Chiara Samori and Cristian Torri

Abstract Lipid extraction is a critical step in the development of biofuels from microalgae. The use of toxic and polluting organic solvents should be reduced and the sustainability of the extraction procedures improved in order to develop an industrial extraction procedure. This could be done by reducing solvent amounts, avoiding use of harmful solvents, or eliminating the solvent at all. Here we describe two new processes to extract hydrocarbons from dried and water-suspended samples of the microalga Botryococcus braunii. The first one is a solvent-based procedure with switchable polarity solvents (SPS), a special class of green solvents easily convertible from a non-ionic form, with a high affinity towards non-polar compounds as B. braunii hydrocarbons, into an ionic salt after the addition of CO2 , useful to recover hydrocarbons. The two SPS chosen for the study, based on equimolar mixtures of 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and an alcohol (DBU/octanol and DBU/ethanol), were tested for the extraction efficiency of lipids from freeze-dried B. braunii samples and compared with volatile organic solvents extraction. The DBU/octanol system was further evaluated for the extraction of hydrocarbons directly from algal culture samples. DBU/octanol exhibited the highest yields of extracted hydrocarbons from both freeze-dried and liquid algal samples (16 and 8.2%, respectively, against 7.8 and 5.6% with traditional organic solvents). The second procedure here proposed is the thermochemical conversion of algal biomass by using pyrolysis; this process allowed to obtain three valuable fractions, exploitable for energy purpose, fuel production, and soil carbon storage: a volatile fraction (37% on dry biomass weight), a solid fraction called biochar (38%) and, above all, a liquid fraction named bio-oil (25%), almost entirely composed by hydrocarbon-like material, thus directly usable as fuel.

C. Samori (*) • C. Torri

Interdepartmental Research Centre for Environmental Sciences (CIRSA), University of Bologna, via S. Alberto 163, 48123 Ravenna, Italy e-mail: chiara. samori3@unibo. it; cristian. torri@unibo. it

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_27, 651

© Springer Science+Business Media New York 2013

1 Introduction

The need to replace fossil fuels with fuels derived from renewable biomass is cur­rently focused on biodiesel from oleaginous plant seeds and ethanol from sugar — cane/corn; however, this first-generation biofuels, primarily produced from food crops and mostly oil seeds, are limited in their ability to achieve targets for biofuel production, climate change mitigation, and economic growth; moreover, the recent dramatic increase of food stocks prices has become a worldwide emergency. Because of these environmental and social concerns, the attention is recently shift­ing towards the development of next-generation biofuels mainly produced from non-food feedstock [1], by converting for example the highly abundant and wide­spread non-edible lignocellulosic fraction of plants. A further exploitable source of biofuels relies on the aquatic environment, specifically on micro and macroalgae; lipids, which include acylglycerols and hydrocarbons, represent the most valuable fraction of microalgal biomass as their high energy content per mass unit is similar to conventional fuels. Several oleaginous microalgae (with lipid content exceeding 20% of their dry weight) have been exploited to this purpose [2], and the biodiesel obtained has been claimed to be more convenient than conventional biodiesel from plant seeds [3, 4]. Benefits rising from the utilization of aquatic over terrestrial bio­mass include: (1) higher sunlight use efficiency (about 5% vs. 1.5% [5]), (2) utiliza­tion of marginal areas (e. g. desert and coastal regions), (3) possible coupling with other activities (e. g. wastewater treatment, CO2 sequestration) [6-9], (4) minor dependence on climatic conditions, (5) availability of a larger number of species, and (6) easier genetic manipulation to modify chemical composition (e. g. lipid con­tent) [10]. However, the industrial development of fuels from microalgae is still hampered by higher overall costs with respect to both fossil fuel and first generation of biofuels counterparts: operating open ponds and bioreactors are expensive and the harvesting of algal biomass is energy costly [11]. For this reason, the net energy balance from microalgae cultivation is still debated [12, 13]. Moreover, besides the cost of growing and collecting microalgae, downstream processes are to be taken into account to evaluate the overall productivity. Botryococcus braunii is a freshwa­ter colonial green microalga proposed as a future renewable source of fuels because it is capable of producing high levels of liquid hydrocarbons [14]. There are three main B. braunii races, each one synthesizing different types of olefinic hydrocar­bons: the A, B, and L races. The A race (Fig. 1) accumulates linear olefins, odd numbered from C23 to C31, chiefly C27, C29, and C31 dienes or trienes; some studies have revealed that oleic acid is the direct precursor of these specific olefins [15] and that decarboxylation of very long chain fatty acid derivatives, activated by a b-sub — stituent, is the final step which leads to the formation of the terminal unsaturation [16]. The B race produces polyunsaturated triterpenes (botryococcenes), while the L race synthesizes one single tetraterpenoid hydrocarbon named lycopadiene [17, 18]. Both A and B races contain similar amounts of lipids (approximately 30% on a dry weight basis), but with a very different composition: in the A race hydrocarbons, non-polar lipids and polar lipids are, respectively, 25, 60, and 15% of the total lipids,

Fig. 1 Botryococcus braunii, A race

whereas in the B race the percentages are 71, 9, and 20%, clearly indicating that one quarter of the dried biomass of the B race is composed by hydrocarbons [19].

Specifically for B. braunii, the bulk ofhydrocarbons is located in external cellular pools and it can be recovered from algal biomass by means of physical process, named cold press and typically used to extract more traditional food oils as olive oil, and by means of chemical process (extraction with solvents) or both [20]. The chemical pro­cess, mainly used for the extraction of industrial oils such as soybean and corn oils, is generally based on an extraction with n-hexane, to obtain vegetable oil in higher yields and with a faster and less expensive process [21] . However, the existing solvent approach is characterized by several problematic aspects, such as the high solvent/ biomass ratio, solvent hazard (including solvent toxicity, volatility, and flammability) and large solvent losses (e. g. in the extraction process of soybean oil, n-hexane losses are 1 kg per tonne of beans processed [22]). Because of this general lack of “green­ness” in the chemical extractive processes, in the last years different efforts have been made to reduce the use of toxic and polluting organic solvents and to improve the sustainability of the extraction procedures from aquatic and terrestrial biomass, for example by using supercritical fluids [23, 24].

Here we present two novel methods for the extraction of lipids from B. braunii, comparing the extraction efficiency of the new processes with those of traditional organic solvents. The first method [25] is a solvent-based process, more sustainable than the traditional solvent extraction because it involves the use of switchable polar­ity solvents (SPS) [26, 27], a “new” class of green solvents, considerable as reversible ionic liquids, with the unique and advantageous feature of having switching solubility behaviour, correlated with reversible polarity. This feature can be successfully exploited in practical applications as extraction procedures or chemical reactions, bypassing the cumbersome need to change solvent in each step of the process itself.

The second method is based on the thermochemical conversion of B. braunii biomass by using pyrolysis [28], in order to obtain, directly and in one step process, a liquid fraction rich in lipids, a gaseous fraction useful for energy purposes, and a soil-amending co-product called biochar [29] .