Elsevier

Bioresource Technology

Volume 102, Issue 17, September 2011, Pages 8295-8303
Bioresource Technology

Chemical properties of biocrude oil from the hydrothermal liquefaction of Spirulina algae, swine manure, and digested anaerobic sludge

https://doi.org/10.1016/j.biortech.2011.06.041Get rights and content

Abstract

This study explores the influence of wastewater feedstock composition on hydrothermal liquefaction (HTL) biocrude oil properties and physico-chemical characteristics. Spirulina algae, swine manure, and digested sludge were converted under HTL conditions (300 °C, 10–12 MPa, and 30 min reaction time). Biocrude yields ranged from 9.4% (digested sludge) to 32.6% (Spirulina). Although similar higher heating values (32.0–34.7 MJ/kg) were estimated for all product oils, more detailed characterization revealed significant differences in biocrude chemistry. Feedstock composition influenced the individual compounds identified as well as the biocrude functional group chemistry. Molecular weights tracked with obdurate carbohydrate content and followed the order of Spirulina < swine manure < digested sludge. A similar trend was observed in boiling point distributions and the long branched aliphatic contents. These findings show the importance of HTL feedstock composition and highlight the need for better understanding of biocrude chemistries when considering bio-oil uses and upgrading requirements.

Highlights

Feedstock composition reflected in biocrude chemistry despite similar heating values. ► HTL biocrude N, O heteroatom content exceeds petroleum and requires upgrading. ► Feedstock selection and profiling critical for intended downstream application. ► Complementary analyses provide value-added data for biocrude oil characterization. ► Molecular-level profiling can aid in targeted treatments based on biocrude oil chemistry.

Introduction

Hydrothermal liquefaction (HTL) is a promising technology for converting wastewater biomass into a liquid fuel (Cantrell et al., 2007). HTL has been applied to a wide range of wastewater feedstocks, including swine manure, cattle manure, microalgae, macroalgae, and sludge (Suzuki et al., 1988, Dote et al., 1994, He et al., 2000, Brown et al., 2010, Xiu et al., 2010a, Yin et al., 2010, Biller and Ross, 2011). During HTL, water serves as the reaction medium, alleviating the need to dewater biomass which can be a major energy input for biofuel production. Elevated temperature (200–350 °C) and pressure (5–15 MPa) are used to breakdown and reform biomass macromolecules into biofuel (Peterson et al., 2008), subsequently referred to as biocrude oil. Self-separation of the biocrude oil from water is then facilitated as the reaction solution returns to standard conditions (Peterson et al., 2008). The recovered biocrude oil can be directly combusted or upgraded to approach petroleum oils (Dote et al., 1991, Elliott, 2007, Duan and Savage, 2011a). While the ability of HTL to convert a wide-range of wastewater feedstocks provides a significant waste-disposal benefit, it also presents a major challenge for optimization and downstream processes due to the diverse biocrude oil chemistry that can result.

HTL biocrude oils contain a diverse range of chemical compounds which can include straight and branched aliphatic compounds, aromatics and phenolic derivatives, carboxylic acids, esters, and nitrogenous ring structures (Brown et al., 2010, Xiu et al., 2010a, Yin et al., 2010, Zhou et al., 2010, Biller and Ross, 2011). The class of compounds identified in HTL biocrude oils has been shown to be influenced by the ratio of protein, lipid, and carbohydrate fractions in the initial biomass feedstock (Biller and Ross, 2011). Biocrude oils are often characterized by high heteroatom contents, primarily in the form of oxygenated and nitrogenous compounds. The high heteroatom content is the main distinguishing factor separating bio-oils from petroleum crude oils (Huber et al., 2006, Peterson et al., 2008, Demirbas, 2009) and results in undesirable biofuel qualities such as oil acidity, polymerization, high viscosity, and high-boiling distribution (Adjaye et al., 1992, Speight, 2001). Furthermore, the diverse chemical composition of biocrude oil affects the combustion performance, storage stability, upgrading response, and economic value (Huber et al., 2006). Therefore, the objective of this study was to examine and compare the influence of wastewater feedstock composition for three feedstocks (Spirulina algae, swine manure, and anaerobic digested sludge) on the bulk properties and physico-chemical characteristics of HTL biocrude oil.

The algal species Spirulina was selected for HTL conversion due to increased interest in integrating algal cultivation into wastewater treatment from both a water resource and renewable energy perspective (Pittman et al., 2010). Spirulina can thrive in municipal and agricultural wastewater effluents and the filamentous cell structure facilitates harvesting (Kosaric et al., 1974). The high protein content can also be converted into HTL biocrude oil (Biller and Ross, 2011) and the ability to capture CO2 can be used to reduce a treatment facility’s environmental footprint (Packer, 2009). Additionally, HTL of low-lipid algae such as Spirulina can provide a comparison with contributions focused on HTL of high-lipid species (Dote et al., 1994, Brown et al., 2010), which tend to exhibit relatively low total lipid content when cultured in wastewaters (Pittman et al., 2010). Swine manure was chosen as a representative solid waste generated by agricultural livestock facilities. In the United States alone, livestock manure accounts for ∼250 million tons of dry solids annually (Xiu et al., 2010a) and land application of these wastes has been linked to the spread of hormones, pathogens, and nutrient runoff (Cantrell et al., 2007). Swine manure also contains a moderate lipid and high carbohydrate content to provide a distinct biochemical comparison to Spirulina algae. Anaerobically digested sludge was selected as the final feedstock to compare the effect of high obdurate carbohydrate content on biocrude oil yields and chemistry and evaluate HTL as an alternative to current predominant disposal practices: land application and landfilling. Currently, over 7 million tons of dry anaerobically digested sludge is produced annually with only 60% going towards beneficial use (US Environmental Protection Agency, 1999).

Advanced characterization methods were used to assess the molecular properties of biocrude oil and the influence of feedstock composition. Molecular-level characterization has been critical for designing engineering approaches to catalytically upgrade low-quality “heavy” petroleum oils (Speight, 2001) and will be necessary for transitioning to a biomass-based fuel infrastructure (Huber et al., 2006). Previous examples of molecular characterization used to better understand HTL chemistry include, but are not limited to, observing the conversion of carbohydrates into biocrude oil (Duan and Savage, 2011b), verifying the presence of aromatic hydrocarbons tied to increased bio-oil viscosity and density (Yin et al., 2010), determining the effects of reaction parameters (Xiu et al., 2010a, Yin et al., 2010) and catalyst addition (Duan and Savage, 2011b), and monitoring the deoxygenation of fatty acids through catalytic upgrading (Duan and Savage, 2011a). However, further understanding of the chemistry involved during HTL biomass conversion and upgrading process is needed to develop more efficient and sustainable biofuel and biochemical production methods (Huber et al., 2006).

In this study, both bulk properties (e.g., oil yield, elemental analysis, and heating value) and physico-chemical characteristics (e.g., molecular constituents, functional group allocation, proton and carbon speciation, molecular weight distribution, and boiling point distribution) of biocrude oils produced by liquefaction of different wastewater feedstocks were compared with each other as well as published results for petroleum crudes and tar sand bitumens. Low-boiling compounds were identified by gas chromatography–mass spectroscopy (GC–MS), functional group compositions were examined with Fourier Transform infrared (FTIR) and nuclear magnetic resonance (1H and 13C NMR) spectroscopies, and the molecular weight and boiling point distributions were analyzed with size exclusion chromatography (SEC) and simulated distillation (Sim-Dist), respectively. To our knowledge, this is the first study to compare the chemical characteristics of biocrudes generated from disparate wastewater feedstocks under identical HTL conditions.

Section snippets

Feedstock sources and characterization

Spirulina algae (solids content of 95%) were obtained in dry-powder form from Cyanotech (Kailua-Kona, Hawaii). Swine manure (solids content of 27%) was sampled from grower-finisher pen floors at the Swine Research Center at the University of Illinois at Urbana-Champaign. The pens contained a partially slotted floor for manure collection. Both the swine manure and algal samples were stored at 4 °C prior to processing. Digested anaerobic sludge (solids content of 26%) was collected from the outlet

Feedstock composition

Forage analysis of the feedstocks indicated that Spirulina had the highest overall organic matter content on a dry weight basis (90%) compared to swine manure (84%) and digested sludge (69%) as shown in Table 1. The organic matter in Spirulina was comprised primarily of crude protein (64%) with relatively smaller amounts of crude lipids (5%) and fibrous carbohydrates (2%) accounted for in the neutral detergent fiber (NDF) fraction (e.g., lignin, cellulose, and hemicellulose). Spirulina had a

Conclusion

Detailed multi-method characterization demonstrates that feedstock organic content and nutritional composition greatly affect HTL biocrude oil yields and chemistry, despite having similar bulk elemental distributions. The feedstock nutritional profile was reflected in the heteroatom content, type of compounds, and functionality observed in the resulting HTL biocrude oils. The tie between feedstock nutritional profile and HTL biocrude molecular makeup emphasizes the need for feedstock

Acknowledgements

The research described in this paper has been funded in part by the United States Environmental Protection Agency (EPA) under the Science to Achieve Results (STAR) Graduate Fellowship Program. The EPA has not officially endorsed this publication and the views expressed herein may not reflect the views of the EPA. Financial support was also provided by the Department of Civil and Environmental Engineering at the University of Illinois and the National Science Foundation Division of Chemical,

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