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Torrefaction of herbal medicine wastes: Characterization of the physicochemical properties and combustion behaviors
Bioresource Technology 287 (2019) 121408
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Torrefaction of herbal medicine wastes: Characterization of the physicochemical properties and combustion behaviors
T
⁎
Shanzhi Xina, Fang Huanga, Xiaoye Liua, Tie Mia, , Qingli Xub a b
Hubei Key Laboratory of Industrial Fume and Dust Pollution Control, Jianghan University, Wuhan 430056, China Laboratory of Coal Gasification and Energy and Chemical Engineering Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Herbal medicine waste Torrefaction Physicochemical properties Combustion
To explore the feasibility of using herbal medicine waste (HMW) as solid fuel, HMW was torrefied under different temperatures and atmospheres. The physicochemical properties and combustion behaviors of the torrefied HMW were investigated. Temperature was found to be the most influential factor affecting the torrefaction. Torrefaction improved the hydrophobicity of HMW and decreased the equilibrated moisture uptake from 24.48(0.083) % to 15.22(0.054) %. The HMW samples torrefied under different conditions are easy to ignite. The comprehensive combustibility index (S) of the torrefied HMW increased by 3–5 folds compared to that of the raw sample. In general, the HMW torrefied under lower temperatures and under CO2 and O2 have better flammability. The present results revealed that the torrefied HMW exhibited good combustion characteristics and can thus be used for solid fuel production, such as fuels for co-combustion or raw materials for pelletization.
1. Introduction Traditional Chinese herbal medicines have played an important role in disease treatment and drug development for a thousand years. Currently, there are over 1500 herbal medicine enterprises in China (Mi and Yu, 2012). Pharmaceutical processes produce large amounts of herbal medicine waste (HMW). In recent years, the development of the herbal medicine industry has led to an increase in the amount of HMW discharged by pharmaceutical factories. It is estimated that approximately 12 million tons of HMW are produced annually, and this number continues to increase each year (Guo et al., 2014). At present, HMW is mainly stacked in pharmaceutical factories, or abandoned randomly in the wild and part of it is treated by sanitary landfilling, which can pollute the air, soil and water. Moreover, the medicine enterprises need to pay fees for disposing HMW. Thus, the development of an efficient utilization method for HMW can produce both economic and environmental benefits. HMW is rich in cellulose, hemicellulose, and lignin, which can be converted into biofuels by thermochemical processes. Furthermore, HMW is a highly centralized resource compared to the dispersion of biomass. Previous studies obtained syngas with heating value of ∼6.0 MJ/Nm3 and CO2-rich gas with medium-energy (8.8–10.5 MJ/kg) from the gasification and pyrolysis of herbaceous biomass (Guo et al., 2014; Hlavsová et al., 2016). Wang et al., (2010) obtained bio-oil with a
low oxygen content and a high calorific value (25.94 MJ/kg). Activation carbons were prepared from herbal biomass with the specific surface areas as great as 920–1952 m2/g (Yang and Qiu, 2011; Mi et al., 2015). The char was then used as an absorbent for absorbing sulfamethoxazole (Lian et al., 2014) and lead (Wang et al., 2015). Nevertheless, HMW has a high moisture content (as high as ∼70%) and cannot be directly utilized by thermochemical methods. Therefore, HMW must be dewatered before being processed. One effective way to overcome the drawback is torrefaction, which is also called mild pyrolysis. Torrefaction improves the energy content and energy density of a biomass. Compared to traditional biomass, torrefied biomass ignites more quickly and is an attractive feedstock for gasification and combustion (Bridgeman et al., 2008; Tsalidis et al., 2015). The torrefied oil palm wastes improved the combustion efficiency in a boiler and thus have great potential to be used as solid fuel (Lasek et al., 2017). Meanwhile, the co-combustion of coal and torrefied biomass enabled decreases in the emissions of NO and SO2 (Gil et al., 2015). In addition, the hot flue gas from the combustor are commonly used as a torrefaction agent with respect to the torrefaction efficiency, economic feasibility and the simplicity of production processes (Sellappah et al., 2016; Lasek et al., 2017). The O2 and CO2 in flue gas facilitated the decomposition process and enhanced the heating value of the torrefied char (Uemura et al., 2017). Therefore, torrefaction is an optional method for the disposal of
⁎ Corresponding author at: Hubei Key Laboratory of Industrial Fume and Dust Pollution Control, Jianghan University, 8 Sanjiao Lake Road, Wuhan, Hubei 430056, China. E-mail address: [email protected] (T. Mi).
https://doi.org/10.1016/j.biortech.2019.121408 Received 30 March 2019; Received in revised form 29 April 2019; Accepted 30 April 2019 Available online 02 May 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Chemical composition of the torrefied HMW. raw
230N
260N
290N
260CO
260O
Proximate analysis, wt%, db.
VM FC Ash
83.65 (0.32) 13.49 (0.38) 2.86 (0.1)
81.86 (0.29) 15.66 (0.4) 2.48 (0.11)
78.85 (0.24) 18.66 (0.37) 2.49 (0.11)
75.12 (0.27) 22.54 (0.35) 2.34 (0.09)
81.27 (0.21) 16.05 (0.29) 2.18 (0.1)
81.22 (0.28) 16.60 (0.2) 2.48 (0.09)
Ultimate analysis, wt%, daf.
C H O N S
48.01 (0.12) 6.94 (0.24) 42.85 (0.17) 2.01 (0.14) 0.19 (0.023)
48.85 (0.15) 6.86 (0.08) 42.39 (0.04) 1.82 (0.054) 0.08 (0.025)
50.02 (0.062) 6.62 (0.063) 41.41 (0.12) 1.84 (0.0044) 0.11 (0.0084)
53.29 (0.31) 6.4 (0.023) 38.22 (0.29) 2.0 (0.0076) 0.09 (0.0068)
49.34 (0.13) 6.8 (0.029) 41.89 (0.096) 1.85 (0.093) 0.12 (0.0004)
49.57 (0.13) 6.84 (0.11) 41.60 (0.08) 1.87 (0.0044) 0.12 (0.0023)
18.49 (0.21)
18.75 (0.13)
18.97 (0.095)
20.34 (0.12)
18.92 (0.061)
19.10 (0.16)
HHV (MJ/kg)
db: dry basis; daf: dry ash free basis.
The cellulose, hemicellulose and lignin contents in the raw and torrefied HMW were analyzed according to the Chinese national standard (GB/T 2677.10-1995) and the reported methods with minor modifications (Chen et al., 2018b; Zhang et al., 2018). Firstly, the extractives of the raw HMW were removed by the Soxhlet extraction for 6 h with a toluene/ethanol mixture (2:1, v/v). Afterwards, 2 ± 0.005 g extractive-free samples were placed in a conical flask and mixed with 65 mL of purified water, 0.50 mL of acetic acid, and 0.60 g of NaClO2 (100% basis) at 348 K for 1 h. Then, add 0.50 mL of acetic acid and 0.60 g of NaClO2 for every 1 h until the sample turned white. The weight loss was counted as the fraction of lignin. The lignin-free samples were further soaked in a 6 wt% KOH solution at room temperature for 12 h, followed by 80 °C water bath for 2 h in order to remove hemicellulose. The final residue was regarded as cellulose. The fraction of hemicellulose was calculated by difference. The surface functionality of the raw and torrefied HMW were analyzed via FTIR spectroscopy (VERTEX 70 Bruker, Germany). The wavelengths of the spectra range from 400 to 4000 cm−1. Each spectrum was the accumulation of 120 scans with a resolution of 4 cm−1. The crystallinity changes of the raw and torrefied HMW were measured by using X-ray diffraction (X’Pert PRO, Netherlands). A powder X-ray diffraction detector with Cu-Kα radiation was applied, and the scanning angle (2θ) ranged from 10 to 90° at a scan rate of 4° min−1. The crystallinity index (CrI) was calculated using Segal’s method (Li et al., 2018):
HMW. The torrefied HMW is a solid carbonaceous resource, which can be sued for replacing coal in energy and heat production processes (Proskurina et al., 2017). However, to date, the torrefaction of the highmoisture HMW and the resulting combustion behaviors of solid char have received little attention. In this study, torrefaction of HMW was conducted, and the physicochemical properties and combustion behaviors of torrefied HMW were investigated to explore the feasibility of using HMW as solid fuel. 2. Experiment and methods 2.1. Materials and torrefaction experiments HMW was collected from a traditional Chinese medicine pharmacy in the school infirmary. The moisture, ash, and volatile content of the HMW were determined in accordance with ASTM E871–82, D1102–84, and E872–82, respectively. The fixed carbon content was calculated by difference. The moisture content of the HMW is 68.8%, and the chemical composition of the HMW is listed in Table 1. The raw HMW was torrefied on a horizontal lab scale furnace with a quartz tubular reactor, as the same in our previous work (Xin et al., 2013, 2018). The reactor was first heated to the designated temperature with nitrogen at 500 mL/min. Afterward, 10 g of raw HMW was placed in the quartz ark and quickly pushed into the center of the reactor for 60 min. The torrefaction temperatures were 230 °C, 260 °C and 290 °C, and the torrefied samples were denoted as 230N, 260N and 290N, respectively. In addition, the HMW was torrefied under CO2 and O2 (10% O2, 90% N2) at 260 °C, and the corresponding samples were designated 260CO and 260O, respectively. Each torrefaction experiment was conducted 10 or more times. The measurement uncertainty was evaluated according to the national standards of China (GB/T 27418-2017).
CrI (%) =
(I002 − Iam) × 100% I002
(4)
The elemental composition was analyzed by a CHNS elementary analyzer (Vario Macro cube, Germany). The amount of carbon, hydrogen, nitrogen and sulfur was measured directly. The amount of oxygen was calculated by difference. The higher heating value (HHV) of the char was calculated by Eq. (1) (Li et al., 2015):
where I002 is the maximum peak intensity at ∼22° and Iam is the minimum intensity at 2θ of approximately 18.1°. The water adsorption capacity was tested at a relative humidity (RH) of 95% and a temperature of 30 °C. Approximately 2 g of the predried sample was dispersed evenly on a glass dish. The weight of the sample after moisture uptake was recorded at defined time intervals in 0.5–72 h. The amount of moisture absorbed (AMC) was calculated according to the equation AMC (%) = (mi–m0)/m0 × 100%, where m0 and mi are the weight of the samples before and after moisture uptake, respectively. Each test was performed at least thrice to ensure repeatability.
HHV (MJ/kg) = 0.3383WC + 1.422(WH − WO/8)
2.3. Combustion characteristics of the raw and torrefied HMW
2.2. Analysis of the raw and torrefied HMW
(1)
where WC, WH and WO are the weight fractions of carbon, hydrogen and oxygen, respectively. The mass yield (My) and energy yield (Ey) were calculated on a dry basis using Eqs. (2) and (3):
My = (Mt /Mr ) × 100%
(2)
E y = My × (HHV/HHV) t r
(3)
2.3.1. Combustion characteristic The combustion of the torrefied HMW was conducted on a thermogravimetric analyzer (TGA4000, Perkin Elmer). Approximately 10 mg of the raw and torrefied HMW samples were placed in the crucible and heated from ambient temperature to 800 °C with an air flow rate of 100 mL/min. The heating rate was 10, 15, 20, and 30 °C/min. The temperature at which fuel begins to burn is considered the ignition temperature (Ti), where when the mass loss rate reaches 1%/min in the last stage of combustion is denoted as the burnout temperature (Tb).
where M represents the mass of the sample and the subscripts ‘‘t” and ‘‘r” represents the torrefied and raw HMW samples, respectively. 2
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The Ti and Tb values were determined in accordance with the methods described in previous studies (He et al., 2013). Moreover, the ignition index (Di) and comprehensive combustibility index (S) were used to evaluate the combustion behaviors of the torrefied HMW, which were defined as follows by Eqs. (5) and (6) (Zhang et al., 2016):
Di =
S=
(dw / dt )max Ti Tp (dw / dt )max (dw / dt )mean Ti2 Tb
(5)
(6)
where (dw/dt)max and (dw/dt)mean represent the maximum and mean mass loss rate (%/min), respectively, and Tp is the temperature corresponding to the maximum mass loss rate (°C). 2.3.2. Kinetic parameters from the isoconversional method Generally, the “model-free” kinetic methods enable the evaluation of the activation energy without the premise of assuming the reaction model and thus more accurate. The activation energy obtained from these methods can be used to assess the consistency of the reaction mechanism throughout the decomposition process by comparing the activation energy values at different conversion rates (α). Among the model-free methods, the differential isoconversional methods have been shown to be more accurate than the integral methods (Vyazovkin et al., 2011). In the present study, the Friedman method, one of the most widely used differential methods, was used to determine the kinetic parameters during the combustion of the raw and torrefied HMW samples. The Friedman method was formulated as follows (Vyazovkin et al., 2011):
dα E ln ⎡ ⎛ ⎞ β⎤ = ln[Af (α )] − a ⎢ RT ⎣ ⎝ dT ⎠ ⎥ ⎦
Fig. 1. Mass and energy yields of the HMW torrefied under different conditions.
Ey values, which were 93.42(0.63) % and 95.17(0.007) %, respectively. This phenomenon occurs because O2 has a higher oxidative rate than CO2 or N2 and thus shows a stronger torrefaction severity than CO2 and N2 (Uemura et al., 2015). The present results are similar to the torrefaction of bamboo under CO2 reported by Li et al. (2015). Note that the My and Ey values under different torrefaction agents did not exhibit significant differences. This lack of a significant difference is probably because the promotion of solid conversion by O2 and CO2 is more pronounced only at higher temperatures (Uemura et al., 2015). However, the moisture in the HMW may also affect the decomposition of the biomass components. The energy yield of the torrefied HMW in O2 was 87.39(0.015) %, which is greater than the torrefaction of oil palm fiber pellets (59.50–71.37%) (Chen et al., 2016). This result suggests that a greater amount of energy was preserved in the torrefied char in the presence of O2. Moreover, the torrefied HMW may also be used as a raw material for pelletization.
(7)
where β is the linear heating rate, K/min; f(α) is model-dependent function in the differential form. Ea is the activation energy, J/mol; A is the pre-exponential factor, 1/s; and R is the universal gas constant, 8.314 J/(mol K), and T is the absolute temperature, K. Plotting the left side of Eq. (7) against 1/T at a fixed degree of conversion rate and different heating rates will give a series of straight lines, and the activation energy can be calculated from the slope.
3.1.2. Chemical and organic compositions of the torrefied samples Table 1 lists the proximate and ultimate analyses of the raw and torrefied HMW samples under different conditions. The raw HMW sample is characterized by a high volatile matter (VM) content (83.65%), low fixed carbon (FC) and ash contents. The VM in the raw HMW is much higher than that in common biomass, such as oil palm fiber and shell (70.50–70.71%) (Uemura et al., 2015; Chen et al., 2016), and rice straw (Park et al., 2014). The VM content in the torrefied HMW decreased steadily as the temperature increased from 81.86(0.29) % at 230 °C to 75.12(0.27) % at 290 °C, which is consistent with the mass yield in Fig. 1. Consequently, the FC content of the residue char increased. A previous study found that a high amount of VM in a biomass can produce fuel-rich conditions during combustion and may benefit the decrease in NO (Gil et al., 2015). However, high biomass co-combustion ratios (∼10% and above) will decrease the overall boiler efficiency of coal-fired power plant (Agbor et al., 2014). Therefore, the high content of VM in the torrefied HMW may have positive effects on the NO emissions during combustion. Notably, the VM contents of the HMW torrefied by CO2 and O2 were slightly higher than that of the HMW torrefied by N2. Uemura et al. noted that oxygen shows a strong devolatilization capacity during torrefaction (Uemura et al., 2015). Therefore, the moisture may lower the torrefaction severity compared to the torrefaction of dry biomass and thus reduce the extent of hemicellulose decomposition. As shown in Table 1, the content of carbon in the HMW increased after torrefaction, whereas the hydrogen and oxygen content decreased. The elemental composition of the torrefied HMW at 230 °C was close to
3. Results and discussions 3.1. Physicochemical properties of the torrefied samples 3.1.1. Mass and energy yields The mass yield (My) and energy yield (Ey) of the torrefied HMW under different conditions are shown in Fig. 1. The measurement uncertainty of the mass yield for 230N, 260N, 290N, 260CO and 260O was determined to be 0.68%, 0.37%, 0.34%, 0.63% and 0.33%, respectively. For energy yield, the measurement uncertainty was in the range of 0.007–0.015%. Apparently, the torrefaction temperature exerted a stronger effect on the yields than the torrefaction agent. The My and Ey values both decreased continuously with increasing temperature. The My and Ey values at 230 °C were as high as 95.88(0.68) % and 96.96(0.015) %, and these values slightly decreased by 4–5% as the temperature increased to 260 °C. This result indicated that hemicellulose did not undergo apparent decomposition at temperatures less than 260 °C. However, the decomposition of hemicellulose generally occurs in the range of 220–315 °C, and maximum mass loss occurs at 268 °C (Yang et al., 2007). This finding indicated that the release of moisture during torrefaction hindered the decomposition of hemicellulose. At 290 °C, the My and Ey values decreased remarkably to 76.94(0.34) % and 83.23(0.014) %, respectively, which was attributed to the loss of hemicellulose. As seen in Fig. 1, the torrefaction of HMW in O2 obtained the lowest My and Ey values, while torrefaction in CO2 attained the highest My and 3
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cellulose content was almost unchanged when HMW was torrefied at 230 °C. At the same temperature, the hemicellulose content decreased slightly from 36.97(0.25) % to 32.50(0.18) %. Thus, the decrease in VM and oxygen at 230 °C was attributed to the partial decomposition of hemicellulose. As the temperature increased from 230 to 260 °C, the cellulose content decreased from 22.75(0.17) % to 22.09(0.19) %, while the hemicellulose content decreased substantially from 32.50(0.18) % to 25.01(0.20) %. Consequently, the lignin content increased from 44.75(0.33) % to 52.90(0.39) %. Cellulose and hemicellulose are richer in oxygen than lignin (Xin et al., 2013). Hence, the released volatiles with high O/C and H/C ratios decreased the hydrogen and oxygen contents in the torrefied HMW, as shown in Table 1. At 290 °C, the apparent decomposition of cellulose and hemicellulose resulted in a progressive decrease in their contents. As shown in Fig. 2, torrefaction of HMW in O2 resulted in low cellulose and hemicellulose contents of 19.22(0.11) % and 22.63(0.23) %, respectively. However, their contents in CO2 torrefaction were slightly higher than those in N2 torrefaction. This finding demonstrated that oxygen has the strongest capacity to degrade polysaccharides in biomasses. The above findings are consistent with previous results showing that temperature exerts a greater effect than torrefaction agents.
that of the raw sample. As the temperature increased from 230 °C to 290 °C, the carbon content continuously increased from 48.85(0.15) % to 53.29(0.31) %, whereas the oxygen content decreased from 42.39(0.04) % to 38.22(0.29) %. The atomic H/C and O/C ratios are commonly used as an index to evaluate the energy density of a solid fuel. The atomic ratios of H/C and O/C of the torrefied HMW decreased with increasing temperature, which was attributed to the dehydration, deoxygenation, and dehydrogenation reactions. The torrefied HMW at 290 °C had the lowest H/C and O/C ratios of 1.44 and 0.54, which demonstrated that the temperature is the most influential factor affecting the torrefaction process. Note that the torrefied HMW in different torrefaction agents possess similar elemental compositions. As a result, the H/C and O/C ratios also appeared to be equivalent. Wang et al. found that the properties of sawdust torrefied in O2 less than 6% were similar to those samples torrefied in N2 (Wang et al., 2013). This finding indicates that torrefaction agents exert minor effects on the torrefaction of high-moisture HMW. Moreover, the H/C and O/C ratios from the present study are higher than those in the literature results under similar torrefaction conditions. This finding suggests that the moisture in the biomass is also an important factor that can suppress dehydration, deoxygenation, and dehydrogenation during torrefaction. The HHV of the torrefied HMW is very close to that of the raw sample when torrefied at temperatures less than 260 °C. The HHV notably increased from 18.97(0.095) MJ/kg to 20.34(0.12) MJ/kg as the temperature increased from 260 °C to 290 °C. The results show that the torrefied HMW with a higher FC and carbon content has a higher heating value. Generally, fuels with higher FC and carbon content facilitate the subsequent gasification and combustion processes (Pérez et al., 2012). In this regard, torrefaction of the HMW at higher temperatures is preferable, and torrefied char is suitable for use as solid fuel. Nevertheless, torrefaction at higher temperatures will also reduce the mass and energy yields. At 260 °C, the HMW torrefied in CO2 and O2 have higher heating values than that the HMW torrefied in N2. A previous study found that biomass torrefaction in oxygen-containing atmospheres has a potentially positive impact on processing costs and processing efficiency (Joshi et al., 2015). This finding suggests that torrefaction with flue gas is an option for the disposal of HMW. However, it may be better to partially remove the moisture in the raw HMW prior to torrefaction. Fig. 2 illustrates the change in the organic components of the raw and torrefied HMW. In general, torrefaction decreased the contents of cellulose and hemicellulose in the HMW because they are more likely to undergo thermal decomposition than the other components. The
3.1.3. Chemical structure of the torrefied HMW The torrefaction altered the surface functional groups of the HMW. The strongest peak at 1026 cm−1 in the raw HMW indicates the abundant hydroxyl groups in polysaccharides or the methoxyl and β-O4 linkages in lignin. The absorbance intensity decreased remarkably after torrefaction, which demonstrated the breakage of the CeO bond. However, the intensity increased with a further increase in temperature. This phenomenon is probably due to the increase in the relative content of lignin in the torrefied HMW as the temperature increased from 230 °C to 290 °C. The peaks at 1075 cm−1 and 1152 cm−1 were associated with the CeOeC linkage of hemicellulose units or the glycosidic bond of cellulose. The peak at 1075 cm−1 disappeared, and the peak at 1152 cm−1 exhibited an apparent decrease in the torrefied HMW. This phenomenon demonstrated the notable decomposition of hemicellulose and a part of the cellulose in the course of torrefaction, which is consistent with the previous results (Fig. 2). The bands at 1318 cm−1, 1515 cm−1 and 1630 cm−1 are the characteristic absorption peaks of aromatic structures. The absorbance intensity of each of these peaks increased progressively with increasing torrefaction temperature, which confirmed the synchronous increase in lignin content. The CeOeC linkage of the torrefied HMW also vanished in the presence of CO2 and O2. Moreover, the absorbance intensity of 1026 cm−1 in CO2 and O2 was apparently lower than that in N2. This finding is attributed to the stronger capacity of CO2 and O2 in the deoxygenation and dehydrogenation reactions during torrefaction, which enhanced the decomposition of hemicellulose and cellulose. 3.1.4. Crystal structure analysis Cellulose is the only crystal structure in the biomass, which displays three typical diffraction peaks at 2θ = 22.6 (0 0 2 plane), 16.3 (1 0 1¯ plane) and 14.8 (1 0 1 plane) (Xin et al., 2015). The raw HMW is less ordered with a CrI of 18.93(0.14) % because of the influences of amorphous hemicellulose and lignin. As the torrefaction temperature increased from 230 °C to 290 °C, the intensity of the 0 0 2 peak increased and resulted in a gradual increase in CrI from 32.87(0.21) % to 40.70(0.33) %. This increase in CrI indicates that torrefaction increases the crystallinity of the HMW. The cellulose and hemicellulose are connected together by chemical bonds in the biomass (Sun, 2010). The decomposition of hemicellulose during torrefaction and the loss of amorphous cellulose and a part of lignin will liberate the crystalline fractions of cellulose and thus increase the CrI of the torrefied HMW. In the present study, the CrI changes in accordance with the wet torrefaction of biomass but shows the opposite trends with dry torrefaction (Zheng et al., 2015; Chen et al., 2018a). This implies that the moisture
Fig. 2. Organic components of the raw and torrefied HMW under different conditions (L, lignin; HC, hemicellulose; C, cellulose; and E, extractives). 4
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starts to decompose at a lower temperature but exhibits burnout at a higher temperature. The corresponding DTG curve was classified into four stages: dehydration, volatile release, volatile combustion, and residue char combustion. As seen from Fig. 4, the combustion of the torrefied HMW samples was characterized by two distinct stages, which lie in ∼200–387 °C and 387–554 °C, according to the DTG curves. The maximum weight loss temperature is 330 °C and 451 °C. As the temperature increases, the TG curves move gradually to higher temperatures. From Fig. 4, the weight loss curves of 260CO and 260O basically overlap over the entire temperature range, which indicates that the properties of the HMW torrefied under an oxygen-containing atmosphere are similar. In the first stage, the combustion behaviors of 260CO and 260O are coincident with those of 260 N. However, the combustion of 260 N shifted to higher temperatures in the second stage. Generally, the first stage was due to devolatilization, and the second stage was attributed to the combustion of char. This finding suggests that the torrefaction agents have minor effects on the devolatilization of the HMW during combustion. The thermal decomposition behaviors of biomass components are known to be different (Yang et al., 2007). Therefore, to understand the contributions of the biomass components to the weight loss of the combustion of the torrefied HMW, the DTG curve of 260N was curvefitted. The two main DTG peaks of the torrefied HMW are superimposed by four peaks: 272 °C, 318 °C, 340 °C and 461 °C. A previous study found that the weight losses of hemicellulose, hemicellulose and lignin combustion are concentrated in the temperature ranges of 220–320 °C, 300–360 °C and 200–600 °C, respectively (Cao et al., 2018). Therefore, the first three peaks are attributed to the decomposition of hemicellulose, cellulose, and lignin and may be accompanied by the combustion of volatiles. The peak at 461 °C was assigned to the combustion of residual char, and lignin may continue to decompose. Table 2 shows the combustion characteristic parameters of the torrefied HMW. The ignition temperature (Ti) of the raw HMW was 261.29(0.15) °C, which increased slightly after torrefaction. The Ti of the torrefied HMW increased slightly from 284.78(0.13) °C to 296.35(0.11) °C as the torrefaction temperature increased from 230 °C to 290 °C. The ignition temperatures of common biomasses, such as straw dust, wheat straw, paper sludge and rice straw, were in the range of 271–279 °C (Wang et al., 2009; Xie and Ma, 2013). In this study, the Ti of the torrefied HMW was only 10–20 °C higher than those of biomasses, but it is significantly lower than that of coal (475 °C) (Wang et al., 2009). This finding indicated that the torrefied HMW samples under different conditions are easy to ignite. However, few differences in Ti were observed among the various torrefied HMW samples, especially for those torrefied in N2, CO2 and O2 at 260 °C. This suggests that torrefaction not only removes the moisture from the raw HMW and increases its energy density but also exerts less impact on the ignition characteristics. As seen from Table 2, the ignition index (Di) and the comprehensive combustibility index (S) of the raw HMW are 1.12(0.003) %· °C−2·min−1 and 0.72(0.009) %· °C−3·min−2, respectively. After torrefaction, the Di and S increased significantly, implying that torrefaction also enhanced the combustion properties of the HMW. The combustion of 230N resulted in the highest Di and S values of 2.21(0.0087) %·°C−2·min−1 and 3.05(0.036) %·°C−3·min−2, respectively, and decreased slightly with increasing temperature. In particular, the Ti of the torrefied HMW exhibits the opposite trend with temperature. This phenomenon suggested that the HMW torrefied at a lower temperature has better flammability. However, torrefaction at higher temperatures does not remarkably lower the combustible performance of the HMW. Generally, oxygen-containing agents, such as O2 and CO2, can increase the severity of torrefaction and enhance the deoxygenation and dehydrogenation reactions (Chen and Lin, 2016; Chen et al., 2016). From Table 2, the Ti and Th of 260CO and 260O were lower than those of 260N, but the Di and S of the former were much higher. This finding
Fig. 3. Moisture absorption properties of the torrefied HMW prepared under different conditions.
in HMW may affect the bond dissociation during the torrefaction process. The CrI of torrefied HMW in O2 is the lowest at 34.52(0.19) %, whereas the CrI of the torrefied HMW in CO2 is the highest at 40.29(0.22) %. These results demonstrated that O2 enhances the torrefaction severity and promotes the decomposition of hemicellulose and cellulose. 3.1.5. Water adsorption capacity The hygroscopic behaviors of the torrefied biomass are crucial in terms of transportation and long-term storage and for follow-up thermochemical utilization. Fig. 3 shows the moisture absorption curves of the torrefied HMW. It is apparent that the rate of moisture absorption of the torrefied HMW exhibited a sharp increase in the first 3 h and reduced in the 3–12 h. Afterwards, the moisture uptake reached the equilibrium value as time elapsed. The moisture uptake of the torrefied HMW can be equilibrated in a short time. The maximum AMC of the raw HMW was 24.48(0.083) %, and this value was notably reduced after torrefaction. The AMC decreased gradually from 22.53(0.065) % at 230 °C to 15.22(0.054) % at 290 °C. This finding indicated that torrefaction improves the hydrophobicity of the HMW, which is in agreement with the results of a previous study (Oluoti et al., 2018). Cellulose and hemicellulose are rich in hydroxyl groups, which are capable of forming hydrogen bonds with water. Our previous study found that water can be evolved out chemically at 200 °C from the pyrolysis of cellulose (Xin et al., 2015). Therefore, the loss of hydroxyl groups, along with the decomposition of hemicellulose and cellulose, decreased the water sorption capacity of the torrefied samples. From Fig. 3, the AMC values of the HMW torrefied in CO2 and O2 are 20.73 (0.071) % and 21.61(0.066) %, respectively, which are higher than that of the HMW torrefied in N2. This finding suggests that a greater number of hydroxyl groups were preserved in the torrefied sample in the presence of CO2 and O2. Consequently, the oxygen and hydrogen contents of the HMW torrefied in CO2 and O2 are higher than those in the HMW torrefied in N2 (Table 1). Although the AMC of the torrefied HMW in the present study was slightly higher than that in some published data, the water sorption of the former can be equilibrated in a shorter time (Toscano et al., 2015). 3.2. Combustion characteristics of the torrefied HMW The combustion TG-DTG curves of the raw and torrefied HMW samples at a heating rate of 20 °C/min are shown in Fig. 4. It is apparent that the combustion behaviors of the torrefied HMW differ from those of the raw sample. Compared to the torrefied HMW, the raw HMW 5
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Fig. 4. Combustion mass loss plots of the raw and torrefied HMW at a heating rate of 20 °C/min.
devolatilization and combustion of chars. Hence, the activation energy of the second stage was lower. The higher the torrefaction temperature is, the lower the activation energy. The average activation energy decreased from 87.64 ± 16.48 kJ/mol to 70.07 ± 11.09 kJ/mol when the temperature increased from 230 °C to 290 °C. Yang et al. found that the pyrolysis of lignin and hemicellulose was exothermic, whereas that of cellulose was endothermic (Yang et al., 2007). The decomposition of 290N released more heat and thus lower the activation energy. The average activation energy of the HMW torrefied in CO2 was 91.31 ± 14.73 kJ/mol, which was higher than that of the HMW torrefied in O2 (85.23 ± 17.18 kJ/mol). This difference is because oxygen has the strongest bond breaking capacity (lowest CrI), which makes the torrefied HMW more sensitive to thermal decomposition.
demonstrated that the combustion behaviors of the HMW torrefied in either CO2 or O2 are better than those of the HMW torrefied in N2. In other words, it is feasible to use the combustion flue gas as the agent for the torrefaction of high-moisture HMW. In this study, the S was one order of magnitude higher than that of olives (Cuevas et al., 2019). The above results revealed that the torrefied HMW exhibits good combustion behaviors, which can be used for solid fuel production, such as fuels for co-combustion or raw materials for pelletization. The activation energy changes constantly with increasing α, which demonstrates that the decomposition process is complicated during combustion. Moreover, the activation energy can be divided into two phases. The first stage was α at 0.1–0.6 (∼200–387 °C), and the second stage was α = 0.60–0.95 (∼387–550 °C). The activation energy exhibits the same tendency in the two stages, e.g., increases first and then decreases. However, the activation energy of the first stage is in the range of 58.22–112.68 kJ/mol, which is higher than that of the second stage (50.57–97.12 kJ/mol). Generally, the volatiles in the sample need energy to be released in the initial stage. Then, the volatiles will combust and subsequently release heat, which can in turn accelerate the
4. Conclusions In this study, the torrefaction of HMW and the combustion behaviors of resulting solid were investigated. It was found that torrefaction
Table 2 Combustion characteristic parameters of the raw and torrefied HMW samples. Ti (°C) Raw 230N 260N 290N 260CO 260O
261.29 284.78 288.56 296.35 286.71 286.19
(0.15) (0.13) (0.097) (0.11) (0.077) (0.12)
Tp (°C)
Th (°C)
(dw/dt)max (%·min−1)
Di × 104 (%·°C−2·min−1)
S × 106 (%·°C−3·min−2)
324.26 (0.16) 329.75 (0.14) 331.83 (0.18) 333.43 (0.2) 331.04 (0.13) 329.3 (0.19)
608.5 (0.21) 551.18 (0.14) 556.79 (0.13) 552.61 (0.19) 549.53 (0.17) 548.95 (0.13)
9.53 (0.1) 20.76 (0.081) 18.32 (0.092) 17.89 (0.054) 20.36 (0.095) 20.16 (0.12)
1.12 2.21 1.91 1.81 2.15 2.14
0.72 3.05 2.63 2.56 3.08 3.07
(0.003) (0.0087) (0.0097) (0.0056) (0.01) (0.012)
(0.009) (0.036) (0.035) (0.03) (0.047) (0.059)
Note: Ti, the ignition temperature; Th, the burnout temperature; (dw/dt)max, the maximum mass loss rate; and Tp, the temperature at the maximum mass loss rate. 6
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converts the high moisture HMW to a potential solid fuel with consistent quality. The torrefied HMW are easy to ignite and the comprehensive combustibility index of torrefied samples increased by 3–5 folds. The torrefaction of HMW can be operated in a wide temperature range and agents. However, higher temperature and oxygen-containing agents is preferred. The present study indicated that torrefaction is a promising method for the disposal of HMW.
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Acknowledgements This work was supported by Natural Science Foundation of Hubei Province (No. 2017CFB370), National Natural Science Foundation of China (No. 21806057, 21376084), and The open project of Hubei Key Laboratory of Industrial Fume and Dust Pollution Control (HBIK201802). Notes The authors declare no competing financial interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.121408. References Agbor, E., Zhang, X., Kumar, A., 2014. A review of biomass co-firing in North America. Renew. Sustain. Energy Rev. 40, 930–943. Bridgeman, T.G., Jones, J.M., Shield, I., Williams, P.T., 2008. Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties. Fuel 87, 844–856. Cao, W., Li, J., Martí-Rosselló, T., Zhang, X., 2018. Experimental study on the ignition characteristics of cellulose, hemicellulose, lignin and their mixtures. J. Energy Inst. https://doi.org/10.1016/j.joei.2018.1010.1004. Chen, D., Cen, K., Cao, X., Li, Y., Zhang, Y., Ma, H., 2018a. Restudy on torrefaction of corn stalk from the point of view of deoxygenation and decarbonization. J. Anal. Appl. Pyrolysis 135, 85–93. Chen, D., Gao, A., Ma, Z., Fei, D., Chang, Y., Shen, C., 2018b. 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, W.-H., Lin, B.-J., 2016. Characteristics of products from the pyrolysis of oil palm fiber and its pellets in nitrogen and carbon dioxide atmospheres. Energy 94, 569–578. Chen, W.-H., Zhuang, Y.-Q., Liu, S.-H., Juang, T.-T., Tsai, C.-M., 2016. Product characteristics from the torrefaction of oil palm fiber pellets in inert and oxidative atmospheres. Bioresour. Technol. 199, 367–374. Cuevas, M., Martínez Cartas, M.L., Sánchez, S., 2019. Effect of short-time hydrothermal carbonization on the properties of hydrochars prepared from olive-fruit endocarps. Energy Fuels 33, 313–322. Gil, M.V., García, R., Pevida, C., Rubiera, F., 2015. Grindability and combustion behavior of coal and torrefied biomass blends. Bioresour. Technol. 191, 205–212. Guo, F., Dong, Y., Zhang, T., Dong, L., Guo, C., Rao, Z., 2014. Experimental study on herb residue gasification in an air-blown circulating fluidized bed gasifier. Ind. Eng. Chem. Res. 53, 13264–13273. He, C., Giannis, A., Wang, J.-Y., 2013. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior. Appl. Energy 111, 257–266. Hlavsová, A., Corsaro, A., Raclavská, H., Vallová, S., Juchelková, D., 2016. The effect of feedstock composition and taxonomy on the products distribution from pyrolysis of nine herbaceous plants. Fuel Process. Technol. 144, 27–36. Joshi, Y., Di Marcello, M., Krishnamurthy, E., de Jong, W., 2015. Packed-bed torrefaction of bagasse under inert and oxygenated atmospheres. Energy Fuels 29, 5078–5087. Lasek, J.A., Kopczyński, M., Janusz, M., Iluk, A., Zuwała, J., 2017. Combustion properties of torrefied biomass obtained from flue gas-enhanced reactor. Energy 119, 362–368. Li, M.F., Li, X., Bian, J., Xu, J.K., Yang, S., Sun, R.C., 2015. Influence of temperature on
7
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Torrefaction of herbal medicine wastes: Characterization of the physicochemical properties and combustion behaviors
T
⁎
Shanzhi Xina, Fang Huanga, Xiaoye Liua, Tie Mia, , Qingli Xub a b
Hubei Key Laboratory of Industrial Fume and Dust Pollution Control, Jianghan University, Wuhan 430056, China Laboratory of Coal Gasification and Energy and Chemical Engineering Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Herbal medicine waste Torrefaction Physicochemical properties Combustion
To explore the feasibility of using herbal medicine waste (HMW) as solid fuel, HMW was torrefied under different temperatures and atmospheres. The physicochemical properties and combustion behaviors of the torrefied HMW were investigated. Temperature was found to be the most influential factor affecting the torrefaction. Torrefaction improved the hydrophobicity of HMW and decreased the equilibrated moisture uptake from 24.48(0.083) % to 15.22(0.054) %. The HMW samples torrefied under different conditions are easy to ignite. The comprehensive combustibility index (S) of the torrefied HMW increased by 3–5 folds compared to that of the raw sample. In general, the HMW torrefied under lower temperatures and under CO2 and O2 have better flammability. The present results revealed that the torrefied HMW exhibited good combustion characteristics and can thus be used for solid fuel production, such as fuels for co-combustion or raw materials for pelletization.
1. Introduction Traditional Chinese herbal medicines have played an important role in disease treatment and drug development for a thousand years. Currently, there are over 1500 herbal medicine enterprises in China (Mi and Yu, 2012). Pharmaceutical processes produce large amounts of herbal medicine waste (HMW). In recent years, the development of the herbal medicine industry has led to an increase in the amount of HMW discharged by pharmaceutical factories. It is estimated that approximately 12 million tons of HMW are produced annually, and this number continues to increase each year (Guo et al., 2014). At present, HMW is mainly stacked in pharmaceutical factories, or abandoned randomly in the wild and part of it is treated by sanitary landfilling, which can pollute the air, soil and water. Moreover, the medicine enterprises need to pay fees for disposing HMW. Thus, the development of an efficient utilization method for HMW can produce both economic and environmental benefits. HMW is rich in cellulose, hemicellulose, and lignin, which can be converted into biofuels by thermochemical processes. Furthermore, HMW is a highly centralized resource compared to the dispersion of biomass. Previous studies obtained syngas with heating value of ∼6.0 MJ/Nm3 and CO2-rich gas with medium-energy (8.8–10.5 MJ/kg) from the gasification and pyrolysis of herbaceous biomass (Guo et al., 2014; Hlavsová et al., 2016). Wang et al., (2010) obtained bio-oil with a
low oxygen content and a high calorific value (25.94 MJ/kg). Activation carbons were prepared from herbal biomass with the specific surface areas as great as 920–1952 m2/g (Yang and Qiu, 2011; Mi et al., 2015). The char was then used as an absorbent for absorbing sulfamethoxazole (Lian et al., 2014) and lead (Wang et al., 2015). Nevertheless, HMW has a high moisture content (as high as ∼70%) and cannot be directly utilized by thermochemical methods. Therefore, HMW must be dewatered before being processed. One effective way to overcome the drawback is torrefaction, which is also called mild pyrolysis. Torrefaction improves the energy content and energy density of a biomass. Compared to traditional biomass, torrefied biomass ignites more quickly and is an attractive feedstock for gasification and combustion (Bridgeman et al., 2008; Tsalidis et al., 2015). The torrefied oil palm wastes improved the combustion efficiency in a boiler and thus have great potential to be used as solid fuel (Lasek et al., 2017). Meanwhile, the co-combustion of coal and torrefied biomass enabled decreases in the emissions of NO and SO2 (Gil et al., 2015). In addition, the hot flue gas from the combustor are commonly used as a torrefaction agent with respect to the torrefaction efficiency, economic feasibility and the simplicity of production processes (Sellappah et al., 2016; Lasek et al., 2017). The O2 and CO2 in flue gas facilitated the decomposition process and enhanced the heating value of the torrefied char (Uemura et al., 2017). Therefore, torrefaction is an optional method for the disposal of
⁎ Corresponding author at: Hubei Key Laboratory of Industrial Fume and Dust Pollution Control, Jianghan University, 8 Sanjiao Lake Road, Wuhan, Hubei 430056, China. E-mail address: [email protected] (T. Mi).
https://doi.org/10.1016/j.biortech.2019.121408 Received 30 March 2019; Received in revised form 29 April 2019; Accepted 30 April 2019 Available online 02 May 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Chemical composition of the torrefied HMW. raw
230N
260N
290N
260CO
260O
Proximate analysis, wt%, db.
VM FC Ash
83.65 (0.32) 13.49 (0.38) 2.86 (0.1)
81.86 (0.29) 15.66 (0.4) 2.48 (0.11)
78.85 (0.24) 18.66 (0.37) 2.49 (0.11)
75.12 (0.27) 22.54 (0.35) 2.34 (0.09)
81.27 (0.21) 16.05 (0.29) 2.18 (0.1)
81.22 (0.28) 16.60 (0.2) 2.48 (0.09)
Ultimate analysis, wt%, daf.
C H O N S
48.01 (0.12) 6.94 (0.24) 42.85 (0.17) 2.01 (0.14) 0.19 (0.023)
48.85 (0.15) 6.86 (0.08) 42.39 (0.04) 1.82 (0.054) 0.08 (0.025)
50.02 (0.062) 6.62 (0.063) 41.41 (0.12) 1.84 (0.0044) 0.11 (0.0084)
53.29 (0.31) 6.4 (0.023) 38.22 (0.29) 2.0 (0.0076) 0.09 (0.0068)
49.34 (0.13) 6.8 (0.029) 41.89 (0.096) 1.85 (0.093) 0.12 (0.0004)
49.57 (0.13) 6.84 (0.11) 41.60 (0.08) 1.87 (0.0044) 0.12 (0.0023)
18.49 (0.21)
18.75 (0.13)
18.97 (0.095)
20.34 (0.12)
18.92 (0.061)
19.10 (0.16)
HHV (MJ/kg)
db: dry basis; daf: dry ash free basis.
The cellulose, hemicellulose and lignin contents in the raw and torrefied HMW were analyzed according to the Chinese national standard (GB/T 2677.10-1995) and the reported methods with minor modifications (Chen et al., 2018b; Zhang et al., 2018). Firstly, the extractives of the raw HMW were removed by the Soxhlet extraction for 6 h with a toluene/ethanol mixture (2:1, v/v). Afterwards, 2 ± 0.005 g extractive-free samples were placed in a conical flask and mixed with 65 mL of purified water, 0.50 mL of acetic acid, and 0.60 g of NaClO2 (100% basis) at 348 K for 1 h. Then, add 0.50 mL of acetic acid and 0.60 g of NaClO2 for every 1 h until the sample turned white. The weight loss was counted as the fraction of lignin. The lignin-free samples were further soaked in a 6 wt% KOH solution at room temperature for 12 h, followed by 80 °C water bath for 2 h in order to remove hemicellulose. The final residue was regarded as cellulose. The fraction of hemicellulose was calculated by difference. The surface functionality of the raw and torrefied HMW were analyzed via FTIR spectroscopy (VERTEX 70 Bruker, Germany). The wavelengths of the spectra range from 400 to 4000 cm−1. Each spectrum was the accumulation of 120 scans with a resolution of 4 cm−1. The crystallinity changes of the raw and torrefied HMW were measured by using X-ray diffraction (X’Pert PRO, Netherlands). A powder X-ray diffraction detector with Cu-Kα radiation was applied, and the scanning angle (2θ) ranged from 10 to 90° at a scan rate of 4° min−1. The crystallinity index (CrI) was calculated using Segal’s method (Li et al., 2018):
HMW. The torrefied HMW is a solid carbonaceous resource, which can be sued for replacing coal in energy and heat production processes (Proskurina et al., 2017). However, to date, the torrefaction of the highmoisture HMW and the resulting combustion behaviors of solid char have received little attention. In this study, torrefaction of HMW was conducted, and the physicochemical properties and combustion behaviors of torrefied HMW were investigated to explore the feasibility of using HMW as solid fuel. 2. Experiment and methods 2.1. Materials and torrefaction experiments HMW was collected from a traditional Chinese medicine pharmacy in the school infirmary. The moisture, ash, and volatile content of the HMW were determined in accordance with ASTM E871–82, D1102–84, and E872–82, respectively. The fixed carbon content was calculated by difference. The moisture content of the HMW is 68.8%, and the chemical composition of the HMW is listed in Table 1. The raw HMW was torrefied on a horizontal lab scale furnace with a quartz tubular reactor, as the same in our previous work (Xin et al., 2013, 2018). The reactor was first heated to the designated temperature with nitrogen at 500 mL/min. Afterward, 10 g of raw HMW was placed in the quartz ark and quickly pushed into the center of the reactor for 60 min. The torrefaction temperatures were 230 °C, 260 °C and 290 °C, and the torrefied samples were denoted as 230N, 260N and 290N, respectively. In addition, the HMW was torrefied under CO2 and O2 (10% O2, 90% N2) at 260 °C, and the corresponding samples were designated 260CO and 260O, respectively. Each torrefaction experiment was conducted 10 or more times. The measurement uncertainty was evaluated according to the national standards of China (GB/T 27418-2017).
CrI (%) =
(I002 − Iam) × 100% I002
(4)
The elemental composition was analyzed by a CHNS elementary analyzer (Vario Macro cube, Germany). The amount of carbon, hydrogen, nitrogen and sulfur was measured directly. The amount of oxygen was calculated by difference. The higher heating value (HHV) of the char was calculated by Eq. (1) (Li et al., 2015):
where I002 is the maximum peak intensity at ∼22° and Iam is the minimum intensity at 2θ of approximately 18.1°. The water adsorption capacity was tested at a relative humidity (RH) of 95% and a temperature of 30 °C. Approximately 2 g of the predried sample was dispersed evenly on a glass dish. The weight of the sample after moisture uptake was recorded at defined time intervals in 0.5–72 h. The amount of moisture absorbed (AMC) was calculated according to the equation AMC (%) = (mi–m0)/m0 × 100%, where m0 and mi are the weight of the samples before and after moisture uptake, respectively. Each test was performed at least thrice to ensure repeatability.
HHV (MJ/kg) = 0.3383WC + 1.422(WH − WO/8)
2.3. Combustion characteristics of the raw and torrefied HMW
2.2. Analysis of the raw and torrefied HMW
(1)
where WC, WH and WO are the weight fractions of carbon, hydrogen and oxygen, respectively. The mass yield (My) and energy yield (Ey) were calculated on a dry basis using Eqs. (2) and (3):
My = (Mt /Mr ) × 100%
(2)
E y = My × (HHV/HHV) t r
(3)
2.3.1. Combustion characteristic The combustion of the torrefied HMW was conducted on a thermogravimetric analyzer (TGA4000, Perkin Elmer). Approximately 10 mg of the raw and torrefied HMW samples were placed in the crucible and heated from ambient temperature to 800 °C with an air flow rate of 100 mL/min. The heating rate was 10, 15, 20, and 30 °C/min. The temperature at which fuel begins to burn is considered the ignition temperature (Ti), where when the mass loss rate reaches 1%/min in the last stage of combustion is denoted as the burnout temperature (Tb).
where M represents the mass of the sample and the subscripts ‘‘t” and ‘‘r” represents the torrefied and raw HMW samples, respectively. 2
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The Ti and Tb values were determined in accordance with the methods described in previous studies (He et al., 2013). Moreover, the ignition index (Di) and comprehensive combustibility index (S) were used to evaluate the combustion behaviors of the torrefied HMW, which were defined as follows by Eqs. (5) and (6) (Zhang et al., 2016):
Di =
S=
(dw / dt )max Ti Tp (dw / dt )max (dw / dt )mean Ti2 Tb
(5)
(6)
where (dw/dt)max and (dw/dt)mean represent the maximum and mean mass loss rate (%/min), respectively, and Tp is the temperature corresponding to the maximum mass loss rate (°C). 2.3.2. Kinetic parameters from the isoconversional method Generally, the “model-free” kinetic methods enable the evaluation of the activation energy without the premise of assuming the reaction model and thus more accurate. The activation energy obtained from these methods can be used to assess the consistency of the reaction mechanism throughout the decomposition process by comparing the activation energy values at different conversion rates (α). Among the model-free methods, the differential isoconversional methods have been shown to be more accurate than the integral methods (Vyazovkin et al., 2011). In the present study, the Friedman method, one of the most widely used differential methods, was used to determine the kinetic parameters during the combustion of the raw and torrefied HMW samples. The Friedman method was formulated as follows (Vyazovkin et al., 2011):
dα E ln ⎡ ⎛ ⎞ β⎤ = ln[Af (α )] − a ⎢ RT ⎣ ⎝ dT ⎠ ⎥ ⎦
Fig. 1. Mass and energy yields of the HMW torrefied under different conditions.
Ey values, which were 93.42(0.63) % and 95.17(0.007) %, respectively. This phenomenon occurs because O2 has a higher oxidative rate than CO2 or N2 and thus shows a stronger torrefaction severity than CO2 and N2 (Uemura et al., 2015). The present results are similar to the torrefaction of bamboo under CO2 reported by Li et al. (2015). Note that the My and Ey values under different torrefaction agents did not exhibit significant differences. This lack of a significant difference is probably because the promotion of solid conversion by O2 and CO2 is more pronounced only at higher temperatures (Uemura et al., 2015). However, the moisture in the HMW may also affect the decomposition of the biomass components. The energy yield of the torrefied HMW in O2 was 87.39(0.015) %, which is greater than the torrefaction of oil palm fiber pellets (59.50–71.37%) (Chen et al., 2016). This result suggests that a greater amount of energy was preserved in the torrefied char in the presence of O2. Moreover, the torrefied HMW may also be used as a raw material for pelletization.
(7)
where β is the linear heating rate, K/min; f(α) is model-dependent function in the differential form. Ea is the activation energy, J/mol; A is the pre-exponential factor, 1/s; and R is the universal gas constant, 8.314 J/(mol K), and T is the absolute temperature, K. Plotting the left side of Eq. (7) against 1/T at a fixed degree of conversion rate and different heating rates will give a series of straight lines, and the activation energy can be calculated from the slope.
3.1.2. Chemical and organic compositions of the torrefied samples Table 1 lists the proximate and ultimate analyses of the raw and torrefied HMW samples under different conditions. The raw HMW sample is characterized by a high volatile matter (VM) content (83.65%), low fixed carbon (FC) and ash contents. The VM in the raw HMW is much higher than that in common biomass, such as oil palm fiber and shell (70.50–70.71%) (Uemura et al., 2015; Chen et al., 2016), and rice straw (Park et al., 2014). The VM content in the torrefied HMW decreased steadily as the temperature increased from 81.86(0.29) % at 230 °C to 75.12(0.27) % at 290 °C, which is consistent with the mass yield in Fig. 1. Consequently, the FC content of the residue char increased. A previous study found that a high amount of VM in a biomass can produce fuel-rich conditions during combustion and may benefit the decrease in NO (Gil et al., 2015). However, high biomass co-combustion ratios (∼10% and above) will decrease the overall boiler efficiency of coal-fired power plant (Agbor et al., 2014). Therefore, the high content of VM in the torrefied HMW may have positive effects on the NO emissions during combustion. Notably, the VM contents of the HMW torrefied by CO2 and O2 were slightly higher than that of the HMW torrefied by N2. Uemura et al. noted that oxygen shows a strong devolatilization capacity during torrefaction (Uemura et al., 2015). Therefore, the moisture may lower the torrefaction severity compared to the torrefaction of dry biomass and thus reduce the extent of hemicellulose decomposition. As shown in Table 1, the content of carbon in the HMW increased after torrefaction, whereas the hydrogen and oxygen content decreased. The elemental composition of the torrefied HMW at 230 °C was close to
3. Results and discussions 3.1. Physicochemical properties of the torrefied samples 3.1.1. Mass and energy yields The mass yield (My) and energy yield (Ey) of the torrefied HMW under different conditions are shown in Fig. 1. The measurement uncertainty of the mass yield for 230N, 260N, 290N, 260CO and 260O was determined to be 0.68%, 0.37%, 0.34%, 0.63% and 0.33%, respectively. For energy yield, the measurement uncertainty was in the range of 0.007–0.015%. Apparently, the torrefaction temperature exerted a stronger effect on the yields than the torrefaction agent. The My and Ey values both decreased continuously with increasing temperature. The My and Ey values at 230 °C were as high as 95.88(0.68) % and 96.96(0.015) %, and these values slightly decreased by 4–5% as the temperature increased to 260 °C. This result indicated that hemicellulose did not undergo apparent decomposition at temperatures less than 260 °C. However, the decomposition of hemicellulose generally occurs in the range of 220–315 °C, and maximum mass loss occurs at 268 °C (Yang et al., 2007). This finding indicated that the release of moisture during torrefaction hindered the decomposition of hemicellulose. At 290 °C, the My and Ey values decreased remarkably to 76.94(0.34) % and 83.23(0.014) %, respectively, which was attributed to the loss of hemicellulose. As seen in Fig. 1, the torrefaction of HMW in O2 obtained the lowest My and Ey values, while torrefaction in CO2 attained the highest My and 3
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cellulose content was almost unchanged when HMW was torrefied at 230 °C. At the same temperature, the hemicellulose content decreased slightly from 36.97(0.25) % to 32.50(0.18) %. Thus, the decrease in VM and oxygen at 230 °C was attributed to the partial decomposition of hemicellulose. As the temperature increased from 230 to 260 °C, the cellulose content decreased from 22.75(0.17) % to 22.09(0.19) %, while the hemicellulose content decreased substantially from 32.50(0.18) % to 25.01(0.20) %. Consequently, the lignin content increased from 44.75(0.33) % to 52.90(0.39) %. Cellulose and hemicellulose are richer in oxygen than lignin (Xin et al., 2013). Hence, the released volatiles with high O/C and H/C ratios decreased the hydrogen and oxygen contents in the torrefied HMW, as shown in Table 1. At 290 °C, the apparent decomposition of cellulose and hemicellulose resulted in a progressive decrease in their contents. As shown in Fig. 2, torrefaction of HMW in O2 resulted in low cellulose and hemicellulose contents of 19.22(0.11) % and 22.63(0.23) %, respectively. However, their contents in CO2 torrefaction were slightly higher than those in N2 torrefaction. This finding demonstrated that oxygen has the strongest capacity to degrade polysaccharides in biomasses. The above findings are consistent with previous results showing that temperature exerts a greater effect than torrefaction agents.
that of the raw sample. As the temperature increased from 230 °C to 290 °C, the carbon content continuously increased from 48.85(0.15) % to 53.29(0.31) %, whereas the oxygen content decreased from 42.39(0.04) % to 38.22(0.29) %. The atomic H/C and O/C ratios are commonly used as an index to evaluate the energy density of a solid fuel. The atomic ratios of H/C and O/C of the torrefied HMW decreased with increasing temperature, which was attributed to the dehydration, deoxygenation, and dehydrogenation reactions. The torrefied HMW at 290 °C had the lowest H/C and O/C ratios of 1.44 and 0.54, which demonstrated that the temperature is the most influential factor affecting the torrefaction process. Note that the torrefied HMW in different torrefaction agents possess similar elemental compositions. As a result, the H/C and O/C ratios also appeared to be equivalent. Wang et al. found that the properties of sawdust torrefied in O2 less than 6% were similar to those samples torrefied in N2 (Wang et al., 2013). This finding indicates that torrefaction agents exert minor effects on the torrefaction of high-moisture HMW. Moreover, the H/C and O/C ratios from the present study are higher than those in the literature results under similar torrefaction conditions. This finding suggests that the moisture in the biomass is also an important factor that can suppress dehydration, deoxygenation, and dehydrogenation during torrefaction. The HHV of the torrefied HMW is very close to that of the raw sample when torrefied at temperatures less than 260 °C. The HHV notably increased from 18.97(0.095) MJ/kg to 20.34(0.12) MJ/kg as the temperature increased from 260 °C to 290 °C. The results show that the torrefied HMW with a higher FC and carbon content has a higher heating value. Generally, fuels with higher FC and carbon content facilitate the subsequent gasification and combustion processes (Pérez et al., 2012). In this regard, torrefaction of the HMW at higher temperatures is preferable, and torrefied char is suitable for use as solid fuel. Nevertheless, torrefaction at higher temperatures will also reduce the mass and energy yields. At 260 °C, the HMW torrefied in CO2 and O2 have higher heating values than that the HMW torrefied in N2. A previous study found that biomass torrefaction in oxygen-containing atmospheres has a potentially positive impact on processing costs and processing efficiency (Joshi et al., 2015). This finding suggests that torrefaction with flue gas is an option for the disposal of HMW. However, it may be better to partially remove the moisture in the raw HMW prior to torrefaction. Fig. 2 illustrates the change in the organic components of the raw and torrefied HMW. In general, torrefaction decreased the contents of cellulose and hemicellulose in the HMW because they are more likely to undergo thermal decomposition than the other components. The
3.1.3. Chemical structure of the torrefied HMW The torrefaction altered the surface functional groups of the HMW. The strongest peak at 1026 cm−1 in the raw HMW indicates the abundant hydroxyl groups in polysaccharides or the methoxyl and β-O4 linkages in lignin. The absorbance intensity decreased remarkably after torrefaction, which demonstrated the breakage of the CeO bond. However, the intensity increased with a further increase in temperature. This phenomenon is probably due to the increase in the relative content of lignin in the torrefied HMW as the temperature increased from 230 °C to 290 °C. The peaks at 1075 cm−1 and 1152 cm−1 were associated with the CeOeC linkage of hemicellulose units or the glycosidic bond of cellulose. The peak at 1075 cm−1 disappeared, and the peak at 1152 cm−1 exhibited an apparent decrease in the torrefied HMW. This phenomenon demonstrated the notable decomposition of hemicellulose and a part of the cellulose in the course of torrefaction, which is consistent with the previous results (Fig. 2). The bands at 1318 cm−1, 1515 cm−1 and 1630 cm−1 are the characteristic absorption peaks of aromatic structures. The absorbance intensity of each of these peaks increased progressively with increasing torrefaction temperature, which confirmed the synchronous increase in lignin content. The CeOeC linkage of the torrefied HMW also vanished in the presence of CO2 and O2. Moreover, the absorbance intensity of 1026 cm−1 in CO2 and O2 was apparently lower than that in N2. This finding is attributed to the stronger capacity of CO2 and O2 in the deoxygenation and dehydrogenation reactions during torrefaction, which enhanced the decomposition of hemicellulose and cellulose. 3.1.4. Crystal structure analysis Cellulose is the only crystal structure in the biomass, which displays three typical diffraction peaks at 2θ = 22.6 (0 0 2 plane), 16.3 (1 0 1¯ plane) and 14.8 (1 0 1 plane) (Xin et al., 2015). The raw HMW is less ordered with a CrI of 18.93(0.14) % because of the influences of amorphous hemicellulose and lignin. As the torrefaction temperature increased from 230 °C to 290 °C, the intensity of the 0 0 2 peak increased and resulted in a gradual increase in CrI from 32.87(0.21) % to 40.70(0.33) %. This increase in CrI indicates that torrefaction increases the crystallinity of the HMW. The cellulose and hemicellulose are connected together by chemical bonds in the biomass (Sun, 2010). The decomposition of hemicellulose during torrefaction and the loss of amorphous cellulose and a part of lignin will liberate the crystalline fractions of cellulose and thus increase the CrI of the torrefied HMW. In the present study, the CrI changes in accordance with the wet torrefaction of biomass but shows the opposite trends with dry torrefaction (Zheng et al., 2015; Chen et al., 2018a). This implies that the moisture
Fig. 2. Organic components of the raw and torrefied HMW under different conditions (L, lignin; HC, hemicellulose; C, cellulose; and E, extractives). 4
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starts to decompose at a lower temperature but exhibits burnout at a higher temperature. The corresponding DTG curve was classified into four stages: dehydration, volatile release, volatile combustion, and residue char combustion. As seen from Fig. 4, the combustion of the torrefied HMW samples was characterized by two distinct stages, which lie in ∼200–387 °C and 387–554 °C, according to the DTG curves. The maximum weight loss temperature is 330 °C and 451 °C. As the temperature increases, the TG curves move gradually to higher temperatures. From Fig. 4, the weight loss curves of 260CO and 260O basically overlap over the entire temperature range, which indicates that the properties of the HMW torrefied under an oxygen-containing atmosphere are similar. In the first stage, the combustion behaviors of 260CO and 260O are coincident with those of 260 N. However, the combustion of 260 N shifted to higher temperatures in the second stage. Generally, the first stage was due to devolatilization, and the second stage was attributed to the combustion of char. This finding suggests that the torrefaction agents have minor effects on the devolatilization of the HMW during combustion. The thermal decomposition behaviors of biomass components are known to be different (Yang et al., 2007). Therefore, to understand the contributions of the biomass components to the weight loss of the combustion of the torrefied HMW, the DTG curve of 260N was curvefitted. The two main DTG peaks of the torrefied HMW are superimposed by four peaks: 272 °C, 318 °C, 340 °C and 461 °C. A previous study found that the weight losses of hemicellulose, hemicellulose and lignin combustion are concentrated in the temperature ranges of 220–320 °C, 300–360 °C and 200–600 °C, respectively (Cao et al., 2018). Therefore, the first three peaks are attributed to the decomposition of hemicellulose, cellulose, and lignin and may be accompanied by the combustion of volatiles. The peak at 461 °C was assigned to the combustion of residual char, and lignin may continue to decompose. Table 2 shows the combustion characteristic parameters of the torrefied HMW. The ignition temperature (Ti) of the raw HMW was 261.29(0.15) °C, which increased slightly after torrefaction. The Ti of the torrefied HMW increased slightly from 284.78(0.13) °C to 296.35(0.11) °C as the torrefaction temperature increased from 230 °C to 290 °C. The ignition temperatures of common biomasses, such as straw dust, wheat straw, paper sludge and rice straw, were in the range of 271–279 °C (Wang et al., 2009; Xie and Ma, 2013). In this study, the Ti of the torrefied HMW was only 10–20 °C higher than those of biomasses, but it is significantly lower than that of coal (475 °C) (Wang et al., 2009). This finding indicated that the torrefied HMW samples under different conditions are easy to ignite. However, few differences in Ti were observed among the various torrefied HMW samples, especially for those torrefied in N2, CO2 and O2 at 260 °C. This suggests that torrefaction not only removes the moisture from the raw HMW and increases its energy density but also exerts less impact on the ignition characteristics. As seen from Table 2, the ignition index (Di) and the comprehensive combustibility index (S) of the raw HMW are 1.12(0.003) %· °C−2·min−1 and 0.72(0.009) %· °C−3·min−2, respectively. After torrefaction, the Di and S increased significantly, implying that torrefaction also enhanced the combustion properties of the HMW. The combustion of 230N resulted in the highest Di and S values of 2.21(0.0087) %·°C−2·min−1 and 3.05(0.036) %·°C−3·min−2, respectively, and decreased slightly with increasing temperature. In particular, the Ti of the torrefied HMW exhibits the opposite trend with temperature. This phenomenon suggested that the HMW torrefied at a lower temperature has better flammability. However, torrefaction at higher temperatures does not remarkably lower the combustible performance of the HMW. Generally, oxygen-containing agents, such as O2 and CO2, can increase the severity of torrefaction and enhance the deoxygenation and dehydrogenation reactions (Chen and Lin, 2016; Chen et al., 2016). From Table 2, the Ti and Th of 260CO and 260O were lower than those of 260N, but the Di and S of the former were much higher. This finding
Fig. 3. Moisture absorption properties of the torrefied HMW prepared under different conditions.
in HMW may affect the bond dissociation during the torrefaction process. The CrI of torrefied HMW in O2 is the lowest at 34.52(0.19) %, whereas the CrI of the torrefied HMW in CO2 is the highest at 40.29(0.22) %. These results demonstrated that O2 enhances the torrefaction severity and promotes the decomposition of hemicellulose and cellulose. 3.1.5. Water adsorption capacity The hygroscopic behaviors of the torrefied biomass are crucial in terms of transportation and long-term storage and for follow-up thermochemical utilization. Fig. 3 shows the moisture absorption curves of the torrefied HMW. It is apparent that the rate of moisture absorption of the torrefied HMW exhibited a sharp increase in the first 3 h and reduced in the 3–12 h. Afterwards, the moisture uptake reached the equilibrium value as time elapsed. The moisture uptake of the torrefied HMW can be equilibrated in a short time. The maximum AMC of the raw HMW was 24.48(0.083) %, and this value was notably reduced after torrefaction. The AMC decreased gradually from 22.53(0.065) % at 230 °C to 15.22(0.054) % at 290 °C. This finding indicated that torrefaction improves the hydrophobicity of the HMW, which is in agreement with the results of a previous study (Oluoti et al., 2018). Cellulose and hemicellulose are rich in hydroxyl groups, which are capable of forming hydrogen bonds with water. Our previous study found that water can be evolved out chemically at 200 °C from the pyrolysis of cellulose (Xin et al., 2015). Therefore, the loss of hydroxyl groups, along with the decomposition of hemicellulose and cellulose, decreased the water sorption capacity of the torrefied samples. From Fig. 3, the AMC values of the HMW torrefied in CO2 and O2 are 20.73 (0.071) % and 21.61(0.066) %, respectively, which are higher than that of the HMW torrefied in N2. This finding suggests that a greater number of hydroxyl groups were preserved in the torrefied sample in the presence of CO2 and O2. Consequently, the oxygen and hydrogen contents of the HMW torrefied in CO2 and O2 are higher than those in the HMW torrefied in N2 (Table 1). Although the AMC of the torrefied HMW in the present study was slightly higher than that in some published data, the water sorption of the former can be equilibrated in a shorter time (Toscano et al., 2015). 3.2. Combustion characteristics of the torrefied HMW The combustion TG-DTG curves of the raw and torrefied HMW samples at a heating rate of 20 °C/min are shown in Fig. 4. It is apparent that the combustion behaviors of the torrefied HMW differ from those of the raw sample. Compared to the torrefied HMW, the raw HMW 5
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Fig. 4. Combustion mass loss plots of the raw and torrefied HMW at a heating rate of 20 °C/min.
devolatilization and combustion of chars. Hence, the activation energy of the second stage was lower. The higher the torrefaction temperature is, the lower the activation energy. The average activation energy decreased from 87.64 ± 16.48 kJ/mol to 70.07 ± 11.09 kJ/mol when the temperature increased from 230 °C to 290 °C. Yang et al. found that the pyrolysis of lignin and hemicellulose was exothermic, whereas that of cellulose was endothermic (Yang et al., 2007). The decomposition of 290N released more heat and thus lower the activation energy. The average activation energy of the HMW torrefied in CO2 was 91.31 ± 14.73 kJ/mol, which was higher than that of the HMW torrefied in O2 (85.23 ± 17.18 kJ/mol). This difference is because oxygen has the strongest bond breaking capacity (lowest CrI), which makes the torrefied HMW more sensitive to thermal decomposition.
demonstrated that the combustion behaviors of the HMW torrefied in either CO2 or O2 are better than those of the HMW torrefied in N2. In other words, it is feasible to use the combustion flue gas as the agent for the torrefaction of high-moisture HMW. In this study, the S was one order of magnitude higher than that of olives (Cuevas et al., 2019). The above results revealed that the torrefied HMW exhibits good combustion behaviors, which can be used for solid fuel production, such as fuels for co-combustion or raw materials for pelletization. The activation energy changes constantly with increasing α, which demonstrates that the decomposition process is complicated during combustion. Moreover, the activation energy can be divided into two phases. The first stage was α at 0.1–0.6 (∼200–387 °C), and the second stage was α = 0.60–0.95 (∼387–550 °C). The activation energy exhibits the same tendency in the two stages, e.g., increases first and then decreases. However, the activation energy of the first stage is in the range of 58.22–112.68 kJ/mol, which is higher than that of the second stage (50.57–97.12 kJ/mol). Generally, the volatiles in the sample need energy to be released in the initial stage. Then, the volatiles will combust and subsequently release heat, which can in turn accelerate the
4. Conclusions In this study, the torrefaction of HMW and the combustion behaviors of resulting solid were investigated. It was found that torrefaction
Table 2 Combustion characteristic parameters of the raw and torrefied HMW samples. Ti (°C) Raw 230N 260N 290N 260CO 260O
261.29 284.78 288.56 296.35 286.71 286.19
(0.15) (0.13) (0.097) (0.11) (0.077) (0.12)
Tp (°C)
Th (°C)
(dw/dt)max (%·min−1)
Di × 104 (%·°C−2·min−1)
S × 106 (%·°C−3·min−2)
324.26 (0.16) 329.75 (0.14) 331.83 (0.18) 333.43 (0.2) 331.04 (0.13) 329.3 (0.19)
608.5 (0.21) 551.18 (0.14) 556.79 (0.13) 552.61 (0.19) 549.53 (0.17) 548.95 (0.13)
9.53 (0.1) 20.76 (0.081) 18.32 (0.092) 17.89 (0.054) 20.36 (0.095) 20.16 (0.12)
1.12 2.21 1.91 1.81 2.15 2.14
0.72 3.05 2.63 2.56 3.08 3.07
(0.003) (0.0087) (0.0097) (0.0056) (0.01) (0.012)
(0.009) (0.036) (0.035) (0.03) (0.047) (0.059)
Note: Ti, the ignition temperature; Th, the burnout temperature; (dw/dt)max, the maximum mass loss rate; and Tp, the temperature at the maximum mass loss rate. 6
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converts the high moisture HMW to a potential solid fuel with consistent quality. The torrefied HMW are easy to ignite and the comprehensive combustibility index of torrefied samples increased by 3–5 folds. The torrefaction of HMW can be operated in a wide temperature range and agents. However, higher temperature and oxygen-containing agents is preferred. The present study indicated that torrefaction is a promising method for the disposal of HMW.
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Acknowledgements This work was supported by Natural Science Foundation of Hubei Province (No. 2017CFB370), National Natural Science Foundation of China (No. 21806057, 21376084), and The open project of Hubei Key Laboratory of Industrial Fume and Dust Pollution Control (HBIK201802). Notes The authors declare no competing financial interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.121408. References Agbor, E., Zhang, X., Kumar, A., 2014. A review of biomass co-firing in North America. Renew. Sustain. Energy Rev. 40, 930–943. Bridgeman, T.G., Jones, J.M., Shield, I., Williams, P.T., 2008. Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties. Fuel 87, 844–856. Cao, W., Li, J., Martí-Rosselló, T., Zhang, X., 2018. Experimental study on the ignition characteristics of cellulose, hemicellulose, lignin and their mixtures. J. Energy Inst. https://doi.org/10.1016/j.joei.2018.1010.1004. Chen, D., Cen, K., Cao, X., Li, Y., Zhang, Y., Ma, H., 2018a. Restudy on torrefaction of corn stalk from the point of view of deoxygenation and decarbonization. J. Anal. Appl. Pyrolysis 135, 85–93. Chen, D., Gao, A., Ma, Z., Fei, D., Chang, Y., Shen, C., 2018b. 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, W.-H., Lin, B.-J., 2016. Characteristics of products from the pyrolysis of oil palm fiber and its pellets in nitrogen and carbon dioxide atmospheres. Energy 94, 569–578. Chen, W.-H., Zhuang, Y.-Q., Liu, S.-H., Juang, T.-T., Tsai, C.-M., 2016. Product characteristics from the torrefaction of oil palm fiber pellets in inert and oxidative atmospheres. Bioresour. Technol. 199, 367–374. Cuevas, M., Martínez Cartas, M.L., Sánchez, S., 2019. Effect of short-time hydrothermal carbonization on the properties of hydrochars prepared from olive-fruit endocarps. Energy Fuels 33, 313–322. Gil, M.V., García, R., Pevida, C., Rubiera, F., 2015. Grindability and combustion behavior of coal and torrefied biomass blends. Bioresour. Technol. 191, 205–212. Guo, F., Dong, Y., Zhang, T., Dong, L., Guo, C., Rao, Z., 2014. Experimental study on herb residue gasification in an air-blown circulating fluidized bed gasifier. Ind. Eng. Chem. Res. 53, 13264–13273. He, C., Giannis, A., Wang, J.-Y., 2013. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior. Appl. Energy 111, 257–266. Hlavsová, A., Corsaro, A., Raclavská, H., Vallová, S., Juchelková, D., 2016. The effect of feedstock composition and taxonomy on the products distribution from pyrolysis of nine herbaceous plants. Fuel Process. Technol. 144, 27–36. Joshi, Y., Di Marcello, M., Krishnamurthy, E., de Jong, W., 2015. Packed-bed torrefaction of bagasse under inert and oxygenated atmospheres. Energy Fuels 29, 5078–5087. Lasek, J.A., Kopczyński, M., Janusz, M., Iluk, A., Zuwała, J., 2017. Combustion properties of torrefied biomass obtained from flue gas-enhanced reactor. Energy 119, 362–368. Li, M.F., Li, X., Bian, J., Xu, J.K., Yang, S., Sun, R.C., 2015. Influence of temperature on
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