INTRODUCTION

First-generation, or conventional, biofuels are derived from sugars, starches, or vegetable oils from traditional agricultural crops and waste oils. Given first — generation biofuels’ impact on agricultural crop demand and prices, alternative feedstocks have been sought out. Microalgae have since been identified as a viable second-generation biofuels feedstock
(Figure 10.1). The advantages of using microalgae for biofuel production in comparison with other available feedstocks have been extensively reported.

There are an estimated 100,000 microalgae species, each with specific properties that allow them to exist in nearly every environment on Earth, including arid cli­mates that do not sustain most agricultural crops. There­fore, microalgal production systems need not displace other traditional land-based crops intended for human

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00010-3

FIGURE 10.1 The progression from first — to second-generation biofuels. (For color version of this figure, the reader is referred to the online version of this book.)

or livestock consumption, which in turn greatly reduces the impact to the food distribution chain. Further, micro­algae may be harvested multiple times a year, which greatly increases yearly production yields. The cultiva­tion of microalgae for biofuels production can also be coupled with other beneficial production schemes to improve net income and positively address environ­mental concerns. Some possibilities currently being investigated include the following:

• Reclamation of nutrients such as NH^, NO^, POfi, and others from wastewater, which reduces costs associated with cultivating the algae and treating wastewater (Zhu, 2013; Batten, 2013).

• Utilization of waste CO2 from industrial flue gases, which reduces greenhouse gas emissions while producing biofuel (Gonzalez-Lopez, 2012).

• Cultivation and extraction of value-added metabolites within microalgae intended for biofuel production. In this scenario, the value-added metabolite is extracted prior to, or during, the biofuel production stream. Commercially relevant products include a large range of fine chemicals and bulk products, such as polyunsaturated omega fatty acids, antioxidants, high-value bioactive compounds, natural dyes, sugars, and proteins (Mimouni, 2012; Skjanes et al., 2013).

• After oil and target metabolite extraction, the processed algal biomass can be used as a nutrient-rich livestock feed, or used as sustainable organic fertilizer due to its high N:P ratio (Mulbry, 2005; Stamey, 2012).

Because of this variety of value-added biological de­rivatives, coupled with environmental sustaining

strategies, microalgae intended for biofuel production can potentially revolutionize a large number of biotech­nology areas concurrently, including pharmaceuticals, cosmetics, nutrition and food additives, aquaculture, and pollution prevention.

MICROALGAL BIOFUEL HISTORY

Beginning in the 1950s, Golueke et al. (1957) conduct­ed early work on the anaerobic digestion of microalgal biomass for the production of methane fuel. The energy crisis in 1973 prompted the formation of The National Renewable Energy Laboratory (NREL) under the Jimmy Carter Administration. From 1978 to 1996, NREL con­ducted the most authoritative study to date on the devel­opment of biofuels from algae (Sheehan et al., 1998). The study concluded that under controlled conditions, algae are capable of producing 40 times the amount of oil for biodiesel per unit area of land when compared to terres­trial oilseed crops such as soy and canola, and that the use of wastewater as a nutrient source for algae propa­gation was the most practical approach for near-term production of algal biodiesel (Sheehan et al., 1998; Oswald, 2003). Despite the promise of cost-effective fuel production from microalgae, interest in renewable energy quickly waned as the energy crisis subsided and fuel prices fell. The recent world-wide escalation in oil prices has renewed interest in microalgae as a bio­fuels feedstock.

Since the original NREL study, other groups also have conducted analyses of full-scale algae-to-biodiesel pro­duction (Benemann et al., 1982; Weissman and Goebel,

1987; Beal, 2012a). Although these and other studies have indicated a great potential for profitable biofuel from microalgae, they also highlighted the need for system im­provements, in both cultivation management and pro­cessing schemes to improve yields and reduce costs in order to be competitive with fossil fuels. For example, even when robust algae growth was achieved, inefficient processing techniques such as biomass centrifugation and drying followed by solvent extraction made recovery of biofuels cost-prohibitive. To overcome this barrier, changes to the system have been introduced, including processing techniques that eliminate the need for expen­sive dewatering regimens such as centrifugation and dry­ing of the harvested biomass prior to oil extraction with solvents. One suggested path forward is a solventless wet stream process whereby microalgae are concentrated using pH-driven flocculation using inexpensive lime, fol­lowed by rupturing of the cells by pulsed electric field, and ultimate recovery of released lipids by cross-flow filtration. When coupled with waste streams for CO2 and nutrients, this process has a positive return on invest­ment (Beal, 2012b). Another suggested path forward to­ward practical biofuel extraction from microalgae is the use of hydrothermal liquefaction (HTL) processing. This method eliminates the need for solvents to break open algae cells, instead relying on heat and pressure to remove the water from the biomass. An ancillary benefit of the HTL method is that in addition to lipids, other organic metabolites such as carbohydrates, proteins, and nucleic acids can likewise be converted to biocrude during the HTL process. Thus, a cultivation strategy needs only to focus on the production of biomass rather than inducing the accumulation of lipids at the expense of cellular proliferation. Ultimately, cultivation and pro­cessing strategies should be firmly supported by real­time analysis of fuel precursors such as lipids that can be converted to biodiesel, carbohydrates that can be con­verted to bioethanol, and the organic biomass that can be converted to biocrude. Detailed analytical feedback is necessary to optimize growth conditions to maximize specific biofuel precursors.