(498g) Controlling Phase Stability of Biomass Fast Pyrolysis Bio-Oils

Authors: 
Oasmaa, A., VTT Technical Research Centre of Finland
Garcia-Perez, M., Washington State University

AIChE Annual Meeting 2015, T4A08 Thermochemical Conversion of Biomass I

Controlling Phase Stability of Biomass Fast Pyrolysis Bio-oils

Anja Oasmaa, Tom Sundqvist, Eeva Kuoppala, Manuel Garcia-Perez, Yrjö Solantausta, Christian Lindfors

Fast pyrolysis bio-oil is entering into the heating oil market to replace fossil fuel oils. First commercial size pyrolysis plant is under commissioning in Joensuu, Finland, by Fortum and the next one is under commissioning by Empyro in the Netherlands. Large-scale combustion of bio-oil has been demonstrated and standards and norms are being prepared. Information on critical fuel oil properties related to bio-oil combustion is available. However, there is insufficient data on the phase stability of bio-oils and especially how to predict and prevent phase-separation. It is important to control this property because phase-separated bio-oil will cause e.g. pumping problems, blockages, and irregular combustion.

Fast pyrolysis bio-oil is a chemically diverse mixture consisting of thermal degradation products of lignocellulose. The complexity of chemical compounds in bio-oil varies from simple oxygenated C1 compounds to high molecular weight oligomers. Bio-oil contains oxygenated functionalities, such as hydroxyl, carbonyl and carboxyl in both aliphatic and aromatic structures. The homogeneity of bio-oil depends on the ability of the dissimilar molecules to dissolve each other. The water-soluble and water-insoluble compounds in bio-oil can either be in one homogeneous phase or form two individual phases, to which we refer to as phase separation. Phase separation can occur immediately after condensation of the pyrolysis vapours to bio-oil due to certain pyrolysis conditions or type of raw material, or after years of ageing because of changes in composition caused by re-polymerization reactions.

In this study we present how the phase separation of fast pyrolysis bio-oils is related to the chemical composition, and show that the probability of phase separation can be predicted with a numerical stability index based on the chemical composition. The chemical composition of the bio-oils studied was characterized using a solvent extraction scheme that describes the composition of bio-oil as a blend of three macro fractions: C1-C6 oxygenated molecules (named co-solvents), water-insoluble molecules and water-soluble polar molecules (including water but excluding the co-solvents), e.g. anhydrosugars. The results show that the required amount of the co-solvent to dissolve both fractions and keep the oil homogeneous varies depending on the chemical composition. The minimum amount of co-solvent for homogeneous bio-oils was observed to be from 15 to 30 wt% depending on the composition of bio-oil. The co-solvent content in fresh bio-oils was typically from 20 to 30 wt% and decreased to as low as 5 wt% after prolonged ageing. Correlation of chemical composition and homogeneity of fresh and aged bio-oils are shown in ternary phase diagrams. Addition experiments were made with model compounds to cover a larger part of the phase diagram. The model compounds were water, a model solvent mixture and an isolated water-insoluble fraction. The numerical stability index was generated based on the ternary phase diagram.