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A physicochemical method for increasing methane production from rice straw: Extrusion combined with alkali pretreatment
Applied Energy 160 (2015) 39–48
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A physicochemical method for increasing methane production from rice straw: Extrusion combined with alkali pretreatment Yalei Zhang ⇑, Xiaohua Chen, Yu Gu, Xuefei Zhou State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The first report of using extrusion
combined with NaOH pretreatment for methane production from LB. Pretreatment with an ALR of 3.0% at 35 °C for 48 h increased methane production from rice straw by 54%. The ER efficiency improved from 38.9% to 59.9% using extrusion combined with NaOH pretreatment. The changes in the physicochemical characterization of rice straw were investigated.
a r t i c l e
i n f o
Article history: Received 7 January 2015 Received in revised form 31 August 2015 Accepted 3 September 2015
Keywords: Extrusion combined with alkali pretreatment Lignocellulosic biomass Rice straw Physicochemical pretreatment Methane
a b s t r a c t Pretreatment is a crucial processing step in the conversion of lignocellulosic biomass (LB) into methane by anaerobic digestion. A physicochemical LB pretreatment method, i.e., using an extruder to reduce the biomass size prior to sodium hydroxide (NaOH) pretreatment, was reported. The optimal condition for economic feasibility and pretreatment efficiency was an alkaline loading rate of 3.0% at 35 °C for 48 h. Under this condition, the methane production from the rice straw that was processed by extrusion combined with NaOH pretreatment was 54.0% higher than that of a control sample. The energy recovery (ER) efficiency improved from 38.9% to 59.9% using the combination pretreatment. The mechanisms that caused the significant improvement in the methane production and ER efficiency in the extrusion–NaOH pretreatment were investigated. The pretreatment changed the physical properties (water-holding capacity, specific porosity, specific surface area and crystallinity index), the chemical composition (lignin, benzene–ethanol extractives and hot-water extractives) and the chemical structure, which increased degradation of holocelluloses and other difficulty biodegradable compounds. Ó 2015 Elsevier Ltd. All rights reserved.
Abbreviations: AD, anaerobic digestion; ADF, acid detergent fiber; ADL, acid detergent lignin; AIA, acid insoluble ash; ALR, alkaline loading rate; BEE, benzene–ethanol extractives; BMP, biological methane potential; C/N, carbon-to-nitrogen ratio; CrI, crystallinity index; ER, energy recovery; FTIR, Fourier transform infrared; HHV, higher heating value; HWE, hot-water extractives; LB, lignocellulosic biomass; LHV, lower heat value; NaOH, sodium hydroxide; NDF, neutral detergent fiber; SEM, scanning electron microscope; SLR, solid-to-liquid ratio; SP, specific porosity; SPSS, statistic package for social science; SSA, specific surface area; STP, standard temperature and pressure; TS, total solids; VS, violate solids; WHC, water-holding capacity; XRD, X-ray diffraction. ⇑ Corresponding author. Tel.: +86 21 65985811; fax: +86 21 65986960. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.apenergy.2015.09.011 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
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Y. Zhang et al. / Applied Energy 160 (2015) 39–48
1. Introduction In 2012, the world energy consumption was 12476.6 Mtoe, of which petroleum, coal and natural gas consumption were 4130.5, 3730.1 and 2987.1 Mtoe, respectively. Fossil fuel consumption accounted for 86.9% of the world energy consumption, whereas renewable energy accounted for a mere 1.9% [1]. The high demand for energy, the unstable and uncertain availability of fossil fuels, and concern over global climate warming have produced an increasing urgency for the development of new, sustainable and renewable energy that can displace traditional fossil energy [2]. Lignocellulosic biomass (LB), with a worldwide annual production of 200 billion tons, has become a primary candidate for biofuel production [3,4]. The development of new technologies for renewable energy production from LB, especially agricultural residues, is very promising for the following reasons: (1) LB is abundant and renewable; (2) the non-edible parts, i.e., the stalks and leaves, of agricultural residues are used for fuel production, which does not compete with food production; and (3) LB use affects both energy consumption and the environment because renewable energy is recovered from agricultural residues while treating waste biomass [4,5]. Anaerobic digestion (AD) offers the advantages of a low-cost, mature, and stable technology and in particular, produces a high energy recovery (ER) from LB [6,7]. AD produces a methane-rich biogas that can provide heating, for power generation or as a vehicle fuel to replace fossil fuels [8]. However, LB has a complex structure that results in poor hydrolysis and biodegradability during the AD process [3]. The primary chemical composition of LB is cellulose, hemicelluloses and lignin. Cellulose and hemicelluloses can be converted into methane, whereas lignin cannot be degraded by anaerobic microbes [9]. Moreover, lignin is hydrophobic and resistant to microbial attack, which limits the accessibility of holocelluloses (cellulose and hemicelluloses) to anaerobic microbes, which lowers the substrate availability and methane production [4]. Pretreatment is a crucial processing step in which anaerobic microbes convert LB into methane. The physical properties, chemical composition and chemical structure of LB can be altered using several pretreatment methods that increase the accessibility of holocelluloses to enzymes, thereby enhancing the digestibility of LB and increasing methane production [3,10]. Alkaline pretreatment is one of the current leading pretreatment methods and offers several benefits such as the solubilization of lignin and hemicelluloses, the destruction of the ester bond of lignin–carbohydrate complexes and a decrease in the cellulose crystallinity; the residual alkali also provides alkalinity for the subsequent AD process [6,7,11]. A literature review shows that sodium hydroxide (NaOH) is a common alkali that is widely used in alkaline pretreatment [12,13]. Different LBs that are pretreated with NaOH for methane production are summarized in Table 1. The primary influence factors for the NaOH pretreatment are the temperature, the time, the solid-to-liquid ratio (SLR), and the alkaline loading rate (ALR). Most studies have focused on these operating factors over a wide range of temperatures (0–200 °C), times (10 min to 21 d), SLRs (1:0.8–1:19) and ALRs (1–152%) [14–19]. In general, pretreatment increases methane production (from 0% to 174.2%) from LB compared with untreated samples [13,20–25]. Biomass size reduction always occurs prior to NaOH pretreatment and affects the pretreatment. The extruder, which is more efficient than other physical pretreatment methods, has recently been used to enhance methane production from LB. The methane production increased significantly by 18–70% using extrusion as a pretreatment to test five agricultural residues [26]. A similar result was reported of an increase in methane production 72.2% by extrusion pretreated rice straw over that of the untreated rice straw [9].
However, to the best of our knowledge, there have been no previous reports of a pretreatment of extrusion combined with NaOH for improving methane production from LB. Rice straw is the major component of agricultural residues. The annual yield of rice straw in the world is approximately 731 million tons [5]. Rice straw has a high silica content and is therefore unsuitable for use in animal feeding, pulping and papermaking [27]. Rice straw has a good potential for methane production. In this study, the effect of extrusion combined NaOH pretreatment on rice straw methane production was studied and the effect of other pretreatments such as milling and milling combined NaOH pretreatment also has been analyzed for comparison. The contributions of cellulose, hemicelluloses and other components to methane production from rice straw using different pretreatment methods were investigated. The changes in the physical properties, chemical composition and chemical structure of rice straw from pretreatment were also examined. 2. Materials 2.1. Feedstock and inoculum Rice straw was obtained from a rice field in Yancheng city, Jiangsu province, China. Table 2 presents the proximate, ultimate and compositional properties and the heat value of the rice straw. The rice straw was dried at room temperature (25 °C) to a moisture content below 10%. The resultant rice straw was stored in vacuum bags for later pretreatment. The inoculum used in this study was anaerobic digested sludge that was taken from an anaerobic digester that was fed with dairy manure in Chongming Island, Shanghai, China. The anaerobic digester was a 300-m3 continuously stirred digester that had been stably operated at 35 °C for more than 3 years. 2.2. Extrusion combined with alkali pretreatment The procedure for the extrusion combined with alkali pretreatment is presented in Fig. 1. In brief, a twin-screw extruder (JXM80, Jinwor Machinery Co., Ltd, Nanjing, China) and NaOH were used as the biomass-size-reduction machine and the alkaline reagent, respectively. The dried rice straw was pretreated with the extruder and then passed through a 0.45-mm sieve. The resulting rice straw was pretreated with a NaOH solution. The pretreatment parameters were as follows: a treatment temperature of 35 °C, a treatment time of 3–120 h, a SLR of 1:6 and an ALR of 1.5–6.0%. The variation in the pH and the chemical composition during the alkali pretreatment were analyzed using the following procedure: eight 50-mL plastic centrifuge tubes were prepared with different ALRs (1.5%, 3.0%, 4.5% and 6.0%) for the alkali pretreatment, and the reaction was terminated after 3, 6, 12, 24, 48, 72, 96 and 120 h for sampling and subsequent analysis. 2.3. Methane production from rice straw by AD The biological methane potential (BMP) of untreated and pretreated rice straw was evaluated using 500-mL batch glass digesters, which each had a 400-mL working volume and a 100-mL head space. First, 10.0 g of volatile solids (VS) from a rice straw sample were weighed and added to each digester. Second, the required amount of sludge was fed into each digester with a substrate-to-inoculum ratio of 1:1 based on the VS. Urea was used to adjust the carbon-to-nitrogen ratio (C/N) of the mixture to 25. Third, distilled water was added to each digester to reach the working volume. The desired pH range of 6.8–7.2 range for AD
Table 1 Summaries of the biomass size reduction machine and NaOH pretreatment on methane production from different LB. LB
Main compositions
NaOH pretreatment parameters
HCEL (% TS)
Lignin (% TS)
Machine
Particle size (mm)
Temperature (°C)
Time
SLR (w/ w)
ALR (%)
SMP (mL/g)
Increasea (%)
SMPUS (mL/ g)
Rice straw
30.0
29.8
6.5
Extruder
<0.45
35
48 h
1:6
3
288
54
187
Rice straw
38.9
24.0
5.6
Force mill
<1
33.4 49.1
28.2 34.1
7.4 6.5
Hammer mill Kitchen blender
5–10 1
5d 10 min 21 d 1h
1:9
Rice straw Wheat straw
35.1
25.6
7.5
Force mill
<1
Corn stover
37.5
30.0
8.4
Hammer mill
5–10
5d 10 min 3d
1:9 1:9 1:9
3 5 6 1 10 4 5 2
Corn stover Corn stover Corn stover
37.5 40.7 NR
30.0 22.5 NR
8.4 21.7 23.55
5–10 <5 1
3d 24 h 3h
1:7.3 1:1 1:10
2 1; 2.5; 5 20d; 60d
24 122 44 14; 48; 23 43; 67 111.6 20 56; 75 67; 56 73.4 0; 3; 37 10.7; 40.6d
Corn stover Ensiled sorghum
42.3 47.5
29.8 27.4
13.4 7.0
Hammer mill NR Thomas Wiley mill Kneading machine Cutting mill
74.1 132.7 520b 232; 302; 252 291; 341 165.9 94.1 207.9; 233.0 221.6; 207.2 211 267; 276; 372b 276; 350
59.8 59.8 360b 204
Wheat straw
37 200 20 ± 2 40; 100; 160 40; 100 37 200 10; 20 30; 50 20 ± 2 20 ± 0.5 20; 35
Ensiled sorghum Sunflower stalks Sunflower stalks Switchgrass Grass silage
48.9
35.1
4.1
Kitchen blender
1
3d 24 h 24 h 12 h 1h
1:8 NR NR NR –c
34 23.1 NR 34.3
20.8 10.6 NR 29.6
29.7 29.9 NR 8.6
Cutting mill Cutting mill Blender Blender
2–3 2–3 <78 10
20 40 55 55 40; 100; 160 40; 100 30; 55; 80 55 100 100
24 h 24 6h 12 h
NR –e 1:11.7 1:4
Asparagus stem Wheat plant
34.6 43
21.2 15
13.2 18.1
Hammer mill NR
0.28–0.45 <1
5d 1h
1:10 1:19
Birch Spruce OPEFB PPR
41.0 43.0 32.4 47.7
27.9 20.8 8.7 25.6
29.68 28.83 24.6 23.6
Ball mill Ball mill NR Hammer mill
<0.8 <0.8 <0.42 6–12
35 25; 50 75; 100 100 5 100 20
6 4; 10 4; 10 4; 10 1 10 4 4 5.5 1; 2.5 5; 7.5 6 152d
2h 2h 1h 4d
–f –f 1:19 1:7.3
–f –f 152d 3; 5; 7
15; 48 4; 8 3; 12 10; 19 12; 11; 0 29; 32 17; 35; 25 36 13.3 10; 23 38; 39 38.4 47; 41 54; 5.3 84 63 100 102; 114; 94
Pine wood
38.2
24.1
34.4
NR
<1
0 100
60 min 10 min
1:19
152d
287.0; 272.6 287; 311 292; 298 293; 316 300; 299; 261 346; 356 225; 259; 240 262 332 359.5; 401.8 449.5; 452.5 242.3 386; 369 404; 276 460 50 404 256.6; 271.9; 246.8 178.2 142.1d
0.25–1; 5–20 1
1:0.8 –c
Methane yield
Reference
118.6 174.2d
This study [33] [10] [19]
78.4
[14]
132.7
[18]
122.7 272b 249
[7] [11] [17]
249; 184 266
[16] [12]
268
[19]
192 193 293 326.8
[22] [21] [15] [8]
175.0 262
[6] [24]
250 30 202 127.2
[20]
65
[13]
Y. Zhang et al. / Applied Energy 160 (2015) 39–48
Biomass size reduction
CEL (% TS)
[23] [25]
ALR: alkaline loading rate, g NaOH/100 g TS; CEL: cellulose; HCEL: hemicelluloses; NR: not reported; OPEFB: oil palm empty fruit bunches; PPR: poplar processing residues; SLR: solid-to-liquid ratio, based on TS of substrate; SMP: specific methane production, based on VS; SMPUS: specific methane production of untreated sample. a Compared to untreated sample. b Biogas production, mL/g VS. c Total solid content of 160 g TS/L. Added water weight was not reported, thus the SLR could not be calculated. d Values calculated from the reference. e Total solid content of 35 g TS/L. Added water weight was not reported, thus the SLR could not be calculated. f Milled birch and spruce were pretreated with 7% w/w NaOH solution. Both the substrate weight and the solution volume were not reported, thus the SLR and ALR could not be calculated.
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Y. Zhang et al. / Applied Energy 160 (2015) 39–48
Table 2 Proximate, ultimate and compositional properties and heat value of rice straw used in this study. Valuesa
Properties parameter
Unit
Proximate properties
VS Ash
% TS % TS
85.7 ± 0.4 14.3 ± 0.4
Ultimate properties
Carbon (C) Nitrogen (N) C/N
% TS % TS
41.7 ± 0.1 0.6 ± 0.0 69.5 ± 0.2
Compositional propertiesb
Celluloseb (ADF–ADL) Hemicellulosesb (NDF–ADF) Ligninb (ADL–AIA)
% TS % TS
30.0 ± 1.1 29.8 ± 0.9
Heat value
Lower calorific value (LCV)
MJ/kg TS
% TS
6.5 ± 0.4 14.79 ± 0.1
a
The values are a mean of triplicate measurements. Cellulose was determined as the difference between acid detergent fiber (ADF) and acid detergent lignin (ADL), hemicelluloses as the difference between neutral detergent fiber (NDF) and acid detergent fiber (ADF), and lignin as the difference between acid detergent lignin (ADL) and acid insoluble ash (AIA).
bags was determined using a 100-mL glass syringe. The biogas volume was then converted to the volume under standard temperature and pressure (STP) conditions. The biogas composition was analyzed using a gas chromatograph (GC-14B, Shimadzu, Japan). 2.4.2. Physical properties of untreated and pretreated samples The heat value was measured using a bomb calorimeter (XRY-1A, Mechanical and & Electrical Technology Co., Ltd., Shanghai, China). The WHC and the specific porosity (SP) were determined as reported previously [28]. A Brunauer–Emmett–Teller surface area analyzer (3H-2000BET-A, Beishide Instrument S&T Co., Ltd, Beijing, China) was used to measure the specific surface area (SSA). The overall crystallinity of samples was characterized using a XRD instrument (D8 Advance, Bruker, Germany) with Cu Ka radiation (k = 1.54 Å) at 40 kV and a tube current of 40 mA.
b
was obtained by measuring and adjusting the pH of each bottle was measured and adjusted to 7.0 ± 0.1 by adding 2 M HCl or 2 M NaOH. Finally, the digesters were flushed with nitrogen gas for 1 min to remove oxygen, tightly sealed with rubber stoppers, and then incubated in a work-in incubator at 35 °C. A blank control was conducted under the same conditions to remove endogenous methane production from the sludge.
2.4. Analytical methods 2.4.1. Biogas analysis The produced biogas was collected through a vapor trap in an aluminized polyethylene gas sampling bag (1 L, Shanghai Eler Co., Ltd., Shanghai, China). The volume of biogas in the gas sampling
2.4.3. Chemical compositions of untreated and pretreated samples An elemental analysis was performed using an elemental analyzer (Vario EL III, Elementar, Germany). The total solids (TS), VS and ash were determined as reported previously [9]. A PHS-3E pH meter (Shanghai Lei Instrument Factory, Shanghai, China) was used to measure the pH. The chemical compositions of cellulose, hemicelluloses, lignin, hot-water extractives (HWE) and benzene–ethanol extractives (BEE) were determined as reported previously [29,30]. 2.4.4. Changes in chemical structure The changes in the functional groups were investigated using Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra were obtained using a FTIR spectrophotometer (Nicolet 5700, Thermo Electron Corporation, USA). The cleaned and dried samples were milled into powders and dispersed in a matrix of potassium bromide (KBr), followed by compression at approximately 1 MPa to form transparent pellets. The pellets were scanned approximately 30 times from 4000 to 500 cm1 at a resolution of 4 cm1.
Elemental analysis
Chemical structure
FTIR
Raw material Untreated rice straw Pretreated with a twin-screw extruder
Specific surface area
Extrusion-pretreated rice straw power BET
Rice straw pretreated with extrusion combined with NaOH
Specific porosity Biomass crystallinity
Soaked in NaOH solution
XRD
Anaerobic digestion at 35 ºC
Bomb calorimeter Oven dried Calcination
CHNO LHV Moisture Ash
Digested with neutral detergent
Neutral detergent fiber Digested with acid detergent
Acid detergent fiber Hydrolyzed with 72% sulfuric acid
Acid detergent lignin Cellulose accessibility
WHC
Calcination
Anaerobic Biogas digested residues
Acid insoluble ash
Fig. 1. Schematic illustration of the experimental procedure in this study.
Y. Zhang et al. / Applied Energy 160 (2015) 39–48
2.5. Statistical analysis The software Statistical Package for Social Science (SPSS) 17.0 was used to calculate the standard deviations and significant differences. 3. Results and discussion 3.1. Changes in cellulose, hemicelluloses and lignin contents and pH values during extrusion combined with alkali pretreatment Apart from the biomass size reduction (Table 1), the primary operating parameters for the alkali pretreatment were the treatment temperature, the treatment time, the SLR and the ALR [18]. Higher SLRs and ALRs would be needed when using large amounts of water and NaOH reagent, i.e., the NaOH reagent would require recycling or disposal, which could pollute the environment [31]. A previous study showed that the true alkali consumption for wheat straw was 4.72 g NaOH/100 g TS in 24 h, and the maximum alkali consumption was 5.5 g NaOH/100 g TS over a pretreatment period of 30 days [31]. Another study in the literature reported that a 6% ALR was the optimal dose for four NaOH doses (4%, 6%, 8% and 10%) when NaOH was used to treat corn stover to improve biodegradability and biogas production [32]. Thus, a low SLR (1:6) and ALR (1.5–6.0%) were used in this study to avoid producing a waste chemical solution and to reduce the treatment cost. The pH was analyzed as an indirect measure of the NaOH consumption during the pretreatment process. During the treatment time of 3–12 h, the pH was almost constant for the four ALRs; however, the pH decreased dramatically for the ALRs of 1.5%, 3.0%, 4.5% and 6.0% at 24, 48, 72 and 72 h, respectively (Fig. 2). This result showed that a higher ALR required a longer time for NaOH consumption and that the treatment time for the four ALR values did not exceed 72 h. Table 1 shows that the treatment time generally decreased as the treatment temperature increased. A shorter treatment time enhances pretreatment efficiency and economic feasibility. However, increasing the energy consumption and the complexity of the operation at a high pretreatment temperature would restrict the practical application of the pretreatment for large-scale anaerobic digesters [8]. Thus, a relatively low temperature (35 °C) was used in this study. In addition, mesophilic (35 °C) AD is dominant in large-scale digesters; therefore, using the waste heat from the digester effluent can save energy because an additional energy input is not required, thus increasing the ER of an existing digester. Fig. 2 shows the cellulose, hemicelluloses, and lignin contents (based on the VS of the original material) for the extrusion–alkali pretreated rice straw at different operating conditions. The contents of cellulose were almost constant relative to the raw rice straw for all of the pretreatments. The hemicelluloses content decreased more than the cellulose content. Hemicelluloses have a lower molecular weight than cellulose and heterogeneous structures and are therefore easily hydrolysable. The solubilization of hemicelluloses may also be explained by the disruption and breaking of hydrogen bonds by an alkaline solution [3]. In contrast, the decrease in the lignin content was much higher than that of the cellulose and the hemicelluloses contents. The lignin removal rate increased from 8.0% to 39.5% as the ALR was varied from 1.5% to 6.0% after 120 h of pretreatment. Both the treatment time and the ALR significantly affected lignin solubilization. For treatment times below 12 h, no effects on lignin solubilization were observed, regardless of the ALR. When the treatment time exceeded 48 h for ALR values of 3.0–6.0%, the lignin removal rate did not increase further. The lignin removal rate was in accordance with the pH change, which may indicate alkali consumption. The lignin
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removal was attributed to the breakage of alkali-labile linkages between monomers or the saponification of the ester bonds that crosslink xylan hemicelluloses and lignin [10,31]. 3.2. Anaerobic digestion There are typically four steps involved in the AD of a LB, such as rice straw, for methane production: the solubilization of rice straw organic matter, the hydrolysis of solubilized rice straw organic matter, the acidification of the hydrolyzed products, and methane production. Cellulose, hemicelluloses and lignin associate with each other: cellulose and hemicelluloses, in particular, are firmly packed by lignin, which provides a primary protective barrier for holocelluloses against anaerobic bacterial attack [4,10]. Therefore, the digestibility of LB is closely linked to the removal/solubilization of lignin. As discussed in Section 3.1, the chemical composition of rice straw, especially the lignin content, changed after pretreatment. To maintain its elemental compositions and heat value (based on the VS), rice straw was directly digested for AD without any rinsing or detoxification after pretreatment [6,11,32]. The effects of pretreatment on methane production and ER from rice straw by AD were investigated. 3.2.1. Effects of extrusion combined with alkali pretreatment on methane production from rice straw Fig. 3 presents the effects of the treatment time and the ALR on methane production from rice straw using extrusion combined with alkali pretreatment. For an ALR of 1.5%, there were insignificant differences in the methane yields from the pretreated rice straw for treatment times ranging from 3 to 120 h. This result indicated that lowering the ALR could not open the ‘‘acetyl valve” and partially open the ‘‘lignin valve”, making the feedstock less accessible to anaerobic bacteria, and thus, methane production was not increased [8]. This result was in agreement with the lignin removal rate for an ALR of 1.5% (Fig. 2) and also indicated that lignin solubilization is important for enhancing methane production from lignin cellulosic biomass. Methane production from the pretreated rice straw increased significantly as the ALR and treatment time were increased. For ALRs of 3.0–6.0%, the methane yields from the pretreated rice straw increased slightly for treatment times between 3 and 24 h, improved dramatically for treatment times between 24 and 48 h, and remained relatively stable for treatment times between 48 and 120 h. These results showed that higher ALRs of 3.0–6.0% with shorter treatment times of 3–24 h were not as effective in increasing methane production. A shorter treatment time cannot guarantee complete chemical reaction between NaOH and rice straw. The statistical analysis indicated that ALRs of 3.0–6.0% with treatment times of 48–120 h resulted in significantly higher methane production from rice straw compared to that obtained using an ALR of 1.5%; thus, these ALR values and treatment times were concluded to be effective. However, the methane yield was not significantly different for ALRs of 3.0%, 4.5% and 6.0%. For economic feasibility and pretreatment efficiency, an ALR of 3.0% and a treatment time 48 h are recommended for practical application. 3.2.2. Comparison of effects on anaerobic digestion for different rice straw pretreatment methods It is not clear whether the higher methane yield from rice straw using extrusion combined with alkali pretreatment can be attributed to physical effects (extrusion), chemical effects (NaOH pretreatment), or synergistic effects from the combined extrusion and NaOH pretreatment. Thus, four different pretreatments were conducted to address this issue: milling, extrusion, milling combined with NaOH pretreatment, and extrusion combined with
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Y. Zhang et al. / Applied Energy 160 (2015) 39–48
Fig. 2. Changes in cellulose, hemicelluloses and lignin contents and pH values during extrusion combined with alkali pretreatment.
Fig. 3. Effects of treatment time and ALR on the methane production from rice straw by extrusion combined with alkali pretreatment.
alkali pretreatment. All of the pretreated rice straw consisted of small particles with sizes below 0.45 mm. Milling pretreatment was conducted using a laboratory hammer mill (JP-350A-8, Jiupin Industry & Trade Co., Ltd., Shanghai, China). As expected, the results obtained using the biomass size reduction machine and milling combined with NaOH pretreatment were the same as those obtained using extrusion combined with NaOH pretreatment (temperature 35 °C; time 48 h; SLR 1:6; ALR 3.0%). Fig. 4a shows that as the fermentation process proceeded, the daily biogas production from rice straw using different pretreatment methods first increased, peaked and then decreased. The primary difference among the rice straws from different pretreatments was in the peak values. The maximum peak value was obtained using extrusion combined with NaOH pretreatment. The difference among the peak values indicated that extrusion combined with
NaOH pretreatment was more effective for biogas production than the other pretreatments. Fig. 4b shows the cumulative biogas productions from rice straw based on the daily biogas production for different pretreatment methods. The statistical analysis demonstrated that the total biogas production from extrusion and milling combined with NaOH was significantly higher than that for the other pretreatments. This result indicated combining extrusion with alkali pretreatment had a synergistic effect on biogas production and was not purely physical or chemical effects. Fig. 4c shows the cumulative methane content obtained for different pretreatment methods. In the first 10 days, the methane contents for the four pretreatments were relatively low because of the rapid growth of fermentative and acetogenic bacteria that converted organic matter into intermediate metabolites (e.g., carbon dioxide, hydrogen and acetic acid) [11]. The cumulative methane content then
45
Daily biogas production (mL/g VS)
Y. Zhang et al. / Applied Energy 160 (2015) 39–48
(a) Milling
60
Extrusion M+NaOH E+NaOH
40
20
0 10
0
20
30
40
Cumulative biogas production (mL/VS)
Digestion time (d)
(b)
10d
30d
40d
400 300 200
ðC6 H10 O5 Þn þ nH2 O ! 3nCO2 þ 3nCH4 5 5 ðC5 H8 O4 Þn þ nH2 O ! nCO2 þ nCH4 2 2
100
ð1Þ ð2Þ
0 Milling
Cumulative methane contents
20d
500
into non-biodegradable substances (primarily lignin and ash) and biodegradable substances, which can further be classified into substances that are readily biodegradable (cellulose, hemicelluloses, pectin, proteins, etc.) and substances that are difficult to biodegrade (waxes, tannins, etc.) [7,30]. LB has a complex chemical composition; thus, the methane yield from LB is based on the TS or VS in many studies in the literature. Most of the cellular contents are water soluble and partially readily biodegradable, making it difficult to distinguish between those cellular contents that are converted into methane and those that are water soluble. In contrast, anaerobic bacteria can readily degrade cellulose and hemicelluloses, which if will convert into methane, associate with lignin and remain as anaerobic digested residues. Therefore, the total contribution of cellulose, hemicelluloses and other components to methane production from rice straw was investigated for different pretreatment methods. The theoretical methane production of cellulose and hemicelluloses was calculated using Eqs. (1) and (2). At STP conditions, 415 mL and 424 mL of methane were produced per g of the cellulose and hemicelluloses, respectively, which were degraded by AD. Fig. 5a shows that the cellulose, hemicelluloses and other components contributed 77.0–107.9, 66.3–82.9 and 43.7–97.3 mL of CH4/VS for the four pretreatments. Cellulose and hemicelluloses contributed 66.2–76.7% of the methane produced for the four pretreatments. That is, the higher the biodegradation of the cellulose and hemicelluloses, the higher was the methane production after pretreatment.
(c)
10d
Extrusion 20d
M+NaOH 30d
E+NaOH 40d
55%
50%
45%
40% Milling
Extrusion
M+NaOH
E+NaOH
(b)
Fig. 4. Anaerobic digestion of rice straw by different pretreatment methods. M + NaOH: milling combined with NaOH pretreatment; E + NaOH: extrusion combined with NaOH pretreatment.
3.2.3. Contributions of cellulose, hemicelluloses and other components to methane production from rice straw by different pretreatment methods LBs, such as rice straw, can be divided into two components: a cell wall that consists primarily of cellulose, hemicelluloses, and lignin, and cellular contents consisting of non-structural carbohydrates, sugars, pectin, tannin, proteins, lipids, waxes, minerals, etc. [30]. In terms of anaerobic biodegradability, LB can be divided
Energy (MJ/kg)
gradually increased to between 50% and 55% for all of the pretreatments because of the growth of methanogenic bacteria that consumed the metabolites to produce methane.
15
10
5
0 Rice straw
Milling
Extrusion M+NaOH
E+NaOH
Fig. 5. Contributions of cellulose, hemicelluloses and other components to methane production (a) and ER (b) from anaerobic digestion of rice straw by different pretreatment methods. M + NaOH: milling combined with NaOH pretreatment; E + NaOH: extrusion combined with NaOH pretreatment.
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3.2.4. ER from anaerobic digestion of rice straw by different pretreatment methods There are two types of heat values: the higher heating value (HHV) and the lower heating value (LHV). The HHV is the gross heat value or the total energy content and corresponds to the latent heat of the water vapor when the biomass is burnt in air. However, the latent heat of the water vapor cannot be utilized effectively in practice. Therefore, the LHV is used as a measure of the available energy of rice straw [2]. The LHV for rice straw (based on VS) and methane (at STP) were 17.254 MJ/kg and 35.898 MJ/m3, respectively. The energy output values from the AD of rice straw by milling, extrusion, milling combined with NaOH pretreatment, and extrusion combined with NaOH pretreatment were 6.7, 8.2, 8.5 and 10.3 MJ/kg VS, respectively, which corresponded to ER efficiencies of 38.9%, 47.6%, 49.1% and 59.9%, respectively (Fig. 5b). When LB is pretreated for methane production, the energy consumption for the pretreatment process, especially for hydrothermal, thermal, and combined thermal-alkali pretreatments, should be considered in view of the ER efficiency [33,34].
3.3. Investigation of mechanism of extrusion combined with alkali pretreatment to enhance methane production from rice straw 3.3.1. Comparison of physical properties of rice straw for different pretreatment methods Fig. 6 shows the physical properties of rice straw including the WHC, SP, SSA and CrI. The WHC for rice straw that was pretreated by extrusion combined with NaOH pretreatment increased by 51%, compared with the samples that were pretreated with milling, implying that more surfaces were exposed after the extrusion– NaOH pretreatment, in accordance with the increase in the SSA. The SP increased for the three pretreatments other than the milling pretreatment, but the reasons for the increase may have been different for the different pretreatments. In the extrusion pretreatment, the extruder physically caused the rice straw to expand; for milling combined with NaOH pretreatment, the NaOH swelled the fiber; and for extrusion combined with NaOH pretreatment, physicochemical effects caused the expansion of the rice straw. The CrI is an important indicator of LB digestibility. The CrI of rice straw decreased significantly (by 48.37%) after the extrusion pretreatment. The extruder functioned as a biomass size reduction machine and applied vigorous shear forces, rapid heat transfer, effective biomass mixing, and dynamic compression, resulting in the defibration and fibrillation of the LB. The depolymerization of cellulose was attributed to the decrease in the CrI. The CrI
WHC
SSA
CrI
10
1.5
8
1.2
6
0.9
4
0.6
2
0.3
0
0 Milling
Extrusion M+NaOH
Specific surface area (m2/g) Crystallinity index
Specific porosity (mL/g) Water-holding capacity (g/g)
SP
E+NaOH
Fig. 6. Physical properties of rice straw pretreated by different pretreatment methods. M + NaOH: milling combined with NaOH pretreatment; E + NaOH: extrusion combined with NaOH pretreatment.
decreased further for the extrusion combined with NaOH pretreatment, which could be attributed to the removal of amorphous substances, such as lignin and the acetyl group, or the preferential hydrolysis of the amorphous cellulose over that of the crystalline cellulose [10,25], which was in agreement with the change in the chemical composition (Fig. 2). A peeling reaction occurred between NaOH and rice straw, which also broke down the crystalline cellulose and enlarged the porosity and inner surface areas, which was in agreement with the changes in the SP and SSA (Fig. 6). The lower CrI value for the rice straw indicated that more amorphous cellulose was digested by the anaerobic bacteria, which increased the cellulose degradation (Fig. 5a) and biogas production (Fig. 4b). The heterogeneous nature of LB means that other components, such as lignin and hemicelluloses, also affect the digestibility of these materials in addition to the CrI. 3.3.2. Effects of different pretreatment methods on changes in chemical composition of rice straw Compared with the crystalline cellulose content, the lignin content had a relatively higher negative impact on methane production from LB [4]. Lignin significantly limited the amount of cellulose available for degradation and posed a major barrier to LB decomposition. Table 3 shows significant decreases in the lignin content for both the milling combined with NaOH and extrusion combined with NaOH pretreatments, which explained why more methane was produced with these pretreatments than that for the milling pretreatment. The lignin content was also related to the cellulose accessibility: Fig. 6 shows that a lower lignin content corresponded to a higher cellulose accessibility. Table 3 shows that the HWE increased by 42.0–46.5% after the milling combined with NaOH and the extrusion combined with NaOH pretreatments. The HWE consisted of sugar, starch, pectin, tannin, cyclitol, and inorganic matter [29]. Most of these substances have simple chemical structures and low molecular weights and are thus readily biodegradable. The newly produced HWEs were mainly derived from the decomposition of holocelluloses and lignin, as indicated by the changes in the chemical composition (Table 3). This increase in the HWE content contributed to the increase of methane production from rice straw after extrusion combined with NaOH pretreatment, in agreement with Fig. 5(a), which shows that other components contributed to methane production. The HWE in rice straw from milling combined with NaOH pretreatment was not significantly different from the HWE level obtained using extrusion combined with NaOH pretreatment; however, the contribution of the other components to the methane production in the milling combined with NaOH pretreatment was 32.5 mLCH4/g VS lower than that from the extrusion combined with NaOH pretreatment. This result was attributed to that more difficult to biodegrade compounds were structurally disrupted by the extruder and then converted into readily biodegradable compounds. Therefore, both the physical effect (extrusion) and chemical effect (NaOH pretreatment) resulted in higher methane production from rice straw using extrusion combined with NaOH pretreatment. Unlike the HWE, the BEE decreased when NaOH pretreatment was used. BEE are hydrophobic and are normally not readily
Table 3 Chemical composition of rice straw pretreated by different methods. Component
Milling
Extrusion
M + NaOH
E + NaOH
Cellulose (% TS) Hemicelluloses (% TS) Lignin (% TS) HWE (% TS) BEE (% TS)
30.0 ± 1.1 29.8 ± 0.9 6.5 ± 0.4 22.6 ± 1.3 7.1 ± 0.5
29.8 ± 1.0 29.7 ± 1.9 6.5 ± 0.4 22.8 ± 1.1 7.1 ± 0.5
28.4 ± 0.9 25.8 ± 1.2 5.0 ± 0.3 32.1 ± 1.4 3.5 ± 0.1
28.7 ± 1.2 25.4 ± 0.7 4.8 ± 0.1 33.1 ± 1.6 3.3 ± 0.2
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Y. Zhang et al. / Applied Energy 160 (2015) 39–48
b 1372 1726 1635 1606 1515 1428
2900 2920
3300
3000
898 1163
3420
3600
a
2700
2400
2100
1800
1500
1200
900
600
Wave numbers /cm-1 Fig. 7. FTIR spectra of rice straw: (a) milling pretreatment; and (b) extrusion combined with NaOH pretreatment.
biodegradable by anaerobic bacteria. BEE primarily consists of waxes, fattiness, resins, tannins, and pigments [29]. The waxes on the LB surface make degradation by anaerobic bacteria difficult. Reducing the amount of waxes can improve the cellulose accessibility and expose more cellulose to anaerobic bacteria. This result may also explain the increase in the cellulose accessibility (Fig. 6) and methane production from rice straw by extrusion combined with NaOH pretreatment.
3.3.3. Change in chemical structures of rice straw during extrusion combined with alkaline pretreatment The FTIR spectra of the rice straw that was pretreated by milling and extrusion combined with NaOH were also analyzed (Fig. 7). The band at 3348 cm1 corresponded to OH-stretching, and the absorption intensities decreased for the samples that were pretreated with NaOH in combination with extrusion, demonstrating that the cellulose hydrogen bonds were destroyed [10]. The band at approximately 2919 cm1 was assigned to CH2-stretching, and the absorption intensities decreased for the samples that were pretreated with NaOH combined with extrusion, indicating a decrease in the aliphatic fractions of the waxes [35]. The band at 2900 cm1 was attributed to C–H-stretching, and the absorption intensities decreased for the samples that were pretreated with NaOH combined with extrusion, demonstrating the partial release of the methyl, methylene, and methane groups of cellulose [7]. The band at 1726 cm1 represented CO-stretching, and the decrease in the absorption intensities indicated de-esterification from extrusion combined with NaOH pretreatment of the rice straw. The aromatic skeletal vibration ranging from 1465 to 1610 cm1 was assigned to lignin and indicated that lignin was dissolved during extrusion combined with NaOH pretreatment [25]. The band intensities at 898 cm1 that were associated with the b-(1 ? 4)-glycosidic linkages were lower for the samples from the extrusion combined with NaOH pretreatment than those for the samples that were pretreated by milling, indicating the cleavage of acetyl groups [10]. The above results showed that (1) substances that were difficult to biodegrade (waxes) and non-biodegradable substances (lignin) by AD were partially solubilized/degraded; (2) the association of lignin with holocelluloses decreased; and (3) cellulose and hemicelluloses were partially degraded by extrusion combined with NaOH pretreatment. These structural changes in the rice straw that was pretreated using extrusion combined with NaOH treatment further confirmed the changes in the physical properties (Fig. 6) and the chemical composition (Table 3). These changes enabled the rice straw to be easily accessed, attacked and digested by anaerobic bacteria and enhanced the biodegradability of the rice straw and methane production.
4. Conclusions We investigated extrusion combined with alkaline pretreatment for methane production from rice straw. An ALR of 3.0%, a treatment temperature of 35 °C, and a treatment time of 48 h were recommended. Under these conditions, methane production increased by 54.0% over that obtained from samples pretreated by milling. An ER efficiency of 59.9% was obtained for rice straw that was pretreated by extrusion combined with alkali treatment. The increase in methane production and the ER were attributed to changes in the physical properties, chemical composition and chemical structure of LB, demonstrating the synergistic effects of combining extrusion with alkali pretreatment. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Nos. 20976139, 21246001, 51138009), the National Key Technologies R&D Program of China (No. 2012BAJ25B02), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRY11004), New Century Excellent Talents in University (NCET-11-0391) and the Project of Shanghai Science and Technology Commission (No. 14XD1403700). References [1] BP. Statistical review of World energy 2013.; 2013. [2] Kaparaju P, Serrano M, Thomsen AB, Kongjan P, Angelidaki I. Bioethanol, biohydrogen and biogas production from wheat straw in a biorefinery concept. Bioresour Technol 2009;100:2562–8. [3] Hendriks A, Zeeman G. Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresour Technol 2009;100:10–8. [4] Monlau F, Sambusiti C, Barakat A, Guo XM, Latrille E, Trably E, et al. Predictive models of biohydrogen and biomethane production based on the compositional and structural features of lignocellulosic materials. Environ Sci Technol 2012;46:12217–25. [5] Fatih Demirbas M, Balat M, Balat H. Biowastes-to-biofuels. Energy Convers Manage 2011;52:1815–28. [6] Chen X, Gu Y, Zhou X, Zhang Y. Asparagus stem as a new lignocellulosic biomass feedstock for anaerobic digestion: increasing hydrolysis rate, methane production and biodegradability by alkaline pretreatment. Bioresour Technol 2014. [7] Zheng M, Li X, Li L, Yang X, He Y. Enhancing anaerobic biogasification of corn stover through wet state NaOH pretreatment. Bioresour Technol 2009;100: 5140–5. [8] Xie S, Frost J, Lawlor P, Wu G, Zhan X. Effects of thermo-chemical pretreatment of grass silage on methane production by anaerobic digestion. Bioresour Technol 2011;102:8748–55. [9] Chen X, Zhang Y, Gu Y, Liu Z, Shen Z, Chu H, et al. Enhancing methane production from rice straw by extrusion pretreatment. Appl Energy 2014;122:34–41.
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[10] He Y, Pang Y, Liu Y, Li X, Wang K. Physicochemical characterization of rice straw pretreated with sodium hydroxide in the solid state for enhancing biogas production. Energy Fuels 2008;22:2775–81. [11] Zhu J, Wan C, Li Y. Enhanced solid-state anaerobic digestion of corn stover by alkaline pretreatment. Bioresour Technol 2010;101:7523–8. [12] Sambusiti C, Ficara E, Malpei F, Steyer J, Carrère H. Influence of alkaline pretreatment conditions on structural features and methane production from ensiled sorghum forage. Chem Eng J 2012;211:488–92. [13] Salehian P, Karimi K, Zilouei H, Jeihanipour A. Improvement of biogas production from pine wood by alkali pretreatment. Fuel 2013;106:484–9. [14] Chandra R, Takeuchi H, Hasegawa T, Kumar R. Improving biodegradability and biogas production of wheat straw substrates using sodium hydroxide and hydrothermal pretreatments. Energy 2012;43:273–82. [15] Jin G, Bierma T, Walker PM. Low-heat, mild alkaline pretreatment of switchgrass for anaerobic digestion. J Environ Sci Health, Part A 2014;49:565–74. [16] Xiao X, Zhang R, He Y, Li Y, Feng L, Chen C, et al. Influence of particle size and alkaline pretreatment on the anaerobic digestion of corn stover. BioResources 2013;8:5850–60. [17] You Z, Wei T, Cheng JJ. Improving anaerobic co-digestion of corn stover using sodium hydroxide pretreatment. Energy Fuels 2014. [18] Zheng M, Li L, Li X, Xiong J, Mei T, Chen G. The effects of alkaline pretreatment parameters on anaerobic biogasification of corn stover. Energy Sources, Part A: Recov, Utilization, Environ Effects 2010;32:1918–25. [19] Sambusiti C, Monlau F, Ficara E, Carrère H, Malpei F. A comparison of different pre-treatments to increase methane production from two agricultural substrates. Appl Energy 2013;104:62–70. [20] Mirahmadi K, Kabir MM, Jeihanipour A, Karimi K, Taherzadeh MJ. Alkaline pretreatment of spruce and birch to improve bioethanol and biogas production. BioResources 2010. [21] Monlau F, Aemig Q, Barakat A, Steyer J-P, Carrère H. Application of optimized alkaline pretreatment for enhancing the anaerobic digestion of different sunflower stalks varieties. Environ Technol 2013;34:2155–62. [22] Monlau F, Barakat A, Steyer J, Carrere H. Comparison of seven types of thermochemical pretreatments on the structural features and anaerobic digestion of sunflower stalks. Bioresour Technol 2012;120:241–7.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A physicochemical method for increasing methane production from rice straw: Extrusion combined with alkali pretreatment Yalei Zhang ⇑, Xiaohua Chen, Yu Gu, Xuefei Zhou State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The first report of using extrusion
combined with NaOH pretreatment for methane production from LB. Pretreatment with an ALR of 3.0% at 35 °C for 48 h increased methane production from rice straw by 54%. The ER efficiency improved from 38.9% to 59.9% using extrusion combined with NaOH pretreatment. The changes in the physicochemical characterization of rice straw were investigated.
a r t i c l e
i n f o
Article history: Received 7 January 2015 Received in revised form 31 August 2015 Accepted 3 September 2015
Keywords: Extrusion combined with alkali pretreatment Lignocellulosic biomass Rice straw Physicochemical pretreatment Methane
a b s t r a c t Pretreatment is a crucial processing step in the conversion of lignocellulosic biomass (LB) into methane by anaerobic digestion. A physicochemical LB pretreatment method, i.e., using an extruder to reduce the biomass size prior to sodium hydroxide (NaOH) pretreatment, was reported. The optimal condition for economic feasibility and pretreatment efficiency was an alkaline loading rate of 3.0% at 35 °C for 48 h. Under this condition, the methane production from the rice straw that was processed by extrusion combined with NaOH pretreatment was 54.0% higher than that of a control sample. The energy recovery (ER) efficiency improved from 38.9% to 59.9% using the combination pretreatment. The mechanisms that caused the significant improvement in the methane production and ER efficiency in the extrusion–NaOH pretreatment were investigated. The pretreatment changed the physical properties (water-holding capacity, specific porosity, specific surface area and crystallinity index), the chemical composition (lignin, benzene–ethanol extractives and hot-water extractives) and the chemical structure, which increased degradation of holocelluloses and other difficulty biodegradable compounds. Ó 2015 Elsevier Ltd. All rights reserved.
Abbreviations: AD, anaerobic digestion; ADF, acid detergent fiber; ADL, acid detergent lignin; AIA, acid insoluble ash; ALR, alkaline loading rate; BEE, benzene–ethanol extractives; BMP, biological methane potential; C/N, carbon-to-nitrogen ratio; CrI, crystallinity index; ER, energy recovery; FTIR, Fourier transform infrared; HHV, higher heating value; HWE, hot-water extractives; LB, lignocellulosic biomass; LHV, lower heat value; NaOH, sodium hydroxide; NDF, neutral detergent fiber; SEM, scanning electron microscope; SLR, solid-to-liquid ratio; SP, specific porosity; SPSS, statistic package for social science; SSA, specific surface area; STP, standard temperature and pressure; TS, total solids; VS, violate solids; WHC, water-holding capacity; XRD, X-ray diffraction. ⇑ Corresponding author. Tel.: +86 21 65985811; fax: +86 21 65986960. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.apenergy.2015.09.011 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
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Y. Zhang et al. / Applied Energy 160 (2015) 39–48
1. Introduction In 2012, the world energy consumption was 12476.6 Mtoe, of which petroleum, coal and natural gas consumption were 4130.5, 3730.1 and 2987.1 Mtoe, respectively. Fossil fuel consumption accounted for 86.9% of the world energy consumption, whereas renewable energy accounted for a mere 1.9% [1]. The high demand for energy, the unstable and uncertain availability of fossil fuels, and concern over global climate warming have produced an increasing urgency for the development of new, sustainable and renewable energy that can displace traditional fossil energy [2]. Lignocellulosic biomass (LB), with a worldwide annual production of 200 billion tons, has become a primary candidate for biofuel production [3,4]. The development of new technologies for renewable energy production from LB, especially agricultural residues, is very promising for the following reasons: (1) LB is abundant and renewable; (2) the non-edible parts, i.e., the stalks and leaves, of agricultural residues are used for fuel production, which does not compete with food production; and (3) LB use affects both energy consumption and the environment because renewable energy is recovered from agricultural residues while treating waste biomass [4,5]. Anaerobic digestion (AD) offers the advantages of a low-cost, mature, and stable technology and in particular, produces a high energy recovery (ER) from LB [6,7]. AD produces a methane-rich biogas that can provide heating, for power generation or as a vehicle fuel to replace fossil fuels [8]. However, LB has a complex structure that results in poor hydrolysis and biodegradability during the AD process [3]. The primary chemical composition of LB is cellulose, hemicelluloses and lignin. Cellulose and hemicelluloses can be converted into methane, whereas lignin cannot be degraded by anaerobic microbes [9]. Moreover, lignin is hydrophobic and resistant to microbial attack, which limits the accessibility of holocelluloses (cellulose and hemicelluloses) to anaerobic microbes, which lowers the substrate availability and methane production [4]. Pretreatment is a crucial processing step in which anaerobic microbes convert LB into methane. The physical properties, chemical composition and chemical structure of LB can be altered using several pretreatment methods that increase the accessibility of holocelluloses to enzymes, thereby enhancing the digestibility of LB and increasing methane production [3,10]. Alkaline pretreatment is one of the current leading pretreatment methods and offers several benefits such as the solubilization of lignin and hemicelluloses, the destruction of the ester bond of lignin–carbohydrate complexes and a decrease in the cellulose crystallinity; the residual alkali also provides alkalinity for the subsequent AD process [6,7,11]. A literature review shows that sodium hydroxide (NaOH) is a common alkali that is widely used in alkaline pretreatment [12,13]. Different LBs that are pretreated with NaOH for methane production are summarized in Table 1. The primary influence factors for the NaOH pretreatment are the temperature, the time, the solid-to-liquid ratio (SLR), and the alkaline loading rate (ALR). Most studies have focused on these operating factors over a wide range of temperatures (0–200 °C), times (10 min to 21 d), SLRs (1:0.8–1:19) and ALRs (1–152%) [14–19]. In general, pretreatment increases methane production (from 0% to 174.2%) from LB compared with untreated samples [13,20–25]. Biomass size reduction always occurs prior to NaOH pretreatment and affects the pretreatment. The extruder, which is more efficient than other physical pretreatment methods, has recently been used to enhance methane production from LB. The methane production increased significantly by 18–70% using extrusion as a pretreatment to test five agricultural residues [26]. A similar result was reported of an increase in methane production 72.2% by extrusion pretreated rice straw over that of the untreated rice straw [9].
However, to the best of our knowledge, there have been no previous reports of a pretreatment of extrusion combined with NaOH for improving methane production from LB. Rice straw is the major component of agricultural residues. The annual yield of rice straw in the world is approximately 731 million tons [5]. Rice straw has a high silica content and is therefore unsuitable for use in animal feeding, pulping and papermaking [27]. Rice straw has a good potential for methane production. In this study, the effect of extrusion combined NaOH pretreatment on rice straw methane production was studied and the effect of other pretreatments such as milling and milling combined NaOH pretreatment also has been analyzed for comparison. The contributions of cellulose, hemicelluloses and other components to methane production from rice straw using different pretreatment methods were investigated. The changes in the physical properties, chemical composition and chemical structure of rice straw from pretreatment were also examined. 2. Materials 2.1. Feedstock and inoculum Rice straw was obtained from a rice field in Yancheng city, Jiangsu province, China. Table 2 presents the proximate, ultimate and compositional properties and the heat value of the rice straw. The rice straw was dried at room temperature (25 °C) to a moisture content below 10%. The resultant rice straw was stored in vacuum bags for later pretreatment. The inoculum used in this study was anaerobic digested sludge that was taken from an anaerobic digester that was fed with dairy manure in Chongming Island, Shanghai, China. The anaerobic digester was a 300-m3 continuously stirred digester that had been stably operated at 35 °C for more than 3 years. 2.2. Extrusion combined with alkali pretreatment The procedure for the extrusion combined with alkali pretreatment is presented in Fig. 1. In brief, a twin-screw extruder (JXM80, Jinwor Machinery Co., Ltd, Nanjing, China) and NaOH were used as the biomass-size-reduction machine and the alkaline reagent, respectively. The dried rice straw was pretreated with the extruder and then passed through a 0.45-mm sieve. The resulting rice straw was pretreated with a NaOH solution. The pretreatment parameters were as follows: a treatment temperature of 35 °C, a treatment time of 3–120 h, a SLR of 1:6 and an ALR of 1.5–6.0%. The variation in the pH and the chemical composition during the alkali pretreatment were analyzed using the following procedure: eight 50-mL plastic centrifuge tubes were prepared with different ALRs (1.5%, 3.0%, 4.5% and 6.0%) for the alkali pretreatment, and the reaction was terminated after 3, 6, 12, 24, 48, 72, 96 and 120 h for sampling and subsequent analysis. 2.3. Methane production from rice straw by AD The biological methane potential (BMP) of untreated and pretreated rice straw was evaluated using 500-mL batch glass digesters, which each had a 400-mL working volume and a 100-mL head space. First, 10.0 g of volatile solids (VS) from a rice straw sample were weighed and added to each digester. Second, the required amount of sludge was fed into each digester with a substrate-to-inoculum ratio of 1:1 based on the VS. Urea was used to adjust the carbon-to-nitrogen ratio (C/N) of the mixture to 25. Third, distilled water was added to each digester to reach the working volume. The desired pH range of 6.8–7.2 range for AD
Table 1 Summaries of the biomass size reduction machine and NaOH pretreatment on methane production from different LB. LB
Main compositions
NaOH pretreatment parameters
HCEL (% TS)
Lignin (% TS)
Machine
Particle size (mm)
Temperature (°C)
Time
SLR (w/ w)
ALR (%)
SMP (mL/g)
Increasea (%)
SMPUS (mL/ g)
Rice straw
30.0
29.8
6.5
Extruder
<0.45
35
48 h
1:6
3
288
54
187
Rice straw
38.9
24.0
5.6
Force mill
<1
33.4 49.1
28.2 34.1
7.4 6.5
Hammer mill Kitchen blender
5–10 1
5d 10 min 21 d 1h
1:9
Rice straw Wheat straw
35.1
25.6
7.5
Force mill
<1
Corn stover
37.5
30.0
8.4
Hammer mill
5–10
5d 10 min 3d
1:9 1:9 1:9
3 5 6 1 10 4 5 2
Corn stover Corn stover Corn stover
37.5 40.7 NR
30.0 22.5 NR
8.4 21.7 23.55
5–10 <5 1
3d 24 h 3h
1:7.3 1:1 1:10
2 1; 2.5; 5 20d; 60d
24 122 44 14; 48; 23 43; 67 111.6 20 56; 75 67; 56 73.4 0; 3; 37 10.7; 40.6d
Corn stover Ensiled sorghum
42.3 47.5
29.8 27.4
13.4 7.0
Hammer mill NR Thomas Wiley mill Kneading machine Cutting mill
74.1 132.7 520b 232; 302; 252 291; 341 165.9 94.1 207.9; 233.0 221.6; 207.2 211 267; 276; 372b 276; 350
59.8 59.8 360b 204
Wheat straw
37 200 20 ± 2 40; 100; 160 40; 100 37 200 10; 20 30; 50 20 ± 2 20 ± 0.5 20; 35
Ensiled sorghum Sunflower stalks Sunflower stalks Switchgrass Grass silage
48.9
35.1
4.1
Kitchen blender
1
3d 24 h 24 h 12 h 1h
1:8 NR NR NR –c
34 23.1 NR 34.3
20.8 10.6 NR 29.6
29.7 29.9 NR 8.6
Cutting mill Cutting mill Blender Blender
2–3 2–3 <78 10
20 40 55 55 40; 100; 160 40; 100 30; 55; 80 55 100 100
24 h 24 6h 12 h
NR –e 1:11.7 1:4
Asparagus stem Wheat plant
34.6 43
21.2 15
13.2 18.1
Hammer mill NR
0.28–0.45 <1
5d 1h
1:10 1:19
Birch Spruce OPEFB PPR
41.0 43.0 32.4 47.7
27.9 20.8 8.7 25.6
29.68 28.83 24.6 23.6
Ball mill Ball mill NR Hammer mill
<0.8 <0.8 <0.42 6–12
35 25; 50 75; 100 100 5 100 20
6 4; 10 4; 10 4; 10 1 10 4 4 5.5 1; 2.5 5; 7.5 6 152d
2h 2h 1h 4d
–f –f 1:19 1:7.3
–f –f 152d 3; 5; 7
15; 48 4; 8 3; 12 10; 19 12; 11; 0 29; 32 17; 35; 25 36 13.3 10; 23 38; 39 38.4 47; 41 54; 5.3 84 63 100 102; 114; 94
Pine wood
38.2
24.1
34.4
NR
<1
0 100
60 min 10 min
1:19
152d
287.0; 272.6 287; 311 292; 298 293; 316 300; 299; 261 346; 356 225; 259; 240 262 332 359.5; 401.8 449.5; 452.5 242.3 386; 369 404; 276 460 50 404 256.6; 271.9; 246.8 178.2 142.1d
0.25–1; 5–20 1
1:0.8 –c
Methane yield
Reference
118.6 174.2d
This study [33] [10] [19]
78.4
[14]
132.7
[18]
122.7 272b 249
[7] [11] [17]
249; 184 266
[16] [12]
268
[19]
192 193 293 326.8
[22] [21] [15] [8]
175.0 262
[6] [24]
250 30 202 127.2
[20]
65
[13]
Y. Zhang et al. / Applied Energy 160 (2015) 39–48
Biomass size reduction
CEL (% TS)
[23] [25]
ALR: alkaline loading rate, g NaOH/100 g TS; CEL: cellulose; HCEL: hemicelluloses; NR: not reported; OPEFB: oil palm empty fruit bunches; PPR: poplar processing residues; SLR: solid-to-liquid ratio, based on TS of substrate; SMP: specific methane production, based on VS; SMPUS: specific methane production of untreated sample. a Compared to untreated sample. b Biogas production, mL/g VS. c Total solid content of 160 g TS/L. Added water weight was not reported, thus the SLR could not be calculated. d Values calculated from the reference. e Total solid content of 35 g TS/L. Added water weight was not reported, thus the SLR could not be calculated. f Milled birch and spruce were pretreated with 7% w/w NaOH solution. Both the substrate weight and the solution volume were not reported, thus the SLR and ALR could not be calculated.
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Table 2 Proximate, ultimate and compositional properties and heat value of rice straw used in this study. Valuesa
Properties parameter
Unit
Proximate properties
VS Ash
% TS % TS
85.7 ± 0.4 14.3 ± 0.4
Ultimate properties
Carbon (C) Nitrogen (N) C/N
% TS % TS
41.7 ± 0.1 0.6 ± 0.0 69.5 ± 0.2
Compositional propertiesb
Celluloseb (ADF–ADL) Hemicellulosesb (NDF–ADF) Ligninb (ADL–AIA)
% TS % TS
30.0 ± 1.1 29.8 ± 0.9
Heat value
Lower calorific value (LCV)
MJ/kg TS
% TS
6.5 ± 0.4 14.79 ± 0.1
a
The values are a mean of triplicate measurements. Cellulose was determined as the difference between acid detergent fiber (ADF) and acid detergent lignin (ADL), hemicelluloses as the difference between neutral detergent fiber (NDF) and acid detergent fiber (ADF), and lignin as the difference between acid detergent lignin (ADL) and acid insoluble ash (AIA).
bags was determined using a 100-mL glass syringe. The biogas volume was then converted to the volume under standard temperature and pressure (STP) conditions. The biogas composition was analyzed using a gas chromatograph (GC-14B, Shimadzu, Japan). 2.4.2. Physical properties of untreated and pretreated samples The heat value was measured using a bomb calorimeter (XRY-1A, Mechanical and & Electrical Technology Co., Ltd., Shanghai, China). The WHC and the specific porosity (SP) were determined as reported previously [28]. A Brunauer–Emmett–Teller surface area analyzer (3H-2000BET-A, Beishide Instrument S&T Co., Ltd, Beijing, China) was used to measure the specific surface area (SSA). The overall crystallinity of samples was characterized using a XRD instrument (D8 Advance, Bruker, Germany) with Cu Ka radiation (k = 1.54 Å) at 40 kV and a tube current of 40 mA.
b
was obtained by measuring and adjusting the pH of each bottle was measured and adjusted to 7.0 ± 0.1 by adding 2 M HCl or 2 M NaOH. Finally, the digesters were flushed with nitrogen gas for 1 min to remove oxygen, tightly sealed with rubber stoppers, and then incubated in a work-in incubator at 35 °C. A blank control was conducted under the same conditions to remove endogenous methane production from the sludge.
2.4. Analytical methods 2.4.1. Biogas analysis The produced biogas was collected through a vapor trap in an aluminized polyethylene gas sampling bag (1 L, Shanghai Eler Co., Ltd., Shanghai, China). The volume of biogas in the gas sampling
2.4.3. Chemical compositions of untreated and pretreated samples An elemental analysis was performed using an elemental analyzer (Vario EL III, Elementar, Germany). The total solids (TS), VS and ash were determined as reported previously [9]. A PHS-3E pH meter (Shanghai Lei Instrument Factory, Shanghai, China) was used to measure the pH. The chemical compositions of cellulose, hemicelluloses, lignin, hot-water extractives (HWE) and benzene–ethanol extractives (BEE) were determined as reported previously [29,30]. 2.4.4. Changes in chemical structure The changes in the functional groups were investigated using Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra were obtained using a FTIR spectrophotometer (Nicolet 5700, Thermo Electron Corporation, USA). The cleaned and dried samples were milled into powders and dispersed in a matrix of potassium bromide (KBr), followed by compression at approximately 1 MPa to form transparent pellets. The pellets were scanned approximately 30 times from 4000 to 500 cm1 at a resolution of 4 cm1.
Elemental analysis
Chemical structure
FTIR
Raw material Untreated rice straw Pretreated with a twin-screw extruder
Specific surface area
Extrusion-pretreated rice straw power BET
Rice straw pretreated with extrusion combined with NaOH
Specific porosity Biomass crystallinity
Soaked in NaOH solution
XRD
Anaerobic digestion at 35 ºC
Bomb calorimeter Oven dried Calcination
CHNO LHV Moisture Ash
Digested with neutral detergent
Neutral detergent fiber Digested with acid detergent
Acid detergent fiber Hydrolyzed with 72% sulfuric acid
Acid detergent lignin Cellulose accessibility
WHC
Calcination
Anaerobic Biogas digested residues
Acid insoluble ash
Fig. 1. Schematic illustration of the experimental procedure in this study.
Y. Zhang et al. / Applied Energy 160 (2015) 39–48
2.5. Statistical analysis The software Statistical Package for Social Science (SPSS) 17.0 was used to calculate the standard deviations and significant differences. 3. Results and discussion 3.1. Changes in cellulose, hemicelluloses and lignin contents and pH values during extrusion combined with alkali pretreatment Apart from the biomass size reduction (Table 1), the primary operating parameters for the alkali pretreatment were the treatment temperature, the treatment time, the SLR and the ALR [18]. Higher SLRs and ALRs would be needed when using large amounts of water and NaOH reagent, i.e., the NaOH reagent would require recycling or disposal, which could pollute the environment [31]. A previous study showed that the true alkali consumption for wheat straw was 4.72 g NaOH/100 g TS in 24 h, and the maximum alkali consumption was 5.5 g NaOH/100 g TS over a pretreatment period of 30 days [31]. Another study in the literature reported that a 6% ALR was the optimal dose for four NaOH doses (4%, 6%, 8% and 10%) when NaOH was used to treat corn stover to improve biodegradability and biogas production [32]. Thus, a low SLR (1:6) and ALR (1.5–6.0%) were used in this study to avoid producing a waste chemical solution and to reduce the treatment cost. The pH was analyzed as an indirect measure of the NaOH consumption during the pretreatment process. During the treatment time of 3–12 h, the pH was almost constant for the four ALRs; however, the pH decreased dramatically for the ALRs of 1.5%, 3.0%, 4.5% and 6.0% at 24, 48, 72 and 72 h, respectively (Fig. 2). This result showed that a higher ALR required a longer time for NaOH consumption and that the treatment time for the four ALR values did not exceed 72 h. Table 1 shows that the treatment time generally decreased as the treatment temperature increased. A shorter treatment time enhances pretreatment efficiency and economic feasibility. However, increasing the energy consumption and the complexity of the operation at a high pretreatment temperature would restrict the practical application of the pretreatment for large-scale anaerobic digesters [8]. Thus, a relatively low temperature (35 °C) was used in this study. In addition, mesophilic (35 °C) AD is dominant in large-scale digesters; therefore, using the waste heat from the digester effluent can save energy because an additional energy input is not required, thus increasing the ER of an existing digester. Fig. 2 shows the cellulose, hemicelluloses, and lignin contents (based on the VS of the original material) for the extrusion–alkali pretreated rice straw at different operating conditions. The contents of cellulose were almost constant relative to the raw rice straw for all of the pretreatments. The hemicelluloses content decreased more than the cellulose content. Hemicelluloses have a lower molecular weight than cellulose and heterogeneous structures and are therefore easily hydrolysable. The solubilization of hemicelluloses may also be explained by the disruption and breaking of hydrogen bonds by an alkaline solution [3]. In contrast, the decrease in the lignin content was much higher than that of the cellulose and the hemicelluloses contents. The lignin removal rate increased from 8.0% to 39.5% as the ALR was varied from 1.5% to 6.0% after 120 h of pretreatment. Both the treatment time and the ALR significantly affected lignin solubilization. For treatment times below 12 h, no effects on lignin solubilization were observed, regardless of the ALR. When the treatment time exceeded 48 h for ALR values of 3.0–6.0%, the lignin removal rate did not increase further. The lignin removal rate was in accordance with the pH change, which may indicate alkali consumption. The lignin
43
removal was attributed to the breakage of alkali-labile linkages between monomers or the saponification of the ester bonds that crosslink xylan hemicelluloses and lignin [10,31]. 3.2. Anaerobic digestion There are typically four steps involved in the AD of a LB, such as rice straw, for methane production: the solubilization of rice straw organic matter, the hydrolysis of solubilized rice straw organic matter, the acidification of the hydrolyzed products, and methane production. Cellulose, hemicelluloses and lignin associate with each other: cellulose and hemicelluloses, in particular, are firmly packed by lignin, which provides a primary protective barrier for holocelluloses against anaerobic bacterial attack [4,10]. Therefore, the digestibility of LB is closely linked to the removal/solubilization of lignin. As discussed in Section 3.1, the chemical composition of rice straw, especially the lignin content, changed after pretreatment. To maintain its elemental compositions and heat value (based on the VS), rice straw was directly digested for AD without any rinsing or detoxification after pretreatment [6,11,32]. The effects of pretreatment on methane production and ER from rice straw by AD were investigated. 3.2.1. Effects of extrusion combined with alkali pretreatment on methane production from rice straw Fig. 3 presents the effects of the treatment time and the ALR on methane production from rice straw using extrusion combined with alkali pretreatment. For an ALR of 1.5%, there were insignificant differences in the methane yields from the pretreated rice straw for treatment times ranging from 3 to 120 h. This result indicated that lowering the ALR could not open the ‘‘acetyl valve” and partially open the ‘‘lignin valve”, making the feedstock less accessible to anaerobic bacteria, and thus, methane production was not increased [8]. This result was in agreement with the lignin removal rate for an ALR of 1.5% (Fig. 2) and also indicated that lignin solubilization is important for enhancing methane production from lignin cellulosic biomass. Methane production from the pretreated rice straw increased significantly as the ALR and treatment time were increased. For ALRs of 3.0–6.0%, the methane yields from the pretreated rice straw increased slightly for treatment times between 3 and 24 h, improved dramatically for treatment times between 24 and 48 h, and remained relatively stable for treatment times between 48 and 120 h. These results showed that higher ALRs of 3.0–6.0% with shorter treatment times of 3–24 h were not as effective in increasing methane production. A shorter treatment time cannot guarantee complete chemical reaction between NaOH and rice straw. The statistical analysis indicated that ALRs of 3.0–6.0% with treatment times of 48–120 h resulted in significantly higher methane production from rice straw compared to that obtained using an ALR of 1.5%; thus, these ALR values and treatment times were concluded to be effective. However, the methane yield was not significantly different for ALRs of 3.0%, 4.5% and 6.0%. For economic feasibility and pretreatment efficiency, an ALR of 3.0% and a treatment time 48 h are recommended for practical application. 3.2.2. Comparison of effects on anaerobic digestion for different rice straw pretreatment methods It is not clear whether the higher methane yield from rice straw using extrusion combined with alkali pretreatment can be attributed to physical effects (extrusion), chemical effects (NaOH pretreatment), or synergistic effects from the combined extrusion and NaOH pretreatment. Thus, four different pretreatments were conducted to address this issue: milling, extrusion, milling combined with NaOH pretreatment, and extrusion combined with
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Y. Zhang et al. / Applied Energy 160 (2015) 39–48
Fig. 2. Changes in cellulose, hemicelluloses and lignin contents and pH values during extrusion combined with alkali pretreatment.
Fig. 3. Effects of treatment time and ALR on the methane production from rice straw by extrusion combined with alkali pretreatment.
alkali pretreatment. All of the pretreated rice straw consisted of small particles with sizes below 0.45 mm. Milling pretreatment was conducted using a laboratory hammer mill (JP-350A-8, Jiupin Industry & Trade Co., Ltd., Shanghai, China). As expected, the results obtained using the biomass size reduction machine and milling combined with NaOH pretreatment were the same as those obtained using extrusion combined with NaOH pretreatment (temperature 35 °C; time 48 h; SLR 1:6; ALR 3.0%). Fig. 4a shows that as the fermentation process proceeded, the daily biogas production from rice straw using different pretreatment methods first increased, peaked and then decreased. The primary difference among the rice straws from different pretreatments was in the peak values. The maximum peak value was obtained using extrusion combined with NaOH pretreatment. The difference among the peak values indicated that extrusion combined with
NaOH pretreatment was more effective for biogas production than the other pretreatments. Fig. 4b shows the cumulative biogas productions from rice straw based on the daily biogas production for different pretreatment methods. The statistical analysis demonstrated that the total biogas production from extrusion and milling combined with NaOH was significantly higher than that for the other pretreatments. This result indicated combining extrusion with alkali pretreatment had a synergistic effect on biogas production and was not purely physical or chemical effects. Fig. 4c shows the cumulative methane content obtained for different pretreatment methods. In the first 10 days, the methane contents for the four pretreatments were relatively low because of the rapid growth of fermentative and acetogenic bacteria that converted organic matter into intermediate metabolites (e.g., carbon dioxide, hydrogen and acetic acid) [11]. The cumulative methane content then
45
Daily biogas production (mL/g VS)
Y. Zhang et al. / Applied Energy 160 (2015) 39–48
(a) Milling
60
Extrusion M+NaOH E+NaOH
40
20
0 10
0
20
30
40
Cumulative biogas production (mL/VS)
Digestion time (d)
(b)
10d
30d
40d
400 300 200
ðC6 H10 O5 Þn þ nH2 O ! 3nCO2 þ 3nCH4 5 5 ðC5 H8 O4 Þn þ nH2 O ! nCO2 þ nCH4 2 2
100
ð1Þ ð2Þ
0 Milling
Cumulative methane contents
20d
500
into non-biodegradable substances (primarily lignin and ash) and biodegradable substances, which can further be classified into substances that are readily biodegradable (cellulose, hemicelluloses, pectin, proteins, etc.) and substances that are difficult to biodegrade (waxes, tannins, etc.) [7,30]. LB has a complex chemical composition; thus, the methane yield from LB is based on the TS or VS in many studies in the literature. Most of the cellular contents are water soluble and partially readily biodegradable, making it difficult to distinguish between those cellular contents that are converted into methane and those that are water soluble. In contrast, anaerobic bacteria can readily degrade cellulose and hemicelluloses, which if will convert into methane, associate with lignin and remain as anaerobic digested residues. Therefore, the total contribution of cellulose, hemicelluloses and other components to methane production from rice straw was investigated for different pretreatment methods. The theoretical methane production of cellulose and hemicelluloses was calculated using Eqs. (1) and (2). At STP conditions, 415 mL and 424 mL of methane were produced per g of the cellulose and hemicelluloses, respectively, which were degraded by AD. Fig. 5a shows that the cellulose, hemicelluloses and other components contributed 77.0–107.9, 66.3–82.9 and 43.7–97.3 mL of CH4/VS for the four pretreatments. Cellulose and hemicelluloses contributed 66.2–76.7% of the methane produced for the four pretreatments. That is, the higher the biodegradation of the cellulose and hemicelluloses, the higher was the methane production after pretreatment.
(c)
10d
Extrusion 20d
M+NaOH 30d
E+NaOH 40d
55%
50%
45%
40% Milling
Extrusion
M+NaOH
E+NaOH
(b)
Fig. 4. Anaerobic digestion of rice straw by different pretreatment methods. M + NaOH: milling combined with NaOH pretreatment; E + NaOH: extrusion combined with NaOH pretreatment.
3.2.3. Contributions of cellulose, hemicelluloses and other components to methane production from rice straw by different pretreatment methods LBs, such as rice straw, can be divided into two components: a cell wall that consists primarily of cellulose, hemicelluloses, and lignin, and cellular contents consisting of non-structural carbohydrates, sugars, pectin, tannin, proteins, lipids, waxes, minerals, etc. [30]. In terms of anaerobic biodegradability, LB can be divided
Energy (MJ/kg)
gradually increased to between 50% and 55% for all of the pretreatments because of the growth of methanogenic bacteria that consumed the metabolites to produce methane.
15
10
5
0 Rice straw
Milling
Extrusion M+NaOH
E+NaOH
Fig. 5. Contributions of cellulose, hemicelluloses and other components to methane production (a) and ER (b) from anaerobic digestion of rice straw by different pretreatment methods. M + NaOH: milling combined with NaOH pretreatment; E + NaOH: extrusion combined with NaOH pretreatment.
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3.2.4. ER from anaerobic digestion of rice straw by different pretreatment methods There are two types of heat values: the higher heating value (HHV) and the lower heating value (LHV). The HHV is the gross heat value or the total energy content and corresponds to the latent heat of the water vapor when the biomass is burnt in air. However, the latent heat of the water vapor cannot be utilized effectively in practice. Therefore, the LHV is used as a measure of the available energy of rice straw [2]. The LHV for rice straw (based on VS) and methane (at STP) were 17.254 MJ/kg and 35.898 MJ/m3, respectively. The energy output values from the AD of rice straw by milling, extrusion, milling combined with NaOH pretreatment, and extrusion combined with NaOH pretreatment were 6.7, 8.2, 8.5 and 10.3 MJ/kg VS, respectively, which corresponded to ER efficiencies of 38.9%, 47.6%, 49.1% and 59.9%, respectively (Fig. 5b). When LB is pretreated for methane production, the energy consumption for the pretreatment process, especially for hydrothermal, thermal, and combined thermal-alkali pretreatments, should be considered in view of the ER efficiency [33,34].
3.3. Investigation of mechanism of extrusion combined with alkali pretreatment to enhance methane production from rice straw 3.3.1. Comparison of physical properties of rice straw for different pretreatment methods Fig. 6 shows the physical properties of rice straw including the WHC, SP, SSA and CrI. The WHC for rice straw that was pretreated by extrusion combined with NaOH pretreatment increased by 51%, compared with the samples that were pretreated with milling, implying that more surfaces were exposed after the extrusion– NaOH pretreatment, in accordance with the increase in the SSA. The SP increased for the three pretreatments other than the milling pretreatment, but the reasons for the increase may have been different for the different pretreatments. In the extrusion pretreatment, the extruder physically caused the rice straw to expand; for milling combined with NaOH pretreatment, the NaOH swelled the fiber; and for extrusion combined with NaOH pretreatment, physicochemical effects caused the expansion of the rice straw. The CrI is an important indicator of LB digestibility. The CrI of rice straw decreased significantly (by 48.37%) after the extrusion pretreatment. The extruder functioned as a biomass size reduction machine and applied vigorous shear forces, rapid heat transfer, effective biomass mixing, and dynamic compression, resulting in the defibration and fibrillation of the LB. The depolymerization of cellulose was attributed to the decrease in the CrI. The CrI
WHC
SSA
CrI
10
1.5
8
1.2
6
0.9
4
0.6
2
0.3
0
0 Milling
Extrusion M+NaOH
Specific surface area (m2/g) Crystallinity index
Specific porosity (mL/g) Water-holding capacity (g/g)
SP
E+NaOH
Fig. 6. Physical properties of rice straw pretreated by different pretreatment methods. M + NaOH: milling combined with NaOH pretreatment; E + NaOH: extrusion combined with NaOH pretreatment.
decreased further for the extrusion combined with NaOH pretreatment, which could be attributed to the removal of amorphous substances, such as lignin and the acetyl group, or the preferential hydrolysis of the amorphous cellulose over that of the crystalline cellulose [10,25], which was in agreement with the change in the chemical composition (Fig. 2). A peeling reaction occurred between NaOH and rice straw, which also broke down the crystalline cellulose and enlarged the porosity and inner surface areas, which was in agreement with the changes in the SP and SSA (Fig. 6). The lower CrI value for the rice straw indicated that more amorphous cellulose was digested by the anaerobic bacteria, which increased the cellulose degradation (Fig. 5a) and biogas production (Fig. 4b). The heterogeneous nature of LB means that other components, such as lignin and hemicelluloses, also affect the digestibility of these materials in addition to the CrI. 3.3.2. Effects of different pretreatment methods on changes in chemical composition of rice straw Compared with the crystalline cellulose content, the lignin content had a relatively higher negative impact on methane production from LB [4]. Lignin significantly limited the amount of cellulose available for degradation and posed a major barrier to LB decomposition. Table 3 shows significant decreases in the lignin content for both the milling combined with NaOH and extrusion combined with NaOH pretreatments, which explained why more methane was produced with these pretreatments than that for the milling pretreatment. The lignin content was also related to the cellulose accessibility: Fig. 6 shows that a lower lignin content corresponded to a higher cellulose accessibility. Table 3 shows that the HWE increased by 42.0–46.5% after the milling combined with NaOH and the extrusion combined with NaOH pretreatments. The HWE consisted of sugar, starch, pectin, tannin, cyclitol, and inorganic matter [29]. Most of these substances have simple chemical structures and low molecular weights and are thus readily biodegradable. The newly produced HWEs were mainly derived from the decomposition of holocelluloses and lignin, as indicated by the changes in the chemical composition (Table 3). This increase in the HWE content contributed to the increase of methane production from rice straw after extrusion combined with NaOH pretreatment, in agreement with Fig. 5(a), which shows that other components contributed to methane production. The HWE in rice straw from milling combined with NaOH pretreatment was not significantly different from the HWE level obtained using extrusion combined with NaOH pretreatment; however, the contribution of the other components to the methane production in the milling combined with NaOH pretreatment was 32.5 mLCH4/g VS lower than that from the extrusion combined with NaOH pretreatment. This result was attributed to that more difficult to biodegrade compounds were structurally disrupted by the extruder and then converted into readily biodegradable compounds. Therefore, both the physical effect (extrusion) and chemical effect (NaOH pretreatment) resulted in higher methane production from rice straw using extrusion combined with NaOH pretreatment. Unlike the HWE, the BEE decreased when NaOH pretreatment was used. BEE are hydrophobic and are normally not readily
Table 3 Chemical composition of rice straw pretreated by different methods. Component
Milling
Extrusion
M + NaOH
E + NaOH
Cellulose (% TS) Hemicelluloses (% TS) Lignin (% TS) HWE (% TS) BEE (% TS)
30.0 ± 1.1 29.8 ± 0.9 6.5 ± 0.4 22.6 ± 1.3 7.1 ± 0.5
29.8 ± 1.0 29.7 ± 1.9 6.5 ± 0.4 22.8 ± 1.1 7.1 ± 0.5
28.4 ± 0.9 25.8 ± 1.2 5.0 ± 0.3 32.1 ± 1.4 3.5 ± 0.1
28.7 ± 1.2 25.4 ± 0.7 4.8 ± 0.1 33.1 ± 1.6 3.3 ± 0.2
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b 1372 1726 1635 1606 1515 1428
2900 2920
3300
3000
898 1163
3420
3600
a
2700
2400
2100
1800
1500
1200
900
600
Wave numbers /cm-1 Fig. 7. FTIR spectra of rice straw: (a) milling pretreatment; and (b) extrusion combined with NaOH pretreatment.
biodegradable by anaerobic bacteria. BEE primarily consists of waxes, fattiness, resins, tannins, and pigments [29]. The waxes on the LB surface make degradation by anaerobic bacteria difficult. Reducing the amount of waxes can improve the cellulose accessibility and expose more cellulose to anaerobic bacteria. This result may also explain the increase in the cellulose accessibility (Fig. 6) and methane production from rice straw by extrusion combined with NaOH pretreatment.
3.3.3. Change in chemical structures of rice straw during extrusion combined with alkaline pretreatment The FTIR spectra of the rice straw that was pretreated by milling and extrusion combined with NaOH were also analyzed (Fig. 7). The band at 3348 cm1 corresponded to OH-stretching, and the absorption intensities decreased for the samples that were pretreated with NaOH in combination with extrusion, demonstrating that the cellulose hydrogen bonds were destroyed [10]. The band at approximately 2919 cm1 was assigned to CH2-stretching, and the absorption intensities decreased for the samples that were pretreated with NaOH combined with extrusion, indicating a decrease in the aliphatic fractions of the waxes [35]. The band at 2900 cm1 was attributed to C–H-stretching, and the absorption intensities decreased for the samples that were pretreated with NaOH combined with extrusion, demonstrating the partial release of the methyl, methylene, and methane groups of cellulose [7]. The band at 1726 cm1 represented CO-stretching, and the decrease in the absorption intensities indicated de-esterification from extrusion combined with NaOH pretreatment of the rice straw. The aromatic skeletal vibration ranging from 1465 to 1610 cm1 was assigned to lignin and indicated that lignin was dissolved during extrusion combined with NaOH pretreatment [25]. The band intensities at 898 cm1 that were associated with the b-(1 ? 4)-glycosidic linkages were lower for the samples from the extrusion combined with NaOH pretreatment than those for the samples that were pretreated by milling, indicating the cleavage of acetyl groups [10]. The above results showed that (1) substances that were difficult to biodegrade (waxes) and non-biodegradable substances (lignin) by AD were partially solubilized/degraded; (2) the association of lignin with holocelluloses decreased; and (3) cellulose and hemicelluloses were partially degraded by extrusion combined with NaOH pretreatment. These structural changes in the rice straw that was pretreated using extrusion combined with NaOH treatment further confirmed the changes in the physical properties (Fig. 6) and the chemical composition (Table 3). These changes enabled the rice straw to be easily accessed, attacked and digested by anaerobic bacteria and enhanced the biodegradability of the rice straw and methane production.
4. Conclusions We investigated extrusion combined with alkaline pretreatment for methane production from rice straw. An ALR of 3.0%, a treatment temperature of 35 °C, and a treatment time of 48 h were recommended. Under these conditions, methane production increased by 54.0% over that obtained from samples pretreated by milling. An ER efficiency of 59.9% was obtained for rice straw that was pretreated by extrusion combined with alkali treatment. The increase in methane production and the ER were attributed to changes in the physical properties, chemical composition and chemical structure of LB, demonstrating the synergistic effects of combining extrusion with alkali pretreatment. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Nos. 20976139, 21246001, 51138009), the National Key Technologies R&D Program of China (No. 2012BAJ25B02), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRY11004), New Century Excellent Talents in University (NCET-11-0391) and the Project of Shanghai Science and Technology Commission (No. 14XD1403700). References [1] BP. Statistical review of World energy 2013.
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