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Co-pelletization of sewage sludge and biomass: The energy input and properties of pellets
Fuel Processing Technology 132 (2015) 55–61
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Co-pelletization of sewage sludge and biomass: The energy input and properties of pellets Hui Li a,b, Long-Bo Jiang a,c, Chang-Zhu Li a,⁎,1, Jie Liang c,⁎⁎,1, Xing-Zhong Yuan c, Zhi-Hua Xiao c, Zhi-Hong Xiao a, Hou Wang c a b c
Institute of Bio-energy, Hunan Academy of Forestry, Changsha 410004, PR China Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, PR China College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e
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Article history: Received 10 May 2014 Received in revised form 10 December 2014 Accepted 11 December 2014 Available online 10 January 2015 Keywords: Sewage sludge Biomass Co-pelletization Compression energy consumption Moisture adsorption
a b s t r a c t Presently, most pellets are made of lignocellulose biomass. However, the rocketing development of demands and fierce competition of feedstock between biomass industries have resulted in local biomass supply insufficiency and rising prices of the raw material. Therefore, other kinds of raw materials should be involved in the production of pellets to improve the stable supply of feedstock. In this study, the pelletization of biomass materials mixed with sewage sludge (SS) was investigated. The effects of process parameters on energy consumption and pellet properties were studied. It was shown that the compression energy consumption was affected by applied pressure, die temperature and water content. Both temperature and water content had an influence on the above pellet properties to a different extent, which was found to be independent of pressure. Optimal parameters (55 MPa for pressure, 90 °C for temperature and 10–15% for water content) were recommended for co-pelletization. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The utilization of forestry and agricultural biomass residues for sustainable heat and power production is an important part of future energy concepts [1]. The growing use of biomass boilers, instead of fossil-fuel boilers, has facilitated an expansive market of solid biofuels. Pellets are made of biomass, which can reduce the costs of handling, transportation and storage [2]. There are more than 442 pellet plants in the world (except mini workshops) with an annual yield of 14 million tons [3]. However, the rapid growth of wood pellet production and the competition of raw material have caused a marked decrease of the preferred raw materials for the production of pellets, such as cutter shavings and sawdust. In China, the price of sawdust (water content around 40%) has increased to higher than 400 RMB/ton, especially in some area in Hunan and Guangdong where the price has been higher than 600 RMB/ton. The increase of raw material price weakened market competitiveness of pellets. Several reports have focused on other raw materials or mixtures of raw materials which could be solutions [3–8]. Miranda et al. [5] studied
⁎ Corresponding author. Tel./fax: +86 731 85578702. ⁎⁎ Corresponding author. Tel./fax: +86 731 88821697. E-mail addresses: [email protected] (C.-Z. Li), [email protected] (J. Liang). 1 C.Z. Li and J. Liang contribute equally to this work.
http://dx.doi.org/10.1016/j.fuproc.2014.12.020 0378-3820/© 2014 Elsevier B.V. All rights reserved.
the properties of blends of pelletized residues from olive pomace and pyrenean oak. It was shown that the supplement of pyrenean oak residues to olive pomace samples can facilitate the effectiveness of pellet compression without a significant change of its thermal properties. Razuan et al. [6] reported the pelletization of palm kernel cake. The addition of palm kernel cake into sawdust can enhance the density and tensile strength of pellets at elevated temperature (80–100 °C) and pressure (64.38 MPa) [6]. Stahl and Berghel [3] investigated the co-pelletization of sawdust mixed with rapeseed cake. The energy consumption and pellet durability were decreased with rapeseed cake content in feedstock increased [3]. It is environment friendly to reuse non-wood biomass waste into pellets producing, which can also help pellet manufacturer hedge the risk of raw-material market. In the present study, the sewage sludge (SS), a residual waste of sewage plants, was applied to mix with biomass material for pelletization. SS is a large volume byproduct from the municipal wastewater treatment plants. The treatment and disposal of SS have been one of the most severe environmental problems in the world. In China, 20.76 million tons SS containing 80% water has been generated in 2010, which is expected to increase in the near future [9]. However, the present SS disposal procedures are unsustainable, due to the land limitations and secondary pollution caused by large amounts of leachate. For instance, EU directives have restricted the use of SS in agriculture and imposed a ban on the landfill [10]. The Chinese government has stipulated that only SS with a water content of less than 60% is permitted in landfill [11].
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Meanwhile, SS also contains a significant amount of organic compounds which are principally proteins and carbohydrates. The denaturation of protein and the gelatinization of carbohydrate in the sludge could serve as a natural binder which can improve the strength of fiber grid in pellet [12]. Dospoy et al. [13] firstly managed to produce sludge derived fuel from paper sludge, powder coal, and plastics and applied in the market. Other studies have been conducted, focusing on the physical properties and combustion characteristics of sludge refuse-derived fuel (RDF). Chen et al. [14] investigated the characteristics of a sludge RDF and the combustion behaviors. Wzorek [10] studied the physical and chemical properties and the calorific values of three types of sludge RDF which were applied to the cement clinker manufacturing process. Zhao et al. [9] developed an RDF technology, including the steam explosion pretreatment, mechanical dewatering, natural drying and pelleting, to produce solid fuel from SS. Hou et al. [15] estimated the heating value and investigated combustion characteristics of densified RDF produced from oil sludge. However, few studies have been conducted on the functions of process parameters for sludge pelleting. The control of pressure, die temperature, and water content have been considered as three important parameters to affect the pelletization process of biomass and properties of pellets [16–21]. Therefore, it is necessary to study the influence of process parameters on sludge derived pellet. In the present work, the effects of process parameters on energy consumption and product properties for co-pelletization of SS and biomass were carried out. The pellets produced from different biomass material (Chinese fir, camphor and rice straw) with the pressure (28–110 MPa), temperature (30–150 °C) and water content (5–25%) were examined in terms of energy consumption, pellet density, volume expansion, maximum breaking force, and moisture uptake, etc.
were weighed and mixed manually with a constant ratio of 50 wt.%. The percentage of the blends was calculated on a dry weight basis. A single factor variable method were applied to investigate the variables of pressure, temperature, and water content which were conducted at 90 °C with 15% water content, 55 MPa with 15% water content, and 55 MPa with 90 °C, respectively. Each experiment was repeated seven times. Forces and displacement curves in pelletization were recorded to calculate the energy consumption. The energy consumptions associated with compaction were obtained by integrating the curve. The mass, length, and diameter of each pellet were measured immediately after it was removed from the cylinder to calculate pellet density. The measure of mass, length and diameter was repeated after storing the pellets inside a sealed bottle at 4 °C for two weeks to calculate expansion ratio. The maximum breaking force of pellets was analyzed by the same single pellet press unit assembled with a hemisphere-end rod [22]. The moisture uptake of pellets was measured in a humidity chamber (GT-TH-S-150Z, China), which was set at 30 °C and 90% relative humidity. Prior to the moisture uptake test, the pellets were dried in a convection oven at 105 °C for 24 h. The weight of the sample was measured every 20 min at the first 4 h. The kinetic analysis of moisture uptake was represented using the ASABE S448.1 formulation for thin-layer drying as below [25]. Equilibrium moisture uptake and length expansion were measured after 48 h absorption.
2. Materials and methods
2.3. Physicochemical analysis
2.1. Materials
Ultimate analysis results were obtained by CHNOS Elemental Analyzer Vario EL III (Elementar Analysen systeme GmbH, Germany). Proximate analysis was carried out according to the Chinese Standard Practice for the Proximate Analysis of solid biofuels (GB/T287312012). The amounts of cellulose, hemicellulose and lignin were obtained by automated fiber extraction analyzer (Gerhardt fibretherm FT12, Germany) [26]. Protein was calculated by multiplying the concentration of organic nitrogen by 6.25 as reported by Shao et al. [27]. The higher heating value (HHV) was determined using an oxygen bomb calorimeter (SUNDY SDACM5000, China) from three replicates.
The dewatered SS and three kinds of biomass were collected as the raw material. The sludge was obtained from an urban sewage plant in Changsha City, China. Chinese fir sawdust, camphor sawdust and rice straw were prepared to represent softwood, hardwood, and grass, respectively (Table 1). After dried in an oven at 40 °C for 48 h, raw materials were pulverized and sieved in order to obtain particle size fraction below 0.45 mm. The water content of samples was adjusted by adding predetermined amount of deionized water. The mixed samples were kept in a sealed plastic container at 4 °C until pelletization.
−kt
ðM−Me Þ=ðM i −M e Þ ¼ e
ð1Þ
where M is the instantaneous moisture content, Me is the equilibrium moisture content, and Mi is the initial moisture content. The coefficient k is the absorption constant, and t is the exposure time (min).
3. Results and discussion 2.2. Pelletization and product analysis 3.1. Energy consumption A single pellet press unit was used for pelletization. The assembly consists of two parts: (1) a cylinder with 7.00 mm in inside diameter and 70.00 mm in height; (2) a piston with 6.90 mm in diameter and 90.00 mm in length. The detailed description of pelletization unit and the test procedure can be found in the previous publication of authors [22–24]. Approximately 1.00 g of mixed raw materials was filled in the hole of the cylinder to make a single pellet. Sludge and single biomass sample
Error bars in the figures represented standard deviations of seven replicates. As shown in Fig. 1a–c, co-pelletization of sludge and biomass could reduce the heterogeneous nature of biomass sample and the inconsistency of wood pellets in its mechanical properties. The compression energy consumptions for Chinese fir-SS pellet (CFSP), camphor-SS pellet (CSP) and rice straw-SS pellet (RSSP) increased with the increase of pressure at the die temperature of 90 °C. The
Table 1 Properties of raw sewage sludge and biomass sample. Analysis
Sludge Chinese fir Camphor Rice straw a
Ultimate analysis (wt.%)
Proximate analysis (wt%)
Chemical analysis (wt.%)
C
H
N
S
Moisture
Volatile
Fixed carbon
Ash
Protein
Hemicellulose
Cellulose
Lignin
36.11 49.08 48.18 45.04
5.25 5.96 6.09 5.05
6.50 0.63 0.70 1.06
1.03 0.00 0.00 0.00
5.42 7.63 6.67 6.56
57.22 74.49 79.02 64.59
6.09 16.68 12.53 13.51
31.27 1.20 1.78 15.34
35.5 –a –a –a
–a 12.05 20.82 24.60
–a 36.22 38.87 41.33
–a 27.61 24.40 9.22
Not measured.
HHV (MJ/kg) 15.59 18.38 18.40 14.64
H. Li et al. / Fuel Processing Technology 132 (2015) 55–61
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Fig. 2. Compression energy consumption of pellets as a function of sludge ratio.
inner surface, which limited the relative movement between particles. As a result, the compression energy consumption in the case of 25% water content (see Fig. 1c) augmented significantly. Furthermore, the die temperature also had a significant effect on compression energy consumption. As shown in Fig. 1b, the compression energy consumption decreased with the increase of temperature. The denaturation temperature of protein was above 57 °C, while the mean lignin softening temperature was higher than 75 °C [12,28]. In the present study, the lignin and protein contained in raw material couldn't be softened or denatured completely at the die temperature below 70 °C. Therefore, the sludge-biomass samples were still rigid as the high elasticity modulus of particles, resulting in the higher compression energy consumption of pellets produced below 70 °C. The activation of lignin, protein and carbohydrate with increasing temperature improved the plasticity of particles, which thus contributed to the decrease of energy consumption during the compression. For comparison, the compression energy consumptions for sludgebiomass pellets of different sludge ratio produced at 55 MPa, 90 °C and 15% water content were presented in Fig. 2. As shown in the figure, the compression energy consumption of biomass decreased distinctly after mixing with sludge, which indicated that the addition of sludge had a notable influence on the reduction of the compression energy consumption. Furthermore, the higher the sludge ratio was, the lower the compression energy consumption was. The energy consumptions
Fig. 1. Compression energy consumption of pellets as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
water content of mixed raw material also had a marked influence on compression energy consumption. The compression energy consumption was decreased when water content increased in the range from 5% to 20%. It could be due to the lubrication and adhesive action of water among the feedstock particles [2]. However, protein and carbohydrate were squeezed out of the particles with excessive moisture, which were denatured and gelated at the compression temperature. It consequently increased the viscosity between the pellet and the die
Fig. 3. Typical compression vs. displacement graph of three mixed feedstocks.
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of pellets with 50% sludge ratio were much lower than those of the pure biomass pellets. Therefore, the sludge ratio was controlled at 50% in this study according to a comprehensive consideration of the energy consumption and pellet properties described in a previous paper [24]. It was reported by Lam et al. [29] that the compression behavior of steam-treated materials could be divided into three regions. However, the pellets produced in this study did not exhibit the region II (see Fig. 3). It is similar to the properties of untreated sample in the work
of Lam et al. [29]. The Region I corresponded to the particle packing region where the large and small particles were rearranged. The Region III represented the second compression phase of the non-modified chemical components (e.g. cellulose) which would appear deformation and mechanical interlocking [29]. Attention should be paid to the lower value of compression energy consumption of RSSP. In Fig. 3, the slope of the axial compression force and displacement in region III increased with the increasing cellulose content of biomass (Table 1), resulting in
Fig. 4. Pellet density as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
Fig. 5. Volume expansion as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
H. Li et al. / Fuel Processing Technology 132 (2015) 55–61
the lower compression energy consumption. Meanwhile, the initial slope in region I of RSSP was lower. It was reported by Assor et al. [30] and Lam et al. [29] that hemicelluloses was the visco-elastic component which introduced viscous effect between the amorphous lignin and the crystalline cellulose in the fiber. Therefore, the higher amount of hemicelluloses in rice straw led to an increase in plasticity of the samples, which thus contributed to the decreased energy consumption during the compression.
Fig. 6. Maximum breaking forces of pellets as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
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3.2. Pellet density and volume expansion Fig. 4a and b indicated that higher density of pellet could be obtained above 55 MPa and 70 °C. However, further increment of pressure and temperature had a slight effect on the pellet density. It was speculated that the natural binding components such as starch, protein, lignin, and water soluble carbohydrate in the SS and biomass materials could be squeezed out of the particles under the pressure above 55 MPa.
Fig. 7. Moisture absorption of CFSP pellets as a function of pelletization parameters (a) pressure, (b) temperature, and (c) water content. All pellets were dried at 105 °C for 48 h before being placed in the humidity chamber set at 30 °C and 90% RH.
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Moreover, the protein denaturation and lignin softening would occur at the temperature above 70 °C [12,28], which contributed to the forming of solid bridges [2]. Therefore, sludge particles could be squeezed into the gaps and voids of biomass particles. The denatured protein and softened lignin could play roles as binders. However, the pellet density increased slightly with incremental pressure and temperature (above 55 MPa and 70 °C). This might be caused by the increasing elasticity of denatured protein, which reduced the effectiveness of lignin softening. Water acts as both a binder and lubricant in the pelleting process. Fig. 4c showed that the densities of pellets reached a maximum at the suitable water content, which was consistent with previous studies [17,31]. Water content out of this range would lead to lower density of pellets. The optimum water content for co-pelletization in this study was 10–15%. Comparing Fig. 4a–c, it was found that the pellet density was independent of the pressure above 55 MPa and temperature above 70 °C, but was highly affected by the water content increasing in the range of 5–25%. It was shown in Fig. 5 that the volume expansion was related to pellet density. Furthermore, the lower of volume expansion was obtained when the pellet density was higher. The lengths of pellets ranged from 18.24 mm to 24.32 mm which were depended on the applied pressure, die temperature, water content and types of biomass, etc. The diameter of pellets ranged from 7.41 mm to 7.57 mm. It was slightly larger than the diameter of the die hole (7.00 mm), indicating that pellets expanded after they were released from the die. As illustrated in Fig. 5, all the pellets had distinct expansions which indicated that elasticity generated among particles. The higher volume expansion (e.g. temperature at 30 °C, 50 °C case and 5% water content case) indicated that the compression deformation of the mixing particles was not entirely plastic. Moreover, the volume expansion was observed to be unrelated to the pressure from 28 MPa to 110 MPa, but highly dependent on temperature and water content (see Fig. 5). The volume expansion (mainly length expansion) would lead to pellet density decreasing, mechanical hardness declining and dust formation. In this study, die temperature of 90–130 °C and water content of 10–20% for co-pelletization were recommended to reduce the volume expansion during the two-week storage. During this temperature range, water (i.e. free water) was diffused into the middle lamella to activate the protein and lignin as a natural binder [32]. 3.3. Maximum breaking force Maximum breaking force is an important factor, which determines the dust formation and the mechanical strength of the pellets. As depicted in Fig. 6, the maximum breaking force was independent of the pressure above 41 MPa, but was highly influenced by the temperature and water content. The maximum breaking force increased with the increasing temperature and water content below 15%, which would decrease with higher water content. As described in Section 3.2, during compression, the lignin, protein, starch and carbohydrate in the raw materials might acted as a natural binder to bond between particles under the appropriate pressure, temperature and water content. Furthermore, attention should be paid to the remarkably higher maximum breaking force of RSSP in 10% water content runs. It might
be caused by the lower glass transition temperature of lignin in rice straw [33]. The glass transition temperature of the lignin would be decreased with increasing water content, resulting in an improvement of breaking force below 15% water content [2,32]. However, the water content above 20% would lead to a decrease of maximum breaking force due to the incompressibility of water, which might prevent complete flattening and the release of natural binders from the particles [24,31]. 3.4. Moisture adsorption Fig. 7 depicted the typical plots of the moisture adsorption of CFSP pellets versus time during moisture uptake in the humidity chamber. The moisture uptake ability was found to be independent of applied pressure, but was affected by the die temperature and water content. This was confirmed by the absorption rate (k) listed in Table 2, which was an absorption constant modeled through the ASABE S448.1 formulation. The model demonstrated a good fit with R2 values ranging from 0.962 to 0.987. The pellet made at 30 °C exhibited a higher absorption rate (0.01201 min−1) when compared with pellets produced at 150 °C with the absorption rate of 0.00779 min−1. It was mainly ascribed to the weak bond within particles and the corresponding voids and gaps structure. Equilibrium moisture contents (Me) of pellets at different process parameters were calculated after placing in the humidity chamber for 48 h. The equilibrium moisture contents of co-pellets of sludge and biomass were independent of pressure, temperature and water content, but were depended on the types of biomass. The Me for RSSP pellets approached 16%, whereas the value for Me dropped to about 13.2% and 13.6% for CFSP and CSP pellets, respectively. It may be attributed to the higher content of hydrophilic hemicelluloses [34] and lower content of hydrophobic lignin [35] in rice straw (see Table 1). The slightly higher Me obtained at temperature above 130 °C might be caused by the larger quantity of polar groups exposed during protein denaturation, which accompanied with the salt-induced protein aggregation to cause the stronger adsorption [36]. It was worth to note that RSSP always had better properties at the following condition: low pressure, low temperature or inappropriate moisture, when compared with CFSP and CSP. For instance, RSSP had the lower compression energy consumption, higher pellet density, lower volume expansion and lower length expansion compared with CFSP and CSP processed at the same condition, especially at lower pressure and lower temperature. It might be attributed a lower lignin glass transition temperature of rice straw [33]. 4. Conclusions The optimal quality of pellets could be produced at 55 MPa applied pressure and 90 °C die temperature. The water content was suggested to be 10–15%. The activities of water, lignin, protein and carbohydrate were the main reason for diversity of energy input and properties of pellets during pelletization, storage and water uptake. Those four components could act as natural binders with suitable ratio. Otherwise, they could form a viscosity between the pellet and the die inner surface to
Table 2 Absorption rate and the corresponding R-square of pellets at different conditions. Applied pressure (MPa)
Absorption rate, k (min−1)
R-square
Die temperature (°C)
Absorption rate, k (min−1)
R-square
Moisture content (%)
Absorption rate, k (min−1)
R-square
28 41 55 69 83 110
0.00931 0.00829 0.00792 0.00783 0.00826 0.00826
0.982 0.984 0.980 0.982 0.983 0.986
30 50 70 90 110 130 150
0.01201 0.01085 0.00932 0.00861 0.00866 0.00840 0.00779
0.973 0.977 0.984 0.981 0.983 0.982 0.978
5 10 15 20 25
0.01302 0.00923 0.00812 0.00856 0.00930
0.962 0.987 0.982 0.977 0.975
H. Li et al. / Fuel Processing Technology 132 (2015) 55–61
enhance the compression energy consumption. The increased elasticity of protein and carbohydrate offset the decreased plasticity of particles which was caused by the softening and melting of lignin at higher compression temperature. The content of hydrophilic hemicelluloses and hydrophobic lignin, along with the degree of protein denaturation could significantly affect the moisture adsorption of pellets. Acknowledgments The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21407046, No. 31470594, No. 51479072), the Natural Science Foundation of Hunan Province, China (No. 13JJ4118), and the Key Laboratory of Renewable Energy Chinese Academy of Sciences (No. y407k91001). References [1] W. Stelte, C. Clemons, J.K. Holm, A.R. Sanadi, J. Ahrenfeldt, L. Shang, U.B. Henriksen, Pelletizing properties of torrefied spruce, Biomass & Bioenergy 35 (2011) 4690–4698. [2] N. Kaliyan, R.V. Morey, Natural binders and solid bridge type binding mechanisms in briquettes and pellets made from corn stover and switchgrass, Bioresource Technology 101 (2010) 1082–1090. [3] M. Ståhl, J. Berghel, Energy efficient pilot-scale production of wood fuel pellets made from a raw material mix including sawdust and rapeseed cake, Biomass & Bioenergy 35 (2011) 4849–4854. [4] T. Brlek, L. Pezo, N. Voća, T. Krička, Đ. Vukmirović, R. Čolović, M. Bodroža-Solarov, Chemometric approach for assessing the quality of olive cake pellets, Fuel Processing Technology 116 (2013) 250–256. [5] T. Miranda, J.I. Arranz, I. Montero, S. Román, C.V. Rojas, S. Nogales, Characterization and combustion of olive pomace and forest residue pellets, Fuel Processing Technology 103 (2012) 91–96. [6] R. Razuan, K.N. Finney, Q. Chen, V.N. Sharifi, J. Swithenbank, Pelletised fuel production from palm kernel cake, Fuel Processing Technology 92 (2011) 609–615. [7] Y. Elmay, G. Trouvé, M. Jeguirim, R. Said, Energy recovery of date palm residues in a domestic pellet boiler, Fuel Processing Technology 112 (2013) 12–18. [8] S. Obidziński, Analysis of usability of potato pulp as solid fuel, Fuel Processing Technology 94 (2012) 67–74. [9] P. Zhao, S. Ge, K. Yoshikawa, An orthogonal experimental study on solid fuel production from sewage sludge by employing steam explosion, Applied Energy 112 (2013) 1213–1221. [10] M. Wzorek, Characterisation of the properties of alternative fuels containing sewage sludge, Fuel Processing Technology 104 (2012) 80–89. [11] J. Jiang, X. Du, S. Yang, Analysis of the combustion of sewage sludge-derived fuel by a thermogravimetric method in China, Waste Management 30 (2010) 1407–1413. [12] N. Kaliyan, R.V. Morey, Densification characteristics of corn stover and switchgrass, Transactions of the ASABE 52 (2009) 907–920. [13] R.L. Dospoy, C.E. Raleigh, C.D. Harrison, D.J. Akers, Pelletized fuel composition and method of manufacture, US Patent (1998) 5743924. [14] W.S. Chen, F.C. Chang, Y.H. Shen, M.S. Tsai, The characteristics of organic sludge/ sawdust derived fuel, Bioresource Technology 102 (2011) 5406–5410. [15] S.S. Hou, M.C. Chen, T.H. Lin, Experimental study of the combustion characteristics of densified refuse derived fuel (RDF-5) produced from oil sludge, Fuel 116 (2014) 201–207. [16] P. Gilbert, C. Ryu, V. Sharifi, J. Swithenbank, Effect of process parameters on pelletisation of herbaceous crops, Fuel 88 (2009) 1491–1497.
61
[17] C. Serrano, E. Monedero, M. Lapuerta, H. Portero, Effect of moisture content, particle size and pine addition on quality parameters of barley straw pellets, Fuel Processing Technology 92 (2011) 699–706. [18] M.T. Carone, A. Pantaleo, A. Pellerano, Influence of process parameters and biomass characteristics on the durability of pellets from the pruning residues of Olea europaea L, Biomass & Bioenergy 35 (2011) 402–410. [19] D. Bergström, S. Israelsson, M. Öhman, S.A. Dahlqvist, R. Gref, C. Boman, I. Wästerlund, Effects of raw material particle size distribution on the characteristics of Scots pine sawdust fuel pellets, Fuel Processing Technology 89 (2008) 1324–1329. [20] T. Filbakk, G. Skjevrak, O. Høibø, J. Dibdiakova, R. Jirjis, The influence of storage and drying methods for Scots pine raw material on mechanical pellet properties and production parameters, Fuel Processing Technology 92 (2011) 871–878. [21] T.A. Lestander, M. Finell, R. Samuelsson, M. Arshadi, M. Thyrel, Industrial scale biofuel pellet production from blends of unbarked softwood and hardwood stems—the effects of raw material composition and moisture content on pellet quality, Fuel Processing Technology 95 (2012) 73–77. [22] H. Li, X. Liu, R. Legros, X.T. Bi, C.J. Lim, S. Sokhansanj, Pelletization of torrefied sawdust and properties of torrefied pellets, Applied Energy 93 (2012) 680–685. [23] C. Wang, J. Peng, H. Li, X.T. Bi, R. Legros, C.J. Lim, S. Sokhansanj, Oxidative torrefaction of biomass residues and densification of torrefied sawdust to pellets, Bioresource Technology 127 (2013) 318–325. [24] L.B. Jiang, J. Liang, X.Z. Yuan, H. Li, C.Z. Li, Z.H. Xiao, H.J. Huang, H. Wang, G.M. Zeng, Co-pelletization of sewage sludge and biomass: the density and hardness of pellet, Bioresource Technology 166 (2014) 435–443. [25] P.S. Lam, S. Sokhansanj, X.T. Bi, C.J. Lim, S. Melin, Energy input and quality of pellets made from steam-exploded Douglas fir (Pseudotsuga menziesii), Energy & Fuels 25 (2011) 1521–1528. [26] M.T. Miranda, J.I. Arranz, S. Rojas, I. Montero, Energetic characterization of densified residues from Pyrenean oak forest, Fuel 88 (2009) 2106–2112. [27] L. Shao, T. Wang, T. Li, F. Lu, P. He, Comparison of sludge digestion under aerobic and anaerobic conditions with a focus on the degradation of proteins at mesophilic temperature, Bioresource Technology 140 (2013) 131–137. [28] D. Porter, F. Vollrath, Water mobility, denaturation and the glass transition in proteins, Biochimica et Biophysica Acta 1824 (2012) 785–791. [29] P.S. Lam, P.Y. Lam, S. Sokhansanj, X.T. Bi, C.J. Lim, Mechanical and compositional characteristics of steam-treated Douglas fir (Pseudotsuga menziesii L.) during pelletization, Biomass & Bioenergy 56 (2013) 116–126. [30] C. Assor, V. Placet, B. Chabbert, A. Habrant, C. Lapierre, B. Pollet, P. Perre, Concomitant changes in viscoelastic properties and amorphous polymers during the hydrothermal treatment of hardwood and softwood, Journal of Agricultural and Food Chemistry 57 (2009) 6830–6837. [31] N. Kaliyan, R.V. Morey, Factors affecting strength and durability of densified biomass products, Biomass & Bioenergy 33 (2009) 337–359. [32] P.Y. Lam, P.S. Lam, S. Sokhansanj, X.T. Bi, C.J. Lim, S. Melin, Effects of pelletization conditions on breaking strength and dimensional stability of Douglas fir pellet, Fuel 117 (2014) 1085–1092. [33] W. Stelte, C. Clemons, J.K. Holm, J. Ahrenfeldt, U.B. Henriksen, A.R. Sanadi, Thermal transitions of the amorphous polymers in wheat straw, Industrial Crops and Products 34 (2011) 1053–1056. [34] L.M. Zhang, T.Q. Yuan, F. Xu, R.C. Sun, Enhanced hydrophobicity and thermal stability of hemicelluloses by butyrylation in [BMIM]Cl ionic liquid, Industrial Crops and Products 45 (2013) 52–57. [35] S.H. Blanquet, D. Zheng, N.L. Ferreira, C. Lapierre, S. Baumberger, Effect of pretreatment and enzymatic hydrolysis of wheat straw on cell wall composition, hydrophobicity and cellulase adsorption, Bioresource Technology 102 (2011) 5938–5946. [36] G. Jovanovich, M.C. Puppo, S.A. Giner, M.C. Anon, Water uptake by dehydrated soy protein isolates: comparison of equilibrium vapour sorption and water imbibing methods, Journal of Food Engineering 56 (2003) 331–338.
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Co-pelletization of sewage sludge and biomass: The energy input and properties of pellets Hui Li a,b, Long-Bo Jiang a,c, Chang-Zhu Li a,⁎,1, Jie Liang c,⁎⁎,1, Xing-Zhong Yuan c, Zhi-Hua Xiao c, Zhi-Hong Xiao a, Hou Wang c a b c
Institute of Bio-energy, Hunan Academy of Forestry, Changsha 410004, PR China Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, PR China College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e
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Article history: Received 10 May 2014 Received in revised form 10 December 2014 Accepted 11 December 2014 Available online 10 January 2015 Keywords: Sewage sludge Biomass Co-pelletization Compression energy consumption Moisture adsorption
a b s t r a c t Presently, most pellets are made of lignocellulose biomass. However, the rocketing development of demands and fierce competition of feedstock between biomass industries have resulted in local biomass supply insufficiency and rising prices of the raw material. Therefore, other kinds of raw materials should be involved in the production of pellets to improve the stable supply of feedstock. In this study, the pelletization of biomass materials mixed with sewage sludge (SS) was investigated. The effects of process parameters on energy consumption and pellet properties were studied. It was shown that the compression energy consumption was affected by applied pressure, die temperature and water content. Both temperature and water content had an influence on the above pellet properties to a different extent, which was found to be independent of pressure. Optimal parameters (55 MPa for pressure, 90 °C for temperature and 10–15% for water content) were recommended for co-pelletization. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The utilization of forestry and agricultural biomass residues for sustainable heat and power production is an important part of future energy concepts [1]. The growing use of biomass boilers, instead of fossil-fuel boilers, has facilitated an expansive market of solid biofuels. Pellets are made of biomass, which can reduce the costs of handling, transportation and storage [2]. There are more than 442 pellet plants in the world (except mini workshops) with an annual yield of 14 million tons [3]. However, the rapid growth of wood pellet production and the competition of raw material have caused a marked decrease of the preferred raw materials for the production of pellets, such as cutter shavings and sawdust. In China, the price of sawdust (water content around 40%) has increased to higher than 400 RMB/ton, especially in some area in Hunan and Guangdong where the price has been higher than 600 RMB/ton. The increase of raw material price weakened market competitiveness of pellets. Several reports have focused on other raw materials or mixtures of raw materials which could be solutions [3–8]. Miranda et al. [5] studied
⁎ Corresponding author. Tel./fax: +86 731 85578702. ⁎⁎ Corresponding author. Tel./fax: +86 731 88821697. E-mail addresses: [email protected] (C.-Z. Li), [email protected] (J. Liang). 1 C.Z. Li and J. Liang contribute equally to this work.
http://dx.doi.org/10.1016/j.fuproc.2014.12.020 0378-3820/© 2014 Elsevier B.V. All rights reserved.
the properties of blends of pelletized residues from olive pomace and pyrenean oak. It was shown that the supplement of pyrenean oak residues to olive pomace samples can facilitate the effectiveness of pellet compression without a significant change of its thermal properties. Razuan et al. [6] reported the pelletization of palm kernel cake. The addition of palm kernel cake into sawdust can enhance the density and tensile strength of pellets at elevated temperature (80–100 °C) and pressure (64.38 MPa) [6]. Stahl and Berghel [3] investigated the co-pelletization of sawdust mixed with rapeseed cake. The energy consumption and pellet durability were decreased with rapeseed cake content in feedstock increased [3]. It is environment friendly to reuse non-wood biomass waste into pellets producing, which can also help pellet manufacturer hedge the risk of raw-material market. In the present study, the sewage sludge (SS), a residual waste of sewage plants, was applied to mix with biomass material for pelletization. SS is a large volume byproduct from the municipal wastewater treatment plants. The treatment and disposal of SS have been one of the most severe environmental problems in the world. In China, 20.76 million tons SS containing 80% water has been generated in 2010, which is expected to increase in the near future [9]. However, the present SS disposal procedures are unsustainable, due to the land limitations and secondary pollution caused by large amounts of leachate. For instance, EU directives have restricted the use of SS in agriculture and imposed a ban on the landfill [10]. The Chinese government has stipulated that only SS with a water content of less than 60% is permitted in landfill [11].
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Meanwhile, SS also contains a significant amount of organic compounds which are principally proteins and carbohydrates. The denaturation of protein and the gelatinization of carbohydrate in the sludge could serve as a natural binder which can improve the strength of fiber grid in pellet [12]. Dospoy et al. [13] firstly managed to produce sludge derived fuel from paper sludge, powder coal, and plastics and applied in the market. Other studies have been conducted, focusing on the physical properties and combustion characteristics of sludge refuse-derived fuel (RDF). Chen et al. [14] investigated the characteristics of a sludge RDF and the combustion behaviors. Wzorek [10] studied the physical and chemical properties and the calorific values of three types of sludge RDF which were applied to the cement clinker manufacturing process. Zhao et al. [9] developed an RDF technology, including the steam explosion pretreatment, mechanical dewatering, natural drying and pelleting, to produce solid fuel from SS. Hou et al. [15] estimated the heating value and investigated combustion characteristics of densified RDF produced from oil sludge. However, few studies have been conducted on the functions of process parameters for sludge pelleting. The control of pressure, die temperature, and water content have been considered as three important parameters to affect the pelletization process of biomass and properties of pellets [16–21]. Therefore, it is necessary to study the influence of process parameters on sludge derived pellet. In the present work, the effects of process parameters on energy consumption and product properties for co-pelletization of SS and biomass were carried out. The pellets produced from different biomass material (Chinese fir, camphor and rice straw) with the pressure (28–110 MPa), temperature (30–150 °C) and water content (5–25%) were examined in terms of energy consumption, pellet density, volume expansion, maximum breaking force, and moisture uptake, etc.
were weighed and mixed manually with a constant ratio of 50 wt.%. The percentage of the blends was calculated on a dry weight basis. A single factor variable method were applied to investigate the variables of pressure, temperature, and water content which were conducted at 90 °C with 15% water content, 55 MPa with 15% water content, and 55 MPa with 90 °C, respectively. Each experiment was repeated seven times. Forces and displacement curves in pelletization were recorded to calculate the energy consumption. The energy consumptions associated with compaction were obtained by integrating the curve. The mass, length, and diameter of each pellet were measured immediately after it was removed from the cylinder to calculate pellet density. The measure of mass, length and diameter was repeated after storing the pellets inside a sealed bottle at 4 °C for two weeks to calculate expansion ratio. The maximum breaking force of pellets was analyzed by the same single pellet press unit assembled with a hemisphere-end rod [22]. The moisture uptake of pellets was measured in a humidity chamber (GT-TH-S-150Z, China), which was set at 30 °C and 90% relative humidity. Prior to the moisture uptake test, the pellets were dried in a convection oven at 105 °C for 24 h. The weight of the sample was measured every 20 min at the first 4 h. The kinetic analysis of moisture uptake was represented using the ASABE S448.1 formulation for thin-layer drying as below [25]. Equilibrium moisture uptake and length expansion were measured after 48 h absorption.
2. Materials and methods
2.3. Physicochemical analysis
2.1. Materials
Ultimate analysis results were obtained by CHNOS Elemental Analyzer Vario EL III (Elementar Analysen systeme GmbH, Germany). Proximate analysis was carried out according to the Chinese Standard Practice for the Proximate Analysis of solid biofuels (GB/T287312012). The amounts of cellulose, hemicellulose and lignin were obtained by automated fiber extraction analyzer (Gerhardt fibretherm FT12, Germany) [26]. Protein was calculated by multiplying the concentration of organic nitrogen by 6.25 as reported by Shao et al. [27]. The higher heating value (HHV) was determined using an oxygen bomb calorimeter (SUNDY SDACM5000, China) from three replicates.
The dewatered SS and three kinds of biomass were collected as the raw material. The sludge was obtained from an urban sewage plant in Changsha City, China. Chinese fir sawdust, camphor sawdust and rice straw were prepared to represent softwood, hardwood, and grass, respectively (Table 1). After dried in an oven at 40 °C for 48 h, raw materials were pulverized and sieved in order to obtain particle size fraction below 0.45 mm. The water content of samples was adjusted by adding predetermined amount of deionized water. The mixed samples were kept in a sealed plastic container at 4 °C until pelletization.
−kt
ðM−Me Þ=ðM i −M e Þ ¼ e
ð1Þ
where M is the instantaneous moisture content, Me is the equilibrium moisture content, and Mi is the initial moisture content. The coefficient k is the absorption constant, and t is the exposure time (min).
3. Results and discussion 2.2. Pelletization and product analysis 3.1. Energy consumption A single pellet press unit was used for pelletization. The assembly consists of two parts: (1) a cylinder with 7.00 mm in inside diameter and 70.00 mm in height; (2) a piston with 6.90 mm in diameter and 90.00 mm in length. The detailed description of pelletization unit and the test procedure can be found in the previous publication of authors [22–24]. Approximately 1.00 g of mixed raw materials was filled in the hole of the cylinder to make a single pellet. Sludge and single biomass sample
Error bars in the figures represented standard deviations of seven replicates. As shown in Fig. 1a–c, co-pelletization of sludge and biomass could reduce the heterogeneous nature of biomass sample and the inconsistency of wood pellets in its mechanical properties. The compression energy consumptions for Chinese fir-SS pellet (CFSP), camphor-SS pellet (CSP) and rice straw-SS pellet (RSSP) increased with the increase of pressure at the die temperature of 90 °C. The
Table 1 Properties of raw sewage sludge and biomass sample. Analysis
Sludge Chinese fir Camphor Rice straw a
Ultimate analysis (wt.%)
Proximate analysis (wt%)
Chemical analysis (wt.%)
C
H
N
S
Moisture
Volatile
Fixed carbon
Ash
Protein
Hemicellulose
Cellulose
Lignin
36.11 49.08 48.18 45.04
5.25 5.96 6.09 5.05
6.50 0.63 0.70 1.06
1.03 0.00 0.00 0.00
5.42 7.63 6.67 6.56
57.22 74.49 79.02 64.59
6.09 16.68 12.53 13.51
31.27 1.20 1.78 15.34
35.5 –a –a –a
–a 12.05 20.82 24.60
–a 36.22 38.87 41.33
–a 27.61 24.40 9.22
Not measured.
HHV (MJ/kg) 15.59 18.38 18.40 14.64
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Fig. 2. Compression energy consumption of pellets as a function of sludge ratio.
inner surface, which limited the relative movement between particles. As a result, the compression energy consumption in the case of 25% water content (see Fig. 1c) augmented significantly. Furthermore, the die temperature also had a significant effect on compression energy consumption. As shown in Fig. 1b, the compression energy consumption decreased with the increase of temperature. The denaturation temperature of protein was above 57 °C, while the mean lignin softening temperature was higher than 75 °C [12,28]. In the present study, the lignin and protein contained in raw material couldn't be softened or denatured completely at the die temperature below 70 °C. Therefore, the sludge-biomass samples were still rigid as the high elasticity modulus of particles, resulting in the higher compression energy consumption of pellets produced below 70 °C. The activation of lignin, protein and carbohydrate with increasing temperature improved the plasticity of particles, which thus contributed to the decrease of energy consumption during the compression. For comparison, the compression energy consumptions for sludgebiomass pellets of different sludge ratio produced at 55 MPa, 90 °C and 15% water content were presented in Fig. 2. As shown in the figure, the compression energy consumption of biomass decreased distinctly after mixing with sludge, which indicated that the addition of sludge had a notable influence on the reduction of the compression energy consumption. Furthermore, the higher the sludge ratio was, the lower the compression energy consumption was. The energy consumptions
Fig. 1. Compression energy consumption of pellets as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
water content of mixed raw material also had a marked influence on compression energy consumption. The compression energy consumption was decreased when water content increased in the range from 5% to 20%. It could be due to the lubrication and adhesive action of water among the feedstock particles [2]. However, protein and carbohydrate were squeezed out of the particles with excessive moisture, which were denatured and gelated at the compression temperature. It consequently increased the viscosity between the pellet and the die
Fig. 3. Typical compression vs. displacement graph of three mixed feedstocks.
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of pellets with 50% sludge ratio were much lower than those of the pure biomass pellets. Therefore, the sludge ratio was controlled at 50% in this study according to a comprehensive consideration of the energy consumption and pellet properties described in a previous paper [24]. It was reported by Lam et al. [29] that the compression behavior of steam-treated materials could be divided into three regions. However, the pellets produced in this study did not exhibit the region II (see Fig. 3). It is similar to the properties of untreated sample in the work
of Lam et al. [29]. The Region I corresponded to the particle packing region where the large and small particles were rearranged. The Region III represented the second compression phase of the non-modified chemical components (e.g. cellulose) which would appear deformation and mechanical interlocking [29]. Attention should be paid to the lower value of compression energy consumption of RSSP. In Fig. 3, the slope of the axial compression force and displacement in region III increased with the increasing cellulose content of biomass (Table 1), resulting in
Fig. 4. Pellet density as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
Fig. 5. Volume expansion as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
H. Li et al. / Fuel Processing Technology 132 (2015) 55–61
the lower compression energy consumption. Meanwhile, the initial slope in region I of RSSP was lower. It was reported by Assor et al. [30] and Lam et al. [29] that hemicelluloses was the visco-elastic component which introduced viscous effect between the amorphous lignin and the crystalline cellulose in the fiber. Therefore, the higher amount of hemicelluloses in rice straw led to an increase in plasticity of the samples, which thus contributed to the decreased energy consumption during the compression.
Fig. 6. Maximum breaking forces of pellets as a function of pelletization parameters: (a) pressure, (b) temperature, and (c) water content.
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3.2. Pellet density and volume expansion Fig. 4a and b indicated that higher density of pellet could be obtained above 55 MPa and 70 °C. However, further increment of pressure and temperature had a slight effect on the pellet density. It was speculated that the natural binding components such as starch, protein, lignin, and water soluble carbohydrate in the SS and biomass materials could be squeezed out of the particles under the pressure above 55 MPa.
Fig. 7. Moisture absorption of CFSP pellets as a function of pelletization parameters (a) pressure, (b) temperature, and (c) water content. All pellets were dried at 105 °C for 48 h before being placed in the humidity chamber set at 30 °C and 90% RH.
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Moreover, the protein denaturation and lignin softening would occur at the temperature above 70 °C [12,28], which contributed to the forming of solid bridges [2]. Therefore, sludge particles could be squeezed into the gaps and voids of biomass particles. The denatured protein and softened lignin could play roles as binders. However, the pellet density increased slightly with incremental pressure and temperature (above 55 MPa and 70 °C). This might be caused by the increasing elasticity of denatured protein, which reduced the effectiveness of lignin softening. Water acts as both a binder and lubricant in the pelleting process. Fig. 4c showed that the densities of pellets reached a maximum at the suitable water content, which was consistent with previous studies [17,31]. Water content out of this range would lead to lower density of pellets. The optimum water content for co-pelletization in this study was 10–15%. Comparing Fig. 4a–c, it was found that the pellet density was independent of the pressure above 55 MPa and temperature above 70 °C, but was highly affected by the water content increasing in the range of 5–25%. It was shown in Fig. 5 that the volume expansion was related to pellet density. Furthermore, the lower of volume expansion was obtained when the pellet density was higher. The lengths of pellets ranged from 18.24 mm to 24.32 mm which were depended on the applied pressure, die temperature, water content and types of biomass, etc. The diameter of pellets ranged from 7.41 mm to 7.57 mm. It was slightly larger than the diameter of the die hole (7.00 mm), indicating that pellets expanded after they were released from the die. As illustrated in Fig. 5, all the pellets had distinct expansions which indicated that elasticity generated among particles. The higher volume expansion (e.g. temperature at 30 °C, 50 °C case and 5% water content case) indicated that the compression deformation of the mixing particles was not entirely plastic. Moreover, the volume expansion was observed to be unrelated to the pressure from 28 MPa to 110 MPa, but highly dependent on temperature and water content (see Fig. 5). The volume expansion (mainly length expansion) would lead to pellet density decreasing, mechanical hardness declining and dust formation. In this study, die temperature of 90–130 °C and water content of 10–20% for co-pelletization were recommended to reduce the volume expansion during the two-week storage. During this temperature range, water (i.e. free water) was diffused into the middle lamella to activate the protein and lignin as a natural binder [32]. 3.3. Maximum breaking force Maximum breaking force is an important factor, which determines the dust formation and the mechanical strength of the pellets. As depicted in Fig. 6, the maximum breaking force was independent of the pressure above 41 MPa, but was highly influenced by the temperature and water content. The maximum breaking force increased with the increasing temperature and water content below 15%, which would decrease with higher water content. As described in Section 3.2, during compression, the lignin, protein, starch and carbohydrate in the raw materials might acted as a natural binder to bond between particles under the appropriate pressure, temperature and water content. Furthermore, attention should be paid to the remarkably higher maximum breaking force of RSSP in 10% water content runs. It might
be caused by the lower glass transition temperature of lignin in rice straw [33]. The glass transition temperature of the lignin would be decreased with increasing water content, resulting in an improvement of breaking force below 15% water content [2,32]. However, the water content above 20% would lead to a decrease of maximum breaking force due to the incompressibility of water, which might prevent complete flattening and the release of natural binders from the particles [24,31]. 3.4. Moisture adsorption Fig. 7 depicted the typical plots of the moisture adsorption of CFSP pellets versus time during moisture uptake in the humidity chamber. The moisture uptake ability was found to be independent of applied pressure, but was affected by the die temperature and water content. This was confirmed by the absorption rate (k) listed in Table 2, which was an absorption constant modeled through the ASABE S448.1 formulation. The model demonstrated a good fit with R2 values ranging from 0.962 to 0.987. The pellet made at 30 °C exhibited a higher absorption rate (0.01201 min−1) when compared with pellets produced at 150 °C with the absorption rate of 0.00779 min−1. It was mainly ascribed to the weak bond within particles and the corresponding voids and gaps structure. Equilibrium moisture contents (Me) of pellets at different process parameters were calculated after placing in the humidity chamber for 48 h. The equilibrium moisture contents of co-pellets of sludge and biomass were independent of pressure, temperature and water content, but were depended on the types of biomass. The Me for RSSP pellets approached 16%, whereas the value for Me dropped to about 13.2% and 13.6% for CFSP and CSP pellets, respectively. It may be attributed to the higher content of hydrophilic hemicelluloses [34] and lower content of hydrophobic lignin [35] in rice straw (see Table 1). The slightly higher Me obtained at temperature above 130 °C might be caused by the larger quantity of polar groups exposed during protein denaturation, which accompanied with the salt-induced protein aggregation to cause the stronger adsorption [36]. It was worth to note that RSSP always had better properties at the following condition: low pressure, low temperature or inappropriate moisture, when compared with CFSP and CSP. For instance, RSSP had the lower compression energy consumption, higher pellet density, lower volume expansion and lower length expansion compared with CFSP and CSP processed at the same condition, especially at lower pressure and lower temperature. It might be attributed a lower lignin glass transition temperature of rice straw [33]. 4. Conclusions The optimal quality of pellets could be produced at 55 MPa applied pressure and 90 °C die temperature. The water content was suggested to be 10–15%. The activities of water, lignin, protein and carbohydrate were the main reason for diversity of energy input and properties of pellets during pelletization, storage and water uptake. Those four components could act as natural binders with suitable ratio. Otherwise, they could form a viscosity between the pellet and the die inner surface to
Table 2 Absorption rate and the corresponding R-square of pellets at different conditions. Applied pressure (MPa)
Absorption rate, k (min−1)
R-square
Die temperature (°C)
Absorption rate, k (min−1)
R-square
Moisture content (%)
Absorption rate, k (min−1)
R-square
28 41 55 69 83 110
0.00931 0.00829 0.00792 0.00783 0.00826 0.00826
0.982 0.984 0.980 0.982 0.983 0.986
30 50 70 90 110 130 150
0.01201 0.01085 0.00932 0.00861 0.00866 0.00840 0.00779
0.973 0.977 0.984 0.981 0.983 0.982 0.978
5 10 15 20 25
0.01302 0.00923 0.00812 0.00856 0.00930
0.962 0.987 0.982 0.977 0.975
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enhance the compression energy consumption. The increased elasticity of protein and carbohydrate offset the decreased plasticity of particles which was caused by the softening and melting of lignin at higher compression temperature. The content of hydrophilic hemicelluloses and hydrophobic lignin, along with the degree of protein denaturation could significantly affect the moisture adsorption of pellets. Acknowledgments The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21407046, No. 31470594, No. 51479072), the Natural Science Foundation of Hunan Province, China (No. 13JJ4118), and the Key Laboratory of Renewable Energy Chinese Academy of Sciences (No. y407k91001). References [1] W. Stelte, C. Clemons, J.K. Holm, A.R. Sanadi, J. Ahrenfeldt, L. Shang, U.B. Henriksen, Pelletizing properties of torrefied spruce, Biomass & Bioenergy 35 (2011) 4690–4698. [2] N. Kaliyan, R.V. Morey, Natural binders and solid bridge type binding mechanisms in briquettes and pellets made from corn stover and switchgrass, Bioresource Technology 101 (2010) 1082–1090. [3] M. Ståhl, J. Berghel, Energy efficient pilot-scale production of wood fuel pellets made from a raw material mix including sawdust and rapeseed cake, Biomass & Bioenergy 35 (2011) 4849–4854. [4] T. Brlek, L. Pezo, N. Voća, T. Krička, Đ. Vukmirović, R. Čolović, M. Bodroža-Solarov, Chemometric approach for assessing the quality of olive cake pellets, Fuel Processing Technology 116 (2013) 250–256. [5] T. Miranda, J.I. Arranz, I. Montero, S. Román, C.V. Rojas, S. Nogales, Characterization and combustion of olive pomace and forest residue pellets, Fuel Processing Technology 103 (2012) 91–96. [6] R. Razuan, K.N. Finney, Q. Chen, V.N. Sharifi, J. Swithenbank, Pelletised fuel production from palm kernel cake, Fuel Processing Technology 92 (2011) 609–615. [7] Y. Elmay, G. Trouvé, M. Jeguirim, R. Said, Energy recovery of date palm residues in a domestic pellet boiler, Fuel Processing Technology 112 (2013) 12–18. [8] S. Obidziński, Analysis of usability of potato pulp as solid fuel, Fuel Processing Technology 94 (2012) 67–74. [9] P. Zhao, S. Ge, K. Yoshikawa, An orthogonal experimental study on solid fuel production from sewage sludge by employing steam explosion, Applied Energy 112 (2013) 1213–1221. [10] M. Wzorek, Characterisation of the properties of alternative fuels containing sewage sludge, Fuel Processing Technology 104 (2012) 80–89. [11] J. Jiang, X. Du, S. Yang, Analysis of the combustion of sewage sludge-derived fuel by a thermogravimetric method in China, Waste Management 30 (2010) 1407–1413. [12] N. Kaliyan, R.V. Morey, Densification characteristics of corn stover and switchgrass, Transactions of the ASABE 52 (2009) 907–920. [13] R.L. Dospoy, C.E. Raleigh, C.D. Harrison, D.J. Akers, Pelletized fuel composition and method of manufacture, US Patent (1998) 5743924. [14] W.S. Chen, F.C. Chang, Y.H. Shen, M.S. Tsai, The characteristics of organic sludge/ sawdust derived fuel, Bioresource Technology 102 (2011) 5406–5410. [15] S.S. Hou, M.C. Chen, T.H. Lin, Experimental study of the combustion characteristics of densified refuse derived fuel (RDF-5) produced from oil sludge, Fuel 116 (2014) 201–207. [16] P. Gilbert, C. Ryu, V. Sharifi, J. Swithenbank, Effect of process parameters on pelletisation of herbaceous crops, Fuel 88 (2009) 1491–1497.
61
[17] C. Serrano, E. Monedero, M. Lapuerta, H. Portero, Effect of moisture content, particle size and pine addition on quality parameters of barley straw pellets, Fuel Processing Technology 92 (2011) 699–706. [18] M.T. Carone, A. Pantaleo, A. Pellerano, Influence of process parameters and biomass characteristics on the durability of pellets from the pruning residues of Olea europaea L, Biomass & Bioenergy 35 (2011) 402–410. [19] D. Bergström, S. Israelsson, M. Öhman, S.A. Dahlqvist, R. Gref, C. Boman, I. Wästerlund, Effects of raw material particle size distribution on the characteristics of Scots pine sawdust fuel pellets, Fuel Processing Technology 89 (2008) 1324–1329. [20] T. Filbakk, G. Skjevrak, O. Høibø, J. Dibdiakova, R. Jirjis, The influence of storage and drying methods for Scots pine raw material on mechanical pellet properties and production parameters, Fuel Processing Technology 92 (2011) 871–878. [21] T.A. Lestander, M. Finell, R. Samuelsson, M. Arshadi, M. Thyrel, Industrial scale biofuel pellet production from blends of unbarked softwood and hardwood stems—the effects of raw material composition and moisture content on pellet quality, Fuel Processing Technology 95 (2012) 73–77. [22] H. Li, X. Liu, R. Legros, X.T. Bi, C.J. Lim, S. Sokhansanj, Pelletization of torrefied sawdust and properties of torrefied pellets, Applied Energy 93 (2012) 680–685. [23] C. Wang, J. Peng, H. Li, X.T. Bi, R. Legros, C.J. Lim, S. Sokhansanj, Oxidative torrefaction of biomass residues and densification of torrefied sawdust to pellets, Bioresource Technology 127 (2013) 318–325. [24] L.B. Jiang, J. Liang, X.Z. Yuan, H. Li, C.Z. Li, Z.H. Xiao, H.J. Huang, H. Wang, G.M. Zeng, Co-pelletization of sewage sludge and biomass: the density and hardness of pellet, Bioresource Technology 166 (2014) 435–443. [25] P.S. Lam, S. Sokhansanj, X.T. Bi, C.J. Lim, S. Melin, Energy input and quality of pellets made from steam-exploded Douglas fir (Pseudotsuga menziesii), Energy & Fuels 25 (2011) 1521–1528. [26] M.T. Miranda, J.I. Arranz, S. Rojas, I. Montero, Energetic characterization of densified residues from Pyrenean oak forest, Fuel 88 (2009) 2106–2112. [27] L. Shao, T. Wang, T. Li, F. Lu, P. He, Comparison of sludge digestion under aerobic and anaerobic conditions with a focus on the degradation of proteins at mesophilic temperature, Bioresource Technology 140 (2013) 131–137. [28] D. Porter, F. Vollrath, Water mobility, denaturation and the glass transition in proteins, Biochimica et Biophysica Acta 1824 (2012) 785–791. [29] P.S. Lam, P.Y. Lam, S. Sokhansanj, X.T. Bi, C.J. Lim, Mechanical and compositional characteristics of steam-treated Douglas fir (Pseudotsuga menziesii L.) during pelletization, Biomass & Bioenergy 56 (2013) 116–126. [30] C. Assor, V. Placet, B. Chabbert, A. Habrant, C. Lapierre, B. Pollet, P. Perre, Concomitant changes in viscoelastic properties and amorphous polymers during the hydrothermal treatment of hardwood and softwood, Journal of Agricultural and Food Chemistry 57 (2009) 6830–6837. [31] N. Kaliyan, R.V. Morey, Factors affecting strength and durability of densified biomass products, Biomass & Bioenergy 33 (2009) 337–359. [32] P.Y. Lam, P.S. Lam, S. Sokhansanj, X.T. Bi, C.J. Lim, S. Melin, Effects of pelletization conditions on breaking strength and dimensional stability of Douglas fir pellet, Fuel 117 (2014) 1085–1092. [33] W. Stelte, C. Clemons, J.K. Holm, J. Ahrenfeldt, U.B. Henriksen, A.R. Sanadi, Thermal transitions of the amorphous polymers in wheat straw, Industrial Crops and Products 34 (2011) 1053–1056. [34] L.M. Zhang, T.Q. Yuan, F. Xu, R.C. Sun, Enhanced hydrophobicity and thermal stability of hemicelluloses by butyrylation in [BMIM]Cl ionic liquid, Industrial Crops and Products 45 (2013) 52–57. [35] S.H. Blanquet, D. Zheng, N.L. Ferreira, C. Lapierre, S. Baumberger, Effect of pretreatment and enzymatic hydrolysis of wheat straw on cell wall composition, hydrophobicity and cellulase adsorption, Bioresource Technology 102 (2011) 5938–5946. [36] G. Jovanovich, M.C. Puppo, S.A. Giner, M.C. Anon, Water uptake by dehydrated soy protein isolates: comparison of equilibrium vapour sorption and water imbibing methods, Journal of Food Engineering 56 (2003) 331–338.