The circular economy represents a novel paradigm to conciliate both economic growth and sustainable development goals ^[1] for the conversion of communities to a less carbon-intensive energy system ^[2]. It recommends reducing waste by recycling and reusing products for closing materials and energy loops. The use of wastes like vegetal residual biomass, from a circular economy perspective ^[3], not only avoids their disposal costs [4,5,6]^[4][5][6] but creates an optimized way to realize a carbon-neutral energy system ^[2]. Several studies have demonstrated that vegetal residual biomass is an important energy resource in terms of quantity and quality [7,8^{[7][8][9][10][11]},9,10,11], representing a large feedstock for clean energy production ^[12], with a significant role in reducing the environmental impacts arising from the use of fossil fuel sources ^[13]. In this context, the concept of energy communities, which are based on self-production and self-consumption ^[14], has emerged ^[15], and biomass, being locally and ubiquitously available, is very suitable. The agricultural sector produces large quantities of processing residues, and olive growing [16,17,18]^[16][17][18] represents one of the principal areas for the economy of the Mediterranean basin ^[19], with plantations that cover significant lands in the basin. Usually, the unproductive branches of olive trees are cut every two years, leading to a large amount of olive tree pruning (OTP) as waste has to be removed from the fields to avoid the circulation of vegetal pests ^[20].

2. Process Analysis

2.1. Combustion

Picchi et al. [22]^[21] examined the physical and chemical features of OTP for direct combustion, concluding that OTP seems the most suitable (compared to other pruning materials) for direct combustion as it contains lower concentrations of critical compounds, such as N, S, and Cl. Chip and pellet production represents a low-cost way to internalize a potential cost of waste disposal, turning it into an energy resource. This process can be implemented near the area of pruning collection because of the ubiquity of the biomass, avoiding road transportation and packaging for external sales, with a benefit for local farmers. In order to obtain a high-quality pellet from OTP, high temperatures, low moisture content (less than 15% [52]^[22]), and reduced particle dimension represent crucial factors, while the compression force is not so significant. In the case of olive trees affected by Xylella fastidiosa, moisture content is minimal because of the action of the bacterium that dries the lymphatic vessels, leading to the death of the plant. The pelleting process slightly improves the calorific value of olive wood [53]^[23]. Kougioumtzis et al. [54]^[24] compared combustion in an industrial boiler of OTP to sunflower husk (SH) pellets: lower emissions of CO and NOx were found for OTP pellets, while dust emissions were high for both fuels, suggesting that particulate matter abatement equipment should be installed in the combustion facilities. OTP seems suitable for direct combustion with respect to other common European orchards crops, like vine, apple, pear, and hazelnut, probably because the olive crops are cultivated in a less intensive way and, thus, receive fewer chemical inputs [22]^[21], fulfilling set specifications for direct combustion. If, on one hand, OTP satisfies the industrial pellets specification given in the European Standard EN ISO 17225-2:2021, then, on the other hand, this standard does not make it suitable for residential uses given the high ash and nitrogen contents [55,56]^[25][26].

2.2. Gasification

Gasification is a technology that realizes a partial oxidation of hydrocarbons based on a controlled amount of steam or oxygen at high temperatures (>700 °C) [59,60,61,62]^{[27][28][29][30]}. It does not lead to combustion, and the products are syngas (H₂ and CO mostly) and byproducts in the form of condensable organic compounds [53]^[23]. Gasification applied to biomass has attracted considerable interest because of the use of a new substrate for this technology [63]^[31]. Focusing on the gasification of OTP, Vera et al. [64]^[32] modelled a downdraft gasifier and a gas engine grid connected on a small-scale plant, able to produce 110 kW of thermal power and 70 kW of electric power when fueled with 105 kg/h of biomass operating in steady-state conditions. The LHV of the syngas was 3.7 MJ/kg because of the high OTP ash content (8.7%), and it was a relatively low value due to the high air-to-OTP ratio (2.7) that amplified N₂ formation. A better performance in terms of syngas LHV was found in a pilot plant located in Andalusia (Spain) through the cogeneration of thermal and electric power through a downdraft gasifier, gas cooling–cleaning stage, and spark ignition engine with a modified carburetor that showed an LHV of 4.8 MJ/kg [65]^[33].

.3. Pyrolysis

2.3. Pyrolysis

Pyrolysis works in an inert atmosphere under the absence of oxygen, capturing the off-gases arising from the thermal decomposition of biomass. The matter is divided into smaller sizes under specified operating conditions [69]^[34]. Pyrolysis is a very complex process that involves a huge number of chemical reactions within seconds or minutes [70,71]^[35][36]. Products are mainly bio-oils made of hydrocarbon molecules arising from condensed hot vapors, biochar rich in carbon, and bio-syngas [53]^[23]. Biochar is mostly used as a soil amendment for agricultural and environmental purposes, while charcoal is used for heat or as a reducing agent in metallurgical applications, as well as an adsorbent material. Zambon et al. [52]^[22] obtained biochar via the pyrolysis of OTP pellets with an LHV and HHV of 30.5 and 31.7 MJ/kg, respectively, with a mean conversion rate of 0.21. In a study by Calahorro et al. [72]^[37], olive wood sawdust, branch barks, leaves, and twigs (small branches of 1 cm) were subjected to pyrolysis under operating conditions of 400, 500, and 600 °C; a 10 °C/min heating rate; a 20 min residence time; and a 200 cm³/min N₂ stream. The high ash and volatile content, together with the low process yield, made the resulting charcoal a low-quality product. The same authors tested charcoal obtained via the pyrolysis of wood (sawdust or cubes), twigs, and branch bark, excluding leaves that were more appropriate to be used for feed cattle: the results showed that charcoal can be recognized as suitable for the manufacture of briquettes. A pyrolysis-based circular system from OTP arising from a 10 ha olive grove produced 8.5 t of bio-oil (LHV of 31 MJ/kg), 9.9 t of syngas, and 7.4 t of biochar (LHV 29 MJ/kg) [63]^[31]. A microwave-assisted process of OTP pyrolysis was analyzed by Bartoli et al. [73]^[38]: among products, biochar had calorific power up to 25 MJ/kg, while bio-oils showed interesting biochemical compounds, like acetic acid, furans, and aromatics. This last finding represented, for the authors, a potential for reducing the disposal environmental risks of these chemicals and fuels.

2.4. HydroThermal Carbonization

HydroThermal Carbonization (HTC), also known as wet pyrolysis, is a form of thermochemical conversion through pressurized water under sub-critical conditions (usually between 180 and 280 °C) and autogenous saturated vapor conditions (10–80 bars), originating from residual biomass into highly dense carbonaceous materials [78]^[39] with a high heating value (HHV) and a high carbon content [79,80,81,82]^{[40][41][42][43]}. This process generally includes hydrolysis, dehydration, and decarboxylation. At high temperatures and pressures, water experiences a dramatic change in properties and acts more as an organic solvent with an increased ion product that promotes reactions, usually catalyzed by acids or bases, favoring biomass decomposition through hydrolysis, dehydration, and decarboxylation reactions [83^[44][45],84], followed by condensation, as well as aromatization reactions [66]^[46]. Volpe et al. [85]^[47] carried out a study to compare HTC and torrefaction in a 50 mL batch reactor and low-temperature pyrolysis (LTP) in a fixed bed reactor with OTP, with the aim of producing performing solid biofuels. The results demonstrated that the hydrothermally obtained biochar (hydrochar hereinafter) had a higher energy densification, whereas the torrefied biochar had a higher mass yield.

3. Products

3.1. Solid Materials

High porosity, high carbon content, high surface area, low thermal conductivity, renewability, high stability, and bulk density make char a sustainable coal-like solid: it has less calorific value than standard coal, but produces less ashes during the combustion process [90]^[48]. Several studies investigated the effects of residence time and reaction temperature on hydrochar mass yields and features [83,91^{[44][49][50][51]},92,93], asserting that the principal contribution to biomass degradation and, thus, the increase in the calorific value, is represented by the reaction temperature rather than the residence time [94]^[52]. Lucian et al. [91]^[49] noted an improvement in the heating value of olive trimming from 22.6 to 27.8 MJ/kg when the temperature of HTC was raised from 180 to 250 °C. The high reaction temperature and energy consumption represent an issue with the advance of HTC technology. Conventional batch reactors couple pressure and temperature at saturated states. Yu et al. [27]^[53] have developed Decoupled Temperature and Pressure Hydrothermal (DTPH) reactions through a method that decreased the temperature of the HTC reaction of lignocellulosic biomass (poplar leaves and rice straw). The results allowed us to realize HTC at a temperature of 200 °C in spite of the lower bound of 230 °C adopted in the conventional process. The scientific community is concentrating on developing supercapacitors with activated carbon derived from olive pruning that can provide an improvement in energy storage in terms of the specific area and surface composition [72]^[37]. The results of OTP application as a supercapacitor electrode [95,96]^[54][55] represent a promising method of developing competitive energy storage devices based on agro-industrial wastes. Activated carbon, obtained through KOH [95]^[54], reached a BET surface of 4083 m²/g and, when applied as a supercapacitor electrode, generated a high specific capacitance of 264.4 F/g at a current density of 0.5 A/g, with high values of energy density (17.8 Wh/kg) and power density (65 W/kg). An excellent performance supercapacitor was demonstrated by electrode materials derived from OTP with chemically activated carbon working as electrode material and PVA-KOH hydrogel working as an electrolyte: they present a capacitance of 1.15 F at 5 mA, a voltage of 1.2 V, and equivalent series resistance of 1.42 Ω [96]^[55]. OTP-activated carbon was also applied as a detoxifying agent, allowing the elimination of inhibitory compounds prior to fermentation of the hydrolyzed liquid for ethanol production [86]^[56]. The removal quantities of inhibitor compounds were 89.2%, 91.8%, and 32.6% for polyphenols, furfural and hydroxymethylfurfural, respectively. Biochar pellets arising from wood pellets have been found to produce an HHV equal to 31.5 MJ/kg [97]^[57]. Biochar production via biomass pyrolysis is a practical and attractive process for storing carbon and lowering greenhouse gas (GHG) emissions, and its stability (carbon recalcitrance) represents a significant characteristic that determines the carbon sequestration capacity [69]^[34]. Char is expected to have an even larger market in the next few years: the breakeven selling price of Co-HTC hydrochar was found USD 117 per ton for a 110 Mwe, and sensitivity studies indicated that it can reach USD 106 per ton for a higher capacity plant [89]^[58]. The lignocellulosic composition of OTP presents high thermal stability, resulting in higher mass yield (approx. 50%) and fixed carbon (9%) with respect to protein-based and fruit wastes [78]^[39].

3.2. Liquid Materials

OTP lignin extracted using deep eutectic solvents (DES) is a promising environmental-friendly method [103]^[59]; the product of reaction shows a high antioxidant activity. As a sugar-rich matter with low lignin content and high cellulose content, olive pruning debris represent an excellent substrate for bioethanol production. With respect to the first and third generation of bioethanol production from lignocellulosic biomass, it has the advantage of reducing the cost of raw materials and proposing suitable solutions for environmental problems when agro-industrial wastes are processed [104]^[60]. Actually, the process includes four stages: pretreatment, hydrolysis, fermentation, and ethanol concentration [53,105]^[23][61]. The pretreatment of lignocellulosic biomass is a crucial step in both technical and economic terms [39]^[62], being necessary because of its intrinsic recalcitrant nature to degradation, in turn necessary for the production of valuable chemicals [106]^[63]. The beginning of the pretreatment stage is the breakdown of hemicellulose to sugars, followed by the opening of the structure of the cellulose. Lignin can be extracted from cellulose through an alkaline solution with oxidizing agents, such as H₂O₂ [107]^[64]. This process reduces the volume of the hydrolysis reactor and increases sugar content, reducing energy demand during cellulose hydrolysis. Major pretreatments include ultrasound, ozonation, steam explosion, extrusion, diluted-acid hydrolysis [108,109,110,111,112^{[65][66][67][68][69][70]},113], alkaline peroxide pretreatment, autohydrolysis or liquid hot water pretreatment [53^[23][71],114], electron beam, gamma ray, microwave, high hydrostatic pressure, high-pressure homogenization, and pulsed-electric field [106]^[63]. Two of the most widely adopted pretreatment methods are steam explosion and liquid hot water: results published by Romero-Garcìa et al. [39]^[62] show that they performed similarly, even though the second one yielded the highest overall sugar recovery, i.e., 92%, at a lower operation temperature (180 °C) versus 80.4% for steam explosion at 220 °C. Ethanol production resulted in a solution of about 4.4% (v/v) with a yield that was slightly better for steam explosion-pretreated samples, i.e., 72%, compared to 63% in liquid hot water samples, albeit at different temperatures (220 °C against 200 °C). Mineral acid hydrolysis can penetrate lignin without pretreatment and more quickly than enzymatic hydrolysis; on the other hand, it occurs under mild conditions of temperature (between 40 and 50 °C) and pH (around 5.0) [53]^[23].

3.3. Gaseous Materials

A low C/N ratios and high concentrations of nitrogenous matter make olive pruning suitable for anaerobic digestion. Nevertheless, only the finest particles of pruning debris are considerable for methane production. They are produced through a fractionation process, followed by batch anaerobic digestion at 38 °C. The process is recognized to be highly energy efficient, with the highest methane yield achieved equal to 176.5 Nm³ per t of volatile solids [55]^[25]. Biomass typically contains 6% hydrogen by weight and lends itself to both thermal and biological conversion processes to this energy vector: direct gasification and pyrolysis represent the thermal way to produce hydrogen, while fermentation and bio-photolysis are the biological paths. Other routes [121]^[72] are new technologies such as microwave gasification, solar gasification, integrated pyrolysis-gasification, plasma gasification, and catalytic gasification. Wood gasification on a fixed bed without a catalyst showed a hydrogen yield of 7.7% at 550 °C ^[12]. A hydrogen-rich syngas produced from the lignocellulosic biomass via catalytic gasification was investigated by Ghodke et al. [121]^[72]. They carried out an investigation into the performance of several lignocellulosic biomass gasification systems with and without catalysts. An aspect to be considered for the gasification process is tar production. Tars are high-molecular-weight hydrocarbons, constituting undesirable by-products of gasification. Methods to minimize their formation are catalysis, pretreatment technologies, and the optimal design of both gasifier and operating conditions ^[12]. Several catalysts (as oxides of calcium) increased syngas quality and quantity, reducing tar and carbon deposition; alkali and alkaline earth metal catalysts significantly reduced tar production, as well as resistance to the carbon deposition, while Ni and alkaline metals were used as standalone catalysts in dry and steam gasification and gave a good performance in the hydrogen concentration in syngas [121]^[72]. A gasification method using a metal oxide sorbent, such as calcium oxide, water gas shift (WGS), integrated with steam-hydrocarbon reaction, and CO₂ absorption in a single reactor received considerable attention: the presence of a metal oxide sorbent can involve an in situ CO₂ capture, and, if properly designed, the exothermic CO₂ absorption can be coupled with the endothermic biomass gasification reaction ^[12]. This principle is at the basis of HyPr-RING (hydrogen production via reaction-integrated novel gasification), a technology that adopts chemical looping with the calcium cycle, in which CaO (or Ca(OH)₂) captures CO₂ during coal gasification to form CaCO₃ and release heat for gasification to produce near pure hydrogen in one gasifier [122]^[73]. 

54. Conclusions

All thermochemical treatments of OTP change the initial structure of the biomass, allowing products to be evaluated as energy fuel. Torrefaction represents a good compromise between mass yield and LHV, but it is only a profitable process for energy purposes if the solid phase is considered as a valuable output. Among the three processes considered for OTP, namely pyrolysis, torrefaction, and HTC, the first one shows the best performance for both char HHV and syngas yield under different operating conditions, even though it is a process that concentrates inorganic matter in the char, resulting in higher ash concentration with respect to HTC and torrefaction. Regarding the energy analysis results, the optimal energy balance is obtained through chips and pellets and, thus, the use of combustion as a process of wood conversion. Nevertheless, OTP pellets also have high ash content due to the leaf and soil contamination arising from the harvesting stage, leading to a slight decrease in the boiler efficiency. The choice of chips would reduce the supply chain by one step compared to pellets, avoiding the industrial processing necessary to compress wood chips, but on the other hand, the storage of chips can lead to biological deterioration phenomena, which risk reducing part of the biomass in terms of effective weight, making it unusable. The energy balance is also influenced by the pretreatment necessary to dry the biomass. HydroThermal Carbonization does not require drying of the biomass like combustion, gasification, or pyrolysis. Thus, an important aspect to be taken into account is the water content: the higher the water content of the wood, the lower its LHV. As for hydrochar and biochar, their characterization is fundamental for industry and the environment: a biochar with low carbon content and high ash content is not suitable for use in energy products. On the other hand, a biochar that shows a high adsorption capacity and high surface area is highly suitable for agriculture and wastewater treatment. The amount of alkali and alkaline earth metals is related to the ash percentage in the raw feedstock; thus, the challenge is to reduce their presence in ash composition, making char highly advantageous when used for energy production. In a circular economy perspective, the efficient use of wastewater arising from the HTC process represents a challenge for the application of this technology on an industrial scale. Recirculating water from the HTC process could solve this problem, reducing wastewater treatment cost and recovering heat. Actually, excluding combustion, gasification seems to be the most appropriate technology to optimize olive wood for the advantage of enabling the cogeneration of electricity and heat. In addition, natural gas or oil-fired boilers do not continuously operate and lead to significant combustion problems stemming from the high production of NOx and CO; with a gasifier, the production of pollutants is greatly reduced because of the lower percentage of nitrogen. While gasification represents the energy optimization of olive wood biomass among conversion technologies, the enhancement occurs in biochar because it represents a value-added product, the quality and versatility of which can find a wide market of applications, ranging from soil conditioners to additives for supercapacitor construction. In particular, biochar is an attractive material because it has low thermal conductivity, high surface area, high porosity, high stability, and high carbon content. Bioethanol is a potentially strategic fuel, but the use of two enzymes and two fermentation steps does not make bioethanol production from olive wood economically feasible. Technologies for producing hydrogen-rich syngas show signs of promise, but they still need to be improved in terms of selectivity, efficiency, and cost-effectiveness.