Improving Anaerobic Codigestion of Corn Stover Using Sodium

Dec 4, 2013 - Muhammad Hassan , Muhammad Umar , Tursun Mamat , Furqan Muhayodin , Zahir Talha , Esmaeil Mehryar , Fiaz Ahmad , Weimin Ding , and ...
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Improving Anaerobic Codigestion of Corn Stover Using Sodium Hydroxide Pretreatment Zhaoyang You,†,§ Taoyuan Wei,‡,§ and Jay J. Cheng*,§ †

Jiangsu Key Laboratory of Industrial Water-Conservation & Emission Reduction, Nanjing University of Technology, Nanjing, Jiangsu 210039, China ‡ College of Urban Construction, Wuhan University of Science and Technology, Wuhan, Hubei 430065, China § Department of Biological and Agricultural Engineering, North Carolina State University, Campus Box 27625, Raleigh, North Carolina 27695-7625, United States ABSTRACT: NaOH pretreatment of corn stover was investigated for anaerobic codigestion of corn stover with swine manure to shorten digestion time and improve biogas yield. Different NaOH concentration (2%, 4%, and 6%) at various temperatures (20 °C, 35 °C, and 55 °C) and 3 h of pretreatment time were tested for corn stover pretreatment. A C/N ratio of 25:1 in the substrates (corn stover and swine manure) was employed in the codigestion test. The results showed that the lignin removal rate of 54.57% to 79.49% was achieved through the NaOH pretreatment. The highest biogas production rate was obtained from the corn stover pretreated at 6% NaOH at 35 °C produced for 3 h, which was 34.59% higher than that from the untreated raw corn stover. The increase of methane yield was from 276 to 350 mL/g VS. On the average, the reducing sugar content of corn stover decreased to 126.7 mg/g after digestion. Digestion time (T80) of pretreated corn stover was shortened from 18 days to 12−13 days. NaOH pretreatment not only effectively shortened the digestion time for anaerobic codigestion of corn stover with swine manure by removing the lignin from the corn stover but also improved biogas yield of corn stover. The pretreatment condition of 6% NaOH at 35 °C for 3 h is recommended for the pretreatment of corn stover.

1. INTRODUCTION Biogas is a renewable energy source, which can be used as a replacement of fossil fuels in power and heat production. It can also be used as automobile fuel. Biogas has been considered as one of the most energy-efficient and environmentally beneficial technology for bioenergy production. It is generated through anaerobic digestion of renewable resources, such as agricultural residues, grasses, woody materials, and other organic waste materials which are abundant and renewable annually in nature.1−4 Many countries in the world have realized the importance to replace the fossil fuels by developing anaerobic digestion technology of agricultural residues. Approximately 4,000 agricultural biogas production units are operated on German farms at the end of 2008.5 China has set a goal of building 60 million household-scale anaerobic digesters in rural areas by 2020.6 The technology of anaerobic digestion for generating biogas is a series of biological processes in which microbes break down biodegradable materials to methane and carbon dioxide in the absence of oxygen. The biological processes of anaerobic digestion can be divided into four phases: hydrolysis, acidogenesis, acetogenesis, and methanation. Hydrolyzing and fermenting microbes are responsible for the initial attack on polymers and monomers to produce mainly acetate and hydrogen and some volatile fatty acids such as propionate and butyrate. The volatile fatty acids are converted into acetate and hydrogen by acetogenic bacteria. Methanogenic microorganisms then produce methane from acetate, hydrogen, and carbon dioxide at the end of the degradation chain. Hydrolysis stage is important for methane production in anaerobic digestion and its rate varies with different raw organic materials. © 2013 American Chemical Society

Some organic compounds which are insoluble in water such as cellulose are usually decomposed slowly into monomers in several days, whereas the hydrolysis of soluble carbohydrates only needs a few hours.5 Corn stover, which contains rich cellulose and hemicellulose, is one of the largest residues from agriculture. It has a great potential for biogas production because of its abundance and high carbohydrate content. Corn stover has a carbohydrate content of about 60% of the dry matter.7 It is estimated that over 100 million tons of corn stover can be collected annually and used for biofuel production.4 However, the biodegradability of corn stover needs to be substantially improved before corn stover can be efficiently converted to biogas in anaerobic digestion. This is because corn stover, like many other lignocellulosic materials, has a complex and tight structure, which makes it difficult for the anaerobic microorganisms to attack and decompose. Lignocellulose of plant materials consists of three major components: cellulose, hemicellulose, and lignin. They form a recalcitrant structure that resists the access of hydrolytic enzymes to cellulose and hemicelluloses for fermentable sugar or biogas production. Thus, the anaerobic digestion of natural corn stover usually has a low biogas yield and needs a long digestion time. An effective pretreatment is required for corn stover to reduce the recalcitrance by altering the chemical and structural features of the corn stover before it is processed in anaerobic digestion for biogas production. The main purposes of the pretreatment are to separate the lignin Received: August 19, 2013 Revised: November 30, 2013 Published: December 4, 2013 549

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methods which can not only shorten pretreatment and digestion time but also improve the biogas and methane yields of the corn stover. Buffer was used to control the pH value at around neutral to avoid the inhibition of the methanogenic activities. At the same time, high volume ratio of inoculum (50% of inoculum to swine manure) was used to start the anaerobic codigestion experiment to boost microbial activities and shorten digestion time.

from cellulose and hemicelluloses in the lignocellulose, reduce cellulose crystallinity, and increase the porosity of the material, so the hydrolytic enzymes can access their substrates (cellulose and hemicelluloses) in the following anaerobic digestion.8−12 Many technologies (physical, chemical, and biological pretreatments) have been extensively investigated for the pretreatment of lignocellulosic materials in recent years.13,14 Sodium hydroxide (NaOH) pretreatment, a chemical pretreatment method, has been proven to be a simple but effective method for improving the biodegradability of corn stover. NaOH pretreatment disrupts lignin structure, decreases the crystallinity of cellulose, and increases the porosity of the lignocellulosic materials. The accessibility of the cellulolytic microbes to the substrates (cellulose and hemicelluloses) can be substantially enhanced after the NaOH pretreatment of corn stover. Pang et al. indicated that the lignin content in corn stover decreased by 4.3%−39.2%, the hot-water extractives increased by 64.8%, and considerable lignocellulose was decomposed after the NaOH pretreatment.6 The efficiency of lignin degradation varies with the NaOH concentration. Zhu et al. found that lignin degradation of corn stover could be increased from 9.1% to 46.2% when NaOH concentration increased from 1.0% to 7.5% in the pretreatment.15 The changes in the corn stover after NaOH pretreatment include fiber swelling, decrease of the binding of lignin with carbohydrates, and increase of degradability and solubility of lignin and carbohydrates. These changes together contribute to the improvement of the biodegradability of corn stover. Compared with anaerobic digestion of raw corn stover, the efficiency of the anaerobic digestion of NaOH-pretreated corn stover can be substantially improved. When corn stover was pretreated with NaOH at 20 °C for 3 days, the highest biogas and methane yields reached 420.6 mL/g VS and 233.0 mL/g VS, respectively.16 Zhu et al. also showed that the highest biogas yield of 372.4 L/kg VS was obtained with 5% NaOH pretreated corn stover, which was 37.0% higher than that of the untreated corn stover.15 Pang et al. also reported that anaerobic digestion of corn stover pretreated with 6% NaOH achieved 48.5% more biogas production and 71.0% more bioenergy gain than the digestion of untreated corn stover when the loading rate was 65 g/L.6 However, a higher NaOH concentration of 7.5% caused fast production of volatile fatty acids during the hydrolysis and acidogenesis stages, which might result in inhibition of the methanogenesis.15 Anaerobic codigestion of animal manure and agricultural residues is an advanced technology for biogas production and has attracted extensive research in recent years.17,18 Li et al. studied anaerobic codigestion of corn stover with cattle manure and found that 4.9%−7.4% higher biogas yield was obtained at the feeding concentration of 65 g/L. The yield improvement is mainly attributed to better balanced nutrients and increased buffering capacity.19 Although anaerobic codigestion of NaOH-pretreated corn stover and animal manure could improve the efficiency of the digestion in comparison with the anaerobic digestion of untreated corn stover or animal manure alone, the main challenge of the codigestion is the cost-effectiveness of the pretreatment and anaerobic codigestion. In this study, corn stover was pretreated with NaOH for 3 h to improve its biodegradability and then codigested with swine manure to balance the nutrients in the digestion (the C/N ratio of 25:1 in the substrate) and also increase the buffering capacity. The objective of this study was to find the cost-effective and efficient

2. MATERIALS AND METHODS 2.1. Materials. The corn stover (Agventure Variety R9534VBW) in the test was collected in early October 2011 from a farm belonging to Novozymes North America, Inc. in Franklinton, North Carolina, USA. It was air-dried and then milled with a Thomas Wiley laboratory mill (model No. 4) with a 1-mm screen. The milled corn stover was collected in plastic bags, sealed, and stored at room temperature. Swine manure used in the test was collected from the swine unit at the Lake Wheeler Road Field Laboratory (LWRFL) of North Carolina State University. It was stored in a refrigerator at 4 °C before used in the test. The inoculums were taken from a working anaerobic digester treating corn stover and swine manure in Dr. Cheng’s laboratory in the Biological and Agricultural Engineering Department of North Carolina State University. The digester had been operated as a completely mixed and semicontinuous system at 35 °C for 12 months when the culture from the digester was taken as the inoculums for the test in this study. The chemical composition of the materials used in the test was shown in Table 1. Table 1. Chemical Composition of the Materials Used in the Anaerobic Codigestion Test property

corn stover

swine manure

inoculum

MLSS,a g/L TC,b % TKN,c % reducing sugar, mg/g lignin, % NH4, mg/L COD,d mg/L

NDe 30.51 0.52 516.74 23.55 ND ND

ND 0.12 0.04 ND ND 189.67 9280.43

20.70 0.32 0.06 ND ND 154.20 17160.40

a Mixed liquid suspended solids. bTotal carbon. cTotal Kjeldahl nitrogen. dChemical oxygen demand. eNot determined.

2.2. NaOH Pretreatment. Corn stover was dried in the drying oven at 105 °C for 8 h before the test. Ten g of dried corn stover and 100 mL of NaOH at desired concentration (2%, 4%, or 6%) were put into a 150 mL serum bottle. The serum bottle was then sealed and crimped to prevent water evaporation for the pretreatment of the corn stover in a shaking water-bath for 3 h at temperature of 20 °C, 35 °C, or 55 °C. Duplicate samples were conducted for the test. After the pretreatment, the pretreated corn stover was recovered by centrifugal separation, and 100 mL DI water was used to remove NaOH residue for 5 times.7 The separated corn stover was dried in the drying oven at temperature of 105 °C for 8 h. Then 0.3 g of pretreated corn stover was taken for composition analysis, and the rest was stored in a sealed plastic bag at 4 °C for the following anaerobic codigestion experiments. 2.3. Anaerobic Codigestion. The NaOH-pretreated corn stover was then anaerobically digested in the serum bottles with 150 mL of working volume (actual volume was 165 mL). Untreated raw corn stover was also anaerobically digested as a 550

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control. A C/N ratio of 25:1 in the substrates (corn stover and swine manure), which is believed to be the optimal ratio for microbial growth in anaerobic digestion, was applied in the test. Again, duplicate samples were conducted in the test. The precalculated amount of corn stover was fed into each serum bottle (digester) filled with 60 mL of swine manure and 60 mL of inoculum which was taken from the operating anaerobic codigester in the lab as seed in all test serum bottles. A phosphate buffer (monopotassium phosphate and 0.1 mol/L NaOH) was used for maintaining a neutral pH (around 7.0) in the anaerobic digestion process. The prepared digesters were then placed in a bath shaker for anaerobic digestion test at 35 °C. The shaking speed was 120 rpm. 2.4. Analytical Methods. The chemical composition of NaOH-pretreated and untreated raw corn stover were analyzed in the test. Lignin of the corn stover was measured according to the Laboratory Analytical Procedures (LAP) established by National Renewable Energy Laboratory (NREL).21 The total reducing sugars of corn stover were measured using the DNS (3,5-dinitrosalicylic acid) method.22 The daily biogas production from each anaerobic digester was recorded by water displacement method, and the related cumulative biogas production was calculated. Each biogas production datum was collected as the average of duplicate samples. The biogas samples were taken from the gas collection line and analyzed for the methane content using a gas chromatograph (GC) (GC17A, Shimadzu) equipped with a thermal conductivity detector (TCD). The total carbon (TC), total nitrogen (TN), and the total Kjeldahl nitrogen (TKN) were determined with an autoanalyzer (Bran & Luebbe Digital Autoanalyzer III system). During the combustion process, elemental nitrogen is converted to N2 and NOx. In the catalytical heater, NOx gases are reduced to N2 which is read using TN detection. Meanwhile, carbon is read using IR detection and reported as total carbon (EPA Method 415.1 (1979) and 9060A). COD was measured using a HACH Dr/ 2010 spectrophotometer.

Table 2. Lignin Content of Corn Stover Pretreated with NaOH pretreatment temperature NaOH concn (%) 2

4

6

biomass recovery,a % lignin content, % lignin removal rate,b % biomass recovery, % lignin content, % lignin removal rate, % biomass recovery, % lignin content, % lignin removal rate, %

20 °C

35 °C

55 °C

53.04 ± 1.08

55.98 ± 0.06

57.34 ± 0.91

17.18 ± 1.02

13.66 ± 1.41

12.40 ± 0.86

54.57

67.93

70.71

59.24 ± 0.54

50.54 ± 0.32

51.09 ± 1.99

15.77 ± 2.11

13.33 ± 0.52

9.99 ± 1.13

60.81

71.74

78.59

57.61 ± 0.27

47.28 ± 0.11

49.46 ± 0.56

13.70 ± 1.72

10.34 ± 1.17

10.08 ± 0.35

66.89

79.49

79.09

a

Recovered mass of corn stover after the pretreatment with NaOH. Removal rate = (lignin of raw corn stover − lignin of pretreated corn stover)/lignin of raw corn stover ×100%.

b

seemed to level off. Further increase of NaOH concentration or temperature did not significantly increase the lignin removal. Zhu et al. did a similar test and got the highest lignin removal of 46.2% for corn stover (milled to 5 mm) in a pretreatment with 7.5% NaOH at 20 °C for 24 h.15 In our study, only 3 h of pretreatment time was used, and the highest lignin removal rate obtained was 79.49% at 55 °C. This is because high temperature accelerated molecule diffusion and collision during the NaOH pretreatment and intensified alkaline lignin dissolution reactions. As a result, lignin structure was substantially broken, and the crystallinity of cellulose decreased in a short period of time. In our study, a lignin removal rate of 66.89% was obtained at the pretreatment conditions of 6% NaOH and 20 °C for 3 h, which was much higher than 46.2% reported by Zhu et al. for the pretreatment conditions 7.5% NaOH at 20 °C for 24 h.15 The size of the corn stover might have played a significant role in lignin removal. The size of corn stover in our pretreatment test was 1 mm, while it was 5 mm in the study of Zhu et al.15 3.2. Anaerobic Codigestion. Biogas yield of anaerobic digestion was used to evaluate the effect of pretreated corn stover with the raw corn stover in the test. The biogas yield was measured from the next day after inoculation. All samples in the test appeared similar performance of gas production, which generated high and stable biogas yield on the early stage, and then decreased to lower and stable yield in the later stage. Figure 1 shows the daily biogas production (yield) from the anaerobic codigestion of swine manure and corn stover pretreated with 6% NaOH at different temperatures (20 °C, 35 °C, and 55 °C). As shown in the figure, corn stover pretreated at 20 °C generated 47 mL/d biogas on the first day, and then it sharply increased to the first peak of 102 mL/d biogas yield on the second day. However, biogas yield on the third day decreased to 65 mL/d due to acidification which made excessive fatty acids at the phase of hydrolysis and acidogenesis and resulted in a sharp drop of pH value to below 6. The low pH inhibited the

3. RESULTS AND DISCUSSION 3.1. Effect of Pretreatment. Lignin is a complex aromatic polymer present in the cell walls of plants, which resists against microbial access into the interior of the wall structure. Removing lignin from corn stover can improve the accessibility of the hydrolytical anaerobic microbes during the digestion. The lignin content of the raw and pretreated corn stovers were analyzed to determine the lignin removal of the pretreatment. The lignin removal rate was calculated with the amount of lignin in the corn stover before and after the pretreatment. The results of the pretreatment with different NaOH concentrations and at different temperatures are shown in Table 2. The lignin content of the untreated raw corn stover was 23.84%. As shown in Table 2, lignin content of the corn stover decreased substantially after the NaOH pretreatment. The results indicated that NaOH pretreatment could substantially remove lignin from corn stover, and a higher removal rate was achieved at higher NaOH concentration at different temperatures (20 °C, 35 °C, and 55 °C). Temperature is another important factor for lignin removal in the pretreatment. Lignin removal rate significantly increased with the increase of temperature from 20 to 55 °C at different NaOH concentration. However, when the lignin removal rate reached close to 80% (pretreatment conditions of 4% NaOH at 55 °C, 6% NaOH at 35 °C, and 6% NaOH at 55 °C), the removal rate 551

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Figure 2. Comparison of total biogas production from anaerobic codigestion of corn stover pretreated at different NaOH concentration and temperature with that from untreated corn stover. 2%, 4%, and 6% mean corn stover pretreated with 2%, 4%, and 6% NaOH, respectively; 20 °C, 35 °C, and 55 °C mean the temperature for the pretreatment; untreated means raw corn stover.

Figure 1. Daily biogas yield from anaerobic codigestion of swine manure and corn stover pretreated with 6% NaOH at different temperatures (20 °C, 35 °C, and 55 °C). Untreated raw corn stover (RCS) was used as a control.

activity of methanogenic microbes. After that biogas was steadily generated at about 40 mL/d to 70 mL/d in the following 10 days. After 13 days, biogas yield slowly decreased, and only 2 mL/d biogas was collected by the 30th day. A similar biogas production pattern was observed for the anaerobic codigestion of corn stover pretreated at 6% NaOH and 35 °C and 6% NaOH and 55 °C. Slightly higher biogas yield on the early stage and a slightly lower biogas yield on the later stage were observed in these cases, compared with that at 6% NaOH and 20 °C. The daily biogas yield from the untreated raw corn stover (RCS) was only about 20−60 mL/d at the early stage which was obviously lower than that from the pretreated corn stover at the same stage. However, biogas production from the RCS maintained at around 8 mL/d from the 17th through the 30th days, which was obviously higher than that from the pretreated corn stover (about 2 mL/d) in the same period of the digestion. This is probably because cellulose and hemicelluloses of the raw corn stover were tightly wrapped by lignin at the beginning, which prevented microbes from accessing to the cellulosic substrates and decomposing cellulose and hemicelluloses, resulting in a poor anaerobic digestion on the early stage. As the proceeding of the digestion process, the cellulose and hemicelluloses in the raw corn stover were slowly accessed by the microbes, resulting in a higher biogas production rate on the later stage. Methane content is an important parameter for the quality of biogas. Our methane content analysis indicates that methane content in the biogas produced from pretreated corn stover is different at different digestion stages. The average methane content of 44.3%−47.5% was observed at the early phase of the anaerobic codigestion, while it was 67.5%−69.1% at the later stage of the digestion. At the early stage of the digestion, fast hydrolysis and acidogenesis of the substrates (cellulose and hemicelluloses) produced excessive carbon dioxide, and methanogenic microbes could not keep up the speed to convert the carbon dioxide to methane, resulting in a low methane content in the biogas. The total biogas production was calculated by accumulating the daily biogas yield for 30 days. The results indicate that corn stover pretreated with NaOH produced more biogas than the raw corn stover (Figure 2). The average cumulative biogas production was 838 mL, 862 mL, and 872 mL, respectively, from the corn stover pretreated with 2%, 4%, and 6% NaOH, compared with only 717 mL from the untreated corn stover.

The highest total biogas yield was 965 mL in the case that corn stover was pretreated with 6% NaOH at 35 °C, which was 34.59% higher than that from the untreated corn stover. The results indicate that the pretreatment makes more substrates (cellulose and hemicelluloses) available for conversion to biogas during the anaerobic codigestion. On the contrary, the high content of lignin in the untreated raw corn stover impeded the microbial degradation process, which resulted in the lower biogas yield. Among the pretreatment conditions tested in this study, the highest biogas yield was obtained from the corn stover pretreated at 6% NaOH and 35 °C. The methane yield per gram of corn stover is an important parameter to estimate the net energy production of the corn stover digestion. Methane yield was calculated with methane content and daily biogas yield. After anaerobic codigestion of 30 days, the methane yield per gram of pretreated corn stover was 350 mL/g VS (volatile solids) at the pretreatment conditions of 6% NaOH and 35 °C, compared with only 249 mL/g VS for untreated corn stover (Figure 3). In general,

Figure 3. Methane yield of anaerobic codigestion of corn stover pretreated at different NaOH concentration (2%, 4%, and 6%) and temperature (20 °C, 35 °C, and 55 °C) in comparison with that of untreated corn stover.

methane yield of pretreated corn stover increased by 10.68%− 39.74%, compared with that of untreated raw corn stover. This means that NaOH pretreatment is an effective way to obtain higher net energy production through anaerobic codigestion of corn stover. 3.3. DT80 of Digestion. Digestion time can be an indicator for the efficiency of anaerobic codigestion of corn stover with 552

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swine manure. In our study the digestion test was conducted for 30 days. In practice it is not economical to run anaerobic digesters at a long retention time because very little biogas would be generated on the later stage. Digestion time (DT80) was defined as the number of days required to reach 80% of potential biogas generation.23 In our study, biogas yield from corn stover in 30 days was considered as potential biogas generation because biogas yield was close to zero from most samples after 30 days. DT80 of raw and pretreated corn stovers is calculated, and the results are shown in Figure 4. It was observed that the DT80 of Figure 5. Changes of reducing sugars in corn stover pretreated at different NaOH concentration (2%, 4%, and 6%) and temperature (20 °C, 35 °C, and 55 °C) in comparison with that of untreated corn stover before and after anaerobic digestion.

mg/g) after anaerobic digestion. This indicated that more cellulose and hemicelluloses in the pretreated corn stover were converted into biogas than those in untreated raw corn stover. D-value, which means the difference of reducing sugars before and after digestion, was used for evaluating the extent of digestion. The larger D-value means more cellulose and hemicelluloses were digested and converted into biogas in anaerobic digestion. In this study, the reducing sugar content of the pretreated corn stover was 595.3−732.4 mg/g and only 308.7 mg/g in the untreated corn stover. The corn stover pretreated with 4% NaOH at 35 °C achieved a D-value of 732.4 mg/g and a high biogas production accordingly. As the control, the D-value for the untreated corn stover was only 308.7 mg/g with the lowest biogas production of 717 mL. This indicates that the loss of cellulose and hemicelluloses of corn stover had been converted to biogas, and the large loss of D-value means more biogas production. This is another indication that NaOH pretreatment can help corn stover achieve high conversion of reducing sugars to biogas.

Figure 4. Anaerobic digestion time (DT80) of corn stover pretreated at different NaOH concentration (2%, 4%, and 6%) and temperature (20 °C, 35 °C, and 55 °C) in comparison with that of untreated corn stover.

raw corn stover was 18 days, which was obviously shorter than those numbers reported in the literature (22−35 days).20,23 Corn stover loading should be one of the main factors affecting the process of digestion. Zheng et al. reported 24 days of the DT80 at the corn stover loading of 35 g/L but had 35 days of the DT80 at higher loading of 65 g/L in the digestion.23 This indicates that the higher loading would need more time to digest the substrates. Corn stover pretreated wtih NaOH needs lower digestion time (DT80) (about 12−13 days), about 33.33% lower compared with untreated corn stover. This is because most of the lignin in corn stover was broken during NaOH pretreatment and cellulose crystallinity was reduced, which made it easier for the microbes to get access to cellulose and hemicelluloses and decompose them. Thus, NaOH pretreatment of corn stover could bring significant economical benefit for improving biogas production efficiency by shortening digestion time. 3.4. Changes of Reducing Sugars. Reducing sugars generated from the hydrolysis of cellulose and hemicelluloses were investigated to observe the changes of cellulose and hemicelluloses of corn stover after anaerobic digestion and to evaluate the completeness of the digestion. The loss of reducing sugars correlated with biogas generation when the sugars were converted to biogas by the anaerobic microorganisms. Before digestion, reducing sugar content in corn stover was 723.9 mg/g−831.0 mg/g in the NaOH-pretreated corn stover, which was about 207.1 mg/g−314.3 mg/g higher than that in the raw corn stover (516.7 mg/g) (Figure 5). However, the reducing sugar content of pretreated corn stover decreased to 98.53 mg/g−156.7 mg/g (average 126.3 mg/g) after the anaerobic digestion and was about 14.85%−52.64% (average 39.30%) lower than that for untreated raw corn stover (208.0)

4. CONCLUSIONS NaOH pretreatment of corn stover for lignin removal not only shortened the digestion time of the biomass but also improved the biogas yield of the corn stover. Lignin content decreased from 23.84% to 10.08%−17.18% and lignin removal rate of 54.57%−79.49% were achieved in the NaOH pretreatment in this study. After anaerobic codigestion with swine manure, corn stover pretreated with NaOH had higher biogas yield than untreated raw corn stover, and the highest methane yield of 350 mL/g VS was obtained at the pretreatment conditions of 6% NaOH and 35 °C. Digestion time (DT80) was shortened from 18 days to 12−13 days, about 33.33% reduction, compared with untreated raw corn stover. Reducing sugar content after digestion was obviously decreased for the corn stover; the Dvalue of reducing sugars was in agreement with the biogas yield of the digestion. The best pretreatment condition observed in this study was 6% NaOH at 35 °C for 3 h.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 919-515-6733. Fax: 919-515-7760. E-mail: jay_ [email protected]. Notes

The authors declare no competing financial interest. 553

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(21) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D. Determination of Structural Carbohydrates and Lignin in Biomass, Laboratory Analytical Procedure; National Renewable Energy Laboratory: Golden, CO, 2008. (22) Xu, J.; Cheng, J. J. Pretreatment of Switchgrass for Sugar Production with the Combination of Sodium Hydroxide and Lime. Bioresour. Technol. 2011, 102, 3861−3868. (23) Zheng, M.; Li, X.; Li, L. Enhancing Anaerobic Biogasification of Corn Stover through Wet State NaOH Pretreatment. Bioresour. Technol. 2009, 100, 5140−5145.

ACKNOWLEDGMENTS The authors would like to thank Novozymes North America, Inc. for providing corn stover for this research.

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REFERENCES

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dx.doi.org/10.1021/ef4016476 | Energy Fuels 2014, 28, 549−554