Lipid content in microalgae determines the quality of biocrude and Energy Return On Investment of hydrothermal liquefaction
Graphical abstract
Introduction
Biofuels are a promising renewable energy source and potential substitutes for fossil fuels, and their adoption may also help alleviate environmental problems such as global warming and climate change [1], [2]. Unlike other renewable energy sources such as solar energy and wind power, biofuels are carbon substances that can directly substitute for traditional petroleum products such as transportation fuels [3]. Among various feedstocks for biofuels, microalgae have gathered much attention as an advanced biomass for the production of biofuels and chemicals. Microalgae grow rapidly and efficiently fix carbon dioxide in the atmosphere, and can be a source of various biofuels ranging from solid to gaseous fuels [2], [4]. However, there are numerous technical challenges that must be overcome for microalgae to become a commercially viable feedstock. Among these, the wet condition of algal biomass after harvesting contributes to a significant portion of the production cost of biofuels from algal biomass, since it requires drying of the biomass prior to lipid extraction or conversion for high yield [5]. Even with recent successes with direct transesterification of wet algal biomass with acid catalysts, there are still problems such as excessive use of solvents and catalysts [6], [7]. Furthermore, the lipid content of microalgae dictates the yield and efficiency of transesterification, as well as the economics of microalgae-based biodiesel production [8].
In order to overcome these technical challenges, various thermochemical conversions of microalgal biomass such as pyrolysis and hydrothermal liquefaction (HTL) have also been investigated [2], [9], [10]. In particular, HTL of microalgae in subcritical water was found to be effective for the production of biocrude due to the soft physical nature of microalgal biomass, which obviates the need for pretreatment such as crushing, in contrast to other biomass such as lignocellulosic biomass. Biocrude is an oily phase containing diverse hydrocarbons that can be converted into various fuels using existing facilities in petrorefinery [9], [11], and it can be produced with higher liquid yield than the original lipid content of microalgae [9]. Lab-scale [12], [13], [14], [15], [16], [17], [18] and pilot-scale [19], [20], [21] HTL studies using various microalgal strains have produced biocrude yields around 30–50% (dry cell weight basis) regardless of the biochemical composition of microalgae (biomass-agnostic). Since HTL directly treats wet biomass without drying, the entire production chain of microalgal biofuels can be improved in terms of energy balance, economic viability, and environmental impact [22], [23]. HTL is therefore gathered considerable attention as a promising unit process for microalgae-based biofuel production. However, a detailed understanding of the chemical components of biocrudes, the influence of the quality of biocrudes on subsequent catalytic upgrading, and the Energy Return On Investment (EROI) of HTL of microalgal biomass has yet to be attained.
While 300–350 °C is generally considered as the temperature of HTL because of the highest total yield of biocrude [13], [14], [16], [17], the total biocrude yield alone cannot be taken as a sole indicator of the efficiency and feasibility of HTL. Although HTL at higher temperatures can generate larger amounts of biocrudes, it also requires greater process energy for increasing the temperature of wet biomass. This means that careful consideration of the EROI of HTL as a function of reaction temperature is required. In addition, biocrude produced by HTL at high temperature contains a large amount of polyaromatic compounds (asphaltene) containing high levels of heteroatoms such as nitrogen and sulfur [13], [16], [24], which should be removed to meet the environmental regulations on fuels. Nitrogen and sulfur can be transformed to nitrogen oxides (NOx) and sulfur oxides (SOx) upon combustion. Unfortunately, it is difficult to upgrade heavy asphaltenes via catalytic conversion processes due to significant fouling and poisoning of catalysts [24], [25], [26]. Some previous attempts on catalytic upgrading of biocrudes showed only limited success in the removal of heteroatoms [27], [28], [29]. Other studies tried to reduce the level of heteroatoms in biocrude during HTL, but various attempts such as pretreatment [30], [31] or addition of catalysts [32] and hydrogen gas [33] also showed limited results.
In the present work, we conducted a series of experiments to determine the influence of the biochemical composition of microalgae and the HTL reaction temperatures on the quality of biocrude and the EROI of the HTL process. The effects of HTL temperature were thoroughly investigated by measuring various physicochemical properties such as asphaltene/non-asphaltene content, heteroatom (O, N, and S) concentration, effective hydrogen-to-carbon ratio (H/Ceff), and thermal stability. Furthermore, we investigated the EROI of HTL of microalgal biomass with different lipid contents. The EROI of HTL and the quality of biocrude of Nannochloropsis oceanica (N. oceanica) with a high lipid content were superior to those of Golenkinia sp. with a low lipid content. The microalgal strain with high lipid content showed maximum EROI (4.91) after HTL at 200 °C, while the low-lipid strain showed maximum EROI (2.70) at 300 °C.
Section snippets
Materials
N. oceanica and Golenkinia sp. cultivated in large-scale open raceway ponds were kindly provided by NLP Co. Ltd. (Hadong, Korea). N. oceanica is a marine alga that has a high lipid content, and Golenkinia sp. is a freshwater alga that has a low lipid content. Dichloromethane (DCM, 99.8%) and n-hexane (HPLC grade) were purchased from Samchun Chemical (Pyeongtaek, Korea). Methanol, chloroform (HPLC grade), sulfuric acid (95–97%), and heptadecanoic acid (⩾98%) were purchased from Sigma–Aldrich
Biochemical compositions of microalgae
Table 2 shows the biochemical compositions of the two microalgae used for HTL. The total and saponifiable lipid contents of N. oceanica were almost 2-fold larger than those of Golenkinia sp. The amount of unsaponifiable lipids (total lipids – saponifiable lipids) of N. oceanica (9.1%) and Golenkinia sp. (8.7%) were similar. In contrast, the protein content was lower in N. oceanica (36.3%) than in Golenkinia sp. (45.1%). These two species had similar contents of total carbohydrates and ash.
Total biocrude yield after HTL
Fig. 1
Conclusions
While a high temperature (i.e., 300–350 °C) is generally accepted as the optimal reaction temperature for HTL of microalgal biomass, our results indicate that the low quality biocrude with high concentration of asphaltene and heteroatoms is produced from HTL of microalgal biomass at a high temperature. This was more pronounced when biocrude was prepared from the microalga (Golenkinia sp.) with low lipid content than when it was produced from the microalga (N. oceanica) with high lipid content.
Acknowledgements
This work was supported by the Advanced Biomass R&D Center (ABC) of Korea Grant funded by the Ministry of Science, ICT and Future Planning (ABC-2010-0029728).
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2022, Energy and AICitation Excerpt :The energy consumption in low-pressure pipeline transport is collected from literature [30,32] while the energy consumption in CO2 distribution is calculated coupling the pump efficiency and depth of the bioreactor [8]. The LCI data of Nannochloropsis growth were collected coupling lipid content with specific productivity by literature [2,11,12,33-36] and actual Nannochloropsis oceanica cultivation plant in China [1]. The influence coefficient of the external irradiation intensity, irradiation time, temperature was involved in AI model coupling CO2 distribution and nutrient supply.