oil phase. With 40% by weight glycerol co-solvent, the highest yield of light bio-crude oil (73% by weight of the dry algae basis) was observed at 350 °C for a 30 min. reaction time. This suggests that around 66% of organic compounds in the algae biomass and crude glycerol were converted into bio-crude oil of quality sufficient to be upgraded into advanced biofuels.
Zheng Cui received her bachelor’s degree in chemical engineering in 2015 from East China University of Science and Technology (Shanghai) and NMSU through their dual degree program, before staying at NMSU to pursue a PhD degree. After graduation, Zheng hopes to join a chemical engineering firm where she can continue her work on the problem of finding the best pathway for producing alternative renewable energy resources to lessen our dependence on traditional fossil fuels.