# Thermochemical Conversion of Olive Oil Industry Waste: Circular Economy through Energy Recovery
## 1 Introduction  
Olive oil production is an activity of vital economic importance in Mediterranean countries [1]. Portugal is a reference in modern olive growing, and is expected during next decade to be the third largest producer of olive oil in the world and the seventh largest in terms of occupied area [2]. This growth is sustained by the evolution in the country and, in particular, in Alentejo, Southern Portugal, with the contribution of Alqueva dam that provided the irrigation for intensive crops [3]. For example, in 2018, which saw adverse weather conditions, the results presented in SIAZ--Olive Oil and Table Olives Information System-[[https://www.gpp.pt--indicated](https://www.gpp.pt--indicated)]([https://www.gpp.pt--indicated](https://www.gpp.pt--indicated)) a 30% drop in oil production compared to the previous campaign. Despite this, production was 4% higher than the average production inthe last four campaigns. Currently, as the ninth producer in the world, Portugal has 361 thousand hectares of olive groves, and 135 thousand tons of oil extracted. Olive oil production is associated with the generation of residues, olive pomace, and red waters, which present potentially toxic compositions and are harmful to ecosystems when improperly discharged into the environment [4]. Due to the high levels of oils and fats, the chemical deficiency of oxygen, total solids, polyphenols, and red waters, in order to be able to be sent either to emissaries or directly to the receiving environment, need extremely efficient treatments, with significant investment and operating costs that can make the mills unfeasible. Within these residues, olive pomace has viable forms of recovery already known and in use [5; 6; 7]. This olive pomace, resulting from the recently introduced two-stage centrifugation system, is a semi-solid, moderately acidic residue formed by pieces of olive stone, olive pulp, and water [8]. Its composition varies according to the variety of olives, fruit ripeness, climatic conditions, and cultivation practices [9]. In general, it consists of high amounts of water (60-70%), residual oil retained in the pulp (2.5-3%), inorganic compounds, and appreciable amounts of lignin, cellulose, and hemicellulose, as well as other organic matter including proteins, polyalcohols, fatty acids, sugars, polyphenols, and pigments [9; 10]. This organic load is responsible for its phytotoxic and antimicrobial characteristics, as well as its high moisture content, making it difficult to handle, store, and transport [11; 12]. In recent years, many efforts have been made to find a way to efficiently recover olive pomace in order to add commercial value to this waste [13; 14]. Within the forms of recovery already studied and mentioned in the literature, bioconversion of these residues into fertilizers can be highlighted, including application in the production of animal feed, the use as a substrate for the production of bioethanol/biomethane and biohydrogen, and bioconversion in some biopolymers or enzymes for other industries [15]. Despite all the advantages that are described in the literature for these conversion technologies, as capable of processing residual biomass mainly due to its ability to homogenize different types of raw materials with different qualities, these technologies still present restrictions, namely with regard to its scalability, and the fact that large production units must operate continuously. Many recent developments have been achieved, mainly with regard to process control and stability. However, it cannot yet be considered a mature technology, so a large investment in R&D is still needed. Torrefaction and pyrolysis are thermochemical conversion technologies that present as main difference the temperature range where occur, being, respectively, 220 \({}^{\circ}\)C to 320 \({}^{\circ}\)C for torrefaction, and 320 \({}^{\circ}\)C to 600 \({}^{\circ}\)C for pyrolysis [16].