Subscriber access provided by UNIV OF NEWCASTLE
Article
Efficient Biogas and Ethanol Production from Safflower Straw Using Sodium Carbonate Pretreatment Seyed Sajad Hashemi, Keikhosro Karimi, Mohammad Javad Nosratpour, and Ilona Sarvari Horvath Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02468 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Efficient Biogas and Ethanol Production from Safflower Straw Using Sodium Carbonate Pretreatment Seyed Sajad Hashemi1, Keikhosro Karimi1,2, Mohammad Javad Nosratpour1, Ilona Sárvári Horváth3
1
Department of Chemical Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran
2
Industrial Biotechnology Group, Research Institute for Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran 3
Swedish Centre for Resource Recovery, University of Borås, 501 90 Borås, Sweden
1 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Abstract Safflower straw, an agricultural lignocellulosic waste, was used for biogas and bioethanol production. Prior to bioconversion, the straw was pretreated with 0.5 and 1 mol/L sodium carbonate (Na2CO3) at 120, 150, and 180 oC for 1–5 h, resulting in a liquid fraction containing hemicellulosic sugars and a solid fraction mainly containing cellulose. Two scenarios were followed: (I) subjecting both solid and liquid fractions to biogas production and (II) subjecting the solid fraction to bioethanol production and the liquid fraction to biogas production. The highest yield of glucose was 91.4%, achieved after pretreatment at 180 oC with 0.5 mol/L Na2CO3 for 5 h, compared to that of 22.1% for the untreated substrate. The ethanol yield of untreated straw was 11.1%, and it was improved to 58.1% after alkali pretreatment at 180 oC with 1 mol/L Na2CO3 for 2 h. The highest methane yield obtained was 139.6 NmL/gVS after pretreatment at 120 oC with 0.5 mol/L Na2CO3 for 1 h, while that from the untreated straw was only 92.1 NmL/gVS. On the other hand, from the liquid fraction a maximum 88.2 NmL/gVS of methane was produced after pretreatment with 0.5 mol/L Na2CO3 at 180 oC for 2 h. The highest equivalent gasoline (based on 1 ton of safflower straw) for scenario (I) was 113.9 L obtained by pretreatment at 120 oC with 0.5 mol/L Na2CO3 for 1 h. This value for scenario (II) was 100.0 L, observed after pretreatment at 180 oC with 1 mol/L Na2CO3 for 2 h. The equivalent gasoline for untreated straw for scenarios (I) and (II) was 93.6 and 17.1 L, respectively. Keywords: Safflower; biogas; ethanol; Na2CO3 pretreatment; equivalent gasoline
2 ACS Paragon Plus Environment
Page 2 of 32
Page 3 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1. Introduction Due to population growth and industrialization of human societies, global demand for energy has increased.1 Currently, about 88% of the demand is met using nonrenewable fossil fuel resources consisting of natural gas as well as crude oil and coal.2 However, fossil fuel resources are limited and regarding the current rate of energy consumption, it is estimated that oil, natural gas, and coal resources will be exhausted by the next 45, 60, and 120 years, respectively.3 Furthermore, the consumption of fossil fuels has led to the emission of tremendous amounts of greenhouse gases into the atmosphere, which is expected to cause dangerous weather changes during the coming years.4 The limited availability of fossil fuels in addition to the increasing rate of energy consumption and related environmental problems has made it necessary to find alternative, sustainable sources for energy production. Among all sustainable energy resources, biofuels like biogas and bioethanol are among the most important renewable and eco-friendly energy resources.1,5 However, biofuels produced from starch-based and sugar-based agricultural products led to a food-fuel conflict. Therefore, inexpensive, abundant, and renewable lignocellulosic biomass resources are suggested as a reliable raw material for biofuel production.6 Lignocellulosic materials, containing cellulosic and hemicellulosic sugars, are suitable for biofuel production. Nevertheless, their very complex and recalcitrant structure makes them especially resistant against enzymatic and microbial hydrolysis. An efficient single-stage pretreatment leading to the reduction of cellulose crystallinity together with lignin and hemicellulose removal is therefore needed before enzymatic hydrolysis and anaerobic digestion.7,8 Different pretreatment approaches, including physical, chemical, physicochemical, and biological processes, were assessed to enhance the yield of biogas and ethanol production
3 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
from lignocellulosic materials.9 Although some favorable indications in fuel production from cellulosic biomass were observed, pretreatment is still a costly stage in the process. Hence, the application of low-cost, recoverable, and eco-friendly chemicals for the pretreatment step is therefore required.10 Alkaline processes are among the most efficient chemical pretreatment methods, especially in the case of hardwood and agricultural waste. The major impacts of alkaline pretreatment include: removal or modification of lignin and hemicellulose, which leads to increased porosity; reduction of the crystallinity and degree of polymerization of cellulose; and increased water swelling capacity.11,12 Additionally, some other advantages connected to this treatment are decreased hemicellulose destruction and minimized formation of inhibitors. Sodium hydroxide is the most commonly used alkaline agent in the pretreatment of lignocellulosic materials. However, sodium carbonate (Na2CO3), which is a weak base, rather inexpensive, widely available, and an environmentally friendly substance, and has been previously applied for efficient pretreatment of lignocellulosic materials.13 Moreover, it is easier to recover than sodium hydroxide.14 This alkaline agent has been shown to improve enzymatic hydrolysis as well as anaerobic digestion and fermentation of some lignocellulosic materials such as wheat, rice, and corn straw.13,15-17 Khaleghian et al.18 surveyed the effect of sodium carbonate pretreatment on the ethanol yield of rice straw; 83.2% of the theoretical yield was achieved after the treatment, while this value was 39.8% for the untreated sample. Dehghani et al.19 examined the effect of Na2CO3 pretreatment on methane production from rice straw, and they have found that the methane production increased from 130 NmL/gVS (for untreated sample) to 292 NmL/ gVS after pretreatment performed under optimum conditions (0.5 mol/L sodium carbonate at 110 °C for 2 h).
4 ACS Paragon Plus Environment
Page 4 of 32
Page 5 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
After completion of the pretreatment, a slurry, containing a liquid and a solid fraction, is produced. Due to the pretreatment, vast portion of carbohydrates enter into the liquid phase in a form of solubilized monomers or short chain carbohydrates, mainly as a result of dissolution of hemicelluloses. Saccharomyces cerevisiae, often used for alcoholic fermentation in industrial processes, is not able to produce ethanol from the 5-carbon sugars present in this liquid phase.20,21 Recently, the production of bioethanol and biogas utilizing this liquid fraction has been investigated. Safari et al.22 used the liquid fraction obtained after acidic pretreatment of pinewood to produce biogas, which in the best case led to a 1150% increase in total energy production compared to that from the untreated pinewood. This study focuses on maximum biofuel production using safflower straw as a raw material. Na2CO3 pretreatment is chosen prior to biogas and ethanol production. In order to maximize energy output, the liquid fraction obtained after the pretreatment was subjected to anaerobic digestion. In addition, the solid fraction of pretreatment was exploited for either biogas or bioethanol production, and the produced amounts of biofuels were then expressed in terms of gasoline equivalents. Accordingly, two types of energy production scenarios were investigated: (I) biogas production from both the liquid and the solid fractions obtained after the pretreatment, and (II) production of biogas from the liquid fraction and production of ethanol from the solid fraction after the pretreatment. 2. Material and methods 2.1. Raw material Safflower straw was collected from Lavark field (Isfahan University of Technology plantation, Najafabad, Isfahan, Iran) and dried in the shade for three days at ambient temperature. The straw was then milled and screened to achieve particle size less than 1 using
5 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
20 and 80 mesh (DG, scientific products Co., Iran). The raw material was kept in special plastic bags at ambient temperature until use. 2.2. Pretreatment Pretreatment experiments were carried out using 0.5 and 1.0 mol/L Na2CO3 solutions at temperatures of 120, 150, and 180 °C for 1, 2 and 5 h. All pretreatments were performed in a high pressure stainless steel reactor (Steel San’at, Isfahan, Iran) with working volume of 500 mL, equipped with a pressure indicator and a thermometer.23 The reactor was loaded with 10 g of dry safflower straw together with 110 g of Na2CO3 solution and then heated in an oil bath (One 10, Memmert, Germany) at a rate of 3 °C/min to achieve the temperature needed. The reaction slurries were gently mixed manually every 10 min during the pretreatment. After finishing the experiment, the reactor was transferred into an ice bath to cool to about 50 °C. The pretreatment mixture was then subjected to centrifugation at 4000 rpm for 5 min, aiming to separate the liquid and solid fractions. The solid fraction was then washed with distilled water until a pH value of 7 was obtained, dried at ambient temperature, put into a resealable plastic bag, and kept at 4 °C until further investigations. The liquid fraction was neutralized using phosphoric acid until a pH of 7 was reached, and kept in the laboratory refrigerator until further use. 2.3. Enzymatic hydrolysis Two commercial enzymes, Cellic® CTec2 and Cellic® HTec2, kindly provided by Novozymes, Denmark, were used for enzymatic hydrolysis of treated and untreated materials. Activities of these enzymes were 125 and 23 FPU/mL, respectively, measured by the method presented by Ghose.24 After mixing of enzymes Cellic® CTec2 and Cellic® HTec2 in volume proportion of 9:123, an activity of 114.8 FPU/mL was achieved. Hydrolysis of the solid fraction of treated samples and the untreated straw was performed in 118 mL glass bottles in a shaker incubator at 120 rpm and 45 °C for 72 h. A mixture of 50 g/L substrate 6 ACS Paragon Plus Environment
Page 6 of 32
Page 7 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
was prepared in 50 mmol/L sodium citrate buffer (pH 4.8). The working volume of the enzymatic hydrolysis media was 20 mL. Then, an enzyme mixture corresponding to a load of 10 FPU per gram of dry straw and 0.5 g/L sodium azide (as an antibacterial agent) were added to each bottle. Liquid samples were taken for sugar analyses after 12, 24, and 72 h hydrolysis. The yield of glucose and xylose were calculated according to Eqs. (1) and (2), respectively25: Glucose yield % =
Xylose yield % =
Glucose produced g/L × 100 1 1.111 × glucan in sample g/L
Xylose produced g/L × 100 2 1.136 × xylan in sample g/L
where conversion factors of 1.111 and 1.136 were used for glucan and xylan hydration to glucose and xylose, respectively. 2.4. Simultaneous saccharification and fermentation (SSF) Solid fractions of pretreatments as well as untreated straw were subjected to SSF for ethanol production at 37 °C and 130 rpm under anaerobic conditions for 72 h. The fermenting microorganism was a flocculation strain of Saccharomyces cerevisiae (CCUG 53310, Culture Collection, University of Gothenburg, Sweden). A fermentation medium containing 5 g L−1 yeast extract, 3.5 g L−1 K2HPO4, 7.5 g L−1 (NH4)2SO4, 0.75 g L−1 MgSO4·7H2O, 1 g L−1 CaCl2·2H2O, and 50 g L−1 of straw sample was prepared in 50 mmol/L citrate buffer in a 118 mL glass bottle with a working volume of 40 mL. After adjusting the pH to 5 using 2 M NaOH solution, the mixture was autoclaved at 121 °C for 20 min. After cooling to room temperature in a biosafety cabinet, 1 g/L microorganism (based on dry weight) and an enzyme mixture (the same as for enzymatic hydrolysis) with a load of 10 FPU/g dry substrate were added to each bottle. Samples were taken after 24 and 72 h for ethanol analyses.26 Ethanol yield was calculated according to Eq. (3)25:
7 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Ethanol yield % =
Ethanol produced g/L × 100 3 Glucan % × 50 g/L × 0.51 × 1.111
2.5. Biogas production Anaerobic digestion of both liquid and solid fractions obtained after the pretreatments as well as untreated safflower straw was carried out at mesophilic conditions according to the method described by Hansen et al.27 The inoculum was obtained from a 7000 m3 anaerobic digester (Isfahan Municipal Wastewater Treatment Plant, Isfahan, Iran) operating at mesophilic (37 °C) conditions. Dark glass bottles with 118 mL capacity were used as bioreactors. Each bottle was filled with 20 mL inoculum as well as an appropriate amount of substrate to achieve a volatile solids (VS) mass ratio for inoculum and substrate of 2:1. Then, 5 mL of deionized water added to the bottles to keep a working volume of 25 mL. A sample containing 20 mL of inoculum and 5 mL of deionized water was used as a blank in order to determine the amount of methane produced by the inoculum. All reactors were then sealed using butyl rubber stoppers and aluminum caps. To establish anaerobic growth conditions, the headspace of the bottles was purged with N2 gas for 2 min before the reactors were placed into an incubator at 37 °C. The reactors were then shaken manually every day during the digestion period. All setups were performed in triplicate. The produced biogas was determined using gas chromatography measurements performed every 3 days within the first 15 days and then every 5 days during the next 30 days of the experimental period.28 The methane yield was calculated based on the volume (mL) of methane produced per gram of voletile solid (g VS) of the substrate (dry basis).
2.6. Analytical Methods The composition of the native and treated straw samples was determined using NREL/TP-510-42618 method.29 Total solids (TS), volatile solids (VS), and ash content were 8 ACS Paragon Plus Environment
Page 8 of 32
Page 9 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
determined according to Sluiter et al method.30 The amount of extractives in untreated straw was analyzed by the NREL/TP-51-42619 method.31 The composition of gas produced in the anaerobic digestion (methane and carbon dioxide) was determined using a gas chromatograph (SP-3420A, Beijing Beifen Ruili Analytical Instrument Co., China) equipped with a column (Porapak Q column, Chrompack, Germany) and thermal conductivity detector (TCD, SP-3420A, Beijing Beifen Ruili Analytical Instrument Co., China). The detector, injector, and column temperature were adjusted to 150, 100, and 40 °C, respectively. Nitrogen with flow rate of 20 mL/min was used as a carrier gas. All biogas yields are presented at standard conditions. Ethanol and sugars, including glucose, xylose, arabinose, galactose, and mannose, were determined by high performance liquid chromatography (HPLC) using RI detector (Jasco International Co., Tokyo, Japan). The concentration of ethanol in the SFF broth was determined on an Aminex HPX-87H column (Bio-Rad, Richmond, CA, USA) at 60 ºC with 0.6 mL/min eluent of 5 mmol/L sulfuric acid. An ion-exchange Aminex HPX-87P column (Bio-Rad, USA) at 85 ºC and flow rate of 0.6 mL/min deionized water as a mobile phase was employed to measure all sugars’ concentrations. All quantifications were based on external calibration, using analytical grade of ethanol and sugars. 2.7. Swelling capacity The water swelling capacity of the untreated and pretreated straw was measured using the method presented by Jeihanipour et al.
32
An amount of 0.1 g of each sample (based on dry
weight) was placed in a small material bag and immersed in distilled water for 1 h. Then, the swollen straw was weighed and the water absorbency was calculated using Eq. (4).
Water swelling capacity =
w' − w) 4 w)
9 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
where w1 and w2 are the weights of the dry and swollen materials, respectively. 2.8. Equivalent gasoline In order to optimize the production of biofuels and compare different pretreatment conditions, the gasoline equivalents were calculated based on the amounts of produced ethanol and/or biogas from one ton of safflower straw after all pretreatment conditions. All the data associated with total solids and volatile solids as well as solid and liquid recoveries after the pretreatment were used to estimate the amount of equivalent gasoline. For 1000 kg safflower straw, 11000 kg sodium carbonate solution was used, and the average liquid recovery after the pretreatments was considered to be 80% (nearly 8800 L). Heat value of biogas, ethanol, and gasoline was estimated at 36.1 MJ/m3, 21.2, and 32 MJ/L, respectively.33 2.9. Statistical analysis Statistical validation of the results was carried out using ANOVA test and general linear model (GLM) using Minitab 17.1.0 software (Minitab Inc., State College, USA). Tukey method with 95% confidence interval was implemented in order to estimate and compare significant differences among the means. There are no appreciable differences among the means with the same lettered group at a 5% probability level (p < 0.05). 3. Results and discussion 3.1. Effects of pretreatments on safflower straw composition Composition of native safflower is presented in Table 1. To the best of our knowledge, there is no reported data on safflower straw composition in the literature, and according to our investigations, safflower straw is mainly composed of glucan, xylan, and lignin. The concentrations of other hemicellulosic sugars, including arabinose, galactose, and mannose, are very low (
