The idea of producing biofuels from microalgae comes from the 1960s (Oswald and Golueke, 1960); now, given the high price of petroleum and the global-warming problem, at­tention has refocused on this idea. Microalgae have several advantages over crops in the pro­duction of biofuels: They have high productivity and do not compete for fertile land or water, thus do not affect the food supply nor other crop products. Microalgae have been proposed as the unique third-generation biofuel source (Chisti, 2007). However, for this to become a re­ality, the production of microalgae still has to demonstrate its sustainability, in addition of being produced on a large scale and at a comparably low price, as with traditional crops like soya, corn, or palm. As an example, palm oil is produced at a volume of 40 million t/year and has a market value of €0.5/kg. To replace only 5% of the U. S. demand for transport fuel, it would be necessary to produce more than 66,000 kt/year of oil-rich biomass at production costs below $400/t (Chisti, 2007). Moreover, to replace all transport fuels in Europe with bio­diesel from microalgae, 9.25 million ha (almost the surface area of Portugal) would be needed, assuming a productivity of 40,000 L/ha year (Wijffels and Barbosa, 2010).

To produce microalgae-based biofuels that are able to compete in the worldwide energy markets, it is essential to minimize the energy and nutrient input along with their cost, in addition to optimizing the culture yield and developing adequate transformation routes that allow the valorization of the entire biomass according to the biorefinery concept (see Figure 14.7). For the production of biomass, the use of wastewater is required as the nutrient (nitrogen and phosphorous) source, in addition to free CO2 from flue gases as the carbon source, resulting in purified water and profits obtained from the wastewater treatment pro­cess (Jorquera et al., 2010; Norsker et al., 2010; Acien et al., 2012a). With regard to valorization, the microalgae biomass produced under high-productivity conditions is composed of

FIGURE 14.7 Block diagram of the process for the production of biofuels from microalgae using wastewater and flue gases.

proteins (30-50%), carbohydrates (20-30%), lipids (10-30%), and ash (5-10%) (Vargas et al., 1998; Chisti, 2007). Biodiesel can be obtained from the saponifiable lipids (approximately 50% of total lipids), whereas bioethanol can be produced from fermentable sugars (approximately 30% of total carbohydrates); thus a mere 20-30% of the biomass would be used if only bio­diesel and bioethanol production were carried out. The remaining biomass waste has been suggested as useful in biogas production; however, the economic value of biogas is low due to its low calorific value, CO2 content, and gas nature.

Other than this, biofuel production by hydrothermal liquefaction of the entire biomass has been likewise proposed (Biller and Ross, 2012). In a general scheme, the microalgae could be used to produce biodiesel by extraction/transesterification processes, and waste biomass could be fermented anaerobically to produce biogas, which in the end could be used as both energy and as a CO2 source. Alternatively, amino acids (Romero et al., 2012) and/or bioethanol could be produced from microalgae biomass (John et al., 2011). The biodiesel pro­duction capacity of microalgae is assumed to be up to 35,000 L/ha/year (Rodolfi et al., 2009), whereas the production of bioethanol can reach values up to 38,000 L/ha/year (Harun et al., 2010), although these values have not yet been demonstrated on an industrial scale.

Whatever the transformation route to produce biofuels from microalgae biomass, it is clear that the production step has to be positive in terms of energy balance, in addition to being cheap—a value of $0.5/kg being widely agreed as the upper limit. Recently, several economic analysis approximations of biofuel production from microalgae have been published (Douskova et al., 2009; Norsker et al., 2010; Singh and Gu, 2010; Wijffels et al., 2010; Williams and Laurens, 2010). Due to the lack of both existing facilities and a defined technology, only approximations can be made, all of which include significant uncertainty. Microalgae biomass production costs for different scenarios have recently been analyzed (Acien et al., 2012b). The base scenario considered is the operation of a 100-ha facility consisting of raceway reactors with a V/S ratio depth of 0.2 m3/m2, operated in continuous mode at 0.2 L/day. The power consumption dedicated to mixing is 2 W/m3, while an energy consumption of 0.1 kWh/m3 is assumed for harvesting using a flocculation-sedimentation step, followed by centrifugation. The use of pure raw materials (CO2 and fertilizers) is considered, a biomass productivity of 20 g/m2 day being assumed for the year overall. From these data a production cost of $1.12/kg is reached, a major percentage corresponding to raw material cost due to the use of pure CO2 and fertilizers but especially due to the cost of using pure CO2 (see Figure 14.8).

The second major contribution to overall production is depreciation, especially the cost of harvesting equipment, meaning the sedimenter and centrifugation units, amounting to 59.8% of the total equipment cost. Regarding the utility cost, this mainly corresponds to the power consumption for both operating the photobioreactors and harvesting, water cost being neg­ligible in spite of water evaporation losses of 30,000 m3/ha year. From these data, it is con­cluded that to reduce the biomass production cost and approach the target value of $0.5/kg, it is mandatory to improve CO2 use efficiency or even to replace it using flue gases. Moreover, clean water can be replaced by wastewater, thus avoiding the use of fertilizers. Under these conditions, the production cost reduces to $0.55/kg, which approaches the target value of $0.50/kg.

Considering similar conditions (free flue gases and wastewater), production costs of $0.70/kg have been reported using closed photobioreactors, whereas this value increased up to $1.3/

when using open raceways due to their lower productivity (Norsker et al., 2010). To reduce the production cost below this value, it is necessary to improve the productivity of the system to approximate the maximum theoretical values, which have only been demonstrated under fully controlled conditions at a low scale. Therefore, increasing productivity to 40 g/m2 day, the production cost reduces to $0.21/kg, and considering a maximal productiv­ity of 60 g/m2 day under optimal location and operating conditions, the production cost could be reduced as far as $0.14/kg (Acien et al., 2012a). Recently it has been reported that to be competitive with petroleum at $100/barrel, the biomass with a 40% oil content will need to be produced at $0.16/kg if no credit is allowed for the residual biomass, or at $0.25/kg if a credit is allowed for the nutrients in the residual biomass (Chisti, 2012).

To break the bottleneck for microalgae production used in energy production, it is essential to develop more productive photobioreactor systems while reducing their cost dramatically. The productivity of open raceways varies widely according to the location, strain, and operating conditions; long-term productivity in commercial raceways is lower than 47 t/ha year, although values of up to 91 t/ha year (Borowitzka, 1999) have been reported. Design and operation optimization for open raceways in order to improve their efficiency and productivity is currently performed starting from the basics: fluid dynamics and mass transfer characterizations (Mendoza et al., 2012; Sompech et al., 2012; Chiaramonti et al.,

2013) . Regarding the photobioreactor cost, it has been reported that to guarantee an econom­ical production design for energy products, the investment costs cannot exceed €40/m2
(Hankamer et al., 2007). The cost of open raceways is in the $13/m2 range, which includes the compacted earth, lining, baffles, and paddlewheel, but this cost can be much higher if special designs or plastic-cover structures are used. In addition, this cost does not take into account the harvesting process: The machinery required to collect microalgae biomass from diluted cultures has been demonstrated to be highly expensive. Considering a scaled-up size of 100 ha, the total investment cost has been reported as varying from $48/m2 for open raceways to $66/m2 for tubular photobioreactors (Norsker et al., 2010).

From these data, it can be concluded that although microalgae are not yet produced on a large scale for energy purposes, recent advances allow us to be optimistic and to expect this process to develop in a sustainable and economical way within the next 10 to 15 years (Wijffels and Barbosa, 2010).