Characterization of hydrochar pellets from hydrothermal carbonization

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Biofuels and Biomass

Characterization of hydrochar pellets from hydrothermal carbonization of agricultural residues Youjian Zhu, Yaohui Si, Xianhua Wang, Wennan Zhang, Jingai Shao, Haiping Yang, and Hanping Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02484 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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Characterization of hydrochar pellets from hydrothermal carbonization of agricultural residues Youjian Zhu1,2#, Yaohui Si1#, Xianhua Wang1, Wennan Zhang3, Jingai Shao1, Haiping Yang1*, Hanping Chen1 1

State Key Laboratory of Coal Combustion, Huazhong University of Science and

Technology, Wuhan 430074, Hubei Province, China 2

School of Energy and Power Engineering, Zhengzhou University of Light Industry,

Zhengzhou, Henan 450002, China 3

Department of Chemical Engineering, Mid Sweden University, 85170 Sundsvall,

Sweden

* Corresponding author. Haiping Yang; 1037 Luoyu Road, Wuhan, Hubei 430074, P.R. China; E-mail address: [email protected]; Tel: +86 27 87542417; fax: +86 27 87545526. #

These two authors contribute equally to this work.

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Abstract In this work, the effects of operating conditions of hydrothermal carbonization on the hydrochar pelletization and combustion characteristics were investigated. Corn stalk was carbonized under different conditions and then pelletized to obtain the hydrochar pellets. It was found that hydrothermal temperature and residence time greatly affect the pellet quality. When the temperature was raised up to 240 °C with the residence time longer than 60 min, the heating values of hydrochar were close to or even higher than that of lignite. After hydrothermal treatment, 73.71-94.71% K and 91.81-94.32% Cl contained in the feedstock were removed, indicating a low fouling and slagging tendency when the pellets are used in combustion. The compressive strength and durability increased firstly with increasing temperature and then decreased with further increasing the temperature from 240 to 300 °C. The influence of residence time showed a similar trend and the compressive strength and durability reached its maximum value at the temperature of 240 °C and residence time of 60 min. The hydrophobicity of the hydrochar pellets increased with increasing the temperature and residence time. Hydrochar pellets obtained at the temperature of 240 °C with residence time of 60 min gives the best performance, which can meet the requirement of industrial fuel pellet. Finally, the combustion characteristics were investigated by thermogravimetric analysis and the results indicated that hydrochar pellets were combusted in a comparatively mild way with a high thermal efficiency. As a general conclusion, the hydrochar pellets have


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much better qualities than the raw corn stalk, facilitating the transportation, long-term storage and combustion application.

Key words hydrothermal carbonization; agricultural residues; inorganic species; pelletization; combustion


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1. Introduction Agricultural residues are abundant and widely distributed in China as well as many other countries. The latest official statistics shows that the theoretical reserve of agricultural residues in China is 1.09 billon ton per year. The traditional treatment methods such as open burning, pulverization and return to field cause environmental pollutions and leave a huge amount of energy unutilized[1]. Researches regarding efficient and environmental-friendly treatment of agricultural residues have gained increasing attentions in the past decades. There are many technologies turns agricultural biomass into thermal energy or bio-products[2,

3]

. Among them, combustion is a

straightforward method to easily, reliably and effectively convert solid fuel to heat and electricity [4]. The combustion/co-combustion of agricultural residues can replace coal to a certain degree and also mitigate CO2 emission and environmental pollutions issues resulting from coal combustion. However, agricultural biomass generally has a low bulk and energy density, high alkalis and chlorine contents, and the availability is much dependent on seasonal and geographical conditions[5]. These issues make transportation difficult and restrict the efficient and large-scale utilization.

Densification of biomass residues into a regular shape product, e.g. pellets or briquettes, can increase the bulk density with uniform shape and size[6]. Such a homogeneous solid fuel gives rise to a great reduction in transportation costs and also much easier handling and

feeding

in

application.

However,

agricultural

biomass

has

a

low

friability/grindability and consumes large amounts of energy during the densification 4 ACS Paragon Plus Environment

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process. The compressive strength and durability of agro-pellets are generally low due to its highly fibrous structures[7], and the quality deteriorates rapidly under high moisture environment due to its hydrophilic nature. Pellets with high moisture content favour fungal growth, which causes fuel decomposition, and self-ignition at an extreme condition in transportation and storage

[8]

. Moreover, most ash such as alkalis and

chlorides remain in the pellets and present a potential risk in later combustion application. Therefore, appropriate pre-treatment methods are needed to improve the quality of fuel before densification.

Torrefaction is a promising thermal treatment method to upgrade biomass into a homogeneous and energy dense solid products[9,

10]

, which can be divided into two

modes, wet and dry torrefaction. Dry torrefaction is an energy intensive process and the resultant biochar is difficult to be pelletized. Moreover, the relative ash content of the biochar is generally higher than that in the original biomass fuel, which suggests more severe ash related issues[9]. Wet torrefaction, which is also known as hydrothermal carbonization(HTC), is a thermochemical conversion process in water medium under high pressure at a relatively low temperature (150-260 °C). This process simulates the natural coalification in coal petrology. The resulting solid phase product, namely hydrochar, presents a similar fuel quality to that of lignite

[11, 12]

. Nizamuddin et al.[13]

reported that the heating values of hydrochar, obtained after hydrothermal carbonization of palm shells at 220-290 °C, were in the range of 21-27 MJ/kg. It increased substantially as compared to palm shells and was close to that of lignite. Meanwhile, the 5 ACS Paragon Plus Environment

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fibrous structure was destroyed during the treatment, resulting in an enhanced grindability[14]. Moreover, the hydrophobicity can be greatly improved after hydrothermal treatment due to the elimination of hydrophilic group such as hydroxyl functional group. Thus, in comparison to agro-pellet, an excellent fuel, hydrochar pellet, can be produced via combined hydrothermal treatment with pelletization[8, 11, 15]. Liu et al.

[11]

reported that the mechanical strength of the hydrochar was greatly enhanced by

approximately 1-4 times compared to the corresponding raw biomass fuels, and the equilibrium moisture content of the hydrochar was decreased significantly to less than 2 wt%. Kambo et al.[8] reported that miscanthus had a good grindability and hydrophobicity after hydrothermal carbonization. The produced hydrochar presented comparable physicochemical properties to lignite. It can be expected that hydrothermal treatment of agricultural biomass in combination with pelletization provides a better alternative for its large scale utilization [11].

In the HTC process, the operating parameters, such as reaction temperature, residence time and pressure, have notably influences on the properties of the resultant hydrochar[16, 17]. Hoekman et al.[18] found that, during the HTC of a mixture of Jeffrey Pine and White Fir, the gas and liquid yields increased but the char yield decreased with increasing hydrothermal temperature and time. Nizamuddin et al.[14] reported that hydrochar yield decreased from 62.4 to 43 % when hydrothermal temperature was increased from 220 to 290 °C, but the carbon content and higher heating value increased from 46.52 to 63.77 % and from 20.8 to 26.8 MJ/kg, respectively. This is due to the 6 ACS Paragon Plus Environment

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enhanced degradation of biomass component and the change of hydrochar structure at higher temperature[19]. It can be anticipated that the fuel properties (e.g. component and structure) of the hydrochar greatly affect the following densification process. However, the current studies mainly focus on the effects of operating conditions on the properties of resultant hydrochar. Little attention was paid to the effect of hydrothermal conditions on the following densification process and also the physical properties of the hydrochar pellets.

With regard to the behaviour of inorganic species of biomass during the HTC process, Reza et al.[20] indicated that 5.1-59.5 wt% of ash was removed in HTC depending on the fuel compositions and operating conditions. Smith et al.[21] reported that 60-97% of K and 46-79% of Na were removed from biomass after HTC. The removal of other inorganic species such as Mg, Ca and P was relatively low in comparison to K and Na. Mäkelä et al.[22] indicated that ash behaviour during HTC treatment was closely dependent on the feedstock type and hydrothermal treatment conditions. During HTC treatment of paper sludge, they found that the ash yield was controlled by the solubility of calcium carbonate, which was the main mineral component in paper sludge[23]. The above mentioned studies mainly focus on woody biomass. On the other hand, agricultural residues have a high K and Cl content, leading to severe ash-related problems in combustion such as slagging, sintering, agglomeration, defluidization, fouling and corrosion[24, 25]. Previous studies claimed that the vast majority of K and Cl in agricultural biomass are water soluble and can be removed by water washing[26]. 7 ACS Paragon Plus Environment

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However, the transformation behaviour of K and Cl under different hydrothermal conditions are still not clear and corresponding researches are needed.

In this study, a typical agricultural biomass residue, corn stalk, was used to produce hydrochar under different conditions. The hydrochar samples were then pelletized and evaluated in terms of ash composition, pellet density, compressive strength, durability, hydrophobicity and also combustion characteristics. The densification mechanism was also investigated with the aids of SEM and FTIR analysis of the hydrochar pellets.

2. Experimental section 2.1 Fuels A typical agricultural residue, cornstalk (CS), collected in Henan Province, China, was used in this study. The selected CS is one of the most abundant agricultural resources in the central part of China and is considered to be an alternative solid fuel to replace coal for heat and electricity generation

[27, 28]

. The received fuel was air-dried and milled to

get a particle size of less than 0.6 mm. The milled fuel was dried overnight in an oven at 105 °C and then stored in a desiccator for the following experiments. 2.2 Hydrothermal carbonization HTC was conducted in a 100 ml pressurized stirred tank reactor (SLM-100, Shenglang Instrument Company, Beijing, China). CS was dispersed in distilled water with a mass ratio of 1:10 and then transferred into the reactor. An Ar flow rate of 100 ml/min was used firstly to purge the reactor for 10 min. Then the reactor was heated to the desired 8 ACS Paragon Plus Environment

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temperature (180, 240, 300 °C) at a rate of 5 °C/min and hold for a certain time (30, 120, 240 min) referred to as the reaction residence time. After reaction, the reactor was cooled down to room temperature by a fan and cooling water. The resultant products, consisting of a liquid solution and a solid residue, were transferred carefully into a beaker and then separated by vacuum filtration. The obtained solid residue, namely hydrochar sample, was dried overnight in an oven at 55 °C and weighed by electrical balance to obtain the mass yield. All the experiments were repeated three times to get the average values. The hydrochar samples obtained at different temperatures and residence times were denoted as HTC-T-t, where T and t represents the operating temperature and residence time, respectively. For example, HTC-240-120 means that CS was carbonized at 240 °C with a residence time of 120 min. 2.3 Pelletization of the raw and hydrochar materials A universal material testing machine (WDW3200, Kexin Corporation, China) with a piston mold was used to produce fuel pellets. The testing machine and the schematic diagram of the piston mold are presented in Figure 1. The detailed information of the equipment was reported elsewhere[7]. Approximately 1 g feedstock sample was placed in a cylindrical mold of 8 mm in diameter, and pelletized at a compressive force of 13.6 MPa and a compression rate of 10mm/min with a holding time of 2 minutes. After pelletization, the pellet was ejected from the other end of the mold at a compression rate of 5 mm/min. The data of the pressure and displacement were collected and recorded


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using a Power-Test software (SANS, China) to calculate the energy consumption during the pelletization process, including both the compression and ejection. 2.4 Characterization of the pellets The proximate, ultimate analyses and heating values of the samples were analysed by using a Setaram Labsys Evo1150 simultaneous thermal analyzer (Lyons, France), a EL2 type elemental analyzer (Vario, Germany) and an automatic calorimeter (model 6300, U.S.), respectively. The sample was burned at 550 °C and the resultant ash was analysed by X-ray fluorescence to obtain the ash composition (EAGLE III, EDAX Inc., U.S.).

The mass density of the pellets is defined as the ratio of the mass to the volume. The weight of the pellet was measured using an analytical balance (Mettler ToledoMS204S) with an accuracy of 0.0001 g. The volume of the pellet was determined by measuring the height and diameter of the pellet using a digital calliper (Mastercraft, 58-6800-4). The energy density, which is a measure of how much energy is contained per unit volume of a fuel[29], was calculated by multiplying the mass density of the pellet with the high heating value (HHV) of the pellet.

Compression strength is used to obtain the internal bonding strength or the maximum force a pellet can withstand before its rupture via a compression test[7]. The compressive strength of the pellets was tested using a cylindrical metal probe (20 mm diameter). Each pellet was placed individually in a horizontal direction in the universal material 10 ACS Paragon Plus Environment

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testing machine (WDW3200, Kexin Corporation, China) with a compression rate of 2 mm/min until the pellet was crushed[7,

30]

. The compressive strength is calculated

according to the following equation:

σ=

2F π dl

(1)

Where F represents the maximum compressive force, d and l represents the diameter and length of the pellet, respectively.

Durability is defined as the ability of pellets to remain intact during transportation and handling by measuring the amount of fine particles produced after subjecting the pellets to mechanical or pneumatic agitation[8]. The durability of biomass pellets was tested herein using a tumbling can method[7, 31]. Approximately 30 pellets were placed in a 2 mm sieve where they were vibrated for 10 min at 100 rpm. After the tumbling, the pellets were sieved and weighed. The durability (I, %) is calculated according to the following equations.

I = 100 − (mi − mf ) / mi ×100%

(2)

where mi (g) and mf (g) are the initial and final masses of the pellets, respectively. The hydrophobicity of the pellets was measured using a moisture absorption test[7, 31]. The pellets were exposed to a controlled environment (relative humidity of 70% at 30 °C) and the weight of the pellets was measured every 15 min for the first hour and every hour for the remaining 8 hours. The change in the weight was expressed as the


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moisture absorption rate of the pellets. For all the physical characterization of the pellets, at least two replicates were conducted to assess the reproducibility of the results. 2.5 SEM and FTIR characterization Surface morphology of corn stalk and the hydrochar pellets were analyzed using a Quanta 200 environmental scanning electron microscope (ESEM). The samples were sprayed with Au and then analysed in the secondary electron mode. Fourier transform infrared spectrometer (VERTEX 70 FT-IR, Bruker Corporation, Germany) with an ATR accessory (SPECAC Corporation, Germany) was used to analyze the functional group changes during HTC. 2.6 Thermogravimetric analysis Combustion test was carried out using Setaram Labsys Evo1150 simultaneous thermal analyzer in an air atmosphere. In light of the size of the crucible, the pellet was firstly broken into small particles of approximately 20 mg. These particles were loaded into the crucible and then heated from room temperature to 850 °C at a heating rate of 10 °C/min. An air flow rate of 100 ml/min was maintained during the whole combustion test. Prior to the test, a baseline test without sample was conducted to account for changes in the apparent weight resulted from buoyancy effects[32]. The weight loss curve was later corrected by subtracting the baseline. Repeated runs were conducted for each combustion test to validate the reproducibility of the results.


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3. Results and discussion 3.1 Fuel properties The proximate, ultimate analyses and heating values of corn stalk and the hydrochar samples are presented in Table 1. It can be seen that the hydrochar mass yield from HTC treatment of CS decreases remarkably from 70.57% to 33.40% with the increase of temperature. The residence time has a less influence on the mass yield than the temperature. Similar results were reported in previous work[33]. The properties of hydrochar remain almost same as corn stalk at low HTC temperature of 180 °C. With the increase of temperature or residence time, the fixed carbon content increases substantially up to ~39.55 wt% against a decrease in volatile content. The same trend can been seen in the ultimate analysis with respect to the C content against O, and the ratios of O/C and H/C also decrease significantly with the temperature or the residence time. Similar results were observed by Nizamuddin et al[14, 34].

Under hydrothermal condition, the subcritical water becomes more reactive and facilitates the hydrolysis of macromolecules compounds and the cleavage of oxygencontaining functional group (e.g. ether and ester bonds among monomeric sugars)

[8]

,

leading to the weight loss of solid products and the reduction in the O and H contents. The hydrochar properties varies to a great extent depending on the operation parameters such as temperature, residence time, pressure, solid load and PH values of the solutions[17,

35]

. Funke et al[35] indicated that hemicellulose was almost completely

hydrolyzed around 180 °C, and the onset temperature of hydrolysis of cellulose and 13 ACS Paragon Plus Environment

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lignin are 220 and 200 °C, respectively. Raze et al[20] found that no hemicellulose could be found after HTC at 200 °C. This was attributed to the cleavage of the b-(1-4) glycosidic bonds of hemicellulose into sugar monomers, which further degrade into furfurals and other compounds, including 5-HMF (hydroxyl methyl furfural)[20]. Therefore, the weight loss at 180 °C is mainly attributed to the hydrolysis and decomposition of hemicellulose. As the temperature increased to 240 °C, cellulose degrades into intermediate compounds, e.g. hexoses, polysaccharides, furfurals, and 5HMF (hydroxyl methyl furfural)[20]. These intermediate compounds may undergo dehydration, decarboxylation, polymerization, aromatization and other reactions to form gas and liquid products as well as hydrochar[36]. Meanwhile, some lignin was fragmented and dissolved in the aqueous phase to from phenolic derivatives, which repolymerized subsequently with other water-soluble compounds to form hydrochar microspheres[37]. However, this is a diffusion controlled process and a longer residence time is needed to dissolve more lignin in the aqueous phase. Thus, the hydrochar mass yields decreased with increasing residence time as shown from this study. Dinjus et al.[38] reported that 180–250 °C is too low for a strong chemical modification of lignin and only a small fraction of lignin was degraded in this temperature range. Further weight loss at temperature up to 300 °C can be attributed to the degradation of nondissolved lignin under severe condition [12, 33].

The chemical composition and the characteristic parameters of the CS ashes and hydrochar samples are presented in Table 2. The CS ash is mainly composed of K2O, 14 ACS Paragon Plus Environment

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SiO2 and Cl with small amount of CaO, MgO, Al2O3, SO3 and P2O5. After hydrothermal treatment, the contents of K2O and Cl decrease dramatically from 25.50% and 11.31% to 1.49~6.58% and 0.55~0.9%, respectively. The contents of CaO, MgO, Al2O3 and SO3 vary slightly with a clear increase of SiO2 and P2O5. The removal efficiency of Cl is quite high (91.81-95.45%) and is influenced slightly by the operating condition. However, the removal efficiency of K is lower in comparison to Cl and increases clearly with the increases of temperature and residence time. Previous research found that nearly all the Cl is presented as soluble salts in biomass fuels and can be removed efficiently by water washing[26, 39]. In comparison, although the majority of K exists in water soluble form, certain amount of K is presented in organic forms, e.g. alkaline carboxylates or phenol associated K[40, 41]. The organic-bound K decomposes under high temperature and/or long residence time and is eventually released to the gas/liquid phase. Moreover, the generated organic acid during the HTC process can also increase the solubility of inorganics components (e.g. alkali and alkaline earth metal species) [8, 42].

In biomass thermochemical conversion process, alkali metals (K and Na) are readily released to gas phase and cause fouling and corrosion

[43, 44]

. The problems become

more severe in the presence of large amount of Cl in the fuel

[45]

. High K content in

biomass fuel also give rise to the formation of low-melting temperature eutectic mixture with SiO2, resulting in severe ash sintering and slagging in the reactor. Corn stalk has an extremely high content of K, Si and Cl and the aforementioned issues are deemed to occur without effective countermeasures[46]. After hydrothermal treatment, most of K 15 ACS Paragon Plus Environment

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and nearly all Cl are removed, which is a great benefit for utilization of agricultural biomass residues. In this paper, an empirical alkali index is employed herein to predict fouling and slagging during the thermochemical conversion process

[47, 48]

. An alkali

index above 0.17 kg alkali/GJ suggests that fouling and slagging possibly occur and above 0.34 kg alkali/GJ the occurrence of fouling and slagging are inevitable[48]. Corn stalk has an alkali index of 0.9 kg alkali/GJ suggesting serious problems of fouling and slagging. For HTC-180-60 and HTC-240-30, the alkali index is 0.19-0.21 kg alkali/GJ, indicating the fouling and slagging tendency is greatly reduced in comparison to corn stalk. The alkali index is further reduced to 0.02-0.13 kg alkali/GJ with increasing hydrothermal temperature and residence time, indicating a low possibility of fouling and slagging. It can be concluded that hydrothermal treatment can effectively reduce the K and Cl content and therefore reduce fouling and slagging during thermochemical conversion. 3.2 Physical properties of the hydrochar pellets Mass and energy density Table 3 shows the physical properties of corn stalk pellet and the derived hydrochar pellets. Corn stalk pellet has a mass density of 934 kg/m3, which is slightly lower than the threshold (1000 kg/m3) of industrial fuel pellet

[49]

. After hydrothermal treatment,

the mass density of all the hydrochar pellets is higher than 1000 kg/m3. Meanwhile, the mass density of the hydrochar pellets increases obviously with temperature and reaches its maximum value at 240 °C and then slightly decreases with further increasing the 16 ACS Paragon Plus Environment

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temperature to 300 °C. A longer residence time results in a higher mass density but its influences on mass density is less pronounced compared to hydrothermal temperature. Energy density of the hydrochar pellets increases by 31-104% compared to corn stalk pellets, and reaches the maximum value of 31.15 GJ/m3 for HTC-240-120. The increment of mass density can be explained by the enhanced grindability after hydrothermal carbonization[15], which makes the resultant material highly friable and easy to crush[8]. Mass and energy density of pellets are the two important parameters from the view of transportation and handling. The transportation and storage of a fuel with high mass and energy density is less expensive as it occupies less space

[50]

,

therefore resulting in a substantial decreases in the cost of handling and transportation.

Compressive Strength and Durability As seen in Table 3, CS pellet has a low compressive strength and durability of 2.88 MPa and 82.49%, respectively, which increase to a great extent after hydrothermal treatment. The compressive strength and durability reach 8.51 MPa and 95.39% at 180 °C, and increase slightly at 240 °C but decreases with further increasing the temperature to 300 °C. For residence time, the influence is more noticeable as seen in Table 2. The compressive strength and durability increase slightly in the residence time of 30 min but significantly in the residence time of 60 min. However, further increasing the residence time to 120 min leads to lower compressive strength and durability. The maximum compressive strength and durability of the hydrochar pellet were obtained for the residence time of 60 min at a hydrothermal temperature of 240 °C. Pellets treated at 17 ACS Paragon Plus Environment

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a too high temperature and a too long residence time e.g. HTC-240-120 and HTC-30060, were found to have worse mechanical properties. These two pellet samples show a high brittleness and tend to crack and produce relatively wide gaps between particles during the densification process (as shown in Figure 3), indicating a low compressive strength. This might be explained that more fine particles with a diameter of less than 100 µm is produced at higher hydrothermal temperature, and the resultant pellets tended to shatter into powder during the durability test[8].

Energy consumption Energy consumption of pelletization is 38.15 kJ/kg for CS, but increases to 43.1-52.25 kJ/kg for the hydrochar with HTC temperature from 180 to 240 °C as seen in Table 3. The effects of residence time show a similar trend. In the HTC process, most of hemicellulose and part of cellulose had broken down into fragments by hydrolysis and transformed into aqueous phase. The degradation of hemicellulose and cellulose from hydrothermal treatment reduces the viscoelasticity of the pellet, which results in a higher energy consumption[51]. Lam et al.[51] reported that the resultant hydrochar had a higher friction coefficient than the original biomass. The higher friction coefficient also contributes to higher energy consumption of hydrochar.

Hydrophobicity As seen in Figure 2, Corn stalk shows a highly hygroscopic behaviour and has an equilibrium moisture content of approximately 15wt%. HTC treatment significantly 18 ACS Paragon Plus Environment

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improves the hydrophobicity of the pellets by reducing the moisture content to less than 10 wt%. The equilibrium moisture content of the hydrochar pellets decreases with increasing the temperature and residence time. All the hydrochar pellets except for HTC-240-30 have an equilibrium moisture content of less than 10 wt%, suggesting that these pellets can meet the requirement of commercial pellets[49]. For HTC-300-60 and HTC-240-120 pellets, an extremely low moisture content of ~2wt% was obtained. Among the polymeric components of biomass, hemicellulose has the highest capacity of water adsorption with cellulose in a less content, while lignin has little tendency of water sorption [40]. The high hydrophobicity of the hydrochar is probably due to the loss of hydrophilic function groups (e.g. hydroxyl and carboxyl functional group) from degradation of hemicellulose and cellulose during the HTC process[8, 29, 52]. The results shown in Figure 2 suggest that all the hydrochar pellets, except for HTC-240-30, are highly hydrophobic in nature and therefore can be stored safely in a humid environment for a long time. 3.3 Surface morphology The surface morphology of corn stalk and the hydrochar are presented in Figure 3. Mechanical interlocking of the fibres and bulk particles can be observed for pellet of corn stalk , which leads to the formation of solid bridges between particles. Moreover, natural binders (e.g. lignin) are squeezed out of the matrix cells and formed glassy coating on the surface of the pellet during the compaction process. This glassy coating can act as solid bridges which bind the particles together. Although the pelletization was 19 ACS Paragon Plus Environment

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conducted under room temperature in this study, local temperature may be as high as 100-200 °C due to friction between particles[53]. The local temperature is close to the glass transition temperature of lignin, implying the binding functionality of lignin under room-temperature pelletization. However, rough surface and clear inter-particle gaps can be observed for corn stalk pellets, indicating a poor adhesion between particles due to the lack of a strong binding force between particles[51, 52]. This is consistent with the low compressive strength and durability of corn stalk pellet, as described in section 3.3.

For hydrochar pellets, no clear inter-particle gap can be observed, and the pellet surface is smoother than corn stalk pellet. Dinjus et al.[38] reported that a very small fraction of the lignin was degraded at 180-250°C. Under low HTC temperature (180 and 240 °C), most of hemicellulose and some cellulose have been degraded after HTC treatment while most of lignin is preserved. This facilitates the formation of solid bridges between particles. Moreover, certain amount of polar organic compounds like furan and phenolic resins will be formed and deposited on the surface of the hydrochar after hydrothermal treatment[54]. Hoekman et al.[55] found that the cross-linked oligomers and polymers, also known as furan resins, were formed via complex reactions between the intermediate products (e.g. derivatives of furans and other similar chemicals) during the HTC process. The formed resins, which have been commonly used as adhesives and strengtheners for industrial wood products, can bridge the adjacent particles by forming liquid bridge during pelletization, resulting in a high compressive strength

[56, 57]

. In

addition, the hydrochar becomes highly friable after HTC, which facilitates the 20 ACS Paragon Plus Environment

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Energy & Fuels

formation small particles during pelletization, and results in strong electrostatic attraction (e.g. H-bonding and van der Waal's forces)

[52]

as well as an enhanced

compressive strength and durability[8].

A certain amount of pores and small cracks are observed for HTC-240-120 and HTC300-60. The presence of cracks implies that the fibres are not strong enough to bind large particles when solid bridges are formed [16]. Similar phenomenon was found by Hoekman et al[55], who reported that hydrochar pellets from Tahoe Mix were brittle with a low durability when the HTC temperature is above 275 °C. A possible reason is that the majority of lignin is degraded under intense hydrothermal condition and lose the glass transition behaviour due to the change in structure, which results in low durability. This can be supported by Lei et al.[37]. They indicated that the original framework structure of lignin was almost completely disrupted and formed an interconnected porous network structure under intense condition. 3.4 Organic structure of pellet surface Figure 4 shows the ATR-FTIR spectroscopy of corn stalk and hydrochars under different hydrothermal conditions. The absorption band at 3400-3000 cm−1, representing the stretching vibration of the –OH in hydroxyl and carboxyl groups [33], becomes weak after HTC treatment. This is attributed to the occurrence of dehydration reactions which breaks the hydroxyl and carboxyl group. Meanwhile, the intensity of the band decreases with increasing of the temperature and residence time, indicating a higher extent of


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

dehydration at higher temperature and longer residence time. This is in agreement with the decrease of H/C and O/C atomic ratios as mentioned above. The decrease of hydroxyl and carboxyl contents suggests an improvement of hydrophobicity, which has been explained by the enhanced hydrophobicity in section 3.3. The absorption band at 3000 to 2800 cm−1 is linked to -CH stretching vibration in aliphatic structures, showing a strong link between hydrophobic extractives such as waxes and oils[7]. The presence of extractives inhabits the hydrogen bonding between adjacent particles and therefore results in a pellet with low compression strength[7]. After the hydrothermal treatment, the intensity of the band weakens obviously, indicating a degradation of aliphatic hydrocarbon compounds. The peak at 1610 cm−1 corresponds to the –C=C stretching in the aromatic ring carbons[34]. The peak of the – C=C group increases after hydrothermal treatment, especially at high hydrothermal temperature, indicating the occurrence of aromatisation reaction. This can be supported by evident aromatization during hydrothermal dewatering of lignite and peat above 270 °C[35]. The round peak in the region of 1100-950 cm−1 could be attributed to O-C stretching in aliphatic ether or alcohol[58,

59]

, which was decreased with increasing

reaction temperature and residence time because of the dehydration reaction[33]. 3.5 Combustion characteristics The weight loss and derivative weight loss curves of corn stalk pellet and the hydrochar pellets are presented in Figure 5. It can be seen that the weight loss curves of the hydrochar pellets shift to higher temperature to a bigger extent at higher hydrothermal 22 ACS Paragon Plus Environment

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temperature and longer residence time. An initial weight loss is observed at 25-200 °C in Figure 5(b), which can be attributed to the evaporation of moisture. It should be noted that the initial weight losses of the hydrochar pellets are negligible due to a higher hydrophobicity compared to corn stalk pellet[58].

The combustion process can be divided into two stages, corresponding to the two peaks in the DTG curve of Figure 5(b). Stage I and II represent the release and combustion of volatile species and the combustion of the residual char, respectively. For HTC-240-120 and HTC-300-60, these two peaks merge together as one peak and no clear boundary can be observed. The characteristics parameters of the combustion are presented in Table 4. With increasing of the temperature and residence time, the ignition temperature (Ti) and maximum weight loss temperature (Tmax1) increase, and the maximum weight loss rate ((dm/dt)max1) decreases in Stage I. This can be attributed to the decomposition of hemicellulose and cellulose during the HTC process [26], which causes a substantial release of the volatile. Additionally, the hydrochar pellets have a higher mass and energy density, and thus a more compact structure than corn stalk pellet, leading to slow release of the volatile due to the large diffusion resistance for heat and mass transfer inside pellet. In the second stage, Tmax2, Tb and (dm/dt)max2 of hydrochar pellets are higher than those of corn stalk pellets, more remarkably for the hydrochar pellets obtained at higher hydrothermal temperature and longer residence time. As a conclusion for the combustion characteristics, the hydrochar pellets present a thermochemical


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behaviour close to coal in comparison to biomass, which can be burned easily in a controllable manner to be able to achieve a high thermal efficiency[11].

4. Conclusions In this work, a typical agricultural biomass, corn stalk, was carbonized under different conditions and then pelletized to obtain the hydrochar pellets. The hydrochar pellet was characterized based on the fuel and physical properties, SEM and FTIR measurements and thermogravimetric analysis with respect to the hydrothermal temperature and residence time in comparison with corn stalk pellet. The results show that the heating values of hydrochar were close to or even higher than lignite when the temperature and the residence time were higher than 240 °C and 60 min, respectively. After hydrothermal treatment, 73.71-94.71% K and 91.81-94.32% Cl contained in corn stalk were removed, indicating a low fouling and slagging tendency in later combustion application. The pellet quality is very much dependent on hydrothermal temperature and residence time. The compressive strength and durability increased firstly with increasing temperature and then decreased with further increasing the temperature from 240 to 300 °C. The influence of residence time showed a similar trend and the compressive strength and durability reached its maximum value at the residence time of 60 min when the temperature was 240 °C. The hydrophobicity of the hydrochar pellets increased with increasing the temperature and residence time. Hydrochar pellets obtained at the temperature of 240 °C with residence time of 60 min gives the best performance, which can meet the requirement of industrial fuel pellet. The characteristic 24 ACS Paragon Plus Environment

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Energy & Fuels

bonding mechanism of hydrochar pellets can be attributed to the formation of polar organic compounds, like furan and phenolic resins, and finer particles (for pelletization) after hydrothermal treatment. Finally, hydrochar pellets can be burned easily in a controllable manner with a high thermal efficiency in combustion application. Hydrothermal carbonization combined with pelletization can remarkably improve the fuel quality of agricultural biomass such as corn stalk, facilitating its transportation, long-term storage and combustion application.


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Acknowledgments The present work was financially supported by National Natural Science Foundation of China (51706210, 51476067 and 51622604), Science and Technology Department of Henan province (172102210549), Provincial Key Research Project of higher education institutions in Henan (15600097). The authors are also grateful for the assistance on the sample testing provided by the Analytical and Testing Center of Huanzhong University of Science and Technology.


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References [1] Wei Yang, Youjian Zhu, Wei Cheng, Huiying Sang, Hanshen Xu, Haiping Yang, Hanping Chen. Appl Energy 2018, 215: 106-115. [2] Ayhan Demirbaş. Energy Convers. Manage. 2001, 42(11): 1357-1378. [3] K. R. Thines, E. C. Abdullah, N. M. Mubarak, M. Ruthiraan. Renew Sustain Energy Rev 2017, 67: 257-276. [4] Peter McKendry. Bioresour. Technol. 2002, 83(1): 47-54. [5] Youjian Zhu, Wei Yang, Jiyuan Fan, Tao Kan, Wennan Zhang, Heng Liu, Wei Cheng, Haiping Yang, Xuehong Wu, Hanping Chen. Appl Energy 2018, 230: 925-934. [6] Wei-Hsin Chen, Jianghong Peng, Xiaotao T. Bi. Renew Sustain Energy Rev 2015, 44: 847-866. [7] Yaohui Si, Junhao Hu, Xianhua Wang, Haiping Yang, Yingquan Chen, Jingai Shao, Hanping Chen. Energy Fuels 2016, 30(7): 5799-5808. [8] Harpreet Singh Kambo, Animesh Dutta. Appl Energy 2014, 135: 182-191. [9] Quang-Vu Bach, Øyvind Skreiberg. Renew Sustain Energy Rev 2016, 54: 665-677. [10] Qiang Hu, Haiping Yang, Hanshen Xu, Zhiqiang Wu, C. Jim Lim, Xiaotao T. Bi, Hanping Chen. Energy Convers. Manage. 2018, 161: 205-214. [11] Zhengang Liu, Augustine Quek, R. Balasubramanian. Appl Energy 2014, 113: 1315-1322. [12] Zhengang Liu, Augustine Quek, S. Kent Hoekman, R. Balasubramanian. Fuel 2013, 103: 943-949. [13] Sabzoi Nizamuddin, Natesan Subramanian Jaya Kumar, Jaya Narayan Sahu, Poobalan Ganesan, Nabisab Mujawar Mubarak, Shaukat Ali Mazari. The Canadian Journal of Chemical Engineering 2015, 93(11): 1916-1921. [14] Sabzoi Nizamuddin, Natesan Subramanian Jayakumar, Jaya Narayan Sahu, Poobalan Ganesan, Abdul Waheed Bhutto, Nabisab Mujawar Mubarak. Korean J. Chem. Eng. 2015, 32(9): 1789-1797. [15] M. Toufiq Reza, Joan G. Lynam, Victor R. Vasquez, Charles J. Coronella. Environmental Progress & Sustainable Energy 2012, 31(2): 225-234. [16] Sabzoi Nizamuddin, Muhammad Tahir Hussain Siddiqui, Humair Ahmed Baloch, Nabisab Mujawar Mubarak, Gregory Griffin, Srinivasan Madapusi, Akshat Tanksale. Environmental Science and Pollution Research 2018, 25(18): 17529-17539. [17] Sabzoi Nizamuddin, Humair Ahmed Baloch, G. J. Griffin, N. M. Mubarak, Abdul Waheed Bhutto, Rashid Abro, Shaukat Ali Mazari, Brahim Si Ali. Renew Sustain Energy Rev 2017, 73: 1289-1299. [18] S. Kent Hoekman, Amber Broch, Curtis Robbins. Energy Fuels 2011, 25(4): 18021810. [19] K. R. Thines, E. C. Abdullah, N. M. Mubarak. Microporous Mesoporous Mater. 2017, 253: 29-39. [20] M. Toufiq Reza, Joan G. Lynam, M. Helal Uddin, Charles J. Coronella. Biomass Bioenergy 2013, 49: 86-94. 27 ACS Paragon Plus Environment

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[21] Aidan M. Smith, Surjit Singh, Andrew B. Ross. Fuel 2016, 169: 135-145. [22] Mikko Mäkelä, Andrés Fullana, Kunio Yoshikawa. Energy Convers. Manage. 2016, 121: 402-408. [23] Mikko Mäkelä, Kunio Yoshikawa. Energy Convers. Manage. 2016, 121: 409-414. [24] Britt-Marie Steenari, Anna Lundberg, Helena Pettersson, Magda Wilewska-Bien, David Andersson. Energy Fuels 2009, 23(11): 5655-5662. [25] Dan Bostrom, Nils Skoglund, Alejandro Grimm, Christoffer Boman, Marcus€ Ohman, Markus Brostrom, Rainer Backman. Energy Fuels 2012, 26: 85-93. [26] Maria Zevenhoven, Patrik Yrjas, Bengt-Johan Skrifvars, Mikko Hupa. Energy Fuels 2012, 26(10): 6366-6386. [27] Wei Yang, Youjian Zhu, Wei Cheng, Huiying Sang, Haiping Yang, Hanping Chen. Energy Fuels 2017, 31(7): 7493-7501. [28] Dingding Yao, Qiang Hu, Daqian Wang, Haiping Yang, Chunfei Wu, Xianhua Wang, Hanping Chen. Bioresour. Technol. 2016, 216: 159-164. [29] Tapas C. Acharjee, Charles J. Coronella, Victor R. Vasquez. Bioresour. Technol. 2011, 102(7): 4849-4854. [30] Wolfgang Stelte, Jens K. Holm, Anand R. Sanadi, Søren Barsberg, Jesper Ahrenfeldt, Ulrik B. Henriksen. Biomass Bioenergy 2011, 35(2): 910-918. [31] Nalladurai Kaliyan, R. Vance Morey. Biomass Bioenergy 2009, 33(3): 337-359. [32] Youjian Zhu, Peter J. Ashman, Chi Wai Kwong, Dingbiao Wang, Rocky de Nys. Energy Fuels 2015, 29: 5047-5055. [33] Pin Gao, Yiyuan Zhou, Fang Meng, Yihui Zhang, Zhenhong Liu, Wenqi Zhang, Gang Xue. Energy 2016, 97: 238-245. [34] Nizamuddin Sabzoi, Jaya Kumar Natesan Subramanian, Sahu Jaya Narayan, Ganesan Poobalan, Mubarak Nabisab Mujawar, Mazari Shaukat Ali. The Canadian Journal of Chemical Engineering 2015, 93(11): 1916-1921. [35] Funke Axel, Ziegler Felix. Biofuels, Bioproducts and Biorefining 2010, 4(2): 160177. [36] Shuqing Guo, Xiangyuan Dong, Tingting Wu, Caixia Zhu. Energy Convers. Manage. 2016, 123: 95-103. [37] Yanqiu Lei, Haiquan Su, Rongkai Tian. RSC Advances 2016, 6(109): 107829107835. [38] E. Dinjus, A. Kruse, N. Tröger. Chemical Engineering & Technology 2011, 34(12): 2037-2043. [39] Wu-Jun Liu, Wen-Wei Li, Hong Jiang, Han-Qing Yu. Chem. Rev. 2017, 117(9): 6367-6398. [40] Simone C. van Lith, Peter A. Jensen, Flemming J. Frandsen, Peter Glarborg. Energy Fuels 2008, 22(1598–1609). [41] Joakim M. Johansen, Jon G. Jakobsen, Flemming J. Frandsen, Peter Glarborg. Energy Fuels 2011, 25: 4961–4971. [42] Harpreet Singh Kambo, Animesh Dutta. Energy Convers. Manage. 2015, 105: 746755. 28 ACS Paragon Plus Environment

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[43] Maryori Díaz-Ramírez, Christoffer Boman, Fernando Sebastián, Javier Royo, Shaojun Xiong, Dan Boström. Energy Fuels 2012, 26(6): 3218-3229. [44] Yanqing Niu, Houzhang Tan, Shi'en Hui. Prog. Energy Combust. Sci. 2016, 52: 161. [45] Mikko Hupa, Oskar Karlström, Emil Vainio. P. Combust. Inst 2017, 36(1): 113134. [46] Liang Wang, Johan E. Hustad, Morten Grønli. Energy Fuels 2012, 26: 5905−5916. [47] B. M. Jenkins, L. L. Baxter, T. R. Miles, T. R. Miles. Fuel Process. Technol. 1998, 54(1): 17-46. [48] Changkook Ryu, Yao Bin Yang, Adela Khor, Nicola E. Yates, Vida N. Sharifi, Jim Swithenbank. Fuel 2006, 85(7): 1039-1046. [49] A. García-Maraver, V. Popov, M. Zamorano. Renewable Energy 2011, 36(12): 3537-3540. [50] Nalladurai Kaliyan, R. Vance Morey. Biomass Bioenergy 2009, 33: 337-359. [51] Pak Sui Lam, Pak Yiu Lam, Shahab Sokhansanj, Xiaotao T. Bi, C. J. Lim. Biomass Bioenergy 2013, 56: 116-126. [52] Ilman Nuran Zaini, Srikandi Novianti, Anissa Nurdiawati, Adrian Rizqi Irhamna, Muhammad Aziz, Kunio Yoshikawa. Fuel Process. Technol. 2017, 160: 109-120. [53] Nalladurai Kaliyan, R. Vance Morey. Bioresour. Technol. 2010, 101(3): 10821090. [54] Zhengang Liu, Fu-Shen Zhang. Energy Convers. Manage. 2008, 49(12): 34983504. [55] S. Kent Hoekman, Amber Broch, Andrew Warren, Larry Felix, James Irvin. Biofuels 2014, 5(6): 651-666. [56] Stefaan J. R. Simons. Chapter 27 Liquid bridges in granules. In Handbook of Powder Technology, Salman, A. D.; Hounslow, M. J.; Seville, J. P. K., Eds. Elsevier Science B.V.: 2007; Vol. 11, pp 1257-1316. [57] Shunyan Wu, Shouyu Zhang, Caiwei Wang, Chen Mu, Xiaohe Huang. Fuel Process. Technol. 2018, 171: 293-300. [58] Hui Li, Siyuan Wang, Xingzhong Yuan, Yanni Xi, Zhongliang Huang, Mengjiao Tan, Changzhu Li. Bioresour. Technol. 2018, 249: 574-581. [59] Jamie Minaret, Animesh Dutta. Bioresour. Technol. 2016, 200: 804-811.


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Figures and Tables

Figure 1 Schematic diagram of compression system: (a) the mechanical press machine and (b) the piston mold


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Figure 2 Water absorption characteristics of corn stalk and the hydrochar pellets 20

CS HTC-240-30 HTC-240-120

16

Moisture content, %

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

HTC-180-60 HTC-240-60 HTC-300-60

12

8

4

0 0

100

200

300

400

Time, min


500

600

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Figure 3 SEM images of corn stalk and the hydrochar pellets: (a) CS; (b) HTC-180-60; (c) HTC -240-30; (d) HTC -240-60; (e) HTC -240-120; (f) HTC-300-60


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Figure 4 ATR-FTIR spectra of corn stalk and the hydrochar pellets

O-H

C=O C=C

C-H

C-O

HTC-300-60 HTC-240-120

Absorbance, /%

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

HTC-240-60

HTC-240-30 HTC-180-60

CS

4000

3000

2000

1000 -1

Wave number/cm


O-H

Energy & Fuels

Figure 5 TG and DTG curves of corn stalk and the hydrochar pellets:(a) TG, (b) DTG, in an air flow rate of 100ml/min at a heating rate 20 °C/min

100

TG, %

80

60

CS HTC-180-60 HTC-240-30 HTC-240-60 HTC-240-120 HTC-300-60

40

20

0 100

200

300

400

500

600

o

Temperatures, C

(a) 25

CS HTC-180-60 HTC-240-30 HTC-240-60 HTC-240-120 HTC-300-60

20

DTG, %/min

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

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15

10

5

0 100

200

300

400 o

Temperatures, C

(b)


500

600

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Energy & Fuels

Table 1. Proximate, ultimate analyses and heating values of corn stalk and the hydrochar samples CS mass yield, % 100 LHV, MJ/kg 16.35 proximate analysis (wt % dba) fixed carbon 16.39 volatile matter 77.82 Ash 5.79 ultimate analysis (wt % dba) C 45.41 H 6.61 N 1.3 S 0.34 O, by difference 40.55 O/C 0.67 H/C 1.75 a

HTC180-60 70.75 18.47

HTC240-30 46.96 19.02

HTC240-60 45.21 22.27

HTC240-120 42.37 25.67

HTC300-60 34.4 26.31

18.33 75.7 5.97

18.68 75.74 5.58

24.62 69.86 5.52

36.75 57.88 5.37

39.55 55.21 5.24

47.22 6.37 1.24 0.32 38.88 0.62 1.62

47.81 6.62 1.35 0.33 38.31 0.6 1.66

50.37 6.68 1.27 0.26 35.9 0.53 1.59

56.72 6.72 1.46 0.36 29.37 0.39 1.42

58.84 6.44 1.36 0.25 26.64 0.34 1.31

db=dry basis


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Table 2. Chemical composition characteristic parameters of corn stalk and the hydrochar samples CS

HTC-180-60

HTC-240-30

HTC-240-60

HTC-240-120

HTC-300-60

K2Oa

25.50

6.50

6.58

5.07

1.72

1.49

CaOa

6.57

6.00

6.25

7.47

7.03

8.87

MgOa

3.60

2.04

1.93

2.28

2.31

2.35

Al2O3a

1.78

3.68

5.79

3.63

6.75

2.77

SiO2a

41.56

73.30

62.31

61.10

64.10

63.15

P2O5a

5.89

2.70

10.72

14.29

14.29

17.27

SO3a

3.79

4.88

5.64

5.42

3.24

3.38

Cla

11.31

0.90

0.79

0.73

0.55

0.71

RKb

0

73.71

75.12

81.03

93.75

94.71

RClc

0

91.81

93.26

93.83

95.45

94.32

AId

0.90

0.21

0.19

0.13

0.04

0.02

a

wt%. b the removal ratio of K, %. c the removal ratio of Cl, %. d alkali index, kg alkali/GJ.


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Energy & Fuels

Table 3. Physical properties of corn stalk and the hydrochar pellets Mass density

Energy density

compressive strength

Durability

Energy consumption

(kg/m3)

(GJ/m3)

(MPa)

%

(J/g)

CS

934±23

15.27±0.38

2.88±0.25

82.49±3.66

38.15±1.09

HTC-180-60

1080±13

19.95±0.16

8.51±0.34

95.39±2.27

43.10±0.99

HTC -240-30

1107±11

21.07±0.21

4.32±0.27

88.83±2.68

40.10±0.82

HTC -240-60

1194±13

26.61±0.29

8.64±0.4

98.82±2.01

46.44±0.97

HTC -240-120

1213±18

31.15±0.46

6.64±0.44

94.37±2.31

49.26±1.43

HTC-300-60

1143±17

30.08±0.45

7.69±0.29

92.37±2.645

51.26±1.28

samples


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Table 4 Characteristics parameters from the combustion process of corn stalk and hydrochar pellets Samples

Stage I

Stage II

Ti

Tmax1

(dm/dt)max1

Tmax2

(dm/dt)max2

Tb

CS

226

297

19.11

413

6.72

455

HTC-180-60

242

338

17.45

467

6.13

523

HTC-240-30

243

341

13.95

422

10.98

529

HTC-240-60

253

329

4.61

480

10.38

561

HTC-240-120

275

-

-

459

10.61

591

HTC-300-60

296

-

-

457

13.85

575

a

Ti is the ignition combustion, °C. b Tmax1 is the temperature of the maximum reaction rate of stage I, °C. c

(dm/dt)max1 is the maximum reaction rate of stage I, %.min-1. d Tmax2 is the temperature of the maximum reaction rate of stage II, °C. e (dm/dt)max2 is the maximum reaction rate of stage II, %.min-1. f Tb is the burnout temperature, °C.


Characterization of hydrochar pellets from hydrothermal carbonization

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