Pretreatment of Corn Stover for Methane Production with the

Aug 24, 2015 - †Biomass Energy and Environmental Engineering Research Center, College of Chemical Engineering, and ‡College of Life Science and ...
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Pretreatment of Corn Stover for Methane Production with the Combination of Potassium Hydroxide and Calcium Hydroxide Lin Li,† Chang Chen,*,‡ Ruihong Zhang,§ Yanfeng He,† Wen Wang,† and Guangqing Liu*,† †

Biomass Energy and Environmental Engineering Research Center, College of Chemical Engineering, and ‡College of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China § Department of Biological and Agricultural Engineering, University of California, Davis, California 95616, United States ABSTRACT: To improve the biogasification of corn stover (CS) for high-efficiency anaerobic digestion, the single alkaline pretreatment and the combined alkaline pretreatment of KOH and Ca(OH)2 were carried out. The results showed that the effect of KOH pretreatment was much better than Ca(OH)2 pretreatment, while the combination of 0.5% KOH and 2.0% Ca(OH)2 was comparable to the effect of 2.0% KOH pretreatment, achieving a cumulative methane yield of 271.38 mL gVS−1, which was 77.01% higher than untreated CS. Considering that Ca(OH)2 is much cheaper than KOH, it was feasible to use 2.0% Ca(OH)2 to reduce KOH to 0.5% and achieve similar results. Besides, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses gave evidence for morphology and composition changes of CS after pretreatment. The results collectively indicated that combined alkaline pretreatment could reduce the KOH dose, achieve similar pretreatment effects, and might be a promising method to pretreat agricultural wastes for efficient anaerobic digestion (AD) in future industrial applications. eventually increase the digestibility of cellulose.13 Sodium hydroxide (NaOH) pretreatment has been extensively studied and found very useful; however, it is difficult to be recycled. A high load of NaOH may cause severe environmental problems, such as water pollution and soil salinization.14 Considering that potassium hydroxide (KOH) is also a strong base and potassium after pretreatment can be recycled and used as a fertilizer,14 KOH has been tried to pretreat wheat straw in our previous study and achieved 77.5% higher methane yield at a loading of 5% compared to untreated wheat straw.15 A relatively high price of KOH, however, may limit its application. Calcium hydroxide [Ca(OH)2] is a much cheaper alkaline, and our previous research found that the cumulative methane yield from 4% NaOH- and 2% Ca(OH)2-pretreated CS was comparable to that from 6% NaOH-pretreated CS.16 There have been few studies paying attention to the pretreatment of CS with the combination of KOH and Ca(OH)2 thus far. It might be possible to use Ca(OH)2 partly instead of KOH to reduce cost and gain higher AD efficiency. The purpose of this research was to (1) compare the AD performance on KOH- and Ca(OH) 2-treated CS, (2) develop an effective method of combined alkaline (CA) pretreatment to maximize the methane production, and (3) investigate structural changes after single alkaline (SA) pretreatment and CA pretreatment.

1. INTRODUCTION Corn stover (CS) is an abundant biomass and inevitable coproduct of corn grain production. In China, around 270 million tons of CS was generated in 2013.1 Although there were some methods available for CS reutilization, almost half of CS has not been effectively used and abandoned as solid waste and even burnt in the open field directly, which is causing severe environmental problems.2 It is extremely urgent to find an effective utilization of CS in China. Anaerobic digestion (AD) is a series of reactions, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis. It has been proven to be a fantastic means of using waste biomass to produce clean biogas, liquid fertilizers, valuable digested residues, and soil conditioners.3 CS is gaining great interest as a feedstock of AD to relieve environmental and energy crises in recent years because of its high carbohydrate content.4,5 More than 50 biogas plants using CS as feedstock had been established in China by 2012.6 There still exists large room to improve the biodegradability of CS because of its recalcitrance, which limits the enzymatic hydrolysis by microbes during AD.7,8 Thus far, many substraterelated factors have been proposed to lead to biomass recalcitrance, such as cellulose crystallinity, cellulose degree of polymerization, hemicellulose content, lignin barrier, substrate available surface area, feedstock particle size, and cell wall thickness.9,10 Pretreatment is an important step to overcome the recalcitrance of CS and increase its biodegradability before being fed into an anaerobic digester.11 Many pretreatment methods have been investigated, such as steam explosion, microwave treatment, ionizing radiation, acid pretreatment, and alkaline pretreatment.12 By comparison, alkaline pretreatment has been proven to be an efficient, simple, and energy-saving method. Alkaline could effectively remove most of the lignin and part of the hemicelluloses, resulting in separation of structural linkages between lignin and carbohydrates, and © 2015 American Chemical Society

2. MATERIALS AND METHODS 2.1. Feedstock and Inoculum. CS was provided by a farm in Deqingyuan Company, Beijing, China, and air-dried at room temperature. CS was rubbed by a 9SC-360 kneading machine (Shuncheng, China) and then cut with scissors to a length of 1.5−2 Received: May 26, 2015 Revised: August 24, 2015 Published: August 24, 2015 5841

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⎛ a b 3c ⎞ CnHaOb Nc + ⎜n − − + ⎟H 2O ⎝ 4 2 4⎠ ⎛n ⎛n a b 3c ⎞ a b 3c ⎞ → ⎜ + − − ⎟CH4 + ⎜ − + + ⎟CO2 ⎝2 ⎝2 8 4 8⎠ 8 4 8⎠

cm. The inoculum was active sludge obtained from anaerobic digesters at Nanwu biogas plant in Beijing, China. 2.2. Pretreatment of CS. To shorten the digestion time and increase the biodegradation of CS, KOH and Ca(OH)2 were used to pretreat CS. The pretreatment experiments were categorized into two groups: the SA pretreatment and CA pretreatment. The SA pretreatment was performed to compare the effects of KOH (0.5, 1.0, 1.5, 2.0, 2.5, and 3.0%, w/v) and Ca(OH)2 (0.5, 1.0, 1.5, 2.0, 2.5, and 3.0%, w/v) on the digestion of CS. For CA pretreatment, three concentrations of KOH (0.5, 1.0, and 1.5%, w/v) were combined with three doses of Ca(OH)2 (1.0, 1.5, and 2.0%, w/v), respectively. Alkaline pretreatment were carried out using 1.5 L plastic boxes. In each box, 40 g of CS was mixed evenly with 334 mL of alkaline solution to adjust the moisture content up to 90%, which was calculated by eq 1.17 Then, the samples were placed in an incubator (20 °C) for 24 h and manually shaken at 4 h intervals.

+ c NH3

TMY (mL g VS−1) =

(

22.4 × 1000

n 2

+

a 8

−

b 4

−

3c 8

) (4)

12n + a + 16b + 14c

Bd = EMY/TMY × 100%

(5)

2.6. Kinetic Models. The cumulative methane yield of each test was fitted to a modified Gompertz equation25 (eq 6) to investigate the AD performance for methane production

B = B0 exp{− exp[μm e/B0 (λ − t ) + 1]}

(6) gVS−1),

B0 refers to where B is the cumulative methane production (mL the ultimate methane yield (mL gVS−1), μm is the maximum methane production rate (mL gVS−1 day−1), e is equal to 2.718 218 28, λ represents the lag phase time (days), and t stands for the digestion time (days). 2.7. Statistical Analysis. All of the pretreatment and AD tests were performed 3 times. The significant difference was statistically analyzed using one-way analysis of variance (ANOVA). The software OriginPro 8.0 (OriginLab, Northampton, MA) was used for graph and data processing.

moisture content (%) = [1 − dry weight of CS/(weight of CS + water added)] × 100% (1)

2.3. AD. 3 g of CS [volatile solids (VS) basis] was mixed with the same amount of sludge in a 1 L-digester according to the substrate/ inoculum (S/I) ratio of 1:1 (based on VS). Distilled water was then added to fill up to a working volume of 500 mL. Two blank digesters, which contained the same amount of sludge and water, were used to correct the background gas production. The initial pH was adjusted to 6.8 ± 0.1 by hydrochloric acid. Each bottle was capped with a rubber stopper and purged with 99.0% argon for 3 min to create an anaerobic environment. The prepared digesters were placed in an incubator for AD at mesophilic temperature (37 °C). All digesters were shaken manually 3 times per day for 1 min. Biogas samples were collected for composition analysis every day during the first week of digestion and twice a week thereafter. pH, total alkalinity (TA), and total ammonia nitrogen (TAN) in the effluent were analyzed at the end of digestion. 2.4. Analytical Methods. The total solids (TS), VS, pH, and TA were measured according to standard methods.18 The TAN concentration was measured by the HACH test kit. Elemental compositions (C, H, N, and S) were analyzed by an organic element analyzer (Vario EL cube, Germany). The oxygen content was determined by assuming C + H + O + N = 99.5% on a VS basis.19 The compositions of cellulose, hemicellulose, and lignin were determined by an A2000 fiber analyzer (ANKOM, Macedon, NY) according to the method by Van Soest et al.20 Daily biogas production was determined by measuring the pressure in the headspace using a 3151 WAL-BMP-Test system pressure gauge (WAL Mess- und Regelsysteme GmbH, Germany)16 and calculating based on the reported formula.15 The biogas composition was analyzed by a GC7890B (Agilent Technologies, Santa Clara, CA). Helium was used as the carrier gas. The operational temperatures at the detector and oven were 220 and 60 °C, respectively. Hitachi S-4700 scanning electron microscopy (SEM) was used to observe the morphology of CS. The raw CS and pretreated CS were measured by X-ray diffraction (XRD) using an X-ray power diffractometer (Bruker D8 Advance) with a sealed tube Cu Kα source at 40 kV and 40 mA. The samples were scanned in the 2θ range from 5° to 65°. The crystallinity index (CrI) of each sample was calculated according to eq 221 CrI = [(I002 − Iamorphous)/I002] × 100%

(3)

3. RESULTS AND DISCUSSION 3.1. Characteristics of Feedstock. The characteristics of CS and inoculum are shown in Table 1. The TS, VS, and VS/ Table 1. Characteristics of CS and Inoculum parameter TS (%)a VS (%)a VS/TS (%) cellulose (%)b hemicellulose (%)b lignin (%)b C (%)b H (%)b O (%)b N (%)b S (%)b C/N a

CS 93.41 88.69 94.95 40.12 32.84 6.10 43.64 5.79 43.89 1.16 0.38 37.62

± ± ± ± ± ± ± ± ± ± ± ±

inoculum 0.10 0.30 0.28 1.20 1.84 0.63 0.17 0.02 0.09 0.06 0.08 0.96

6.30 ± 0.02 3.08 ± 0.02 48.89 ± 0.22 NDc ND ND 31.33 4.23 ND 2.85 ND 11

As total weight of sample. bAs TS of sample. cND = not detectable.

TS of the raw CS were 93.41, 88.69, and 94.95%, respectively, indicating a high organic content, which was preferred by methane generation.23 Moreover, the contents of cellulose, hemicellulose, and lignin were 40.12, 32.84, and 6.10%, respectively. According to the elemental contents of CS, the organic substrates of CS could be represented as a formulation of C43.89H69.88O33.10N. The TMY for CS was calculated by eq 4 to be 431.23 mL gVS−1. 3.2. SA Pretreatment Effect on AD of CS. Figure 1 shows cumulative methane yields of SA-pretreated and raw CS during the digestion. It was found that all SA-pretreated CS achieved higher methane production than the untreated CS. Different concentrations of KOH-treated CS showed highly significant (α < 0.01) improvements (14.3−95.7%) compared to the untreated CS (150.78 mL gVS−1). With the increase of the KOH dose, the cumulative methane yields increased gradually

(2)

where I002 is the intensity of the 002 peak at 2θ = 22.4° and Iamorphous refers to the intensity of the background scatter at 2θ = 18.7°. 2.5. Theoretical Methane Yield (TMY) and Biodegradability (Bd). TMY of CS was calculated on the basis of elements, according to eqs 3 and 4.22,23 Bd of the CS could be calculated according to eq 5.24 5842

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Figure 1. Cumulative methane yields of pretreated CS: (A) KOH-treated CS and (B) Ca(OH)2-treated CS.

and reached the top value of 295.00 mL gVS−1 after 2.5% KOH pretreatment, which had highly significant improvements (α < 0.01) compared to 172.30, 191.71, 208.52, and 273.10 mL gVS−1 at KOH doses of 0.5, 1.0, 1.5, and 2.0%, respectively. When the KOH concentration increased to 3.0%, however, the cumulative methane yield declined to 275.02 mL gVS−1, which had a significant decrease (α < 0.05) compared to that of 2.5%. The cumulative methane yields were 153.80, 175.72, 202.42, 190.23, 210.71, and 184.29 mL gVS−1 from 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0% of Ca(OH)2-pretreated CS, respectively. The highest cumulative methane yield was detected from 2.5% Ca(OH)2pretreated CS, which showed extremely remarkable (α < 0.01) improvement compared to others. There was no significant difference (α > 0.05) observed between the cumulative methane yield from 0.5% Ca(OH)2-pretreated and untreated CS, implying that a low concentration of Ca(OH)2 had no obvious improvement on AD performance. 3.3. Performance of CA Pretreatment. The cumulative methane yields of CA-pretreated CS during AD are shown in Figure 2. All of the CA-pretreated CS showed highly significant (α < 0.01) improvements (21.39−88.13%) on the methane yield in comparison to untreated CS. Obviously, the cumulative methane yield improved with the increase of the Ca(OH)2 concentration from 1.0 to 2.0% combined with 0.5% KOH.

When the KOH dose increased to 1.0%, the Ca(OH)2 concentration had little influence on methane production. The maximum cumulative methane yield of 288.33 mL gVS−1 was obtained at CA pretreatment of 1.5% KOH and 2.0% Ca(OH)2, which is comparable to that of 2.5% KOH-pretreated CS (291.15 mL gVS−1). That is to say, it is feasible to use 2.0% Ca(OH)2 to reduce KOH to 1.5% to achieve similar results. Another interesting result was obtained at the CA pretreatment of 0.5% KOH and 2.0% Ca(OH)2. The cumulative methane yield was 271.38 mL gVS−1, which was similar to that of 2.0% KOH (268.85 mL gVS−1). It suggested that 2.0% Ca(OH)2 solution could replace 1.5% KOH. A 2.0% Ca(OH)2 combined with 0.5 or 1.5% KOH-pretreated CS only showed 16.95 mL gVS−1 of difference on the cumulative methane yield. The price of KOH is about 13 times higher than that of Ca(OH)2. Considering pretreatment cost and efficiency of AD, 0.5% KOH combined with 2.0% Ca(OH)2 was regarded as an optimum pretreatment condition. 3.4. Kinetics of the Cumulative Methane Yield. The cumulative methane yield was fitted by the modified Gompertz model for further analysis (Figure 3). The parameters obtained from the model are presented in Table 2. For all digestion

Figure 2. Cumulative methane yields of untreated and CA-pretreated CS.

Figure 3. Kinetics of cumulative methane yields after different pretreatments. 5843

DOI: 10.1021/acs.energyfuels.5b01170 Energy Fuels 2015, 29, 5841−5846

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Energy & Fuels Table 2. Kinetic Parameters of the Modified Gompertz Model modified Gompertz model CS

B0

μm

λ

R2

EMY (mL gVS−1)

TMY (mL gVS−1)

Bd (%)

untreated 2.0% KOH 2.5% KOH 0.5% KOH + 2.0% Ca(OH)2 1.5% KOH + 2.0% Ca(OH)2

167.23 257.95 275.45 259.91 276.06

7.43 32.11 35.04 22.52 27.78

1.65 −0.18 0.20 0.17 0.17

0.998 0.982 0.982 0.985 0.985

153.26 268.85 291.15 271.38 288.33

431.23 431.23 431.23 431.23 431.23

35.54 62.35 67.52 62.93 66.86

Table 3. Composition (% Dry Basis) of CA-Pretreated CS and Indicators after AD alkaline loadinga raw CS 0.5% K 0.5% K 0.5% K 1.0% K 1.0% K 1.0% K 1.5% K 1.5% K 1.5% K a

+ + + + + + + + +

1.0% 1.5% 2.0% 1.0% 1.5% 2.0% 1.0% 1.5% 2.0%

Ca Ca Ca Ca Ca Ca Ca Ca Ca

cellulose (%) 40.12 44.40 44.45 42.66 42.59 45.57 44.20 45.97 43.89 44.75

± ± ± ± ± ± ± ± ± ±

1.20 0.37 0.26 1.68 0.36 0.85 1.37 1.55 0.23 1.28

hemicellulose (%) 32.84 12.24 11.59 11.14 10.92 12.05 11.25 11.80 11.08 11.09

± ± ± ± ± ± ± ± ± ±

1.84 0.52 0.51 0.69 0.21 0.42 1.24 0.85 0.41 0.14

lignin (%)

pH

TA (mg of CaCO3 L−1)

TAN (mg L−1)

± ± ± ± ± ± ± ± ± ±

6.98 6.96 6.96 6.97 6.97 6.97 6.98 6.99 6.97 6.93

2150 1900 2100 1950 2025 1825 1850 2000 2100 1750

573 336 285 312 285 321 318 291 315 363

6.10 3.35 1.57 2.41 1.55 1.90 1.77 2.42 2.95 1.78

0.63 0.81 0.06 0.61 0.18 0.23 0.83 1.53 0.74 0.57

K stands for KOH, and Ca represents Ca(OH)2.

Figure 4. SEM images of CS samples at magnification of 800 times: (A) raw CS, (B) 2.0% Ca(OH)2-pretreated CS, (C) 2.0% KOH-pretreated CS, (D) 0.5% KOH + 2.0% Ca(OH)2-pretreated CS.

experiments, correlation coefficients (R2) ranged from 0.982 to 0.998, which indicated that the Gompertz equation was wellfitted to the cumulative methane yield curve in this research. The B0 values obtained from the model seemed to be slightly lower than the experimental methane yield (EMY) at each pretreatment condition. Both KOH- and CA-treated samples showed great enhancements of μm and B0 compared to untreated CS. An enhanced μm represents a higher maximum methane production rate, and

an increased B0 means a higher ultimate methane yield. The lag phase time (λ) is another important indicator for AD. On the basis of the modified Gompertz model, λ significantly decreased after pretreatment, which indicated a faster startup. All of these parameters obtained from the modified Gompertz model collectively suggested a better performance of AD after CA pretreatment. It was also shown that Bd of CS was 62.35, 67.52, 62.93, and 66.86% for 2.0 and 2.5% KOH, combination of 0.5% KOH and 2.0% Ca(OH)2, and combination of 1.5% KOH and 5844

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pretreated CS. As shown in Figure 5, raw CS, CS after SA pretreatment of 2.0% KOH, and CA pretreatment of 0.5%

2.0% Ca(OH)2 pretreatment conditions, respectively. All of them showed highly significant enhancements (α < 0.01) compared to untreated CS (35.32%). Relatively high Bd values implied that CA pretreatment could be a promising method to improve the biogasification efficiency and might be an alternative to single KOH pretreatment in the future. 3.5. Biodegradation Mechanism of CS. 3.5.1. Influence of CA Pretreatment on CS. The compositions of CS after CA pretreatment are shown in Table 3. The content of hemicellulose decreased sharply from 32.84% in the raw CS to about 11% after pretreatment. Hemicellulose removal has proven to be contributing to increase the mean pore size of the substrate and, therefore, improve digestibility of cellulose.26 Lignin removal is another vital goal for pretreatment. The removal of lignin appears to involve changes in physical, microscopic, and morphological properties of the cellulose fibers.19,27 The content of lignin was 6.10% in raw CS and declined to 1.55− 3.35% after CA pretreatment. No obvious change was observed in the cellulose content. Results above indicated that CA pretreatment mainly degraded hemicellulose and lignin. The pH value, TA, and TAN were important indicators on the stability of the anaerobic process and were measured after digestion (Table 3). The pH values of CA-pretreated CS were in the range of 6.93−6.99, which fell in the generally preferred pH range of 6.8−8.2.28 TA indicates the capacity to neutralize acids, which could provide protection against rapid change in the pH value.29 As presented in Table 3, TA concentrations for AD operations ranged from 1750 to 2150 mg of CaCO3 L−1, implying that CA pretreatment did not affect the system stability because TA of a stable digestion normally ranged from 1000 to 3000 mg of CaCO3 L−1.30 The TAN concentration could contribute to the alkalinity and help to maintain the pH value. As shown in Table 3, TAN of raw CS was 573 mg L−1 and decreased to below 363 mg L−1 in CA-pretreated samples; all of the digesters did not appear with ammonia inhibition phenomenon because it was well-known that the digestion will be inhibited when the TAN concentration in the digester is higher than 1700 mg L−1.31 As mentioned above, all of these results contributed to the same conclusion that digesters were stable and proper for AD after CA pretreatment. 3.5.2. SEM Analysis. To visualize the change of the microstructure and morphology, the SEM images of raw CS, 2.0% Ca(OH)2-, 2.0% KOH-, and CA of 0.5% KOH and 2.0% Ca(OH)2-pretreated CS were examined (Figure 4). The raw CS exhibited complete skeletal structure, with a regular and smooth texture. When pretreated with 2.0% Ca(OH)2, the surface of CS became partly broken and some holes appeared. The 2.0% KOH-pretreated and 0.5% KOH combined with 2.0% Ca(OH)2-pretreated CS showed similar SEM images. The surface of CS became uneven, and serious broken crevices could also be observed. The damage was serious enough to expand the surface and loosen the fibers. This phenomenon could be ascribed to the broken down lignin−carbohydrate linkages and dissolution of lignin. By pretreatment, a larger specific surface area of CS was therefore obtained. A larger surface area results in easier cellulose degradation, allowing for a larger interface between CS and microbes during the methanogenesis process. The results proved that CA pretreatment was effective in destroying the microstructure and, consequently, resulted in significantly higher methane yields compared to raw CS. 3.5.3. XRD Analysis. XRD was tested to investigate the changes of cellulose crystalline structures in untreated and

Figure 5. XRD patterns of CS.

KOH and 2.0% Ca(OH)2 existed similar XRD patterns. For lignocellulosic biomass, the CrI implied the relative amount of crystal cellulose,21 which was strongly influenced by biomass composition. The CrI values of CS pretreated by 2.0% KOH and a combination of 0.5% KOH and 2.0% Ca(OH)2 were calculated to be 48.66 and 48.42%, respectively. Both of them were higher than that of raw CS (35.76%). This might be because SA and CA pretreatment mainly removed hemicellulose and lignin, both of which are non-crystalline. Their CrI values were very close, indicating that 2.0% KOH and CA pretreatment of 0.5% KOH and 2.0% Ca(OH)2 achieved similar structural changes. This is a good explanation of the similar cumulative methane yield from 2.0% KOH- and a combination of 0.5% KOH and 2.0% Ca(OH)2-pretreated CS.

4. CONCLUSION This study compared the difference efficiency of AD after single KOH and Ca(OH)2 pretreatment. The feasibility of using Ca(OH)2 to reduce the dosage of KOH to maintain the AD efficiency was investigated. The results showed that single Ca(OH)2 pretreatment could not achieve significant methane improvement compared to KOH, while the combination of 0.5% KOH and 2.0% Ca(OH)2 showed an amazing result that the cumulative methane yield was comparable to 2.0% KOH pretreatment. This condition was also proven to be an optimum, reaching a cumulative methane yield of 271.38 mL gVS−1 and an increased Bd from 35.54 to 62.93%. Morphology and composition changes of CS after pretreatment were proven by SEM and XRD analyses. These results collectively proved that Ca(OH)2 is helpful to reduce the loading of strong but relatively expensive KOH in the pretreatment of CS, contributing to a novel cost-effective method, and might be promising for industrial applications and worthwhile for further study.

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AUTHOR INFORMATION

Corresponding Authors

*C. Chen. Telephone/Fax: +86-10-6444-2375. E-mail: [email protected]. *G. Liu. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5845

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ACKNOWLEDGMENTS This research was supported by the National Hi-tech R&D Program of China (863 Program, 2012AA101803) and the Fundamental Research Funds for the Central Universities (YS1407).

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NOMENCLATURE AD = anaerobic digestion Bd = biodegradability CA = combined alkaline C/N = carbon/nitrogen CS = corn stover EMY = experimental methane yield SA = single alkaline SEM = scanning electron microscopy TA = total alkalinity TAN = total ammonia nitrogen TMY = theoretical methane yield TS = total solids VFA = volatile fatty acids VS = volatile solids XRD = X-ray diffraction

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b01170 Energy Fuels 2015, 29, 5841−5846