Preliminary assessments of microalgae direct transesterification for biodiesel production

Biodiesel is a renewable fuel which can be produced from a wide range of biomasses. In recent years, the possibility to produce it from non-food feedstock such as microalgae has been proposed. It is well known that: i) microalgae may accumulate significant quantities of triglycerides; ii) they are fast-growing photosynthetic organisms capable of sequestering CO2; iii) they can be cultivated in fresh and sea water. Nevertheless the potential of biodiesel production from microalgae, the industrial development still asks investigation. The present contribution reports about recent results of a research program in Napoli on the biodiesel production from microalgae. The study carried out so far have highlighted the success with Stichococcus strains as microalgal feedstocks to produce fatty acid methyl esters (FAMEs) according to traditional processes: lipid extraction coupled with alkaline catalyzed transesterification of lipids (Olivieri et al., 2011, 2013). Results of the direct alkaline transesterification process of Stichococcus bacillaris algal strains are here presented. Tests were carried out under wide intervals of operating conditions: pre-contact time (time of contact of biomass with alkaline methanol at room temperature), catalyst concentration, methanol/biomass weight ratio, reaction temperature, reaction time, biomass water concentration. Effects of operating conditions on FAME yield were assessed

INTENSIVE CULTURE OF AUTOTROPHIC MICROALGAE FOR ENERGY VECTOR PRODUCTION

The supply of renewable feedstocks for the production of convenience goods combined with the carbon capture and storage is considered a promising solution to both fossil resources depletion and global warming control. Photosynthetic microorganisms, e.g. microalgae, are good candidates for this challenging bet. Indeed, autotrophic microalgae fix CO2 and are feedstocks for several industries involved in human nutrition, animal nutrition, cosmetics, high-added value molecules, pharmaceuticals, biofuels and wastewater treatments. A critical issue for the intensive production of microalgae is the design of low cost and high efficiency photobioreactors. Main critical issues for the exploitation of microalgae as energy vectors are the biomass dryer and lipid extraction. This PhD thesis was focused on the development of solutions for the critical issues listed above. The main activities were: selection of microalgal strains representative of the main phylum; the optimization of the microalgal cultures by the selection of photobioreactor design and operating conditions (effects of photobioreactor hydrodynamics, light intensity and trend, CO2 in the gas phase, medium composition and medium pH on microalgal cultures in cylindrical and parallelepiped shape photobioreactors); the characterization of the composition of microalgae during the day/night cycle as an effect of the circadian behaviour of microalgae; the characterization of the microalgal growth rate under controlled operating conditions; the characterization of the photochemical process; the optimization of the direct transesterification on wet and dried microalgae to produce biodiesel. Main results were reported. The CO2 concentration typical of power plant exhaust gas (up to 15-18%) enhances lipid productivity, notwithstanding the inhibition effects on pigment synthesis. The optimal pH has been found to be 7.0, but satisfactory microalgae and lipid productivities have been obtained at low pH. The ability of microalgae to be active still a low pH suggests the real possibility to adopt acid conditions to preserve cultures from contamination. Continuous light for 24 hour did not affect the biomass and lipid productivity. The kinetic characterization of the photosynthetic reaction centres in microalgae by means of fluorescence methodology pointed out that: at irradiance lower than 1000 μE m-2 s-1 the photochemical process is controlled by the photons capture while at higher irradiance the photoinhibition competes with the photochemical quenching. The time-scale of repairing process is larger than the other photochemical process whatever the investigated irradiance. The optimization of the alkaline direct transesterification was carried out on Stichococcus bacillaris and validate on microalgal strains characterized by a different cell wall structure. Triglycerides were not converted without an alkaline catalyst and approached a maximum conversion at a catalyst concentration of 1.5% NaOH (w/w). Under alkaline conditions the pre-mixing time did not affect bio-oil yield; the bio-oil yield increased with temperature and approached a maximum at around 65°C; the bio-oil yield did not change significantly with the methanol to biomass weight ratio when the methanol is not limiting; the bio-oil yield gradually increased within the first minutes of reaction. Biomass drying was observed to play an important role in direct transesterification: the bio-oil yield reduced with an increase in biomass water content. A higher bio-oil yield was obtained increasing the methanol/wet biomass ratio. Under alkaline catalyzed conditions the direct transesterification process was more efficient than the acidic ones. The characterization of the specific growth rate and the biomass composition during the day/night cycle of Nannochloropsis sp. was carried out by means of a turbidosotat photobioreactor. Two irradiation strategy were been investigated :"constant irradiance day/night cycle" 16 h of light at 600 μE m-2 s-1 and 8 h of dark; "circadian cycle" 16 h of irradiation at intensity characterized by time-sinusoidal path and maximum of 1500 μE m-2 s-1 and 8 h of dark. The role of circadian clock and of the cell division of Nannochoropsis sp. has been highlighted during tests. The fraction of carbohydrate, protein and lipid of the biomass changes over the day: the lower value has been measured after the cell division, while the maximal values were obtained before cell division. The harvesting during the day of the biomass should be carried out when the concentration of the selected constituent is high. A model to describe the change of the specific growth rate of microalgae during the day was been proposed and validated. The optimization on new photobioreactor systems needs to take into account the different time scale of the photochemical process and the instantaneous growth rate of microalgae. Analysis of the microalgal circadian clock is necessary to understand the right instant of the day at which to harvest the biomass with the set composition. It is also possible suggesting to develop systems to maximize the production of microalgal fractions (carbohydrates, lipids, proteins) according to the application of the process. However, the exploitation of only one microalgal fraction may not be in agreement with the maximum economic profit of the process

A techno-economic analysis of biodiesel production from microalgae

The preliminary assessment of a cost-effective flow-sheet for the production of biodiesel from microalgae lipid fraction was carried out. The study was based on approximated cost-estimation methods integrated with the simulation software Aspen Plus (R). Several scenarios were investigated to compare costs regarding the main steps of the biodiesel production process. Ranges of input variables from downstream literature and experimental data were used to simulate the sections that define concrete process routes for production of algal biofuels: the extraction of the lipid fraction from aqueous solution, the lipid transesterification, and the methyl esters (FAMEs) and glycerol recover. The design variables were selected so as to correspond to the main degrees of freedom of the process: number of equilibrium stages, solvent recycle flow rate, transesterification time, methanol to triacylglycerols ratio, plate number and reflux ratio of the distillation unit for methanol recovery. The cost estimation for the conceptual design of the flowsheet dedicated to the FAMEs production from microalgae was carried out according to Happel's method. Data were worked out to assess the venture profit and the selling price (s) for mass unit of FAMEs. The minimization of s was adopted as the objective function. A preliminary determination of plausible values of the cost for unit of mass of biodiesel was attempted as a function of operating conditions

A techno-economic analysis of biodiesel production by transesterification of microalgal lipids

A key issue of the industrialization of microalgae-to-biofuel processes is the characterization/improvement of unit operations involved in the downstream process. Typically, the process includes: biomass harvesting and concentration, oil extraction, transesterification, and fatty acid methyl-esters (FAMEs) purification. This work regards the assessment of a cost-effective flowsheet for the production of biodiesel from microalgal lipid fraction. The study was based on approximated cost-estimation methods integrated with the simulation software Aspen Plus®. Several scenarios were investigated regarding the main steps of the biodiesel production process. The attention was focused on sections dedicated to: the extraction of the lipid fraction from aqueous solution, the lipid transesterification, and the recover and concentration of both FAMEs and glycerol. Features of the produced FAMEs were set according to the ENI EN 14213 and ENI EN 14214 standards. Main assumptions of the simulation were: i) the triglycerides (TAGs) fraction of microalgae set at 25%w; ii) free fatty acids (FFA) <0.05%w. The production cost for unit of biodiesel mass was adopted as objective function. The attention was focused on the assessment of the share to process costs related to each process step

A techno-economic analysis of biodiesel production by transesterification of microalgae lipids

The present work reports on the preliminary assessment of a cost-effective flow-sheet for the production of biodiesel from microalgae intensive cultivation. The study was based on approximated cost-estimation methods integrated with the simulation software Aspen Plus®. Several scenarios were investigated regarding the lipid extraction, transesterification and solvents recovery steps. The design variables were selected so as to correspond to the main degrees of freedom of the process as number of equilibrium stages, solvent recycle flow rate, transesterification time, methanol to triacylglycerols ratio, plate number and reflux ratio of the distillation unit for methanol recovery. A preliminary determination of plausible values of the cost for unit of mass of biodiesel was attempted as a function of operating conditions

Biodiesel Production in Outdoor Cultures of Scenedesmus vacuolatus

The supply of renewable feedstocks for the production of convenience goods combined with the carbon capture and storage is considered a promising solution to both fossil resources depletion and global warming control. Photosynthetic microorganisms, e.g. microalgae, are good candidates for this challenging bet. The culture performances remarkably reduce when microalgal cultures move from laboratory to industrial scale. The basic requisite for the development of large-scale production of oil from microalgae is to make the process environmentally and economically sustainable/feasible. A key issue for the process based on outdoor photobioreactors (PBR) is the efficient utilization of the photosynthetically active radiation (PAR). The present work reports the results of outdoor cultures of Scenedesmus vacuolatus strain ACUF 053/95 in 1.7 L inclined bubble column (IBC) photobioreactors characterized by 250 cm2 irradiated surface. Cultures were carried out in outdoor: i) during the May-July period under shadow conditions, irradiance maximum of 450 μE/(m2 s); ii) during the May-July period under direct sun light, irradiance maximum of 2100 μE/(m2 s); iii) during the September-November period under direct sun light, irradiance maximum of 2000 μE/(m2 s). Harvested microalgae were processed to characterize the biodiesel composition thought direct alkaline transesterification. The biomass concentration at steady state conditions was kept within the range 3-4 g/L. Biomass volumetric productivity ranged in the interval 0.17-0.22 g/(L d) - area specific productivity of 11-15 g/(m2 d) - depending on the operation mode

Outdoor cultures of Scenedesmus vacuolatus: biomass and biodiesel production

Biofuels production coupled with carbon dioxide sequestration by means of photosynthetic microrganisms appeared a promising process since the end of the last century. Microalgae biomass may be processed to produce liquid fuels: bio-oil is extracted from microalgae and it may be either adopted as crude fuels or transesterified to biodiesel. A twofold advantage characterizes the potential success of microalgae as biofuel feedstocks. The first advantage: biodiesel-microalgae production rate may be 1–3 orders of magnitude larger than that from oil crops . The second advantage: microalgal biomass fixes a large amount of CO2 – 1.83 kg of CO2 per kg of dry microalgae – and strongly contributes to the reduction of greenhouse gas emissions. However, yield and productivity of biomass and total lipid content are strongly affected by nutrient and light supply strategy. The present work reports the results of outdoor cultures of Scenedesmus vacuolatus in 1.7 L inclined bubble column (IBC) photobioreactors characterized by 250 cm2 irradiated surface. IBC were operated according to Olivieri et al. (2012). The temperature was set at 23°C. A 2% CO2 gas stream (50 nL/h flow rate) was sparged at the bottom of the IBC. pH was kept constant at about 7.0 by the CO2 feeding. Bold Basal Medium (BBM) - NaNO3 as nitrogen source - was adopted. Cultures were carried out outdoor: i) a test campaign during the May-July period under shadow conditions, irradiance maximum of 450 microE/(m2 s); ii) a test campaign during the September-November period under direct sun light, irradiance maximum of 2000 microE/(m2 s). Cultures were operated under fed-batch or semi-continuous mode. 30% of suspension was withdrawn – and substituted with fresh medium - with a frequency of 1-2 times a week. Two procedures were adopted to induce nitrogen-starvation stress: I) the BBM feeding was stopped for two weeks and the nitrogen content was continuously monitored; II) the gas flow rate was stopped for few hours to separate the biomass by sedimentation and the clarified culture liquid was replaced with fresh BBM without nitrogen source

Effects of photobioreactors design and operating conditions on Stichococcus bacillaris biomass and biodiesel production

Indoor cultures of Stichococcus bacillaris were carried out to investigate the effect of pH, CO2, and hydrodynamics on biomass and biodiesel productivity in a 0.6L vertical bubble column (VBC) and a 1.7L inclined bubble column (IBC) photobioreactor. The temperature and the irradiance level were set at 23°C and 300micronE/(m2s), respectively. The pH of the liquid culture was in the range 3.0-8.5. The CO2 concentration in the gas phase was increased up to 15%, typical of exhaust gas from power plants. The increased CO2 concentration in the gas phase stimulated the microalgal growth. Tests carried out at different pH and spreading a 5% CO2 gas stream showed that the biomass productivity was maximum at a pH of about 7.0. Acid and alkaline conditions can also be adopted with an approximate 25% decrease in biomass productivity. The comparison between a IBC and a VBC reactor operated under the same conditions pointed out that the performances of IBC were higher than those of VBC: 0.26gbiomass/(Ld) and 0.032gFAME/(Ld) for IBC, and 0.124gbiomass/(Ld) and 0.010gFAME/(Ld) for VBC. IBC was characterized by an intensive liquid circulation that promotes a continuous renewal of microalgal cells in the photic zone. �� 2013 Elsevier B.V

Direct transesterification of Stichococcus bacillaris for biodiesel production

The potential of microalgae to capture CO2 and to produce bio-oil is known since the last century. Typically, the process to produce biodiesel from microalgae includes the following steps: microalgae growth in photobioreactor systems, biomass harvesting and lyophilization, extraction of lipids by solvent, transesterification of extracted triglycerides. The direct transesterification of the microalgae has been proposed as an alternative way to produce biodiesel. The lipid fraction is transesterified without any preliminary extraction. The present contribution aims at the characterization of the direct transesterification of Stichococcus bacillaris. The attention has been focused on the effects on methylester yield of: concentration of the catalyst dissolved in methanol, pre-contacting time of the cells with alkaline methanol, biomass/alcohol ratio, time and temperature of transesterification, and water content of the biomass

Scenedesmus vacuolatus cultures for possible combined laccase-like phenoloxidase activity and biodiesel production

A key aspect of the industrial development of microalgal production processes is the excessive cost of biomass production. A solution is a combination of biodiesel production and wastewater treatment. The microalga Scenedesmus has a high lipid content and a potential extracellular phenoloxidase activity, which could improve the phycoremediation of phenolic pollutants. In this work, the most suitable growth conditions to obtain this twofold aim were analyzed. First, different strains of Scenedesmus vacuolatus microalga were tested at different pH, salinity and CO2 concentration in the gas phase. The two most promising strains were then cultivated in autotrophic and heterotrophic conditions, and were investigated in terms of efficient nitrogen removal, fatty acid profile and maximized extracellular phenoloxidase activity in the medium. The results showed two extreme conditions: (1) biomass productivity doubled when photobioreactors were sparged with 5% CO2 supplemented air with respect to cultures sparged with air (the steady state values of strain 53 were 0.138 g L−1 day−1 in the presence of air, and 0.243 in the presence of CO2 addition), and N-starvation under 5% CO2enhanced the transesterified fraction of lipids (strain 53 FAME fraction in the presence of N-starvation was 33%, in the presence of nitrogen FAME fraction was 22%); (2) phenoloxidase activity was completely suppressed by presence of 5% CO2 in the gas phase (strain 53 0.21 U mL−1), indicating clear catabolite repression for the induction of this enzyme in the algal metabolism