Comparison of Dry and Wet Torrefaction for Biochar Production from Olive Leaves and Olive Pomace

Abstract

This work investigated the effect of experimental conditions of dry and wet torrefaction on the properties of olive leaves and olive pomace. Torrefaction improved the fuel properties of olive waste. According to Van Krevelen parameters (O/C and H/C ratios), torrefied biomass, tested as solid biofuel, achieved a similar quality threshold to lignite. For example, dry torrefaction conducted at 230 °C for 80 min reduced the O/C and H/C ratios of olive leaves from 0.51 and 1.51 for raw biomass to 0.25 and 1.17 for torrefied biomass, respectively. Under the same conditions, the O/C and H/C ratios of olive pomace were also reduced from 0.34 and 1.60 to 0.27 and 1.36, respectively. Calorific values of raw olive leaves and olive pomace amounted to 18.0 and 23.2 MJ/kg, respectively. Following dry torrefaction and biomass conversion into biochar, calorific values of olive leaves and olive pomace increased by 24% and 14% up to 22.2 and 26.3 MJ/kg through dry torrefaction, compared with 17% and 23% increments up to 21.1 and 28.5 MJ/kg through wet torrefaction, respectively. Interestingly, biomass processing through wet torrefaction performed in a fluidized bed powered by superheated steam could be completed 8- to 12-fold more rapidly than dry torrefaction. SEM analysis indicated a breakdown of the surface structure of olive waste following the torrefaction process. According to the Brunauer–Emmett–Teller (BET) method, total pore surface areas of biochar obtained from wet torrefaction of olive pomace and olive leaves amounted to 3.6 m²/g and 0.8 m²/g, with total pore volumes amounting to 0.0225 cm³/g and 0.0103 cm³/g, respectively. Maximal contents of 5-hydroxymethylfurfural and furfural in liquid by-products from dry torrefaction amounted to 1930 and 1880 mg/1 kg, respectively. Alternately, in liquid by-products from wet torrefaction, concentrations of these high-value compounds remained very low.

1. Introduction

Agricultural and agro-industrial wastes represent a promising source of biofuel that is available in considerable quantities, especially in rural areas. Processing of agricultural waste into solid biofuel facilitates waste disposal while contributing to the reduction in greenhouse gas emissions, since biofuel produced via this route may be considered CO₂-neutral. For certain categories of agricultural waste, conversion into solid biofuel may represent the most economically feasible disposal option. In particular, in the olive oil sector, torrefaction may constitute a viable treatment process to reduce the toxicity of olive pomace. In Mediterranean countries, the development of disposal routes for by-products from olive oil production is of particular importance, since these countries account for 97% of the world’s olive oil production, with 80–84% being produced in countries of the European Union [1], and olive oil production in Europe amounting to 2,133,800 tons in 2024 [1]. Among other Mediterranean countries, the largest producers of olive oil are Morocco and Algeria, with average annual olive oil production in Morocco amounting to 141,600 tons in recent years, notwithstanding a sharp decrease to 90,000 tons in 2024 [2]. In Russia, olive oil is not yet produced in large quantities, but climatic conditions in some regions may allow for the cultivation of olive trees, resulting in increased interest in this product. Following olive oil production processes, the major share of phenolic compounds originally contained in the olive fruit (~98%) becomes concentrated in the by-products of olive factories [3]. Hence, uncontrolled disposal of olive oil production waste may cause significant negative impacts on the environment due to the high antimicrobial and phytotoxic effects of these phenolic compounds [4].

Composting technologies have not yet provided satisfactory results for the detoxification of olive waste due to adverse effects arising from their antimicrobial and phytotoxic properties [5,6,7]. Furthermore, combustion of olive oil production waste, including co-combustion with coal, may generate adverse effects related to the emission of air pollutants, corrosion of boiler equipment, and lower combustion efficiency compared with coal combustion [8,9]. Alternately, gasification may belong to the most promising future technologies for processing olive oil production residues [5]. Yet, immediate challenges facing the development of gasification technology include, among others, the optimization of conditioning of raw material, aiming at improving the overall energy balance of the process [5]. For example, in the process of biofuel production through gasification of olive oil production residues, significant costs are related to drying and grinding of the raw material [10]. As a consequence, waste of the olive oil industry must undergo pretreatment prior to gasification to obtain a product with reduced moisture content, improved grindability, and higher energy content than original biomass. Alternately, torrefaction may offer a viable low-cost biomass pretreatment option aiming at biomass valorization as biofuel [11,12,13]. Torrefaction is an efficient thermochemical pretreatment process, which may conserve up to 90% of the original energy content of biomass [14,15]. During torrefaction, significant amounts of moisture and oxygen are removed from biomass, while lignocellulosic components become partially degraded, releasing volatile substances. The solid product remaining after torrefaction may be named ‘biocoal’, ‘roasted biomass’, or ‘biochar’, though the latter term ‘biochar’ may also refer to the solid material obtained through the process of pyrolysis in general. While torrefaction may be considered a variant of pyrolysis in the broader sense, the pyrolysis process may also take place at higher temperatures, yielding lower shares of biochar of higher quality for a broad range of applications, along with higher shares of liquid and gaseous products. The physical and chemical properties of biochar may vary broadly depending on temperature, time, and processing conditions [16,17]. Several pathways exist for the valorization of biochar, in particular (1) solid biofuel and raw material for the production of synthesis gas in gasification processes and (2) soil improver in agriculture [18,19,20,21,22].

The torrefaction process may be applicable for biochar production from biomass residues derived from olive cultivation and olive oil production [23,24]. When considering the valorization of biochar obtained from olive oil production residue as biofuel, issues may arise from high nitrogen content in olive pruning residues. These issues may be resolved by mixing olive pruning residues together with olive pomace, with the latter biomass containing lower amounts of nitrogen [25]. Biomass torrefaction for solid biofuel production is not yet a mature technology. At this stage, commercial torrefaction plants are being implemented in different parts of the world, but only a few units have a production capacity of more than 2 tons per hour [26]. A limiting factor impeding commercial development of torrefaction technology is the high cost of the process. According to estimates from Thengane et al. [27], the production costs of torrefied biomass may be approximately divided as follows: 45% for the pretreatment of raw materials, in particular for biomass drying, 3–13% for labor costs, 3–10% for energy costs, 12–17% for depreciation costs, and 22–40% for other costs.

In the course of the torrefaction process, the three main components of biomass (cellulose, hemicellulose, and lignin) undergo thermal decomposition to varying degrees, resulting in the formation of non-condensable gases (CO, CO₂) along with condensable volatile substances [28,29,30]. Besides water, the condensable volatile fraction also contains valuable chemicals, such as furfural, acetic acid, methanol, and formic acid [30]. For example, 5-hydroxymethylfurfural (HMF) and furfural are of particular interest for the production of bio-based chemicals [31,32], whose extraction and recovery may significantly improve the economic attractiveness of torrefaction technology.

The torrefaction process can be carried out in different reaction mediums, including inert gas, gas with low oxygen content, aqueous medium, or a water vapor (steam) environment [31,33,34,35,36,37]. The selection of the reaction medium may have a strong influence on process kinetics, as well as on the characteristics of torrefaction products.

The aim of this study is to conduct a comparative analysis of dry and wet torrefaction processes for the conversion of olive production residues, including olive leaves and olive pomace, into biochar along with the recovery of furfural from the liquid by-product of torrefaction (condensate). Important differences between dry and wet torrefaction are expected, since reaction mediums differ widely, with the dry torrefaction process taking place in contact with gaseous products of torrefaction and the wet torrefaction process taking place in a superheated steam environment.

In our study, the solid product of torrefaction will be labeled as ‘biochar’ in a broader sense. Yet, in a number of its characteristics, such as carbon content and volatile substances, this biochar obtained through torrefaction (low-temperature pyrolysis) is of inferior quality than biochar obtained through conventional pyrolysis at higher temperatures of 500–700 °C. On the other hand, the yield of solid products is higher after torrefaction, compared with pyrolysis.

2. Materials and Methods

2.1. Materials

Dried olive leaves from Egypt (Figure 1a) were provided by the LLC ‘Great Wind’, 125167 Moscow, Russia, and crushed into particles no larger than 1.0 mm. Olive pomace (Figure 1b) was provided by the Agrotechnology Academy of V. I. Vernadsky Crimean Federal University. Olive pomace had an initial moisture content of 60% and was dried to a moisture content of 3.71%.

Sample analysis was performed according to the following standards: EN 14775:2009 [38], EN 14774-3:2009 [39], EN 15104:2011 [40], and EN 15148:2009 [41]. The following equipment was used for biomass characterization: low-temperature laboratory electric furnace SNOL 67/350, electric muffle furnace SNOL 10/11-B (SnolTherm UAB, Narkünai, Lithuania), CHNS analyzer TruSpec Micro (LECO, St Joseph, MI, USA), and bomb calorimeter ABK-1 (Retech, 111024 Moscow, Russia). Chemical characteristics of biomass samples are presented in

Table 1. Chemical and fuel properties of olive leaves and olive pomace treated through dry torrefaction.

Biomass Type	Measured Parameter	Raw Biomass	Operational Conditions of Dry Torrefaction
			200 °C 80 min	200 °C 120 min	230 °C 60 min	230 °C 80 min
Olive leaves	Moisture, %	5.67	5.5	5.33	4.64	4.15
	Ash, %	8.41	11.7	12.21	12.58	11.96
	S, %	0.09	0.08	0.09	0.05	0.06
	C, %	46.4	54.9	56.8	57.5	57.1
	H, %	6.23	5.68	5.57	5.67	5.55
	N, %	1.56	1.88	1.99	2.01	2.07
	O, %	31.64	20.26	23.58	17.55	19.11
	LHV, MJ/kg Mass yield, %	17.96 100	18.01 84.2	21.7 73.8	22.19 80.7	22.23 68.4
	Atomic O/C ratio	0.51	0.28	0.31	0.23	0.25
	Atomic H/C ratio	1.61	1.24	1.18	1.18	1.17
Olive pomace	Moisture, %	3.71	1.75	1.51	2.2	1.67
	Ash, %	4.69	4.13	4.50	4.93	4.76
	S, %	0.02	<0.01	<0.01	<0.01	<0.01
	C, %	57.0	59.4	61.0	63.4	63.3
	H, %	7.60	7.14	7.25	7.46	7.20
	N, %	0.99	0.62	0.65	0.87	0.63
	O, %	25.98	26.95	25.08	21.13	22.43
	LHV, MJ/kg Mass yield, %	23.16 100	24.20 89.3	25.46 76.5	26.46 86.4	26.33 70.8
	Atomic O/C ratio	0.34	0.34	0.31	0.25	0.27
	Atomic H/C ratio	1.60	1.44	1.43	1.41	1.36

. The lower heating value (LHV) of olive pomace was 1.3-fold higher than that of olive leaves, whereas nitrogen content was 1.6-fold lower. SEM images (Scanning Electron Microscopy) were taken at 1000× magnification (Jeol JCM-6000, JEOL Ltd., Tokyo, Japan).

The content of cellulose, hemicellulose, and lignin in olive leaves has been determined in previous works by Lama-Muñoz et al. [42]. For cultivated plants of different varieties, the content of these components varied within very narrow limits, with cellulose, hemicelluloses, and lignin accounting for about 6, 11, and 47% of olive leaf biomass, respectively. Extracted olive pomace typically displays cellulose, hemicellulose, and lignin contents in the ranges 13.8–30.0%, 18.5–32.2%, and 30.0–41.6%, respectively [43].

The Agrotechnology Academy of V. I. Vernadsky Crimean Federal University hosts several hundred olive trees of different varieties, with mixtures of different olive varieties being used for the production of olive oil. Cellulose, hemicellulose, and lignin contents in olive pomace amounted to 22%, 25%, and 36%, respectively.

2.2. Thermogravimetric Analyses

The characteristics of the torrefied sample, including the lower heating value (LHV), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), were determined using a thermal analyzer (SDTQ 600, TA Instruments, New Castle, DE, USA). According to TGA performed in an artificial atmosphere composed of a mixture of nitrogen, oxygen, and argon, when heating olive leaves and olive pomace at a rate of 40 °C/min within the temperature range 0–210 °C, sample mass loss remained below 5% for both biomasses, whereas in the range 210–300 °C, mass loss reached 35% and 45%, respectively (Figure 2). Furthermore, according to DSC analysis performed under similar conditions, the curve of heat flow versus temperature of biomasses displayed a series of two peaks: On the left side, a weaker and rounder peak may be correlated with the combustion of volatile substances, observed around 340 and 360 °C for olive leaves and olive pomace, respectively. On the right side, a sharper peak, detected at 450 and 620 °C for olive leaves and olive pomace, respectively, may correspond to the combustion of char residue. Furthermore, taking into consideration that the primary objective for the torrefaction process is obtaining the maximum yield of biochar with a high carbon content and calorific value, the results obtained from differential scanning calorimetry may suggest that torrefaction of olive leaves and olive pomace should be conducted at temperatures lower than 300 °C.

2.3. Torrefaction Procedure

Dry torrefaction of biomass was carried out in a ‘hearth-type’ reactor (Figure 3), as described in a previous publication [44]. The high-temperature heat carrier powering the torrefaction process was polyalkylbenzene, obtained by alkylating benzene with linear alpha-olefins, and appearing as a transparent liquid, which ranged in color from light yellow to light brown, consisting of synthetic high-molecular-weight alkyl-aromatic compounds. The maximum operating temperature of the heat carrier was 340 °C. However, accounting for potential risks of local overheating of the heat carrier, the maximal operational temperature of dry torrefaction was limited to 230 °C. Using this equipment, dry torrefaction of olive leaves and olive pomace was studied at 200 °C for durations of 80 and 120 min and at 230 °C for durations of 60 and 80 min, respectively.

The operation principle of the fluidized bed reactor used for wet torrefaction experiments (Figure 4) has been described in a previous publication [45]. The installation consists of a reactor for torrefaction in the fluidized bed, a hopper for input of feedstock biomass, an outlet for the recovery of biochar resulting from biomass treatment, a cyclone for efficient separation between biochar particles exiting the reactor and the gas–steam flow, and a condenser allowing the recovery of the liquid fraction from the gas–steam mixture. The system is equipped with an electric boiler and an electrical steam superheater. The amount of waste loaded into the reactor was 1 kg. Processing temperatures of wet torrefaction were set at 220 °C, 260 °C, and 295 °C, respectively. The steam consumption rate went up to 20 kg/h, with steam pressure increasing up to 0.2 MPa, while steam temperature reached 500 °C following the superheating process. The duration of the torrefaction process amounted to 10 min.

The analysis of condensable gaseous products of torrefaction was conducted using the following methodology: The sample of condensate was homogenized for 5 min by means of a laboratory rotary homogenizer to obtain a homogeneous substance; subsequently, 5 mL of sample material was taken with an automatic dispenser and transferred into a 50 mL volumetric flask, which was filled up to the mark with 80% acetonitrile solution (Chromatek, CFA). The resulting solution was treated for 30 min at room temperature in an ultrasonic bath, after which 2 mL of treated sample was withdrawn with a syringe and filtered through a membrane syringe filter (13 mm diameter, 0.22 µm mesh size, PTFE) into a chromatographic vial. Experiments were repeated three times.

Quantitative analysis of 5-hydroxymethyl-2-furfural and furfural was performed using reverse-phase high-performance liquid chromatography (RP-HPLC) on a Thermo Ultimate 3000 chromatograph (Thermo Fischer Scientific, Waltham, MA, USA) equipped with DAD-3000 diode array detector. The separation of components was performed on an Hypersil Gold C18 column (4.6 × 250 mm, 5 µm) in isocratic mode, and 4.5% acetonitrile solution of HPLC Plus Gradient grade (Carlo Erba Reagents Srl, Milan, Italy) was used as the mobile phase. Following the release of target components, the acetonitrile content in the mobile phase was increased to 100% in order to remove accumulating impurities from the column. The flow rate of the mobile phase was 0.9 mL/min, and the column temperature was maintained at 30 °C. The injection volume was 5 µL. Spectra were recorded in the wavelength range 230–430 nm. Detection was carried out at a wavelength of 275 nm for furfural analysis and 280 nm for 5-hydroxymethyl-2-furfural, respectively.

Results were processed in the software Chromeleon 7.2.8. The concentration of target compounds (g/kg) was calculated based on peak areas in relation to external standards according to the following formula:

$$X={\displaystyle \frac{C\times S_1\times V\times 1000}{S_2\times m}},$$

(1)

where C—concentration of the corresponding standard solution;

• S₁—peak area of the component being determined in the analyzed sample;
• V—dilution volume of the sample;
• S₂—peak area of the component being determined in the standard sample;
• m—sample weight, g;
• 1000—coefficient expressing the concentration in g/kg.

3. Results and Discussion

Biochar obtained from olive leaves and olive pomace after dry or wet torrefaction is presented in Figure 5. Analysis results of physicochemical and thermal parameters of olive leaves and olive pomace, including raw biomasses and biomasses treated through dry torrefaction, are presented in Table 1 for dry torrefaction and in

Table 2. Chemical and fuel properties of olive leaves and olive pomace treated through wet torrefaction.

Biomass Type	Measured Parameter	Raw Biomass	Operational Conditions of Wet Torrefaction
			220 °C 10 min	260 °C 10 min	295 °C 10 min
Olive leaves	Moisture, %	5.67	3.42	3.49	3.18
	Ash, %	8.41	11.57	13.75	14.37
	S, %	0.09	0.05	0.07	0.07
	C, %	46.4	52.0	54.6	54.4
	H, %	6.23	6.05	5.82	5.78
	N, %	1.56	1.60	1.60	1.55
	O, %	31.64	25.31	20.67	20.55
	LHV, MJ/kg Mass yield, %	17.96 100	20.36 77.3	20.98 72.2	21.10 60.7
	Atomic O/C ratio	0.51	0.37	0.28	0.28
	Atomic H/C ratio	1.61	1.40	1.28	1.28
Olive pomace	Moisture, %	3.71	1.59	1.88	1.54
	Ash, %	4.69	5.59	7.94	9.86
	S, %	0.02	<0.01	<0.01	<0.01
	C, %	57.0	53.5	66.3	66.8
	H, %	7.60	6.87	7.78	6.86
	N, %	0.99	0.85	1.02	1.07
	O, %	25.98	21.59	15.07	13.86
	LHV, MJ/kg Mass yield, %	23.16 100	23.03 75.3	28.51 70.2	27.67 58.2
	Atomic O/C ratio	0.34	0.30	0.17	0.16
	Atomic H/C ratio	1.60	1.54	1.41	1.23

for wet torrefaction.

The fuel parameters of the atomic H/C and O/C ratios, calculated according to biomass composition (Table 1 and Table 2), were compiled into a Van Krevelen diagram [46] (Figure 6). Biochar obtained from olive leaves through dry torrefaction displayed lignite-like characteristics. The fact that olive leaves achieved the lignite quality threshold after torrefaction may be explained by their lower lignin content and higher cellulose and hemicellulose contents, rendering them more amenable to torrefaction. Interestingly, while the O/C ratios of biomass decreased following either dry or wet torrefaction, the H/C ratios remained high. A significant improvement in fuel properties could be achieved through both dry and wet torrefaction. Nevertheless, it is probable that higher temperatures, corresponding to conventional pyrolysis taking place around 500 °C, would be required in order to further decrease H/C ratios.

From a chemical point of view, the decrease in O/C ratios following torrefaction treatment of biomass may be attributed to the generation of volatiles rich in oxygen, such as CO, CO_2, and H₂O [47]. Furthermore, the more limited decrease in H/C ratios may be related to the fact that the increase in carbon content was only moderate in comparison with other elements, since hydrogen-containing hydrocarbon gases are generated mainly at higher pyrolysis temperatures above 350 °C, beyond the range of the torrefaction process, which may be considered as low-temperature pyrolysis [48]. Martín-Lara et al. [24] found that the O/C and H/C ratios of olive tree pruning residue amounted to 1.02 and 0.17, respectively, with torrefaction at 200 °C achieving a slight reduction in these ratios, down to 0.89 and 0.16, respectively. Chen et al. [48] also noticed gradual increase in carbon contents as well as decreased oxygen contents along with increasing torrefaction temperature. Ohliger et al. [49] found that, following torrefaction of beech wood performed at 280 °C for 40 min, the oxygen content of the treated biomass decreased from 43.1% to 33.8%, while the carbon content increased from 49.8% to 59.4%. Sabil et al. [50] reported that, after torrefaction at 300 °C, the oxygen content of palm shell decreased from 47–50% to 38–43%, and the C/O ratio increased from 0.9–1.0 to 1.3–1.4, respectively.

Based on data of biomass composition and fuel parameters, in order to evaluate the effects of the torrefaction process on biomass composition and properties, the change in parameters of torrefied biomass (biochar), relative to raw biomass, was calculated for both processes of dry and wet torrefaction and is compiled in Figure 7.

Both dry and wet torrefaction resulted in increased LHV, but the increase was more marked for olive leaves than for olive pomace, owing to the higher cellulose and hemicellulose content of olive pomace, as described previously. Both dry and wet torrefaction generally resulted in decreased O, S, and moisture contents, as well as increased C contents, with the effects being generally more marked at higher treatment severities, i.e., higher temperatures and retention times. The effects of dry and wet torrefaction on the treated biomasses were variable, with little effect observed in the wet torrefaction process, while the dry torrefaction process resulted in either increased or decreased N contents for olive leaves and olive pomace, respectively. Taking into consideration this pattern of increased or decreased N content may be critical while investigating the application of biochar as a fertilizer. While the application of biochar as a fertilizer and soil improver may require higher N content, the application of biochar as a fuel may require lower N content. Therefore, a useful strategy may be to select and mix biomasses displaying different properties in order to fine-tune the torrefaction process according to the intended end use of biochar.

Finally, increased ash contents were observed for both olive leaves and olive pomace in wet torrefaction, whereas for olive pomace, increased ash content occurred only in dry torrefaction. Increased ash contents, reflecting higher amounts of minerals contained in the biochar, may be detrimental to biochar use as a fuel. Nevertheless, increased contents of minerals may be an advantage when using biochar for other applications as a soil improver or as a catalyst.

The torrefaction process may also alter the surface structure of biomass, especially for biomass samples subjected to wet torrefaction. Microphotographs of the surface of the biomass samples are presented as follows: Figure 8a, original olive leaf sample (magnification ×2000), average pore size 1.854 μm; Figure 8b, olive leaf sample after wet torrefaction at 295 °C for 10 min (magnification ×5000); Figure 8c, original olive pomace sample; and Figure 8d, olive pomace sample after wet torrefaction at 295 °C for 10 min (magnification ×5000), average pore size 4.68 μm.

The specific surface area of biochar was measured using the Autosorb iQ porosity and specific surface analyzer (Anton Paar, Graz, Austria). The total pore surface areas of biochar obtained from wet torrefaction of olive pomace and olive leaves according to the Brunauer–Emmett–Teller (BET) method were 3.6 m²/g and 0.8 m²/g, with total pore volumes amounting to 0.0225 cm³/g and 0.0103 cm³/g, respectively. Hence, biochar porosity properties suggest the possible use of biochar as a bioadsorbent, such as for the removal of polyphenols from wastewater of olive oil production facilities [51]. However, biochar activation may still be required to achieve more efficient removal of these polyphenols [52,53,54].

The contents of furfural (FU) and 5-hydroxymethylfurfural (5-HMF) in liquid products of dry and wet torrefaction are presented in

Table 3. Furfural contents in condensate from dry torrefaction.

Processing Conditions of Dry Torrefaction			Contents and Yield of 5-HMF and FU in Torrefaction Condensate
Biomass Type	Processing Temperature	Processing Time	Content of 5-HMF, mg/kg	Yield of 5-HMF, %	Content of FU, mg/kg	Yield of FU, %
Olive leaves	200 °C	80 min	284.0	3.3	341.4	4.58
		120 min	174.3	2.0	127.8	1.71
	230 °C	60 min	163.1	1.9	208.8	2.45
		80 min	165.3	1.9	143.6	1.7
Olive pomace	200 °C	80 min	1930.6	0.88	1880.3	0.752
		120 min	915.1	0.42	370.8	0.015
	230 °C	60 min	1465.8	0.66	189.0	0.08
		80 min	685.1	0.31	251.4	0.1

and

Table 4. Furfural contents in condensate from wet torrefaction.

Processing Conditions of Wet Torrefaction			Contents and Yield of 5-HMF and FU in Torrefaction Condensate
Biomass Type	Processing Temperature	Processing Time	Content of 5-HMF, mg/kg	Yield of 5-HMF, %	Content of FU, mg/kg	Yield of FU, %
Olive leaves	220 °C	10 min	48.8	0.06	43.5	0.58
	260 °C	10 min	34.4	0.04	48.7	0.065
	295 °C	10 min	181.9	2.13	151.6	2.03
Olive pomace	220 °C	10 min	11.0	0.005	26.8	0.01
	260 °C	10 min	9.5	0.0043	49.3	0.02
	295 °C	10 min	8.2	0.0037	91.6	0.04

, respectively.

Borrero-López et al. [55] observed that FU yield obtained through pyrolysis of olive waste may only be related to hemicellulose content, so the FU yield relative to hemicellulose content may be calculated from Equation (2). Also, 5-HMF content may only be related to cellulose content, so the 5-HMF yield relative to cellulose content may be calculated from Equation (3).

$${Yield}_{FU}={\displaystyle \frac{{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}_{ FU}\times {\mathrm{V}}_{liquid}}{{\mathrm{m}}_{\mathrm{h}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{u}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{e}}}},$$

(2)

$${Yield}_{5-HMF}={\displaystyle \frac{{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}_{ 5-\mathrm{H}\mathrm{M}\mathrm{F}}\times {\mathrm{V}}_{liquid}}{{\mathrm{m}}_{\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{u}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{e}}}}$$

(3)

As follows from Table 3, following dry torrefaction of olive leaves and olive pomace, the maximum yield of 5-HMF reached 3.3% and 0.88%, respectively. In both cases, the yield of 5-HMF decreased along with increasing torrefaction temperature and treatment duration, whereas the maximum yield of FU reached 4.58%, and followed the same pattern of steady decrease along with increasing torrefaction temperature and treatment duration. Hence, the yields of 5-HMF following dry torrefaction of olive leaves and olive pomace were comparable to the yield of this product obtained from hydrothermal carbonization of olive pits at temperatures of 160–240 °C for 1–6 h obtained by Borrero-López et al. [55]. Yet, the yield of FU obtained by these authors was several times higher than the yield resulting from dry torrefaction of olive leaves and olive pomace.

Following wet torrefaction of olive leaves (Table 4), the yields of 5-HMF and FU were lower than those of dry torrefaction and increased along with increasing torrefaction temperature. Furthermore, as a result of wet torrefaction of olive pomace, the yields of 5-HMF were more than 100-fold lower than those of dry torrefaction, and the yields of FU were approximately twice lower than those of dry torrefaction (Table 4). The reason behind the lower yields of 5-HMF and FU in the time-efficient wet torrefaction process performed in a fluidized bed powered by superheated steam may be the short residence time of the biomass in the reaction zone, amounting to 10 min only.

4. Conclusions

Comparative studies of the dry torrefaction process conducted in a ‘hearth’-type reactor in an environment of gaseous torrefaction products as well as wet torrefaction in a fluidized bed reactor powered with superheated steam have demonstrated that both processes may facilitate the production of biochar from the waste of the olive oil sector, namely olive leaves and olive pomace. The calorific value of the obtained biochar was comparable to that of lignite.

According to Van Krevelen parameters, biochar obtained from olive leaves, owing to higher cellulose and hemicellulose contents, displayed more favorable biofuel properties than biochar obtained from lignin-rich olive pomace. Interestingly, dry torrefaction of olive leaves also resulted in higher N and ash contents in biochar, which may be beneficial for potential application as a biofertilizer or bioadsorbent. Differences between biomasses and torrefaction processes greatly affecting the characteristics of biochar open the avenue of fine-tuning these parameters in order to obtain biochar with optimal properties, depending on the targeted usage.

The possible advantages of the wet torrefaction process in a fluidized bed powered with superheated steam are as follows: (1) ensuring the fire safety of the process, (2) allowing an increase in process temperature, and (3) reducing processing time due to the use of water vapor as an efficient heat transfer medium, along with fluidized bed technology enhancing the rate of heat and mass transfer processes. The latter factors resulted in an 8-fold to 12-fold reduction in processing times at operational temperatures of 230 °C, and 200 °C, respectively. Yet, considering the economic feasibility of the wet torrefaction process, the production of superheated steam may induce higher costs and energy requirements for the torrefaction process. Nevertheless, superheated steam may also be applied for biomass drying and save the costs of a separate biomass drying stage, which would enhance the energy balance and economic efficiency of the process. Therefore, wet torrefaction may be more suitable for the treatment of biomasses with high moisture contents [56]. Furthermore, wet torrefaction in a fluidized bed powered with superheated steam may generate porous biochar displaying suitable properties for use as a bioadsorbent, though with a much lower adsorption capacity than biochar from conventional pyrolysis performed at higher temperatures, with potential application for the removal of polyphenols from wastewater generated by olive oil production. Comparing dry and wet torrefaction with regard to the porosity of the obtained biochar and overall energy balance of the process would be of interest for further studies. Important processing stages to consider are the use of the condensate as a biofertilizer for crop fertigation, as well as efficient energy recovery via an optimized heat exchanger attached to the condenser stage, which may be required for the prevention of environmental pollution and efficient recovery of the liquid by-products.

Nevertheless, in contrast to wet torrefaction, dry torrefaction may hold the advantage of generating a higher yield of 5-hydroxymethylfurfural and furfural within the liquid by-products of the process. Yet, further investigations may be useful to evaluate economic profitability in relation to the valorization of these high-value biochemicals.

In light of these preliminary results, future studies may investigate complete circular economy concepts based on the valorization of olive oil wastes, encompassing the production of biochar and its application as a soil improver, along with the recovery of high-value biochemicals from liquid by-products of the torrefaction process.