Effect of hydrothermal carbonization on the properties, devolatilization, and combustion kinetics of Chilean biomass residues

Biomass and Bioenergy 130 (2019) 105387

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Research paper

Effect of hydrothermal carbonization on the properties, devolatilization, and combustion kinetics of Chilean biomass residues

T

E. Monederoa,∗, M. Lapuertab, A. Pazoa, L.A. Díaz-Roblesc, E. Pino-Cortésd, V. Camposc, F. Vallejoc, F. Cubillosc, J. Gómezc a

Universidad de Castilla-La Mancha, Instituto de Investigación en Energías Renovables, 02006, Albacete, Spain Universidad de Castilla-La Mancha, Escuela Técnica Superior de Ingenieros Industriales, 13071, Ciudad Real, Spain c Universidad de Santiago de Chile, Departamento de Ingeniería Química y Programa Centro de Valorización de Residuos y Economía Circular, Chile d Pontifica Universidad Católica de Valparaíso, Escuela de Ingeniería Química, Chile b

ARTICLE INFO

ABSTRACT

Keywords: Hydrothermal carbonization Hydrochar properties Combustion behavior Pyrolysis kinetics TGA Biomass residues

This work evaluates the hydrothermal carbonization (HTC) process as a method to upgrade the quality of biomass residues to be used as fuels in gasification or combustion processes. Seven residues from Chilean biomass were characterized thermochemically before and after being processed by HTC. Additionally, the kinetics of devolatilization and combustion were studied. HTC produces biomass with lower ash content, higher carbon content and higher heating value than the original biomass. Herbaceous wastes showed lower heating values (LHV) around 20% higher after HTC process, while increases around 10% in woody and agroindustrial wastes and corn (even being herbaceous wastes) were observed. The chlorine values obtained after HTC indicate the possibility of using the herbaceous, woody and industrial wastes studied as fuels without chlorine related problems. The activation energy (Ea) values from cellulose and hemicellulose decomposition were higher after HTC process, while lower Ea values from lignin decomposition were found. The combustion characteristic temperatures, ignition temperature (Ti), peak temperature (Tp), and burnout temperature (Tb), were delayed towards higher temperatures with HTC process for all residues. Moreover, the reactivity (R) and combustibility index (S) were lower after HTC, indicating slower combustion for the samples after HTC. Finally, the results show that HTC is a promising process to homogenize the kinetic parameters and the combustion behavior of the samples, thus increasing the interchangeability of the samples in combustion or gasification systems.

1. Introduction Biomass from residues of agriculture or forestry products are a complex bioenergy resource that can be processed by mechanical, thermal, chemical or biological process to obtain a variety of products. Possible applications include their use as fuels for domestic boilers or stoves, industrial heat, district heating, thermal and power plants, etc. (energy sector) or for engines, fuel cells, etc. (transport sector) [1]. In Chile, mainly in the south regions, there is abundance of agricultural residual biomass, which is not efficiently used. Usually, it is burned on agricultural land, increasing the levels of air pollution as well as the risk of wild fires (mainly in summer and autumn) [2], or in combustion systems, where the high moisture and ash content or the heterogeneity of these residues make their combustion process inefficient. Because of their low price and availability, the Chilean residential sector use biomass as a fuel for cooking and heating. However,

∗

the household stoves used have poor thermal insulation [3]. Moreover, the stove's air inlets are usually blocked, slowing down the combustion and increasing the particulate matter emissions, and thus, the air pollution [4–6]. As a consequence of this, air pollution has become a severe problem in the south of Chile [7]. According to the World Health Organization, a high mortality rate in this country is due to this problem [8]. The use of new technologies to upgrade the biomass properties, before their energy use, is very promising. In recent years, hydrothermal carbonization (HTC) has gained interest as a method to improve biomass properties (moisture, heating value, carbon content, etc.), as shown in both experimental [9,10] and modeling works [11]. Consequently, the biomass adequacy as biofuel increases due to its higher efficiency and suitability. This improvement depends on the process conditions and the biomass source [9–11]. Hydrothermal carbonization consists in the thermal treatment of organic substances such

Corresponding author. E-mail address: [email protected] (E. Monedero).

https://doi.org/10.1016/j.biombioe.2019.105387 Received 5 June 2019; Received in revised form 5 September 2019; Accepted 18 September 2019 0961-9534/ © 2019 Elsevier Ltd. All rights reserved.

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as saccharides (sucrose, glucose or starch) mixed with water at temperatures in the 150–350 °C range, which results in water-soluble organic substances and carbon enriched solid products [12]. Biomass can be subjected to different thermochemical processes, including combustion, gasification and pyrolysis. The main difference between them is the amount of oxygen present during the process. The devolatilization conditions, and specifically the kinetics of this process, plays and important role, strongly affecting the yield of char and its reactivity [13,14]. Together with the physical and chemical properties of the biomass, the knowledge of the devolatilization kinetics is helpful to optimize the reactor design [14–17]. Termogravimetric analysis (TGA) is a method frequently used to analyze the kinetics of devolatilization, and the combustion parameters of biomass samples, which helps to predict their reactivity and thermal behavior [18,19]. From the devolatilization study, the kinetic constants can be determined by means of mathematical models [13,20], while the parameters that determine the combustion tendency of the fuels can be obtained from thermal combustion studies [16,19,21–23]. The objective of this work is to evaluate HTC as a method to improve the properties of residual Chilean biomass samples, and to determine the effect of HTC on the kinetic parameters of the devolatilization process, as well as on the combustion behavior of the biomass tested. Moreover, the devolatilization and combustion behavior of the samples tested have been compared to determine their possible combination or interchangeability in combustion or gasification systems. The material used was selected based on their availability, and include: pinus radiate sawdust, which represents 59.1% of the forest plantations in Chile [24]; grape cake, a residue from wine companies which in 2016 was about 152000 t [25]; olive cake, a residue from olive companies which in 2016 was about 115000 t [26]; wheat straw from the main cereal crop in Chile which provides about 1000000 t of wheat straw/ year [26]; oat husk and straw whose production is about 900000 t/year; raps bran, with a production about 550000 t/year; and corn straw from the main horticultural crops, with a production of 1300000 t/year [27].

2.2. Hydrothermal carbonization

2. Material and methods

For the devolatilization study, about 150 mg of each sample was used in the tests. Devolatilization was carried out in a Netzsch STA 409 Luxx Thermobalance in a nitrogen atmosphere with a purge flow rate of 40 ml/min. Different heating rates (10, 30, 50 K/min) were used to heat the samples from room temperature to 1100 K. The thermograms obtained from the thermogravimetric curves (TG), and their first derivatives (DTG), were used to determine the relevant devolatilization kinetic parameters. A model based on the accumulated amount of released volatile matter was used to reproduce the TG experimental data and to obtain the kinetic parameters Ea and k0i [13,20]. The data used as starting points from the model were those obtained in Ref. [13]. According to this, three mass-loss events (defined by three parallel first-order reactions [16,23,35–37]) are distinguished during the heating process of the samples, which are considered as non-interacting [21,22,38,39]. The first event corresponds to the release of moisture and low boiling point organic materials, and lasts up to approximately 400 K. The second event, between 450 and 650 K, is related to hemicellulose and cellulose decomposition [40–42], and thus, the lightest volatile compounds are released. In some cases, in this second event, a small shoulder is observed together with the main peak in the DTG thermogram. This event probably corresponds to the decomposition of the hemicellulose present in the biomass. Some authors have indicated that the degree of overlapping between the two peaks (cellulose-hemicellulose) depends on the mineral matter present in the material, which catalyses the thermal decomposition of the sample [43,44]. Finally, the third event, from 650 K to the end, is commonly identified as the lignin decomposition, and this event takes place slowly and in a broad temperature range [40]. Fig. 2 shows the events identification in the thermograms. The equations and model used were described previously in Refs. [13,20], and are shown below:

The hydrothermal carbonization was performed in a Parr reactor model CIT-HiPR-SF5L with a capacity of 3.78 dm3 and equipped with valves for loading samples, a pressure manometer, a stirrer, and a PID temperature controller. Fig. 1 shows the experimental set-up. Each experiment was carried out by loading 100 g of dry biomass and distilled water, with a biomass/water ratio of 1:12. After loading the reagents, the top of the reactor was closed. Gaseous nitrogen was then injected into the reactor to purge the oxygen and reach the pyrolysis conditions for the HTC process. All tests were carried out at 220 ± 2 °C, and at water saturation pressure (at this temperature) for 1 h of main reaction time. After that, the reactor was cooled down through the available cooling coil by feeding in cold water to quickly lower temperature to 30 °C. Then, the pyrolysis gas was released through the reactor upper valve and the lid was opened. The solid product formed (hydrochar) was taken out from the reactor after being filtered by an in-situ cloth filter and washed with water (approximately 10 dm3) to avoid water soluble compounds. All samples were dried in a forced air oven for 24 h at 105 °C to ensure a complete moisture elimination, and they were stored in airtight bags for further analysis. 2.3. Thermochemical characterization The thermochemical characterization of the samples is shown in Table 2. Representative samples were used according to EN ISO 14780 [28]. Previous to characterization, each sample was grounded to pass a 0.5 mm screen with a cutting mill. Analyses were based on the international standards for solid fuels [29]: for moisture content [30], for ash content [31], for volatile matter [32], for carbon, hydrogen, and nitrogen content [33], for chlorine, and sulphur [34], for heating value. 2.4. Devolatilization kinetics

2.1. Materials Seven agricultural, woody and industrial biomass residues from Chile before and after being processed by hydrothermal carbonization were used for the experiments. Table 1 shows the samples and the nomenclature used throughout the manuscript. Table 1 Nomenclature. Samples before hydrothermal carbonization Biomass origin

Sample

Nomenclature

Woody Herbaceous

Radiata pine sawdust Raps bran Oats husk and straw Corn leaves and steams Wheat straw Olive cake Grape cake

WP HR HO HC HW IO IG

Agroindustrial wastes

Samples after hydrothermal carbonization Biomass origin

Sample

Nomenclature

Woody Herbaceous

Radiata pine sawdust Raps bran Oats husk and straw Corn leaves and steams Wheat straw Olive cake Grape Cake

HTC-WP HTC-HR HTC-HO HTC-HC HTC-HW HTC-IO HTC-IG

Agroindustrial wastes

2

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Fig. 1. Experimental set-up.

dVi = ki (Vi dt

Vi (t ))

(1)

where Vi(t) is the accumulated amount (wt. %) of released volatile matter from event i up to time t, Vi is the ultimate yield of volatile matter from event i up to t = , and ki is the rate constant of event i:

ki = k 0i exp

Ei RT

(2)

where R is the gas constant, T the temperature, k 0i is the preexponential factor, and Ei is the activation energy for event i. When the heating rate, H, and Ei are both much lower than 1, it has RT been proved [40] that a good approximation for Equation (1) could be expressed as:

Vi

Vi (t ) Vi

= exp

k 0i RT 2exp( Ei/ RT ) Ei H

(3)

Fig. 2. Identification of devolatilization process at each event.

The activation energy (Ea) and the pre-exponential factor (k0i) for each event are the only unknowns in Equation (3). They can be obtained by fitting the modelled volatile evolution and the experimental data collected at the different heating rates by minimization of errors.

combustion temperatures, sample reactivity, and combustibility, which are related to the combustion kinetics of the fuels and can be used to predict their thermal behavior. However, although the results could vary depending on reaction conditions, heating rate, instrument and methodology used, the obtained combustion parameters provide additional information about the fuel, and are helpful to complete their characterization [46–48]. In this work, the samples were burned in an air atmosphere. Approximately, 30 mg of each sample was heated at

2.5. Fuel combustion kinetics TGA was also used to study the combustion kinetics, as it is usual in the literature [45]. The resulting TG and DTG thermograms can be used to determine the main combustion parameters such as weight loss rate,

Table 2 The proximate analysis, elemental analysis and heating values of samples before and after HTC. WP Proximate analysis Ash 0.21 Volatile matter 86.35 Fixed carbon 13.44 Ultimate analysis Carbon 51.55 Hydrogen 5.95 Nitrogen 0.06 Sulphur 0.08 Chlorine 0.01 Oxygen 42.35 Heating values HHV 20.11 LHV 18.81

HR

HO

HC

HW

IO

IG

HTC-WP

HTC-FR

HTC-HO

HTC-HC

HTC-HW

HTC-IO

HTC-IG

Unit

6.25 77.89 15.87

4.44 77.79 17.77

3.87 76.88 19.25

8.35 73.07 18.58

9.63 69.95 20.73

4.16 72.7 23.14

0.02 80.16 19.75

4.04 70.89 25.14

3.49 71.12 25.39

0.38 71.12 28.00

4.07 72.45 23.48

12.03 62.94 25.03

1.86 56.44 41.70

wt.% d.b. wt.% d.b. wt.% d.b.

51.22 6.03 4.93 0.67 0.03 37.13

46.39 5.48 0.63 0.05 0.22 40.77

49.3 5.69 1.21 0.09 0.35 43.36

46.02 5.48 0.63 0.1 1.33 36.6

57.54 6.59 1.2 0.08 0.1 34.49

50.01 5.81 1.11 0.18 0.04 42.85

57.22 5.71 0.05 0.02 0.003 36.99

63.33 6.44 3.51 0.43 0.03 26.26

59.02 5.86 0.61 0.05 0.07 34.39

63.12 6.56 1.70 0.08 0.02 28.52

51.67 5.6 0.65 0.07 1.11 40.9

58.91 5.69 1.28 0.08 0.03 34.01

69.51 5.8 1.56 0.1 0.01 23.02

wt.% wt.% wt.% wt.% wt.% wt.%

21.34 20.03

18.66 17.46

18.52 17.28

19.93 18.74

19.71 18.28

20.05 18.78

22.28 21.04

28.15 26.76

22.87 21.60

24.80 23.38

21.36 20.14

21.63 20.39

25.47 24.22

MJ/kg d.b. MJ/kg d.b.

Wt %:weight %; d.b.:dry basis; HHV: Higher heating value; LHV: Lower heating value.

3

d.b. d.b. d.b. d.b. d.b. d.b.

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50 K/min up to 378 K, and then the temperature was kept constant for 30 min to ensure the moisture release. In a second phase, the sample was heated again at 50 K/min up to 973 K, and then temperature was kept constant for 60 min. Relevant combustion parameters such as the ignition temperature (Ti), the peak temperature (Tp), the burnout temperature (Tb), as well as the maximum combustion rate, were determined from the thermograms and the differential thermograms obtained through the methodology described in Refs. [16,21–23]. The reactivity (R) was determined according to the following equation [49]:

1 dm m dt

R=

H/C atomic ratios decrease with HTC, due to the dehydration and decarboxylation reactions during the carbonization process, although the dehydration leads to a higher decrease of H/C atomic ratios [52]. According to Ref. [52], due to the decrease in the number of low energy O–C and H–C bonds, and to the increase in the number of high energy C–C bonds, the energy density of the biomass feedstock was improved after HTC. Moreover, HTC decreases the ash content and increases the carbon content with respect to the original biomass. Therefore, biomass with higher LHV was obtained after HTC. For the HTC condition studied, the herbaceous wastes, raps, oat, and wheat showed LHV around 20% higher after HTC process, while an increase of around 10% was observed in woody, agroindustrial wastes, and corn (despite being a herbaceous waste). All these results suggest HTC as a technology for biomass upgrading. However, results suggest that the HTC process must be optimized for each biomass, because the hydrochar properties depend on both the biomass feedstock and the process conditions [53]. Regarding the elemental composition, HTC has revealed as an optimal process to remove some undesired elements [52,54]. Zhao et al. [52] suggested that during HTC process, some of the Cl, S, and N could be transformed from organic to inorganic state due to the reactions (hydrolysis, dehydration, aromatization, decarboxylation, and condensation polymerization) happening in the HTC process. Moreover, during the process, the water-soluble harmful elements (Cl, S, K, Na, etc.) are moved out from hydrochar. Elemental analysis showed that, with the exception of raps, for which no difference in the chlorine content before and after HTC was observed, around 70–95% of Cl was removed after the process. The Cl removal efficiency was related to the biomass composition as explained before. It is worth noting that Cl is a source of dioxin formation in furnaces, and promotes corrosion and deposit formation in combustion devices [55], and then, its reduction improves the quality of hydrothermally carbonized biomass. The values obtained after HTC (< 0.1%) indicate the possibility of using herbaceous (usually with very high Cl contents, which adds operational problems when used), woody and industrial wastes as fuels without chlorine related problems [56]. As for nitrogen, no clear trend related to HTC process was observed. The industrial and herbaceous wastes showed the highest total nitrogen contents in both the original feedstock and the hydrochar produced. However, there are studies indicating that temperatures above 180 °C during HTC process favor the N retention in hydrochars [57].

(4)

where m is the mass of the sample. The comprehensive combustibility index (S) was calculated according to Refs. [50,51] as follows: d m m 0) 100

d m m0) 100

dt

dt max Ti2 Tb

S=

where

d m m0) 100

mean

and

dt

(5)

d m m0) 100

max

are the maximum weight

dt mean

loss rates and the average weight loss rate (wt.%/min), respectively, and the temperature is expressed in Kelvin (K). Both, higher R and higher S index imply faster combustion, but the combustibility index considers both the combustion kinetics and the main combustion temperatures. This means that for a given reactivity, the combustibility increases when the main combustion temperatures are lower. Therefore, this index provides complementary information to evaluate the combustion kinetics. 3. Results and discussion 3.1. Thermochemical properties A precise knowledge of the ultimate and proximate analyses and of the heating value of biomass and hydrochar is essential to ensure their efficient utilization as fuels. Table 2 shows the characterization of the samples before and after HTC, and Fig. 3 shows the Van Krevelen diagrams. As it was shown in a previous work [11], both the O/C and

Fig. 3. Van Krevelen diagram for all samples. Arrows indicate that decarboxylation mainly decreases O/C atomic ratio, and dehydration decreases both H/C and O/C atomic ratios.

4

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Table 3 Accumulated volatile yield of sample events (V , wt. %). °C/min Event 1

10 30 50

V1

Mean value Event 2

10 30 50

V2

Mean value Event 3

10 30 50 Mean value

V3

WP

HTC-WP

HR

HTC-HR

HO

HTC-HO

HC

HTC-HC

HW

HTC-HW

IO

HTC-IO

IG

HTC-IG

4.0 4.3 3.4

1.5 1.3 2.2

5.0 4.4 6.4

0.9 1.3 1.8

5.1 5.2 5.6

1.7 2.3 2.0

4.6 5.6 5.9

3.3 2.6 2.2

4.8 5.6 4.6

1.9 1.8 2.2

6.8 4.7 4.8

1.4 2.7 3.2

4.4 4.0 8.2

2.1 2.9 2.2

3.9

1.7

5.3

1.3

5.3

2.0

5.4

2.7

5.0

2.0

5.4

2.4

5.5

2.4

79.6 75.3 73.7

69.5 55.9 63.8

67.6 64.8 62.8

61.0 56.3 59.4

78.9 79.0 79.2

70.7 66.3 61.7

80.1 78.7 78.7

71.8 65.0 71.7

79.1 78.5 79.9

74.7 70.9 64.8

62.6 63.0 62.9

60.1 56.3 52.9

69.7 69.2 65.2

49.2 44.4 49.5

76.2

63.0

65.0

58.9

79.1

66.2

79.2

69.5

79.1

70.1

62.8

56.4

68.0

47.7

16.4 20.4 22.9

29.0 42.8 34.4

27.4 30.8 30.9

38.1 42.4 38.8

16.0 15.8 15.3

27.6 31.5 36.3

15.3 15.7 15.4

24.9 32.5 26.1

16.2 16.0 15.2

23.5 27.4 33.0

30.6 35.4 32.4

31.6 41.0 43.8

25.6 26.8 26.6

48.7 52.9 48.4

19.9

35.4

29.7

39.8

15.7

31.8

15.5

27.8

15.8

27.9

32.8

38.8

26.3

50.0

3.2. Devolatilization kinetics of samples before and after HTC The samples of biomass before and after HTC process were heated in the TGA at different heating rates (5, 30, 50 °C/min). Table 3 shows the values of accumulated volatile yield (V ) for the different heating rates and for each mass loss event (see Fig. 2) and for each sample. In general, a decrease in the mass loss values in the second event and an increase in the third one, were observed when increasing the heating rate. Significant differences in the accumulated volatile yield at each event between samples were observed, mainly at event 2 (V2 ) and event 3 (V3 ) . These differences could be due to their variable chemical and elemental composition, and to their cellulose, hemicellulose and lignin contents. Therefore, the accumulated volatile yield depends on the sample composition and its devolatilization temperature. Generally, wood samples have lower cellulose and hemicellulose, and higher lignin content (and thus, lower V2 , and higher V3 ) than herbaceous samples [58]. In the case of raps, despite being a herbaceous biomass, they include grains (apart from straw), and thus, the presence of triglycerides (due to its high molecular weight) seems to decrease V2 and increase V3 . The same occurs with the industrial wastes, whose V2 and V3 are similar to those of raps. In the case of olive cake, these results can be explained because of both the triglyceride content and the higher lignin content, and in the grape cake, because of its particular composition [58]. Moreover, a decrease in V2 and an increase in V3 after HTC was observed for all biomass samples, as explained below. Fig. 4 shows the DTG thermograms for three of the samples tested, before and after HTC process, as an example. This figure shows a shift of the volatile curves towards higher temperatures as the heating rate increases, probably due to the inertia of the devolatilization process. Different shapes on the DTG profiles of HTC-samples were observed, especially in HTC-oat, which could indicate some chemical changes in the samples, as a consequence of the hydrothermal carbonization. Moreover, it is worth remarking that DTG profiles for HTC samples clearly shift towards higher temperatures. Consequently, since the temperature for each event is the same before and after HTC, a decrease in the accumulated volatile yield for the first and second events, and an increase for the third event, were observed after HTC (Table 3). This could also be associated with some hydrolyzation of cellulose, hemicellulose and lignin (depending on the biomass composition and HTC conditions) [56], and with the polymers with high molecular weight formed (increasing event 3) during HTC process [59]. This could affect the combustion or gasifier design due to the higher temperatures needed to volatilize the different compounds when using HTC samples. Regarding the kinetic parameters, Table 4 shows the Ea and k0i values obtained by fitting the modelled devolatilization curves to the experimental ones through each mass-loss event for each sample and

Fig. 4. DTG curves for three biomass samples before and after HTC process.

heating rate. These values were obtained by least squares regression solved with a Newton algorithm. The instantaneous volatile yield can be simulated from the kinetic parameters as described in Ref. [13]. As an example, Fig. 5 shows the good match between the modelled and 5

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and after HTC, were observed for pine sawdust (woody biomass) and raps (herbaceous oleaginous biomass), from 63 to 141 kJ/mol and from 67 to 132 kJ/mol, respectively. For the second event (release of hemicellulose-cellulose and other semi-light volatile compounds), woody biomass showed the highest Ea, followed by herbaceous biomass and industrial wastes, while for the third event (release of lignin and heavy volatile compounds) woody biomass showed the lowest Ea. In general, both the activation energy and the pre-exponential factor of the samples after HTC process was higher in the second event and lower in the third one. This trend could be related to the chemical transformations occurred during HTC process, such as the hydrolysis of cellulose, hemicellulose, and lignin, and the polymerization reactions which provide compounds with higher molecular weight and with higher energy C–C bonds [60]. Therefore, the compounds formed would need higher Ea to be released. In contrast with the herbaceous and industrial wastes, the Ea for the third event in the woody biomass is higher after HTC. As mentioned before, this could be related to its composition and to the reactions that arise during HTC process. In addition, it is worth remarking that all samples (before and after HTC) showed lower activation energies in the third event than in the second one. This could be due to the decrease in the thermal degradation rate of lignin and high molecular weight compounds with increasing temperature [61]. Finally, the Ea and k0i values of the original biomass (before HTC) obtained in this work are similar to those shown in the literature [13,20,46,62,63]. However, those for biomass after HTC could not be compared because none was found. Therefore, the Ea and k0i values obtained could be used as a database to simulate the gasification or combustion processes, and would be helpful for designing an adequate technology for the efficient conversion of these biofuels. On the other hand, comparing the kinetic parameters for any type of biomass and event before and after HTC, a higher homogeneity in Ea values after HTC process, especially for events 2 and 3, was observed. The Ea standard deviation values for events 2 and 3 were 23 and 10 kJ/ mol, respectively, before HTC, and 15 and 6 kJ/mol, respectively, after HTC. This would enable the interchangeability of these fuels in a combustion or gasification process.

Table 4 First-order kinetic parameters of three events considered in the devolatilization model. k0i (min−1)

Ei (kJ/mol) Sample

Event 1

Event 2

Event 3

Event 1

Event 2

Event 3

WP HTC-WP HR HTC-HR HO HTC-HO HC HTC-HC HW HTC-HW IO HTC-IO IG HTC-IG

63.4 141.8 67.1 132.6 67.1 75.8 67.2 67.7 66.6 67.8 68.0 73.5 85.8 69.9

96.7 109.8 42.8 80.4 65.7 111.0 49.5 95.6 69.1 92.2 52.4 103.9 24.8 70.2

21.2 39.0 38.1 20.6 36.4 26.9 40.1 28.1 27.8 25.6 18.0 18.5 43.8 21.7

1.3E+09 1.0E+11 1.0E+09 9.5E+08 9.9E+08 2.1E+10 9.8E+08 9.3E+08 9.8E+08 1.6E+09 9.9E+08 3.1E+09 9.8E+08 1.0E+09

3.1E+07 2.9E+08 9.4E+02 7.9E+05 1.6E+05 4.3E+08 5.4E+03 1.7E+07 3.2E+05 1.5E+07 4.8E+03 7.9E+07 1.7E+00 8.0E+04

1.2 37.7 49.0 1.4 20.6 4.6 47.5 4.5 3.9 3.1 0.6 0.6 106.0 1.3

experimental accumulated volatile yield curves obtained from pine (WP) before and after HTC at the three heating rates studied. Moreover, Fig. 6 shows the devolatilization curves of three different types of biomass (before and after HTC) at heating rates of 10 °C/min, 30 °C/ min, and 50 °C/min. It can be observed that the model used is able to differentiate between them. As proved previously [13,20], the model used also predicts the experimental shift of the devolatilization curves for increasing heating rates. Significant differences in Ea and k0i values between samples before and after HTC process were observed (Table 4). All biomass before HTC process showed similar Ea values in the first event (mainly dehydration and release of lighter compounds) probably due to the similar moisture content of the samples. After HTC process, although the moisture content is lower, the Ea in this event is higher for all samples, and with significant differences between them, probably due to the elemental and chemical composition of each sample, and the chemical transformations produced during HTC process. The highest differences, before

Fig. 5. Experimental (exp)/modelled (mod) comparison between accumulated volatile yield curves of pine sawdust samples before and after HTC process at different heating rates.

6

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Fig. 6. Experimental/modelled comparison between accumulated volatile yield curves of three of the samples tested before and after HTC at heating rates 10, 30 and 50 °C/min.

3.3. Fuel combustion kinetics

process implies higher difficulty for the samples to ignite. The two other temperatures determined, peak and burnout temperature, also showed an increase after HTC. This indicates that longer reaction time is needed to achieve the maximum decomposition rate and a longer combustion process (because of lower mass loss rate, as observed in Fig. 7 and Table 5) [65]. Regarding reactivity (R), samples before HTC present higher R. The reason is the higher volatile content of these samples, favoring a higher maximum combustion rate. However, this high volatile content of the samples before HTC could cause unstable combustion and flame, thus leading to larger heat losses. Finally, the comprehensive combustibility index (S), which considers both the sample reactivity and the combustion temperatures Ti and Tb, although follows similar trend to R, shows lower differences between samples before and after HTC. Therefore, the lower reactivity and higher Ti and Tb of the samples after HTC leads to expected slower and less violent combustion, and thereby, to more stable flames [47]. Zhengang et al. [48] states that, as far as an adequate weight loss rate is kept, a higher combustion temperature implies improvements in the combustion safety, increasing combustion efficiency and decreasing pollutant emissions. In summary, the use of hydrolized biomass as a fuel could be safe and efficient at the same time. Moreover, a considerable improvement in the homogenization of the combustion behavior of biomass after HTC was observed. The standard deviation (taking in consideration all biomass samples, and thus, their different origin) in Ti, Tp, Tb values before HTC were 34, 34, 32 °C, respectively, while after HTC they were 34, 13, 27 °C, respectively. These results suggest the use of biomass after HTC as solid fuels, and guarantees a good interchangeability in gasification or combustion systems.

Fig. 7 shows the TG and DTG profiles of the samples before and after HTC. HTC process caused significant changes in both profiles. For all samples, the TG and the DTG thermograms were shifted towards higher temperatures after HTC process. Moreover, the higher volatile content and the lower fixed carbon in the samples before HTC favor sharper devolatilization DTG peaks. This could be explained because the devolatilization phase and volatile combustion phase take place along with the char combustion phase. However, in samples after HTC, as the volatile content decreases and fixed carbon increases, two peaks can be distinguished in most of the DTG thermograms (depending on HTCsample properties), indicating that the devolatilization and combustion phases occur more separately with respect to the char combustion phase. HR, HC, HW, and IO are the samples where the formation of two separate peaks is more noticeable. The peak separation depends on the sample carbonization during the HTC process [47]. The maximum combustion rate, the characteristic temperatures, the reactivity, and the comprehensive combustibility index are presented in Table 5. Regarding the biomass origin, in general, woody biomass present higher ignition, peak, and burnout temperatures, 282, 327, and 369 °C, respectively, than herbaceous biomass or industrial wastes. IG (probably because of its composition) was the biomass with the lowest Ti and Tp, but highest Tb, indicating that a longer combustion process was needed. Independently of the origin, an increase in Ti (temperature which indicates the probability to minimize fire and explosion when using solid biofuels [64]) after HTC process was observed for all samples. This increase could be due to the lower volatile content of the fuels after HTC. This makes HTC fuels less reactive (Table 5) and thus, higher temperatures are necessary to start ignition [64]. Therefore, HTC 7

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Fig. 7. TG/DTG curves for combustion profiles of samples before and after HTC. (a): WP; (b): HR; (c) HO; (d): HC; (e): HW; (f): IO; (g): IG.

4. Conclusions

exponential factor of the samples after HTC process were higher in the first and second events and lower in the third one (except for pine, where the latter was higher). Therefore, the compounds formed after HTC would need higher Ea to be released. The Ea and k0i values obtained could be used to predict the thermal behavior of the biomass formed after HTC, which could help improve the technology for the efficient conversion of these fuels. Regarding the combustion behavior, the combustion characteristic temperatures, Ti, Tp, Tb, were higher than those for the biomass subjected to HTC, for all types of biomass. This implies higher difficulty of the samples to ignite, longer reaction time to achieve the maximum decomposition rate, and longer combustion process. Moreover, the reactivity (R) and combustibility index (S) were lower after HTC, indicating slower combustion for the samples after

Hydrothermal carbonization process seems to be a promising method to upgrade the biomass quality. A noticeable increase in carbon content and heating value was observed after HTC, which depends on both the chemical composition of the fuels and the HTC conditions. Moreover, a decrease in ash content and harmful elements as chlorine (between 70 and 95%) was also observed. The chlorine values obtained after HTC (< 0.1%) indicate the possibility of using herbaceous, woody, and agroindustrial wastes as fuels without chlorine related problems. For the kinetic devolatilization parameters, the differences observed are probably due to the different composition of the samples and the HTC process conditions. The activation energy and the pre8

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Table 5 Combustion parameters for all biomass before and after HTC. SAMPLE

DTGmax (wt. %/min)

Ti (°C)

Tp (°C)

Tb (°C)

R (min−1)

S x 10−6 (min−2K−3)

WP HTC-WP HR HTC-HR HO HTC-HO HC HTC-HC HW HTC-HW IO HTC-IO IG HTC-IG

45.0 43.5 22.1 16.2 40.2 26.4 43.8 20.6 41.1 27.5 41.6 19.7 22.0 13.6

287 307 247 265 245 282 229 273 230 265 243 282 175 202

327 347 345 357 266 347 277 299 277 277 275 357 211 237

369 370 406 419 345 387 332 379 335 377 335 392 417 517

0.9 0.8 0.4 0.3 0.6 0.5 0.7 0.3 0.6 0.4 0.6 0.3 0.5 0.2

2.01 1.33 1.00 0.49 2.31 0.88 2.65 0.71 2.24 1.15 2.01 0.50 1.68 0.51

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