Fuel production from algal biomass is most commonly assumed to involve cultivation of microalgal species that have high lipid productivity and the processing of the lipid to biodiesel. In this case, production of biofuel requires the algae to be concentrated and subsequently treated to cause the release of the intracellular lipids. The concentration, or harvest, step involves the separation and typically drying of the algal cells to prepare them for lipid collection. Lipid collection usually is accomplished by rupturing the algal cells. Subsequent extraction of the biomass might be required for economical oil recovery. Thus, biodiesel production from algae requires two distinct separation steps—harvest and product collection—regardless of whether growth occurs in open or closed photobioreactors.
The important feature in harvest and extraction is that the algae and the lipids are insoluble in water. The technical problem in the production of biodiesel is simply producing a pure, dry triacylglycerol stream for subsequent processing to biofuels. Because the algal biomass and the algal oils are immiscible in water, harvest can be completely spontaneous, and there is no key thermodynamic separation energy to be overcome. The constraints on the system are purely engineering-related, and better engineering can reduce the energy expenditure required for separating the algal biomass from the culture water and drying it for subsequent oil collection. Relatively low algal biomass concentrations and the small size of microalgae make separation challenging and energy intensive. A meta-analysis of published studies shows that more than 40 percent of the total energy required for biodiesel production can be attributed to harvest and product collection (Clarens et al., 2010). (See Chapter 4 for details on energy use.)
Purity of the algal lipid is an important parameter for processing into liquid transportation fuel. Inorganic materials that stay with the oil are a concern, and the method of harvest and collection can influence the impurity levels. Inorganic salts and phospholipids are two known impurities that could affect processing. Inorganic salts are in the culture medium and occur naturally in algae, but they also can be introduced as flocculants.
2.3.1 Harvesting and Dewatering Methods
Microalgal cultures are about neutrally buoyant suspensions of microscopic particles. As noted earlier in this chapter (see Tables 2-1, 2-2, and 2-4), algal cell biomass is most commonly reported to be up to about 0.4 grams per liter in open ponds and 3 g/L in photobioreactors, though concentrations up to 40 grams per liter have been reported (Brennan and Owende, 2010). These concentrations require that almost a liter of water be removed from the algae to produce a few grams of dry biomass. The pumping and processing of water are energy intensive, and reducing the energy required to collect the algae directly affects the sustainability of microalgae cultivation.
Microalgae are grown as insoluble particles in an aqueous medium. Furthermore, the lipids present in the algae are similarly immiscible in water. In principle, the separation of algal oils from the aqueous growth media can be spontaneous and require little energy. In practice, the separation of algae from the growth media and the separation of lipids from algal biomass in a timely manner is energy intensive. Reducing the energy required can be accomplished through improvements in the algal strains, through engineering improvements, and through favorable interplay of the two. As an example, improvements to algae that increase the density of cells in the culture, in principle, reduce the amount of water that has to be eliminated during recovery. Reducing the water processed would, all other things being equal, reduce the energy expended during algae collection.