Though only a limited amount of work has been done to date, it is clear that hydrothermal catalytic conversion of algae can produce hydrocarbons for liquid biofuels. Thus, there is tremendous potential for this field and the outlook is bright. The majority of the work to date on producing liquid fuels from hydrothermal conver­sion of aquatic biomass has focused on homogeneous catalysis by metal salts or alkali. More recent studies, however, are beginning to examine heterogeneous cata­lysts due to advantages in separation and selectivity of the catalyst. More work is needed to identify better het­erogeneous catalysts for these applications. In particular, the development of nonprecious metal-based catalysts would provide a major advance.

CONCLUSIONS

Microalgae are a promising source of clean, renew­able biofuel. Not only can it be grown and produced on a large scale, it can be grown in virtually every part of the world including locations that are considered to be otherwise unsuitable to agricultural production and thus lie dormant. However, several challenges remain to its full execution: (1) the successful production of
feedstock on a large scale; (2) the development of pro­cessing methods that are cost-effective and leave intact the desired molecular end-products; and (3) a richer un­derstanding of microalgal chemistry and product accu­mulation during both growth and processing phases. Whether employing open ponds or PBRs for biomass production, cultures must be carefully monitored to maintain the desired composition of the culture. Factors such as nutrient loads, mixing and light source, and con­taminants all drive the production of biomass and thus biofuel precursors. There is a growing trend toward pro­cessing microalgae directly from the aqueous stream, eliminating costly drying steps and conserving water. As such, HTL is an emerging process that converts biomass to biocrude in hot, compressed water, thereby eliminating the need for drying or organic solvents. Further, all organic components serve as the feedstock for the HTL process rather than discreet components, such as lipids for biodiesel or ethanol for bioethanol. Biohydrogen is another provocative fuel derived from microalgae. Whatever the feedstock and biofuel process, additional improvements to each of the technologies are required to make the production of renewable fuels from microalgae cost-effective. These improvements can only result from systems using real-time analytical feedback to inform growth and processing and from innovations derived from a multidisciplinary approach.