Article pubs.acs.org/EF
Extrusion as Pretreatment for Boosting Methane Production: Effect of Screw Configurations Radziah Wahid,*,†,‡ Maibritt Hjorth,† Simon Kristensen,† and Henrik Bjarne Møller† †
Department of Engineering, Aarhus University, Blichers Allé 20, DK 8830 Tjele, Denmark Faculty of Chemical Engineering, Universiti Teknologi Mara, 40450 Shah Alam, Malaysia
‡
ABSTRACT: A major barrier that limits degradation of the lignocellulosic materials during biogas production is the structure itself. Without pretreatment, enzymatic attack during hydrolysis is not effective and leads to a poor yield of biogas. Thus, in this study, the effect of extrusion as pretreatment on wheat straw and deep litter was evaluated. Five screw configurations of the extruder were tested, namely, mild kneading, long kneading, reverse, kneading and reverse, and kneading with reverse. Sugar availability and biogas potentials from extruded samples were examined. Energy consumption, barrel temperature, and residence time of samples during extrusion were measured. The results showed an increment in methane yields of about 4−29 and 1−16% of extruded samples after 28 and 90 days of anaerobic digestion, respectively. A strong positive correlation (R2 = 0.70) was observed between ultimate sugar availability and methane yields at 28 days. Increases in sugar availability (7−42%) accelerate degradation of the biomasses at the early digestion phases, resulting in higher yield of methane. Extrusion was less effective on deep litter because of the soft texture and possibility of sugar hydrolysis during storage.
1. INTRODUCTION Biogas production through anaerobic digestion offers great potential for substituting fossil fuels, because methane can be used in heat and power generation and for vehicle fuel. Greenhouse gases (GHGs) can be reduced using local resources, and digestate from an anaerobic digester is a valuable fertilizer because of the high nitrogen content. Lignocellulosic biomass has huge potential as a substrate for biogas production because of its availability and carbon resource.1 However, factors such as cellulose crystallinity, complex structure of lignin, and coating of cellulose microfibrils by hemicelluloses are the physical barriers that limit degradation of insoluble polymer during anaerobic digestion.2 Hence, application of pretreatment on lignocellulosic biomass is crucial. Pretreatment alters the size and structure of lignocellulosic materials as well as chemical composition and improves hydrolysis of the carbohydrate fraction to simple sugars during anaerobic digestion.3,4 Physical, thermal, and chemical pretreatments demonstrate an increase in sugar recovery of raw biomass; however, these methods have several drawbacks, such as high costs and energy consumption, degradation of products, and production of fermentation inhibitors.3−6 Extensive studies are still continuing, focusing on the optimal pretreatment process with low energy consumption and low operation costs. Extrusion is a thermomechanical process, and its potential as a pretreatment for lignocellulosic materials has been investigated.7 During the process, biomass is subjected to heating, mixing, and shearing, which increase the surface area, pore size, and disrupt the structure of the lignocellulosic materials.8 Extrusion is favorable because it is a continuous process and suitable for large-scale production.2 An extruder typically consists of three sections, namely, feed, transition, and compression and metering.2 The quality of extruded materials is relying on extruder parameters, such as extruder type, screw © 2015 American Chemical Society
configurations, feed moisture content, screw speed, feed rate, and barrel temperature.9 Screw speed and barrel temperature are the parameters that are mainly manipulated during extrusion. Chen et al.10 and Karunanithy and Muthukumarappan8 evaluated the effects of screw speed and barrel temperature on sugar recovery of extruded rice straw10 and corn stover.8 In the studies, screw speed and barrel temperature were varied between 25 and 150 rotations per minute (rpm) and between 25 and 160 °C, respectively. Chen et al.10 observed that screw speed and barrel temperature had no effect on the sugar concentration of rice straw. In contrast, the sugar yield from an extruded corn stover was higher, with a screw speed of 75 rpm and barrel temperature of 125 °C.8 Manipulation of screw configurations during extrusion is less exploited, and it may has a significant impact on the physical manipulation, from mild to harsh screw treatment. Information on extruder application for biogas production is still scarce because most publications are focusing on bioethanol purposes.7,10−16 Few studies reported an increase in the biogas yield when extrusion was used as pretreatment for different biomasses. Hjorth et al.7 found that extrusion accelerates degradation of five agricultural biomasses because 18−70% of the methane yield was increased within the initial 28 days. A similar observation by Menardo et al.16 as the degradation time of extruded maize, ryegrass, and rice straw was faster compared to raw samples. Extrusion also increased 33 and 72% of the specific methane yield of rice straw compared to milled and raw samples, respectively.15 Special Issue: 2nd International Scientific Conference Biogas Science Received: January 26, 2015 Revised: April 20, 2015 Published: April 21, 2015 4030
DOI: 10.1021/acs.energyfuels.5b00191 Energy Fuels 2015, 29, 4030−4037
Article
Energy & Fuels
Figure 1. Photographs of the (a) Xinda 65 mm twin screw co-extruder with a split barrel and (b) screw configurations of the extruder.
Figure 2. Screw configurations of the extruder at different treatments. Description of kneading example, kneading 30°/7/64(2): 30°, angle between kneas; 7, number of individual kneas; 64, kneading element length (mm); and (x), number of replicates.
The aim of this study was to examine the effect of screw configurations of a co-rotating extruder on wheat straw and deep litter for biogas potentials. Five configurations were tested, and the configurations were manipulated to cause various types of shear on the materials. Wheat straw and deep litter at 40% of the dry matter content were examined. Biogas yield and sugar availability from untreated and extruded samples were analyzed.
Temperature, residence time, and energy consumptions of the extruder were also measured.
2. MATERIALS AND METHODS 2.1. Biomass. Two biomasses were tested in the study, i.e., wheat straw and deep litter. Deep litter was prepared manually to ensure sample homogeneity and reproducibility. Cattle manure (CM) was obtained from Research Centre Foulum (Aarhus University, Denmark), and wheat straw was collected near Viborg (Denmark). About 4031
DOI: 10.1021/acs.energyfuels.5b00191 Energy Fuels 2015, 29, 4030−4037
Article
4032
0.8 0.8 0.7 1.6 3.5 0.9 0.6 1.1 0.9 0.5 0.1
a
41.2 40.7 39.6 39.8 42.6 42.0 39.4 41.0 38.6 40.5 88.9
A1, mild kneading (straw); B1, long kneading (straw); C1, reverse (straw); D1, kneading and reverse (straw); E1, kneading with reverse (straw); A2, mild kneading (deep litter); B2, long kneading (deep litter); C2, reverse (deep litter); D2, kneading and reverse (deep litter); E2, kneading with reverse (deep litter); and ND, not determined. ± indicates standard deviation of the mean.
± ± ± ± ± ± ± ± ± ± 314.4 302.6 313.5 327.3 323.1 304.4 306.9 294.0 291.6 300.5 7.8 20.9 2.9 29.4 9.6 7.4 29.5 11.7 14.1 7.9 ± ± ± ± ± ± ± ± ± ± 260.2 255.5 258.3 277.3 274.4 246.6 260.8 246.4 245.6 250.7 0.2 0.8 0.4 0.7 0.5 1.0 1.4 0.4 1.1 0.2 ± ± ± ± ± ± ± ± ± ± 10.8 11.7 10.5 11.1 11.2 10.7 10.5 10.0 10.3 10.3 0.8 3.5 1.0 1.9 0.9 0.7 0.2 0.4 0.8 1.3 ± ± ± ± ± ± ± ± ± ± 40.1 38.6 39.9 38.6 40.0 41.1 42.0 37.7 38.6 38.8 0.8 3.7 1.0 1.9 1.0 0.8 0.2 0.4 0.9 1.4 ± ± ± ± ± ± ± ± ± ± 41.9 40.5 41.8 40.4 41.8 43.8 45.0 40.4 41.4 41.3 295.8 ± 7.3 300.1 ± 13.1 289.9 ± 14.3 283.1 ± 20.2 278.2 ± 24.1 289.9 ± 9.6 305.6 ± 2.9 281.4 ± 14.1 297.9 ± 6.6 285.2 ± 11.0 ND 228.4 ± 5.8 227.6 ± 6.7 220.0 ± 11.6 215.6 ± 14.7 214.5 ± 17.7 231.1 ± 5.8 245.5 ± 0.9 221.7 ± 8.4 236.4 ± 5.3 225.8 ± 11.2 ND 8.9 ± 0.4 9.3 ± 0.6 7.9 ± 1.1 7.8 ± 0.8 8.7 ± 0.5 9.6 ± 0.6 8.5 ± 0.1 8.8 ± 0.6 9.0 ± 0.5 8.9 ± 0.3 ND ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A1 B1 C1 D1 E1 A2 B2 C2 D2 E2 raw straw
39.4 38.8 37.9 38.1 40.7 39.3 36.8 38.2 35.9 38.1 78.3
VS (%) biomass
0.7 0.8 0.6 1.5 3.4 0.9 0.6 1.0 0.8 0.5 0.1
methane yield at 28 days (NL of CH4 kg−1 of VS) dry matter (%) methane yield at 90 days (NL of CH4 kg−1 of VS) methane yield at 28 days (NL of CH4 kg−1 of VS) sugar availability (mg g−1) dry matter (%)
input samples
Table 1. Characteristics of Input versus Output Samples of Extrusion for Wheat Straw and Deep Littera
VS (%)
sugar availability (mg g−1)
output samples
methane yield at 90 days (NL of CH4 kg−1 of VS)
40 kg of each biomass was prepared in total. Biomasses were treated immediately after the specified storage time. Average total solid (TS) and volatile solid (VS) for raw wheat straw were 89 and 78%, while they were 8 and 6% for cattle manure, respectively. Wetted straw (biomass 1) was a mixture of wheat straw and water. The wheat straws were initially chopped to a particle size of 20−25 mm using a quad shaft shredder equipped with an electric control panel (UNTHA RS 30 4-S, Kuchl, Austria). After the addition of water to a TS level of 40%, a cement mixer (ATIKA, Profi 145 S, Germany) was used to ensure uniform mixing of the chopped straw and water. Mixing occurred for 30 min. The biomasses were stored in 6 kg batches in sealed airtight plastic bags for 24 h, under an ambient temperature. Deep litter (biomass 2) was prepared by mixing wheat straw and cattle manure with a ratio of 1:1. Initially, wheat straws were chopped to 100−200 mm and mixed with cattle manure to a TS level of 40% in a cement mixer for 30 min. The mixtures were stored in sealed airtight plastic bags for 72 h, with approximately 6 kg in each, under an ambient temperature. The temperature was measured everyday and was recorded between 37 and 40 °C. After 72 h, the biomass was ground to a 20−25 mm particle size using the shredder (UNTHA RS 30 4-S, Kuchl, Austria) to mimic the animal-treading effect on the soaked deep litter. 2.2. Extruder Treatment. A co-rotating twin screw feeder with a 0.25 m3 hopper equipped with one horizontal paddle mixer (PSHJ-65, Xinda Corporation, Jiangyin, Jiangsu, China) was used in the study. Figure 1a shows the photograph of the Xinda extruder with the label of important components, and Figure 1b demonstrates the screw configurations of the extruder. The screw diameter was 65 mm, and the length/diameter ratio of the barrel was 40. Feeding was performed in a continuous stream with a volumetric feeder at 18 (±2) kg h−1. The screw velocity was 600 rpm, causing the retention time of the biomass to be 30−120 s. The following screw elements were the applied variables for the screw configurations: kneading (30°/7/64, 45°/5/56, 45°/5/88, 60°/4/56, and 90°/5/56, Xinda Corporation) kneading reverse (45°/5/56L), and reverse element (56/28L). Five combinations of the screws, resulting in five screw configurations, were tested (Figure 2). Energy consumption [accumulated kilowatt hour (kWh)] of the extrusion screw engine, screw of volumetric feeder, and control system was logged continuously. The friction-induced temperature increase of the barrel was logged continuously at 10 positions along the extrusion barrel. Treatment capacity was logged continuously with a balance under the output collection container. The residence time of the biomass within the extruder was recorded during normal operation by feeding 20 g of blue color straw directly into the extrusion inlet and measuring the time until the biomass output changed color. Once the extrusion process temperature was stable, three paired input and output samples were collected with 2− 10 min intervals. 2.3. Sample Analysis. All analyses were performed on the three treatment replicates. Samples were stored at −20 °C before analysis. Characteristics of input and output samples from extrusion were summarized in Table 1. 2.3.1. TS and VS. The TS content was described as the weight loss upon drying the samples at 105 °C until a constant weight. VS was defined as the weight loss upon burning the dried samples at 550 °C until a constant weight. The TS and VS of samples were analyzed following the procedure described by the American Public Health Association (APHA).17 2.3.2. Sugar Availability Analysis. The accessibility of the carbohydrates in the biomass was determined by quantifying the sugar released upon an induced enzymatic hydrolysis. A total of 2 g of biomass was dissolved in 0.1 L of a pH 4.8 citrate buffer. Cellulose was included as an external standard.18 Cellulase (Celluclast 1.5 L, Novozymes) was added to 464 mg of a solution with an activity of 798 EGU g−1. Mannase (Novozym 51054, Novozymes) was added to 104 mg of a solution with an activity of 1000 kVHCE g−1. Azide was added for disinfection to avoid microbial degradations to 2 mL of a 1% solution. The hydrolysis was run in an incubator shaker (Innova 43, New Brunswick Scientific Co., Enfield, CT) at 150 rpm at 50 °C until
10.4 28.1 8.6 24.3 7.9 10.3 35.0 12.2 18.0 10.6
Energy & Fuels
DOI: 10.1021/acs.energyfuels.5b00191 Energy Fuels 2015, 29, 4030−4037
Article
Energy & Fuels
Figure 3. Methane yield for biomass 1 (wheat straw) at (a) 28 days and (c) 90 days and for biomass 2 (deep litter) at (b) 28 days and (d) 90 days. Vertical lines indicate ± standard deviation of the mean. A1, mild kneading (straw); B1, long kneading (straw); C1, reverse (straw); D1, kneading and reverse (straw); E1, kneading with reverse (straw); A2, mild kneading (deep litter); B2, long kneading (deep litter); C2, reverse (deep litter); D2, kneading and reverse (deep litter); and E2, kneading with reverse (deep litter). a constant supernatant sugar concentration was reached after 72 h. Subsequently, non-dissolved biomass was removed by centrifugation. Sugar monomers were derivatized by adding dinitrosalicylic acid (DNS), heating to 100 °C for 5 min, and quantifying at 540 nm, according to Wood et al.19 Glucose standards at 0.25−1.0 g L−1 were prepared; thus, the sum of all types of sugar monomers were expressed in glucose units.20 2.3.3. Biogas. Inoculum was collected from a mesophilic postdigester at the full-scale biogas plant in Research Center Foulum, Aarhus University, Denmark. This reactor was operated at an elevated total solid level of 8−9%, because it was fed with high levels of extruder-pretreated (MSZ B 110e, Lehman Maschinenbau GmbH, Germany) lignocellulose-rich biomass. Inoculum was stored 3 weeks at 35 °C to ensure that the biogas production from inoculum was minimized. The inoculum was sieved to remove the larger particles. The average TS and VS of the inoculum were 5.0 and 3.8%, respectively. Average pH of inoculum was 7.9; total nitrogen (TN) was 2.0 g L−1; and volatile fatty acid (VFA) was 0.23 g L−1. The batch test was performed as described by Møller et al.21 A total of 200 g of inoculum was added in each 500 mL infusion bottle, followed by the addition of substrate with a ratio of 1:1 (VSsubstrate/VSinoculum). A control with only inoculum was included. The bottles were incubated at 35 °C for 90 days. The biogas volume was measured 10 times
during the period. The measurement of the biogas volume was performed by inserting a needle connected to a tube with inlet to a column filled with acidified water (pH < 2) through the butyl rubber. The biogas produced was calculated by the water displaced until the two pressures (column and headspace in bottles) were equal. Biogas compositions were analyzed using gas chromatography (7890A, Agilent Technologies, Santa Clara, CA). Methane produced from each sample was corrected by subtracting the volume of methane produced from the inoculum control. Specific methane yields were expressed in NL of CH4 kg−1 of VS (NL = normalized liter, with gas volume corrected to 0 °C and 1.013 bar). 2.4. Data Analysis. Pearson correlation22 was used to examine the relationship of sugar availability and methane production. Factorial design with a significance level of 0.05 was used to evaluate the relationship between the screw treatment replicates of biomass 1 and biomass 2 samples based on sugar availability and methane yield at 28 and 90 days. The analysis was performed using Assistat software, version 7.7 beta.23 The methane yield production at 90 days was modeled by fitting the experimental data to nonlinear regression models (eq 1)24
BMPt = B0 (1 − exp−kt ) 4033
(1) DOI: 10.1021/acs.energyfuels.5b00191 Energy Fuels 2015, 29, 4030−4037
Article
Energy & Fuels
Table 2. Mean Difference Values of Sugar Availability and Methane Yield at 28 and 90 Days for All Five Screw Configurationsa factor 1 (biomass)
factor 2 (screw configuration)
parameter
1
2
A
B
C
D
E
sugar availability (% difference) methane yield at 28 days (% difference) methane yield at 90 days (% difference)
130.4 a 120.2 a 110.4 a
116.8 b 107.9 b 103.1 b
114.3 110.4 105.7
126.1 109.2 103.0
123.3 114.5 107.5
131.9 116.4 106.5
122.4 119.8 111.0
a
1, straw/biomass 1; 2, deep litter/biomass 2; A, mild kneading; B, long kneading; C, reverse; D, kneading and reverse; and E, kneading with reverse. For factors 1 and 2, mean values bearing different lowercase letters (a and b) in the same row are significantly different at p < 0.05.
where BMPt denotes the cumulative methane yield (NL of CH4 kg−1 of VS) at time (t) expressed in days, B0 was the ultimate methane yield (NL of CH4 kg−1 of VS), k is the cumulative methane yield rate constant, expressed in reciprocal of time (day−1), which was substratespecific and gave information about the time required to achieve a certain fraction of B0.24 The squared correlation coefficient (R2) was used to evaluate the precision of the model. k values of different screw configurations were compared statistically using the factorial experiment. The energy balance for electricity of the extrusion process was calculated using eq 2 energy balance = ((Eextruded − Eraw ) − Eprocess)/ Eraw × 100 (2) where Eextruded is the electrical energy from methane produced of extruded biomasses, Eraw is the electrical energy from methane produced of raw biomasses, and Eprocess is the energy consumption during extrusion. The electrical energy content in the methane from the batch test was calculated, and details were explained by Hjorth et al.7
3. RESULTS AND DISCUSSION 3.1. Biomass Effect. The purpose of the extrusion, to increase the methane production of lignocellulose-rich biomass, was confirmed; results showed that extruded samples produced higher methane yields than untreated throughout the initial 28 days of anaerobic digestion (panels a and b of Figure 3 and Table 1). The ultimate methane production of raw wheat straw was on average 289 NL of CH4 kg−1 of VS at elevated levels compared to the literature value of 180−275 NL of CH4 kg−1 of VS.25 This may be explained by the inoculum origin, because the lignocellulose level is above typical operations, and thus, the inoculum is likely well-adapted to lignocellulose degradation. At 28 days, the methane yield from extruded samples was increased significantly: 12−29% for the straw (biomass 1) and 4−11% for the deep litter (biomass 2) (panels a and b of Figure 3). Lower relative increases in the ultimate methane yield after 90 days were observed: 1−16% increases for the straw and 4− 5% increases for the deep litter (panels c and d of Figure 3). Screw configurations, however, had no significant effects toward the methane yield at 28 and 90 days for both biomasses (Table 2). The increase in methane production because of the extrusion was supported by an observed significant increase in the hydrolysis of carbohydrates, i.e., the sugar availability (panels a and b of Figure 4). A significant correlation was observed between the sugar availability and methane yield at 28 days (Figure 5). Higher ultimate sugar availability in samples led to a significant increase in methane production velocity. The thermomechanical extrusion increased the amount of easily available sugar in lignocellulose, which is an energetically attractive carbon source for the anaerobic microorganisms, and caused the biogas production velocity to increase.
Figure 4. Sugar availability for (a) biomass 1 (wheat straw) and (b) biomass 2 (deep litter) at different screw configurations. Vertical lines indicate ± standard deviation of the mean. A1, mild kneading (straw); B1, long kneading (straw); C1, reverse (straw); D1, kneading and reverse (straw); E1, kneading with reverse (straw); A2, mild kneading (deep litter); B2, long kneading (deep litter); C2, reverse (deep litter); D2, kneading and reverse (deep litter); and E2, kneading with reverse (deep litter).
Percentage increases in sugar availability for straw ranged from 21 to 42%, while for deep litter, the increase varied from 7 to 26%. These values were found lower compared to lime pretreatment of wheat straw and crop residue bagasse (90 and 77%)26 and steam explosion of wheat straw, about 46−94% of the sugar availability yield.27 A higher increment of sugar availability observed in a previous study might be due to different parameters used. Beltrame et al.27 used a higher temperature (>200 °C) during steam explosion of wheat straw than the present study ( 0.90. Mean k values of raw samples was used in the statistical analysis because small deviation (standard deviation < 0.003) was observed among the samples (Table 3). The results indicated that k values of extruded samples were significantly increased compared to raw samples for both biomasses. The kneading and reverse configuration (D) had a greater effect toward straw and deep litter as higher biodegradability was examined in comparison to other configurations. In addition, an increase in biodegradability of extruded deep litter was observed when the long kneading configuration (B) was applied. Extrusion thus appears to speed the biological degradation rate of the physically degraded organic compounds, likely because more easily biodegradable biomass was available. 3.2. Extruder Treatment. Pretreatment of materials should be economical; thus, low energy consumption is preferable. The energy consumption by the extruder was within the range of 226−324 kilowatt hour per tonne (kWh t−1) (Table 4). Indeed, energy consumption was high, and a more specialized extruder is mandatory to ensure economical balance at full-scale operation. This has, however, been observed viable in other studies, under conditions approximately comparable to the mild kneading screw configuration (A) tested in this study.7 The lowest electricity consumption for the entire extruder operation of straw was 232 kWh t−1, and the highest was 324 kWh t−1 (Table 4). Pretreatment of deep litter had a tendency to consume less electricity (2−23%) than wet straw, from 226 to 317 kWh t−1. This might be due to the softer texture of the deep litter. The maximum friction-induced temperature along the barrel varied from 80 to 105 °C (Table 2). According to Appels et al.,28 this temperature was within the low-thermal pretreatment range (
