ARTICLE pubs.acs.org/EF
Real-Time Viscosity Measurements during the Accelerated Aging of Biomass Pyrolysis Oil Michael W. Nolte and Matthew W. Liberatore* Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
bS Supporting Information ABSTRACT: An oak bio-oil was aged at 90 °C using various times and methods. A novel method for aging bio-oils under shear is introduced and compared to standard (quiescent) aging experiments. In a hermetically sealed concentric cylinder rheometer, aging with shear for 8, 16, and 24 h showed increases in viscosity of 57, 300, and 720%, respectively. A similar increase in viscosity was observed after quiescently aging of sealed samples in a forced air oven (100, 120, and 740% after 8, 16, and 24 h, respectively). Another aging experiment under shear consisted of three 8 h aging steps with intermediate viscosity measurements. Viscosity increases were comparable to the 8, 16, and 24 h tests. A control experiment in the rheometer without shear found the increase in viscosity to be 3050% less than the sheared experiments. The number-average molecular weight increased as samples were heattreated at 90 °C for longer times. The water content showed small increases and decreases with aging, which was attributed to the heterogeneity of the sample. Real-time viscosity measurements during the 90 °C aging step found that the rate of viscosity growth decreased over time. An exponential decay function estimated the viscosity to be 90% of the steady-state viscosity after ∼3 days at 90 °C.
’ INTRODUCTION In 2003, 190 million dry tons of biomass contributed about 2.9 quadrillion British thermal units (BTUs) (quads) to the energy supply (∼3% of U.S. energy consumption). The role of biomass in energy production can increase greatly considering that half of the nearly 2300 million acres of U.S. land has the potential to grow biomass.1 Pyrolysis increases the energy density of the biomass by forming it into a liquid intermediate or fuel. A 2010 techno-economic study reported pyrolysis to have the lowest capital and operating costs in comparison to gasification and biochemical conversion of biomass.2 The pyrolysis process involves heating the biomass at a high rate (>1000 °C s1) in the absence of oxygen to final temperatures of 400600 °C.3,4 In the reactor, the biomass (composed of cellulose, hemicellulose, and lignin) degrades, depolymerizes, and volatilizes. The pyrolysis vapors are filtered to remove entrained char and ash and then rapidly condensed to yield bio-oil.3 Bio-oil is a viscous dark, brown liquid with a smoky aroma and contains over 300 different organic compounds.4,5 Elemental analysis shows that bio-oils are mainly composed of carbon (44 47 wt %), oxygen (4648 wt %), and hydrogen (67 wt %), as well as nitrogen (00.2 wt %), sulfur (
