Energy Fuels 2010, 24, 6601–6608 Published on Web 11/04/2010
: DOI:10.1021/ef101173r
Viscosity of Biomass Pyrolysis Oils from Various Feedstocks Michael W. Nolte and Matthew W. Liberatore* Department of Chemical Engineering, Colorado School of Mines (CSM), Golden, Colorado 80401, United States Received August 31, 2010. Revised Manuscript Received October 11, 2010
Bio-oil is a renewable energy source that is produced from the pyrolysis of lignocellulosic biomass. The pyrolysis oils are emulsion-like fluids, containing aqueous and phenolic phases, and can be more than 400 times more viscous than water at 25 °C. A series of rheological tests were performed on a set of bio-oils from different feedstocks and pyrolysis conditions. In general, the viscosity of the oils was independent of the shear rate (i.e., Newtonian). However, some of the hardwood samples shear thin at lower temperatures (-5 °C) and high shears (>100 s-1). Oscillatory frequency sweeps were also performed. All of the oil samples were found to be viscous liquids, and the loss modulus (G00 ) was orders of magnitude greater than the storage modulus (G0 ). A strong dependence of viscosity upon the temperature showed that the viscosity of poplar and oak 500 °C oils increased over 220-fold between 55 and -5 °C. Water content and acidity were also measured and compared to viscosity. The water content was found to have a stronger effect on viscosity than acidity. Generally, the oils that had higher water contents had lower viscosities. Viscosity does not correlate with the acid number or pH. While the acid number and pH are independent measurements of the acidity of the bio-oils, no correlation between the acid number and pH was observed. The microstructure of the oils was investigated using optical microscopy and small-angle neutron scattering. Optical microscopy did not show discrete boundaries between the aqueous and organic phases. The neutron-scattering profiles showed that a fractal structure is present in two of the three oils studied.
and softwoods,5-7 forestry residues,8,9 rice husk,10 as well as many others. A study of eight biomass samples (switchgrass, hardwood, bagasse, corn stover, Avicel microcrystalline cellulose, 0.1% K2CO3-doped Avicel (K-Avicel), white oak, and wheat straw) found that the different feedstocks yielded between 46 and 67 wt % of the original biomass as oil, while Avicel produced 81%.11 Other typical oil yields have been reported to be between 60 and 95%.2 Unfortunately, bio-oils have drawbacks that prevent wide use as a fuel. Namely, bio-oil has a high water content (WC, ∼15-30%) and high oxygen content (∼44-60%),12 which reduce the heating value of the oil. Bio-oil is also acidic
1. Introduction Bio-oil, produced from the pyrolysis of lignocellulosic biomass, is a renewable energy source that is gaining more attention as a replacement for petroleum-based fuel oils. Bio-oils, also known as biomass pyrolysis oils and other similar names, are produced from the thermochemical conversion of the three main cellulosic biomass components: cellulose, hemicellulose, and lignin. The biomass is heated at a high rate (>1000 °C/s) in the absence of oxygen to a final temperature of 400-600 °C to maximize liquid yields.1,2 The residence time of the pyrolysis reactor is kept to less than 1 s, and the outlet vapor is filtered to remove char and ash particles and then rapidly condensed to give bio-oil.1 The resulting product is a dark brown liquid with a distinct smoky aroma containing over 300 different organic compounds, which can be grouped into 8 macro-chemical families.2,3 Elemental analysis shows that the oils are mainly carbon (44-47 wt %), oxygen (46-48 wt %), and hydrogen (6-7 wt %), with nitrogen (0-0.2 wt %), sulfur (100 s-1). The major factors affecting viscosity were temperature and WC. As the viscosity of an oil increases, the temperature dependence of the viscosity becomes stronger. The most viscous oils, poplar and oak 500 °C, were 250 and 220 times higher in viscosity at -5 °C than at 55 °C, respectively, while one of the least viscous oils, oak 600 °C, was 7 times thicker in the same temperature range. In contrast, between 5 and 55 °C, water was 3 times thicker and gasoline was only twice as thick. The temperature dependence is an important property to consider with regard to pumping, if the oil is exposed to the local climate and temperature changes throughout the year. Larger WCs were found to lower the viscosities of the oils. That is, the water dilutes the other more viscous components. A power law was able to fit the viscosity-WC relationship for the one- and two-phase oils, as well as other oil samples from the literature. The coefficients for the power law at 25 °C were found to be η=9300WC-3.8.
Acknowledgment. The authors thank all who have provided oil samples, including Dr. John Ortega, Greg Schlichting, and Dr. Andrew Herring at the Colorado School of Mines (CSM) and Dr. Bob Baldwin and Dr. Stefan Czernik at the National Renewable Energy Laboratory (NREL). We thank Dr. Keith Neeves for assistance with the optical microscope. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. Supporting Information Available: Viscosity flow curves for the other oil samples, WC dependence at -5 and 55 °C, and tabulated viscosity values at 100 s-1 and -5, 10, 40, and 55 °C. This material is available free of charge via the Internet at http://pubs.acs.org.
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