Full Length ArticleComparative studies on liquefaction of low-lipid microalgae into bio-crude oil using varying reaction media
Introduction
Recently, microalgae have attracted much attention as a sustainable source for the 3rd generation bio-fuel production. Compared to lignocellulosic biomass, microalgae have unique advantages such as high lipid content, non-arable land use, and substantial environmental benefits through the capture of atmospheric CO2 [1], [2]. The commonly used conversion technique (i.e., transesterification) for microalgae requires high-lipid strains; however, these strains tend to have lower biomass productivities than those of low-lipid microalgae [3]. In order to obtain high lipid yield, a stressed condition, like nitrogen depletion, is a necessity for microalgae cultivation, which could not only lower biomass productivity and net energy yield but also make this process very susceptible to contamination [4]. Therefore, it would be beneficial to use high-productivity low-lipid microalgae as the feedstock for producing bio-crude oil. Another problem for transesterification is that only lipid fraction of microalgae can be converted, and thus protein and carbohydrates fraction will be ended up as waste. According to previous studies, the whole microalgae biomass including lipid, protein, and carbohydrates can contribute to the oil formation by thermochemical conversion technologies [5], [6].
Pyrolysis and hydrothermal liquefaction are regarded as two major thermochemical conversion techniques for microalgae [7]. As a comparison, hydrothermal liquefaction (HTL) is a more suitable approach for wet biomass due to the non-requirement for feedstock drying/dewatering step [8]. In addition, the bio-crude oils obtained from pyrolysis often contain higher oxygen contents than those obtained from HTL process, which could negatively affect the energy density of oil products [9]. Nevertheless, there are several challenges of microalgal HTL still ahead, such as harsh reaction conditions, relatively lower oil yield, and poor bio-crude oil quality [10].
To address the above challenges, numerous catalysts have been applied in the HTL treatment [11], [13], [14], [15]. The advantages of using catalyst for microalgal liquefaction may be summarized as follows: (i) promotes decomposition of biomass macromolecules into smaller molecules; and (ii) improves bio-crude oil properties [9]. Based on the previous literature, effect of catalyst on the quantity and quality of bio-crude oils is not only dependent on the type and dosage of the catalyst but also on the feedstock characteristics [5], [11]. Biller and Ross [5] found that the use of sodium carbonate as a catalyst negatively affected bio-crude oil production obtained from low-lipid strain (Porphyridium cruentum and Spirulina). In contrast, Ross et al. [12] reported that the acid catalysts demonstrated positive effects on the yield and quality of bio-crude oil produced from HTL of Chlorella vulgaris (a low-lipid strain). As indicated by Yang et al. [1], the addition of organic/inorganic acid exhibited different catalytic performances through liquefaction treatment. In this present study, in consideration of low lipid content in the feedstock, four different acid catalysts including organic and inorganic acids rather than base catalysts were adopted.
Apart from catalysts, various organic solvents (e.g., methanol, ethanol, and acetone) have been used as the reaction medium in the liquefaction process [16], [17], [18]. In literature, pure water is normally employed as the reaction medium in the microalgal liquefaction owing to its inherent benefits at higher temperatures and pressures, such as low dielectric constant and high ionic product [19]. However, Guo et al. [4] performed the mass balance analysis for the liquefaction in water, and the results showed that more than one-third of carbon in the feedstock were preferentially transferred into water phase rather than oily phase. Another advantage of using organic solvent as the reaction medium is that a higher bio-crude oil yield can be obtained at moderate reaction conditions [10]. The type of solvent, particularly its polarity, plays a significant role in algal liquefaction with respect to the products distribution and properties of bio-crude oils [10], [17]. Furthermore, a variety of reaction conditions such as residence time, biomass/solvent mass ratio, and reaction temperature have significant effects on the microalgal liquefaction [18].
To the best of our knowledge, no systematic research has performed to identify the most effective reaction medium for converting low-lipid microalgae into bio-crude oil. Therefore, in this present study, nine different reaction media including water, water with four different acid catalysts (formic acid, acetic acid, sulfuric acid, and hydrochloride acid), and varying organic solvents (methanol, ethanol, ethyl acetate, and acetone) were investigated for the liquefaction of a low-lipid strain. The most effective reaction medium determined was then selected to explore the effects of residence time, biomass/solvent mass ratio, and reaction temperature on the products distribution. Furthermore, physical and chemical properties of bio-crude oil were characterized.
Section snippets
Materials
Chlorella was purchased from PureBulk, Inc (Roseburg, USA) as food-grade powder. The microalgae sample had a moisture and ash content of 3.48 wt% and 7.15 wt%, respectively. The ultimate analysis on a dry basis was as follows: 46.54% C, 7.37% H, 8.59% N, 0.48% S, and 29.86% O (calculated by difference). The higher heating value of raw material was 20.97 MJ/kg. The biochemical composition of microalgae were lipid, protein, and carbohydrates with 6.10 wt%, 53.66 wt%, and 33.09 wt%, respectively.
HTL media screening
Initial studies were carried out at 275 °C for 60 min, with a biomass/solvent mass ratio of 1/5 for screening nine different reaction media including water, water with acid catalyst (formic acid, acetic acid, sulfuric acid, or hydrochloride acid), or organic solvent (methanol, ethanol, ethyl acetate, or acetone).
As shown in Fig. 2, the type of reaction medium considerably affected the products yield. For the organic acid-catalyzed HTL, the bio-crude oil yield was significantly higher (38.0 wt%
Conclusions
In this study, the effects of different reaction media (including water, water with formic acid/acetic acid/sulfuric acid/hydrochloric acid, methanol, ethanol, acetone, and ethyl acetate) on the products distribution from low-lipid microalgae were investigated. The results showed that methanol was identified as the most effective reaction medium for the production of bio-crude oil from low-lipid microalgae. Afterwards, the effects of other reaction conditions (residence time, biomass/solvent
Acknowledgements
The authors would like to acknowledge the funding from BioFuelNet Canada, a Network of Centres of Excellence and from NSERC through the Discovery Grants awarded to Dr. Xu and Dr. Bassi.
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