Downstream processing of marine microalgae for the commercial scale production of biofuels.

Abstract

Several species of microalgae have lipid yields that are about 4 to 5 times of that from the highest oil bearing terrestrial plants such as oil palm. Furthermore, marine species can offer the additional advantage of not competing with farm produce for fresh water or arable land. These advantages make marine microalgae appear to be attractive as a feedstock for the production of biofuels; however, despite intensive research, the production cost of microalgal biomass remains high at about A$ 10 kg⁻¹. In comparison, plantation oil such as canola can be available of about $1 L⁻¹ and this large cost difference makes algal biofuels uncompetitive. Hence, the sustainability of microalgal biofuel production depends very much on the technical, energetic and economic issues involved with production. Microalgal lipids and carbohydrates are major feedstock for renewable fuels and the downstream processing can broadly be classified into four steps: harvesting, dewatering, cell disruption and extraction of the desirable products. The specific objectives for the PhD study were harvesting and cell disruption of microalgae; these objectives were structured towards the development of pilot scale production of marine microalgae and included technical, energetic and economic evaluations. Below are details for these objectives: I. Harvesting: the objective was to increase the biomass concentration from about 0.2 kg m⁻³ to about 20 kg m⁻³. Such level of concentration would be suitable for the next unit of processing such as cell disruption or secondary dewatering. Flocculation (electro- and microbial-) were applied as the harvesting methods with induced mechanical mixing and electrode separation as the novelties in energy optimization techniques. The scope of this study included the technical investigation and development of these harvesting methods, evaluation of their energy requirements, plant designs and economic analyses; all these areas were deemed to be necessary for the determination of the viability of such processes. II. Cell disruption: disruption processes with potential for the commercial scale biofuel production were investigated with emphasis on the process reliability, energy requirement and disruption efficiencies. The novelty was the determination of disruption energy efficiencies by comparing the process energy input with the theoretically derived values of cell disruption. III. Cell mechanics: mechanical cell wall properties that affect the disruption process were investigated. Atomic force microscopy was used to evaluate the disruption energy requirements and determine the efficiencies of various cell disruption processes. The novelty was the measurement of disruption energy requirement for individual cells. From the experimental value obtained, the specific disruption energy on a per kg basis was calculated to determine the energy efficiencies of various disruption methods. It was found that the mixing of microalgal media during electroflocculation is essential for the reduction in electrical energy requirement, and hydraulic baffles can provide an energy efficient technique for such purpose. The energy required for such mixing is 3.2 kJ kg⁻¹ of the equivalent dry mass based on the design criteria that the value of Camp number is between 10⁴ to 10⁵ and the velocity gradient is between 100 s⁻¹ to 10 s⁻¹. The cost for harvesting by microbial flocculation, including energy, raw material and capital depreciation, was estimated to be A$ 0.26 kg⁻¹ of the equivalent dry mass. On the other hand, the cost of harvesting by electroflocculation, including electrical energy, aluminium dissolution and capital depreciation, was estimated to be $ 0.185 kg⁻¹ of the equivalent dry biomass. The energy consumptions by both types of flocculation have the potential to be further optimised. For the extraction of lipids for the production of biodiesel, mechanical cell disruption methods are preferred due to the lower risk of contamination of products and the ease of scale up. The drawback is that current mechanical disruption processes have high specific energy consumption that is in excess of that can be available from the combustion of the entire cell mass. The disruption energy may be optimized by selecting processes that are relatively energy efficient and the combination of cell disruption with solvent extraction. The disruption energy as measured by the use of Atomic Force Microscopy revealed that an average value of 17.4 pJ was required for the disruption of an individual Tetraselmis sp. cell, this value is equivalent to specific disruption energy of 673 J kg⁻¹ of dry microalgal biomass. In comparison, disruption by hydrodynamic cavitation, one of the most energy efficient technique, requires specific disruption energy input of 33 MJ kg⁻¹ of the dry biomass. This large difference indicates the low efficiency in the mechanical cell disruption and more innovation is required for the sustainability of such processes in the production of biofuels. This thesis advances the knowledge in the harvesting and cell disruption of microalgae from the laboratory to pilot scale. In the area of harvesting, advances were made in the scale up on the plant design, energy optimisation and economics of microbial- and electro-flocculation; while in the area of cell disruption, advances were achieved on the understanding of the energy requirements for large scale mechanical cell disruption processes, and the fundamental cell mechanics with respect to disruption. All these work has been presented as journal publications as detailed in the Preface of this thesis.