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Highlights in Research & Development

Reliable Characterization for Pyrolysis Bio-Oils Leads to Enhanced Upgrading Methods

NREL's efforts to standardize techniques for bio-oil analysis inform upgrading processes to economically produce renewable fuels and chemicals from biomass.

NREL engineer Jessica Olstad examines pyrolysis oil samples at the thermochemical lab at the FTLB at NREL's campus in Golden, Colorado.

Reliable chemical characterization of bio-oils will enable upgrading research and allow for detailed comparisons of bio-oils produced at different facilities. Photo by Dennis Schroeder, NREL 20404

Concerns over finite petroleum reserves and associated environmental impacts have motivated the search for economically viable routes to produce biofuels and renewable chemicals from biomass. Pyrolysis is a promising thermochemical route, but it produces a liquid bio-oil product that is extremely complex and must be processed further, or upgraded, to produce either a finished product or an intermediate stream that can be sent to a refinery. To improve these upgrading processes, reliable analyses of bio-oil are needed, specifically chemical characterization techniques that can quantify both functional groups and individual compounds present in the bio-oil.

Recently NREL, in collaboration with Pacific Northwest National Laboratory and Oak Ridge National Laboratory, led an international Inter-Laboratory Study (ILS) on the validation of chemical characterization techniques for pyrolysis bio-oils to quantify the variability of the methods. Several methods were tested and the ILS results are some of the first examples of successful validation of chemical characterization techniques for bio-oils.

The two most promising results occurred with carbonyl and carboxylic acid titrations. For the first time, these titration methods yielded acceptable inter-laboratory variabilities, on the order of 5% relative standard deviation, proving the methods are robust. In the case of the carbonyl titration technique, NREL researchers developed a new method for quantifying carbonyls at an elevated temperature (80°C versus room temperature), which has proven to be more accurate and much faster (2 hours versus 24+ hours), allowing for significantly higher sample throughput.

To improve upgrading processes, it is critical to be able to accurately and reliably quantify carbonyl groups and carboxylic acids in bio-oil. Carbonyl groups are responsible for the aging of raw pyrolysis bio-oils, which leads to an increase of viscosity during storage, and carbonyls have also been shown to cause reactor plugging during high-temperature hydrotreating. In addition, pyrolysis bio-oils are acidic, causing corrosion issues, and the acid content must be lowered during upgrading. These new methods are industrially relevant, as they can be run in a production facility and allow for quick, same-day turnaround.