Effect of operating conditions of thermochemical liquefaction on biocrude production from Spirulina platensis

https://doi.org/10.1016/j.biortech.2011.02.057Get rights and content

Abstract

This study investigated the optimum thermochemical liquefaction (TCL) operating conditions for producing biocrude from Spirulina platensis. TCL experiments were performed at various temperatures (200–380 °C), holding times (0–120 min), and solids concentrations (10–50%). TCL conversion at 350 °C, 60 min holding time and 20% solids concentration produced the highest biocrude yield of 39.9% representing 98.3% carbon conversion efficiency. Light fraction biocrude (B1) appeared at 300 °C or higher temperatures and represented 50–63% of the total biocrude. Biocrude obtained at 350–380 °C had similar fuel properties to that of petroleum crude with energy density of 34.7–39.9 MJ kg−1 compared to 42.9 MJ kg−1 for petroleum crude. Biocrude from conversion at 300 °C or above had 71–77% elemental carbon, and 0.6–11.6% elemental oxygen and viscosities in the range 40–68 cP. GC/MS of biocrude reported higher hydrocarbons (C16–C17), phenolics, carboxylic acids, esters, aldehydes, amines, and amides.

Research highlights

► Optimum liquefaction conditions were 350 °C temperature, 60 min time, 20% solids. ► Biocrude yield at optimum thermochemical liquefaction (TCL) was 39.9%. ► Carbon conversion efficiency for most TCL runs were >93%. ► Biocrudes obtained at 350–380 °C had fuel properties close to the petroleum crude.

Introduction

Microalgae are attractive feedstocks for biofuel production because of their high productivities (40–80 tons ha−1 year−1), high lipid content (30–60%) (Wijffels et al., 2010), their ability to grow in contaminated waters, their ability to sequester atmospheric CO2, and because they can be cultivated on marginal lands and variable climatic conditions. Because microalgal biofuel development does not compete with food production systems, this feedstock has further appeal. In order to reduce overall energy and cost input, it is beneficial to process algae without complete drying, which can be accomplished by the thermochemical liquefaction (TCL) process. The process converts organic constituents of algae into a liquid biocrude that can be refined to gasoline like fuels. In addition, a major part of the N and P from the original biomass is recovered in the aqueous phase co-product and can be used in downstream algae cultivation systems (Jena et al., 2011).

Hot compressed water is a highly reactive medium because of changes in properties such as solubility, density, dielectric constant and reactivity as water approaches its critical point (374 °C, 22.1 MPa). These enhance depolymerization and repolymerization of lignins, celluloses, lipids, proteins and carbohydrates, transforming them into biocrude (also referred to as biooil in the literature), gas and char. Multiple reactions occur in three steps, namely, hydrolysis, depolymerization and repolymerization/self-condensation reactions (Yin et al., 2010). In lignocellulosic biomass, the lignin and cellulose components are hydrolyzed into unit structures of sugar monomers (Yin et al., 2010), whereas protein molecules are hydrolyzed into aminoacids followed by deamination and decarboxylation reactions to complex hydrocarbons (Sato et al., 2004). The biocrude is a dark viscous liquid with an energy value 70–95% of that of petroleum fuel oil (Brown et al., 2010, Dote et al., 1994, He et al., 2000, Minowa et al., 1998). Thermochemical liquefaction has been investigated for producing liquid fuels from several types of biomass including lignocellulosic feedstocks (Minowa et al., 1998), swine manure (He et al., 2000), macroalgae (Zhou et al., 2010) and microalgae (Brown et al., 2010, Dote et al., 1994, Ross et al., 2010; Zou et al., 2009; Yang et al., 2004). TCL process is known to be effective for biomass feedstocks, including algae, which have lower percentage of net lipids. The oil is produced not only from the conversion of triglycerides but also from all other components such as proteins, fibers and carbohydrates that constitute the whole biomass (Duan and Savage, 2011). TCL also provides a larger quantity of oil product relative to other methods. Conversion of microalgae biomass was reported to be a function of operating variables such as temperature, holding time, and presence of catalysts and co-solvents (Duan and Savage, 2011, Huang et al., 2011, Zou et al., 2009). Highest TCL yield of 97% was reported for the liquefaction of Dunaliella tertiolecta at optimized operating conditions (Zou et al., 2009).

Although some information is available on liquefaction of algae, further studies are needed to fill the following gaps and inconsistencies in the literature: (1) wide variation in biocrude yield and composition reported in the literature. For example, maximum biocrude yield was reported as 9.0% for Spirulina and 13.6% for Chlorella vulgaris (Ross et al., 2010), 42.0% for D. tertiolecta and 64.0% for Botryococcus braunii (Dote et al., 1994). Another study (Huang et al., 2011) reported biocrude yield from liquefaction of Spirulina to be 35–45%; (2) studies have not attempted to vary key operating parameters with a view to identify an optimum. Particularly, all studies have been conducted at 10% or lower solids content, which we believe to be too low for economic scale-up. With developments of attached growth algal cultivation and advanced harvesting technologies, cell concentration of 20% or more can be achieved easily (Grima et al., 2003) and conversion processes need to be evaluated under these conditions; (3) most of these studies reported in literature (Biller and Ross, 2011, Brown et al., 2010, Ross et al., 2010, Zou et al., 2009) have used small reactor volume of less than 100 mL with dry biomass of <10 g. We believe that larger process volumes are required to obtain realistic estimates of yield and qualities of biocrude as further scale-up is evaluated and (4) there is no information on long term storage properties of biocrude obtained from algal biomass in any of the above studies. To fill these gaps, we studied TCL of the microalga, Spirulina platensis, in 500–750 mL reactant volume (100–250 g dry biomass solids) at varying temperatures, holding times, and solids concentrations. We report biocrude yield, its properties, and characteristics of other co-products to better describe the effect of operating conditions.

Section snippets

Raw materials

S. platensis biomass was provided by Earthrise Nutritionals LLC (Calipatria, CA) in dry powder form with defined properties (Table 1), and was stored in airtight packages at room temperature prior to use. Laboratory grade acetone (99.5% purity) was purchased from Sigma Aldrich. Nitrogen and helium gases were obtained from the Universal Gas and Electric Corp. (USA).

Thermochemical liquefaction (TCL) and separation of products

TCL experiments were performed in a 1.8-L batch reactor system (Parr Instruments Co. Moline, PA) at different temperatures (200,

Feedstock composition

Spirulina had higher organic matter (78.15% volatiles and 15.25% fixed carbon) that comprised 93.40% of the total biomass weight and had 6.60% ash content. The major inorganic composition of the ash consisted of 1.98% K, 0.12% Ca, 1.18% P, 0.07% Na, 0.05% Mg, and 0.05% Fe. The ash content of Spirulina was lower compared to many other lignocellulosic biomass materials such as rice husk, rice straw, and oil-palm shell (Minowa et al., 1998) and other microalgae biomass such as Nanochloropsis

Conclusions

This study has shown that up to 39.9% yield of biocrude could be produced from the TCL of microalgae S. plantensis. Biocrude obtained from TCL runs at 350–380 °C had fuel properties similar to that of petroleum crude and could be further refined to a liquid transportation fuel. Our results have shown that TCL process parameters of 350 °C temperature, 60 min holding time, and 20% solids was optimum for liquefaction of S. platensis. The biocrude obtained under different conditions had 50–63% light

Acknowledgements

This research was conducted under the financial support of the United States Department of Energy and the State of Georgia. The authors are grateful to Earthrise Nutritionals LLC for the supply of Spirulina feedstock. The authors thank Roger Hilten, Joby Miller and Sarah Lee for their assistance in the laboratory analyses.

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