In an effort to process wet algal biomass directly, eliminate organic solvent use during lipid extraction, and recover nutrients (e.g., N, P, and glycerol) for reuse, we developed a catalyst-free, two-step technique for algal biodiesel production. In the first step, wet algal biomass (ca. 80% moisture) reacts in subcritical water to hydrolyze intracellular lipids, conglomerate cells into an easily filterable solid that retains the lipids, and produce a sterile, nutrient-rich aqueous phase. In the second step, the wet fatty acid-rich solids undergo supercritical in situ transesterification (SC-IST/E) with ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs). Chlorella vulgaris grown sequentially under photo-and heterotrophic conditions served as the lipid-rich feedstock (53.3% lipids as FAEE). The feedstock and process solids were characterized for lipid components using highly automated microscale extraction and derivatization procedures and high-temperature gas chromatography. Hydrolysis was examined at 250 °C for 15 to 60 min; solids recovered by filtering contained 77-90% of the lipid originally present in the algal biomass, mainly in the form of fatty acids. The effects of reaction time (60 or 120 min), temperature (275 or 325 °C), and ethanol loading (approximately 2-8 w/w EtOH/solids) were examined on the yield and composition of biodiesel produced from the SC-IST/E of the hydrolysis solids. Longer time, higher temperature, and greater ethanol loading tended to increase crude biodiesel and FAEE yields, which ranged from about 56-100% and 34-66%, respectively, on the basis of lipid in the hydrolysis solids. Isomerization and decomposition of unsaturated FAEEs was quantified, and its effect on fuel yield is discussed.

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The high fertilizer demand for biodiesel production from microalgae is a significant challenge facing the commercialization of this promising technology. We investigated a processing strategy called hydrothermal carbonization (HTC) to convert wet algal biomass into a lipid‐rich hydrochar and aqueous phase (AP) co‐product. By reacting biomass at 200°C for 15 min, about 50% of the algae biomass became a solid hydrochar and roughly 40–70% of the C, N, and P in the reactant material dissolved into the AP. For the first time, an AP co‐product of this nature was analyzed by HPLC, GC‐MS and FT‐ICR‐MS to identify and characterize the dissolved organic matter. Using a unique marine bi‐culture suspected to contain a green algae (Nannochloris) and a cyanobacteria (Synechocystis), we demonstrated that this AP co‐product can support biomass growth better than a medium containing only inorganic nutrients. To manage unwanted contamination and optimize AP utilization, we employed a two‐stage growth process and fed‐batch additions of the AP co‐product. The effect of media recycling and nutrient supplementation, as well as a production model for a large‐scale facility, are discussed. Our work suggests that HTC can play a critical role in making algal biorefineries more sustainable by obviating biomass drying for fuel processing and recycling nutrients. © 2013 American Institute of Chemical Engineers Environ Prog, 32: 962–975, 2013

Biomass can be reformed into higher-value fuels using hydrothermal processes that employ high-temperature and high-pressure water as a reaction medium. Hydrothermal processing obviates feedstock drying and can achieve high energy efficiencies through heat integration. Hydrothermal liquefaction occurs under mild conditions (250–350 °C) in which biomass hydrolyzes rapidly and reacts to form a viscous bio-crude oil. At higher temperatures (350–500 °C), catalysts may be employed to promote the formation of CH4-rich gas in the process of catalytic hydrothermal gasification. Supercritical conditions (500–800 °C) may be used to achieve a H2-rich gas through supercritical water gasification (SCWG). The reaction chemistry underlying these hydrothermal processes is complex and not fully understood, but the influence of temperature, pressure, feedstock concentration, and the presence of catalysts on this chemistry has been extensively studied. In this chapter, we review hydrothermal processing of biomass, with a focus on the chemistry that describes biomass conversion under various hydrothermal conditions. Special attention is given to the relatively recent interest in processing aquatic feedstocks, such as algae, in a hydrothermal environment.

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