Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemical Engineering

Major Professor

George P. Philippidis, Ph.D.

Co-Major Professor

John N. Kuhn, Ph.D.

Committee Member

Aydin K. Sunol, Ph.D.

Committee Member

Sarina Ergas, Ph.D.

Committee Member

Venkat R. Bhethanabotla, Ph.D.

Keywords

Activated char, Algae, Hydrochar, Hydrothermal carbonization, Lipid extraction

Abstract

The extensive use of fossil fuels has increased the CO2 emissions, a phenomenon that has increased the demand for renewable energy and resources. Algae, photosynthetic aquatic organisms, have recently caught attention due to their high growth rate and the high CO2 capture ability. Biofuels and bioproducts from algae are considered promising alternatives because of their lower carbon footprint. Lipids are by far the most popular algal product, as they find applications in both fuels, such as biodiesel, and products, such as nutraceuticals and cosmetics. However, after lipid extraction, conversion of the residual algal biomass to value-added products is viewed as essential for strengthening and diversifying the economics of algae biorefineries. Hence, the overall objective of this research is to develop sustainable algal products, such as hydrochar and activated char, from lipid-extracted algal biomass to replace fossil-derived materials, such as charcoal and activated carbon.

In the first phase, mechanical, thermochemical, and electromagnetic cell disruption methods were tested to maximize lipid extraction from wet biomass of Picochlorum oculatum, an industrially promising marine alga. Among mechanical methods, grinding with and without liquid nitrogen was employed, while among thermochemical methods sulfuric and nitric acid were tested. As for electromagnetic methods, ultrasonication was performed using an ultrasound bath. After cell disruption, two different solvent systems were tested for lipid extraction, namely chloroform and methanol (2:1) and hexane and isopropanol (3:2). Among the cell disruption methods, grinding with liquid nitrogen was the most effective method with a 14.8% lipid yield (content) without affecting the lipid composition that is important for biodiesel production.

Following lipid extraction, lipid-extracted algae (LEA) was obtained and thermochemically converted into hydrochar through a promising method named hydrothermal carbonization (HTC), which represents the second phase of this project. The effect of temperature, reaction time, and solids loading on hydrochar yield were investigated. Response surface methodology was applied to study the effects of temperature (180-220°C), reaction time (1-3 h), and solids loading (8-15%). The optimum hydrochar yield was ~37% and was obtained at 180 oC after a reaction time of 3 h at a solids loading of 15%. The solid product, algal hydrochar, is a material rich in oxygen functional groups and carbon (up to 67.3%) and hence a promising candidate for remediation of wastewaters.

In addition to hydrochar, hydrothermal carbonization also generates an aqueous phase rich in nitrogen and other elements that need to be treated before discharge into water bodies. The aqueous phase was found to contain high concentrations of nitrogen (2.4 g/L), phosphorus (1.75 g/L), organic carbon (20.9 g/L), and trace elements, which are useful nutrients for algae. Utilization of the aqueous phase in algae cultivation can potentially reduce algae production cost, while mitigating waste generation. Hence, in the third phase of this study, the potential of the generated aqueous phase to serve as a source of nutrients for lipid production from Chlorella vulgaris was assessed.

The fourth phase of the project examined chemical activation of algal hydrochar with potassium hydroxide (KOH) to produce value-added activated char by increasing its surface area and pore size. A systematic approach was employed to study the effects of different activation factors such as temperature, impregnation ratio (mass ratios of KOH and hydrochar), and activation time on the characteristics of activated char. Temperatures of 400-600 0C, ratios of 0.25- 1.0, and activation times of 30-60 min were employed. The maximum Brunauer-Emmett-Teller (BET) surface area of 847 m2/g was achieved at 600 oC after 30 min at a ratio of 1:1.

Finally, in the fifth phase of the project, a techno-economic analysis (TEA) of the integrated carbonization/activation process was carried out to assess the economic viability of the process and its potential for commercial development. Using Aspen Plus simulation to analyze process economics, the minimum selling price of activated char was determined to be $2,300/ton.

Overall, the development of hydrochar and activated char from lipid- extracted algae paves a path towards a more cost-competitive and sustainable algae biorefinery. Recommended next steps include testing of catalysts to enhance the HTC process and assessment of the efficacy of hydrochar and activated char in carbon capture applications and removal of organic pollutants, such as siloxanes, from landfill gas.

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