Development of direct conversion method for microalgal biodiesel production using wet biomass of Nannochloropsis salina
Introduction
Concerns regarding depletion of easily accessible conventional petroleum reserves have lead to increased interests in alternative sources for transportation fuel. Even though shale gas has been recently getting widespread attention as a possible future energy source due to its wide availability, it is neither renewable nor carbon neutral (Shirvani et al., 2011). Biofuel is one possible solution which is eco-friendly and sustainable. Microalgae are one of the most promising precursors for biofuel, as microalgae can accumulate high level of lipids which can be converted into biodiesel. Microalgal biodiesel has several notable benefits compare to biodiesel from crop oil or bioethanol from grains or cellulosic biomass. Microalgae can produce much greater quantity of biomass per land area and time, compared to that of the land plants. In terms of biodiesel productivity, microalgae can yield 10 times greater lipid production than jatropha and 50 times that of soybean (Amaro et al., 2011, Chisti, 2007, Halim et al., 2012, Mata et al., 2010, Scott et al., 2010). Moreover, microalgae can utilize waste water and nutrient resources that do not compete with human food production (Mata et al., 2010). However, a number of technical and economical hurdles must be overcome in order to achieve successful industrialization and commercialization of microalgal biodiesel (Halim et al., 2012). Lipid extraction and conversion steps are the major obstacles for the commercialization of microalgal biodiesel because of the high cost and energy input required. Extraction step takes more than 50% of total energy consumption even though drying step was omitted, which generally takes about 80% of total energy consumption. Ignoring cultivation, downstream steps takes 60% total production cost (Kim et al., 2013, Lardon et al., 2009, Wahidin et al., 2014).
Among various lipid extraction/conversion methods for bio-feedstock including microalgae and crops, in situ transesterification (also called direct conversion or direct transesterification) is one of the most promising process for producing biodiesel in a fatty acid alkyl ester (FAAE) form (Griffiths et al., 2010). FAAE can be obtained by transesterification of triacylglyceride (TAG) or esterification of free fatty acid (FFA) with short chain alcohol. This process is different from traditional extraction processes in that, the lipid is extracted by alcohol and undergoes esterification reaction simultaneously. Methanol and ethanol are the most frequently used for transesterification, and the reaction results in the production of fatty acid methyl ester (FAME) or fatty acid ethyl ester (FAEE) respectively (Park et al., 2014a, Yusoff et al., 2014). Traditionally, extraction and conversion steps were independent and separate processes: lipid is first extracted from microalgal cells via one process, and undergoes transesterification in the second process. However, in situ transesterification can produce biodiesel with a single reactor using microalgae cells, not extracted lipid, as feed material. Therefore, this can be significantly reduce the energy consumption (Kim et al., 2013, Park et al., 2014c). In situ transesterification is also known for showing higher productivity compared to most extraction/conversion processes as well (Cavonius et al., 2014, Griffiths et al., 2010). Some past research have studied in situ transesterification of microalgal cells. One study transesterified various strains of dried algal cells under different conditions to examine how much FAME can be produced (Wahlen et al., 2011). Another group compared the two step extraction/conversion methods and in situ transesterification with Schizochytrium limacinum by using various co-solvents (Johnson and Wen, 2009). One study using Nannochloropsis biomass attempted to incorporate an extra treatment using microwave and ultrasound radiation to improve the performance (Koberg et al., 2011).
However, traditional in situ transesterification also has several disadvantages, with a major one being that an excessive amount of solvent and catalyst are required (Wahlen et al., 2011). In normal transesterification reaction, 3 alcohol molecules are theoretically needed to transesterify one TAG molecule, but in practicality 6 alcohol molecules are required for direct reaction between alcohol and pure oil (Fukuda et al., 2001). However, for in situ transesterification case, greater than 100:1 mol ratio is typically used in previous studies, since alcohol serves as not only just the reactant but the solvent for lipid extraction as well (Velasquez-Orta et al., 2012). Therefore, excessive alcohol requirement is due to the lipid extraction part of the process. Moreover, cells include not only lipid but many other impurities that can hinder the reaction, such as cell debris and chlorophyll (Park et al., 2014b). Therefore, excessive reactant is needed to drive the reaction toward biodiesel production for the in situ transesterification of microalgae.
Another disadvantage commonly present in most dry biomass based oil extraction processes, is a requirement of energy intensive cell drying step after harvesting the biomass (Kim et al., 2013). Drying steps are often performed before most downstream processes in order to achieve more effective extraction, faster reaction rate and higher conversion yield (Johnson and Wen, 2009, Kim et al., 2013). However, it is questionable if the drying step can improve the overall process economy due to the additional energy and cost incurred (Canakci and Van Gerpen, 1999). For that reason a number of studies evaluated downstream processes that can be performed using wet biomass. Some examples include transesterifying the cells in supercritical conditions (Levine et al., 2010, Patil et al., 2011), or using microwave to disrupt the cell wall prior to in situ transesterification (Cheng et al., 2013). However, these methods still require additional costs and high energy, which defeats the original purpose of using wet biomass. Therefore, transesterification of wet biomass without additional treatment should be explored in order to achieve improved economical outlook.
There has been a number of previous research regarding the optimization of the in situ transesterification of microalgal biomass, but most of these used dry cells as a feedstock. In this study, the optimization of in situ transesterification of Nannochloropsis salina was investigated using wet biomass of the algae. Development of the process that uses wet algal biomass is particularly important, as life-cycle assessment of microalgal biodiesel reported that wet microalgae route greatly outperforms dry route in energy balance (Lardon et al., 2009). Since the goal of this study is to minimize the cost of the entire process, no additional step or method, such as supercritical condition or microwaves were used. The study optimized for the lowest reaction time, temperature, and amount of solvent and catalyst. The effects of solvent and moisture of cells were also studied. The optimized reaction condition was scaled up from 2 ml to 1 L scale in order to verify whether the optimized conditions are applicable on larger scale as well. The findings in this work are expected to greatly aid the development of successful algae biodiesel production process.
Section snippets
Materials
N. salina biomass cultivated in 200 ton raceway ponds was provided by NLP, a South Korean microalgae cultivation company. The microalgal sample was harvested by continuous centrifugation and frozen at −70 °C for long-term storage. It was thawed right before the series of experiments. HPLC-grade methanol, ethanol, and chloroform were purchased from Merck. Sulfuric acid (98%) and eicosane (⩾99.5%) were purchased from Sigma–Aldrich, and used as homogenous catalyst for transesterification and
Comparison between pure alcohol and alcohol–chloroform as solvent
According to previous research, using co-solvent with alcohol can improve the performance of in situ transesterification, due to the fact that TAG has low solubility in pure alcohols. In particular, several studies report that chloroform has the highest performances among the various co-solvents due to the fact that it is highly miscible with both alcohols and lipids (Im et al., 2014, Johnson and Wen, 2009). To verify whether chloroform can positively influence in situ esterification using wet
Conclusion
The optimal conditions for the in situ transesterification of wet N. salina in lab scale were found by adjusting solvent, acid concentration, temperature, time, and moisture. It was found that 10 ml of methanol or 5 ml of ethanol is proper for 1 g of wet biomass, without co-solvent. Under 1 h reaction at 100 °C, 2.5% and 5% of sulfuric acid (v/v) seems sufficient for complete conversion in methanol and ethanol, respectively. These conditions were applicable at 0.5–1 L solvent systems as well. These
Acknowledgements
This work was supported by the Advanced Biomass R&D Center (ABC) as the Global Frontier Project funded by the Ministry of Science, ICT and Future Planning (ABC-2010-0029728). We also thank to NLP kindly provided the biomass for this study.
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