Abstract
Fast pyrolysis bio-oils are known to age upon storage at room temperature, resulting in changes to both physical properties (increase in viscosity) and chemical composition (decrease in carbonyl content). A widely used accelerated aging test consists of holding samples at 80 °C for 24 hours, with viscosity measurement before and after heat treatment. Unfortunately, the viscosity measurement has high variability, and cannot be applied to samples that have phase separated. Here, we show that carbonyl content is a much better metric for tracking bio-oil aging. Furthermore, results from different accelerated aging protocols (for varying times at both 40 °C and 80 °C) are compared to actual room temperature storage for over 3 years. Based on this, we show that the accepted accelerated aging test (80 °C for 24 hours) is too severe a treatment, and results in more extensive aging than would occur with over 3 years of storage at room temperature. A new aging protocol is proposed: heat treatment at 80 °C for 2 hours, with carbonyl quantification before and after. This protocol correlates to room temperature storage for 1–3 months. Finally, samples were also kept in cold storage (at both 9 °C and −17 °C) for over 3 years. Unexpectedly, these samples also showed a substantial reduction in carbonyl content (by up to 25%), indicating that bio-oil aging still progresses at low temperatures. Both physical and chemical changes will occur in samples in cold storage, which has implications for the archiving of bio-oil samples.
Highlights
In recent years, the necessity of renewable energy technologies has grown, as the inevitable economic and environmental impacts of climate change have become widely accepted
Viscosity measurement was not possible for samples that had phase separated, which is a common occurrence upon bio-oil aging
We propose that carbonyl titration be used in place of the viscosity measurement as a metric to track bio-oil aging, and only carbonyl titration results will be presented for the remainder of this manuscript
Summary
The necessity of renewable energy technologies has grown, as the inevitable economic and environmental impacts of climate change have become widely accepted. The viscosity measurement is prone to high variability.[9] Some FP bio-oils are more reactive than others and can phase separate during the aging process. All measurements were performed in triplicate, and error bars (taken as the standard deviation) are included in Fig. 1–4; these errors are very low, typically below 5% RSD, which has been shown previously in the interlaboratory validation of this carbonyl titration method.[6] Of all the samples analyzed, the Oak oil was the only sample to remain single phase throughout aging at 80 C.
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