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Effects of additives and hydrothermal pretreatment on the pelleting process of rice straw: Energy consumption and pellets quality
Industrial Crops & Products 133 (2019) 178–184
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Effects of additives and hydrothermal pretreatment on the pelleting process of rice straw: Energy consumption and pellets quality
T
⁎
Xianfei Xiaa, Ke Zhangb, , Hongru Xiaoa, Suwei Xiaoa, Zhiyu Songa, Zhengyu Yangc a
Nanjing Research Institute for Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA c Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02120, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Rice straw Additives Pelleting Hydrothermal pretreatment Energy consumption Pellet Quality
Pelleting is an appealing approach to enhance energy density of biomass for renewable fuel production. To overcome these two challenges: high energy consumption of pelleting and low product calorific value of pellets, the pretreatment of raw biomass is a necessary process prior to pelleting. In this study, the rice straw was pretreated by hydrothermal carbonization (HTC) firstly, then the economic additives: crude glycerin and paraffin were added. Afterward, the pretreated biomass was pelletized using mixed-level orthogonal array design to evaluate energy consumption and pellets quality. Finally, the pelleting condition was optimized based on the analysis of range and variance, thermogravimetric analysis and scanning electron microscope (SEM) evaluation. Experimental results indicated that additive type was a more influential factor than HTC pretreatment and adding ratio in terms of reducing energy consumption and improving pellet quality. Compared to the nonpretreated rice straw, pelleting energy of the selected hydrothermally pretreated sample with 4% paraffin addition reduced 37.26%, pellet density (ratio of pellet mass to volume) enhanced by 7.71%, and calorific value (heat released by combustion of unit pellet mass) increased by 68.10%, while the tensile strength (the crushing resistance and mechanical stability of the pellets) just loss by 4.33%. Combustion and micro-bonding characteristics also show that this rice straw pellet fuel has a great potential as an alternative fuel for coal.
1. Introduction With the growing global energy and environment concerns, the surging demand for renewable energy encourages to replace fossilbased fuels using agricultural wastes (Cheng et al., 2017; Yan et al., 2016). There are over 20 types of straw biomass with the annual yield of more than 700 million tons in China.(Zhao et al., 2016). However, most straw biomass is abandoned or uncontrolled burned (Wang et al., 2017; Zhao et al., 2017). Moreover, straw biomass has an innately lower energy density resulting in the low efficiency of combustion and transportation than woody biomass. Pelleting of straw biomass is an appealing process offering high productivity and suitable fuels with better density and strength (Xia et al., 2014, 2016). The biomass produced by this way is more convenient for transportation and burning (Song et al., 2014). Nevertheless, pelleting of rice straw has two technical challenges: high energy consumption (more than 90 kW⋅h/t for rice straw pelleting process) and low product calorific value (15 MJ/kg usually). To overcome these two challenges, the pretreatment of raw biomass prior to pelleting is an essential approach. Physical and
⁎
chemical pretreatments are two conventional methods. Physical pretreatment usually refers to adding binders, while chemical pretreatment includes pyrolysis and hydrothermal carbonization (Stelte et al., 2013). There is some literature related to the addition of binders for the improvement of pelleting. Kong et al. (2013) discovered that the pellets quality could be improved by adding linen fibers. Rosin et al. (2014) mixed biomass samples with various additives such as starch, molasses, high conversion soaker cracking residues, slaked lime, ash and montan resin to improve the pellets density. Obidziński (2014) studied the influence of potato pulp content on the power demand of the oat bran pelleting process, indicating that increasing the potato pulp content from 15 to 20% caused a reduction of the power demand. Tilay et al. (2015) found that the strength and durability of canola meal pellets were improved when binder and lubricant were added. Shang et al. (2014) investigated the effect of rapeseed oil on wood chips densification process and indicated that rapeseed oil reduced the static friction and stabilized pellet production, but the pellet quality, strength, and density were negatively affected. Zannikos et al.(2013) used waste plastic as an additive to enhance the calorific value and emissions of
Corresponding author at: Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA. E-mail address: [email protected] (K. Zhang).
https://doi.org/10.1016/j.indcrop.2019.03.007 Received 28 June 2018; Received in revised form 16 January 2019; Accepted 3 March 2019 0926-6690/ © 2019 Published by Elsevier B.V.
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Fig. 1. Schematic illustration of the pelleting platform.
crude glycerin and paraffin. Afterward, the pretreated biomass was pelletized by mixed-level orthogonal array design to evaluate energy consumption and pellets quality. Finally, the pelleting condition was optimized based on the analysis of range and variance. The objective of this study is to investigate the effects of additives and hydrothermal pretreatment on the pelleting process of rice straw in terms of energy consumption and pellets quality.
sawdust pellets. Although additives can efficiently improve the quality of pellets of biomass, the additives need to be cost-effective and could enhance both physical and chemical properties of the raw biomass, as well as decrease the friction during the pelleting process for low energy consumption. Furthermore, the additives in the pelleting process need to have no negative impact on the environment. Crude glycerin and paraffin are high quality and efficient additives. In 2015, the total yield of crude glycerin was about 0.27 million tons in China. However, due to the technical restriction, crude glycerin cannot be efficiently utilized and the large amount crude glycerin could lead to environmental pollution. The calorific value of crude glycerin is about 27 MJ/kg (Lu et al., 2014). Paraffin is a colorless and white mixture made from crude petroleum. Besides physical pretreatment of biomass, hydrothermal carbonization is an effective chemical pretreatment method to improve the pellets properties. Hydrothermal pretreatment, known as auto-hydrolysis, it is controllable with short reaction period. Hydrothermal pretreatment is friendly to the environment because the water is used as a solvent instead of other chemical reagents. It is divided into hydrothermal gasification, hydrothermal liquefaction, and HTC according to reaction time and temperature (Li et al., 2014). Reza et al. (2013 and 2014) used HTC biochar as a binder to enhance energy density and durability of biomass pellets. Kambo and Dutta (2014) indicated that density and volumetric energy density of biomass pellets produced by HTC were significantly higher comparing to the non-torrefied pellets or torrefied pellets. Moreover, these pellets showed the increasing of hydrophobicity and reduction of ash content. Liu et al. (2014) reported that HTC biomass pellets showed a trend of increasing calorific value but decreasing ash content and compressive strength. However, the present hydrothermal carbonization research almost focused on woody biomass, little information is available for straw biomass with low lignin content. Moreover, there is no report on the combination of physical and chemical pretreatments (additives and HTC) for rice straw pelleting in terms of energy consumption and pellets quality. To fill this gap in the literature, the rice straw was treated by the combination of HTC and adding economic additives included
2. Method and evaluation 2.1. Material preparation Rice straw biomass (Nangeng 46) was obtained in Jiangyan, China in 2016 fall. The air-dried rice straw was ground to 2 mm with the density is 75.3 kg/m3. Then the ground rice straw was stored in sealed plastic bags for further use. All chemicals used in this research were purchased from Sigma Chemical Co. (St. Louis, MO). 2.2. Additives Crude glycerin and paraffin were added into rice straw with a ratio of 1%, 4%, 7% and 10%. Both additives only produce CO2 and H2O after co-combustion with biomass (Narra et al., 2010; Slinn et al., 2008). Furthermore, crude glycerin is a by-product of biodiesel. One ton of biodiesel will generate 100 kg of crude glycerin based on current process (Slinn et al., 2008). As the increasing demand for biodiesel, the yield of crude glycerin also increased recently. Crude paraffin wax offers many advantages such as high oil content, low price and the high calorific value of approximately 48 MJ/kg (Narra et al., 2010). Meanwhile, paraffin wax emulsion is favorable for the mixing with rice straw in this study. 2.3. Hydrothermal pretreatment The rice straw was pretreated using HTC at 240 °C for 30 min. Production from HTC consisted of solid (coke), liquid (sugar, acetic 179
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computer and calculated by Eq. (1).
acid, and TOC) and gaseous products (mainly CO2). In this study, the solid product was used for the pelleting study.
E= 2.4. Pelleting platform and process
σt =
(3)
2.6.5. Thermogravimetric analysis The purpose of thermogravimetric analysis was to investigate the relationship between the sample’s mass loss and the increasing temperature. The result of the thermogravimetric analysis is a thermogravimetric curve (TG curve), and the first derivative of TG curve to temperature is a differential thermogravimetric curve (DTG curve). Thermogravimetric analysis is an important tool to investigate the combustion kinetics characteristics of the biomass pellet. The volatile loss index Rv and the combustion characteristic composite index P can be got from the thermogravimetric analysis results and calculated based on Eq. (4) and (5) respectively. A thermogravimetric analyzer (Mettler Toledo Co. in Switzerland TGA/SDTA 851e) was carried out at the temperature range of 0–700 °C, with the temperature rising rate of 20 °C/min, and the air flow speed is 80 mL/min.
Factor
Paraffin Crude glycerin – –
4m πDL
2.6.4. Calorific value measurement The apparatus used to test the calorific value was an automatic rapid calorimeter (Shanghai OURUI instruments Equipment co., LTD). A certain length of conductive wire was convolved around the weighed pellet, then the sample was fixed in the oxygen bomb, finally, filling oxygen and installing the oxygen bomb into the calorimeter to measure calorific value.
Table 1 Factors and levels of mixed-level orthogonal arrays design.
Untreated Treated – –
(2)
Where, ρ is the relaxed density, g/cm3; m is the mass of the pellet, g; D is the diameter of the pellet, cm; L is the length of the pellet, cm.
2.6.1. Energy consumption measurement Specific energy consumption measurement was calculated based on the energy consumption of per gram rice straw during pelleting process. It is obtained by the displacement-stress curve recorded by the
1 4 7 10
2F πdl
ρ = m / Vp =
The pelleting platform shown in Fig. 1 was used to compress the rice straw to make pellets. Several experiments were conducted to evaluate the densification process in this subsection. The testing items include specific energy consumption, tensile strength, and pellet density. Moreover, thermogravimetric parameters, calorific value, and SEM image were also obtained.
1 2 3 4
(1)
2.6.3. Relaxed density measurement Relaxed density was analyzed according to the standard of NY/T 1881.1–2010 (Densified biofuel – Test methods Part 1: General principle, China). The rice straw was cooled for 24 h in the air. Subsequently, the mass was measured by an electronic scale, while the length and diameter of the pellet were also measured. The density of the sample is calculated based on Eq. (3).
2.6. Experiment testing indicators
Additive type (C)
σ (x ) dx
Where, σt is the tensile strength, Pa; F is the maximum compressing force, N; d is the diameter of the pellet, m; l is the length of the pellet, m.
The experimental factors and their levels were: 1) additives ratio is 1%, 4%, 7% and 10%; 2) hydrothermal carbonization and treated and untreated rice straw; 3) additive: paraffin and crude glycerin. For preliminary experiments, the mixed-level orthogonal table L8 (4124) was presented. The mixed-level orthogonal array is an experimental design method dealing with multi-factors and multi-levels, it could solve the problem of inconsistency of factor level, and has the advantages of less testing time and high testing accuracy. Each run was carried out five replications and calculated the average value. The arranged mixed-level table is shown in Table 1; letters A, B, and C are denoted to additive ratio, hydrothermal carbonization, and additive type, respectively. D is the dummy level to estimate the random error of the experiment.
Hydrothermal carbonization (B)
l
2.6.2. Tensile strength measurement Tensile strength was measured to evaluate pellets quality in terms of the crushing resistance and mechanical stability of the pellets. The tensile strength was determinated using a testing platform equipped with a universal testing machine and the corresponding loading platform. The pressing shaft of this platform moved at a certain speed (usually 5 mm/min) until the pellet crushed, then recording the crushed force as maximum compressing force F. (Lu et al., 2014). Then the tensile strength is calculated by Eq. (2).
2.5. Experimental design
Additive ratio (A)
∫0
Where, E is specific energy consumption, J/kg; W is the total energy consumption of a pellet, J; m is the weight of the pellet, g; σ is the pelleting pressure, Pa; x is the displacement, m; l is the maximum compressive displacement, m; S is the cross-section area of the pressing shaft, m2.
A self-made pelleting platform was applied to produce the pellets as shown in Fig. 1. The platform includes a pelleting device and an electronic universal testing machine. The pelleting apparatus consisted of pressing shaft, guiding device, mold sleeve, mold, heating device, mold supporter, backstop, thermocouple and foundation. The heating element system can rapidly heat up the mold. The electronic universal testing machine is provided by HRJ, Jinan, China. The maximum pelleting load is 100 kN and the extrusion speed is 0.01–1000 mm/min. The pelleting consisted of seven steps: 1) adjusting the pelleting device to ensure the pressing shaft is aligned with the cavity of the mold; 2) moving the back stop to block the mold outlet; 3)heating the die to the target temperature, and then add the weighed sample into the mold; 4) compressing the samples at a certain speed (50 mm/min in this study) until the predetermined pressure is reached; 5) maintaining the pressure for some time; 6) removing the back stop from the mold outlet, and 7) finally extruding the pellet out of the mold. The control computer recorded the stress, displacement and deformation data during the whole process (Song et al., 2014). The experiment was carried out at 120 °C, 60 MPa, 50 mm/min for 60 s. Moisture content of the pelleting samples was 10%, and the length to diameter ratio of the mold was 4.
Level
W S = m m
Rv =
(dm / dt )max T max ΔT
(4) 2
Where, Rv is volatile loss index, mg/(min•K ); dm/dt is the maximum precipitation rate of volatile, mg/min; Tmax is the temperature 180
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corresponding to the maximum precipitation rate of volatile, °C; Δ T is temperature range from the beginning of volatile devolatilization to the maximum precipitation rate, °C.
(dm1/ dt )max(dm1/ dt )mean P= Te 2Th
Table 2 The results of mixed-level orthogonal arrays design for HTC pretreated sample. Run
Factor A
(5)
Where, P is the combustion characteristic composite index, mg2/(min2 K2); (dm1/dt)max is the maximum combustion rate, mg/min; (dm1/dt)mean is the average combustion rate, mg/min; Te is ignition temperature, °C; Th is burnout temperature, °C. the closer of the biomass pellet P value is to coal, the better its comprehensive combustion characteristics are.
1 2 3 4 5 6 7 8
2.6.6. SEM analysis Scanning electron microscope (SEM, JSM-6300) was used to analyze the internal bonding condition of the pellets made by the rice straw in different ways at 15–30 kV and the magnification factor is 200˜1000. The sample was dried for 24 h under the temperature of 105 °C, and then break it into two parts. Cutting out the samples on edge and keeping the samples in the middle as the final sample, with the size of about 3 × 3×3 mm3.
a b c d e f g h
3. Results and discussion
a
1 1 2 2 3 3 4 4
B
1 2 1 2 1 2 1 2
Response b
C
1 2 1 2 2 1 2 1
c
D
d
1 2 2 1 1 2 2 1
SECe /J·g−1 (Y1)
RDf /g·cm−3 (Y2)
TSg /MPa (Y3)
CVh /MJ·kg−1 (Y4)
40.127 41.262 33.471 35.360 33.063 24.720 27.195 18.995
1.159 1.130 1.119 1.100 0.989 1.109 0.816 1.024
2.267 1.053 1.823 0.931 0.914 0.921 0.694 0.742
15.459 19.157 16.894 20.542 16.132 25.206 16.761 25.431
additive ratio. hydrothermal carbonization. additive type. dummy level. specific energy consumption. relaxed density. tensile strength. calorific value.
Table 3 Analysis of range results of rice straw pellets.
The imagines of HTC pretreated rice straw and pellets are shown in Fig. 2. Since the increasing carbon content of biomass, Fig. 2 shows that rice straw after HTC pretreatment had a darker color than non-pretreated biomass. Specific energy consumption (SEC), relaxed density (RD), tensile strength (TS) and calorific value (CV) of pretreated pellets were measured and the results of mixed-level orthogonal arrays design are shown in Table 2. After HTC pretreatment and pelleting, SEC of sample ranged from 24.720 to 40.127 J/g; RD ranged from 0.816 to 1.159 g/cm3; TS of sample ranged from 0.694 to 2.267 MPa; CV ranged from 16.132 to 25.431 MJ/kg. In contrast, the non-pretreated rice straw without additives was also evaluated for the specific energy consumption of 50.089 J/g, relaxed density of 1.051 g/cm3, tensile strength of 1.130 MPa and calorific value of 14.753 MJ/kg.
Response
SECe
f
RD
TS
g
CVh
a b c d e f g h
Factor Level
Aa
Bb
Cc
Dd
k1 k2 k3 k4 Delta Rank k1 k2 k3 k4 Delta Rank k1 k2 k3 k4 Delta Rank k1 k2 k3 k4 Delta Rank
40.69 34.42 28.89 23.09 17.60 1 1.1445 1.1095 1.0490 0.9200 0.2245 1 1.660 1.377 0.917 0.718 0.942 1 17.31 18.72 20.67 21.10 3.79 2
33.46 30.08 – – 3.38 3 1.0208 1.0907
29.33 34.22 – – 4.89 2 1.1027 1.0088
31.89 31.66 – – 0.22 4 1.068 1.0435
0.070 3 1.4243 0.9118 – – 0.5125 3 16.31 22.58 – – 6.27 1
0.094 2 1.4383 0.8978 – – 0.5405 2 20.75 18.15 – – 2.60 3
0.0245 4 1.2132 1.1227 – – 0.0905 4 19.39 19.50 – – 0.11 4
R'
R'
R'
R'
additive ratio. hydrothermal carbonization. additive type. dummy level. specific energy consumption. relaxed density. tensile strength. calorific value.
3.1. Analysis of range The analysis of range is calculated based on the difference between the maximum and minimum of the averages for each factor. The results are illustrated in Table 3. Fig. 2. Hydrothermal carbonization pretreated rice straw and pellets. a) Hydrothermal carbonization pretreated rice straw, b) Samples of the pelleting raw biomass, c) Hydrothermal carbonization and non-pretreated pellets.
3.1.1. Specific energy consumption In Table 2, energy consumption of the pretreated rice straw is between 18.995 J/g and 41.262 J/g while the non-pretreated rice straw is 181
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50.089 J/g. It is notable that the energy consumption decreased significantly after HTC pretreatment. Analysis of range results in Table 3 show that the impact of factors on specific energy consumption (SEC) in decreasing order is additive ratio (A) > additive type (C) > hydrothermal carbonization (B). Therefore, the optimally factorial combination is A1B1C2 referring to pellets without HTC adding 1% crude glycerin had lowest SEC among all factorial combinations.
Table 4 Overall evaluation of rice straw pellets.
3.1.2. Relaxed density and tensile strength In Table 2, relaxed density and tensile strength of the pretreated rice straw are respective 0.816–1.159 g/cm3, and 0.694–2.267 MPa, while the non-pretreated rice straw has the relax density of 1.051 g/cm3 and tensile strength of 1.130 MPa. This result suggests that the relaxed density and tensile strength changed significantly compared to the nonpretreated straw. Based on the agricultural trade standard of NY/T 1878–2010 (Specification for Densified Biofuel) in China, the density of the herbaceous granular biomass solid fuel should not be less than 1000 kg/m3. Lu et al. (2014) pointed out that tensile strength of the straw pellet biofuel should not be lower than 0.81 MPa. Thus, the pellet is qualified only if the density and tensile strength are over 1 g/cm3 and 0.81 MPa respectively. Some of the testing conditions failed to meet the standards. Table 3 shows that the impact of factors on relaxed density (RD) and tensile strength (TS) follows a similar trend as SEC: additive ratio > additive type > hydrothermal carbonization, thus the optimally factorial combination is A1B2C1 for the relaxed density and is A1B1C1 for the tensile strength.
SECapoint Y’j1
RDbpoint Y’j2
TScpoint Y’j3
CVdpoint Y’j4
Overall point Y’
1 2 3 4 5 6 7 8
47.34 46.04 56.75 53.72 57.45 76.84 69.85 100.00
100.00 97.50 96.55 94.91 0.00 95.69 0.00 88.35
100.00 46.45 80.41 41.07 40.30 40.63 0.00 0.00
60.79 75.33 66.43 80.78 63.43 99.12 65.91 100.00
72.44 65.20 72.35 67.54 44.32 80.05 40.73 77.67
a
specific energy consumption. relaxed density. tensile strength. calorific value.
b c d
Table 5 Range analysis of overall evaluation point of rice straw pellets. Response
Factor
Overall point Y’
3.1.3. Calorific value In Table 2, the calorific value of the pretreated rice straw ranges from 15.459 MJ/kg to 25.431 MJ/kg while the non-pretreated rice straw is 14.753 MJ/kg. The calorific value of rice straw pellets increased significantly after HTC pretreatment. Analysis of range results in Table 3 show that the impact of factors on calorific value (CV) in decreasing order is hydrothermal carbonization (B) > additive ratio (A) > additive type (C), while the optimally factorial combination is A4B2C1.
a
Level
Aa
Bb
Cc
Dd
k1 k2 k3 k4 Delta R' Rank
68.82 69.95 62.19 59.20 10.75 3
57.46 72.62 – – 15.16 2
75.63 54.45 – – 21.18 1
65.49 64.58 – – 0.94 4
additive ratio. hydrothermal carbonization. additive type. dummy level.
b c d
Table 6 Variance analysis of overall evaluation. Sources a
A Bb Cc Error Total
3.2. Optimization of experimental conditions 3.2.1. Optimal pretreated method developing In summary, there are four evaluation indexes for the rice straw pelleting quality, and analysis of range results shows that the optimally factorial combinations are significantly different when the different optimal objective was selected. Therefore, the following method was selected to deal with this kind of problem. First, the experimental values were dealt with the dimensionless method and mapped to the 0–100 numeric spaces to solve the problem of dimensional inconsistency. For specific energy consumption, the minimum value was denoted as 100 points, the other is the product of Y1min/Y1 to the minimum item point. For relaxed density, tensile strength and calorific value, the maximum value was denoted as 100 points, the other is the product of Y2/Y2max, Y3/Y3max, Y4/Y4max to the maximum item point. If the relaxed density or tensile strength value was lower than 1 g/cm3 or 0.81 MPa, the related point is 0. Furthermore, based on the importance of each evaluation index to the overall result, the weight coefficient was determined, then the overall evaluation point was calculated based on Eq. (6) and the scoring results are shown in Table 4. Finally, analysis of variance and range was carried out and the optimal condition was selected.
Y ’ = b1 Y ’ j1 + b2 Y ’ j2 + b3 Y ’ j3 + b4 Y ’ j 4
Run
a b c d e f g
DFd
Adj SSe
Adj MSf
F-Value
P-Value
Significance
3 1 1 2 7
161.21 459.35 897.18 1.84 1519.58
53.736 459.348 897.185 0.921
58.34 498.68 974.01
0.017 0.002 0.001
Sg S S
additive ratio. hydrothermal carbonization. additive type. degree of freedom. adjusted sum of square. adjusted mean of square. significant at 0.05 level.
investigate the effects of additive type, hydrothermal carbonization, and additive ratio on the overall quality of pellets. Range results show that the influence of factors on overall point in decreasing order is additive type (C) > hydrothermal carbonization (B) > additive ratio (A), and the optimally factorial combination is A2B2C1. Range results are additionally supported by variance analysis in Table 6. Variance analysis shows all of three factors had a significant effect on the overall point. However, the most influential factor is additive ratio due to highest F-value of 974.01 and lowest corresponding P-value of < 0.01 comparing to other two factors. Optimal factor combination indicates the selected pretreated condition (hydrothermally pretreated rice straw with 4% paraffin addition) had higher density, tensile strength and calorific value, as well as lower energy consumption comparing to other factorial combinations. Verification of the optimal condition was carried out and the comparison data was shown in Table 7. Comparison of the non-pretreated
(6)
Where, Y’ is the overall evaluation point; Y’ji is the evaluation point, g; bi is the weight coefficient, b1 = 30, b2 = 20, b3 = 20, and b4 = 30. The analysis results of range and variance results were listed in Tables 5 and 6. The range of overall evaluation point was analyzed to 182
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Table 7 Comparison of the non-pretreated and optimally factorial combination. Run
SECa(J/g)
Variation range
RDb(g/cm3)
Variation range
TSc(MPa)
Variation range
CVd(MJ/kg)
Variation range
Non-pretreated Optimal
50.089 31.427
– −37.26%
1.051 1.132
– +7.71%
1.130 1.081
– −4.33%
14.753 24.800
– +68.10%
a b c d
specific energy consumption. relaxed density. tensile strength. calorific value.
non-pretreated pellet were compared in Fig. 4 at 100 μm level. The microstructure of the hydrothermal pretreated rice straw pellets with 4% paraffin was more close-knit than the non-pretreated rice straw pellets, and the clearance of particle diameter is smaller (Stelte et al., 2011). The hydrothermal pretreated rice straw pellets show an improved micro-bonding pattern. Moreover, non-pretreated rice straw pellets illustrate a lamellar and smooth surface. In contrast, hydrothermal pretreated pellets show a compact and rough pattern. These are the evidence that HTC pretreatment breaks down the cell structure of biomass to release the cell components for glue-like mass in the pelleting process. These results are in good agreement with Zannikos et al. (2013) and Cheng et al. (2017). In summary, the hydrothermal pretreated rice straw pellet with 4% paraffin addition has a lower energy consumption (31.427 J/g), higher relaxed density (1.132 g/cm3) and calorific value (24.8 MJ/kg), and the tensile strength meet the relevant standards. Furthermore, the combustion and micro-bonding pattern is appealing. Compared to the energy consumption of rice straw pelleting process (36.691 J/g) and calorific (15.05 MJ/kg) value reported in literature (Hu et al., 2013 and Liu et al., 2014), the pelleting process has been greatly improved. Therefore, HTC is a rational approach to produce high-quality rice straw biomass pellets. The possible reason is that the structure of cellulose, hemicellulose and lignin in straw were completely destroyed after hydrothermal carbonization. Meanwhile, a large amount of volatile matter was released during the hydrothermal process leading to increased carbon content in straw. Therefore, the straw particles became more brittle and the energy density was substantially increased. These changes resulted in the increase of relaxed density, calorific value and the decrease of crushing resistance. Moreover, paraffin as an additive played an effective lubricant role in the reeducation of friction. In addition, with the assistance of paraffin binder, microstructure of the hydrothermally pretreated pellet was improved. Therefore, pelleting
and optimally factorial combination suggests that energy consumption of the optimal condition was 37.26% less than that of the non-pretreated condition, and pellet density increased 7.71%, the calorific value increased 68.10%, while the tensile strength decreased 4.33% only. Hu et al.(2013) reported that specific energy consumption of the rice straw pelleting process was 36.691 J/g. Liu et al. (2014) indicated that calorific value of the hydrothermal pretreated agro-residues pellet was 15.05 MJ/kg. Thus, key evaluation indexes of the selected optimal condition are higher than those reported in the literature. 3.2.2. Thermogravimetric analysis Thermogravimetric testing of the rice straw pellets (non-pretreated and optimal condition respectively) was carried out and the TG curve and DTG curves were shown in Fig. 3. Fig. 3a shows that the combustion process included moisture loss, volatile devolatilization and combustion of the solid carbon. The DTG curve (Fig. 3b) show two significant peaks. The first peak represents the combustion of the volatile component and the related extreme point means the maximum weight loss rate point. The second peak represents the combustion of the solid carbon. From the TG and DTG curve, although the ignition temperature of the pellet decreased, the temperature of the maximum weight loss rate point and the burnout point increased after the rice straw was hydrothermally pretreated. Combustion characteristic composite index (calculated by TG and DTG curve) of the optimal condition pellet was 0.97 × 10−9 while the non-pretreated condition was 4.31 × 10−9. The optimal condition pellet was considerably closed to the index of coal (0.81 × 10−9) (Liang et al., 2008), combustion characteristic of the optimal condition pellets was excellent. 3.2.3. SEM analysis SEM images of pretreated pellets with the optimal condition and
Fig. 3. TG curve and DTG curve of the pellets made from the non-pretreated and optimal condition. a) TG curve and b) DTG curve. 183
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Fig. 4. SEM images of pretreated rice straw pellets with the optimal condition and non-pretreated pellets. a) Non-pretreated condition pellets, and b) optimal condition pellet.
quality of the raw biomass was substantially upgraded and the pelleting energy consumption was greatly reduced.
Li, Y., Li, X., Shen, F., Wang, Z., Yang, G., Lin, L., Zhang, Y., Zeng, Y., Deng, S., 2014. Responses of biomass briquetting and pelleting to water-involved pretreatments and subsequent enzymatic hydrolysis. Bioresour. Technol. Rep. 151, 54–62. Liang, A., Hui, S., Xu, T., Zhao, X., 2008. TG- DTG analysis and combustion kinetics characteristic study on several kinds of biomass. Renew. Energy Resour. 26, 56–61. Liu, Z., Quek, A., Balasubramanian, R., 2014. Preparation and characterization of fuel pellets from woody biomass, agro-residues and their corresponding hydrochars. Appl. Energy 113, 1315–1322. Lu, D., Tabil, L.G., Wang, D., Wang, G., Emami, S., 2014. Experimental trials to make wheat straw pellets with wood residue and binders. Biomass Bioenergy 69, 287–296. Narra, S., Tao, Y., Glaser, C., Gusovius, H., Ay, P., 2010. Increasing the calorific value of rye straw pellets with biogenous and fossil fuel additives. Energy Fuel 24, 5228–5234. Obidziński, S., 2014. Pelleting of biomass waste with potato pulp content. Int. Agrophys. 28, 85–91. Reza, M.T., Lynam, J.G., Uddin, M.H., Coronella, C.J., 2013. Hydrothermal carbonization: fate of inorganics. Biomass Bioenergy 49, 86–94. Reza, M.T., Uddin, M.H., Lynam, J.G., Coronella, C.J., 2014. Engineered pellets from dry torrefied and HTC biochar blends. Biomass Bioenergy 63, 229–238. Rosin, A., Schr Der, H.W., Repke, J.U., 2014. Briquetting press as lock-free continuous feeding system for pressurized gasifiers. Fuel 116, 871–878. Shang, L., Nielsen, N.P.K., Stelte, W., Dahl, J., Ahrenfeldt, J., Holm, J.K., Arnavat, M.P., Bach, L.S., Henriksen, U.B., 2014. Lab and Bench-Scale pelleting of torrefied wood chips—process optimization and pellet quality. Bioenergy Res. 7, 87–94. Slinn, M., Kendall, K., Mallon, C., Andrews, J., 2008. Steam reforming of biodiesel byproduct to make renewable hydrogen. Bioresour. Technol. Rep. 99, 5851–5858. Song, X., Zhang, M., Pei, Z.J., Wang, D., 2014. Ultrasonic vibration-assisted pelleting of wheat straw: a predictive model for energy consumption using response surface methodology. Ultrasonics 54, 305–311. Stelte, W., Holm, J.K., Sanadi, A.R., Barsberg, S.O.R., Ahrenfeldt, J., Henriksen, U.B., 2011. A study of bonding and failure mechanisms in fuel pellets from different biomass resources. Biomass Bioenergy 35, 910–918. Stelte, W., Nielsen, N.P.K., Hansen, H.O., Dahl, J., Shang, L., Sanadi, A.R., 2013. Pelletizing properties of torrefied wheat straw. Biomass Bioenergy 49, 214–221. Tilay, A., Azargohar, R., Drisdelle, M., Dalai, A., Kozinski, J., 2015. Canola meal moistureresistant fuel pellets: study on the effects of process variables and additives on the pellet quality and compression characteristics. Ind. Crop. Prod. 63, 337–348. Wang, Y., Wu, K., Sun, Y., 2017. Pelletizing properties of wheat straw blending with rice straw. Enehy Fuel 31, 5126–5134. Xia, X., Sun, Y., Wu, K., Jiang, Q., 2014. Modeling of a straw ring-die briquetting process. Bioresources 9, 6316–6328. Xia, X., Sun, Y., Wu, K., Jiang, Q., 2016. Optimization of a straw ring-die briquetting process combined analytic hierarchy process and grey correlation analysis method. Fuel Process. Technol. 152, 303–309. Yan, L., Street, J.T., Steele, P.H., Entsminger, E.D., Guda, V.K., 2016. Activated carbon derived from pyrolyzed pinewood char using elevated temperature, KOH, H3PO4, and H2O2. Bioresources 11, 10433–10447. Zannikos, F., Kalligeros, S., Anastopoulos, G., Lois, E., 2013. Converting biomass and waste plastic to solid fuel briquettes. J. Renew. Energy 2013, 1–9. Zhao, C., Ma, Z., Shao, Q., Li, B., Ye, J., Peng, H., 2016. Enzymatic hydrolysis and physiochemical characterization of corn leaf after H-AFEX pretreatment. Enegry Fuel 30, 1154–1161. Zhao, C., Qiao, X., Cao, Y., Shao, Q., 2017. Application of hydrogen peroxide presoaking prior to ammonia fiber expansion pretreatment of energy crops. Fuel 205, 184–191.
4. Conclusions Rice straw was pelleted after the combining of HTC pretreatment and adding economic additives to evaluate energy consumption and pellets quality. The following conclusions were drawn: (1) The quality of rice straw pellets was substantially upgraded by the combining of HTC pretreatment and adding economic additives. Additive ratio, hydrothermal carbonization, and additive type have a significant influence on specific energy consumption, relaxed density, tensile strength and calorific value of the rice straw pelleting process. Additive type is the most influential factor, following hydrothermal carbonization and additive ratio. (2) The optimal condition is hydrothermally pretreated rice straw pelleting with 4% paraffin addition. In this scenario, energy consumption of pelleting was 37.26% less than that of the non-pretreated condition, pellet density enhanced by 7.71%, and calorific value increased by 68.10%, while the tensile strength just loss by 4.33%. In addition, combustion and micro-bonding characteristics also show that this rice straw pellet fuel has a great potential as an alternative fuel for coal. Acknowledgments This work was supportedby The Project of Six Talents Peak of Jiangsu Province (2010-JXQC-080), The Natural Science Foundation of Jiangsu Province (BK2011706) and the Innovation Project of Chinese Academy of Agricultural Sciences- Harvest of Fruits, Vegetables and Tea. References Cheng, S., Wei, L., Julson, J., Muthukumarappan, K., Kharel, P.R., Boakye, E., 2017. Hydrocarbon bio-oil production from pyrolysis bio-oil using non-sulfide Ni-Zn/Al2O3 catalyst. Fuel Process. Technol. 162, 78–86. Hu, J., Lei, T., Shen, S., Zhang, Q., 2013. Specific energy consumption regression and process parameters optimization in wet-briquetting of rice straw at normal temperature. Bioresources 8, 663–675. Kambo, H.S., Dutta, A., 2014. Strength, storage, and combustion characteristics of densified lignocellulosic biomass produced via torrefaction and hydrothermal carbonization. Appl. Energy 135, 182–191. Kong, L., Xiong, Y., Tian, S., Li, Z., Liu, T., Luo, R., 2013. Intertwining action of additional fiber in preparation of waste sawdust for biofuel pellets. Biomass Bioenergy 59, 151–157.
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Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Effects of additives and hydrothermal pretreatment on the pelleting process of rice straw: Energy consumption and pellets quality
T
⁎
Xianfei Xiaa, Ke Zhangb, , Hongru Xiaoa, Suwei Xiaoa, Zhiyu Songa, Zhengyu Yangc a
Nanjing Research Institute for Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA c Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02120, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Rice straw Additives Pelleting Hydrothermal pretreatment Energy consumption Pellet Quality
Pelleting is an appealing approach to enhance energy density of biomass for renewable fuel production. To overcome these two challenges: high energy consumption of pelleting and low product calorific value of pellets, the pretreatment of raw biomass is a necessary process prior to pelleting. In this study, the rice straw was pretreated by hydrothermal carbonization (HTC) firstly, then the economic additives: crude glycerin and paraffin were added. Afterward, the pretreated biomass was pelletized using mixed-level orthogonal array design to evaluate energy consumption and pellets quality. Finally, the pelleting condition was optimized based on the analysis of range and variance, thermogravimetric analysis and scanning electron microscope (SEM) evaluation. Experimental results indicated that additive type was a more influential factor than HTC pretreatment and adding ratio in terms of reducing energy consumption and improving pellet quality. Compared to the nonpretreated rice straw, pelleting energy of the selected hydrothermally pretreated sample with 4% paraffin addition reduced 37.26%, pellet density (ratio of pellet mass to volume) enhanced by 7.71%, and calorific value (heat released by combustion of unit pellet mass) increased by 68.10%, while the tensile strength (the crushing resistance and mechanical stability of the pellets) just loss by 4.33%. Combustion and micro-bonding characteristics also show that this rice straw pellet fuel has a great potential as an alternative fuel for coal.
1. Introduction With the growing global energy and environment concerns, the surging demand for renewable energy encourages to replace fossilbased fuels using agricultural wastes (Cheng et al., 2017; Yan et al., 2016). There are over 20 types of straw biomass with the annual yield of more than 700 million tons in China.(Zhao et al., 2016). However, most straw biomass is abandoned or uncontrolled burned (Wang et al., 2017; Zhao et al., 2017). Moreover, straw biomass has an innately lower energy density resulting in the low efficiency of combustion and transportation than woody biomass. Pelleting of straw biomass is an appealing process offering high productivity and suitable fuels with better density and strength (Xia et al., 2014, 2016). The biomass produced by this way is more convenient for transportation and burning (Song et al., 2014). Nevertheless, pelleting of rice straw has two technical challenges: high energy consumption (more than 90 kW⋅h/t for rice straw pelleting process) and low product calorific value (15 MJ/kg usually). To overcome these two challenges, the pretreatment of raw biomass prior to pelleting is an essential approach. Physical and
⁎
chemical pretreatments are two conventional methods. Physical pretreatment usually refers to adding binders, while chemical pretreatment includes pyrolysis and hydrothermal carbonization (Stelte et al., 2013). There is some literature related to the addition of binders for the improvement of pelleting. Kong et al. (2013) discovered that the pellets quality could be improved by adding linen fibers. Rosin et al. (2014) mixed biomass samples with various additives such as starch, molasses, high conversion soaker cracking residues, slaked lime, ash and montan resin to improve the pellets density. Obidziński (2014) studied the influence of potato pulp content on the power demand of the oat bran pelleting process, indicating that increasing the potato pulp content from 15 to 20% caused a reduction of the power demand. Tilay et al. (2015) found that the strength and durability of canola meal pellets were improved when binder and lubricant were added. Shang et al. (2014) investigated the effect of rapeseed oil on wood chips densification process and indicated that rapeseed oil reduced the static friction and stabilized pellet production, but the pellet quality, strength, and density were negatively affected. Zannikos et al.(2013) used waste plastic as an additive to enhance the calorific value and emissions of
Corresponding author at: Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA. E-mail address: [email protected] (K. Zhang).
https://doi.org/10.1016/j.indcrop.2019.03.007 Received 28 June 2018; Received in revised form 16 January 2019; Accepted 3 March 2019 0926-6690/ © 2019 Published by Elsevier B.V.
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Fig. 1. Schematic illustration of the pelleting platform.
crude glycerin and paraffin. Afterward, the pretreated biomass was pelletized by mixed-level orthogonal array design to evaluate energy consumption and pellets quality. Finally, the pelleting condition was optimized based on the analysis of range and variance. The objective of this study is to investigate the effects of additives and hydrothermal pretreatment on the pelleting process of rice straw in terms of energy consumption and pellets quality.
sawdust pellets. Although additives can efficiently improve the quality of pellets of biomass, the additives need to be cost-effective and could enhance both physical and chemical properties of the raw biomass, as well as decrease the friction during the pelleting process for low energy consumption. Furthermore, the additives in the pelleting process need to have no negative impact on the environment. Crude glycerin and paraffin are high quality and efficient additives. In 2015, the total yield of crude glycerin was about 0.27 million tons in China. However, due to the technical restriction, crude glycerin cannot be efficiently utilized and the large amount crude glycerin could lead to environmental pollution. The calorific value of crude glycerin is about 27 MJ/kg (Lu et al., 2014). Paraffin is a colorless and white mixture made from crude petroleum. Besides physical pretreatment of biomass, hydrothermal carbonization is an effective chemical pretreatment method to improve the pellets properties. Hydrothermal pretreatment, known as auto-hydrolysis, it is controllable with short reaction period. Hydrothermal pretreatment is friendly to the environment because the water is used as a solvent instead of other chemical reagents. It is divided into hydrothermal gasification, hydrothermal liquefaction, and HTC according to reaction time and temperature (Li et al., 2014). Reza et al. (2013 and 2014) used HTC biochar as a binder to enhance energy density and durability of biomass pellets. Kambo and Dutta (2014) indicated that density and volumetric energy density of biomass pellets produced by HTC were significantly higher comparing to the non-torrefied pellets or torrefied pellets. Moreover, these pellets showed the increasing of hydrophobicity and reduction of ash content. Liu et al. (2014) reported that HTC biomass pellets showed a trend of increasing calorific value but decreasing ash content and compressive strength. However, the present hydrothermal carbonization research almost focused on woody biomass, little information is available for straw biomass with low lignin content. Moreover, there is no report on the combination of physical and chemical pretreatments (additives and HTC) for rice straw pelleting in terms of energy consumption and pellets quality. To fill this gap in the literature, the rice straw was treated by the combination of HTC and adding economic additives included
2. Method and evaluation 2.1. Material preparation Rice straw biomass (Nangeng 46) was obtained in Jiangyan, China in 2016 fall. The air-dried rice straw was ground to 2 mm with the density is 75.3 kg/m3. Then the ground rice straw was stored in sealed plastic bags for further use. All chemicals used in this research were purchased from Sigma Chemical Co. (St. Louis, MO). 2.2. Additives Crude glycerin and paraffin were added into rice straw with a ratio of 1%, 4%, 7% and 10%. Both additives only produce CO2 and H2O after co-combustion with biomass (Narra et al., 2010; Slinn et al., 2008). Furthermore, crude glycerin is a by-product of biodiesel. One ton of biodiesel will generate 100 kg of crude glycerin based on current process (Slinn et al., 2008). As the increasing demand for biodiesel, the yield of crude glycerin also increased recently. Crude paraffin wax offers many advantages such as high oil content, low price and the high calorific value of approximately 48 MJ/kg (Narra et al., 2010). Meanwhile, paraffin wax emulsion is favorable for the mixing with rice straw in this study. 2.3. Hydrothermal pretreatment The rice straw was pretreated using HTC at 240 °C for 30 min. Production from HTC consisted of solid (coke), liquid (sugar, acetic 179
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computer and calculated by Eq. (1).
acid, and TOC) and gaseous products (mainly CO2). In this study, the solid product was used for the pelleting study.
E= 2.4. Pelleting platform and process
σt =
(3)
2.6.5. Thermogravimetric analysis The purpose of thermogravimetric analysis was to investigate the relationship between the sample’s mass loss and the increasing temperature. The result of the thermogravimetric analysis is a thermogravimetric curve (TG curve), and the first derivative of TG curve to temperature is a differential thermogravimetric curve (DTG curve). Thermogravimetric analysis is an important tool to investigate the combustion kinetics characteristics of the biomass pellet. The volatile loss index Rv and the combustion characteristic composite index P can be got from the thermogravimetric analysis results and calculated based on Eq. (4) and (5) respectively. A thermogravimetric analyzer (Mettler Toledo Co. in Switzerland TGA/SDTA 851e) was carried out at the temperature range of 0–700 °C, with the temperature rising rate of 20 °C/min, and the air flow speed is 80 mL/min.
Factor
Paraffin Crude glycerin – –
4m πDL
2.6.4. Calorific value measurement The apparatus used to test the calorific value was an automatic rapid calorimeter (Shanghai OURUI instruments Equipment co., LTD). A certain length of conductive wire was convolved around the weighed pellet, then the sample was fixed in the oxygen bomb, finally, filling oxygen and installing the oxygen bomb into the calorimeter to measure calorific value.
Table 1 Factors and levels of mixed-level orthogonal arrays design.
Untreated Treated – –
(2)
Where, ρ is the relaxed density, g/cm3; m is the mass of the pellet, g; D is the diameter of the pellet, cm; L is the length of the pellet, cm.
2.6.1. Energy consumption measurement Specific energy consumption measurement was calculated based on the energy consumption of per gram rice straw during pelleting process. It is obtained by the displacement-stress curve recorded by the
1 4 7 10
2F πdl
ρ = m / Vp =
The pelleting platform shown in Fig. 1 was used to compress the rice straw to make pellets. Several experiments were conducted to evaluate the densification process in this subsection. The testing items include specific energy consumption, tensile strength, and pellet density. Moreover, thermogravimetric parameters, calorific value, and SEM image were also obtained.
1 2 3 4
(1)
2.6.3. Relaxed density measurement Relaxed density was analyzed according to the standard of NY/T 1881.1–2010 (Densified biofuel – Test methods Part 1: General principle, China). The rice straw was cooled for 24 h in the air. Subsequently, the mass was measured by an electronic scale, while the length and diameter of the pellet were also measured. The density of the sample is calculated based on Eq. (3).
2.6. Experiment testing indicators
Additive type (C)
σ (x ) dx
Where, σt is the tensile strength, Pa; F is the maximum compressing force, N; d is the diameter of the pellet, m; l is the length of the pellet, m.
The experimental factors and their levels were: 1) additives ratio is 1%, 4%, 7% and 10%; 2) hydrothermal carbonization and treated and untreated rice straw; 3) additive: paraffin and crude glycerin. For preliminary experiments, the mixed-level orthogonal table L8 (4124) was presented. The mixed-level orthogonal array is an experimental design method dealing with multi-factors and multi-levels, it could solve the problem of inconsistency of factor level, and has the advantages of less testing time and high testing accuracy. Each run was carried out five replications and calculated the average value. The arranged mixed-level table is shown in Table 1; letters A, B, and C are denoted to additive ratio, hydrothermal carbonization, and additive type, respectively. D is the dummy level to estimate the random error of the experiment.
Hydrothermal carbonization (B)
l
2.6.2. Tensile strength measurement Tensile strength was measured to evaluate pellets quality in terms of the crushing resistance and mechanical stability of the pellets. The tensile strength was determinated using a testing platform equipped with a universal testing machine and the corresponding loading platform. The pressing shaft of this platform moved at a certain speed (usually 5 mm/min) until the pellet crushed, then recording the crushed force as maximum compressing force F. (Lu et al., 2014). Then the tensile strength is calculated by Eq. (2).
2.5. Experimental design
Additive ratio (A)
∫0
Where, E is specific energy consumption, J/kg; W is the total energy consumption of a pellet, J; m is the weight of the pellet, g; σ is the pelleting pressure, Pa; x is the displacement, m; l is the maximum compressive displacement, m; S is the cross-section area of the pressing shaft, m2.
A self-made pelleting platform was applied to produce the pellets as shown in Fig. 1. The platform includes a pelleting device and an electronic universal testing machine. The pelleting apparatus consisted of pressing shaft, guiding device, mold sleeve, mold, heating device, mold supporter, backstop, thermocouple and foundation. The heating element system can rapidly heat up the mold. The electronic universal testing machine is provided by HRJ, Jinan, China. The maximum pelleting load is 100 kN and the extrusion speed is 0.01–1000 mm/min. The pelleting consisted of seven steps: 1) adjusting the pelleting device to ensure the pressing shaft is aligned with the cavity of the mold; 2) moving the back stop to block the mold outlet; 3)heating the die to the target temperature, and then add the weighed sample into the mold; 4) compressing the samples at a certain speed (50 mm/min in this study) until the predetermined pressure is reached; 5) maintaining the pressure for some time; 6) removing the back stop from the mold outlet, and 7) finally extruding the pellet out of the mold. The control computer recorded the stress, displacement and deformation data during the whole process (Song et al., 2014). The experiment was carried out at 120 °C, 60 MPa, 50 mm/min for 60 s. Moisture content of the pelleting samples was 10%, and the length to diameter ratio of the mold was 4.
Level
W S = m m
Rv =
(dm / dt )max T max ΔT
(4) 2
Where, Rv is volatile loss index, mg/(min•K ); dm/dt is the maximum precipitation rate of volatile, mg/min; Tmax is the temperature 180
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corresponding to the maximum precipitation rate of volatile, °C; Δ T is temperature range from the beginning of volatile devolatilization to the maximum precipitation rate, °C.
(dm1/ dt )max(dm1/ dt )mean P= Te 2Th
Table 2 The results of mixed-level orthogonal arrays design for HTC pretreated sample. Run
Factor A
(5)
Where, P is the combustion characteristic composite index, mg2/(min2 K2); (dm1/dt)max is the maximum combustion rate, mg/min; (dm1/dt)mean is the average combustion rate, mg/min; Te is ignition temperature, °C; Th is burnout temperature, °C. the closer of the biomass pellet P value is to coal, the better its comprehensive combustion characteristics are.
1 2 3 4 5 6 7 8
2.6.6. SEM analysis Scanning electron microscope (SEM, JSM-6300) was used to analyze the internal bonding condition of the pellets made by the rice straw in different ways at 15–30 kV and the magnification factor is 200˜1000. The sample was dried for 24 h under the temperature of 105 °C, and then break it into two parts. Cutting out the samples on edge and keeping the samples in the middle as the final sample, with the size of about 3 × 3×3 mm3.
a b c d e f g h
3. Results and discussion
a
1 1 2 2 3 3 4 4
B
1 2 1 2 1 2 1 2
Response b
C
1 2 1 2 2 1 2 1
c
D
d
1 2 2 1 1 2 2 1
SECe /J·g−1 (Y1)
RDf /g·cm−3 (Y2)
TSg /MPa (Y3)
CVh /MJ·kg−1 (Y4)
40.127 41.262 33.471 35.360 33.063 24.720 27.195 18.995
1.159 1.130 1.119 1.100 0.989 1.109 0.816 1.024
2.267 1.053 1.823 0.931 0.914 0.921 0.694 0.742
15.459 19.157 16.894 20.542 16.132 25.206 16.761 25.431
additive ratio. hydrothermal carbonization. additive type. dummy level. specific energy consumption. relaxed density. tensile strength. calorific value.
Table 3 Analysis of range results of rice straw pellets.
The imagines of HTC pretreated rice straw and pellets are shown in Fig. 2. Since the increasing carbon content of biomass, Fig. 2 shows that rice straw after HTC pretreatment had a darker color than non-pretreated biomass. Specific energy consumption (SEC), relaxed density (RD), tensile strength (TS) and calorific value (CV) of pretreated pellets were measured and the results of mixed-level orthogonal arrays design are shown in Table 2. After HTC pretreatment and pelleting, SEC of sample ranged from 24.720 to 40.127 J/g; RD ranged from 0.816 to 1.159 g/cm3; TS of sample ranged from 0.694 to 2.267 MPa; CV ranged from 16.132 to 25.431 MJ/kg. In contrast, the non-pretreated rice straw without additives was also evaluated for the specific energy consumption of 50.089 J/g, relaxed density of 1.051 g/cm3, tensile strength of 1.130 MPa and calorific value of 14.753 MJ/kg.
Response
SECe
f
RD
TS
g
CVh
a b c d e f g h
Factor Level
Aa
Bb
Cc
Dd
k1 k2 k3 k4 Delta Rank k1 k2 k3 k4 Delta Rank k1 k2 k3 k4 Delta Rank k1 k2 k3 k4 Delta Rank
40.69 34.42 28.89 23.09 17.60 1 1.1445 1.1095 1.0490 0.9200 0.2245 1 1.660 1.377 0.917 0.718 0.942 1 17.31 18.72 20.67 21.10 3.79 2
33.46 30.08 – – 3.38 3 1.0208 1.0907
29.33 34.22 – – 4.89 2 1.1027 1.0088
31.89 31.66 – – 0.22 4 1.068 1.0435
0.070 3 1.4243 0.9118 – – 0.5125 3 16.31 22.58 – – 6.27 1
0.094 2 1.4383 0.8978 – – 0.5405 2 20.75 18.15 – – 2.60 3
0.0245 4 1.2132 1.1227 – – 0.0905 4 19.39 19.50 – – 0.11 4
R'
R'
R'
R'
additive ratio. hydrothermal carbonization. additive type. dummy level. specific energy consumption. relaxed density. tensile strength. calorific value.
3.1. Analysis of range The analysis of range is calculated based on the difference between the maximum and minimum of the averages for each factor. The results are illustrated in Table 3. Fig. 2. Hydrothermal carbonization pretreated rice straw and pellets. a) Hydrothermal carbonization pretreated rice straw, b) Samples of the pelleting raw biomass, c) Hydrothermal carbonization and non-pretreated pellets.
3.1.1. Specific energy consumption In Table 2, energy consumption of the pretreated rice straw is between 18.995 J/g and 41.262 J/g while the non-pretreated rice straw is 181
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50.089 J/g. It is notable that the energy consumption decreased significantly after HTC pretreatment. Analysis of range results in Table 3 show that the impact of factors on specific energy consumption (SEC) in decreasing order is additive ratio (A) > additive type (C) > hydrothermal carbonization (B). Therefore, the optimally factorial combination is A1B1C2 referring to pellets without HTC adding 1% crude glycerin had lowest SEC among all factorial combinations.
Table 4 Overall evaluation of rice straw pellets.
3.1.2. Relaxed density and tensile strength In Table 2, relaxed density and tensile strength of the pretreated rice straw are respective 0.816–1.159 g/cm3, and 0.694–2.267 MPa, while the non-pretreated rice straw has the relax density of 1.051 g/cm3 and tensile strength of 1.130 MPa. This result suggests that the relaxed density and tensile strength changed significantly compared to the nonpretreated straw. Based on the agricultural trade standard of NY/T 1878–2010 (Specification for Densified Biofuel) in China, the density of the herbaceous granular biomass solid fuel should not be less than 1000 kg/m3. Lu et al. (2014) pointed out that tensile strength of the straw pellet biofuel should not be lower than 0.81 MPa. Thus, the pellet is qualified only if the density and tensile strength are over 1 g/cm3 and 0.81 MPa respectively. Some of the testing conditions failed to meet the standards. Table 3 shows that the impact of factors on relaxed density (RD) and tensile strength (TS) follows a similar trend as SEC: additive ratio > additive type > hydrothermal carbonization, thus the optimally factorial combination is A1B2C1 for the relaxed density and is A1B1C1 for the tensile strength.
SECapoint Y’j1
RDbpoint Y’j2
TScpoint Y’j3
CVdpoint Y’j4
Overall point Y’
1 2 3 4 5 6 7 8
47.34 46.04 56.75 53.72 57.45 76.84 69.85 100.00
100.00 97.50 96.55 94.91 0.00 95.69 0.00 88.35
100.00 46.45 80.41 41.07 40.30 40.63 0.00 0.00
60.79 75.33 66.43 80.78 63.43 99.12 65.91 100.00
72.44 65.20 72.35 67.54 44.32 80.05 40.73 77.67
a
specific energy consumption. relaxed density. tensile strength. calorific value.
b c d
Table 5 Range analysis of overall evaluation point of rice straw pellets. Response
Factor
Overall point Y’
3.1.3. Calorific value In Table 2, the calorific value of the pretreated rice straw ranges from 15.459 MJ/kg to 25.431 MJ/kg while the non-pretreated rice straw is 14.753 MJ/kg. The calorific value of rice straw pellets increased significantly after HTC pretreatment. Analysis of range results in Table 3 show that the impact of factors on calorific value (CV) in decreasing order is hydrothermal carbonization (B) > additive ratio (A) > additive type (C), while the optimally factorial combination is A4B2C1.
a
Level
Aa
Bb
Cc
Dd
k1 k2 k3 k4 Delta R' Rank
68.82 69.95 62.19 59.20 10.75 3
57.46 72.62 – – 15.16 2
75.63 54.45 – – 21.18 1
65.49 64.58 – – 0.94 4
additive ratio. hydrothermal carbonization. additive type. dummy level.
b c d
Table 6 Variance analysis of overall evaluation. Sources a
A Bb Cc Error Total
3.2. Optimization of experimental conditions 3.2.1. Optimal pretreated method developing In summary, there are four evaluation indexes for the rice straw pelleting quality, and analysis of range results shows that the optimally factorial combinations are significantly different when the different optimal objective was selected. Therefore, the following method was selected to deal with this kind of problem. First, the experimental values were dealt with the dimensionless method and mapped to the 0–100 numeric spaces to solve the problem of dimensional inconsistency. For specific energy consumption, the minimum value was denoted as 100 points, the other is the product of Y1min/Y1 to the minimum item point. For relaxed density, tensile strength and calorific value, the maximum value was denoted as 100 points, the other is the product of Y2/Y2max, Y3/Y3max, Y4/Y4max to the maximum item point. If the relaxed density or tensile strength value was lower than 1 g/cm3 or 0.81 MPa, the related point is 0. Furthermore, based on the importance of each evaluation index to the overall result, the weight coefficient was determined, then the overall evaluation point was calculated based on Eq. (6) and the scoring results are shown in Table 4. Finally, analysis of variance and range was carried out and the optimal condition was selected.
Y ’ = b1 Y ’ j1 + b2 Y ’ j2 + b3 Y ’ j3 + b4 Y ’ j 4
Run
a b c d e f g
DFd
Adj SSe
Adj MSf
F-Value
P-Value
Significance
3 1 1 2 7
161.21 459.35 897.18 1.84 1519.58
53.736 459.348 897.185 0.921
58.34 498.68 974.01
0.017 0.002 0.001
Sg S S
additive ratio. hydrothermal carbonization. additive type. degree of freedom. adjusted sum of square. adjusted mean of square. significant at 0.05 level.
investigate the effects of additive type, hydrothermal carbonization, and additive ratio on the overall quality of pellets. Range results show that the influence of factors on overall point in decreasing order is additive type (C) > hydrothermal carbonization (B) > additive ratio (A), and the optimally factorial combination is A2B2C1. Range results are additionally supported by variance analysis in Table 6. Variance analysis shows all of three factors had a significant effect on the overall point. However, the most influential factor is additive ratio due to highest F-value of 974.01 and lowest corresponding P-value of < 0.01 comparing to other two factors. Optimal factor combination indicates the selected pretreated condition (hydrothermally pretreated rice straw with 4% paraffin addition) had higher density, tensile strength and calorific value, as well as lower energy consumption comparing to other factorial combinations. Verification of the optimal condition was carried out and the comparison data was shown in Table 7. Comparison of the non-pretreated
(6)
Where, Y’ is the overall evaluation point; Y’ji is the evaluation point, g; bi is the weight coefficient, b1 = 30, b2 = 20, b3 = 20, and b4 = 30. The analysis results of range and variance results were listed in Tables 5 and 6. The range of overall evaluation point was analyzed to 182
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Table 7 Comparison of the non-pretreated and optimally factorial combination. Run
SECa(J/g)
Variation range
RDb(g/cm3)
Variation range
TSc(MPa)
Variation range
CVd(MJ/kg)
Variation range
Non-pretreated Optimal
50.089 31.427
– −37.26%
1.051 1.132
– +7.71%
1.130 1.081
– −4.33%
14.753 24.800
– +68.10%
a b c d
specific energy consumption. relaxed density. tensile strength. calorific value.
non-pretreated pellet were compared in Fig. 4 at 100 μm level. The microstructure of the hydrothermal pretreated rice straw pellets with 4% paraffin was more close-knit than the non-pretreated rice straw pellets, and the clearance of particle diameter is smaller (Stelte et al., 2011). The hydrothermal pretreated rice straw pellets show an improved micro-bonding pattern. Moreover, non-pretreated rice straw pellets illustrate a lamellar and smooth surface. In contrast, hydrothermal pretreated pellets show a compact and rough pattern. These are the evidence that HTC pretreatment breaks down the cell structure of biomass to release the cell components for glue-like mass in the pelleting process. These results are in good agreement with Zannikos et al. (2013) and Cheng et al. (2017). In summary, the hydrothermal pretreated rice straw pellet with 4% paraffin addition has a lower energy consumption (31.427 J/g), higher relaxed density (1.132 g/cm3) and calorific value (24.8 MJ/kg), and the tensile strength meet the relevant standards. Furthermore, the combustion and micro-bonding pattern is appealing. Compared to the energy consumption of rice straw pelleting process (36.691 J/g) and calorific (15.05 MJ/kg) value reported in literature (Hu et al., 2013 and Liu et al., 2014), the pelleting process has been greatly improved. Therefore, HTC is a rational approach to produce high-quality rice straw biomass pellets. The possible reason is that the structure of cellulose, hemicellulose and lignin in straw were completely destroyed after hydrothermal carbonization. Meanwhile, a large amount of volatile matter was released during the hydrothermal process leading to increased carbon content in straw. Therefore, the straw particles became more brittle and the energy density was substantially increased. These changes resulted in the increase of relaxed density, calorific value and the decrease of crushing resistance. Moreover, paraffin as an additive played an effective lubricant role in the reeducation of friction. In addition, with the assistance of paraffin binder, microstructure of the hydrothermally pretreated pellet was improved. Therefore, pelleting
and optimally factorial combination suggests that energy consumption of the optimal condition was 37.26% less than that of the non-pretreated condition, and pellet density increased 7.71%, the calorific value increased 68.10%, while the tensile strength decreased 4.33% only. Hu et al.(2013) reported that specific energy consumption of the rice straw pelleting process was 36.691 J/g. Liu et al. (2014) indicated that calorific value of the hydrothermal pretreated agro-residues pellet was 15.05 MJ/kg. Thus, key evaluation indexes of the selected optimal condition are higher than those reported in the literature. 3.2.2. Thermogravimetric analysis Thermogravimetric testing of the rice straw pellets (non-pretreated and optimal condition respectively) was carried out and the TG curve and DTG curves were shown in Fig. 3. Fig. 3a shows that the combustion process included moisture loss, volatile devolatilization and combustion of the solid carbon. The DTG curve (Fig. 3b) show two significant peaks. The first peak represents the combustion of the volatile component and the related extreme point means the maximum weight loss rate point. The second peak represents the combustion of the solid carbon. From the TG and DTG curve, although the ignition temperature of the pellet decreased, the temperature of the maximum weight loss rate point and the burnout point increased after the rice straw was hydrothermally pretreated. Combustion characteristic composite index (calculated by TG and DTG curve) of the optimal condition pellet was 0.97 × 10−9 while the non-pretreated condition was 4.31 × 10−9. The optimal condition pellet was considerably closed to the index of coal (0.81 × 10−9) (Liang et al., 2008), combustion characteristic of the optimal condition pellets was excellent. 3.2.3. SEM analysis SEM images of pretreated pellets with the optimal condition and
Fig. 3. TG curve and DTG curve of the pellets made from the non-pretreated and optimal condition. a) TG curve and b) DTG curve. 183
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Fig. 4. SEM images of pretreated rice straw pellets with the optimal condition and non-pretreated pellets. a) Non-pretreated condition pellets, and b) optimal condition pellet.
quality of the raw biomass was substantially upgraded and the pelleting energy consumption was greatly reduced.
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4. Conclusions Rice straw was pelleted after the combining of HTC pretreatment and adding economic additives to evaluate energy consumption and pellets quality. The following conclusions were drawn: (1) The quality of rice straw pellets was substantially upgraded by the combining of HTC pretreatment and adding economic additives. Additive ratio, hydrothermal carbonization, and additive type have a significant influence on specific energy consumption, relaxed density, tensile strength and calorific value of the rice straw pelleting process. Additive type is the most influential factor, following hydrothermal carbonization and additive ratio. (2) The optimal condition is hydrothermally pretreated rice straw pelleting with 4% paraffin addition. In this scenario, energy consumption of pelleting was 37.26% less than that of the non-pretreated condition, pellet density enhanced by 7.71%, and calorific value increased by 68.10%, while the tensile strength just loss by 4.33%. In addition, combustion and micro-bonding characteristics also show that this rice straw pellet fuel has a great potential as an alternative fuel for coal. Acknowledgments This work was supportedby The Project of Six Talents Peak of Jiangsu Province (2010-JXQC-080), The Natural Science Foundation of Jiangsu Province (BK2011706) and the Innovation Project of Chinese Academy of Agricultural Sciences- Harvest of Fruits, Vegetables and Tea. References Cheng, S., Wei, L., Julson, J., Muthukumarappan, K., Kharel, P.R., Boakye, E., 2017. Hydrocarbon bio-oil production from pyrolysis bio-oil using non-sulfide Ni-Zn/Al2O3 catalyst. Fuel Process. Technol. 162, 78–86. Hu, J., Lei, T., Shen, S., Zhang, Q., 2013. Specific energy consumption regression and process parameters optimization in wet-briquetting of rice straw at normal temperature. Bioresources 8, 663–675. Kambo, H.S., Dutta, A., 2014. Strength, storage, and combustion characteristics of densified lignocellulosic biomass produced via torrefaction and hydrothermal carbonization. Appl. Energy 135, 182–191. Kong, L., Xiong, Y., Tian, S., Li, Z., Liu, T., Luo, R., 2013. Intertwining action of additional fiber in preparation of waste sawdust for biofuel pellets. Biomass Bioenergy 59, 151–157.
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