Biofuel co-products as livestock feed - Opportunities and challenges

Micro-algae for fuel and use of spent biomass for feed and for other uses 4331978). Air drying and sun drying is the cheapest methodfor drying algal biomass, but it requires a large dryingsurface and has a long drying time (Prakash et al., 1997).Lundquist et al. (2010) recommended online extractionof oil from wet biomass, avoiding the steps of drying andextraction, which could reduce operating costs by 20 to25 percent.Extraction of micro-algal lipidsCell disruption is an important step in recovering intracellularproducts from micro-algae, and so properties of the cellwall play an important role in the extraction process. Someof the commonly used methods for cell disruption includemechanical disruption, like bead-beating, ultrasound andsteam extraction (Mata, Martins and Caetano., 2010) andnon-mechanical disruption, including application of organicsolvents and addition of inorganic acids and alkali for pretreatmentprocessing. The most convenient method wouldbe to efflux the metabolites or constituents of micro-algalcells using solvents, without disrupting cellular functions.Two-phase solvent mixtures, such as methanol-ethanol/hexane co-solvent systems, are advantageous, wherebymore polar solvents are used to disrupt the membranewhile the extracted lipids enter the non-polar solvent phase.This reduces the phase separation step during processing,thereby making solvent extraction more convenient andeconomical (Hejazi and Wijffels, 2004).Green solvents such as ionic liquids and switchablepolarity solvents can be exploited for extraction of lipidsfrom micro-algae (Samori, Samori and Fabbri, 2010; Salvoet al., 2011). Ionic liquids are non-aqueous solutions ofrelatively large asymmetric organic cations coupled with asmall inorganic or organic anion salt that remain liquid atmoderate to room temperatures. The ionic solvents havehydrophilic ionic liquid and polar covalent molecules forboth extraction and partitioning of lipids from micro-algalcells. Salvo et al. (2011) used a hydrophilic ionic liquid,1-butyl-3-methylimidazolium, for a single-step extractionprocess involving lysis of micro-algal cell walls and separationof cellular lipids. After the auto-phase separation, the lowerhydrophilic ionic phase can be re-used for extraction ofmicro-algal cells.Switchable solvents have physical properties, such aspolarity, solubilizing capacity, viscosity and conductivity,that can be converted from one form to other. The mainadvantages of switchable solvents is that many processes,such as extraction, phase separation and purification,can be achieved with one single agent (Phan, 2008).Hydrocarbon yields were higher when a switchable polaritysolvent system containing 1,8-diazobicyclo-[5.4.0]-undec-7-ene and alcohol was used for extraction in B. brauniicompared with conventional solvent extraction by n-hexane(Samori, Samori and Fabbri, 2010).CONVERSION OF ALGAL LIPIDS AND BIOMASSTO BIO-ENERGYTrans-esterificationThe extracted micro-algal oil can be converted to biodieselby trans-esterification. The trans-esterification processinvolves reaction of an alcohol with the triglycerides, formingfatty acid alkyl esters, in the presence of a catalyst.Based on the type of catalyst used, the trans-esterificationprocess can be acid or base catalysed, and involve enzymaticconversion. In acid-catalysed reactions, HCl, H 2 SO 4 orH 3 PO 4 is used for trans-esterification, while in base catalysisstrong bases like KOH or NaOH are commonly used. Basecatalysis has many advantages over the acid-catalysedreaction since it is conducted at low temperature and pressure,and it has a high conversion yield and provides directconversion to biodiesel without intermediate compounds.Balancing the advantages are several drawbacks, includingbeing energy intensive, with problems associated withremoval and treatment of alkaline catalyst from the finalproduct. These problems could be solved by the use of biocatalystslike lipases, but large-scale demonstration has notbeen reported (Svensson and Adlercreutz, 2008).In situ acid-catalysed trans-esterification processes forbiofuel production have been explored but the limiting factoris the high moisture content of algal biomass, affectingthe conversion. The identification of lipid composition is animportant criterion to assess the suitability of algal oil forhigh quality biodiesel production. Some of the importantfuel properties considered for biodiesel include density,viscosity, flash point, ester value, cetane number and combustionheat (Mutanda et al., 2011). In the study conductedby Francisco et al. (2010) on micro-algal strains of Chlorellaspp., Dunaliella spp., Phaeodactylum spp., Aphonothecespp., Phormidium spp. and Scenedesmus spp., it wasfound that the properties of biodiesel obtained from thesestrains were found to be similar to the American Society forTesting and Materials (ASTM) and European Union standards(Table 5).Trans-esterification of algal lipids generates glycerol asthe major co-product. Glycerol is an industrially importantTABLE 5Characteristics of biodieselPropertiesBiodiesel frommicro-algal oilDiesel fuelDensity (kg/L) 0.864 0.838Viscosity (Pa/s) 5.2 × 10 -4(40ºC)1.9 – 4.1 ×10 -4(40ºC)Flash point (°C) 65–115 75Solidifying point (°C) -12 -50 – -10Cold filter plugging point (°C) -11 -3.0 (-6.7 max.)Acid value (mg KOH/g) 0.374 0.5 max.Heating value (MJ/kg) 41 40 – 45HC (hydrogen to carbon) ratio 1.18 1.18Source: Oilgae, 2010.