Tratamientos termoquímicos para valorización energética de residuos biomásicosviabilidad técnica y comparativa económica

Abstract

The European Union's commitment through the Horizon Europe program brings together the challenges of today's society to implement a circular economy model. This program includes a section focused on the use of biomass residues to produce new biofuels, chemical products and different renewable materials. Under this premise, in this work the energetic recovery of two biomass residues (olive tree pruning and compost that does not meet specifications for sale) was proposed. Hydrothermal carbonization was selected as the treatment to evaluate. Moreover, a comparison with other thermal treatments widely studied such as pyrolysis and torrefaction was carried out. First, the optimization of the reaction parameters (temperature and time) for the hydrothermal carbonization of olive tree pruning was experimentally carried out. For that, nine reaction conditions were evaluated (220, 250 and 280 °C were selected as reaction temperatures and 3, 6 and 9 hours as reaction times). The results allow to affirm that it is possible to obtain a suitable hydrochar for being used as solid biofuel. Besides, to obtain this hydrochar it is not necessary to treat the biomass at reaction temperatures higher than 250 °C, nor to apply a reaction time greater than 3 hours, since the energy consumption necessary to reach these parameters is not compensated by the improvement of the properties of the solid product. Once it is experimentally verified that a lignocellulosic biomass such as olive tree pruning is suitable for obtaining a product useful for being used as solid biofuel, the same procedure is proposed for an organic waste with a difficult management (off-specification compost). Due to its composition, this product by itself does not present a good conversion, so it is blended with olive tree pruning at different proportions to manage both residues together. The hydrochar of the blend of 25% compost with 75% olive pruning presents the best results in terms of conversion and physicochemical properties. In this way, the amount of this compost, which has no other use currently, can be reduced. A comparison between dry and wet thermal treatments (i.e., hydrothermal carbonization, pyrolysis and torrefaction) was also proposed. The objective was to compare the obtained products in terms of characteristics of the obtained product and the energy consumption of every process. For a lignocellulosic biomass with a moderate moisture content, such as olive pruning, torrefaction turned out to be the most appropriate thermal treatment in terms of energy consumption and mass yield obtained. However, the pyrolysis treatment showed a higher carbonization degree of the product, but with a lower mass yield and a much higher energy consumption than the previous one. For hydrothermal carbonization, much more significant variations were observed depending on the severity of the process. As a last point, carrying out a preliminary techno-economic study to compare all the treatment alternatives was proposed. This analysis, performed with the extrapolated data collected at a laboratory scale, concluded that, if we only consider the production of the solid char, without adding value to the other generated fractions (liquid and gaseous), none of the alternatives is currently profitable, with net present values (NPV) between -35 and -45 M€. This fact demonstrates the great challenge posed by the evolution towards a more sustainable circular economy. Among all, the hydrothermal carbonization of compost and olive tree pruning turned out to be the most profitable treatment at an economic level. All the conclusions obtained throughout the chapters exposed in this thesis allow opening new research routes to obtain many valuable streams from waste. The results also reveal the present and future challenges that need to be faced to promote the concept of circular economy so necessary in our society nowadays.