EFRI HyBi: The Science and Engineering of Microalgae Hydrothermal Processing
Global climate change, finite petroleum reserves, and geopolitical instability all point to the wisdom of developing transportation fuels from renewable sources. Bio-ethanol from corn and biodiesel from soy and canola oils are the most well-developed biofuels today, but both use agricultural resources (e.g., land, freshwater, fertilizer) that could otherwise be used to nourish people. Next-generation biofuels include ethanol and “green” gasoline from non-food lignocellulosic biomass and biodiesel and “green” diesel from oils extracted from non-edible plants (e.g., jatropha nuts, microalgae). Numerous small businesses have formed to develop and commercialize these different approaches, and significant research efforts dealing with these topics are underway globally at university, government, and corporate R&D labs.
We plan to explore a different approach for converting biomass to liquid fuels. Our approach is hydrothermal liquefaction and the biomass source is microalgae. This combination has received very little attention to date, but holds tremendous promise for producing liquid hydrocarbons that can be transported and made available to the consumer within the existing fuel distribution infrastructure.
Microalgae have several advantages relative to terrestrial lignocellulosic biomass for hydrocarbon production. Compared to terrestrial plants, algae are more photosynthetically efficient, can be grown on currently abandoned or unproductive land, and can utilize brackish, salt, or wastewaters for which they can provide essential treatment services. As a result, land requirements are lower and the environmental issues related to global changes in land-use patterns, which are concerns for terrestrial energy crops, are less problematic for algae cultivation. Finally, the large-scale cultivation of microalgae can be integrated with electricity generation (flue gas CO2 fed to algae), and wastewater treatment (“contaminants” are nutrients for the microalgae). Thus, large-scale cultivation of microalgae for biofuels could bring along the added benefits of CO2 conversion to fuels and wastewater treatment.
Since microalgae do not need to generate the same physical support structure as terrestrial plants, they have no need for lignin, a key structural component of grasses and woody biomass. Breaking down lignin (and cellulose) is one of the main barriers to producing biofuels from lignocellulosic biomass. Thus, the absence of these recalcitrant structural materials in microalgae is a significant advantage. Microalgae are composed of proteins, carbohydrates (predominantly polysaccharides), and lipids. The relative amounts of these different components depend on the species and conditions used for their growth. Several species of microalgae possess a high triglyceride content, which can be converted to liquid fuels. The use of microalgae for liquid fuels that has received the most attention to date is biodiesel production. This process begins with first removing water from the algal biomass and then feeding the dried algae to an oil press to mechanically recover a portion of the oil. Then, an organic solvent (typically hexane) is used to extract the remaining lipids from the algae cells. The triglycerides are then separated from the hexane and transesterified with methanol via a base-catalyzed reaction. The products are fatty acid methyl esters (e.g., biodiesel) and glycerol as a byproduct. This transesterification technology is the same as that used for converting oils from terrestrial plants such as soybeans, seeds, or nuts.
The idea of converting algal biomass to oils that could replace petroleum dates back to at least 1944. Though much time has passed, the vision is not yet a reality. More than 100 start-up companies are working in this field, but there is not yet a commercially viable approach. Key engineering problems in the algae-to-fuels arena using conventional approaches are generally seen as sustainably producing high-oil-yielding algae strains on a large scale and extracting the oil from the algae on a large scale. If a scalable and technologically and economically viable process for converting wet algal biomass to liquid hydrocarbons were developed, it would transform the algae-to-hydrocarbon arena. We believe that integrated hydrothermal, catalytic, and microbial processing can be such a technology.
Hydrothermal processing, which involves the application of heat (~ 300 °C) and pressure in an aqueous medium, mimics the processes that nature used to transform ancient plant material into the crude oil reservoirs we have today. Liquid water under these conditions has properties that are very different from those of liquid water at ambient conditions. The elevated temperature diminishes the hydrogen bond network and reduces the dielectric constant (left panel of Fig. 1), so many small organic compounds become highly soluble in high-temperature water. They become completely miscible in supercritical water. The center plot of Fig. 1 shows experimental data for benzene. From a solubility perspective, one can think of high-temperature liquid water as having properties similar to a polar organic solvent such as acetone. The right-most graph shows that the ion product (KW = [OH-][H+]) for liquid water at about 250°C is nearly three orders of magnitude higher than it is for ambient liquid water. Accordingly, high-temperature liquid water boasts higher “natural” H+ and OH concentrations than ambient liquid water. The combination of the high native H+ and OHconcentrations and the elevated temperature makes hydrothermal processing even more effective for hydrolysis and dehydration reactions, which are known to be acid or base catalyzed.
As outlined in recent reviews, hydrothermal processing has unique advantages for wet biomass. These advantages are related to both the chemistry (e.g., rapid hydrolysis, enhanced deoxygenation) and engineering considerations (e.g., higher energy efficiency). Heating an aqueous slurry of microalgae in the liquid phase causes the biomass to break down into smaller molecules. Among the reactions occurring is hydrolysis of the protein, polysaccharide, and lipid components to form various amino acids, sugars, and fatty acids. A bio-oil is produced. The few previous reports on the hydrothermal processing of microalgae indicate that 30 – 60% of the algal biomass can be converted to bio-oils. The oils have a high heteroatom content (about 10 - 20 wt% O, 6 wt% N) and an energy content of about 30 - 50 kJ/g. The molecular components of the oil include fatty acids, alkanes, and aromatics. Unlike conventional biodiesel synthesis, hydrothermal processing does not require algae with a high lipid content because the non-lipid portions can also be converted into bio-oil. Indeed, the amount of oil generated can be greater than the original hydrocarbon content in the algae.
Hydrothermal processing obviates the need for feedstock dewatering, drying, and lipid extraction. In general, hydrothermal processing of biomass with at least 30 wt% moisture requires less energy than drying the biomass because the enthalpy of saturated liquid water at 300 °C is about half that of water vapor at 25 °C. In addition, hydrothermal processing allows for energy recovery in the process because the hot reactor effluent stream can be used to preheat the ambient temperature feed stream and thereby reduce the overall external energy requirement for the process. In related work on wet gasification, Ro et al. conclude that the process is net-energy positive when the feedstock is above 2 wt% solids and appropriate heat recovery is in place. Conventional biomass conversion processes that employ drying use lower temperatures, and energy recovery is much more difficult. Finally, the operating cost associated with the elevated pressure is minimal because the feed stream is a liquid, which can be economically moved and pressurized with pumps.
We aim to discover the science and develop the engineering strategies needed to integrate hydrothermal, catalytic, and microbial processes for the economical and environmentally responsible production of hydrocarbons from renewable microalgae biomass. We focus on areas where the least is known about the foundational science and areas where technological breakthroughs are needed. Throughout the entire project, we take a systems perspective and assess the life-cycle impacts that accompany different processing choices. This systems perspective and LCA component is critical because it will allow us to identify the key environmental issues at an early stage. Note that the interagency Biomass Research and Development Board has identified in its National Biofuels Action Plan the “Action Areas” of Sustainability and Conversion Science and Technology as being critical. Our proposed work is in precisely these areas of critical importance.
Vision and Objectives
We envision an integrated hydrothermal, catalytic, and microbial processing strategy that can effectively produce a liquid hydrocarbon fuel from wet biomass feedstocks. The hydrothermal step can break down the biomacromolecules in the wet biomass and promote some heteroatom removal. The catalytic step can perform, in the aqueous phase, the chemical transformations needed to produce molecules with the composition and structure for use as liquid transportation fuels. The microbial step can use residual organics and nutrients in the aqueous effluent and the residual biosolids to produce additional wet biomass that can be co-fed to the hydrothermal reactor. Research is required to understand and develop each of these steps and to understand the environmental impacts of process decisions from a system-wide life-cycle perspective.
The specific objectives are:
1. Determine the reaction kinetics, products, pathways, and mechanisms occurring during the hydrothermal liquefaction of microalgae
2. Evaluate and discover catalytic materials and the catalytic science needed to engineer the molecular composition of the hydrocarbon products from hydrothermal liquefaction
3. Evaluate and discover microbial routes for converting byproducts from hydrothermal liquefaction to additional hydrocarbons
4. Evaluate the system sustainability performance using a life-cycle based model that incorporates upstream (algae production) and downstream (nutrient/water reclamation) processes to optimize the design of integrated hydrothermal-catalytic-microbial processes, and compare this system to conventional and other bio-based hydrocarbon production systems.