Resumen de Obtenció de productes de valor afegit a partir de residus agrícoles mitjançant tractaments de piròlisi i torrefacció
Within the transition towards a circular bioeconomy, this study is focused on obtaining value-added products from agricultural wastes through pyrolysis and torrefaction treatments in a waste biorefinery context. Thus, in this work, the wastes are considered as by-products.
Specifically, it has been worked with olive mill waste (OMW), grape pomace (GP) and coffee silverskin (CSS), which are respectively the main solid wastes from olive oil, wine making and coffee roasting processes. The thermochemical treatments have been performed in an auger pilot plant (15 kg/h), with the collaboration of ENERG-bas company, that provided the products, and in a lab size auger reactor (0.3 kg/h), from EBRI institution (Aston University, U.K.), during a stay abroad. The resulting liquid and solid (biochar) fractions have been studied to determine their potential applications. Pyrolysis liquid from OMW (pilot plan, 400 ºC) was composed of two phases, an aqueous phase (AP) containing acetic acid, monosaccharides and phenolic derivatives, and a non-aqueous phase (NAP) mainly composed of phenolic derivatives, fatty acids and their methyl esters. In order to separate the AP and NAP compounds in interesting chemical groups, a methodology based on acid-base extractions (performed with hexane at pH 12, followed by an ethyl acetate extraction at pH 6) was successfully developed.
Within these compounds, phenolics were the most interesting ones due to their antioxidant properties, so the suitability of two methods (Folin-Ciocalteu and DPPH) to quantify them in thermochemical liquids were studied using GP liquids (pilot plant, 225 °C and 400 °C). The results showed that phenolics from 400 ºC liquids can be successfully measured by these methods; however, the ones from 225 ºC liquid should be measured by DPPH since the high content of reducing sugars present in 225 ºC liquid, might interfere in Folin-Ciocalteu measurement. Moreover, 400 ºC was considered a suitable temperature to obtain phenolics, which come not only from the GP composition, but from lignin devolatilization reactions during the thermochemical process.
Regarding the CSS products (pilot plant and lab size reactor, 280 °C, 400 °C and 500 °C), pyrolysis liquids were considered as a potential source of phenolics (mainly the 280 °C one) and caffeine, with 400 °C AP phase having the highest concentration; CSS biochars showed its potential use as energy source and as absorbent of organic pollutants in water, especially cationic and aromatic molecules; and gas fraction was considered as a heat source for biomass drying before pyrolysis. Thus, it has showed that CSS could be completely valorised through thermochemical treatments, which would allow to achieve zero-waste in the coffee roasting industry, being CSS the only waste of the process.
Furthermore, OMW, GP and CSS thermochemical products from both reactors were compared, since using different size reactors could affect the properties and so, applications, of the resulting products. No major differences were observed between biochars, making the biorefinery of these wastes more feasible.; however, in the pyrolysis liquids, the ones from the pilot plant were richer in 2,6-dimethoxy-phenols and phenolics para-substituted by carbonyl groups.
Thus, this thesis has been focused on the valorisation of OMW, GP and CSS, through thermochemical treatments, within an integrated biorefinery context. Specifically, pyrolysis liquids have shown to be a potential source of chemicals, while biochars can be used as solid biofuel, among other potential high-valued applications.
