Effect of short-time hydrothermal carbonization on the properties of

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Biofuels and Biomass

Effect of short-time hydrothermal carbonization on the properties of hydrochars prepared from olive-fruit endocarps Manuel Cuevas, Maria Lourdes Martinez Cartas, and Sebastian Sánchez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03335 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

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Effect of short-time hydrothermal carbonization on the properties of hydrochars prepared from olive-fruit endocarps Manuel Cuevas Aranda, Mª Lourdes Martínez Cartas*, Sebastián Sánchez Villasclaras Department of Chemical, Environmental and Materials Engineering, University of Jaén, Spain * Corresponding author Tel. +34 953648541 E-mail address: [email protected] (M.L. Martínez Cartas) Postal address: Av. de la Universidad s/n–23700 Linares (Jaén) Spain.

ABSTRACT: Endocarps of the olive fruit were subjected to short-time hydrothermal carbonization (SHTC) to study the effect of temperature and holding time on the properties of hydrochars. In general, the increase of these variables improved the calorific value and the combustion properties of the hydrochars, and decreased their capacity to adsorb moisture and the ash content. Best hydrochar was produced at 225 °C for 10 min, showing maximum values of higher calorific value (23.4 MJ/kg), energy efficiency (74.7%) and comprehensive combustibility index (6.14 × 10–7 min–2 ºC–3), higher values than those were managed working at 250 ºC. The increase in energy efficiency at 225 °C at longer reaction residence times was related to repolimerization phenomena studied by mean of the analysis of the structural components of the biomass (fiber analysis), SEM inspection and Fourier-transform infrared spectroscopy (FTIR).

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Finally, the pyrolysis of the olive endocarp could be mathematically modeled, from thermogravimetric analysis, concluding that this process requires higher temperatures to remove volatile materials of the biomass, in comparison with the SHTC.

KEYWORDS: Olive fruit endocarps; Hydrothermal carbonization; Hydrochar; Fuel quality; Pyrolysis simulation.

INTRODUCTION The residual lignocellulosic biomass is an energy resource that arouses an increasing interest due to factors such as its renewable nature, wide geographical distribution, low prices at resource and environmental advantages, compared to fossil fuels, in relation to the problem of global warming. Numerous authors have studied the potential of residual lignocellulosic biomass as an energy source.1,2 At present, approximately 15% of the primary energy consumed in the world is obtained through renewable sources, although this percentage is likely to increase in the future and in the European Union (EU) it should reach, at least, 27% in 2030.3 This can only be achieved by an efficient utilization of the renewable energy resources available in each geographic area, including residual lignocellulosic biomass. The thermochemical conversion of green biomass, through combustion or gasification, presents some drawbacks, among which its low calorific value, high content of volatile materials and ash, and high capacity for moisture adsorption. These problems could be tackled by subjecting the materials to different types of treatment. The hydrothermal carbonization (HTC) of the biomass is a treatment consisting of heating biomass in the presence of water between 180º C and 250 ºC.4 In this way,


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certain components (aqueous extractives, hemicelluloses...) are totally or partially removed from the resulting solid (hydrochar), increasing its calorific value. The HTC is developed through a complex set of chemical reactions that involve depolymerization of polysaccharides and lignin, thermal degradation of monosaccharides and even backpolymerization. The solid residue from hydrothermal conversion of biomass can result from two different reaction pathways.5 The first one, via solid state, produces “primary char”, while in the second reaction pathway the solid residue (“secondary char”) results from polymerization in the fluid phase of molecules previously released from the parent material into such phase. The secondary char shows a sphere-like geometry.6–8 The HTC of biomass, compared to the pyrolysis (900 ºC)9 or even to the dry torrefaction (300 ºC),10 could remove from the biomass those fractions of lower calorific value working at lower temperatures, which would be energetically beneficial. The improvement in the energy densification must take place looking for a balance between the increase of the calorific value and the decrease in the hydrochar yield in order to the energy yields are not too low. Thus, Zhang et al.11 indicated that the optimal temperature for the HTC of corncob residues was that (230 ºC) that maximized the energy yield, achieving values above 70%. The modifications experienced by the hydrochars cause changes in their behavior in combustion that could be studied by thermogravimetric analysis in air. In this way, data such as the ignition temperature (Ti), the shutdown temperature (Tf) or the comprehensive combustibility index (S) could be determined.12 On the other hand, the hydrochars produced by HTC are generally more hydrophobic than the original biomasses and, therefore, contain lower amounts of adsorbed water, which reduces energy losses during the combustion process and it improves the biological stability of the material during storage.13


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The olive grove is a very widespread crop in different countries with a Mediterranean climate. In Spain, 2.6 million hectares were destined to this crop in 2016.14 This fact allowed to reach an olive production of close to 8 million tons. The industrial processing of olives produces olive oil and table olives. In both cases, the removal of the olive stone is a simple operation that generates large amounts of endocarps, which could be energy-enhanced using thermochemical routes.15,16 HTC of olive industry residues is quite developed in literature,17–19 although the hydrothermal carbonization of olive endocarps is yet a poorly researched process. Using olive stones, Álvarez-Murillo et al.20 studied the effect of the ratio biomass/water, temperature and residence time (or holding time) on the hydrochar yield and the higher calorific value (HHV) of the solids resulting from the process. In the work, significant increases of the HHV were achieved but using very high holding times (20 h) and reaching low hydrochar yields (39.8%), which means energy yields close to 55%. A holding time of 20 h, in practice, imply working with a very voluminous and expensive reactor, so it could be interesting to study how the HTC of the olive endocarps takes place using holding times of a few minutes (short-time hydrothermal carbonization, SHTC). The main objective of the present work is to analyze the combustible properties of the hydrochars obtained by submitting olive endocarps to SHTC, with holding times at the maximum temperature between 0 and 10 minutes. Information of great practical interest, not found in bibliography, related to combustion behavior of hydrochars has been included (Ti, Tf and S), as well as FTIR and SEM determinations on these biofuels to clarify the changes experienced by the raw material during the SHTC treatment. Also, this study compares the effect that hydrothermal carbonization and pyrolysis have on the main solid fractions that make up the olive endocarp (hemicellulose, cellulose and lignin) in order to verify which treatment allows to eliminate low-calorific


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components at lower temperatures. To do this, the pyrolysis of the raw material will be modeled mathematically via thermogravimetric analysis. Comparison between dry and wet pyrolysis of lignocellulosic material has been already reported,21,22 however this comparative study has still not been carried out using exclusively olive-fruit endocarps.

MATERIALS AND METHODS Raw Material. Fragmented olive-fruit endocarps from the fruit variety ‘Picual’ were provided by the oil processing company ‘S.C.A. San Juan’ located in the province of Jaén, Spain. The biomass was air-dried at room temperature to equilibrium moisture content and endocarps were milled using a laboratory blade mill (Retsch GMBH, Germany). The resulting solids were sieved to a particle size of 0.300–0.425 mm to minimize the effects of heat and mass transfer limitations during the hydrothermal process. Finally the biomass was washed with destillated water (solid/liquid ratio of 1:4 based only on dry weight) for 20 minutes at 20 ºC, air-dried, homogenized in a single lot, and stored in a dark and dry place until use. In order to name this last material, from now the term 'raw material' or the initials FWOE (fragmented and washed olive-fruit endocarps) will be used. Short-time Hydrothermal Carbonization (SHTC). The hydrothermal experiments were carried out in a batch reactor (Parr model 4522, Moline, IL, USA). In each experiment, the reactor was charged with 50 g of FWOE and 300 mL of distilled water. The tests were carried out without introducing any gas for the elimination of oxygen from the reactor because it was considered that the liquid water covering the solid, would limit the oxidation reactions of the biomass. For this same reason, it was avoided using inertization gases (N2, Ar...) that would make the process more expensive. After


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the hermetic closure of the reactor, its interior was heated at a speed of 5.8 ± 0.1 ºC/min (Figure 1). To study the effect of the reaction temperature, the biomass-water system was heated up to reaching six different temperatures in the range 175–250 °C. When the desired temperature was achieved, the reactor was quickly removed from the heating system and cooled in a water batch to below 100 °C in less than 10 min (Figure 1). The treated solids (hydrochars) were separated from liquids by vacuum filtration. Solids were washed with distillated water (2000 mL), air-dried at room temperature until the equilibrium moisture content was achived. Finally, they were weighed and kept for further analysis. The hydrochars were designated as HC-175, HC-190, HC-200, HC210, HC-225 and HC-250 in accordance with the reaction temperature of 175, 190, 200, 210, 225, and 250 ºC, respectively. With a view to study the effect of holding time in short-time hydrothermal treatments, two experiments were carried out in which the reactor was maintained at 225 ºC for 5 and 10 minutes, and whose hydrochars were designated HC-225-5 and HC-225-10, respectively. The hydrochar yield (HY) was calculated according to eq. (1).

Hydrochar yield (HY) 

Mass of dry hydrochar 100 Mass of dry FWOE

(1)

Figure 1

Analysis of Structural Components. The structural composition of both the raw material and the hydrochars was determined in duplicate by gravimetric methods: moisture (TAPPI T 12 os-75), acid-insoluble lignin (AIL, %) by TAPPI T 222 os-74, and neutral detergent fiber (NDF, %) and acid detergent fiber (ADF, %) by the method


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of Van Soest and Robertson.23 The percentages of hemicelluloses and cellulose were calculated from the AIL, NDF and ADF values according to eqs. (2) and (3). Hemicelluloses (%) = NDF – ADF

(2)

Cellulose (%) = ADF – AIL

(3)

Higher Heating Value and Thermal, Ultimate and Proximate Analyses. Higher heating value (HHV) of FWOE and hydrochars was determined conforming to ASTM Standard method D5865 by using a Parr calorimeter, model 6400 (Moline, IL, USA). The thermal behavior of solid samples was evaluated by TG analysis making use of a differential thermal analyzer (TDA/SDTA 851e, Mettler Toledo, USA). The TG curves were acquired by heating the sample from ambient temperature to 900 °C at a constant heating rate of 10 °C/min. Air gas flow of 150 mL/min was used as the purge gas to provide an oxidative atmosphere for combustion assays, while N2 gas flow of 150 mL/min was used to provide an inert atmosphere for pyrolysis experiments. Tests in oxidative atmosphere were also carried out using a differential scanning calorimeter (DSC 822e, Mettler Toledo, USA). Two characteristic temperatures of combustion, the ignition temperature (Ti) and the shutdown temperature (Tf), were determined by thermogravimetic analysis in air using the methodology exposed by He et al.24 (to determine Ti), and by direct reading of the temperature for which the thermal flow passes from exothermic to endothermic at the end of combustion (to Tf). Comprehensive combustibility index (S) was used to evaluate the reactivity of samples.8 S values were calculated using the eq. (4).

S

dw / dt max dw / dt mean T 2T i f

(4)


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Where (dw/dt)max and (dw/dt)mean represent the maximum and mean rates of weight loss (wt.%/min), respectively. The carbon, hydrogen, nitrogen and sulfur percentages in the samples were determined in duplicate according to ASTM Standard method D5142-09 by using a CHNS elemental analyzer (Thermo Finnigan Flash EA1112, Waltham, MA, USA). The oxygen content was calculated by difference of the ash and CHN content from the total. Ash content was determined in triplicate by gravimetric analysis (TAPPI T 15 os-58). Volatile matter and fixed carbon contents were determined by the ASTM D 5142-02a methods. SEM Observations and FTIR Analysis. The surface morphology of the samples was examined using a high resolution scanning electronic microscope (FE-SEM, Merlin from Carl Zeiss, Oberkochen, Germany). Before that, the samples were metallized with carbon for their observation. FTIR spectra were recorded on a spectrometer (Vertex 70, Bruker, Billerica, MA, USA) using the KBr disc method, with a resolution of 4 cm–1 and 100 scans. Equilibrium Moisture Content. The equilibrium moisture content (EMC) of all samples was measured by the static desiccators’ technique. For each test, an amount of approximately 3 g of the material to be tested was first spread on a glass Petri dish and dried at 105 ºC for 24 h. Next, the dry solids were exposed to an environment with constant humidity and temperature over a long period of time, until the moisture in the solid reached an equilibrium value. The humidity in the chamber was maintained at a constant value (50%) by keeping the air at 35 ºC in equilibrium with an aqueous solution that was saturated with magnesium nitrate.25 Three replicates were measured for each set of solids.


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Pyrolysis of Olive-fruit Endocarps. The pyrolysis process of the raw material was modeled mathematically from the thermogravimetric data obtained under a nitrogen atmosphere. The proposed model, previously used for other biomasses,26 considers that the weight change endured by the FWOE can be interpreted by the sum of the gravimetric changes suffered by the fractions of hemicellulose, cellulose and lignin, according to the three reactions described by the eqs. (5–7). In our work, the calculations were made at temperatures above 100 °C to eliminate from the model the effect associated with the loss of free water from the sample. In order to symplify the model, extractives were incorporated in the hemicellulose components, so these two fractions were added and are hereafter designated as hemicellulose. 1 Hemicellulose  1 Volatiles1 + 1 Char1

(5)

2 Cellulose  2 Volatiles2 + 2 Char2

(6)

3 Lignin  3 Volatiles3 + 3 Char3

(7)

In the eqs. (5–7) 'Volatilesi' and 'Chari' (i = 1–3) represent the gaseous compounds and the solid formed by the degradation of each reagent (Hemicellulose, α1; Cellulose, α2; and Lignin, α3) and the Greek letters β and γ are the yield coefficients of each reaction, namely the maximum weight obtainable by each reaction. Thus, βi and γi (i = 1–3) would give the mass fractions of volatile and carbonaceous wastes, respectively, that would be obtained at infinite time by pyrolysis of the three fractions considered. Assuming that each fraction is transformed according to a kinetic of n-th order and the velocity constant is modified with the temperature considering an Arrhenius type equation, the eqs. (8–10) would describe how the fractional conversion of each


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biopolymer (X, ratio between the mass of solid reacted at any time and the corresponding initial mass of this solid reactant) is modified throughout the pyrolysis.   E1  dX 1   1  X 1 n1  k 10  exp dt  R T 

(for hemicellulose pyrolysis)

(8)

  E2 dX 2  k 20  exp dt  R T

(for cellulose pyrolysis)

(9)

(for lignin pyrolysis)

(10)

   1  X 2 n 2 

  E3  dX 3   1  X 3 n 3  k 30  exp dt  R T 

Where ki0 and Ei are, respectively, the pre-exponential factor and the apparent activation energy of each reaction (i = 1–3), R is the universal gas constant and T is the absolute temperature. The total material balance leads to equation (11), which enables the calculation of the weight loss of solid along the known pyrolysis fractional conversions (Xi) and coefficients β.

wcalculated = 1 – (β1·X1 + β2·X2 + β3·X3)

(11)

The determination of the kinetic parameters was carried out applying the following steps: a. To assume values for ki0, Ei, ni y i, (i = 1–3) b. To integrate the equations (8–10) by the explicit Runge–Kutta 4th order method, and to determine wcalculated by the equation (11). c. To calculate the following la siguiente goal function, eq. (12),

O.F. 

 w m

n

1

1

exp mn

calc  wmn



2

(12)


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d. To minimize the goal function using the optimization method of the function Solver in a Microsoft Excel spreadsheet. To indicate the goodness of fit, we used the statistic χ2, eq. (13). 2 = O.F./(N–P)

(13)

Where N and P indicate, respectively, the number of points and fitted parameters.

RESULTS AND DISCUSSION Effect of Short-time Hydrothermal Carbonization on Chemical Structure of Biomass. The composition (dry basis) of FWOE used in this study was 25.4  1.4% hemicelluloses, 37.0  1.9% cellulose, 32.7  0.6% acid-insoluble lignin, 0.71  0.04% ash, and 4.2% extractives (calculated by difference). In general, an increase in SHTC temperature increased the decomposition of biomass components. This behavior can be observed in Figure 2a where a decrease in hydrochar yield (HY), from 96.6% to 61.3%, is shown when increasing reactor temperature from 175 ºC to 225 ºC. However, the HY value remained practically constant when the temperature rose from 225 °C to 250 °C (HY = 61.5% for the last temperature). This behavior could be explained by the formation of secondary char. Lucian et al.7 reported, for the hydrothermal carbonization of the organic fraction of municipal solid wastes, that between 240 and 280 °C the back polymerization of organics from the liquid aqueous phase to the solid phase becomes more and more important compensating the mass loss from the primary char and thus compensating the HY value. When the temperature was kept at 225 ºC and the holding time was increased from 0 to 10 minutes, a slight drop in HY from 61.3% to 59.9%, were observed (Figure 2b). The recovery solid percentages in tests carried out at the two


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highest temperatures (225 ºC and 250 ºC) were consistent with those data obtained in other works,27,28 confirming hydrochar yields of 58.4% and 58% for hydrothermal treatments of almond-tree prunings at 230 ºC × 5 min and birch at 225 ºC × 30 min, respectively. The loss of solid material during the HTC can be mostly attributed to hemicelluloses and extractives decomposition. The hemicelluloses percentage in hydrochars dropped lightly, from 23.0% to 19.1%, when the maximum reaction temperature increased from 175 ºC to 200 ºC (Figure 2a). However, the loss of the polysaccharide was intensified above 210 °C, so that almost no hemicelluloses were detected in the hydrochars HC-225 and HC-250 (Figures 2a and 2b). Cellulose and lignin suffered less changes in comparison with hemicellulose during hydrothermal treatment. This fact caused, in general, an increase in the percentages of cellulose and lignin when the temperature was increased. This is clearly shown in Figure 2a for treatment temperatures of 175 ºC and 250 ºC, where AIL percentages of 34.6% (HC175) and 51.3% (HC-250) were reached. The content in acid-insoluble lignin also increased, from 44.0% to 54.2%, when the reactor temperature was kept at 225 ºC and the holding time was varied from 0 to 10 minutes. Calculating the recovery percentage of AIL (as grams of AIL in hydrochars per 100 grams of AIL in FWOE) it was observed that its value was higher than 97% for the SHTC processes carried out between 175 ºC and 210 ºC (Figure 2a). However, the parameter fell rapidly, to 82.3%, for the treatment developed at 225 °C with holding time of 0 min and then increased for the hydrochars obtained in more severe conditions: 91.2% (HC-225-5), 99.1% (HC225-10) and 96.4% (HC-250). The complex behavior of lignin during hydrothermal treatments can be explained by the existence of reactions of depolymerization and repolymerization. In this sense, Trajano et al.29 reported that lignin-carbohydrate


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interactions significantly influence lignin deconstruction during a hydrothermal treatment. It is interesting to point out that the maximum recovery percentage of AIL at 225 °C was achieved with a short holding time, 10 minutes, which was essential to complete the aforementioned repolymerization reactions. On the other hand, the percentage of cellulose in hydrochars increased with SHTC temperature from 41.8% (HC-175) to a peak value of 55.9% (HC-225), and then it decreased till 52.7% (HC250) might indicate the beginning of a significant hydrolysis of cellulose. The drop of cellulose content in hydrochars was also observed when raw material was treated at 225 ºC and the holding time was increased from 0 to 10 minutes (Figure 2b). In a previous work, Zhang et al.11 informed that the hydrothermal hydrolysis of microcrystalline cellulose begins to be significant at 230 ºC. Figure 2

Ultimate and Proximate Analysis and Higher Heating Value. The elemental analysis carried out on FWOE and the hydrochars did not detect the presence of sulfur in any sample. In general, as can be seen in Table 1, the increase in temperature and time of the SHTC process did not affect appreciably the hydrogen and nitrogen percentages of the hydrochars, but it did lead to an increase in its carbon percentage and a decrease in the oxygen content. The determined carbon content was maximum in case of the solid HC-225-10. According to the proximate analysis, the SHTC treatment applied on olive endocarps increased the percentage of fixed carbon and decreased the percentages of volatile matter and ash of the resulting solids, which would benefit the combustion of the biomass. Thus, the percentages of fixed carbon of the hydrochars HC-225-10 and


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HC-250 were 77.26% and 82.16% higher, respectively, than the fixed carbon of the FWOE (Table 1). In relation to the ash content, a minimum value of 0.14% was reached for hydrochars HC-225-5 and HC-250. The reduction of ash by hydrothermal treatment, has been highlighted by different authors who relate the phenomenon to metallic cation leaching processes.28 Table 1 Energy density ratio (EDR) in Table 1 was calculated by dividing the HHV of each sample between the HHV of the raw material, while the energy yield was gotten multiplying the EDR by HY. The hydrothermal treatment carried out at 225 °C with a holding time of 10 min achieved the highest values of HHV (23.43 MJ/kg) and EDR (1.25), reaching a local maximum in energy yield (74.7%). These maximum values could be explained taking into account that lignin is the structural material with the highest calorific value and that in hydrochar HC-225-10 a local maximum was reached in the recovery of AIL by repolymerization reactions. The values obtained at 225 ºC-10 min can be compared with those obtained by Kambo and Dutta13 applying rapid hydrothermal carbonization (225 ºC for 15 min) on Miscanthus giganteus, a raw material with HHV value (18.47 MJ/kg) similar to the FWOE. The hydrochar produced from Miscanthus had HHV and energy yield values of 23.0 MJ/kg and 76.5%, respectively. Thermal Analysis. The TG-DTG-DSC analysis carried out in an oxidizing atmosphere is an interesting technique to evaluate the behavior of the biomass when it is subjected to combustion. Figure 3a shows the DTG curves for the FWOE and three hydrochars (HC-225, HC-225-10 and HC-250) in the range 150–600 ºC. The data for the remaining temperatures are not shown because above 600 °C no relevant signals appeared, while


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below 150 °C the signals were due to the drying of the solids. Furthermore, Figure 3b shows the thermal flows of exothermic character obtained by combustion of the previous biomasses. The analysis of the DTG curves showed (Figure 3a) that the combustion of all the analyzed samples took place in two different zones that would correspond to the homogeneous combustion of the volatile materials released from the biomass (zone 1, 150–400 ºC) and the heterogeneous combustion of chars originated in the first stage (zone 2, 400–550 ºC). For the FWOE, two peaks are visible in the first combustion zone, which could be associated with the volatilization of hemicelluloses (6.41%/min at 277 ºC) and cellulose (6.21%/min at 313 ºC). Hemicelluloses are more readily decomposed than cellulose in thermal pyrolysis,30 which means that the DTG peak of cellulose decomposition appears at higher temperatures than hemicellulose DTG peak. For the FWOE in the combustion zone 2, two peaks also appeared, at 439 ºC and 467 ºC, which could correspond to the heterogeneous combustion of the chars generated from volatilization of polysaccharides and lignin fractions, respectively. Figure 3

The SHTC treatments caused important changes in the combustion properties of the hydrochars. For zone 1, the peak associated with the hemicelluloses (277 ºC) it became a small shoulder for the HC-225 solid and practically disappeared in the materials HC-225-10 and HC-250 (Figure 3a), which is according to the results on hemicelluloses losses previously discussed. The increase of the temperature and the holding time of the SHTC treatment intensified the signal of the peak associated with the volatilization of cellulose (around 313 ºC) going from a value of 6.21%/min (FWOE) to 14.76%/min (HC- 225) and 15.40%/min (HC-225-10). However, the hydrothermal carbonization carried out at 250 °C caused a decrease in the signal (12.2%/min for HC-250). It is interesting to note that in the DTG curve for hydrochar


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HC-225-10 a peak was visible at 339 °C (5.67%/min) which increased in intensity and shifted slightly towards lower temperatures for the HC-250 solid (7.15%/min at 331 ºC). This signal, which remains hidden under one shoulder for hydrochars obtained below 225 °C, could correspond to the partial volatilization of the secondary char. In relation to the second combustion zone, corresponding to the heterogeneous oxidation of the chars, important changes of the DTG curves were also observed (Figure 3a). The peak at 439 °C visible to FWOE became a small shoulder for the hydrochars, possibly due to the change of non-volatile cellulose to volatile cellulose as a consequence of the SHTC treatment, which caused the mass degradation to move from zone 2 (around 439 ºC) to zone 1 (around 313 ºC). The DTG peaks that appear in zone 2 above 450 ºC could be related to the heterogeneous combustion of aromatic structures. Figure 3a indicates that increases in SHTC temperature make the hydrochars more reactive during the char combustion stage, leading to an increase in the DTG signals, especially for the hydrochar HC-250 (8.40%/min at 467 ºC). For this hydrochar it was also observed that its DTG peak suffed a shift to the left. The behavior of these signals could be related to the formation of secondary char during hydrothermal treatment. Thus, Lucian et al.7 informed that the hydrothermal carbonization of solid urban wastes at 240 ºC maximized the formation of a secondary char of high reactivity in combustion. This material could lose its structural stability at higher temperatures, 6 which could explain the shift to the left of the DTG peak for the hydrochar HC-250. Regarding to the net energy flows associated with the combustion of biomasses (Figure 3b), there was a displacement of signals from zone 2 of combustion to zone 1. Thus, for the FWOE, the net emission of heat in the area 1 was low (23% of the total), manifesting as a small peak (3.88 W/kgdry solid) at 329 ºC, while in the second combustion zone the greatest amount of heat was released reaching a maximum thermal


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flow of 10.56 W/kgdry solid at 437 °C (Figure 3b). However, for the solids HC-225, HC225-10 and HC-250 the net energies released in zone 1 represented 60.9%, 63.0% and 54.3% of the total issued, respectively. The maximum emission peak in zone 1 (14.80 W/kgdry solid at 337 ºC) was reached for hydrochar HC-225-10, which would be the material with the highest volatile cellulose content. Table 2 was obtained from the data provided by the TG analysis, which includes some characteristic parameters of the combustions of both FWOE and six hydrochars. The FWOE had the lowest value of ignition temperature (251.4 ºC), García Torrent et al.31 obtained, for olive pit, a Ti value (248.55 ºC) very similar to the one obtained in this work. The increase of the temperature of the SHTC treatment caused the elevation of Ti, reaching the maximum values of 299.9 ºC and 299.0 ºC for hydrochars HC-225-5 and HC-225-10, respectively. The increase in Ti is beneficial, since it implies a lower risk of self-ignition of the biomass. However, hydrochar HC-250 had a lower value of Ti (294.5 ºC), which could be due to the intense destruction of the biomass structure at 250 ºC, which generated new materials easier to volatilize than those produced at 225 ºC. The shutdown temperature (Tf) showed an evolution similar to that of Ti, reaching again a maximum value for the hydrochar HC-225-5. From the values of Ti and Tf it was possible to determine the holding time necessary for the combustion of the biomass, obtaining values between 24.6 min (HC-175) and 20.4 min (HC-250). Finally, the comprehensive combustibility index (S) was calculated according to what is established in equation (4). The data in Table 2 show how the FWOE and the solid HC-175 presented the lowest values of S, 3.80 × 107 min–2 °C–3 and 3.77 × 107 min–2 °C–3 respectively. For the rest of the hydrochars, the S values increased with the increase of the temperature and the holding time of the SHTC treatment until a maximum of 6.19 × 107 min–2 ºC–3 was obtained for the solid HC-225-10, while for the material HC-250 the


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value of S dropped to 5.25 × 107 min–2 °C–3 as a consequence of the strong decrease in the value of (dw/dt)max (Table 2). Table 2 Hydrophobicity of Hydrochars. One of the most important advantages of HTC process is that hydrochars can be more hydrophobic materials than original biomasses. This behavior makes that treated biomass can have a long-term storage without suffering biological degradation and, besides, it could be burned with higher reactivity and thermal efficiency. The equilibrium moisture content (EMC, calculated as mg of adsorbed water per gram of dry solid) can be used as indicator of hydrophobicity of a solid material.32 The effects of SHTC temperature and holding time on the hydrophobicity of hydrochars are represented in Figure 4, which shows that all tested samples approach their equilibrium moisture contents before 216 h. The EMC values decreases with either increasing SHTC temperature or holding time. Thus, EMC data for raw material was 81.2 mg water/g dry solid whilst equilibrium moisture contents for hydrochars HC-175, HC-200, HC-225 and HC-250 were equal to 80.2, 69.4, 48.5 and 32.5 mg water/g dry solid, respectively. Another way, when the temperature was kept at 225 ºC and the reaction time increased from 5 to 10 minutes, EMC values of 40.4 and 37.7 mg water/g dry solid were obtained, respectively (Figure 4). These values are in accordance with wet basis moisture content values, between 7.5% (for raw material) and 3.1% (for HC-250), and they highlight the strong effect of hydrothermal carbonization on the surface morphology of biomass.

Figure 4


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FTIR Analysis and SEM Observations. The FTIR analysis applied to the hydrochars shows the chemical changes experienced by the original biomass during the SHTC treatment, which could be related to the changes in its composition and hydrophobic character. The FTIR spectra of the FWOE and hydrochar HC-175 were practically the same (Figure 5), however the materials produced with hydrothermal treatments above 175 °C underwent changes in the intensity and position of some spectral peaks. The wide band between 3730 and 3050 cm–1 (Figure 5, position 1) is usually associated with strain vibrations in O-H bonds, which could come from both the water in the samples and the bonds present in groups functional hydroxy and carboxy.33,34 The increase in temperature and hydrothermal carbonization time caused a loss of intensity and symmetry of the band, which could be indicating the elimination of hydroxyl groups of the hydrochars, and with it, an increase of its hydrophobic character. In the FTIR spectra, three other signals were found (positions 3, 7 and 9) that could be related to functional groups of hemicelluloses and celluloses. a. Position 3 (1733 cm–1): This band could be attributed to vibrational stretching of the C=O present in acetoxy groups of xylans.35Above 175 ºC the band intensity decreased as the temperature and holding time increased, which agrees with the previously set out loss of hemicelluloses. The SHTC process carried out at the highest temperatures caused the appearance of a new band around 1705 cm–1, just on the right of position 3 for HC-225-10 and HC-250, which may be due to the generation of new materials as a result of condensation reactions between degradation products from carbohydrates and lignin.36 b. Position 7 (1236 cm–1): The signal, which could be associated with C-O-C bonds of acetyl groups present in hemicelluloses,37 weakens with the increase in the reaction temperature.


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c. Position 9 (1032 cm–1): This signal has been related by different authors with the C-O bond in alcohols present in polysaccharides.38 Although it does not disappear, the peak at 1032 cm–1 weakens with the increase in the severity of the SHTC treatment (Figure 5). Figure 5 The spectral bands for the raw material indicated in Figure 5 by positions 2 (3000–2800 cm–1), 4 (1593 cm–1), 5 (1506 cm–1), 6 (1474–1404 cm–1) and 8 (1111 cm– 1)

would show changes suffered by lignin.34–39 In general, an increase of all the signals

was observed with the increase in temperature and reaction times, which would be in correspondence with the increases of the percentage of AIL of the hydrochars previously discussed. SEM inspection of the solid materials resulting from SHTC process applied on FWOE revealed substantial changes in the surface morphology of the hydrochars (Figure 6). The hydrochar produced at low temperature (HC-175) maintained a welldefined structure (Figure 6a), formed by geometric cells of approximately 20 μm in width. Figure 6b was performed with the hydrochar HC-225 and shows how the total decomposition of the hemicellulose, together with the possible thermal softening of lignin, caused a great alteration of the external biomass structure, although they are still perceptible the edges of some cells of the primitive cell wall. Figure 6b also shows the formation of hydrochar microspheres of various sizes and shapes. In solid HC-225-10 (Figure 6c) there are no morphological features of the original biomass structure. Comparing figures 6b and 6c, it can be seen that at 225 °C the rise in the holding time, from 0 to 10 minutes, causes an important increase in the diameter of the microspheres, which in some cases exceeded 40 μm. Volpe et al.6 observed that, throughout the


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hydrothermal carbonization of olive trimmings and olive pomace at 250 ºC during 30 minutes, microspheres whose size range varied from few microns to to hundred of microns were formed. The Figures 6b and 6c show how other new smaller microspheres that favor the growth of the hydrochar when fused with it were settling on the surface of these microspheres. These new materials could be produced by reactions in the liquid phase among phenolic compounds40 or compounds of degradation of monosacaharides.8 Hydrochar HC-250 (Fig. 6d) had many surface smooth bubbles with diameter mostly less than 2 µm. The production of this type of microspheres to 250 ºC might be explained by the significant hydrolysis that suffers the cellulose to this temperature. From the glucose, via hydroxymethyl-furfural, different authors have explicated the formation of microspheres of char secondarily.8.41 Figure 6

Hydrothermal Carbonization vs Pyrolysis. Comparison of Effects on Structural Components of Olive-fruit Endocarp. To compare the losses of hemicellulose, cellulose and lignin caused by pyrolysis and hydrothermal carbonization, the pyrolysis process of the raw material was modeled mathematically according to the method previously described in Materials and Methods section. Extractives were incorporated in the hemicellulose components and in this way the recalculated percentages of hemicellulose, cellulose and lignin in the FWOE were 29.81%, 37.26% and 32.93%, respectively. Therefore, α1, α2 and α3, in the eqs. (5–7), were 0.2981, 0.3726 and 0.3293, respectively. The kinetic parameters offered by Microsoft Excel are shown in Table 3, while in Figure 7a the experimental data are compared with those determined by applying the


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theoretical model, appreciating a remarkable convergence between both, obtaining a value of χ2 equals to 9.31·10–5. Blázquez García et al.26 working with two-phase olive mill solid waste, reported apparent activation energies of 198.7, 129.2 and 65.7 kJ/mol for the pyrolysis of hemicellulose, cellulose and lignin fractions, respectively. These values are close to those achieved in our study, except for the case of cellulose. However, apparent activation energies between 150 and 250 kJ/mol have been reported in the literature for the cellulose fraction,42 and between 18 and 65 kJ/mol for lignin.43 The values of β in Table 2 would come to show the great capacity of lignin to generate char, while most of the hemicellulose (98.1%) and cellulose (86.0%) would be transformed into volatile materials. For lignin, the high order of reaction (7.46) could indicate that this fraction, of very complex aromatic structure, is transformed through a mechanism in which multiple chemical reactions would intervene, while the low value of the pre-exponential factor (3.407 · 103 s–1) would indicate a low reaction rate. Table 3 From Table 3 parameters and eqs. (8–10) of the model, it was possible to describe the behavior of each biomass fraction during the pyrolysis of the raw material (Figure 7b, 7c and 7d). In the figures, a distinction is made between amounts of unreacted biopolymer and amounts of volatile matter and char generated. Hemicellulose suffers a rapid transformation to volatile materials, while cellulose, and especially lignin, slowly pyrolyzes, generating greater quantities of char. Comparing total gravimetric recovery data gotten at 225 °C from Figures 2a and 7a (61.3% and 2%, respectively), the greater capacity of the hydrothermal carbonization to eliminate components of the raw material at low temperatures compared to pyrolysis has been checked. Thus, for example, the hemicellulose fraction of the olive endocarp disappeared around 400 °C using pyrolysis (Figure 7b), while rapid hydrothermal


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carbonization allowed to achieve this same objective at significantly lower temperatures (225 °C, Figure 2b). Similar results have been reported when comparing HTC and pyrolysis of olive waste by Volpe et al.21 Figure 7

CONCLUSIONS This work provides detailed information on the rapid hydrothermal carbonization of the endocarps of the olive fruit. It has been shown that the modification of holding time, in the 0–10 min interval, significantly affects the quality of the hydrochars. Thus, at 225 °C, the recovery yield of AIL increased considerably, due to repolimerization reactions, with the increase in holding time until reaching a value close to 100% for times of 10 minutes. The hydrochar HC-225-10 showed the maximum values of HHV (23.43 MJ/kg), EDR (1.25) and energy yield (74.69%), even higher than those found for the solid produced at 250 ºC. Also, with the hydrochar HC-225-10 important improvements were obtained when parameters of great interest in combustion were determined, such as ash content and moisture adsorption capacity, which were 80.3% and 60.0% lower, respectively, than the raw material. In addition, the material HC-225-10 presented the highest value of comprehensive combustibility index, which would indicate that it is the material with the best combustion characteristics. The study has proven the greater capacity of hydrothermal carbonization versus pyrolysis, to eliminate the components of reduced calorific value (extractives and hemicelluloses) at low temperatures from the raw material.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel. +34 953648541 ORCID© Manuel Cuevas Aranda: 0000-0003-4160-2174 Mª Lourdes Martínez Cartas: 0000-0001-8175-5269 Sebastián Sánchez Villasclaras: 0000-0003-1993-0227 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors thank the technical and human support of the CICT of the University of Jaén (UJA, MINECO, Junta de Andalucía, FEDER), as well as the oil processing company 'S.C.A. San Juan' (Jaén, Spain) for the supply of olive endocarps.

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Figure Captions

Figure 1: Temperature profile of the reactor for four SHTC processes:  190 ºC, 0 min;  225 ºC, 0 min;  225 ºC, 5 min;  225 ºC, 10 min


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Figure 2. Short-time hydrothermal carbonization of FWOE: Effect of reaction temperature (a) and holding time (b) on hydrochar yield (HY, ), hydrochar composition (percentages of AIL , cellulose , and hemicelluloses ) and AIL recovery (). AIL: acid-insoluble lignin.

Figure 3. DTG curves (a) and exothermic heat flow curves from DSC analyses (b) for FWOE and hydochars under an oxidative atmosphere.

Figure 4. Moisture adsorption curves for original and hydrothermal treated olive fruit endocarps (raw material, ; HC-175, ; HC-200, ▲; HC-225, ; HC-225-5, ; HC-225-10, ; HC250, ).

Figure 5. FTIR spectra of FWOE (fragmented and washed olive-fruit endocarps) and hydrochars HC-175, HC-200, HC-225, HC-225-10 and HC-250.

Figure 6. SEM images of hydrochars HC-175 (A), HC-225 (B), HC-225-10 (C) and HC-250 (D).

Figure 7. Simulation of the evolution of the total mass (a), hemicelluloses (b), cellulose (c), and lignin (d) during the FWOE pyrolysis at heating rate of 10 °C/min.

Table Titles

Table 1. Ultimate/Proximate analysis and Energy yield of original and treated olive-fruit endocarps (on dry basis)


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Table 2. Combustion parameters of FWOE and hydrochars. Table 3. Kinetic parameters obtained for the pyrolysis of olive endocarps according to the model proposed.

Footnotes

Table 1: aSulfur

was not detected. VM: volatile matter; FC: fixed carbon; HHV: higher heating value. EDR: Energy Densification Ratio; nd:

not determined. Table 2: a

Ti, ignition temperature (ºC). Tf, burnout temperature (ºC). c (dw/dt) max and (dw/dt)mean, maximum and mean rate of weight loss, respectively (wt.%/min). d S, comprehensive combustibility index (min–2 ºC–3). b


32

Page 33 of 42

Figure 1

250 200 Temperature (ºC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

150 100 50 10

20

30 40 50 Time (min)

60

70


33

Energy & Fuels

Figure 2

a 90

60

80

40

70

20

60

0

50

E C17 H 5 C19 H 0 C20 H 0 C21 H 0 C22 H 5 C25 0

80

AIL recovery (%)

100

H

FW

O

HY and composition (%)

100

Sample

100

100

80

90

60

80

40

70

20

60

0

AIL recovery (%)

b HY and composition (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 42

50 HC-225

HC-225-5 HC-225-10 Sample


34

Page 35 of 42

Figure 3

16

a

FWOE HC-225 HC-225-10 HC-250

313 ºC

DTG (%/min)

12

467 ºC

339 ºC 8

277 ºC 439 ºC

485 ºC

4

0 150 200 250 300 350 400 450 500 550 600 Temperature (ºC) 16 Heat flow (W/kg dry solid)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

12

b

FWOE HC-225 HC-225-10 HC-250

8

4

0 150 200 250 300 350 400 450 500 550 600 Temperature (ºC)


35

Energy & Fuels

Figure 4

90 Adsorbed water (mg/g dry solid)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 42

60

30

0 0

50

100 150 Time (h)

200

250


36

Page 37 of 42

Figure 5

1

2 Raw material

Absorbance (u.a.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3456 789

HC-175 HC-200 HC-225 HC-225-10 HC-250

3700

3100

2500

1900

1300

700

Wavenumber (cm-1)


37

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 42

Figure 6

a

20m m 20

b

20 m

c

d

2 m

40 m


38

Page 39 of 42

Figure 7

Char1

a

w: [VALOR DE Y]

1

0.4

b

Hemicell ulose

Yield (g/graw material)

weight fraction (w) = m/m0

1.2

0.8 0.6 0.4 0.2

0.3 Volatiles1

0.2 0.1

225 ºC

0

0 100

0.4

300 500 700 Temperature (ºC)

Cellulose

100

900

300 500 700 Temperature (ºC)

0.4

c

0.3


Volatiles2

0.2 0.1

Char2

900

d

Lignin


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.3 0.2

Volatiles3

0.1 Char3

0

0 100

300

500

700

900

100

Temperature (ºC)


300

500

700

900

Temperature (ºC)

39

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 42

Table 1

Table 1. Ultimate/Proximate analysis and Energy yield of original and treated olive-fruit endocarps (on dry basis) Hydrochars FWOE Ultimate analysis (%)a

Proximate analysis (%)

HHV (MJ/kg) EDR Energy yield (%) aSulfur

HC-175

HC-190

HC-200

HC-210

HC-225

HC-225-5

HC-225-10

HC-250

C

49.65

49.12

49.43

49.34

49.94

51.77

54.37

54.81

52.76

H

6.83

6.73

6.30

6.67

6.52

6.60

6.68

6.65

6.74

N

0.13

0.13

0.14

0.13

0.18

0.18

0.16

0.16

0.17

O

42.68

43.73

44.04

43.64

43.29

41.26

38.63

38.24

40.19

VM

82.42

82.34

nd

81.79

nd

75.21

74.07

FC

13.06

13.32

nd

14.22

nd

18.23

22.62

23.15

23.79

Ash

0.71

0.28

0.25

0.21

0.19

0.20

0.15

0.14

0.14

18.78

20.50

20.61

20.88

21.90

22.55

23.43

22.56

1.00

1.09

1.10

1.11

1.17

1.17

1.20

1.25

1.20

100.00

105.47

96.38

93.59

82.85

71.46

72.95

74.69

73.89

21.9

79.31

75.65

was not detected. VM: volatile matter; FC: fixed carbon; HHV: higher heating value. EDR: Energy Densification Ratio; nd: not determined.


40

Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 2

Table 2. Combustion parameters of FWOE and hydrochars. Sample Ti a Tf b (dw/dt)max c (dw/dt)mean c

S d × 107

FWOE

251.4

479.4

6.41

1.80

3.80

HC-175

254.9

499.0

6.51

1.88

3.77

HC-200

280.9

507.0

9.92

2.00

4.96

HC-225

298.2

511.4

14.76

1.73

5.61

HC-225-5

299.9

504.5

15.01

1.85

6.12

HC-225-10

299.0

503.2

15.40

1.81

6.19

HC-250

294.5

496.9

12.23

1.85

5.25

a

Ti, ignition temperature (ºC). Tf, burnout temperature (ºC). c (dw/dt) max and (dw/dt)mean, maximum and mean rate of weight loss, respectively (wt.%/min). d S, comprehensive combustibility index (min–2 ºC–3). b


41

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 42

Table 3

Table 3. Kinetic parameters obtained for the pyrolysis of olive endocarps according to the model proposed. Kinetic parameters i

ki0 (1/s)

Ei (kJ/mol)

ni

Hemicelluloses

0.981

6.393·1015

208.8

1.83

Cellulose

0.860

1.675·1015

183.2

4.60

Lignin

0.650

3.407·103

64.0

7.46

Component


42

Effect of short-time hydrothermal carbonization on the properties of

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