Elsevier

Bioresource Technology

Volume 110, April 2012, Pages 617-627
Bioresource Technology

Impact of reaction conditions on the simultaneous production of polysaccharides and bio-oil from heterotrophically grown Chlorella sorokiniana by a unique sequential hydrothermal liquefaction process

https://doi.org/10.1016/j.biortech.2012.01.047Get rights and content

Abstract

A two-step sequential hydrothermal liquefaction (SEQHTL) model for simultaneous extraction of polysaccharide at the first step followed by bio-oil in the second was established. The effects of reaction temperature, residence time, and biomass/water ratio on the product distribution of each SEQHTL step were evaluated. Maximum yield (32 wt.%) of polysaccharides was obtained at 160 °C, 20 min and 1:9 biomass/water ratio. Considering the operation cost and bio-oil yield (>30%); 240 °C, 20 min and 1:9 biomass/water ratio was preferred as ideal SEQHTL condition for bio-oil extraction. SEQHTL always produced ∼5% more bio-oil and ∼50% less bio-char than direct hydrothermal liquefaction (DHTL). Free fatty acid content of the bio-oils exhibited a sharp decrease with increase in temperature. Comparative analysis of the energy input and net energy balance showed that SEQHTL requires ∼15% less MJ/kg bio-oil than DHTL. Energy recovery rate for SEQHTL is nearly 4% higher than the DHTL.

Highlights

Polysaccharides and bio-oil are extracted concomitantly by a novel sequential hydrothermal liquefaction (SEQHTL). ► By SEQHTL, optimal bio-oil production is achieved at much lower temperature than direct hydrothermal liquefaction (DHTL). ► SEQHTL produces 50% less bio-char than DHTL. ► SEQHTL requires less energy and has higher thermal efficiency than DHTL.

Introduction

Owing to its faster growth rate, maximum productivity, higher lipid content (30–60%), higher photosynthetic efficiency, the ability of adapting high saline environment, large scale cultivation without competition of arable land algal biomass has gained a considerable attention as a potential feedstock for bio-fuels production (Wijffels et al., 2010). However, of the significant challenges in producing bio-fuels from algal biomass is the lack of, efficient conversion technology. Though there are several conversion technologies available, most of them are at lab scale and much more expensive than transesterification of plant seed oil. Several researchers have pursued the lipid extraction strategies from algae for bio-fuels production, while physical, chemical, thermal, and biological techniques have been widely exploited (Ranjan et al., 2010).

Organic solvent extraction is the most traditional method to extract lipid components from algae started from 1959 by Bligh and Dyer (Bligh and Dyer, 1959), while some other solvent extraction approaches have also been developed for various algae, including solvent-based extraction with microwave and ultrasound (Tukai et al., 2002), accelerated solvent extraction (Klaus, 1998), subcritical water extraction (Herrero et al., 2006) and supercritical fluid extraction (Metzger and Largeau, 2005) used as selective extraction. However, organic solvents extraction involves toxic solvents which limit the biomass usage for co-products recovery environmentally. Also, for better lipid recovery with organic solvent extraction, an energy-consuming dewatering process of the wet biomass needs to be considered. Apart from organic solvent extraction; pyrolysis, gasification, and hydrothermal liquefaction (HTL) are the three main thermal conversion approaches, while pyrolysis and gasification have been widely employed to convert lingo-cellulosic materials to produce bio-fuels. But for algal biomass conversion, a significant roadblock in both pyrolysis and gasification is the high moisture content of the biomass. Significant energy requires in dewatering the biomass make these method economically unsuitable for algal biofuel production. Therefore, unless an inexpensive dewatering or extraction process is developed, pyrolysis and gasification will not be a cost-competitive in a near future.

Recently several literatures have cited the usefulness of hydrothermal liquefaction of wet algal biomass for biofuel production not only from the drying cost point of view but also from the benefits of using water as an extraction solvent. Water under high temperature showed several unique properties which make it an excellent environment friendly solvent for bio-fuels and co-products production from the algal biomass. With increase in temperature water under autogenic pressure starts to behave more like an organic solvent and exhibits gamut of unique properties which makes it an excellent candidate for selective extraction. The influence of temperature on the extraction properties of water has reviewed in details by Toor et al. (2011). At 300 °C, water exhibits a density and polarity similar to those of acetone at room temperature (Pitzer, 1983). These properties of the high temperature water make it an excellent candidate for extracting lipids from the algal biomass.

Algal direct hydrothermal liquefaction (DHTL) research has received extensive attention since 1994 started from Dote et al. (1994). During the early stage algal liquefaction research, some Japanese researchers worked on algal liquefaction with sodium carbonate as the catalyst, which had demonstrated algal bio-oil is comparable to the petrol fuel from energy and environment perspective (Dote et al., 1994, Minowa et al., 1995). Hereafter, especially starting from 2008, a large number of researchers started to probe on the algal bio-fuel production within hydrothermal liquefaction technology. Both of microalgae (Duan and Savage, 2010, Jena et al., 2011, Minowa et al., 1995, Ross et al., 2010, Zou et al., 2010) and macroalgae (Anastasakis and Ross, 2011, Aresta et al., 2005) have been tested as the feedstock to produce bio-fuels. Bio-oil or bio-crude, bio-char, water extractives, and bio-gas are the major products from algal liquefaction process, usually can be achieved from 200–400 °C within 10–60 min. As mentioned in several papers, reaction temperature, residence time, biomass/water ratio play a significant role in the quantity and quality of bio-oil. Bio-oil liquefied from Dunaliella tertiolecta was observed increasing along with the climb of temperature from 280 to 380 °C and the prolong of residence time from 10 min till 50 min (Zou et al., 2010). Chaetomorpha linum was processed at 250, 300, 350, and 395 °C for 60 min (Aresta et al., 2005), in spite of the total bio-oil yield change with temperature, the amount of fatty acids in bio-oil was also found reduced resulting from the decomposition of oil at higher temperature. Macro-alga Laminaria saccharina was studied with different reaction conditions reporting the biomass/water ratio were extremely crucial because of water acts as both a hydrogen donor and a solvent for hydrolyzing the carbohydrates present in algae (Anastasakis and Ross, 2011).

Furthermore, at different stages of hydrothermal liquefaction multiple reaction mechanism takes place. Hydrothermal liquefaction is first initiated at lower temperature by the hydrolysis of biomass components followed by dehydration and deoxygenation, then with further increase in temperature and residence time more complex reactions like polymerization and polycondensation take place. Liquefaction reaction can be tuned by tuning the hydrothermal parameters (mainly reaction temperature, residence time, and biomass/water ratio) and can be utilized to enhance the production and quality of the desired co-products and bio-fuels. As, developing co-product is one of the plausible strategies to make algal biofuel economically viable therefore, exploring hydrothermal liquefaction as an useful conversion tool for developing different co-products along with bio fuel from algal biomass is worth studying.

In this context, we developed a modified hydrothermal liquefaction strategy (Chakraborty et al., 2012), which aims at tuning the hydrothermal reaction temperature to isolate polysaccharides from the algae at low temperature followed by a typical HTL step of the extracted biomass to obtain a bio-oil rich in lipids from Chlorella sorokiniana biomass. Particularly, we aim at isolating polysaccharides because algal polysaccharides are very promising source of value-added co-product as they are more tolerant to hydrothermal liquefaction conditions than other components of the biomass as co-product. Furthermore, it has been reported from the energy perspective, the removal of carbohydrates will contribute to the economical algal bio-oil production owing to carbohydrates have a negative energy balance for its fairly low bio-oil yield and enormous energy input (Biller and Ross, 2011). Polysaccharides are reported mainly contributed to the production of bio-char of lower economic importance. However, extraction of algal polysaccharides in its native form has large industrial potential, depending upon the algal strain. It is known that a wide range of industrial products can be developed from algal polysaccharides.

Therefore, to better understand the influence of different hydrothermal parameter on the sequential hydrothermal liquefaction (SEQHTL) and to identify its strength and weaknesses in compare to DHTL, in this paper we focuses on three aspects; the effect of reaction condition on the two steps liquefaction products yields including reaction temperature, residence time and biomass/water ratio, comparative analysis of the quality and yield of bio-oil, bio-char, and water extractives (WEs) produced by DHTL and SEQHTL, and to evaluated the relative energy consumption and net energy balance of SEQHTL method in comparison to DHTL of bio-oil. Furthermore, we attempt to explain the possible hydrothermal liquefaction mechanisms which might involve in the products distribution and quality under different liquefaction conditions.

Section snippets

Raw materials

The green alga C. sorokiniana (UTEX 1602) was obtained from the Culture Collection of Alga at the University of Texas (Austin, TX, USA). Heterotrophic stock culture was maintained at 30 °C in Kuhl medium (Kuhl and Lorenzen, 1964) supplemented with 10 g/l glucose. All media were autoclaved at 121 °C for 20 min before use.

Algal biomass preparation

The inocula was prepared in 500 ml flasks containing 200 ml Kuhl medium supplemented with 20 g/l glucose, then fermentation of heterotrophic C. sorokiniana was performed in a 5 L

First step of SEQHTL reaction – polysaccharides extraction

Target products of the first step of SEQHTL are polysaccharides. Therefore, an in-depth study was conducted to understand the impact of three reaction conditions; temperature, residence time and biomass/water ratio on the yield of polysaccharides. Researcher used severity factor which combined the effects of retention time and reaction temperature to measure the hydrothermal treatment intensity (Miyazawa et al., 2008). Higher temperature and longer residence time lead to higher severity factor.

Conclusion

Removal of algal polysaccharides before biomass liquefaction increased the bio-oil yield at lower temperature and reduced bio-char formation. From thermal efficiency point of view C. sorokiniana seems to be a better candidate for bio-fuel production than its counterpart. SEQHTL is producing bio-oil and polysaccharides at the similar energy cost as required by DHTL to produce bio-oil only. These results are encouraging related to co-product development. Developing co-product along with biofuel

Acknowledgements

This work is supported partially by the Department of Energy (DOE) and by Washington State University Agricultural Research Center. The authors would like to thank Dr. James V. O’Fallon for fatty acid analysis. And the authors also like to thank Yubin Zheng and Tingting Li for their help in developing algal culture.

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    Both the authors contributed equally.

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