Crucial correction of the Faix method for the determination of carbonyls in pyrolysis and hydrothermal liquefaction bio-oils with high nitrogen content

Accurate and reliable determination of carbonyl content in bio-oils from pyrolysis and hydrothermal liquefaction (HTL) of biomass is crucial not only for estimating the storage stability of the sample but also for creating an adequate design of the hydrotreatment process allowing long-term operation. The composition of bio-oils from various feedstocks can, however, significantly differ from woody bio-oils, which were utilized to develop the Faix method, the most frequently used for carbonyl determination. In this study, we present a limitation of this method when used for the analysis of bio-oils prepared from protein-rich biomass. These bio-oils are known to have a high nitrogen content, which can be present in the form of possibly interfering compounds. In this study, nitrogen interferences were identified, and a correction step mitigating the underestimation of carbonyl content was introduced. This novel approach was tested for 13 crude bio-oils prepared by pyrolysis or HTL from 10 different feedstocks with a wide range of nitrogen content. Besides crude bio-oils, five hydrotreated HTL oils from sewage sludge were also analyzed. For bio-oils or hydrotreated products with increased nitrogen content but containing less than 0.5 mmol/g of carbonyls, without the correction step, the method can provide zero and in extreme cases even negative values. Statistical analysis of results showed that the introduction of the correction step can be crucial for bio-oils containing more than 2 wt% of nitrogen and especially hydrotreated bio-oils from protein-rich biomass.