The correlation of physicochemical properties and combustion performance of hydrochar with fixed carbon index

Bioresource Technology 294 (2019) 122053

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The correlation of physicochemical properties and combustion performance of hydrochar with fixed carbon index

T

Xiwei Xu1, Ren Tu1, Yan Sun, Yujian Wu, Enchen Jiang , Yulin Gong, Yan Li ⁎

College of Materials and Energy in South, China Agricultural University, Guangzhou 510640, China

GRAPHICAL ABSTRACT

Hydrothermal carbonization performance and the correlation of pelletizaiton and combustion of hydrochar with fixed carbon index (FCI)

ARTICLE INFO

ABSTRACT

Keywords: Hydrothermal carbonization Hydrochar Fixed carbon index (FCI) Pelletization Combustion

Hydrothermal carbonization (HTC) is effective method for improving fuel properties of biomass. Investigating the relationship between the HTC severity and the physicochemical properties of hydrochar is beneficial for the large-scale utilization. The fixed carbon index (FCI) based on the hydrothermal carbonization severity is introduced to predict the physicochemical properties, pelletization and combustion performance of hydrochar. The results showed the relationship between decarbonization, dehydrogenation, deoxygenation and FCI fits exponential function. It was predicted that the hydrochar pellets with FCI = 0.15–0.45 possessed the highest bulk density (> 1175 kg/m3), the lowest specific energy consumption (< 16.07 kJ/kg) and the strongest radial compressive strength (> 10.7Mpa). Moreover, the activation energy of hydrochar combustion in FCI (0.15–0.25) is higher (the maximum is 216 kJ/mol). The study provides based datas for predicting the fuel properties of hydrochar and obtains high quality solid fuel.

1. Introduction Hydrothermal carbonization (HTC) is a pretreatment technology for bio-waste at a relatively low temperature (180–250 °C) under autogenous pressure. And the effectivity of HTC is high due to the no-limited transfer of heat and mass. In particular, the high-moisture bio-

waste such as sewage sludge, kitchen waste, microalgae and animal manure could be treated by HTC without pre-drying. Compared with other pretreatment, HTC favors for the hydrolysis of cellulose, hemicellulose, and lignin (Chen et al., 2012) and obtaining hydrochar with low atomic H/C and O/C ratios (Xu et al., 2018), which is beneficial for the improvement of hydrochar quality as solid fuel (Xu et al., 2019).

Corresponding author. E-mail address: [email protected] (E. Jiang). 1 Contributed equally. ⁎

https://doi.org/10.1016/j.biortech.2019.122053 Received 18 June 2019; Received in revised form 19 August 2019; Accepted 20 August 2019 Available online 24 August 2019 0960-8524/ © 2019 Published by Elsevier Ltd.

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Meanwhile, the grindability and hydrophobicity of biomass are enhanced after HTC (Chai and Saffron, 2016), which is advantage for the storage and transportation of biomass solid fuel. HTC is considered as a promising technology for obtaining renewable energy and chemicals instead of coal and fossil fuel. The rice husk can be obtained in many agricultural countries and the yield reached 1.7*108t /year. Tobacco rod stands for biomass which is rich in cellulose (Chen et al.,2018a). The camellia shell belonged to the hard-shell biomass. The three biomasses are widely distributed all over the world and are promising renewable resource for fuel. Agricultural and forestry wastes pellets is widely used in the industry for supplying heat or clean gas due to its promising fuel properties, such as renewable, clean and low cost (Chen et al., 2018b). However, the largescale application in industry of agricultural and forestry wastes pellets is restricted due to the drawback such as the high moisture absorption, low energy density, high energy consumption during pelletization and unstable combustion properties of pellets (Zhou et al., 2014). Therefore, it is necessary to overcome those drawbacks and improve the fuel properties of biomass pellets. It is well accepted that the fuel properties of bio-waste such as hydrophobicity, grindability, density and heat value can be improved after HTC pretreatment (Yan et al., 2017). However, the energy consumptions during pelletization are significantly increased after HTC pretreatment, while the density and strength of the hydrochar pellets are decreased compared with untreated bio-waste (Kambo et al., 2014). Therefore, it is necessary to discuss the relationship between the HTC severity and the properties of pelletization and combustion in order to improve the quality of pellet and decrease the products cost at the same time. To identify the HTC severity, Kent hoekman finds that severity factor (SF) is a useful metric for characterizing reaction conditions in different reactor systems during HTC (Hoekman et al., 2017). Jeder investigates that the severity factor is a useful tool for producing hydrochars and derived carbon materials (Jeder et al., 2018). Lee et al. (2012) calculates a severity factor based on the reaction time and temperature and presentes the relationship between elemental content and energy distribution of hydrochar from torrefied biomass. Chen et al. (2014) introduced the torrefaction severity index (TSI) to explain the influence of torrefaction severity on the pretreatment performance of biomass. Although there are a large number of researches about HTC severity (Hoekman et al., 2017), the studies about correlation between HTC severity and properties of hydrothermal carbon and pelletization and combustion characteristics of hydrochar is not clear. Therefore, it is urgently-needed to provide the relationship between the HTC severity and the properties of pelletization and combustion for the large-scale utilization of biomass. Moreover, the FCI, which shows the main inherent properties of hydrochar, can be used to reflect the physicochemical properties accurately. And FCI changed with the HTC severity. Therefore, in the article, the fixed carbon index (FCI) of hydrochar based on the HTC severity is introduced to predict the physicochemical properties, the pelletization and combustion performance of hydrochar obtained from three different bio-waste (camellia shell, rice husk and tobacco stalk). In particular, by establishing the relationship between decarbonization, dehydrogenation, deoxygenation, HHV (high heat value) improvement, energetic retention efficiency and FCI, the energy utilization of the hydrothermal carbonization system had been considered. Meanwhile, the establishment of the correlation between FCI and bulk density, energy consumption and radial compressive strength of pellets to provide a basic law for the preparation of high-quality solid fuel with lower cost via HTC. Moreover, the correlation between combustion activation energy and FCI is also investigated. The article provided the information for a deep insight for obtaining the solid pellet fuel with high quality and low cost based on the prediction of FCI.

2. Material and methods 2.1. Material preparation Three different biomass such as camellia shell, rice husk and tobacco stalk are used to produce hydrochar and pellets. Camellia shell bought from Guangzhou; Rice husk bought from Guangzhou; Tobacco stalks are purchased from Yunnan. All biomass materials are crushed to 20–60 mesh by an YB-1000A pulverizer which is produced by Yongkang Sufeng Industry for subsequent hydrothermal carbonization experiments as well as pelletization experiments and performance tests. The cellulose content of camellia shell, rice husk, tobacco stalk is 18.90 wt%, 11.20 wt%, 13.14 wt%, respectively. And the hemicellulose content is 10.31 wt%, 27.44 wt% and 33.17 wt%, respectively. And the lignin content is 34.81 wt%, 31.35 wt%, 38.11 wt% for camellia shell, rice husk, tobacco stalk, respectively. Three main components are determined by National Renewable Energy Laboratory (NREL) standard method (Sluiter et al., 2019). 2.2. Hydrothermal carbonization (HTC) Hydrothermal carbonization is carried on a 500 ml autoclave reactor produced by Shanghai Pengyi Instrument Co., Ltd. The HTC temperature is at 150 °C, 175 °C, 200 °C, 225 °C, 250 °C separately, and kept for 30 min. About 30 g of camellia shell (CS) or rice husk (RH) or tobacco stalk (TR) and 300 ml of deionized water are added to the autoclave at a ratio of about 1:10. Stirring speed is 300 rpm. The high purity nitrogen is use to remove air from the autoclave. After the reaction, the autoclave is placed in air, naturally cooling to below 85 °C. And the solid–liquid mixture is separated using a vacuum suction filter. The solid product is then dried at 105 °C for 24 h and labeled as CS-150 (175, 200, 225, 250), RH-150 (175, 200, 225, 250), TR-150 (175, 200, 225, 250) based on the HTC temperature. 2.3. Pelletization The process of pelletization is similar in the previous research (Tu et al., 2018). The main steps are briefly described as follow. Hydrochar is pelleted in a mold with a diameter of 10 mm and a length of 70 mm. When the temperature of the grinding tool reaches 120 °C, about 1 g of hydrochar is quickly added into the mold and kept at a maximum pressure of 5 kN for 5 s. Five pellets are prepared for all biomass and hydrothermal carbon samples to ensure accurate experimental results. Relaxed density (ρ), Specific energy consumption (E) can be calculated by the following formula:

= 4m / D 2 L where, ρ is the relaxed density (g/cm3), m is the mass of the pellet (g), L is the length of the pellet (cm), D is the diameter of the pellet (cm).

E = W/ m =

fds m

where, E, W, m, f, s is the specific energy consumption of the compression process (kJ/kg), total energy consumption (J), particle mass (g), pressure (kN) and position Move (mm), respectively. 2.4. Combustion and kinetic analysis The combustion and kinetic analysis is similar in the previous research (Tu et al., 2018). Combustion kinetic parameters in this paper is obtained from NETZSCH Thermodynamics Analysis. The main steps are briefly described as follow. The reaction equation of solid heterogeneous reaction is generally calculated as follow:

d /dT = A/ exp( E/RT)

F( )

where α, β, E, A, T, R, F(α) are the percentage of conversion of 2

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reactants to products; the heating rate; apparent activation energy; preexponential factor; absolute temperature; the gas constant; the kinetics model, respectively.

is relatively severer compared to cellulose and lignin at a high temperature (Chen et al., 2010). Meanwhile, the content of lignin in camellia shell is also relatively plentiful and lignin has a higher resistance to thermal degradation. This is the reason that weight loss rate of camellia shell is low at over 175 °C. The relationship between solid yield and FCI is shown in the supplemental materials. Regression lines, fitted model, fitted equation and the correlation coefficient R2 are all in the figure. It can be seen that the correlation between solid yield of three different biomass samples and FCI is coincident with exponential equation y = a1 − b1c1x. The parameter c1 determines the sensitivity of the solid yield to FCI. The smaller the c1 is, the faster the mass loss rate is under the low-temperature hydrothermal carbonization, and the slower the mass loss rate is at the high-temperature hydrothermal carbonization. The value of c1 depends on the characteristics of different biomass materials. The fitted line of the camellia shell shows a very slow weight loss rate at high temperature stage due to the relatively highest content of lignin, which has a higher resistance to thermal degradation. It is worth mentioning that the mass loss rate of rice husk is the slowest at the low temperature HTC stage, and the solid residue is the highest due to the highest ash content in rice husk. The relationship between the gas yield and FCI is shown in supplemental materials. The gas yield also has a nonlinear correlation with FCI. The gas yield of three different biomass samples fits well with the x) index model of y = a2*(1−e(-b ) (R2 > 0.94) based on FCI. As the 2 degree of HTC deepens, the gas yield increases rapidly and then gradually stabilizes. The model parameter b2 is the main control parameter of the gas yield to the sensitivity of FCI. The smaller the b2 is, the faster the gas yield increases under low temperature during HTC. The gas yield of tobacco stalk is similar with rice husk at the low-temperature stage, but producing gas rate is slightly lower than rice husk at the high-temperature stage. It is possible that the content of hemicellulose and lignin are higher in tobacco stalk than rice husk. In the high-temperature stage, more acetic acid-based condensable volatile liquid products are produced from hemicellulose and lignin, causing that the content of non-condensable gases produced by tobacco is lower than rice husk (Bu et al., 2014). Above all, the solid and gas yield had good correlation with FCI (R2 > 0.94). Therefore, FCI can be considered as a reliable indicator to either describe or predict the performance of biomass HTC.

2.5. Definition of FCI

FCI =

FCt FCm FC250 FCm

where FCI presents hydrothermal carbonization severity. Where FC250 represents the value of the fix carbon content at the 250 °C. FCt represents the value of the fix carbon at the t/°C; FCm represents the value of the fix carbon in the raw material. Based on the aforementioned definition, the value of FCI is in the range of 0–1. 2.6. Product analysis for physical and chemical properties of hydrochar and pellets The analysis for physical and chemical properties of hydrochar and pellets is similar in the previous research (Chen et al., 2012). All product analysis is repeated three times. The element analysis is tested by CHN analyzer (EA-CHONS, Thermo Scientific FLASH 2000) The heat high value analysis is carried out by using an YX-ZR Skyhawk Automatic Calorimeter (Changsha Youxin Instrument Manufacturing Co., Ltd.). The Proximate analysis are performed by using an automatic industrial analyzer (Changsha Youxin Instrument Manufacturing Co., Ltd.) with GB/T28731-2012 as reference. 3. Result and discussion 3.1. Hydrothermal carbonization behavior of three biomass The distribution of hydrothermal products is shown in Table 1. The relationship between solid yield, gas yield and FCI is shown in supplemental materials, respectively. It can be seen from Table 1 that the solid yields of camellia shell, rice husk and tobacco stalk at temperatures of 150–250 °C are 42.5–70.67%, 45–77.33%, 35.34–63.67%, respectively. The lowest solid yield is obtained at 250 °C, reaching 35.34–45%. This result indicates that nearly 55–64.66% of the organic components are decomposed during the HTC. Compared with the tobacco stalk and the rice husk, the solid yield of camellia shell is near 40% when the temperature is over 175 °C, and the thermal decomposition degree of the organic matter is the largest. It is possible that the content of hemicellulose in camellia shell is the highest (i.e. 29.52%) among the three biomass and the thermal degradation of hemicellulose

3.2. The fuel properties of hydrochar 3.2.1. Properties of hydrochar The chemical and physical properties of hydrochar products are shown in Table 2. It can be seen that the volatile matter percentages in camellia shell, rice husk and tobacco stalk are in the range of 65.16–51.19%, 65.25–44.54, 72.64–54.81%, respectively, accounting for the highest proportion. With the increase of hydrothermal carbonization temperature, the content of fixed carbon appears to be strictly increasing. The content of fixed carbon of camellia shell, rice husk and tobacco stalk increased from 19.5% to 39.45%, 10.18 to 29.24%, 15.81 to 37.22%, respectively. It implies that tobacco stalk has the highest level of energy accumulation during hydrothermal carbonization. The percentages of ash in camellia shell, rice husk and tobacco stalk are in the range of 3.11–6.81%, 16.03–23.54%, 2.1–4.91%, respectively. It is worth noting that the ash content in the rice husk is much higher than that of the other biomass. The properties of hydrochar have been profoundly affected by the different chemical composition and structure of biomass.

Table 1 the distribution of hydrothermal products. Sample

Solid

Oil

Gas

CS-150 CS-175 CS-200 CS-225 CS-250 RH-150 RH-175 RH-200 RH-225 RH-250 TR-150 TR-175 TR-200 TR-225 TR-250

70.67 47.67 46 45 42.5 77.33 69.33 65 59.67 45 63.67 60 56.67 52.34 35.34

17 33.12 32.57 28.33 17.33 13 15.67 15 15.67 23.67 22 28 26.67 23.34 38

12.33 19.21 21.43 26.67 40.17 9.67 15 20.34 24.67 31.33 14.34 12 16.67 24.33 26.67

3.2.2. Element removal via HTC As shown in Table 3, with the increase of hydrothermal carbonization temperature, the carbon content of three biomass samples showed a gradually increasing trend. It indicated that the hydrochar obtained from CS, RH, TR has the similar fuel characteristics to coal due 3

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Table 2 Proximate analysis of hydrochar and raw material.

Table 4 Energy density and HHV analysis.

Sample

Moisture (%)

Volatile matter (%)

Fixed carbon (%)

Ash (%)

CS CS-150 CS-175 CS-200 CS-225 CS-250 RH RH-150 RH-175 RH-200 RH-225 RH-250 TR TR-150 TR-175 TR-200 TR-225 TR-250

12.22 ± 0.23 3.04 ± 0.18 2.57 ± 0.14 1.82 ± 0.62 1.59 ± 0.43 2.52 ± 0.34 6.11 ± 0.52 2.5 ± 0.41 10.64 ± 0.23 5.89 ± 0.82 3.95 ± 0.81 3.42 ± 0.57 6.64 ± 0.63 5.60 ± 0.51 5.41 ± 0.22 4.54 ± 0.43 3.70 ± 0.69 3.07 ± 0.65

65.16 ± 2.62 70.90 ± 2.43 66.65 ± 2.54 61.43 ± 1.46 58.34 ± 1.54 51.19 ± 1.45 65.25 ± 1.21 70.04 ± 2.52 65.53 ± 2.13 61.1 ± 1.64 57.79 ± 1.67 44.54 ± 2.42 72.64 ± 1.53 73.98 ± 1.44 73.38 ± 1.45 71.50 ± 1.53 63.76 ± 2.41 54.81 ± 2.66

19.50 20.62 26.06 33.06 36.92 39.45 10.18 13.06 13.82 14.76 20.45 29.24 15.81 16.79 18.03 21.86 27.69 37.22

3.11 ± 0.45 5.42 ± 0.23 4.70 ± 0.24 3.67 ± 0.34 3.12 ± 0.68 6.81 ± 0.36 16.03 ± 0.77 15.13 ± 0.72 11.36 ± 0.98 16.59 ± 0.24 18.31 ± 0.68 23.54 ± 0.42 4.91 ± 0.24 3.63 ± 0.15 3.18 ± 0.13 2.10 ± 0.42 4.85 ± 0.34 4.90 ± 0.66

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.21 1.42 0.86 1.34 1.29 0.92 0.56 0.42 0.36 1.42 1.67 0.36 0.42 0.31 0.85 0.69 0.43 0.32

Table 3 Element analysis and DC, DH, DO during HTC. Sample

CS CS-150 CS-175 CS-200 CS-225 CS-250 RH RH-150 RH-175 RH-200 RH-225 RH-250 TR TR-150 TR-175 TR-200 TR-225 TR-250

Element analysis (%) H (%)

O (%)

N (%)

DC (%)

DH (%)

DO (%)

CRE (%)

48.47 51.05 52.75 58.00 60.49 64.41 42.84 42.89 41.83 43.52 46.25 50.66 44.85 46.57 48.71 51.43 55.22 64.10

5.29 6.14 6.18 6.04 5.86 5.56 6.54 6.56 6.21 5.95 5.66 4.97 6.70 6.92 6.97 6.73 6.46 6.31

44.96 41.51 39.73 34.47 32.24 28.59 49.41 49.22 50.86 49.30 46.84 42.89 45.75 43.78 41.64 38.96 35.24 25.79

1.26 1.31 1.31 1.48 1.40 1.36 1.18 1.30 1.09 1.23 1.24 1.46 2.47 2.57 2.55 2.73 2.90 3.66

0 25.57 48.12 44.95 43.84 43.52 0 22.59 32.31 33.97 35.59 46.79 0 33.89 34.83 35.01 35.55 49.49

0 17.99 44.35 47.46 50.14 55.31 0 22.45 34.13 40.86 48.33 65.80 0 34.29 37.64 43.14 49.60 66.76

0 34.76 57.88 64.73 67.73 72.98 0 22.97 28.63 35.15 43.43 60.94 0 39.07 45.39 51.74 59.68 80.08

100 74.43 51.88 55.04 56.16 56.48 100 77.42 67.70 66.03 64.42 53.21 100 66.11 65.16 64.98 64.44 50.51

Enhancement factor

Energy densification efficiency (%)

HHV improvement (%)

Heat value (MJ/mol)

CS CS-150 CS-175 CS-200 CS-225 CS-250 RH RH-150 RH-175 RH-200 RH-225 RH-250 TR TR-150 TR-175 TR-200 TR-225 TR-250

1.00 1.15 1.25 1.41 1.46 1.49 1.00 1.03 1.05 1.10 1.19 1.37 1.00 1.07 1.11 1.28 1.49 1.87

100 81.08 59.36 64.81 65.62 63.25 100 79.55 73.01 71.67 70.77 61.45 100 67.93 66.62 72.69 78.09 65.97

0 14.74 24.53 40.89 45.83 48.83 0 2.87 5.30 10.27 18.61 36.57 0 6.69 11.03 28.28 49.20 86.68

16.294 18.695 20.291 22.956 23.761 24.251 15.348 15.788 16.162 16.924 18.204 20.960 13.828 14.753 15.353 17.738 20.631 25.814

R2 is a bit lower (R2 > 9.0). Those results indicats that the release rate of these three elements has a very strict correlation with FCI. In general, the dehydrogenation, deoxygenation or decarbornization extremely rapid increase in low temperature stage, and gradually stabilizes at high temperature hydrothermal carbonization stage. This result can be explained by the fact that the dehydrogenation, deoxygenation and decarburization of biomass are easy and rapid via dehydration and decarboxylation at low temperature hydrothermal carbonization stage (Sevilla et al., 2009; Chen et al., 2015). However, at high temperature stage, thermosensitive organic component such as hemicellulose and cellulose gradually decrease, and the content of lignin that is difficult to decompose gradually increases. Therefore, the removal rate of C, H, O is low at the high temperature stage. It is worth mentioning that the parameter b4 in the fitting equation is the main control parameter of the release rate of carbon, hydrogen and oxygen in at low temperature hydrothermal carbonization stage. The larger the b4 is, the faster the release rate will be. It is obvious in Fig. 2 that the order for removal rate of C, H and O is tobacco stalk > camellia shell > rice husk. This result implies that the tobacco stalk is more prone to dehydration and decarboxylation than the other biomass samples in the low temperature stage. Meanwhile, the parameter a4 represents the maximum of removal C, H and O, which depends on the severity of hydrothermal carbonization. The larger the value of a4 is, the higher removal rate of a certain element is during high temperature stage. It is obvious that the elemental removal in the Fig. 1a, 1b and 1c is in the rank of DO > DH > DC regardless of biomass species, which is consistent with the research of Zhang (Zhang et al., 2018). This result implies that HTC has a more significantly effect on O and H removal than on C due to the releasing of both moisture (Chen et al., 2015) and light volatiles during HTC (Cai et al., 2016) through dehydration, dihydroxylation (Matsubara et al., 2004), devolatilization (Cai et al., 2016), and decomposition of hemicellulose (Parajo et al., 1998; Chen et al., 2015). Moreover, the carbon content increases as well as energy density via DO and DH during HTC. The strong correlation of carbon recovery efficiency with FCI is shown in Fig. 1d. The carbon recovery efficiency (CRE) tends to decrease as the degree of hydrothermal carbonization deepens regardless of biomass species. This indicates that the rate of mass loss of biomass during hydrothermal carbonization exceeds the rate of carbon growth. Meanwhile, as the rate of mass loss slows down at high temperature hydrothermal carbonization stage, the carbon recovery efficiency also exhibits a slow decrease rate. The main reason for this trend is the process of HTC contained the biomass decomposition and generation of pseudo-lignin during

Derived calculation parameter

C (%)

Sample

DC is Decarbonization, DH is Dehydrogenation, and DO is Deoxygenation. CRE is Carbon recovery efficiency.

to chemical dehydration and decarboxylation with the releasing of H2O and CO2 (Kim et al., 2014). The carbon content in tobacco stalk increased from 44.85% to 64.10% after HTC. And the deoxygenation of camellia shell, rice husk and tobacco stalk are 72.98%, 60.94% and 80.08%, respectively. It is clear that the carbon growth rate and the oxygen removal rate for tobacco stalk are the highest at high temperature stage. The results are consistent with the increase of calorific value of hydrochar from tobacco stalk with temperature in the Table 4. It is possible that the content of lignin and cellulose in tobacco stalk is the highest, compared with rice husk and camellia shell. It is accepted that the O/C in hemicellulose, cellulose and lignin is 0.8, 0.83, 0.43–0.52, respectively (Arin et al., 2004). The O/C in tobacco stalk decreases significantly due to the removal of most hemicellulose and cellulose during hydrothermal carbonization. This result also is agreement with the lowest solid yield of tobacco stalk in Fig. 1. The relationship between decarbonization, dehydrogenation, deoxygenation and FCI of three biomass samples is shown in Fig. 1a–c. During the hydrothermal carbonization process, the decarbornization, dehydrogenation and deoxygenation of biomass all satisfy the exponential model y = a3(1−e(-b3x)), although the correlation coefficient 4

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Fig. 1. The correlation of decarbonization, dehydrogenation, deoxygenation, carbon recovery efficiency and FCI.

hydrothermal carbonization. At the first stage, a large amount of hemicellulose in the biomass is decomposed during the HTC below 200 °C and a small amount of cellulose is decomposed. The growth rate of carbon is much lower than the rate of mass loss of biomass. Therefore, the carbon recovery efficiency presents a rapidly decreasing trend. Then, when the temperature is higher than 200 °C, a large amount of cellulose is decomposed significantly, resulting in a significant polycondensation and oxidation reaction of the decomposition products from hemicellulose and cellulose, which promotes the formation of pseudo-lignin (Sannigrahi et al., 2011). At this stage, the growth rate of carbon and solid products exceeds the loss rate of biomass, which brings about a small rising peak in carbon recovery efficiency at around 225 °C. Decarbonization, deoxygenation, dehydrogenation are important indicators for quantifying the mass loss of carbon, hydrogen and oxygen in biomass during HTC at different temperature (Chen et al., 2016). The mass calculation formula for carbon elements is as follow:

McT (g ) = Mb

SYT

YcT

10

CRE is the carbon recovery efficiency, and the calculation formula for CRE is defined as:

CRE (%) = YcT /Ycs

where Ycs is the carbon (%) in camellia shell. 3.2.3. The energy properties analysis of hydrochar With the increase of hydrothermal carbonization temperature, the heat value increases regularly. Therefore, the correlation between the energetic retention efficiency and FCI is established, which is presented in Fig. 2a. The correlation of the energetic retention efficiency with FCI is in agreement with the power function model y = a5 + b5x + c5x2 + d5x3. From the nonlinear fitting equations, it indicates that the energetic retention efficiency decreases with the severity of hydrothermal carbonization, and then gradually flattens. This result implies the decreasing degree of solid yield is beyond the increasing degree of HHV in the low-temperature hydrothermal stage. However, it is opposite during the high-temperature stage. Referring to Fig. 2b, the decreasing degree of solid yield is nonlinear with the severity of hydrothermal carbonization, while the enhancement factor increases linearly with the severity of hydrothermal carbonization. This is the main reason for the fluctuation of energy recovery efficiency with FCI. Fig. 3b, strongly linear distributions (R2 > 0.97) of the enhancement factor of three biomass materials versus FCI are exhibited. The enhancement factor fits the linear model y = a6 + b6x. The slope b6 represents the increasing rate of enhancement factor with FCI. The slopes of the fitted models of camellia shell, rice husk and tobacco stalk are 0.44512, 0.37235, 0.85092, respectively. Therefore, the order of sensitivity to FCI is tobacco stalk > camellia shell > rice husk. This is mainly due to the fact that the

4

where McT represents the mass of carbon in hydrochar obtained by hydrothermal carbonization at the specific temperature (T). Mb is the weight of sample, SYT is the solid yield (hydrochar yield) at the specific temperature T. And YcT is the carbon content in hydrochar obtained at the specific temperature (T). The calculation formula for DC is defined as:

DC (%) = (1

McT / Mc0)

SYT

100

where DC, Mc0 are the decarbonization and the weight of carbon in the raw materials, respectively. DH and DO can also be calculated following the same equals. 5

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Fig. 3. Relationship between bulk density and energy consumption and FCI.

higher content of hemicellulose and cellulose in the tobacco stalk, which leads to an extremely sensitive reaction during the thermal reaction. Rice husks are extremely insensitive to thermal reactions due to the high ash content. The correlation of carbon recovery efficiency (CRE) versus energetic retention efficiency (ERE) is established in Fig. 2c. The carbon recovery efficiency and energetic retention efficiency are sensitive with FCI, and both of them increased linearly with the severity of hydrothermal carbonization. The slopes in Fig. 2c show that the growth rate of ERE and CRE is similar regardless of biomass species. It is noting that the slopes value in Fig. 2c are less than 1. This result implies that C is not the only source of energy retention efficiency. When the carbon content increases by 1%, the energy retention efficiency is only 0.72–0.82%. Since ERE has a strong linear correlation with CRE, these two indicators have the same change trend with FCI. The calculation formula for enhancement factor (EF) is defined as: Enhancement factor = HHVT/HHVswhere HHVT is the heat value of hydrochar obtained by hydrothermal carbonization at the specific temperature (T). HHVs is the heat value of biomass. ERE is the energy retention efficiency, and the calculation formula for ERE is defined as:

Fig.2. The correlation of energetic retention efficiency and enhancement factor and FCI and the relationship between ERE and CRE.

ERE (%) = EF

SYT

where EF is the enhancement factor.

6

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Fig. 4. Relationship between radial compressive strength of pellets and FCI (A:Rice husk B: Camellia shell C: tobacco stalk D: radial compressive strength of pellets with FCI).

3.3. The pelletization properties of hydrochar

economic benefits of biomass pellet as solid fuel. The relationship between bulk density, energy consumption and FCI is presented in Fig. 3. Error bars represent for the standard deviations of 5 replicates. It can be seen from Fig. 3A that the density of pellets increases with the degree of hydrothermal carbonization, and it tends to rise firstly and then

3.3.1. Bulk density and energy consumption during pelletization The density of pellets and the energy consumption are two important parameters for measuring the performance of pelletization and 7

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Fig. 5. Combustion curves of biomass and hydrochar.

(b0 + b1(x + a) + b2(x + a)2). The mathematical model is similar to the parabolic model, with a peak in its curve. The model gives a reliable prediction about the bulk density of hydrochar pellet with degree of hydrothermal carbonization. Moreover, the maximum of bulk density also depends on the biomass species (Peng et al., 2013). In general, the high of lignin content is in biomass, the larger the FCI is in order to reach the maximum of bulk density. As can be seen from Fig. 3B, the energy consumption of pellets appears to be completely opposite to the bulk density. This result implies that the pellet with high bulk density can be obtained via low energy consumption at low FCI in the range of 0.25–0.35. The models of bulk density and energy consumption provide effective prediction for the large-scale industry application of biomass pellets as solid fuels. Similarly, the energy consumption of the forms particles is also very strong correlated with FCI, fitting the nonlinear model y = a8*x(b8x^(-c8)). The coefficient is R2 > 0.9, regardless of biomass species. The minimum of energy consumption can rank as rice husk < tobacco stalk < camellia shell, depending on the biomass species. For the three biomasses, the energy consumption shows extremely obvious upward trend with the temperature of hydrothermal carbonization due to the increased friction in the press channels at higher torrefaction degrees (Rudolfsson et al., 2017). Meanwhile, the specific energy consumption of biomass pelllets has a strong correlation with the bulk density. During the low-temperature hydrothermal carbonization, the frictional force is small when the biomass is compressed under the pressure in the mold due to the bonding effect of lignin and the binding force of some other small molecular substances. Therefore, the lowest specific energy consumption is obtained at the hydrothermal carbonization temperature at about 170°C

decrease. Meanwhile, the bulk density of the biomass is higher than the one of hydrochar pellets obtained from HTC at 150 °C due to dehydration of biomass during low-temperature HTC. And the water content of biomass is an important factor, which significantly influences the bulk density of pellet. In general, biomass with low water content is more difficult to palletization due to the role of lubrication and bonding of water. Surprisingly, all of hydrochar pellets have a peak bulk density among 170 and 200 °C due to the glass transition of lignin above 140 °C (Reza et al., 2012). In the range of 170 to 200 °C, lignin plays the role of binder. Meanwhile, process, the compression during the pelletization also increases the temperature of the bio-char particles and promotes the softening of lignin, resulting in the bonding of hydrochar particles (Li et al., 2012). And other major components such as hemicellulose and cellulose did not undergo significant decomposition, resulting in an increase in the bulk density of the pellets. It is also accepted that the hydroxyl functional groups in the skeleton of lignin and hemicellulose can produce large of new H- bonds during the pelletization, which results in the increases of the pellets density (Larsson et al., 2013). Moreover, cellulose is also the important part for forming a solid bridge, which will increase the bulk density via interlock mechanism. In conclusion, the binder of lignin and the hydrogen bonding of cellulose and hemicellulose leads to the appearance of highest density in 170–200 °C. Therefore, the trend of the density of pellets is inevitably present in the real industrial production. When the HTC temperature is higher than 200 °C, a large amount of hemicellulose and part of the cellulose are decomposed (Bach et al., 2016). Resulting in a significant reduction in the bulk density of the pellets From the curves of the bulk density with FCI in Fig. 3A, it can be found that biomass generally follows a model y = (x + a)/ 8

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Table 5 Combustion characteristics of biomass and hydrochar. sample

RH RH-150 RH-175 RH-200 RH-225 RH-250 CS CS-150 CS-175 CS-200 CS-225 CS-250 TR TR-150 TR-175 TR-200 TR-225 TR-250

Curve parameter of TG

The combustion parameter

Temperaturezone of mass loss (°C)

Temperature of Peak/°C

Mass loss (%)

mass loss rate (wt%/min)

Ignition temperation T i /K

Index of combustible characteristics Cr/ 102−7·(K−2 ·min -1)

Integrated combustion characteristics index SN/ (10−11 ·K−3 ·min−2)

262.3–358.6 358.6–445.2 284.8–362.2 362.2–481.3 298–365.8 365.8–502.5 300.7–373.6 373.6–501.5 294.3–362.1 362.1–506.2 283–354.1 354.1–501.4 253.5–370.1 370.1–453.9 453.9–531.4 244.2–349 349–569.5 266–348.1 348.1–568 280.2–352.8 352.8–576.4 276.3–315.6 315.6–502.4 264.7–302.4 302.4–493.6 262.9–300.4 300.4–411.5 411.5–504.6 277–315 315–449.9 449.9–532.6 273.7–343.1 343.1–452.1 279.2–312.7 312.7–531.8 258–297.5 297.5–533.8 255.8–281.1 281.1–433.1 433.1–549.7

296 420.2 313.7 445.6 315.7 467.4 316.3 473.2 310.2 472.5 300.8 464.4 277.4 429.4 465.4 272.3 406.3 304.6 390.7 301.7 410.5 292.9 384.5 279.1 370.8 277.1 362.8 433.1 296.7 414.5 465.2 307.0 406.4 292.7 397.2 270.5 403.5 264.7 280.5 433.1

49.11 26.31 39.92 28.20 40.57 27.25 37.48 28.68 32.40 36.47 19.39 47.14 45.69 27.25 8.26 45.08 38.15 35.63 41.64 30.63 42.43 23.24 56.12 6.19 68.5 41.57 18.62 10.6 42.19 25.63 3.76 44.16 24.68 44.02 29.88 42.17 28.84 15.21 47.74 20.56

8.32 4.13 11.37 3.18 17.77 2.69 15.29 3.06 12.75 3.66 3.65 4.47 8.63 8.35 2.24 5.34 6.67 6.61 6.69 6.38 11.51 10.43 16.31 8.49 17.04 27.49 2.42 8.13 26.25 3.15 1.33 10.45 3.8 30.05 14.13 23.61 10.5 13.05 17.59 2.81

535.45

1.21

1.66

557.95

1.40

1.61

578.15

1.91

2.28

561.85

1.83

2.06

567.45

1.47

1.65

556.15

0.56

0.57

526.65

1.34

1.54

517.35

1.12

1.52

539.15

0.93

1.45

553.35

1.47

2.45

588.75

1.64

4.43

537.85

2.43

5.47

536.05

3.98

5.41

550.15

3.42

3.98

546.85

1.39

1.89

552.35

3.86

8.66

531.15

3.55

5.44

528.95

2.69

3.99

3.3.2. Strength of pellets The correlation of radial compressive strength, displacement, as well as radial compressive strength versus FCI is shown in Fig. 4A–D, respectively. Moreover, the brittleness of the pellets is enhanced by the HTC treatment. It is possible that the pseudo-lignin carbon microspheres are formed from the migration and polymerization of organic matters from the decomposition of cellulose and hemicellulose during HTC. And the pseudo-lignin is accumulated on the surface of the hydrochar (Sannigrahi et al., 2018). Meanwhile, it is worth noting that the samples with the maximum radial compressive strength for the rice husk and tobacco stalk appears at 200 °C. While it is 175 °C for camellia shell due to the lower cellulose and hemicellulose content. When temperature is over 175 °C, which is higher than the degradation temperature of the degradation of hemicellulose and cellulose. Therefore, it is beneficial for forming solid bridge and supplied H-bonds as adhere, significantly increase. Therefore, the radial compressive strength of the high-temperature pellets decreases rapidly. The relationship between the radial compressive strength of the pellets and FCI is shown in Fig. 5D. The growth trend for the radial compressive strength is similar to the bulk density of the pellets, indicating that the higher of bulk density is, the higher of the radial

Table 6 kinetics parameter of combustion of biomass and hydrochar. sample

The kinetics parameter of combustion E(KJ/mol) A/s−1

n

R2

RH RH-150 RH-175 RH-200 RH-225 RH-250 CS CS-150 CS-175 CS-200 CS-225 CS-250 TR TR-150 TR-175 TR-200 TR-225 TR-250

70.94 74.66 112.11 62.92 43.16 31.56 25.77 35.25 38.09 37.68 37.20 34.70 176.62 197.82 210.42 216.59 70.53 33.34

2.38 2.43 3.63 2.15 1.47 0.52 0.96 0.74 1.01 0.66 0.098 0.002 5.46 5.60 2.48 5.64 2.83 0.73

0.996 0.995 0.995 0.996 0.993 0.999 0.997 0.999 0.999 0.998 0.998 0.998 0.996 0.997 0.993 0.994 0.995 0.998

6.17E + 03 2.09E + 04 5.25E + 07 7.08E + 02 6.92E + 00 2.95E − 01 2.04E − 01 1.35E + 00 3.02E + 00 1.41E + 00 1.58E + 00 4.90E + 00 1.58E + 15 5.62E + 16 1.12E + 13 2.19E + 18 1.62E + 04 7.59E − 01

9

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carbonization. For the three biomasses, there are 2–3 stages during the combustion. Moreover, the peak temperatures for every stage is concentrated around 270–310 °C and 420–470 °C, respectively. It is believed that the first stage is due to the decomposition and devolatilization of light composition as well as the combustion of the volatiles. The second stage corresponds to the thermal carbonization of more complex components and the combustion of charcoal. The burning of rice husk and camellia shell is distinctly different. The maximum of weight loss rate of rice husk exists in the first stage, while it exists in the second stage for camellia shell due to the higher content of fixed carbon. The combustion rate of fixed carbon of rice husk decreases due to the high content of ash. Therefore, the combustion mainly concentrates on the generation and combustion of volatile matter. It is worth noting that the weight loss at the first stage significantly decreases for the hydrochar obtained under different temperature due to the degradation of large amount of hemicellulose and part of cellulose. With the deepening of hydrothermal carbonization, the weight loss and maximal weight loss rate at the second stage increases. Moreover, the comprehensive combustion characteristic index SN slightly changes with the deepening of the degree of hydrothermal carbonization. It is worth mentioning that there is a significant difference in the Sn for hydrochar from different biomass. The Sn of the hydrochar from tobacco stalk, rice husk and camellia shell fluctuates in the range of 3.98*10−11 K−3·min−2–8.66*10−11 K−3·min−2, 0.57*10−11 K−3 min−2–2.28*10−11 K−3·min−2 and 1.52*10−11 K−3·min−2–5.47*10−11 K−3 min−2, respectively, depending on the hydrothermal carbonization temperature. This result also corresponds to the highest volatile content of the tobacco stalk in Table 2. However, the high VM results in unstable flame and combustion and brings about large heat loss (He et al., 2013). Therefore, the tobacco stalk is not suitable raw materials for producing good solid fuel compared with other two biomass. 3.4.2. The combustion kinetics The combustion kinetics parameter of biomass and hydrochar are shown in Table 6. It is obvious that activation energy E raised first and then decreased. It is possible that biomass mainly undergoes drying and hemicellulose decomposition during HTC at low temperature. Therefore, the relative content of cellulose increases, resulting in the rising of combustion activation energy. However, when the HTC temperature increases, the content of cellulose decreases, resulting in a significant decrease of activation energy. In order to predict the development of E with degree of HTC, a correlation model between combustion activation energy and FCI is established in Fig. 6. It fits the nonlinear model y = a10x(b10x^(-c10)) and the order of combustion activation energy is TR > RH > CS. This is consistent with the order of the ignition points of hydrochar in Table 5, indicating that the hydrochar with low activation energy has a lower ignition point.

Fig. 6. Relationship between activation energy and FCI.

compressive strength is. It is possible that the bonding among particles with high bulk density is much stronger. And the solid bridge structure forms among the particles is much easier, so the compressive strength is larger. The relationship between radial compressive strength with FCI is agreement with the model y = a9x(b9x^(-c9)). The correlation is R2 > 0.9, which gives a feasible predictor for the radial compressive strength with degree of HTC. The maximum of radial compressive strength is determined by the value of two parameters b9 and c9. The larger c9, the faster the increase of radial compressive strength. The model shows that the compressive strength is in the rank of tobacco stalk > camellia shell > rice husk. Combined with the bulk density and energy consumption of pellets in Fig. 4, the tobacco stalk has the largest bulk density, the lowest energy consumption, and the highest radial compressive strength among the three-biomass samples. This result indicates that the biomass hydrochar with high cellulose content has the best pelletization properties. Meanwhile, by establishing the correlation between the properties of pelletization with FCI, it can be predicted that the suitable hydrothermal carbonization temperature is in the range of 175–200 °C with high bulk density of the pellets, low energy consumption and strong the radial compressive strength. Those results provide theoretical guidance for the preparation of high-quality solid fuels from biomass via HTC.

3.5. Discussion In the article, the fixed carbon index based on the hydrothermal carbonization severity is introduced to predict the fuel properties, pelletization and combustion performance of biomass and get high value solid fuel from biomass. The relationship between the chemicalphysical properties of hydrochar, pelletization properties and combustion performance and FCI is investigated. The results show that the correlations between solids and gas yield, enhancement factors, energy retention efficiency, decarburization, dehydrogenation, deoxygenation, carbon recovery efficiency, bulk density, energy consumption, compressive strength, activation energy and fixed carbon index fits well. Especially, the characteristic parameters of the regression lines suggest that element removal from hydrochar can be ranked as DO > DH > DC. Although the fuel quality increases with FCI, the energy retention efficiency decreases. The correlation is useful for balancing the energy efficiency and fuel quality. In addition, the

3.4. The combustion behavior of hydrochar 3.4.1. The combustion properties analysis Fig. 5 and Table 5 show the combustion characteristics and parameters of biomass and hydrochar. In table 5, it is obvious that the properties of hydrochar varied with the degreed of hydrothermal 10

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hydrochar obtained in the range of (FCI = 0.15–0.45) can be applied in producing pellet fuel due to the highest bulk density, the lowest specific energy consumption and the strongest radial compressive strength. Moreover, the combustion characteristics of hydrochar and the combustion activation energy are correlated to FCI. The activation energy of hydrochar obtained at FCI (0.15–0.25) is higher, which is not conducive to combustion. Therefore, it is beneficial to obtain the solid fuel from biomass with high-quality and low energy consumption when FCI is 0.25–0.45.

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4. Conclusion In conclusion, the relationship between energy efficiency, properties of pelletization and combustion characteristics and FCI was investigated in order to predict the fuel properties, pelletization and combustion performance of biomass and get high value solid fuel with high-quality solid fuel and low energy consumption from biomass. The results also suggest that the when FCI is 0.25–0.45, the solid fuel from biomass is with high-quality and low energy consumption. The research provides theoretical guidance for the preparation of pellet fuel based on the FCI of hydrochar. Acknowledgements Supported by Chinese National Natural Science Foundation (Grant No. 51706075); ChineNational Natural Science Foundation (Grant NO. 51576071); Science and Technology Planning Project of Guangdong Province, China (Grant No. 2016A020210073), the Science and Technology Planning Project of Guangdong Province, China (Grant No. 2015B020237010); the Science and Technology Planning Project of Guangzhou City, China (Grant No. 201906010042); Guangdong Key Laboratory of Clean Energy Technology, China (Grant No. 2017B030314127). References Arin, G., Demirbas, A., 2004. Mathematical modeling the relations of pyrolytic products from lignocellulosic materials. Energy Sources 26, 1023–1032. Bach, Q.V., Trinh, T.N., Tran, K.Q., et al., 2016. Pyrolysis characteristics and kinetics of biomass torrefied in various atmospheres. Energ. Conver. Manage S0196890416303478. Bu, Q., Lei, H., Wang, L., Wei, Y., Zhu, L., Zhang, X., et al., 2014. Bio-based phenols and fuel production from catalytic microwave pyrolysis of lignin by activated carbons. Bioresour. Technol. 162, 142–147. Cai, J., Li, B., Chen, C., Wang, J., Zhao, M., Zhang, K., 2016. Hydrothermal carbonization of tobacco stalk for fuel application. Bioresour. Technol. 220, 305–311. Chai, L., Saffron, C.M., 2016. Comparing pelletization and torrefaction depots: Optimization of depot capacity and biomass moisture to determine the minimum production cost. Appl. Energy 163, 387–395. Chen, Y., Chen, W., Lin, B., Chang, J., Ong, H.C., 2016. Impact of torrefaction on the composition, structure and reactivity of a microalga residue. Appl. Energy 181, 110–119. Chen, D.Y., Gao, A.J., Ma, Z.Q., Fei, D.Y., Chang, Y., Shen, C., 2018a. In-depth study of rice husk torrefaction: Characterization of solid, liquid and gaseous products, oxygen migration and energy yield. Bioresour. Technol. 253, 148–153. Chen, D.Y., Gao, A.J., Cen, K.H., Zhang, J., Cao, X.B., Ma, Z.Q., 2018b. Investigation of biomass torrefaction based on three major components: Hemicellulose, cellulose, and lignin. Energ. Convers. Manage. 169, 228–237. Chen, W., Lu, K., Tsai, C., 2012. An experimental analysis on property and structure

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