A Systems Approach to Bio-Oil Stabilization - Final Technical Report
The objective of this project is to develop practical, cost effective methods for stabilizing biomass-derived fast pyrolysis oil for at least six months of storage under ambient conditions. The U.S. Department of Energy has targeted three strategies for stabilizing bio-oils: (1) reducing the oxygen content of the organic compounds comprising pyrolysis oil; (2) removal of carboxylic acid groups such that the total acid number (TAN) of the pyrolysis oil is dramatically reduced; and (3) reducing the charcoal content, which contains alkali metals known to catalyze reactions that increase the viscosity of bio-oil. Alkali and alkaline earth metals (AAEM), are known to catalyze decomposition reactions of biomass carbohydrates to produce light oxygenates that destabilize the resulting bio-oil. Methods envisioned to prevent the AAEM from reaction with the biomass carbohydrates include washing the AAEM out of the biomass with water or dilute acid or infusing an acid catalyst to passivate the AAEM. Infusion of acids into the feedstock to convert all of the AAEM to salts which are stable at pyrolysis temperatures proved to be a much more economically feasible process. Our results from pyrolyzing acid infused biomass showed increases in the yield of anhydrosugars by greater than 300% while greatly reducing the yield of light oxygenates that are known to destabilize bio-oil. Particulate matter can interfere with combustion or catalytic processing of either syngas or bio-oil. It also is thought to catalyze the polymerization of bio-oil, which increases the viscosity of bio-oil over time. High temperature bag houses, ceramic candle filters, and moving bed granular filters have been variously suggested for syngas cleaning at elevated temperatures. High temperature filtration of bio-oil vapors has also been suggested by the National Renewable Energy Laboratory although there remain technical challenges to this approach. The fast pyrolysis of biomass yields three main organic products: condensable vapors, non-condensable gases, and liquid aerosols. Traditionally these are recovered by a spray quencher or a conventional shell and tube condenser. The spray quencher or condenser is typically followed by an electrostatic precipitator to yield 1 or 2 distinct fractions of bio-oil. The pyrolyzer system developed at Iowa State University incorporates a proprietary fractionating condenser train. The system collects the bio-oil into five unique fractions. For conditions typical of fluidized bed pyrolyzers, stage fractions have been collected that are carbohydrate-rich (anhydrosugars), lignin-rich, and an aqueous solution of carboxylic acids and aldehydes. One important feature is that most of the water normally found in bio-oil appears in the last stage fraction along with several water-soluble components that are thought to be responsible for bio-oil aging (low molecular weight carboxylic acids and aldehydes). Research work on laser diagnostics for hot-vapor filtration and bio-oil recovery centered on development of analytical techniques for in situ measurements during fast pyrolysis, hot-vapor filtration, and fractionation relative to bio-oil stabilization. The methods developed in this work include laser-induced breakdown spectroscopy (LIBS), laser-induced incandescence (LII), and laser scattering for elemental analysis (N, O, H, C), detection of particulates, and detection of aerosols, respectively. These techniques were utilized in simulated pyrolysis environments and applied to a small-scale pyrolysis unit. Stability of Bio-oils is adversely affected by the presence of particulates that are formed as a consequence of thermal pyrolysis, improving the CFD simulations of moving bed granular filter (MBGF) is useful for improving the design of MBGF for bio-oil production. The current work uses fully resolved direct numerical simulation (where the flow past each granule is accurately represented) to calculate the filter efficiency that is used in the CFD model at all flow speeds. This study shows that fully-resolved direct numerical simulation (DNS) is successful in calculating the filter efficiency at all speeds. Aldehydes and acids are thought to play key roles in the stability of bio-oils, so the catalytic stabilization of bio-oils was focused on whether a reaction approach could be employed that simultaneously addressed these two types of molecules in bio-oil. Our approach to post treatment was simultaneous hydrogenation and esterification using bifunctional metal/acidic heterogeneous catalyst in which reactive aldehydes were reduced to alcohols, creating a high enough alcohol concentration so that the carboxylic acids could be esterified.
- Research Organization:
- Iowa State Univ., Ames, IA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- DOE Contract Number:
- FG36-08GO18205
- OSTI ID:
- 1055935
- Report Number(s):
- DOE/GO18205
- Country of Publication:
- United States
- Language:
- English
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