Torrefaction of wheat-barley straw: Composition and toxicity of torrefaction condensates

Until now, few studies on the valorisation of torrefaction condensable volatiles (condensates) have been performed. The composition and toxicity of torrefaction condensable volatiles determine their possible applications. Therefore, a study on the composition of wheat-barley straw torrefaction condensates, combined with toxicity tests, was performed. This analysis was mainly aimed at the utilisation of these condensates as an anaerobic digestion substrate. The torrefaction process was performed in a batch-scale reactor at temperatures from 240 °C to 320 °C. The condensates were analysed using GC/MS, and quantitative analysis was based on the calibration curves (external standard method). Toxicity tests were performed for vascular plants (Lemna sp. Growth Inhibition Test), saltwater bacteria (Microtox® bioassay) and freshwater crustaceans (Daphtoxkit F magna® survival bioassay). The condensates were classified as highly toxic for the tested organisms. Compounds that are inhibitory and/or toxic to anaerobic bacteria were detected in the samples, e.g. furfural, guaiacol, palmitic acid and oleic acid. Taking into account the condensates’ high toxicity for the tested organisms and the presence of anaerobic digestion process inhibitors, there is a significant likelihood that prior detoxification of the condensates is necessary before they can be utilised as anaerobic digestion substrates.