Article pubs.acs.org/EF
Pretreatment of Rice Straw for the Improvement of Biogas Production Maziar Dehghani,† Keikhosro Karimi,*,†,‡ and Morteza Sadeghi† †
Department of Chemical Engineering and ‡Industrial Biotechnology Group, Institute of Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran ABSTRACT: Improvement of biogas production by pretreatment of rice straw with 0.25 and 0.5 M sodium carbonate at 90, 110, and 130 °C and for 1, 2, and 3 h was investigated, and promising results were obtained. All treated and untreated rice straws were digested under mesophilic conditions (37 °C). Increasing the concentration of sodium carbonate showed significant improving effects, whereas the treatment time showed less impact. The best results, obtained by pretreatment with 0.5 M sodium carbonate at 110 °C for 2 h, resulted in the production of 292 mL of CH4 per gram of VS, whereas untreated rice straw produced 130 mL of CH4 per gram of VS. Compositional, SEM, and FTIR analyses confirmed that structural modification, lignin removal, and cellulose crystalline reduction are responsible for the improvement.
1. INTRODUCTION Nowadays, energy is one of the most important parts of human life. Fossil fuels, major sources of energy worldwide, are coming to an end. Furthermore, the consumption of these types of energy results in many serious environmental and human health problems. Therefore, the development of environmental friendly and renewable fuels is highly encouraged.1,2 Biogas is one of the promising routes to supply the future energy demand. Significant reduction of greenhouse gas emissions and the possibility of production from widely available and low-cost feedstocks are among the advantages of biogas application.3,4 Agriculture residues such as rice straw have a great potential for biogas production. Rice straw is one of the low-cost lignocellulosic waste materials that is widely produced.5 The estimation of annual worldwide rice production in 2014 by the Food and Agriculture Organization of the United Nations was about 744.4 million tones, where 1−1.5 kg straw is produced per each kilogram of rice. Therefore, a large amount of waste rice straw is produced annually that could be converted to biogas.6 Lignocelluloses wastes contain cellulosic and hemicellulosic sugars, which are suitable for biofuel production. However, the cellulose fibers of native rice straw are highly crystalline and protected by lignin and hemicelluloses.3 Because of this recalcitrant structure, carbohydrate parts of the straw are inaccessible to anaerobic bacteria for biogas production, resulting in low yields of methane production.7,8 Thus, a pretreatment process is crucial to increase the biogas production yield.7,9 There are several different processes for the pretreatment, including physical, chemical, and biological techniques.7 Among the chemical pretreatments, alkali treatments are highly effective. Alkali pretreatment can remove lignin and reduce the crystallinity of lignocelluloses. Intraparticle porosity and channel size of lignocelluloses are also increased by alkali treatments.10 However, alkaline pretreatments are typically expensive and accompanied by the formation of hazardous materials. Sodium carbonate is an inexpensive and environmental friendly chemical that could be used for the pretreatment. Promising © 2015 American Chemical Society
results were observed for the improvement of bioethanol production from some lignocellulosic materials such as wheat straw and corn straw after pretreatment with sodium carbonate.7,11−13 However, to our knowledge, this pretreatment has not been evaluated for enhancement of biogas production from lignocelluloses. The aim of this study was to evaluate the effects of sodium carbonate pretreatment for the improvement of biogas production from rice straw. A pressurized reactor was used for the pretreatments at different temperatures, time intervals, and sodium carbonate concentrations. Compositional and structural analyses were performed to indicate the effects of pretreatments on the straw.
2. MATERIALS AND METHODS 2.1. Raw Materials. Rice straw from the cultivar Sazandegi was used in this research, obtained from a rice field located in Lenjan area in Isfahan province, Iran (32°34′ N, 51°32′ E). It was milled with a domestic miller (Feller-Model EG 500) and screened to achieve particles with 177−833 μm (20−80 meshes). 2.2. Pretreatments. A stainless-steel high-pressure-jacketed reactor equipped with a temperature controller and mixer with a 3.5 L working volume was used for all pretreatments (Figure 1). The pretreatment was conducted at three different temperatures (90, 110, and 130 °C), three different retention times (1, 2, and 3 h), and two different concentrations of sodium carbonate (0.25 and 0.5 M). Initially, 2 L of the reactor was filled by sodium carbonate solution in required concentration. The reactor was heated up to around 90 °C by circulating the hot oil in the jacket. To avoid any possible oxidation defect, all the enclosed air was purged using nitrogen. Then, 100 g of rice straw was injected into the reactor (5% solid loading). To avoid solution evaporation and any consequent change in concentration, the increase of reactor pressure to the desire pressure was maintained by nitrogen purging, leading to minimization of the evaporation of solutions to the minimum possible amount. The desired temperature was achieved and controlled by circulation of hot oil. The temperature Received: April 5, 2015 Revised: May 24, 2015 Published: June 1, 2015 3770
DOI: 10.1021/acs.energyfuels.5b00718 Energy Fuels 2015, 29, 3770−3775
Article
Energy & Fuels
Gas analysis was accomplished by a gas chromatograph (Sp-3420A, TCD detector, Beijing Beifen Ruili Analytical Instrument Co.), equipped with a packed column (Porapak Q column, Chrompack, Middelburg, The Netherlands). Nitrogen was the carrier gas at a flow rate of 20 mL/min at 60 °C. A 100 μL pressure-tight gas syringe (VICI, Precision Sampling Inc., USA) was used to take samples from the headspace of the biogas reactors. The samples were then directly injected into the gas chromatograph. To equilibrate the pressure of reactors, excess gas was released. Having the measurements accomplished before and after the releasing of gas, the amount of gas produced over the time period between two measurements was calculated. Pure methane and pure carbon dioxide were used as standard gases, and all reported results of methane volume are based on standard conditions. Structural effects of pretreatments by sodium carbonate were investigated by scanning electron microscopy (SEM) analysis. The samples were mounted on double-sided tape placed on aluminum stubs. The samples were coated with gold (BAL-TEC SCD 005) and analyzed by a scanning electron microscope (PHILIPS, XL30) with an accelerating voltage of 15 kV. The crystallinity and structural analysis of both treated and untreated rice straw were examined by a Fourier transform infrared (FTIR) spectrometer (Bruker Tensor 27 FT-IR) equipped with a universal ATR (attenuated total reflection) accessory and a DTGS detector. Its spectra were obtained on average of 64 scans and a resolution of 2 cm−1 in the range of 600−4000 cm−1.
3. RESULTS 3.1. Biogas Production. Figures 2 and 3 represent methane yield of untreated rice straw and the straw treated at different temperatures, time intervals, and sodium carbonate concentrations after 15, 30, and 50 days of anaerobic digestion. Over the first 15 days of digestion, the pretreatments had no significant effects on the methane productions, whereas significant effects appeared after 30 days. Untreated straw has higher hemicellulose levels than the treated straw, which is more digestible than native cellulose. When the straw was treated, a part of lignin was removed, and the cellulose structure was significantly modified. This could be the reason for higher overall methane yields; however, this improvement was observed after 15 days of digestion because the microbial hydrolysis of cellulose is typically a limiting and timeconsuming step in the anaerobic digestion of cellulose. The native straw, which has higher levels of easily digestible carbohydrate in the form of hemicellulose, showed a higher yield of biomethane in the former stages of digestion. The increase of temperature from 90 to 130 °C and time interval from 1 to 3 h in pretreatment with 0.25 M sodium carbonate improved the methane production (Figure 2). In contrast, treatment with 0.5 M sodium carbonate, prolonging the pretreatment, and temperature did not show a significant improvement in methane production (Figure 3). The straw pretreated with 0.5 M sodium carbonate at 110 °C for 2 h demonstrated the highest improvement among the pretreatments, resulting in production of 292 mL/g VS methane (125% improvement compared with the untreated straw). The treatment with 0.25 M sodium carbonate at 90 °C for 1 h led to production of 184 mL/g VS methane, which was the lowest improvement (41% improvement compared with the untreated straw) among all pretreatment used in this study. 3.2. Effects of Pretreatments on the Composition of Rice Straw. The composition of treated and untreated rice straw is represented in Table 1. The untreated mainly contained carbohydrates (70.2%), lignin (18.3%), and ash (5.3%). Lignin and ash contents were significantly reduced by
Figure 1. Scheme of pretreatment reactor. of 90, 110, and 130 °C was reached after 2, 10, and 15 min, respectively. A mixer at 150 rpm ensured the uniformity of concentration and temperature throughout the reactor. At the desired retention time, a 100 mL sample was taken and immediately cooled down to room temperature by cold water. The solid fraction was separated by filtration and washed with distillated water to obtain a clean pretreated rice straw with neutral pH. The pretreated straw was dried at room temperature for 3 days and kept in plastic until use. 2.3. Biogas Production. All treated and untreated rice straws were prepared in 118 mL bottles with rubber septa to be further digested under mesophilic conditions (37 °C) according to the method and conditions presented by Shafiei et al.14 Each bottle contains 0.25 g of volatile solid (VS) of substrate, 5 mL of distilled water, and 20 mL of inoculum obtained from a 7500 m3 biogas digester (Isfahan Municipal Wastewater Treatment Plant, Isfahan, Iran). To reach anaerobic conditions, pure N2 was flushed at headspace for about 2 min. During the experimental periods, the methane and carbon dioxide contents of the reactors were measured on a regular basis. All experimental setups were run in triplicate. 2.4. Analytical Methods. Total solid (TS), VS, and ash contents of all treated and untreated materials were determined through drying at 105 °C for about 3 h and subsequent heating at 550 °C for 1 h, according to the standard method presented by Sluiter et al.15 Carbohydrate (cellulose and hemicellulose) and lignin contents of all treated and untreated rice straw were determined according to the standard procedure provided by NREL.16 In this method, a two-step acid hydrolysis with concentrated and diluted sulfuric acid was performed to liberate sugars from the cellulose and hemicellulose. The formed sugars were then quantified by HPLC. The acid-soluble lignin content was determined using UV spectroscopy at 205 nm, and acidinsoluble lignin content was measured by burning the samples at 575 °C. All lignin and carbohydrate analyses were performed in duplicate. Biogas production including methane and carbon dioxide was analyzed every 3 days at the beginning (up to 15 days) and every 5 days for the rest of the experiments. 3771
DOI: 10.1021/acs.energyfuels.5b00718 Energy Fuels 2015, 29, 3770−3775
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Figure 2. Yield of methane production from rice straw pretreated with 0.25 M Na2CO3 for (a) 1 h, (b) 2 h, and (c) 3 h as well as untreated straw after 15 days (white-filled columns), 31 days (light-gray-filled columns), and 47 days (dark-gray-filled columns) of anaerobic digestion.
Figure 3. Yield of methane production from rice straw pretreated with 0.5 M Na2CO3 for (a) 1 h, (b) 2 h, and (c) 3 h as well as untreated straw after 15 days (white-filled columns), 31 days (light-gray-filled columns), and 47 days (dark-gray-filled columns) of anaerobic digestion.
rice straw with 0.5 M sodium carbonate at 110 °C for 2 h, at which the highest methane yield was obtained. The crystallinity index for cellulose is defined as the ratio of peaks at 1430 and 898 cm−1, in which the bands denote cellulose I and cellulose II, respectively.17 Crystallinity index was 0.718 for the untreated sample, which was reduced to 0.679 by pretreatment with 0.5 M sodium carbonate at 110 °C for 2 h. Total crystallinity index, defined as the ratio of peaks at 1375 and 2900 cm−1,17 was 1.841 and 1.889 for the untreated and treated rice straws, respectively. According to the FTIR results, the amount of lignin in the treated rice straw was also lower compared to that of untreated sample. SEM images of the untreated straw and the straw treated with 0.5 M sodium carbonate at 110 °C for 2 h were used to visualize the possible structural modification caused by the pretreatment (Figure 5). As can be inferred from the figure, the
pretreatments at all conditions, whereas the carbohydrates fractions were increased. The analyses demonstrated that increasing the sodium carbonate concentration from 0.25 to 0.5 M as well as the temperature from 90 to 130 °C resulted in more lignin and ash removal. Table 2 illustrates the percentage of VS and TS of the untreated and treated rice straws. Untreated sample VS was 86%, which was increased by the pretreatments. The results of VS and TS correspond to the carbohydrate analyses in Table 1. These results also show that prolongation of the pretreatment led to the increase of VS content, indicating higher carbohydrate material content in the treated materials. 3.3. Effects of Pretreatment on Rice Straw Structure. Effects of the alkali pretreatment on the structure of rice straw were investigated through FTIR analysis. Table 3 and Figure 4 represent FTIR analysis results for the untreated and treated 3772
DOI: 10.1021/acs.energyfuels.5b00718 Energy Fuels 2015, 29, 3770−3775
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Energy & Fuels Table 1. Chemical Composition of Untreated and Treated Straws pretreatment conditions sodium carbonate (M)
time (h)
temperature (°C)
0.25 0.25 0.25 0.5 0.5 0.5
2 2 2 2 2 2 untreated
90 110 130 90 110 130
ash (%) 2.1 1.9 1.8 1.9 1.4 1.5 5.3
± ± ± ± ± ± ±
0.1 0.2 0.1 0.1 0.2 0.1 0.2
lignin (%) 12.8 11.5 10.7 11.3 9.7 9.2 18.3
± ± ± ± ± ± ±
carbohydrates (%)
0.5 1.0 0.7 0.4 0.8 0.2 0.5
80.1 82.3 80.0 80.4 81.2 80.5 70.2
± ± ± ± ± ± ±
1.4 0.5 2.0 2.0 1.5 0.5 1.0
Table 2. TS (%) and VS (%) Contents of Untreated and Treated Straws pretreatment conditions sodium carbonate (M)
temperature (°C)
time (h)
TS (%)
VS (%)
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
90 90 90 110 110 110 130 130 130 90 90 90 110 110 110 130 130 130 untreated rice straw
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
94.7 95.0 94.4 95.3 95.2 95.2 96.6 96.0 96.0 94.9 94.6 95.0 96.3 95.9 96.0 96.5 96.2 95.6 94.8
91.3 91.7 91.2 87.3 88.0 88.6 87.7 87.9 88.6 90.8 90.9 91.2 91.9 92.1 92.1 92.2 91.9 90.8 86.0
Figure 4. FTIR spectra of (1) rice straw treated with 0.5 M sodium carbonate at 110 °C for 2 h and (2) untreated rice straw.
4. DISCUSSION Rice is a plant that is cultivated in large capacity in different parts of the world, and its straw is mainly useless. Rice straw contains cellulosic and hemicellulosic sugars, which are suitable for biofuel production.18,19 However, the yield of biogas from the native straw is inefficient. The resistance of the straw to biodegradation is related to the compact structure, high crystallinity of cellulose, and presence of lignin. In the present study, structural modification was performed by sodium carbonate pretreatment under different process conditions. The pretreatment with Na ions under alkali conditions at elevated temperature is very complicated, and a large number of reactions may take place. Dissolution of carbohydrates,
treated sample had undergone significant structural changes. The untreated straw surface seems to be very compact and inaccessible. The pretreated sample seems to have an openedup, disintegrated, and accessible structure.
Table 3. Characteristics and Variations of Bands in FTIR Spectra of Treated and Untreated Straws band intensity wave number (cm−1)
functional group
3175 2918 1726 1627 1598 1512 1465 1423 1430 1375 1335 1315 1158 896
−OH stretching intramolecular hydrogen bonds C−H stretching CO stretching of acetyl or carboxylic acid CC stretching of the aromatic ring CC CC stretching of the aromatic ring asymmetric bending in C−H3 C−H2 symmetric bending C−H2 bending C−H bending −OH (in plane bending) C−H2 wagging C−O−C asymmetric stretching asymmetric stretching, out-of-phase ring stretching (cellulose)
band assignment
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cellulose II cellulose hemiellulose and lignin lignin lignin lignin lignin cellulose cellulose cellulose cellulose cellulose cellulose cellulose
untreated rice straw
pretreated rice straw
0.124 0.005 0.010 0.017 0.015 0.017 0.013 0.016 0.016 0.017 0.017 0.018 0.026 0.022
0.114 0.005 0.006 0.009 0.009 0.013 0.013 0.016 0.016 0.015 0.015 0.015 0.023 0.023 DOI: 10.1021/acs.energyfuels.5b00718 Energy Fuels 2015, 29, 3770−3775
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pretreatment with sodium carbonate would increased methane production. According to the results of pretreatments, the increase in sodium carbonate concentration from 0.25 to 0.5 M had the highest impact on methane production yield, compared to the impact of 20 °C temperature rises from 90 to 110 °C or 110 to 130 °C and 1 h time interval prolongation from 1 to 2 h or 2 to 3 h. There are some studies which confirmed improvement of methane production from lignocellulosic materials by alkali pretreatments, such as Salehian et al.3 that used NaOH pretreatment and achieved 181.2% improvement in methane production yield from pinewood under the best pretreatment conditions (8% w/w NaOH at 100 °C for 10 min). Also, in the Zhong et al. study,4 biogas production from corn straw was about 17% higher after alkali pretreatment compared with the biological pretreated straw. Besides being a highly efficient means of pretreatment, sodium carbonate is more environmental friendly and available at lower prices in comparison with other alkali agents.
5. CONCLUSIONS The present study demonstrated that it is possible to produce appreciable amounts of biogas from rice straw after pretreatment with sodium carbonate. However, the efficiency of the treatment highly depends on time, sodium carbonate concentration, and temperature of the pretreatment. The highest improvement was observed after pretreatment with 0.5 M sodium carbonate at 110 °C for 2 h. The main reasons for biogas improvement were significant reduction in the crystallinity of cellulose and lignin content.
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Figure 5. SEM images of (a) untreated rice straw and (b) straw pretreated with 0.5 M Na2CO3 at 110 °C for 2 h.
AUTHOR INFORMATION
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formation of alkali-stable end groups (peeling-off reactions), hydrolysis of carbohydrates and acetyl groups, transformation of cellulose I into cellulose II, and decomposition of solubilized carbohydrates are among the reactions. In contrast, fragmentations, degradation and, delignification, and condensations of lignin are among the reactions in the pretreatment with Na ions under alkali conditions. Moreover, like other alkaline treatments, sodium carbonate treatment is an effective method to break the ester bonds between lignin, hemicellulose, and cellulose, which resulted in release of lignin.9 Furthermore, the sodium ions diffuse into the lignocellulose structure and change the polyionic characters of the biomass. These ions act as a countercharge to carboxylate ions in the biomass. This polyionic character of the pretreated materials promotes swelling of the substrate. Therefore, the accessibility of carbohydrates to microorganisms is increased.10 The best improvement was achieved by pretreatment with 0.5 M sodium carbonate at 110 °C for 2 h, resulting in a 125% improvement in the yield of methane production. The analysis showed that lignin removal, crystallinity reduction, transformation of cellulose I to cellulose II, and structural modification were responsible for the improvements. Lower biomethane yield at severe conditions (e.g., pretreatment at 130 °C for 3 h with 0.5 M Na2CO3) may be related to the formation of alkali-stable end groups (peeling reactions of carbohydrate end groups).10 Furthermore, the results of the current work indicated that reduction of cellulose crystallinity is also possible by this pretreatment.7 All in all, it could be claimed that alkali
Author Contributions
M.D. did all the experiments, and the manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Institute of Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan, Iran.
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NOMENCLATURE TS = total solid VS = volatile solid FTIR = Fourier transform infrared spectroscopy SEM = scanning electron microscope HPLC = high-performance liquid chromatography GC = gas chromatography REFERENCES
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DOI: 10.1021/acs.energyfuels.5b00718 Energy Fuels 2015, 29, 3770−3775
