Suitability of hydrothermal carbonization to convert water hyacinth to added-value products

Renewable Energy 146 (2020) 1649e1658

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Renewable Energy journal homepage: www.elsevier.com/locate/renene

Suitability of hydrothermal carbonization to convert water hyacinth to added-value products a

n a, *, B. Ledesma a, A. Alvarez S. Roma , C. Coronella b, S.V. Qaramaleki b a b

Department of Applied Physics, University of Extremadura, Avda. Elvas, s/n, 06006, Badajoz, Spain Department of Chemical and Materials Engineering, MS 170, University of Nevada Reno, 1664 North Virginia Street, Reno, NV 89557, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 April 2019 Received in revised form 26 June 2019 Accepted 30 July 2019 Available online 1 August 2019

Water hyacinth hydrothermal carbonization was studied under different temperature (160e250 C), time (30e120 min) and biomass/water ratio (10e50%) conditions. The research was designed following response surface methodology, which was very useful to infer interactions between variables and to develop models predicting the system behaviour with good accuracy. Output functions were solid yield, hydrochar C and N content, as well as their captures, and heating value. It was found that while temperature was the most influential variable promoting HTC reactions, time and even biomass load were decisive to provide particular C and N captures; based on these results, reaction mechanisms were discussed. On the other hand, 2D graphs allowed to build different scenarios in which target properties might be achieved under a wide range of dissimilar conditions, leading to process optimization. The study was complemented by exploring hydrochar surface properties by N2 adsorption at 77 K, SEM micrography and XPS analyses. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Hydrothermal carbonization Water hyacinth Nitrogen recovery Carbon densification Response surface methodology

1. Introduction Water Hyacinth (WH, Eichornia crassipes) is universally regarded as one of the more serious world's invasive plants. Native from the Amazon river basis, this plant was introduced into many other countries as an ornamental plant for water gardens. Its quickly spread have made this plant to be a major weed in many areas such as southern US states from Florida to California [1]. Other countries such as Spain, India, Bangladesh and African countries like Nigeria or Victoria lake bordering countries [2] also present massive infestation of WH. This floating plant tends to form mats on the water surface and can quickly dominate aquatic systems because of rapid leaf production, fragmentation of daughter plants, and copious seed production and germination. WH causes problems for humans by obstructing navigable waterways, impeding drainage, fouling hydroelectric generators and water pumps, and blocking irrigation channels. Ecosystems are seriously affected by WH; it deteriorates water quality by lowering pH, dissolved oxygen, and light levels, and increasing CO2 tension and turbidity. This affects the health of

* Corresponding author.
n). E-mail address: [email protected] (S. Roma https://doi.org/10.1016/j.renene.2019.07.157 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

fish, decreasing income and food security, while decaying plants make water unfit for drinking by humans, livestock, and wildlife [3]. In addition, WH areas can serve as a breeding place for organisms carrying malaria, river blindness and other diseases. Several biological, physical and chemical methods have been tried for the control and eradication of WH [4]; however, despite a large amount of man power and money have been spent on WH; this weed is still far from being completely eliminated and current methods lead to nothing but temporary removal. In this frame, research on ways of providing added-value to this waste have been regarded as a powerful motivation to enhance the control and recovery of this specie. In this frame, a large body of literature has reported the suitability of WH as a source of chemicals such as enzimes (cellulase, bblucosidase, xylanase) and organic acids (levulinic acid, shikimic acid or xylitol) [3]. In the field of material applications, WH has been proven to be a promising low cost biosorbent; owing to the high ash content, some specific adsorption interactions can be enhanced in water systems, as it has been found for some heavy metals [5]. Other application that has gained prominence is soil amendment; due to its high proportion of macronutrients (such as nitrogen, phosphorus or potassium), WH can be suitable for the production of vermicompost [6]. WH fibers can also be used in arquitecture or craftwork [4].

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In the energy field, several processes have been studied to convert WH into biofuels. Mostly, WH has been tested as substrate for biogas production; in this line, some promising results have been reported when the weed is used in combination with other biomass resources (anaerobic co-digestion processes) such as cow manure and elephant grass [7], fresh rumen residue [8] or alone [9]. Bioethanol production has also been proven from pristine and pretreated WH [10]. More recently, the production of biohydrogen from WH has also been investigated yielding inspiring results [11]. The production of briquettes from WH for combustion processes at domestic (cooking) [12] and industrial scale [13,14] has also been reported; in the latter case, it was necessary to blend WH with other materials (coal or wood) in order to provide suitable combustion parameters. The production of solid carbonaceous fuels (char) from WH has received less attention; Thomas and Eden [15], reported that the high investments and technological level necessary for WH pyrolysis made this process an unfavourable alternative for this material, mainly due to the need of cost intensive drying. The search on alternative routes for its processing might therefore be very interesting. Hydrocarbonization (HTC) has received increasing attention during last years because, amongst other significant advantages, it allows the use of high moisture biomass. By HTC, biomass is heated in the presence of water usually under moderate temperature conditions (180e250
C) and at a sufficient pressure (autogeneous) to keep the water in liquid state. The inherited advantage of using wet feedstock in HTC process stems from the fact that the reaction medium is subcritical water. Under these conditions, water dielectric constant drops drastically (the more as the temperature is increased), and so does the solubility of biomass components. The hydrolysis of biomass constituents gives rise to the formation of other compounds (such as monomers, amino acids, carboxylic acids …), which further react following variable pathways. As a whole, HTC is a very complex process in which a large variety of reactions have been identified so far (dehydration, decarboxylation, decarbonylation, deamination, polymerization, condensation, repolymerization …) whose prominence is highly dependent on reaction conditions [16]. As a result, a solid product (hydrochar, HC) with a higher C content as compared with the precursor is obtained; the produced HC is highly hydrophobic therefore instead of an extremely energy intensive drying process a simple physical dewatering (e.g. filter press) would be enough to remove the process water. Also, a liquid phase rich in organics and a little amount of gas (mainly CO2) are obtained. Thermodynamically, the lack of a phase change allows for increased heat recovery; that is, energetic sinks associated to the evaporation of water are avoided, which results in a more efficient processing of biomass. Another highlighting advantage especially in relation to high moisture materials stands on the fact that product streams are completely sterilized with respect to any possible pathogens including biotoxins, bacteria or viruses. Apart from the carbon densification, the fate of nitrogen during HTC has recently attracted the attention of researchers [17]. The capture of N on the HC can be advantageous because of its importance as nutrient (used for example in soil amendment processes) or in other applications such as adsorption or as electrode material in supercondensers; N can be beneficial because of the lone electron pair availability, which can enhance specific interactions [18] and additional redox reactions in the case of supercondensers [19]. Also, for some HC applications such as biofuel use, a very low N content is required, in order to minimize the formation of NOx during combustion processes. Despite its importance, the mechanisms associated to N compounds during HTC are not yet fully understood. WH has a moderate protein amount (around 20%) [20],

and can therefore be considered as major source of N and might therefore be suitable to yield high N content HCs. Notwithstanding the push that has been put on the research of HTC of humid biomass, investigations on the HTC of WH are scarce. A few works have been devoted to study this process as a suitable pretreatment to decrease the lignin content and thus make it more suitable for biogas production, because of the enhancement of hydrolysis enzyme action due to greater accessibility to cellulose. However, these pieces of research used fixed HTC conditions and did not perform an optimization of the pretreatment neither identified interactions between variables [21,22], which are for sure present during HTC processes [23]. Gao et al. [24] investigated the effect of time (0.5e24 h) during WH HTC at 240
C on the surface properties of HCs. None of these works paid attention to the biomass N distribution on the phases originated as a consequence of HTC. In this work, we use HTC as an effective way of converting WH into a lower moisture material with enhanced carbon content and/ or greater nitrogen proportion. For the first time, we provide a systematic study in which the influence of temperature, time and ratio biomass/water on the process reactivity and HC features (heating value, elemental analyses and surface properties) are analyzed. We investigate HTC as a tool for the sequestration of atmospheric CO2 (the one grabbed by the plant by photosynthesis) and to obtain an efficient deposited form of carbon, at the same time that we also focus on the N recovery. Based on the results obtained, we discuss on the mechanisms involved during HTC and on the conditions, which lead to process optimization. 2. Experimental 2.1. Precursor WH was gathered from river Guadiana basin (Badajoz, Spain). Stem, leaves and roots were manually separated and stem was chosen as precursor for this study, because of its relative greater abundance. According to its proximate analysis, this material has 88.9% wt. of moisture, which justifies the use of HTC, since other process would be economically very unfavorable. Volatile, ash and fixed carbon contents are, respectively, 9.6, 1.07 and 0.44% wt. The immediate analyses of the HCs was determined from thermal degradation profiles under inert and oxidizing atmospheres (thermobalance Setsys Evolution, Setaram), using Ar and Air, respectively. Heating rate was 5
C min1 and the carrier gas flow was 100 cm3 min1 in both cases. Water Hyacinth elemental composition was determined in dry basis with an elementary analyzer (Eurovector EA 3000), according to the norm CEN/TS 15104 (for determining the content of C, H and N) and CEN/TS 15289 (for S). 2.2. HTC processes. Design of experiments Since the calculation of solid yield was based on the dry matter of WH therefore, prior to experiments, WH was dried in an oven (100
C) to remove residual moisture, grounded and sieved to a particle size of 0.28e0.5 mm. This step was also necessary to obtain consistent and comparable result by tracking the exact amount of WH used in each experimental run. The size was chosen based on previous experiments which showed a negligible change on the thermal behavior of WH in the range 0.09e2.00 mm; after grinding, this size was the most abundant. HTC processes were performed in a 0.015 L stainless steel microcolumn reactor (weight of the dried and empty column: 221.7 g), which allowed the flash injection of water-biomass mixtures. In each experiment, variable amounts of deionized water and


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WH were used, providing an interval of biomass load conditions in the range 10e50 (wt. %, biomass to water ratio). The mixture entered the reactor once it had reached the target temperature (160e250 C) and was left during a variable time period (15e232 min). After that, it was cooled down by ice bath and when it reached room temperature the column was weighted again (closed filled column), the sample was taken out and liquid and solid phases were separated by filtration (Whatman filter paper no. 3). Solid HCs were rinsed in deionized water and then dried in an oven and stored for further analyses. Drying was necessary to avoid deviations on subsequent analysis such as HHV or N2 adsorption, due to the possible HC water adsorption from the environmental air, that might be different from one day to another. The experimental margins were chosen based on previous results obtained by the research group from preliminary studies with WH. It is highlighting that this specie was more reactive as compared with other biomass feedstock already investigated by the authors such as walnut shell or cellulose [23,25]. Once the intervals of each variable were chosen, the number and specific conditions of each variable were determined following a Central Composition Design (CCD), which is very useful since it provides an even distribution of the experimental points (a more detailed description can be found from the approach described by Montgomery [26]). Then, one second order model was created for each parameter of interest (Y, output function) namely the solid yield, carbon content, nitrogen content, heating value and carbon and nitrogen capture (both defined as the net amount of element retained in the HC in relation to its initial quantity). For each of these outputs, the values of the function coefficients (A0-A9) were calculated (eqn. (1))

Y ¼ A0 þ A1 R þ A2 T þ A3 t þ A4 RT þ A5 Rt þ A6 tT þ A7 R2 þ A8 T 2 þ A9 t 2 (1) Where R stands for biomass/water ratio (wt. %), T represents temperature (
C) and t corresponds to time (min). The experimental fitting was made using IBM SPSS statistical software package, following LevenbergeMarquardt algorithm, and the resulting equations were plotted using Wolfram Mathematica 8 software. Each run was named according to the nomenclature T/t/R, where each digit represents temperature (
C), time (min) and biomass to water ratio (wt., %), respectively.

2.3. Characterization Hydrochars (HCs) were characterized in terms of their solid yield (SY, %) calculated by mass balance. This parameter was determined as the ratio between the initial biomass mass (m0) and the final mass of the HC (mf), after its separation by filtration from the liquid phase (the increase in the filter paper after filtering was also included, after drying it). The High Heating Value (HHV) was determined using a bomb calorimeter, Parr). Elemental analyses for carbon, hydrogen, nitrogen, and oxygen were carried out with an elementary analyzer (Eurovector EA 3000), according to the norm CEN/TS 15104 (for determining the content of C, H and N) and CEN/TS 15289 (for S). The surface structure of selected HCs was further characterized by N2 adsorption at 77 K (Autosorb-1, Quantachrome) and Scanning Electron Microscopy (SEM, Se3600N Microscope, Hitachi). In the first case, samples were previously outgassed at 70
C overnight. From experimental adsorption data, characteristic textural parameters (specific surface, and volumes of micro and mesopores) were calculated by appropriate methods [27]: Brunauer-EmmettTeller (BET) and Dubinin-Radushkevitch methods were used to

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determine the values of BET surface (m2g1) and micropore volume (cm3 g1), while the mesopore volume was calculated as the difference between total pore volume (i.e., at P/P0 ¼ 0.99) and micropore volume. In the second case, SEM samples were prepared by depositing 50 mg of sample individually on Al studs, covered with conductive adhesive carbon tapes and then coating with RdePd for 1 min to prevent charging during observations. Imaging was done in the high vacuum mode at an accelerating voltage of 20 kV, using secondary electron detector (SED). Finally, X-Ray Photoelectron Microscopy (XPS, K-Alpha) analyses were conducted in order to infer information about the components and chemical bonding of HCs N-surface functionalities. A monochromatic radiation Al Ka source was used, while binding energies for high resolution spectra were calibrated by setting C 1s to 285.0 eV. 3. Discussion 3.1. Hydrocarbonization reactivity, carbon and energy densification The results of solid yield (SY, %), liquid yield (LY, %), proximate analysis (%) and heating value (MJ kg1) of the HCs obtained from WH stem under varying temperature, HTC time and biomass/water ratio conditions are listed in Table 1. Gas yield, as determined according to the procedure described in section 2.3, was in all cases lower than 1%, and has not been included in the table. In this table, the values of C and N capture, defined as the net quantity of element retained in the solid phase (i.e., the hydrochar, HC) have been included. The table also includes the analyses made on the precursor; as compared to other classical lignocellulosic biomass sources, WH stands out for its low C, and high O and N content. Although the chemical composition of WH depends strongly on its environment, the literature reports C and N values in the range 18.5e29.7 and 0.74e2.76 [2] in dry basis, respectively (although it is not usually stated whether one fraction (stem or leaf) is studied, or if both, blended, are analyzed). Apart from the trends obtained from HTC processes, it is important to highlight the strong interaction between variables: one can see that the effect of one of them depends on the other two variables, as it will be thoughtfully described along the manuscript. The effect of the variables on the SY will be studied first. Using experimental data, 2D level curves have been plotted in Fig. 1(aei). In general, the effect of two variables has been represented using one fixed variable at a time, at each row: Ratio (a-c), Temperature (d-f) and time (g-i). In each row, three different values of the fixed variable have been included. This methodology has been adopted for all 2D level plots. HTC temperature has been referred to as the most important parameter influencing HTC processes, enhancing biomass compound volatilization and dissolution, with a clear effect on the SY. The most of the pieces of research on HTC have reported that increasing temperature always involve a decrease on this parameter [28,29], although the severity of its effect may be joined to the values of other variables. This is also found from our results; on the one hand, the model coefficients, provided as supplementary material (Table S1) show that this variable is the one with the strongest effect on the SY, although interactions T-t and T-R exists. On the other hand, a detailed examination of SY 2D level graphs show how the same temperature shift can influence greatly or scantly, depending on the system conditions. For instance, it is interesting to notice that using a greater load of biomass makes the process more sensitive to temperature and relatively immune to time; if one compares Fig. 1a-1 c, in which different biomass ratios have been included, sharp differences in the slopes of the isolines can be observed. In this way, if high loads


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Table 1 Solid Yield (SY, %), High heating value (HHV, MJ kg1), elemental analyses (%), and C and N capture values (% retained in the solid phase) for WH hydrochars. Sample

T ºC

time (min)

Ratio (%)

SY (%)

C (%)

H (%)

N (%)

O (%)

Ccapture (%)

Ncapture (%)

HHV(MJ kg1)

WH 178/36/18 178/36/42 232/36/18 178/99/18 232/36/42 178/99/42 232/99/18 232/99/42 208/68/10 160/68/30 208/15/30 208/68/49 205/68/30 205/120/30 208/68/30 205/68/30 205/68/30 205/68/30

e 178 178 232 178 232 178 232 232 205 160 205 205 250 205 205 205 205 205

e 36 36 36 99 36 99 99 99 68 68 15 68 68 120 68 68 68 68

e 18 42 18 18 42 42 18 42 10 30 30 49 30 30 30 30 30 29

e 69.33 68.71 60.99 50.42 26.90 66.26 46.19 47.07 48.12 76.92 75.83 60.69 40.88 49.17 54.72 53.06 54.17 50.56

42.08 41.51 42.36 43.28 44.39 42.27 51.68 53.12 44.09 40.92 41.73 43.62 53.20 50.03 45.76 46.23 46.46 46.64 42.08

5.28 5.17 5.42 5.36 5.42 5.16 5.07 4.91 5.46 5.15 5.29 5.23 5.23 5.19 5.17 5.23 5.23 5.23 5.28

1.79 1.87 2.02 1.98 2.31 2.28 2.94 3.54 2.00 2.00 2.05 2.44 2.73 3.04 2.59 2.68 2.87 2.71 1.79

50.85 51.45 50.20 49.38 47.88 50.29 40.31 38.43 48.45 51.93 50.93 48.71 38.84 41.74 46.48 45.86 45.44 45.42 50.85

0.80 0.78 0.71 0.60 0.33 0.77 0.65 0.68 0.58 0.86 0.86 0.72 0.59 0.67 0.68 0.67 0.69 0.64 0.80

0.54 0.56 0.53 0.43 0.27 0.65 0.59 0.72 0.42 0.67 0.67 0.64 0.48 0.65 0.61 0.62 0.67 0.59 0.54

13.90 17.16 16.86 17.19 18.47 18.35 16.83 21.18 21.83 18.95 17.08 16.99 16.77 21.84 18.94 16.87 18.67 19.26 19.30

Fig. 1. Influence of experimental variables on Solid Yield (%).


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(R: 42%) are used, temperature is the major variable controlling HTC extent (see almost vertical lines on Fig. 1c, and SY values varying in the range 30e90%). In contrast, time has a moderate influence for lower R values (see Fig. 1a), for which there is a clear interaction time-temperature. Some remarks on the process kinetics can also be suggested; if one follows the evolution of SY with time, two effects can be observed: a) A higher temperature promotes SY decay, at the first stages of the process. For example, one can compare graphs 1.d-1.f at 30 min, and see that for any ratio, SY values decrease as temperature gets higher. Previous pieces of research have given evidence on the role of temperature to accelerate carbonization reactions, providing faster kinetics [30]. Rising temperature would enhance hydrolysis reactions, which can initiate at mild temperature conditions (>150 C) on hemicellulosic fractions and result in the complete removal of these compounds [31]. The rate of hydrolysis reactions is primarily determined by the diffusion within the biomass matrix [32], promoted by increasing temperature. The effect of decreasing SY for increasing temperature is therefore clear for low HTC times; however, for longer times (i.e., 99 min, see Fig. 1i), one can find a large surface of temperature and ratio combinations for which SY is constant, provided temperature reaches a certain value. b) Under certain circumstances, biomass load can also have an effect on SY. If a low temperature is used (i.e., 178
C, plot 1.d), a lower R value involves a more rapid decrease of SY with time (see green arrow in this Figure, indicative of a significant change of SY, from >80% at 40 min, to <40% at 120 min). In fact, a detailed inspection of this Figure allows to see that there is a critical value of time (close to 40 min) for which the effect of time and biomass load on the SY diverge at times below 40 min SY is highly time dependent while after 40 min time becomes relatively less effective. Instead biomass to water ratio becomes more influential (see green area in the same Figure). In other words, at low temperatures, the effect of time on SY vanishes as the load gets higher. One direct explanation for the effect found at lower temperatures can be related to the larger availability of water at lower ratios. According to Le Chatelier principle, a greater amount of water would enhance hydrolysis reactions, which can initiate at mild temperature conditions (>150 C) on hemicellulosic fractions and result in the complete removal of these compounds [31]. As these reactions take place, hydronium ions are formed and variety of organic compounds are incorporated to the liquid phase (acetic acid, lactic acid, hydroxymethyl furfural, furfural, pentoxes, hexoses …) [32]. The kinetics of these processes, at the first stages of decomposition, can also be affected by the biomass loads, as it can be inferred from the increasing verticality of the isolines for greater ratios. For example, for R ¼ 18% (1.a), a value of SY between 60 and 70% is found at 180 C in 50 min, and thereafter, as the reaction proceeds, this parameter drops gradually up to values lower than 50%, probably as a result of further decomposition reactions such as decarboxylation and depolymerization, some of them enhanced or catalyzed by by-products generated in the former stage (i.e., hydronium ions and other acids). The occurrence of these reactions might be somehow hindered for high loads due to a greater mass transfer resistance, in such a way that the use of longer times do not involve a greater reaction extent. Another plausible explanation might be related to a higher prominence of recondensation reactions between HTC products due to a more concentrated liquid phase at greater ratios and therefore enhancement of adsorption of

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these products onto the HC surface. The system behaves significantly different if high temperatures are used, or, said in another way, if the temperature allowing the participation of reactions including degradation of the more resistant fractions of biomass (such as cellulose of lignin), as well as protein hydrolysis, is provided. Under these conditions, the ratio has a clear effect on the solid yield especially if short times are used (plots 1.f and 1.g). This might be explained on the basis that at these conditions liquid phase properties affecting HTC pathways (pH, mass density, ionic constant, concentration of acids, heat transfer characteristics, etc.) [33], may be also influenced by a greater mass load. For longer times, recondensation reactions and adsorption at high loads might be source of increasing SY values. Since a larger concentration of products from dehydration and decarboxylation is expected for greater loads, nucleation might be enhanced, resulting in the formation of cross linked polymers [16]. This effect (higher SY as time is enlarged) is only found at high temperatures and is consistent with the findings of Yu et al. [34], who suggested that products from hemicellulose (expected to be the only fraction to decompose at lower temperatures) do not undergo repolymerization reactions. This is very interesting and allows a highlighting advantage regarding production costs: depending on the HC target characteristics and operation limitations, one can decide if obtaining a greater SY is worthy in relation to the expenses is related to process prolongation. The effect of biomass load, rarely investigated, is of great importance with regards to costs in industrial applications, especially if as it is the case of this work, the precursor is a high moisture waste generated in great quantities that has to be quickly processed to decrease its biological activity and be stored in safety conditions. Gai et al. [35] investigated the effect of mass load during the hydrothermal liquefaction (HTL) of microalgae Chlorella under varying biomass/water ratios (15e25%) and found, in agreement with our results, that lower loads reduced the solid yield, which they attributed to enhanced extraction by a denser solvent medium. The major influence of temperature is also visible from C analyses. Higher thermal levels involve in general a decrease on the net amount of solid-phase carbon (its proportion in the solid phase increases but the decrease in SY brings out an overall decrease of the quantity of carbon present in the system as solid phase carbon). These effects can be observed from Table 1, where the carbon content of the HC has been expressed in two forms: a) as C (%), determined from elemental analyses, and b) carbon capture, Ccap (%), calculated as the amount of carbon captured in the HC in relation to the amount of carbon in the feedstock, considering the solid yield. While the results of first one (a) have been represented

Fig. 2. Influence of experimental variables on SY (%) and carbon proportion (%) for experiments developed under fixed a) loads (R ¼ 18%) and b) times (t ¼ 99 min).

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in Fig. 2, carbon capture plots have been provided as supplementary material (Fig. S2); instead, some representative graphs have been built in order to clarify the interpretation of trends, as it is described later on. The proportion of carbon on the HC (Fig. S2) can be of interest in order to evaluate its suitability as solid biofuel, since it is directly related to the HHV. In general, it can be found from our results and from the literature that a greater carbon densification depends primarily on the temperature. Although some interactions between variables exist, in general temperature is the controlling one and it is not possible to attain a certain proportion of C (and HHV) unless a minimum temperature is attained. Using a higher biomass load can decrease the relevance of temperature under determinant circumstances (low dwell time) (Figure S2c). In this work, the study of biomass ratio allowed to obtain interesting findings in relation to C densification, which might be very interesting towards a real implementation of the process at larger scale. In order to facilitate the reader's interpretation, graphs of the effect of experimental variables on the C proportion (Fig. S2) and SY (Fig. 1) were overlapped, and are shown in Fig. 2. Both Figures (2a and 2 b) show that there is a wide range of experimental conditions which result in an HC with varying C proportions but guaranteeing an almost constant SY (of 40e50%); in particular from Fig. 2 a one can deduce that there is a variety of combinations T-t yielding HCs with C proportions in the range 40e57.5%, as long as the load is constant (18%). In this way, if there is any temperature limit in the plant (for example, one cannot exceed 200
C because we are using residual heat from a particular device), we can optimize the time, since periods greater than 90 min would not result in a significant C enhancement nor in a greater HC production. Analogously, for a fixed residence time, there is a wide combination of couples T-R which would result in HCs with varying C proportions (37.5e57.5%) but similar SY values; in this case, temperature would control the C proportion. Although the proportion of carbon on the HC is extremely important, the examination of Ccapture (%) can also be very interesting (Fig. S2); it corresponds to the real amount of C from biomass that has remained on the HC and can therefore very useful in terms of reaction efficiency. In general, at higher temperatures less carbon is retained in the solid phase, and a greater quantity is released to the aqueous phase as degradation products in the liquid phase or gas products (most of it expected to be as CO2). In general, lowest temperatures resulted in 90e100% carbon capture, while using highest temperatures under certain conditions led to an enhanced migration of carbon from the precursor (values lower than 30%). However, our results indicate that this rule cannot be applied under all conditions, as biomass load and especially time can make this trend to be different or even reversed. In this section, we try to describe these effects and to relate them to the reaction mechanisms. Despite stating preferable pathways during HTC is very complex, van Krevelen diagram (plot of H/C versus O/C) can provide some insight about the extent of reaction [36]. This representation allows identifying dehydration, decarboxylation and demethylation mechanisms, by the estimation of the slope of lines from the precursor and the corresponding HCs; moreover, the extent of HTC is associated with the decrease of both ratios, which, for coal, are located next to the axes intersection. Fig. 3 compiles the results obtained from Ccapture (fixed temperature plots, Fig. S2 d-f, were chosen) with Van Krevelen diagram. In the 2D level curves as well as in Van Krevelen diagram, different color circles have been included, representative of particular HTC runs (all collected in Table 1). In Fig. 3, the colors of the circles representing different HTC conditions were defined as: a) green, ensemble of points for which

Fig. 3. C capture (%) level curves with location (red, green and purple points) of particular HTC runs (aec) and Van Krevelen diagram showing the H/C and O/C ratios of these runs. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

the extent of HTC was lower, as deduced from their location in Van Krevelen diagram (closer to WH); c) red, points which are located in an intermediate position and c) purple, experiments for which a greater decrease on both ratios was found, indicating their composition more similar to coal. It was found that in general, green points corresponded to high solid yield values and higher carbon captures (note the location of green points in 2D level graphs as compared with other points); analogously, red and purple points were associated to lower carbon capture values (also SY values). From the detailed examination of Fig. 3 (and Table 1) the following conclusions can be inferred: a) Van Krevelen diagram suggests that the main degradation mechanisms for HTC was dehydration; however, some participation of decarboxylation might be also suggested since the arrow joining pristine WH with each run can exhibit a different orientation; for example, from the comparison of 178/36/18 and 232/36/18 an enhancement decarboxylation at higher temperatures can be deduced. In coherence, this is accompanied by a lower SY and Ccapture for the experiment made at 232
C. b) The extension of residence time can change the role of temperature, in other words, in contrast with short time reactions, more carbon is retained at the solid phase for higher temperatures if long treatments are used. This is probably indicative of the participation of solid char formation under these conditions, as it has been previously suggested: in fact, at 205 and 232
C Ccapture eventually levels off, indicating that whether there are still degradation reactions, some carbon compounds are also being formed and deposited in the HC surface, and both effects occur in an extension that makes that although SY decreases, the carbon amount on the solid phase remains moderately similar. c) The use of longer times clearly helps the process to yield an HC with a closer composition to coal, i.e., the effect described in b) about char formation is more likely associated to purple


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points in Van Krevelen diagram (compare runs 205/68/30 and 205/120/30). These purple points also show a consistent slight increase in SY for longer times, in coherence with the adsorption of C compounds on the HC. This effect is also found for low temperatures, although it is less marked (see 178/36/42 and 178/99/42). d) Biomass load can also have an effect on Ccapture, which is different depending on the experimental conditions; and shows a trend similar than that described for C (%), although the effects are more marked, as a result of the SY importance in the former parameter. e) Increasing the amount of biomass can enhance the extent of the process (points move towards the axes) and may cause a greater prominence of decarboxylation (see points 232/36/ 18 and 232/36/42 and points 178/36/18 and 178/36/42). In both cases, this effect also corresponded with lower SY and Ccapture values. The oxygen and hydrogen decrease with temperature have also been found as a general trend in the literature, the former being associated with the most significant contribution to the solid yield decrease [37]. Table 1 lists the values of HHV of the HCs obtained. The increase on HHV as compared to parent materials during HTC has been well documented; greater temperatures are generally associated to enhance HHV values, as a consequence of larger C content [25,30]. From our results, it was found that a strong synergy between time and temperature exists; in such a way that the effect of time is more marked if greater temperatures are used, and also, the effect of temperature is very scarce for low time periods but becomes more important for longer HTC treatments. Values in the range 16.9e21.9 MJ kg1 were obtained, which mean a 10e14 fold increase in relation with original WH stem (pristine) and a rise of 1.2e1.5 times, in relation to dried precursor. These ranges are similar or greater than those reported in the literature. The carbon densification achieved for water hyacinth can then open the possibility of upgrading this weed, although, at this point, the scalability of the process for this material should be further studied. In this regard, several hydrothermal treatment plants are currently under operation, at different industrial scales, in different areas of the world, and with different goals [38]. For instance, some companies (one example is Renmatrix, in USA and Canada, or GF biochemical in USA and Germany) work on the production of platform chemicals (sugars or high-interest compounds such as levulinic acid) by hydrothermal liquefaction from pure biomass. In the case of HTC and the specific use of this technique for heterogeous biomass, in the line of the present study, there are also some companies under operation, at different scales and using several feedstock; some examples are INGENIA (Spain), AVA and Grenol (Germany), Suncoal (Germany), Terranova (China) … From the authors viewpoint, the HCs can be a substitute for fossil coal, as long as several challenges are solved, apart from the fact that so far the price of coal is still low (reality that might be changed during next decades, according to all estimations). An important challenge to foster a greater presence of HTC on the energy paradigm are the recovery of interest-compounds and nutrients from the liquid phase (and managing of this liquid to make it clean for the environment). Also, helping a circular green economy joining farmers, energy managers and HC suppliers is essential to approach this reality. 3.2. Fate of nitrogen It has been generally reported that the way in which N is

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bounded in the precursor (which in turn depends on the specific feedstock) strongly influences its subsequent degradation upon HTC. It is accepted that proteins in biomass are a major source of N (organic N) which, during HTC are hydrolyzed into amino acids at temperatures as low as 120
C [39]. These amino acids can further be converted to other amino acids or can take part on decarboxylation and deamination reactions to yield amines, ammonia or organic acids; moreover, N-cyclic compounds can be formed via Maillard reactions [28]. Additionally, N in biomass can also be found as oxidized N (nitrate or nitrite) [40]. In Fig. 4 the N (%) data obtained for the different runs (also collected in Table 1) have been plotted. Ncapture 2D level graphs have been provided as supplementary material (Fig. S4). The N capture values achieved by WH HCs are significantly higher than those obtained under similar conditions for other high protein content biomass sources such as microalgae Chlorella [23], or swine manure [38]. Our results also showed that HTC can be tuned up to either decrease or increase the N proportion of the HC in relation to the precursor. As a general rule, there is agreement on the fact that increasing temperature corresponds to a greater extent of protein breakdown, thus yielding a lower N content in the solid HC [38,40,41]; this occurs at the first stages of the process. However, as the reaction proceeds, additional phenomena contributing to N capture can get a significant prominence, depending on the system conditions. In this line, previous pieces of research have proposed the existence of adsorption processes [41,42]. For example, under acidic conditions (enhanced at greater temperature) deamination of amino acids is prompted, yielding a greater proportion of ammoniacal N or nitrate ions. The first one might undergo adsorption onto the HC, since electrostatic attractive interactions between the char and the adsorbate will be favored, provided that the HC has negative surface charge due to ionization as a result of its surface acidity [43]. The potential adsorption of ammonia on the HC surface can be regarded as a highlighting advantage, since it might boost HC application as a slow release fertilizer [44]. On the other hand, nitrate adsorption via specific interactions might also be considered [45], although the latter compounds are expected to decrease its concentration in the liquid phase at longer reaction times [38]. Finally, another contribution to N capture on the solid HC has been related to nitrate salt precipitation, especially for rising temperatures and times [17]. These authors reported a drastic reduction on the N content of HCs at the beginning of the reaction, followed by a slight modification (up or down depending on the feedstock) with time. Gai et al. [35] obtained an increase on the N content of the chlorella HCs for long treatments in the range 30e90 min) at 250 C. On the other hand, graphs 4 (a-i) show a radial trend, with a different slope (sharper gradient) for greater temperature (d-f) or time values (g-i). This means that both time and temperature have to attain a cut value in order to allow to obtain of a certain N proportion on the HC. Once that value is surpassed, both properties have a positive influence on the N content of the HC. That might indicate that enhanced time (and associated effects such as adsorption or precipitation of N salts) is only important if a minimum temperature is reached; said in other words, unless temperature is enough to produce compounds that are prone to be adsorbed on the HC we will not find this effect. Analogously, increasing temperature will lead to a migration of N to the liquid phase which can be reversed provided a minimum dwell time is applied. According to the values of normalized coefficients shown in Table S1, N is the only parameter studied in which time is more influent than temperature. Ratio biomass/water does not have a remarkable influence on this parameter on a wide range of operating conditions as inferred from Fig. 4(aec); in which the shape of the isolines are equal and

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Fig. 4. Influence of experimental variables on N proportion (%) on the solid phase.

the values of N (%) have similar ranges. However, the inspection of Fig. 4g-h can suggest that R is decisive for enhancing N content under particular circumstances. From the detailed examination can be noticed that the red area is shifted up and to the right (i.e., to greater biomass loads and temperatures) as time is extended. This means that R has a positive effect on this parameter and also that the maximum N proportion will be achieved only if a certain ratio is used. For instance, if a high N proportion is aimed, and therefore long times are applied (i.e., 99 min, Fig. 4i), then, it is not possible to get N ¼ 3.5% unless a biomass ratio of 32% is used. Following the trend of Ncapture (%) on the solid phase (Fig. S4) can also be very useful. Apart from giving information about the process efficiency, it allows the direct estimation of the amount of N that migrated to the liquid phase (100-Ncapture, since no N is expected as gas product under these conditions), which can also find applications in many sectors (production of chemicals or nutrient medium for plant growing, for example) [28]. Ncapture results indicate that the amount of N remaining on the solid phase is very dependent on HTC conditions (values ranging between 20 and 70%). In general, rising temperatures promote the migration of N to the liquid phase, for all ratios, unless long times are used (see Figure S4a-c). Finally, the effect of using greater loads and time is

quite clear: it always helps the HC enrichment on N; reasons for these are associated to the joint effect of greater N proportion and SY, and have previously been discussed. In case that a high N migration on the liquid phase is required, not only the N quantity but also the form in which this element is bonded should be studied. For example, previous manuscripts have reported a noticeable impact of this variable on the speciation of the nitrogen and can lead to the formation of unwanted species which at certain concentrations might be harmful to the environment. Although this is beyond the scope of this work, attention should be paid to this issue in future works. 3.2.1. Surface properties The surface structure of the HCs was studied by means of N2 adsorption at 77 K and SEM imaging, while XPS provided additional information on surface N functionalities. To the best of the authors’ known, none of these techniques have been used before to explore WH HCs. Nitrogen adsorption analyses at 77K were performed on selected HCs; the isotherms have not been included here for the sake of brevity, while characteristic textural parameters obtained from N2 adsorption experimental data (SBET, Vmi, Vme and %SEXT) have been collected in Table S2, provided as Supplementary


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Fig. 5. SEM micrographs of HC 232/99/42 under different magnifications: a) 400; b) 1800 and c) 5000.

material. In general, and in consistence with previous works [18,25], the HCs had a very low porosity development, which is directly related to the permanence of degradation products on the material surface. In consistence, values of SBET were very low, in the range 2e27 m2 g1. A detailed examination of the shape of the isotherms allowed to infer that the materials presented slight differences in the pore size distribution. Samples prepared during longer processing time and at higher temperatures had a greater micropore contribution; one can see that, although all isotherms correspond to mesoporous materials and can be classified as type IV (BBDT classification), samples prepared under these conditions (232/99/42, 232/99/18) exhibit a slightly greater adsorption volume at low relative pressures, associated to micropore filling. This might be related to a better diffusion of HTC products through the HC matrix. Although it is very scant, the porosity of the HCs might be further developed by means of physical activation under different atmospheres (air, CO2 and steam) [43]; other possibilities [18], include the addition of chemicals (KOH, H3PO4 etc.) to the reaction media in order to further the formation of carbonates which can therefore be removed by pyrolysis. These processes can inspire new routes to produce adsorbents and will be investigated in future works. Surface morphology SEM analyses revealed that the precursor structure was maintained after HTC. A rough and heterogeneous surface was found in all cases and no remarkable differences were found between samples made under different conditions. Firstly, Fig. 5 a shows a typical HC particle, in which one can see the existence of some irregular cavities as well as the presence of white particles on the HC surface; these particles can be associated with the mineral content of the precursor which, as a consequence of the thermal treatment, are sintered or precipitate on the HC surface. Further elemental energy-dispersive X-ray (EDX) analyses confirmed that the HC mineral composition mainly consisted of Ca, P, Mg, Si and Al. In addition, the HCs presented sphere-like microparticles, commonly attributed to the degradation of cellulosic compounds, such as glucose [19]; owing to their hydrophobic nature, these products combine themselves in the form of spheres, to minimize the surface in contact with water. This hydrophobicity is consistent with the occurrence of physical dehydration reactions (dewatering), in which bound water is ejected from the biomass during HTC. Finally, the chemical nature of the nitrogen groups was examined by XPS high-resolution spectroscopy. High resolution N 1s spectra of selected samples (170/36/18, 205/68/30 and 205/120/30) have been provided as supplementary material in Fig. S5. From the analyses, three main peaks were found and were assigned to the following functionalities: quaternary-N (401.1 ± 0.2 eV) in N-cyclic compounds, NeH-R bonds in amides (399.0e400.0 eV) and CeNH2

Table 2 Relative amounts of nitrogen moieties.

205/120/30 205/68/30 178/36/18

NeH Amine

NeH Amide

N-cyclic

0.609 0.539 0.228

1.863 1.558 1.175

1.197 1.023 0.856

bonds in amides (398.7e398.8.0 eV). The relative amount of the identified nitrogen moieties has been included in Table 2. However, the authors also suggest the presence of pyridinic-N (398.6 ± 0.2 eV) and pyrrolic-/pyridonic-N (400.4 ± 0.2 eV) because of the proximity of the signals (a slight shift of the spectra or overlapping might be considered since these compounds have been identified as the most abundant N forms in chars) [46]. Apparently, a greater concentration of N-compounds was found in the case of samples prepared under more severe conditions, in coherence with elemental N characterization: greater temperatures enhance protein and, from them, amino acid hydrolysis into ammonia and other N-organic compounds probably via Maillard reactions and then, these compounds are adsorbed onto the HC surface as long as enough time is provided. Finally, no peak attributable to oxidized pyridinic nitrogen was found in any of the HCs, as suggested in previous papers [19]. On the other hand, the greater HC aromatization as deduced from the rise in N-cyclic compounds is consistent with Van Krevelen diagram results. 4. Conclusions Hydrothermal carbonization can provide a sustainable green route to produce added value materials from Water Hyacinth. It was found that the experimental conditions used (temperature, time and ratio biomass/water), had an influence on the process reactivity as well on the material physico-chemical properties. Heating values up to 21.8 MJ kg1 were obtained; the results also showed that optimal conditions to achieve high C capture values were associated to less aggressive conditions and involved a decrease in the heating value, mainly owing to the important role of solid yield. On the other hand, while under any experimental condition C proportion was always improved after HTC, in the case of N, the process resulted in a decrease or an increase on this element depending on the severity; time was particularly important to allow N recovery on the HC which most mainly associated to the combined effect of adsorption and salt precipitation. Acknowledgements The authors are grateful to MINECO and the Junta de

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Extremadura, for financial help by projects CTM2016-75937-R and IB16108, respectively, and also to the program ”Ayudas a grupos de la Junta de Extremadura-Fondos FEDER”.

[22]

Appendix A. Supplementary data

[23]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.07.157.

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[25]

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